Stabilization of the Tervalent Nickel Complex with meso-5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecane by Axial Coordination of Anions in Aqueous Solution

The electrochemical oxidation of NiL²⁺ (where L = meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) in aqueous solutions in the presence of sulfate, phosphate, chloride, and phthalate yields Ni^IIILX₂. The stability constants for the axial coordination of the anions X are estimated from the observed redox potentials and from kinetic measurements. The complexes [Ni^IIIL(SO₄)H₂O]ClO₄ and [Ni^IIIL(H₂PO₄)₂]ClO₄ were precipitated. The crystal structure of the latter complex was determined. The complex crystallizes in the triclinic space group P1̅ with a = 11.074 (3) Å, b = 9.386 (2) Å, c = 14.299 (2) Å, α = 88.55 (1)°, β = 106.33 (2)°, γ = 111.46 (2)°, and two molecules in the unit cell; least-squares refinement based on 4282 reflections led to a conventional R of 0.046. The nickel is located in the plane determined by the four ligating nitrogen atoms, and the configuration of the remainder of the macrocyclic ligand is essentially identical with that reported for the divalent nickel complex with this ligand. The kinetics of decomposition of the complexes were studied and found to be complicated. The results indicate that the complexes Ni^IIIL(SO₄)2⁻ and Ni^IIIL(H₂PO₄⁻)2⁺ decompose via Ni^IIIL³⁺ and perhaps also partially via Ni^IIILSO₄⁺ and Ni^IIILH₂PO₄²⁺. On the other hand decomposition of the complex Ni^IIILCl₂⁺ is predominantly via Ni^IIILCl²⁺ with partial decomposition via Ni^IIILCl₂⁺.