Catalytic system mononuclear

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

Limitations. At the moment TAMREAC is limited to mononuclear catalytic systems. It does not take into account stereochemistry, and the molecules or complexes are represented in only two dimensions. The number of nonhydrogen atoms is limited to 30 (see also Section V). 

The second class of catalysts are zinc(II) mono- or dialkoxides obtained from polyhydric phenols and dialkylzinc with partly polymeric stmctures. This system, extensively studied by Kuran [84], is an optimization of the water/diethylzinc and polyphenol/diethylzinc systems developed by Inoue [85]. The use of soluble zinc phenoxides and their analogous cadmium complexes as catalyst for the copolymerization of CO2 and epoxide was studied extensively by the Darensbourg group [86]. This work focused on the use of mononuclear phenoxide derivatives with bulky substituents, e. g., phenyl- and fe/t-butyl groups, on the aromatic ring to a homogeneous catalytic system and thus enhance the activity of the Zn phenoxides. The catalysts developed are stabilized through ancillary neutral... 

But Wilke, Bogdanovic and co-workers have shown that, in the case of the catalytic system r-allyl-nickelhalide/aluminum alkyl, the addition of bulky phosphines such as tricyclohexylphosphine, triisopropylphosphine or di(t-butyl)ethylphosphine, directs the dimerization reaction to 2,3-dimethylbutene ab route), with a selectivity of 60—80%, An X-ray structural analysis of the active species has revealed a mononuclear, approximately square-planar nickel complex with a PR3, and a... 

Figure 2 shows that the catalytic system PE-gr-PAAA Co(II)-Et2AlCl is more active than its homogeneous analog based on the chelate of Co(II) with N-acetylbenzamide. It is noteworthy that the immobilized mononuclear chelates of Co(II) with PE-gr-PAVK and PE-gr-PAAA have a comparable activity. This indicates that the nature of the chelate unit (N,0- and 0,0-, respectively) has inappreciable influence on the activity of the complexes in the polymerization of butadiene. At the same time, the binuclear Co(II) chelate with PE-gr-PSAA exhibits a lower (by a factor of 5-6) activity than its mononuclear analogs. 

Finally, the dendrimer or star may be surface or internally functionalized or decorated with metal-containing motifs. This type of dendrimer is widely investigated with a view to preparing novel catalytic systems in which a dendritic effect results in enhanced or modified catalytic properties with respect to a similar number of mononuclear complexes. As many of these species are strictly organometaUic they do not fall properly within the scope of this volume. 

With regard to fast time scales, they are often treated as equilibrium processes in the physical sciences. In organometallic chemistry, equilibria are omnipresent, for example, between axial and equatorial geometric isomers of a monosubstituted mononuclear trigonal-bipyramidal complex. For homogeneous catalytic systems, equilibria between intermediates on the cycle and reservoirs have been experimentally verified. Also, equilibria between catalyst precursors and intermediates and between catalyst precursors and nonactive complexes have been observed (23). 

Using and P NMR in the model [Pd]/Ph2S2/(OtPr)3P catalytic system trans-and ds-mononuclear and dinudear metal complexes were detected. The equilibrium constant for the dissociation of the dinudear metal complexes was measured, K= 0.066 0.007 M at 30 °C [119]. Although the equilibrium is shifted towards dinudear complexes, in excess of the (OiPr)3P ligand the mononudear trans-[Pd (SPh)2((OiPr)3P)2] complex has been isolated and characterized by X-ray analysis (Figure 3.1). 

Nuclearity has a strong impact on reactivity, as evidenced by the different reactivity of mono versus binuclear copper oxidation enzymes (aromatic vs aliphatic oxidation) or the specific roles of the different Zn centers in alkaline phosphatase. Some nuclearities (e.g., binuclear metal complexes) correspond to thermodynamic sinks. In that case, preorganization is not required. Less stable nuclearities require a platform in order to be maintained during the catalytic process. Mononuclear systems correspond to this situation. In particular, monocopper centers need to be embedded to be protected. The funnel complex strategy was developed for that purpose. Porphyrin complexes can be protected from deactivation pathways by grafting them... 

Cluster complexes are being studied in the hope that they might provide novel catalytic systems. In a mononuclear complex potential coordination sites must... 

In 1984, one of us first reported on the use of a Ru3(CO)i2-based catalytic system for the carbonylation of nitroarenes to carbamates [170]. This report was followed by several others, mostly from our and Grate s groups [171-176]. Since the catalytic systems are very similar, they are discussed together. Note that, although Ru3(CO)i2 is always used as a catalyst precursor, it is known that, under the reaction conditions, it is in equilibrium with the mononuclear Ru(CO)5 and it has become now clear that it is this last compound which is mainly responsible for catalytic activity [177]. However, Ru(CO)s is an unstable volatile liquid, which tends to aggregate to Ru3(CO)i2 within a few hours at room temperature and is thus an unsuitable starting catalyst for most applications. Anyway, in the only experiments we performed by using Ru(CO)s as catalyst, we obtained results essentially identical to those obtained under the same conditions... 

Catalytic systems based on rhodium clusters or [Rh(CO)4] for the carbonylation of nitro compounds to carbamates have been described in Chapter 3. The application of the same or similar systems to the reduction of nitro compounds to anilines have been described in Chapter 4. Initial work has shown that rhodium clusters Rh4(CO)i2 and Rh<5(CO)i6, but even several mononuclear compounds such as Rh(CO)2(acac) (acac = acetylacetonate), are active catalyst precursor for the carbonylation of nitrobenzene to carbamates, when promoted by an heterocyclic nitrogen base [56, 140, 187, 188]. Later, Liu and Cheng and us independently reported that even higher catalytic activities could be obtained by the use of preformed [PPN][Rh(CO)4] (PPN = (PPh3)2N ) [189, 190]. We have also conducted a mechanistic study of the catalytic cycle using this last complex [1, 192] and the initial stage of the reaction has also been reinvestigated by Liu et al. [193]. Since no mechanistic study has been yet reported on the cluster-based systems, we will first discuss the [PPN][Rh(CO)4] system and then draw a comparison with the other systems. 

In addition to molybdic acid, there have been reports of several other catalytic systems that cause mechanistically relative transformations of aldoses involving a 1,2-shift of their carbon skeleton. They include the nickel(II)-ethylenediamine [43] and cobalt(II)-ethylenediamine complexes [44] as well as the calciiun(II), strontium(II),lanthanum(III) and neodymium(III) cations in both aqueous and alcoholic alkaline solutions [45,46]. Unlike the BiTik reaction, all these transformations exploit rather equimolar amounts of the catalysts so that they result in thermodynamic equilibria of the catalytic complexes with the epimeric aldoses. Moreover, due to steric restrictions, the aforementioned alkaline earth and rare earth cations, which form mononuclear tetradentate complexes with aldoses. 

Several catalytic systems use mononuclear or cluster carbonyls in the presence of a base such as KOH or NEta. The solvent is usually aqueous methanol or ethoxyethanol since pure water gives low activity. Temperatures are in the... 

The bridges of 5.70 are easily cleaved by the substrate to give mononuclear catalytic intermediates. For hydroamidation reactions, Ru-based precatalysts are generally required. An effective catalytic system for such reactions is 5.16 (see Section 5.1.2), in combination with phosphines and substituted pyridines. 

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