Ligand-Based Reduction of CO2 to CO Mediated by an Anionic Niobium Nitride Complex

摘要

There are several motivations for producing chemicals from CO2nwhenever possible, and in particular, CO is a promising target beingnboth a versatile chemical precursor and a fuel.n1nReducing metalncomplexes capable of O-atom abstraction from CO2 typically leadnto the formation of strong metal oxygen bonds that represent anchallenge to catalytic turnover.n2nIn a special case for which thenbinding of CO2 evidently involves insertion into a Cu B linkage,ncatalytic turnover producing CO was accomplished by takingnadvantage of the ultimate delivery of oxygen into a stable B O Bnreservoir.n3nThis is to be compared with electrocatalytic methodsnfor CO2 reduction to CO in systems that likely involve a directninteraction between CO2 and the metal center at some point in thencatalytic cycle.n4nAnother commonly observed trend in metal-nmediated CO2 reduction is disproportionation reactivity leading tonthe formation of CO and CO3n2n, highlighted by the reaction ofnLi2[W(CO)5] with CO2 to give W(CO)6 and Li2CO3.n5nWith regardnto organic systems, conversion of CO2 to CO has recently beennachieved with an N-heterocyclic carbene serving as the catalyst,nactivating the CO2 through nucleophilic attack, and a variety ofnaldehydes serving as the oxygen acceptors to form carboxylic acids.n6nTaking a different approach, we have sought to develop anligand-based7,8nCO2 reduction protocol in which the initial bindingnevent is to a terminal nitrido nitrogen atom, rather than directly tonthe redox-active metal itself; this approach avoids the formationnof strong oxometal bonds while ultimately making possible oxidenion transfer to ariable externally added acceptors. This paradigmnsuggested itself through the twin discoveries of facile CO2 uptakenby a terminal vanadium nitride anion complex and correspondingnniobium nitride anion synthesis via isocyanate decarbonylation.n9,10nAccordingly, herein we present a sequence of reactions constitutingna cycle for CO2 reduction to CO, using a terminal niobium nitridenanion complex as a stable platform that is maintained throughout.