Self-assembly of carboxylic substituted PTM radicals: From weak ferromagnetic interactions to robust porous magnets Nans Roques a , Daniel Maspoch a , Angela Datcu a , Klaus Wurst b , Daniel Ruiz-Molina a , Concepcio ´ Rovira a , Jaume Veciana a, * a Institut de Cie `ncia de Materials de Barcelona (CSIC), Campus Universitari, 08193 Bellaterra, Catalonia, Spain b Institut fu ¨ r Allgemeine Anorganische und Theoretische Chemie, Universitat Innsbru ¨ ck, 6020, Innrain 52 a, Austria Received 14 September 2006; accepted 22 September 2006 Available online 30 September 2006 Abstract An overview of the work that have been developed over the last six years in our group on the use of polychlorotriphenylmetyl radicals (PTM) functionalized by carboxylic groups to access to purely organic/molecular magnetic materials is reported. From the seminal work on the monocarboxylic PTM (Section 2), of great importance to determine both the ability of these molecules to form intermolecular H- bonds and the nature of the intermolecular interactions mediated through the resulting supramolecular motifs, we will move to the self- assembly of PTM radicals functionalized with two and three carboxylic groups (Section 3). In those cases, the self-assembly of the para- magnetic units yield robust and porous magnetic structures, associating in some cases magnetic ordering to the latest remarkable char- acteristics. The last part of the review will present the latest results obtained with the idea to increase both the structural and magnetic dimensionality in purely organic PTM-based materials using a PTM radical functionalized by six carboxylic groups (Section 4). New trends and challenges for this research line, concerning the design and synthesis of new PTM radicals, as well as the obtaining of PTM based sensors or multifunctional materials will be presented in the concluding section (Section 5). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: PTM radicals; Nanoporous materials; Supramolecular chemistry; Pure organic magnet 1. Introduction Over the last two decades there has been increasing interest to control the intermolecular interactions and, therefore, the mutual orientation of organic spin-bearing units (or tectons) to design purely organic/molecular mag- netic materials [1]. The discovery of the first organic ferro- magnet by Kinoshita and co-workers [2], the b form of the p-nitrophenyl nitronyl nitroxide (1) in 1991, which orders magnetically at 0.6 K opened new perspectives in this field. This fortuitous result stimulated the preparation of succes- sive purely organic aromatic derivatives of nitroxides and nitronyl nitroxides ferromagnets 2–11 (Chart 1) [3,4]. Although the majority order below 1 K, some exceptions include the neutral oligo-nitroxide radical 12 discovered by Tholence and co-workers [5] that orders as a ferromag- net at 1.48 K and two radical cations salts, [C 60 ][TDAE] (13) and [BBDTA]GaCl 4 (14) reported by Thompson and co-workers [6] and by Awaga et al. [7], which order at 16 and 6.7 K, respectively. More recently, an interesting approach proposed by Rawson and co-workers and based on the supramolecular arrangement of thiazyl radicals has yielded two new organic magnets 15 and 16, which order at 36 and at 1.32 K as weak magnets, respectively [8]. How- ever, even though all of them behave as organic/molecular ferromagnets, many difficulties have impeded the establish- ment of a methodology for an ultimate design of organic magnets. First, there is an intrinsic difficulty in establishing ferromagnetic interactions between spins belonging to neighbouring open-shell species, owing to the natural 0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.09.065 * Corresponding author. Tel.: +34 93 5801853; fax: +34 93 5805729. E-mail address: [email protected](J. Veciana). www.elsevier.com/locate/poly Polyhedron 26 (2007) 1934–1948
Transcript
www.elsevier.com/locate/poly
Polyhedron 26 (2007) 1934–1948
Self-assembly of carboxylic substituted PTM radicals: Fromweak ferromagnetic interactions to robust porous magnets
Nans Roques a, Daniel Maspoch a, Angela Datcu a, Klaus Wurst b,Daniel Ruiz-Molina a, Concepcio Rovira a, Jaume Veciana a,*
a Institut de Ciencia de Materials de Barcelona (CSIC), Campus Universitari, 08193 Bellaterra, Catalonia, Spainb Institut fur Allgemeine Anorganische und Theoretische Chemie, Universitat Innsbruck, 6020, Innrain 52 a, Austria
Received 14 September 2006; accepted 22 September 2006Available online 30 September 2006
Abstract
An overview of the work that have been developed over the last six years in our group on the use of polychlorotriphenylmetyl radicals(PTM) functionalized by carboxylic groups to access to purely organic/molecular magnetic materials is reported. From the seminal workon the monocarboxylic PTM (Section 2), of great importance to determine both the ability of these molecules to form intermolecular H-bonds and the nature of the intermolecular interactions mediated through the resulting supramolecular motifs, we will move to the self-assembly of PTM radicals functionalized with two and three carboxylic groups (Section 3). In those cases, the self-assembly of the para-magnetic units yield robust and porous magnetic structures, associating in some cases magnetic ordering to the latest remarkable char-acteristics. The last part of the review will present the latest results obtained with the idea to increase both the structural and magneticdimensionality in purely organic PTM-based materials using a PTM radical functionalized by six carboxylic groups (Section 4). Newtrends and challenges for this research line, concerning the design and synthesis of new PTM radicals, as well as the obtaining ofPTM based sensors or multifunctional materials will be presented in the concluding section (Section 5).� 2006 Elsevier Ltd. All rights reserved.
Keywords: PTM radicals; Nanoporous materials; Supramolecular chemistry; Pure organic magnet
1. Introduction
Over the last two decades there has been increasinginterest to control the intermolecular interactions and,therefore, the mutual orientation of organic spin-bearingunits (or tectons) to design purely organic/molecular mag-netic materials [1]. The discovery of the first organic ferro-magnet by Kinoshita and co-workers [2], the b form of thep-nitrophenyl nitronyl nitroxide (1) in 1991, which ordersmagnetically at 0.6 K opened new perspectives in this field.This fortuitous result stimulated the preparation of succes-sive purely organic aromatic derivatives of nitroxides andnitronyl nitroxides ferromagnets 2–11 (Chart 1) [3,4].
0277-5387/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.
Although the majority order below 1 K, some exceptionsinclude the neutral oligo-nitroxide radical 12 discoveredby Tholence and co-workers [5] that orders as a ferromag-net at 1.48 K and two radical cations salts, [C60][TDAE](13) and [BBDTA]GaCl4 (14) reported by Thompson andco-workers [6] and by Awaga et al. [7], which order at 16and 6.7 K, respectively. More recently, an interestingapproach proposed by Rawson and co-workers and basedon the supramolecular arrangement of thiazyl radicals hasyielded two new organic magnets 15 and 16, which order at36 and at 1.32 K as weak magnets, respectively [8]. How-ever, even though all of them behave as organic/molecularferromagnets, many difficulties have impeded the establish-ment of a methodology for an ultimate design of organicmagnets. First, there is an intrinsic difficulty in establishingferromagnetic interactions between spins belonging toneighbouring open-shell species, owing to the natural
N. Roques et al. / Polyhedron 26 (2007) 1934–1948 1935
predilection for antiparallel spin alignments in these mole-cules. Second, there are complications that usually arisewhen a transmission of such intermolecular ferromagneticinteractions, along two or three dimensions of the solidand over a long range, is desired. And third, it is impossibleto predict the ultimate structure of even the simplest crys-talline solids from a knowledge of their chemical composi-tion [9].
In order to minimize these inconveniences, crystal engi-neering tools are needed. Crystal engineering is the design,construction and study of crystal structures from molecularcomponents built with predesigned intermolecular interac-tions [10]. Besides p-stacking between verdazyl radicals,that has allowed the transmission of very strong magneticexchange interactions [11], hydrogen bonds (H-bonds)have been mostly studied because their directional andoften predictive nature may allow controlling long-rangesupramolecular order in solid state [12]. H-bonds areformed by strong donor–acceptor functionalities (OH,NH2, CO2H, CONH2) and with weak donors (C„C–H,C6H5, C@C–H, C(sp3)–H) and acceptors (CN, NO2, halo-gen, p). In such a context, Veciana and co-workers [13]proposed the idea to use these H-bonds to control theself-assembly of open-shell building-blocks which wereused also successfully by Sugawara and co-workers [14].Such a study was initiated by studying the family of aro-matic nitronyl nitroxide derivatives with one or twohydroxylic groups on the aromatic ring as functionalgroups, susceptible to form hydrogen bond interactions.Success of this approach was reflected in two differentorganic ferromagnets based on molecules 17 [15] and 18
[16] that are substituted with one and two hydroxylic
groups, and which orders at 0.45 and 0.50 K, respectively(Chart 2). Furthermore, additional examples belonging tothis family substituted with one or several H-bond donorssuch as phenol [17], boronic acid [18], imidazole [19], benz-imidazole [20], triazole [21], uracil [22], pyrazole [23], phe-nyl acetylene [24] and benzoic acid [25] have provided aninteresting assortment of magnetic behaviours.
In addition to the hydroxylic groups, carboxylic groupsare also important for crystal engineering because they alsoform strong and directional O–H� � �O hydrogen bonds. H-bonded patterns in carboxylic derivatives in solid statehave been described in detail by Leiserowitz [26]. The mostfrequent and dominant H-bonded motif is the syn–syn cen-trosymmetric dimer. However, other patterns in the formof infinite non-cyclic H-bonded catemer motifs and morerare cyclic trimeric and tetrameric motifs can also beenfound [27,28].
Given this background, our group has initiated a newexperimental approach based on the synthesis and studyof organic/molecular magnetic materials based on thesupramolecular arrangement of polychlorotriphenylmethyl
1936 N. Roques et al. / Polyhedron 26 (2007) 1934–1948
(PTM) radicals functionalized with carboxylic groups inthe para- and meta-positions of the aromatic rings withrespect of the central methyl carbon (Chart 3). The interestfor these molecules is multiple according to magnetic, geo-metrical and structural considerations. First, they exhibit ahigh thermal and chemical stability thanks to the presenceof six bulky chlorine atoms that are shielding the centralcarbon atom, where the most part of the spin density islocalized. Second, from a structural point of view, therigidity and bulkiness of this family of open-shell molecules[29] was expected not only to prevent close packing and toform robust networks [30], but also to minimize undesiredthrough space magnetic interactions in the solid state.Moreover, for example, PTMTC radical can be consideredas an expanded versions of the trimesic acid molecule [31],where the benzene-1,3,5-triyl units has been replaced by ansp2 hybridized carbon atom decorated with three 4-substi-tuted 2,3,5,6-tetrachlorophenyl rings. Therefore, accordingto its related trigonal symmetry and functionality and tothe molecule rigidity, PTMTC radical was expected to yielda purely organic magnetic open-framework, a goal withinthe sole crystal engineering field [32]. The last relevant facetof these molecules is related to the possibility to function-alize these radicals with carboxylic acid groups. As hasbeen stated above, among the numerous functional groupsused to build self-assembled networks, carboxylic acidgroups combine the capability to form strong and direc-tional H-bonds with their ability to promote magneticinteractions between radical species [25]. These functionalgroups are also particularly appealing in order to use in afurther step: PTM radicals as ligands to access to porousand magnetic metal-organic coordination polymers [33],following a strategy combining the so-called ‘‘reticular syn-thesis’’ [34] together with the well-known ‘‘metal-radicalapproach’’ [35].
When we decided to start this work, only three examplesof carboxylic substituted PTM radicals were described inthe literature [36], and only two of them were characterizedmagnetically [36b,36c]. They were obtained in low overallyields following multistep synthetic methodologies, andnone of them was characterized by X-ray diffraction. Asa consequence, the work undertaken was also particularlychallenging related to the synthetic and crystal engineeringfields.
Chart 3
Here, an overview of the work that has been developedover the last six years in our group on this research line ispresented. From the seminal work on the monocarboxylicPTM (Section 2), of great help to determine both the abilityof these molecules to form intermolecular H-bonds and thenature of the intermolecular interactions mediated throughthe resulting supramolecular motifs, we will move to theself-assembly of PTM radicals functionalized with twoand three carboxylic groups (Section 3). In both cases,the self-assembly of the paramagnetic units yield robustand porous magnetic organic/molecular structures, associ-ating in some cases magnetic ordering to the latest remark-able characteristics. The last part of the review will presentthe latest results obtained with the idea to increase both thestructural and magnetic dimensionality in purely organicPTM-based materials using a PTM radical functionalizedby six carboxylic groups (Section 4). New trends and chal-lenges for this research line, concerning the design and syn-thesis of new PTM radicals, as well as the obtaining ofPTM based sensors or multifunctional materials will bepresented in the concluding section (Section 5).
2. The simplest model: self-assembly of monocarboxylic
PTMMC radical
The first stage of our study was focused to investigatethe capacity of carboxylic acid functions of PTM radicalsfor the formation of intermolecular H-bonds. Althoughthe ability of carboxylic groups to form H-bonds waswidely reported [26,27,31], the neighbouring chlorineatoms located at the aromatic rings of these radicals shouldcause a high sterical impediment that could frustrate theformation of any direct H-bonds between carboxylicgroups. To answer this question, we selected the monocar-boxylic PTMMC radical previously synthesized by Bal-lester et al. in 1982 [36a]. The synthesis, crystallizationand structural and magnetic characterization of this simpleradical with only one carboxylic moiety would certainlyhelp us to identify the formation of H-bonds as well asthe nature and the strength of the expected magneticexchange interactions mediated through these supramolec-ular interactions.
PTMMC was obtained following the synthetic proce-dure that is presented in Scheme 1 [36]. Briefly, this
.
Fig. 1. Temperature dependences of the magnetic susceptibility, v, ofpolycrystalline samples of a (D) and b (s) phases of radical PTMMCwhich are plotted as v Æ T vs. T. Solid lines are the best fit of theexperimental data. [R2
2ð8Þ] H-bonded dimers in a phase of PTMMC (top)yield weak ferromagnetic interactions while disruption of this synthon byethanol molecules in b phase of PTMMC (bottom) is traduced by theonset of weak antiferromagnetic interactions at low temperatures.
N. Roques et al. / Polyhedron 26 (2007) 1934–1948 1937
synthesis involves five crucial points: (i) the formation ofthe triphenyl methane skeleton (molecule 24), via a Grig-nard reaction followed by the reduction of the triphenyl-carbinol derivative 23 obtained after this reaction; (ii) thechlorination of the methyl group, which is accompaniedby the chlorination of the central carbon atom (molecule25), that makes necessary an additional step for removingthis chlorine; (iii) the chlorination of the three phenyl rings,to obtain the perchlorinated skeleton (molecule 27); (iv) theconversion of the trichloromethyl group into carboxylicacid (molecule 28); and finally, (v) the oxidation step toform the PTMMC radical.
The crystallization of this radical was performed by dif-fusion of hexane into a methylene chloride solution ofPTMMC [25e]. This procedure yielded an a-phase of theradical, in which the PTMMC radicals are associated intoH-bonded centrosymmetric [R2
2ð8Þ] dimeric synthonsdefined by O� � �O distances of 2.676 A and O–H� � �O anglesof 144� (Fig. 1) [37,38]. In this synthon, both carboxylategroups of PTMMC are twisted by 88� with respect to thephenyl plane to which they are bonded. We believe that thischaracteristic is mostly due to the previously predicted ste-ric hindrance from the neighbouring chlorine atoms to thecarboxylic groups. Crystal packing of these dimers is thengoverned by interdimer Cl� � �Cl short contacts that yieldto the formations of tapes running in a plane, while addi-tional Cl� � �Cl contacts are responsible of the 3-D closepacking. A detailed revision of all supramolecular contactsis given in Table 1 (see also Fig. 2a for atom labelling).
Taking into account the basic H-bonded dimeric organi-zation of PTMMC radicals found in a-phase, and that theshortest distance between the central methyl carbons that
Scheme 1. Synthes
bear most of the spin density is just found within thesedimers at a length of 15.36 A, the analysis of the magneticproperties provided the ideal scenario to study the trans-mission of magnetic exchange interactions between H-bonded PTM radicals. SQUID measurements revealed aparamagnetic behaviour in the 6–200 K temperature range,and the onset of weak ferromagnetic interactions below6 K (Fig. 1). To confirm this behaviour, ESR measure-ments were performed on an oriented single crystal of thea phase and revealed the same trend. Hence, weak domi-nant ferromagnetic interactions are present in this phase.
is of PTMMC.
Table 1Supramolecular interactions and magnetic properties in carboxylic substituted PTM radicals
a Abbreviations o (ortho), m (meta) and p (para) positions of the phenyl rings with respect to the central carbon of carboxylic substituted PTM radicals.
1938 N. Roques et al. / Polyhedron 26 (2007) 1934–1948
The H-bonded dimers could be responsible for the appear-ance of the ferromagnetic interaction within the dimers. Asonly Cl� � �Cl contacts are present between the dimers andthe distances between them are quite large, it is possiblethat the dimers behave as magnetically independent spe-cies. Therefore, the Bleaney-Bowers equation was used tofit the magnetic data and gave the following exchange cou-pling constant, J/kB = 0.5 ± 0.1 K. This value was laterconfirmed by fitting single crystal ESR data which gave asimilar value, J/kB = 1.6 ± 0.2 K.
Beside the formation of direct intermolecular H-bonds,a second interesting feature on the crystallization of car-boxylic-based PTM radicals was discovered whenPTMMC radical was crystallized by slow evaporation ofa radical solution in ethanol. From this crystallization, asecond b-phase was obtained: the [R2
2ð8Þ] synthonsobserved in the a-phase are disrupted by ethanol mole-cules. Indeed, change in the solvent nature and the possi-bility offered by ethanol to form H-bonds with carboxylicgroups conduces to the disruption of any direct H-bondbetween PTMMC radicals (Fig. 1). Similar to the firstphase, the packing of the PTM units is assumed via addi-
tional Cl� � �Cl short contacts that lead to a close packedstructure (Table 1 and Fig. 2b).
From a magnetic point of view, the frustration of directH-bonds between PTMMC radicals allowed us to furtherinvestigate the transmission of the magnetic interactionsbetween these radicals. In principle, if H-bonded synthonsare mostly responsible for the ferromagnetic interactionmeasured in a phase, this interaction should not be presentin the b-phase. Magnetic measurement on this sampleshowed a paramagnetic behaviour down to low tempera-ture, where a decrease of the v Æ T value is observed(Fig. 1). This behaviour was fitted to the Curie-Weiss lawyielding a Weiss constant of h = �0.80 K, indicating thepresence of dominant antiferromagnetic interactions.Cl� � �Cl contacts may be responsible for this weak antifer-romagnetic interactions, as seen in other chlorinatedtriphenylmethyl radical derivatives [39]. Hence, the com-parison of the solid-state magnetic data of both phasesclearly concludes that the propagation of ferromagneticinteractions mostly occurs through the pathways definedby the intermolecular H-bonds between the carboxylicfunctions of PTM radicals.
Fig. 2. Representative ORTEP views for the different crystal structures. (a) a phase of PTMMC, (b) b phase of PTMMC, (c) PTMDC (POROF-1),(d) PTMTC (POROF-2), (e) [PTMHC Æ (Et2O)3], and (f) [PTMHC Æ (THF)6]. Thermal ellipsoids are set at 30% of probability and solvates solventmolecules have been omitted for clarity.
N. Roques et al. / Polyhedron 26 (2007) 1934–1948 1939
3. Extending the H-bonded dimensionality: self-assembly of
di- (PTMDC) and tricarboxylic (PTMTC) radicals
The ability of PTMMC radicals to interact ferromagnet-ically through H-bonds prompted us to go a step furtherbuilding extended purely organic/molecular structures, inwhich the transmission of ferromagnetic interactions couldbe propagated along the three dimensions and, therefore,induce a long-range magnetic ordering. The natural andrational step to extend the H-bonds in these radicals isthe addition of more carboxylic functions to the skeletonof the PTM radicals. Thus, in a first approach, we designedtwo PTM radicals with two (PTMDC) and three (PTMTC)carboxylic functions to the para-positions of the aromaticrings with respect of the central carbon atom (Chart 3).From a structural point of view, both PTMDC andPTMTC radicals can be considered expanded versions ofisophtalic and trimesic acids, respectively, where the ben-zene-1,3-diyl or the benzene-1,3,5-triyl unit has beenreplaced by an sp2 hybridized carbon atom decorated withthree perchlorophenyl rings substituted with carboxylicgroups. And since isophtalic and trimesic acid moleculeshave been successfully used as building-blocks to create awide diversity of extended H-bonded organic structures, asimilar behaviour could be expected for both radicals[31,40]. Furthermore, the related trigonal symmetries andfunctionalities of both organic radicals also provides atypical template for getting cavities or channels, which
envisions the synthesis of a series of purely organic/molecular magnetic structures with additional porositycharacteristics.
A previous synthesis of the PTMDC radical togetherwith preliminary magnetic characterizations was reportedin 1994 by Domingo et al. Nevertheless, the overall yieldof this multistep synthesis was rather low (12%) and mag-netic characterizations, even if showing the onset of weakferromagnetic interactions at low temperature, were per-formed on an amorphous sample of the radical [36b].
PTMDC was prepared following a new three-step pro-cedure, starting from compound 29 (Scheme 2). Dichlo-romethyl groups were introduced by reacting 29 withchloroform following a Friedel–Crafts reaction, usingAlCl3 as catalyst (78%), and were subsequently convertedto carboxylic groups by a hydrolysis–oxidation reactionusing oleum 20% (31%). Finally, the subsequent treatmentof the hydrocarbon precursor 31 with excess of NaOH, I2,and HCl allowed the recovering radical (90%) [41]. Despitethe low yield observed for the second step, this straightfor-ward synthetic approach gave a higher overall yield thanthe synthesis previously reported for the molecule (21%).
With the experience acquired with the crystallizationprocesses of PTMMC radical and in order to avoid anydisruption of the expected H-bonded network, the crystal-lization of PTMDC was carried out without using solventhighly susceptible to generate H-bonds, such as alcohols.Thus, by reproducing the same crystallization conditions
Fig. 3. Crystal structures of POROF-1 and POROF-2. Repetitive [R66ð24Þ]
hexamers (center) are connected through [R22ð8Þ] synthon in POROF-1
(left) while this unique motif is responsible of the formation of 2-D porousH-bonded layers in POROF-2.
Fig. 4. Space-filling view along the b axis of the large nanocontainersformed along the 1-D channels in POROF-1.
Scheme 2. Synthesis of PTMDC.
1940 N. Roques et al. / Polyhedron 26 (2007) 1934–1948
that yielded the a-phase of PTMMC, a robust porousextended H-bonded network (POROF-1, where POROFrefers to as Pure Organic Radical Open-Framework), thatcombines the presence of highly non-polar nanocontainersconnected by polar narrow windows, was obtained. Thusin the solid state, the intermolecular H-bonded connectionsbetween PTMDC radicals create a primary structure con-sisting of 2-D H-bonded sheets, where each molecule ofPTMDC adopts two H-bonded motifs, the expecteddimeric [R2
2ð8Þ] and the unusual hexameric [R66ð24Þ] motifs
(Fig. 3)1 [28]. The first [R22ð8Þ] synthon is almost identical to
that observed in the a-phase of PTMMC with O� � �O dis-tances of 2.677 A and O–H� � �O angles of 164�. On the
1 Cyclic hydrogen bonded supramolecular motifs, as well as polymerichydrogen bonded catemers, are particularly rare. See Ref. [27].
other hand, the new centrosymmetric cyclic [R66ð24Þ] motif
is formed by six mutually H-bonded carboxylic groups ofPTMDC radicals with O� � �O distances of 2.692 and O–H� � �O angles of 169�. Precisely, this hexameric [R6
6ð24Þ]motif is the repetitive unit in POROF-1, which involvesthe participation of one of two carboxylic groups ofPTMDC radical. The second carboxylic group of eachPTMDC radical forms the dimeric motif, which acts as aconnecting element. Indeed, the formation of the dimerpermits to connect each hexamer with six more identicalunits in an hexagonal topology, extending the infinitelyH-bonded net along the two dimensions (Fig. 3).
These 2-D layers pack together in a shifted mannerthrough several weak Cl� � �Cl contacts (Table 1 andFig. 2c), leading an organic/molecular open-frameworkstructure with 1-D channels formed by narrowed polarwindows and larger hydrophobic cavities, where a sphere10 A in diameter can fit inside them (Fig. 4). The smaller
Fig. 6. View of the channel-like system of POROF-2 filled with idealspheres with 5.2-A diameter.
Fig. 5. Temperature-dependence of the magnetic susceptibility ofPOROF-1.
N. Roques et al. / Polyhedron 26 (2007) 1934–1948 1941
windows with a highly hydrophilic environment, due to thepresence of the carboxylic groups at the inner walls, have adiameter of 5 A, considering van der Waals radius. Thecombination of supercages and windows gives way to sol-vent-accessible voids in the crystal structure that amountup to 31% (5031 A3 per unit cell) of the total volume(16158 A3). Furthermore, the fact that POROF-1 is com-posed of radical tectons leads a magnetic character to thisopen-framework structure, which exhibits weak antiferro-magnetic interactions between radical units at low temper-atures (Fig. 5).
The new synthetic approach developed to synthesizePTMDC also allowed the access to the PTM radical withthree carboxylic groups at the para-positions of phenylrings. The PTMTC radical was prepared following a simi-lar three-step procedure, starting from compound 32
(Scheme 3), and with a remarkable high overall yield(74%).
The crystallization of this radical was again carried outby diffusing a layer of hexane into a dichloromethane solu-tion of PTMTC. The crystal packing of PTMTC moleculesforms a robust porous extended H-bonded 2-D network(POROF-2), which combines the presence of highly polartubular channels and very interesting magnetic properties,such as long-range magnetic ordering at low temperatures[42]. As shown in Fig. 3, the repetitive unit is the identicalcyclic [R6
6ð24Þ] motif identical to those found in POROF-1,and herein formed by six mutually H-bonded molecules ofPTMTC with O� � �O distances of 2.657 A and O–H� � �O
Scheme 3. Synthes
angles of 169�. Since every radical unit contains three car-boxylic groups, each PTMTC molecule participates in theconstruction of three identical hexameric units that propa-gates along a plane (Fig. 3). Remarkable is the fact that thestacking of layers through Cl� � �Cl contacts (Table 1 andFig. 2d) occurs in an eclipsed fashion generating a 3-Dstructure that exhibits regular tubular channels, where asphere of 5.2 A in diameter can fit inside them (Fig. 6),without significant constrictions in contrast withPOROF-1. In addition, such channels are surrounded bya second set of small pores with a diameter of 3.3 A. Thecombination of both originates solvent-accessible voids inthe crystal structure that amount up to 15% (450 A3 perunit cell) of the total volume. Interestingly, the locationof carboxylic groups at the inner walls of the largest chan-nels furnishes them with a highly polar and hydrophilicenvironment. This fact may stand for the lack of guest sol-vent non-polar molecules within the channels when crystal-lized from dichloromethane and hexane.
Variable temperature magnetic susceptibility data for acrystalline sample of POROF-2 revealed a paramagneticbehaviour from 300 to 5 K, where the v Æ T value increaseson cooling down to 2 K according with the presence of
is of PTMTC.
1942 N. Roques et al. / Polyhedron 26 (2007) 1934–1948
weak ferromagnetic interactions (Fig. 7). This behaviourwas fitted to the Curie–Weiss law with a Weiss constantof h = +0.2 K. To investigate the existence of a long-rangemagnetic ordering at very low temperatures, variable tem-perature magnetic susceptibility experiments down to70 mK were performed in a dilution cryostat. A consider-able increase of the v Æ T value up to a maximum peakaround 110 mK was observed on cooling down below2 K, which reveals the transition to a ferromagneticordered state at very low temperatures. The intensity ofthe peak decreases whereas its maximum shifts slightly tohigher temperatures on increasing the external appliedmagnetic field. For instance, for an applied magnetic fieldof 20 Oe a value of 2.2 emu K mol�1 was obtained, whereasfor an external field of 50 Oe the value is reduced to1.4 emu K mol�1. This behaviour originates in the satura-tion of magnetization for fields of few hundred Oe. Magne-tization curves were measured above and below the criticaltemperature and are illustrated in Fig. 7b. At 1.35 K,
Fig. 7. Magnetic properties of POROF-2. (a) v Æ T as a function oftemperature for different applied magnetic fields: empty circles (s),H = 200 Oe; filled triangles (m), H = 500 Oe; and empty squares (h),H = 1000 Oe. (b) Magnetization curves as a function of the appliedmagnetic field measured at 80 mK. Inset, detail of the hysteresis curve at80 mK.
POROF-2 remains in the paramagnetic region, and there-fore, the magnetization curve has a slight gradient. Onthe contrary, the curve at 80 mK, even if it is very closeto the critical temperature, traces a hysteretic loop charac-teristic of a soft ferromagnet. The magnetization is almostsaturated at about 400 Oe, and though the coercive force isof the order of 50 Oe (see inset of Fig. 7b) the remanentmagnetization at zero field is of about 35% of the satura-tion value.
4. A highly carboxylic building-block: self-assembly of
hexacarboxylic PTMHC radical
As a natural evolution due to the interest rose up by thework performed on the previous carboxylic substitutedradicals, we decided to increase again the number of car-boxylic acid functions and prepare a PTM radical withsix carboxylic groups in meta-positions, studying its supra-molecular organization and magnetic properties. In addi-tion to the synthetic challenge that implies the synthesisand characterization of this novel radical [43,44], changesin both the number (six) and location (meta-positions) ofthe carboxylic groups along the PTM moiety was expectedto afford new 3-D assemblies, as recently demonstrated inthe case of triphenylmethane-based polyphenols [45]. Inaddition, the presence of six carboxylic groups in PTMHCwas also expected to enhance the number of magneticexchanges pathways between PTM radicals by endorsingthe H-bonded network.
PTMHC was prepared following an adaptation of thesynthetic methodology used to obtain PTMDC andPTMTC radicals (Scheme 4). While the initial part of thissynthetic route was identical, the increase of the numberof carboxylic groups in 37 compared to the correspondingdi and tri-acid molecules 31 and 34 makes impossible thedirect formation of the radical starting from this molecule[46]. To overcome this inconvenience, the hexa-acid 37 wasfirst converted into the corresponding hexa-ester 38, andthen oxidized to give the hexa-ester radical 39. Methylgroups were finally easily removed to obtain PTMHC witha good overall yield (30%).
The same principles applied for the crystallization of theprevious radicals were considered for the crystallization ofPTMHC. Multiple attempts of crystallization were per-formed and different crystallization techniques used inorder to disfavour the formation of H-bonds betweenPTMHC and solvent molecules and, therefore, create afully 3-D H-bonded organic/molecular material. Thusfar, however, all these tries have failed, mainly because ofthe lack of solubility of the highly polar radical PTMHCin non-oxygenated solvents. Also, other common tech-niques such as gas phase neutralization, and based on a dif-fusion of HCl vapours into a saturated solution of thePTMHC hexacarboxylate, did not afford any satisfactoryresult.
An alternative method for crystallizing highly carboxylicsubstituted molecules was reported by Darlow in 1961
Scheme 4. Synthesis of PTMHC.
Fig. 8. Crystal structure of [37 Æ (H2O)1.5]. Packing of molecules of 37
creates a 3-D H-bonded structure that reveals the presence of polarchannels in which water molecules are localized. Water molecules arerepresented as spheres for clarity.
N. Roques et al. / Polyhedron 26 (2007) 1934–1948 1943
[43a]. By recrystallizing benzene hexacarboxylic acid inconcentrated nitric acid, a solvent free structure of thiscompound was obtained. Following this methodology, wetried the crystallization of the non-radical 37 as a firstapproach [47]. As expected, the use of this method affordedan organic solid with a formula of [37 Æ (H2O)1.5] thatshows a fully 3-D H-bonded structure with 1-D channels.Noteworthy is the fact that this H-bonded network is par-tially disrupted by water molecules, but this disruption isnot enough to prevent the formation of a 3-D H-bondedstructure. In the structure, the solvent molecules arelocated in the 1-D channels as isolated water moleculesor water tetrahedrons (Fig. 8). Overall, this structure pos-sesses an accessible volume near 8.5% when the water mol-ecules are omitted, that corresponds to a solvent accessiblevolume of 568.1 A3 versus the volume of the total unit cell(6564.0 A3).
If successful with PTMHC, a similar crystallizationcould lead us to the obtaining of a pure organic/molecularporous material with a 3-D H-bonded structure similar tothe water solvate of 37, with foreseeable magnetic proper-ties. However, and in spite of the fact that the fully chlori-nated PTM radical was already shown to be stable in hotconcentrated nitric acid during several days [48], PTMHCwas oxidized with nitric acid to give the corresponding dia-magnetic fuchsone 40, probably because of the good solu-bility of this compound in nitric acid at high temperatureswhich contrasts with the low solubility of the fully chlori-nated PTM radical (Scheme 5).
Besides the failures of previous crystallizations, a partialsuccess was later on obtained when a mixture of diethyl
ether and n-hexane was used. Although diethyl ether mol-ecules can form H-bonds with carboxylic acid groups, thecrystallization of PTMHC using these solvents yields anorganic/molecular [PTMHC Æ (Et2O)3] solid with an H-bonded structure that is only partially disrupted by solventmolecules (Table 1 and Fig. 2e). Thus, the primary struc-ture is formed by the H-bonded connections of PTMHCradicals into 2-D H-bonded corrugated layers. As shownin Fig. 9, the repeating unit consists of an unusual
Scheme 5. Oxidation of PTMHC to the corresponding fuchsone.
1944 N. Roques et al. / Polyhedron 26 (2007) 1934–1948
[R66ð48Þ] H-bonded motif formed by six PTMHC molecules
(Fig. 9a). In this motif, each radical is H-bonded to twoneighbouring radicals through one carboxylic group withO� � �O distances of 2.696 A and O–H� � �O angles of 164�.It is important to note the similarities of these H-bondswith the ones found in POROF-2. The remaining carbox-ylic group localized in the same phenyl ring is also involvedin one H-bond with a diethyl ether molecule. Since everyradical unit contains six carboxylic groups (three areinvolved in H-bonds with PTMHC molecules and threein H-bonds with solvent molecules) each radical moleculeparticipates in the construction of three identical hexamericunits, resulting in open polar windows that propagatealong the H-bonded layers (Fig. 9b). Three diethyl ethermolecules are localized on each side of each polar windowcontributing to isolate one layer from the two neighbouringones (the shortest radical-radical interlayer distance is6.892 A). Interdigitation of diethyl ether molecules belong-ing to neighbouring layers avoids then the formation of afully 3-D H-bonded structure, leading to a ABC arrange-ment of these layers (Fig. 9c).
The disruption of the direct H-bonds between PTMHCradicals was further confirmed when PTMHC was crystal-lized from a mixture of THF and n-hexane. The higherability of THF molecules to form H-bonds [27,49] in com-parison with diethyl ether yielded a new phase[PTMHC Æ (THF)6], in which no direct intermolecular H-bonds between PTMHC radicals can be found (Table 1and Fig. 2f). Indeed, this crystal shows the particularitythat all six carboxylic acid groups of PTMHC radicalsare involved in the formation of H-bonds with one THF(Fig. 10a). Consequently, the resulting ‘‘supramolecularradical–THF clusters’’ [PTMHC Æ (THF)6], avoid anydirect H-bonds between radical molecules pushing themfar away. The shortest distance between radical centers,7.514 A, is observed in supramolecular columns runningalong the c axis (Fig. 10b). Close packing of supramolecu-lar radical-THF clusters takes place through several weakH-bonds between neighbouring THF molecules leadingto a PTMHC-templated honeycomb-like arrangement ofthe solvent molecules (Fig. 10c).
Variable temperature magnetic susceptibility data foras-synthesized crystalline sample of [PTMHC Æ (Et2O)3]showed a very similar magnetic behaviour than POROF-
2 (Fig. 11). At high temperatures (between 20 and300 K), this compound behaves as a pure paramagnet.However, below 20 K, the v Æ T value smoothly increasesupon decreasing temperature to reach a value of0.395 emu K mol�1at 1.8 K. This result is again in agree-ment with the presence of very weak intermolecular ferro-magnetic interactions between H-bonded radical moleculesand, presumably, to the presence of a long-range magneticordering at very low temperatures. On the contrary, andalso as expected, [PTMHC Æ (THF)6] behaves as aparamagnet with only very weak intermolecular antiferro-magnetic interactions (through space) at very low tempera-tures, in agreement with the lack of direct H-bondsbetween PTMHC radicals in that solvate (Fig. 11).
5. Self-assembly of carboxylic substituted PTM radicals:
concluding remarks, new trends and challenges
In this short review, we have demonstrated that poly-chlorinated triphenylmethyl radicals, properly functional-ized with one or more carboxylic functions, are excellentorganic building-blocks for the preparation of purelyorganic/molecular magnetic molecular materials. Whilethe self-assembly of PTMMC yield to the formation ofsupramolecular dimers and to the onset of weak ferromag-netic interactions between the radical units, increase in thenumber of carboxylic acid functions in PTMDC andPTMTC has allowed the access to the first examples ofpurely organic-radical open-frameworks, POROF-1 andPOROF-2, that are associating astonishing structural char-acteristics together with relevant magnetic properties, suchas long-range magnetic ordering. Indeed, POROF-1, builtby self-assembly of PTMDC molecules, is a paramagneticmaterial that presents a high porosity, with highly non-polar nanocontainers connected through polar narrowwindows. High porosity is also observed in the case ofPOROF-2, in which the 2-D self-assembly of PTMTC mol-ecules and the packing of the resulting H-bonded layersyield a hydrophilic porous architecture. However, com-pared to POROF-1, this supramolecular nanoporouspurely organic ‘‘zeolite-like’’ material exhibits a long-rangeferromagnetic ordering that behaves as a soft-magnet.
Two are the most important features to explain theresults obtained for POROFs materials. First, the
Fig. 9. Crystal structure of [PTMHC Æ (Et2O)3]. (a) R66ð48Þ hexamer and
(b) extended hydrogen-bonded corrugated layer. (c) ABC arrangement ofthe layers that are isolated one from each other by solvent molecules. Et2Omolecules are represented as spheres for clarity.
N. Roques et al. / Polyhedron 26 (2007) 1934–1948 1945
carboxylic groups on these radicals have been shown asgood superexchange pathways for attaining magnetic cou-plings. And second, the geometry and the rigidity of themolecules that together with the paramagnetic characterof carboxylic substituted PTM radicals offer magneticand porous materials with a purely organic nature.
Attempts to take advantage of these characteristics tobuild a 3-D H-bonded porous and magnetic structureusing PTMHC have given interesting results: even if,mainly for solubility problems, the access to a PTM-based3-D H-bonded structure is still not achieved, the presenceof ferromagnetic interactions in the diethyl ether solvateof this radical together with the structural analogybetween its structure and the one of POROF-2 are partic-ularly promising. Moreover, two points are particularlyrelevant for magnetic considerations: (i) increase in thenumber of carboxylic groups is traduced by the lack ofCl� � �Cl contacts in the solid state, contacts that are usu-ally responsible for antiferromagnetic interactions in thistype of radicals [39]; and (ii) change in the carboxylicgroups relative position compared to PTMMC andPTMTC (meta versus para) does not modify the natureof the intermolecular magnetic interactions, that are stillferromagnetic. Thus, attempts to crystallize PTMHC arestill under progress, but the last point and the possibilityto access to new POROFs or new metal-organic materials[33], only changing the relative positions of carboxylicgroups, make particularly attractive the synthesis of mol-ecules 41, 42 and 43 (Chart 4). The later compound, withits distorted tetrahedral geometry, appears also to be agood candidate and a good alternative to PTMHC inorder to access to 3-D H-bonded porous and magneticPTM based materials.
The remarkable properties and characteristics ofPOROF materials could also be exploited to address sev-eral challenging aspects for both crystal engineering andmolecular magnetism fields [50]. The synergism of magneticproperties and porous materials together with the molecu-lar characteristics purely organic materials opens a newroute to the development of Purely Organic Multifunc-tional Molecular Materials. For instance, along with thepossibility to act as zeolite-like materials, POROFs materi-als offer excellent conditions to encapsulate different func-tional systems with conducting, optical, chiral and NLOproperties. . . using, for instance, functional molecules astemplates during the crystallization of the radicals [51]. Inconsequence, the resulting material would combine themagnetic properties of the framework and the inherentproperties and applications of the encapsulated functionalmolecules.
Another field of research for which this type of materi-als could be particularly interesting is their use as purelyorganic sensors. For instance, and since the magneticproperties are close dependant to the presence of intermo-lecular H-bonds between PTM units into the crystal struc-ture, insertion of polar molecules in the solvent-free polarchannels of POROF-2 could be expect to yield to weakstructural changes (because of the numerous chlorine–chlorine contacts present in the material) and to strongchanges in the magnetic properties, by disruption of theH-bonded two dimensional network. In a second time,removal of the solvent molecules only heating the solvatedmaterial could be expect to give back the starting
Fig. 10. Crystal structure of [PTMHC Æ (THF)6]. (a) View of the supramolecular [PTMHC Æ THF6] cluster (THF molecules are represented as spheres forclarity). (b) Top view of a supramolecular column resulting from the packing of the supramolecular clusters along the c axis. (c) Self-assembly of thecolumns yield a honeycomb-like arrangement of the THF molecules around the molecules of PTMHC.
Fig. 11. Magnetic characterization of solvates [PTMHC Æ (THF)6] (h)and [PTMHC Æ (Et2O)3] (s) in the presence of their mother liquors.Product of the magnetic susceptibility with the temperature as a functionof the temperature in the lower limit.
1946 N. Roques et al. / Polyhedron 26 (2007) 1934–1948
POROF-2 material with soft-magnet properties, openingthe line towards purely organic magnetic solvent sensors[33c,52].
Chart 4
Actually, work on these two challenging research lines isunderway in our laboratory. Further experimentationsaimed not only at the generation of new magnetic and por-ous molecular materials using PTMDC, PTMTC orPTMHC radicals but also at the obtaining of new organicbuilding-blocks by the chemical modification of the poly-chlorotriphenylmethyl skeleton, such as molecules 41–43,are also in progress. Thus, the use of these radicals as build-ing-blocks is expected to allow the design of novel architec-tures with interesting porosity characteristics, magneticproperties and surprising topologies. In conclusion, suchnew radical units, along with PTMDC, PTMTC andPTMHC radicals, are expected to increase the new familyof magnetic purely organic/molecular (POROF) radicalopen-frameworks.
Acknowledgements
This work was supported by the Direccion General deInvestigacion (Spain) under project EMOCIONa(CTQ2006-06333/BQU), Generalitat de Catalunya(2001SGR00362), EU under a Marie Curie Research Train-ing Network (contract ‘‘QuEMolNa’’ number MCRTN-
.
N. Roques et al. / Polyhedron 26 (2007) 1934–1948 1947
CT-2003-504880), as well as by NoE MAGMANet (con-tract 515767-2). D.M. is grateful to the Generalitat deCatalunya for both predoctoral and postdoctoral grants.N.R. thanks the MCRTN for its postdoctoral contract.A.D. is grateful to the MCRTN for a predoctoral grant.
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