One metal is enough: a nickel complex reduces nitrate anions to nitrogen gas

Abstract

A stepwise reduction sequence from nitrate to dinitrogen gas at a single nickel center was discovered. A PNP nickel scaffold (PNP⁻ = N[2-PⁱPr₂-4-Me-C₆H₃]₂) emerged as a universal platform for the deoxygenation of NO_x substrates. In these reactions carbon monoxide acts as the oxygen acceptor and forms CO₂ to provide the necessary chemical driving force. Whereas the first two oxygens are removed from the Ni-nitrate and Ni-nitrite complexes with CO, the deoxygenation of NO requires a disproportionation reaction with another NO molecule to form NO₂ and N₂O. The final deoxygenation of nitrous oxide is accomplished by the Ni–NO complex and generates N₂ and Ni–NO₂ in a relatively slow, but clean reaction. This sequence of reactions is the first example of the complete denitrification of nitrate at a single metal-site and suggests a new paradigm of connecting CO and NO_x as an effective reaction pair for NO_x removal.