REACTIONS OF ML4CL2 (M = MO, W L = PME3, PMEPH2) WITH EPOXIDES, EPISULFIDES, CO2, HETEROCUMULENES, AND OTHER SUBSTRATES - A COMPARATIVE-STUDY OF OXIDATIVE ADDITION BY OXYGEN ATOM, SULFUR ATOM, OR NITRENE GROUP TRANSFER

A comparative survey of the reactivity of the divalent molybdenum and tungsten chloro-phosphine complexes ML4Cl2 (M = Mo, W; L = PMe3, PMePh2) toward oxidation by a variety of oxygen atom, sulfur atom, and nitrene donors is presented. In general, reactions result in net two-electron oxidation of the metal center, producing metal oxo, sulfido, and imido complexes. The reactions can also be described as oxidative addition reactions, in many cases oxidative addition of C=X double bonds. Reactions are apparently thermodynamically driven by the propensity of Mo and W to form strong multiple bonds with oxygen, sulfur, and nitrogen. ML4Cl2 compounds react with ethylene oxide and ethylene sulfide to produce oxo and sulfido tris(phosphine) species, M(E)L3Cl2 (E = O, S), in equilibrium with oxo and sulfido ethylene species M(E)(CH2=CH2)L2Cl2. Isocyanates (RN=C=O; R = (t)Bu, p-tolyl) and (t)BuN=C=N(t)Bu react to form imido tris(phosphine) and imido carbonyl or imido isonitrile complexes, respectively. Phosphine sulfides are desulfurized forming sulfido complexes, but phosphine oxides are unreactive. The pi-acids formed in these reactions-for instance, CO from cleavage of RNCO-bind more strongly to the tungsten(IV) versus the molybdenum(IV) oxo, sulfido, and imido products. Similarly, the equilibria for pi-acid coordination are more favorable when the ligand is PMePh2 than when L = PMe3. For all of the complexes, reactions are slowed by free phosphine, consistent with a mechanism involving an initial dissociation of a phosphine ligand followed by trapping of the coordinatively unsaturated species by the oxidizing substrate. Ligand loss from ML4Cl2 is rapid for L = PMePh2 at ambient temperatures but slower for L = PMe3, with half-lives for PMe3 loss of 18 min at 24-degrees-C for Mo(PMe3)4Cl2 and 6 min at 69-degrees-C for W(PMe3)4Cl2. For the molybdenum complexes MoL4Cl2 (L = PMe3, PMePh2), dimerization to the known Mo(II) quadruply bound species Mo2L4Cl4 is competitive with oxidation at the metal center. In reactions involving stronger oxidants (SO2, DMSO, and N2O), the formation of trivalent species ML3Cl3 is often observed, indicating that chlorine atom transfer processes also occur.