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The heterolytic cleavage of H2into H+and H−, and the subsequent transfer of these hydrogen species to polarized CX groups, is a useful strategy to obtain high selectivities in catalytic hydrogenation reactions, which are one of the most important chemical processes. In the homogeneous field, this transformation is catalyzed by a wide variety of metal–ligand complexes, leading to the selective reduction of ketones, aldehydes, and imines, to name a few. In many cases, a ligand–metal cooperative mechanism promotes the activation of the hydrogen molecule; that is, a Lewis basic site of the ligand binds the proton, and simultaneously, a coordinated metal atom acts as a Lewis acid and takes the hydride. Similar to homogeneous catalysts based on organometallic complexes, in the field of metal nanoparticle catalysis the use of supports, ligands and additives can promote heterolytic H2splitting by a cooperative mechanism with the metal, and thus produce the selective reduction of thermodynamically more stable unsaturated groups. This successful approach has allowed very high selectivities to be obtained, with both supported and non-supported metal nanoparticles (MNPs). This Perspective aims to carry out a critical review of recent examples on heterolytic cleavage of dihydrogen mediated by MNPs, making a selection of the most representative works in search for general features and new steps to be followed. As will be shown, promising advances have been made through the use of supported MNPs functionalized by organic ligands, which combine the advantages of homogeneous and heterogeneous catalysts (activity, selectivity, stability and recyclability), and thus represent a new research area with tremendous industrial interest.

The main challenge in the hydrogenation of alkynes into (E)- or (Z)-alkenes is to control the selective formation of the alkene, avoiding the over-reduction to the corresponding alkane. In addition, the preparation of recoverable and reusable catalysts is of high interest. In this work, we report novel nickel nanoparticles (Ni NPs) stabilized by three different imidazolium-amidinate ligands (ICy·(Ar)NCN; L1: Ar = p-tol, L2: Ar = p-anisyl and L3: Ar = p-ClC6H4). The as-prepared Ni NPs were fully characterized by (HR)-TEM, XRD, WASX, XPS and VSM. The nanocatalysts are active in the hydrogenation of various substrates. They present a remarkable selectivity in the hydrogenation of alkynes towards (Z)-alkenes, particularly in the hydrogenation of 3-hexyne into (Z)-3-hexene under mild reaction conditions (room temperature, 3% mol Ni and 1 bar H2). The catalytic behaviour of Ni NPs was influenced by the electron donor/acceptor groups (–Me, –OMe, –Cl) in the N-aryl substituents of the amidinate moiety of the ligands. Due to the magnetic character of the Ni NPs, recycling experiments were successfully performed after decantation in the presence of an external magnet, which allowed us to recover and reuse these catalysts at least 3 times preserving both activity and chemoselectivity.

Air-stable nanoparticles stabilized with secondary phosphine oxides (SPOs) can have a remarkable role as catalysts. Since size and activity depend both on the cluster-ligand interaction and the nature of the employed ligand, the present work provides a fresh perspective on the prospects of bonding analysis, rationalizing for the first time the physical nature of interactions between silver nanoclusters (SNCs) and secondary phosphine oxides (SPOs) in the light of energy decomposition (EDA-NOCV) and noncovalent interaction (NCI) analyses, using the silver core of the X-ray structure for a highly symmetric, ligand-decorated Ag44cluster available in the literature. Our findings reveal that phosphines containing aliphatic substituents are more stabilized when interacting with Ag cluster, in contrast to aromatic groups, and that SPOs containing aromatic moieties become aligned to the silver surface upon adsorption. This alignment of aromatic substituents substantially contribute to dispersion interactions, which is observed by both EDA-NOCV and NCI analyses, showing ligand to metal donations and fragment polarizations, as well as metal to ligand backdonations in some cases. Adsorption conformations bearing hydrogen bonds to the surface were found to be the most favorable ones, and models for SPO-cluster hydrogen transfer are in line with previous experimental findings on gold nanoparticles. The present contribution will help pave the way for the rational development of new SPO-protected nanoparticles and nanoclusters, which may become cheaper and more efficient catalysts in the future.

The catalytic activity of a series of Ru-PNP pincer ligand complexes was studied in the direct amination of alcohols with ammonia. It turned out that all complexes of PNP ligands bearing a secondary amine showed no activity in this hydrogen-shuttling reaction sequence, while all complexes of homologous ligands bearing a tertiary amine gave active catalysts. Further comparative studies on catalysts bearing an acridine-based PNP pincer ligand and a PNP ligand of the Xantphos family provided valuable mechanistic insight that led to the design of a highly active catalyst. It appears that in the group of ligands studied here only ligands that do not form stable Ru-amido complexes are active alcohol amination catalysts.

A secondary phosphine oxide (SPO) ligand (tert-butyl(phenyl)phosphine oxide) was employed to generate an Ir–SPO complex which shows a particular ability to activate dihydrogen under mild conditions without the help of an external base or additive. Such an iridium(i) complex serves as a precursor for homogeneous catalysis since under H2it is converted to a mixture of several iridium(iii) hydride species that are the active catalysts. This system was found to be a highly active catalyst for the hydrogenation of substituted aldehydes, giving very high conversions and chemoselectivities for a wide range of substrates. The SPO ligand presumably plays a key role in the catalytic process through heterolytic cleavage of H2by metal–ligand cooperation. In addition, an exhaustive characterization of the different iridium hydride species was performed by 1D and 2D NMR spectroscopy. The oxidative addition of H2to the Ir(i)–SPO complex is highly stereoselective, as all generated Ir(iii) hydrides are homochiral. Finally, the crystal structure, as determined by X-ray diffraction, of a dinuclear iridium(iii) hydride complex is described.

This review highlights the use of a bisphosphine ligand group in homogeneous catalysis. This ligand group is known as the Xantphos-like ligands, which are based on the generic formula of 2,2′-bis(phosphino)diaryl ether. The synthesis of the main members is summarized and emphasis is given to ligands that can be easily synthesized, also on large scale. The catalytic applications are not comprehensively presented, but instead, about 20% of the examples from literature have been extracted, aiming at the demonstration of diversity, peculiar examples, historical development, design vs.trial and error, and new reactions. Metals ranging from group 7 to 11 have been used in a wide variety of reactions: hydroformylation, alkoxycarbonylation, hydrocyanation, cross-coupling reactions for many C–C and C–X bond forming reactions, carbonylative coupling reactions, phosphinations, etc.