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.