Reactions of New Osmium−Dihydride Complexes with Terminal Alkynes: Metallacyclopropene versus Metal−Carbyne. Influence of the Alkyne Substituent

Reaction of OsH 2 ( κ 2 -OCOCH 3 ){ κ 1 -OC(O)CH 3 }(P i Pr 3 ) 2 ( 1 ) with 1/2HBF 4 ·OEt 2 leads to the dimer [{OsH 2 ( κ 2 -OCOCH 3 )(P i Pr 3 ) 2 } 2 ( μ -OCOCH 3 )]BF 4 ( 2 ), while the reaction with HBF 4 ·OH 2 gives the aquo-derivative [OsH 2 ( κ 2 -OCOCH 3 )(H 2 O)(P i Pr 3 ) 2 ]BF 4 ( 3 ). The structure of 2 in the solid state has been determined by an X-ray diffraction study. The structure consists of two OsH 2 ( κ 2 -OCOCH 3 )(P i Pr 3 ) 2 units connected through an acetate bridge. Complex 3 reacts with phenylacetylene and 1,1-diphenyl-2-propyn-1-ol to give the metallacyclopropene complexes ( 4 ) and ( 7 ), respectively. The structure of 4 in the solid state has been determined by an X-ray diffraction study. The geometry around the metal center can best be described as a pentagonal bipyramid with the two phosphorus atoms of the phosphines occupying apical positions. The equatorial plane is defined by the hydride, the acetate ligand, and the two carbon atoms of the metallacyclopropene. Reaction of 3 with tert -butylacetylene or trimethylsilylacetylene affords the carbyne complexes [OsH( κ 2 -OCOCH 3 )(⋮CCH 2 R)(P i Pr 3 ) 2 ]BF 4 [R = CMe 3 ( 8 ), H ( 9 )], respectively. Deprotonation of 8 and 9 with KOH gives the vinylidene derivatives OsH( κ 2 -OCOCH 3 )( C CHR)(P i Pr 3 ) 2 [R = CMe 3 ( 10 ), H ( 11 )]. The carbyne analogue of 8 and 9 bearing a phenyl group, [OsH( κ 2 -OCOCH 3 )(⋮CCH 2 Ph)(P i Pr 3 ) 2 ]BF 4 ( 5 ), can be obtained upon protonation of OsH( κ 2 -OCOCH 3 )( C CHPh)(P i Pr 3 ) 2 ( 6 ) with HBF 4 ·OEt 2 . In agreement with the experimental results, DFT (B3PW91) calculations show that the cyclopropene product is thermodynamically preferred for phenylacetylene, while the carbyne isomer is preferred for tert -butylacetylene.