Bulky N-heterocyclic carbene and pyridine donor adducts of Co(II) bromide: Influence on reactivity of stoichiometry, sterics and donor capability

The reactivity of cobalt dibromide with the bulky N-heterocyclic carbenes IMes and IPr has been explored [IMes = N,N0-bis(2,4,6-trimethylphenyl)imidazol- 2-ylidene; IPr = N,N0-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], with the aim of generating synthetically versatile complexes of the types (NHC)CoBr ₂ and (NHC)₂CoBr₂. Differences between the two carbene systems illustrate the key role of ligand sterics in this synthetic approach, while the importance of donor strength is emphasized by comparative studies with the related sterically encumbered pyridine ligand 2,6-Mes ₂py. Thus, IMes and IPr are differentiated by the inability of the latter, bulkier, donor to form a 2:1 adduct, while 2,6-Mes₂py gives rise to CoeN bonds only in the presence of a suitable halide abstraction agent.