Green and Brown Isomers of the ((S)-1-Amino-2-propanethiolato-N,S)(1,4,7,10-tetraazacyclododecane)cobalt(III) Ion and Crystal Structure of the Green Isomer

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Abstract
The [Co(S-apS)(cyclen)]2+ (S-apS=(S)-1-amino-2-propanethiolate ion, cyclen=1,4,7,10-tetraazacyclododecane) complex was prepared and separated into two isomers (green and brown) by column chromatography. The crystal structure of the green isomer, [Co(S-apS)(cyclen)]ZnCl4·H2O was determined by X-ray analysis. Crystal data are as follows: monoclinic, space group P21, a=9.382(2), b=13.124(3), c=8.934(2) Å, β=91.08(3)°, and Z=2. The complex ion has a cis-octahedral geometry with the cyclen ligand coordinated in a folded, butterfly-like configuration and the bidentate S-apS ligand in a δ-gauche form. One of the two “planar” NH hydrogens of the macrocycle with respect to the S-apS ligand is directed opposite the S-apS chelate ring (exo), while the other “planar” and the remaining two “apical” NH hydrogens are directed to the S-apS ring (endo). The green isomer can be represented by endo(S,NH), where NH denotes the planar NH hydrogen cis to the sulfur atom. The other brown isomer was assigned as the exo(S,NH) configuration, where the S and NH2 coordination sites of S-apS in the green isomer are interchanged with each other, remaining the configuration of the macrocycle unchanged. Reversible isomerization between the two isomers occurred in aqueous solution and the kinetics was studied at 25.4 °C in the pH range of 5.98–6.97. The isomerization obeyed a rate law of the form, R=k[OH−]·[complex], and was suggested to take place with synchronous inversion at the two nitrogen atoms of the macrocycle. The relative abundance of the two isomers at equilibrium is 1.0, and the forward and reverse second-order rate constants at 25.4 °C are 6.5×103 mol−1 dm3 s−1.

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