Molecular hydrogen complex

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

Kubas, G.J., R.R. Ryan, B.N. Swanson, P.J. Vergatini, J. Wasswerman, Molecular hydrogen complexes Coordination of a sigma bond to transition metals.. Am. Chem. Soc. 120,1988. 

Consider finally the equilibrium between molecular hydrogen complexes and the monatomic species. Although the existence of stable H complexes with n > 2 is by no means excluded—indeed, the platelets discussed in Chapter 7 seem to be such complexes with a very large n—we shall limit our discussion here to the case where only one kind of two-hydrogen complex, which we shall call a molecule and designate by H2, needs to be considered. We shall assume, in accordance with theoretical predictions (Van de Walle et al., 1988a,b Chang and Chadi, 1989 and earlier studies) that the stable state of H2 is neutral for all positions of the Fermi level in the gap there seems to be no experimental reason to doubt this.-In any case, equilibrium of the reaction... 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

The biggest divergence between our mechanism and the one in Figure 2 is the intermediacy of molecular hydrogen complexes, which are often found in other catalytic cycles [65], and had been predicted to exist by previous theoretical studies [66], The formation of stable molecular hydrogen intermediates will have important consequences for the mechanism, as we shall demonstrate. 

Catalyst-Enamide Transition Molecular Hydrogen Complex State Complex... 

It is often suggested that the activation of molecular hydrogen may take place via the formation of a molecular hydrogen complex [M(H2)] [75-77] which may further undergo either oxidative addition giving a metal... 

Fig. 2. Molecular hydrogen complex (structure 2) in [NiFe] hydrogenase. The oxidation states are Ni(II) and Fe(II). Distances are given in A. Atoms marked with were kept frozen from the X-ray structure during the optimization.

Kubas GJ. Molecular hydrogen complexes coordination of a a bond to transition metals. Acc Chem Res 1988 21 120-128. 

In the molecular dihydrogen species W(CO)3(PR3j2(H2) first characterized by Kubas et al. in both X-ray and neutron difiraction studies (1,2), the dihydrogen is bonded in rj sideways fashion to the d metal center. Over 50 compounds involving many of the transition metals have since been synthesized by various workers. In addition, reanalysis(13) of existing hydrides such as FeH4(PR3)3 have now been found to be formulated as molecular hydrogen complexes, i.e., as Fe(H2)(H)2(PR3)3-... 

The possible role of molecular H2 complexes in transition metal-catalyzed hydrogenations has been contemplated. Known hydrogenation catalysts form molecular Hj complexes for example, [RuH(BINAP)2] forms the molecular hydrogen complex [RuH(ij -H2)(BINAP)2] Three roles for nonclassical hydrides in hydrogenation reactions can be envisaged (1) they may be viewed as arrested intermediates to classical metal hydrides that react with substrates, (2) they may react directly with substrates, and... 

Experimental evidence exists for the direct reaction of some nonclassic hydrides with bases the nonclassical hydride (rather than the classical hydride) of [IrH(H2)(bq)L2]" is deprotonated by RLi and [CpRufdmpeXHz)] is deprotonated by Et3N in preference to its equilibrium dihydride form . However, direct involvement of nonclassical hydrides in stoichiometric or catalytic reduction of substrates is still not demonstrated a stoichiometric hydrogenation of 1-hexene has been reported using a dinuclear Ru-molecular hydrogen complex. ... 

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