Cu(HCO₂)₂L (L = pyrazine, 4,4′-bipyridine): Employing the formate anion as a building block in three-dimensional coordination polymers

The formate anion, HCO₂−, has been used infrequently as a building block in molecular magnetic materials. We have synthesized and structurally and magnetically characterized two new Cu(HCO₂)₂L compounds, where L represents pyrazine (pyz) and 4,4′-bipyridine (bipy). The single crystal structure of Cu(HCO₂)₂(pyz), 1, has been established by both X-ray (295 K) and neutron diffraction (20 K). The compound consists of tetragonally-elongated CuN₂O₄ octahedra made up of four bridging formate anions and two neutral pyz ligands. The 3D polymeric network is comprised of 2D Cu(HCO₂)₂ layers that are fused together by the linear pyz spacers, which form Cu–pyz-Cu chains. Cu(HCO₂)₂(bipy), 2, is chiral and has a more complex framework than 1. The CuO₄N₂ octahedra align in two unique orientations relative to one another, owing to the 4₁ and 2₁ screw axes that lie along the a and b-axes. The octahedra are connected via four bridging HCO₂− anions and two bridging bipy ligands, resulting in a unique 3D scaffold structure. The magnetic behavior of 1 and 2 indicates antiferro- and ferromagnetic interactions, respectively, and the exchange couplings in both 1 and 2 are well reproduced by a 1D spin Hamiltonian. Spin dimer analysis was carried out to evaluate the relative strengths of the various spin exchange paths. It is found that the interaction through HCO₂− is comparatively weak, and the strong Cu²⁺ spin exchange interactions are mediated by the pyz and bipy ligands via the σ-pathway. A possible reason as to why 2 displays ferromagnetic coupling is proposed.