Glyoxylate as a reducing agent for manganese(III) in salen scaffold: a kinetics and mechanistic study

Abstract

The kinetics of oxidation of glyoxylic acid (HGl) by Mn^III(salen)(OH₂)₂⁺ ((H₂salen = N,N′- bis(salicylidene)ethane-1,2-diamine) is investigated at 30.0–45.0°C, 1.83 ≤ pH ≤ 6.10, I = 0.3 mol dm⁻³(NaClO₄). The products are identified as formic acid, CO₂ and Mn^II with the reaction stoichiometry, |Δ[Mn^III]/Δ[HGl]| = 2. The overall reaction involves fast equilibrium pre-association of Mn^III(salen)(OH₂)₂+ with HGl and its conjugate base Gl⁻ forming the corresponding inner sphere complexes (both HGl and Gl^- being the monohydrate gem-diol forms) followed by the slow electron transfer steps. In addition, the second order electron transfer reactions involving the inner-sphere complexes and HGl/Gl^- are also observed. The rate, equilibrium constants and activation parameters for various steps are presented. Mn^III(salen)(OH₂)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for Mn^III(salen)(OH₂)(HGl)⁺ (10⁵k_et = 2.8 ± 0.3 s^-1 at 35.0°C) reflecting the involvement of proton coupled electron transfer mechanism in the latter case. A computational study of the structure optimization of the complexes, trans-Mn^III(salen)(OH₂)₂⁺, trans-Mn^III(salen)(OH₂)(Gl), and trans- Mn^III(salen)(OH₂)(HGl)⁺ (all high spin Mn^III(d⁴) systems), reveals strongest axial distortion for the (aqua)(Gl) complex ; HGl bound to MnIII centre by the C=O function of the carboxyl group in the (aqua)(HGl) complex facilitates the formation of a hydrogen bond between the proton of the carboxyl group and the coordinated phenoxide moiety ((O-H. . .O hydrogen bond distance 1.745 Å) and the gem-diols are not involved in H-bonding in either case. A rate comparison for the second order paths: Mn^III(salen)(OH₂)(HGl)/Gl),^+/0+ HGl/Gl^- → products, shows that HGl for the (aqua)(HGl) complex is a better reducing agent than Gl^- for the (aqua)(Gl) complex (^kHG ~ 5 ^kGl). The high values of activation enthalpy (ΔH^≠ = 93–119 kJ mol⁻¹) are indicative of substantial reorganization of the bonds as expected for inner-sphere ET process.