Strong H...F hydrogen bonds as synthons in polymeric quantum magnets: structural, magnetic, and theoretical characterization of [Cu(HF)(pyrazine)]SbF, [CuF(HF)(FH)(pyrazine)].(SbF), and [CuAg(HF)(pyrazine)](SbF).
Three Cu{sup 2+}-containing coordination polymers were synthesized and characterized by experimental (X-ray diffraction, magnetic susceptibility, pulsed-field magnetization, heat capacity, and muon-spin relaxation) and electronic structure studies (quantum Monte Carlo simulations and density functional theory calculations). [Cu(HF{sub 2})(pyz){sub 2}]SbF{sub 6} (pyz = pyrazine) (1a), [Cu{sub 2}F(HF)(HF{sub 2})(pyz){sub 4}](SbF{sub 6}){sub 2} (1b), and [CuAg(H{sub 3}F{sub 4})(pyz){sub 5}](SbF{sub 6}){sub 2} (2) crystallize in either tetragonal or orthorhombic space groups; their structures consist of 2D square layers of [M(pyz){sub 2}]{sup n+} that are linked in the third dimension by either HF{sub 2}{sup -} (1a and 1b) or H{sub 3}F{sub 4}{sup -} (2). The resulting 3D frameworks contain charge-balancing SbF{sub 6}{sup -} anions in every void. Compound 1b is a defective polymorph of 1a, with the difference being that 50% of the HF{sub 2}{sup -} links are broken in the former, which leads to a cooperative Jahn-Teller distortion and d{sub x{sup 2}-y{sup 2}} orbital ordering. Magnetic data for 1a and 1b reveal broad maxima in x at 12.5 and 2.6 K and long-range magnetic order below 4.3 and 1.7 K, respectively, while 2 displays negligible spin interactions owing to long and disrupted superexchange pathways. The isothermal magnetization, M(B), for 1a and 1b measured at 0.5 K reveals contrasting behaviors: 1a exhibits a concave shape as B increases to a saturation field, B{sub c}, of 37.6 T, whereas 1b presents an unusual two-step saturation in which M(B) is convex until it reaches a step near 10.8 T and then becomes concave until saturation is reached at 15.8 T. The step occurs at two-thirds of M{sub sat}, suggesting the presence of a ferrimagnetic structure. Compound 2 shows unusual hysteresis in M(B) at low temperature, although x vs T does not reveal the presence of a magnetic phase transition. Quantum Monte Carlo simulations based on an anisotropic cubic lattice were applied to the magnetic data of 1a to afford g = 2.14, J = ?13.4 K (Cu-pyz-Cu), and J = ?0.20 K (Cu?F {hor_ellipsis} H {hor_ellipsis} F?Cu), while x vs T for 1b could be well reproduced by a spin-1/2 Heisenberg uniform chain model for g = 2.127(1), J{sub 1} = ?3.81(1), and zJ{sub 2} = ?0.48(1) K, where J{sub 1} and J{sub 2} are the intra- and interchain exchange couplings, respectively, which considers the number of magnetic nearest-neighbors (z). The M(B) data for 1b could not be satisfactorily explained by the chain model, suggesting a more complex magnetic structure in the ordered state and the need for additional terms in the spin Hamiltonian. The observed variation in magnetic behaviors is driven by differences in the H {hor_ellipsis} F hydrogen-bonding motifs.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); National Science Foundation (NSF); Research Corp.; Glasston Found.; Seaborg Inst.; EPSRC-UK
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 962072
- Report Number(s):
- ANL/MSD/JA-64564; JACSAT; TRN: US200919%%374
- Journal Information:
- J. Am.. Chem. Soc., Vol. 131, Issue 19 ; 2009; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- ENGLISH
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