STEREOCHEMISTRY OF (R)-2-METHYLAZIRIDINE COMPLEXES OF COBALT(III) AND DIMERIZATION OF THE LIGAND

A cobalt(III) complex containing (R)-2-methylaziridine (R-meaz), [Co(R-meaz)(NH3)(5))](3+), was prepared and the two diastereomers arising from the presence of the chiral nitrogen atom (N(R) and N(S)) were separated by column chromatography. Molecular mechanics calculations estimated the N(R)-isomer to be more stable. This result was supported by the x-ray structure determination of the more abundant (ca. 94%) isomer, N(R)-[Co(R-meaz)(NH3)(5)]Br-3.H2O. Crystal data: monoclinic, P2(1), a = 7.357(1), b = 9.780(1), c = 10.426(1) Angstrom, beta = 93.58(1)degrees, V = 748.7(3) Angstrom(3), Z = 2. Kinetic studies of isomerization (epimerization) between the two isomers revealed that inversion at the nitrogen center was very slow (5 x 10(-2) M-1 s(-1) at 25 degrees C). The small rate constant seems to be related to the strained three-membered structure of the meat ligand. The reaction of Na-3[Co(NO2)(6)] and R-meaz yielded a complex containing two dimerized R-meaz chelates, trans-[Co(NO2)(2)(di-R-meaz)(2)](+) (di;R-meaz=RR)-alpha;2-dimethyl-1-aziridineethanamine) The crystal structure of trans-[Co(NO2)(2) (di-R-meaz)(2)]ClO4.H2O was established by x-ray crystallography. Crystal data: orthorhombic, P2(1)2(1)2(1),a = 11.784(6), b = 21.023(9), c = 8.608(7) Angstrom, V = 2133(2) Angstrom(3), Z = 4.