Ionothermal Synthesis of the First Luminescent Open-Framework Manganese Borophosphate with Switchable Magnetic Properties

A new open-framework manganese borophosphate, KMnBP2O7(OH)2, has been prepared under ionothermal conditions in the ionic liquid 1-butyl-1-methylpyrrolidinium bromide. Single-crystal X-ray diffraction analysis showed that KMnBP2O7(OH)2 [monoclinic, P21/c, a = 6.659(1), b = 12.049(2), c = 9.790(2) angstrom, beta = 109.12(3)degrees, V = 742.2(3) angstrom 3, and Z = 4] has an unprecedented open-framework structure. {MnO5(OH)} octahedra and {BO3(OH)} and {PO3(OH)}/{PO4} tetrahedra link to form a 3D framework with distorted eight-membered-ring channels along the crystallographic [001] direction. The negative charges of the inorganic framework are balanced by K+ ions located in the channels. At room temperature, KMnBP2O7(OH)2 shows a bright orange-red luminescence that redshifts with decreasing temperature. The luminescence lifetimes were determined to be 0.17 ms at room temperature and 3.49 ms at 77 K. Below 3 K, d5 Mn2+ of two neighboring, edge-sharing {MnO5OH}2 units couple antiferromagnetically, and the magnetic state can be best described as a singlet spin system. These singlets continuously turn into triplets when increasing the magnetic field up to 7 T at 2 K due to relatively weak spinspin coupling through two identical 98.92(6)degrees MnOMn superexchange paths, turning the system into a metastable ferromagnetic state.