Catalysts open metal sites

Phan, N. T. S., Nguyen, T. T., Nguyen, K. D., and Vo, A. X. T. 2013. An open metal site metal-organic framework Cu(BDC) as a promising heterogeneous catalyst for the modified Friedlander reaction. AppZ. Catal. A. 464-465 128-135. 

Here, L is a mobile ligand which can leave the metal site (M) open briefly for reaction with A in the initial step of the catalytic cycle. The transformation of the M A complex into products completes the cycle. The equilibrium in step (1) lies far to the left in most cases, because the ligands protect the metal centers from agglomeration. Thus, the concentration of M is very small, and the total concentration of catalyst is cMr = cm a + cm l- The rate law which arises from this mechanism is... 

The open nature of the metal site limits catalyst site control by CpA initiators. Polymerization of propene proceeds with weak chain end control at low temperatures. The highest stereoselectivity reported is (mmmm) — 0.77 using Me2Si(Flu)(N-t-Bu)ZrCl2. 

The metallocene catalyst with cationic nature and spatially opened active site provides favorable condition for the incorporation of p-alkylstyrene (p-ms) to polyolefins. The p-ms groups can be easily metallated to produce "stable" polymeric anions for graft-from polymerization. With the coexist of anion-polymerizable monomers, we have prepared many graft copolymers, such as PE-g-PS, PE-g-PMMA, PE-g-PAN, PP-g-PS, PP-g-PB, PP-g-PI and PP-g-PMMA. 

Like electrophilic addition to diazo compounds [7] from which diazonium ions and, subsequently, carbocations are generated, transition-metal compounds that can act as Lewis acids are potentially effective catalysts for metal carbene transformations. These compounds possess an open coordination site that allows the formation of a diazo carbon-metal bond with a diazo compound and, after loss of dinitrogen, affords a metal carbene (Scheme 5.2). 

At low conversion and for catalysts with equal metal content, the TOF for RO was found to decrease with increasing acidity. Addition of acid sites to a metal/zeolite catalyst apparently not only opens an additional catalytic pathway but also modifies the metal sites (73). An alternative explanation of this observation, namely, deactivation of metal sites for RO by the benzene that is formed by RE, was rejected on the basis of experiments wherein catalysts were tested by TPD of adsorbed MCP. In this case no RE takes place and no benzene is present but he TPD products of adsorbed MCP from acidic Pd/HY and neutralized Pd/NaY were found to be entirely different (314). 

However, when several open coordination sites (sites normally occupied by water) on the metal catalyst are available the following mechanism appears to be operative (Hoffmann and Jacob, 1984 Conklin and Hoffmann, 1988a-c) ... 

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