Dicarbonyl{[2-(diphenylphosphino)ethyl]cyclopentadienyl} Group VI Metal Hydrides, Halides, and Anions: Precursors for Olefin Epoxidation Catalysts

Oxidative decarbonylation of (eta(5)-C5R5)Mo-(CO)(3)X and isoelectronic four-legged piano-stool Mo(II) precursors to homogeneous olefin epoxidation catalysts has garnered significant attention as an alternative to organorhenium oxides RReO3. An emerging theme has been the introduction of donor-functionalized cyclopentadienyl ligands to tune catalyst performance. However, the utility of the [2-(diphenylphosphino)ethyl]cyclopentadienyl (Cp-PPh) ligand under oxidative decarbonylation conditions has not been explored. The application of Mo(VI) and W(VI) compounds containing mono-and bidentate phosphine oxide ligands as epoxidation catalysts suggests that screening MoX(CO)(2)(eta(5):eta(1)-Cp-PPh) as catalyst precursors is a worthy objective. To this end, HM(CO)(2)(eta(5):eta(1)-Cp-PPh) (M = Cr (1), Mo (2), W (3)) were synthesized; the hydrides 1 and 3 are of interest, since 2 is an established precursor for ionic hydrogenation catalysts. Hydrogen-halogen exchange using 1-3 afforded MX(CO)(2)(eta(5):eta(1)-Cp-PPh) (M = Cr, X = I (4); M = Mo, X = Cl (5), Br (6), I (7); M = W, X = Br (8)), while deprotonation of 1-3 provided [K(18C6)][M(CO)(2)(eta(5):eta(1)-Cp-PPh)] (M = Cr (9), Mo (10), W (11)). Complexes 1 and 3-11 have been characterized in solution and by X-ray crystallography. Treatment of 5-7 with t-BuOOH resulted in active cyclooctene and 1-dodecene epoxidation catalysts, with conversion curves and activities similar to those afforded by MoCl(CO)(3)(eta(5)-C5R5) and MoR(CO)(3)(eta(5)-C5R5) precursors.