Stimuli‐Responsive Equilibrium in N‐Heterocyclic Carbene‐Supported Transition Metal Complexes: Selective Isolation of Multinuclear Species

Abstract The Ag I –N‐heterocyclic carbene (NHC) complexes ( 3a , b ) derived from bis(NHC) ligand 1 , which exhibit a temperature‐ and concentration‐dependent equilibrium between the trinuclear and tetranuclear forms, has been successfully isolated, which is slow on NMR time scale. Intriguingly, by freezing this slow equilibrium, facilitated by strong and stable H‐bonding, we could achieve selective isolation of the tetranuclear species ( 3a , b ‐ II ). On the other hand, encapsulation of an anionic guest, ClO 4 , led to exclusive formation of the guest encapsulated trinuclear Ag I ‐NHC complex, 3a ‐ I . This is unprecedented and marks the first instance where selective isolation of both the species involved in an equilibrium using a C‐donor ligand could be attained. In contrast, distinct behavior was observed with the Au I ion, possessing the same linear geometry, however, with stronger M–C NHC bonds. In this context, a stable non‐exchangeable ≈1:1 mixture ( 4a , b ) of the tri‐ and tetranuclear Au I ‐NHC complexes were obtained via transmetalation of both equilibrating ( 3a , b ) and the H‐bonded tetranuclear Ag I ‐NHC complexes ( 3a , b ‐ II ). Moreover, by aptly adjusting the reaction parameters of transmetalation process, the trinuclear Au I ‐NHC complex ( 4a , b ‐ I ) could indeed be obtained selectively. Overall, this study highlights a pioneering approach for the control of an equilibrium between different nuclearity metallo‐supramolecular species engaging C‐donor ligands.