Influence of Coordinating and Non-Coordinating Anions and of aMethoxy Substituent on the Formation of Copper-BasedCoordination Assemblies

Jinkui Tang,† José Sánchez Costa,† Andrej Pevec,‡ Bojan Kozlevcar,‡ 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, AškerceVa 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, andDepartment 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 structurallycharacterized. Slight variations of one of the building blocks (namely, the ligand or the counterion) lead to drastic changes in thenature 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 polynuclearcopper(II) compounds have been investigated, which show that the different clusters exhibit antiferromagnetic interactions.

Introduction

The formation of cluster coordination complexes is nowubiquitous in chemistry,1–3 mainly stimulated by the field ofmolecular magnetism.4–6 The reaction of transition metal ionswith small bridging ligands (such as chloride, azide, hydroxide,methoxide, and so on) may result in the formation of metallo-supramolecular architectures exhibiting remarkable metal-metalinteractions.7–10 The growth to infinite networks is normallyblocked by terminal monodentate or polydentate ligands, whichusually act also as bridging ligands.11 Cubanes are amazingmetalloclusters that have received great attention during the pasttwo decades, for their potential physical properties12–14 and fortheir application as biomimetic models of nitrogenases andhydrogenases.15–17

Recently, some of us have reported the use of the ligand2-methoxy-6-(pyridine-2-ylhydrazonomethyl)phenol (Hmphp,Chart 1)18 for the generation of a unique fused double-stranded[MnII

3] dihelicate.19 The potential of this ligand to produceinteresting supramolecular arrays is now evaluated with othermetal ions. In the present study, the coordination of Hmphp todifferent copper(II) salts is explored. In particular, the effect ofthe anion, coordinating or noncoordinating, on the coordinationarrangement between the metal and the organic ligand isexamined. It appears that the bridging ability of the anion hasa drastic effect on the resulting coordination framework. In thatcontext, the role of the methoxy group of Hmphp is alsoassessed. The presence of this methoxy substituent has beenfound to be crucial for the formation of the manganese(II)dihelicate, described earlier.19 Therefore, the coordination ofthe ligand without the methoxy group, namely, 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp, Chart 1),20,21 to copper is

also examined. The magnetic properties of all the new coppercompounds are investigated in detail.

Experimental Section

All chemicals were of reagent grade and were used as commerciallyobtained. Elemental analyses for C, H, and N were performed with aPerkin-Elmer 2400 analyzer. Fourier tranform infrared (FTIR) spectrawere recorded with a Perkin-Elmer Paragon 1000 FTIR spectropho-tometer, equipped with a Golden Gate ATR device, using the reflectancetechnique (4000-300 cm-1). The ligands Hmphp18 and Hphp21 wereprepared following procedures described in the literature. X-bandelectron paramagnetic resonance (EPR) measurements were performedat 77 K in the solid state and in frozen solutions on a Jeol RE2x electronspin 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 solutionwas left unperturbed to allow a slow evaporation of the solvent. Afterthree days, dark blue block-shaped crystals, suitable for X-ray diffractionanalysis, 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 absorptionbands 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, 0.50mmol) was added to a solution of Hphp (53 mg, 0.25 mmol) inmethanol (10 mL). The resulting green solution was left unperturbedto allow a slow evaporation of the solvent. After two days, dark greenblock-shaped crystals, suitable for X-ray diffraction analysis, wereobtained. Yield ) 50 mg (58% based on the ligand). Anal. Calcd. forC13H14ClCuN3O2 (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),

* 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

CRYSTALGROWTH& DESIGN

2008VOL. 8, NO. 3

1005–1012

10.1021/cg700993s CCC: $40.75 2008 American Chemical SocietyPublished on Web 02/12/2008

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) inmethanol (10 mL). The resulting green solution was left unperturbedto allow a slow evaporation of the solvent. After three days, dark greenprismatic crystals, suitable for X-ray diffraction analysis, were obtained.Yield ) 45 mg (45% based on the ligand). Anal. Calcd. forC52H48Cl4Cu4N12O24 (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, 0.25mmol) in methanol (10 mL). The resulting dark green solution wasleft unperturbed to allow a slow evaporation of the solvent. After threedays, dark green block-shaped crystals, suitable for X-ray diffractionanalysis, 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 IRabsorption 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 measurementswere carried out using a Quantum Design MPMS-5 5T SQUIDmagnetometer at various fields in the temperature range 5–300 K. Datawere corrected for the diamagnetic contributions estimated from Pascal’stables.24

X-ray Crystallographic Analysis and Data Collection. Themolecular structure of complexes 1-4 were determined by single-crystalX-ray diffraction methods. Crystallographic data and refinement detailsare given in Table 1.

X-ray crystallographic data for 1 were collected on a Nonius KappaCCD diffractometer with graphite-monochromated Mo KR radiation(λ ) 0.71073 Å) at 150 K. A suitable crystal (blue block) was affixedto the end of a glass fiber using silicone grease and transferred to thegoniostat. DENZO-SMN25 was used for data integration andSCALEPACK25 corrected data for Lorentz-polarization effects. Thestructures were solved by direct methods and refined by a full-matrixleast-squares method on F2 using the SHELXTL crystallographic

software package.26,27 All non-hydrogens were refined anisotropically.All hydrogens were found in difference Fourier maps and placedgeometrically on their riding atom.

X-ray crystallographic data for 2 were collected on a Nonius KappaCCD difractometer at 150(2) K. A Cryostream cooler (Oxford Cryo-systems) was used for cooling the sample. Graphite monochromatedMo KR radiation (λ ) 0.71073 Å) was employed. The structure wassolved by direct methods implemented in SHELXS-9727 and refinedby a full-matrix least-squares procedure based on F2 using SHELXL-97.26

Intensity data and cell parameters for 3 were recorded at roomtemperature on a Bruker AXS Smart 1000 single-crystal diffractometer(Mo KR radiation) equipped with a CCD area detector. The datareduction 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 withanisotropic atomic displacements with the exception of the perchlorateion. The hydrogen atoms were included in the refinement at idealizedgeometries (C-H 0.95 Å) and refined “riding” on the correspondingparent atoms. The weighting scheme used in the last cycle of refinementwas w ) 1/[σ2Fo

2 + (0.0161P)2] (where P ) (Fo2 + 2Fc

2)/3). Moleculargeometry 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 andmounted to the glass fiber using the oil drop method32 and data werecollected at 193 K using a Nonius KappaCCD diffractometer withgraphite monochromatised Mo KR-radiation. The intensity data werecorrected 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. Allthe aromatic six rings were refined as rigid groups. Part of the wateroxygen atoms were refined in isotropically. The water H atoms werenot located. The other H atoms were geometrically fixed and allowedto ride on the attached atoms.

Crystallographic data (excluding structure factors) for the structuresreported have been deposited with the Cambridge Crystallographic DataCentre as supplementary publications nos. CCDC 663145 (1), 663146(2), 663147 (3), and 663148 (4) and can be obtained free of charge athttp://www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cam-

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)

1 2 3 4

formula C15H22Cl3Cu2N3O5 C13H14ClCuN3O2 C52H48Cl4Cu4N12O24 C52H62B2Cu4F14N12O15Sifw (g mol-1) 557.76 343.27 1620.98 1665.12cryst size (mm3) 0.62 × 0.45 × 0.30 0.25 × 0.20 × 0.20 0.13 × 0.11 × 0.09 0.20 × 0.20 × 0.20cryst color blue green dark green greentemperature (K) 150(2) 150(2) 293(2) 173(2)cryst syst, space group monoclinic, P21/n monoclinic, P21/c tetragonal, P4j21/c triclinic, P1ja (Å) 15.152(3) 13.3013(4) 12.159(2) 14.936(3)b (Å) 8.296(2) 7.5995(2) 12.159(2) 15.712(3)c (Å) 16.555(3) 14.3627(2) 20.958(3) 17.855(4)R (deg) 90.00 90.00 90.00 72.45(3)� (deg) 95.61(3) 110.4358(13) 90.00 69.04(3)γ (deg) 90.00 90.00 90.00 62.25(3)volume (Å3) 2071.0(7) 1360.46(7) 3098.5(7) 3416.3(16)Z 4 4 2 2calcd density (g cm-3) 1.779 1.676 1.737 1.632F(000) 1116 700 1640 1716abs coeff (mm-1) 2.472 1.805 1.619 1.354θ for data collection (deg) 1.74–27.49 3.08–27.46 1.94–27.49 2.75–25.00rflns collected (Rint) 24389 (0.0749) 5714 (0.0252) 13748 (0.0820) 36749 (0.0579)data/params 4714/257 3091/191 3541/200 11449/763goodness of fit on F2a 1.292 1.016 1.030 1.084R1b (wR2)c [Fo > 4σ(Fo)] 0.0398 (0.1277) 0.0338 (0.0888) 0.0508 (0.0795) 0.0974 (0.2337)largest diff peak and hole (e Å3) 1.039 and -2.128 0.691 and -0.469 0.702 and -0.432 1.167 and -1.095

a Goodness-of-fit S ) [Σw(Fo2 - Fc

2)2/(n - p)]1/2, where n is the number of reflections and p is the number of parameters. b R1 ) Σ|Fo| -|Fc|/Σ|Fo|. c wR2 ) [Σ[w(Fo

2 - Fc2)2]/Σ[w(Fo

2)2]]1/2.

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

bridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, 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 equivof Hmphp in methanol produces a dinuclear copper complexthat crystallizes in the monoclinic P21/n space group (Figure1). Crystallographic data and associated experimental detailsfor 1 are given in Table 1. The molecular structure of 1 is shownin Figure 1, and selected bond lengths and angles are sum-marized in Tables 2 and 3, respectively. 1 consists of twodifferent copper(II) centers that are doubly bridged by a chloridoand a phenoxido ligand. Cu1 is in an almost perfect square-pyramidal coordination environment (τ5 ) 0.06).33 Cu1 is boundto two N atoms and one O atom from a deprotonated mphpunit which acts as a chelating, planar ligand. The plane of thesquare-pyramid is completed by a bridging chloride anion. TheCu-N bond lengths are in the range of 1.970(3)–1.981(3) Å,in agreement with those found in similar CuN2Cl2O chromo-phores.34,35 The in-plane Cu-O and Cu-Cl bond lengths arealso within the range of distances described in the literature forrelated coordination environments.36,37 The apical position ofthe square-pyramid is occupied by a chlorido ligand at a normalapical distance of 2.737(1) Å.37,38 The basal angles vary from81.86(13) to 96.84(10)°, reflecting a minor distortion of thesquare plane, most likely due to the bite angle of the pyridine/hydrazino chelating unit of the ligand (the angle N1-Cu1-N8is 81.86(13)°) and to the bridging chloride anion (the angleCl2-Cu1-N1 is 96.84(10)°). The copper atom Cu2 has adistorted octahedral geometry. The four equatorial sites aroundCu2 are occupied by two ligand donor atoms, namely, thebridging phenoxido atom O16 and the methoxy atom O17, thebridging chloride Cl2 and one monodentate chlorido ligand Cl3.The axial positions of the octahedron are completed by onewater and one methanol molecule. All coordination distancesare in normal ranges, consistent with those observed forcomparable copper(II) coordination environments.39,40 Thedistortion of the octahedral geometry presumably rises from thebridging chlorido ligand Cl2 (the angle Cl2-Cu2-Cl3 is110.22(5)° and the angle Cu2-Cl2-Cu1 is 82.74(5)°). TheCu1Cu2 distance is 3.256(1) Å.

In a previous study, the reaction of Hmphp with Mn(ClO4)2

produced a fused doubled-stranded dihelicate.19 The formationof this remarkable supramolecular architecture is apparentlyobtained by click-assembly between two helicates. The resultingtrinuclear dihelicate is connected through the central MnII ion

Figure 1. Atomic displacement plot (30% probability level) of themolecular structure of [Cu2(mphp)Cl3(MeOH)(H2O)] (1). The latticemethanol molecule and the hydrogen atoms have been removed forclarity.

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 2.737(1) Cu2-Cl2 2.651(1)Cu1-Cl2 2.254(1) Cu2-Cl3 2.250(1)Cu1-N1 1.981(3) Cu2-O16 2.037(3)Cu1-N8 1.970(3) Cu2-O17 2.327(3)Cu1-O16 1.949(3) Cu2-O19 2.012(3)

Cu2-O21 1.974(3)

2

Cu1-N1 1.988(2) Cu1-N3 1.963(2)Cu1-O1 1.903(2) Cu1-Cl1 2.268(1)Cu1-Cl1_a 2.858(1)

3

Cu1-N1 1.949(4) Cu1-N3 1.974(6)Cu1-O1 1.983(4) Cu1-O1_b 2.706(4)Cu1-O2_c 2.299(5)

4

Cu1-N81 1.977(5) Cu2-N21 1.993(8)Cu1-N88 1.946(9) Cu2-N28 1.939(1)Cu1-O17 2.001(9) Cu2-O17 1.993(9)Cu1-O77 1.975(7) Cu2-O37 1.987(9)Cu1-O18 2.293(8) Cu2-O38 2.299(8)Cu1-O37 2.753(7) Cu2-O57 2.705(7)Cu3-N41 1.991(5) Cu4-N61 1.988(1)Cu3-N48 1.956(1) Cu4-N68 1.943(2)Cu3-O37 1.977(7) Cu4-O57 1.977(1)Cu3-O57 2.044(1) Cu4-O77 2.002(8)Cu3-O58 2.323(1) Cu4-O17 2.703(7)Cu3-O77 2.753(9) Cu4-O78 2.245(8)

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 81.86(13) O16-Cu2-Cl2 76.95(9)N8-Cu1-O16 90.95(13) Cl2-Cu2-Cl3 110.22(5)O16-Cu1-Cl2 89.07(9) Cl3-Cu2-O17 99.57(9)Cl2-Cu1-N1 96.84(10) O17-Cu2-O16 73.32(11)N8-Cu1-Cl2 172.06(10) O21-Cu2-O19 176.41(12)N1-Cu1-O16 168.63(13)

2

N1-Cu1-N3 81.27(9) N3-Cu1-O1 91.91(8)O1-Cu1-Cl1 90.37(6) Cl1-Cu1-N1 95.85(7)N3-Cu1-Cl1 173.98(7) N1-Cu1-O1 170.91(9)

3

O1-Cu1-N1 90.6(2) N1-Cu1-N3 82.0(2)N3-Cu1-O1_c 98.1(2) O1_c-Cu1-O1 88.7(2)O2_c-Cu1-O1_b 145.5(1)

4

N81-Cu1-N88 82.8(3) N88-Cu1-O77 90.1(3)O77-Cu1-O17 88.6(3) O17-Cu1-N81 98.1(3)O18-Cu1-O37 145.3(3)N21-Cu2-N28 81.1(5) N28-Cu2-O17 91.7(5)O17-Cu2-O37 89.0(3) O37-Cu2-N21 97.8(3)O38-Cu2-O57 147.1(3)N41-Cu3-N48 81.7(4) N48-Cu3-O37 90.9(4)O37-Cu3-O57 88.0(3) O57-Cu3-N41 98.7(3)O58-Cu3-O77 144.4(3)N61-Cu4-N68 82.4(6) N68-Cu4-O57 91.3(6)O57-Cu4-O77 90.1(4) O77-Cu4-N61 95.8(4)O17-Cu4-O78 146.6(3)

Dinuclear 1D Polymer and Cubane Cu Structures Crystal Growth & Design, Vol. 8, No. 3, 2008 1007

which exhibits bonding interactions with four methoxy donorsbelonging to four different mphp ligands.19 From the copperstructure described above (Figure 1), it appears that the methoxysubstituent of the ligand mphp also plays a role in the generationof the dinuclear unit. To verify this assumption, the reaction ofthe ligand without the methoxy substituent, namely, 6-(pyridine-2-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 ofHphp in methanol (the same experimental conditions have beenused to prepare 1 and 2; see Experimental Section) produces amononuclear copper(II) complex, that is, 2, whose molecularstructure is illustrated in Figure 2. 2 crystallizes in themonoclinic P21/c space group. Details for the structure solutionand refinement are summarized in Table 1, and selected bondlengths and angles are given in Tables 2 and 3, respectively.The copper(II) ion is pentacoordinated in a square-pyramidalenvironment (τ5 ) 0.05),33 with a N2Cl2O donor set. Actually,the coordination environment around the copper(II) center in 2is analogous to the one around Cu1 in the dinuclear complex 1(Figure 3). Indeed, the metal ion is bound to two N atoms andone O atom from a deprotonated php unit and to two chlorideanions. The coordination bond lengths are comparable to thoseobserved for Cu1 in 1 (see Table 2). The coordinationgeometries of both coppers are almost identical with τ valuesof 0.06 (complex 1) and 0.05 (complex 2), respectively (seeTable 3). In conclusion, the addition of a 2-methoxy substituentto the phenol ring of Hphp confers a dinucleating character tothe resulting ligand Hmphp. Interestingly, the chloride ionbridging the two copper ions in 1 is now connected to anadjacent mononuclear unit, generating a one-dimensional co-ordination polymer (see Figure S1, Supporting Information),like the one observed for [Cu(php)Cl](H2O)2 by Padhye andco-workers.20

In the case of complex 1, the coordinating chloride anionsapparently are preventing the formation of an extended structure.

In the recently reported trimanganese(II) supramolecular struc-ture, non-coordinating anions, namely, perchlorates, have beenused. The next logical step to further investigate the coordinatingproperties of Hmphp with copper is to use the non-coordinatingperchlorate as a counterion with this metal ion.

Crystal structure of [Cu4(mphp)4](ClO4)4 (3). The reactionof 2 equiv of copper(II) perchlorate with 1 equiv of Hmphp inmethanol generates the tetranuclear copper(II) complex 3. Asanticipated, the sole use of non-coordinating anions allows theformation of a larger core, namely, a cubane (Figure 4 andFigure S2, Supporting Information). 3 crystallizes in thetetragonal P4j21/c (No. 114) space group (Table 1). Selectedinteratomic 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, sym-metry-related copper atoms and four deprotonated, tetradentatemphp ligands (Figure 4 and Figure S2, Supporting Information).Each copper(II) ion is in an octahedral coordination environ-ment. The basal plane of the octahedron is formed by two Natoms and one bridging phenoxido-O atom belonging to onedeprotonated mphp ligand and one O atom from a bridgingphenoxido group of a second mphp ligand. The axial positionsare occupied by one methoxy O atom and the phenoxido Oatom of a third mphp ligand. As a result, the copper center iscoordinated by two parallel ligands and one perpendicular ligand(the angle between the two planes is 88.9°). Actually, the parallelligands are π-π stacked with centroid-to-centroid separationdistances of 3.578(4) Å (Figure 5). The Cu-N and Cu-O bondlengths are in normal ranges for a CuN2O4 chromophore withan elongated octahedral geometry.41,42 The basal angles varyfrom 82.0(2) to 98.1(2)°. The distortion of the octahedron isrevealed by the angle O2_c-Cu1-O1_b of 145.5(1)°. Thisdeformation of the cubane cluster is most likely due to stericconstraints rising from its assembly. Consequently, the Cu4O4

core displays two different Cu · · ·Cu separation distances, thatis, Cu1 · · ·Cu1_a ) 3.290(1) Å and Cu1 · · ·Cu1_b ) 3.531(1)Å, which define two different magnetic coupling pathwaysbetween the copper(II) ions (see Magnetic Properties section).

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

Figure 3. Comparison of the coordination environments around theCu1 ions in complexes 1 and 2.

Figure 4. Atomic displacement plot (10% probability level) of the cation[Cu4(mphp)4]4+ of 3. The perchlorate anions and the hydrogen atomshave been removed for clarity. Symmetry operations: a ) y, -x, -z;b ) -x, -y, z; c ) -y, x, -z.

1008 Crystal Growth & Design, Vol. 8, No. 3, 2008 Tang et al.

Crystal Structure of [Cu4(mphp)4](BF4)2 · (SiF6) · (H2O)7

(4). Reaction of 2 equiv of copper(II) tetrafluoridoborate with1 equiv of Hmphp in methanol yields complex 4 whose cationicpart is a cubane-type cluster (Figure 6) analogous to the one ofcomplex 3. However, contrary to 3, the Cu4O4 core of 4 doesnot exhibit S4 symmetry. As a result, the tetranuclear complexis constituted of four different Cu atoms having slightly distinctoctahedral geometries. Each copper ion is in an N2O4 distortedenvironment formed by a N2O donor set provided by onedeprotonated mphp ligand, one phenoxido-O atom from asecond mphp ligand, and a methoxy O atom belonging to athird mphp ligand. Selected bond lengths and angles are listedin Tables 2 and 3, respectively. All Cu-N and Cu-O distancesare within the range of distances described in the literature forrelated cubane structures.43 As for 3, the distortions of theoctahedra are probably due to constraints caused by stericinteractions between the four ligands wrapped around the Cu4O4

core. Interestingly, the crystal packing of 4 reveals the presenceof two different anions, that is, tetrafluorido borate andhexafluoridosilicate. The formation of SiF6

2- anions results fromthe gradual ligand-assisted decomposition of BF4

- followed bythe attack of the resulting fluoride ions on the glass surfaceof the reaction vessel. Such degradation of tetrafluorido anions

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 ofthe solid-state structure is clearly illustrated by the fact that thecation of 4 is formed by four different copper centers, while 3consists of crystallographically equivalent copper(II) ions.

Magnetic Properties. The magnetic properties of all fourdinuclear and polynuclear compounds have been investigatedin some detail.

The �M and �MT versus T plots for the dinuclear coppercomplex 1, recorded under a constant magnetic field of 0.1 T,are shown in Figure 7. When the temperature is lowered, �M

increases and reaches a maximum around 75 K. Then, the �M

value smoothly decreases until 25 K, where a rather steepincrease is observed, most likely due to the presence ofparamagnetic impurities. At room temperature the �MT productis 0.76 cm3 ·K ·mol-1, which is in good agreement with theexpected value for two isolated copper(II) ions (Figure 7). Whenthe sample is cooled, the XMT value decreases continuously toreach a value close to zero below 20 K, suggesting anantiferromagnetic S ) 0 ground state. To estimate the magnitudeof the antiferromagnetic coupling, the magnetic susceptibilitydata were fitted to the Bleaney–Bowers44 equation (eq S1,Supporting Information) for two interacting copper(II) ions usingthe Hamiltonian H ) -J S1 ·S2. The least-squares fitting of thedata 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(�calcd - �obs)2/Σi(�obs)2). The solid line in Figure 7 correspondsto the theoretical curve obtained using the above parameters.As aforementioned, the coordination environment is square-based pyramidal around Cu1 and distorted octahedral aroundCu2. In both cases, the unpaired electron occupies mainly thedx2–y2 orbitals. The bridging phenoxido oxygen atom occupiesan equatorial position for both copper atoms with a largeCu1-O-Cu2 angle of 109.5°. This structural arrangementallows a strong overlap between the symmetric and antisym-metric combinations of the two dx2–y2 orbitals belonging to thecopper centers and the oxygen p orbital. Such orbital overlapsfully agree with the antiferromagnetic response of compound1. It appears that the coupling pathway through the halide bridgeis minor, probably as the result of the small Cu-Cl-Cu angleof 82.74(5)° and weak interactions with the magnetic d orbital[Cu2-Cl2 ) 2.651(1) Å]. Nevertheless, the bridging chloridoligand has a significant effect on the magnetic properties of 1because it is obviously involved in the coordination geometriesobserved for both copper ions, which are responsible for the

Figure 5. Schematic representation of the cation [Cu4(mphp)4]4+ of 3showing the two pairs of π-π stacked ligands (one pair is shown inspace 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 hexafluoridosilicateanions and the hydrogen atoms have been omitted for clarity.

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

Dinuclear 1D Polymer and Cubane Cu Structures Crystal Growth & Design, Vol. 8, No. 3, 2008 1009

strong antiferromagnetic exchange. The EPR spectrum (70 K)of a polycrystalline sample of complex 1 shows an intensesymmetrical signal with g ) 2.20. The signal of the forbiddentransition is absent, which suggests that the intermolecularinteractions dominate over the intramolecular ones. The EPRspectrum of a frozen solution (70 K) of 1 in methanol exhibitsa 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 methanolmolecules. These EPR data therefore indicate that the dinuclearcomplex 1 is not stable in solution.

No significant magnetic coupling is observed for compound2 during the magnetization measurements. From a magneticpoint of view, the copper(II) centers can be considered asisolated, as has been also found for [Cu(php)Cl](H2O)2 byPadhye and co-workers.20 The solid-state EPR spectrum of 2at 70 K clearly shows an axial anisotropy with g factors typicalfor a copper(II) ion in the ground-state dx2–y2, with g|| ) 2.20and g⊥ ) 2.08. In frozen methanolic solution (70 K), an axialsignal, consistent with a square-pyramidal copper(II) species isobserved (g|| ) 2.28 and g⊥ ) 2.05). In this spectrum, the low-field signal shows the hyperfine splitting (A|| ) 167 G) expectedfrom 63Cu.

The temperature dependence (range 5–300 K) of the magneticsusceptibility of complexes 3 and 4 under a constant appliedfield of 0.1 T is illustrated in Figure 8. For both cubanecompounds, the �M value reaches a maximum around 120 K,followed by a decrease until 25 K, where �M experiences a steepincrease due to the presence of paramagnetic impurities (Figure8). The �MT value at room temperature is 1.20 and 1.21cm3 ·K ·mol-1 for 3 and 4, respectively. These values are lowerthan those expected for four uncoupled copper(II) ions (1.5cm3 ·K ·mol-1 for g ) 2.0), suggesting the existence ofantiferromagnetic interactions in both systems. As is evidencedin Figure 8, the �MT value of both 3 and 4 decreases with thetemperature, the complexes reaching an ST ) 0 state at verylow T. The magnetic responses are almost identical for bothcomplexes, indicating that the magnetic interactions are gov-erned by intracluster exchange couplings. The magnetic behaviorof cubane-type structures can be described by six couplingconstants45,46 (see Figure 9). In the present case, the symmetryof the two systems allows consideration of only two couplingconstants, namely, J and J′, characterizing the two differentbridges. To interpret this behavior, the experimental data were

analyzed using the isotropic Heisenberg–Dirac-van VleckHamiltonian model24 (eq 1), with four paramagnetic centersplaced in a cubane-like structure.

H)-J′(S1 · S2 + S3 · S4)- J(S1 · S3 + S1 · S4 + S2 · S3 + S2 · S4)

(1)

In this Hamiltonian, J′ corresponds to the exchange couplingconstant within the Cu1-Cu1b and Cu1a-Cu1c pairs incomplex 3 (see Figure 9) and within the Cu1-Cu3 andCu2-Cu4 pairs in 4 (see Figure 9). J symbolizes the couplingconstant within the Cu1-Cu1_a, Cu1-Cu1_c, Cu1_b-Cu1_aand Cu1_b-Cu1_c pairs (complex 3) and within the Cu1-Cu2,Cu1-Cu4, Cu3-Cu2, and Cu3-Cu4 pairs (complex 4). Theresulting expression for the molar magnetic susceptibility canbe obtained from van Vleck’s equation (see Supporting Infor-mation). Considering the temperature independent paramagnet-ism (TIP) and paramagnetic impurities (fraction F), the resultingexpression is given in eq 2.

�exp ) (1-F)�tetra +F�para +TIP (2)

The cubane core of 3 exhibits four separation distancessuperior to 2.7 Å (Cu1-O1_b, Cu1_a-O1_c, Cu1_b-O1 andCu1_c-O1_a; see Figure 9). As a result, the coupling interac-tions including those weak contacts are not significant; therefore,the corresponding J′ value (see Figure 9) is considered to benegligible and is thus fixed to 0. The relevant parametersobtained for 3 by nonlinear fitting of eq 2 to the experimentaldata 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 agreementfactor defined as Σi(�calcd - �obs)2/Σi(�obs)2. The same math-ematical model is applied to complex 4 whose Cu4O4 core isanalogous to the one of 3 (Figures 4 and 6). The best fit isfound 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, SupportingInformation). The good agreement between the experimentaland the calculated values is illustrated in Figure 8.

These magnetic values observed can be rationalized by takinginto account some structural parameters (Cu · · ·Cu distances andCu-O-Cu angles). For both complexes 3 and 4, each copperatom of the Cu4O4 unit is triply bridged to the other metalcenters through phenoxido oxygen atoms. No significant spindensity is expected for the Cux · · ·Cuy pairs, which are bridgedby apical O atoms, because this coordination position corre-sponds to the dz

2 orbital. Consequently, the interactions

Figure 8. Plots �MT vs T and �M vs T per mol of complex 3 (0) andcomplex 4 (O). The red solid lines are a fit to the experimental datafor 3; the orange solid lines are a fit to the experimental data for 4 (seetext).

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_c-O1_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 couplingscheme is applied for the analogous complex 4 (see Figure S3,Supporting Information).

1010 Crystal Growth & Design, Vol. 8, No. 3, 2008 Tang et al.

Cu1-Cu1b and Cu1a-Cu1c in 3 and Cu1-Cu3 and Cu2-Cu4in 4 are most likely insignificant (see Figure 9 and Figure S3,Supporting Information).47,48 However, the Cu · · ·Cu interactionsthrough Cu-O-Cu entities involving bridging O atoms locatedin the basal planes of both metal ions can be expected to besignificant. In this case, two coupling pathways are possible.The first pathway brings in two short Cu-O bonds [bonddistances inferior to 2.002(4) Å] and a Cu-O-Cu angle of112.10(2)° for 3 and 113.0(4)° for 4. The second pathway isrealized via one short and one long Cu-O bonds [bond lengthssuperior to 2.705(4) Å] and with a small angle of 87.86(2)°(Figure 9). For the first coupling pathway, the short distancesand the large angles are expected to lead to strong antiferro-magnetic interactions. Hence, due to the copper symmetry, theunpaired electron around copper(II) is described by a magneticorbital built from the dx2–y2 orbital pointing toward its four nearestneighbors.48 Thus, the angle driving the magnetism for com-pounds 3 and 4 are well above the transition angle, namely,104°, established experimentally for polynuclear complexescontaining Cu4O4 cubane entities.48 For the second couplingpathway, the long bond lengths and the small angles areexpected 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 showa broad signal with g ) 2.13 and g ) 2.06, respectively. Asexpected, in frozen methanolic solutions, both complexes exhibitidentical 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 thevalues of the g-tensors (g|| > g⊥ > 2.00) are indicative of adx2–y2 ground state. The well-resolved structures in the parallelorientations (A|| ) 180 G for 3 and 4) are typical formononuclear copper(II) species with axial symmetry. These EPRdata suggest that the cubane cores of 3 and 4 are partiallydissociated in methanol. Actually, the exact mass spectrometryanalysis of a methanolic solution of 3 (see Figure S4, SupportingInformation) shows an isotopic pattern corresponding to the two-charged cubane core [Cu4(mphp)2(mphp-H)2]2+ which has losttwo protons. The comparison of the solid-state and the solution(in methanol) UV–vis spectra of 4 (Figure S5, SupportingInformation) suggests that more than one species is present insolution (i.e., in addition to the cubane one).

Conclusion

A series of four different copper(II) coordination compoundshave been prepared. The slight coordination variations inducedby minor changes in one of the building blocks generatesignificantly distinct architectures. For instance, the introductionof a methoxy substituent on the phenol ring of the ligand6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp), resulting inthe 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-coordinatinganions, such as ClO4

-, instead of coordinating chloride ionsgives rise to the formation of a cubane structure, that is,[Cu4(mphp)4](ClO4)4.

The importance of the supramolecular potential of the ligandHmphp is currently examined with other metals. Other slightmodifications of the building blocks are investigated. Forinstance, the addition of different substituents on the ligandHmphp, the use of other counterions such as PF6

- and Ph4B-

and of different solvents are now studied.

Acknowledgment. Support by the Graduate Research SchoolCombination “Catalysis”, a joint activity of the graduate research

schools NIOK, HRSMC, and PTN, and the COST programAction D35/0011 is acknowledged. Coordination of some ofour research by the FP6 Network of Excellence “Magmanet”(contract number 515767) is also kindly acknowledged. Finan-cial support from the Ministry of Higher Education, Scienceand Technology, Republic of Slovenia, through Grant P1-0175,is gratefully acknowledged. We would like to thank Hans vanden Elst (bio-organic synthesis research group; Leiden Instituteof Chemistry) for his precious help with exact mass measure-ments.

Supporting Information Available: Figure S1 showing the one-dimensional chain of complex 2; Figures S2 and S3 showing the Cu4O4

cores of complexes 3 and 4, respectively; equations applied to fit themagnetic experimental data. This material is available free of chargevia the Internet at http://pubs.acs.org.

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