Solid-State 93Nb, 19F, and 113Cd Nuclear Magnetic Resonance Study of Niobium Oxyfluorides: Characterization of Local Distortions and Oxygen/Fluorine Ordering

Abstract

The local structures and oxygen/fluorine ordering of Cdpy 4 NbOF 5 (py = C 5 H 5 N) and [pyH] 2 [Cdpy 4 (NbOF 5 ) 2 ] have been investigated with 1 9 F, 1 1 3 Cd, and 9 3 Nb solid-state NMR spectroscopy. The 9 3 Nb magic-angle spinning (MAS) NMR spectra acquired at an ultrahigh magnetic field strength (19.6 T) and at ultrahigh spinning speeds (∼43 kHz) of both compounds are dominated not only by the quadrupolar interaction but also by the chemical shielding (CS) interaction, consistent with highly asymmetric environments for niobium. A methodology is presented to extract the nuclear quadrupole coupling constant C Q , the asymmetry parameter η, and the isotropic chemical shift δ i s o in the presence of a large chemical shift anisotropy (CSA). The CS tensor was then obtained from the simulations of the 9 3 Nb (I = 9/2) static NMR spectrum. The separations between the centerband of the central transition and sidebands of the ′ 3/2 ↔ ′ 5/2 satellite transition in the 9 3 Nb MAS NMR spectra are sensitive to the value of C Q ( 9 3 Nb), while the line shapes are mainly determined by η and the relative orientation between the electric field gradient (EFG) and CS tensors. Thus, simulations of the MAS NMR spectra provided additional constraints on the fitting of all these parameters. The 9 3 Nb MAS NMR spectra acquired at lower field and lower spinning speeds, where the spinning sidebands are not separated from the centerband, were also reproduced in the simulations. The following parameters were determined for the two compounds at room temperature: C Q = 36.0(5)/33.2(5) MHz, η = 0.50(5)/0.40(5), δ i s o ( 9 3 Nb) = -1310(10)/-1320(10) ppm, Ω = 1000(20)/1100(20) ppm, K = 1.00(5)/0.8(1), a = 0(20)°/ 60(20)°, β = 0(2)°/5(2)°, y = 0(20)°/0(20)° for Cdpy4NbOF5/[pyH] 2 [Cdpy 4 (NbOF 5 ) 2 ], respectively. A very distorted multiplet in the solid-state 1 9 F MAS NMR spectra was observed, which arises from J-coupling [ 1 J( 9 3 Nb, 1 9 F) = 362(2) and 350(5) Hz, for Cdpy 4 NbOF 5 and [pyH] 2 [Cdpy 4 (NbOF 5 ) 2 ], respectively] and residual dipolar coupling between the 1 9 F and 9 3 Nb nuclei. Only one 1 1 3 Cd NMR resonance is observed for Cdpy 4 -NbOF 5 , which, together with the 1 9 F NMR, suggests that the O-substitution is not random. On the basis of the 1 1 3 cd NMR, the NbOF 5 octahedra are most likely oriented in opposite directions in different chains so as to allow the acentric NbOF 5 octahedra to occupy a center of inversion in the crystal structure of this compound determined by X-ray diffraction (the interchain cancellation mechanism proposed by Heier et al., in J. Solid State Chem. 1997, 133, 576-579).