Donor‐Stabilised [SbF₄]⁺: SbF₅ as a Fluoride‐Ion Donor

Abstract

Antimony pentafluoride is a strong Lewis acid and fluoride-ion acceptor that has not previously demonstrated any discreet fluoride-ion donor properties. The first donor-stabilised [SbF 4 ] + cations were prepared from the autoionisation of SbF 5 in the presence of bidentate N-donor ligands 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) as their [SbF 6 ] À salts. The [SbF 4 (NÀ N)][Sb 2 F 11 ] (NÀ N = bipy, phen) salts were synthesised by the addition of one equivalent of SbF 5 •SO 2 to [SbF 4 (NÀ N)][SbF 6 ] in liquid SO 2. The salts show remarkable stability and were characterised by Raman spectroscopy and multinuclear NMR spectroscopy. The crystal structures of [SbF 4 (phen)][SbF 6 ] • 3CH 3 CN and [SbF 4 (phen)][SbF 6 ] • 2SO 2 were determined, showing distorted octahedral cations. DFT calculations and NBO analyses reveal that significant degree of electron-pair donation from N to Sb stabilizes [SbF 4 ] + with the SbÀ N bond strength being approximately two thirds of that of the SbÀ F bonds in these cations and the cationic charge being primarily ligand-centred.Antimony pentafluoride, SbF 5 , is one of the strongest F À acceptors currently known, forming highly stable [Sb n F 5n + 1 ] À (n = 1-4) anions. Its solutions in anhydrous HF (aHF) are some of the strongest recorded Bronsted superacids. Antimony pentafluoride is also the benchmark for Lewis superacidity, as molecular Lewis acids with fluoride-ion affinities (FIAs) greater than monomeric SbF 5 in the gas phase (calcd. 501 kJ mol À 1 , exptl. 506 � 60 kJ mol À 1 ) are considered Lewis superacids. [1][2][3] The formation of polynuclear anions, however, can drive the already extreme FIA of SbF 5 even higher. [4] Thus, it is unsurprising that, aside from self-association of solid and liquid SbF 5 to form cis-linked SbF 6 octahedra, the ability of SbF 5 to act as a discrete F À donor is hitherto unknown. However, the lighter pnictogen pentafluorides, PF 5 and AsF 5 , have both previously demonstrated the ability to act as F À donors and form [PnF 4 ] + cations (Pn = P and As). [5,6] In fact, the entire series of [PX 4 ] + (X = F, [5] Cl, [7] Br, [8] I [9,10] ) and their arsonium analogues are known. [6,[11][12][13][14][15][16] Conversely, only [NF

Insights on the Lewis Superacid Al(OTeF5)3: Solvent Adducts, Characterization and Properties**

“…for the autoionization of SbF 5 . [21] The anticipated salt [Al(OTeF 5 ) 2 (bipy) 2 ][Al(OTeF 5 ) 4 (bipy)] crystallizes in the triclinic space group P (cf. Figure 5 ).…”

Fluoride-Ion Donor Properties of AsF5

“…We recently showed that donor-stabilized [SbF 4 ] + cations could be accessed through the ligand-induced autoionization of SbF 5 in the presence of the bidentate nitrogen bases 1,10phenanthroline (phen) and 2,2′-bipyridine (bipy) in SO 2 ClF. 12 Similar reactivity has also been observed with PF ] salts. 13,14 The group-5 transition metal pentafluorides MF 5 (M = Nb or Ta) are also known to undergo facile ligand-induced autoionization with a number of mono-and bidentate donors.…”

“…18 A similar level of thermal stability was also observed for [SbF 4 (phen)] + and [SbF 4 (bipy)] + salts. 12…”

“…Similar behavior was observed for the 19 F resonances of the [SbF 4 (phen)] + cation. 12 The F ax and F eq environments of [AsF 4 (phen)] + (Figure 4b) also appear as board quadrupolar collapsed singlets (F ax , −39.0 ppm, Δν 1/2 = 510 Hz; F eq , −79.0 ppm, Δν 1/2 = 550 Hz) in the 19 The 19 F NMR spectrum of (AsF 5 ) 2 •bipy (Figure 5) was recorded in CH 3 CN at −40 °C and shows a partially quadrupolar collapsed doublet at −49.0 ppm (8F; Δν 1/2 = 300 Hz) and a partially quadrupolar collapsed quintet at −73.9 ppm (2F; Δν 1/2 = 410 Hz). This indicates an AX 4 spin system and is consistent with two equivalent octahedrally coordinated arsenic centers with one fluorido ligand trans to the nitrogen donor and four cis to the nitrogen donor.…”

Insights on the Lewis Superacid Al(OTeF5)3: Solvent Adducts, Characterization and Properties

“…A similar approach was recently reported by Gerken et al for the autoionization of SbF5. [16] The anticipated salt [Al(OTeF5)2(bipy)2][Al(OTeF5)4(bipy)] crystallizes in the triclinic space group P1 ̅ (cf. Figure 6).…”