Synthesis and electrochemical properties of oxo-bridged manganese dimers incorporating alkali and alkaline earth cations

Abstract

The dimer [( 1)Mn III] 2(μ-O) ( 3) was synthesized by oxygenation of the mononuclear Mn(II) complex in CH 3CN. The ligand used was 3,3′-17-crown-6-SAL-3-CH 3-OPHEN ( 1). Dimer 3 reacted with KPF 6 and Ba(SO 3CF 3) 2 in DMF forming 3·2KPF 6 and 3·2Ba(SO 3CF 3) 2 and these were isolated and characterized by spectroscopic and electrochemical means. All of the dimers participated in a reversible two-electron transfer process that produced the corresponding Mn II,Mn II (μ-oxo) dimer. Inclusion of cations into the crown ether portions of the dimer caused that formal reduction potential of 3 to shift to more positive values: E o ( 3)= −120 mV, CH 2Cl 2; E o ( 3·2KPF 6)= 0 mV, CH 3CN; E o ( 3·2Ba(SO 3CF 3) 2)= 180 mV, CH 3CN all versus SSCE. Results of UV-Vis experiments suggested that the shift in formal reduction potential was caused primarily by an electrostatic effect and not a perturbation of the d orbitals on the Mn(III) centers.