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

Abstract

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-(μ-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.