Importance of halogen ··· halogen contacts for the structural and magnetic properties of CuX ₂(pyrazine-N,N'-dioxide) (H ₂O) ₂ (X = Cl and Br)

The structural and magnetic properties of the newly crystallized CuX ₂(pyzO)(H ₂O) 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) Å ³ 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) Å ³ for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu ²⁺ 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 Cu ²⁺ ion is contained in the CuX ₂(H ₂O) ₂ plane and the X··· X contact distances are short. The magnetic data were fitted to a Heisenberg 1D 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 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)(H ₂O) ₂ is magnetically ordered with T _N = 1.06(1) K, while the signature for long-range magnetic order in CuBr2(pyzO)(H ₂O) ₂ was much less definitive down to 0.26 K. The results for the CuX ₂(pyzO)(H ₂O) ₂ complexes are compared to the related CuX ₂(pyrazine) materials.