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Importance of Halogen···Halogen Contacts for the Structural and Magnetic Properties of CuX2(pyrazine-N,N′-dioxide)(H2O)2 (X = Cl and Br)

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journal contribution
posted on 2012-02-20, 00:00 authored by John A. Schlueter, Hyunsoo Park, Gregory J. Halder, William R. Armand, Cortney Dunmars, Karena W. Chapman, Jamie L. Manson, John Singleton, Ross McDonald, Alex Plonczak, Jinhee Kang, Chaghoon Lee, Myung-Hwan Whangbo, Tom Lancaster, Andrew J. Steele, Isabel Franke, Jack D. Wright, Stephen J. Blundell, Francis L. Pratt, Joseph deGeorge, Mark M. Turnbull, Christopher P. Landee
The structural and magnetic properties of the newly crystallized CuX2(pyzO)­(H2O)2 (X = Cl, Br; pyzO = pyrazine-N,N′-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) Å, b = 5.5560(2) Å, c = 10.4254(5) Å, β = 115.400(2)°, and V = 892.21(7) Å3 for X = Cl and a = 17.3457(8) Å, b = 5.6766(3) Å, c = 10.6979(5) Å, β = 115.593(2)°, and V = 950.01(8) Å3 for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu2+ ions linked through bidentate pyzO ligands. These chains are joined together through OH···O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu–X···X–Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu–X···X–Cu supersuperexchange because the magnetic orbital of the Cu2+ ion is contained in the CuX2(H2O)2 plane and the X···X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J1D/kB = −11.1(1) (Cl) and −45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J1D is determined to be −11.5(2) (Cl) and −46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J1D/kB up to −51.2 K at 0.84 GPa, suggesting a shortening of the Br···Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at ∼3.5 GPa. Muon-spin relaxation measurements indicate that CuCl2(pyzO)­(H2O)2 is magnetically ordered with TN = 1.06(1) K, while the signature for long-range magnetic order in CuBr2(pyzO)­(H2O)2 was much less definitive down to 0.26 K. The results for the CuX2(pyzO)­(H2O)2 complexes are compared to the related CuX2(pyrazine) materials.