Models for the Molybdenum(VI/V) Centers of the Molybdenum Hydroxylases and Related Enzymes. Geometry, Electronic Structure, and EPR g-Tensor Predictions from ab Initio and Semiempirical Molecular Orbital Studies

The geometries and electronic structures of several molybdenum(V) and molybdenum(VI) complexes with tetradentate N₂S₂ and N₂O₂ ligands (LH₂ = N,N′-bis(2-mercaptophenyl)-N,N′-dimethyl-1,2-diaminoethane, N,N′-bis(2- hydroxyphenyl)-N,N′-dimethyl-1,2-diaminoethane) which have been proposed as models for the molybdenum- (VI/V) centers of the molybdenum hydroxylases and related enzymes have been predicted using ab initio and INDO molecular orbital methods. Ab initio and INDO geometrical predictions have been compared with known X-ray crystal structures for three complexes and have been found to be in excellent agreement. In addition, the EPR g tensors of the molybdenum(V) complexes have been estimated using INDO methods. The results support previously proposed structures for both the model complexes and the enzyme molybdenum centers and are in good agreement with EPR and EXAFS experimental data for these systems. The results indicate the ligand has a profound effect on the geometry and electronic structures of the complexes. Insight into the electronic factors responsible for the EPR parameters of the complexes is presented, and application to the enzyme molybdenum centers is explored.