Homogeneous catalysts evolution

The square planar [NiX(bdppm)]+ complex was found to react with CO at room temperature and atmospheric pressure in EtOH—H20 solution according to equation (168). The nickel(O) carbonyl complex containing the protonated ligand Hbdppm reacts in turn with acids with evolution of H2 (equation 169). Thus the complex [NiX(bdppm)]+ acts as an effective homogeneous catalyst of the water-gas reaction.1373... 

The brief review emphasizes the useful catalytic activities of metal oxides, i.e., Ru02, towards 02-evolution but points to their limitations as a result of surface recombination with intermediate H-atoms. Possible routes to circumvent these difficulties could involve elimination of surface H-atom through the application of homogeneous H2-evolution catalysts (see Sect. 4.3), and compartmentalization of the oxidation catalyst from the H2-evolution catalyst, i.e., liposomes. Alternatively, reduction of other substrates rather than water i.e. C02, could lead to intermediate carbonous species being insensitive to oxidation by intermediate O-species. 

Photosensitized generation of hydrido-metal complexes in aqueous media provides a general route for H2-evolution, hydrogenation of unsaturated substrates (i.e. olefins, acetylenes), or hydroformylation of double bonds, see Scheme 2. Co(II) complexes, i.e. Co (II)-fn s-bipyridine, Co(bpy) +, or the macrocyclic complex Co(II)-Me4[14]tetraene N4, act as homogeneous H2-evolution catalysts in photosystems composed of Ru(bpy) + (or other polypyridine (Ru(II) complexes) as photosensitizers and triethanolamine, TEOA, or ascorbic acid, HA-, as sacrificial electron donors [156,157], Reductive ET quenching of the excited photosensitizer... 

After getting somewhat acquainted with PET in natural and artificial systems, the reader may become aware of its tremendous implications by studying the next article. There, heterogeneous and homogeneous catalysts as well as biocatalysts are described for artificial photosynthetic applications dealing with H2 evolution, COz fixation, hydrogenation, and hydroformylation processes. 

Fig. 4 Starting with a subset of appropriate protein scaffolds and chemically diverse homogeneous catalysts, a chemogenetic diversity matrix can be used to screen for improved characteristics. Designed evolution consists of iterative rounds of screening and selection of the chemogenetic diversity that is introduced, at least partially, according to the structural information available...

Since the initial papers on RU4, this complex has been extensively characterized and shown to function when immobilized on carbon nanotubes as a water electrooxidation catalyst [92], in solution as a homogeneous catalyst for visible-light-driven water oxidation [93], and when interfaced with [Ru(bpy)3] +-sensitized Ti02 surfaces [94], RU4 has shown no evidence of hydrolytic decomposition to the metal oxides (RUO2, WO3) in any of these studies. The mechanism of water oxidation by [Ru(bpy)] + has also been probed in some depth and the principal catalytic cycle for water oxidation involves sequential oxidation of the resting oxidation state of RU4, which is Ru(IV)4, to the oxidation state Ru(V)4 [95], followed by O2 evolution. 

In order to achieve a true comparison between both catalytic systems, colloidal and molecular, which display very different reaction rates, a series of experiments were carried out with the homogeneous molecular system, decreasing the catalyst concentration in the studied allylic alkylation reaction. The reaction evolution is monitored taking samples at different reaction times and analysing each of them by NMR spectroscopy (to determine the conversion) and HPLC chromatography with chiral column (to determine the enantioselectivity of I and II). For molecular catalyst systems, the Pd/substrate ratio was varied between 1/100 and 1/10,000. For the latter ratio, the initial reaction rate was found comparable to that of the colloidal system (Figure 2a), but interestingly the conversion of the substrate is quasi complete after ca. 100 h in... 

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