Binding, activation, and transformation of carbon dioxide mediated by anionic metal complexes

Abstract

The vanadium nitride complex [Na][NV(N['Bu]Ar) 3] reacts with CO to produce the vanadium tris-anilide complex V(N['Bu]Ar)3 and NaNCO. This is the first example of complete denitrogenation of a termainal nitride complex with generation of a lower coordinate metal complex. This reactivity contrasts sharply with the reactivity of the niobium analogue, where the nitride anion complex [Na][NNb(N['Bu]Ar) 3] is synthesized from the reductive decarbonylation of the niobium(IV) isocyanate complex (OCN)Nb(N['Bu]Ar) 3. Electrochemical studies of the niobium(IV) and vanadium(IV) isocyanate (OCN)V(N['Bu]Ar) 3 complexes are presented. The reactivity of the vanadium carbamate complex [(THF) 2Na][O2CNV(N['Bu]Ar) 3] with electrophilic reagents is presented. The carbamate complex reacts readily with silylation and alkylation reagents to form the carbamate ester complexes of the type ROC(O)NV(N['Bu]Ar) 3. The vanadium carbamate complex reacts with SO2 via a decarboxylation pathway to produce the sulforyl imido complex [Na][O 2SNV(N['Bu]Ar)3], the solid-state structure of which is presented. The reactivity of the vanadium carbamate complex with typical dehydrating reagents, e.g organic acid anhydrides, is shown to proceed cleanly when cobaltocene, acting as an in situ reductant, is present to form the vanadium(IV) isocyanate complex (OCN)V(N['Bu]Ar) 3. The synthesis and structure of the bimetallic complex (TPP)MnOC(O)NV(N['Bu]Ar) 3 (TPP = tetraphenylporphyrin) is presented. Although thermally stable, the complex undergoes a photochemical transformation that forms the vanadium isocyanate complex and putative OMn(TPP), which reacts with triphenylphosphine in the reaction mixture to produce triphenylphosphine oxide. The synthesis the niobium carbamate complex [Na][O 2CNNb(N['Bu]Ar) 3] from the reaction of [Na][NNb(N[Bu]Ar) 3] with CO2 is presented. Its solid-state structure in the form of the ionpair [(12-crown-4) 2Na][O2CNNb(N['Bu]Ar) 3] has been determined. Reaction of the niobium carbamate complex with organic acid anhydrides results in the production of five-coordinate carboxylate, acetate complexes (RC(O)O)(OCN)Nb(N['Bu]Ar) 3. The reduction of these complexes by two electrons results in the regeneration of the niobium nitride complex (60-80% yield) with concomitant release of CO (30-60% yield). This three-step process represents a highly controlled conversion of CO2 to CO via a ligand-based strategy. The reactivity of CO2 with anionic complexes featuring terminal multiply bonded ligands is extended to the oxo anion complex [(Et 2O)2Li][OTi(N['Bu]Ar) 3] resulting in the formation of the carbonate complex ([Li][O 2COTi(N['Bu]Ar) 3]) 6. The binding of CO2 to the oxo complex is reversible when 12-crown-4 is bound to the lithium countercation or if the complex is dissolved in THF. The thermodynamic parameters for the CO2 binding equilibrium have been measured. Exchanging the lithium countercation for sodium or potassium results in a significant weakening of the CO2 binding ability of the oxo complex.