Multicopper(II) Pyromellitate Compounds: Self-Assembly Synthesis

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Multicopper(II) Pyromellitate Compounds: Self-Assembly Synthesis, Structural Topologies, and Magnetic Features Yauhen Y. Karabach,† Alexander M. Kirillov,† Matti Haukka,‡ Joaquin Sanchiz,§ Maximilian N. Kopylovich,† and Armando J. L. Pombeiro*,†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 4100–4108

Centro de Quı´mica Estrutural, Complexo I, Instituto Superior Te´cnico, TU Lisbon, AV. RoVisco Pais, 1049-001, Lisbon, Portugal, Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, FIN-80101, Joensuu, Finland, and Departamento de Quı´mica Inorga´nica, UniVersidad de La Laguna, 38200 La Laguna, Tenerife, Spain. ReceiVed May 28, 2008

ABSTRACT: Self-assembly syntheses based on the reactions, in aqueous solution at room temperature, of copper(II) nitrate, triethanolamine (H3tea), pyromellitic acid (H4pma), and ammonia or its derivatives (methylamine, ethylamine) give rise to the new Cu(II) 2D [Cu2(µ6-pma)(NH3)4]n · 2nH2O (1) and 3D [Cu2(µ-H2O)2(µ4-pma)(MeNH2)4]n (2) coordination polymers and the dimer [Cu2(µ2-pma)(H3tea)2(EtNH2)2] (3). A crucial synthetic and structural role is played by ammonia and its derivatives, acting also as pH regulators. 1-3 have been isolated as air-stable crystalline solids and characterized by IR spectroscopy and elemental and singlecrystal X-ray diffraction analyses, the latter showing the formation of infinite 2D (1) and 3D (2) metal-organic frameworks or discrete dimeric blocks (3) adopting distinct structural topologies. The structures are further extended through numerous H-bonding interactions to give 3D (1, 2) and 1D (3) hydrogen bonded supramolecular assemblies. Magnetic susceptibility measurements reveal weak antiferromagnetic coupling between the copper(II) ions. Compound 1 follows the Bleaney-Bowers dinuclear model, 2 the antiferromagnetic chain model, and 3 the Curie-Weiss law. The weak magnetic coupling is explained in terms of the poor overlap between the magnetic orbitals centered at the copper(II) ions through the bridging ligands. Introduction The synthesis of new copper-containing coordination polymers and supramolecular networks has become an important research field in view of their growing interest in areas of inorganic and coordination chemistry, crystal engineering and host-guest chemistry, as well as on account of various potential applications as new materials such as absorbents, catalysts, gasstorage systems, molecular conductors and magnets.1 Recently, we have developed2a a simple self-assembly approach toward the synthesis of various crystalline multicopper(II) compounds, which consists in the combination in water of a copper salt, a main chelating aminopolyalcohol ligand, a pH-regulator, and an auxiliary ligand or spacer. Since the Cu(II) compounds obtained thereof have shown some interesting structural,2 catalytic,2b,c,e magnetic,2a and host-guest2a,d features, the current work aims at further exploiting that self-assembly synthetic strategy toward searching for a flexible design of new multicopper compounds, by considering a number of factors that can potentially determine their structures and properties. Hence, in our previous studies,2 we obtained the compounds with distinct topologies by varying the combinations of the main chelating ligands (aminopolyalcohols, typically triethanolamine, H3tea) and spacers (benzenepolycarboxylic acids), as well as the types of alkali metal hydroxides used as pH-regulators. As a continuation of this project, the present work shows the possibility to modify the product structure utilizing different amine-type pH-regulators, namely ammonia, methylamine, and ethylamine, which are further involved in coordination, thus acting also as structure determinant agents. The choice of ammonia and its derivatives was governed by their availability, simplicity, and coordinating ability toward copper centers.3,4 * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +351 218419237. Fax: +351 218464455. † Instituto Superior Te´cnico. ‡ University of Joensuu. § Universidad de La Laguna.

Besides, pyromellitic (1,2,4,5-benzenetetracarboxylic) acid (H4pma) was applied as a recognized spacer5 in view of its versatile coordination modes, capacity to form multidimensional structural patterns, affinity for H-bonds, and ability to transmit magnetic interactions. Thus, we describe herein how a slight modification of an amine-type pH-regulator leads to three distinct self-assembled multicopper(II) compounds: the 2D [Cu2(µ6-pma)(NH3)4]n · 2nH2O (1) and 3D [Cu2(µ-H2O)2(µ4-pma)(MeNH2)4]n (2) coordination polymers, and the dimer [Cu2(µ2-pma)(H3tea)2(EtNH2)2] (3), all derived from pyromellitic acid and the corresponding amines. The characterization of these compounds by single-crystal X-ray diffraction, their structural features and magnetic properties are also reported. Experimental Section Materials and Methods. All synthetic work was performed in air and at room temperature. All chemicals were obtained from commercial sources and used as received. C, H and N elemental analyses were carried out by the Microanalytical Service of the Instituto Superior Te´cnico. Melting points were determined on a Kofler table. Infrared spectra (4000-400 cm-1) were recorded on a BIO-RAD FTS 3000MX instrument in KBr pellets. Magnetic susceptibility measurements on polycrystalline samples were carried out in the temperature range 2.0-300 K by means of a Quantum Design SQUID magnetometer operating at 1 T; at low temperatures, measurements were also performed at an applied field of 0.1 T. Diamagnetic corrections of the constituent atoms were estimated from Pascal’s constants, experimental susceptibilities were also corrected for the temperature-independent paramagnetism [60 × 10-6 cm3 mol-1 per Cu(II)] and the magnetization of the sample holder. General Synthetic Procedure for 1-3. To an aqueous solution (10.0 mL) containing Cu(NO3)2 · 2.5H2O (1.00 mmol, 233 mg) and HNO3 (1.00 mmol) [the acid was added to avoid spontaneous hydrolysis of the metal salt] were added dropwise triethanolamine (1.00 mmol, 130 µL), an excess of an aqueous solution of amine [ammonia (33% in H2O) (15.7 mmol, 1.0 mL) for 1, methylamine (35% in H2O) (10.1 mmol, 1.0 mL) for 2, or ethylamine (70% in H2O) (12.4 mmol, 1.0 mL) for 3] and pyromellitic acid (127 mg, 0.50 mmol), in this order

10.1021/cg8005597 CCC: $40.75  2008 American Chemical Society Published on Web 09/23/2008

Multicopper(II) Pyromellitate Compounds and with continuous stirring at room temperature. The resulting reaction mixture was stirred overnight and then filtered off. The filtrate was left to evaporate in a beaker at ambient temperature. Blue X-ray quality crystals were formed in 1-2 weeks, then collected and dried in air to furnish compounds 1-3 in ca. 50% yield (based on copper(II) nitrate). [Cu2(µ6-pma)(NH3)4]n · 2nH2O (1). Mp > 300 °C (dec.); FT-IR (KBr): 3549 (s), 3438 (s), 3331 (s) and 3124 (s) ν(NH) + ν(H2O), 2994 (w), 2920 (w) and 2852 (w) ν(CH), 1621 and 1584 (vs, one broadband with two maxima), and 1489 (m) νas(COO), 1411 (vs) and 1367 (vs) νs(COO), 1329 (m), 1284 (vs) 1230 (m), 1141 (w), 1101 (m), 1057 (s), 970 (m), 863 (m), 764 (s), 693 (m), 623 (m) and 466 (w) cm-1; elemental analysis calcd (%) for C10H18Cu2N4O10 (481.3): C 24.95, H 3.77, N 11.64; found C 24.90, H 3.57, N 11.39. [Cu2(µ-H2O)2(µ4-pma)(MeNH2)4]n (2). Mp ≈ 200 °C (dec.); FTIR (KBr): 3435 (s sh) and 3356 (s br), and 3149 (w) ν(NH)+ν(H2O), 2974 (w), 2931 (w) and 2898 (w) ν(CH), 1631 (s) and 1583 (m sh), and 1493 (w) νas(COO), 1415 (s sh) and 1369 (s br) νs(COO), 1323 (w), 1140 (m), 1080 (s) and 1061 (s sh), 1016 (m), 901 (m), 789 (w), 764 (w), 677 (m), 582 (w), 530 (m) and 488 (w) cm-1; elemental analysis calcd (%) for C14H26Cu2N4O10 (537.5): C 31.29, H 4.88, N 10.42; found C 31.71, H 5.28, N 10.11. [Cu2(µ2-pma)(H3tea)2(EtNH2)2] (3). Mp ≈ 200 °C (dec.); FT-IR (KBr): 3356 (s), 3293 (s), 3255 (s) and 3073 (s br) ν(NH)+ν(OH), 2974 (w) and 2878 (w) ν(CH), 1600 (m sh) and 1562 (s), and 1492 (s) νas(COO), 1420 (m sh) and 1384 (vs br) νs(COO), 1211 (m), 1141 (m), 1080 (s) and 1065 (s sh), 901 (m), 840 (w), 789 (m), 762 (m), 679 (s), 585 (w), 533 (m) and 484 (m) cm-1; elemental analysis calcd (%) for C26H46Cu2N4O14 (MW 765.8): C 40.78, H 6.05, N 7.32; found C 40.46, H 6.35, N 7.51. X-Ray Crystallography. The X-ray diffraction data of 1-3 were collected with a Nonius Kappa CCD diffractometer using Mo KR radiation. The Denzo-Scalepack6 program package was used for cell refinements and data reductions. All the structures were solved by direct methods using the SHELXS-977 or SIR978 programs with the WinGX9 graphical user interface. An analytical absorption correction (de Meulenaer Tompa)10 was applied to 1. An empirical absorption correction was applied to 2 and 3 using SADABS version 2.10 program.11a Structural refinements were carried out with SHELXL97.11b The H2O in 1, NH2 and H2O in 2, NH2 and OH in 3 hydrogen atoms were located from the difference Fourier map but constrained to ride on their parent atom (Uiso ) 1.5 (parent atom)). Other hydrogen atoms were positioned geometrically and were also constrained to ride on their parent atoms, with C-H ) 0.95 Å and N-H ) 0.91 Å (in 1), and C-H ) 0.95-0.98 Å (in 2 and 3), with Uiso ) 1.2-1.5 Ueq (parent atom). Crystal data and details of data collection for 1-3 are reported in Table 1.

Crystal Growth & Design, Vol. 8, No. 11, 2008 4101 Table 1. Crystal Data and Structure Refinement Details for 1-3

empirical formula fw T (K) λ (Å) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (mg/m3) µ(Mo KR) (mm-1) no. of collected reflns no. of unique reflns RInt Final R1a, wR2b (I g 2 σ) R1, wR2 (all data) GOF on F2 Largest diff. peak and hole, e Å-3 a

1

2

3

C5H9CuN2O5 240.68 120(2) 0.71073 Triclinic P1j 7.2921(6) 7.5161(7) 7.6282(5) 88.900(5) 78.627(6) 80.188(4) 403.84(6) 2 1.979 2.697 7263 1838 0.0648 0.0333, 0.0824

C7H13CuN2O5 268.73 120(2) 0.71073 Triclinic P1j 7.5307(2) 8.7478(3) 8.7673(4) 100.595(2) 93.595(2) 115.240(2) 507.01(3) 2 1.760 2.158 9734 2329 0.0339 0.0329, 0.0844

C26H46Cu2N4O14 765.75 120(2) 0.71073 Triclinic P1j 8.6030(5) 9.1945(4) 10.9382(6) 87.167(3) 71.930(3) 79.282(3) 808.16(7) 1 1.573 1.388 14478 3684 0.0535 0.0374, 0.0750

0.0419, 0.0862 0.0446, 0.0914 0.0647, 0.0841 1.065 1.055 1.057 0.734, -0.604 0.484, -0.750 0.434, -0.410

R1 ) Σ|Fo| - |Fc|/Σ|Fo|. b wR2 ) Σ[w(Fo2 - Fc2)2]/ Σ[w(Fo2)2]]1/2.

Scheme 1. Self-Assembly Syntheses of 1-3

Results and Discussion Synthesis and Spectroscopic Characterization. Recently, we have reported the self-assembly synthesis of copper(II) 2D [Cu2(µ-H2tea)2{µ3-Na2(H2O)4}(µ6-pma)]n · 10nH2O (A)2d and 1D [Cu2(H3tea)2(µ4-pma)]n] (B)2e coordination polymers, undertaken by combining, in aqueous solution, copper(II) nitrate, triethanolamine (H3tea), pyromellitic acid (H4pma) and an alkali metal hydroxide (NaOH for A and LiOH for B), applied as metal source, main chelating ligand, spacer and pH-regulator, respectively.2a By modifying this synthetic procedure and using, instead of alkali metal hydroxides, either ammonia, methylamine or ethylamine as pH-regulators (Scheme 1), we have obtained the new multicopper(II) compounds [Cu2(µ6-pma)(NH3)4]n · 2nH2O (1), [Cu2(µ-H2O)2(µ4-pma)(MeNH2)4]n (2), and [Cu2(µ2pma)(H3tea)2(EtNH2)2] (3) (Scheme 2). During the self-assembly synthesis the pH values varied from ca. 1.0 to 1.1 upon addition of H3tea to an acidic copper(II) nitrate solution, and then to ca. 10 (for 1) or 12 (for 2 and 3) after introducing an excess (ca. 10-16 equiv relative to Cu(NO3)2) of the corresponding amines. The slow evaporation in air of the obtained highly basic solutions lead to the crystallization of 1-3, which were isolated as deep blue crystalline solids in ca. 50% yields (based on copper(II) nitrate) and characterized by IR spectroscopy,

elemental and single-crystal X-ray diffraction analyses. It should be noted that the crystallization of 1-3 is not efficient or does not occur unless a high excess of amine is used. The obtained compounds are insoluble in water and in common organic solvents; they are stable in air and upon heating decompose at temperatures starting from above 300 (1) or 200 (2, 3) °C. Compounds 1-3 appear to be rather different from those with pyromellitate ligands which have been previously synthesized by us2d,e and others.5,13 This is because of the dual role of ammonia and its derivatives, since they behave not only as regulators of pH but also as terminal ligands, thus affecting the metal coordination environment and the structure thereof. In contrast to dimer 3, polymers 1 and 2 do not comprise triethanolamine moieties. Nevertheless, the use of H3tea is also crucial for the synthesis of 1 and 2, since, in the absence of triethanolamine, their isolation failed. Since H3tea is an effective chelating ligand to Cu(II) in solution, it can presumably behave as a template toward the formation of 1 and 2.

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Scheme 2. Schematic Representations of 1-3a

a

Numbers indicate the corresponding extensions of polynuclear chains.

The IR spectra of 1-3 exhibit related features with typical vibrations due to the pyromellitate and amine ligands, coordinated (2) and crystallization (1) water molecules, and the triethanolamine moiety (3). Thus, three or four typically strong bands in the wide 3550-3070 cm-1 range are assigned to ν(NH) and ν(H2O/OH) vibrations, whose broad character is associated with extensive hydrogen bonding interactions. Two sets of high or medium intensity bands are observed in the 1630-1490 and 1420-1365 cm-1 ranges due to the asymmetric and symmetric modes of carboxylate groups, respectively. The ∆ν values of ca. 180-220 cm-1, which concern the difference between the strongest νas(COO) and νs(COO) absorptions in compounds 1-3, are larger than ∆ν of 95 cm-1 reported for tetrasodium pyromellitate,5b,j thus indicating a monodentate coordination fashion of the carboxylate groups. The unprotonated character of two COO- groups in 3 is in accord with the absence of the band at 1740-1720 cm-1, typical for a protonated COOH moiety.2a,5j The elemental analyses of compounds 1-3 confirm their analytical purity and are consistent with formulations (Scheme 2) authenticated by X-ray diffraction studies, as indicated below. Description of X-Ray Crystal Structures. Although all three compounds crystallize in the triclinic space group P1j, possess comparable unit cell dimensions (Table 1), and are composed of related {Cu2(µ-pma)(amine)4} building blocks, their crystal structures are quite distinct. Hence, compounds 1 and 2 are 2D and 3D coordination polymers, respectively, whereas 3 is a dimer. The ellipsoid plots of structural fragments of 1 and 2 and molecular structure of 3 are shown in Figure 1. The crystal packing diagrams of 1-3 are depicted in Figures 2 and 3 (see also Supporting Information for additional Figures S1-S6). The selected bond distances and angles as well as hydrogen bonding parameters are given in Tables 2 and 3, respectively. The

bonding parameters in 1-3 are within typical values previously reported for copper compounds bearing pyromellitate,5 ammine,3 methylamine,4 ethylamine,4 or triethanolamine2,14 moieties. All the symmetry transformation codes along the discussion below are those of Tables 2 and 3. [Cu2(µ6-pma)(NH3)4]n · 2nH2O (1). The crystal structure of 1 (Figure 1a) is built from repeated symmetry equivalent Cu1 atoms that are interconnected through two bridging O1 atoms of carboxylate groups of different pma molecules, thus forming dimeric [Cu2(µ-Opma)2]2+ cores. They are multiply interlinked further through O3 pma atoms leading to the infinite 2D gridtype metal-organic framework running along the b axis (Figure 2a). Hence, each of the centrosymmetric pyromellitate(4-) spacers acts in a µ6-bridging mode connecting to six different Cu1 atoms through two pairs of carboxylate oxygen atoms O1 and O3 (Figure 1a). Each Cu1 atom is five coordinated and its tetragonal-pyramidal geometry is filled by two Opma atoms [Cu1-O1i 1.996(2) Å; Cu1-O3 1.947(2) Å] and two ammine N atoms [Cu1-N1 1.999(2) Å; Cu1-N2 1.976(2) Å] in basal positions, while an elongated [Cu1-O1 2.308(2) Å] apical site is occupied by the O1 atom of other pma moiety. It should be mentioned that in 1 the coordination mode of each O1 pma oxygen atom, that is bound simultaneously to two Cu atoms (Figure 1a, Scheme 2), is unusual and has not yet been observed in any transition metal pyromellitate compound.13 The binding of the NH3 and pma ligands is further stabilized by strong intramolecular H-bonds N(1)-H(1A) · · · O(4)ii [2.990(3) Å] and N(2)-H(2C) · · · O(4) [2.932(3) Å] (Figure 1a). The [Cu2(µ-Opma)2]2+ core is planar and possesses the Cu1 · · · Cu1i separation of 3.404(2) Å, whereas the representative O1i-Cu1-O1 and Cu1-O1-Cu1i bond angles are 75.73(8) and 104.27(8)°, respectively. The separations between neighboring dimeric cores depicted in Figure 2a are 7.2921(6) and

Multicopper(II) Pyromellitate Compounds

Crystal Growth & Design, Vol. 8, No. 11, 2008 4103

Figure 1. Structural fragments of (a) 1, (b) 2, and (c) 3 with the partial atom labeling and selected intramolecular H-bonds (dashed lines). Hydrogen atoms (apart from those involved in H-bonds) and crystallization water molecules are omitted for clarity. Displacement ellipsoids are drawn at the 50% (a, b) and 30% (c) probability level. Copper, green; nitrogen, blue; oxygen, red; carbon, gray ellipsoids; hydrogen, gray balls.

Figure 2. Fragments of the crystal packing diagrams of (a) 1, (b) 2, and (c) 3 representing the structural arrangement within one metal-organic layer (views along the b (a) and a axis (c), or arbitrary view (b)). Letters A, B, C correspond to the repeating cavities. Hydrogen atoms (1-3) and crystallization water molecules (1) are omitted for clarity. Copper cores, green; ligands, black.

7.6282(5) Å, being the a and c unit cell dimensions, respectively (Table 1). The 2D layers comprise cavities (see A, B, C in Figure 2a) formed by repeating 16-membered Cu-pma-Cu-pma rings through connective O(3)-Cu(1)-O(1)i [85.01(8)°] angles. [Cu2(µ-H2O)2(µ4-pma)(MeNH2)4]n (2). The crystal structure of 2 (Figure 1b) contains one centrosymmetric pma ligand per two symmetry nonequivalent Cu atoms which, however, adopt similar coordination environments. Both Cu1 and Cu2 atoms lie on the inversion centers and possess distorted octahedral geometries filled by the following symmetry generated pairs of mutually trans ligands: methylamine [avg. Cu-N 2.011(2) Å] and carboxylate groups [avg. Cu-O 2.009(2) Å] in equatorial sites, and bridging water molecules [avg. Cu-O 2.417(2) Å] in apical sites. Multiple H-bonds between H2O or NH2 groups and carboxylate oxygen atoms are observed (Table 3, Figure S5, Supporting Information). The pyromellitate(4-) spacers act in a µ4-mode linking four different Cu atoms and providing the formation of a 2D layered structure (Figure 2b). It is further extended to a third dimension by means of numerous µ-H2O

ligands which interconnect Cu atoms from different layers [Cu1ii-O7-Cu2 130.49°] (Figure S3, Supporting Information), thus giving rise to an infinite 3D metal-organic framework (Figure 3). This framework can be described as a collection of three kinds of crossing polymeric motifs: ∼Cu1-pma-Cu1pma∼, ∼Cu2-pma-Cu2-pma∼ and ∼Cu1-H2O-Cu2-H2O∼ with repeating periods of 11.194(3), 11.189(3), and 4.389(2) Å, respectively (Figure 3). Besides, the 3D network reveals the presence of cavities (see A in Figure 2b) within the repeating 20-membered Cu1-pma-Cu2-H2O-Cu1-pma-Cu2-H2O cycles. [Cu2(µ2-pma)(H3tea)2(EtNH2)2] (3). In contrast to compounds 1 and 2, the crystal structure of 3 is not polymeric and apart from ethylamine comprises triethanolamine ligands (Figure 1c). The dimeric structure is composed of two symmetry equivalent monomeric [Cu(H3tea)]2+ units linked by the centrosymmetric pyromellitate(4-) ligand acting in a µ2-bridging mode through two para carboxylate groups (O5-C9-O6). The other two carboxylate moieties (O7-C12-O8) remain unco-

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Figure 3. Crystal packing diagram of 2 (arbitrary view) showing the infinite 3D metal-organic framework. All hydrogen atoms and methyl groups of MeNH2 are omitted for clarity. Cu1 cores, blue; Cu2 cores, green; µ-H2O molecules, red; pyromellitate spacers, dark gray. Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1-3a 1 Cu(1)-O(1) Cu(1)-O(1)i Cu(1)-O(3) Cu(1)-N(1) Cu(1)-N(2)

2.308(2) 1.996(2) 1.947(2) 1.999(2) 1.976(2)

Cu(1) · · · Cu(1)i N(2)-Cu(1)-N(1) O(3)-Cu(1)-O(1)i O(1)i-Cu(1)-O(1) Cu(1)-O(1)-Cu(1)i

Cu(1)-O(1) Cu(1)-O(7)i Cu(1)-N(2) Cu(2)-O(5) Cu(2)-O(7) Cu(2)-N(6)

2.026(2) 2.401(2) 2.004(2) 1.992(2) 2.433(2) 2.017(2)

Cu(1) · · · Cu(2) Cu(1)ii-O(7)-Cu(2) O(1)-Cu(1)-N(2) O(1)-Cu(1)-O(7)i O(5)-Cu(2)-N(6) O(5)-Cu(2)-O(7)iii

Cu(1)-O(1) Cu(1)-O(2) Cu(1)-O(3) Cu(1)-O(5) Cu(1)-N(1)

2.008(2) 2.352(2) 2.464(2) 1.975(2) 2.037(2)

Cu(1)-N(4) N(1)-Cu(1)-O(2) N(4)-Cu(1)-O(2) O(3)-Cu(1)-O(5) O(5)-Cu(1)-N(4)

3.404(2) 92.14(10) 85.01(8) 75.73(8) 104.27(8)

2 4.389(2) 130.49(8) 91.97(9) 85.15(7) 85.96(8) 93.14(7)

3 1.982(2) 79.08(7) 100.64(8) 89.61(8) 90.67(8)

a Symmetry transformations used to generate equivalent atoms: for 1: (i) - x + 1, - y, - z + 1; for 2: (i) x, y, z - 1; (ii) x, y, z + 1; (iii) x + 2, - y + 1, - z + 2.

ordinated and are unprotonated, thus neutralizing the overall positive charge of the two Cu fragments separated by 11.171(3) Å. The distorted octahedral coordination geometry around each Cu1 atom is filled by the O1 and N1 H3tea atoms [Cu-O1 2.008(2) Å; Cu-N1 2.037(2) Å], the ethylamine N atom [Cu1-N4 1.982(2) Å] and one pma oxygen atom [Cu1-O5 1.975(2) Å] in equatorial sites, whereas the axial positions are occupied by the remaining triethanolamine OH groups with considerably lengthened coordination bonds [Cu1-O2 2.352(2) Å; Cu1-O3 2.464(2) Å]. The major deviation from the octahedral geometry concerns the N1-Cu1-O2 and N4Cu1-O2 angles of 79.08(7) and 100.64(8)°, respectively. The binding of the tetradentate H3tea ligand involves three fivemembered chelate rings with the N-Cu-O bite angles ranging from 78.84(7) to 84.72(7)° [avg. 80.88(7)°]. Two O(3)-H(3O) · · · O(6) [2.644(2) Å] and two N(4)-H(4C) · · · O(7) [3.023(3) Å] intramolecular H-bonds connect either one OH group of H3tea or the EtNH2 moiety with all four carboxylate groups of pma, thus strengthening its linkage to the Cu units (Figure 1c).

Comparison of Architectures in 1-3. By considering the size of equivalent fragments of the crystal packing diagrams (Figure 2), the distinct character of the structural architectures (topologies) in 1-3 within one metal-organic layer can be recognized (Figure 4). Thus, the µ6-mode of pma and the involvement of carboxylate groups in the formation of dimeric [Cu2(µ-Opma)2]2+ cores (altogether three of five coordination sites around equal Cu atoms are occupied by pma) define the highly pma saturated grid-type topology of 2D sheets in 1. In contrast, the metal-organic layers of 2 possess only half of pma saturation. These features of 2 arise from the existence of two nonequivalent copper cores that have different orientations and only two from six coordination sites are occupied by µ4pma spacers. Besides, the more efficient packing of 1 is in agreement with the Kitaigorodsky packing indices12 (KPI; that is, the ratio between the occupied volume and the total unitcell volume) that is higher in 1 (78.6%) vs 2 (74.2%). In 3, the pma saturation and topology of dimeric [Cu2(pma)] blocks is somehow related to those of 1, although without the formation of polymeric metal-organic network. Compound 1 exhibits both the shortest [6.520 Å] and longest [11.546 Å] Cu-pma-Cu separations through o- or p-carboxylate groups, respectively (Table 4). In 1-3, the shortest Cu · · · Cu separations between two collateral layers (the second layer lies behind those depicted in Figures 2 and 4) are 5.228, 4.389, and 8.460 Å, respectively. In summary, all of the compounds 1-3 possess distinct structural patterns initially defined by the type of pH-regulator used in the self-assembly synthesis (Scheme 1). Supramolecular Features in 1-3. The presence of numerous NH3/NH2 moieties (H-bond donors) and crystallization (1) or coordinated (2) H2O molecules, or OH groups (3) (both H-bond donors and acceptors) in the compounds, in combination with multiple carboxylate oxygen atoms of pyromellitate(4-) ligands (H-bond acceptors), leads to an extensive hydrogen bonding (Table 3, Figures S4-S6 in the Supporting Information), thus contributing to a structure-stabilizing effect and resulting in the formation of H-bonded supramolecular architectures. Hence, in 1 the neighboring 2D metal-organic layers are disposed relatively close (avg. separation is about half of the b unit cell dimension), giving rise to an extensive direct interlayer H-bonding. Besides, the presence of interlayer crystallization water molecules and their involvement into H-bonding provides an additional linkage of collateral

Multicopper(II) Pyromellitate Compounds

Crystal Growth & Design, Vol. 8, No. 11, 2008 4105 Table 3. Hydrogen Bond Geometry (Å, deg) in 1-3a

D-H · · · A

d (D · · · A)

∠ (D-H · · · A)

O(5)-H(5O) · · · O(2) O(5)-H(5P) · · · O(2)i N(1)-H(1A) · · · O(4)ii N(1)-H(1C) · · · O(2)iii N(1)-H(1B) · · · O(5)iii N(2)-H(2A) · · · O(4)iv N(2)-H(2B) · · · O(5)iii N(2)-H(2C) · · · O(4)

2.842(3) 2.934(3) 2.990(3) 2.962(3) 3.303(3) 3.033(3) 3.097(3) 2.932(3)

154.6 166.4 149.1 148.0 126.5 172.5 146.7 131.9

O(7)-H(7O) · · · O(3)i O(7)-H(7P) · · · O(4)ii N(2)-H(2A) · · · O(5)i N(2)-H(2B) · · · O(3)iii N(6)-H(6B) · · · O(1)iv N(6)-H(6A) · · · O(3)v N(6)-H(6B) · · · O(7)

2.702(3) 2.661(3) 2.958(3) 3.334(3) 3.193(3) 3.041(3) 3.117(3)

165.4 168.8 171.3 133.6 161.5 174.1 112.2

O(1)-H(1O) · · · O(8)i O(2)-H(2O) · · · O(7)i O(3)-H(3O) · · · O(6) O(3)-H(3O) · · · O(5) N(4)-H(4C) · · · O(7) N(4)-H(4C) · · · O(2)i N(4)-H(4D) · · · O(8)i

2.537(2) 2.659(2) 2.644(2) 3.147(2) 3.023(3) 3.246(3) 3.183(3)

170.2 164.3 164.1 120.1 161.5 110.8 138.2

symmetry code 1 (i) -x + 1, -y - 1, -z + 2; (ii) x + 1, y, z; (iii) x, y + 1, z; (iv) -x, -y + 1, -z + 1

2 (i) -x + 1, -y, -z + 1; (ii) -x + 2, -y + 1, -z + 2; (iii) -x + 1, -y, -z; (iv) x, y, z + 1; (v) -x + 2, -y, -z + 1

3

a

(i) -x + 1, -y + 1, -z + 1

Some H-bonds may not be within the limits of classical H-bonds and thus should be considered as “weak hydrogen bonds”.15

Figure 4. Schematic comparative representation of the architectures in 1-3 within one metal-organic layer (size equivalent fragments are shown) depicted in Figure 2. Copper cores, green sticks/crosses; pyromellitate spacers, black crosses.?> Table 4. Selected Cu · · · Cu Separations (Å) in Compounds 1-3a compound Cu-pma-Cu(within one layer) Cu · · · Cu(collateral layers) 1 2 3

6.520 (11.546) b 6.881 (11.192) b (11.171) b

5.228 4.389 8.460

a The values correspond (where appropriate) to the shortest separations within those metal-organic layers represented in Figures 2 and 4. b In parentheses is the value of the longest separation (through p-carboxylate groups).

layers, thus sewing them into a 3D supramolecular assembly (Figure S4, Supporting Information). In 2, the already 3D polymeric structure is further stabilized by numerous H-bonds between H2O, NH2 and COO- groups (Figure S5, Supporting Information). In 3, the neighboring dimeric blocks are held together via multiple N-H · · · O and O-H · · · O contacts (Table 3), leading to the formation of 1D H-bonded double chains with the ∼Cu2-pma-Cu2-pma∼ motif (Figure S6, Supporting Information). The long distance between those chains prevents the existence of interchain H-bonding contacts and thus the structure extension to further dimensions. Magnetic Properties. The magnetic properties of compounds 1, 2, and 3 under the form of χMT vs T plot in the 2-300 K range are shown in Figures 5 and 6 [χM is the magnetic susceptibility per two Cu(II) ions for 1 and per one Cu(II) ion

for 2 and 3]. At room temperature, the χΜT values are the expected ones for two isolated spin doublets for 1 and for one spin doublet for 2 and 3. Upon cooling, χΜT remains almost constant up to 50 K, and then decreases at lower temperatures, thus all the compounds displaying an overall weak antiferromagnetic coupling between the copper(II) ions. However, there is no maximum in the χΜ vs T plot. The crystal structure of 1 is made up from dinuclear [Cu2(µ-Opma)2]2+ cores and the magnetic susceptibility data can be analyzed by means of the Bleaney-Bowers equation for a copper(II) dimer, eq 1,16 where J is the intramolecular exchange coupling constant, g the Lande´ factor, β the Bohr magneton and k the Boltzman’s constant, the spin Hamiltonian defined as H ) -J(S1S2)

χM )

6 Nβ2g2 3kT 3 + exp(-J ⁄ kT)

[

]

(1)

The best least-squares fit parameters are g ) 2.077(9), J ) -2.03(1) cm-1, and R ) 0.9994, the calculated curve matching very well the magnetic susceptibility data in the whole temperature range (see Figure 5a). Compound 2 has a 3D structure, but the Cu(II) · · · Cu(II) distances within the layers are so long that the only active magnetic exchange pathway is defined by means of the µ-H2O bridges which interconnect Cu(II) ions from different layers

4106 Crystal Growth & Design, Vol. 8, No. 11, 2008

Karabach et al. Scheme 3. Schematic View of the Relative Orientation of the dx2-y2 Orbitals of the Copper(II) Ions with Respect to the Bridging µ-Opma Atoms in 1

data can be analyzed by means of the Bonner and Fisher expression for antiferromagnetic chains,16 eq 2 with x ) |J|/ kT.

χM )

Figure 5. χMT vs T plot for (a) 1 and (b) 2: (o) experimental data, the blue line is the best fit curve (see text). The magnetic susceptibility is referred to two copper(II) ions for 1 and one copper(II) ion for 2.

(Figure 3 and Figure S7, Supporting Information). Under this approach we have copper(II) uniform chains and the magnetic

0.25 + 0.074975x + 0.075235x2 Nβ2g2 kT 1.0 + 0.9931x + 0.172135x2 + 0.757825x3 (2)

The best fit parameters are g ) 2.19(1), J ) -0.70(1) cm-1, and R ) 0.9965, the calculated curve matching very well the magnetic susceptibility data in the whole temperature range (see Figure 5b). Although compound 3 is a copper(II) dimer, the Cu(II) · · · Cu(II) distance is rather long [Cu(1) · · · Cu(2) ) 11.171(1) Å] and the efficiency for electronic communication of the long pma bridge is so low that the magnetic susceptibility data do not follow the Bleaney-Bowers dinuclear model, and the weak antiferromagnetic coupling is better interpreted by means of the Curie-Weiss law. Under that approach the best fit parameters are C ) 0.411 cm3 mol-1, Θ ) -0.68(4) K, and R ) 0.99998. The weak antiferromagnetic coupling between the copper(II) ions in compound 1 can be explained by means of the poor overlap between their SOMOs through the double µ-Ocarboxylate bridge.16 The environment of the copper(II) ions in compound 1 is tetragonal-pyramidal [the basal plane defined by the two Cu-N and the two short Cu-O distances] and the magnetic orbitals have dx2-y2 character with some admixture of the dz2 due to the trigonal distortion of the copper(II) environment, τ

Figure 6. Temperature dependence of χ-1 and χT for 3. The black line is the Curie-Weiss fit for χ-1.

Multicopper(II) Pyromellitate Compounds

Crystal Growth & Design, Vol. 8, No. 11, 2008 4107

Table 5. Relevant Data for the Structural and Magnetically Characterized µ-Ocarboxylate Dinuclear Copper(II) Complexes compounda 7

[CuL (MeCO2)]2 [CuL1(MeCO2)]2 [CuL2(MeCO2)]2 [CuL3(MeCO2)]2 [CuL4(MeCO2)]2 · 2H2O [CuL5(MeCO2)] · 2MeOH [CuL6(MeCO2)]2 · H2O · EtOHc [Cu(PhCONHCH2CO2)(H2O)]2 · 2H2O [Cu(tzq)2(HCO2)]2(µ-HCO2)2 · 4H2O Compound 1

Ro/Å

φ/°

J (cm-1)

2.490(1) 2.665(4) 2.577(2) 2.512(5) 2.498(8) 2.495(6) 2.446(2) 2.651(1) 2.37(1) 2.331(4) 2.308(2)

95.34(5) 96.3(5) 96.1(1) 96.9(2) 98.1(3) 98.3(5) 95.7(1) 102.6(1) 101.0(5) 102.2(2) 104.3(1)

-0.25 -1.84 -1.51 -1.33 -1.54 (-2.26) b -1.50 (-7.88) b +0.63 -2.15 -0.52 -2.03

ref 20 22 22 17, 23 23 24 25 18, 19 21 this work

a HL1 ) N-(5-Bromosalicylidene)-N-methylpropane-1,3-diamine, HL2 ) N-methyl-N′-(5-nitrosalicydene)propane-1,3-diamine, HL3 ) N-methyl-N′salicydenepropane-1,3-diamine, HL4 ) N-(5-methoxysalicydene)-N-methylpropane-1,3-diamine, HL5 ) N,N′-[bis(2-o-hydroxybenzylidene-amino)ethyl]ethane-1,2-diamine, HL6 ) N-(2-hydroxy-1,1-dimethylethyl)salicyleneamine, HL7 ) 7-amino-4-methyl-5-azahept-3-en-2-onate. b Magnetic behavior described as alternating chain. c This is the unique compound whose Cu2O2 system is non centrosymmetric.

) 0.10 [τ ) 0 for a perfect tetragonal-pyramidal environment and τ ) 1 for a perfect trigonal-bipyramidal one].17 The bridging µ-Opma atoms fill a basal position with respect to one copper(II) ion and an apical position with respect to the other Cu(II) ion of the [Cu2(µ-Opma)2]2+ core. With this orientation of the bridge the dx2-y2 magnetic orbitals have a parallel arrangement and the µ-Opma bridges provide a very poor overlap between them (Scheme 3). Comparable values for the coupling constant have been previously reported for compounds with similar arrangement of the [Cu2O2] core (Table 5). It can be seen that the weak magnetic interaction found for compound 1 is in the expected range.18-26 The magnetic coupling in compound 2 is also very small, and it is also due to a very poor overlap between the magnetic orbitals of the copper(II) ions. The geometry of the copper(II) ions in 2 is elongated-octahedral, the equatorial plane being defined by the two Cu-N and the two short Cu-O distances. The two bridging water molecules fill the axial positions. The SOMOs of copper(II) ions in this geometry are mostly of the dx2-y2 character and the µ-H2O bridges link axial positions, where there is a very little spin density.16 This situation leads to the observed very weak magnetic interaction. The magnetic coupling in 3 is very small as expected from the large separation between the copper(II) ions. Conclusions The present work has opened up a new self-assembly synthetic strategy toward multicopper(II) compounds that is based on the use of ammonia, methylamine and ethylamine as versatile dual role pH-regulators. Such a route broadens the generality of our previously reported self-assembly approach,2a leading to three new multicopper(II) compounds 1-3 with pyromellitate spacers. These crystalline materials can be readily obtained in aqueous medium, under ambient conditions and using rather simple and cheap commercially available chemicals. A slight modification in the pH-regulator affects strongly the composition, structure, and topology of the obtained compounds. All the compounds exhibit very weak magnetic interactions. In 1 and 2, the relative configuration of the bridging groups with respect to the magnetic orbitals provides a poor overlap between them, while in the case of compound 3 it is simply due to the very long Cu · · · Cu distance. The study also expands the use of pyromellitic acid as a convenient spacer5 toward the design of diverse metal-organic frameworks and widening their family. Besides, a particular feature of the 2D and 3D coordination polymers 1 and 2 consists in their stability, insolubility and some porosity, encouraging the search for applications in, for example, heterogeneous catalysis and materials chemistry. Further research will also be

focused on the extension of the series of amines as convenient and versatile pH-regulators for the self-assembly synthesis of a diversity of metal-organic frameworks. Acknowledgment. This work has been partially supported by the Foundation for Science and Technology (FCT) and its POCI 2010 programme (FEDER funded). J.S. acknowledges the Servicio General de Medidas Magne´ticas of the Universidad de La Laguna for the magnetic measurement facilities. Supporting Information Available: X-ray crystallographic file for the structure determinations of 1-3, and Figures S1-S7 showing additional crystal packing diagrams of 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG8005597