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

• Published in: Chemistry : a European journal Vol. 25; no. 6; pp. 1588 - 1594
• Main Authors: Álvarez‐Rodríguez, Lucía, Cabeza, Javier A., García‐Álvarez, Pablo, Gómez‐Gallego, Mar, González‐Álvarez, Laura, Merinero, Alba D., Sierra, Miguel A.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 28.01.2019
• Subjects: Carbenes; carbonyl complexes; Chemistry; Chromium; Coordination compounds; Crossovers; Fischer carbenes; germylenes; Heat exchange; insertion; ring expansion; Substitution reactions; Temperature effects; Tungsten; insertion; carbonyl complexes; germylenes; ring expansion; Fischer carbenes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201805406

The formation of the five‐membered‐ring germylene complexes [M(CO)5{Ge(tBu2bzamC(OEt)Me)tBu}] (3M; M=Cr, W), which occurs readily at room temperature from the germylene Ge(tBu2bzam)tBu (1tBu) and Fischer carbenes [M(CO)5{C(OEt)Me}] (2M; M=Cr, W), has been found to be reversible. Upon heating at 60 °C, complexes 3M undergo epimerization to an equilibrium mixture of 3M and 3′M. At that temperature, the chromium epimers (but not the tungsten ones) release CO to end in the mixed germylene–Fischer carbene complexes [Cr(CO)4{C(OEt)Me}{Ge(tBu2bzam)tBu}] (cis‐4Cr and trans‐4Cr). The latter decompose at 120 °C to [Cr(CO)5{Ge(tBu2bzam)tBu}] (6Cr). Because the formation of cis‐4Cr and trans‐4Cr from 3Cr or 3′Cr requires the presence of free 1tBu and 2Cr in the reaction solutions, the reactions of 1tBu with 2M to give 3M (and 3′M at 60 °C) should be reversible. This proposal has been proven by germylene‐exchange crossover reactions in which free 1tBu and [M(CO)5{Ge(tBu2bzamC(OEt)Me)CH2SiMe3}] (5′M; M=Cr, W) were formed when complexes 3M were treated at room temperature with the germylene Ge(tBu2bzam)CH2SiMe3 (1tmsm). A clear differential behavior between N‐heterocyclic carbenes (NHCs) and amidinatogermylenes (1tBu and 1tmsm) 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. Reversible ring expansion: The formation of the five‐membered‐ring germylene complexes 3M (M=Cr, W) from germylene 1tBu and electrophilic Fischer carbenes (2M) is reversible (see graphic). In fact, complex 3Cr leads to the CO substitution products cis‐4Cr and trans‐4Cr when it is heated at 60 °C.