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

Abstract

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)×10³ dm³ mol^-1 s^-1 at 298 K, I = 0.2 (NaCF₃SO₃). The activation parameters, ΔH‡ 22±4 kJ mol^-1 and ΔS‡ -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) Ǻ.