Molecular switch triggered by solvent polarity: Synthesis, acid-base behavior, alkali metal ion complexation, and crystal structure

Abstract

The synthesis and characterization of the new tetraazamacrocycle L, bearing two 1,1′-bis(2-phenol) group as side-arms, is reported. The basicity behavior and the binding properties of L toward alkali metal ions were determined by means of potentiometric measurements in ethanol/water 50:50 (v/v) solution (298.1 ± 0.1 K, I= 0.15 moldm-3). The anionic H-1L- species can be obtained in strong alkaline solution, indicating that not all of the acidic protons of L can be removed under the experimental conditions used. This species behaves as a tetraprotic base (log K1 = 11.22, log K2 = 9.45, log K3 = 7.07, log K4 = 5.08), and binds alkali metal ions to form neutral [MH-1L] complexes with the following stability constants: log KLi = 3.92, log KNa = 3.54, log KK = 3.29, log KCs = 3.53. The arrangement of the acidic protons in the H-1L- species depends on the polarity of the solvents used, and at least one proton switches from the amine moiety to the aromatic part upon decreasing the polarity of the solvent. In this way two different binding areas, modulated by the polarity of solvents, are possible in L. One area is preferred by alkali metal ions in polar solvents, the second one is preferred in solvents with low polarity. Thus, the metal ion can switch from one location to the other in the ligand, modulated by the polarity of the environment. A strong hydrogen-bonding network should preorganize the ligand for coordination, as confirmed by MD simulations. The crystal structure of the [Na(H-1L)]·CH3CN complex (space group P21/c, a = 12.805(1), b = 20.205(3), c = 14.170(2) Å, β = 100.77(1)°, V = 3601.6(8) Å3, Z = 4, R= 0.0430, wR2 = 0.1181), obtained using CH2Cl2/CH3CN as mixed solvent, supports this last aspect and shows one of the proposed binding areas.