Conjugated systems pyridine ligands

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. 

In 1955, Terent ev and Mogilyanskii reported the catalytic oxidative coupling of aniline to azobenzene with a yield of 88%, mediated by copper chloride in pyridine (which acts as both a metal ligand and solvent) in the presence of molecular oxygen [1]. This system was subsequently used for the generation of various conjugated and nonconjugated main-chain aromatic azo polymers from primary aromatic diamines [2]. In contrast, the use of CUCI-O2 with phenol produces tars [3] as a result of the inherent properties of this reactant. Indeed, while the metal salt produces iV-centered aniline radicals that dimerize to form azobenzene... 

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