Spin modulation and electrochemical behavior of a five-coordinate cobalt(III) salen complex

<p>We report the synthesis of a Co(III) complex with the five-coordinate salen-type ligand (N,N′-bis(3,5-di-<i>tert</i>-butyl-2-hydroxybenzyliden)-1,7-diamino-4-methyl-4-azaheptane). This complex is stable in air with a trigonal bipyramidal geometry and we show spectroscopically and computationally that a high-spin triplet ground state is preferred. This spin state is readily modulated by introduction of an exogenous ligand (pyridine, acetonitrile) to yield a six-coordinate complex with low-spin ground state. The five-coordinate complex exhibits solvent- and ligand-dependent electrochemical behavior in solution for the Co^II/III transition and undergoes a one-electron ligand oxidation to generate a phenoxyl radical species that is relatively stable in the absence of oxygen. We show that this phenoxyl radical species is a Class I mixed-valence compound that can undergo photoinduced inner-sphere charge transfer with the neighboring phenoxide. This process is mediated by the Co(III) center which acts as a bridge. Understanding this behavior will lead to a better understanding of a dicobalt bis-salen analog previously reported by our group as a proton reduction catalyst.