Catalysts binuclear

Subsequent research efforts have been focused on discovering more efficient catalytic processes benefiting from cooperative effects between active centers in multinuclear complexes. The idea is to ultimately mimic the advantageous enzyme characteristics. Thus, for instance, Li, Stem, and Marks [303] reported that in their earlier work they demonstrated that the constrained geometry of binuclear catalyst + binuclear co catalyst combination... 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

The above proposed process can be expected to easily put into practice as ammonia is abuandant as the main feedstock for fertilizer. Nevertheless, there is also a problem that Co(NH3)6 is apt to be oxidized to Co(NH3)6 which is unable to form the peroxo binuclear complex and ineffective to O2 solubility enhancement, thus reaction (4) is inhibited. But Co will be relatively stable, and Co may be reduced to Co " by H2O [12]. As a result, a regenration method has also been proposed by using the activated carbon as the catalyst[7], in which Co(NH3)6 dissociation into Co " and NH3 occurs on the activated carbon surface followed by reduction of Co with H2O into Co, O2 and H. ... 

The cyclooctapyrroles shown in Figure 55 appear predestined to form binuclear metal complexes since the loop-shaped conformation of these macrocycles exhibits two structurally identical, helical N4 cavities. Enantiomers of such complexes, which are presumably generally very stable towards racemization owing to the rigidity of the molecule imposed by the incorporation of the metal, are of interest as possible models for binuclear metalloenzymes and as potential catalysts in asymmetric synthesis. The first two ligands as well as their recently obtained palladium complexes601 were... 

Mono and binuclear platinum(II) complexes with diphosphines have been reported as catalysts in the hydroformylation reaction. Dppp and related diphosphines are used as ligands in platinum/Sn systems for the hydroformylation of different substrates.99-107... 

Rhodium compounds have also been used as catalysts since the late 1960s and mechanistic studies date from the 1970s.534,578-582 The binuclear rhodium complex [(Ph3P)4Rh2(//-OH)2] was found to be an effective catalyst for the reductive carbonylation of nitrobenzenes to carbamate esters. Electron-withdrawing groups at the para-position enhance the reactivity of the substrate.583... 

Ally 1-tin compounds are employed as more reactive allylating agents. Because of their high reactivity, less active catalysts (TX species having mild Lewis acidity) or less reactive substrates are often required (Scheme 23).88,89 In addition to carbonyl compounds as substrates, allylation reactions of imines have been also reported.90 Also, a binuclear TiIV Lewis acid has been developed (compound (C) in Scheme 23), which shows higher catalytic activity than the mononuclear analogue (D) because of bidentate coordination to the carbonyl moiety of the substrate.91... 

Binuclear Al111 complexes for MPV reduction have been developed. In the presence of 5mol.% of the bidentate catalyst (Scheme 69, compound (R)), the reduction proceeds smoothly at room... 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

The binuclear precursor (di-,u-chloro-bis-[ /4-2,5-norbomadiene]-rhodium(I)) = [(Rh(NBD)Cl]2 is well suited for the in-situ preparation of a variety of homogeneous hydrogenation catalysts, if tertiary phosphines (here PMe3, PMe2Ph,... 

Lanthanide(III) isopropoxides show higher activities in MPV reductions than Al(OiPr)3, enabling their use in truly catalytic quantities (see Table 20.7 compare entry 2 with entries 3 to 6). Aluminum-catalyzed MPVO reactions can be enhanced by the use of TFA as additive (Table 20.7, entry 11) [87, 88], by utilizing bidentate ligands (Table 20.7, entry 14) [89] or by using binuclear catalysts (Table 20.7, entries 15 and 16) [8, 9]. With bidentate ligands, the aluminum catalyst does not form large clusters as it does in aluminum(III) isopropoxide. This increase in availability per aluminum ion increases the catalytic activity. Lanthanide-catalyzed reactions have been improved by the in-situ preparation of the catalyst the metal is treated with iodide in 2-propanol as the solvent (Table 20.7, entries 17-20) [90]. Lanthanide triflates have also been reported to possess excellent catalytic properties [91]. 

The basic study was performed on copper complexes with N,N,N, N1-tetramethylethane-1,2-diamine (TMED), which were known to be very effective oxidative coupling catalysts (7,12). From our first kinetic studies it appeared that binuclear copper complexes are the active species as in some copper-containing enzymes. By applying the very strongly chelating TMED we were able to isolate crystals of the catalyst and to determine its structure by X-ray diffraction (13). Figure 1 shows this structure for the TMED complex of basic copper chloride Cu(0H)Cl prepared from CuCl by oxidation in moist pyridine. 

There are at least three reasons for attempting to prepare solid-phase catalysts that resemble enzymes. Synthetic procedures would generally be simplified. Catalytic groups are fixed on the support so that they cannot interact with one another, for example, thiols cannot deactivate by forming disulfides and metal ions cannot deactivate by forming binuclear structures. Finally, if the successful catalyst is eventually made, it will almost certainly be used in heterogeneous systems. 

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