SYNTHESIS, RESOLUTION AND KINETICS OF ELECTRON SELF-EXCHANGE OF HIGH-SPIN MANGANESE(II)/(III) CAGE COMPLEXES

The ligand sarcophagine (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) rapidly reacts with [Mn(OH2)6]2+ to form the nearly colourless [Mn(sar)]2+ ion, which can readily be oxidized to the bright orange [Mn(sar)]3+ ion (E-degrees'+0.53 V v. n.h.e. in 0.1 mol l-1 CF3SO3H at 295 K). A single-crystal structure determination on [Mn(sar)] (NO3)3, space group I42dBAR, a 15.549(6), c 19.014(6) angstrom, R 0-051, R. 0-049 for 608 'observed' reflections, shows the coordination geometry of the manganese(III) ion, with site symmetry 2, to be subject to a Jahn-Teller distortion, giving three pairs of Mn-N bond lengths of 2.18(1), 2.13(1) and 2.08(1) angstrom. The (Mn(sar)]3+ ion is stable in strongly acidic aqueous solutions, but in solutions of pH > 3 undergoes deprotonation and subsequent disproportionation reactions. The manganese(II) complex ion is less stable in acidic solutions, undergoing hydrolysis at a rate showing a first-order dependence on both proton and chloride ion concentrations. Both [Mn(sar)]2+ and [Mn(sar)]3+ can be obtained in chiral forms, and the rate constant for electron self-exchange obtained by polarimetric measurements on solutions of mixtures of [Mn(sar)]2+ and [Mn(sar)]3+ of opposite chirality is 30 dm3 mol-1 s-1 at 298 K, I = 0.1 M.