j ourna l h omepage: www.elsev ier .com/ locate /ccr
eview
tructural chemistry of uranium phosphonates
eiting Yanga, T. Gannon Parkerb, Zhong-Ming Suna,∗,1
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street,hangchun, Jilin 130022, ChinaDepartment of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306, United States
rticle history:eceived 4 April 2015ccepted 19 May 2015vailable online 30 May 2015
a b s t r a c t
Uranium phosphonates, an important class of actinide-organic coordination polymers, exhibit an excep-tionally diverse and broad range of crystal structures. A variety of structure topologies have beenidentified for hexavalent uranium phosphonates, including cage clusters, chains, ribbons and tubes,sheets, and three-dimensional frameworks. In contrast, only a handful of tetravalent uranium phos-
eywords:raniumhosphonate ligandsrystal structuresecondary building units
phonates are known. This review presents a comprehensive account of the crystal structures of uraniumphosphonates and the various building units (e.g. discrete monomers, polymeric units, infinite chains,and sheets) that result from the numerous coordination modes of phosphonate ligands with uranium.
Uranium is the heaviest naturally occurring element, and it is theost thoroughly studied of the actinides. As the main component
n nuclear fuel, it plays essential roles in the generation of elec-ricity in nuclear power plants and the manufacturing of nucleareapons. Elemental uranium is chemically active and it is easily
xidized to various oxidation states, from +2 (which is exceedinglyare in the solid state and nonexistent in solution) up to +6 (theost common oxidation state in nature), each of which is asso-
iated with characteristic coordination environments. It can form variety of compounds through chemical interactions with inor-anic species and organic ligands. Inorganic uranium compounds,uch as uranium minerals and oxides, fluorides, silicates, sulfates,hosphates, vanadates, and borates in synthesized phases, haveeen thoroughly investigated [1–4]. These materials are criticallyelevant to mining and extraction processes, to separations pro-edures, and to the long term storage of nuclear fuels [5,6]. Onhe other hand, organic ligands bearing O-donor and N-donor sitesxhibit high binding affinity for uranium ions, thereby resulting inarious uranium bearing hybrid materials. In particular, uraniumarboxylates and phosphonates, which exhibit diverse structuresnd intriguing chemical and physical properties, have been pre-ared using a number of synthetic methodologies, and many ofhese materials have been structurally characterized using singlerystal X-ray diffraction. A few excellent reviews written by Loiseaut al. [7], Wang and Chen [8], Cahill and coworkers [9–11] andlearfiled and coworkers [12] cover a range of topics on the crystalhemistry of uranium coordination polymers and uranium hybridaterials.As is the case with most other uranium materials, U(VI) phos-
honates are the most prevalent in the literature due to thetability of the hexavalent state of uranium under ambient condi-ions. Only six U(IV) phosphonates and one mixed-valent U(IV/VI)
ethylenediphosphonate are known. To our knowledge, no ura-ium phosphonates with other valences have been reported. Therevailing uranium species is the uranyl dioxocation (UO2
2+),hich is comprised of a central U(VI) ion axially coordinated by
wo kinetically inactive oxo atoms. This axial coordination motifestricts coordination to the equatorial plane, which is capablef supporting 4–6 donor atoms. Since coordination of the uranylation is limited to the equatorial plane, square bipyramidal,entagonal bipyramidal, and hexagonal bipyramidal geometriesre typically observed in U(VI) compounds, and these can fur-her polymerize to form a variety of polynuclear uranyl species.hese diverse geometries, together with their numerous possibleligomers, allow uranyl phosphonates with unique structures toe generated. The phosphonate ligand, R-PO3H2, with its spher-
cal phosphonate group and three possible states of protonation,xhibits flexible ligating abilities for binding metal ions. Further-ore, the modulation and adjustment of the organic residue of
hosphonate ligands enables the resulting compounds to have richtructural diversities. Readers are encouraged to read the contrib-tions on metal phosphonate chemistry by Clearfield and Demadis13], Shimizu et al. [14] and Mao [15] in which uranium phospho-ates have also been covered or reviewed briefly.
The first uranium phosphonate, synthesized with (hydrox-methyl)phosphonic acid, was reported in 1986 [16]. In the 1990s,learfield and coworkers [17–22] reported a series of uraniumhenylphosphonates with chain or tube structures. After that, theccurrence of uranium phosphonates was sporadic [23,24]. Theumber of reported uranium phosphonates has skyrocketed due
n part to innovative synthetic strategies, which include modula-ion of the phosphonate ligand, incorporation of transition metals,ddition of co-ligands, and introduction of structure-directinggents. Various alkylphosphonate and arylphosphonate ligands
ry Reviews 303 (2015) 86–109 87
with different functionalities including hydroxyl, halogen, andcarboxylate moieties have been utilized. Heterometallic speciesinclude 3d, 4f, and 5f metals, and they contribute to not only therich structure geometries, but also intriguing properties. Shorthandnotations for the phosphonate ligands are given in Scheme 1. Vari-ous structure-directing agents including metal cations, ammoniumcations, hydronium cations, and even ionic liquids and crown ethershave been used in these syntheses. Aromatic amines can also serveas templates and co-ligands to direct the resulting structures. Theirabbreviations are reported at the beginning of the review. In 2012,Knope and Cahill [11] summarized structural chemistry of uranylphosphonate materials and described some representative exam-ples. After that, the number of uranium phosphonates increased bynearly four times. More importantly, U(IV) phosphonates, whichhave not been reviewed previously, will be discussed in this paper.This review thus comes at a befitting time in the development ofuranium phosphonate chemistry.
A key factor that is responsible for the oligomerization of UO22+
species is hydrolysis. It mainly depends on the pH value of thesolution and the concentration of UO2
2+. In general, at low pH(<3) and/or low U(VI) concentration (<10−3 M), the monomericspecies are predominant in solution. With increasing pH and/orU(VI) concentration, the UO2
2+ ions tend to hydrolyze and polymer-ize, leading to the formation of polynuclear uranyl species. Besides,the temperature can also influence the hydrolysis by affecting thesolution pH and ionic strength. Detailed discussions on this topiccan be found in the excellent review written by Loiseau et al. [7],thus we will not discuss it further.
Three bipyramidal coordination geometries of uranium(VI)species have been observed for known uranyl phosphonates. Ofthese, the pentagonal bipyramid is the most commonly encoun-tered (79%), then the square bipyramid (23%) and finally thehexagonal bipyramid (9%). Apart from serving as primary build-ing blocks in uranyl phosphonates, these bipyramidal polyhedracan also polymerize via corner-sharing and/or edge-sharing inter-actions, leading to various building units including two types ofdimers, one trimer, three kinds of tetramers, one octamer, six typesof rings, five kinds of chains, and one sheet (Fig. 1). These poly-merized uranyl species may act as secondary building units (SBUs)toward the construction and syntheses of uranyl phosphonates.Interestingly, no combination of square and hexagonal bipyramidshas been observed. Detailed investigations of reported uranyl phos-phonates reveal that about 50% adopt discrete mononuclear units,16% contain dimeric SBUs, and 14% consist of two kinds of SBUsoccurring in the same structure. Few examples feature higher thantrimeric SBUs. Indeed, most of these SBUs have been observed inU(VI) minerals and inorganic compounds as well as in carboxyl-ates. Only the tetrameric SBUs, one type of chain and the sheet areunique for U(VI) phosphonates. While several U(IV) phosphonateshave been reported, only four coordination motifs – UO6, UO7, UO8,and UO9 polyhedra – have been identified (Fig. 2).
In this contribution, a comprehensive and updated overview ofthe structural chemistry of uranium phosphonates, including themajor U(VI) compounds, several U(IV) compounds, and one mixedU(IV/VI) compound, is given. Various molecular structures andextended networks of uranium phosphonates are presented here,most of them synthesized by hydrothermal methods under acidicconditions (pH < 3). The classification of different uranyl phospho-nates is based on the rigidity and attached carboxylate functionalgroups of the phosphonate ligands. Firstly homometallic uranylalkylphosphonates (Section 2) and arylphosphonates (Section 3)are summarized, and then uranyl carboxyphosphonates (Section
4). Heterometallic uranyl phosphonates will be presented in Sec-tion 5 followed by tetravalent uranium phosphonates in Section 6.The rearranged chemical formulae are used in the text and tablesto facilitate comparisons. In the tables, the compounds are listed
88 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
for co
iwwplp
2
papmiaoatTcacbtias
Scheme 1. Phosphonate ligands used
n the order of increasing dimensionality, from clusters to frame-orks, and those containing similar SBUs are grouped. This paperill primarily deal with the coordination chemistry of uranium andhosphonic acids. Thus discussions of local structural metrics (bond
engths and angles) will be omitted. Compounds of uranium withhosphonate ester ligands will not be covered.
. Uranyl compounds with aliphatic phosphonate ligands
The aliphatic species used in the syntheses of uranyl phos-honate compounds include linear alkyl and cyclic phosphoniccids. The former subclass is prolific and comprises monophos-honic and diphosphonic acids. The most common linearonophosphonate ligands are methylphosphonic acid (MeP) and
ts halogen- and hydroxyl-substituted derivatives, CH2ClPO3H2nd CH2(OH)PO3H2. Five low-dimensional structures have beenbtained using these ligands (Table 1). The diphosphonate ligandsre based on saturated alkyl chains (C1–C3) with each end func-ionalized by a phosphonate group capable of binding uranyl ion(s).hirty-nine compounds with various structures ranging from cagelusters to frameworks have been prepared due to the flexibilitynd functionalization of the alkyl chain backbone (Table 2). The onlyyclic phosphonate ligand features triazacyclononane as the back-one with three CH2PO3H2 groups. It is incorporated into a uranyl
riphosphonate sheet structure. The applied synthetic strategiesnclude typical hydrothermal methods, ionothermal methods, andqueous solution processes. In addition, various N-bearing organicpecies (amines, pyridines, imidazoles, and ionic liquids) have been
nstruction of uranium phosphonates.
used to direct the assemblies by serving as structure-directingagents and/or counter-ions.
2.1. Uranyl compounds with aliphatic monophosphonate ligands
Methylphosphonic acid is the simplest organic phosphonateligand, and it has been used for the syntheses of three uranylcompounds [23,25]. All three uranyl methylphosphonates havelayered structures and incorporate uranyl pentagonal bipyramids.In (H2pip)[(UO2)(MeP)(HMeP)]2, mononuclear UO2O5 polyhedraare linked by methylphosphonate ligands to form the layeredassembly, the negative charge of which is compensated by proton-ated piperazine cations (Fig. 3a). In another piperazine-templateduranyl methylphosphonate (H2pip)[(UO2)F(MeP)]2, a chain SBU isformed by corner-sharing uranyl pentagonal bipyramids (Fig. 3b).In the neutral compound (UO2)(MeP) [25], the uranyl pentagonalbipyramids share edges to condense into another type of chain SBU(Fig. 3c).
By substituting one methyl H atom on this ligand with a Clatom or an OH group, different structures can be obtained. Com-plexation of uranyl with (chloromethyl)phosphonic acid resultsin the formation of the layered structure (UO2)(PO3CH2Cl) [26],which bears similarities to (UO2)(MeP) [25]. When (hydrox-ymethyl)phosphonic acid is used as the ligand, a chain structure of
[(UO2)(H2O)2(PO3CH2OH)]2·6H2O is formed [16] in which the iso-lated uranium center is in a seven-coordinate environment definedby two oxo atoms, three oxygen atoms from the ligands, and twocoordinated water molecules.
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 89
Fig. 1. Various uranyl SBUs in reported uranium(VI) phosphonates.
Fig. 2. UO6, UO7, UO8 and UO9 coordination polyhedra in tetravalent uranium phosphonates.
Table 1List of uranyl compounds with aliphatic monophosphonate ligands.
90 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
Table 2List of uranyl compounds with aliphatic diphosphonate ligands.
Compounds Structure features Uranyl SBUs Ref.
[(UO2)18(O2)18(OH)2(MDP)6(P2O7)2]34− Cage cluster UO2O6 edge-sharing double 5-membered ring and 10-membered ring [27][(UO2)24(O2)24(MDP)12]48− Cage cluster UO2O6 edge-sharing 4-membered ring [27](UO2)(H2O)(H2MDP)·nH2O (n = 2, 4) 1D UO2O5 monomer [28](H2en)2[(UO2)2(H2O)2(MDP)2]·0.5H2O 1D UO2O5 monomer [29](Hphen)[(UO2)(H2O)(HMDP)] 1D UO2O5 monomer [29]Ba2[(UO2)2F4(MDP)]·5.75H2O 1D UO2O3F2 monomer [30](Hbipy)[(UO2)(H2O)(HMDP)] 2D UO2O5 monomer [29](H2DACBO)[(UO2)2(HMDP)2]·2H2O 2D UO2O5 monomer [29]Ag[(UO2)(HMDP)] 2D UO2O5 monomer [31]Ag2[(UO2)(MDP)] 2D UO2O5 monomer [31]NH4[(UO2)(H2O)(HMDP)]·3H2O 2D UO2O5 monomer [28]Ba[(UO2)(MDP)]·1.4H2O 2D UO2O4 monomer [30](UO2)2(H2O)3(MDP)·H2O 2D UO2O4 and UO2O5 monomer [32][Ag2(H2O)1.5][(UO2)2F2(MDP)]·0.5H2O 2D UO2O3F2 edge-sharing dimer [31]Ba3[(UO2)4F6(MDP)2]·6H2O 2D UO2O3F2 edge-sharing dimer and UO2O3F2 monomer [30](UO2)(H2O)(H2MDP) 3D UO2O5 monomer [32](H2en)[(UO2)(MDP)]·H2O 3D UO2O5 monomer [33](UO2)(H2O)(H2MDP)·H2O 3D UO2O5 monomer [28](NEt4)K[(UO2)3(H2O)2(MDP)2]·1.5H2O 3D UO2O5 edge-sharing dimer and UO2O5 monomer [33]U16(hedp)8 Cage cluster UO2O6 edge-sharing 4-membered ring and 8-membered ring [34]U16(hedp)8P4 Cage cluster UO2O6 edge-sharing 4-membered ring and UO2O5 edge-sharing dimer [34]U20(hedp)10 Cage cluster UO2O6 edge-sharing 5-membered ring and 10-membered ring [34]U24(hedp)12 Cage cluster UO2O6 edge-sharing 4-membered ring [34]U40(hedp)20 Cage cluster UO2O6 edge-sharing 4-membered ring [34]U64(hedp)32 Cage cluster UO2O6 edge-sharing 4-membered ring [34](Hbpi)[(UO2)(H2O)(Hhedp)]·H2O 1D UO2O5 monomer [35](Hbpi)[(UO2)(H2O)(Hhedp)] 2D UO2O5 monomer [35](H2dib)0.5[(UO2)(H2O)(Hhedp)] 2D UO2O5 monomer [35](UO2)(H2O)(H2hedp)·2H2O 2D UO2O5 monomer [35](H2bipy)[(UO2)3(H2O)4(hedp)2]·2H2O 3D UO2O5 monomer [35](H3O)2[(UO2)3(H2O)3(hedp)2]·2H2O 3D UO2O5 monomer [35](H2bbi)0.5[(UO2)(H2O)(Hhedp)] 2D UO2O5 monomer [36](UO2)(H2O)(H2EDP) 2D UO2O5 monomer [37](NEt4)2[(UO2)3(HEDP)2(H2EDP)]·4H2O 2D UO2O4 monomer [37](H2bipy)[(UO2)(EDP)] 2D UO2O4 monomer [37](H2DABCO)2[(UO2)5(EDP)2(HEDP)2]·2H2O 3D UO2O4 monomer [38](Hphen)2[(UO2)2(H2EDP)3] 3D UO2O5 monomer [37](Bmim)[(UO )(HPDP)] 2D UO2O monomer [39]
UO2O
2
ipfcs(tbtrScu4t(
tcfifTisa
2
(Bmim)2[(UO2)5(HPDP)4] 3D
.2. Uranyl compounds with aliphatic diphosphonate ligands
Methylenediphosphonic acid (MDP) has been intensively stud-ed for its utilization in the construction of various uranylhosphonate structures including clusters, chains, layers, and evenrameworks [27–33]. It is noted that the clusters in this sub-lass contain peroxo units which are largely responsible for thetructures adopted [27]. [(UO2)18(O2)18(OH)2(MDP)6(P2O7)2]34−
U18Py2PCP6) contains 18 uranyl hexagonal bipyramids. Eight ofhese are located toward the top of the cluster forming dou-le 5-membered rings through edge-sharing interactions, anden are located on the bottom of the cluster in a 10-membereding with two pyrophosphate groups in the central regions.ix MDP groups connect the top and bottom portions of thisluster (Fig. 4a). In [(UO2)24(O2)24(MDP)12]48− (U24PCP12), fourranyl bipyramids share edges defined by peroxide, giving a-membered ring. Six of these 4-membered rings are linkedhrough MDP groups, giving a total of 24 bipyramids in the clusterFig. 4b).
The second family of uranyl methylenediphosphonates fea-ures three types of 1D structures [28–30], all of whichontain isolated uranium centers in pentagonal bipyramids. Therst example adopts a trinuclear uranyl phosphonate motif
ormed by one uranyl bipyramid and one MDP ligand [28].
he ligand binds three uranium atoms in a bis-bidentate fash-on through four phosphonate oxygen atoms (Fig. 5a). In theecond example, (H2en)2[(UO2)2(H2O)2(MDP)2]·0.5H2O (Uethyl)nd (Hphen)[(UO2)(H2O)(HMDP)] (Uphen) [29], a similar uranyl
4
5 edge-sharing tetramer and UO2O4 monomer [39]
phosphonate motif has been isolated, but different linear arrange-ments form by different connections between the motifs (Fig. 5b). Inthis structure, tetradentate MDP connects two uranyl centers, andthe anionic chain crystallizes with protonated ethylenediamine or1,10-phenanthroline as counter-cations. The last illustration cor-responds to a uranyl oxyfluoride ribbon [30], in which UO2O5F2pentagonal bipyramidal monomers are linked together by MDPgroups. Each MDP ligand bridges and chelates four uranium atoms.
Due to the nature of the uranyl dication, layered assem-blies dominate the structures of uranyl methylenediphosphonates[28–32]. Mononuclear uranyl pentagonal bipyramids are involvedin three types of layered structures, and all of them con-tain the typical trinuclear uranyl phosphonate motifs [28,29,31].Another 2D example, Ba[(UO2)(MDP)] (Ba-1), exhibits isolatedUO2O4 square bipyramids bridged by MDP ligands (Fig. 10a)[30]. The structure of (UO2)2(H2O)3(MDP)·H2O is interestingin that the uranyl centers occur in both square bipyramidaland pentagonal bipyramidal geometries [32]. Each diphospho-nate linker binds four UO2O4 polyhedra to form corrugatedsheets where the remaining two phosphonate oxygen atomschelate the uranyl pentagonal bipyramids. A special feature ofthis pentagonal bipyramidal motif is that it contains three coor-dinating water molecules in the equatorial plane. In layeredstructures of [Ag2(H2O)1.5][(UO2)2F2(MDP)]·0.5H2O (Ag-2) [31]
and Ba3[(UO2)4F6(MDP)2]·6H2O (Ba-2) [30], edge-sharing dimersof uranyl centers share two bridging F atoms. In Ag-2, these uranyldimers are chelated and cross-linked by phosphonate moietiesto form anionic sheets with Ag+ cations and water molecules
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 91
Fig. 3. The layered structures (and corresponding simplified graphical representations) of (H2pip)[(UO2)(MeP)(HMeP)]2 (a) [23], (H2pip)[(UO2)F(MeP)]2 (b) [23], and(UO2)(MeP) (c) [25]. Color codes for all the figures in this review: green, U; magenta, P; gray, C; red, O; blue, N; purple, F; the remaining one corresponding to the heterometalatom.
y2PCP
ottl
Fs
Fig. 4. Cage clusters of U18P
ccupying the interlayer space. In Ba-2, both the uranyl dimers andhe UO2O3F2 monomers are chelated by the diphosphonate ligandso form zigzag sheets. Ba2+ cations occupy the space between theayers and balance the charge (Fig. 6).
ig. 5. (a) The chain and simplified graphical representation in (UO2)(H2O)(H2MDP)·nH2Oimplified graphical representation of Uethyl and Uphen contain the motifs [29].
6 (a) and U24PCP12 (b) [27].
In addition to 1D and 2D architectures, 3D framework struc-tures involving MDP ligands have also been reported that exhibitfour structural varieties [28,32,33]. Three of them employ the com-mon trinuclear uranyl phosphonate motifs. It is worth mentioning
formed by the trinuclear uranyl phosphonate motifs [28]. (b) Chain structure and
92 W. Yang et al. / Coordination Chemist
Fa
t[tc1ots[upbbct
pwtaafceoab
F
ig. 6. The uranyl methylenediphosphonate layer incorporating uranyl monomersnd dimers in Ba3[(UO2)4F6(MDP)2]·6H2O [30].
hat among them, the chiral structure (H2en)[(UO2)(MDP)]·H2O33] features multi-dimensional channels along the [1 0 0] direc-ion with the pore size of 1.1 nm × 0.6 nm and another nanoporoushannel in the [0 0 1] direction with the measured size of.0 nm × 1.0 nm (Fig. 7). These channels are occupied by dis-rdered protonated ethylenediamine cations. When using theetraethylammonium cation as the template, another chiral poroustructure, (NEt4)K[(UO2)3(H2O)2(MDP)2]·1.5H2O, was prepared33]. In this structure, the uranium atoms appear as monomericranyl pentagonal bipyramids and dimers of edge-sharing uranylentagonal bipyramids. The uranyl monomers are linked togethery the diphosphonate ligands to form sheets. The sheets are furtherridged by the uranyl dimers to produce a 3D framework, whichontains large channels with pore sizes of 1.0 nm × 1.1 nm alonghe b axis.
The methylene group of methylenediphosphonic acid allows forossible modification by replacement of the two hydrogen atomsith functional groups. By adding hydroxyl and methyl groups
o the methylene carbon, (1-hydroxyethane-1,1-diyl)diphosphoniccid (hedp) is formed, and it is has been used for the preparation of
series of uranyl diphosphonates ranging from cage clusters to 3Dramework structures. Burns and coworkers [34] reported six cagelusters built by 16–64 uranyl ions and 8–32 phosphonate link-rs. The structure of U (hedp) contains two 4-membered rings
16 8f uranyl peroxide hexagonal bipyramids located at the ends ofn elongated cage and an 8-membered ring of uranyl hexagonalipyramids encircling the equatorial region. These uranyl rings are
ig. 7. A view of the channels of (H2en)[(UO2)(MDP)]·H2O along the c axis [33].
ry Reviews 303 (2015) 86–109
bridged by hedp ligands. The cluster U20(hedp)10 displays struc-tural similarities to U16(hedp)8, but it consists of two 5-memberedcapping rings of uranyl bipyramids and one 10-membered equato-rial ring. U16(hedp)8P4 is similar to U16(hedp)8 in the polar regionsof the cluster with 4-membered uranyl rings. However, in the equa-torial region of U16(hedp)8P4, four edge-sharing dimers of uranylpentagonal bipyramids are linked through PO4 tetrahedra. Clus-ters U24(hedp)12, U40(hedp)16, and U64(hedp)32 form a series oftopologically related clusters of increasing size, and all of them arebuilt from 4-membered rings of uranyl peroxide bipyramids. Clus-ter U24(hedp)12 has the same topology as U24PCP12 [27], exceptthat the bridging phosphonate linkers are different.
Significant structural correlations can be observed withextended uranyl diphosphonate compounds prepared with MDPand hedp ligands. Most of them contain the trimeric uranyl phos-phonate motif and some have the same structure topology. Forinstance, the chain structure of (Hbpi)[(UO2)(H2O)(Hhedp)]·H2O(UP-1) [35] is similar to that in Uethyl and Uphen [29](Fig. 5b). (Hbpi)[(UO2)(H2O)(Hhedp)] (UP-2), (H2dib)0.5[(UO2)(H2O)(Hhedp)] (UP-3), and (H2bbi)0.5[(UO2)(H2O)(Hhedp)](UP-7) [35,36] possess the same layered topologies as that in(Hbipy)[(UO2)(H2O)(HMDP)] [29]. The protonated organic speciesbpi, dib, and bbi exist between the layers of UP-2, UP-3, and UP-7,respectively, leading to different interlayer distance of 7.1, 3.4, and3.0 A (Fig. 8). (H2bipy)[(UO2)3(H2O)4(hedp)2]·2H2O (UP-5) [35]incorporates a heptanuclear uranyl phosphonate motif with a U:Pratio of 3:4, which consists of three uranyl pentagonal bipyramidschelated and bridged by two hedp ligands. These heptanuclearuranyl phosphonate motifs are connected with each other to formthe whole framework with channels along the a axis (Fig. 9a). In theabsence of organic templates, (UO2)(H2O)(H2hedp)·2H2O (UP-4)and (H3O)2[(UO2)3(H2O)3(hedp)2]·2H2O (UP-6) were obtainedby adjusting the pH of the reaction mixture [35]. The structureof UP-4 also features the trinuclear uranyl phosphonate motifsbut forms a different layer from UP-2. The framework of UP-6also exhibits the same heptameric motif as those in UP-5. Themotif adopts a boat-configuration, while in UP-5 it appears as achair-configuration (Fig. 9). Two types of channels along [0 0 1]direction are formed in UP-6. The opening aperture of the largechannels is approximately 5.3 A × 6.0 A, whereas the smaller oneis about 2.0 A × 2.0 A (Fig. 9b). The H3O+ cations located in thechannels can be selectively exchanged by monovalent cations.
By increasing the length of the alkyl chain, flexible 1,2-ethylenediphosphonic acid (EDP) is produced, which serves asthe linker for the construction of five uranyl phosphonatesincluding three sheets and two 3D frameworks [37,38]. In(UO2)(H2O)(H2EDP) (EDP-U1), the EDP ligand is doubly proton-ated and chelates three uranyl pentagonal bipyramids, forminga layered structure. (NEt4)2[(UO2)3(HEDP)2(H2EDP)]·4H2O (EDP-U2) and (H2dipy)[(UO2)(EDP)] (EDP-U3) possess similar layeredarrangements, which consist of uranyl square bipyramids andtetradentate EDP linkers (Fig. 10b). NEt4
+ cations and protonated4,4′-bipyridine cations occupy the interlayer space and balance thecharge. The 2D structure can be seen as chains of uranyl squarebipyramids and bridging phosphonate moieties linked by ethylenechains. These sheets are similar to Ba-1 [30], in which the bridgingligand is MDP (Fig. 10a). (H2DABCO)2[(UO2)5(EDP)2(HEDP)2]·2H2O(EDP-U5) also incorporates isolated uranyl square bipyramidswhich are connected by pentadentate EDP ligands to form a 3Dporous framework with channels filled by protonated DABCOcations. (Hphen)2[(UO2)2(H2EDP)3] (EDP-U4) also features a 3Dframework topology with large elliptical channels along the c-axis
(1.3 nm × 1.1 nm) (Fig. 11). Protonated phen molecules occupy thechannels and serve as templating counter-ions. In this compound,U(VI) adopts a seven-fold coordination mode, and the EDP ligandsserve as bidentate and tridentate linkers for the uranyl centers.
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 93
ted by
ff[t([mt(eiga(es
Fig. 8. The uranyl diphosphonate layers are separa
1,3-Propylenediphosphonic acid (PDP) has also been utilizedor the isolation of uranyl phosphonate compounds [39]. Differingrom the typical hydrothermal synthesis of uranyl phosphonates,Bmim][(UO2)(HPDP)] was obtained under ionothermal condi-ions using the ionic liquid 1-butyl-3-methylimidazolium chloride[Bmim][Cl]) as the solvent. This structure is 2D and similar to Ba-130] and EDP-U2 [37] in that uncommon uranyl square bipyra-
ids are bridged by phosphonate groups to form chains, buthe bridging linker is a propylene group in [Bmim][(UO2)(HPDP)]Fig. 10c) instead of a methylene group as in Ba-1 [30] or anthylene group as in EDP-U2. The Bmim counter-cations residen the interlayer space between the sheets. When an analo-ous reaction was carried out using the hydrothermal method,
3D framework structure of [Bmim]2[(UO2)5(HPDP)4] formedFig. 12). The structure contained uranium atoms in tetramers ofdge-sharing uranyl pentagonal bipyramids and isolated uranylquare bipyramids. The tetramers are linked by PDP ligands to
Fig. 9. The 3D uranyl phosphonate structure of UP-5 with it
different imidazoles in UP-2, UP-3, and UP-7 [35].
form sheets, which are further linked together by UO2O4 moi-eties, ultimately resulting in a 3D porous structure. The pore sizesmeasure 7.5 A × 12.1 A and the channels are occupied by Bmimcations.
1,4-Butylenediphosphonic acid (BDP) was also used for thepreparation of uranyl compounds, but only bimetallic zinc uranyldiphosphonates have been observed. These structures will be dis-cussed in Section 5.1.
Unlike the commonly used linear aliphatic phosphonic acids,
((1,4,7-triazonane-1,4,7-triyl)tris(methylene))triphosphonic acid(nopt) features triazacyclononane as the backbone, which isfunctionalized with three CH2PO3H2 groups. It is the onlytriphosphonic acid that has been successfully used for the
s chair motif (a) and UP-6 with its boat motif (b) [35].
94 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
Fi[
s[ptmt(
3
ip
The most common and simplest aromatic phosphonic acidis phenylphosphonic acid (PhP), which was among the first lig-
ig. 10. Similar layered structures formed by uranyl square bipyramids andncreasingly long diphosphonate ligands in Ba-1 (a) [30], EDP-U2 (b) [37], andBmim][UO2(HPDP)] (c) [39] as well as the identical graphical representation (d).
ynthesis of uranyl phosphonates. In 2006, Zheng and coworkers24]repared a layered structure, (UO2)(H4nopt)·H2O, by usinghis ligand. In this structure, the isolated uranyl pentagonal bipyra-
ids and PO3H moieties form infinite chains which are joined byriazacyclononane groups resulting in an overall sheet topologyFig. 13).
. Uranyl compounds with aromatic phosphonate ligands
Phosphonates with an aromatic backbone represent anothermportant class of ligands for the preparation of uranyl phos-honate compounds. These ligands are typically phenyl or benzyl
Fig. 11. 3D framework structure with channels in EDP-U4 [37].
Fig. 12. 3D framework structure of [Bmim]2[(UO2)5(HPDP)4] showing channelsalong the a axis. Bmim cations have been omitted for clarity [39].
moieties functionalized with phosphonate groups. They aredivided into monophosphonates (phenylphosphonic acid (PhP)and benzylphosphonic acid (PhMeP)) and diphosphonates (1,4-phenylenediphosphonates (bbp), 1,2-phenylenediphosphonates(o-bbp) and 4,4′-biphenyl-bis-phosphonic acid (bpbp)). As shownin Table 3, all of the twelve reported uranyl monophosphonatesin this section are low-dimensionality structures. It is notablethat interesting phase transitions occur among the 1D uranylphenylphosphonate structures. For uranyl phenylenediphospho-nates (Table 4), pillared structures are favored by using the rigidphenyl ring as spacers to separate the uranyl partial structures,which often contain similar uranyl phosphonate motifs.
3.1. Uranyl compounds with aromatic monophosphonate ligands
ands used for the synthesis of uranyl hybrid materials. In the
Fig. 13. The sheet topology present in (UO2)(H4nopt)·H2O [24].
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 95
Table 3List of uranyl compounds with aromatic monophosphonate ligands.
990s, Clearfield and coworkers [17–22] reported a series of uranylhenylphoshonates with tubular and linear chain structures. Theyxhibit fascinating phase transitions, the prime example of whichs (UO2)(PhP)·0.7H2O (�-UPP), in which six chains of edge-sharingranyl pentagonal bipyramids are connected by the PO3 moietieso form tubes (Fig. 14). The phenyl groups of the PhP ligandsre located on the outside of the tubes, leading to isolated chan-els with a calculated diameter of 7.3 A. The tubular �-phase,UO2)3(PhP)2(HPhP)2·H2O (�-UPP), is constructed by edge-sharingranyl pentagonal bipyramid-based chains, isolated UO2O6 hexag-nal bipyramids, and phosphonate moieties. The phenyl groups arerranged on the periphery of the channels and maintain the tubulartructure by pi stacking interactions. Heating this phase to 170 ◦Cields another nanotubular isomer, C-- -UPP, which has a structurehat is very similar to �-UPP, but it consists of closer packing of theubes. The phase transition between the two phases is reversible.nterestingly, when the two phases were refluxed in water, �-UPP
as formed.The linear chain uranyl phenylphosphonate [(UO2)(H2O)
HPhP)2]2·8H2O (�-UPP) was prepared in aqueous solution. In this
tructure, the PO3H moieties bridge the adjacent UO2O5 polyhedra,nd the phenyl groups adopt a cis conformation by being nearlyerpendicular to the chains. Under moderate humidity, this non-
uminescent �-UPP can transform into a luminescent �-UPP phase
able 4ist of uranyl compounds with aromatic diphosphonate ligands.
Compounds Structure features Uranyl SBU
(NEt4)[(UO2)(H2O)(H2.5bbp)2] 1D UO2O5 monCs3.62H0.38[(UO2)4F2(bbp)(H2bbp)3] Tubular UO2O5 cornCs[(UO2)(Hbbp)] 2D UO2O4 monAg2[(UO2)(H2bbp)2] 2D UO2O4 mon(UO2)(H2O)(H2bbp)·H2O 2D UO2O5 mon(H2phen)[(UO2)(bbp)] 2D UO2O5 edg[(H3O)12C4][(UO2)(H1.5bbp)]2 2D UO2O5 edg(H3O)4[(UO2)4F4(bbp)2]·H2O 2D UO2O3F2 ed[(H3O)@18C6][(UO2)2F(H2bbp)2]·2H2O 2D UO2O4F cor[K@18C6][(UO2)2F(H2bbp)2]·4H2O 2D UO2O4F cor[(H3O)@15C5][(UO2)2F(H2bbp)2]·4H2O 2D UO2O4F cor(UO2)4(H2O)(Hbbp)2(H2bbp)·5H2O 2D UO2O5 corn[(H3O)12C4][(UO2)3(H1.5bbp)2(H2bbp)] 3D UO2O4 mon(Emim)2[(UO2)2(H2bbp)3]·2H2O 3D UO2O5 mon(Hbipy)2[(UO2)2(H2bbp)3]·2H2O 3D UO2O5 mon(H2temp)[(UO2)2(H2bbp)3]·2H2O (temp = pip, DABCO) 3D UO2O5 mon[(UO2)(H2bbpF2)(H2O)]2·H2O 3D UO2O5 mon[Sr(H2O)3][(UO2)2(OH)2(H2O)(bbp)]·3H2O 3D UO2O5 edg(H2temp)[(UO2)3(bbp)2]·H2O (temp = bipy, phen) 3D UO2O5 edg(H3O)2[(UO2)6(H2O)2(bbp)(Hbbp)2(H2bbp)2] 3D UO2O5 edg[Ba(H2O)3][(UO2)3(H2O)(bbp)2]·5H2O 3D UO2O5 edg(NMe4)[(UO2)3F(H2O)(Hbbp)2]·0.5H2O 3D UO2O4F ed(NEt2Me2)[(UO2)3F(H2O)(Hbbp)2] 3D UO2O4F ed(NEtMe3)[(UO2)3F(H2O)(Hbbp)2] 3D UO2O4F ed(UO2)4(H2O)(o-bbpH)2(o-bbpH2)·5H2O 2D UO2O5 edg(UO2)(H2O)(o-bbpH2)·H2O 2D UO2O5 monA2[(UO2)2F(Hbpbp)(H2bpbp)]·2H2O (A = Cs+, Rb+) Tubular UO2O4F ed
-sharing dimer and UO2O5 monomer [36]rner-sharing and edge-sharing chain and UO2O3F2 monomer [23]-sharing dimer and UO2O4 monomer [23]
by undergoing a bond breaking/reformation procedure. In �-UPP,the phenyl groups are arranged in a trans conformation in the oppo-site direction of the uranyl phosphonate chains. In the presence ofalkali cations in solution, both �-UPP and �-UPP are transformedto the tubular �-UPP phase (Fig. 14).
Three chain structures were synthesized by changing thesolvent. The reaction of uranyl cations with phenylphospho-nic acid in ethanol produced UO2(HPhP)2·2CH3CH2OH, whichincorporates UO2O4 square bipyramids bridged by phospho-nate ligands [18]. Reaction in the ionic liquid [Bmim][Cl] ledto [Bmim][(UO2)(PhP)(HPhP)] and [Bmim]2[(UO2)4Cl4(PhP)3][39]. The former possess a chain structure similar to(UO2)(HPhP)2·2CH3CH2OH [18]. The latter can be viewed aschains of the former decorated by UO2O2Cl2 polyhedra. In bothof the structures, Bmim cations occupy the void space betweenparallel chains and balance the charge.
In addition to 1D topologies, three anionic sheet structureswere also isolated using this ligand in the presence of tib, bpiand pip, respectively, which are located in the interlayer spaceand balance the charge (Fig. 15). In (H2tib)[(UO2)3(PhP)4]·2H2O
(UPhP-tib) (Fig. 15a) [40], two of the three unique uranium cen-ters adopt pentagonal bipyramidal geometries, which form adimer via edge-sharing interactions. These dimers are bridgedand chelated by phosphonate ligands, resulting in a chain
e-sharing dimer [44]e-sharing dimer and UO2O4 monomer [48]e-sharing tetramer and UO2O5 monomer [47]e-sharing dimer and UO2O5 monomer [44]ge-sharing and corner-sharing tetramer [41]ge-sharing and corner-sharing chain and UO2O5 monomer [41]ge-sharing and corner-sharing tetramer and UO2O5 edge-sharing dimer [45]e-sharing tetramer [49]omer [49]
ge-sharing dimer [43]
96 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
s of va
sTgaifaUiubUbci
gp[Olcgp
F
Fig. 14. The structures and phase transition
tructure (Chain I) that is typical among uranyl phosphonates.he third unique uranyl center adopts a square bipyramidaleometry, which links adjacent chains to form the layeredrrangement. (Hbpi)[(UO2)2(H2O)(PhP)2(HPhP)] (UPhP-bpi) [36]s comprised of two 1D components connected to each other,orming a layered structure (Fig. 15b). One is a type I chain,nd the other chain is formed by the alternating connection ofO2O5 polyhedra and phosphonate moieties. The last structure
s (H2pip)[(UO2)3F4(H2O)(PhP)2]·2H2O [23], which contains twonique U(VI) centers in UO2O2F3 and UO2O2(H2O)F2 pentagonalipyramidal environments (Fig. 15c). The edge-sharing dimers ofO2O2F3 polyhedra are linked by the UO2O2(H2O)F2 pentagonalipyramids through corner-sharing interactions, forming infinitehains. The phenylphosphonate ligands join the chains, resultingn layers separated by piperazine.
Replacing the phenyl group of PhP with a benzyl groupives benzylphosphonic acid (PhMeP) [23]. Only one exam-le of a uranyl benzylphosphonate compound, (H2pip)2(UO2)5(PhMeP)6(HPhMeP)2] (UPNO-4), has been reported by’Hare. This compound contains similar uranyl phosphonate
ayers as those in UPhP-tib, which are comprised of uranyl chainsonnected by uranyl square bipyramids. In UPNO-4, the benzylroups occupy the interlayer space along with the templatingiperazine cations.
ig. 15. The uranyl phosphonate layers in UPhP-tib (a) [40], UPhP-bpi (b) [36], and (H2pi
rious uranyl phenylphosphonates [17–21].
3.2. Uranyl compounds with aromatic diphosphonate ligands
The functionalization of benzene with two phospho-nate groups leads to three diphosphonic acid isomers:1,2-Phenylenediphosphonic acid (o-bbp), 1,3-Phenylened-iphosphonic acid (m-bbp) and 1,4-Phenylenediphosphonic acid(bbp). 1,4-Phenylenediphosphonic acid is the most intensivelyinvestigated of the three, and twenty-seven uranyl phosphonatecompounds including two 1D, eight 2D, and thirteen 3D struc-tures have been prepared [39,41–48]. Pillared structures oftenoccur by using the rigid phenyl ring as spacers to separate theuranyl phosphonate partial structures. Various cationic species,including alkaline earth metals, amines, ionic liquids, and crownether complexes, can serve as structure-directing agents. Onlytwo examples of uranyl 1,2-phenylenediphosphonate compoundshave been reported [49], and no uranyl phosphonates preparedwith 1,3-phenylenediphosphonic acid have been reported.
A remarkable elliptical nanotubular structure is fea-tured by the aromatic diphosphonate compound Cs3.62H0.38[(UO2)4F2(bbp)(H2bbp)3] (CsUbbp-1) [41], which is composed
of uranyl dimers of UO2O4F pentagonal bipyramids with bridg-ing F atoms. These corner-sharing dimers are chelated andbridged by the phosphonate moieties, forming chains (Chain II).These chains are linked by the phenyl rings to make elliptical
p)[(UO2)3F4(H2O)(PhP)2]·2H2O (c) [23]. The phenyl rings are omitted for clarity.
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 97
s (and
csnn(ARotbosPoo
lb[[it1[shppcc
spsab
Fig. 16. The tubular structures and their uranyl phosphonate chain component
hannels with dimensions of approximately 1 nm × 2 nm. Fig. 16ahows how the packing of the tubes generate additional chan-els. Cs+ ions reside both within the nanotubules and betweeneighboring nanotubules. 4,4′-Biphenylenebisphosphonic acidbpbp) was also used to construct nanotubular uranyl compounds2[(UO2)2F(Hbpbp)(H2bpbp)]·2H2O (A = Cs, Rb+) (CsUbpbp-1 andbUbpbp-1) [43]. In these structures, the corner-sharing dimersf UO2O4F pentagonal bipyramids are linked by PO3 moietieso form chains (Chain III). Two of these chains are joined byiphenyl groups, creating elliptical channels with dimensionsf approximately 1 nm × 1.5 nm (Fig. 16b). The uranyl chain islightly different than Chain II in CsUbbp-1 [42] which containsO3 groups that chelate and bridge the dimers. In CsUbpbp-1, onef the PO3 moieties does not participate in connecting the chainsr the dimers.
It is interesting to note that when type II chains wereinked by phenyl rings along one direction, layered assem-lies of [(H3O)@18C6][(UO2)2F(H2bbp)2]·2H2O (U18C6-1),K@18C6][(UO2)2F(H2bbp)2]·4H2O (U18C6-2), [(H3O)@15C5](UO2)2F(H2bbp)2]·4H2O (U15C5-1) were produced [46]. Templat-ng hydronium-crown ethers and potassium-crown ethers occupyhe interlayer space and balance the charge of the structure. Using2-crown-4 as the structure-directing agent, a layered structure,(H3O)12C4][(UO2)(H1.5bbp)]2(U12C4-1), and a 3D frameworktructure, [(H3O)12C4][(UO2)3(H1.5bbp)2(H2bbp)] (U12C4-2),ave been prepared. In U12C4-1, type I chains are connected byhenyl rings to form sheets. A similar sheet structure can also berepared using 1,10-phenantroline as the template [42]. U12C4-2onsists of the less common uranyl square bipyramidal monomersonnected by phosphonate moieties.
(H3O)4[(UO2)4F4(bbp)2]·H2O [47] is unique in that its pillaredtructure is formed by the cross-linkage of uranyl phos-
honate sheets with the rigid phenylene spacers. In theseheets, two UO2O3F2 pentagonal bipyramids share two Ftoms, resulting in dimers. The dimers are joined togethery PO3 moieties. When the similar F-free sheets are linked
their graphical representations) in CsUbbp-1 (a) [42] and CsUbpbp-1 (b) [43].
on both sides, the pillared layered 3D framework structureof [Sr(H2O)3][(UO2)2(OH)2(H2O)(bbp)]·3H2O [44] is observed(Fig. 17).
The first uranyl phosphonate compounds synthesized usingthe bbp ligand were reported by Adelani and Albrecht-Schmitt[48], and five 3D uranyl diphosphonates with pillared struc-tures were described. In (H2temp)[(UO2)3(bbp)2]·H2O (temp = bipy(UBBISP1), phen (UBBISP5)), the type I chains are connected byuranyl square bipyramids, resulting in sheets. These sheets arepillared by phenyl rings to create the 3D framework. The frame-works can also be seen as sheets, that are similar to those inU12C4-1 [46], joined by UO2O4 polyhedra. If the bridged uranylcenters are replaced by monomeric UO2O5 pentagonal bipyramids,then [Ba(H2O)3][(UO2)3(H2O)(bbp)2]·5H2O is formed [44]. For(H2temp)[(UO2)2(H2bbp)3]·2H2O (temp = pip (UBBISP2), DABCO(UBBISP4)), a similar chain component constructed by uranyl pen-tagonal bipyramids and PO3 moieties can be isolated. The chainsare linked by phenyl rings to form the canted pillared structures.(Hbipy)2[(UO2)2(H2bbp)3]·2H2O (UBBISP3) is an isomorph of therecently reported [Emim]2[(UO2)2(H2bbp)3]·2H2O [39]. It is note-worthy in these compounds that some organic N-bearing speciescan lead to the same structure while others may lead to differentstructure types.
[(UO2)(H2O)(H2F2bbp)]2·H2O [47] adopts a 3D frameworktopology and crystallizes in the tetragonal space group P42/nmc,which is uncommon for uranyl phosphonates. The mononuclearuranyl pentagonal bipyramids are bridged by PO3 moieties, creat-ing a tube with an 8-membered ring opening (Fig. 18). These tubesare separated by the phenylene spacers, thereby forming roundchannels. Two of the carbon atoms of the phenyl rings are fluori-nated in situ, and these fluorine atoms point into the channels.
As is the case with most uranyl 1,4-phenylenediphosphonates,
which accommodate pillared structures by using thephenylene spacer as the column, (H3O)2[(UO2)6(H2O)2(bbp)(Hbbp)2(H2bbp)2] (Ubbp-1) [47], (NMe4)[(UO2)3F(H2O)(Hbbp)2]·0.5H2O (Me4Ubbp) [41], (NEtMe3)[(UO2)3F(H2O)(Hbbp)2]
98 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
Fig. 17. Layered structure of (H3O)4[(UO2)4F4(bbp)2]·H2O (a) and framework structure ophonate layer (b) [47].
Ft
((1btp
Functionalization of the phosphonate ligand with a carbox-
F
ig. 18. The 3D framework of [(UO2)(H2O)(H2F2bbp)]2·H2O along c axis showinghe channels [47].
U6bbp) [45], and (NEt2Me2)[(UO2)3F(H2O)(Hbbp)2]Et2Me2Ubbp) [41] all adopt 3D pillared structures. In Ubbp-, edge-sharing tetranuclear and mononuclear uranyl pentagonal
ipyramids are chelated and bridged by the PO3 moieties, leadingo a uranyl sheet (Fig. 19a). The sheets are pillared by the rigidhenylene spacers to form the overall 3D structure. U6bbp and
ig. 19. The uranyl phosphonate layers isolated from Ubbp-1 (a) [47], Me4Ubbp (b), and
f [Sr(H2O)3][(UO2)2(OH)2(H2O)(bbp)]·3H2O (c) comprise the similar uranyl phos-
Me4Ubbp are isostructural and templated by different quaternaryammonium cations. The pillared layer of U6bbp incorporates edge-sharing dimers of UO2O5 and tetramers of UO2O4F pentagonalbipyramids via edge-sharing interactions through the O atoms andcorner-sharing interactions through the F atoms (Fig. 19b). Theseuranyl species are connected by diphosphonate ligands, forminga 3D framework with channels occupied by Me4N or Me3EtNcations. In Et2Me2Ubbp, two UO2O4F pentagonal bipyramids sharea common edge of two O atoms, forming a dimer. The dimersfurther condense into a chain by sharing the F atoms. The chainsand mononuclear UO2O5 pentagonal bipyramids are bridged bythe phosphonate groups, generating a corrugated layer (Fig. 19c).The layers are connected by phenyl rings creating channels filledby NEt2Me2 cations.
Only two homometallic uranyl phosphonates have beenreported using the 1,2-phenylenediphosphonate ligand [49], andboth are layered structures (Fig. 20). (UO2)4(H2O)(o-bbpH)2(o-bbpH2)·5H2O contains a rare tetramer formed by one uranylhexagonal bipyramid surrounded by three uranyl pentagonalbipyramids through edge-sharing interactions. These tetramersare chelated and bridged by the diphosphonate ligands, leadingto infinite sheets. The other compound (UO2)(H2O)(o-bbpH2)·H2Oincorporates mononuclear uranium centers in pentagonal bipyra-mids which are chelated and bridged by the phosphonate ligandto form the layers. Notably, the uranyl centers can be partiallyreplaced by Pu(IV) without changing the structure.
4. Uranyl compounds with carboxyphosphonate ligands
ylate group leads to a bifunctional carboxyphosphonate linkerthat exhibits multitopic coordination possibilities for bindingmetal centers. Indeed, the use of carboxyphosphonate ligands
Et2Me2Ubbp (c) [41] which are pillared by phenyl rings to form the 3D structures.
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 99
pH2)·
fhwbeub(aatIu
4l
bachso
TL
Fig. 20. The sheets in (UO2)4(H2O)(o-bbpH)2(o-bb
or the isolation of homometallic and heterometallic uranylybrids has proved very successful. Although some relatedorks on this subject have been covered in the contribution
y Loiseau et al. [7], a comprehensive summary of this cat-gory is needed. Table 5 includes the reported homometallicranyl carboxyphosphonates. The organic ligands involve flexi-le phosphonoacetic acid (PPA) and 2-phosphonopropanoic acidPPP), as well as rigid 3-carboxyphenylphosphonic acid (3-CPP)nd 4-carboxyphenylphosphonic acid (4-CPP). Based on hard/softcid/base theory [50], the uranyl cations preferentially bind tohe harder phosphonate group over the softer carboxylate group.n fact, most homometallic uranyl carboxyphosphonates possessnbound carboxylate oxygen atoms.
.1. Uranyl compounds with aliphatic carboxyphosphonateigands
Phosphonoacetic acid is the most extensively studiedifunctional organic linker for the construction of homometallicnd heterometallic uranyl hybrids. Due to the characteristic
oordination manner of the UO2
2+ cation, the majority of reportedomometallic uranyl phosphonoacetates is dominated by 2Dtructures [52–57]. As far as we know, only one 1D structure andne 3D structure have been reported using a carboxyphosphonate
able 5ist of homometallic uranyl carboxyphosphonate compounds.
Compounds Structure features
(UO2)(H2O)(HPPA)·2H2O 1D
Na[(UO2)(PPA)]·2H2O 2D
A[(UO2)(PPA)]·nH2O (A = Cs, NH4, Na, n = 0, 1, 2) 2D
ligand, compared to twelve layered structures. The only 1Dexample, (UO2)(H2O)(HPPA)·2H2O [51], is comprised of uranylpentagonal bipyramids ligated by both the phosphonate andcarboxylate groups, forming a neutral chain structure with theunbound carboxylate O atom being protonated. If the chains arejoined by the remaining carboxylate O atom, then the layeredstructure of Na[(UO2)(PPA)]·2H2O (NaUPAA-1) is formed [52].A[(UO2)(PPA)]·nH2O (A = Na, Cs, NH4) [53–55] possesses a verysimilar layered arrangement to NaUPAA-1 in the sense that PPAchelates and bridges four uranyl centers. The only difference isthe arrangement of the three PO3 and two CO2 moieties in thesurrounding area of the unique uranium center. As a result, nosuch chain like (UO2)(H2O)(HPPA)·2H2O can be isolated fromA[(UO2)(PO3CH2CO2)]·nH2O.
In K[(UO2)2(H2O)(PPA)(HPPA)]·H2O (KUPAA) [53], the corner-sharing dimers of uranyl pentagonal bipyramids are linkedby PPA ligands to form sheets. Half of the carboxylategroups in this structure are chelated to uranium centersand the other half are dangling into the interlayer. In addi-tion to these uranyl corner-sharing dimers, UO2O5 monomershave been observed in (UO2)4(H2O)4(PPA)2(HPPA)·3H2O [54],
(H2dipy)[(UO2)3(H2O)(PPA)2(HPPA)]·3H2O and (H2Temp)[(UO2)3(H2O)(PPA)2(HPPA)]·2H2O (Temp = bpe or dpe) [57]. Neverthe-less, the variation of metal–ligand connection in these structures
100 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
lCdla
scssbtyc
4l
actcbcKlwAhhlpupcuct
Cusiistd
Fig. 21. The sheet in (NH4)2[(UO2)3(PPP)2(HPPP)]·H2O [54].
eads to different layered arrangements. The 3D structure ofs3[(UO2)4(PPA)2(H0.5PPA)2]·nH2O [53] contains corner-sharingimers of uranyl pentagonal bipyramids as its SBUs. These units are
inked by PPA, creating channels with dimensions of 9.3 A × 14.0 Along the c axis.
If one H atom of the bridging methylene group of PPA is sub-tituted by a methyl group, PPP is obtained, and only one uranylompound, (NH4)2[(UO2)3(PPP)2(HPPP)]·H2O [54], with a layeredtructure, has been reported using this ligand (Fig. 21). Corner-haring trimers of uranyl pentagonal bipyramids are bridged byoth the phosphonate and the carboxylate moieties of the ligando form the sheets. The methyl groups and the unbound carbox-late units extend into the interlayer space where the ammoniumations reside and balance the charge of the structure.
.2. Uranyl compounds with aromatic carboxyphosphonateigands
Two isomers of aromatic carboxyphenylphosphoniccid, 3-carboxyphenylphosphonic acid (3-CPP) and 4-arboxyphenylphosphonic acid (4-CPP) have been used forhe syntheses of uranyl compounds. Seven homometallic uranylarboxyphosphonates ranging from clusters to frameworks haveeen reported [58–60]. Two hybrid uranyl carboxyphosphonateage clusters, K18Li12[(UO2)2(O2)2(OH)(3-CPPH)]10·nH2O (A) and3Li21[(UO2)3(O2)3(OH)(3-CPPH)]8·nH2O (B), have been crystal-
ized from aqueous solution under different pH values, and theyere built from uranyl peroxide units and 3-CPP [58]. Cluster
is composed of an edge-sharing 10-membered belt of uranylexagonal bipyramids capped by two 5-membered rings of uranylexagonal bipyramids on both ends of the resulting cage via
inkage of 3-PPP groups. The overall cluster contains 20 uranyleroxide units and 10 3-CPP ligands, while cluster B consists of 24ranyl units and 8 3-CPP ligands. 16 uranyl hexagonal bipyramidsolymerize to an 8-membered uranyl double ring via edge- andorner-sharing interactions. On either end, two 4-memberedranyl rings are arranged and linked by the 3-CPP ligands. For bothlusters, K+ and Li+ cations are located either within or betweenhe cages to balance the charge.
Two chain structures, (UO2)(4-CPPH2)2·2H2O (Ucpp) [59] ands2[(UO2)2F(4-CPPH)(4-CPPH2)3] (CsUcpp-1) [60], were isolatedsing the 4-CPP ligand. In Ucpp, only PO3 moieties link the uranylquare bipyramids to form the chain, which is similar to thatn (UO2)(HPhP)2·2CH3CH2OH. The unbound carboxylate portion
nteracts via interchain hydrogen-bonding to give rise to a layeredtructure. In CsUcpp-1, the uranyl phosphonate chain is similaro that found in CsUbbp-1, and it is formed by corner-sharingimers of UO2O4F pentagonal bipyramids connected through the
Fig. 22. The open-framework structure of Cs3[(UO2)(CPP)]·7H2O [60].
phosphonate groups. The carboxylate groups are unbound andpoint between the chains like those found in Ucpp. The overallcharge balance is maintained by Cs+ cations.
Another cesium uranyl carboxyphenylphosphonate,Cs4[(UO2)6(4-CPPH)8]·7H2O, forms a layered structure in whichedge-sharing dimers of uranyl pentagonal bipyramids are linkedonly by a PO3 portion, leaving the protonated carboxylate groupspointing into the interlayer space [60]. In the presence of Ba2+
instead of Cs+, uranyl chains are created by corner-sharing inter-actions between UO2O3F2 pentagonal bipyramids. These chainsare connected by phosphonate groups, which gives rise to thesheet structure of Ba[(UO2)F(4-CPPH)]2·2H2O [60]. Like otherlow-dimensionality uranyl carboxyphenylphosphonates obtainedunder low pH (<3), the carboxylate moiety of 4-CPP ligands inBaUcpp-1 projects into the interlayer.
The same ligand also affords the 3D framework structureCs3[(UO2)(4-CPP)]3·7H2O [60]. This structure incorporates UO2O5pentagonal bipyramidal monomers linked by both the phospho-nate and carboxylate groups, creating voids filled by cesium ionsand disordered water molecules (Fig. 22).
5. Heterometallic uranyl phosphonates
The incorporation of a second metal atom into uranyl phos-phonates to obtain heterometallic phosphonates (Table 6) andcarboxyphosphonates (Table 7) has been quite successful inexpanding the family of uranyl-bearing materials. The obtainedstructures vary from 0D clusters to 3D frameworks. So far, nearlya third of the known uranium phosphonates are heterometallichybrids. The incorporated heteroatoms include transition metals(TM), lanthanides (Ln), and actinides (An). The majority of theseheterometallic uranyl phosphonates are 3d/5f species (84%). Itshould be stated that a true heterometallic compound should con-tain both of the metal centers coordinated by the functional groupsof a single organic ligand. In this sense, some cases are not typicalheterometallic compounds, as the second metal center is strictlycharge balancing. Nevertheless, we have included such materialsin this category considering their context for other related hete-rometallic structures, especially those in which the second metalcenters play multitopic roles including linker and charge balancer.
5.1. Heterometallic uranyl compounds with phosphonate ligands
This subclass associates with twenty TM2+/UO22+, one
Ln3+/UO22+ and three isostructural An4+/UO2
2+ compounds
(Table 6). 75% heterometallic transition metal uranyl phospho-nates are incorporated by Zn heteroatoms [40,61–64]. Most ofthe reported zinc uranyl phosphonates utilize the double saltZn(UO2)(OAc)4·7H2O as both U(IV) and Zn(II) resources in the
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 101
Table 6List of heterometallic uranyl phosphonate compounds.
Compounds Structure features Uranyl SBUs Ref.
Zn2(phen)2(UO2)2(H2O)3(hedp)2·3H2O 2D UO2O5 monomer [61]Zn2(bipy)2(UO2)2(H2O)2(hedp)2 2D UO2O5 monomer [61](Hbi)[Zn0.5(UO2)2(H2O)3(hedp)(H2hedp)]·3H2O 3D UO2O5 monomer [61](Hpi)[Zn(UO2)2(H2O)4(hedp)(Hhedp)]·H2O 3D UO2O5 monomer [61]Zn(bipy)(UO2)(EDP) 2D UO2O4 monomer [62]Zn(H2tib)(UO2)2(EDP)(HEDP)(H2EDP)0.5·3H2O 3D UO2O5 edge-sharing dimer [63]Zn(phen)2(UO2)2(HPDP)2·2H2O 2D UO2O4 monomer [64]Zn(bipy)(UO2)(PDP) 2D UO2O4 monomer [63]Zn(bipy)(H2O)(UO2)(PDP) 2D UO2O5 edge-sharing dimer [63]Zn2(phen)4(UO2)3(BDP)(HBDP)2·4H2O 2D UO2O5 edge-sharing dimer and UO2O4 monomer [63]Zn2(bipy)2(UO2)3(HBDP)2(H2BDP)2 3D UO2O5 edge-sharing dimer and UO2O4 monomer [63]Zn(phen)(H2O)2(UO2)3(BDP)2 3D UO2O5 edge-sharing dimer and UO2O4 monomer [62]Zn(pi)2(UO2)(PhP)2 2D UO2O4 monomer [40]Zn(dib)(UO2)(PhP)2·2H2O 3D UO2O4 monomer [40]Zn(bipym)0.5(UO2)(Hbbp)(H2bbp)0.5 1D UO2O5 edge-sharing dimer [64]Mn2(H2O)2(bipym)(UO2)4(bbp)3·2H2O 3D UO2O5 edge-sharing dimer [64]Ni2(H2O)2(bipym)(UO2)3(bbp)(Hbbp)2·6H2O 3D UO2O5 edge-sharing dimer and UO2O4 monomer [64]Cu(bipym)(UO2)(bpbp)·H2O 2D UO2O4 monomer [65]Cu2(H2O)2(bipym)(UO2)4(Hbpbp)4 3D UO2O4 monomer [65]
snussms
hdIZnoti(lctoh
TL
Cu2(H2O)2(bipym)(UO2)4F2(Hbpbp)2(H2bpbp)2·6H2O 3D
Ce(H2O)(UO2)(o-bbpH)2·H2O 2D
An(H2O)2(UO2)(HMDP)2 (An = Np, Pu, Th) 3D
yntheses. Interestingly, some of these zinc uranyl phospho-ates involve structural motifs that are similar to homometallicranyl phosphonates. The zinc-centered polyhedral moietieserve as either the bridge, linking the uranyl phosphonate partialtructures into higher dimensionality arrangements, or as ter-inal coordination groups decorating the uranyl phosphonate
tructures.Four zinc uranyl compounds were obtained with the
edp ligand (Fig. 23) [61]. They all comprise the uranyliphosphonate layers, which are isotypes of UP-4 [35].
n Zn2(phen)2(UO2)2(H2O)3(hedp)2·3H2O (ZnUP-1) andn2(bipy)2(UO2)2(H2O)2(hedp)2 (ZnUP-2), the layers are coordi-ated by Zn-centered polyhedra via the phosphonate moietiesn both sides. The chelated phen or bipy molecules maintainhe coordination environment of Zn atoms. On the other hand,n (Hbi)[Zn0.5(UO2)2(H2O)3(hedp)(H2hedp)]·3H2O (ZnUP-3) andHpi)[Zn(UO2)2(H2O)4(hedp)(Hhedp)]·H2O (ZnUP-4), the adjacentayers are bridged by 4-connected ZnO4(H2O)2 octahedra or 3-
onnected ZnO3(H2O)2 trigonal bipyramids, respectively, forminghe 3D framework. The channels are occupied by protonated bir pi cations, respectively. Even more interesting is that the 3Domometallic uranyl structures of UP-5 and UP-6 [35] can also
able 7ist of heterometallic uranyl carboxyphosphonates.
Compounds Structure features
M(H2O)6[(UO2)2(H2O)2(PPA)2]·4H2O (M = Zn, Fe) 1D
Cu(H2O)2(UO2)(OH)(PPA) 1D
M(H2O)6[(UO2)(PPA)]2·8H2O (M = Fe, Co) 2D
M(H2O)4[(UO2)(H2O)(PPA)]2 (M = Zn, Co, Cd) 2D
Zn(H2O)6[(UO2)4(PPA)2(HPPA)2]·5H2O 2D
Cu(UO2)2(H2O)3(PPA)2 2D
Cd3(UO2)6(H2O)13(PPA)6·6H2O 3D
Co(H2O)4[(UO2)(PPA)]2·4H2O 3D
(H3O)[Cu2(UO2)2(H2O)2(PPA)3] 3D
Zn2(H2O)2(UO2)2(PPA)2(HPPA)·3H2O 3D
Mn(H2O)6[Mn3(UO2)5(H2O)6(PPA)6]·5.75H2O 3D
[Mn(H2O)4]2(UO2)3(O2)(PPA)2 3D
Co(H2O)4[(UO2)6(O2)(OH)3(H2O)3(PPA)2]2·3H2O 3D
M2[(UO2)6(O3)(OH)(H2O)2(PPA)3]·16H2O (M = Mn, Co, and Cd) 3D
be seen as such layers joined by 4-connected UO2O4(H2O) or3-connected UO2O3(H2O)2 pentagonal bipyramids, respectively.
There are in total eight zinc uranyl phosphonates with increas-ing alkyl chain lengths in the phosphonate ligand that havebeen reported. Zn(bipy)(UO2)(EDP) (ZnUEDP-bipy) [62] containsa uranyl phosphonate layer isotypic to EDP-U2 [37]. The layersare modified by Zn(bipy) moieties through Zn O P and Zn-O = U connections producing the overall layered arrangement. InZn(H2tib)(UO2)2(EDP)(HEDP)(H2EDP)0.5·3H2O (ZnUEDP-tib) andZn(bipy)(H2O)(UO2)(PDP) (ZnUPDP-bipy1) [63], the edge-sharingdimers of uranyl pentagonal bipyramids are chelated and bridgedby diphosphonate ligands forming sheets which are further coor-dinated by Zn(tib) or Zn(bipy)(H2O) groups. The heterometallicuranyl phosphonate sheets in ZnUEDP-tib are bridged by additionalEDP linkers, creating the 3D framework with channels occupiedby coordinated tib molecules. Both Zn(bipy)(UO2)(PDP) (ZnUPDP-bipy2) [63] and Zn(phen)2(UO2)2(HPDP)2·2H2O (ZnUPDP-phen)[62] adopt overall 2D structures and contain isolated uranyl
square bipyramids. For ZnUPDP-phen, the chains of uranylsquare bipyramids and PO3 moieties are cross-linked by pro-pylene spacers, thereby leading to a different uranyl sheetthan [Bmim][(UO2)(HPDP)] [39], which has the propylene chains
UO2O5 and UO2O6 edge-sharing octamer [70–72]UO2O4 monomer [73]UO2O5 edge-sharing dimer [74]UO2O5 edge-sharing dimer and UO2O4 monomer [73]UO2O5 and UO2O6 edge-sharing dimer [75]
102 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
Fig. 23. The similar uranyl phosphonate layers can be connected by zinc units to create overall 2D structures of ZnUP-1 and ZnUP-2, and 3D frameworks of ZnUP-3 andZ ].
aiuSZZpsmfBbftp
poZtdcic
Zt
UO2 /3d phosphonates Mn2(H2O)2(bipym)(UO2)4(bbp)3·2H2Oand Ni2(H2O)2(bipym)(UO2)3(bbp)(Hbbp)2·6H2O were obtainedunder similar conditions (Fig. 25) [64]. In the former, the edge-sharing dimers of uranyl pentagonal bipyramids are linked by
nUP-4, or connected by uranyl units forming frameworks of UP-5 and UP-6 [35,61
rranged in a parallel fashion. When the diphosphonate ligands lengthened to BDP, both the uranyl dimers of edge-sharingranyl pentagonal bipyramids and the UO2O4 groups serve as theBUs in Zn2(bipy)2(UO2)3(HBDP)2(H2BDP)2 (ZnUBDP-bipy) [63],n2(phen)4(UO2)3(BDP)(HBDP)2·4H2O (ZnUBDP-phen1) [63], andn(phen)(H2O)2(UO2)3(BDP)2 (ZnUBDP-phen2) [62]. In ZnUBDP-hen1, the pendent Zn(phen)2 groups are coordinated to bothides of its uranyl diphosphonate sheets. In ZnUBDP-bipy, Zn(bipy)oieties link adjacent uranyl phosphonate layers to form a 3D
ramework. In ZnUBDP-phen2, the dimeric SBUs are bridged byDP, forming sheets that are similar to ZnUEDP-tib and ZnUPDP-ipy1. These sheets are bridged by uranyl square bipyramids,orming a 3D framework. The Zn(phen) moieties are coordinatedo the framework via rare U O Zn O U connections. Coordinatedhen molecules reside in the channels.
As with the syntheses of bimetallic zinc uranyl alkylphos-honates, Zn(UO2)(OAc)4·7H2O was used in the presencef imidazoles to obtain Zn(pi)2(UO2)(PhP)2 (ZnUPhP-1) andn(dib)(UO2)(PhP)2·2H2O (ZnUPhP-2) (Fig. 24) [40]. In ZnUPhP-1,he chains of UO2O4 and phenylphosphonate ligands are coor-inated by Zn atoms through two PO3 groups. The Zn atom isoordinated by two bimolecules, forming chains. If dib was usednstead of bi, the heterometallic chains would be allowed to
onnect, and the layered structure of ZnUPhP-2 would result.
Only one zinc uranyl phenylenediphosphonate,n(bipym)0.5(UO2)(Hbbp)(H2bbp)0.5, has been isolated byhe Burns group [64]. In this structure, the typical type I
uranyl chains are joined by the phenylene spacers, creatingthe uranyl phosphonate sheets. The [Zn2(bipym)]4+ subunitsare linked by the diphosphonate ligands into another lay-ered assembly. These two phosphonate sheets are positionedalternately and connected to each other by Zn O P link-age, creating an overall 3D framework. Two 3D bimetallic
2+
Fig. 24. (a) The chain structure of ZnUPhP-1 with Zn centers are coordinated bybi molecules. (b) Similar chains are connected by dib molecules forming layeredstructure of ZnUPhP-2 [40].
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 103
Fig. 25. The 3D structures of [Mn(H2O)]2(bipym)(UO2)4(bbp)3·2H2O (a) and [Ni(H2O)]2(bipym)(UO2)3(bbp)(Hbbp)2·6H2O (b) [64].
Fig. 26. Structures of three copper uranyl diphosphonates constructed by bpbp ligands (a) Cu (H O) (bipym)(UO ) F (Hbpbp) (H bpbp) ·6H O, (b)C
Pgmgbbgawc
stCC(oasoptsuats2b
to dimers via edge-sharing interactions. The Ce(IV) centers areeight-coordinate, and two CeO8 dodecahedra share an edge, form-ing a dimer. Two types of dimers are ligated by the diphosphonate
u2(H2O)2(bipym)(UO2)4(Hbpbp)4, and (c) Cu(bipym)(UO2)(bpbp)·H2O [65].
O3 moieties, forming sheets which are pillared by rigid phenylroups to generate the 3D framework. The [Mn2(H2O)2(bipym)]4+
oiety is incorporated into the framework by three phosphonateroups. In the latter, the edge-sharing dimers of uranyl pentagonalipyramids and mononuclear square bipyramids are chelated andridged by the PO3 groups to make a uranyl sheet. Rigid phenylroups pillar these sheets to form the 3D framework. The Ni atomsre chelated and linked by bipym to form the linear subunits,hich are incorporated into the framework as anchors in the
hannels.The use of 4,4′-biphenylenebisphosphonic acid for the
yntheses of heterometallic uranyl phosphonates producedhree multidimensional copper uranyl diphosphonates:u2(H2O)2(bipym)(UO2)4F2(Hbpbp)2(H2bpbp)2·6H2O (1),u2(H2O)2(bipym)(UO2)4(Hbpbp)4 (2), and Cu(bipym)(UO2)bpbp)·H2O (3) (Fig. 26) [65]. The overall 3D framework structuref 1 consists of nanotubular uranyl diphosphonate subunits thatre bridged by [Cu2(H2O)2(bipym)]4+ moieties. The nanotubularubunit is composed of edge-sharing dimers of UO2O4F pentag-nal bipyramids instead of the corner-sharing dimers of uranylolyhedra in CsUbpbp. The structure of 2 is similar to 1, except forhe fact that the nanotubular unit is constructed solely of UO2O4quare bipyramids and diphosphonate groups. A similar chain ofranyl square bipyramids can also be isolated from compound 3,nd they are linked by rigid biphenyl groups in a parallel pattern
o form a sheet. The dimeric Cu2O4(bipym)2 moieties link theheets to make the whole 3D structure. This is different from 1 and, which contain double-chelating bipym secondary ligands. Theipym only chelates one Cu center in 3.
2 2 2 2 4 2 2 2 2 2
For rare earth-bearing uranyl phosphonates, onlyCe(H2O)(UO2)(o-bbpH)2·H2O has been isolated [49], and itfeatures a layered structure (Fig. 27). In this structure, the U(VI)atoms are in a pentagonal bipyramidal environment, and condense
Fig. 27. The layered structure of Ce(H2O)(UO2)(o-bbpH)2·H2O with two dimericunits [49].
104 W. Yang et al. / Coordination Chemist
F
lo
AwSfeUaaasw
further condense to form a uranyl sheet through corner-sharing
FC
ig. 28. The framework of An(H2O)2(UO2)(HMDP)2 (An = Th, Np, Pu) [66,67].
igands to form a layered arrangement. The phenylene groupsccupy the interlayer space.
Three mixed-actinoid phosphonates with identical compositionnIV(H2O)2UO2(HMDP)2 (An = Th, Np, Pu) and isotypic structuresere synthesized using a hydrothermal method by Albrecht-
chmitt and coworkers [66,67]. The structure exhibits a complex 3Dramework, in which U(VI) is found in a square bipyramidal geom-try and An(IV) is in a distorted dodecahedral environment. TheO6 and AnO8 polyhedra are bridged by monoprotonated MDP lig-nds to form the framework (Fig. 28). These compounds representn exceedingly rare example that contains different oxidation state
ctinoids with the divergent structural and solution chemistry, andhow that the preparation of mixed-actinoid uranyl phosphonatesith different oxidation states is possible.
ig. 29. The chain in (UO2)(H2O)(HPPA)·2H2O and M(H2O)6[(UO2)2(H2O)2(PPA)2]·4H2Ou(H2O)2(UO2)(OH)(PPA) (b) [52] or by other metal centers to form sheets of M(H2O)4[(U
ry Reviews 303 (2015) 86–109
5.2. Heterometallic uranyl compounds with carboxyphosphonateligands
An overwhelming majority of bimetallic uranyl carboxyphos-phonates have thus far incorporated transition metals (Table 7).The preparation of heterometallic 3d-uranyl carboxyphosphonateshas been successful due to the different binding preferences ofthe phosphonate and carboxylate groups of the ligand, wherethe PO3 portion prefers to bind to a uranyl center, while thecarboxylate tends to bind to the transition metal. In general, itis consistent with hard/soft acid/base theory [50]. However, acarboxylate group can ligate both transition metals and U(VI)centers, as can the phosphonate portion. In some cases, nei-ther the phosphonate nor carboxylate coordinates the transitionmetal center, which only serves as the counter ion. As shownin Fig. 29a, M(H2O)6[(UO2)2(H2O)2(PPA)2]·4H2O (M = Zn, Fe)[68,69] form uranyl phosphonate chains, which are isotypic with(UO2)(H2O)(HPPA)·2H2O [51]. The unbound carboxylate portion isdeprotonated, and the negative charge of the anionic chains is com-pensated by fully hydrated [Zn(H2O)6]2+or [Fe(H2O)6]2+ cations. Ifthe chain is functionalized by square planar copper centers, the1D heterometallic structure Cu(H2O)2(UO2)(OH)(PPA) [52] forms(Fig. 29b). Alternatively, the chains may be linked by bridging tran-sition metal centers, thereby forming sheet structures like those inM(H2O)4[(UO2)(H2O)(PPA)]2 (M = Zn, Cd, Co) (Fig. 29c) [68,70,71].
Similarly, the anionic layers in KUPAA [53] can beseparated by fully hydrated Zn(H2O)6 cations, resultingin bimetallic Zn(H2O)6[(UO2)4(PPA)2(HPPA)2]·5H2O [68].M(H2O)6[(UO2)(PPA)]2·8H2O (M = Fe, Co) [69,70] contain sim-ilar uranyl phosphonate layers to Cs[(UO2)(PPA)] [53] (Fig. 30a),and fully hydrated Fe(H2O)6
2+ or Co(H2O)62+cations reside in
the interlayer space. Interestingly, such layers can be bridged byCo-centered octahedra via U O Co O U connection, producingthe 3D framework structure of Co(H2O)4[(UO2)(PPA)]2·4H2O[70] (Fig. 30b). Another cobalt uranyl carboxyphosphonate,Co(H2O)4[(UO2)6(O2)(OH)3(H2O)3(PPA)2]2·3H2O, adopts a verycomplex 3D framework structure [70]. Edge-sharing tetramers ofuranyl pentagonal bipyramids are connected by uranyl hexagonalbipyramids via edge-sharing interactions, forming chains which
interactions and bridging PPA ligands (Fig. 31a). These uranylsheets are linked by UO2O5 monomers to create the overall 3D
(M = Zn, Fe) (a) [51,68,69], which is connected by Cu units forming ribbons ofO2)(H2O)(PPA)]2 (M = Zn, Cd, Co) (c) [68,70,71].
W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109 105
Fig. 30. The sheets found in Cs[(UO2)(PPA)]·2H2O and M(H2O)6[(UO2)(PPA)]2·8H2O (M = Fe, Co) (a) [69], which can be bridged by Co units to form the 3D framework ofCo(H2O)4[(UO2)(PPA)]2·4H2O (b) [70].
F hat isf
sc
3diltg
ig. 31. One uranyl sheet in Co(H2O)4[(UO2)6(O2)(OH)3(H2O)3(PPA)2]2·3H2O (a) torming the 3D structure (b) [70].
tructure, in which Co(H2O)4 moieties are coordinated to uranylations in the sheets (Fig. 31b).
Mn(H2O)6[Mn3(UO2)5(H2O)6(PPA)6]·5.75H2O is also a complexD structure [69], where corner-sharing trimers and edge-sharingimers of uranyl pentagonal bipyramids are linked by PPA, lead-
ng to uranyl layers. The Mn units serve as linkers to connect theayers to form the overall 3D structure and also as counter-ionso compensate the negative charge (Fig. 32a). Another 3D man-anese uranyl phosphonoacetate [Mn(H2O)4]2(UO2)3(O2)(PPA)2
Fig. 32. The 3D framework structures in Mn(H2O)6[Mn3(UO2)5(H2O)6
connected by uranyl pentagonal bipyramidal monomers and Co(H2O)4 moieties
[69] is interesting in that edge-sharing dimers of uranyl pen-tagonal bipyramids are bridged by uranyl square bipyramids viaedge-sharing interactions to form uranyl chains. These chainsare connected by Mn-centered moieties through Mn O P andMn O U interactions, leading to the 3D heterometallic uranyl
phosphonoacetate (Fig. 32b).
The most remarkable heterometallic uranyl car-boxyphosphonates are those with the formulaM2[(UO2)6(O3)(OH)(H2O)2(PPA)3]·16H2O (M = Mn, Co, and Cd)
(PPA)6]·5.75H2O (a) and [Mn(H2O)4]2(UO2)3(O2)(PPA)2 (b) [69].
106 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
F[
[cwcteaMoib
Cttib
F
ig. 33. One cavity in M2[(UO2)6(O3)(OH)(H2O)2(PPA)3]·16H2O (M = Mn, Co, and Cd)70–72].
70–72] owing to the high-symmetry and porosity. The structurerystallizes in the highly symmetrical cubic space group Im-3,hich is exceedingly rare for uranyl compounds. An octanuclear
luster with a nearly planar structure is formed by six uranyl pen-agonal bipyramids and two UO2O6 hexagonal bipyramids throughdge-sharing interactions. A cavity is created by six uranyl clustersrranged on each of the faces of the cavity and bridged by PPA and
2+ cations (Fig. 33). The PPA ligands link the cavities to form theverall 3D framework. The cavities, which are approximately 16 An diameter, and the channels between the cavities are occupiedy water molecules.
The cadmium uranyl phosphonoacetated3(UO2)6(H2O)13(PPA)6·6H2O [71,72] is also remarkable in
hat it crystallizes in the rhombohedral space group R-3 and fea-ures a nano-sized channel along the c-axis, which is uncommonn uranyl phosphonate compounds (Fig. 34). Uranyl pentagonalipyramidal monomers are linked by both the PO3 and CO2
ig. 34. The flower-shaped channels in Cd3(UO2)6(H2O)13(PPA)6·6H2O [71,72].
Fig. 35. The cage structure of NiUcppe [74].
portions of the ligands, producing a 1D flower-shaped channelalong the c axis. Clusters of disordered face-sharing Cd-centeredpolyhedra are trapped in the channels by coordinated carboxylateoxygen atoms and water molecules.
Incorporating transition metals into uranyl car-boxyphenylphosphonates yields three heterometallic compounds(H3O)4[Ni(H2O)3]4{Ni[(UO2)(2-CPP)]3(PO4H)}4·2.72H2O(NiUcppe) [74], [Cu3(H2O)4][(UO2)(2-CPP)(2-CPPH)]2 (UCuCPPE-1), and (H3O)2{[Cu(H2O)]2(UO2)3(3-CPP)4}·3H2O (UCuCPPE-2)[73]. As expected, in these mixed-metal uranyl cay-boxylphenylphosphonates, the uranyl centers exclusively showaffinity for the phosphonate moiety, while the carboxylate moi-ety binds the transition metal cations. The cluster of NiUcppeis composed of six edge-sharing dimers of uranyl pentagonalbipyramids, eight NiO8 octahedra, twelve 2-CPP ligands, and fourin situ-generated PO4 tetrahedra (Fig. 35). The uranyl dimersare bridged by phosphonate and phosphate groups, while fourNi centers are positioned at the tetrahedral corners linking theuranyl dimers to create the {(UO2)12Ni4} core. Four additionalNi atoms are coordinated on the exterior of the core through thecarboxylate portion. In UCuCPPE-1, the isolated UO2O4 squarebipyramids are bridged by PO3 groups of 2-CPP to form chainsthat are further linked by CuO4 square planar and CuO5 squarepyramidal units through carboxylate moieties, thus forming thesheet structure. The phenyl rings are situated between the layers.Another copper uranyl carboxyphenylphosphonate adopts a 3Dframework topology formed by the 3-CPP ligand. In this structure,the edge-sharing dimers of uranyl pentagonal and isolated squarebipyramids are linked by phosphonate moieties, forming a sheetsimilar to that found in UPhP-tib [40]. These sheets are linked bydimeric copper square pyramidal units via carboxylate groups, togive rise to the overall framework structure. The negative chargeof the framework is balanced by hydronium ions.
The only mixed-metal 4f/5f example isLn(H2O)(UO2)2(PPA)(HPPA)2·2H2O (Ln = Sm, Er, Tb, Yb) [75], whichfeatures a 2D structure that contains edge-sharing dimers of UO2O5and UO2O6 polyhedra (Fig. 36). These dimeric units are chelated and
bridged by PO3 groups to form uranyl sheets. LnO8 polyhedra arecoordinated to these sheets by binding to both the phosphonate and
W. Yang et al. / Coordination Chemist
co
6p
pasd
F[
Fig. 36. The 2D structure of Ln(H2O)(UO2)2(PPA)(HPPA)2·2H2O [75].
arboxylate moieties. An intriguing effect of uranyl sensitizationf Sm3+ is displayed by Sm(H2O)(UO2)2(PPA)(HPPA)2·2H2O.
. Uranium(IV) and mixed-valent uranium(IV,VI)hosphonates
In contrast to the widely investigated uranium(VI) phos-
honates, tetravalent uranium phosphonates have received lessttention, partly due to the reducibility, relatively low stability,paring solubility and lower mobility of U4+. U(IV) is easily oxi-ized to U(VI) and needs to be stored under an inert atmosphere.
ig. 37. (a) Cluster structure of U6(H2O)m(o-bbpH)6(NO3)n6−n [77], (b) chain of U(o-bbpH
45].
ry Reviews 303 (2015) 86–109 107
Syntheses of tetravalent uranium materials have typically been car-ried out at nearly room temperature under reductive conditionsto avoid oxidation. In this way, three U(IV) phosphonates wereobtained (Table 8). Notably, Albrecht-Schmitt et al. isolated threetetravalent uranium phosphonates using hydrothermal methodswith U(VI) as the resource, which was partially reduced to U(IV) inthe process.
Due to its larger ionic radius, tetravalent uranium often adoptsrelatively high coordination number of eight and nine. However,six-coordinate and seven-coordinate uranium polyhedra also occurin uranium(IV) phosphonates. Four kinds of phosphonate ligands,including one alkylphosphonic acid (MDP) and three arylphospho-nic acids (PhP, bbp and o-bbp), have been successfully used for thesyntheses of uranium(IV) phosphonates, which range from clustersto framework structures [32,45,49,76,77]. Hexanuclear clustersU6(H2O)m(o-bbpH)6(NO3)n
6−n [77] are formed by six UO9 polyhe-dra bridged by six monoprotonated 1,2-phenylenediphosphonateligands (Fig. 37a). In this cluster, the uranium center is nonacoordi-nated by oxygen atoms. Five of these are from the diphosphonateligands, and the remaining four are from the chelating nitrategroups and water molecules, thereby creating a tricapped trigonalprism. This ligand also serves as the linker bridging UO8 squareantiprisms to form the linear structure of U(o-bbpH2)2·1.5H2O(Fig. 37b) [49]. When using 1,4-phenylenediphosphonic acid as theligand, a 3D framework of U2(Hbbp)2(H2bbp) was formed [45], inwhich the uranium centers are found as seven-coordinate, capped
trigonal prisms (Fig. 37d). Another 3D structure also containsseven-coordinate uranium centers, described as square antiprismswith missing vertices that are linked by methylenediphosphonateligands [32]. The only tetravalent uranium monophosphonate is
2)2·1.5H2O [49], (c) sheet of U(PhP)2 [76], and (d) framework of U2(Hbbp)2(H2bbp)
108 W. Yang et al. / Coordination Chemistry Reviews 303 (2015) 86–109
(PhP)2 [76]. The lamellar structure of U(PhP)2 consists of UO6ctahedra bridged by phenylphosphonate ligands in the ab planeFig. 37c). The phenyl groups are situated in the interlamellar space.
The only mixed-valent U(IV,VI) phosphonate was isolatedsing the methylenediphosphonate ligand under hydro-hermal conditions using S-2-butanol as the reducing agent,hich partially reduced U(VI) to U(IV) [78]. The structure of
H3O)2[(UO2)3U(H2O)2(MDP)3]·6H2O consists of four uniqueranium centers. Three hexavalent U centers are in the formf UO2O4 square bipyramidal monomers that are bridged byDP ligands forming sheets. The unique U(IV) centers span
he sheets via coordination by six phosphonate O atoms andwo water molecules, thus giving rise to a 3D frameworkith small cavities. Lattice water molecules and hydronium
ons reside in the void space to balance the charge of theramework.
. Conclusion
This review describes the diversity of uranium phosphonatesn solid state structures. Various inorganic building motifs forexavalent uranium have been discussed, from discrete bipyrami-al geometries to polymeric nuclearities (dinuclears, trinuclears,etranuclears, octanuclear and rings), and even up to infinite chainnd sheet sub-units. Additionally, a variety of phosphonate lig-nds, including alkylphosphonates and arylphosphonates, withdditional functional groups have been identified, and they exhibitumerous coordination modes. These inorganic and organic build-
ng units have been combined in different coordination manners,esulting in the rich structural diversity of uranyl phosphonates.urthermore, the effects of heterometals, co-ligands, and structure-irecting agents further enrich the library. Insight into this revieweveals that most of the reported uranium phosphonates wererepared within only the last few years. The phosphonate lig-nds used herein are very limited in terms of the library ofast organic phosphonic acids which display a nearly limitlessange of linker geometries through sophisticated substitutionnd functionalization. Despite the fact that a respectable cat-log of uranyl structural building motifs has been identified,ore complex building motif topologies should be accessible by
ptimizing synthetic parameters. Moreover, only tentative stud-es of tetravalent uranium phosphonates have been performedo far. Taking these features into account, one may anticipatehat the diversity of uranium phosphonates will continue toxpand, especially when applying synthetic methodologies typi-ally reserved for producing transition metal–organic frameworks,uch as microwave, ultrasonic, electrochemical, mechanochemical,nd high-throughput syntheses, postsynthetic modification, in situigand reaction, etc. Further exploration on uranium phospho-
ate materials contributes not only to the general understandingf the nature of uranium chemical crystallography, but also tohe development of actinide functional materials for practicalpplications.
[
[
UO7 monomer [45]
Acknowledgments
This work was supported by the National Nature ScienceFoundation of China (Nos. 21301168, 21171162, U1407101), JilinProvince Youth Foundation (20130522123JH, 20130522132JH),and SRF for ROCS (State Education Ministry). We are very grate-ful to Prof. Thomas E. Albrecht-Schmitt and his group for their helpin the revision of this manuscript.
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