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

Oxidative decarbonylation of (η5-C5R5)­Mo­(CO)3X 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
(CpPPh) 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(η5:η1-CpPPh) as catalyst precursors
is a worthy objective. To this end, HM­(CO)2(η5:η1-CpPPh) (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(η5:η1-CpPPh) (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(η5:η1-CpPPh)] (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(η5-C5R5) and MoR­(CO)3(η5-C5R5) precursors.