Oxidative activation oxophilicity

The catalytic process is also achieved in the Pd(0)/Cr(II)-mediated coupling of organic halides with aldehydes (Scheme 33) [74], Oxidative addition of a vinyl or aryl halide to a Pd(0) species, followed by transmetallation with a chromium salt and subsequent addition of the resulting organo chromate to an aldehyde, leads to the alcohol 54. The presence of an oxophile [Li(I) salts or MesSiCl] allows the cleavage of the Cr(III) - 0 bond to liberate Cr(III), which is reduced to active Cr(II) on the electrode surface. 

Rhenium is one of the oxophilic atoms effective for oxidation reactions. ReOx species are likely to have chemical interaction with various oxide supports and exhibit unique catalytic properties that cannot be observed on monomeric rhenium oxides. A new active six-membered octahedral Re cluster in zeolite pores (H-ZSM-5 [HZ]) is produced from inactive [Re04] monomers in situ under selective propene oxidation to acrolein (C3H6+02 - CH2=CHCH0+H20) in the presence of ammonia that is not involved in the reaction equation [16], The cluster is transformed back to the original inactive monomer in the absence ammonia. Note that coexistence of spectator NH3 is indispensable for the selective oxidation. 

In conclusion, the computational study of ternary Pt-Ru-X alloys suggests that future strategies toward more active electrocatalysts for the oxidation of methanol should be based on a modification of the CO adsorption energy of Pt (ligand effect), rather than on the enhancement of the oxophilic properties of alloy components (enhanced bifunctional effect). 

The starting material bis(pinacolato)diboron is a poor Lewis acid and 1 B-NMR of KOAc and B2bin2 in DMSO-d6 shows no evidence of the coordination of the acetoxy anion to a boron atom leading to a tetrahedral activated species. However, the formation of an (acetato)palladium(II) complex after the oxidative addition of the halide influences the reaction rate of the transmetalation step. The Pd-O bond, which consists of a hard Lewis base with a soft Lewis acid, is more reactive than a Pd-X (X=Br, I) bond. In addition, the high oxophilicity of boron has to be considered as a driving force for the transmetalation step, which involves an acetato ligand. 

Asymmetric cyanosilylation of ketones and aldehydes is important because the cyanohydrin product can be easily converted into optically active aminoalcohols by reduction. Moberg, Haswell and coworkers reported on a microflow version of the catalytic cyanosilylation of aldehydes using Pybox [5]/lanthanoid triflates as the catalyst for chiral induction. A T-shaped borosilicate microreactor with channel dimensions of 100 pm X 50 pm was used in this study [6]. Electroosmotic flow (EOF) was employed to pump an acetonitrile solution of phenyl-Pybox, LnCl3 and benzal-dehyde (reservoir A) and an acetonitrile solution of TMSCN (reservoir B). LuC13-catalyzed microflow reactions gave similar enantioselectivity to that observed in analogous batch reactions. However, lower enantioselectivity was observed for the YbCl3-catalyzed microflow reactions than that observed for the batch reaction (Scheme 4.5). It is possible that the oxophilic Yb binds to the silicon oxide surface of the channels. 

Titanium. The high reducing ability and the pronounced oxophilicity of early transition metals in low oxidation states act jointly as a formidable driving force in many transformations. However, such processes are usually hampered by the fact that the metal oxides or alkoxides formed as the inorganic by-products usually resist attempted re-reductions to the active species and thus render catalysis a difficult task. 

Pt-based electrocatalysts are usually employed in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMSC). In direct-methanol fuel cells (DMFCs), aqueous methanol is electro-oxidized to produce COj and electrical current. To achieve enhanced DMFC performance, it is important to develop electrocatalysts with higher activity for methanol oxidation. Pt-based catalysts are currently favored for methanol electro-oxidation. In particular, Pt-Ru catalysts, which gave the best results, seem to be very promising catalysts for this application. Indeed, since Pt activates the C-H bounds of methanol (producing a Pt-CO and other surface species which induces platinum poisoning), an oxophilic metal, such as Ru, associated to platinum activates water to accelerate oxidation of surface-adsorbed CO to... 

The oxophilicity difference between Ru and Os may also account for the observed better performance of Ru/Pt(lll) for methanol oxidation at potentials below 0.21V71,72 However, the Os/Pt(lll) system is more active for methanol oxidation than Ru/Pt(l 11) at higher potentials.19... 

In terms of the catalytic aspects of this review, we have found that the oxidation state of the deposited nanoislands, as precisely determined by separate XPS measurements and observed by in situ STM imaging, contributed significantly to the overall activity of examined bimetallic surfaces. For instance, it was found that Ru nanoislands showed oxide formation at lower potentials than Os nanoislands. Therefore, we conclude that the oxophilicity difference between Ru and Os may account for the observed better performance... 

The conversion never reaches 100% whatever the reaction time, suggesting deactivation of the catalyst. Indeed, after addition of the epoxide, the intensity of signal B decreases with time. Similarly, the initial red-orange colour fades with time and vanishes at the end of substrate conversion. This colour may be attributed to the active complex. The catalytic site proposed above involves an oxophilic Ni centre. Deactivation might arise from oxidation of this active complex and simultaneous epoxide deoxygenation yielding the small amount of allylic tertiary alcohol la observed (Table 3). 

For Class B (substitution labile) metal complexes, reequilibration to more thermodynamically favorable coordination modes will be very rapid relative to immobilization. Such behavior is typical of first-row TM complexes. In addition, these ions are usually very oxophilic, so the metal complexes are typically subject to ICC interactions with oxide materials. Since these metal ions are generally immobilized under conditions of thermodynamic control, all pertinent speciation equilibria, including ICC reactions (Section III.B), must be considered in order to understand or predict the outcome of immobilization reactions. It is essential to understand the relevant equilibria if direct imprinting of active site structures is to be successful. The studies of Klonkowski et al. (210-213), for example, underscore this point Sol-gel immobilization of copper complexes bearing silylated amine and ethylenediamine ligands were shown by EPR to result in multiple copper environments, suggesting competition between immobilization and ICC reactions. 

Most of the olefin complexes examined in this study exhibit an unspectacular reactivity towards molecular oxygen, i. e. either ligand exchange reactions, 0-0-bond activation by highly oxophilic metals Sc, Ti, and V, or even complete absence of any reaction are observed (eg. even Cu(C2H4) is unreactive). However, in the case of the iron complexes extensive oxidation reactions are observed. Indeed, not only olefins attached to an iron cation react effectively with molecular oxygen, even stable molecules like benzene and acetone are rapidly oxidized in the presence of Fe+. 

An efficient ethanol electrooxidation catalyst should combine at least two features (i) high tolerance to CO and other intermediate species generated over the surface of the electrocatalyst during alcohol electrooxidation and (ii) ability to break the C-C bond of the ethanol molecule under mild conditions. The most relevant features for the designing of CO tolerant electrocatalysts have been described above namely, Pt modification with more oxophilic metals such as Ru, Mo or Sn renders the best electrocatalysts. This is because such oxophilic atoms promote the formation of -OfT. species (involved in the CO j oxidation reaction) at potentials that are more negative than that on pure Pt (Eq. 9.17). Among those, Sn-modified Pt electrocatalysts are the most active formulations. There is also widespread consensus that the PtsSn phase is the most active one in the CO reaction and early stages of the ethanol electrooxidation process. ... 

While CO is an intermediate to CO2, it readily builds up on the surface and poisons most of the active metals such as Pt. As such, Pt is typically alloyed with a more oxophilic metal such as Ru to promote the adsorption and dissociation of water thus creating bifunctional sites on the surface. The OH groups that result interrupt the CO adlayer and readily oxidize CO to CO2. The addition of Ru also helps to weaken the Pt-CO bond thus enhancing CO desorption [106]. 

The active nitrene intermediate should insert into a saturated C-H bond [8], excluding also that reaction 27 proceeds via the corresponding N-(2-aminobenzoyl)amidc. Nucleophilic nitrene complexes are known to react with the carbonyl group of aldehydes and ketones to yield the corresponding imines and a metal-oxo complex [91], However, the driving force of this reaction is the oxophilicity of early transition metals used in [91], whereas the catalyst used in this work are derivatives of the late-transition metals. Oxo derivatives of these metals in the low oxidation state are ver> rare. The mechanism followed by this reaction requires further investigation in order to be clarified. 

PtRu, the current choice for HOR catalysts, exhibits excellent CO tolerance, which could be ascribed to a decrease in CO binding energy on R due to electronic substrate effects and to the oxidation of chemisorbed carbon monoxide being catalyzed at low potentials by the activation of H2O [82], Ternary Pt-based catalysts have also been investigated in which a third oxophilic component such as Sn, Ir, Rh, Os, Mo, W, WO3, or Re is added to promote CO oxidation at lower potentials [83]. 

To overcome CO deactivation, alloys of Pt with more oxophilic elements have been investigated as methanol electrooxidation catalysts. PtRu bifunctional catalysts are presently the most active for methanol oxidation. It is believed that Ru serves the role of removing COads as CO2 [93] ... 

In alkaline media, Ni-based supports were also explored in conjunetion with PtRu and PtRuMo electrocatalysts [200-202]. Pt Ru compositions between 1.1 1 and 2.1 1 atomic ratio supported on Ni were found to yield the lowest faradaic resistances for the oxidation of 1 M ethanol in 1 M NaOH, determined by eleetrochemical impedance spectroscopy [200]. It was speculated that the role of Ni support extends beyond purely mechanical passive interaction with the catalyst, and Ni could contribute to the electrocatalytic activity by its surface and electronic properties as an oxophilic element. Further studies are required in this area. 

The addition of Ni as the second or third element is claimed to increase the activity of Ft electrocatalysts. The main advantage of the introduction of this metal is the reduction of the ethanol oxidation potential, coupled with the rise in current density. A literature search reveals that Sn and Ni can introduce electronic modifications into Ft [50,51], in other words, these metals decrease the energy of the chemisorption of ethanol and its oxidation intermediates, such as CO. The participation of the Sn site in a bifunctional mechanism is also claimed, in order to explain the enhanced activity of the Ft-Sn/C catalyst [50,52]. The effect that Ni introduction has on Sn-sites must also be considered. The addition of a third metal boosts the oxophilic character of the surface, thus raising the... 

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