Coordinated diimine ligands, oxidation products

The tris(ethylenediamine)ruthenium(III) species is obtained by oxidation of [Ru(en)3]2+ with, for example, iodine4 or bromine.6 The oxidizing agent and conditions employed must be chosen carefully to avoid further oxidation of the ethylenediamine ligand to coordinated diimine.7 In the present procedure solid silver anthranilate is used to oxidize [Ru(en)3] [ZnCl4], and [Ru(en)3] Cl3 is isolated. In this heterogeneous procedure the desired [Ru(en)3]Cl3 is the only soluble product and can easily be separated from the insoluble silver, silver chloride, and zinc dianthranilate. Other less soluble [Ru(en)3]3+ compounds can be obtained easily from the soluble chloride. 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

A large amount of work has been devoted to N-binding macrocyclic complexes of Ni, Cu and Fe, which yield imine ligand products. Bidentate amine ligands, mainly ethylenediamine (en), have been used with Ru and Os complexes. The oxidation of coordinated ethylenediamine and related ligands stops at the diimine stage and does not continue to the dinitrile. The a,a -diimine entity -N=C-C = N- formed in the four-electron oxidation is particularly stable (93). 

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