Kinetics and mechanism of the reaction of sulphur(IV) with <i>trans-bis</i>(aqua)(N,N' -salicy lidiniminato-<i>ortho-</i>phenylenediamine cobaltate (III) : The <i>trans</i> effect of <i>S</i>-bonded sulphite

Abstract

The reaction of <i>trans</i>-(aqua)(hydroxo)(N,N'-salicylidiniminato-<i>ortho</i>-phenylenediamine) cobaltate(III) with SO₂ results in the formation of <i>S</i>-bonded monosulphito complex, <i>trans</i>-[Co(salophen) (OH₂)(SO₃)]^- as the primary product; the Co^III-S bond formation is concerted with Co^III-<i>O</i> bond breaking. At 25°C, <i>I </i>= 0,5 mol dm^-3 (NaNO₃), the rate and activation parameters for this process are: <i>k^SO2 </i>= (4.8)<i> </i>× 10⁸ dm³ mol^-1 s^-1, Δ<i>H</i>^±<i> </i>= 23.6±3.2 kJ mol^-1 , Δ<i>S</i>^± = - 0.14±16.7 J K^-1 mol^-1. The aqua-sulphito complex further undergoes substitution of the aqua ligand by imidazole and SO₃^2-; the formation of the imidazole complex, <i>trans</i>-[Co(salophen)(imH)(SO₃)ris, however, reversible. The rate data indicate that the <i>trans </i>effect of the S-bonded sulphite in <i>trans</i>-[Co(salophen)(OH₂)(SO₃)]^- is only marginally stronger for the aqua ligand substitution by imidazole and SO₃^2- than in the case of its <i>salen </i>analogue reported earlier. The kinetic inertness of <i>trans</i>[Co(salophen)(OH₂)(SO₃)]^- towards H⁺ catalysed aquation in comparison to its salen analogue (<i>k</i>_H^salen/<i>k</i>_H^salophen= 10⁴ at 25°C) demonstrates the effects of rigidity and extended electron delocalization by the salophen moiety. The rate constants of base hydrolysis of <i>trans</i>[Co(salophen)(OH)(SO₃)]^2- at [OH^-]_T = 0.01-0.5 , <i>I= </i>0.5 mol dm^-3 (25°C) obeys <i>k</i>_obs = <i>k_o </i>+ <i>k</i>_OH [OH^-] with <i>k</i>₀<i> </i>= (0.89±0.04 ) × 10^-3s^-1 and <i>k</i>_OH<i> </i>=(6.4±0.6) × 10^-3 dm³ mol^-1 s^-1. A finite value of <i>k</i>_o<i> </i>indicates that the <i>trans-</i>labilising action of the coordinated OH group is stronger than that of the bound SO₃^2-.It is suggested that the second order rate constant <i>k</i>_OH, is associated with a mechanism which involves the formation of a reactive C-conjugate base from the aldimine function , H-C=N-R.