Reversible reduction of coordinated carbon monoxide by permethylzirconocene hydrides. Synthesis and reactivity of a bridging diruthenium zirconoxy carbene complex

Abstract

Reaction of the binuclear ruthenium complex [CpRu(CO) 2] 2, 2, (Cp = η 5-C 5H 5) with Cp * 2ZrH 2, 1, (Cp * = η 5-C 5(CH 3) 5) produces the carbene complex CP 2Ru 2(CO) 2(μ-CO)(μ-CHOZr(H)Cp * 2), 3, in which the zirconoxy carbene occupies a bridging position, and the ruthenium Cp ligands are mutually trans. The complex undergoes a fluxional process on the NMR time scale at room temperature whereby the inequivalent pairs of Cp and Cp * ligands are exchanged. A value of Δ G‡ = 14.0(2) kcal mol −1 has been determined for this process from the coalescence temperature (−4°C at 90 MHz). The hafnium analogue exhibits a similar value (14.4(2) kcal mol −1 at +5°C). The ZrH addition to the carbonyl is reversible at 25°C, regenerating 2 and the zirconium hydride, which can be trapped with reagents such as CO, CH 3I and CF 3CO 2H. Lewis acids, on the other hand, induce the transfer of a second hydride from zirconium with concurrent CO cleavage, yielding the bridging methylene complex Cp 2Ru 2(CO) 3(μ-CH 2). A modified procedure for the conversion of [CpRu(CO) 2] 2 to Cp 2Ru 2(CO) 3(CH 2) with LiHBEt 3 in non-polar solvents is also reported.