Spectroscopic and Theoretical Studies of Ruthenium Complexes with a Noninnocent N2S2 Ligand in Different Redox States

Abstract

Herein we report an electronic structure investigation of neutral and oxidized Ru complexes containing a redox noninnocent N₂S₂ ligand derived from o-phenylenediamide (L1). UV–vis spectroelectrochemistry (SEC) studies were conducted on the square pyramidal complex [Ru^II(L1)(PPh₃)] (1) and the six-coordinate complexes [Ru^II(μ-BH₃)(L1)(PPh₃)] (2) – which has BH₃ bound in a metal–ligand cooperative (MLC) fashion across Ru and L1 – and [Ru^II(L1)(PPh₃)(MeCN)] (3). The SEC results yielded spectra assigned to singly and doubly oxidized 1 and 3, revealing electronic structure changes as a function of oxidation state and in response to the presence and absence of bound MeCN. By contrast, the SEC results of 2 showed that it rapidly loses MLC-bound BH₃ upon oxidation. The SEC results for 1 and 3 were compared to single-crystal XRD data and UV–vis, EPR, and P K-edge, S K-edge, and Ru L₃-edge X-ray absorption spectroscopy (XAS) data collected on isolated samples of chemically oxidized 3. The data revealed that the first two oxidations are primarily localized on the ligand, which was supported by DFT and TDDFT calculations. DFT calculations for the doubly oxidized species revealed a singlet ground state with a singlet–triplet gap of 8.9 kcal/mol. CASPT2 calculations corroborated the DFT calculations and further revealed that the singlet ground state is multiconfigurational with 21% radical character. Collectively, the results establish redox formalisms and the underlying electronic structure of Ru complexes containing a noninnocent tetradentate ligand in different oxidation states.

Herein we report an electronic structure investigation of neutral and oxidized Ru complexes containing a redox noninnocent N₂S₂ ligand. Spectroscopic data, structural studies, and theoretical calculations indicate that oxidations are primarily localized on the N₂S₂ ligand.