The Synthesis, Separation and Structures of Three [Co(cyclen)((S)-AlaO)]2+ Isomers. The Alkaline Hydrolysis of [Co(cyclen)((S)-AlaO)]2+

Abstract

The reaction of [Co(cyclen)(OH₂)OH]²⁺ (cyclen = 1,4,7,10-tetraazacyclododecane) with (S)-alanine at pH 7·2 gives a mixture of three [Co(cyclen)((S)-AlaO)]²⁺ isomers (1)–(3). These have been isolated by using both cation ion-exchange chromatography (Dowex 50 W×2, HCl eluent) and reversed phase ion-pair chromatography (C₁₈, p-toluenephosphate in MeOH/H₂O eluent). By using a combination of ¹H n.m.r. techniques (n.O.e. and COSY) for solutions in (CD₃)₂SO the syn(N),anti(O) (1), syn(O), anti(N) (2) and syn(N), syn(O) (3) configurations have been assigned to these isomers. These have been confirmed by single-crystal X-ray analysis: [Co(cyclen)((S)-AlaO)] I₂.H₂O, isomer (1), P4₃2₁2, a = b = 8·55150(10), c 51·8693(11) Å, Z 8, R 0·0343; [Co(cyclen)((S)-AlaO)] (ClO₄)₂.H₂O, isomer (2), P2₁2₁2₁, a 8·499(3), b 14·538(5), c 16·592(4) Å, Z 4, R 0·0388; [Co(cyclen)((S/R)-AlaO)] ZnBr₄, a 1 : 1 mixture containing both (S)-alanine (isomer (3)) and (R)-alanine, P2₁/c, a 7·618(2), b 13·806(4), c 19·094(7) Å, Z 4, R 0·0726. In alkaline solution (0·1–1·0 M NaOH, 25·0°C, I = 1·0 M (NaClO₄)), equilibration between (1), (2) and (3) is faster than hydrolysis to give cis-[Co(cyclen)(OH)₂]⁺+(S)-AlaO^-. Time zero spectroscopic observation (300 nm) allowed the equilibrium constant, K, for the reaction [Co(cyclen)((S)-AlaO)]²⁺ + OH^- ↔ [Co(cyclen – H)((S)-AlaO)]⁺ +H₂O to be determined as 1·05 M^-1 at 25·0°C and I = 1·0 M. The hydrolysis reaction follows the rate law k_obs = kKK₁K₂[OH^-]²/(1+K[OH^-]+KK₁K₂[OH^-]²) with k = 1·0 s^-1 corresponding to rate-determining loss of (S)-AlaO^- from the ring-opened complex, [Co(cyclen – H)((S)-AlaO)OH].