Isomerism and Solvent Interaction in Octamethyl Calix[4]pyrrole Complexed with Formate

We investigate the binding motifs of host-guest complexes of the anion receptor octamethyl calix[4]pyrrole (omC4P) with the formate anion using cryogenic ion vibrational spectroscopy in concert with density functional theory. The resulting infrared spectrum in vacuo is compared to that in deuterated acetonitrile and acetone solutions. The combination of the strong host-guest interaction and the charge distribution that the formate ion presents to the chemical environment results in complex behavior of the NH stretching features in the two solvents. The formate-omC4P complex has three low energy isomers in vacuo: (i) one with an oxygen atom of formate interacting with three of the NH groups of omC4P and the other oxygen atom interacting with the remaining NH group; (ii) one with a single oxygen atom of formate interacting with all four NH groups of omC4P; and (iii) one with each oxygen atom interacting with two NH groups. Each complex geometry lowers the C_4v symmetry of the receptor to C₁, C_s, or C_2v, respectively, and both symmetry breaking and isomerism are reflected in the pattern and broad line shapes of the NH stretching modes of omC4P.