Tweezer-like complexes of crown ethers with divalent metals: probing cation-size-dependent conformations by vibrational spectroscopy in the gas phase

Crown ethers constitute central building blocks for the synthesis of molecular tweezers capable of trapping cationic species. In this study, isolated ternary complexes comprising two [18]crown-6 (18c6) ether molecules and one divalent cation of varying size (Cu2+, Ca2+, Ba2+) are investigated by means of laser vibrational action spectroscopy and computations. In the ternary (18c6)2-Cu2+ complex, one of the crown units folds tightly around the cation, while the second crown ether unit binds peripherally. Such asymmetrical binding manifests itself as a bimodal splitting of the vibrational bands measured for the complex. The size of the cation in the Ca2+ and Ba2+ complexes leads to a progressively more symmetrical coordination of the two crown ether molecules with the metal. In particular, in the spectrum of the (18c6)2-Ba2+ complex, the two components of the vibrational bands are merged into single-maximum envelopes. This is consistent with a C2 arrangement predicted by the computation, in which the cation coordinates with the two crown ether units in a fully symmetrical way.