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

•Syntheses of anionic/charge neutral 1:1 adducts of the NHC IPr with Co(II).•Elucidation of the importance of sterics in complexation through studies of IMes.•Evaluation of donor strength in complexation through studies of 2,6-Mes2py.

The reactivity of cobalt dibromide with the bulky N-heterocyclic carbenes IMes and IPr has been explored [IMes = N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], with the aim of generating synthetically versatile complexes of the types (NHC)CoBr2 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-Mes2py. 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 Co–N bonds only in the presence of a suitable halide abstraction agent.

Graphical abstractThe roles of ligand donor strength, steric bulk and reaction stoichiometry on the course of the reaction of CoBr2·2THF with the N-heterocyclic carbenes IMes and IPr, and with 2,6-dimesityl pyridine have been investigated.Figure optionsDownload full-size imageDownload as PowerPoint slide