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

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).