Thermal and photochemical reactions ofbis(diamine)(sulfito)cobalt(III) complexes: effect ofchelate-ring size

Dash, Anadi C. ; Jena, Kishore C. ; Roy, Ansuman ; Mukherjee, Dipti ; Aditya, Sukumar (1997) Thermal and photochemical reactions ofbis(diamine)(sulfito)cobalt(III) complexes: effect ofchelate-ring size Journal of the Chemical Society Dalton Transactions, 1997 (14). pp. 2451-2462. ISSN 1472-7773

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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)(SO3S)]+ by N, 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 (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 CoIIIand SIV 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 (CoIII–SO3+ → CoIII–OSO2+) also occurred at comparable rates [kiso = (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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