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

The hepta-coordinated isomeric M(NO)Cl-3((PNP)-P-H) complexes {M = Mo, 1a(syn,anti); W, 1b(syn,anti), (PNP)-P-H = (iPr(2)PCH(2)CH(2))(2)NH, (H-N atom of (PNP)-P-H syn and anti to the NO ligand)} and the paramagnetic species M(NO)Cl-2((PNP)-P-H) (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](-) = [(iPr(2)PCH(2)CH(2))(2)N](-)} were reacted with CO2 at room temperature with CO2 approaching the M=N 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)-((PNP)-P-H) {M = Mo, 5a(cis,trans); W, 5b(cis,trans)}, obtained by H-2 addition to 3a,b, insert CO2 (2 bar) at room temperature into the M-H bond generating isomeric mixtures of the eta(1)-formato complexes M(NO)(CO)((PNP)-P-H)(eta(1)-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%.