The Dihydride−Osmium(IV) Complex [OsH2(κ2-O2CCH3)(H2O)(PiPr3)2]BF4 as a Precursor for Carbon−Carbon Coupling Reactions

The dihydride−osmium(IV) complex [OsH 2 ( κ 2 -O 2 CCH 3 )(H 2 O)(P i Pr 3 ) 2 ]BF 4 ( 1 ) reacts with acetylene (1 atm) at 0 °C to give polyacetylene and the vinyl−carbyne derivative [Os(CH CH 2 )( κ 2 -O 2 CCH 3 )(⋮CCH 3 )(P i Pr 3 ) 2 ]BF 4 ( 2 ). Both polyacetylene and 2 can also be obtained by reaction of [OsH( κ 2 -O 2 CCH 3 )(⋮CCH 3 )(P i Pr 3 ) 2 ]BF 4 ( 3 ) with acetylene. Under an acetylene atmosphere, complex 2 yields polyacetylene in a slow but constant manner. Complex 2 reacts with carbon monoxide to give initially the carbene derivative [Os( κ 2 -O 2 CCH 3 ){ C(CH CH 2 )CH 3 }(CO)(P i Pr 3 ) 2 ]BF 4 ( 4 ), by migratory insertion of the carbyne ligand of 2 into the Os−vinyl bond. Under a carbon monoxide atmosphere, complex 2 is unstable and evolves into [Os( κ 2 -O 2 CCH 3 )(CO) 2 (P i Pr 3 ) 2 ]BF 4 ( 5 ). Treatment of 2 with KOH in methanol produces the deprotonation of the carbyne ligand and the formation of the vinyl−vinylidene derivative Os(CH CH 2 )( κ 2 -O 2 CCH 3 )( C CH 2 )(P i Pr 3 ) 2 ( 6 ), which reacts with carbon monoxide to give the butadienyl compound Os{C(CH CH 2 ) CH 2 }{ κ 1 -OC(O)CH 3 }(CO) 2 (P i Pr 3 ) 2 ( 7 ) by migratory insertion of the vinylidene ligand into the Os−vinyl bond. The structure of 7 has been determined by X-ray diffraction analysis. The geometry around the osmium atom can be rationalized as a distorted octahedron with the phosphine ligands occupying opposite positions. The remaining perpendicular plane is formed by the butadienyl ligand, the acetate, and the carbonyl groups mutually cis disposed. The Os−butadienyl distance is 2.195(5) Å, whereas the torsion angle in the diene is 50.7(8)°. The butadienyl ligand of 7 acts as a diene in Diels−Alder reactions. Thus, the addition of dimethyl acetylenedicarboxylate and maleic anhydride to benzene solutions of 7 affords the corresponding cycloaddition products ( 8 ) and ( 9 ). Treatment of 8 with HBF 4 ·OEt 2 gives 5 and dimethyl 1,4-cyclohexadiene-1,2-dicarboxylate.