Reversible Carbene Insertion into a Ge-N Bond and Insights into CO and Carbene Substitution Reactions Involving Amidinatogermylenes and Fischer Carbene Complexes.

The formation of the five-membered-ring germylene complexes [M(CO) {Ge(tBu bzamC(OEt)Me)tBu}] (3 ; M=Cr, W), which occurs readily at room temperature from the germylene Ge(tBu bzam)tBu (1 ) and Fischer carbenes [M(CO) {C(OEt)Me}] (2 ; M=Cr, W), has been found to be reversible. Upon heating at 60 °C, complexes 3 undergo epimerization to an equilibrium mixture of 3 and 3' . At that temperature, the chromium epimers (but not the tungsten ones) release CO to end in the mixed germylene-Fischer carbene complexes [Cr(CO) {C(OEt)Me}{Ge(tBu bzam)tBu}] (cis-4 and trans-4 ). The latter decompose at 120 °C to [Cr(CO) {Ge(tBu bzam)tBu}] (6 ). Because the formation of cis-4 and trans-4 from 3 or 3' requires the presence of free 1 and 2 in the reaction solutions, the reactions of 1 with 2 to give 3 (and 3' at 60 °C) should be reversible. This proposal has been proven by germylene-exchange crossover reactions in which free 1 and [M(CO) {Ge(tBu bzamC(OEt)Me)CH SiMe }] (5' ; M=Cr, W) were formed when complexes 3 were treated at room temperature with the germylene Ge(tBu bzam)CH SiMe (1 ). A clear differential behavior between N-heterocyclic carbenes (NHCs) and amidinatogermylenes (1 and 1 ) in their reactivity against group 6 metal Fischer carbene complexes is demonstrated. The higher electron-donor capacity of amidinatogermylenes with respect to NHCs and the bias of the former to get involved in ring expansion processes are responsible for this differential behavior.