Substitution reactions at the Ni(III) cation in macrobicyclic complexes – Reaction of metal ion complexes prepared from ligands L1 (17-oxa-1,4,8,11-tetraazabicyclo[6.5.6]nonadecane) and L2 (17-oxa-1,4,8,11-tetraazabicyclo[10.5.2]nonadecane) with chloride ion

The potentially penta-coordinating ligand L 1 has been synthesized by reaction of the bridge bis-(2-(methylsulfonyl)oxyethyl)ether with the tetraazamacrocycle cyclam (cyclam = 1,4,8,11-tetraazatetradecane). The copper(II) complex was characterized initially and subsequent demetallation provided pure L 1 . Electrochemical and chemical oxidation of the nickel(II) species yields a Ni(III) ion (low-spin d 7 , rhombic, g xx 2.195, g yy 2.189, g zz 2.027) of sufficient stability in acidic solutions for kinetic measurements to be undertaken of substitution reactions at the sixth site. The isomeric ligand L 2 , has been synthesized previously, and reactions of the [Ni(L 1 )(solv)] 3+ and [Ni(L 2 )(solv)] 3+ ions with chloride have also been investigated. In both reactions, an unusually high acid dissociation constant is observed, associated with the proposed replacement of the apical ether oxygen by a water molecule that is bound both to the metal centre and hydrogen bonded to the ether. The structure [Ni(L 2 )](PF 6 ) 2 (monoclinic, P2 1 /c, a = 10.699(2), b = 13.244(4), c = 16.603(2) Å, β = 92.10(1)°) converged at R = 0.055 (R' = 0.057) for 325 parameters using 3293 reflections with I > 2σ(I). In this complex, the detachment of the ether O donor to yield a square-planar complex is confirmed. Comparisons are made with substitution rates at other Ni(III) macrocyclic ions.Key words: nickel(III), substitution, macrobicyclic ligand, synthesis, kinetics.