Ditopic ligand

Other coordination modes of trans-diammac have been identified where one (154) or both (155) primary amines are free from the metal.721 725 An extension of this concept involves attachment of active functional groups such as crown ethers selectively at one primary amine to generate ditopic ligands capable of electrochemically sensing alkali metal ions through their inductive effect on the Co11111 redox potential. One example is provided by (156) further, the 15-crown-5 and 18-crown-6 analogs were also prepared.726... 

The hydrolysis of p-nitrophenyl acetate and bis(p-nitrophenyl phosphate) are frequently used to probe hydrolytic activity. A problem with some other dinuclear systems is that the Zn units are held together by bridging ligands which can be cleaved on reaction with the substrate.440 This is not the case in a ditopic ligand such as those designed by Lippard and co-workers based on Kemp s triacid imide with a xylyl spacer.441,442 Both zinc dimers and mixed metal dimers were formed and a structure characterized with a bridging phosphodiester (Figure 6). 

Figure 7.6 Structure of a bent ditopic ligand prepared by Wang et al. (2005) and a schematic representation of a metal-templated macrocycle.

Metal Translocation Based on the Fe "/Fe" Couple The first example of redox-driven translocation of a metal center was based on the Fein/Fen couple and took place in ditopic ligands containing (i) a tris-hydroxamate compartment and (ii) a tris-(2,2 -bipyridine) compartment.5... 

On addition of 1 equivalent of Fenl to a MeOH/H20 solution of the ditopic ligand 5, the solution takes a light brown color with formation of a rather intense band centered... 

Figure 2.8 Redox-driven translocation of a copper center, based on the Cu"/Cu change. The Cu11 ion stays in the tetramine compartment of the ditopic ligand 10, whereas the Cu1 ion prefers to occupy the bis-(2,2 -bipyridine) compartment. The translocation of the copper center between the two compartments is fast and reversible when carried out through the Cun-to-Cu1 reduction with ascorbic acid and Cu -to-Cu" oxidation with H202, in a MeCN solution.

The dinuclear Sr complex of the ditopic ligand 17 increases the rate of basic ethanolysis ofthe malonate derivative 19 by a surprising 5700-fold, but increases by only 9.5-fold the rate of cleavage of 14 [28]. It is remarkable that such a huge rate-enhancement occurs under extremely dilute conditions, namely 15 pM 19 and 30 pM 17-Sr2. A slightly lower rate enhancement is observed in the presence of 17-Ba2. It seems likely that under the dilute conditions of the catalytic experiments several crown-complexed metal species occur simultaneously (Scheme 5.4). Given the plethora of species involved in such a complicated system of multiple equilibria, quantitative kinetic treatment is out of reach. Nevertheless, a comparison with the reactivity of model compounds, particularly that of the malonate derivative 20, provides insight into the composition of the reactive intermediate (Table 5.8),... 

The low rate-enhancement brought about by the dinuclear complex in the reaction of 14 suggests a very modest formation, in the very dilute solution, of the productive intermediate 17 [SrOEt] [SrO2CR], a supramolecular complex composed of one molecule of ditopic ligand 17, two Sr ions, one EtO ion, and one substrate molecule. In contrast, the absence of any difference between mononuclear and dinuclear catalyst in the cleavage of 20 demonstrates that only one metal... 

Ditopic Ligands for the Construction of Bidentate Phosphine Ligands... 

These assembly ligands will be tested in suitable catalytic reactions that leave the assemblies intact. Salt-forming reactions are not attractive as the salts might interact with the assembly, nor is the use of catalytic metals that compete with the assembly metal for the salen type positions in the ditopic ligand ideally, all potential problems can be avoided if the same metal could be used. Rhodium-catalyzed hydroformylation of 1-octene is a suitable reaction, with the only disadvantage that high pressures are needed, but hydrogen or CO do not interfere with our assemblies. Metal salts do not interfere with the rhodium hydrides involved in the hydroformylation catalysis, as for instance the most effective industrial process today for propene hydroformylation... 

The molecular structure of 40 (Figure 10.14) shows two independent ditopic ligands. The zinc metal is four-coordinate to the Schiff base ligands (each of them... 

The molecular structure of 44 (Figure 10.17) revealed that two ditopic ligands are coordinated to Zn. As expected, coordination takes place through the nitrogen and... 

Escudero-Adan, E.C., Freixa, Z. and van Leeuwen. P.W.N.M. (2007) Catalysis by design Wide-bite-angle diphosphines by assembly of ditopic ligands for selective rhodium-catalyzed hydroformylation. Angeiv. Chem., Int. Ed., 46. 7247—4750. 

DITOPIC LIGAND (10) AND PALLADIUM OR PLATINUM COMPLEXES 11a,b. THE COORDINATION GEOMETRY AT THE METAL CENTERS AND THE RIGIDITY OF THE LIGANDS DETERMINE THE FINAL STRUCTURE. 

The advent of anion recognition has opened up the possibility of developing ditopic ligands for extraction of metal salts,... 

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