Synthesis and Characterization of VV(3,6-DBSQ)(3,6-DBCat)2, a d0 Metal Complex with Dioxygenase Catalytic Activity

Transition-metal complexes containing redox-active quinoid ligands are of interest because of their catalytic capabilities in multielectron, substrate-activation reactions such as dioxygenase catalysis using O2. The new catecholate complex VV(3,6-DBSQ)(3,6-DBCat)2 (where 3,6-DBSQ = 3,6-di-tert-butylsemiquinone and 3,6-DBCat = 3,6-di-tert-butylcatecholate) was synthesized by combining VO(acac)2 with 1 equiv of 3,6-DBBQ (where 3,6-DBBQ = 3,6-di-tert-butylbenzoquinone) and 2 equiv of H2(3,6-DBCat) in dry methanol under an inert atmosphere. The resultant complex was characterized by single-crystal X-ray diffraction, elemental analysis, near-IR, UV/vis, and electron paramagnetic resonance (EPR) spectroscopy. The crystallography as well as the near-IR and EPR studies suggest that the radical spin is localized on the 3,6-DBSQ ligand at room temperature, making VV(3,6-DBSQ)(3,6-DBCat)2 a type 1 mixed-valence complex. Initial dioxygenase catalysis studies reveal that VV(3,6-DBSQ)(3,6-DBCat)2 is a good dioxygenase precatalyst for the substrate H2(3,6-DBCat) with O2 in ca. 600 total turnovers to >93% intra- and extradiol products with only 1−2% of the undesired benzoquinone autoxidation product.