Molecular and Electronic Structure of a Reduced Schiff Base Cryptand: Characterization by X-ray Crystallography and Optical and EPR/ENDOR Spectroscopy

Abstract

The macrobicyclic Schiff base cryptand, 1, with a m-phenyl group in each of the arms was reduced in tertrahydrofuran with the alkali metals Na through Cs to yield mono-, di-, and trianions. The crystal structure of a salt of 1-, formed by reduction of 1 with potassium metal in mixed dimethyl ether−methylamine solutions, shows that K+ is not encapsulated in the cavity of the cryptand. Instead, it forms methylamine-separated ion pairs arranged in symmetric fashion to give overall C3 symmetry. Solution studies by optical and EPR/ENDOR spectroscopies revealed complex ion pair equilibria that are compatible with external contact ion pair and solvent-separated ion pair formation. The rate of electron (and cation) transfer between strands is <4 × 107 s-1 for contact pairs, but faster for solvent-separated pairs. The addition of cryptand [2.2.2] to complex K+ breaks up the contact ion pairs and yields behavior similar to that of solvent-separated ion pairs. Cyclic voltammetry revealed three partially reversible reduction waves. Both the dianion and trianion were formed in solution by reduction with potassium and studied by optical and EPR spectroscopies. Two compounds, 2 and 3, which model a single strand of the macrobicycle, were used to deconvolute the spectra of 1-. The monoanions, 2- and 3-, are in equilibrium with a diamagnetic dimer that may be related to the pinacolate structure of the analogous ketyl dimers.