Tuning of Metal鈥揗etal Interactions in Mixed-Valence States of Cyclometalated Dinuclear Ruthenium and Osmium Complexes Bearing Tetrapyridylpyrazine or -benzene

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• 作者：Takumi Nagashima ; Takuya Nakabayashi ; Takashi Suzuki ; Katsuhiko Kanaizuka ; Hiroaki Ozawa ; Yu-Wu Zhong ; Shigeyuki Masaoka ; Ken Sakai ; Masa-aki Haga
• 刊名：Organometallics
• 出版年：2014
• 出版时间：September 22, 2014
• 年：2014
• 卷：33
• 期：18
• 页码：4893-4904
• 全文大小：639K
• ISSN：1520-6041

文摘

New dinuclear ruthenium or osmium complexes with cyclometalated bonds in either tridentate bridging (BL) or ancillary ligands (L), [(L)M(BL)M(L)] (where M = Ru, Os; L = bis(N-methylbenzimidazolyl)pyridine, -benzene; BL= tetrapyridylpyrazine (tppz), -benzene (tpb)), were synthesized, and their mixed-valence-state characteristics were investigated. All of the complexes showed successive one-electron redox processes, each of which correspond to M(II/III) (M = Ru, Os) or ligand reduction waves. In addition, an M(III/IV) couple was observed in cyclometalated [M₂(bis(benzimidazolyl)benzene)₂(BL)] complexes (M = Ru, Os). Effects of the cyclometalated bonds on the redox behaviors and the accessibility to the mixed-valence M(II)鈥揗(III) dinuclear complexes are discussed. Introduction of a cyclometalated bond induced a large negative potential shift in the redox potentials of dinuclear ruthenium and osmium complexes, depending on either bridging or ancillary sites of the cyclometalated bonds: the change falls within the range of 鈭?.0 to 鈭?.2 V for the bridging sites and 鈭?.65 to 鈭?.7 V for the ancillary ones. This large negative potential shift arises from the strong electron-donating property of the phenyl anion in a metal鈥揅 bond. Replacing the ruthenium by osmium in the dinuclear complexes with the same bridging ligand results in an increase of the potential separation (螖E(1)) and the comproportionation constant (K_com) of the mixed-valence complexes having the tppz bridging ligand (螖E(1) and K_com values: Os > Ru); however, complexes having the tpb bridging ligand showed the opposite trend (螖E(1) and K_com: Os < Ru). In addition to the results of EPR and DFT calculation, it was found that the orbital energy levels of the central metal ion (namely, either Ru or Os) in the mixed-valence complex determines the degree of orbital mixing between metal d蟺 orbitals and bridging-ligand 蟺 or 蟺* orbitals, which leads to either hole- or electron-transfer exchange mechanisms.