PROPERTIES AND CRYSTAL-STRUCTURE OF SQUARE-PLANAR -HEXAAZATRICYCLO[13.3.1.1(8,12)]ICOSANE)NICKEL(II) PERCHLORATE AND KINETICS OF ITS ISOMERIZATION TO THE R,R,S,S CONFIGURATION

The four-co-ordinate nickel(II) macrocyclic complex (R,S,R,S)-[NiL][Cl O4](2), where L is ,6,8,12,15-hexaazatricyclo[13.3.1.1(8,12)]icosane, has been isolated. If shows a significantly weaker ligand held with an increased absorption coefficient and more positive electrochemical ox idation and reduction potentials compared with those of the R,R,S,S is omer. Its Crystal structure was determined by X-ray crystallography. I t shows a square-planar geometry with a large tetrahedral distortion o f the four nitrogen donors (maximum +/- 0.135 Angstrom). The average N i-N bond distance is 1.938(3) Angstrom for-the secondary nitrogens-and 1.969(3) Angstrom for the tertiary. The complex isomerizes to the R,R ,S,S complex at 50 degrees C in an aqueous solution containing 9.1% (v /v) MeCN. The first-order rate constant k(obs) is 4.16 x 10(-5) s(-1) at pH 7 and I = 1.0 mol dm(-3) (LiClO3). The value of k(obs) is indepe ndent of pH and buffer concentration, but increases with increase in w ater content and decrease in ionic strength. The activation parameters are Delta H-double dagger = 33 +/- 1 kcal mol(-1) and Delta S-double dagger = 24 cal K-1 mol(-1) at 25 degrees C. It is proposed that isome rization of the R,S,R,S to the R,R,S,S complex proceeds via axial co;o rdination of water followed by a Ni-N(tertiary) bond dissociation assi sted by a water molecule in the second co-ordination sphere acting as a general acid.