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

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 kobs = 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].