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

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 ).