Revisiting the Role of Hydrogen Bonding in the Strong Dimer Superexchange of a 2D Copper(II) Halide Honeycomb-Like Lattice: Structural and Magnetic Study

The title compound H2L(CuCl3H2O)Cl (H2L = 1-(4'-pyridinium)pyridin-4-of-ium), 1) was synthesized and investigated structurally and magnetically as well as via a first-principles, bottom-up theoretical analysis of the potential magnetic superexchange pathways. Compound 1 can be described structurally as a well-isolated, distorted 2D-honeycomb lattice with two potential exchange pathways: a dimeric interaction via hydrogen-bonded pairs of (CuCl3H2O) ions and a chain structure via bridging chloride ions. Surprisingly, the experimental magnetic data are best fitted using both a simple dimer model with a Curie-Weiss correction for interdimer exchange (J(dimer) = -107.4(1) K, theta = -1.22(4) K) and a strong-rung ladder model (J(rung) = -105.8(7) K, J(rail) = 2(7) K). Theoretical analysis at the UB3LYP/6-31+G(d) level supports the strong exchange observed through the [CuCl4(H2O)]2- dimer moiety superexchange pathway (-102 K = -71 cm(-1)). However, the apparent vanishingly small exchange through the single halide bridge is merely a brute average of competing ferromagnetic (FM) (+24.8 K = +17.0 cm(-1)) and antiferromagnetic (AFM) (-21.0 K = -14.6 cm(-1)) exchange interactions. Our computational study shows that these fitting parameters carry no physical meaning since a honeycomb plaquette must be taken as magnetic building block for 1. The competition between FM and AFM pair interactions leads to geometrical frustration in 1 and could induce interesting magnetic response at low temperatures, if the magnetic exchange is adequately tuned by modifying substituents in ligands and, in turn, interactions within the crystal packing.