Exchange interaction between S = 1/2 centers bridged by multiple noncovalent interactions: Contribution of the individual chemical pathways to the magnetic coupling

Abstract

We report EPR and magnetic measurements of diaqua-(pyridine-2,6-dicarboxylato)Copper(II) together with computational calculations. This compound consists of an extended lattice of magnetically equivalent copper(II) ions in which the copper centers are linked by hydrophobic interactions such as π–π, π–metal, and Y–X…π, and hydrogen bonds involving different topologies, viz., [sbnd]Cu[sbnd]Oeq…Oeq[sbnd]Cu[sbnd], [sbnd]Cu[sbnd]Oeq…Oap[sbnd]Cu[sbnd], and [sbnd]Cu[sbnd]Oeq…O[dbnd]C[sbnd]Oeq[sbnd]Cu[sbnd]. The powder EPR spectrum shows nearly axial symmetry with non-resolved hyperfine structure with the copper nucleus, suggesting the presence of intercenter isotropic exchange interactions. Single crystal EPR experiments show a single lorentzian resonance line for all the magnetic field orientations investigated, typical of an extended exchange coupled system. Susceptibility measurements showed weakly antiferromagnetically coupled Cu(II) ions (J = −0.79 (4) cm−1). Using Anderson's exchange narrowing model for the collapse of the hyperfine structure, EPR yielded |J| = 0.6 (1) cm−1. The distinct contributions of the different chemical pathways to the experimentally determined J-value are analyzed through first principle computational calculations.