Given the limited product variety
of electrocatalytic CO2 reduction reactions solely from
CO2 and H2O as the reactants, it is desirable
to expand the product scope by
introducing additional reactants that provide elemental diversity.
The integration of inorganic heteroatom-containing reactants into
electrocatalytic CO2 reduction could, in principle, enable
the sustainable synthesis of valuable products, such as organonitrogen
compounds, which have widespread applications but typically rely on
NH3 derived from the energy-intensive and fossil-fuel-dependent
Haber–Bosch process for their industrial-scale production.
In this Perspective, research progress toward building C–N
bonds in N-integrated electrocatalytic CO2 reduction is
highlighted, and the electrosyntheses of urea, acetamides, and
amines are examined from the standpoints of reactivity, catalyst structure,
and, most fundamentally, mechanism. Mechanistic discussions of C–N
coupling in these advances are emphasized and critically evaluated,
with the aim of directing future investigations on improving the product
yield and broadening the product scope of N-integrated electrocatalytic
CO2 reduction.
The development of benign methylation
reactions utilizing CO2 as a one-carbon building block
would enable a more sustainable
chemical industry. Electrochemical CO2 reduction has been
extensively studied, but its application for reductive methylation
reactions remains out of the scope of current electrocatalysis. Here,
we report the first electrochemical reductive N-methylation reaction
with CO2 and demonstrate its compatibility with amines,
hydroxylamines, and hydrazine. Catalyzed by cobalt phthalocyanine
molecules supported on carbon nanotubes, the N-methylation reaction
proceeds in aqueous media via the chemical condensation of an electrophilic
carbon intermediate, proposed to be adsorbed or near-electrode formaldehyde
formed from the four-electron reduction of CO2, with nucleophilic
nitrogenous reactants and subsequent reduction. By comparing various
amines, we discover that the nucleophilicity of the amine reactant
is a descriptor for the C–N coupling efficacy. We extend the
scope of the reaction to be compatible with cheap and abundant nitro-compounds
by developing a cascade reduction process in which CO2 and
nitro-compounds are reduced concurrently to yield N-methylamines with high monomethylation selectivity via the overall
transfer of 12 electrons and 12 protons.
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