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.

Two 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 as PowerPoint slide