Nearest- and Next-Nearest-Neighbor Ru(II)/Ru(III) Electronic Coupling in Cyanide-Bridged Tetra-Ruthenium Square Complexes

• Published in: Inorganic chemistry 2011-09, Vol.50 (17), p.8274-8280
• Main Authors: Lin, Ju-Ling, Tsai, Chia-Nung, Huang, Sheng-Yi, Endicott, John F, Chen, Yuan-Jang, Chen, Hsing-Yin
• Format: Article
• Language: English
• Subjects: Cyanides - chemistry; Electrochemistry; Electrons; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Quantum Theory; Ruthenium - chemistry; Spectrophotometry, Ultraviolet; Spectroscopy, Near-Infrared
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic200806a

Electrochemical properties of cyanide-bridged metal squares, [Ru4]4+ and [Rh2–Ru2]6+, clearly demonstrate the role of the nearest (NN) metal moiety in mediating the next-nearest neighbor (NNN) metal-to-metal electronic coupling. The differences in electrochemical potentials for successive oxidations of equivalent Ru(II) centers in [Ru4]4+ are ΔE 1/2 = 217 mV and 256 mV and are related to intense, dual metal-to-metal-charge-transfer (MMCT) absorption bands. This contrasts with a small value of ΔE 1/2 = 77 mV and no MMCT absorption bands observed to accompany the oxidations of [Rh2–Ru2]6+. These observations demonstrate NN-mediated superexchange mixing by the linker Ru of NNN Ru(II) and Ru(III) moieties and that this mixing results in a NNN contribution to the ground state stabilization energy of about 90 ± 20 meV. In contrast, the classical Hush model for mixed valence complexes with the observed MMCT absorption parameters predicts a NNN stabilization energy of about 6 meV. The observations also indicate that the amount of charge delocalization per Ru(II)/Ru(III) pair is about 4 times greater for the NN than the NNN couples in these CN-bridged complexes, which is consistent with DFT modeling. A simple fourth-order secular determinant model is used to describe the effects of donor/acceptor mixing in these complexes.