MODELS FOR THE BINDING-SITE IN BROMOPEROXIDASE - MONONUCLEAR VANADIUM(V) PHENOLATE COMPLEXES OF THE HYDRIDOTRIS(3,5-DIMETHYLPYRAZOLYL)BORATE LIGAND

Vanadium (3,5-dimethylpyrazolyl)borate complexes with a series of phenolate ligands have been prepared and characterized. The single-crystal X-ray structure of the p-bromophenol derivative tris[(3,5-dimethylpyrazolylborato]bis(p-bromophenoxy)oxovanadium(V) has been determined by standard methods and refined to an unweighted R factor of 0.052. The dark green triclinic crystals belong to space group PlBAR with cell dimensions of a = 12.140 (3) angstrom, b = 13.515 (4) angstrom, c = 10.811 (3) angstrom, alpha = 107.96 (2)-degrees, beta = 116.15 (2)-degrees, and gamma = 75.78 (3)-degrees. Phenolate coordination stabilizes the V(V) oxidation state to the extent that the V(IV) complexes are easily air oxidized. Near linear correlations are observed between the Hammett sigma constants for the para substituents on the phenoxy rings and physicochemical parameters such as LMCT band position and redox potentials. The phenoxy complexes also undergo relatively facile hydrolysis to produce the oxo-bridged dimer, [LVO(OH)]2O. The implications of this work to the binding site model proposed for the V(V)-dependent bromoperoxidase enzyme from marine algae are discussed.