THE SYNTHESIS, SEPARATION AND STRUCTURES OF 3 [CO(CYCLEN)((S)-ALAO)](2- THE ALKALINE-HYDROLYSIS OF [CO(CYCLEN)((S)-ALAO)](2+)() ISOMERS )

The reaction of [Co(cyclen)(OH2)OH](2+) (cyclen = 1,4,7,10-tetraazacyc lododecane) with (S)-alanine at pH 7 2 gives a mixture of three [Co(cy clen)((S)-AlaO)](2+) isomers (1)-(3). These have been isolated by usin g both cation ion-exchange chromatography (Dowex 50 Wx2, HC1 eluent) a nd 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), sy n(O),anti(N)(2) and syn(N),syn(O) (3) configurations have been assigne d 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 0.072 6. In alkaline solution (0.1-1.0 M NaOH, 25.0 degrees C, I = 1.0 M (Na ClO4)), equilibration between (1), (2) and (3) is faster than hydrolys is to give cis-[Co(cyclen) (OH)2](+) +(S)-AlaO-. Time zero spectroscop ic observation (300 nm) allowed the equilibrium constant, K, for the r eaction [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 a nd I = 1.0 M. The hydrolysis reaction follows the rate law k(obs) = kK K(1)K(2) [OH-](2)/(1 + K [OH-]+KK1K2[OH-](2)) with k = 1.0 s(-1) corre sponding to rate-determining loss of (S)-AlaO(-) from the ring-opened complex, [Co(cyclen-H)((S)-AlaO)OH].