Tunability of the MIIMIII/MII2 and MIII2/MIIMIII (M = Mn, Co) couples in bis-μ-O,O′-carboxylato-μ-OR bridged complexes

Abstract

A comparison of the electrochemical properties of a series of dinuclear complexes [M₂(L)(RCO₂)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, CH₃, CF₃ or 3,4-dimethoxybenzoate demonstrates: (i) The electron-withdrawing sulfonyl groups in the backbone of bpsmp⁻ stabilize the [M₂(bpsmp)(RCO₂)₂]⁺ complexes in their M^II₂ oxidation state compared to their [M₂(bpbp)(RCO₂)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^IIM^III ↔ M^II₂ processes range from 0.31 to 1.41 V for the [Co₂(L)(RCO₂)₂]^+/2+ complexes and from 0.54 to 1.41 V for the [Mn₂(L)(RCO₂)₂]^+/2+ complexes versusAg/AgCl for E^°_ox(M^IIM^III/M^II₂). The extreme limits are defined by the complexes [M₂(bpbp)(CH₃CO₂)₂]⁺ and [M₂(bpsmp)(CF₃CO₂)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^IIM^III/M^II₂) couple of the Mn and Co complexes, respectively. The second one-electron process, M^IIM^III ↔ M^III₂ was also observed in some cases. The lowest potential recorded for the E°(M^III₂/M^IIM^III) couple was 0.63 V for [Co₂(bpbp)(CH₃CO₂)₂]²⁺ and the highest measurable potential was 2.23 V versusAg/AgCl for [Co₂(bpsmp)(CF₃CO₂)₂]²⁺.