Terminal and bridging fluorine ligands in TiF₄ as studied by ¹⁹F NMR in solids

• 12 isotropic signals for 12 crystallographic sites of fluorine in TiF₄.

• 6 signals at ca. 460 ppm bear significantly large CSA ∼1000 ppm.

• The observed chemical shifts are consistent with those calculated by DFT-GPAW.

• Rotation-synchronized DANTE was applied to obtain each spinning sideband pattern.

• Anomalous ¹⁹F chemical shifts are rationalized on the basis of the Ramsey's eq.

Abstract

To examine bonding nature of fluorine ligands in a metal coordinated system, ¹⁹F high-resolution solid-state NMR has been applied to TiF₄, which bears both bridging and terminal fluorines. Observed 12 isotropic signals are assigned to 12 crystallographically different fluorines (6 terminal and 6 bridging fluorines) in TiF₄ by referring to the calculated isotropic shifts using density functional theory (DFT). The isotropic chemical shift (δ_iso) for terminal F (F_T) appears at high frequency (420–480 ppm from δ(CCl₃F) = 0 ppm) with large shielding anisotropy Δσ ∼ 850 ppm. Whereas the δ_iso and Δσ values for bridging F (F_B) are moderate; δ_iso ∼ 0–25 ppm and Δσ ∼ 250 ppm. The origin of the observed high-frequency shift for F_T is ascribed to the second-order paramagnetic shift with increased covalency, shorter Ti–F bonds, and smaller energy difference between the occupied and vacant orbitals. Examination of the orientation of the shielding tensor relative to the molecular structure shows that the most deshielded component of the shielding tensor is oriented along the Ti–F bond. The characteristic orientation is consistent with a Ti–F σ bond formed by d_YZ of Ti and p_z of F. Further, we show that the selectively observed spinning sideband patterns and the theoretical patterns with the calculated Δσ and η (shielding asymmetry) values are not consistent with each other for F_B, indicating deficiency of the present DFT calculation in evaluating Δσ.

Introduction

In various fluorine-containing coordination complexes with transition metals, fluorine acts as a bridging ligand between two metals. For example, in a dimeric species (M₂F₂L₆²⁺; M = Co(II) and Cu(II) and L denotes another ligand), two ML₃ units are connected by two bridging fluorines (abbreviated to F_B here) to form trigonal bipyramidal or square pyramidal structures [1]. A tetrameric species (M₄F₄L₁₂⁴⁺; M = Mn(II), Co(II), Ni(II), and Cd(II)) consists of four ML₃ units at the alternating corners of a cubic connected with each other via three F_B atoms at the remaining corners to form the octahedral MF₃L₃ structure. A chain-type compound has the octahedral MF₄L₂ structural unit as the repeating unit with the four fluorines shared by neighboring units. Other important structural types of transition metal fluoro-compounds and fluoride salts are reviewed by Leblanc et al. [2].

In addition to the F_B species, a terminal fluorine (abbreviated to F_T here) has also been long known, and chemical nature of the F_B and F_T species has been investigated. As for a few examples, Reinen et al. have examined the binding properties of F_B and F_T in M(III)L₆ complexes (M = Cr(III), Fe(III), and Mn(III)) by a combined vibronic coupling and angular overlap analysis with DFT calculations [3]. It was concluded that the total bond strength and the ionic contributions to the bond energy are F_T ≫ F_B, and F_B induces much stronger π-bonds than F_T does. A high electrostatic Dy-F_T bond was also shown by high-resolution luminescence spectroscopy and correlated with the single-molecule magnet behavior through experimental magnetic susceptibility data and ab initio calculations, leading a large axial crystal-field splitting of the J = 15/2 ground state [4]. It was also shown that the F⁻ ligands in Cu(II)-F_T in dinuclear Cu(II) complexes based on a diazecine ligand behave as “acceptors” for hydrogen bonding [5]. Among these, we are particularly interested in the reported low-frequency ¹⁹F isotropic NMR chemical shift of F_T in a palladium (II) fluoride complexes with an anionic fluoro-substituted diarylamido/bis(phosphine) pincer as a ligand [6]. The chemical shift is −414.3 ppm in C₆D₆ from δ(CFCl₃) = 0 ppm, which appears to be at the low-frequency end of the range of chemical shifts for metal fluorides. The low-frequency shift was related to the nature of the ligand trans to the fluorine.

Among various fluorine-containing coordination complexes with transition metals, fluorine-titanate(IV) complexes are known to form various supramolecular crystal assembly, which is recently reviewed by Davidovich [7]. In this work, we chose to examine TiF₄, which bears both F_T and F_B [8], by ¹⁹F high-resolution solid-state NMR. Both isotropic chemical shift (δ_iso) and chemical shielding anisotropy (CSA) of ¹⁹F for F_T and F_B are of concern.

We show that, by employing a high magnetic field of 14 T with fast magic-angle spinning (MAS) (the MAS spinning frequency ν_R = 50 kHz), highly resolved 12 signals for all crystallographically different 12 F sites are obtained. The 12 isotropic chemical shifts are then assigned to the 12 fluorines with the aid of quantum-chemical calculation. The observed isotropic shifts linearly correlated well with those calculated ones. The large high-frequency shift observed for F_T is discussed on the basis of the covalency/ionicity of the Ti–F bonding and the Ti–F bond distance. The calculated ¹⁹F shielding anisotropy Δσ values are anomalously large for F_T (ca. 1000 ppm) and the lowest-frequency (the most shielded) component of the shielding tensor lies along the Ti–F bond direction. This orientation of the shielding tensor is used to consider the molecular orbitals responsible for the Ti–F bonding. Further, the individual spinning-sideband pattern for each fluorine is selectively observed by using the rotation-synchronized Delays Alternating with Nutation for Tailored Excitation (rs-DANTE [9]). By comparison among the selectively observed sideband patterns and the calculated ones, we show that the experimental Δσ values are much smaller than those calculated especially for F_B.