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

A new open‐framework manganese borophosphate, KMnBP 2 O 7 (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 KMnBP 2 O 7 (OH) 2 [monoclinic, P 2 1 / c , a = 6.659(1), b = 12.049(2), c = 9.790(2) Å, β = 109.12(3)°, V = 742.2(3) Å 3 , and Z = 4] has an unprecedented open‐framework structure. {MnO 5 (OH)} octahedra and {BO 3 (OH)} and {PO 3 (OH)}/{PO 4 } 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, KMnBP 2 O 7 (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, d 5 Mn 2+ of two neighboring, edge‐sharing {MnO 5 OH} 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 spin–spin coupling through two identical 98.92(6)° Mn–O–Mn superexchange paths, turning the system into a metastable ferromagnetic state.