Reassignment, Isolation, Spectral and X‐ray Characterization of Diastereoisomers of the Macrocyclic Ligand 3,10‐meso‐3,5,7,7,10,12,14,14‐Octamethyl‐1,4,8,11‐tetraazacyclotetradecane and their NiII and CuII Complexes

Abstract

AbstractReduction of the macrocyclic ligand, 3,10‐meso‐3,5,7,7,10,12,14,14‐octamethyl‐1,4,8,11‐tetraazacyclotetradecane‐4,11‐diene dihydroperchlorate (L), with sodium borohydride yielded three [Lb (1), Lc (2), and Ld (3)] of four possible diastereoisomers. The diastereoisomeric macrocycles and their NiII and CuII complexes have been characterized and confirmed by IR, UV/Vis, and NMR spectroscopy as well as X‐ray crystallography. The chiral carbon centers and methyl groups at the 5 and 12 positions of 1, 2 and 3 have the 5S,12R‐C‐meso, 5S,12S‐C‐racemic, and 5R,12R‐C‐racemic configurations, respectively. In accordance with the X‐ray structure determination, the geometry around the NiII ion in the complexes is approximately square‐planar and the CuII ion has an octahedral geometry. The N–H configuration of [NiLc](ClO4)2(H2O)0.5 (5) is trans‐II, different from the most thermodynamically stable trans‐III configuration that is found in [NiLb](ClO4)2 (4), [CuLb(H2O)2](ClO4)2 (7), and [CuLd(ClO4)2] (8). UV/Vis spectra reveal that NiII complexes exhibit almost 100 % planar species in aqueous solution resulting from steric effects produced by numerous methyl groups, especially axial orientations and distorted structures. In six coordinate CuII complexes, acetonitrile acts as a better axial donor than the water molecule; the weaker axial bonding is a result of steric interactions that lead to stronger in‐plane coordinations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)