Well-isolated pyrazine-bridged copper(II) chains: synthesis and magneto-structural analysis

A pair of well-isolated 1-D-QHAF compounds with the molecular formula [Cu(pz)(L)₂(H₂O)₂](ClO₄)₂ with L = 3-Cl-4-hydroxypyridine (<b>1</b>) and 3-Br-4-hydroxypyridine (<b>2</b>) (<i>catena-(diaqua-bis(3-chloro/bromo-4-pyridono-κO)(μ-pyrazine)copper(II) diperchlorate</i>) were synthesized via slow-evaporation crystallization and characterized structurally and magnetically to develop magneto-structural correlations. Single-crystal X-ray diffraction at 120 K provided the structures of <b>1</b> and <b>2</b> which comprise six-coordinate Cu(II) pyrazine-bridged chains propagating parallel to the crystallographic <i>a</i>-axis in the triclinic space-group <i>P</i>-1. Chains are offset from each other in the <i>c</i>-direction by the bulk of L and along the d_z2 axis occupied by a semi-coordinate water molecule engaged in hydrogen bonding with the perchlorate gegenion. Magnetic susceptibility experiments showed that <b>1</b> and <b>2</b> behave as quasi-Heisenberg 1-D-AFMs with <i>J</i> ≈ 8 K and no sign of 3 D ordering down to 1.8 K. Aspects of crystal engineering are addressed to understand the potential to design systems with desired magnetic properties in light of comparisons with a variety of pyrazine-bridged chains already in the literature. The effect of L and the general structural scaffold are investigated to this end. Powder X-Band EPR experiments were performed to determine the electronic environment of the Cu(II) ion and average g-factor. The pyrazine-bridged chains in <b>1</b> with the AFM electronic spins represented as glowing arrows.