Metal-bound H-atoms

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

Fourteen years passed between the report of the boride from Schmid and cow-orkers, and that of the next fully encapsulated boron atom in a discrete molecular environment Shore s group described [HRu6(CO)nB], the octahedral structure (Fig. 1) being confirmed by the results of an X-ray diffraction studyj The metal-bound H atom is readily removed, and Shore also reported the isolation of [(Ph3P)2N][Ru6(CO)nB].f Independently, we reported the synthesis and spectroscopic characterization of the salt [Me3NH][Ru6(CO)i7B]. These studies opened the door to the development of the chemistry of interstitial borido clusters. 

Metal bound H-atoms undergo a number of unique intermolecular and intramolecular exchange processes. In the words of Greg Kubas Transition metal complexes containing rf -H2 ligands and hydride ligands are unquestionably the most dynamic ligand systems known [4]. 

In general, the rapid exchange of metal-bound H-atoms benefits from the nature of the Is valence orbital of hydrogen. The spherical shape of the Is orbital allows H-atom exchange to occur by associative mechanisms (with low barriers) in which H-H bond formation coincides with metal-hydrogen bond breaking. 

Fig. 3 CSD intermolecular searches on H---0 interactions in (M)C-H---O and [(M)C-H]+- [O]" systems [metal bound C-atoms with M=first row transition metal)...

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

The key steps of a concerted three-center reductive elimination mechanism (Fig. 4, path b) are dissociation of the ligand L trans- to the hydrocarbyl R (step b-i), a concerted M-C bond cleavage and C-X bond formation (step h 2). and a displacement of the organic product R-Z by the ligand L (step b i). The reaction leads to the product of cis-elimination of R-Z with the retention of the configuration of the metal-bound carbon atom. 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

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