Kinetics and mechanism of the reaction of sulphur(IV) with N,N′-ethylene-bis(sali-cylidiniminato)manganese-(III) in aqueous solution

✍ Scribed by A. C. Dash; A. Das

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 173 KB
• Volume: 31
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/(sici)1097-4601(1999)31:9<627::aid-kin4>3.0.co;2-s

✦ Synopsis

The reaction of (diaqua)(N,NЈ-ethylene-bis(salicylidiniminato)manganese(III) with aqueous sulphite buffer results in the formation of the corresponding mono sulphito complex, [Mn(Salen)(SO 3 )] Ϫ (S-bonded isomer) via three distinct paths: (i) Mn(Salen)(OH 2 ) 2 ϩ ϩ HSO 3 Ϫ : (k 1 ); (ii) Mn(Salen)(OH 2 ) 2 ϩ ϩ SO 3 2Ϫ : (k 2 ); (III) Mn(Salen)(OH 2 )(OH) ϩ SO 3 2Ϫ : (k 3 ) in the stopped flow time scale. The fact that the mono sulphito complex does not undergo further anation with SO 3 2Ϫ /HSO 3 Ϫ may be attributed to the strong trans-activating influence of the S-bonded sulphite. The values of the rate constants (10 Ϫ2 k i /dm 2 mol Ϫ1 s Ϫ1 at 25ЊC, I ϭ 0.3 mol dm Ϫ3 ), ⌬H i # /kJ mol Ϫ1 and ⌬S i # /J K Ϫ1 mol Ϫ1 respectively are: 2.97 Ϯ 0.27, 42.4 Ϯ 0.2, Ϫ55.3 Ϯ 0.6 (i ϭ 1); 11.0 Ϯ 0.8, 33 Ϯ 3, Ϫ75 Ϯ 10 (i ϭ 2); 20.6 Ϯ 1.9, 32.4 Ϯ 0.2, Ϫ72.9 Ϯ 0.6 (i ϭ 3). The trend in reactivity (k 2 Ͼ k 1 ), a small labilizing effect of the coordinated hydroxo group (k 3 /k 2 Ͻ 2), and substantially low values of ⌬S # suggest that the mechanism of aqua ligand substitution of the diaqua, and aqua-hydroxo complexes is most likely associative interchange (I a ). No evidence for the formation of the O-bonded sulphito complex and the ligand isomerization in the sulphito complex, (Mn III -OSO 2 : Mn III -SO 3 ), ensures the selectivity of the Mn III centre toward the S-end of the S IV species.

The monosulphito complex further undergoes slow redox reaction in the presence of excess sulphite to produce Mn II , S 2 O 6 2Ϫ and SO 4 2Ϫ . The formation of dithionate is a consequence of the fast dimerization of the SO 3Ϫ . generated in the rate determining step and also SO 4 2Ϫ formation is attributed to the fast scavenging of the SO 3Ϫ . by the Mn III species via a redox path. The internal reduction of the Mn III centre in the monosulphito complex is insignificant. The redox reaction of the monosulphitomanganese(III) complex operates via two major paths, one involving HSO 3Ϫ and the other SO 3 2Ϫ . The electron transfer is believed to be outersphere type. The substantially negative values of activation entropies (⌬S # ϭ Ϫ(1.3 Ϯ 0.2) ϫ 10 2 and Ϫ(1.6 Ϯ 0.2) ϫ 10 2 J K Ϫ1 mol Ϫ1 for the paths involving HSO 3Ϫ and SO 3 2Ϫ respectively) reflect a considerable degree of ordering of the reactants in the act of electron transfer.