Active catalysts complexes

A final example of homogeneous catalysis is the use of metallocene catalyst systems in chain growth polymerization processes. The metallocene, which consists of a metal ion sandtviched between two unsaturated ring systems, is activated by a cocatalyst. The activated catalyst complexes with the monomer thereby reducing the reaction s energy of activation. This increases the rate of the reaction by up to three orders of magnitude. 

Fig. 1. Stereoisomeric forms of the [Ni°(ri2-butadiene)2L] active catalyst complex la of the C8-cyclodimer reaction channel and the related stereoisomers of the r 3,ri1(C1), 2a, and bis(r 3), 4a, octadienediyl-Ni11 species. SF and OF denotes the coordination of two c/s-butadienes (cc), of two tr

The last two decades have seen enormous developments in catalyst discovery and optimization tools, notably in the area of high-throughput experimentation (HTE) and process optimization (5). However, the basic concept used for exploring the catalyst space in homogeneous catalysis has not changed Once an active catalyst complex is discovered, small modifications are made on the structure to try and screen the activity of neighboring complexes, covering the space much like an ink drop spreads on a sheet of paper. This is not a bad method, but can we do better with the new tools that are available today ... 

RNA catalysis and in vitro selection are ever increasing in scope, and the method presented in Section 8.3.6.1 is by no means the only alternative for separating reacted/active- catalyst complexes. Most research groups have used this type of partitioning procedure, based on some type of biotin-product capture by streptaviclin. Other partitioning methods are possible and this step in the overall RNA catalysis selection cycle is where many new innovations need to occur to advance the field. 

Pyridine, a tertiary amine, yields a very active catalyst complex, [Py2Cun(OH)Cl] (7), for the coupling reaction giving high rates of oxidation at low temperatures. However, molecular weights of the azopolymers preparable in this medium proved to be somewhat limited (see Table I) owing to the generally low solubility of the azopolymers formed. 

Comparable reaction conditions were applied in the coupling of activated and non-activated arylchlorides with styrene or 2-ethylhexyl acrylate, using the palladium carbene catalysts shown in Scheme 6.6. While compounds 22 and 23 were found to be highly active catalysts, complex 21 was thermally unstable and decomposed to palladium black during the catalysis.1671 The yield and selectivity were only moderate in DMA, but results improved markedly when the reaction was carried out in [(C4)4N]Br. 

Apart from the requirement of cw-chelating neutral ligands (L2), the high-activity catalyst complexes, L2PdX2, also require weakly or noncoordinating anions (X ) [10, 13],... 

Another effect is obvious from experiments with a Al/Fe ratio of 400 as soon as all the aluminum centers carry one longer hydrocarbon chain, further ethene polymerization proceeds only sluggishly. This indicates the formation of an active catalyst complex between MAO - methyl-substituted aluminoxane - and diimine pyridine iron with the formed larger alkyl-substituted aluminoxane, such an associate between catalyst and co-catalyst cannot form so readily or is less accessible and the activity is much lower. 

Ligands for active catalyst complexes ate available and scalable systems using very low levels of catalysts have been demonstrated by ATRP Solutions while preparing their expansive catalogue. (63)... 

First, the several possible forms of the active catalyst complex are investigated in order to infer the thermodynamically favorable one, which is followed by the investigation of their propensity to undergo oxidative coupling. Based on the precedence to known olefin-Ni° complexes [6,7], the two butadienes can be coordinated in bis(t ) and tx, ri mode for la, while tris-(ethylene)-Ni°, tris(ri -butadiene)-Ni ,bis(tx -butadiene)(ethylene)-Ni°,bis(ethylene)(ri -butadiene)-Ni° andbis(t -butadiene)(ri -butadiene)-Ni°,bis(ethylene)(ri -butadiene)-Ni , (t -butadiene)(ethylene)(tx -butadiene)-Ni° compounds are possible 16e" and 18e species, respectively, of lb. In general, ethylene complexation is found to have a higher stability relative to the coordination of butadiene, which, for example, amounts to 6.6 kcal moT (AG) for tris(ethylene)-Ni° vs tris(ri -trans-butadiene)-Ni species [11]. This is essentially attributed to unfavorable steric interactions, which act to prevent the preferred trigonal planar conformation for the latter species. 

Generally speakiiig. transition metal catalysed polymerizaticHi cannot be performed in aqueous media since water destroys active catalyst complexes. However, there are a few monomers whidi have been ptdymerized in pure wat - via transition metal catalyzed reactions. The following discussion of these polymerizations have been divided into vinyl polymerizations and ring-opening metatiiesis polymerizations (ROMP). 

In the interfacial synthesis, toluene and aqueous s ium hydroxide are used as the biphasic reaction medium, palladium-(4-dimethylaminophenyl) diphenylphospine complex as the surface-active catalyst complex, and DSS as die emulsifier. The nature of the phosphine ligand is important. Little or no reaction occurs with triethoxylphenylphosphine and triphenylphosphine as the ligand. Presumably, (4-dimethylaminophenyl)diphenylphosphine is effective in promoting the reaction because it is surface-active. A cationic surfactant, e.g., cetyltrimethylammonium bromide, may be used in place of DSS as the emulsifier. In that event, however, the surfactant also functions as a phase-transfer agent, and the overall reaction has a phase-transfer component in addition to the interfadal component. 

Furthermore, the two isomeric butenylnickel(II) forms of the active catalyst complex, the anti and syn forms, can differ in their reactivity depending on the catalyst structure. The key to understanding the cis-trans regulation lies in the different reactivities of the anti- and syn-butenylnickel(II) forms of the active catalyst complex, both in relation to their interconversion and together with the associated anti-syn equilibrium. 

The initiation period indicates that TPP by itself is probably not the active catalyst. Rather, the TPP must first complex or react with some other constituent to produce an activated catalytic complex. Substantial initiation periods are observed for low phenol concentrations (high R, high epoxy content), yet almost no initiation period for high phenol concentrations (low R, low epoxy content) are observed. This suggests that a rate-limiting step in the formation of the activated catalyst complex occurs as some type of interaction between the TPP and phenol. Such an interaction would occur more rapidly for higher PN concentrations and more slowly for lower PN concentrations. 

The catalytic activity of the families of cationic complexes 17-19 and 20-22 toward the iROP of l-LA was evaluated as well (Table 28.4). We reasoned that the weakly coordinating anion H2N B(C F5)3 2 and the absence of additional solvent on the metal centers should enhance the Lewis acidity of the cations and produce highly active catalysts. Complexes 23-25, which contain THF, exhibited good activities but limited control under the chosen conditions. 

Simultaneous normal and reverse initiation (SR NI) ATRP (Fig. 1.17c) was developed to allow the precursors of highly active catalytic complexes to be added to the reaction in the higher oxidation state and at lower concentration. SR NI ATRP comprises a dual initiation system i.e. standard free radical initiators and initiators comprising a transferable atom or group in conjunction with the stable precursor of an active catalyst complex. This initiation system can be used to prepare any type of polymer that can be obtained by normal ATRP, and can be conducted in bulk, solution, emulsion, miniemulsion, and by heterogeneous polymerization. 

In a standard ATRP reaaion, a solution of the lower oxidation state copper complex is added to the reaction medium or formed in situ from a Cu salt and an N-containing ligand. This can present a problem if active catalyst complexes are formed because the readily oxidized activator complex can react with oxygen, present in the system as an impurity, and be quickly deactivated. However, this spontaneous oxidation could increase the efficiency of initiation from R-X since termination reaaions required to form the equivalent of the persistent radical could be avoided or reduced. ... 

An improved reverse ATRP was developed to take advantage of the ability to use more active catalyst complexes, that is, more readily oxidized complexes,without inaeasing the concentration of copper in the reaction. This procedure was called a simultaneous reverse and normal initiation procedure (SR NI), since the activator and a small fraction of the initiating chains were formed in a reverse ATRP reaction, while the majority of the growing chains were initiated from the added normal ATRP initiator molecule. A limitation of SR NI clarified after development of 2D chromatography is the presence of a small fraaion of polymer chains initiated by the added free radical initiator. ... 

For ligands forming 1 1 complexes with copper ions, the activity of the catalyst is proportional to p /p whereas the tendency of the Cu complex to disproportionate in aqueous solution depends on the ratio p /(p ) [L]. Thus, a map can be constructed that can be used to select a ligand for aqueous ATRP that forms an active catalyst complex yet remains stable toward disproportionation. Various redox processes related to ATRP and the Cu disproportionation reaction, including the effect of both ligand and solvent, were reviewed. The... 

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