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 Wx2, HC1 eluent) and reversed phase ion-pair chromatography (C-18, p-toluenephosphate in MeOH/H2O eluent). By using a combination of H-1 n.m.r. techniques (n.O.e. and COSY) for solutions in (CD3)(2)SO 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)] I-2.H2O, isomer (1), P 4(3)2(1)2, a = b = 8.55150(10), c 51.8693(11) , Z 8, R 0.0343; [Co(cyclen)((S)-AlaO)] (ClO4)(2).H2O, isomer (2), P 2(1)2(1)2(1), a 8.499(3), b 14.538(5), c 16.592(4) Angstrom, Z 4, R 0.0388; [Co(cyclen)((SIR)-AlaO)] ZnBr4, a 1:1 mixture containing both (S)-alanine (isomer (3)) and (R)-alanine, P2(1)/c, a 7.618(2), b 13.806(4), c 19.094(7) Angstrom, Z 4, R 0.0726. In alkaline solution (0.1-1.0 M NaOH, 25.0 degrees 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- reversible arrow [Co(cyclen - H)((S)-AlaO)](+)+H2O to be determined as 1.05 M-1 at 250 degrees C and I = 1.0 M. The hydrolysis reaction follows the rate law k(obs) = kKK(1)K(2) [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].