NiII(1,4,8,11-tetraazacyclotetradecane)2+, Ni IIL2+, is a good electrocatalyst for the oxidation of CH3NH2 and (CH3)2NH but not of (CH3)3N. The oxidation kinetics of the amines by Ni IIIL(H2O)23+ indicate that the amines are good axial ligands to the tervalent nickel complex. The complexes NiIIIL[N(CH3)iH3-i](H 2O)3+ are stronger oxidants than the complexes Ni IIIL[N(CH3)iH3-i]2 3+. The oxidation is base-catalyzed and obeys a second-order rate law in NiIIILX2. It is proposed that the key step is Ni III-L(H2O)[N(CH3)iH 2-i]2+ + NiII- 1LX2 → LNi II-N(=CH2)(CH3)i-1H2-1 + NiIIL2+ + H3O+ + 2 X. Naturally, N(CH3)3 is not oxidized by this mechanism. Of special interest is the observation that the axial ligands CH3NH2 and (CH3)2NH are oxidized by the central cation, while the cyclam ligand, which has four secondary amine groups bound to the nickel(III) ion, and axially bound pendant primary amine groups, which are covalently linked to the macrocyclic ligand, are relatively stable. This difference in the behavior of axially bound amine groups is attributed to the free rotation of the axially bound N(CH3)iH3-i ligands that is required for the oxidation to proceed.
- Pendant primary amines