Multimodal Structure Solution with ¹⁹F NMR Crystallography of Spin Singlet Molybdenum Oxyfluorides

Complex crystal structures with subtle atomic-scale details are now routinely solved using complementary tools such as X-ray and/or neutron scattering combined with electron diffraction and imaging. Identifying unambiguous atomic models for oxyfluorides, needed for materials design and structure-property control, is often still a considerable challenge despite their advantageous optical responses and applications in energy storage systems. In this work, NMR crystallography and single-crystal X-ray diffraction are combined for the complete structure solution of three new compounds featuring a rare triangular early transition metal oxyfluoride cluster, [Mo₃O₄F₉]^5-. After framework identification by single-crystal X-ray diffraction, 1D and 2D solid-state ¹⁹F NMR spectroscopy supported by ab initio calculations are used to solve the structures of K₅[Mo₃O₄F₉]·3H₂O (1), K₅[Mo₃O₄F₉]·2H₂O (2), and K₁₆[Mo₃O₄F₉]₂[TiF₆]₃·2H₂O (3) and to assign the nine distinct fluorine sites in the oxyfluoride clusters. Furthermore, ¹⁹F NMR identifies selective fluorine dynamics in K₁₆[Mo₃O₄F₉]₂[TiF₆]₃·2H₂O. These dual scattering and spectroscopy methods are used to demonstrate the generality and sensitivity of ¹⁹F shielding to small changes in bond length, on the order of 0.01 Å or less, even in the presence of hydrogen bonding, metal-metal bonding, and electrostatic interactions. Starting from the structure models, the nature of chemical bonding in the molybdates is explained by molecular orbital theory and electronic structure calculations. The average Mo-Mo distance of 2.505 Å and diamagnetism in 1, 2, and 3 are attributed to a metal-metal bond order of unity along with a 1a²1e⁴ electronic ground state configuration for the [Mo₃O₄F₉]^5- cluster, leading to a rare trimeric spin singlet involving d² Mo⁴⁺ ions. The approach to structure solution and bonding analysis is a powerful strategy for understanding the structures and chemical properties of complex fluorides and oxyfluorides.