High steric constraints and molecular distortion in methyl-substituted amide-based paracyclophanes and the binuclear Cu2+ complexes: X-ray structures, NMR and absorption spectra

Chelating paracyclophanes that are sterically constrained to a great extent have been synthesized and characterized by X-ray crystallography and NMR s pectroscopy: the macrocycles studied are 2,9,18,25-tetraoxo-4,7,20,23-tetra kis(carboxtnetgtk)-1,4,7,10, 17,20,23,26-octaaza[10.10]paracyclophane, abbr eviated as (Lpd)H-4, and its 2,5-dimethyl-p-phenylene and tetramethyl-p-phe nylene derivatives, abbreviated as (Ldmpd)H-4 and (Ltmpd)H-4, respectively. Steric interaction between tetramethylphenylene and amide groups in the te tramethyl derivative defines the conformation of the macrocyclic cavity, an d causes unusual spectroscopic and chemical properties including the extrem e line-broadening of H-1 NMR signals and the low basicity of amino nitrogen ; such properties are not observed for the other macrocycles, in which ster ic interaction between phenylene and amide groups is less effective. The co mplexation of the highly strained ligand (Ltmpd)H-4 with Cu2+ ions has been studied by X-ray crystallography and solution electronic spectroscopy. The macrocycle forms a binuclear complex of [Cu-2(LH-4)](4-) type in which fou r amide nitrogen atoms are deprotonated and each metal ion is coordinated t o two amide nitrogen atoms and two amino nitrogen atoms. In the binuclear c helate molecule, the severe contraction of the macrocyclic rings forces the phenylene groups distorted to a boat form, due to the steric effect of the tetramethyl substituents. As a result, the metal-ligand charge-transfer in teraction in the binuclear complex differs from that in the mononuclear che late of the same macrocycle. (C) 2001 Elsevier Science B.V. All rights rese rved.