Selected paper development of highly active IrPNP catalysts for hydrogenation of carbon dioxide with organic bases

Article abstract of DOI:10.1246/bcsj.20150311

Methoxy-substituted PNP-iridium(III) complexes and pyrazine-based PNP-iridium(III) complexes were developed and used to hydrogenate carbon dioxide in the presence of triethanolamine as a base. The methoxy-substituted PNP-hydridodichloridoiridium complex (C-HCl₂) showed the highest turnover number, 160000; this is the highest value ever reported with an organic base in an aqueous medium. The reactivities of these complexes, derived from their ligand modification, were further studied. The results were as follows. (i) The pyrazine-based PNP-trihydridoiridium complex undergoes release of dihydrogen to afford dihydridoamido complex, possibly because of easy dearomatization of the pyrazine ring. This process was reversible, i.e., B-H_2amido can readily be converted back to B-H₃ on exposure to dihydrogen. (ii) The p-methoxy-substituted dihydridochlorido complex showed facile disproportionation of the chloride anion on the iridium center; this is attributed to the electron-donating nature of the methoxypyridine backbone.