Chemistry of Molybdenum with Hard-Soft Donor Ligands. 2. Molybdenum(VI), -(V), and -(IV) Oxo Complexes with Tridentate Schiff Base Ligands

The synthesis and characterization of a number of oxomolybdenum(VI), -(V), and -(IV) complexes with salicylaldehyde thiosemicarbazones as the SNO-donor metal-binding substrates are described. The Mo(VI) complexes are cis-dioxo, Mo02(SNO), with one coordination site available for substrate binding. The Mo(V)-oxo complexes are of the type MoO(SNO)X2(X = Cl, Br) with a distorted-octahedral geometry. The Mo(IV)-oxo complexes, MoO(SNO), are derived from the corresponding Mo-(Vl)-dioxo complexes by oxo abstraction with PPh3. The cyclic voltammograms for Mo02(SNO) complexes are typically irreversible, involving Mo(VI)/Mo(V) and Mo(V)/Mo(IV) reductive responses. The Mo(V)-oxo complexes display unique voltammograms exhibiting Mo(V)/Mo(IV), Mo(IV)/Mo(III), and Mo(V)/Mo(VI) couples. The Mo(IV)-oxo complexes are oxidized to Mo(V)-oxo complexes and undergo one-electron reduction to the corresponding Mo(III) species. EPR results indicate that the isotropic metal hyperfine coupling constant of Mo(V)-oxo complexes decreases with an increase of covalent character in going from chlorine to their bromine analogues. The spin-orbit coupling of bromide ligands appears to be the dominant factor contributing to the drastic change in isotropic g values. © 1989, American Chemical Society. All rights reserved.