Homogeneous cobalt catalyzed reductive formylation of N-heteroarenes with formic acid

•A novel homogeneous cobalt complex for reductive formylation of N-heteroarenes to generate N-formyltetrahydroquinolines.•Formic acid as an abundant and easy to handle transfer hydrogenation reagent under cobalt catalysis.•Low catalyst loading, high efficiency and broad substrate scope.•Metal ligand cooperated dehydrogenation of formic acid.