Carbon Dioxide Activation by a Palladium Terminal Imido Complex

Abstract

We recently reported the first example of a palladium(ii) terminal imido complex. We proposed that this complex features exceptional high nucleophilicity at the nitrogen atom and a peculiar zwitterionic electronic structure with an anti-bonding highest-occupied molecular orbital (HOMO). This complex swiftly activated moderately acidic CH, OH, and NH bonds and also reacted with dihydrogen. However, unambiguous nucleophilic reactivity with substrates not featuring a hydrogen atom could not be observed. Herein, we now show that this nucleophilic complex also reacts with CO2 to give a ring-strained four-membered palladium(ii) carbamate complex. Remarkably, the same product is obtained in the reaction of the related bisamido complex, albeit at a slower reaction rate. Density functional theory calculations indicate that the addition of CO2 does not proceed via initial 1,2-addition across the Pd–N bond, but instead through nucleophilic attack by the imido (amido respectively) nitrogen atom.

Recent Advances in the Chemistry of Metal Carbamates

“…In this case, the geometry of the ligand forces the nitrogen atom to remain coordinated after CO 2 addition, thus stabilizing a rare example of monoanionic N,O-chelating carbamato ligand (D/1-n). The insertion of CO2 into M−N bonds has also been reported for Sc(III) [217], Ti(IV) [218][219][220], Ni(II) [221], Pd(II) [222] and Ir(III) [223,224] imido complexes. The dianionic carbamyldiide ligand thus generated usually remains coordinated to the metal via N,O (Scheme 21a).…”

“…The insertion of CO 2 into M−N bonds has also been reported for Sc(III) [ 217 ], Ti(IV) [ 218 , 219 , 220 ], Ni(II) [ 221 ], Pd(II) [ 222 ] and Ir(III) [ 223 , 224 ] imido complexes. The dianionic carbamyldiide ligand thus generated usually remains coordinated to the metal via N,O ( Scheme 21 a).…”

“…The dianionic carbamyldiide ligand thus generated usually remains coordinated to the metal via N,O (Scheme 21a). Additionally, in this case, a mechanism involving the direct nucleophilic attack of nitrogen to carbon dioxide appears to be favored over a [2 + 2] cycloaddition [185,222]. The cyclic metallacarbamato ligand may undergo further reactions, i.e., a second CO2 insertion to produce a bis-carbamato (azadicarboxylato) ligand Some of the late transition metal complexes featuring a monodentate carbamate are quite unstable towards decarboxylation, either in solution or in the solid state, especially under vacuum [190][191][192]198,200].…”

Terminal Imido Complexes of the Groups 9–11: Electronic Structure and Developments in the Last Decade

“…Figure 4). The imido complex is very nucleophilic as evidenced by the swift coordination of potassium cations to the imide, [97] yet was shown to give the iminophosphorane as well with trimethylphosphine. The activation of H−H, O−H, N−H and C−H bonds via 1,2 addition as well as catalytic hydrogenation of azides with dihydrogen was demonstrated [29] …”

Heron8 – The 8th Heron Conference on Reactive Intermediates and Unusual Molecules

“…DFT calculations indicate that the addition of CO 2 does not proceed via initial 1,2addition across the Pd-N bond, but instead through nucleophilic attack by the imido (amido) nitrogen atom. [8]…”