Inorganica Chimica Acta 373 (2011) 68–72

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Inorganica Chimica Acta

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Synthesis, structure and DFT calculation of a hexanuclear mixed-valencecopper cluster supported by 2,3-disulfidobenzoate and 3-carboxybenzene-1,2-bis(thiolate)

Kuntal Pal a, Satoshi Takamizawa b, Kazushi Mashima a,⇑a Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japanb Department of Nanosystem Science, Graduate School of Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, Kanagawa 236-0027, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 October 2010Received in revised form 8 March 2011Accepted 18 March 2011Available online 25 March 2011

Keywords:Homoleptic clusterH-bonding networkMixed-valenceDithiolene

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.03.055

Abbreviations: DSB, 2,3-disulfidobenzoate; CBT, 3-late); Cutd, tetrahedral coordination geometry of coppcoordination geometry of copper atom; PPh4, tetrap[(Cu(DSB)(CBT))2(Cu2Br)2][PPh4]2.⇑ Corresponding author. Fax: +81 6 6850 6249.

E-mail addresses: staka@yokohama-cu.ac.jp (Schem.es.osaka-u.ac.jp (K. Mashima).

A homoleptic hexanuclear Cu cluster, [(Cu(DSB)(CBT))2(Cu2Br)2][PPh4]2 (1-PPh4) [DSB = 2,3-dis-ulfidobenzoate; CBT = 3-carboxybenzene-1,2-bis(thiolate)] was synthesized as dark green crystals bythe reaction of CuCl2 with 2,3-dimercaptobenzoic acid in acetate buffer solution. The X-ray crystal studyof 1-PPh4 revealed its unique structural features: (1) one of two types of crystallographically distinct Cucenters adopted a square planer geometry and the other center had a tetrahedral geometry, and (2) inter-molecular H-bonding interactions connected between carboxylic acid group of CBT and the carboxylategroup of DSB led to the construction of an unprecedented topologic architecture of a zigzag patternedinfinite sheet. In addition, taking into account the total charge of the molecule, which contained 2,3-dis-ulfidobenzoate and 3-carboxybenzene-1,2-bis(thiolate), and the diamagnetic nature of 1-PPh4, 1-PPh4

led to it is assignment as a mixed-valence Cu(I)/Cu(III) cluster. Such mixed valence states of Cu atomswere also examined by density functional theory calculation.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Transition metal complexes of dithiolene ligands have attractedcontinuous interest over the last few decades because of their ver-satile coordination and redox properties [1]. Depending on thecoordination geometry and oxidation states of the transition metalion, dithiolene complexes have been explored in terms of funda-mental coordination chemistry, material components, and bio-mimicking model studies of Mo and W-containing enzymes [2,3].In general, the initial raw transition metal ions tend to adopt asquare planer geometry in bis(dithiolene) complexes [M(dithio-lene)2]n� (M = Fe, Co, Ni; n = 1 or 2) [4,5]. Although the chemistryof Cu(II) and Cu(III) dithiolene complexes has the same geometricalpreference that of other initial raw transition metals [4,5], that ofCu(I)–dithiolene complexes is entirely different from that of Cu(II)and Cu(III), but Cu(I) center in Cu(I)–dithiolene complexes prefer-entially adopts tetrahedral and trigonal planer geometries similarto Cu(I)–monodentate thiolate complexes [6,7]. Because a sulfur

ll rights reserved.

carboxybenzene-1,2-bis(thio-er atom; Cusq, square planerhenylphosphonium; 1-PPh4,

. Takamizawa), mashima@

center favored the l3 bridging coordination due to the soft–softinteraction between Cu(I) and S atoms, complexes containing theCu(I)–S bond were dominated by the formation of either discretemultinuclear complexes or infinite materials. A number of multi-nuclear Cu(I)–S clusters supported by a variety of monodentatethiolate ligands have been reported [6] leading to a betterunderstanding of the diverse stoichiometry of Cu:S in relation toCu–metallothioneins [7], and symmetric 1,1- and 1,2-dithiolene li-gands have been used to synthesize multinuclear Cu(I)–dithioleneclusters supported by thiophenedithiolate, 1,2-dicyanoethanedi-thiolate (mnt) and 2,2-dicyano-1,l-ethenedithiolate (i-mnt) ligands[8]. In such clusters, {Cu(I)(dithiolene)}1� units were served asbuilding blocks to assemble multinuclear {Cu(I)(dithiolene)}n

n�

clusters in which the third coordination site of trigonal planergeometry around the Cu(I) center is satisfied by a l3 bridging link-age of the sulfur atom. The use of a dissymmetric dithiolene ligandled to the synthesis of a few [M(dithiolene)2]n� [5]; however,mixed-valence Cu clusters with dissymmetric dithiolene ligands,however, have not been reported. Herein, we report the synthesisand structural characterization of a new hexa-nuclear mixed-valence Cu cluster supported by asymmetric dithiolene ligand(2,3-dimercaptobenzoic acid) to form a unique 2D zigzag H-bond-ing network in solid state. The electronic structure of the mixed-valence Cu cluster was estimated using in the density functionaltheory (DFT) calculations.

K. Pal et al. / Inorganica Chimica Acta 373 (2011) 68–72 69

2. Experimental

2.1. Materials and physical methods

All manipulations for air- and moisture-sensitive compoundswere carried out using the standard Schlenk techniques under ar-gon. Solvents were distilled under an atmosphere of argon usingstandard procedures. Anhydrous CuCl2, PPh4Br and lithium metalwere purchased from Aldrich. 2,3-Dimercaptobenzoic acid wasprepared according to the literature procedure [9]. Elemental anal-ysis was performed on a Perkin–Elmer 2400 microanalyzer at theFaculty of Engineering Science, Osaka University.

2.2. Synthesis

2.2.1. Synthesis of [(Cu(DSB)(CBT))2(Cu2Br)2][PPh4]2 (1-PPh4)2,3-Dimercaptobenzoic acid (1 mmol, 0.186 g) was added to a

solution of LiOMe (4 mmol, 0.152 g) in MeOH (5 mL). The solutionwas stirred for 1 h at room temperature, to which methanolic solu-tion (5 mL) of anhydrous CuCl2 (2 mmol, 0.268 g) was added drop-wise. The color of the solution changed from yellow to dark red.After 1 h, the reaction mixture was allowed to expose to the airfor 10 min and the color of the solution was changed to dark green.After the reaction mixture was filtered, removal of solvent undervacuum afforded dark green solids. To a solution of the resultingdark green solids in methanol (10 mL) was layered on a solutionof PPh4Br (0.5 mmol, 0.21 g) and acetic acid (0.1 mL, 1.6 mmol) inMeOH (20 mL). pH of the solution was checked using pH paperas 5–6, dark green crystals of 1-PPh4 were precipitated after threedays, washed with MeOH and Et2O, and dried under vacuum. Theyield of the crystals was 52%. Anal. Calc. for C76H54Br2Cu6O8P2S8:C, 44.24; H, 2.76. Found: C, 44.11; H, 2.78%.

Scheme 1. Synthesis of 1-PPh4. (i) 2 equivalents CuCl2 in MeOH, (ii) expose to airfor 10 min, (iii) 4 equivalents of PPh4Br in MeOH, and (iv) acetic acid in MeOH.

2.3. Computational details

All calculations were performed with the GAUSSIAN 03 package(revision B.04) [10]. Molecular orbitals were visualized using GaussView 4.1. Geometry optimizations, single point calculations, andpopulation analysis of the molecular orbitals were carried out atthe density functional theory (DFT) level with Becke’s three-parameter hybrid exchange functional [11], the non-local correla-tion provided by the Lee, Yang, and Parr expression, and the Vosko,

Table 1Crystallographic data parameters for complex 1-PPh4.

Complex 1-PPh4

Empirical formula C76H54Br2Cu6O8P2S8

Formula weight 1954.79Crystal system monoclinicSpace group P2/nTemperature (K) 100Z 2a (Å) 14.907(5)b (Å) 15.568(5)c (Å) 5.936(5)c (�) 105.341V (Å3) 3567(2)Dcalc (g m�3) 1.820F(0 0 0) 1952Absorption (mm�1) 3.214Theta range (�) 3.08–27.44Reflections collected 56 570Reflections unique 8123Goodness-of-fit (GOF) on F2 1.030R1

a (wR2b) (%) 0.0649 (0.1488)

a R1 =P

||F0| � |Fc||/P

|F0|.b wR2 = {

P[w(F02 � Fc2)2]/

P[w(F0

2)2]}1/2.

Wilk, and Nuair 1980 correlation functional (III) local (B3LYP)6�31g⁄+ basis set [12] were used for H, C, O, Br and S atoms. TheLANL2DZ [13] basis set and LANL2 pseudo-potentials of Hey andWadt [14] were used for the Cu atom. The initial geometry of theanions of 1 was obtained from the crystal structure and the geom-etry was optimized without any symmetry constraints. The opti-mized minima were characterized by harmonic vibrationfrequency calculation where minima have no imaginary frequency.The coordinates of optimized geometry has been given in Support-ing information (Table S1).

2.4. Crystallographic measurements

Suitable diffraction quality crystals of 1-PPh4 were obtainedfrom the crystallization described in its synthesis. The crystalwas measured and data was collected using Rigaku RAXIS RAPIDX-ray instrument. Details of X-ray measurement have been de-scribed in Supporting information. The structure was solved using

Fig. 1. Crystal structure of the anionic part of 1-PPh4 with atom number schemeexcept carbon atoms. ‘Asterisk’ stands for symmetry equivalent. Selected bonddistances (Å): Cu(1)–S(1) = 2.208(5), Cu(1)–S(2) = 2.183(6), Cu(1)–S(3) = 2.190(5),Cu(1)–S(4) = 2.206(6), Cu(2)–O(1) = 2.122(4), Cu(2)–Br(1) = 2.408(1), Cu(3)–Cu(2) = 2.919(2), O(2)–C(7) = 1.268(6), O(3)–C(14) = 1.242(7), Cu(2)–S(2) = 2.260(8), Cu(2)–S(4) = 2.262(8), Cu(3)–S(1) = 2.262(7), Cu(3)–S(3) = 2.276(7),Cu(3)–O(3) = 2.120(4), Cu(3)–Br(1) = 2.413(1), Cu(1)–Cu(1)⁄ = 3.922(5), O(1)–C(7) = 1.229(7), O(4)–C(14) = 1.274(7).

Fig. 2. Intermolecular H-bonding interactions. Various perspective views of the zigzag 2-dimensional intermolecular hydrogen bond network for 1-PPh4, (a) H-bondinginteraction between –COOH group of one molecule and –COO�1 of another molecule. (b) H-bonding interactions of each molecule with neighbor four molecules making a 2Dsheet. (c) Two parallel sheet separated by a distance of 3.558 Å and the position of cation in between two sheet shown as polyhedron. The separation between two sheets is3.558.

70 K. Pal et al. / Inorganica Chimica Acta 373 (2011) 68–72

SIR97 [15] and refined using SHELXL-97 [16]. All non-hydrogenatoms were refined with anisotropic displacement parameters.All H-atoms were included in the refinement on calculated posi-tions riding on their carrier atoms. Crystal structure was viewedusing ORTEP [17]. Selected crystallographic parameters are listedin Table 1.

3. Results and discussion

Because 2,3-dimercaptobenzoic acid [9] has different pKa valuesdue to its thiolate and carboxylic acid groups, it was expected tocoordinate as a bidentate ligand in an ene-dithiolate form (3-carb-oxybenzene-1,2-bis(thiolate) = CBT) under controlled bufferconditions or as a multidentate ligand with a chelating 1,2-ene-dithiolate and carboxylate (2,3-disulfidobenzoate = DSB) upon fulldeprotonation (Scheme 1). In acetate buffer methanol solution, adianionic hexanuclear cupper cluster, [(Cu(CBT)(DSB))2

(Cu2Br)2][(PPh4)2] (1-PPh4) was obtained by the sequential reac-tions outlined in Scheme 1. The addition of 3 equivalents of LiOMeto 2,3-dimercaptobenzoic acid in methanol quantitatively afforded[Li3(DSB)], which was treated with the solution of anhydrous CuCl2

in methanol to generate a dark red colored solution. Upon expo-sure to air, the red solution changed immediately to dark greendue to the oxidation of Cu(II) to Cu(III) by aerial oxygen. Such aerial

oxidation of Cu(II) to Cu(III) has been reported for some dithio-lene–Cu complexes [4,5]. To pH of the solution was adjusted to5–6 by adding small amount of acetic acid (pH of the solutionwas checked using pH paper), and bulky cation PPh4Br to the greensolution of Cu(III)–dithiolene in methanol. After allowing the solu-tion to stand for 3 days, fine dark green crystals of 1-PPh4 were ob-tained in 52% yield. The 1-PPh4 complex is insoluble in commonorganic solvent even in DMF and DMSO, preventing spectroscopiccharacterization of 1-PPh4 complex in solution.

Fig. 1 shows the molecular structure of 1-PPh4 determined byX-ray crystallography. Compound 1-PPh4 was crystallized in themonoclinic space group of P21/n, in which each unit cell containstwo anionic hexanuclear copper clusters (1) and four tetraphenyl-phosphonium cations. The anionic part of 1-PPh4 contains twotypes of Cu atoms with different coordination environments: oneis a square planar Cu ion (abbrev. as Cusq) and the other is a tetra-hedral Cu ion (abbrev. as Cutd), and hence, each anion (1) has a to-tal of four Cutd and two Cusq atoms. Two different forms ofdeprotonated ligands, where DSB and CBT, respectively, are in-volved in constructing the cluster. Cusq adopts (Cu1 and Cu1⁄ inFig. 1) a perfect square planer geometry surrounded by four Satoms from CBT and DSB ligands to form a {CusqS4} unit. The meanCu–S bond distance and dithiolene bite angle are 2.197(2) Å and90.45�, respectively. For the orientation of dissymmetric dithioleneligands, each Cusq possesses two dithiolene ligands in a syn fashion,

Fig. 3. Pictorial representation of LUMO and LUMO+1 molecular orbitals.

K. Pal et al. / Inorganica Chimica Acta 373 (2011) 68–72 71

being in contrast to the anti orientation preferred for Cu(II)/Cu(III)–dithiolene complexes with dissymmetric dithiolene ligands [5].The dihedral angle and distance between two parallel best planesof {CusqS4} were 0.00(3)� and 3.580 Å, respectively.

Tetrahedral Cutd atoms (Cu2, Cu2⁄, Cu3, and Cu3⁄ in Fig. 1) weresurrounded by one carbonyl oxygen atom, two l3 linked sulfuratoms, and one l2 bromide atom. Two neighboring Cutd centers be-tween two {CutdS2OBr} units were bridged by a bromide ion (Cutd–Br–Cutd = 74.49(3)�) to form a {(Cutd)2Br} unit. The mean Cutd–Sl3

bond distance and S–Cutd–S angles were 2.262(3) Å and 124.93(6)�,respectively. It is notable that there was no bonding interaction be-tween any two adjacent Cu centers within the molecule as evi-denced by the long distances of Cu(1)� � �Cu(1⁄) (2.919(12) Å) andCu(2)� � �Cu(3) (3.922(5) Å). The carboxylate/carboxylic acid groupswere used to satisfy the fourth coordination site of the Cutd centerto form a {(Cu(S)2O)2Br} unit.

In the unit cell packing (Fig. 2), each carboxylate group had ashort distance to a neighboring carboxylic acid group of anothermolecule (O(4)–O(2) = 1.721(4) Å),1 indicating that the anionic partof the cluster (1) has a total of four intermolecular H-bonding inter-actions with four neighborhood anionic clusters (1) as depicted inFig. 2. As a consequence of such intermolecular hydrogen bonding,1-PPh4 could not be dissolved in a common organic solvent andits spectroscopic characterization, including electrochemical mea-surement to determine the oxidation state of two different Cu cen-ters, was hampered. Thus, based on the geometries of the Cuatoms, the two different deprotonated ligands (CBT and DSB), thediamagnetic nature of the molecule, and the total charge balance,the oxidation states of two kinds of Cu centers were determined tobe a mixture of four Cu(I) and two Cu(III), where the Cusq atom ofthe {CuS4} unit has a (+3) oxidation state and the Cutd atom has a(+1) oxidation state.2

The alignment of 1-PPh4 in unit cells is shown in Fig. 2. If weconsidered 1 to be a rectangular box, the H-bonds were directedtowards the four opposite corner of the box so as to form a 2Dinfinite sheet along the b and c axes of unit cell. Such a directedH-bonding network afforded an unprecedented topologic architec-ture of a zigzag patterned infinite sheet of the anionic part of1-PPh4 (Fig. 2). The thickness of the zigzag sheet was 3.580 Å.Tetraphenylphosphonium cations occupied the space betweentwo parallel sheets of the anionic part of 1-PPh4 and the distancebetween two parallel sheets was 3.558 Å.

Due to the presence of two electronically different Cu atomswithin the cluster, the nature of the cluster was evaluated basedon ground-state electronic structure calculations performed atthe DFT (B3LYP) level to determine the energies and compositionsof their molecular orbitals (MOs). The initial geometry was ob-tained from the crystal structure and the geometry was optimizedin the gas phase. The optimized geometry of 1 was in good agree-ment with that obtained from the X-ray crystal structure. An addi-tional single-point calculation in gas phase was performed on thegas phase-optimized geometry for MO analysis. Each MO was as-signed to a type on the basis of its composition and by visualinspection of its localized orbital. The coordinate frames for bothcompounds were assigned by a visual inspection of the dz2 anddxy orbitals and the plane of {CusqS4} unit was assigned as xy plane.It is very difficult to construct the exact MO diagram and theircompositions for multinuclear clusters with different types of li-

1 Even, only one hydrogen atom was located in the difference Fourier map at usuaOH position of O(2) atom, no such electron density was found at O(4). This wasinitially refined with an O–H distance restraint of 0.82 (1) Å and later allowed to rideon the O atom with Uiso(H) = 1.5 Ueq(O).

2 At slightly acidic pH (5–6), coordinated dithiolene ligand is very active forintramolecular redox reaction where metal center get reduced and thiolate groupoxidized to di-sulfido. Such redox reaction might happening during the course oreaction by which Cu(II) get reduced to Cu(I).

l

f

gands. MO analysis, however, can predict the oxidation states oftwo different types of Cu atoms. The highest occupied molecularorbital (HOMO), HOMO-1 and next several low-lying HOMOs weremostly populated by ligand p orbitals from different ligands. MOorbitals originating from d orbitals of both Cusq and Cutd centerswere low-lying and deeply buried HOMOs, which are more stablethan ligand p orbitals. We found that only doubly degenerate(DE = 3.9 kcal) lowest unoccupied molecular orbitals (LUMO andLUMO+1) occurred due to the significant contribution of dx2–y2orbital of Cusq atoms. These results clearly indicated that the Cutd

atom had fully occupied five d orbitals in low energy states. TheCusq center had four low-laying occupied d orbitals (dxz, dyz, dxy,dz2) and one empty high-energy dx2–y2 orbital (Fig. 3). Twoempty dx2–y2 orbitals from two different {CusqS4} units were al-most degenerate (19.1 and 15.2 kcal). The possibility of thepresence of ligand base radical such as Cu(II)-(DSB)(CBT�) or Cu(II)-(DSB�)(CBT) radical can be excluded according to the reportedbroken symmetry DFT calculations for [Cu(III)(L)2]�1 (L = S2C2(CN)2,S2C6H6) which clearly indicated that those complexes consist of atrivalent metal ion with d8 configuration and two S,S coordi-nated-1,2-dithiolate(2-) ligands [18]. Accordingly, Cutd center haved10 electronic configuration and (+1) oxidation state can be

72 K. Pal et al. / Inorganica Chimica Acta 373 (2011) 68–72

assigned. Cusq centers had a low spin d8 electronic configurationwith a (+3) oxidation state. This result is very consistent with theexperimental observations.

4. Conclusions

We synthesized a hexanuclear Cu cluster 1-PPh4 supported by2,3-disulfidobenzoate and 3-carboxybenzene-1,2-bis(thiolate) inacetate buffer. Structural characterization determined that the an-ionic cluster 1-PPh4 contains two types of deprotonated forms of2,3-dimercaptobenzoic acid and two different electronic types ofCu centers. In solid state, 1-PPh4 showed strong intermolecularH-bonding interactions that uniquely build a 2D zigzag sheetstructure. The diamagnetism and total charge balance suggestedthat 1-PPh4 contains two Cu(III) and four Cu(I) centers with a dif-ferent coordination geometry. Such mixed-valence state was con-firmed by DFT calculation.

Acknowledgments

K.P. acknowledges a fellowship from the Japan Society for thePromotion of Science (JSPS) and the Grant-in-Aid for Scientific Re-search provided by JSPS. This work is financially supported by aGrant-in-Aid from the Ministry of Education, Culture, Sports, Sci-ence, and Technology, Japan.

Appendix A. Supplementary material

CCDC 790336 contains the supplementary crystallographic datafor this paper. Details of crystallographic study for 1-PPh4 and thegeometry optimized coordinate of the anionic part of 1-PPh4 areavailable. These data can be obtained free of charge from The Cam-bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-ta_request/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2011.03.055 Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.ica.2011.03.055.

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