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) towards having Fe(II) character according to Mössbauer and IR spectroscopies. According to DFT modeling, this change in electronic structure is correlated to pyramidalization of the Fp fragment away from a planar orientation and to disruption of semi-bridging CO interactions typically found in (NHC)CuFp complexes. The latter change can be attributed, in part, to the presence of contra-electrostatic C-H⋯Cu anagostic interactions that outcompete semi-bridging Cu⋯CO interactions due to geometric constraints. These combined factors are particularly dramatic for a (NHC)CuFp derivative encapsulated by α-cyclodextrin, resulting 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.