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 F-19 NMR spectroscopy supported by ab initio calculations are used to solve the structures of K-5[Mo3O4F9]center dot 3H(2)O (1), K-5[Mo3O4F9]center dot 2H(2)O (2), and K-16[Mo3O4F9](2)[TiF6](3)center dot 2H(2)O (3) and to assign the nine distinct fluorine sites in the oxyfluoride clusters. Furthermore, F-19 NMR identifies selective fluorine dynamics in K-16[Mo3O4F9](2)[TiF6](3)center dot 2H(2)O. These dual scattering and spectroscopy methods are used to demonstrate the generality and sensitivity of F-19 shielding to small changes in bond length, on the order of 0.01 angstrom 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 angstrom and diamagnetism in 1, 2, and 3 are attributed to a metal-metal bond order of unity along with a 1a(2)1e(4) electronic ground state configuration for the [Mo3O4F9](5-) cluster, leading to a rare trimeric spin singlet involving d(2) 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.