Interaction of N,N'-ethylene bis(salicylamide) with iron (III): A magneto-structural, electrochemical, and mechanistic investigation

Abstract

The reaction of N,N&cent;-ethylene bis(salicylamide) (H₂SALM) with ion(lll) results in the formation of a 1: 1 complex in solution. The rate and activation parameters for the reactions, Fe(OH₂)+ H₂SALM &reg; Fe(OH₂)₂(SALMH)²⁺, and Fe(OH₂)₅,(OH)²⁺ + H₂SALM &reg; Fe(OH₂)₂(SALMH)²⁺ are: <em>k</em>(25&deg;C)/dm³ mol^-1 s^-1 = 2.1 &plusmn; 0.6, (2.67 &plusmn; 0.1) x 10³; &Delta;<em>H&nbsp;</em>(kJ mol^-1) = 97 &plusmn; 8, 64 &plusmn; 1; &Delta;S'(J K^-1 mol^-1) = 87 &plusmn; 28, 35 &plusmn;4 (<em>I</em>= 1.0 NaCIO₄, 8% v/v MeOH/H₂O) respectively; the mechanism is <em>I_a. </em>for hexa-aquairon(III) and <em>I</em>_d for hydroxopentaaquairon(III). The substantially large values of the activation parameters as compared to those reported for the water exchange reactions of the iron (III) species are accountable in terms of the solvent shell reorganisation in the process. The species Fe(OH₂)₂(SALMH)²⁺ undergoes facile deprotonation of the phenolic and amide functions which strongly suggests that these remain coordinated to the iron(lll) centre (N bonding for -C=ONH-) in the complex. While the complexation reaction displays a single relaxation mode in the stopped flow time scale, the amide deprotonation of the complex obeys biphasic kinetics (Fe(SALM)⁺&reg; <em>Fe(SALM</em>-H), <em>k_t; </em>Fe(SALM-H) &reg; Fe(SALM-2H)^-, <em>k_s) </em>with general base catalysis (k_H2O&lt; k_OH&lt; <em>k</em>_tris<em>' </em>&lt; k_H2BO3^-, tris = trishydroxymethylaminomethane, <em>k </em>= <em>k_t, </em>or <em>k_s,). </em>The complex isolated in the solid state as [Fe(SALM)(NO₃)(OH₂)]H₂O displays I. R. bands characteristic of H₂O and NO₃; coordinated to iron(III) centre. Its m_eff = 5.96 B M and 4 line (broad) X-band ESR spectrum are characteristics of high spin iron(III) with five unpaired electrons. The cyclic voltamogram ( Ag/AgCI reference) of the insitu generated complex, Fe(SALM)⁺ (<em>p</em>H = 5.01, 10% v/v MeOH/H₂O, 1.0 KNO₃, 25&deg;C) shows quasi-reversible redox process with <em>E</em>_pc and <em>E_ac. </em>as -0.391V and -0.326V respectively.