Transition metal cluster complexes, hydrogen

Hydrogen-Deuterium Exchange in Hydrido Transition Metal Cluster Complexes... 

A few years ago Smalley and coworkers were able to obtain detailed experimental information about the reactivity of specific transition metal clusters with hydrogen molecules (1). The results for copper and nickel clusters were essentially as expected from the known results for surface and metal complex activities. For copper no clusters were able to dissociate whereas for nickel all clusters were active with a slow, steady increase of activity with cluster size. For the other transition metals studied, cobalt, iron and niobium, a completely different picture emerged. For these metals a dramatic sensitivity of the reactivity to cluster size was detected. No convincing explanation for these surprising results has hitherto been suggested. It should be added that there are no dramatic differences in the activity towards Hg for the metal surfaces (or the metal complexes) of nickel on the one hand and iron, cobalt and niobium on the other. 

Metal carbonyls have many derivatives obtained by (formal) substitution of a carbonyl for another ligand, including hydride. The interest in the complexes arose because they were considered as potential models for hydrogen on metal surfaces ( 7.3.1). In general this is valid, in that the frequencies fall in similar ranges to those found on metal surfaces, the detailed interaction with the surface is distinctly different from that in a transition metal cluster and the analogy is imperfect. 

An illustration of a ship-in-a-bottle synthesis is given below. Other examples that follow illustrate (a) transition metal complexes bonded in nearly uniform sites in a zeolite, constituting one of the best-defined supported catalysts, and (b) transition metal clusters in a zeolite that are simple and uniform enough to allow precise characterization by x-ray absorption spectroscopy and density functional theory, with results that provide fundamental new, and apparently general, understanding of the metal-support interface in supported metals and the chemistry of hydrogen spillover. 

Hydrogen is found in transition metal cluster directly attached to metal atoms, in some few cases as terminal ligand but more frequently bridging two, three, or more metal atoms. Examples of the uncommon terminal hydrogen ligands are found in the complex H2 0s3(C0)u illustrated in Fig. 3.3. 

Measured heats of hydrogenation of metal-metal bonded complexes (M2(CO)gCp2] are -3.3, +6.3 and -1.5 kcal/mol, for M Cr, Ifo and W, respectively. Further studies of reactions of gas phase transition metal cluster ions with hydrocarbons have been reported. ... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

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