Reversible CO2 binding triggered by metal-ligand cooperation in a rhenium(I) PNP pincer-type complex and the reaction with dihydrogen

Herein we report the reversible binding of CO2 to a very rare dearomatized PNP pincer complex (cis[Re(PNPtBu *)(CO)(2)] 2). The [1,3]-addition of CO2 to the pincer complex is triggered by metal-ligand cooperation via an aromatization/dearomatization sequence to form cis-[Re(PNPtBu-COO)(CO) 2] (3) via Re-O and C-C bond formation. The reversible binding was demonstrated by the exchange reaction of (CO2)-C-13/CO2 from the isotopically labelled compound cis-[Re(PNPtBu-(COO)-C-13)(CO)(2)] (3a). Furthermore, complex 3 reacts with H-2 to give free CO2 and the aromatized hydride complex cis-[Re(PNPtBu)(CO)(2)H] (4), which undergoes the reverse reaction to re-form 3 under CO2 pressure at elevated temperature. Alternatively, 4 reacts, in a low temperature pathway, via the insertion of CO2 into the Re-H bond to give the formate complex cis-[Re(PNPtBu)(CO)(2)(OOCH)] (5)]. Remarkably, complex 5 catalyses efficiently the dehydrogenation of formic acid under base-free conditions.