Molecular and electronic structure of a reduced Schiff base cryptand: Characterization by x-ray crystallography and optical and EPR/ENDOR spectroscopy

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 C-3 symmetry. Solution studies by optica l and EPRI ENDOR spectroscopies revealed complex ion pair equilibria that r ue compatible with external contact ion pair and solvent-separated ion pair formation. The rate of electron (and cation) transfer between strands is < 4 x 10(7) s(-1) for contact pairs, but faster for solvent-separated pairs. The addition of cryptand [2.2.3] to complex K+ breaks up the contact ion pa irs and yields behavior similar to that of solvent-separated ion pairs. Cyc lic voltammetry revealed three partially reversible reduction waves. Both t he dianion and trianion were formed in solution by reduction with potassium and studied by optical and EPR spectroscopies. Two compounds, 2 and 3. whi ch 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 d iamagnetic dimer that may be related to the pinacolate structure of the ana logous ketyl dimers.