Investigations of the Coordination Chemistry of Molybdenum with Facultative Tetradentate Ligands Possessing N₂S₂ Donor Sets. 2. Preparation, Chemical Characterization, and Electrochemical Study of the Molybdenum(IV)-, Molybdenum(V)-, and Molybdenum(VI)-Oxo Complexes

The chemistry of the tetradentate N₂S₂ donor ligands N,N′-dimethyl-N,N′-bis(2-mercaptoethyl)ethylenediamine (LH₂) and N,N′-dimethyl-N,N′-bis(2-mercaptoethyl)-1,3-propanediamine (L′H₂) with molybdenum in the VI, V, and IV oxidation states is described. In methanol solution Mo(VI)-oxo reagents combine with LH₂ and L′H₂ to give [MoO₂L] and [MoO₂L′]. Reduction of these complexes with HSPh in methanol yields [Mo₂O₃L₂] and [H₂Mo₂O₄L′₂]. The more common monooxo-bridged Mo(V) dimer [Mo₂O₃L′₂] may be prepared by the reaction of [MoO₂L′] with PMe₂Ph in dry THF. The Mo(IV) species [MoOL] and [MoOL′] are obtained by treatment of the Mo(VI) species with excess PMe₂Ph in THF solution. The sulfido-bridged dimers [Mo₂S₄L′₂]²⁻ and [MoO₂S₂L′₂]²⁻ have also been prepared. The infrared, electronic, and proton magnetic resonance spectra of these complexes are noted. The electrochemical properties of the complexes are discussed in detail. The significant differences in the redox characteristics of [MoO₂L] and [MoO₂L′] are reflected in the contrasting reactivity patterns with substituted hydrazines. Whereas [MoO₂L] reacts with H₂NNR₂ and H₂NNHR species to give the common monosubstituted derivatives [MoO(N₂R₂)L] and the disubstituted diazene complexes [Mo(N₂R)₂L], [MoO₂L′] reacts only with benzoylhydrazides (C₆H₅CONHNH₂) to yield the potentially seven-coordinate species [MoO(C₆H₅CONNH)L′].