Latent-catalyst complexes

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

These new sulfur bearing complexes can potentially be used as a thermo-switchable latent catalyst. 

The two most common BF3 amine catalysts used commercially to cure epoxies are boron trifluoride monoethylamine, BF3 NH2C2H5, and boron trifluoride piperidine, BF3 NHCsHi0, complexes. Such complexes are latent catalysts at room temperature but enhance epoxide group reactivity at higher temperatures. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Previous Work on Latent Catalysts for Epoxy Resins. Numerous patents (, , ) have been issued in recent years on the development of latent catalysts for DGEBA (i.e., diglycidylether of bisphenol a ) resins, but most fulfill only a few of the conditions outlined above. One of the most successful of these has been the boron trifluoride-monoethylamine complex ( ). However, one of the serious disadvantages of this particular latent catalyst is the poor electrical properties at elevated temperatures of the epoxy resin in the cured state (7 ). 

These complexes showed higher thermal stabihty in toluene at 80 °C than the Hoveyda first-generation catalyst, with half-lives ranging from 3 to 6 h, depending on the nature of the Schiff base-derived hgand. They also showed latent catalyst behavior, as only moderate-to-low olefin metathesis activity was observed at room temperature in CM and RCM [44]. On the other hand, these complexes were active in the ROMP of cyclooctene and cyclopentene. The NHC-containing catalyst was found to be especially efficient, leading to a TOP of 667 min at room temperature [43]. 

A widely used strategy for latent catalyst activation is the addition of a chemical entity or a co-catalyst to promote the formation of the active species. Co-catalysts are usually a carbene source (such as diazo compounds) or an acidic compound (Bronsted or Lewis) that stimulates the dissociation of hgands from the ruthenium complex to generate an active species. 

Stable, one-package systems can be developed with many catalytic curing agents such as the boron trifluoride complexes. Tertiary amines and amine salts have pot lives generally ranging from 2 to 24 hours. The latent catalysts are activated by heat and cause a disassociation of the active catalyst from the blocking group. 

In 2014, two independent reports from Poland disclosed the preparation of ruthenium-alkylidene complexes chelated via a phenoxide anion [29)7 After activation with hydrogen chloride or other suitable acidic additives, these stable catalyst precursors became efficient promoters for various CM, RCM, and enyne metathesis reactions, including butenolysis. It is noteworthy that they were soluble in neat dicyclopentadiene, thereby enabling their use as latent catalysts for the ROMP of this highly reactive monomer. 

Hexamethylenetetramine. Hexa, a complex molecule with an adamantane-type stmcture, is prepared from formaldehyde and ammonia, and can be considered a latent source of formaldehyde. When used either as a catalyst or a curative, hexa contributes formaldehyde-residue-type units as well as benzylamines. Hexa [100-97-0] is an infusible powder that decomposes and sublimes above 275°C. It is highly soluble in water, up to ca 45 wt % with a small negative temperature solubiUty coefficient. The aqueous solutions are mildly alkaline at pH 8—8.5 and reasonably stable to reverse hydrolysis. 

Latent images or faint images in silver metal or other materials can be amplified by redox chemistries other than metal deposition. Several dye-forming redox chemistries have been discovered in which metal complexes serve as catalysts, catalyst precursors or one of the redox partners. The applications of coordination compounds in physical development and image amplification systems are therefore quite broad and diverse. 

In these processes, metal complexes find a number of uses as light-sensitive latent-image-catalyst formers, catalyst replacements for image silver in low-silver systems, and oxidants for various developers in image amplification baths. 

The most popular catalysts for epoxy resins are tertiary amines, tertiary amine salts, boron trifluoride complexes, imidazoles, and dicyandiamide. Many of these catalysts provide very long pot lives (months) at room temperatures and require elevated temperatures for reaction with the epoxy groups. These catalysts are often referred to as latent hardeners. 

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