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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.

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.

Chiral iridium nanoparticles (IrNPs) were synthesized by H2reduction of (1,5-cyclooctadiene)(methoxy)iridium(i) dimer ([Ir(OMe)(COD)]2) in the presence of an asymmetric secondary phosphine oxide (4,5-dihydro-3H-dinaphtho[2,1-c:1′,2′-e] phosphepine-4-oxide, L). This highly reproducible and simple procedure furnished small, well dispersed and soluble nanoparticles of 1.4 (0.2) nm, which were found to be active catalysts for the enantioselective hydrogenation of prochiral ketones. This study represents the first example of asymmetric hydrogenation catalyzed by SPO-ligated metal nanoparticles and also the first example of asymmetric hydrogenation catalyzed by non-supported chiral IrNPs. The IrNPs were characterized by the use of a wide variety of techniques, such as TEM, HRTEM, EDX, XPS, ATR FT-IR, ECD, and MAS-NMR spectroscopy with and without 1H–13C cross-polarization (CP).

Platinum complexes modified with a chiral non-racemizing SPO preligand 1have been applied in the hydration of aromatic nitriles. [Pt(1)3Cl]Cl formed readily from Pt(COD)Cl2. The chiral secondary phosphine oxide complex showed moderate activity in the hydration of para- and meta-substituted benzonitriles, but failed in converting the ortho-substituted derivatives. The hydride complex PtH(PR2OH)(PR2O–HOR2P) (PR2OH = 1) formed from Pt(PPh3)4and 1, and the cationic complex derived from [Pt(1)3Cl]Cl viadirect chloride abstraction with AgNO3were proven to be considerably more active, allowing us to extend the scope to the hydration of ortho-substituted aromatic nitriles, including axially chiral [1,1′-binaphthalene]-2,2′-dicarbonitrile. In the hydration of the racemic dinitrile, successful kinetic resolution has been achieved. The catalysts derived from non-racemizing 1are the first chiral transition metal–SPO complexes that provide kinetic resolution in the hydration of a racemic chiral nitrile.

The synthesis of ruthenium nanoparticles (RuNPs) using secondary phosphine oxides (SPOs) as ligands is reported. These easily accessible ligands allow the formation of small nanoparticles in the size range of 1–2 nm which display a high efficiency for hydrogenation of aromatics with TOFs up to 2700 mol h−1.

The application of ruthenium nanoparticles (RuNPs) stabilized by the N-heterocyclic carbenes (NHC) N,N′-di(tert-butyl)imidazol-2-ylidene (ItBu) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene IPr as catalysts in the hydrogenation of several substrates is reported under various reaction conditions (solvent, substrate concentration, substrate/metal ratio, temperature). The RuNHC nanoparticles are active catalysts in the hydrogenation of aromatics and show an interesting ligand effect, RuIPr NPs being generally more active than RuItBu.

The self-assembly of Secondary Phosphine Oxides (SPOs) into anionic bidentate chelates was used to construct unique systems for metal catalyzed transfer hydrogenation of ketones in isopropanol. Chelating bidentate or tridentate ligands were formed by assembly of secondary phosphine oxides through hydrogen bonding in the presence of rhodium trichloride as demonstrated by means of NMR spectroscopy and X-ray diffraction. When a chiral version of an SPO was used in asymmetric transfer hydrogenation of isopropanol and acetophenone, an enantiomeric excess of 89% was achieved. The presence of at least two ligands in the catalytically active species was confirmed by a positive non-linear effect. DFT calculations were applied to characterize several intermediates for the isopropanol dehydrogenation to produce a rhodium hydride complex and acetone. A transition state for the hydrogen-transfer was fully characterized, which revealed that the process occurs viaa concerted outer-sphere mechanism.