Synthesis, Resolution and Kinetics of Electron Self-Exchange of High-Spin Manganese(II)/(III) Cage Complexes

Abstract

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°′+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 I 42d, a 15.549(6), c 19.014(6) Ǻ, R 0.051, Rw 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)Ǻ. 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^-1s^-1 at 298 K, I = 0.1 M.