Kinetics and mechanism of aquation reactions of dihalo-, diacetonitrile- and bromoacetonitrile-(3,3′,3″-triaminotripropylamine)cobalt(III) perchlorate

Abstract

The octahedral cobalt(III) complexes with the tripodal quadridentate amine 3,3′,3″-triaminotripropylamine [trpn = N(CH 2CH 2CH 2NH 3) 3], [Co(trpn)Cl 2]ClO 4, [Co(trpn)Br 2ClO 4, [Co(trpn)F 2]CO 4·3/2H 2O, [Co(trpn)(CH 3CN) 2](ClO 4) 3 and [Co(trpn)(CH 3CN)Br]BrClO 4 have been synthesized. The acid hydrolysis of [Co(trpn)Cl 2] + and the rate of bromide substitution in [Co(trpn)(CH 3CN)Br] 2+, clearly demonstrate the extreme lability of these complexes. The aquation of [Co(trpn)F 2] + ion was examined in both neutral and acidic medium. In neutral aqueous medium, the primary aquation determined at 25°C and μ = 0.5 M, gave k 1 = 1.28 × 10 −3 s −1, Δ H† 1 = 12.8 kcal mol −1 and Δ S† 1 − 28.9 cal K −1 mol −1, while for the secondary aquation step (25°C and μ = 1.0 M) the acid-independent pathway gave k 2 = 1.4 × 10 −4 s −1, Δ H† 2 = 21.3 kcal mol −1 and Δ S = −4.7 cal K −1 mol −1. The acid hydrolysis of [Co(trpn)(CH 3CN) 2] 3+ was studied over the pH range 1–4 and the temperature range 8–20°C. The hydrolysis proceeds in two consecutive steps : [Co(trpn)(CH 3CN) 2] 3+ k 1→ [Co(trpn)(CH 3CN)(H 2O)] 3+ 2→ [Co(trpn)(H 2O) 2] 3+. Analysis of the kinetic data gave k 1 = 1.2 × 10 −2 s −1(20°C), Δ H† 1 = 16.0 kcal mol −1 and Δ S 1‡ = 12.7 cal K −1 mol −1, and k 2 = 2.38 × 10 −3 s −1 (20°C), Δ H 2‡ = 14.9 kcal mol −1 and Δ S‡ 2 = − 19.4 cal K −1 mol −1. The results of the kinetics of these substitution reactions are discussed in relation to data for the other tetraamine cobalt(III) complexes.