Amide-Directed Formation of Five-Coordinate Osmium Alkylidenes from Alkynes

[Image: see text] The amide-directed synthesis of five-coordinate osmium alkylidene derivatives from alkynes is reported. These types of complexes, which have been elusive until now because of the tendency of osmium to give hydride alkylidyne species, are prepared by reaction of the dihydride OsH(2)Cl(2)(P(i)Pr(3))(2) (1) with terminal alkynes containing a distal amide group. Complex 1 reacts with N-phenylhex-5-ynamide and N-phenylhepta-6-ynamide to give OsCl(2){=C(CH(3))(CH(2))(n)NH(CO)Ph}(P(i)Pr(3))(2) (n = 3 (2), 4 (3)). The relative position of carbonyl and NH groups in the organic substrates has no influence on the reaction. Thus, treatment of 1 with N-(pent-4-yn-1-yl)benzamide leads to OsCl(2){=C(CH(3))(CH(2))(3)NHC(O)Ph}(P(i)Pr(3))(2) (4). The new compounds are intermediate species in the cleavage of the C–C triple bond of the alkynes. Under mild conditions, they undergo the rupture of the C(α)–CH(3) bond of the alkylidene, which comes from the alkyne triple bond, to afford six-coordinate hydride–alkylidyne derivatives. In dichloromethane, complex 2 gives a 10:7 mixture of OsHCl(2){≡C(CH(2))(3)C(O)NHPh}(P(i)Pr(3))(2) (5) and OsHCl(2){≡CCH(CH(3))(CH(2))(2)C(O)NHPh}(P(i)Pr(3))(2) (6). The first complex contains a linear separation between the alkylidyne C(α) atom and the amide group, whereas the spacer is branched in the second complex. In contrast to the case for 2, complex 4 selectively affords OsHCl(2){≡C(CH(2))(3)NHC(O)Ph}(P(i)Pr(3))(2) (7). In spite of their instability, these compounds give the alkylidene–allene metathesis, being a useful entry to five-coordinate vinylidene complexes, including the dicarbon-disubstituted OsCl(2)(=C=CMe(2))(P(i)Pr(3))(2) (8) and the monosubstituted OsCl(2)(=C=CHCy)(P(i)Pr(3))(2) (9).