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In aqueous acetonitrile containing a phosphate buffer, 4-methoxybenzenediazonium ion is reduced by one or more of the partially aquated cations derived from tetrakis(acetonitrile)Cu(I) cation in this medium. Investigation of the reaction mechanism indicates the rate determining step to be the association of the diazonium ion with the hydrogenphosphate dianion to give an adduct which then undergoes reduction by Cu(I). The reaction gives a range of products which have been identified and quantified by GC. One of these, 4-methoxyphenol was unexpected in the reducing conditions; its presence could be explained by the disproportionation of a 4-methoxyphenylcopper(II) complex giving bis(4-methoxyphenyl)copper(III) which reacts with water to produce the phenol and an equivalent amount of methoxybenzene. A scheme is proposed which accounts for all the observed products and computer modelling gives a satisfactory description of the distributions of the five major products as functions of the relative proportions of the reactants for dilute conditions and those where the reductant is in excess. When the diazonium ion is in excess, the behaviour of the model and the experimental reactant accountability suggest the occurrence of additional reactions which give products unobserved by GC.

Relative to the rate observed for the hexa-aqua ion, Cu(OH(2))(6)(2+), chelation of the copper catalyst by certain bidentate ligands enhances the rate of hydroxydediazoniation reaction (Sandmeyer hydroxylation); the ligands also provide a source of hydrogen in competitive hydrodediazoniation (H-transfer) reactions. By using the cyclisation of 2-benzoylphenyl radical as a radical clock, it has been possible to evaluate absolute rate constants for both processes effected by a variety of complexes involving one or two bidentate ligands (2-aminocarboxylate, 2-hydroxycarboxylate, 1,3-dicarboxylate, 1,2-diamine). The radical exhibits electrophilic character in both processes. The pattern of behaviour observed suggests the rate determining step in hydroxylation is reaction of the aryl radical at the metal centre to form an organocopper adduct which is rapidly converted into products. The relative reactivities of different complexes are explained qualitatively in terms of variations in the ligand field and Jahn-Teller distortion splittings of the copper d orbitals. Hydrodediazoniation is an S(H)2 H-abstraction process. Generally, coordination by Cu(2+) deactivates the first added ligand relative to its reactivity as a free species in the same state of protonation. For the majority of complexes studied, the relative reactivity as H-donors of 1 : 1 and 1 : 2 complexes is statistically determined but an additional electronic effect is discerned for doubly charged ions.