Tuning the Inclusion Properties and Solid-State Reactivity of Second Sphere Adducts Using Conformationally Flexible Bidentate Ligands

Second-sphere coordination refers to any intermolecular interaction with the ligands directly bound to the primary coordination sphere of a metal ion. In this article, we have successfully applied the second-sphere coordination approach in the construction of versatile host frameworks that can accommodate various guest molecules. We have used a family of bidentate flexible molecules as second-sphere ligands, and the tetrachlorometalate anion [MCl4]2– (where M = Cu, Co, Cd, Zn, and Hg) as the primary coordination sphere to synthesize new second sphere adducts. By introducing an alkyl spacer −(CH2)n– (n = 1, 2, 3, 4) to bibenzylamine (L0), the ligands L1, L2, L3, and L4 with higher degree of flexibility were synthesized. Different guest molecules such as alcohol, acetic acid, acrylic ester, or acetonitrile can be included in the host framework self-assembling diprotonated L1–L4 and [MCl4]2–, leading to a novel type of supramolecular assemblies: CH3CH2OH⊂[L2]2H+·[CuCl4]2– (2), CH3OH⊂[L3]2H+·[MCl4]2– (3), CH3COOH⊂[L3]2H+·[CuCl4]2– (4), CH2CHCOOCH3⊂[L3]2H+·[MCl4]2– (5–7), CH3CN·H2O⊂[L4]2H+·[MCl4]2– (8–9), and CH3OH⊂[L4]2H+·[MCl4]2– (10). L2 forms the quasi-chelating charge-assisted N–H···Cl hydrogen bonds with [MCl4]2– that can transform in the solid-state to a chelated coordination complex following a mechanochemical dehydrochlorination reaction. By increasing the number of methylene groups, ligands L3 and L4 exhibit considerable conformational diversity due to the higher flexibility induced by the backbone chains. The −(CH2)n– spacer lengths of the ligands influences the structural dimensionality, and its solid-state mechanochemical reactivity preventing the transformation from salt [L3–4]2H+·[MCl4]2– to the chelating coordination complex [(MCl2)(L3–4)]. Moreover, the thermal stability of the second sphere adducts has been monitored by thermogravimetric analyses and X-ray powder diffraction (PXRD). We demonstrate that some of the second sphere adducts are dynamic, showing reversible guest release/uptake involving crystalline-to-amorphous-to-crystalline phase transformations. Quantum\Mechanical (QM) demonstrate that ligands with backbone lengths longer than −(CH2)2– are reticent to react via dehydrochlorination reaction because of the backbone chain length, the symmetry and orientation of the frontier molecular orbitals (FMOs), while for the −(CH2)2–, the length and orientation of the FMOs is optimal for the reaction to occur.