Kinetics of the formation, reactivity and crystal structure of the sulfite-bridged cation di-µ-hydroxo-µ-(sulfito-O,O′)-bis[(1,4,7-triazacyclononane)chromium(III)

Abstract

The kinetics of the formation of [(tacn)Cr(µ-OH)2(µ-OSO2)Cr(tacn)]2+(tacn = 1,4,7-triazacyclononane) from trans-[(H2O)(tacn)Cr(µ-OH)2Cr(tacn)(H2O)]4+ in aqueous buffered sulfite solution have been studied spectrophotometrically. The formation takes place in two steps. The first step which is a rapid uptake of SO2 was studied at 532 nm in the range 1.82 ⩽ pH ⩽ 5.93, 0.03 ⩽[S]t⩽ 0.05 mol dm–3([S]t=[SO2]+[HSO3–]+[SO32–]) and 19.9 ⩽θ⩽ 29.9 °C. The rate constant for SO2 uptake at 25.2 °C was found to be 2.10 ± 0.09 dm3 mol–1 s–1 with ΔH‡= 94 ± 2 kJ mol–1 and ΔS‡= 110 ± 8 J K–1 mol–1. The second step in the formation is the pH-independent isomerization of the trans-aqua-sulfito species, to the cis analogue [kiso=(4.5 ± 0.1)× 10–3 s–1, 25.2 °C]; followed by a fast unobserved ring-closure step. The sulfido-bridged product undergoes hydroxo bridge cleavage in acid solution (0.030 ⩽[H+]⩽ 1.0 mol dm–3) to form cis-[(H2O)(tacn)Cr(µ-OH)(µ-O2SO)Cr(tacn)(H2O)]3+ in which the sulfite bridge remains intact [k=(6.1 ± 0.1)× 10–2 s–1, 30.0 °C]. The crystal structure of [(tacn)Cr(µ-OH)2(µ-O2SO)Cr(tacn)][ClO4]2·3H2O shows that the sulfite moiety is bridging the two metal centres via two of its oxygen atoms with partial inversion of the sulfur.