Precursor activated complex

These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

In the skeleton of many chelating diphosphines, the phosphorus atoms bear two aryl substituents, not least because the traditional route to this class of compounds involves the nucleophilic substitution with alkali metal diarylphosphides of enantiopure ditosylates derived from optically active natural precursors, approach which is inapplicable to the preparation of P-alkylated analogs. The correct orientation of these aryl substituents in the coordination sphere has been identified as a stereo chemically important feature contributing to the recognition ability of the metal complex [11,18-20]. 

One key aspect of SOMC is the determination of the structure of surface complexes at a molecular level one of the reasons being that our goal is to assess structure-activity relationships in heterogeneous catalysis, which requires a firm characterization of active sites or more exactly active site precursors. While elemental analysis is an essential first step to understand how the organometallic complex reacts with the support, it is necessary to gather spectroscopic data in order to understand what are the ligands and... 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

The efficiency of the more readily accessible Cp 2Sm and Cp 2Sm(thf)2 complexes was also recognized [128, 129[. Further studies indicate that active catalyst precursors include organolanthanide complexes of the general formula Cp 2Ln-R (Ln = La, Nd, Sm, Yh, Lu) with R = H, T),- C5H5, CH(TMS)2, N(TMS)2 [131[. Generation of the catalytically active species is believed to occur via protonolysis of the Ln-R bond by the amine (Eq. 4.19). 

The most active catalyst precursors are the monocyclopentadienyl complexes [Me2Si(C5Me4)(t-BuN)]LnN(TMS)2 [132]. Comparative reactivities for the IH of H2NCH2C(Me)2CH2CH=CH2 to give 2,4,4-trimethylpyrrolidine are given in Table 4-1. 

Yoshida and Otsuka found that platinum(O) complexes [PtLj] (26) (a L = PEtj b L = P Prj) and rhodium hydrido complexes such as [RhHLj] (L = P Prs 33, PEts), [RhiHidr-NJlPCyslJ (34), tram-[RhH(N2)(PPh Bu2)J, and [RhH(P Bu3)J, all of which carry electron-donating alkylphosphine ligands, can catalyze the water gas shift reaction under fairly mild conditions (100-150°C CO 20 kg/cm ) (Eq. 6.32) [23, 60]. Among these complexes, [RhH(P Pr3)3] (33) was the most active catalyst precursor. Several complexes were isolated from or detected in the reaction mixture... 

Crabtree s catalyst is an efficient catalyst precursor for the selective hydrogenation of olefin resident within nitrile butadiene rubber (NBR). Its activity is favorably comparable to those of other catalyst systems used for this process. Under the conditions studied the process is essentially first order with respect to [Ir] and hydrogen pressure, implying that the active complex is mononuclear. Nitrile reduces the catalyst activity, by coordination to the metal center. At higher reaction pressures a tendency towards zero order behavior with respect to catalyst concentration was noted. This indicated the likelihood of further complexity in the system which can lead to possible formation of a multinuclear complex that causes loss of catalyst activity. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

To make the ideas sharper consider the case of two quasi degenerate quantum states of the active precursor and successor complexes. The discussion made around equation (57) holds true here too. The activated complex will be the place of a coherent electro-nuclear fluctuation that will go on forever, unless there are quantum states belonging to the relaxation channels of Hc(i) and Hc(j). Note that the mechanisms of excitation to get into the quantum activated complex and those required to relax therefrom are related to the actual rate, while the mechanism of interconversion is closely connected with an... 

We reach the same conclusion (Eq. 5.8a) if we treat the reaction sequence according to the activated complex theory (ACT), often also called the transition state theory. The particular surface species that has formed from the interaction of H+, OH, or ligands with surface sites is the precursor of the activated complex (Fig. 

Activated complex theory for the surface-controlled dissolution of a mineral far from equilibrium. A is the precursor, i.e., a surface site that can be activated to A. The latter is in equilibrium with the precursor. The activation energy for the conversion of the precursor into the product is given by AG. ... 

The surface concentration of the particular surface species, Cj, corresponds to the concentration of the precursor of the activated complex. Note that we use braces ( > and brackets [ ] to indicate surface concentrations [mol nr2] and solute concentrations [M], respectively. 

However, we have to reflect on one of our model assumptions (Table 5.1). It is certainly not justified to assume a completely uniform oxide surface. The dissolution is favored at a few localized (active) sites where the reactions have lower activation energy. The overall reaction rate is the sum of the rates of the various types of sites. The reactions occurring at differently active sites are parallel reaction steps occurring at different rates (Table 5.1). In parallel reactions the fast reaction is rate determining. We can assume that the ratio (mol fraction, %a) of active sites to total (active plus less active) sites remains constant during the dissolution that is the active sites are continuously regenerated after AI(III) detachment and thus steady state conditions are maintained, i.e., a mean field rate law can generalize the dissolution rate. The reaction constant k in Eq. (5.9) includes %a, which is a function of the particular material used (see remark 4 in Table 5.1). In the activated complex theory the surface complex is the precursor of the activated complex (Fig. 5.4) and is in local equilibrium with it. The detachment corresponds to the desorption of the activated surface complex. 

In MeOH, Pd - H+ species are unstable and have the tendency to deproto-nate with reduction to less active dimeric Pd(I) and Pd(0) complexes, which may lead to degeneration of the catalyst with formation of inactive palladium metal and free ligands, which in turn may give less active bis-chelate complexes [Pd(P-P)2]2+ [55,61]. Possible deactivation paths have been delineated in [17]. In order to maintain or improve the catalytic activity, the precursor is used together with an oxidant and an excess of acid (usually BQ/Pd = 100 - 200 and acid/Pd = 10 - 20) [15,47]. 

Figure 1. Potential energy plot of the reactants (precursor complex) and products (successor complex) as a function of nuclear configuration Eth is the barrier for the thermal electron transfer, Eop is the energy for the light-induced electron transfer, and 2HAB is equal to the splitting at the intersection of the surfaces, where HAB is the electronic coupling matrix element. Note that HAB << Eth in the classical model. The circles indicate the relative nuclear configurations of the two reactants of charges +2 and +5 in the precursor complex, optically excited precursor complex, activated complex, and successor complex.

Cai Y, Jin J, Tomomori-Sato C, Sato S, Sorokina 1, Parmely TJ, Conaway RC, Conaway JW (2003) Identification of new subunits of the multiprotein maimnalian TRRAP/TIP60-containing histone acetyltransferase complex. J Biol Chem 278 42733 2736 Cao X, Sudhof TC (2001) A transcriptionally [correction of transcriptively] active complex of APP with Fe65 and histone acetyltransferase Tip60. Science 293 115-120 Cao X, Sudhof TC (2004) Dissection of amyloid-beta precursor protein-dependent transcriptional transactivation. J Biol Chem 279 24601-24611... 

During the same period, Nolan showed also that indenylidene complexes are active catalyst precursors in the RCM of dienes. The reactions were performed on the NMR scale and moderate to good yields were obtained for diethyl diallylmalonate. 

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