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)(OH 2) + 2 ((H 2 salen = N,Nbis(salicylidene)ethane-1,2-diamine) is investigated at 30.0-45.0 • C, 1.83 ≤ pH ≤ 6.10, I = 0.3 mol dm −3 (NaClO 4). The products are identified as formic acid, CO 2 and Mn II with the reaction stoichiometry, | [Mn III ]/ [HGl]| = 2. The overall reaction involves fast equilibrium pre-association of Mn III (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. Mn III (salen)(OH 2)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for Mn III (salen)(OH 2)(HGl) + (10 5 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-Mn III (salen)(OH 2) + 2 , trans-Mn III (salen)(OH 2)(Gl), and trans-Mn III (salen)(OH 2)(HGl) + (all high spin Mn III (d 4) systems), reveals strongest axial distortion for the (aqua)(Gl) complex ; HGl bound to Mn I I I 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)(OH 2)(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 (H = = 93-119 kJ mol −1) are indicative of substantial reorganization of the bonds as expected for inner-sphere ET process.

Kinetics and mechanistic study of the reduction of $$\hbox {Mn}^{\mathrm{III}}$$ Mn III by oxalate in Salophen scaffold: relevance to oxalate oxidase

“…The mass spectrum was recorded on a Macromass Q-TOF ESI-MS mass spectrometer. 1 Figure S3a, Supplementary Information) as observed for this complex in CD 3 OD by Ciringh et al 8 The weak and broad imine proton resonance reported by Ciringh et al, (at −398 ppm) 8 lies far beyond the accessible range probed in the present work. The chemical shifts of Mn III Schiff base complexes have been attributed to the contact term which occurs through phenoxide oxygen and the imine group.…”

“…The stock solution of NaClO 4 was prepared and standardized as described earlier. 1 The stock solution of the complex (5 × 10 −3 mol dm −3 , pH 5, 10% v/v MeOH/water) was protected from light and stored in a refrigerator at ∼ 20 • C when not in use. It was not allowed to age for more than 24 h.…”

“…The very slow reactions (k obs ≤ 4 × 10 −5 s −1 ) were followed by initial rate method. 1 The rate data at 25 • C are collected in Table 1. The rate data at (30-40) • C and some representative kinetic plots are presented in the supporting information in the Table S1(a,b) and Figure S5(a-c), Supplementary Information, respectively, in Supplementary Information.…”

“…Previously we have reported the redox reactions of Mn III (Salen) + by glyoxylate 1 and oxalate. 2 The redox process involves fast pre-equilibrium of the glyoxalate and oxalate species followed by slow electron transfer reactions between Mn III (Salen)X (X = glyoxylate, HOX − , OX 2− ) and the corresponding acids, and their anions.…”

Ligand substitution and electron transfer reactions of trans-(diaqua)(salen)manganese(iii) with oxalate: an experimental and computational study

“…However, the studies devoted to understand the mechanisms of ligand substitution and redox reactions of Mn III are limited. In view of the importance of oxalate in biology and Mn III (salen) as a suitable oxidant for oxalate oxidation, we, in continuation of our other investigations 16,17 of the mechanism of ligand substitution and electron transfer at the Mn III centre, present here a thorough study on trans-Mn III (salen)(OH 2 ) 2 + + oxalate system over extended pH and temperature ranges. To the best of our knowledge there is no earlier literature report on such a study, although oxidation of oxalate by higher valent manganese has been the topic of several investigations in the past.…”