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

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 N-3(-), NCS-, SO32--HSO3-, anation of trans-[Co(tn)(2)(OH)(SO3-S)] by SO32- 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 ten) complexes. Expanding the chelate-ring si:le 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 trails 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+ --> (CoOSO2+)-O-III) also occurred at comparable rates [k(iso) = (4.1 +/- 0.8) x 10(4) and (3.2 +/- 1.3) x 10(4) s(-1) at 25 degrees 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.