Bidentate-based systems

For the abatement of NO with NH3 in the absence of O2, ZV catalysts give low, but stable activity. The activity is strongly enhanced in the presence of O2- This is a common feature to all SCR catalysts, including the ZSM-5 based system [30]. In the presence of O2. our results show the formation of bidentate nitrates and, on NH3 addition, their transformation into chelating nitrates. Our results also show that the... 

Phosphorus-based systems constitute an important family of ligands in homogeneous catalysis. Even though the use of basic phosphines, which increases the nucleophilicity of the rhodium center, was initially considered a very promising strategy, their instability under the harsh conditions of the process constituted a major drawback for their application. Attempts to circumvent the decomposition were undertaken by using bidentate diphosphines, which stabilize the active rhodium species by the chelate effect (vide supra). 

Recently, a new bidentate hemispherical chelating bisphosphite ligand based on a calixarene backbone has been designed for linear selective hydroformylation of alkenes (Scheme 9) [54], Excellent levels of regioselectivity have been observed, and even the intrinsic branched-selective hydroformylation of styrene could be overruled by this system. However, the system suffers from low catalytic activity. 

The dithioacid family of ligands includes dithiocarbamates (R2N CS2 ), xanthates (RO-CS2 ), thioxanthates (RS CS2 ) and dithiocarboxylates (R CS2 ), which have been described in CCC(1987, Section 47.8.10).1 The ligands are prepared by addition of a suitable nucleophile to the carbon center in CS2, usually in the presence of a base. It has been established for some decades that they bind to Co almost exclusively as bidentate chelates, including S-donors acting as bridging ligands in dinuclear systems. 

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

BLM transport systems for ferrioxamine B were also devised based on first coordination shell recognition via ternary complex formation utilizing vacant coordination sites on the Fe(III) center (Fig. 29) (199). The tetra-coordinated substrate complex selectively transported was partially dechelated diaqua-ferrioxamine B and coordinately unsaturated di-hydroxamato iron(III) complexes, which utilized a hydrophobic membrane bound bidentate chelator as a carrier for selective transport. Active transport for these systems was accomplished using a pH gradient (199). 

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