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

Herein we report an electronic structure investigation of neutral and oxidized Ru complexes containing a redox noninnocent N₂S₂ ligand derived from <i>o</i>-phenylenediamide (<b>L1</b>). UV-vis spectroelectrochemistry (SEC) studies were conducted on the square pyramidal complex [Ru^II(<b>L1</b>)(PPh₃)] (<b>1</b>) and the six-coordinate complexes [Ru^II(μ-BH₃)(<b>L1</b>)(PPh₃)] (<b>2</b>) - which has BH₃ bound in a metal-ligand cooperative (MLC) fashion across Ru and <b>L1</b> - and [Ru^II(<b>L1</b>)(PPh₃)(MeCN)] (<b>3</b>). The SEC results yielded spectra assigned to singly and doubly oxidized <b>1</b> and <b>3</b>, 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 <b>2</b> showed that it rapidly loses MLC-bound BH₃ upon oxidation. The SEC results for <b>1</b> and <b>3</b> 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 <b>3</b>. 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.