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

作者：Kar Akshaya K; Acharya Achyutananda; Pradhan Guru C; Dash Anadi C^*

来源：Journal of Chemical Sciences, 2014, 126(3): 547-559.

DOI：10.1007/s12039-014-0605-0

• Glyoxylate
• Mn-III(salen)
• electron transfer
• kinetics
• DFT

The kinetics of oxidation of glyoxylic acid (HGl) by Mn-III(salen)(OH ((H(2)salen = N,N'- bis(salicylidene)ethane-1,2-diamine) is investigated at 30.0-45.0A degrees C, 1.83 a parts per thousand currency sign pH a parts per thousand currency sign 6.10, I = 0.3 mol dm(-3)(NaClO4). The products are identified as formic acid, CO2 and Mn-II with the reaction stoichiometry, |Delta[Mn-III]/Delta[HGl]| = 2. The overall reaction involves fast equilibrium pre-association of Mn-III(salen)(OH 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)(+) (10(5) k (et) = 2.8 +/- 0.3 s(-1) at 35.0A degrees 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, trans-Mn-III(salen)(OH2)(Gl), and trans- Mn-III(salen)(OH2)(HGl)(+) (all high spin Mn-III(d(4)) 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 aEuro broken vertical bar O hydrogen bond distance 1.745 ) 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 (k (HG) similar to 5 k (Gl)). The high values of activation enthalpy ( Delta H (not equal)= 93-119 kJ mol(-1)) are indicative of substantial reorganization of the bonds as expected for inner-sphere ET process.

• 出版日期2014-5