Aquo-pentamine co(III) complexes as models for carbonic anhydrase

Abstract

The hydrolysis of 4-nitrophenyl acetate by metal complexes Co(en) 2(imH)H 2O 3+, Co(en) 2(bzmH)H 2O 3+, and Co(en) 2(imCH 3)H 2O 3+, (imH = imidazole, bzmH = benzimodazole, imCH 3 = methyl imidazole) has been investigated in the pH range 5.4–8.9. The small difference in nucleophilic reactivity in the pH range 5.4–6.7 is assumed to be due to hydrogen bonding abilities of the imidazole and substituted imidazole ligands and small p K a differences ( k 2(imH) = 2.2 × 10 −2 M −1 sec −1, K 2(bzmH) = 5.68 × 10 −2 M −1 sec −1, k 2(imCH 3) = 1.35 × 10 −2 M −1 sec −1, 40°C, l = 0.3 NaClO 4 p K a(imH) = 6.2, p K a(imCH 3) = 6.2 and p K a(bzmH) = 5.9). In the pH range 7.8–8.9, the differences in nucleophillic reactivity ( k 3(imH) = 85.5 × 10 −2 M −1 sec −1, k 3(bzmH) = 33.4 × 10 −2 M −1 sec −1, 40°C, I = 0.3 NaClO 4) are reconciled with a significant steric factor outweighing the acidity of the benzimidazole complex. In the pH region 6.7–7.7, the deviation from linearity is presumably due to both hydroxo and imido ligands functioning as nucleophiles, the latter being about 40 times stronger than the former.