From molecules to the crystalline solid: Secondary hydrogen-bonding interactions of salt bridges and their role in magnetic exchange

The primary two-point hydrogen-bond contact of p-substituted benzamidinium cations to benzoate anions in [D-6]DMSO forms a salt bridge as described by a classical Hammett relation. At carboxylate: amidinium proportions greate r than 1:1, amidinium hydrogen atoms external to the salt bridge associate to carboxylate. This complementary interaction of four protons and four lon e pairs of the amidinium-carboxylate salt bridge creates closure of the hyd rogen-bonding network in 3-amidinium benzoate, Structural characterization of this solid shows that the primary amidinium-carboxylate interaction of t he salt bridge, which leads to zigzag tapes, is augmented by lateral hydrog en bonding, forming ladder structures of oppositely oriented salt bridges. The ladders crosslink tapes, thus setting the three-dimensional structure o f the system. We have exploited this secondary hydrogen-bonding interaction to construct layered magnetic solids based on the salt bridge formed betwe en 3-cyanobenzamidinium and 2,2,4,4-tetramethylpyrroline-N-oxyl-3-carboxyla te. In this solid, the nitroxyl radical moiety participates in the hydrogen -bonding network, truncating salt bridge ladder formation by capping (salt bridge), ladder segments. These segments are linked to each other via the s econdary hydrogen-bonding interaction of the salt bridge to form isolated l inear chains of nitroxyl radicals running diagonally within the layer. Magn etic susceptibility studies show that the nitroxyl spin can propagate throu gh the secondary hydrogen bond. When a water molecule in the solid formed f rom benzamidinium and 2,2,5,5-tetramethyl-3-carboxypyrroline-1-oxyl interru pts the pathway, magnetic coupling of the spins is obstructed. These result s show that the hydrogen bonds external to salt bridges can both set struct ure and mediate magnetic organization, establishing the salt bridge as a us eful synthon in the design of layered magnetic materials.