Periodic Trends of Ions Bound in Molecular Cages: Mn+-[2.2.2]-Benzocryptand Complexes in the Gas Phase.

Abstract

Cryptands are three-dimensional molecular cages that selectively bind cations in their interiors, discriminating between atomic cations based on their size and charge state. We report a detailed study of the IR and UV spectroscopy and structures of Mⁿ⁺-[2.2.2]-benzocryptand (BzCrypt) complexes with Mⁿ⁺ = Na⁺, K⁺, Rb⁺, Ca²⁺, Sr²⁺, and Ba²⁺ under cryo-cooled conditions in the gas phase, to discern periodic trends in the absence of thermal, solvent, or packing effects. This work builds off an initial report that compared the gas phase structures of K⁺-BzCrypt and Ba²⁺-BzCrypt (Foley et al., J. Phys. Chem. A 2023, 127, 6227), where the ions are of the right size to fit inside the cage optimally. The electronic origins of the UV spectra shift by more than 1000 cm^-1 with ion size and charge, with strong electronic origins and modest low frequency Franck-Condon activity arising from the confined motion of the ion in response to electronic excitation of the aromatic chromophore. In the infrared, the embedded ion produces significant shifts and anharmonic couplings in the alkyl CH stretch infrared spectrum (2800-3000 cm^-1) that reflect the way in which the cryptand cage changes its shape to maximize its binding with ions of different size and charge. We employ an anharmonic local mode model of the manifold of CH stretch transitions to assign the experimentally observed structures. All six ions are bound inside the cage primarily by the six O atoms in the three diether bridges, with secondary binding to the two tertiary amines. Mid-IR spectra in the 1000-1700 cm^-1 region are dominated by transitions that carry significant C-O stretch character. We develop a local mode model of the CO stretch vibrations whose frequencies are shown to vary significantly with the Mⁿ⁺ charge state but respond only weakly to the conformation of the cage as long as primary binding is to all six O atoms. Based on a complete search of the conformational potential energy surface, we track the calculated relative energies of a set of 16 low-energy conformations as a function of the imbedded cation. The assigned structures are either the calculated global minima for that Mⁿ⁺-BzCrypt complex or its closest competitor. When cations that are smaller than optimal are bound inside the cryptand cage (Na⁺, Ca²⁺, Sr²⁺), the 'end-to-end' N···N distance is shortened significantly from its optimal value (6.06 in K⁺-BzCrypt and 5.96 Å in Ba²⁺-BzCrypt) by twisting the phenyl ring from its parallel optimal configuration to nearly perpendicular to the N···N axis, reducing the N···N distance to 5.24, 5.31, and 5.47 Å in the observed Ca²⁺-, Na⁺-, and Sr²⁺-BzCrypt complexes. Finally, Rb⁺ takes up the same structure as in K⁺-BzCrypt, with both UV and IR spectra that are closely similar to those in the latter complex. Thus, the [2.2.2]-benzocryptand cage accommodates a slightly larger ion without significant structural rearrangement.