Anisole as an ambidentate ligand: Ab initio molecular orbital study of alkali metal cations binding to anisole

We present optimized geometries and binding energies for alkali metal cation complexes with anisole (methoxybenzene). Results are obtained for Li+ through Cs+ at the RHF/6-311G* and MP2/6-311+G* levels of theory, with K+, Rb+, and Cs+ represented by relativistic ECPs and associated valence basis sets. RHF/6-311G* frequencies are used to verify that the optimized geometries are minima and to calculate binding enthalpies. The effects of basis set superposition error (BSSE) are estimated at both the RHF and MP2 levels. We find that the alkali metals bind to anisole in two ways, either predominantly through interactions with the aromatic ring or with the ether oxygen. For binding to the ring, we obtain BSSE-corrected MP2/6-311+G* binding enthalpies (in kcal/mol) of -38.1 (Li+), -23.6 (Na+), -18.3 (K+), -15.4 (Rb+), and -13.6 (Cs+). The average distances (in Angstrom) between the ring carbons and the cations are 2.33 (Li+), 2.79 (Na+), 3.20 (K+), 3.44 (Rb+), and 3.70 (Cs+). For binding to the ether oxygen, the BSSE-corrected MP2/6-311G* binding enthalpies (in kcal/mol) are -37.6 (Li+), -25.2 (Na+), -19.4 (K+), -16.4 (Rb+), and -14.3 (Cs+). The distances (in Angstrom) between the ether oxygen and the cations are 1.82 (Li+), 2.24 (Na+), 2.62 (K+), 2.87 (Rb+), and 3.10 (Cs+). Although the differences in binding energy between the two sites are small, the cations generally prefer to bind to the oxygen.