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 MnIII(salen)(OH $_{2})_{2}^{+}$ ((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 MnII with the reaction stoichiometry, |Δ[MnIII]/Δ[HGl]| = 2. The overall reaction involves fast equilibrium pre-association of MnIII(salen)(OH $_{2})_{2}^{+}$ 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. MnIII(salen)(OH2)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for MnIII(salen)(OH2)(HGl)+ (105 k 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-MnIII(salen)(OH $_{2})_{2}^{+}$ , trans-MnIII(salen)(OH2)(Gl), and trans- MnIII(salen)(OH2)(HGl)+ (all high spin MnIII(d4) systems), reveals strongest axial distortion for the (aqua)(Gl) complex ; HGl bound to Mn III 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: MnIII(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 (k HG ∼ 5 k Gl). 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.