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].
