pubs.acs.org/IC Published on Web 06/17/2010 r 2010 American Chemical Society Inorg. Chem. 2010, 49, 6565–6574 6565 DOI: 10.1021/ic100500p Carboxylate Tolerance of the Redox-Active Platform [Ru( μ-tppz)Ru] n , where tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine, in the Electron-Transfer Series [(L)ClRu( μ-tppz)RuCl(L)] n , n =2þ, þ, 0, -,2-, with 2-Picolinato, Quinaldato, and 8-Quinolinecarboxylato Ligands (L - ) Tanaya Kundu, † Biprajit Sarkar, ‡ Tapan Kumar Mondal, § Jan Fiedler, ^ Shaikh M. Mobin, † Wolfgang Kaim,* ,‡ and Goutam Kumar Lahiri* ,† † Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India, ‡ Institut f € ur Anorganische Chemie, Universit € at Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany, ^ J. Heyrovsk  y Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolej skova 3, CZ-18223 Prague, Czech Republic, and § Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700032, India Received March 16, 2010 The neutral title complexes [(L 1-3 )ClRu II ( μ-tppz)Ru II Cl(L 1-3 )] [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine with L 1 = 2-picolinate, L 2 = 2-quinolinecarboxylate (quinaldate) and with the hitherto little used L 3 = 8-quinolinecarboxylate] have been structurally characterized as approximately anti- (1 and 3) and syn-configured isomers (2) with L ligand N (1 and 3) or O atoms (2) trans to the pyrazine N atoms of tppz. In contrast to 1 and 2 with five-membered chelate rings, complex 3 (which is isomeric with 2) contains six-membered chelate rings. Each system 1-3 thus features a significantly different coordination situation, and all complexes exhibit a considerably distorted tppz bridge, including a twisted central pyrazine ring. In spite of this, double one-electron reduction of the bridge is always possible, as is evident from electron paramagnetic resonance (EPR) and UV/vis spectroelectrochemistry. Two separate (ΔE ∼ 0.4 V and K c ∼ 10 7 ) one-electron oxidations occur on the metals, producing invariably EPR-silent (4K) Ru III Ru II inter- mediates with moderately intense near-IR absorptions, ranging from 1500 to 1900 nm. IR spectroelectrochemistry of the carboxylato carbonyl stretching bands did not result in any noticeable shift, in contrast to what was observed with dipyridyl ketones and related coligands. Density functional theory (DFT)/time-dependent DFT calculations confirm the experimental structures and explain the noted spectroscopic trends: destabilized and closer-spaced frontier orbitals for 3 over 2, with the comparison to 1 suggesting that the configuration is a major factor. Nevertheless, the rather unperturbed electronic structure of the [Ru( μ-tppz)Ru] n entity, despite different coordination situations at the metal sites, is remarkable and suggests further use of this entity as a robust, carboxylate-tolerant redox-active platform in extended frameworks. Introduction The establishment of pyrazine-mediated intramolecular electron transfer in the mixed-valent Ru III Ru II states of the Creutz-Taube ion (A) 1 and of its cyano analogue (B) 2 has initiated continuous efforts to understand the valence loca- lization/delocalization phenomena in mixed-valent diruthe- nium complexes, using a variety of pyrazine-derived bridging ligands. 3 Theoretical, 4 methodical, 5 and conceptual advances have been made. 6 The molecular bridge-mediated electronic interaction between redox-active transition-metal ions has significantly contributed to a general understanding of the redox reactivity 3t,u,6 and to speculation about its potential applications in information transfer 7 and energy-relevant research. 8 In the context of the suitability of pyrazines and other 1,4- diazines as electron-transfer-supporting bridging systems, 9 the potential of the bis-tridentate, redox noninnocent, and inherently nonplanar bridging ligand 2,3,5,6-tetrakis(2-pyr- idyl)pyrazine (tppz) toward electron-transfer between mixed- valent diruthenium termini containing ancillary ligands *To whom correspondence should be addressed. E-mail: kaim@ iac.uni-stuttgart.de (W.K.), lahiri@chem.iitb.ac.in (G.K.L.). (1) (a) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1969, 91, 3988. (b) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1973, 95, 1086. (2) Scheiring, T.; Kaim, W.; Olabe, J. A.; Parise, A. R.; Fiedler, J. Inorg. Chim. Acta 2000, 125, 300.