Catalytic Properties of Coordination Polymers

PEI can be applied as a polymer matrix because the PEI molecules contain amino groups which are capable of bonding with group VIII metal salts. Catalytic properties of coordination compounds of PEI have been thoroughly studied [57-62] The structure or active sites of PEI complexes with nickel, cobalt, rhodium and palladium ions is presented as a five-membered chelate ring  

PEI-RhCl3 and PEI-RUCI3 complexes exhibit different catalytic activity with regard to a mixture of the cis- and trans isomers of pentadiene [61]. The former complex is more selective for pentene (0.94) than the latter. A quantitative aniline yield results from the reduction of nitrobenzene in the presence of PEI complexes with Ni(II), Co(ll), Sn(II), Pd(II) and Rh(III) [61]. The reduction proceeds rapidly at 20- 70°C and at 1-25 atm pressure of H2 both in a solvent and without. Besides aniline, cyclohex-ylamine is produced by further hydrogenation of the aromatic ring in the presence of the PEI-Rh(III) complex. Polymer-metal catalysts do not lose their catalytic stability after repeated application. For example, when eight hydrogenation reactions were catalyzed with PEI-Pd(II), in each case a 100% product yield of aniline was attained. 

The preparation of palladium catalysts from polyethyleneimine-silicagel composites was reported [65]. PEI is first absorbed on silica beads or silica gel. Then the absorbed 

The catalyst was synthesized as follows. Polymer amino groups were saturated (to 5%) with Pd(II) ions which were then reduced by NaBH4 dissolved in water or in an organic solvent (e.g. formalin). The thus obtained Pd/PEl/silica beads and Pd/PEI/silica gel powders had pore areas of 50 and 500 mVg, respectively. These catalysts were employed for reduction of nitrobenzene in ethanol. Catalysts of both types were easily separated from reaction products by decanting and could be used repeatedly. 

In particular, in the presence of the AN-40/Pd(ll) complex, allylbenzene is mainly isomerized, whereas it is hydrogenated in the presence of AN-23/Pd(ll). 

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Polymer-supported Lewis acid catalysts based on metals with high coordination numbers, such as Sc, Yb, and Ln, proved to be highly effective in promoting several organic transformations. Umani-Ronchi and coworkers developed polymer-supported indium (I II) Lewis acid [85]. The polymeric In (III) was easily prepared from Amberlyst-Na [86] and In(OTf)3 (Scheme 19.40). They tested the catalytic properties of the polymer-supported indium Lewis acid in the ring... 

Before coordination polymerization was discovered by Ziegler and applied to pro-pene by Natta, there was no polypropylene industry. Now, more than 10 pounds of it are prepared each year in the United States. Ziegler and Natta shared the 1963 Nobel Prize in Chemistry Ziegler for discovering novel catalytic systems for alkene polymerization and Natta for stereoregular polymerization. WeTl see more about Ziegler-Natta polymerization in Chapter 27 when we examine the properties of synthetic polymers in more detail. 

Many researchers have focused on the preparation and catalytic properties of polymer-bound ruthenium and osmium species because of their proven ability to catalyze homogeneous reactions and the vast synthetic chemistry available for their preparation. A series of preformed polymers of [M(bpy)2(pol)nCl]Cl, where M can be a Ru(II) or Os(II) metal center coordinated to 2,2 -bipyridine ligands (bpy) and anchored to a pyridine or imidazole nitrogen of a PVP or poly(N-vinylimidazole) polymer (pol), have been prepared and characterized with respect to charge transport rates and mechanisms in drop-coated films on electrode surfaces. Electrodes coated with films of the ruthenium polymer have been shown to mediate the oxidation of nitrite, and nickel bis(2-hydroxyethyl)dithiocarbamate. ... 

This review deals with the chemistry and coordination complexes of isoelectronic analogues of common oxo-anions of phosphorus such as PO3, POl", RPOl" and R2POy. The article begins with a discussion of homoleptic systems in which all of the 0x0 ligands are replaced by imido (NR) groups. This is followed by an account of heteroleptic phosphorus-centered anions, including [RN(E)P(/<-NR )2P(E)NR]2-, [EP(NR)3]3-, [RP(E)(NR)2] and [R2P(E)(NR )] (E=0,S, Se, Te). The emphasis is on the wide variety of coordination modes exhibited by these poly-dentate ligands, which have both hard (NR) and soft (S, Se or Te) centers. Possible applications of their metal complexes include new catalytic systems, coordination polymers with unique properties, and novel porous materials. 

Oligomerization and polymerization of terminal alkynes may provide materials with interesting conductivity and (nonlinear) optical properties. Phenylacetylene and 4-ethynyltoluene were polymerized in water/methanol homogeneous solutions and in water/chloroform biphasic systems using [RhCl(CO)(TPPTS)2] and [IrCl(CO)(TPPTS)2] as catalysts [37], The complexes themselves were rather inefficient, however, the catalytic activity could be substantially increased by addition of MesNO in order to remove the carbonyl ligand from the coordination sphere of the metals. The polymers obtained had an average molecular mass of = 3150-16300. The rhodium catalyst worked at room temperature providing polymers with cis-transoid structure, while [IrCl(CO)(TPPTS)2] required 80 °C and led to the formation of frani -polymers. 

In the first step, a substrate coordinates to a metal catalyst and forms an intermediate mixed complex (LMS in Scheme 13). The substrate is then activated by metal ions and dissociates from the catalyst. The complex catalyst, having accomplished its purpose, is regenerated to the original complex. The catalytic action of a metal ion depends substantially on the nature of the ligands in the intermediate mixed complex. Certain ligands induce an increase in catalytic activity, while others, e.g. multidentate ligands such as ethylenediaminetetracetic acid, inhibit the catalytic action of a metal ion. Therefore, if a polymer ligand is used as one component of a metal-complex catalyst, its properties may affect the catalytic action of the metal ion. 

PCPs with well-defined pores and surface-isolated Lewis acid sites could potentially serve as size- or shape-selective heterogeneous catalysts, in a similar manner to zeolites.33 43 161-164 The two-dimensional PCP, [Cd(4,4 -bpy)2(H20)2] 2N03 4H20 , was the first example that showed catalytic properties for the cyanosilylation of aldehydes.33 Experimental data in the case of cyanosilylation of imines, which is also performed by the same compound, led to the conclusion that hydrophobic grid cavities bind to the substrate very efficiently to promote a rapid reaction, and that the heterogeneous reaction involves the selective activation of the imino nitrogen by the weak Lewis acid Cdn center.161 In this polymer, the NO3" anions exist in a coordination-free state. This situation contributes to increasing the Lewis acidity of the Cdn centers. 

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