Tunability of the M~(II)M~(III)/M~(II)_2 and M~(III)_2/M~(II)M~(III) (M = Mn, Co) couples in bis-μ-O,O′-carboxylato-μ-OR bridged complexes

摘要

A comparison of the electrochemical properties of a series of dinuclear complexes [M_2(L)(RCO_2)_2]~+ with M = Mn or Co, L = 2,6-bis(N,N-bis-(2-pyridylmethyl)-sulfonamido)-4-methylphenolato (bpsmp~-) or 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert- butylphenolato (bpbp~-) and R = H, CH_3, CF_3 or 3,4-dimethoxybenzoate demonstrates: (i) The electron-withdrawing sulfonyl groups in the backbone of bpsmp~- stabilize the [M_2(bpsmp) (RCO_2)_2]~+ complexes in their M ~(II)_2 oxidation state compared to their [M _2(bpbp)(RCO_2)_2]~+ analogues. Manganese complexes are stabilised by approximately 550 mV and cobalt complexes by 650 mV. (ii) The auxiliary bridging carboxylato ligands further attenuate the metal-based redox chemistry. Substitution of two acetato for two trifluoroacetato ligands shifts redox couples by 300-400 mV. Within the working potential window, reversible or quasi-reversible M~(II)M~(III) ? M~(II)_2 processes range from 0.31 to 1.41 V for the [Co_2(L)(RCO_2)_2]~(+/2+) complexes and from 0.54 to 1.41 V for the [Mn2(L)(RCO_2) _2]~(+/2+) complexes versus Ag/AgCl for E°ox(M ~(II)M~(III)/M~(II)_2). The extreme limits are defined by the complexes [M_2(bpbp)(CH_3CO _2)_2]~+ and [M_2(bpsmp)(CF _3CO_2)_2]~+ for both metal ions. Thus, tuning the ligand field in these dinuclear complexes makes possible a range of around 0.9 V and 1.49 V for the one-electron E°ox(M~(II)M ~(III)/M~(II)_2) couple of the Mn and Co complexes, respectively. The second one-electron process, M~(II)M~(III) ? M~(III)_2 was also observed in some cases. The lowest potential recorded for the E°(M~(III)_2/M ~(II)M~(III)) couple was 0.63 V for [Co_2(bpbp) (CH_3CO_2)_2]2+ and the highest measurable potential was 2.23 V versus Ag/AgCl for [Co_2(bpsmp) (CF_3CO_2)_2]2+.