Deoxygenation of carbon dioxide by electrophilic terminal phosphinidene complexes

Abstract

Deoxygenation of carbon dioxide was achieved using transient terminal phosphinidene chromium and tungsten complexes 2a,b. The overall reaction is exothermic according to DFT calculations on the model terminal P-methyl phosphinidene complex Me-2b; this was also supported by the calculated thermodynamic oxygen-transfer potential. The oxaphosphiran-3-one complex intermediates 3a,b possess an unprecedented bonding situation as some characteristics of a side-on bound carbon dioxide to the (formally) low-coordinated phosphorus centre come to the fore. This is expressed by equidistant P–C and P–O bonds and unusual bond strength relationship, i.e. P–C > P–O, as revealed by the relaxed force constants and other related parameters. The decomposition of 3a,bvia CO extrusion yields terminal phosphinidene oxide complexes 4a,b which dimerise to the final products, the 1,3-dioxa-2,4-diphosphetane complexes 5a,b–8a,b. Additional experimental evidence for the transient formation of phosphinidene oxide complexes 4a,b was obtained by a cross dimerisation experiment using transient chromium and tungsten complexes 2a,b. First comparative investigations on the reaction of Li–Cl phosphinidenoid complex 10 and CO₂ at low temperature revealed the formation of the carbamoyl–phosphane complex 11.