Influence of Coordinating and Non-Coordinating Anions and of a

Feb 12, 2008 - Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Faculty of Chemistry and Chemical ...
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CRYSTAL GROWTH & DESIGN

Influence of Coordinating and Non-Coordinating Anions and of a Methoxy Substituent on the Formation of Copper-Based Coordination Assemblies

2008 VOL. 8, NO. 3 1005–1012

Jinkui Tang,† José Sánchez Costa,† Andrej Pevec,‡ Bojan Kozlevcˇar,‡ Chiara Massera,§ Olivier Roubeau,| Ilpo Mutikainen,⊥ Urho Turpeinen,⊥ Patrick Gamez,† and Jan Reedijk*,† Leiden Institute of Chemistry, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Faculty of Chemistry and Chemical Technology, UniVersity of Ljubljana, AškercˇeVa 5, P.O. Box 537, 1000 Ljubljana, SloVenia, Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, UniVersità degli Studi di Parma, Viale G. Usberti 17/A, 43100 Parma, Italy, UniVersité Bordeaux 1, CNRS-CRPP, 115 aVenue du dr. A. Schweitzer, 33600 Pessac, France, and Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55 (A. I. Virtasenaukio 1), 00014 UniVersity of Helsinki, Helsinki, Finland ReceiVed October 11, 2007; ReVised Manuscript ReceiVed December 16, 2007

ABSTRACT: A series of copper(II) complexes with pyridine-phenol-based N,O/donor ligands has been synthesized and structurally characterized. Slight variations of one of the building blocks (namely, the ligand or the counterion) lead to drastic changes in the nature of the resulting coordination assemblies. Indeed, depending on the anions involved and on a minor modification of the ligand, a dinuclear, a one-dimensional chain, or two cubane copper structures are obtained. The magnetic properties of all polynuclear copper(II) compounds have been investigated, which show that the different clusters exhibit antiferromagnetic interactions. Introduction The formation of cluster coordination complexes is now ubiquitous in chemistry,1–3 mainly stimulated by the field of molecular magnetism.4–6 The reaction of transition metal ions with small bridging ligands (such as chloride, azide, hydroxide, methoxide, and so on) may result in the formation of metallosupramolecular architectures exhibiting remarkable metal-metal interactions.7–10 The growth to infinite networks is normally blocked by terminal monodentate or polydentate ligands, which usually act also as bridging ligands.11 Cubanes are amazing metalloclusters that have received great attention during the past two decades, for their potential physical properties12–14 and for their application as biomimetic models of nitrogenases and hydrogenases.15–17 Recently, some of us have reported the use of the ligand 2-methoxy-6-(pyridine-2-ylhydrazonomethyl)phenol (Hmphp, Chart 1)18 for the generation of a unique fused double-stranded [MnII3] dihelicate.19 The potential of this ligand to produce interesting supramolecular arrays is now evaluated with other metal ions. In the present study, the coordination of Hmphp to different copper(II) salts is explored. In particular, the effect of the anion, coordinating or noncoordinating, on the coordination arrangement between the metal and the organic ligand is examined. It appears that the bridging ability of the anion has a drastic effect on the resulting coordination framework. In that context, the role of the methoxy group of Hmphp is also assessed. The presence of this methoxy substituent has been found to be crucial for the formation of the manganese(II) dihelicate, described earlier.19 Therefore, the coordination of the ligand without the methoxy group, namely, 6-(pyridine-2ylhydrazonomethyl)phenol (Hphp, Chart 1),20,21 to copper is * To whom correspondence should be addressed. E-mail: reedijk@ chem.leidenuniv.nl. † Leiden Institute of Chemistry, Leiden University. ‡ University of Ljubljana. § Università degli Studi di Parma. | Université Bordeaux 1. ⊥ University of Helsinki.

Chart 1

also examined. The magnetic properties of all the new copper compounds are investigated in detail. Experimental Section All chemicals were of reagent grade and were used as commercially obtained. Elemental analyses for C, H, and N were performed with a Perkin-Elmer 2400 analyzer. Fourier tranform infrared (FTIR) spectra were recorded with a Perkin-Elmer Paragon 1000 FTIR spectrophotometer, equipped with a Golden Gate ATR device, using the reflectance technique (4000-300 cm-1). The ligands Hmphp18 and Hphp21 were prepared following procedures described in the literature. X-band electron paramagnetic resonance (EPR) measurements were performed at 77 K in the solid state and in frozen solutions on a Jeol RE2x electron spin resonance spectrometer, using 2,2-diphenyl-picrylhydrazyl (DPPH) (g ) 2.0036) as a standard. Synthesis of [Cu2(mphp)Cl3(MeOH)(H2O)](MeOH) (1). CuCl2 · 2H2O (85 mg, 0.50 mmol) was added to a solution of Hmphp (61 mg, 0.25 mmol) in methanol (10 mL). The resulting dark green solution was left unperturbed to allow a slow evaporation of the solvent. After three days, dark blue block-shaped crystals, suitable for X-ray diffraction analysis, were obtained. Yield ) 60 mg (43% based on the ligand). Anal. Calcd. for C15H22Cl3Cu2N3O5 (fw ) 557.76 g mol-1): C, 32.30; H, 3.98; N, 7.53. Found: C, 32.43; H, 4.11; N, 7.48. Main IR absorption bands for 1 (cm-1): 1622 (vs), 1526 (s), 1455 (s), 1431 (s), 1249 (s), 1220 (vs), 1145 (s), 1099 (m), 1010 (s), 973 (s), 735 (vs), 617 (s). Synthesis of [Cu(php)Cl](MeOH) (2). CuCl2 · 2H2O (85 mg, 0.50 mmol) was added to a solution of Hphp (53 mg, 0.25 mmol) in methanol (10 mL). The resulting green solution was left unperturbed to allow a slow evaporation of the solvent. After two days, dark green block-shaped crystals, suitable for X-ray diffraction analysis, were obtained. Yield ) 50 mg (58% based on the ligand). Anal. Calcd. for C13H14ClCuN3O2 (fw ) 343.27 g mol-1): C, 45.49; H, 4.11; N, 12.24. Found: C, 45.60; H, 4.29; N, 12.11. Main IR absorption bands for 2 (cm-1): 2938 (w), 1622 (vs), 1598 (s), 1538 (s), 1471 (s), 1424 (s),

10.1021/cg700993s CCC: $40.75  2008 American Chemical Society Published on Web 02/12/2008

1006 Crystal Growth & Design, Vol. 8, No. 3, 2008

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Table 1. Crystal Data and Structure Refinement for [Cu2(mphp)Cl3(MeOH)(H2O)](MeOH) (1), [Cu(php)Cl](MeOH) (2), [Cu4(mphp)4](ClO4)4 (3), and [Cu4(mphp)4](BF4)2 · (SiF6) · (H2O)7 (4) formula fw (g mol-1) cryst size (mm3) cryst color temperature (K) cryst syst, space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) volume (Å3) Z calcd density (g cm-3) F(000) abs coeff (mm-1) θ for data collection (deg) rflns collected (Rint) data/params goodness of fit on F2a R1b (wR2)c [Fo > 4σ(Fo)] largest diff peak and hole (e Å3)

1

2

3

4

C15H22Cl3Cu2N3O5 557.76 0.62 × 0.45 × 0.30 blue 150(2) monoclinic, P21/n 15.152(3) 8.296(2) 16.555(3) 90.00 95.61(3) 90.00 2071.0(7) 4 1.779 1116 2.472 1.74–27.49 24389 (0.0749) 4714/257 1.292 0.0398 (0.1277) 1.039 and -2.128

C13H14ClCuN3O2 343.27 0.25 × 0.20 × 0.20 green 150(2) monoclinic, P21/c 13.3013(4) 7.5995(2) 14.3627(2) 90.00 110.4358(13) 90.00 1360.46(7) 4 1.676 700 1.805 3.08–27.46 5714 (0.0252) 3091/191 1.016 0.0338 (0.0888) 0.691 and -0.469

C52H48Cl4Cu4N12O24 1620.98 0.13 × 0.11 × 0.09 dark green 293(2) tetragonal, P4j21/c 12.159(2) 12.159(2) 20.958(3) 90.00 90.00 90.00 3098.5(7) 2 1.737 1640 1.619 1.94–27.49 13748 (0.0820) 3541/200 1.030 0.0508 (0.0795) 0.702 and -0.432

C52H62B2Cu4F14N12O15Si 1665.12 0.20 × 0.20 × 0.20 green 173(2) triclinic, P1j 14.936(3) 15.712(3) 17.855(4) 72.45(3) 69.04(3) 62.25(3) 3416.3(16) 2 1.632 1716 1.354 2.75–25.00 36749 (0.0579) 11449/763 1.084 0.0974 (0.2337) 1.167 and -1.095

a Goodness-of-fit S ) [Σw(Fo2 - Fc2)2/(n - p)]1/2, where n is the number of reflections and p is the number of parameters. |Fc|/Σ|Fo|. c wR2 ) [Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]]1/2.

1371 (m), 1338 (s), 1285 (s), 1203 (s), 1142 (s), 1110 (s), 1038 (s), 1016 (s), 954 (s), 931 (s), 878 (m), 855 (s), 791 (w), 752 (vs), 654 (w), 460 (s), 417 (s), 322 (vs). Synthesis of [Cu4(mphp)4](ClO4)4 (3). Cu(ClO4)2 · 6H2O (186 mg, 0.50 mmol) was added to a solution of Hmphp (61 mg, 0.25 mmol) in methanol (10 mL). The resulting green solution was left unperturbed to allow a slow evaporation of the solvent. After three days, dark green prismatic crystals, suitable for X-ray diffraction analysis, were obtained. Yield ) 45 mg (45% based on the ligand). Anal. Calcd. for C52H48Cl4Cu4N12O24 (fw ) 1621 g mol-1): C, 38.53; H, 2.98; N, 10.37. Found: C, 38.93; H, 3.06; N, 10.56. Main IR absorption bands for 3 (cm-1): 1624 (vs), 1534 (s), 1485 (vs), 1457 (s), 1432 (s), 1349 (w), 1288 (m), 1245 (vs), 1219 (s), 1150 (w), 1089 (vs), 1042 (vs), 973 (s), 841 (m), 776 (s), 729 (vs), 618 (vs), 485 (s). Synthesis of [Cu4(mphp)4](BF4)2(SiF6)(H2O)7 (4). Cu(BF4)2 · 6H2O (173 mg, 0.50 mmol) was added to a solution of Hmphp (61 mg, 0.25 mmol) in methanol (10 mL). The resulting dark green solution was left unperturbed to allow a slow evaporation of the solvent. After three days, dark green block-shaped crystals, suitable for X-ray diffraction analysis, were obtained. Yield ) 35 mg (33% based on the ligand) Anal. Calcd. for C52H62B2Cu4F14N12O15Si (fw ) 1665.12 g mol-1): C, 37.51; H, 3.75; N, 10.09. Found: C, 37.18; H, 4.39; N, 10.08. Main IR absorption bands for 4 (cm-1): 2951 (w), 1626 (vs), 1538 (s), 1482 (s), 1456 (s), 1435 (s), 1352 (w), 1289 (s), 1246 (vs), 1220 (s), 1147 (s),1061 (vs), 970 (s), 843 (w), 725 (s), 476 (w), 323 (w) cm-1. The SiF62- anions most likely developed from the decomposition of BF4- followed by the attack of the resulting fluoride ions on the glass surface of the Erlenmeyer flask used for the reaction.22,23 Magnetic Measurements. Magnetic susceptibility measurements were carried out using a Quantum Design MPMS-5 5T SQUID magnetometer at various fields in the temperature range 5–300 K. Data were corrected for the diamagnetic contributions estimated from Pascal’s tables.24 X-ray Crystallographic Analysis and Data Collection. The molecular structure of complexes 1-4 were determined by single-crystal X-ray diffraction methods. Crystallographic data and refinement details are given in Table 1. X-ray crystallographic data for 1 were collected on a Nonius Kappa CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at 150 K. A suitable crystal (blue block) was affixed to the end of a glass fiber using silicone grease and transferred to the goniostat. DENZO-SMN25 was used for data integration and SCALEPACK25 corrected data for Lorentz-polarization effects. The structures were solved by direct methods and refined by a full-matrix least-squares method on F2 using the SHELXTL crystallographic

b

R1 ) Σ|Fo| -

software package.26,27 All non-hydrogens were refined anisotropically. All hydrogens were found in difference Fourier maps and placed geometrically on their riding atom. X-ray crystallographic data for 2 were collected on a Nonius Kappa CCD difractometer at 150(2) K. A Cryostream cooler (Oxford Cryosystems) was used for cooling the sample. Graphite monochromated Mo KR radiation (λ ) 0.71073 Å) was employed. The structure was solved by direct methods implemented in SHELXS-9727 and refined by a full-matrix least-squares procedure based on F2 using SHELXL97.26 Intensity data and cell parameters for 3 were recorded at room temperature on a Bruker AXS Smart 1000 single-crystal diffractometer (Mo KR radiation) equipped with a CCD area detector. The data reduction was performed using the SAINT and SADABS programs.28 The structure was solved by Direct Methods using the SIR97 program29 and refined on Fo2 by full-matrix least-squares procedures, using the SHELXL-97 program.26 All non-hydrogen atoms were refined with anisotropic atomic displacements with the exception of the perchlorate ion. The hydrogen atoms were included in the refinement at idealized geometries (C-H 0.95 Å) and refined “riding” on the corresponding parent atoms. The weighting scheme used in the last cycle of refinement was w ) 1/[σ2Fo2 + (0.0161P)2] (where P ) (Fo2 + 2Fc2)/3). Molecular geometry calculations were carried out using the PARST97 program30 and the PLATON package.31 A single crystal of 4 was selected for the X-ray measurements and mounted to the glass fiber using the oil drop method32 and data were collected at 193 K using a Nonius KappaCCD diffractometer with graphite monochromatised Mo KR-radiation. The intensity data were corrected for Lorentz and polarization effects and for absorption. The tetrafluoroborate and silicon hexafluoride groups were disordered. These groups were refined in different positions as rigid groups. All the aromatic six rings were refined as rigid groups. Part of the water oxygen atoms were refined in isotropically. The water H atoms were not located. The other H atoms were geometrically fixed and allowed to ride on the attached atoms. Crystallographic data (excluding structure factors) for the structures reported have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications nos. CCDC 663145 (1), 663146 (2), 663147 (3), and 663148 (4) and can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cam-

Dinuclear 1D Polymer and Cubane Cu Structures

Crystal Growth & Design, Vol. 8, No. 3, 2008 1007 Table 2. Selected Bond Lengths (Å) in [Cu2(mphp)Cl3(MeOH)(H2O)](MeOH) (1), [Cu(php)Cl](MeOH) (2), [Cu4(mphp)4](ClO4)4 (3), and [Cu4(mphp)4](BF4)2 · (SiF6) · (H2O)7 (4) 1 Cu1-Cl1 Cu1-Cl2 Cu1-N1 Cu1-N8 Cu1-O16

2.737(1) 2.254(1) 1.981(3) 1.970(3) 1.949(3)

Cu1-N1 Cu1-O1 Cu1-Cl1_a

1.988(2) 1.903(2) 2.858(1)

Cu1-N1 Cu1-O1 Cu1-O2_c

1.949(4) 1.983(4) 2.299(5)

Cu1-N81 Cu1-N88 Cu1-O17 Cu1-O77 Cu1-O18 Cu1-O37 Cu3-N41 Cu3-N48 Cu3-O37 Cu3-O57 Cu3-O58 Cu3-O77

1.977(5) 1.946(9) 2.001(9) 1.975(7) 2.293(8) 2.753(7) 1.991(5) 1.956(1) 1.977(7) 2.044(1) 2.323(1) 2.753(9)

Cu2-Cl2 Cu2-Cl3 Cu2-O16 Cu2-O17 Cu2-O19 Cu2-O21

2.651(1) 2.250(1) 2.037(3) 2.327(3) 2.012(3) 1.974(3)

Cu1-N3 Cu1-Cl1

1.963(2) 2.268(1)

Cu1-N3 Cu1-O1_b

1.974(6) 2.706(4)

Cu2-N21 Cu2-N28 Cu2-O17 Cu2-O37 Cu2-O38 Cu2-O57 Cu4-N61 Cu4-N68 Cu4-O57 Cu4-O77 Cu4-O17 Cu4-O78

1.993(8) 1.939(1) 1.993(9) 1.987(9) 2.299(8) 2.705(7) 1.988(1) 1.943(2) 1.977(1) 2.002(8) 2.703(7) 2.245(8)

2

3

Figure 1. Atomic displacement plot (30% probability level) of the molecular structure of [Cu2(mphp)Cl3(MeOH)(H2O)] (1). The lattice methanol molecule and the hydrogen atoms have been removed for clarity. bridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-1223/336-033; E-mail: deposit@ccdc. cam.ac.uk].

Results and Discussion Crystal structure of [Cu2(mphp)Cl3(MeOH)(H2O)](MeOH) (1). The reaction of 2 equiv of copper(II) chloride with 1 equiv of Hmphp in methanol produces a dinuclear copper complex that crystallizes in the monoclinic P21/n space group (Figure 1). Crystallographic data and associated experimental details for 1 are given in Table 1. The molecular structure of 1 is shown in Figure 1, and selected bond lengths and angles are summarized in Tables 2 and 3, respectively. 1 consists of two different copper(II) centers that are doubly bridged by a chlorido and a phenoxido ligand. Cu1 is in an almost perfect squarepyramidal coordination environment (τ5 ) 0.06).33 Cu1 is bound to two N atoms and one O atom from a deprotonated mphp unit which acts as a chelating, planar ligand. The plane of the square-pyramid is completed by a bridging chloride anion. The Cu-N bond lengths are in the range of 1.970(3)–1.981(3) Å, in agreement with those found in similar CuN2Cl2O chromophores.34,35 The in-plane Cu-O and Cu-Cl bond lengths are also within the range of distances described in the literature for related coordination environments.36,37 The apical position of the square-pyramid is occupied by a chlorido ligand at a normal apical distance of 2.737(1) Å.37,38 The basal angles vary from 81.86(13) to 96.84(10)°, reflecting a minor distortion of the square plane, most likely due to the bite angle of the pyridine/ hydrazino chelating unit of the ligand (the angle N1-Cu1-N8 is 81.86(13)°) and to the bridging chloride anion (the angle Cl2-Cu1-N1 is 96.84(10)°). The copper atom Cu2 has a distorted octahedral geometry. The four equatorial sites around Cu2 are occupied by two ligand donor atoms, namely, the bridging phenoxido atom O16 and the methoxy atom O17, the bridging chloride Cl2 and one monodentate chlorido ligand Cl3. The axial positions of the octahedron are completed by one water and one methanol molecule. All coordination distances are in normal ranges, consistent with those observed for comparable copper(II) coordination environments.39,40 The distortion of the octahedral geometry presumably rises from the bridging chlorido ligand Cl2 (the angle Cl2-Cu2-Cl3 is 110.22(5)° and the angle Cu2-Cl2-Cu1 is 82.74(5)°). The Cu1Cu2 distance is 3.256(1) Å.

4

Table 3. Selected Bond Angles (°) in [Cu2(mphp)Cl3(MeOH)(H2O)](MeOH) (1), [Cu(php)Cl](MeOH) (2), [Cu4(mphp)4](ClO4)4 (3), and [Cu4(mphp)4](BF4)2 · (SiF6) · (H2O)7 (4) 1 N1-Cu1-N8 N8-Cu1-O16 O16-Cu1-Cl2 Cl2-Cu1-N1 N8-Cu1-Cl2 N1-Cu1-O16

81.86(13) 90.95(13) 89.07(9) 96.84(10) 172.06(10) 168.63(13)

O16-Cu2-Cl2 Cl2-Cu2-Cl3 Cl3-Cu2-O17 O17-Cu2-O16 O21-Cu2-O19

76.95(9) 110.22(5) 99.57(9) 73.32(11) 176.41(12)

N3-Cu1-O1 Cl1-Cu1-N1 N1-Cu1-O1

91.91(8) 95.85(7) 170.91(9)

2 N1-Cu1-N3 O1-Cu1-Cl1 N3-Cu1-Cl1

81.27(9) 90.37(6) 173.98(7)

O1-Cu1-N1 N3-Cu1-O1_c O2_c-Cu1-O1_b

90.6(2) 98.1(2) 145.5(1)

N81-Cu1-N88 O77-Cu1-O17 O18-Cu1-O37 N21-Cu2-N28 O17-Cu2-O37 O38-Cu2-O57 N41-Cu3-N48 O37-Cu3-O57 O58-Cu3-O77 N61-Cu4-N68 O57-Cu4-O77 O17-Cu4-O78

82.8(3) 88.6(3) 145.3(3) 81.1(5) 89.0(3) 147.1(3) 81.7(4) 88.0(3) 144.4(3) 82.4(6) 90.1(4) 146.6(3)

3 N1-Cu1-N3 O1_c-Cu1-O1

82.0(2) 88.7(2)

N88-Cu1-O77 O17-Cu1-N81

90.1(3) 98.1(3)

N28-Cu2-O17 O37-Cu2-N21

91.7(5) 97.8(3)

N48-Cu3-O37 O57-Cu3-N41

90.9(4) 98.7(3)

N68-Cu4-O57 O77-Cu4-N61

91.3(6) 95.8(4)

4

In a previous study, the reaction of Hmphp with Mn(ClO4)2 produced a fused doubled-stranded dihelicate.19 The formation of this remarkable supramolecular architecture is apparently obtained by click-assembly between two helicates. The resulting trinuclear dihelicate is connected through the central MnII ion

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Figure 2. Atomic displacement plot (30% probability level) of the molecular structure of [Cu(php)Cl](MeOH) (2). The lattice methanol molecule and the hydrogen atoms have been removed for clarity. The bridging Cl1_a atom belongs to an adjacent complex. Symmetry operation: a ) -x, -½ + y, ½ - z.

Figure 4. Atomic displacement plot (10% probability level) of the cation [Cu4(mphp)4]4+ of 3. The perchlorate anions and the hydrogen atoms have been removed for clarity. Symmetry operations: a ) y, -x, -z; b ) -x, -y, z; c ) -y, x, -z. Figure 3. Comparison of the coordination environments around the Cu1 ions in complexes 1 and 2.

which exhibits bonding interactions with four methoxy donors belonging to four different mphp ligands.19 From the copper structure described above (Figure 1), it appears that the methoxy substituent of the ligand mphp also plays a role in the generation of the dinuclear unit. To verify this assumption, the reaction of the ligand without the methoxy substituent, namely, 6-(pyridine2-ylhydrazonomethyl)phenol (Hphp, Chart 1),21 with copper(II) chloride has been investigated. Crystal structure of [Cu(php)Cl](MeOH) (2). As expected, the reaction of 2 equiv of copper(II) chloride with 1 equiv of Hphp in methanol (the same experimental conditions have been used to prepare 1 and 2; see Experimental Section) produces a mononuclear copper(II) complex, that is, 2, whose molecular structure is illustrated in Figure 2. 2 crystallizes in the monoclinic P21/c space group. Details for the structure solution and refinement are summarized in Table 1, and selected bond lengths and angles are given in Tables 2 and 3, respectively. The copper(II) ion is pentacoordinated in a square-pyramidal environment (τ5 ) 0.05),33 with a N2Cl2O donor set. Actually, the coordination environment around the copper(II) center in 2 is analogous to the one around Cu1 in the dinuclear complex 1 (Figure 3). Indeed, the metal ion is bound to two N atoms and one O atom from a deprotonated php unit and to two chloride anions. The coordination bond lengths are comparable to those observed for Cu1 in 1 (see Table 2). The coordination geometries of both coppers are almost identical with τ values of 0.06 (complex 1) and 0.05 (complex 2), respectively (see Table 3). In conclusion, the addition of a 2-methoxy substituent to the phenol ring of Hphp confers a dinucleating character to the resulting ligand Hmphp. Interestingly, the chloride ion bridging the two copper ions in 1 is now connected to an adjacent mononuclear unit, generating a one-dimensional coordination polymer (see Figure S1, Supporting Information), like the one observed for [Cu(php)Cl](H2O)2 by Padhye and co-workers.20 In the case of complex 1, the coordinating chloride anions apparently are preventing the formation of an extended structure.

In the recently reported trimanganese(II) supramolecular structure, non-coordinating anions, namely, perchlorates, have been used. The next logical step to further investigate the coordinating properties of Hmphp with copper is to use the non-coordinating perchlorate as a counterion with this metal ion. Crystal structure of [Cu4(mphp)4](ClO4)4 (3). The reaction of 2 equiv of copper(II) perchlorate with 1 equiv of Hmphp in methanol generates the tetranuclear copper(II) complex 3. As anticipated, the sole use of non-coordinating anions allows the formation of a larger core, namely, a cubane (Figure 4 and Figure S2, Supporting Information). 3 crystallizes in the tetragonal P4j21/c (No. 114) space group (Table 1). Selected interatomic distances and angles are listed in Tables 2 and 3, respectively. The cubane structure is characterized by a Cu4O4 core assembled from four crystallographically equivalent, symmetry-related copper atoms and four deprotonated, tetradentate mphp ligands (Figure 4 and Figure S2, Supporting Information). Each copper(II) ion is in an octahedral coordination environment. The basal plane of the octahedron is formed by two N atoms and one bridging phenoxido-O atom belonging to one deprotonated mphp ligand and one O atom from a bridging phenoxido group of a second mphp ligand. The axial positions are occupied by one methoxy O atom and the phenoxido O atom of a third mphp ligand. As a result, the copper center is coordinated by two parallel ligands and one perpendicular ligand (the angle between the two planes is 88.9°). Actually, the parallel ligands are π-π stacked with centroid-to-centroid separation distances of 3.578(4) Å (Figure 5). The Cu-N and Cu-O bond lengths are in normal ranges for a CuN2O4 chromophore with an elongated octahedral geometry.41,42 The basal angles vary from 82.0(2) to 98.1(2)°. The distortion of the octahedron is revealed by the angle O2_c-Cu1-O1_b of 145.5(1)°. This deformation of the cubane cluster is most likely due to steric constraints rising from its assembly. Consequently, the Cu4O4 core displays two different Cu · · · Cu separation distances, that is, Cu1 · · · Cu1_a ) 3.290(1) Å and Cu1 · · · Cu1_b ) 3.531(1) Å, which define two different magnetic coupling pathways between the copper(II) ions (see Magnetic Properties section).

Dinuclear 1D Polymer and Cubane Cu Structures

Figure 5. Schematic representation of the cation [Cu4(mphp)4]4+ of 3 showing the two pairs of π-π stacked ligands (one pair is shown in space filling mode). The centroid A · · · centroid A′ distance is 3.578(4) Å.

Figure 6. Atomic displacement plot (10% probability level) of the cation [Cu4(mphp)4]4+ of 4. The tetrafluoridoborate and hexafluoridosilicate anions and the hydrogen atoms have been omitted for clarity.

Crystal Structure of [Cu4(mphp)4](BF4)2 · (SiF6) · (H2O)7 (4). Reaction of 2 equiv of copper(II) tetrafluoridoborate with 1 equiv of Hmphp in methanol yields complex 4 whose cationic part is a cubane-type cluster (Figure 6) analogous to the one of complex 3. However, contrary to 3, the Cu4O4 core of 4 does not exhibit S4 symmetry. As a result, the tetranuclear complex is constituted of four different Cu atoms having slightly distinct octahedral geometries. Each copper ion is in an N2O4 distorted environment formed by a N2O donor set provided by one deprotonated mphp ligand, one phenoxido-O atom from a second mphp ligand, and a methoxy O atom belonging to a third mphp ligand. Selected bond lengths and angles are listed in Tables 2 and 3, respectively. All Cu-N and Cu-O distances are within the range of distances described in the literature for related cubane structures.43 As for 3, the distortions of the octahedra are probably due to constraints caused by steric interactions between the four ligands wrapped around the Cu4O4 core. Interestingly, the crystal packing of 4 reveals the presence of two different anions, that is, tetrafluorido borate and hexafluoridosilicate. The formation of SiF62- anions results from the gradual ligand-assisted decomposition of BF4- followed by the attack of the resulting fluoride ions on the glass surface of the reaction vessel. Such degradation of tetrafluorido anions

Crystal Growth & Design, Vol. 8, No. 3, 2008 1009

Figure 7. Plots χMT vs T (0) and χM vs T (∆) per mol of complex 1. The solid lines are a fit to the experimental data (see text).

has been earlier observed with copper complexes.22,23 The BF4-/ SiF62- ratio of 2:1 in 4 obviously breaks the S4 symmetry observed for the cubane core of 3. This desymmetrization of the solid-state structure is clearly illustrated by the fact that the cation of 4 is formed by four different copper centers, while 3 consists of crystallographically equivalent copper(II) ions. Magnetic Properties. The magnetic properties of all four dinuclear and polynuclear compounds have been investigated in some detail. The χM and χMT versus T plots for the dinuclear copper complex 1, recorded under a constant magnetic field of 0.1 T, are shown in Figure 7. When the temperature is lowered, χM increases and reaches a maximum around 75 K. Then, the χM value smoothly decreases until 25 K, where a rather steep increase is observed, most likely due to the presence of paramagnetic impurities. At room temperature the χMT product is 0.76 cm3 · K · mol-1, which is in good agreement with the expected value for two isolated copper(II) ions (Figure 7). When the sample is cooled, the XMT value decreases continuously to reach a value close to zero below 20 K, suggesting an antiferromagnetic S ) 0 ground state. To estimate the magnitude of the antiferromagnetic coupling, the magnetic susceptibility data were fitted to the Bleaney–Bowers44 equation (eq S1, Supporting Information) for two interacting copper(II) ions using the Hamiltonian H ) -J S1 · S2. The least-squares fitting of the data applying eq S1 leads to J ) -104(1) cm-1, g ) 2.28(1), TIP ) 60 × 10-4 cm3 · mol-1 per CuII and R ) 5 × 10-5 (R ) Σi(χcalcd - χobs)2/Σi(χobs)2). The solid line in Figure 7 corresponds to the theoretical curve obtained using the above parameters. As aforementioned, the coordination environment is squarebased pyramidal around Cu1 and distorted octahedral around Cu2. In both cases, the unpaired electron occupies mainly the dx2–y2 orbitals. The bridging phenoxido oxygen atom occupies an equatorial position for both copper atoms with a large Cu1-O-Cu2 angle of 109.5°. This structural arrangement allows a strong overlap between the symmetric and antisymmetric combinations of the two dx2–y2 orbitals belonging to the copper centers and the oxygen p orbital. Such orbital overlaps fully agree with the antiferromagnetic response of compound 1. It appears that the coupling pathway through the halide bridge is minor, probably as the result of the small Cu-Cl-Cu angle of 82.74(5)° and weak interactions with the magnetic d orbital [Cu2-Cl2 ) 2.651(1) Å]. Nevertheless, the bridging chlorido ligand has a significant effect on the magnetic properties of 1 because it is obviously involved in the coordination geometries observed for both copper ions, which are responsible for the

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Tang et al.

analyzed using the isotropic Heisenberg–Dirac-van Vleck Hamiltonian model24 (eq 1), with four paramagnetic centers placed in a cubane-like structure. H ) -J′(S1 · S2 + S3 · S4) - J(S1 · S3 + S1 · S4 + S2 · S3 + S2 · S4) (1)

Figure 8. Plots χMT vs T and χM vs T per mol of complex 3 (0) and complex 4 (O). The red solid lines are a fit to the experimental data for 3; the orange solid lines are a fit to the experimental data for 4 (see text).

strong antiferromagnetic exchange. The EPR spectrum (70 K) of a polycrystalline sample of complex 1 shows an intense symmetrical signal with g ) 2.20. The signal of the forbidden transition is absent, which suggests that the intermolecular interactions dominate over the intramolecular ones. The EPR spectrum of a frozen solution (70 K) of 1 in methanol exhibits a well-resolved hyperfine structure in the parallel orientation, which is characteristic of a mononuclear copper(II) species. Actually, the g⊥ value of 2.08 and the g|| value of 2.43 with A|| ) 111 G are typical for a copper(II) ion coordinated by methanol molecules. These EPR data therefore indicate that the dinuclear complex 1 is not stable in solution. No significant magnetic coupling is observed for compound 2 during the magnetization measurements. From a magnetic point of view, the copper(II) centers can be considered as isolated, as has been also found for [Cu(php)Cl](H2O)2 by Padhye and co-workers.20 The solid-state EPR spectrum of 2 at 70 K clearly shows an axial anisotropy with g factors typical for a copper(II) ion in the ground-state dx2–y2, with g|| ) 2.20 and g⊥ ) 2.08. In frozen methanolic solution (70 K), an axial signal, consistent with a square-pyramidal copper(II) species is observed (g|| ) 2.28 and g⊥ ) 2.05). In this spectrum, the lowfield signal shows the hyperfine splitting (A|| ) 167 G) expected from 63Cu. The temperature dependence (range 5–300 K) of the magnetic susceptibility of complexes 3 and 4 under a constant applied field of 0.1 T is illustrated in Figure 8. For both cubane compounds, the χM value reaches a maximum around 120 K, followed by a decrease until 25 K, where χM experiences a steep increase due to the presence of paramagnetic impurities (Figure 8). The χMT value at room temperature is 1.20 and 1.21 cm3 · K · mol-1 for 3 and 4, respectively. These values are lower than those expected for four uncoupled copper(II) ions (1.5 cm3 · K · mol-1 for g ) 2.0), suggesting the existence of antiferromagnetic interactions in both systems. As is evidenced in Figure 8, the χMT value of both 3 and 4 decreases with the temperature, the complexes reaching an ST ) 0 state at very low T. The magnetic responses are almost identical for both complexes, indicating that the magnetic interactions are governed by intracluster exchange couplings. The magnetic behavior of cubane-type structures can be described by six coupling constants45,46 (see Figure 9). In the present case, the symmetry of the two systems allows consideration of only two coupling constants, namely, J and J′, characterizing the two different bridges. To interpret this behavior, the experimental data were

In this Hamiltonian, J′ corresponds to the exchange coupling constant within the Cu1-Cu1b and Cu1a-Cu1c pairs in complex 3 (see Figure 9) and within the Cu1-Cu3 and Cu2-Cu4 pairs in 4 (see Figure 9). J symbolizes the coupling constant within the Cu1-Cu1_a, Cu1-Cu1_c, Cu1_b-Cu1_a and Cu1_b-Cu1_c pairs (complex 3) and within the Cu1-Cu2, Cu1-Cu4, Cu3-Cu2, and Cu3-Cu4 pairs (complex 4). The resulting expression for the molar magnetic susceptibility can be obtained from van Vleck’s equation (see Supporting Information). Considering the temperature independent paramagnetism (TIP) and paramagnetic impurities (fraction F), the resulting expression is given in eq 2. χexp ) (1 - F)χtetra + Fχpara + TIP

(2)

The cubane core of 3 exhibits four separation distances superior to 2.7 Å (Cu1-O1_b, Cu1_a-O1_c, Cu1_b-O1 and Cu1_c-O1_a; see Figure 9). As a result, the coupling interactions including those weak contacts are not significant; therefore, the corresponding J′ value (see Figure 9) is considered to be negligible and is thus fixed to 0. The relevant parameters obtained for 3 by nonlinear fitting of eq 2 to the experimental data are J ) -117(1) cm-1, J′ ) 0, F ) 0.063(3) with g ) 2.14(1), R ) 1.6 × 10-4 and with a TIP value of 60 × 10-4 cm3 · mol-1 per copper(II) fixed. R corresponds to the agreement factor defined as Σi(χcalcd - χobs)2/Σi(χobs)2. The same mathematical model is applied to complex 4 whose Cu4O4 core is analogous to the one of 3 (Figures 4 and 6). The best fit is found for J ) -111(1) cm-1, J′ ) 0, F ) 0.016(4) and R ) 1.3 × 10-4 with g ) 2.11(1) (see Figure S3, Supporting Information). The good agreement between the experimental and the calculated values is illustrated in Figure 8. These magnetic values observed can be rationalized by taking into account some structural parameters (Cu · · · Cu distances and Cu-O-Cu angles). For both complexes 3 and 4, each copper atom of the Cu4O4 unit is triply bridged to the other metal centers through phenoxido oxygen atoms. No significant spin density is expected for the Cux · · · Cuy pairs, which are bridged by apical O atoms, because this coordination position corresponds to the dz2 orbital. Consequently, the interactions

Figure 9. Core of complex 3 together with the spin-coupling scheme. Cu1-O1 ) Cu1-O1_c ) Cu1_b-O1_a ) Cu1_b-O1_b ) Cu1_cO1_b ) Cu1_c-O1_c ) 1.983(4) Å; Cu1-O1_b ) Cu1_a-O1_c ) Cu1_b-O1 ) Cu1_c-O1_a ) 2.706(4) Å. A similar spin coupling scheme is applied for the analogous complex 4 (see Figure S3, Supporting Information).

Dinuclear 1D Polymer and Cubane Cu Structures

Cu1-Cu1b and Cu1a-Cu1c in 3 and Cu1-Cu3 and Cu2-Cu4 in 4 are most likely insignificant (see Figure 9 and Figure S3, Supporting Information).47,48 However, the Cu · · · Cu interactions through Cu-O-Cu entities involving bridging O atoms located in the basal planes of both metal ions can be expected to be significant. In this case, two coupling pathways are possible. The first pathway brings in two short Cu-O bonds [bond distances inferior to 2.002(4) Å] and a Cu-O-Cu angle of 112.10(2)° for 3 and 113.0(4)° for 4. The second pathway is realized via one short and one long Cu-O bonds [bond lengths superior to 2.705(4) Å] and with a small angle of 87.86(2)° (Figure 9). For the first coupling pathway, the short distances and the large angles are expected to lead to strong antiferromagnetic interactions. Hence, due to the copper symmetry, the unpaired electron around copper(II) is described by a magnetic orbital built from the dx2–y2 orbital pointing toward its four nearest neighbors.48 Thus, the angle driving the magnetism for compounds 3 and 4 are well above the transition angle, namely, 104°, established experimentally for polynuclear complexes containing Cu4O4 cubane entities.48 For the second coupling pathway, the long bond lengths and the small angles are expected to generate significantly weaker coupling interactions, mostly owing to the long Cu-O distances. The powder EPR spectra of compounds 3 and 4 at 70 K show a broad signal with g ) 2.13 and g ) 2.06, respectively. As expected, in frozen methanolic solutions, both complexes exhibit identical axial spectra (g|| ) 2.25 and g⊥ ) 2.05 for 3 and g|| ) 2.25 and g⊥ ) 2.05 for 4). The shape of the EPR lines and the values of the g-tensors (g|| > g⊥ > 2.00) are indicative of a dx2–y2 ground state. The well-resolved structures in the parallel orientations (A|| ) 180 G for 3 and 4) are typical for mononuclear copper(II) species with axial symmetry. These EPR data suggest that the cubane cores of 3 and 4 are partially dissociated in methanol. Actually, the exact mass spectrometry analysis of a methanolic solution of 3 (see Figure S4, Supporting Information) shows an isotopic pattern corresponding to the twocharged cubane core [Cu4(mphp)2(mphp-H)2]2+ which has lost two protons. The comparison of the solid-state and the solution (in methanol) UV–vis spectra of 4 (Figure S5, Supporting Information) suggests that more than one species is present in solution (i.e., in addition to the cubane one). Conclusion A series of four different copper(II) coordination compounds have been prepared. The slight coordination variations induced by minor changes in one of the building blocks generate significantly distinct architectures. For instance, the introduction of a methoxy substituent on the phenol ring of the ligand 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp), resulting in the ligand 2-methoxy-6-(pyridine-2-ylhydrazonomethyl)phenol (Hmphp), converts the original polymeric complex {[Cu(php)Cl](MeOH)}n into the dinuclear complex [Cu2(mphp)Cl3(MeOH)(H2O)](MeOH). The use of non-coordinating anions, such as ClO4-, instead of coordinating chloride ions gives rise to the formation of a cubane structure, that is, [Cu4(mphp)4](ClO4)4. The importance of the supramolecular potential of the ligand Hmphp is currently examined with other metals. Other slight modifications of the building blocks are investigated. For instance, the addition of different substituents on the ligand Hmphp, the use of other counterions such as PF6- and Ph4Band of different solvents are now studied. Acknowledgment. Support by the Graduate Research School Combination “Catalysis”, a joint activity of the graduate research

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schools NIOK, HRSMC, and PTN, and the COST program Action D35/0011 is acknowledged. Coordination of some of our research by the FP6 Network of Excellence “Magmanet” (contract number 515767) is also kindly acknowledged. Financial support from the Ministry of Higher Education, Science and Technology, Republic of Slovenia, through Grant P1-0175, is gratefully acknowledged. We would like to thank Hans van den Elst (bio-organic synthesis research group; Leiden Institute of Chemistry) for his precious help with exact mass measurements. Supporting Information Available: Figure S1 showing the onedimensional chain of complex 2; Figures S2 and S3 showing the Cu4O4 cores of complexes 3 and 4, respectively; equations applied to fit the magnetic experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

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