Heterobimetallic Complexes with Polar, Unsupported Cu-Fe and Zn-Fe Bonds Stabilized by N-Heterocyclic Carbenes

Heterobimetallic complexes of the formulations (NHC)Cu-FeCp(CO)(2) (NHC = IPr, IMes, SIMes), (IPr)Cu-MoCp(CO)(3), and (IPr)(Cl)Zn-FeCp(CO)(2) were synthesized in high yield from readily available starting materials and characterized crystallographically. The solid-state structures of the Cu-Fe systems reveal close, secondary interactions between Cu and one CO ligand from the [FeCp(CO)(2)] unit that are absent in the Zn-Fe analogue. The heterobimetallic complexes feature short yet polar Cu-Fe, Cu-Mo, and Zn-Fe bonds in which the electrophilic metal (Cu, Zn) is later in the transition series than the nucleophilic metal (Fe, Mo), thereby subverting the more common early-late heterobimetallic paradigm. DFT analyses were used to assess M-M' bond polarity and examine effects on M-M' bonding of systematic modifications to both the nucleophilic and electrophilic fragments. Experimental confirmation of Cu-Fe bond polarity was obtained by analysis of product mixtures resulting from the reactions between (NHC)Cu-FeCp(CO)(2) complexes and MeI, which produced (NHC)Cu-I and Me-FeCp(CO)(2) products.