Dihydride complexes

Unimolecular reductive elimination of a metal (or metal complex) dihydride H2Fe(CO)4 <-> Fe(CO)4 + H2 65... 

Clusters Ru3(CO)l2 and Os3(CO)l2 as well as their substitution products, e.g. [Os3(CO)lo(AN)2], activate pyrrole and its derivatives in many different ways. Thus, dihydrides 43 (R = H, Me) follow from triosmium dodecacarbonyl and pyrrole or 1-methylpyrrole [82JCS(D)2563 84P1175 86JOM(311)371]. Complex 43 (R = H) isomerizes as a result of proton transfer to the more stable species 44 and... 

Both cis- and (rans-structures are possible RuH2(PMe3)4 is cis (Ru-H 1.507, 1.659 A, Ru-P 2.276-2.306 A) [90] while spectra show that RuH2(PF3)4 and others have this configuration. RuH2[PPh(OEt)2]4 is definitely trans (X-ray) with Ru-H 1.6 A, Ru-P 2.272 A. Many diphosphines form dihydrides. Ru(dmpe)2H2 has been a useful starting material for the synthesis of thiolate complexes [91] such as fra s-Ru(SPh)2(dmpe)2. 

Tetraphenylmolybdenocene dihydride Mo(r 5-C5HPh4)CpH2 (45) was formed by addition of diphenylacetylene to MoCpL(PhC CPh)CH3 (L = P(OMe)3) (Eq. 15), presumably via an ot-hydrogen abstraction to an intermediate methylidene hydrido complex, followed by addition of two equivalents of diphenylacetylene and C — H insertion with concomitant elimination of L [57 b],... 

Treatment of mer-IrH3(PPh3)3 with N,N, N"-triphenylguanidine in refluxing toluene afforded the iridium(III) dihydride complex [PhNHC(NPh)2]IrH2... 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Scheme 13 Synthesis of group 14 element polysulfido complexes from the corresponding metal dihydrides...

Scheme 14 Synthesis of antimony polysulfido complexes from the corresponding antimony dihydride...

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

In mechanistic studies, monodeuterated alcohols were obtained by using PrOD (Scheme 14). These results indicate that the intermediate for this transfer hydrogenation was not a dihydride complex but rather a monohydride complex, which was generally accepted by analogous transition-metal-catalyzed reactions [55-57]. 

The direct reductive amination (DRA) is a useful method for the synthesis of amino derivatives from carbonyl compounds, amines, and H2. Precious-metal (Ru [130-132], Rh [133-137], Ir [138-142], Pd [143]) catalyzed reactions are well known to date. The first Fe-catalyzed DRA reaction was reported by Bhanage and coworkers in 2008 (Scheme 42) [144]. Although the reaction conditions are not mild (high temperature, moderate H2 pressure), the hydrogenation of imines and/or enam-ines, which are generated by reaction of organic carbonyl compounds with amines, produces various substituted aryl and/or alkyl amines. A dihydrogen or dihydride iron complex was proposed as a reactive intermediate within the catalytic cycle. 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

Fe-H Complexes in Catalysis ). Similarly, also di-ferf-butyltin dihydride could be dimerized [46]. 

Sharma HK, Arias-Ugarte R, Metta-Magana AJ, Pannell KH (2009) Dehydrogenative dimerization of di-tert-butyltin dihydride photochemically and thermally catalyzed by iron and molybdenum complexes. Angew Chem Int Ed 48 6309-6312... 

Scheme 2.2 The formation of the rhodium dihydride complex via a cyclometallated intermediate...

Ruthenium NHC dihydride complex 26 was found to exhibit interesting reversible hydrogenation/dehydrogenation activity (Scheme 10.11) [35,36]. When excess acetone was used as a hydrogen acceptor, dehydrogenation of several alcohols was achieved (Table 10.5). 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

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