Synthesis, Structure and Redox Properties of Nickel Complexes of Cage Amine Ligands

The syntheses of complexes containing the [Ni(sar)]2+/3+ and [Ni((NH3)2sar)]4+ ions are described along with an X-ray crystal structure analysis of the salt [Ni((NH3)2sar)] (NO3)4.H2O (sar is 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane; (NH3)2sar2+ is its 1,8-diammonio derivative). The Ni(III) ion is a powerful oxidant, with E' = 0.90 V (v. n.h.e. at 298 K, I = 0.2, aqueous trifluoromethanesulfonate medium), but is relatively stable in dilute aqueous acid. Both the Ni(II) and Ni(III) complexes of sar have been resolved into their enantiomeric forms, and their absorption, optical rotatory dispersion and circular dichroism spectra recorded and partly analysed. The electron self-exchange rate between enantiomeric forms in the different oxidation states has been measured by a stopped-flow circular dichroism method, and k(et) = (5.3+/-0.3)x10(3) dm3 mol-1 s-1 at 298 K, I = 0.2 (NaCF3SO3). The activation parameters, DELTAH(double dagger) 22+/-4 kJ mol-1 and DELTAS(double dagger) -100+/-12 J K-1 mol-1, and the rate constants are consistent with the comparatively small rearrangements required in the structures of the two ions relative to the optimal cavity radius for the cage of 2.05(1) angstrom.