Electron transfer reactions of nickel complexes of 1,4,7-triazacyclodecane

Abstract

The preparation of [Ni(1,4,7-triazacyciodecane)2]3+, (Ni(10-aneN3)23+) is described. The existing procedure has been modified leading to good yields of the ligand trihydrochloride. The nickel(II) analogue (reported previously) is oxidised in a facile manner, either by Co3+aq in acidic aqueous media or by NO+ in CH3CN. Since the octahedral NiN6, chromophore is retained upon electron transfer, outer sphere reactions both of the Ni(II) and Ni(III) species have been studied. Rates of oxidation by various nickel(III) macrocycles have been measured and details are provided. Electrochemical oxidation of the Ni(II) complex is consistent with E0(Ni(10-aneN3)23+/2+) = 0.997 V (vs. NHE). The data have been used in a Marcus correlation, leading to the self-exchange rate k11 for the couple (Ni(10-aneN3)23+/2+) = (2 ± 1) × 104 M−1 s−1. This value is compared with other data derived using octahedral Ni(II)/Ni(III) centres. The oxidation of the Ni(II) complex by Co(III)aq has been studied in both protonated and deuterated media. There is no evidence for any proton transfer (from the N—H) being coupled to the electron transfer step. The observed rate constant for the reaction of Co3+ with Ni(II)(10-aneN3)22+ (550 M−1 s−1) may be compared with the calculated outer sphere rate (270 M−1 s−1). An estimate of k11 (CoOH2+/+) ~ 3 M−1 s−1 for the CoOH2+/+ exchange is discussed.

Kinetic studies of the reduction of nickel oxime complexes by tris(1,10-phenanthroline)cobalt(II) and hydroquinone in aqueous media

“…In the reaction with hydroquinone, the observed hydrogenion dependence may be attributed to equilibria involving the reductant. A scheme consistent with the data may be expressed as given in reactions ( 21)- (24), where Hquin' is the H,quin…”

Kinetic stabilization of trivalent nickel complexes with tertiary tetraaza macrocyclic ligands in aqueous solution

“…5 The complex with 1,4,8,11-tetraazacyclotetradecane, cyclam (L 1 ) was shown to stabilize best nickel(). 1,6,7 The redox properties of nickel complexed with L 2 are very similar to those with L 1 . 8,9 Substituents on the macrocyclic backbone affect the redox characteristics by restricting axial ligand binding which usually stabilizes the trivalent oxidation state of nickel complexes, 5,10-13 and by inductive effects.…”

Outer-sphere electron transfer reactions of aqua(5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane-1-acetato) nickel(II) and (III).

“…The [Ni(tacn) 2 ] 3+ cation is a well-known outer-sphere oxidant that is not susceptible to protonation or deprotonation reactions in solution (15 3+ . This is the reverse of reaction [1], and so the overall reaction may be described as a second-order reversible redox: [2] [ (Fig.…”