Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes
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Catalysis Letters Vol. 104, Nos. 1–2, October 2005 ( 2005) DOI: 10.1007/s10562-005-7440-3
Catalytic processes involving dihydrogen complexes and other sigma-bond complexes Gregory J. Kubas* Structural Inorganic Chemistry Group, Chemistry Division, MS J514, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Received 31 August 2004; accepted 24 June 2005
The discovery of dihydrogen complexes, LnM(H2), pointed to direct transfer of hydrogen from coordinated H2 ligands to substrates as an operable pathway in catalysis both in homogeneous and heterogeneous systems. Sigma complexes, LnM(g2-H–X) (X=H, Si, C, etc), are indeed relevant in hydrogenation as well as silane alcoholysis and methane conversion. KEY WORDS: hydrogenation; homogeneous catalysis; heterogeneous catalysis; dihydrogen complexes; hydride; sigma complexes; silane; silane alcoholysis; methane; heterolytic cleavage; oxidative addition; electrophilic complexes; isotopic exchange; hydrogenase.
1. Introduction Catalytic hydrogenations are the largest man-made chemical reactions in the world: all crude oil is treated with hydrogen in hydrotreating processes and billions of tons of ammonia are produced worldwide by the Haber process. Metal hydride complexes formed by oxidative addition (OA) of the H–H bond in H2 had early on been known to be a part of homogeneous catalytic cycles and were well-characterized species. However the discovery by Kubas and coworkers in 1983 of coordination of a nearly intact dihydrogen molecule (H2) to a metal complex has led to a new paradigm in chemistry [1,2].
Molecules containing only strong ‘‘inert’’ r-bonds such as H–H in H2 and C–H in alkanes had previously been believed to be incapable of stable binding to a metal. However, dihydrogen complexes (referred to as g2-H2 or H2 complexes) that were only assumed to be unobservable intermediates in dihydride formation can be isolatable species, as exemplified by the first H2 complex, W(CO)3(PiPr3)2(H2) (figure 1) [1,2]. The H–H distance is elongated to 0.89 A˚ from 0.75 A˚ in free H2, indicating the bond is only partially broken. Hundreds of H2 complexes have now been established and in many cases the nearly intact H2 ligand is reversibly bound as in physisorbed H2. These species are part of a class of compounds called r-complexes, which * To whom correspondence should be addressed. E-mail: [email protected]
refers to the side-on 3-center interaction of the bonding electron pair in H–H or other X–Y bonds with M. One of most important initial questions was whether direct transfer of hydrogens from an g2-H2 ligand rather than a dihydride can take place in catalytic hydrogenation. This would avoid the need for completely breaking the strong H–H bond (104 kcal/mol), giving lower energy pathways for catalysis. Although this is difficult to prove conclusively, evidence now exists that certain reactions, e.g. styrene hydrogenation, involve transfer of metal-bound H2 to substrate. As will be shown, Noyori’s elegant asymmetric catalytic hydrogenation systems that led to the Nobel
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