Self-Assembly and Dynamic Equilibrium of Trinuclear and Tetranuclear Cu(I) Supramolecules Featuring <i>nido</i>-Carborane-Supported <i>N</i>-Heterocyclic Carbene Ligands

The self-assembly of metallo-supramolecules has attracted considerable attention in recent decades. These discrete architectures are primarily driven by coordination interactions, typically involving M-N/O (Werner-type) or M-C (organometallic) bonding. However, the use of M-π interactions for constructing these multinuclear complexes remains largely unexplored. In this work, we report the self-assembly of trinuclear and tetranuclear copper(I) complexes driven by a combination of M-π and M-C coordination interactions. These multinuclear Cu(I)-NHC complexes were synthesized from the <i>nido</i>-carborane-supported <i>N</i>-heterocyclic carbene (NHC) precursors and Cu(I) ions. In solution, a dynamic equilibrium between the trinuclear and tetranuclear species was observed, as confirmed by the variable-temperature NMR spectrum. Van't Hoff analysis revealed that the equilibrium is endothermic (Δ<i>H</i>_eq = 53.6 kJ mol^-1) and entropically driven (Δ<i>S</i>_eq = 158 J mol^-1 K^-1). The solid-state structures of both forms were elucidated through single-crystal XRD analysis. Density functional theory calculations showed that the Cu-C_NHC bonds in these complexes are relatively weak (∼100 kJ mol^-1, approximately one-third of the strength of typical Cu-C_NHC bonds). This is attributed to the strong Coulombic attraction between positively charged Cu(I) and negatively charged <i>nido</i>-carborane ligands (M-π interactions), which significantly reduces the bond strength between Cu(I) and neutral NHC moieties (M-C bonding).