Catalysts multinuclear

Porphyrinic co-complexes as novel multinuclear catalysts for the reduction of dioxygen directly to water 97ACR437. 

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 occurrence of multinuclear catalysts in hydrogenations catalyzed by rhodium-DIOP systems seems unlikely, although the trans-RhCl(CO)(DIOP) complex 43 is dimeric (276), and in basic methanolic solution the 1 1 diphos complex exists as Rh3(diphos)3(OMe)2 + (138a, Section II, B, 1). 

As a matter of fact, it was proven, by variable temperature multinuclear NMR studies, that in the dtbpx-based catalyst, which is the most efficient in producing MP, the ligand maintains cz s-coordination in all the identified intermediates well above room temperature [42,114,115] (see below). 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Titanium alkoxides are also effective and sought-after initiators for the ROP of lactides due to a low toxicity, which minimizes the problems linked to the presence of catalyst residues in commercial PLA products [18, 19]. Despite impressive advancements in the use of Lewis acidic metal initiators in the preparation of PLAs, surprisingly little attention has been paid to the group 4 metal (Ti, Zr, Hf) initiators, probably due to the highly oxophilic nature of M(1V) which has a natural tendency to form aUcoxy-bridged multinuclear complexes. Verkade and coworkers previously demonstrated a series of titanium aUcoxide complexes 118-122 (Fig. 17) that function as moderately efficient initiators in bulk homopolymeization of L-lactide and rac-lactide, some of these initiators displaying a well-controlled polymerization behavior [119]. 

A novel macrocyclic multinuclear acetyUde complex was prepared from o-diethyn -benzene with equimolar [PdCWPEtsli] in the presence of CuCl catalyst in EtzNH at 25 °C gave 21 in 33% yield<99AGIE174>. 

Contrary to previous reports suggesting colloidal metal as the active species in Pt-catalyzed hydrosilylations, the catalyst was found to be a monomeric platinum compound with silicon and carbon in the first coordination sphere.615 The platinum end product at excess olefin concentration contains only platinum-carbon bonds, whereas at high hydrosilane concentration, it is multinuclear and also contains platinum-silicon bonds. An explanation of the oxygen effect in hydrosilylation was also given to show that oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. 

Our knowledge concerning soluble metal complexes with sulfide ions as ligands has increased considerably during the last two decades and this kind of Compound is still of topical interest. Some of the reasons for this are the development of a very flexible and fascinating structural chemistry of multinuclear metal-sulfur complexes, the fact that the active sites of some electron transfer proteins contain metal ions and labile sulfur,41,42 and also the relation of metal-sulfur cluster compounds to some heterogeneous catalysts. In addition, apart from the numerous binary and ternary sulfides which occur in nature, we have at our disposal a rich solid state chemistry of metal sulfides, which has been reviewed elsewhere and will be excluded here.43"17... 

A multinuclear Zn-binaphthoxide complex prepared from Et2Zn and the linked-BINOL 28 has recently been developed as an effective catalyst for the Michael reaction (Scheme 13) [15], The a-hydroxy acetophenone derivative 29 was a suitable substrate, reacting with the Zn complex to afford the configurationally stable Zn enolate. Reaction with a variety of enones proceeded smoothly and... 

Most coordination catalysts have been reported to be formed in binary or ternary component systems consisting of an alkylmetal compound and a protic compound. Catalysts formed in such systems contain associated multinuclear species with a metal (Mt)-heteroatom (X) active bond ( >Mt X Mt—X > or — Mt—X—Mt—X— Mt = Al, Zn, Cd and X = 0, S, N most frequently) or non-associated mononuclear species with an Mt X active bond (Mt = Al, Zn and X = C1, O, S most frequently). Metal alkyls, such as triethylaluminium, diethylzinc and diethylcadmium, without pretreatment with protic compounds, have also been reported as coordination polymerisation catalysts. In such a case, the metal heteroatom bond active in the propagation step is formed by the reaction of the metal-carbon bond with the coordinating monomer. Some coordination catalysts, such as those with metal alkoxide or phenoxide moieties, can be prepared in other ways, without using metal alkyls. There are also catalysts consisting of a metal alkoxide or related compound and a Lewis acid [1]. 

The formation of epoxide polymers with a very high molecular weight by the discussed catalysts containing associated multinuclear species (— Zn-0— Zn O >) indicates that only a small fraction of the metal species in the catalyst is effective for the polymerisation. The broad molecular weight distribution of polymers yielded by these catalysts corresponds to the existence of various active sites [30]. 

Taking into account the associated structures of catalysts for the coordination polymerisation of epoxides and considering that the coordinated epoxide is attacked by the nucleophile from the back side [scheme (1)], the catalyst must engage its two metal atoms in order to make the polymerisation possible. In fact, zinc-based catalysts containing associated multinuclear species (—>Zn-O-Zn-O—>), including those with condensed zinc atoms ( Zn O Zn O ), are all characterised by the appearance of active sites with two zinc atoms bridged via a nucleophilic oxygen atom in which the O atom, bound covalently to the octahedral zinc atom, is coordinated to the adjacent tetrahedral Zn atom (Zn O -Zn). 

Characteristic of these models of active sites is the appearance of the OZn O Zn(0)Et species. Considering the structural properties of the discussed catalysts as well as the polymer chain microstructure and the structure of the end groups of poly(propylene oxide) obtained with the PhOZnOCeHi ( -Bu)OZnEt catalyst, a concerted mechanism of epoxide ring-opening polymerisation with catalysts containing multinuclear species, including those with condensed zinc atoms, has been postulated [65,74] ... 

In connection with the discussed mechanism of epoxide polymerisation in the presence of associated multinuclear catalysts [65], it is worth noting the flip-flap mechanism proposed by Vandenberg to operate in epoxide coordination polymerisation [18,60,61] as the earliest explanatory attempt involving the formation of an intermediate that could accommodate the essentially linear, three-centred transition state necessary for inversion of the configuration at the epoxide ring carbon atom where cleaved. The flip-flap mechanism assumed... 

The mechanism of epoxide polymerisation with Zn-, Al- or other metal-based coordination catalysts containing multinuclear species can be presented schematically as in scheme (4a) (the structures and charge distribution are simplified in the scheme, without differentiation of + or and <5+ or 6 charges) [1] ... 

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