N-doped resin supported cobalt nanoparticles for the catalytic reduction of nitroarenes to corresponding anilines in aqueous medium

A nitrogen-doped polymer supported cobalt catalyst (Co-pol) was synthesized by co-polymerization of a methacrylic β-ketoesterate cobalt complex with amidic co-monomers and subsequent calcination under dihydrogen. Co-pol was employed as active, selective and recyclable catalyst for nitroarene reduction in aqueous medium (Et2O/H2O) at room temperature with NaBH4 as reducing agent. The catalysis was truly heterogeneous, as demonstrated by hot filtration tests, and the catalytic system prevented hydrodehalogenation when halonitroarenes were used as substrates. To keep its activity, Co-pol needed to be regenerated after duty by calcination under dihydrogen. Accurate STEM and EDS analyses detected a mixture of both cobalt(0) and cobalt oxide nanoparticles in the active catalyst and only cobalt oxide nanoparticles in the spent catalyst indicating that the real catalytically active species were cobalt(0) nanoparticles. A beneficial role of nitrogen atoms in the catalytic support was ascertained by synthetizing a new catalyst (Co-pol2) similar to Co-pol, but without nitrogen centers. Co-pol2 turned out much less active than Co-pol, thus suggesting a mechanism consisting of a support-assisted heterolytic splitting of dihydrogen (rather than involving the formation of Co–N interaction, not detected by XPS analysis) as operative in Co-pol system.