Chelating pyridine ligands

Dye Polymers Constructed from Chelating Pyridine Ligands... 

Chelating pyrazole ligands are also known. The pseudo-octahedrally coordinated zinc complex ZnL2(pyridine)2 was formed where L = 5-(4-methoxyphenyl)pyrazole-3-carboxylate.850... 

One important feature of the ligands shown in Fig. 18 is that they both contain the py—CH2—R functional group. Jackson et al. (117) have shown that for several Co(III) systems, these -CH2- protons can exchange in alkaline D20 at rates comparable to those of base hydrolysis. Consequently, there are three possible mechanisms for the OH-dependence in these complexes without NH protons (i) reversible Cr—N bond rupture (118) (ii) conjugate base formation at the methylene protons and subsequent electron delocalization through the chelated pyridine ring (117) and (iii) direct bimolecular attack. 

Pyridine ligands 186 are monodentate, forming complexes of the kind presented by 189. Pyridine is coordinated in the same way in copper acetate 194 [314] and iron rhodanide complexes [315], adducts of cobalt complex with fe(salicylidene)ethyle-nediamine 195 [316] and nickel chelate, formed by tridentate N,S-donor azomethine ligand 196 [317] ... 

Scrimin and co-workers have extensively studied the ester-cleavage abilities of various Cu(II)-chelating bidentate ligand [(2-hydroxymethyl)pyridine] amphi-phile, 9, and related bolaphile, 10, in micellar media [32], The corresponding metallomicelles are powerful catalysts for the cleavage of substrates, e.g. p-nitrophenyl alkanoates, that do not coordinate with the metal-complex core. Subsequent studies demonstrated that tridentate ligand amphiphiles such as 9,... 

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

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