C−H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C−H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C−H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C−H activation until summer 2018.

Acylated amino acid ligands enabled ruthenium(II)-catalyzed C-H functionalizations with excellent levels of meta-selectivity. The outstanding catalytic activity of the ruthenium(II) complexes derived from monoprotected amino acids (MPAA) set the stage for the first ruthenium-catalyzed meta-functionalizations with removable directing groups. Thereby, meta-alkylated anilines could be accessed, which are difficult to prepare by other means of direct aniline functionalizations. The robust nature of the versatile ruthenium(II)-MPAA was reflected by challenging remote C-H transformations with tertiary alkyl halides on aniline derivatives as well as on pyridyl-, pyrimidyl-, and pyrazolyl-substituted arenes. Detailed mechanistic studies provided strong support for an initial reversible C-H ruthenation, followed by a SET-type C-Hal activation through homolytic bond cleavage. Kinetic analyses confirmed this hypothesis through an unusual second-order dependence of the reaction rate on the ruthenium catalyst concentration. Overall, this report highlights the exceptional catalytic activity of ruthenium complexes derived from acylated amino acids, which should prove instrumental for C-H activation chemistry beyond remote functionalization.

Aerobic oxidative CH functionalizations of weakly coordinating benzoic acids have been accomplished with versatile ruthenium(II) biscarboxylates under ambient oxygen or air. Mechanistic studies identified the key factors controlling the elementary step of the oxidation of the ruthenium(0) complex.

Sequential twofold meta-C–H/ortho-C–H functionalization was achieved by means of versatile ruthenium­(II) biscarboxylate catalysis. The double C–H activation proved viable in a one-pot fashion with the assistance of synthetically useful imidates. The operationally simple twofold C–H functionalization occurred with high levels of positional selectivity control and was conducted in a nonsequential manner by the judicious choice of the reaction temperature. Detailed experimental mechanistic studies, including unprecedented electron paramagnetic resonance (EPR) experiments, provided strong support for homolytic C–X bond cleavage and facile C–H ruthenation, while a computational density functional theory (DFT) analysis was supportive of a novel mechanistic scenario involving synergistic catalysis via cyclometalated ruthenium­(III) complexes as key intermediates.

Iron Fe(NHC)(CO)4 complexes were formed by direct reaction of Fe3(CO)12 with equimolecular amounts of NHC imidazolium halide precursors; addition of base was not needed in this reaction. When excess (9:1 ratio) 1,3-dimesitylimidazolium chloride is reacted with the iron cluster Fe3(CO)12, a mixture of Fe(IMes)(CO)4 and Fe(IMes)2(CO)3 is obtained. Single crystals of Fe(IMes)(CO)4 and crystals resulting from the cocrystallization of Fe(IMes)(CO)4 and Fe(IMes)2(CO)3 have been studied by X-ray diffraction. These iron(0) complexes were found to catalyze the reduction of benzaldehydes.