Ligand assisted carbon dioxide activation and hydrogenation using molybdenum and tungsten amides

Abstract

The hepta-coordinated isomeric M(NO)Cl₃(PN^HP) complexes {M = Mo, 1a(syn,anti); W, 1b(syn,anti), PN^HP = (iPr₂PCH₂CH₂)₂NH, (H_N atom of PN^HP syn and anti to the NO ligand)} and the paramagnetic species M(NO)Cl₂(PN^HP) (M = Mo, 2a(syn,anti); W, 2b(syn,anti)) could be prepared via a new synthetic pathway. The pseudo trigonal bipyramidal amides M(NO)(CO)(PNP) {M = Mo, 3a; W, 3b; [PNP]⁻ = [(iPr₂PCH₂CH₂)₂N]⁻} were reacted with CO₂ at room temperature with CO₂ approaching the MN double bond in the equatorial (CO,NO,N) plane trans to the NO ligand and forming the pseudo-octahedral cyclic carbamates M(NO)(CO)(PNP)(OCO) (M = Mo, 4a(trans); W = 4b(trans)). DFT calculations revealed that the approach to form the 4b(trans) isomer is kinetically determined. The amine hydrides M(NO)H(CO)(PN^HP) {M = Mo, 5a(cis,trans); W, 5b(cis,trans)}, obtained by H₂ addition to 3a,b, insert CO₂ (2 bar) at room temperature into the M–H bond generating isomeric mixtures of the η¹-formato complexes M(NO)(CO)(PN^HP)(η¹-OCHO), (M = Mo, 6a(cis,trans); M = W, 6b(cis,trans)). Closing the stoichiometric cycles for sodium formate formation the 6a,b(cis,trans) isomeric mixtures were reacted with 1 equiv. of Na[N(SiMe₃)₂] regenerating 3a,b. Attempts to turn the stoichiometric formate production into catalytic CO₂ hydrogenation using 3a,b in the presence of various types of sterically congested bases furnished yields of formate salts of up to 4%.