Synthesis and characterization of new oligomeric and polymeric complexes based on the [Cu II (bpca)] + unit [Hbpca = bis(2-pyridylcarbonyl)amine] L. Carlucci a,⇑ , G. Ciani a , S. Maggini a , D.M. Proserpio a , R. Sessoli b , F. Totti b a Dipartimento di Chimica Strutturale e Stereochimica Inorganica (DCSSI), Università degli Studi di Milano, Via G. Venezian 21, 20133 Milano, Italy b Dipartimento di Chimica ‘‘U. Schiff’’ and INSTM Research Unit, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy article info Article history: Received 16 March 2011 Received in revised form 7 July 2011 Accepted 12 July 2011 Available online 23 July 2011 Keywords: Copper(II) Bis(2-pyridylcarbonyl)amine Magnetism DFT Crystallography Coordination chemistry abstract Five novel bpca-based Cu(II) polynuclear coordination compounds [Hbpca = bis(2-pyridylcar- bonyl)amine] were prepared using the [Cu(bpca)(H 2 O) 2 ](NO 3 )2H 2 O(1) building block and characterized by single crystal X-ray diffraction. We have also isolated and characterized two new crystal forms of the starting species, with lower water contents. Three of the new products are dinuclear complexes obtained by reacting 1 with different rigid or flexible spacer ligands: [Cu 2 (bpca) 2 (H 2 O) 2 (bipy)](NO 3 ) 2 6H 2 O(2) (bipy = 4,4 0 -bipyridine) and [Cu 2 (bpca) 2 (H 2 O) 2 (bpete)](NO 3 ) 2 xH 2 O (3) [bpete = (E)-1,2-di(pyridin-4- yl)ethane] are linear dumbbell-like species with CuCu separations of 11.075 and 13.275 Å, respectively. The third dinuclear compound, [Cu 2 (bpca) 2 (H 2 O) 2 (bpx)](NO 3 ) 2 8H 2 O(4) [bpx = 1,4-bis((1H-pyrazol-1- yl)methyl)benzene], with the flexible bpx ligand, assumes an unusual S-shaped conformation and shows a quite shorter CuCu contact of 6.869 Å only. We have also obtained a chiral 1D neutral polymeric com- plex, [Cu 3 (bpca) 2 (bipy) 3 (NO 3 ) 4 ]6H 2 O(5), that shows a central linear –Cu–bipy–Cu– chain, with all these Cu atoms connected to two lateral [Cu(bpca)(NO 3 ) 2 ] groups on two opposite sides by means of bipy spacers. An unprecedented type of Cu(II) neutral trinuclear complex, [Cu 3 (bpca) 2 (H 2 O) 2 (NO 3 ) 2 ](6), was obtained which has a centrosymmetric structure with two external [Cu(bpca)(NO 3 ) 2 ] units chelating on a central copper atom via the two pairs of carbonyl groups of the bpca ligands. The central metal is octahedral with two axial water molecules, while the two lateral Cu atoms are in square pyramidal geom- etry; the CuCu separation is 5.205 Å. The magnetic properties of 6 have been rationalized through a ferromagnetic coupling between the central metal ion and the peripheral ones which are coupled by a smaller antiferromagnetic interaction. DFT calculations have been also performed in order to give a better insight into magnetic interactions. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction One of the most useful synthetic strategies in the crystal engi- neering of coordination networks, metal–organic polymers and supramolecular architectures of great interest for their fascinating structures [1] and potential applications [2] consists in the use of preformed metal complexes as building blocks that, due to their functionalities and stereochemistry, can orient and facilitate the formation of target products. Different classes of such building blocks can be envisaged. These include species that can work only as acceptors (coordinatively unsaturated mono- or poly-nuclear complexes or complexes with good leaving groups) [3], only as donors (complexes called metalloligands [4], as many bis-chelated or tris-chelated species, like tris(dipyrrinato)metals [5]) as well as acceptor/donor complexes. To the third class we can ascribe the cation [Cu(bpca)(H 2 O) 2 ] + (bpca = bis(2-pyridylcarbonyl)amidate, see Scheme 1), present in [Cu(bpca)(H 2 O) 2 ](NO 3 )2H 2 O(1) [6], that can receive donor groups easily replacing the coordinated water molecules and, in addition, can work as a chelating agent towards a different metal through the two carbonyl groups of the bpca ligand. Since the discovery by Lerner and Lippard [7] in the 1970s that Cu(II) salts can promote, at room temperature, the hydrolysis of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) to give the bis(2-pyridyl- carbonyl)amidatecopper(II) complex and 2-pyridylformamide, the [Cu(bpca)(H 2 O) 2 ] + species has received great attention and a lot of derivatives have been prepared and characterized. In monomeric [Cu(bpca)(H 2 O) 2 ] + and similar complexes, the three N–donor atoms of the bpca ligand coordinate to the Cu(II) ion while the two O–donor atoms of the carbonyl groups are exo-oriented and not involved in the coordination. The highly stable [Cu(bpca)(H 2 O) 2 ] + unit is an optimal building block for the synthesis of homo- and heterometallic coordination oligomers and polymers or supramolecular architectures. Many intriguing structures have been characterized and interesting properties 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.07.017 ⇑ Corresponding author. Tel.: +39 02 50314446; fax: +39 02 50314454. E-mail address: [email protected](L. Carlucci). Inorganica Chimica Acta 376 (2011) 538–548 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Transcript
Inorganica Chimica Acta 376 (2011) 538–548
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier .com/locate / ica
Synthesis and characterization of new oligomeric and polymeric complexesbased on the [CuII(bpca)]+ unit [Hbpca = bis(2-pyridylcarbonyl)amine]
L. Carlucci a,⇑, G. Ciani a, S. Maggini a, D.M. Proserpio a, R. Sessoli b, F. Totti b
a Dipartimento di Chimica Strutturale e Stereochimica Inorganica (DCSSI), Università degli Studi di Milano, Via G. Venezian 21, 20133 Milano, Italyb Dipartimento di Chimica ‘‘U. Schiff’’ and INSTM Research Unit, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy
a r t i c l e i n f o a b s t r a c t
Article history:Received 16 March 2011Received in revised form 7 July 2011Accepted 12 July 2011Available online 23 July 2011
Five novel bpca-based Cu(II) polynuclear coordination compounds [Hbpca = bis(2-pyridylcar-bonyl)amine] were prepared using the [Cu(bpca)(H2O)2](NO3)�2H2O (1) building block and characterizedby single crystal X-ray diffraction. We have also isolated and characterized two new crystal forms of thestarting species, with lower water contents. Three of the new products are dinuclear complexes obtainedby reacting 1 with different rigid or flexible spacer ligands: [Cu2(bpca)2(H2O)2(bipy)](NO3)2�6H2O (2)(bipy = 4,40-bipyridine) and [Cu2(bpca)2(H2O)2(bpete)](NO3)2�xH2O (3) [bpete = (E)-1,2-di(pyridin-4-yl)ethane] are linear dumbbell-like species with Cu� � �Cu separations of 11.075 and 13.275 Å, respectively.The third dinuclear compound, [Cu2(bpca)2(H2O)2(bpx)](NO3)2�8H2O (4) [bpx = 1,4-bis((1H-pyrazol-1-yl)methyl)benzene], with the flexible bpx ligand, assumes an unusual S-shaped conformation and showsa quite shorter Cu� � �Cu contact of 6.869 Å only. We have also obtained a chiral 1D neutral polymeric com-plex, [Cu3(bpca)2(bipy)3(NO3)4]�6H2O (5), that shows a central linear –Cu–bipy–Cu– chain, with all theseCu atoms connected to two lateral [Cu(bpca)(NO3)2]� groups on two opposite sides by means of bipyspacers. An unprecedented type of Cu(II) neutral trinuclear complex, [Cu3(bpca)2(H2O)2(NO3)2] (6), wasobtained which has a centrosymmetric structure with two external [Cu(bpca)(NO3)2]� units chelatingon a central copper atom via the two pairs of carbonyl groups of the bpca ligands. The central metal isoctahedral with two axial water molecules, while the two lateral Cu atoms are in square pyramidal geom-etry; the Cu� � �Cu separation is 5.205 Å. The magnetic properties of 6 have been rationalized through aferromagnetic coupling between the central metal ion and the peripheral ones which are coupled by asmaller antiferromagnetic interaction. DFT calculations have been also performed in order to give a betterinsight into magnetic interactions.
� 2011 Elsevier B.V. All rights reserved.
1. Introduction
One of the most useful synthetic strategies in the crystal engi-neering of coordination networks, metal–organic polymers andsupramolecular architectures of great interest for their fascinatingstructures [1] and potential applications [2] consists in the use ofpreformed metal complexes as building blocks that, due to theirfunctionalities and stereochemistry, can orient and facilitate theformation of target products. Different classes of such buildingblocks can be envisaged. These include species that can work onlyas acceptors (coordinatively unsaturated mono- or poly-nuclearcomplexes or complexes with good leaving groups) [3], only asdonors (complexes called metalloligands [4], as many bis-chelatedor tris-chelated species, like tris(dipyrrinato)metals [5]) as well asacceptor/donor complexes. To the third class we can ascribe thecation [Cu(bpca)(H2O)2]+ (bpca = bis(2-pyridylcarbonyl)amidate,
ll rights reserved.
: +39 02 50314454.).
see Scheme 1), present in [Cu(bpca)(H2O)2](NO3)�2H2O (1) [6], thatcan receive donor groups easily replacing the coordinated watermolecules and, in addition, can work as a chelating agent towardsa different metal through the two carbonyl groups of the bpcaligand.
Since the discovery by Lerner and Lippard [7] in the 1970s thatCu(II) salts can promote, at room temperature, the hydrolysis of2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) to give the bis(2-pyridyl-carbonyl)amidatecopper(II) complex and 2-pyridylformamide, the[Cu(bpca)(H2O)2]+ species has received great attention and a lotof derivatives have been prepared and characterized.
In monomeric [Cu(bpca)(H2O)2]+ and similar complexes, thethree N–donor atoms of the bpca ligand coordinate to the Cu(II)ion while the two O–donor atoms of the carbonyl groups areexo-oriented and not involved in the coordination. The highlystable [Cu(bpca)(H2O)2]+ unit is an optimal building block for thesynthesis of homo- and heterometallic coordination oligomersand polymers or supramolecular architectures. Many intriguingstructures have been characterized and interesting properties
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(magnetic, photochemical, electrochemical) derived by a direct orindirect metal–metal interactions on the bpca ligand have beenreported.
The labile coordinated water molecules in [Cu(bpca)(H2O)2]+
can be easily substituted leading to accessible coordination sites.They have been replaced partially or entirely by inorganic ororganic fragments like in the systems [Cu(bpca)X(H2O)](X = N(CN)2
[14b], benzoato [10], 2-pyridinecarboxylato [15], 3-chlorobenzoato[13]). These species are either mononuclear or can be consideredoligomers or polymers via weak interactions like H-bonds or verylong Cu� � �donor contacts.
With anions working as bridging ligands oligomeric and poly-meric species have been obtained, like the dinuclear compounds(all containing dianionic bridges) [Cu2(bpca)2(H2O)n(X)]�mH2O(X = CrO4
2�, SO42�, n = 3, m = 1; X = C2O4
2�, n = 2, m = 2 [16];X = C2O4
2�, n = 0, m = 0 [17]) and some species linked by dithio-squarato [18], squarato [19], croconato [20] and isophthalato [21]dianions, the trinuclear complex {[Cu(bpca)(H2O)]2[Cu(1,2-dithio-croconate)2]}�2H2O [22] and the 1D polymers [Cu(bpca)X](X = CN�, N3
� [23], tricyanomethanide [24]).Strictly related building blocks are also those present in [Cu(bp-
Moreover, as already mentioned, the [Cu(bpca)(H2O)2]+ build-ing block can also work as bidentate donor through the two car-bonyl groups of the ligand, as shown by the copper(II) 1Dpolymer [Cu(bpca)]ClO4 [23]. This behavior has also suggested toprepare bis-chelated complexes [M(bpca)2]0/+ (divalent M = MnII,FeII, NiII, CuII, ZnII, RhII [16b,26–29] and trivalent M = FeIII, CoIII, RhIII
[27,30–32]) to be used as metalloligands for joining two metals inthe self-assembly of oligomeric [16b,32–36] and polymeric[25,32,35,37–41] homo- and heterometallic species.
All of these compounds have shown luminescence properties,and weak antiferro- or ferro-magnetic interactions, according tothe symmetry of the magnetic orbitals involved and the ability ofthe bridging ligand to support a magnetic exchange between themetal units.
This paper reports the synthesis and structure of five novelcoordination compounds prepared starting from [Cu(bpca)(H2O)2](NO3)�2H2O (compound 1) [6]. In addition in the course ofthis study we have isolated two new crystal forms of the startingbuilding block, namely [Cu(bpca)(H2O)2](NO3) (10) and [Cu(bpca)(H2O)(NO3)] (100), that are here described. Three of the newproducts are dinuclear (compounds 2, 3, 4) and involve an addi-tional rigid or flexible spacer ligand, such as 4,40-bipyridine (bipy),(E)-1,2-di(pyridin-4-yl)ethene (bpete) or 1,4-bis((1H-pyrazol-1-yl)methyl)benzene (bpx). We have also obtained a new type of1D polymeric species (compound 5) and an unprecedented Cu(II)trinuclear complex (compound 6) whose magnetic properties andDFT calculations are also reported.
2. Experimental
2.1. General procedures and materials
All the commercial reagents and solvents employed were ofhigh-grade purity and supplied by Sigma–Aldrich. They were usedas supplied, without further purification. 1,4-bis((1H-pyrazol-1-yl)methyl)benzene was synthesized from 1,4-di(bromomethyl)ben-zene and sodium pyrazolate in anhydrous THF [42]. The [Cu(bpca)(H2O)2](NO3)�2H2O (1) building block was synthesized accordingto a previously published procedure [6].
Elemental analyses were carried out at the MicroanalyticalLaboratory of the University of Milan. The purity of the bulk micro-crystalline materials obtained from the syntheses was checked byXRPD analyses on a Philips PW1820 diffractometer in the range5–35� of 2h, at room T.
Infrared spectra of 1–6 were recorded with a Perkin-ElmerParagon 1000 spectrophotometer as KBr pellets in the 4000–300 cm�1 region. The magnetic susceptibilities of polycrystallinesamples of 6 were performed in the temperature range 1.9–290 Kwith a Quantum Design SQUIDsusceptometer, using an appliedmagnetic field of 1000 Oe. The complex Gd2(SO4)3�8H2O was usedas susceptibility standard. DFT calculations were performed on 6and computational details are given in SupplementaryInformation.
Measurements of the magnetic susceptibilities of the com-pounds 2 and 5 were performed at room temperature using aMSB–AUTO Sherwood Scientific Magnetic Susceptibility Balance.Diamagnetic corrections of the constituent atoms were estimatedfrom Pascal’s constants.
2.2. Preparation of the compounds
2.2.1. Preparation of [Cu2(bpca)2(H2O)2(bipy)](NO3)2�6H2O (2)Compound 1 (64.0 mg, 0.16 mmol) in a MeOH/H2O (9 mL/4 mL)
mixture was added to a solution of 4,40-bipyridine (12.0 mg,0.08 mmol) in CHCl3 (8 mL). The resulting mixture was stirred for5 h. Then the volatile were evaporated and the solution was leftto concentrate over night. The day after blue plate crystals wereformed, they where filtered and dried in the air. Yield: 41.2 mg(57%). Anal. Calc. for C34H40Cu2N10O18: C, 40.68; H, 4.02; N, 13.95.Found: C, 40.63; H, 4.10: N, 13.84%. leff = 1.79 BM, measured atT = 23.5 �C. IR (KBr): mmax = 3420, 3088, 3042, 2421, 2367, 2344,1717, 1636, 1603, 1448, 1384, 1360, 1289, 1153, 1044, 1025,996, 807, 760, 701, 630, 487, 358, 348 cm�1.
2.2.2. Preparation of [Cu2(bpca)2(H2O)2(bpete)](NO3)2 (3)A solution of (E)-1,2-di(pyridin-4-yl)ethene (7.3 mg,
0.04 mmol) in EtOH (4 mL) was added to a solution of 1(32.0 mg, 0.080 mmol) in H2O (4 mL). The resulting blue solutionwas stirred at room temperature for 2 h and left to concentrateover night. The day after blue crystals were formed. The crystalswere filtered and washed with H2O, EtOH, and dried in the air.Yield: 5.3 mg (16%). Anal. Calc. for C36H30Cu2N10O12: C, 46.91; H,3.28; N, 15.20. Found: C, 47.02; H, 3.25: N, 15.31%. IR (KBr):mmax = 3464, 3100, 3087, 3034, 2431, 2372, 2344, 1702, 1612,1600, 1507, 1384, 1297, 1251, 1207, 1157, 1092, 1024, 840, 762,704, 633, 577, 562, 493, 323, 358 cm�1.
2.2.3. Preparation of [Cu2(bpca)2(H2O)2(bpx)](NO3)2�8H2O (4)A solution of 1,4-bis((1H-pyrazol-1-yl)methyl)benzene
(11.0 mg, 0.04 mmol) in EtOH (4 mL) was combined to a solutionof 1 (32.0 mg, 0.08 mmol) in H2O (4 mL). The resulting solutionwas stirred at room temperature for 2 h and then left to concen-trate slowly. Blue crystals were formed after few days, they were
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filtered and dried in the air. Yield: 39.6 mg (88%). Anal. Calc. forC38H50Cu2N12O20: C, 40.68; H, 4.49; N, 14.98. Found: C, 40.55; H,4.38: N, 15.08%. IR (KBr): mmax = 3116, 3080, 1717, 1635, 1603,1568, 1516, 1448, 1384, 1286, 1167, 1092, 1046, 1024, 975, 911,824, 807, 760, 745, 701, 650, 630, 616, 485, 458, 414, 360 cm�1.
2.2.4. Preparation of [Cu3(bpca)2(bipy)3(NO3)4]�6H2O (5)A solution of Cu(NO3)2�3H2O (10.0 mg, 0.04 mmol) and 4,40-
bipyridine (19.0 mg, 0.12 mmol) in EtOH (5 mL) was added to asolution of 1 (32.0 mg, 0.08 mmol) in H2O (4 mL). The resultingblue solution was stirred for 4 h at room temperature and left toconcentrate slowly. After 3 days a microcrystalline blue materialwas formed. The solid was filtered and washed with H2O, EtOH,and dried in the air. Yield: 46.0 mg (85%). Anal. Calc. for C54H56Cu3-
2.2.5. Preparation of [Cu3(bpca)2(H2O)2(NO3)2] (6)A solution of Cu(NO3)2�3H2O (9.6 mg, 0.040 mmol) in EtOH
(4 mL) was added to a solution of 1 (32.8 mg, 0.081 mmol) inCH3NO2 (8 mL). The resulting blue solution was stirred for 24 hat room temperature. The solution was left to concentrate slowly;after 2 days blue block crystals were formed. The crystals were fil-tered and dried in the air. Yield: 34.8 mg (94%). Anal. Calc. forC24H20Cu3N10O18: C, 31.09; H, 2.17; N, 15.11. Found: C, 31.15; H,2.23: N, 15.26%. IR (KBr): mmax = 3080, 1718, 1406, 1025,763, 703,356 cm�1.
2.3. Crystal structure determinations and refinements
The crystal data for all the compounds are listed in Table 1 andselected bond distances and angles in Table 2. The data collectionswere performed on a SMART-CCD Bruker diffractometer (Mo Kak = 0.71073 Å) at 293 K except 3 that was collected at 150 K.Empirical absorption corrections (SADABS) [43] were applied to alldata. The structures were solved by direct methods (SIR97) [44]and refined by full-matrix least-squares (SHELX97) [45], with WINGX
interface [46]. Anisotropic thermal parameters were assigned toall the non-hydrogen atoms. All hydrogen atoms were placed in
geometrically calculated positions and thereafter refined using ariding model with Uiso(H) � 1.2Ueq(C). All the diagrams were per-formed using MERCURY [47] and TOPOS [48] programs.
Compound 3 presents large solvent-accessible volumes, thatwere assessed using PLATON software [49] and the contribution ofthe disordered solvent (located in the voids) to the diffraction pat-tern was subtracted from the observed data by the BYPASS/Squeeze method as implemented in PLATON [50]. The final R1 for[I > 2r(I)] before ‘‘Squeeze’’ was 0.1807. Compound 5 crystallizesin an acentric space group and is a racemic twin (twinning byinversion) with disordered solvent molecules. The twinning cannotbe fully resolved/refined, hence the use of restrain and the highfinal R value. The twins are in the ratio approx. 3–1 [Flackparameter 0.26(4)]. The refinements with other twinning laws donot improve the results.
3. Results and discussion
3.1. Crystal structures
3.1.1. Comparison of the structures of[Cu(bpca)(H2O)x(NO3)y](NO3)z�nH2O (1: x = 2, y = 0, z = 1, n = 2; 10:x = 2, y = 0, z = 1, n = 0; 100: x = 1, y = 1, z = 0, n = 0)
Compounds [Cu(bpca)(H2O)2](NO3) (10) and [Cu(bpca)(H2O)(NO3)] (100) were serendipitously isolated and structurally charac-terized during this study from the parent compound [Cu(bpca)(-H2O)2](NO3)�2H2O (1). In particular, 10 was obtained as darkgreen crystals by slow evaporation of an aqueous solution of 1,K3[Cr(ox)3]�3H2O and 18-crown-6 ether, while blue crystals of 100
separated from a solution of [Cu3(bpca)2(H2O)2(NO3)2] (compound6, see below) and oxalic acid in CH3NO2/EtOH by slow evaporation.
The structure of compound 1 was reported many years ago [6]and consists of discrete monomeric square pyramidal cationiccomplexes with the bpca ligand and one water molecule in theequatorial plane [Cu–N(amine) 1.938(1), Cu–N(pyridyl)1.994(1) Å, Cu–O 1.966(1) Å] and one water molecule in the axialposition [Cu–O 2.265(4) Å]. Compound [Cu(bpca)(H2O)2](NO3)(10), illustrated in Fig. 1a, contains the same cationic complex asin 1 with similar bond parameters (see Table 2). The differencesarise from the absence of solvated water molecules in 10, that, inturn, results in a different packing and supramolecular array. Thecomplexes are disposed in such a way that the copper atoms formin the axial direction, approximately trans to the coordinated water
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molecule, an additional very weak coordinative bond to a carbonyloxygen of an adjacent complex [Cu1–O1 2.892(2) Å]. These inter-actions originate a chain polymer with intrachain Cu� � �Cu separa-tion of 5.870 Å (see Fig. 2a), extending along c; the equatorialcoordination planes of the monomers are superimposed in a pile,with a slight deviation from parallelism for adjacent units (dihe-dral angle of 20.8�). The chains are joined by H-bond bridgesinvolving the coordinated water molecules and the nitrate anions(Fig. 2a) into 2D layers. These layers stack along a in an ABABsequence with interdigitation (Fig. S1).
The structure of [Cu(bpca)(H2O)(NO3)] (100) contains mononu-clear neutral square pyramidal complexes with the chelated bpca
ligand and an oxygen atom of a nitrate anion j-O coordinated inthe equatorial plane and one water molecule in the axial position(see Fig. 1b). The bond parameters are similar to those observedin 1 and 10 (see Table 2). The complexes are connected by H-bondbridges involving the coordinated water molecules and the car-bonyl oxygen atoms of adjacent units to give double 1D polymericchains (illustrated in Fig. 2b). The packing of these chains is shownin Fig. S2.
3.1.2. Structures of the dinuclear [Cu2(bpca)2(H2O)2(bipy)](NO3)2�6H2O (2), [Cu2(bpca)2(H2O)2(bpete)](NO3)2�xH2O (3) and[Cu2(bpca)2(H2O)2(bpx)](NO3)2�8H2O (4)
We have planned the self-assembly of polynuclear complexesbearing [Cu(bpca)]+ corners by reacting 1 with some neutralN,N0-donor spacer ligands of different length and flexibility to ob-tain new architectures with structures directly dependent on theshape of the additional L ligand and the 1:L ratio. We have thus ob-tained three dinuclear complexes of the type [Cu(bpca)]2L2+ withL = bipy (2), L = bpete (3) and L = bpx (4). Though many dinuclearcomplexes containing two connected [Cu(bpca)]+ units have beendescribed (see above) the species here reported are the first casescontaining neutral spacers instead of dianionic linkers.
Similar strategies were previously employed using other che-lated-CuII corner building blocks, like, for instance, [Cu(2,20-bipy)]2+ [51], in the self-assembly of polynuclear complexes orpolymers.
Taking into account the presence of the exo-oriented pairs ofcarbonyl donor groups these dinuclear [Cu(bpca)]2L2+ species canbe considered as new elongated metalloligands to be employedin the further engineering of novel architectures and networkswith long edges.
Compounds 2 and 3 contain rigid spacers and the dinuclearspecies can be described as dumbbell-like complexes.
The structure of compound 2, [Cu2(bpca)2(H2O)2(bipy)](NO3)2�6H2O, is comprised of centrosymmetrical copper(II) dinu-clear cationic complexes, shown in Fig. 3a, uncoordinated NO3
� an-ions and solvated water molecules. Selected bond distances andangles are given in Table 2. Two parallel [Cu(bpca)(H2O)]+ unitsare joined by the bipy ligand in such a way that the Cu� � �Cu linkagevector, 11.075 Å long, lies almost coplanar with the two equatorialcoordination planes [e.g. Cu� � �Cu�N(amine) 160.3�]. Each copperatom has a square pyramidal coordination geometry with the threenitrogen atoms from the tridentate bpca ligand and one pyridylnitrogen of bipy in the equatorial plane and a water molecule inthe axial position [Cu�O 2.268(1) Å]. The bond parameters are
Fig. 2. (a) View of the 1D chains formed in complex 10 (see text); H-bond bridges involving coordinated water molecules and nitrate anions are shown that generate 2Dlayers. (b) View of one 1D double chain in 100 formed via H-bonds involving coordinated water molecules and carbonyl oxygens of adjacent complexes.
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similar to those observed in the mononuclear species above de-scribed. The two pyridyl rings of the bipy ligand are coplanar andthe dihedral angle between the average bipy plane and the averageequatorial CuN4 coordination plane is 71.1�.
The packing of these dinuclear complexes shows the formationof 1D chains via p–p stacking of bpca phenyl rings of adjacent com-plexes (centroid-to-centroid distance 3.635 Å), as illustrated inFig. 4. The chains are interdigitated in the c axis direction and leavechannels running along c that contain the nitrate anions and theguest water molecules (see Fig. S3). H-bond bridges involving thecoordinated H2O molecules, one of the two carbonyl groups, the ni-trates and the free water molecules form a very complicated 3Dnetwork.
The shortest intermolecular copper� � �copper distance is5.775 Å.
It seemed strange to us that a dinuclear species of thetype [Cu(bpca)]2L2+ based on L = bipy, which is probably the mostwidely employed bridging ligand in crystal engineering, was notreported previously; indeed in two recent papers the structures of{[Cu(l2-bpca)(4,40-bipy)(H2O)]ClO4}2 [52] and [Cu(bpca)(4,40-bipyH)(H2O)(ClO4)]ClO4�H2O [53] have been described, whichcontain only monodentate 4,40-bipy and 4,40-bipyH+ ligands,respectively.
Using as auxiliary ligand the longer rigid bpete spacer we haveisolated [Cu2(bpca)2(H2O)2(bpete)](NO3)2�xH2O (3). The structureof 3, like that of 2, consists of dinuclear copper(II) cationic com-plexes, illustrated in Fig. 3b, and highly disordered free NO3
� an-
ions and water molecules. Selected bond parameters are reportedin Table 2.
The [Cu(bpca)(H2O)] units are bridged by the bpete ligand, witha Cu� � �Cu vector of 13.275 Å that slightly deviates from the averageequatorial CuN4 coordination planes [e.g. Cu� � �Cu�N(amine)157.3� and 161.9�]. The bpete ligand is almost planar and the dihe-dral angles between this average plane and the average equatorialcoordination planes are 78.8� and 79.3�.
The packing is similar to that observed in 2, with longer p–pstacking contacts so that these interactions essentially generatepairs of molecules (see Fig. 5).
At difference from 2 and 3 the spacer ligand used in the synthe-sis of 4, 1,4-bis((1H-pyrazol-1-yl)methyl)benzene (bpx), is highlyflexible, so that both the Cu� � �Cu distance and the mutual orienta-tion of the two terminal [Cu(bpca)]+ groups cannot be predicted.
The structure of [Cu2(bpca)2(H2O)2(bpx)](NO3)2�8H2O (4) con-sists of centrosymmetric dinuclear complexes, free nitrate anionsand water molecules. The complex assumes a unique S-shapegeometry (see Fig. 3c) due to the particular conformation of bpx.The Cu� � �Cu vector is surprisingly short, 6.869 Å, and almost per-pendicular to the average equatorial CuN4 coordination planes[e.g. Cu� � �Cu�N(amine) 84.0�]. The two Cu atoms are mutuallyshielded by the central phenyl ring of bpx (there are two shorterCu� � �C contacts per copper atom of 3.104 Å).
The bpx ligand was rarely used in coordination polymer chem-istry. Two polymeric species were reported exhibiting quite longerM�bpx�M contacts, namely the 1D [Co(bpx)(NO3)2] with a Co� � �Co
Fig. 3. View of the molecular structures of the dinuclear complexes 2 (a), 3 (b) and4 (c).
Fig. 4. Packing of the dinuclear complexes in 2; p–p interactions are evidenced thatproduce 1D chains (in red and blue). (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)
Fig. 5. Packing of the dinuclear complexes in 3; p–p interactions are evidenced thatproduce pairs of complexes.
544 L. Carlucci et al. / Inorganica Chimica Acta 376 (2011) 538–548
distance of 10.728 Å, and the 2D species [Cd2(bpx)3(NCS)2] withCd� � �Cd contacts of 10.567, 10.654 and 11.084 Å [54].
The dinuclear units in 4 form 1D chains via H-bonds involvingthe coordinated water molecules and one of the bpca carbonylgroups of an adjacent complex, as illustrated in Fig. 6; the polymersextend along b. Moreover other H-bonds with the participation ofthe free nitrate and water molecules link the above chains into acomplex 3D array.
3.1.3. Structure of the 1D polymer [Cu3(bpca)2(bipy)3(NO3)4]�6H2O (5)The structure of [Cu3(bpca)2(bipy)3(NO3)4]�6H2O (5) shows a
new way to bind the [Cu(bpca)]+ units. It contains 1D polymericchains all running in the a direction, exhibiting a central linear –Cu–bipy–Cu– polymer, with each Cu atom being connected totwo [Cu(bpca)(NO3)2] groups on two opposite sides via bipyspacers (see Fig. 7). The central Cu atoms display octahedralcoordination geometry, with four equatorial pyridine groups[Cu–N 2.033(6), 2.051(7) Å] in a propeller disposition and twoaxial water molecules with Cu–O Jahn–Teller elongated bondlengths [2.46(1) and 2.47(1) Å]. The two lateral Cu atoms alsoshow octahedral coordination geometry being bound to threeN atoms of bpca [Cu–N 1.91(1), 2.03(1) and 2.03(1) Å] and oneN(pyridine) [Cu–N 1.990(7) Å] in the equatorial plane and totwo j-O coordinated NO3
� anions in the axial direction (withlong Cu–O contacts of 2.58(1) and 2.60 (1) Å). Selected bondparameters are given in Table 2. The Cu� � �Cu separations are11.169 Å along the central chain and 11.149 Å within the lateralarms. Compound 5 crystallizes in the non-centrosymmetric chi-ral space group I2 (non standard setting for C2 No. 5) and allthe polymeric chains are homochiral. The dihedral angle be-tween each lateral Cu(bpca) coordination plane and the averagepolymer plane defined by the metal atoms is 65.8�.
The two types of metal atoms differ also for their local charges,with an excess of negative charge on the lateral Cu atoms and ofpositive charge on the central ones.
The crystals contain two identical sets of polymers related by Icentering.
H-bonds join adjacent polymers belonging to the same set viaC@O� � �HOH� � �O@C and via Cu–OH2� � �(H2O)2� � �H2O–Cu bridges.In this way a 3D network is obtained with the pcu topology (seeFig. S4). The same occurs within the second set of polymersgenerated by I centering. Thus the whole system could be de-
Fig. 6. View of one 1D chain (extending in the b direction) formed by the dinuclear complex 4 via H-bonds involving coordinated water molecules and bpca carbonyl groups.The bpca fragments of adjacent complexes interact through a double C@O� � �H–O bridge (in red). Other H-bonds (in green) are present that link the chains extending thepattern into 3D. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. Two views of the 1D chiral polymeric species in 5, with different rotations about the polymer axis (in the a direction).
L. Carlucci et al. / Inorganica Chimica Acta 376 (2011) 538–548 545
scribed as consisting of two interpenetrating pcu nets (see Fig. S5)except for the presence of few H-bonds (involving the nitrates andthe free water molecules) that link the two networks.
3.1.4. Structure of the trinuclear [Cu3(bpca)2(H2O)2(NO3)2] (6)Many trinuclear homo- and heterometallic complexes contain-
ing the bpca ligand were reported in the last years mainly due tothe interest for their magnetic properties. Almost all are linear spe-cies based on the bis-chelated [M(bpca)2]0/+ metalloligands, withtwo distinct situations: (i) two such entities are chelating via twocarbonyl pairs on a central different metal species, with a backboneof the type [M(bpca)2]–M0–[M(bpca)2] [32,35,37]; (ii) one[M(bpca)2]0/+ complex binds two lateral metal species to give a
M0–[M(bpca)2]–M0 skeleton [33,16b,34]. Two different examplesare also known that contain [M(bpca)] building blocks, with trinu-clear schematic chains of the type [M(bpca)]–X–M0–X–[M(bpca)](X = anionic bridge), namely [Fe(bpca)(CN)3]2Mn(H2O)2(MeOH)2�2H2O [55] and [Cu(bpca)(H2O)X]2Cu�2H2O (X = 1,2-dimercaptocy-clopent-1-ene-3,4,5-trione) [56].
The crystal structure of [Cu3(bpca)2(H2O)2(NO3)2] (6) reveals anovel type of linear trinuclear species, based on mono-chelated[Cu(bpca)]+ units that work as donors with their two carbonylgroups. The complex is centrosymmetric (see Fig. 8) with two lat-eral [Cu(bpca)(NO3)2]� groups that chelate a central copper atombearing also two coordinated water molecules. The central metalatom is octahedrally coordinated while the two lateral Cu atoms
Fig. 8. Molecular structure of the centrosymmetric trinuclear complex 6. Symmetryoperation: 1 � x, 1 � y, �z.
Fig. 9. A single 2D layer in 6 formed via H-bonds that link coordinated nitrates andwater molecules.
Fig. 10. vT vs. T curve for complex (6) (open circles) and best fit curves obtainedwith one J (short dashed line) and two J’s (short and long dashed line) SpinHamiltonian.
Fig. 11. Isosurface plots of magnetic orbitals computed for Mg doped 6 complex.
Fig. 12. Magneto structural correlation: dependence of J1 magnitude respect to theCu(2)–ONO2 distance.
546 L. Carlucci et al. / Inorganica Chimica Acta 376 (2011) 538–548
are in square pyramidal environment. Except for the nitrates andthe coordinated water molecules all the other atoms of the com-plex are almost coplanar. The Cu� � �Cu contact is 5.205 Å long.
The trinuclear species are connected by H-bonds that link thecoordinated nitrates and water molecules to give the 2D layersillustrated in Figs. 9 and S7.
3.2. Magnetic properties of compound 6
The vT versus T dependence in the temperature range 2–300 Kis shown in Fig. 10. At room temperature the vT value is1.28 cm3 K mol�1, and, on decreasing the temperature, it monoton-ically increases reaching 1.68 cm3 K mol�1 at 2 K, The magneticbehavior of 6 was satisfactorily interpreted using the Spin Hamilto-nian (SH) of Eq. (1)
H ¼ J1ðS1 � S2 þ S1 � S20 Þ þ J2ðS2 � S20 Þ ð1Þ
that takes into account the crystallographic Ci symmetry of the mol-ecule, with J1 = �14.1 cm�1 and J2 = 5.7 cm�1, i.e., a ferromagneticinteraction, J1, between adjacent centers and a next-nearest neigh-bor antiferromagnetic, J2 interaction. With these SH parameters theground state of the complex is S = 3/2 with excited doublets at 1.5and 21.1 cm�1, respectively. The low-lying doublet state corre-sponds to the antiferromagnetic arrangement of Cu(1) and Cu(2).
L. Carlucci et al. / Inorganica Chimica Acta 376 (2011) 538–548 547
In order to verify the validity of the used SH, an attempt with a SHwith only J1 has been performed obtaining a less satisfactory result-ing fit (see Fig. 10).
DFT–Broken Symmetry calculations, already communicated byone of us [56] on the whole cluster, confirm the importance of J2,its antiferromagnetic nature and the S = 3/2 as the ground state.The computed values were J1 = �50.7 cm�1 and J2 = 7.7 cm�1. Asemi-quantitative agreement between the computed first neigh-bors exchange interaction, in this case J1, and the experimental va-lue is not uncommon when projected Broken Symmetry–DFTapproach is used for systems containing Copper ions ([57] and ref-erences therein). To get a better insight into the interaction mech-anism ruling the magnetic behavior, we substituted either thecopper(II) ion in position (1) or the one in (2) in the structure of6 with a diamagnetic ion. Differently from what reported in Ref.[50] where a Zn(II) ion was used (yielding computed valuesJ1 = �29.0 cm�1 and J2 = �5.7 cm�1), we chose here to use theMg(II) ion. This choice has been done since its ionic radius(72 pm) is still comparable with the Cu(II) one (73 pm) and be-cause Mg(II) has no electrons in the 3d orbitals which can influencethe magnetic interactions. The computed J’s are in this caseJ1 = �69.1 cm�1 and J2 = +0.06 cm�1, respectively. In Fig. 11 areshown the computed magnetic orbitals. It is evident fromFig. 11a that the Cu(1) (dxy) and Cu(2) ðdx2�y2 Þ orbitals are strictlyorthogonal, explaining the computed strong J1 ferromagnetic cou-pling. The fact that for the Mg doped system a significantly stron-ger J1 is obtained can be ascribed to the lack of d orbitals availablefor delocalization of spin density coming from Cu(1) ion, which isproviding an anti-ferromagnetic contribution. The non-interactingsituation found for J2, when the central copper ion is replaced byMg(II) can again be rationalized with the lack of d electrons inMg(II) ion to promote a super-exchange pathway (see Fig. 11b).
Looking at the magnetic orbitals more carefully, we noticed thatthe p⁄ orbitals of the apical NO3
� ligands seem, surprisingly, tohave a strong effect onto the magnetic pathway. To verify this,we carried out magnetic correlation calculations on the Cu(2)-Cu(1)Mg system assuming the Cu(2)–ONO2 apical distance as thecritical parameter. The results are reported in Fig. 12. Our supposi-tion was successfully supported by the calculations: J1 is expectedto be antiferromagnetic when the dCu–O is shorter than 1.934 Å.From spin population analysis, we observed an increased spindelocalization onto the apical NO3
� passing from a dCu–O = 1.971to 2.600 Å. A strong influence of the crystal packing on the mag-netic properties of compound (6) is therefore expected.
4. Conclusions
We have shown here the possibility to obtain unprecedentedoligomeric and polymeric complexes based on the [Cu(bpca)]+
building block with ditopic N–donor neutral spacer ligands, bothrigid and flexible. Two linear dumbbell-like dinuclear species wereobtained with the rigid bipy (compound 2) and bpete (compound3) ligands while the flexible bpx ligand has produced an unusualS-shaped dinuclear complex (compound 4). Due to the presenceof the exo-oriented pairs of carbonyl donor groups these dinuclearspecies can be considered as new metalloligands that could poten-tially be employed in the engineering of novel architectures andnetworks.
The 1D polymeric product 5, prepared using bipy, exhibits a no-vel structural type with a central linear chain –Cu–bipy–Cu–bipy–bearing on each Cu atom two side arms ending with [Cu(bpca)]units, and is interesting also for the fact that all the chains in thecrystal are homochiral. The unique trinuclear complex 6 differsfrom all the other related trinuclear species in that it derives frommono-chelated Cu(bpca) units instead of bis-chelated [M(bpca)2]0/+
metalloligands. It can be considered the first example of a new familyof complexes with skeleton [Cu(bpca)]M[(bpca)Cu]. DFT calculationsperformed on 6 get more insights in the magnetic interactionsthrough magnetic orbitals analysis and magneto-structuralcorrelations.
Finally, we have reported the X ray structures of two new formsof the starting complex 1 (i.e. 10 and 100, that show decreasing watercontents) for a more complete basic information on these systemsand as a contribution potentially useful to other researchers work-ing in this area.
Acknowledgments
This work was supported by MIUR within the projects PRIN2006 ‘‘POLYM2006: Innovative experimental and theoretical meth-ods for the study of crystal polymorphism—a multidisciplinary ap-proach’’ and PRIN 2008 ‘‘CRYSFORMS Design, properties andpreparation of molecular crystals and co-crystals’’.
Appendix A. Supplementary material
CCDC 817357, 817358, 817359, 817360, 817361, 817362, and817363 contain the supplementary crystallographic data for 10,100, 2, 3, 4, 5, and 6, respectively. These data can be obtained freeof charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-ated with this article can be found, in the online version, atdoi:10.1016/j.ica.2011.07.017.
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