Mononuclear catalysts

Cai et al. (71) examined the use of dinuclear copper complexes as catalysts in the cyclopropanation reaction. Their ligand design, based on the success exhibited by the Aratani system, incorporates a diimine aryloxide. A comparison of the mononuclear catalyst 99 with the corresponding dinuclear catalyst 100 showed certain modest benefits conferred by the latter, Eq. 52. The authors note that these catalysts are effective at ambient temperature but isolated yields are higher at 50°C with no loss in enantioselectivity. 

There is one further caveat. The above arguments have been developed in terms of a mononuclear catalyst. At the very least some entropy corrections will be needed for comparison with bi-nuclear catalysts. [Note These considerations were developed during postdoctoral work at Stanford University with Professor Henry Taube.]... 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

The political justification for transition metal cluster chemistry is the assumption that clusters are models in which metallic properties may be more easily studied than in the metals themselves. These properties include electronic phenomena such as color and conductivities as well as surface phenomena, such as atom arrangements and catalytic activities. Thus, there are two main lines of cluster research. The more academic line leads to the search for new types of clusters and their structure and bonding, whereas the more practical line leads to the investigation of reactivities with the hope that clusters may open catalytic pathways that neither plain metals nor mononuclear catalysts can provide. The interdependence of both lines is obvious. 

In an oversimplification, we can identify the following two ways in which the nature of heterogeneous catalytic reactions may differ from homogeneous catalytic reactions with mononuclear catalysts. 

The ruthenium-based metallodendrimer Go-8 was applied as catalyst in a ring-closure metathesis reaction. The activity per metal center of the dendritic catalysts was found to be comparable to that of the corresponding mononuclear catalyst. Unfortunately, the metathesis reaction conditions were not compatible with the nanofiltration membrane set-up used, since a black precipitate was formed in the vessel containing the catalyst. It was found that the conversion to diethyl-3-cyclopentene dicarboxylate product stopped... 

The first example of asymmetric rhodium-catalyzed hydrogenation of prochi-ral olefins in dendrimer catalysis was reported by Togni et al., who immobilized the chiral ferrocenyl diphosphine Josiphos at the end groups of dendrimers, thus obtaining systems of up to 24 chiral metal centres in the periphery (Fig. 2) [12-14]. The fact that the catalytic properties of the dendrimer catalysts were almost identical to those of the mononuclear catalysts was interpreted as a manifestation of the independence of the individual catalytic sites in the macromolecular systems. 

The mononuclear catalyst [(Boc-Pyrphos)PdCl2], which is very unselective for this transformation [9% enantiomeric excess (ee)], provided the point of reference for the subsequent studies with the dendrimer catalysts. This system and the metalladendrimers PPI(PyrphosPdCl2)4 -PPI(PyrphosPdCl2)64... 

Fig. 8. Energetics of dinitrogen reduction with the mononuclear catalyst 2 (solid lines). The energy D0 of N2 bound to 2 and three unbound H2 molecules has been chosen as the zero-energy reference point AD0 = AScp, B3LYP + AZPE. Note that the non-reacting H2 molecules (e.g., two H2 molecules at the N2 + H2 step) are not mentioned explicitly. The energetics of dinitrogen reduction without the catalyst are depicted for comparison (dotted lines). Here, the D0 energy of N2 and three H2 has been chosen as the energy reference point.

Cationic propagation involving mononuclear catalyst species is illustrated schematically as follows [1] ... 

The possibility of changing the polymerisation mechanism depending on the kind of monomer as well as the catalyst, especially in binary or ternary comonomer systems, is obvious. This may also concern change in the nature of the growing species throughout the propagation of one polymer chain in the presence of both multinuclear catalysts [scheme (33)] and mononuclear catalysts [scheme (34)] ... 

Multinuclear metal complexes that may act as active catalysts or off-cycle species can also be easily identified and studied via ESl-MS. For example, analysis of a simple Pd-catalyzed allylic substitution reaction lead to the discovery of two reversibly formed binuclear bridged palladium complexes (Fig. 6) that act as a reservoir for the active mononuclear catalyst [21], The observation of dimers when using ESl-MS is common and it is crucial to confirm that they truly exist in solution and are not just formed during the ESI process, in this case the detection was supported by P and H NMR studies of stoichiometric reaction mixtures and in situ XAFS experiments [49]. 

Van Koten and coworkers have described the first example of a dendrimer employed as a catalyst snpport (Scheme 10). This system is based npon diaminoarylnickel(II) complexes attached to the dendrimer surface and catalyze the Kharasch-type additions of polyhalogenoalkanes to aUcenes. There is a clear advantage with regard to catalyst recovery, althongh in comparison to the analogous mononuclear catalyst the activity of the supported material was reduced. ... 

Use. The oligomerization of olefins has generally been carried out with zero-valent transition metal complexes (mononuclear catalysts) and usually leads to an array of dienes see 1, 259). Schrauzer et at.1 of the Shell Development Co. reasoned that a dinuclear catalyst such as ZnfCo(CO).,J2 in which the two cobalt centers are connected close to each other will lead to new transition state formation from which different products can form. As a model, they examined the dimerization of nor-hornadiene and with the new catalyst obtained in almost quantitative yield a single dimer, m.p. 65-65.6°, shown unequivocably by elemental analysis (C14H16), infrared, nuclear magnetic resonance, and mass spectrometry to have the structure (2). 

We explored the hydroformylation reaction for monometal model complexes that represent one half of the bimetal catalysts 1 or 5. These tests give us an idea about whether each metal centre is functioning as a conventional mononuclear catalyst or whether there is some cooperativity. Thus the catalytic activity and selectivity of the complexes [RuCl(j/ -Ph2PCH2PPh2)Cp], [RuCl -HC(PPh2)3)Cp] and [ RhCl(CO)2 2] were studied under the same conditions as described above. 

For 1, the same products were observed (Table 1). But TBHP was completely consumed after 2 hours, and the catalyst became inactive. NMR monitoring of the reaction indicated that the mononuclear catalyst was converted to [Fe20(TPA)2Cl2] , which could be independently synthesized and found to be incapable of catalyzing these reactions. In addition, an equivalent of chlorocyclohexane was observed as product. 

Table 2. Comparison of the Selectivities of Various Mononuclear Catalysts for the Halogenation of Alkanes.

Results, using cluster complexes of nickel or iridium, support the proposal that hydrogenation of triple-bonded substrates can be more readily achieved by the use of polynuclear homogeneous catalysts than by use of mononuclear catalysts.The requirement for multinuclear centers to heterogeneously catalyze the hydrogenation of CO has also been proposed. Consequently, complexes of transition metals in which the metal atoms are constrained to be proximate are potentially of interest as catalysts. 

The mononuclear catalyst [Ru(CO)3(dppe)] is of lower activity than monophosphine ruthenium complexes, but of higher selectivity. Platinum complexes promoted with SnCl2 are also of low activity if the phosphine ligand is dppe, but are much more active with dppb, or a related 6/5(phosphinomethyl)cycloalkane ligand, which are known to form bridged structures. ... 

Despite the vast number of outstanding examples of enzymatic catalysis that rely on the collaboration of two or more vicinal metal centers hitherto disclosed, the design and development of efficient binuclear organometallic complexes able to enhance the performance of mononuclear catalyst by means of an intermetallic cooperative process remains widely unexplored [22-24]. In fact, the formation of bi- or polynuclear complexes has been often described as a catalyst deactivation pathway [25-30]. However, the availability of more electron density at the active site, extra coordination positions, and the possibility to develop more preorganized systems that allow for (enantio)selective reactions shows great promise for an improved catalytic performance [9]. 

Iodorhodium(III)mesotetraphenylporphyrin complex 3 was the first reported mononuclear catalyst for cyclopropanation (28). With EDA, this catalyst gave cw-cyclopropane as the dominant product in the cyclopropanation of styrene and bicyclo[2.2.1]hept-2-ene, although catalytic activity as shown by yields of the cyclopropanes and dimerization products was only modest. The oxidation state of the catalytic active form was proposed to be Rh(II). In another study, a perpendicular approach of alkene it system to rhodium-carbene has been proposed to explain the cis-selective cyclopropanation (29). 

Especially interesting are oxidative additions of unsaturated hydrocarbons because they are the basis of many processes as hydrogenations and isomeriz-ations which can be catalyzed by metal clusters with similar and often better results than those obtained with mononuclear catalysts (see Sect. 2.6). 

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