Experimental and Theoretical Characterization of the Magnetic Properties of CuF₂(H₂O)₂(pyz) (pyz = pyrazine): A Two-Dimensional Quantum Magnet Arising from Supersuperexchange Interactions through Hydrogen Bonded Paths

Abstract

The structural, electronic, and magnetic properties of the new linear chain coordination polymer CuF 2 (H 2 O) 2 (pyz) (pyz = pyrazine) were determined by single crystal X-ray diffraction at various temperatures, SQUID magnetometry, pulsed-field magnetization, ESR, muon-spin relaxation (μSR), and electronic structure calculations. Each Cu 2+ ion of CuF 2 (H 2 O) 2 (pyz) is located at a distorted CuF 2 O 2 N 2 octahedron with axial elongation along the Cu−N bonds. These octahedra are tethered together by strong F···H−O hydrogen bonds to yield two-dimensional (2D) square nets in the bc -plane that are linked along the a -direction by pyrazine linkages. Measurements of the g -factor by ESR along with first principles density functional theory electronic structure calculations show that the magnetic orbital of the Cu 2+ ion lies in the CuF 2 O 2 plane thus forming a 2D antiferromagnetic square lattice. A broad maximum observed in χ( T ) at 10 K indicates a modest spin exchange interaction through the Cu−F···H−O−Cu supersuperexchange paths, and a theoretical fit of χ( T ) to a 2D square model gives J 2D / k B = −5.58(1) K (in the convention where J rather than 2 J is used for spin exchange). At lower temperatures, χ( T ) shows a sharp peak at ∼2.6 K, which signals a transition to a long-range magnetic ordering as confirmed by coherent precession of implanted muons. Isothermal M ( B ) measurements made at 0.5 K on an aligned single crystal reveal magnetic saturation, M sat, at 28.8 T ( B ∥ a ) while a higher field of 33.1 T is required to saturate the spins when B is applied perpendicular to the a -axis thus showing clear anisotropy in these orientations. The presence of strong hydrogen bonds in CuF 2 (H 2 O) 2 (pyz) causes the chains to adopt a tilted packing arrangement, thus leading to a novel ground-state likely characterized by spin-canting within the 2D layers and a markedly increased critical temperature relative to the well-known Cu(NO 3 ) 2 (pyz) polymer chain compound.