Carboxylation of N-heterocyclic carbenes with carbon dioxide and their tantalum complexes with high oxidation

•Two tantalum complexes [(IMes:CO2)TaCl5] and [(IPr:CO2)2TaCl3][TaCl6]2 with high oxidation were obtained.•The crystal structures of the obtained tantalum complexes were discussed.•The catalytic properties of two complexes on the cycloaddition between CO2 and propylene oxide were investigated.

Carboxylation of two free N-heterocyclic carbenes (NHCs) 1,3-dimesitylimidazol-2-yliden (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) with CO2 led to two adducts [IMes:CO2] and [IPr:CO2], which were further reacted with TaCl5 to afford the finial metal complexes [(IMes:CO2)TaCl5] (1) and [(IPr:CO2)2TaCl3][TaCl6]2 (2), respectively. NMR spectroscopy, elemental analysis and single crystal X-ray diffraction proved that the carboxylate groups were retained in complexes 1 and 2. Crystallography revealed that the metal centers in complexes 1 and 2 adopted the distorted octahedral and pentagonal bipyramidal geometries, respectively, in which complex 1 was constructed with one [IMes:CO2] adducts, while complex 2 with two [IPr:CO2] adducts. In addition, the catalytic properties of complexes 1–2 on the cycloaddition between CO2 and propylene oxide to synthesize cyclic carbonate at the mild condition were investigated.

Graphical abstractTwo imidazolylidene carboxylates [IMes:CO2] and [IPr:CO2], which were obtained by combination of CO2 with N-heterocyclic carbenes, were reacted with VB group metal TaCl5 to give a monomeric complex [(IMes:CO2)TaCl5] (1) and a dimeric complex [(IPr:CO2)2TaCl3][TaCl6]2 (2) with high oxidation of metal.Figure optionsDownload full-size imageDownload as PowerPoint slide