Stimuli-Responsive Equilibrium in N-Heterocyclic Carbene-Supported Transition Metal Complexes: Selective Isolation of...

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, ClO₄, 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.