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

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 (mu SR), and electronic structure calculations. Each Cu2+ ion of CuF2(H2O)(2)(pyz) is located at a distorted CuF2O2N2 octahedron with axial elonclation along the Cu-N bonds. These octahedra are tethered to-ether by strong F center dot center dot center dot H-O hydrogen bonds to yield two-dimensional (2D) square nets in the bc-plane that are linked alone, 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 chi(T) at 10 K indicates a modest spin exchange interaction through the Cu-F center dot center dot center dot H-O-Cu supersuperexchange paths, and a theoretical fit of chi(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, chi(T) shows a sharp peak at similar to 2.6 K. which si-nals a transition to a lon-ran-e maonetic orderine, 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 parallel to 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.