Da. Buckingham et al., SYNTHESIS AND STRUCTURES OF 5 [CO(MECYCLEN)(S-ALAO)](2- USE OF NOE AND COSY H-1-NMR SPECTROSCOPY FOR STRUCTURAL ASSIGNMENT IN SOLUTION() ISOMERS ), Inorganic chemistry, 34(14), 1995, pp. 3646-3657
The cyclic quadridentate 1-methyl-1,4,7,10-tetraazacyclododecane (Mecy
clen) has been prepared in three simple steps from p-tosylaziridine, m
ethylammonium acetate, and tris(p-tosyl)diethanolamine. Coordination t
o Co(III) gives [Co(Mecyclen)Cl-2]Cl in high yield, which on treatment
with S-alanine (S-AlaOH), or its methyl ester (50 degrees C, pH 7.5-8
.0), gives a mixture of six [Co(Mecyclen)(S-AlaO)](2+) isomers. Five o
f these have been isolated by a combination of cation ion-exchange chr
omatography and selective crystallization. Each has been structurally
assigned using a combination of nOe and COSY H-1 NMR spectroscopies (d
(6)-DMSO solvent) as, in order of decreasing yield from S-AlaOMe, the
following: anti(Me), syn(N), anti(O) (isomer 4); anti(Me), anti(N), sy
n(O) (isomer 5); syn(Me), syn(N), anti(O), (isomer 1); syn(Me), anti(N
), syn(O) (isomer 2); syn(Me), syn(N), syn(O) (isomer 3). The structur
es of 1, 2, and 4 have been confirmed by single-crystal X-ray analysis
: [Co(Mecyclen)(S/R-AlaO)](ClO4)(2) . H2O (50:50 mixture of isomer 1 c
ontaining S-AlaO and isomer 4 containing R-AlaO), monoclinic, P2(1)/n,
a = 14.131(3) Angstrom, b = 8.777(2) Angstrom, c = 17.745(4) Angstrom
, Z = 4, R = 0.0734; [Co(Mecyclen)(S-AlaO)]ZnCl4 . 2H(2)O (isomer 2),
orthorhombic P2(1)2(1)2(1), a = 8.842(2) Angstrom, b = 14.794(7) Angst
rom, c = 17.157(8) Angstrom, Z = 4, R = 0.0415; [Co(Mecyclen)(S-AlaO)]
(ClO4)(2) . H2O (isomer 4); orthorhombic P2(1)2(1)2(1), a = 8.757(2) A
ngstrom, b = 9.684(5) Angstrom, c = 25.298(6) Angstrom, Z = 4, R = 0.0
380. Structural assignment by the H-1 NMR method depends critically on
the presence of separate N-H resonances for the different sites; this
can usually be achieved by addition of DCl to a d(6)-DMSO solution of
the complex.