Reactions of a dihydrogen complex with terminal alkynes:: Formation of osmium-carbyne and -carbene derivatives with the hydridotris(pyrazolyl)borate ligand

The dihydrogen complex [OsTp(eta(2)-H-2)(k(1)-OCMe2)((PPr3)-Pr-1)]BF4 (1; Tp = hydridotris(pyrazolyl)borate) acts with tert-butylacetylene to give the hydride-carbyne derivative [OsHTp((CCH2Bu)-Bu-t)((PPr3)-Pr-i)]BF4 (2). Similarly, the treatment of I with (trimethylsilyl)acetylene in the presence of methanol leads to [OsHTp( CCH3)((PPr3)-Pr-i)]BF4 (3). In chloroform at 60 degrees C, complexes 2 and 3 evolve into the corresponding chloro derivatives [OsClTp( CR)((PPr3)-Pr-i)]BF4 (R = (CH2Bu)-Bu-t (4), CH3 (5)), whereas in acetonitrile the carbenes [OsTp(=CHR)(NCCH3)((PPr3)-Pr-i)]BF4 (R=(CH2Bu)-Bu-t (6), CH3 (7)) are formed-Complex 1 also reacts with phenylacetylene. The reaction initially gives [OsHTp( CCH2Ph)((PPr3)-Pr-i)]BF4 (8). Subsequently, the insertion of a second molecule of alkyne into the Os-H bond of 8 and the migration of the resulting alkenyl group from the metal center to the carbyne carbon atom take place, to afford the carbene [OsTp{=C(CH2Ph)C(Ph)=CH2}((PPr3)-Pr-i)]BF4 (9). The metal center of 9 is saturated by means of a strong agostic interaction (r(bp) = 0.4(1)) between the osmium atom and one of the ortho-CH bonds of the phenyl substituent of the alkenyl unit of the alkenylcarbene ligand. Treatment of 1 with methylpropiolate leads. to the carbene [OsTp{=CHCH2C(O)OMej((PPr3)-Pr-i)]BF4 (10) in a one-pot synthesis. Complexes 2, 4, 9, and 10 have been characterized by X-ray diffraction analysis.