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

Abstract

Herein we report the reversible binding of CO₂ to a very rare dearomatized PNP pincer complex (cis-[Re(PNP^tBu*)(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(PNP^tBu–COO)(CO)₂] (3) via Re–O and C–C bond formation. The reversible binding was demonstrated by the exchange reaction of ¹³CO₂/CO₂ from the isotopically labelled compound cis-[Re(PNP^tBu–¹³COO)(CO)₂] (3a). Furthermore, complex 3 reacts with H₂ to give free CO₂ and the aromatized hydride complex cis-[Re(PNP^tBu)(CO)₂H] (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(PNP^tBu)(CO)₂(OOCH)] (5)]. Remarkably, complex 5 catalyses efficiently the dehydrogenation of formic acid under base-free conditions.