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^–]+[SO₃^2–]) 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 [k_iso=(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.