Base catalysed hydrolysis of β2-cis-(chloro)(benzimidazole)(trienthylenetetramine)cobalt(III)cation: A comparison of the reactivities of the benzimidazole and benzimidazolato species of the complex

Abstract

The base hydrolysis of <em>β</em>₂<em>-cis</em>-(chloro)(benzimidazole)(triethylenetetramine)cobalt(III) obeys the rate law: -dln[complex]_total/dt=(<em>k</em><img src='http://www.niscair.res.in/jinfo/equation86.gif' border=0> [OH^-] + <em>k</em><img src='http://www.niscair.res.in/jinfo/equation87.gif' border=0> <em>k</em>_OH[OH^-]²)/(1 + <em>K</em>_OH[OH^-]) in the range of <em>p</em>H = 7.40-12, for which <em>k</em><em><img src='http://www.niscair.res.in/jinfo/equation86.gif' border=0>, k</em><img src='http://www.niscair.res.in/jinfo/equation87.gif' border=0> and K_OH denote the second order rate constants of the benzimidazole, benzimidazolato form of the complex, and the equilibrium constant for the OH^- promoted formation of the benzimidazolato species from the parent complex respectively. At 25°C and <em>1= </em>0.05 mol dm^-3, <em>k</em><img src='http://www.niscair.res.in/jinfo/equation86.gif' border=0> = (4.79±0.15) x 10⁶ dm³ mol^-1 S^-1, <em>k</em><img src='http://www.niscair.res.in/jinfo/equation87.gif' border=0> = (9.2±0.6) x 10⁴ dm³ mol^-1 S^-1and KOH = (2.64 ± 0.07) x 10⁵ dm³ mol^-1. At [OH^-] = (2.6-11.6) x 10^-3 mol dm^-3 (<em>l </em>= 1.0 mol dm^-3) and 10-30°C, <em>k</em>_obs,= <em>k</em><img src='http://www.niscair.res.in/jinfo/equation87.gif' border=0> [OH^-] is valid with <em>k</em><img src='http://www.niscair.res.in/jinfo/equation87.gif' border=0> (25°C) = (4.42 ± 0.04) x 10⁴ dm³ mol^-1 S^-1, <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0> <em>H</em>^≠ = 67.6 ± 1.0 kJ mol^-1 and <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0> <em>S</em>^≠ = 71 ± 3 JK^-1 mol^-1. There is no general base catalysis discounting the possibility of the rate limiting NH-deprotonation mechanism. Comparison of the rate data shows that the benzimidazole form of the complex ((trien)Co(bzmH)Cl²⁺) base hydrolyses ~ 40 times faster than its benzimidazolato analogue despite the fact that the conjugate base of the latter (net charge zero) is likely to be more reactive than the same derived from the former (net charge + 1). This reactivity trend (i.e. <em>k</em><img src='http://www.niscair.res.in/jinfo/equation86.gif' border=0> > <em>k</em><img src='http://www.niscair.res.in/jinfo/equation87.gif' border=0><em>) </em>reflects the perturbation of the <em>pK </em>of the NH proton of the coordinated trien forming the reactive conjugate base. In the conventional SN₁CB mechanism the reactive conjugate base is believed to be generated by the deprotonation of the planar secondary N - H of the trien ligand <em>trans </em>to the coordinated benzimidazole or its conjugate base (bzm).