Activation of H 2 and SO 2 by Mo and W complexes: first examples of molecular-H 2 complexes, SO 2 insertion into metal-hydride bonds, and homogeneous hydrogenation of SO 2

Abstract

Our recent work on small-molecule activation by low-valent organometallic Mo and W complexes is summarized. Stereochemical control of MoSO 2 coordination geometry has been achieved since the SO 2 in Mo(CO) 2(PPh 3) 2(SO) 2(L) can bind as a σ-base (η 1), as a π-acid(η 2), or as an isomeric mixture of both forms merely by fine-tuning the donor-acceptor nature of the single cis ligand, L. Modification towards bulky phosphines led to a synthetic pathway to the formally 16-electron complexes, M(CO) 3(PR 3) 2(R = Cy or Pr i ), the structures of which show agostic M···HC interaction. This weak interaction can readily be supplanted by others, including the first example of coordination of molecular H 2. Neutron diffraction studies of W(CO) 3(PPr i i 3) 2(η 2-H 2) show that WH = 1.89(1) Å and HH = 0.82(1) Å. 1H NMR of the HD complex provides a useful spectroscopic diagnostic for molecular coordination ( J HD = 33.5 Hz). Complete HD isotopic exchange occurs for D 2 +H 2 complex, even in the solid state. Reductions of SO 2 by hydrides and H 2 have been studied. Addition of SO 2 to solutions of (η 5-C 5R 5)M(CO) 3H [R = H (= Cp) or Me (=Cp *)] produced (η 5-C 5R 5)M(CO) 3(SO 2H), the first example of SO 2 insertion into an MH bond. The S(O)(OH) complexes are thermally unstable, decomposing at 25–75°C to SO 2 reduction products, including water and sulfido-oxo cluster complexes. A dithionite-bridged complex. [Cp *Mo(CO) 3] 2(μ-S 2O 4), has also been obtained from the hydride-SO 2 reaction. Homogeneous catalysis of SO 2 reduction by H 2 has been effected using [Cp *Mo(μ-S)(μ-SH)] 2 catalyst. Clean conversion to sulfur and water occurs in chlorobenzene-butanol at 1–3 atm and 25–100°C at a rate of up to 350 catalyst turnovers h −1. Weak bases such as Et 3N and ROH greatly increase the rate.