Chelating ligands supramolecular assemblies

Figure 1.14 Generation of supramolecular catalysts for asymmetric hydrogenation (a) Assembly of heterodimeric chelating ligands, (b) Structure of the optimal rhodium-diphosphonite complex for asymmetric hydrogenation (other ligands from the metal center omitted for clarity), (c) Enantioselective hydrogenation of functionalized alkenes.

FIGURE 22 Preparation of supramolecular catalysts for hydrocyanation reactions (82) (A) assembly of heterodimeric chelating ligands (B) structure of the optimal nickel-diphosphine complex for hydrocyanation (other ligands of the metal center are omitted for clarity) and (C) hydrocyanation of functionalized styrenes. (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

The first example of chiral tetranuclear supramolecular assembly 5.27 was reported by Saalfrank and coworkers in 1988. Thus the complex [(NH3)4n(Mg (22)5] (5.27) was obtained by serendipity upon treatment of the doubly bidentate ligand 22, which is formed in situ from malonic ester and oxalyl chloride, with MeMgl in aqueous ammonium chloride solution (Scheme 5.13). Later an improved one-pot reaction was used to prepare such tetrahedral complexes of magnesium, manganese, cobalt and nickel by treating the bis(chelate) ligand 22, obtained in situ, with MeLi/MCl2 followed... 

A related class of compounds has been derived by using porphyrins connected to polypyridines such as 2,2 -bipyridine, 1,10-phenantholine and 2,2 6, 2"-terpyridine. These chelating ligands form thermodynamically more stable and inert transition metal complexes than the pyridine ligand, and such porphyrins have been extensively explored for the assembly of photochemically active supramolecular systems, especially by Sauvage et The oblique... 

Figure 7 Crystal structures of prismatic metallo-supramolecular assemblies, (a) Tetrahedral [Ag4(33)4]" + with included CH3CN shown in space-filling mode (b) the topologically complicated complex [Pd4(34)4(N03)2(H20)2] + (c) tetrahedral [(VO)6(CTC-6H)4(Mg(H20)4)3] anion with chelating deprotonated CTC ligands Mg centers shown in green, V in purple and (d) the stella octangula structure of [Pdg(35)8] + shown in space-filling mode with each ligand in a different color.

Assemblies based on 8 and pyridine phosphorus ligands 5-7 were used as supramolecular ligands in the rhodium-catalyzed hydroformylation and showed typical bidentate behavior. The chelating bidentate assembly exhibited lower activities (a factor of three) than the monodentate analogue. Only a slightly higher selectivity for the linear aldehyde was observed. The chiral ligand assemblies based... 

The chelating behavior was also evident from H P-NM R experiments. The addition of triphenylphosphine (a) to a catalyst solution of [HRh(CO)2(13 b)] did not affect the complex. Moreover, the addition of 1 equiv. of b to a solution of HRh(CO)2(13) PPhj resulted in the exclusive formation of [HRh(CO)2(13 b)] upon release of free triphenylphosphine. The chelating effect of the supramolecular ligand assembly effectively competes with triphenylphosphine, leading to exclusive formation of the rhodium complex of 13 b. In the complex [HRh(CO)2(13 b) the supramolecular ligand 13 b coordinates in an equatorial-equatorial fashion to the rhodium metal center, whereas the HRh(CO)2(13)PPh3 exists in a mixture of complexes (ee and ea). 

The Allosteric—Chelate Cooperative Model, a 1, y = 1, c> 0). In this model, both allosteric and chelate cooperativity are taken into account. The factor a can be evaluated by studying the interaction of the receptor with a monovalent ligand. An example of an assembly conforming to the allosteric-chelate cooperative model is offered by the supramolecular complex 19 in Hg. 21, whose formation is discussed herein. The following stabihty constants at 25°C in MeCN/CH2Cl2 50 50 (v/v) have been reported log FQa(iT) = 4.5, log Qa(i8) = 8.6 and log... 

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