Reversible substrate binding at copper centers in neutral copper(i) carbene complexes derived from bis(3-tert-butylimidazole-2-ylidene)methane

Deprotonation of the CH2-bridged bis-imidazolium salt [H2(BIM(tBu))]Br2 followed by reaction with CuBr afforded polymeric [(BIM(tBu))2(CuBr)2]n (3), where BIM(tBu) = bis(3-tert-butylimidazole-2-ylidene)methane. In the solid state, compound 3 is an organometallic polymer with an unusual (-L-Cu-(micro-Br)2-Cu-L-)n type of bonding. The reaction of 3 with nitrogen bases N-tert-butylimidazole or piperidine splits the polymer and afforded the corresponding mononuclear amine adducts [(BIM(tBu))2(CuBr)2(amine)] (6 and 7, respectively). Their molecular structures revealed a unique bonding pattern, which has no precedents in copper-NHC coordination chemistry. The amine binding energies in 6 or 7 of 0.7-7.7 kcal mol(-1), calculated by DFT methods, are consistent with an experimentally observed ligand exchange in solution, fast on the NMR time scale.