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

• Published in: Chemistry of materials Vol. 20; no. 24; pp. 7408 - 7416
• Main Authors: Manson, Jamie L, Conner, Marianne M, Schlueter, John A, McConnell, Amber C, Southerland, Heather I, Malfant, Isabelle, Lancaster, Tom, Blundell, Stephen J, Brooks, Michael L, Pratt, Francis L, Singleton, John, McDonald, Ross D, Lee, Changhoon, Whangbo, Myung-Hwan
• Format: Journal Article
• Language: English
• Published: American Chemical Society 23.12.2008
• Subjects: Characterization of Materials; Magnetic Materials; Polymeric Materials
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm8016566

The structural, electronic, and magnetic properties of the new linear chain coordination polymer CuF2(H2O)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 Cu2+ ion of CuF2(H2O)2(pyz) is located at a distorted CuF2O2N2 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 Cu2+ ion lies in the CuF2O2 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 2J 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 CuF2(H2O)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(NO3)2(pyz) polymer chain compound.