Single-crystal X-ray diffraction study, NMR and electrochemical analysis of a copper(i) 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-[(6-methyl-2,2′-bipyridyl-6-yl)methoxy]calix[4]arene complex: an original M4L2 “hand-to-hand” system

Abstract

Attempts to prepare a supramolecular M₄L₂ box involving two tetra-bipyridyl calix[4]arene units and four copper(I) cations afforded, in fact, complex 2 for which, in the solid state, X-ray diffraction analysis showed that the stoichiometry is indeed M₄L₂, with the two calixarene subunits associated in a “hand-to-hand” mode by complexation via two adjacent bipyridine units to two copper(I) cations, and each group of two residual, adjacent, bipyridine arms is complexed to one copper(I) centre. Mass spectrometry, UV spectroscopy and preliminary ¹H-NMR analysis are in preliminary accordance with a monocalixarenic right/left handed M₂L structure. Nevertheless, some particularities of the NMR spectrum related to Ar-CH₂-Ar and O-CH₂bpy resonance signals, the possible generation of the M₂L species during the mass spectrometry experiment and the lack of sensitivity of the UV-visible titration caused us to proceed to deeper investigations, looking for other associations, notably the M₄L₂ “hand-to-hand” systems. This involved 1D and 2D-NMR spectroscopies, among them ¹H–¹⁵N HMBC, for the reconstruction of the molecule, but also the measurement of self-diffusion coefficients by NMR and electrochemistry. The 2D-NMR analysis and NMR self-diffusion evaluation were in accordance with the M₂L structure, but the electrochemical self-diffusion experiment indicated a M₄L₂ structure.