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

Abstract

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.

Fluoridation of HfO2

“…The low frequency signals are assigned to intralayer fluorine atoms that typically have longer Hf–F bonds. The resonances in Hf–O–F are broader than would be expected in an ordered oxyfluoride, which could be explained by anion disorder and/or a distribution of bond lengths that arises from an incommensurate structural modulation (vide supra) . If all the 19 F signals stem from a single phase, it would translate to an expected bond length range of about 0.28 Å.…”

“…From this basis, it is possible to estimate relative bond distances, that is, ranges of bond distances, from measured chemical shifts in cases where the structure model is unknown or incomplete. As a first-order approximation, we have observed a linear relationship among shift–bond length in covalent and ionic early transition metal fluorides such as TiF 4 , ZrF 4 , NbF 5 , TaF 5 , A 2 ZrF 6 , and A 3 ZrF 7 (A = Li + , Na + , K + , Rb + , Cs + , and NH 4 + ) and even [Mo 3 O 4 F 9 ] 5– cluster compounds, where effects such as the nature of the alkali species or the presence of oxygen can be considered second-order offsets. − Following this pattern for the aforementioned compounds, each transition metal has an apparent constant (in ppm/Å): Ti ≈ 1850 ppm/Å, Zr ≈ 270 ppm/Å, Nb ≈ 1250 ppm/Å, and Ta ≈ 1050 ppm/Å. The sensitivity of the shift to bond distance, combined with the high resolution of fast magic-angle spinning, mean that 19 F NMR can be a powerful probe of the atomic structure in transition metal fluorides.…”

Perovskite-like K3TiOF5 Exhibits (3 + 1)-Dimensional Commensurate Structure Induced by Octahedrally Coordinated Potassium Ions

“…The assignment is made on the basis of the chemical shifts and peak intensities. Generally, fluorine sites in oxyfluoride octahedra that are trans to oxygen have longer M–F bond distances than those that are trans to other fluorine . Longer bond distances translate to weaker paramagnetic shielding that arises from the orbital motion of the valence electrons on fluorine, according to Ramsey’s theory of magnetic shielding. − Thus, the resonance from the axial fluorine atom in the TiOF 5 unit should appear at a lower frequency than the resonances from the equatorial fluorine atoms.…”

“…Generally, fluorine sites in oxyfluoride octahedra that are trans to oxygen have longer M−F bond distances than those that are trans to other fluorine. 43 Longer bond distances translate to weaker paramagnetic shielding that arises from the orbital motion of the valence electrons on fluorine, according to Ramsey's theory of magnetic shielding. 44−46 Spatial relationships between the fluorine sites were probed with two-dimensional dipolar-coupling-mediated 19 F− 19 F correlation spectroscopy (Figure 7).…”

Anionic (dis)order and fluoride dynamics in complex transition metal oxyfluorides from NMR crystallography

“…The first system has relevance to fluoride-doped HfO 2 electronic materials [1]; the second example features a rare triangular metal oxyfluoride cluster, [Mo3O4F9] 5− (Fig. 1) [2]; and the third series of compounds are structurally diverse and provide fundamental insights into competition between centrosymmetric and noncentrosymmetric crystallization [3]. Identifying the anion (dis)order is central to building design rules for noncentrosymmetric crystals with technologically relevant properties.…”