Paramagnetic Zinc(II) Complexes of a Bis(catechol): Dependence of Product Spin State on Tautomerization of the Bis(catechol) Ligand

We report the preparation and characterization of zinc(II) hydrotris(3-cumenyl-5-methylpyrazolyl)borate (LZn) complexes, (LZn)(2)()1'-H and (LZn)(2)()1, of a bis(catechol) ligand. The formation of (LZn)(2)()1'-H, an S = (1)/(2) complex, rather than (LZn)(2)()1, an S = 1 complex, is observed due to tautomerization of a reaction intermediate. The biradical complex, (LZn)(2)()1, can be prepared from (LZn)(2)()1'-H by oxidation, a conversion that is accompanied by a blue-green to red-purple color change and an increase in spin from (1)/(2) to 1. The frozen solution EPR spectrum of the biradical complex (LZn)(2)()1 exhibits zero-field splitting and a Deltam(s)() = 2 transition characteristic of a triplet state. The temperature dependence of the EPR signal intensity is consistent with high-spin coupling of the unpaired electrons of the ligand.