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.