Ligands that actively participate in chemical or redox transformations have expanded the scope of precious and non-precious metal complexes in catalytic reactions, and have given rise to new types of small molecule reactivity. In this talk, I will describe efforts from our group aimed at preparing tetradentate ligands that are both chemical and redox-active. We recently reported a series of Ru(II) complexes containing a triaryl S2N2 ligand called N,N′-bis[2-(methylthio)phenyl]-1,2-diaminobenzene, H2(MeSNNSMe). This ligand can be metallated and sequentially deprotonated to form the 5-coordinate, square pyramidal complex Ru(MeSNNSMe)(PPh3). The 1,2-diaminobenzene sub-unit of the S2N2 ligand is known to be redox-active, and electrochemical studies of Ru(MeSNNSMe)(PPh3) confirmed the presence of multiple redox events consistent with one-electron oxidations of the ligand and metal. In separate experiments, we discovered that treating Ru(MeSNNSMe)(PPh3) with BH3-THF yields Ru[(μ-H)BH2](MeSNNSMe)(PPh3) with BH3 bound across a Ru–N bond. This type of substrate binding across the metal and ligand is referred to metal-ligand cooperativity (MLC) and has important relevance to CO2 reduction chemistries. Analysis of Ru[(μ-H)BH2](MeSNNSMe)(PPh3) by cyclic voltammetry revealed new reversible redox features when compared to Ru(MeSNNSMe)(PPh3), and no significant decomposition was observed over repeated scans despite the presence of MLC-bound BH3. Structural, spectroscopic, and electrochemical data will be presented to describe how MLC binding is affected during redox cycling.

Metal-ligand cooperativity with redox-active tetradentate ligands

255th National Meeting (2018)

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