On the Importance of Unprecedented Lone Pair–Salt Bridge Interactions in Cu(II)–Malonate–2-Amino-5-Chloropyridine–Perchlorate Ternary System

A Cu­(II)–malonate complex
with formula {(C5H6N2Cl)12​[Cu­(1)​(C3H2O4)2]​[Cu­(2)​(C3H2O4)2​(H2O)2]​[Cu­(4)​(C3H2O4)2]​[Cu­(3)​(C3H2O4)2​(H2O)2]​(ClO4)4}n (1) [C5H6N2Cl = protonated 2-amino-5-chloropyridine,
C3H4O4 = malonic acid, ClO4– = perchlorate] has been synthesized from purely
aqueous media simple by mixing the reactants in their stoichiometric
ratio, and its crystal structure has been determined by single-crystal
X-ray diffraction. In 1, copper­(II) malonate units form
infinite 1D polymeric chains, which are interlinked by hydrogen bonds
to generate 2D sheets. These 2D sheets are joined side by side primarily
by various hydrogen bonds to form a 3D structure. A multitude of salt
bridges are formed in this structure, connecting the protonated 2-amino-5-chloropyridines
and the malonate ligands of the polymeric polyanion. Examining this
characteristic of the solid-state architecture, we noticed several
salt-bridge (sb)···π interactions and an unexplored
interaction between the lone pair (lp) of one malonate oxygen atom
and a planar salt bridge. The combination of this interaction with
various other weak intermolecular forces results in a remarkably extended
supramolecular network combining a wide variety of interactions involving
π–systems (Cl···π, π···π)
and salt bridges (sb···π and lp···sb).
We describe the energetic and geometric features of this lone pair–salt-bridge
interaction and explore its impact on the resultant supramolecular
organization using theoretical DFT-D3 calculations.