Magnesium Pincer Complexes and Their Applications in Catalytic Semihydrogenation of Alkynes and Hydrogenation of Alkenes: Evidence for Metal–Ligand Cooperation

Abstract

The development of catalysts for environmentally benign organic transformations is a very active area of research. Most of the catalysts reported so far are based on transition-metal complexes. In recent years, examples of catalysis by main-group metal compounds have been reported. Herein, we report a series of magnesium pincer complexes, which were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H2 via aromatization/dearomatization metal–ligand cooperation was studied. Utilizing the obtained complexes, the unprecedented homogeneous main-group metal catalyzed semihydrogenation of alkynes and hydrogenation of alkenes were demonstrated under base-free conditions, affording Z-alkenes and alkanes as products, respectively, with excellent yields and selectivities. Control experiments and DFT studies reveal the involvement of metal–ligand cooperation in the hydrogenation reactions. This study not only provides a new approach for the semihydrogenation of alkynes and hydrogenation of alkenes catalyzed by magnesium but also offers opportunities for the hydrogenation of other compounds catalyzed by main-group metal complexes.

Efficient (Z)-selective semihydrogenation of alkynes catalyzed by air-stable imidazolyl amino molybdenum cluster sulfides

“…As a very recent example, Milstein and co-workers have evidenced the great potential of Mg pincer complexes as catalysts for the transformation of diphenylacetylene derivatives to the corresponding (Z)-alkenes with good to very good diastereoselectivities. 11 Previously, our groups have also shown that low-valent Mo pincer complexes and cuboidal Mo 3 S 4 clusters are also efficient homogeneous catalysts for this transformation. 12,13 Notably, pincer complexes utilizing sophisticated phosphine ligands dominate this chemistry and more importantly, mechanistic investigations on these systems have revealed unique reaction pathways.…”

“…7,9 In the case of Mg, a metal-ligand cooperation mechanism (MLC) involving the aromatization/dearomatization of the pincer compound is proposed. 11 Interestingly, trinuclear Mo 3 S 4 clusters containing inexpensive diamino ligands also catalyze the semihydrogenation of diphenylacetylene (dpa) with an acceptable (Z)-stereoselectivity (Z/E ca. 6/1) in the absence of a base, albeit harsh conditions were required (12% mol catalyst, 100 bars of H 2 and 150 °C) to achieve moderate conversion (ca.…”

Cooperative Bond Activation and Catalytic CO2 Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene

“…In addition to stoichiometric reactions, the geometrically constrained aluminum and phosphorus compounds serve as efficient catalysts for the dehydrogenation of amines and formic acid and for the hydroboration of imines and ketones, respectively . Recently, magnesium and zinc pincer complexes have been shown to act as efficient catalysts for catalytic hydrogenation reaction, which proceeds by aromatization and dearomatization element–ligand cooperation . The reactivity of these compounds originates from the cooperation of the electrophilic main-group element centers with the nucleophilic ligands.…”

Catalytic, Z-Selective, Semi-Hydrogenation of Alkynes with a Zinc–Anilide Complex

“…Frustrated Lewis pair (FLP) systems, featuring boranes in combination with a Lewis basic moiety, are some of the most successful homogeneous nontransition metals capable of the selective semi-hydrogenation of internal alkynes. − During the revision of this work, Milstein and co-workers reported a Mg-based catalyst for alkyne semi-hydrogenation . Herein, we report the activation of H 2 by the 1,2-addition across a Zn–N bond of a novel zinc anilide complex.…”