Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

Reduction of imines with a reusable bimetallic PdCo-Fe3O4 catalyst at room temperature under atmospheric pressure of H2

Bimetallic nanocatalysts have been used for the development of organic reactions, owing to the synergistic effect between the transition metals. A new procedure for synthesizing amines by the reduction of imines with H₂ at atmospheric pressure and room temperature in the presence of PdCo–Fe₃O₄ nanoparticles is reported. The straightforward procedure, mild reaction conditions, high turnover number, and recyclability extend the scope of this reaction to practical applications.

A catalytic procedure that has mild reaction conditions, high turnover number, and the recyclability of the catalyst is presented, whereby the synthesis of amines through the reduction of imines employing PdCo–Fe₃O₄ under atmospheric pressure of H₂ is achieved.

Selective C-alkylation Between Alcohols Catalyzed by N-Heterocyclic Carbene Molybdenum

The first implementation of a molybdenum complex with an easily accessible bis-N-heterocyclic carbene ligand to catalyze β-alkylation of secondary alcohols via borrowing-hydrogen (BH) strategy using alcohols as alkylating agents is reported. Remarkably high activity, excellent selectivity, and broad substrate scope compatibility with advantages of catalyst usage low to 0.5 mol%, a catalytic amount of NaOH as the base, and H₂ O as the by-product are demonstrated in this green and step-economical protocol. Mechanistic studies indicate a plausible outer-sphere mechanism in which the alcohol dehydrogenation is the rate-determining step.

Ligand assisted carbon dioxide activation and hydrogenation using molybdenum and tungsten amides

Molybdenum and tungsten amides M(NO)(CO)(PNP) {M = Mo, 3a; W, 3b; PNP = (iPr₂PCH₂CH₂)₂N} can activate CO₂ at room temperature forming carbamate species M(NO)(CO)(PNP)(OCO) (M = Mo, 4a(trans); W = 4b(trans). Employing 3a,b stoichiometric hydrogenation of carbon dioxide could be demonstrated.