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 spectroscopy: the macrocycles studied are 2,9,18,25-tetraoxo-4,7,20,23-tetrakis(carboxymethyl)-1,4,7,10,17,20,23,26- octaaza[10.10]paracyclophane, abbreviated as (Lpd)H₄, and its 2,5-dimethyl-p-phenylene and tetramethyl-p-phenylene derivatives, abbreviated as (Ldmpd)H₄ and (Ltmpd)H₄, respectively. Steric interaction between tetramethylphenylene and amide groups in the tetramethyl derivative defines the conformation of the macrocyclic cavity, and causes unusual spectroscopic and chemical properties including the extreme line-broadening of ¹H NMR signals and the low basicity of amino nitrogen; such properties are not observed for the other macrocycles, in which steric interaction between phenylene and amide groups is less effective. The complexation of the highly strained ligand (Ltmpd)H₄ with Cu²⁺ ions has been studied by X-ray crystallography and solution electronic spectroscopy. The macrocycle forms a binuclear complex of [Cu₂(LH_-4)]^4- type in which four amide nitrogen atoms are deprotonated and each metal ion is coordinated to two amide nitrogen atoms and two amino nitrogen atoms. In the binuclear chelate molecule, the severe contraction of the macrocyclic ring 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 interaction in the binuclear complex differs from that in the mononuclear chelate of the same macrocycle.