Synthesis of Chelating Complexes through Solid-State Dehydrochlorination Reactions via Second-Sphere-Coordination Interaction with Metal Chlorides: A Combined Experimental–Molecular Modeling Study

We have applied crystal engineering
as a tool to study the solid-state
transformation from molecular salts to coordination complexes via
mechanochemical dehydrochlorination reactions. The −(CH2)n– (n = 2, 3) alkyl chains were introduced into the bibenzylamine moiety
to form the two nitrogen bases N,N,N′,N′-tetrabenzylethylenediamine
(L1) and N,N,N′,N′-tetrabenzylpropydiamine
(L2), which were self-assembled
with tetrachlorometalates to form a series of supramolecular salts
through second-sphere coordination. Single crystals of salts [L1]­2H+·[CuCl4]2–·solvent (1) and [L2]­2H+·[XCl4]2–·solvent (2–4; X = Cu, Hg, Zn) were obtained and their structures determined by
single-crystal X-ray diffraction. The effect of different alkyl chains
(two and three −CH2– units) on the solid-state
reactivity showed that the chelating complexes resulting from the
mechanochemical dehydrohalogenation reaction depend on the formation
of quasi-chelating hydrogen-bonding salts. Quantum-mechanical
calculations have been used to gain insight in this mechanochemical
dehydrohalogenation reaction, demonstrating that not only is size
matching between reactants is important but also conformational energies,
intermolecular interactions, and the symmetry of frontier molecular
orbitals play an important role.