Ligand heterogeneity

Klabunovskii, E.I., Petrov, Yu.I., and Vedenyapin, A.A. (1974) On regularities of stereospecific catalysis on modified with optically active ligands heterogeneous hydrogenation catalysts, T Soviet-France Seminar on Catalysis, Kiev, Preprint N7. p. 33-39. 

Fig. 4 Non-retained (or split-peak) fractions observed for injections of rabbit immunoglobulin G onto an immobilized protein G column at various flow rates. Source From Non-linear elution effects in split-peak chromatography. II. Role of ligand heterogeneity in solute binding to columns with adsorption-limited kinetics, in J. Chromatogr.

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

The reaction of iodobenzene with acrylate is a good synthetic method for the cinnamate 17[7]. In the competitive reaction of acrylate with a mixture of 0-and /i-iodoanisoles (18 and 19), the o-methoxycinnamate 20 was obtained selectively owing to the molecular recognition by interlamellar montmorillonite ethylsilyldiphenylphosphine (L) as a heterogenized homogeneous catalyst used as a ligand[28]. 

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Allyl Complexes. Allyl complexes of thorium have been known since the 1960s and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a thorium haUde and an aHyl grignard. This synthetic method was utilized to obtain a rare example of a naked aHyl complex, Th(Tj -C2H )4 [144564-74-9] which decomposes at 0°C. This complex, when supported on dehydroxylated y-alumina, is an outstanding heterogeneous catalyst for arene hydrogenation and rivals the most active platinum metal catalysts in activity (17,18). 

Four general methods have been used for obtaining chiral ligands resolution of a racemic mixture, use of a chiral naturally occurring product 33), and asymmetric homogeneous or heterogeneous hydrogenation. 

The method of Ito et at. 50) as applied by Bakos et at. 12) to the reduction of acctylacetone to either ( —)-(2R,4J )- or ( + H25,45)-2,4-pentanediol will serve to illustrate how a chiral heterogeneous catalyst has been used to prepare a chiral homogeneous ligand precursor. 

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

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