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

• Published in: RSC advances Vol. 4; no. 102; pp. 58867 - 58879
• Main Authors: Kar, Akshaya K., Acharya, Achyut N., Mundlapati, V. Rao, Pradhan, Guru C., Biswal, Himansu S., Dash, Anadi C.
• Format: Journal Article
• Language: English
• Published: 01.01.2014
• Subjects: Bonding strength; Carbon dioxide; Computation; Electron transfer; Ligands; Oxalates; Redox reactions; Sodium dodecyl sulfate
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/C4RA10324F

The trans -Mn III (salen)(OH 2 ) 2 + undergoes reversible aqua ligand substitution by HOX − (H 2 salen = N , N ′-bis(salicylidene)ethane-1,2-diamine; HOX − = − O–COCO 2 H) with k 1 /dm 3 mol −1 s −1 ( k −1 /s −1 ) = 11.8 ± 0.7 (0.255 ± 0.02), Δ H ≠ /kJ mol −1 = 54.6 ± 0.8 (64.2 ± 6.7), Δ S ≠ /J K −1 mol −1 = −41.2 ± 2.6 (−40.8 ± 22.7) at 25.0 °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 CO 2 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, C 2 O 4 − ˙ (= CO 2 + CO 2 − ˙) which succeeds in reducing Mn III species much faster than the dimerisation of its congener, CO 2 − ˙ in keeping with the stoichiometry, |[ΔMn III ]/Δ[OX]| = 2. The trans -[Mn III (salen)(OH 2 )(HOX) and its conjugate base, trans -Mn III (salen)(OH 2 )(OX) − are virtually inert to intramolecular reduction of the Mn III centre by the bound oxalate species but undergo facile electron transfer by H 2 OX, HOX − and very slowly by OX 2− 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)(OH 2 )(HOX) ( A ), trans -Mn III (salen)(HOX) 2 − ( B ), trans -Mn III (salen)(OH 2 )(OX) − ( C ), trans -Mn III (salen)(OH 2 )(H 2 OX) + ( E1 ), and trans -Mn III (salen)(HOX)(H 2 OX) ( E2 ) {all high spin Mn III (d 4 )} by Density Functional Theory (DFT) reveals that the structural trans -effect of the unidentately bonded OX 2− in C is the strongest and Mn III assumes five coordination with the H 2 O 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 + H 2 OX, ΔH ≠ /kJ mol −1 (Δ S ≠ /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.