Oxidation of CH₃NH₂ and (CH₃) ₂NH by Ni^III(cyclam)(H₂O)₂ ³⁺ in aqueous solutions

Ni^II(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 Ni^IIIL[N(CH₃)_iH_3-i](H ₂O)³⁺ are stronger oxidants than the complexes Ni ^IIIL[N(CH₃)_iH_3-i]₂ ³⁺. The oxidation is base-catalyzed and obeys a second-order rate law in Ni^IIILX₂. It is proposed that the key step is Ni ^III-L(H₂O)[N(CH₃)_iH _2-i]²⁺ + Ni^II- 1LX₂ → LNi ^II-N(=CH₂)(CH₃)_i-1H_2-1 + Ni^IIL²⁺ + 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)_iH_3-i ligands that is required for the oxidation to proceed.