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.