Mechanism of base promoted reduction of nickel(III) complexes of macrocyclic amines. Coordinated ligand radical intermediate

Abstract

The mechanisms of decomposition of two nickel(III) complexes which contain macrocyclic secondary amine ligands have been studied. When 2 is treated with basic solvents such as pyridine, triethylamine, or water a Ni(II) ligand radical species is formed according to:Chemical and spectroscopic evidence is presented in support of this species. A similar species is observed in the decomposition of 4 by pyridine but is not observed in water although product analyses suggest that a radical species is again formed. Depending upon the solvent 1-25% of the radicals formed decompose with rupture of the macrocyclic ring; the remainder are converted to Ni(II) tetramine complex by hydrogen abstraction from ligand fragments or from a second radical species. The latter process leads to monoimine complex formation. Under certain conditions monoimine may also be formed by oxidation of the Ni(II) ligand radical by Ni(III) complex. Such radical species have been proposed earlier as intermediates in macrocyclic amine complex reactions, but no direct evidence for their formation has previously been obtained.

Kinetics and mechanism of the oxidation of cyclohexanone by a nickel(III) macrocyclic ligand complex in CH3CN

“…In light of the recent work on the base catalysed decomposition of Ni(II1) macrocycles (16)(17)(18)(19)(20) the results of the present work can best be explained by the following general mechani~m.~ 'The proton could also be lost from the nitrogen next to the carbon containing 2 methyl groups to give another isomer of 3. Likewise the radical in 4 could, and very likely is, also on the other nitrogen.…”

“…After this work was begun Barefield and Mocella reported that the base catalyzed decomposition of the same complex proceeded via a Ni(I1)-ligand radical (16). Later several Ni(II1) macrocyclic complexes, including 2, were generated by pulse radiolysis and their decomposition studied (17)(18)(19)(20).…”

“…The rac and meso forms were then separated using sodium oxalate. The [Ni-(rnes0- [14]ane)(CH,CN)~](BF~)~ was prepared by the procedure of Barefield and Mocella with the exception NOBF, was used in place of NOC104 (16).…”

“…16 does not consider the separate deprotonation equilibria given by [2]. Reference 17 does not consider the detailed pathways for decomposition of the deprotonated complex.…”

“…Barefield and Mocella (16) proposed that one typical reaction of the radical, 4, is hydrogen extraction from a H atom donor which in this case is cyclohexanone. The acid inhibition term in the rate expression indicates deprotonation of the ligand is a reversible process and the isotope effect with 2,2,6,6-tetradeuterocyclohexanone indicates [8] is the slow step.…”