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

[Image: see text] Complex OsH(4){κ(3)-P,O,P-[xant(P(i)Pr(2))(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 OsH(3)(SiR(3)){κ(3)-P,O,P-[xant(P(i)Pr(2))(2)]} [SiR(3) = SiEt(3) (2), SiPh(3) (3), SiMe(OSiMe(3))(2) (4)] and H(2). 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(P(i)Pr(2))(2)). This intermediate, which has been trapped to form OsH(4){κ(2)-P,P-[xant(P(i)Pr(2))(2)]}(P(i)Pr(3)) (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)Ph(2)}(2){=C=CHC(OH)Ph(2)}{κ(3)-P,O,P-[xant(P(i)Pr(2))(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)Ph(2)}(2){=C=C=CPh(2)}{κ(3)-P,O,P-[xant(P(i)Pr(2))(2)]} (7). The reaction of 2 with 1-phenyl-1-propyne leads to OsH{κ(1)-C,η(2)-[C(6)H(4)CH(2)CH=CH(2)]}{κ(3)-P,O,P-[xant(P(i)Pr(2))(2)]} (8) and PhCH(2)CH=CH(SiEt(3)).