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A complete series of copper(II) halide complexes (X = F, Cl, Br, I)with a novel Cu(II)–C(sp3) bond

Riichi Miyamoto, Ryoko Santo, Toshio Matsushita, Takanori Nishioka, Akio Ichimura,Yoshio Teki and Isamu Kinoshita*Department of Molecular Materials Science, Graduate School of Science, Osaka CityUniversity, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan.E-mail: isamu@sci.osaka-cu.ac.jp; Fax: +81 6 6690 2753; Tel: +81 6 6605 2546

Received 19th May 2005, Accepted 20th July 2005First published as an Advance Article on the web 15th August 2005

A complete series of copper(II) halide complexes [CuX(tptm)] (X = F (1), Cl (2), Br (3), I (4); tptm =tris(2-pyridylthio)methyl) with a novel CuII–C(sp3) bond has been prepared by the reactions of[Cu(tptm)(CH3CN)]PF6 (5·PF6) with corresponding halide sources of KF or n-Bu4NX (X = Cl, Br, I), and thetrigonal bipyramidal structures have been confirmed by X-ray crystallography and/or EPR spectroscopy. The iodidecomplex 4 easily liberates the iodide anion in acetonitrile forming the acetonitrile complex 5 as a result. The EPRspectra of the complexes showed several superhyperfine structures that strongly indicated the presence of spin densityon the halide ligands through the Cu–X bond. The results of DFT calculations essentially matched with the X-raycrystallographic and the EPR spectroscopic results. Cyclic voltammetry revealed a quasi-reversible reduction wavefor CuII/CuI indicating a trigonal pyramidal coordination for CuI states. A coincidence of the redox potential for all[CuX(tptm)]0/+ processes indicates that the main oxidation site in each complex is the tptm ligand.

IntroductionRecent development of copper complexes has explored therelationship between oxidation state of a copper centre and thegeometry of the complex as well as the type of coordinatingatom.1 As a result, it has become possible to control theoxidation/reduction behaviour precisely using tripodal typeligands such as tris(pyridylmethyl)amine (tpa),2 derivatives ofhydrotris(pyrazolyl)borate (tpb)3 and tris(pyrazolyl)methane(tpm).4 For example, the increment of the chelate rings in tpastabilises the copper(I) state significantly.5 The tpb and tpmligands have similar structures but the negative charge on tpbdestabilises copper(I) state.

Recently, we have designed a novel tripodal ligand, tris(2-pyridylthio)methane (tptmH) and prepared its copper(II) com-plexes [CuL(tptm)]+/0 (L = Br, CH3CN) which have a copper(II)–carbon(sp3) bond as the first example for a copper(II) complex.6,7

The structure of the copper complex of tptm resembles to thatof tpa and the differences between them are mainly the coor-dination atom either by carbon or nitrogen atoms and also thecharges on the ligands. The copper(II)–carbon bond is rare andthere are only two other examples of a copper(II)–carbon bondwhich found in the nitrogen-anchored N-heterocyclic carbene(NHC) complexes8 and the N-confused porphyrin system.9 The

NHC ligands coordinate to a divalent copper ion forming thetrigonal pyramidal complexes, of which the stabilised carbenecoordinate to copper(II). In the case of N-confused porphyrins,the total p system of the N-confused porphyrin ring stabilisesthe copper(II)–carbon(sp2) bond. In [CuL(tptm)]n+, the presenceof three sulfur atoms stabilises the carbanion which affects thewhole properties of the resulting complexes. Accordingly,the exotic system promotes us to investigate the property ofthe copper(II) complexes with the tptm ligand.

In this study, the substitution reaction of 5·PF6 has beenexamined to obtain a complete series of the halide complexes.The redox and spectroscopic properties of [CuX(tptm)] (X = F(1), Cl (2), Br (3), I (4)) as well as the X-ray structures have alsobeen examined to explore the effect of the CuII–C(sp3) bond.In addition, density functional calculation on the basis of themolecular orbital theory was applied to elucidate the effect ofthe sp3 carbanion coordination.

ExperimentalAll chemicals were purchased from Aldrich, Nacalai Tesqueand Wako Pure Chemicals. Complex 5·PF6 was preparedD

OI:

10.1

039/

b50

7073

b

T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5 D a l t o n T r a n s . , 2 0 0 5 , 3 1 7 9 – 3 1 8 6 3 1 7 9

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as reported previously.7 All reagents and solvents were usedwithout further purification. Anhydrous acetonitrile was pur-chased from Nacalai Tesque. IR spectra were recorded on aJASCO FT/IR-420 spectrophotometer in the range of 4000–400 cm−1. Electronic spectra were recorded on a JASCO V-570 spectrophotometer. Electrochemical measurements wereperformed with a BAS CV-50 W voltammetric analyser. Cyclicvoltammograms were recorded at 25 ◦C with a platinum diskworking electrode (φ = 1.6 mm) and a Ag/Ag+ reference elec-trode, dichloromethane and acetonitrile, and 0.1 M n-Bu4NPF6

used as solvent and supporting electrolyte, respectively. All datawere referred to an internal Fc/Fc+ couple, of which the redoxpotential was standardised as 0.400 V vs. SHE. EPR spectra wererecorded on a JEOL JES-FE2XG EPR spectrometer. Elementalanalyses were performed by the Analytical Research ServiceCentre at Osaka City University on FISONS Instrument EA108or Perkin Elmer 240 C elemental analysers.

Preparations of the halogen complexes

[CuF(tptm)] (1). This experiment was performed underrigorously anhydrous condition in an MBRAUN UNIlabOP7 globebox under an argon atmosphere. A solution of2,2,2-crypt, 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane (0.023 g, 0.062 mmol) in anhydrous acetonitrile(5 mL) was stirred with KF (0.009 g, 0.15 mmol) and 5·PF6

(0.032 g, 0.054 mmol) for 2 h. The resulting green mixturewas filtered and the filtrate was allowed to stand for 1 weekat −10 ◦C to afford green crystals of 1 suitable for X-raycrystallography (yield 70%, 0.016 g) (Found: C, 45.18; H, 2.74;N, 9.88. C16H12CuFN3S3 requires C, 45.21; H, 2.85; N, 9.89%);mmax/cm−1 1584s, 1553s, 1417s, 1275s, 1130s, 1092w, 1039m,1005m, 764s, 722s, 668m, 635m, 493w, 436m, 416m (Nujol).

[CuCl(tptm)] (2). To an acetonitrile solution (2 mL) of 5·PF6

(0.0262 g, 0.0442 mmol), tetra-n-butylammonium chloride(0.0126 g, 0.0455 mmol) in acetonitrile (2 mL) was added. Theresulting reddish purple solution was evaporated to drynessand then the residue was re-dissolved in toluene (10 mL). Aresulting insoluble white precipitate, tetra-n-butylammoniumhexafluorophosphate was removed by filtration, then the fil-trate was evaporated to dryness giving a crude product of 2which was purified by a silica-gel column chromatography withdichloromethane and acetone (8:1) as eluant to give a reddishpurple fraction. Removal of the solvent gave a reddish purplesolid of 2 which was recrystallised from dichloromethane andcyclohexane (yield 89%, 0.0174 g) (Found: C, 43.57; H, 2.72; N,9.53. C16H12ClCuN3S3 requires C, 43.53; H, 2.74; N, 9.52%);mmax/cm−1 2725w, 1582s, 1554s, 1418s, 1377s, 1282s, 1154m,1132s, 1089m, 1046m, 1004m, 973w, 840w, 772m, 754s, 723s,634m, 488m, 464w, 411m (Nujol).

[CuBr(tptm)] (3). Complex 3 was prepared in the samemanner as 2 using tetra-n-butylammonium bromide insteadof tetra-n-butylammonium chloride and isolated as purplecrystals in 69% yield (Found: C, 39.48; H, 2.25; N, 8.37.C16H12BrCuN3S3: C, 39.55; H, 2.49; N, 8.65%); mmax/cm−1 1582s,1555s, 1418s, 1378s, 1279s, 1236w, 1153w, 1131s, 1087m, 1046s,1003s, 970w, 889w, 765m, 768s, 752s, 739s, 722s, 634m, 586w,489m, 464w, 411m (Nujol).

[CuI(tptm)] (4). To a purple dichloromethane solution(10 mL) of 5·PF6 (0.511 g, 0.863 mmol), tetra-n-butylammoniumiodide (0.322 g, 0.871 mmol) in dichloromethane (5 mL) wasadded. The resulting blue solution was evaporated to drynessunder reduced pressure and then re-dissolved in toluene (5 mL).After the insoluble white precipitate was filtered off using aglass frit, hexane (50 mL) was then added into the blue filtrateto afford a microcrystalline solid in 70% yield (0.322 g). Thecrystals suitable for X-ray diffraction analysis were obtainedfrom the blue toluene solution of the complex by slow diffusionof cyclohexane layered on the solution (Found: C, 36.22; H, 2.24;

N, 7.88. C16H12CuIN3S3 requires C, 36.06; H, 2.27; N, 7.88%);mmax/cm−1 2725w, 1581s, 1555s, 1536s, 1417s 1377s, 1300w, 1279s,1152m, 1132m, 1087w, 1048m, 1001m, 971w, 843w, 804w, 757s,723s, 635m, 586w, 489w, 463w, 410m (Nujol).

X-Ray crystallography

Data were collected on a Rigaku AFC-7/Mercury CCD area-detector diffractometer with graphite-monochromated Mo-Karadiation (k = 0.7107 A). The CrystalClear10 software was usedfor the collection, processing and correction for Lorentzianand polarisation effects. Absorption corrections were appliedon comparison of multiple symmetry equivalent measurements.The teXsan11 or CrystalStructure12 programmes were used forthe analyses.

Systematic absences in the intensity data for 1 were consistentwith the two space groups Pca21 and Pbcm. The former spacegroup was chosen and confirmed by the successful solutionand refinement of the structure. Efforts to solve in the spacegroup Pbcm were unsuccessful. The structure of 1 was solvedby the SHELX9713 direct method and expanded using Fouriertechniques and refined by full-matrix least squares against F 2

using SHELX97. The tptm ligand in complex 1 was disorderedin positions but only S atoms in the ligand were treated asdisordered because atoms in the disordered pyridine ringswere too close to each other. Only the carbon atoms of thepyridine rings in the orientation with major occupancies weretaken into account and the occupancies of them were setto 1.0. This treatment resulted in prolate or oblate thermalellipsoids for some of carbon atoms on the pyridine ring. Thedisordered S and carbanion C atoms with smaller occupanciesand four carbon atoms in the pyridine rings of tptm were refinedisotropically, while the rest of non-hydrogen atoms were refinedanisotropically. All hydrogen atoms were refined using the ridingmodel.

Systematic absences in the data for 4 were consistent withthe space groups P212121. The structure of 4 was solved by theSIR200214 direct method, expanded using Fourier techniquesand refined by CRYSTALS.15 The complex molecules of 4 weredisordered in positions but only the carbanion C, Cu, I and Satoms were treated as disordered because of the same reason ascomplex 1. The disordered atoms with smaller occupancies wererefined isotropically, while the rest of non-hydrogen atoms wererefined anisotropically. All hydrogen atoms were refined usingthe riding model. A summary of the crystal data, detail of datacollection and refinement are given in Table 1.

Table 1 Crystallographic data of 1 and 4

Compound 1 4

T/K 193 193Formula CuFC16H12N3S3 CuIC16H12N3S3

M 425.02 532.92Crystal system Orthorhombic OrthorhombicSpace group Pca21 (no. 29) P212121 (no. 19)a/A 18.583(7) 9.329(3)b/A 13.026(5) 13.504(4)c/A 14.035(5) 14.721(5)a/◦ 90 90b/◦ 90 90c /◦ 90 90V/A3 3397(2) 1854.6(10)Z 8 4l(Mo-Ka)/mm−1 1.667 3.183Data collected 33171 17994Unique data (Rint) 7031 (0.075) 4210 (0.076)Observed data [I >

2r(I)]4280 2804

Final R1 [I > 2r(I)]a 0.0521 0.060wR2 (all data) 0.0916 0.121

a R1 = ∑||F o| − |F c||/∑|F o|, wR2 = [

∑w(F o

2 − F c2)2/

∑w(F o

2)2]1/2.

3 1 8 0 D a l t o n T r a n s . , 2 0 0 5 , 3 1 7 9 – 3 1 8 6

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CCDC reference numbers 272551–272552.See http://dx.doi.org/10.1039/b507073b for crystallographic

data in CIF or other electronic format.

Computational methods

Electronic structure calculations for 1–4 were performed apply-ing Becke’s three-parameter hybrid function using Lee, Yangand Parr’s correlation function (B3LYP)16 together with theLANL2DZ basis set17 as implemented in the program packagesGaussian 98 and 03.18,19 The fermi contact coupling constantsand the anisotropic spin dipole couplings for 1–3 were calculatedwith the 6-31g(d) basis set. The systems were treated as the C3

point group. Geometries of the complexes were optimised and itwas confirmed that no imaginary frequencies were arisen at allcalculated structures by frequency calculations.

Results and discussionPreparations and characterisations of halide complexes

Treatment of 5·PF6 with tetra-n-butylammonium halides atroom temperature caused the substitution reaction to form thecorresponding halide complexes 1–4 (Scheme 1, eqn. (1)) whosecolour are green, reddish purple, purple and blue, respectively.The reaction proceeded almost quantitatively with an equalmolar amount of the halide sources for 2–4 indicating thestability of the [CuX(tptm)] system. In the case of 1, the producttends to react with co-existing water in the solvent givingan aqueous purple material as well as the complex hydrate.20

To avoid the hydration, the complex 1 was prepared underrigorously anhydrous condition in a globebox under an argonatmosphere, (Scheme 1, eqn. (2)). Excess amount of KF was usedto accelerate the dissolving rate of KF in a reaction mixture withthe help of 2,2,2-crypt.

Scheme 1 Preparation of complexes 1–4.

The softer halide in the complex is irreversibly substituted withthe harder halide indicating the hardness of the copper(II) ion inthe [CuX(tptm)] complex. However, the iodide complex 4 wasobtained with a reasonable yield suggesting the intrinsic stabilityof the trigonal bipyramidal structure of the complex. Generally,copper(II) iodide complexes are stabilised by the coexistence ofthe strongly coordinated ligands,21 even more, it is rare to havea set of copper(II) complexes with the complete series of halidesions. The presence of this series provoked us to investigate thespectroscopic properties as well as the redox properties on thebasis of theoretical consideration.

Single-crystal X-ray structural determinations of halide complex

The crystals of 1 and 4 suitable for single crystal X-rayanalyses were obtained by cooling the solution or diffusion ofcyclohexane into the toluene solution, respectively. For 2 and3, only preliminary analyses could be performed due to theinherent problem of disorder as mentioned afterward. Althoughthe obtained structural data for 2 and 3 were not enoughfor discussions of structural detail, the results clearly showedthe frameworks and the crystal packing of the complexes andrevealed that they were isomorphous with 4.

Complex 1 crystallises in the space group Pca21. Two dis-ordered tilting orientations of the pyridine rings of the tptmligand were observed. This tilting orientations are attributed tothe shorter Cu–C(carbanion) bond resulting that the planes ofN3 and S3 triangles are not superimposed to each other. Becauseof the goodness of the data in spite of the disorder, we could

discuss the further structural detail. There are two independentcomplex molecules in the asymmetric unit. The F–Cu–C axes ofthe complexes are aligned nearly parallel to the c axis and thedirections of the F–Cu–C axes of the independent complexes areopposite to each other (Fig. 1).

Fig. 1 Crystal packing of 1 viewed along (a) the a axis, (b) the b axisin which hydrogen atoms are omitted for clarify. Atoms on positions ofy > 1/2 are omitted for clarity for (b).

The I–Cu–C axis of complex 4 is aligned toward the c axisin the crystal. Most of them pointed to the same direction(Fig. 2) while a part of them headed to the opposite resultingin the positional disorder. The shortest distance between theC atoms of the pyridine rings in the ligand and those of theclosest neighbour molecule is 3.52(1) A suggesting the p–pinteraction causing the distortion of the coordination geometryof the central copper atom from the ideal trigonal bipyramidalarrangement.

By the p–p interaction between the pyridine rings, themolecules of 4 were aligned along with the c axis forming 1-D arrays in the crystal, which is also the same for isomorphous2 and 3. Because the pyridine rings in the complex moleculesthat have minor site occupancies are almost superimposed tothose having major occupancies, the pyridine rings could not betreated as disordered (see Experimental section). This meansthat both orientation of the disordered complex moleculeswhich directed to opposite directions along the I–Cu–C axiscan participate the 1-D p–p interaction network. On the other

D a l t o n T r a n s . , 2 0 0 5 , 3 1 7 9 – 3 1 8 6 3 1 8 1

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Fig. 2 Crystal packing of 4. Hydrogen atoms and atoms with minoroccupancies are omitted for clarity. A dashed line and dots indicate p−pstacking.

hand, no p–p interaction of the pyridine rings was observed inthe crystal of 1. However short distances between the fluorideligands and the carbon atoms in the pyridine rings of theneighbour complex molecules (3.15(2)–3.21(2) A) were observedin 1 probably reflecting the more negatively charged characterof the fluoride than the other halides (Fig. 3).

Fig. 3 ORTEP drawing of 1 with 50% probability ellipsoids. Selectedbond lengths and angles are shown in Table 2.

The coordination geometries of the copper ions in 1 and 4are distorted trigonal bipyramidal (Fig. 4) and their structuresresemble that of 5·PF6 or [CuIIX(tpa)]+ (X = F, Cl, Br).22 Selectedbond lengths and angles were listed in Table 2. In these cases,trigonal bipyramidal structures are flexible and the N–Cu–Nangles in the molecules easily deviate from 120◦ by the influenceof the neighbourhood molecules. The bond lengths of CuII–C are 2.02(2) av., 2.001(8) and 2.004(3) A for 1, 4 and 5·PF6,respectively. In all cases, the copper atoms are located at muchthe centre of the tripodal pyridine arrangement, and 0.097(2) av.,0.176(2) and 0.1303(4) A off from the triangle plane of the threenitrogen atoms of the pyridines toward the halogen atom for 1,4 and 5·PF6, respectively. The frameworks of the complexes aresimilar to that of [CuIIX(tpa)]+, which have trigonal bipyramidal

Fig. 4 ORTEP drawing of 4 with 50% probability ellipsoids.

Table 2 Selected bond lengths (A) and angles (◦) for 1, 4 and 5·PF6

1

aa ba 4 5·PF6

Cu–X 1.874(7) 1.918(8) 2.717(1) 2.077(3)Cu–C1 2.04(1) 2.01(1) 2.001(8) 2.004(3)Cu–N1 2.11(1) 2.05(1) 2.117(7) 2.084(3)Cu–N2 2.072(9) 2.091(9) 2.118(8) 2.074(3)Cu–N3 2.04(1) 2.101(9) 2.116(7) 2.074(3)

X–Cu–C1 176.8(4) 177.2(4) 176.6(3) 179.7(1)X–Cu–N1 91.5(3) 93.5(4) 96.7(2) 93.4(1)X–Cu–N2 92.4(4) 93.6(3) 110.4(3) 93.4(1)X–Cu–N3 92.2(4) 92.9(3) 120.4(3) 94.0(1)C1–Cu–N1 85.6(4) 83.9(5) 85.1(4) 86.5(1)C1–Cu–N2 86.8(4) 88.9(4) 83.5(4) 86.4(1)C1–Cu–N3 90.7(5) 87.9(4) 86.9(4) 86.3(1)N1–Cu–N2 101.6(4) 132.8(3) 110.4(3) 116.6(1)N1–Cu–N3 128.2(3) 125.1(3) 120.4(3) 121.5(1)N2–Cu–N3 129.9(3) 101.0(4) 127.2(3) 129.7(1)

a a and b indicate the independent complex molecules.

geometry with the different charge of the ligand from that oftptm with the coordinating carbanion (Table 2, 3).

Electrochemical and spectroscopic properties

UV-Vis absorption spectra of the halide complexes weremeasured in acetonitrile and dichloromethane (Fig. 5), andthe data were listed in Table 4. The absorption maxima in

Table 3 Selected bond lengths (A) and angles (◦) for [CuX(tpa)]+ (X =F, Cl, Br)a

X = F X = Cl X = Br

Cu–X 1.852(5) 2.249(2) 2.378(2)Cu–N1ax 2.059(8) 2.065(4) 2.038(7)Cu–N2eq 2.019(8) 2.030(5) 2.035(7)Cu–N3eq 2.039(8) 2.164(4) 2.108(7)Cu–N4eq 1.94(1) 2.013(4) 2.054(8)

X–Cu–N1ax 179.7(3) 178.9(1) 179.1(2)X–M–N2eq 98.6(3) 95.4(1) 97.7(2)X–M–N3eq 99.4(3) 101.6(1) 98.8(2)X–M–N4eq 98.3(3) 100.2(1) 100.5(2)N1ax–M–N2eq 81.7(3) 84.7(2) 81.8(3)N1ax–M–N3eq 80.4(3) 79.3(1) 80.9(3)N1ax–M–N4eq 81.6(4) 78.8(2) 80.4(3)N2eq–M–N3eq 117.8(3) 117.2(2) 117.8(3)N2eq–M–N4eq 124.8(3) 125.1(2) 125.1(3)N3eq–M–N4eq 110.5(3) 110.5(2) 109.9(3)

a References 21a,b and c for F, Cl and Br complexes, respectively.

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Table 4 UV-Vis absorption data of complexes 1–4

Dichloromethane Acetonitrile

k/nm e/M−1 cm−1 k/nm e/M−1 cm−1 k/nm e/M−1 cm−1 k/nm e/M−1 cm−1

1 —a 466 549 —b 462 5002 383 1976 527.5 1462 380 1378 524 9923 392.5 2204 541 1993 393 2200 538.5 18734 405.5 2634 570 2926 399 2106 554 1279

a d–d transition appears at 653 nm (e = 177 M−1 cm−1). b d–d transition appears at 680 nm (e = 158 M−1 cm−1).

Fig. 5 UV-Vis spectra of the halide complexes in acetonitrile (top) anddichloromethane (bottom).

dichloromethane agree with those in acetonitrile except for 4.UV-Vis spectrum of 4 in acetonitrile was the same as that of5·PF6 indicating the substitution of acetonitrile for the iodideanion. All these complexes have an absorption maximum around450–600 nm with e value of ca. 103 M−1 cm−1. Both wavelengthand intensity increase in the same order of F < Cl < Br < I andthen the absorptions are assignable to the XMCT transition.Complex 1 exhibits additional absorption maximum at 653 nm(e = 177) in dichloromethane assignable to the d–d transition,which superimpose to XMCT transition for 2–4 and was notdefined.

The EPR spectra of the halide complexes in toluene at123 K show typical resonance for trigonal bipyramidal structure(Fig. 6). The superhyperfine as well as hyperfine splitting in thesespectra indicates that the unpaired electron of each complex isdelocalised over the copper dz2 and halogen p and s orbitals, sincethe dz2 orbital combines with the p and s orbital of the halogenatom to form the Cu–X bond.23 Simulated values for the com-plexes shown in Table 5 indicate that the complexes in toluenemaintain the trigonal bipyramidal structures strictly. The maindifference appears in the hyperfine coupling constants. A(para)sfor copper are 62 and 52 for 3 and 4, respectively, which are muchsmaller than those for 1 and 2. This phenomenon indicates thelager d orbital splitting for 1 and 2 than 3 and 4 (F, Cl > Br, I).

Previously, we have preliminarily examined the redox pro-cesses for 3 and 5·PF6 by cyclic voltammetry.7 There are severalinteresting features for these processes. Both complexes exhibitone-electron oxidation and one-electron reduction processes in

Table 5 Simulated g and A (G) values of complexes 1–4

Complex g Value A/G

1 g|| = 2.017 |ACu| = 102, |AF| = 105g⊥ = 2.155 |ACu| = 65

2 g|| = 2.014 |ACu| = 94, |ACl| = 17g⊥ = 2.124 |ACu| = 47

3 g|| = 2.031 |ACu| = 62, |ABr| = 49g⊥ = 2.126 |ACu| = 54

4 g|| = 2.025 |ACu| = 52, |AI| = 52g⊥ = 2.123 |ACu| = 59

Fig. 6 EPR spectra for complexes 1–4 in toluene at 123 K. All spectraresemble to those of typical trigonal bipyramidal copper complexes.

acetonitrile and dichloromethane. The oxidation process of 5is reversible in dichloromethane, whereas the process becomesirreversible in acetonitrile. The reduction of 3 is electrochemi-cally more reversible in acetonitrile than in dichloromethane. Toclarify these processes, we have measured the electrochemicalprocesses of 1–4.

The [CuX(tptm)] complexes show the systematic redoxbehaviours in cyclic voltammograms (Fig. 7). Each of thecomplexes exhibits a reversible one-electron oxidation to[CuX(tptm)]+ in acetonitrile and dichloromethane except for 4 inacetonitrile. The oxidation potentials for [CuX(tptm)]+/0 couplesare close to each other within the range from 0.427 to 0.455 Vin dichloromethane, the difference being only 28 mV amongthem (Table 6). The multi oxidation peaks of 4 in acetonitrile areconsistent with the sum of the voltammograms for the oxidationof 5 and for a free iodide ion, which are generated by the ligandsubstitution of solvent acetonitrile for iodide ion in the complex.In contrast to the oxidation process, the voltammetric behaviourof the reduction process changed systematically from 4 to 1(from I to F). In dichloromethane, [CuX(tptm)] shows a quasi-reversible CuII/CuI process and the reversibility systematicallyincreases in the order of 1 to 4 (from F to I) with the positive shift

D a l t o n T r a n s . , 2 0 0 5 , 3 1 7 9 – 3 1 8 6 3 1 8 3

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Table 6 Electrochemical data for complexes 1–4

E0 ′/V (DEp/mV)

Complex Solvent CuII/I [CuIIX(tptm)]+/0

1 (X = F) CH2Cl2 −1.321 (Epc),a

−0.347 (Epa)a0.427 (74)

CH3CN −1.226 (Epc),a

−0.426 (Epa)a0.447 (62)

2 (X = Cl) CH2Cl2 −0.989 (Epc),a

−0.347 (Epa)a0.448 (76)

CH3CN −0.717 (Epc),a

−0.404 (Epa)a0.480 (62)

3 (X = Br) CH2Cl2 −0.609 (Epc),a

−0.377 (Epa)a0.453 (80)

CH3CN −0.463 (99) 0.491 (57)4 (X = I) CH2Cl2 −0.536 (Epc),a

−0.125 (Epa)a0.455 (77)

CH3CN −0.237 (67) 0.373 (Epa),a 0.657 (Epa)a

a Irreversible peak potentials with a scan rate of 100 mV s−1.

Fig. 7 Cyclic voltammograms of the halide complexes in acetonitrile(thin line, above) and in dichloromethane (thick line, below).

of the peak potentials. The hard copper(II) ion in [CuX(tptm)] isdifficult to reduce when fluoride coordinates to the copper centre.The same re-oxidation potential for all complexes indicates thepresence of [CuI(tptm)] species produced by the EC processof one electron reduction with succeeding the liberation of Xanions. The CuII/CuI couple for 5 is quite similar to that for[Cu(tpa)(CH3CN)]+ suggesting that the similar copper(I) statefor the tptm complex is concerned.24

The potential of [CuX(tptm)]+/0 couple does not rely on thekind of halide in the complex suggesting the possibility ofthe presence of ligand oxidation process. The EPR spectrumof the electrochemically oxidised 2 at 0.5 V vs. Ag/Ag+ inacetonitrile shows no signals indicating the complex should have

a copper(III) centre or a singly oxidised state of the tptm ligandstrongly coupled with the copper(II) centre in [CuIICl(tptm)]+.25

DFT calculations

The copper(II) complex of tptm contains a novel CuII–C(sp3)bond performing an important role in this system. To clarify thedetail of this feature, DFT calculations were performed for the[CuX(tptm)]+/0 (X = F, Cl, Br, I) complexes and the results werecompared with the electrochemical data, the magnetic propertiesand the X-ray data. The calculated and observed bond lengthsfor 1 and 4 coincide quite well (Table 7). The calculated CuII–C bond lengths and the charges on the central carbon atomfor each [CuX(tptm)] complex show no significant differencesamong them. By contrast, the observed bond lengths of Cu–Nfor 1 and 4 differ in certain extent and the calculated values for1 to 4 elongate systematically.

The calculated spin density exists on the halogen, copperand carbanion atoms, i.e., it exists on the z axis for all halidecomplexes (Table 8). It is consistent with the results of the EPRexperiment in which the unpaired electron of each complex isdelocalised on the copper and halogen atoms (Fig. 8).

The calculated anisotropic spin dipole couplings for the zaxis with 6-31g(d) basis set26 (Table 9) agree with the trend inthe experimental hyperfine coupling constants for copper. The

Table 7 Calculated and observed bond lengths (A)

Bond F (1) Cl (2) Br (3) I (4)

Cu–X 1.944 2.419 2.609 2.8301.874(7),a 1.918(8)a 2.717(1)a

Cu–Cax 2.066 2.063 2.064 2.0652.04(1),a 2.01(1)a 2.006(8)a

Cu–N 2.077 2.134 2.147 2.1612.07(3),a 2.08(2)a 2.117(7)a

Cax–S 1.901 1.890 1.887 1.8841.80(4),a 1.81(5)a 1.81(2)a

a Observed lengths from X-ray crystallographic data. Cu–N and Cax–Slengths are averaged.

Table 8 Selected spin densities for 1–4

Atom 1 2 3 4

X 0.097 0.085 0.091 0.10Cu 0.56 0.48 0.46 0.43Cax 0.21 0.26 0.27 0.28

Fig. 8 Spin density of 3.

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Table 9 The anisotropic spin dipole couplings for the z axis (Gauss)

1 2 3

X 115 (0.64) 8.28 (0.44) 35.8 (0.80)Cu 172 (3.4) 160 (3.16) 145 (3.1)Cax 5.98 (0.12) 8.79 (0.18) 8.212 (0.18)

a Absolute strength (a. u.) in parentheses.

calculated value decreases in the order of X = F, Cl and Br,which coincides with the trend of empirical hyperfine couplingconstants for each copper atom (|ACu| = F > Cl > Br). Likewise,trend of the hyperfine coupling constants for each halide atomis consistent with the calculated value. These agreements comefrom the covalency of the Cu–X bond interaction.23b

Ionisation potentials of [CuX(tptm)] are calculated to be6.43, 6.46, 6.45 and 6.42 eV for X = F, Cl, Br and F,respectively. The similarity of these values is consistent withthe results of cyclic voltammetry for [CuX(tptm)]+/0 processwhich occurs at almost the same potential for all halides.The HOMOs for all [CuX(tptm)] are quite similar to eachother and they are mainly localised on the sulfur atoms andthe central carbon atom of tptm as shown in Fig. 9 for theorbital of 2, [CuCl(tptm)] for example. Interestingly, the chargeson the copper atom slightly decrease upon oxidation for all[CuX(tptm)]+ (Table 10). This is controversy over the oxidationwhich is probably compensated by the migration of charge fromthe sulfur atoms. In consequence, the oxidation processes werestrongly influenced by the coordination of the tptm ligand.

Fig. 9 The HOMO of 2.

Table 10 Mulliken total atomic charges of the complexes for[CuX(tptm)]+/0 states

Charge X Cu Cax N S

1 n = 0 −0.54 0.66 −0.90 −0.21 0.29n = +1 −0.47 0.64 −0.92 −0.22 0.43

2 n = 0 −0.44 0.44 −0.90 −0.17 0.30n = +1 −0.30 0.39 −0.92 −0.19 0.45

3 n = 0 −0.46 0.44 −0.90 −0.16 0.31n = +1 −0.28 0.39 −0.92 −0.19 0.45

4 n = 0 −0.45 0.40 −0.90 −0.15 0.31n = +1 −0.20 0.30 −0.92 −0.17 0.44

ConclusionsA complete series of copper(II) halide complexes, [CuX(tptm)](X = F, Cl, Br, I) were prepared and their structures wereconfirmed by X-ray crystallography as trigonal bipyramidwhich is consistent with the result of EPR spectroscopy. Thevoltammetric results indicate the trigonal pyramidal coordina-tion with the copper(I) states, which make the reduction wavequasi-reversible. The coincidence of all the redox potentials of[CuX(tptm)]+/0 processes agrees with the DFT calculation tellingthe existence of charge migration from the sulfur atoms. Thecharge on the central carbon atom is close to −0.9 and is morethan that on halides, −0.54 for 1 and −0.45 for 4. The X-ray structures were well reproduced by the DFT calculationsand the data indicate the unique character for the carbanioncoordination.

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