Reassignment, Isolation, Spectral and X‐ray Characterization of Diastereoisomers of the Macrocyclic Ligand 3,10‐<i>meso</i>‐3,5,7,7,10,12,14,14‐Octamethyl‐1,4,8,11‐tetraazacyclotetradecane and their Ni^II and Cu^II Complexes

Abstract

Abstract Reduction 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 [L b ( 1 ), L c ( 2 ), and L d ( 3 )] of four possible diastereoisomers. The diastereoisomeric macrocycles and their Ni II and Cu II 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 5 S ,12 R ‐C‐ meso , 5 S ,12 S ‐C‐ racemic , and 5 R ,12 R ‐C‐ racemic configurations, respectively. In accordance with the X‐ray structure determination, the geometry around the Ni II ion in the complexes is approximately square‐planar and the Cu II ion has an octahedral geometry. The N–H configuration of [NiL c ](ClO 4 ) 2 (H 2 O) 0.5 ( 5 ) is trans ‐II, different from the most thermodynamically stable trans ‐III configuration that is found in [NiL b ](ClO 4 ) 2 ( 4 ), [CuL b (H 2 O) 2 ](ClO 4 ) 2 ( 7 ), and [CuL d (ClO 4 ) 2 ] ( 8 ). UV/Vis spectra reveal that Ni II 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 Cu II 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 &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)