Molybdenum(VI)-Dioxo Complexes with Linear and Tripodal Tetradentate Ligands: Models for the Molybdenum(VI/V) Centers of the Molybdenum Hydroxylases and Related Enzymes. 1. Syntheses and Structures

As models for the molybdenum(VI/V) centers of the molybdenum hydroxylases and related enzymes, 15 new Mo(VI)-dioxo complexes (MoO₂L) with tetradentate ligands have been synthesized and characterized. The effects of coordinating groups (N₂S₂, N₂OS, and N₂O₂), chelate ring size (five and six members), ligand geometry (linear and tripodal), and steric bulk have been investigated. X-ray crystal structures have been obtained for seven of the complexes. While minor differences, attributed to these features, are evident, the structures have remarkably similar Mo-ligand bond lengths and bond angles and all have distorted-octahedral geometry. The oxo groups are cis to one another and to the thiolate or phenolate groups of the ligands. The N atoms are approximately trans to the oxo groups, and the Mo—N bonds are relatively long (>2.34 Å), with the bond length correlated with the size of the trans O—Mo—N bond angle. The Mo^O and Mo—S(thiolate) bond lengths are comparable to those determined by EXAFS spectroscopy for the Mo centers of the enzymes. The relevance of the results to the structures of the Mo centers of the enzymes is discussed.