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

The structural and magnetic properties of the newly crystallized CuX2(pyzO)(H2O)(2) = 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) angstrom, b = 5.5560(2) angstrom, c = 10.4254(5) angstrom, beta = 115.400(2)degrees, and V = 892.21(7) angstrom(3) for X = Cl and a = 17.3457(8) angstrom, b = 5.6766(3) angstrom, c = 10.6979(5) angstrom, beta = 115.593(2)degrees, and V = 950.01(8) angstrom(3) 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 center dot center dot center dot O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X center dot center dot center dot 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 center dot center dot center dot X-Cu supersuperexchange because the magnetic orbital of the Cu2+ ion is contained in the CuX2(H2O)(2) plane and the X center dot center dot center dot X contact distances are short. The magnetic data were fitted to a Heisenberg ID uniform antiferromagnetic chain model with J(1D)/k(B) = -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 J(1D) 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 J(1D)/k(B) to -51.2 K at 0.84 GPa, suggesting a shortening of the Br center dot center dot center dot Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at similar to 3.5 GPa. Muon-spin relaxation measurements indicate that CuCl2(pyzO)(H2O)(2) is magnetically ordered with T-N = 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.