Silyl-Osmium(IV)-Trihydride Complexes Stabilized by a Pincer Ether-Diphosphine: Formation and Reactions with Alkynes.

Abstract

Complex OsH4{κ3-P,O,P-[xant(PiPr2)2]} (1) activates the Si-H bond of triethylsilane, triphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane to give the silyl-osmium(IV)-trihydride derivatives OsH3(SiR3){κ3-P,O,P-[xant(PiPr2)2]} [SiR3 = SiEt3 (2), SiPh3 (3), SiMe(OSiMe3)2 (4)] and H2. The activation takes place via an unsaturated tetrahydride intermediate, resulting from the dissociation of the oxygen atom of the pincer ligand 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2). This intermediate, which has been trapped to form OsH4{κ2-P,P-[xant(PiPr2)2]}(PiPr3) (5), coordinates the Si-H bond of the silanes to subsequently undergo a homolytic cleavage. Kinetics of the reaction along with the observed primary isotope effect demonstrates that the Si-H rupture is the rate-determining step of the activation. Complex 2 reacts with 1,1-diphenyl-2-propyn-1-ol and 1-phenyl-1-propyne. The reaction with the former affords Os{C≡CC(OH)Ph2}2{=C=CHC(OH)Ph2}{κ3-P,O,P-[xant(PiPr2)2]} (6), which catalyzes the conversion of the propargylic alcohol into (E)-2-(5,5-diphenylfuran-2(5H)-ylidene)-1,1-diphenylethan-1-ol, via (Z)-enynediol. In methanol, the hydroxyvinylidene ligand of 6 dehydrates to allenylidene, generating Os{C≡CC(OH)Ph2}2{=C=C=CPh2}{κ3-P,O,P-[xant(PiPr2)2]} (7). The reaction of 2 with 1-phenyl-1-propyne leads to OsH{κ1-C,η2-[C6H4CH2CH=CH2]}{κ3-P,O,P-[xant(PiPr2)2]} (8) and PhCH2CH=CH(SiEt3).