Ester and amide functionalized alkylidyne triosmium clusters. rearrangement of acyl-coordinated alkylidene complexe

Abstract

Treatment of H 3Os 3(CO) 9(μ 3-CBr) ( 1) with excess aluminum chloride in dichloromethane under an atmosphere of carbon monoxide, followed by quenching with methanol, provides H 3Os 3(CO) 9(μ 3CCO 2Me) ( 2) (69%). Analogous reactions quenched with diethylamine or N-methylaniline give H 3Os 3(CO) 9(μ 3-CC(O)NEt 2) ( 3) (44%) or H 3Os 3(CO) 9(μ 3-CC(O)NMePH) ( 4) (40%), respectively. The same products are obtained when [H 3Os 3(CO) 9(CCO) +] (generated in situ by protonating H 2Os 3(CO) 9(CCO)) is treated with excess methanol, diethylamine or N-methylaniline. Compund 2 is transesterified to the ethyl H 3Os 3(CO) 9(μ 3-CCO 2Et) ( 5), either when dissolved in concentrated sulfuric acid and quenched with ethanol or when heated in ethanol in the presence of a catalytic amount of sulfuric acid. Pyrolysis of the alkylidyne compounds 2 and 3 in hot toluene causes rearrangement to the alkylidene tautomers H 2Os 3(CO) 9(μ 2-CHC(O)OMe ( 6) or H 2Os 3(CO) 9(μ 3,η 2-CHC(O)NEt 2 ( 7) in high yield; the indicated structure for these compounds involves an alkylidene moiety spanning two osmium atoms with the acyl group bridging to the third osmium center. Under parallel conditions at 125°C, the rearrangement requires 5.5 h for the ester ( 2→6) but only 10 min for the amide ( 3→7). The significance of this rate difference in consideration of mechanisms for the alkylidyne to alkylidene rearrangement is discussed.