Thermal and photochemical reactions of bis(diamine)(sulfito)cobalt(III) complexes: effect of chelate-ring size

Abstract

The kinetics of formation of cis-[Co(tn)₂(OH₂)(OSO₂-O )]⁺ (tn = 1,3-diaminopropane), its acid-catalysed aquation to the parent diaqua complex, anation of trans-[Co(tn)₂(OH₂)(SO₃- S )]⁺ by N₃^-, NCS^-, SO₃^2-–HSO₃^- , anation of trans-[Co(tn)₂(OH)(SO₃-S  )] by SO₃^2- and acid-catalysed aquation of trans-[Co(tn)₂(SO₃-S ) ₂]^- to the corresponding (aqua)(sulfito-S ) complex were investigated and the results compared with analogous data for the corresponding 1,2-diaminoethane (en) complexes. Expanding the chelate-ring size from five to six had virtually no effect on the rate of formation of the sulfito-O complex, but retarded its acid-catalysed aquation. The latter effect was attributed to a pK perturbation; intramolecular hydrogen bonding between the co-ordinated H₂O and sulfite in cis-[Co(tn)₂(OH₂)(OSO₂-O )]⁺ hindered the protonation pre-equilibrium of the sulfito-O complex involved in the acid-catalysed aquation. This is further supported by the fact that there was no ring-size effect on the acid-catalysed aquation of trans-[Co(L–L)₂(OH₂)(OSO₂) ]⁺ (L–L = tn or en). The strong labilising action due to chelate-ring expansion is remarkably attenuated by the trans effect of S-bonded sulfite as observed in the anation of trans-[Co(L–L)₂(OH₂/OH)(SO₃ -S )]^+/0. However, trans-[Co(tn)₂(OH₂)(SO₃- S )]⁺ was found to be prone to intramolecular electron transfer between Co^III and S^IV under thermal conditions unlike its en analogue, further reflecting the ring-size effect. Flash photolysis of trans-[Co(L–L)₂(OH₂)(SO₃ -S )]⁺ (L–L = en or tn) generated the transient trans-[Co(L–L)₂(OH₂)(OSO₂ )]⁺. The photochemical ligand isomerisation of both complexes (Co^III–SO₃⁺ → Co^III–OSO₂⁺) also occurred at comparable rates [k_iso = (4.1 ± 0.8) × 10⁴ and (3.2 ± 1.3) × 10⁴ s^-1 at 25 °C for the en and tn complexes respectively]. Steady-state photolysis at 254 nm indicated that trans-[Co(en)₂(OH₂)(SO₃-S  )]⁺ underwent photoaquation and photoreduction. Strikingly photoreduction could not be detected for this complex at pH > 8.

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