3524 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 9 Notes

http://dx.doi.org/10.5012/bkcs.2011.32.9.3524

Redox-Active Cu(I) Complex with Bi-functionalized Tetrathiafulvalene Ligand

Byung-Do Park, Jang-Hoon Cho, Kyoung-Soon Shin,† Dong-Youn Noh,†

Dohyun Moon,‡ Myoung Soo Lah,§ and Hong-In Lee*

Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University,

Daegu 702-701, Korea. *E-mail: leehi@knu.ac.kr†Department of Chemistry, Seoul Women’s University, Seoul 139-774, Korea

‡Pohang Accelerator Laboratory, Pohang, Kyungbook 790-784, Korea§Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea

Received July 20, 2011, Accepted July 29, 2011

Key Words : Functionalized TTF, Metal-TTF complex, X-ray diffraction

Multifunctionalized tetrathiafulvalenes (TTFs) and theirtransition metal complexes have been widely studied todevelop the materials with electric-magnetic hybrid proper-ties.1 Central concept for achieving the hybrid properties isthe proper packing of the complexes in which TTF planesand metals are suitably arranged to provide the exchangeinteractions of electric π orbitals for conductivity andmagnetic d orbitals for magnetic properties, respectively.TTF should be appropriately functionalized to form such afused metal complex as well as to tune its structure andproperties. Attempts have been made to functionalize TTFs,whose metal complexes maintain electroactivity similarly toTTF, with phosphine,2 dithiolate,1b pyridine,3 and acetyl-acetonate.4

Interest in the usage of carboxylate for the function-alization of TTFs has grown because of its hydrogen-bond-ing ability,5a pH dependence,5b and the forte of formingmany useful metal organic frames (MOFs).6 The metalcoordination compounds with bicarboxylate-TTF or tetra-carboxylate-TTF ligand have been recently reported, inwhich they exhibited the electrochemically and magneticallyactive properties, and the possibilities of potential appli-cations for the battery-electrode materials.7 In those com-plexes, the TTF derivatives bind to metal ions through thecarboxylate groups. In this study, we use a TTF bi-function-alized with sulfide and carboxylate groups (Scheme 1) tostudy its ability to form a metal complex with Cu(I) and toinvestigate the structure and electrochemical activity of themetal complex.

The ligand, 4,5-bis(methylthio)-4',5'-bis(sodiumcarbox-ylate)tetrathiafulvalene (Na2L), was prepared in a form ofsodium salt.5b,8 The purified product used directly for thesynthesis of Cu(I) complex in acetnitrile solution of Cu(I)Br.Elemental and X-ray crystallographic analyses revealed thatthe complex is [Cu(I)(HL)(CH3CN)2] (1). Figure 1 showsthe ORTEP drawing of complex 1. The TTF-derivative

ligand (HL−) bidentately coordinates to the Cu(I) ionthrough two sulfur atoms of the methylthio groups. Two Natoms of the solvent acetonitrile molecules occupy theremaining sites of the distorted tetrahedral geometry. In theother hard-acid metal complexes with the same ligand, themetal ions have been found to bind to the carboxylate groupsof the ligand.7a-c In complex 1, Cu(I), a soft acid, prefers thesoft-base site, methylthio group, of the ligand for complex-ation. One of the carboxylate groups is protonated and thisproton mediates the intramolecular hydrogen bond with2.411(7) Å of the distance between two oxygen atoms of thecarboxylate groups forming seven-membered ring. Thebond angle of N-Cu-N is near tetrahedral (110.9o) but that ofS-Cu-S is smaller (91.50(5)o) due to the five-memberedchelate ring strain. Two S2C3 heterocyclic rings of the TTFmoiety are coplanar and carboxylate groups lie on this plane.The bond length of the central C-C bond is sensitive to theoxidation state of TTF, where the length is within the rangeof 1.33-1.35 Å for the neutral TTF (TTF0) while 1.38-1.40 Åfor one-electron oxidized TTF (TTF+•).5b,7e-f 1.341(8) Å ofthe C5-C6 bond length of complex 1 is consistent with thelength found in the neutral TTF (TTF0). It has been reportedthat the configurations of two S2C3 heterocyclic rings in the

Scheme 1

Figure 1. ORTEP drawing of complex 1 with 30% thermal ellip-soids and atomic numbering scheme. Hydrogen atoms, except forthe carboxylate hydrogen, have been omitted for clarity. Dottedline represents the hydrogen bond. Selected bonds (Å) and angles(o): Cu1-S1, 2.3365(16); Cu1-S2, 2.3343(16); C2-C3, 1.323(8);C5-C6, 1.341(8); C8-C9, 1.356(8); S3-C2, 1.760(5); S3-C5, 1.770(6);S4-C3, 1.757(5); S4-C5, 1.769(6); S5-C6, 1.756(6); S5-C8, 1.750(5);S6-C6, 1.750(6); S6-C9, 1.748(5); N1-Cu1-N2, 110.9(2); N1-Cu1-S2, 107.50(15); N1-Cu1-S1, 118.25(15); N2-Cu1-S1, 108.68(14);N2-Cu1-S2, 119.25(16); S1-Cu1-S1, 91.50(5).

Notes Bull. Korean Chem. Soc. 2011, Vol. 32, No. 9 3525

TTF moiety were boat for TTF0 5b,7a and planar for TTF+•.9

But the planar configuration for TTF0 has also been report-

ed.7f

Molecular packing of complex 1 in the crystal is displayedin Figure 2. Molecular C2 axis is parallel to the crystal c-axisand the complexes run opposite directions side-by-side(Figure 2(a)). The TTF planes lie on either [2 1 0] or [2 -1 0]plane and are packed along the b-axis (Figure 2(b)). Theefficiency of the packing along the b-axis is achieved by theweak π

…π interactions between the layers of the TTF

planes as depicted in Figure 2(c) showing the view normal tothe [2 1 0] plane. The shortest S

…S contact distance of

3.377 Å is found between the sulfur atoms of methylthiogroups and the S

…S contact distances between the TTF

planes are 3.690 and 3.695 Å. These distances are shorter orclose to the van der Waals distance of 3.60 Å.

The redox properties of TTF being readily oxidized to itscation radical (TTF+•) or dication (TTF2+) provide thepotentials for many applications. The electrochemical beha-vior of complex 1 was investigated by using cyclic voltam-metry (CV). Figure 3 shows the CV of complex 1 and Na2Lobtained in CH3CN with 0.1 M [(n-Bu)4N]ClO4 electrolyteat room temperature under N2. Redox potentials weremeasured relative to a Ag/Ag+ reference electrode anddetermined as potential vs ferrocene/ferrocenium (Fc/Fc+)redox couple. For the ligand Na2L, two quasi-reversibleprocesses with E1/2(1) = 0.19 V and E1/2(2) = 0.51 V ascribedto TTF+•/TTF and TTF2+/TTF+•, respectively, are observed

Figure 2. (a) Molecular packing of complex 1. View along thecrystal b-axis. Hydrogen atoms are omitted for clarity. (b) Repre-sentation of TTF planes. View along the c-axis. The 1D packingsbehind the front 1D packings along the b-axis are faded. (c) Viewnormal to [2 1 0] plane. In (b) and (c), hydrogen atoms, acetoni-triles, and carbon atoms of methylthio groups are omitted forclarity.

Figure 3. Cyclic voltammograms of (a) 1 mM Na2L and (b) 1 mM complex 1 in 0.1 M (n-C4H9)4N·ClO4/CH3CN solution. Scanning rate:100 mV/s. (c) Four redox-active forms of complex 1.

3526 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 9 Notes

(Figure 3(a)).7a The redox couples are not much perturbed bycomplexation, implying the electronic structure of the TTFmoiety preserves ((Figure 3(b)). In the CV of complex 1, theirreversible redox process of Cu++/Cu+ is also observed atEpc(M) = −0.73 V and Epa(M) = −0.06 V. Electrochemicalinvestigation demonstrates that complex 1 is a redox-activespecies with four different forms as depicted in Figure 3(c).

In summary, we have used the ligand L2−, a TTF bi-functionalized with sulfide and carboxylate groups, todeveloped a new Cu(I)-TTF complex and characterized itssturcture and electrochemical properties. Single crystal X-ray crystallograpy revealed that the methylthio groups of theligand and the solvent acetonitriles are bound Cu(I) to form adistorted tetrahedral geometry. CV study of the complexdemostrated the four redox-active forms of complex 1. Thecarboxylate groups of complex 1 can further work as bases.We are currently developing the method of an additionalcoordination through the open base sites of complex 1.

Experimental Section

Synthesis. The ligand, 4,5-bis(methylthio)-4',5'-bis(sodi-umcarboxylate)tetrathiafulvalene (Na2L), was prepared in aform of sodium salt according to the previously reportedmethod.5b,8 The purified product used directly for the syn-thesis of Cu(I) complex as follow: 7 mL of 10 mM aceto-nitrile solution of Cu(I)Br (0.015 g, 0.035 mmol) was addeddropwise to 7 mL of 5 mM aqueous solution of LNa2 (0.010g, 0.070 mmol). After standing at room temperature for 48hrs, red needle shape crystals were formed at the interface oftwo solutions and analyzed to be [Cu(I)(HL)(CH3CN)2] (1).Yield: 40% (10 mg). FT-IR (KBr, cm−1): 3425vs, 1618s,1465m, 1400m, 1346m, 1170w, 1070w, 980w. Anal. Calc.for C14H13CuN2O4S6 (%): C, 31.77; H, 2.48; N, 5.29, Found:C, 32.06; H, 2.62; N, 5.45.

X Ray-Crystallographic Data Collection and Refine-

ment of the Structure. X-ray diffraction data were collectedon a 4AMXW ADSC Quantum-210 detector with a Pt-coated Si double crystal monochromator (0.75000 Å) at100(2) K in the Pohang Accelerator Laboratory (PAL),Korea. The crystal structure of complex 1 was solved by thedirect method and refined by full-matrix least-squares calcu-lation with the SHELXTL software package.10 Hydrogenatoms were theoretically added and included in the finalrefinement. Crystal data for 1: crystal size = 0.18 × 0.03 ×0.02 mm3, formula weight = 529.16, crystal system = mono-clinic, space group = P21/c, a = 14.104(3) Å, b = 4.2740(9)Å, c = 33.013(7) Å, β = 91.91(3)°, V = 1988.9(7) Å3, Z = 4,R1(wR2) = 0.0530 (0.1722) for 3,738 reflections [I > 2σ(I)],R1(wR2) = 0.0583 (0.1805) for all reflections, and GOF =1.189. CCDC-833079 contains the supplementary crystallo-graphic data for this paper. The data can be obtained free ofcharge at www.ccdc.cam.ac.uk/conts/retrieving.html or from

the Cambridge Crystallographic Data Centre, 12, UnionRoad, Cambridge CB2 1EZ, UK (fax: +44 1223 336033,e-mail: deposit@ccde.cam.ac.uk).

Acknowledgments. This research was supported by BasicScience Research Program through the National ResearchFoundation of Korea (NRF) funded by the Ministry ofEducation, Science and Technology (2010-0024929). Theauthors acknowledge the PAL for beam line use (2010-3063-10).

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