Properties and crystal structure of square-planar R,S,R,S-(1,3,6,8,12,15-hexaazatricy clo[13.3.1.18,12]icosane)nickel(II) perchlorate and kinetics of its isomerization to the R,R,S,S configuration

Abstract

The four-co-ordinate nickel(II) macrocyclic complex (R,S,R,S)-[NiL][ClO ₄ ] ₂ , where L is 1,3,6,8,12,15-hexaazatricyclo[13.3.1.18,12 ]icosane, has been isolated. It shows a significantly weaker ligand field with an increased absorption coefficient and more positive electrochemical oxidation and reduction potentials compared with those of the R,R,S,S isomer. Its crystal structure was determined by X-ray crystallography. It shows a square-planar geometry with a large tetrahedral distortion of the four nitrogen donors (maximum ± 0.135 Å). The average Ni–N bond distance is 1.938(3) Å for the secondary nitrogens and 1.969(3) Å for the tertiary. The complex isomerizes to the R,R,S,S complex at 50 °C in an aqueous solution containing 9.1% (v/v) MeCN. The first-order rate constant k _obs is 4.16 × 10 ^-5 s ^-1 at pH 7 and I = 1.0 mol dm ^-3 (LiClO ₄ ). The value of k _obs is independent of pH and buffer concentration, but increases with increase in water content and decrease in ionic strength. The activation parameters are ΔH ^‡ = 33 ± 1 kcal mol ^-1 and ΔS ^‡ = 24 cal K ^-1 mol ^-1 at 25 °C. It is proposed that isomerization of the R,S,R,S to the R,R,S,S complex proceeds via axial co-ordination of water followed by a Ni–N(tertiary) bond dissociation assisted by a water molecule in the second co-ordination sphere acting as a general acid.