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) Å, β= 109.12(3)°, V= 742.2(3) Å3, 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)2shows 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, d5Mn2+of two neighboring, edge‐sharing {MnO5OH}2units 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 spin–spin coupling through two identical 98.92(6)° Mn–O–Mn superexchange paths, turning the system into a metastable ferromagnetic state.