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

摘要

The trans-Mn-III(salen)(OH2)(2)(+) undergoes reversible aqua ligand substitution by HOX- (H(2)salen = N,N'-bis(salicylidene)ethane-1,2-diamine; HOX- = O--COCO2H) with k(1)/dm(3) mol(-1) s(-1) (k(-1)/s(-1)) = 11.8 +/- 0.7 (0.255 +/- 0.02), Delta H-not equal/kJ mol(-1) = 54.6 +/- 0.8 (64.2 +/- 6.7), Delta S-not equal/J K-1 mol(-1) = -41.2 +/- 2.6 (-40.8 +/- 22.7) at 25.0 degrees C and I = 0.3 mol dm(-3). The low values of the activation enthalpy and nearly the same and negative values of the activation entropy are ascribed to an associative transition state for this interchange process (I-a mechanism). The redox reaction that follows involves several paths and the products are Mn-II and CO2 identified by ESR spectroscopy and conventional test, respectively. The rate retardation by acrylamide monomer with no perceptible polymerization during the course of the redox reaction supports the involvement of the radical intermediate, C2O4-center dot (= CO2 + CO2-center dot) which succeeds in reducing Mn-III species much faster than the dimerisation of its congener, CO2-center dot in keeping with the stoichiometry, vertical bar[Delta Mn-III]/Delta[OX]vertical bar = 2. The trans-Mn-III(salen)(OH2)(HOX) and its conjugate base, trans-Mn-III(salen)(OH2)(OX)(-) are virtually inert to intramolecular reduction of the MnIII centre by the bound oxalate species but undergo facile electron transfer by H2OX, HOX- and very slowly by OX2- following the reactivity sequence, k(H2OX) > k(HOX) > k(OX) and featuring second order kinetics. The rate retardation by the anionic micelles of SDS (sodium dodecyl sulfate) and rate enhancement by N-3(-) provide supportive evidence in favor of the proposed mechanistic pathways. The structure optimization of trans-Mn-III(salen)(OH2)(HOX) (A), trans-Mn-III(salen)(HOX)(2)(-) (B), trans-Mn-III(salen)(OH2)(OX)(-) (C), trans-Mn-III(salen)(OH2)(H2OX)(+) (E-1), and trans-Mn-III(salen)(HOX)(H2OX) (E-2) {all high spin Mn-III(d(4))} by Density Functional Theory (DFT) reveals that the structural trans-effect of the unidentately bonded OX2- in C is the strongest and Mn-III assumes five coordination with the H2O molecule (displaced from the Mn-III centre), hydrogen bonded to the phenoxide oxygen moiety. The computational study highlights different modes of H-bonding in structures A-E. The activation parameters for the redox reactions, A + HOX- and A + H2OX, Delta H-not equal/kJ mol(-1) (Delta S-not equal/J K-1 mol(-1)): 42.5 +/- 6.2, (-106 +/- 20) and 71.7 +/- 7.7 (+12 +/- 25), respectively, are indicative of different degrees of ordering and reorganization of bonds as expected in the case of a proton coupled electron transfer (PCET) process.