Supramolecular Perturbation of Metal-Metal Bonding in Cyclodextrin-Encapsulated (NHC)Cu-FeCp(CO)₂ Complexes

Although secondary coordination sphere effects on catalytic active sites are widely appreciated, the influence of such interactions on (hetero)binuclear active sites has not been examined comprehensively. Here, the influence of cyclodextrin encapsulation on the (NHC)Cu-FeCp(CO)(2) moiety, which is known to be catalytically active in several transformations, is analyzed in detail. Compared to free (NHC)CuFp complexes (Fp = FeCp(CO)(2)), encapsulated (NHC)CuFp complexes were found to shift from resembling Fe(0) toward having Fe(II) character according to Mossbauer and IR spectroscopies. According to DFT modeling, this change in electronic structure is correlated to the pyramidalization of the Fp fragment away from a planar orientation and to the disruption of semibridging CO interactions typically found in (NHC)CuFp complexes. The latter change can be attributed, in part, to the presence of contra-electrostatic C-HCu anagostic interactions that outcompete semibridging CuCO interactions due to geometric constraints. These combined factors result in Fe(II)-like substitution reactivity of one of the CO ligands that is enabled only within the supramolecular architecture. The data presented herein provide understanding of how (hetero)binuclear reaction centers, especially those involving CO ligands, are influenced by partially covalent interactions from beyond the primary coordination sphere.