Tin(IV) fluoride complexes with neutral phosphine coordination and comparisons with hard N- and O-donor ligands

Abstract

The reactions of trans-[SnF₄(PMe₃)₂] with one, two or three equivalents of Me₃SiO₃SCF₃ (TMSOTF), respectively, in anhydrous CH₂Cl₂ form six-coordinate [SnF_4−n(PMe₃)₂(OTf)_n] (n = 1–3), which have been characterised by microanalysis, IR and multinuclear NMR (¹H, ¹⁹F{¹H}, ³¹P{¹H} and ¹¹⁹Sn) spectroscopy. The crystal structure of [SnF₃(PMe₃)₂(OTf)] reveals the three fluorines are in a mer-arrangement with mutually trans PMe₃ ligands. The multinuclear NMR spectra confirm this structure is retained in solution, and show that [SnF₂(PMe₃)₂(OTf)₂] has trans-phosphines, while [SnF(PMe₃)₂(OTf)₃] has trans PMe₃ groups and hence mer-triflate ligands. The [SnF_4−n(PMe₃)₂(OTf)_n] are unstable in solution and the decomposition products include [Me₃PF]⁺ and the tin(II) complexes [Sn(PMe₃)₂(OTf)₂] and [Sn₃F₅(OTf)], both of the latter identified by their crystal structures. The reaction of trans-[SnF₄(PⁱPr₃)₂] containing the bulkier phosphine, with one and two equivalents of TMSOTf produced unstable mono- and bis-triflates, which the NMR data also suggest contain weakly coordinated triflate, [SnF₃(PⁱPr₃)₂(OTf)] and [SnF₂(PⁱPr₃)₂(OTf)₂], again with axial phosphines, although some OTf dissociation from the former to give [SnF₃(PⁱPr₃)₂]⁺ may occur in solution at room temperature. The new phosphine complexes of SnF₄, trans-[SnF₄(PⁱPr₃)₂] and (cis) [SnF₄(κ²-triphos)] (triphos = CH₃C(CH₂PPh₂)₃) have also been fully characterised, including the crystal structure of [SnF₄(κ²-triphos)]. Attempts to promote P₃-coordination by further treatment of this complex with TMSOTf were unsuccessful. The [SnF₄(L)₂] (L = dmso, py, pyNO, DMF, OPPh₃) complexes, which exist as mixtures of cis and trans isomers, react with one equivalent of TMSOTf, followed by addition of one equivalent of L, to form the ionic [SnF₃(L)₃][OTf] complexes, which were characterised by microanalysis, IR and multinuclear NMR spectroscopy. In nitromethane solution they are a mixture of mer and fac isomers based upon multinuclear NMR data (¹H, ¹⁹F{¹H}, ¹¹⁹Sn). Reaction of [SnF₄(OPPh₃)₂] with two equivalents of TMSOTf and further OPPh₃ produced [SnF₂(OPPh₃)₄][OTf]₂, which is a mixture of cis and trans isomers in solution. The crystal structure of [SnF₂(OPPh₃)₄][OTf]₂ confirms the trans isomer in the solid state, with the triflate ionic. These complexes are rare examples of fluorotin(IV) cations with neutral monodentate ligands.

---

Introduction

In addition to their intrinsic interest, coordination complexes of the main group elements have attracted considerable interest in recent years for a variety of applications, including Lewis acid based and metal free catalysis, development of precursors for the chemical vapour deposition of semiconductor thin films and, particularly in the case of main group fluoride complexes, towards new types of ¹⁸F carriers for positron emission tomography imaging in medicine.¹

The tin(IV) halides, SnX₄ (X = Cl, Br or I) are tetrahedral monomers that are widely used as Lewis acids and as synthons for tin(IV) complexes.^2–5 The complexes are generally formed by adding two neutral donor groups to form six-coordinate cis- or trans-[SnX₄(L)₂] or cis-[SnX₄(bidentate)]. Only very rarely do neutral ligands displace a coordinated halide to form Sn(IV) cations; exceptions are provided by the macrocycles 1,4,7-trimethyl-1,4,7-triazacyclononane, 1,3,5-trimethyl-1,3,5-triazacyclohexane and 1,4,7-trithiacyclononane (L₃), which generate the ‘self-ionisation’ complexes, [SnX₃(L₃)]₂[SnX₆] (X = Cl or Br),^6,7 although more flexible acyclic polydentate ligands, such as MeC(CH₂AsMe₂)₃, behave as bidentate ligands, as in [SnX₄{κ²-MeC(CH₂AsMe₂)₃}].⁸ Formation of tin(IV) cations can be achieved by using halide abstractors such as AlCl₃, CF₃SO₃SiMe₃ (TMSOTf) or Na[BAr^F] (BAr^F = [B{3,5-(CF₃)₂(C₆H₃)}₄]⁻).^9,10

Tin(IV) fluoride contains six-coordinate tin in vertex sharing octahedra¹¹ and although readily hydrolysed, is unreactive towards many neutral ligands. As a result of the lack of commercial availability and this limited reactivity, its coordination chemistry was little explored until recently; only one crystal structure was reported (for the complex [SnF₄(2,2′-bipy)]) and spectroscopic data were limited.^12–17 A key development was the synthesis of [SnF₄(MeCN)₂] from SnF₂ and I₂ in MeCN by Tudela.¹⁸ The other product, SnI₄, does not form a stable MeCN adduct and is readily removed by washing with CS₂. The MeCN is easily displaced from [SnF₄(MeCN)₂] by neutral ligands, and hence this is a valuable synthon for developing wider coordination chemistry of SnF₄ with a range of ligands. Detailed studies of the reactions of [SnF₄(MeCN)₂] with phosphine and arsine oxides produced a series of complexes [SnF₄(R₃EO)₂] (R = Me, Ph; E = P, As) and [SnF₄(L-L)] (L-L = (L-L = o-C₆H₄(P(O)Ph₂)₂, o-C₆H₄(P(O)Me₂)₂ or Ph₂P(O)CH₂P(O)Ph₂),¹⁹ and comparison of spectroscopic and structural data on these and their heavier SnX₄ (X = Cl, Br, I) analogues confirmed SnF₄ as the strongest Lewis acid of the four. Subsequently, these studies were extended to tertiary phosphine complexes, although arsine ligands did not form isolable complexes with SnF₄ (in contrast to their behaviour with other SnX₄).⁸ Further studies of N- and O-donor complexes of SnF₄ were also included, but none of the ligands showed any tendency to displace fluoride from SnF₄.⁸ Fluorotin cations were obtained with the tetradentate N-donor ligand, tris(1-ethyl-benzoimidazol-2-ylmethyl)amine (BIMEt₃) by reaction of SnF₄, BIMEt₃ and appropriate amounts of TMSOTf, when [SnF_4−x(BIMe₃)][OTf]_x (x = 1, 2, 3) were isolated.²⁰ Other fluorotin(IV) cations appear unknown.¹²

Here we report the reactions of phosphine complexes of tin(IV) tetrafluoride with some halide abstractors, and several cationic N- and O-donor ligand complexes, [SnF_4−xL_2+x]^x+, prepared by abstraction of fluoride from [SnF₄L₂] with TMSOTf in the presence of added L.

Experimental

All complex syntheses were carried out using standard Schlenk and vacuum line techniques. Samples were handled and stored in a glove box under a dry dinitrogen atmosphere to exclude moisture. TMSOTf was obtained from Sigma-Aldrich and distilled before use. Tin(II) fluoride was obtained from Alfa Aesar and used as received. Phosphine ligands, OPPh₃ and pyNO were obtained from Sigma-Aldrich or Strem and used as received. Na[BAr^F] was made as described.¹⁰ CH₂Cl₂, DMF, dmso and MeCN were dried by distillation from CaH₂, py, diethyl ether and n-hexane from Na. [SnF₄(MeCN)₂] was made from SnF₂ and I₂ in MeCN as described by Tudela.¹⁸ [SnF₄(PMe₃)₂],⁸ [SnF₄(Py)₂]⁸ and [SnF₄(OPPh₃)₂]¹⁹ were made by literature methods.

Infrared spectra were recorded as Nujol mulls between CsI plates using a Perkin Elmer Spectrum 100 spectrometer over the range 4000–200 cm⁻¹. ¹H, ¹⁹F{¹H}, ³¹P{¹H}, and ¹¹⁹Sn NMR spectra were recorded from CH₂Cl₂/CD₂Cl₂ or CH₃NO₂/CD₃NO₂ solutions using a Bruker AV400 spectrometer and referenced to TMS via the residual solvent resonance, CFCl₃, 85% H₃PO₄, or neat SnMe₄, as appropriate. Typically, [Cr(acac)₃] was added as a relaxation agent when recording the ¹¹⁹Sn spectra. Microanalyses were undertaken by London Metropolitan University or Medac.

[SnF₃(PMe₃)₂(OTf)]

To a solution of [SnF₄(PMe₃)₂] (0.100 g, 0.288 mmol) in CH₂Cl₂ (2 mL) a solution of TMSOTf (0.064 g, 0.288 mmol) in CH₂Cl₂ (2 mL) was added dropwise to form a clear solution. The reaction was stirred for 2 h, the volatiles were then removed in vacuo to leave a solid, which was washed with hexane (3 × 10 mL) and dried in vacuo to form a white powder. Crystals suitable for single crystal X-ray diffraction were grown by layering a CH₂Cl₂ solution of the complex with hexane. Yield 0.083 g, 60%. Required for C₇H₁₈F₆P₂S₂Sn (476.91): C, 17.6; H, 3.8. Found: C, 17.5; H, 4.2%. IR (Nujol/cm⁻¹): ν = 517m, 546m, 573m (Sn–F), 1156 (–OSO₂), 1224, 1261 (CF₃). ¹H NMR (CD₂Cl₂, 298 K): δ = 1.70 (d, ²J_PH = 12 Hz, CH₃). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −78.8 (s, OTf), −126.9 (br s, Sn–F), −149.2 (br s, Sn–F); (CD₂Cl₂, 183 K): δ = −78.5 (s, [3F], OTf), −127.9 (td, [2F], ²J_PF(cis-OTf) = 153 Hz, ²J_FF = 41 Hz, ¹J_¹¹⁹SnF = 3253 Hz, ¹J_¹¹⁷SnF = 3108 Hz, Sn–F_cis-OTf), −151.4 (tt, [F], ²J_{PF(trans-OTf)} = 121 Hz, ²J_FF = 41, ¹J_¹¹⁹SnF = 3013 Hz, ¹J_¹¹⁷SnF = 2893 Hz, Sn–F_trans-OTf). ³¹P{¹H} NMR (CH₂Cl₂, 298 K): δ = −6.01 (s); (CH₂Cl₂, 183 K): δ = −3.2 (td, ²J_PF(cis-OTf) = 153 Hz; ²J_{PF(trans-OTf)} = 121 Hz; ¹J_¹¹⁹SnP = 3412 Hz; ¹J_¹¹⁷SnP = 3263). ¹¹⁹Sn NMR (CH₂Cl₂, 183 K): δ = −599.9 (m).

[SnF₂(PMe₃)₂(OTf)₂]

To a solution of [SnF₄(PMe₃)₂] (0.100 g, 0.288 mmol) in CH₂Cl₂ (2 mL) a solution of TMSOTf (0.128 g, 0.576 mmol) in CH₂Cl₂ (2 mL) was added dropwise to form a clear solution. The reaction was stirred for 2 h. The volatiles were then removed in vacuo to leave a solid, which was washed with hexane (3 × 10 mL) and dried in vacuo to form a white powder. Yield 0.103, 59%. Required for C₈H₁₈F₈O₆P₂S₂Sn (606.98): C, 15.8; H, 3.0. Found: C, 15.7; H, 3.6%. IR (Nujol/cm⁻¹): ν = 531 (s) (Sn–F). ¹H NMR (CD₂Cl₂, 298 K): δ = 1.88 (d, ²J_PH = 12 Hz, CH₃). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −78.0 (s, [6F], OTf), −140 (br s, [2F], Sn–F); (CD₂Cl₂, 183 K): δ = −77.8, −78.4 (OTf⁻), −142.7 (t, ²J_P− = 107 Hz, ¹J_¹¹⁹SnF = 3393 Hz, ¹J_¹¹⁷SnF = 3232, Sn–F). ³¹P{¹H} NMR (CH₂Cl₂, 298 K): δ = 7.3 (br); (CH₂Cl₂, 183 K): δ = 10.2 (t, ²J_PF 107 Hz; ¹J_¹¹⁹SnP = 3654 Hz; ¹J_¹¹⁷SnP = 3484). ¹¹⁹Sn NMR (CH₂Cl₂, 183 K): δ = −609 (tt, ¹J_¹¹⁹SnP = 3654 Hz, ¹J_¹¹⁹SnF = 3393 Hz).

[SnF(PMe₃)₂(OTf)₃]

To a solution of [SnF₄(PMe₃)₂] (0.100 g, 0.288 mmol) in CH₂Cl₂ (2 mL) a solution of TMSOTf (0.192 g, 0.864 mmol) in CH₂Cl₂ (2 mL) was added dropwise to form a clear solution. The reaction was stirred for 10 min, the volatiles were then removed in vacuo to leave a solid, which was washed with hexane (3 × 10 mL) and dried in vacuo to form a white powder. Yield 0.137 g (64%). Required for C₉H₁₈F₁₀O₉P₂S₃Sn (737.07): C, 14.7; H, 2.6. Found: C, 14.3; H, 3.7%. IR (Nujol/cm⁻¹): ν = 510m (Sn–F), 1158m, 1164m (–OSO₂), 1191m, 1202m, 1237m, 1243m (CF₃). ¹H NMR (CD₂Cl₂, 298 K): δ = 2.0 (d, ²J_PH = 13 Hz, CH₃). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −77.6 (s, [9F], OTf), −132.9 (t, ²J_PF = 88 Hz, ¹J_¹¹⁹SnF = 3557 Hz, ¹J_¹¹⁷SnF = 3396 Hz), [F], (Sn–F). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 18.3 (d, ²J_PF = 88 Hz, ¹J_¹¹⁹SnP = 3740 Hz; ¹J_¹¹⁷SnP = 3568 Hz). ¹¹⁹Sn NMR (CH₂Cl₂, 183 K): δ = −617 (m).

[SnF₄(PⁱPr₃)₂]

To a suspension of [SnF₄(MeCN)₂] (0.648 g, 2.34 mmol) in CH₂Cl₂ (5 mL) a solution of PⁱPr₃ (0.750 g, 4.68 mmol) in CH₂Cl₂ was added to form a slightly cloudy solution. The reaction was stirred for 1 h, then the solution was filtered and the filtrate was concentrated in vacuo to yield a white solid, which was washed with hexane (3 × 10 mL) and dried in vacuo. Yield: 0.862 g (71%). C₁₈H₄₂F₄P₂Sn·3/4CH₂Cl₂ (578.84): C, 38.9; H, 7.6. Found: C, 38.8 H, 8.1%. IR (Nujol/cm⁻¹): ν = 535s (Sn–F) ¹H NMR (CD₂Cl₂, 298 K): δ = 1.41 (dd, ³J_PH = 15Hz, ³J_HH = 7 Hz, [18H], CH₃), 2.55 (septet of d, ³J_HH = 7 Hz, ²J_PH = 1 Hz, [3H], CH), ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −99.1 (t, ²J_PF = 124 Hz, ¹J_¹¹⁹SnF = 2477 Hz, ¹J_¹¹⁷SnF = 2363 Hz), ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 30.6 (quint, ²J_PF = 124, ¹J_¹¹⁹SnP = 2920 Hz, ¹J_¹¹⁷SnP = 2792 Hz). ¹¹⁹Sn NMR (CH₂Cl₂, 183 K): δ = −622.0 (quin of t, ¹J_¹¹⁹SnF = 2920 Hz, ¹J_¹¹⁷SnP = 2477 Hz).

[SnF₃(PⁱPr₃)₂(OTf)]

To a solution of [SnF₄(PⁱPr₃)₂] (0.100 g, 0.194 mmol) in CH₂Cl₂ (2 mL) a solution of TMSOTf (0.043 g, 0.194 mmol) was added dropwise to form a clear solution. The reaction was stirred for 1 h, volatiles were then removed in vacuo to yield a white solid, which was washed with hexane (3 × 10 mL) and dried in vacuo. Yield: 0.095 g (76%). Required for C₁₉H₄₂F₆O₃P₂S·1/2CH₂Cl₂ (718.68): C, 32.6; H, 6.0. Found: C, 32.1; H, 6.7%. IR (Nujol/cm⁻¹): ν = 540m, 558m, 571m (Sn–F). 1150m (–OSO₂) 1223, 1260 (CF₃). ¹H NMR (CD₂Cl₂, 298 K): δ = 1.45 (dd, ³J_PH = 15Hz, ³J_HH = 7 Hz, [18H], CH₃), 2.77 (septet of d, ³J_HH = 7 Hz, ²J_PH = 2 Hz, CH), ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −73.5 (br s, ¹J_SnF = 3518 Hz), −78.5 (s, OTf), −116.9 (br s, ¹J_SnF = 3244 Hz). ³¹P{¹H} NMR (CH₂Cl₂, 298 K): δ = 38.2 (q, ²J_PF = 121 Hz, ¹J_¹¹⁹SnP = 2827 Hz, ¹J_¹¹⁷SnP = 2702 Hz), ¹¹⁹Sn NMR (CH₂Cl₂, 183 K): −611.1 (tdt, ¹J_¹¹⁹SnF = 3558, 3200 Hz, ¹J_¹¹⁹SnP = 2856 Hz.

[SnF₂(PⁱPr₃)₂(OTf)₂]

To a solution of [SnF₄(PⁱPr₃)₂] (0.200 g, 0.388 mmol) in CH₂Cl₂ (2 mL) a solution of TMSOTf (0.173 g, 0.778 mmol) was added dropwise to form a clear solution. The reaction was stirred for 1 h, volatiles were removed in vacuo to yield a white solid which was washed with hexane (3 × 10 mL) and dried in vacuo. Yield: 0.231 g (77%). Required for C₂₀H₄₂F₈O₆P₂S₂Sn·CH₂Cl₂ (860.20): C, 29.3; H, 5.2. Found: C, 29.1; H, 5.8%. IR (Nujol/cm⁻¹): ν = 519w, 533w (Sn–F), 1150 (OSO₂), 1234w, 1261w (CF₃). ¹H NMR (CD₂Cl₂, 298 K): 1.49 (dd, ³J_PH = 16Hz [8H], CH₃), 3.05 (septet of d, ³J_HH = 8 Hz, ²J_PH = 3 Hz, [3H], CH). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): −56.9 (br, ¹J_SnF = 4082 Hz), −77.9 (s, OTf). ³¹P{¹H} NMR (CH₂Cl₂, 298 K): 49.4 (t, ²J_PF = 117 Hz, ¹J_¹¹⁹SnP = 2971 Hz, ¹J_¹¹⁷SnP = 2839 Hz), ¹¹⁹Sn NMR (CH₂Cl₂, 183 K): −630 (tt, ¹J_¹¹⁹SnF = 4013, ¹J_¹¹⁹SnP = 2941).

[SnF₄(triphos)]

To a suspension of [SnF₄(MeCN)₂] (0.200 g, 0.722 mmol) in CH₂Cl₂ (2 mL), triphos (0.451 g, 0.722 mmol) was added as a solid and the resulting solution was stirred for 2 h to yield a cloudy solution, which was then filtered. Volatiles were removed from the filtrate in vacuo to yield a white solid, which was washed with hexane (3 × 10 mL) and dried in vacuo to yield a white solid. Crystals suitable for single crystal X-ray diffraction were grown by layering a CH₂Cl₂ solution of the complex with hexane. Yield: 0.326 g (55%). Required for C₄₁H₃₉F₄P₃Sn·1/2CH₂Cl₂ (861.78): C, 57.8; H, 4.7. Found: C, 58.2 H, 4.8%. IR (Nujol/cm⁻¹): ν = 516m, 552m, 564m (Sn–F). ¹H NMR (CD₂Cl₂, 298 K): δ = 0.87 (s, [3H], CH₃), 2.28 (s, [2H]), 2.91–2.95 (m, [2H]), 3.11 (m, [2H]), 7.1–8.1 (m, [30H], Ar–H). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −144.2 (tt, ²J_PF = 98 Hz, ²J_FF = 45, [2F]), −109.7 (m, [2F]). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = −6.9 (dtd, ²J_PF = 121, 98, 51, [2P]), −27.1 (s, [P]). ¹¹⁹Sn NMR (CH₂Cl₂, 183 K): δ = −681.4 (m).

[SnF₄(dmso)₂]

[SnF₄(MeCN)₂] (0.20 g, 0.72 mmol) was suspended in excess dmso and stirred for 5 min. The solvent was removed, and the white powder was dried in vacuo. Yield 0.120 g, 47%. Required for C₄H₁₂F₄O₂S₂Sn (351.0): C, 13.7; H, 3.5%. Found C, 13.1; H, 3.5%. IR (Nujol/cm⁻¹): ν = 936s, 908sh (SO), 573vs, 552sh, 522m (SnF). ¹H NMR (CD₃NO₂, 298 K): δ = 3.1 (s), 3.0 (s). ¹⁹F{¹H} NMR (CD₃NO₂, 298 K): δ = −161.6 (t, ¹J_¹¹⁹SnF = 2030, ²J_FF = 52 Hz), −149.3 (t, ¹J_¹¹⁹SnF = 2020, ²J_FF = 52 Hz), −149.0 (s, ¹J_¹¹⁹SnF = 2466 Hz). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): δ = −778.4.

[SnF₃(dmso)₃][OTf]

[SnF₄(dmso)₂] (0.15 g, 0.43 mmol) was suspended in CH₂Cl₂ (10 mL). To this TMSOTf (0.035 g, 0.43 mmol) in CH₂Cl₂ (5 mL) was added and the solution stirred for 2 h. Dmso (0.04 g, 0.43 mmol) in MeCN (5 mL) was then added and the solution was again stirred for 2 h. The solvent and volatiles were removed and dried in vacuo, which yielded a colourless, gel-like solid which was recrystalised from MeCN/Et₂O to a colourless powder. Yield 0.11 g, 46%. Required for C₇H₁₈F₆O₆S₄Sn (559.2): C, 15.0; H, 3.2. Found C, 14.9; H, 3.2%. IR (Nujol/cm⁻¹): ν = 912 vbr (SO), 574br, 519br (SnF). ¹H NMR (CD₃NO₂, 298 K): δ = 3.11 (s), 3.10 (s), 3.08 (s). ¹⁹F{¹H} NMR (CD₃NO₂, 298 K): δ = −155.2 (s, ¹J_¹¹⁹SnF = 2063 Hz), −153.7 (t, ¹J_¹¹⁷SnF = 1852, ¹J_¹¹⁹SnF = 2181 Hz), −145.8 (d, ¹J_¹¹⁷SnF = 1847, ¹J_¹¹⁹SnF = 2063 Hz, ²J_FF = 59 Hz), −79.5 (s, OTf). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): δ = −744, −735.

[SnF₄(pyNO)₂]

[SnF₄(MeCN)₂] (0.15 g, 0.54 mmol) was dissolved in CH₂Cl₂ and pyNO (0.10 gm, 1.08 mmol) was added. The solution was stirred for 2 h. The white powder was filtered, washed in hexane (3 × 5 mL) and dried in vacuo. Yield 0.125 g, 60%. Required for C₁₀H₁₀F₄N₂O₂Sn (384.9): C, 31.2; H, 2.6; N, 7.3. Found: C, 31.4; H, 2.6; N, 7.1%. IR (Nujol/cm⁻¹): ν = 1202m (NO), 573br (Sn–F). ¹H NMR (CD₃NO₂, 298 K): δ = 8.7 (m, [2H]), 8.2 (m, [1H]), 7.9 (m, [2H]). ¹⁹F{¹H} NMR (CD₃NO₂, 298 K): δ = −168.7 (t, ¹J_¹¹⁹SnF = 2030, ²J_FF = 51 Hz), −166.5 (t, ¹J_¹¹⁹SnF = 2081, ²J_FF = 51 Hz), −164.2 (s, ¹J_¹¹⁹SnF = 1951). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): δ = −778.5.

[SnF₃(pyNO)₃][OTf]

[SnF₄(pyNO)₂] (0.30 g, 0.78 mmol) was suspended in CH₂Cl₂. To this TMSOTf (0.17 g, 0.78 mmol) was added in MeCN (5 mL). The solution was stirred for 2 h. PyNO (0.074 g, 0.78 mmol) was added and the solution was stirred for a further 2 h. The solvent was then removed in vacuo which yielded a colourless, gel-like solid which was recrystallised from MeCN/Et₂O to give a white powder. Yield: 0.22 g, 46%. Required for C₁₆H₁₅F₆N₃O₆Sn·1/2CH₂Cl₂ (652.54): C, 30.4; H, 2.5; N, 6.4. Found: C, 30.3; H, 2.9; N, 6.6%. IR (Nujol/cm⁻¹): ν = 1225m (NO), 574br,s, 517m (Sn–F). ¹H NMR (CD₃CN, 298 K): δ = 8.8 (m, [2H]), 8.2 (m, [1H]), 7.9 (m, [2H]). ¹⁹F{¹H} NMR (CD₃CN, 298 K): δ = −172.4 (s, ¹J_¹¹⁹SnF = 2159 Hz), −171.6 (t, ¹J_¹¹⁹SnF = 2268 Hz, ²J_FF = 59 Hz), −169.9 (d, ¹J_¹¹⁹SnF = 2244 Hz, ²J_FF = 59 Hz), −78.7 (s, OTf). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): δ ∼−770 (multiplets for the two isomers are almost coincident).

[SnF₄(DMF)₂]

[SnF₄(MeCN)₂] (0.20 g, 0.72 mmol) was added to an excess of DMF (5 mL). The suspension was stirred for 2 h. The white precipitate was filtered, washed in hexane (3 × 3 mL) and dried in vacuo. Yield 0.195 g, 80%. Required for SnF₄C₆H₁₄N₂O₂·CH₂Cl₂ (425.8): C, 19.7; H, 3.8; N, 6.6. Found C, 19.5; H, 3.8; N, 7.4%. IR (Nujol/cm⁻¹): ν = 1669 (CO), 585s (Sn–F). ¹H NMR (CD₃NO₂, 298 K): δ = 8.16 (s), 7.91 (s) (H), 3.31 (s), 3.28 (s), 3.15 (s), 3.12 (s) CH₃. ¹⁹F{¹H} NMR (CD₃NO₂, 298 K): δ = −169.5 (t, ¹J_¹¹⁹SnF = 2015 Hz, ²J_FF = 50 Hz), −161.9 (t, ¹J_¹¹⁹SnF = 1907 Hz, ²J_FF = 50 Hz), −161.8 (s). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): n.o.

[SnF₃(DMF)₃][OTf]

[SnF₄(DMF)₂] (0.31 g, 0.91 mmol) was suspended in CH₂Cl₂. To this TMSOTf (0.20 g, 0.91 mmol) was added in CH₂Cl₂. The solution was stirred for 2 h. DMF (0.067 g, 0.91 mmol) in MeCN was then added and the solution was stirred for 2 h. Addition of n-hexane formed a viscous oil from which the solvent was decanted and the residue was washed with further hexane and dried in vacuo, leaving a colourless glassy solid. Yield 0.085 g, 17%. Microanalytical data were not obtained due to the glassy nature of the product. IR (Nujol/cm⁻¹): ν = 1666 (C=O), 582s (Sn–F), 518s (Sn–F). ¹H NMR (CD₃NO₂, 298 K): δ = 8.27 (br s), 8.21 (s), 3.34 (m), 3.18 (m). ¹⁹F{¹H} NMR (CD₃NO₂, 298 K): δ = −169.2 (t, ¹J_¹¹⁹SnF = 2143 Hz, ²J_FF = 55 Hz), −168.8 (s, ¹J_¹¹⁹SnF = 2150 Hz), −164.0 (d, ¹J_¹¹⁹SnF = 1985 Hz, ²J_FF = 55 Hz), −79.5 (s, OTf). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): n.o.

[SnF₃(py)₃][OTf]

[SnF₄(py)₂] (0.10 g, 0.28 mmol) was suspended in CH₂Cl₂ (10 mL). To this TMSOTf (0.063 g, 0.28 mmol) was added in CH₂Cl₂ (5 mL). The solution was stirred for 2 h. Py (0.022 g, 0.28 mmol) was added and the solution was stirred for a further 2 h. The solvent was then removed in vacuo which yielded a colourless, gel-like solid which was recrystallised from MeCN/Et₂O to a white powder. Although we were unable to obtain satisfactory microanalytical data for this complex, the spectroscopic data are in accord with the formulation above. IR (Nujol/cm⁻¹): ν = 568br,s (Sn–F). ¹H NMR (CD₃NO₂, 298 K): δ = 8.9 (m), 8.8 (m), 8.4 (m), 8.3 (m), 7.9 (m), 7.8 (m). ¹⁹F{¹H} NMR (CD₃NO₂, 298 K): δ = −165.8 (m, ¹J_¹¹⁷SnF = 1637 Hz, ¹J_¹¹⁹SnF = 1717 Hz, ²J_FF = 40 Hz), −158.8 (d, ¹J_¹¹⁷SnF = 1658 Hz, ¹J_¹¹⁹SnF = 1737 Hz, ²J_FF = 40 Hz), −157.2 (s, ¹J_¹¹⁷SnF = 1515 Hz, ¹J_¹¹⁹SnF = 1588 Hz), −79.9 (s, OTf). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): n.o.

[SnF₃(OPPh₃)₃][OTf]

[SnF₄(OPPh₃)₂] (0.073 g, 0.097 mmol) was dissolved in CH₂Cl₂ (10 mL). To this TMSOTf (0.20 g, 0.91 mmol) was added in CH₂Cl₂ (5 mL). The solution was stirred for 2 h. To this OPPh₃ (0.027 g, 0.097 mmol) then added and the solution was stirred for 2 h. The solution was concentrated in vacuo and excess hexane was added, the solvent was removed, and the resulting white powder was dried in vacuo. Yield 0.070 g, 63%. IR (Nujol/cm⁻¹): ν = 1145sh, 1059 (P=O), 554m, 537m (Sn–F). ¹H NMR (CD₂Cl₂, 298 K): δ = 7.9–7.3(m). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −142.1 (s), −141.2 (t, ²J_FF = 61 Hz), −134.5(d, ²J_FF = 61 Hz), −79.1 (s, OTf). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 46.0 (s), 43.9 (s), 41.7 (s). ¹¹⁹Sn NMR (CH₃NO₂, 253 K): n.o.

[SnF₂(OPPh₃)₄][OTf]₂

[SnF₄(OPPh₃)₂] (0.085 g, 0.11 mmol) was dissolved in CH₂Cl₂ (5 mL). To this TMSOTf (0.050 g, 0.23 mmol) was added in CH₂Cl₂ (5 mL). The suspension was stirred for 2 h and then OPPh₃ (0.063 g, 0.23 mmol) was added and the solution was stirred for a further 2 h. The solution was concentrated in vacuo and excess hexane was added, the solvent was decanted, and the resulting white powder was dried in vacuo. Yield 0.08 g, 47%. Required for C₇₄H₆₀F₈O₁₀P₄S₂·CH₂Cl₂ (1652.9): C, 54.5; H, 3.8. Found: C, 54.6; H, 3.8%. IR (Nujol/cm⁻¹): ν = 1150(sh), 1060 (P=O), 550s, 537s, 517m (Sn–F). ¹H NMR (CD₃NO₂, 298 K): δ = 7.3–7.9 (m). ¹⁹F{¹H} NMR (CD₃NO₂, 298 K): δ = −123.1 (s, ¹J_¹¹⁷SnF = 1730, ¹J_¹¹⁹SnF = 1812 Hz), −122.4 (s, ¹J_¹¹⁷SnF = 1977, ¹J_¹¹⁹SnF = 2069 Hz), −79.8 (s, OTf). ³¹P{¹H} NMR (CD₃NO₂, 298 K): δ = 50.5 (s, ¹J_SnP = 78 Hz), 48.4 (s, ¹J_SnP = 78 Hz), 47.1 (s, ¹J_SnP = 96 Hz) average ¹¹⁷Sn/¹¹⁹Sn; separate couplings were not resolved. ¹¹⁹Sn NMR (CH₃NO₂, 253 K): n.o.

X-Ray experimental

Crystals of the phosphine complexes were grown by layering CH₂Cl₂ solutions with hexane, while those for the N- and O-donor complexes were obtained by slow evaporation from MeNO₂ solutions in the glovebox. Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å) rotating anode generator with VHF Varimax optics (70 μm focus) with the crystal held at 100 K. Structure solution and refinement were performed using SHELX(S/L)97, SHELX-2013, or SHELX-2014/7.41.^21,22 H atoms bonded to C were placed in calculated positions using the default C–H distance and refined using a riding model. The structure of [SnF₄(py)₂] showed some twinning, which we were not able to fully resolve, hence the final structure quality if less good than for the other complexes reported. Details of the crystallographic parameters are given in Table S1 (ESI†). CCDC reference numbers for the crystallographic information files in cif format are CCDC 2104490 ([Sn(PMe₃)₂(OTf)₂]), 2104491 (Sn₃F₅OTf), 2104492 ([SnF₃(PMe₃)₂(OTf)]), 2104493 ([SnF₄(κ²-triphos)]), 2104976 ([SnF₂(OPPh₃)₄)][OTf]₂), 2106812 ([SnF₄(py)₂] and 2104984 ([SnF₄(pyNO)₂]).†

Results and discussion

The neutral tin(IV) fluoride complex trans-[SnF₄(PMe₃)₂] was synthesised following the literature method⁷ and two new complexes [SnF₄(PⁱPr₃)₂] and [SnF₄(κ²-triphos)] (triphos = CH₃C(CH₂PPh₂)₃) were made similarly. The presence of a quintet in the ³¹P{¹H} and a triplet in the ¹⁹F{¹H} NMR spectrum of [SnF₄(PⁱPr₃)₂] shows this exists only as the trans isomer as found for other tertiary phosphine complexes of SnF₄.⁷ The reaction of [SnF₄(MeCN)₂] with triphos leads to the formation of the complex [SnF₄(κ²-triphos)]. The ³¹P{¹H} NMR spectrum of this complex at 298 K in CH₂Cl₂ solution is a doublet of triplets of doublets at δ = −6.9 ppm and a singlet at δ = −27.1 ppm with integrals in a 2 : 1 ratio, consistent with a κ²-coordinated triphos in solution, and with no evidence of other isomers present. The crystal structure of the complex (Fig. 1) confirms this, and there is no evidence that the free arm can spontaneously displace a fluoride from the metal centre, a result similar to that found in the heavier tin(IV) halide complexes.⁸ The geometry at the tin is distorted octahedral and the Sn–F and Sn–P bond lengths are much as expected.⁸

Fig. 1 Crystal structure of [SnF₄(κ²-triphos)] showing the atom labelling scheme. The ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–P1 = 2.6302(5), Sn1–P2 = 2.6314(5) Sn1–F1 = 1.9670(12), Sn1–F2 = 1.9488(12), Sn1–F3 = 1.9791(12), Sn1–F4 = 1.9307(12), P1–Sn1–P2 = 91.245(14), P1–Sn1–F4 = 172.46(5), P2–Sn1–F2 = 174.74(4), F1–Sn1–F3 = 173.44(5).

Fluoride abstraction from [SnF₄(PR₃)₂] complexes

Since neutral phosphine ligands do not displace fluoride from the tin centre directly, the reactions with the potential fluoride abstraction reagents⁹ Na[BAr^F] and Me₃SiO₃SCF₃ (TMSOTf) were explored. Some reaction was evident with Na[BAr^F], but did not go to completion (cf. ref. 10) and hence studies were focussed on TMSOTf (Scheme 1). The reaction of trans-[SnF₄(PMe₃)₂] with one equivalent of TMSOTf in CH₂Cl₂ leads to the formation of [SnF₃(PMe₃)₂(OTf)]. The crystal structure of this complex (Fig. 2) shows that in the solid state the three fluorines are in a mer-arrangement with OTf trans-F and with mutually trans-PMe₃ ligands. The same isomer is present in [SnCl₃(PMe₃)₂(OTf)],¹⁰ and comparison of the structures shows the d(Sn–P) and d(Sn–O) are only marginally shorter in the fluoride complex.

Scheme 1

Fig. 2 Crystal structure of [SnF₃(PMe₃)₂(OTf)] showing the atom labelling scheme. The ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–P1 = 2.5278(4), Sn1–P2 = 2.5405(4), Sn1–F1 = 1.9761(9), Sn1–F2 = 1.9595(10), Sn1–F3 = 1.9871(9), Sn1–O1 = 2.2445(12), P1–Sn1–P2 = 171.615(13), F1–Sn1–F3 = 171.00(4), O1–Sn1–F3 = 176.66(4).

In general the triflate complexes are less robust than the [SnF₄(PR₃)₂] and resonances of decomposition products, which include [R₃PF]⁺, build up quite rapidly in solution, especially for the mono-fluoride species. In solution at 298 K the [SnF₃(PMe₃)₂(OTf)] exhibits broad ¹⁹F{¹H} and ³¹P{¹H} NMR spectra, probably due to some reversible OTf dissociation. However, cooling the solution to 183 K sharpened these resonances; in the ³¹P{¹H} NMR spectrum there is a triplet of doublets and in the ¹⁹F{¹H} NMR spectrum a triplet of triplets and a triplet of doublets are seen in a 1 : 2 ratio (Table 1 and Fig. 3), as expected if the structure in Fig. 2 was retained in solution. Obtaining ¹¹⁹Sn NMR spectra proved to be difficult from the triflate complexes, even at 183 K, the full couplings were often unclear, although the chemical shifts were identified.

Fig. 3 The ¹⁹F{¹H} NMR spectrum of [SnF₃(PMe₃)₂(OTf)] (CD₂Cl₂, 183 K).

Table 1 Selected multinuclear NMR data^a,b

Compound	δ(^31P{^1H})/ppm	δ(^19F{^1H})^c/ppm	δ(^119Sn)/ppm	^2 J(^31P–^19F)/Hz	^2 J(^19F–^19F)/Hz	^1 J(^119Sn–^31P)/Hz	^1 J(^119Sn–^19F)/Hz
a NMR data from CH2Cl2 (phosphine complexes) or CH3NO2 (N- and O-donor ligands) solution at 298 K. b Full data are in the Experimental section. c Non triflate resonances. d Data from ref. 20. e Data from ref. 18. f Not clear due to near coincident spectra for the two isomers; n.o. = not observed. g Averaged ^117/119Sn couplings since separate resonances not resolved.
trans-[SnF4(PMe3)2]^d	−19.1 (quin)	−132.8 (t)	−628.0 (t of quin)	155		2975	2745
[SnF3(PMe3)2(OTf)]	−3.2 (td)	−127.9 (td)	−599.9 (m)	153	41	3412	3253
		−151.4 (tt)		121			3013
[SnF2(PMe3)2(OTf)2]	10.2 (t)	−142.7 (t)	−609 (tt)	107		3654	3393
[SnF(PMe3)2(OTf)3]	18.3 (d)	−132.9(t)	−617 (m)	88		3740	3557
trans-[SnF4(P^iPr3)2]	30.6 (quin)	−99.1 (t)	−622.0 (quin of t)	124		2477	2920
[SnF3(P^iPr3)2(OTf)]	38.2 (q)	−73.5 (br)	−611.1 (tdt)	121		2856	3518
		−116.9 (br)					3244
[SnF2(P^iPr3)2(OTf)2]	49.4 (t)	−56.9 (br)	−630 (tt)	117	—	2971	4082
[SnF4(dmso)2] cis		−161.6 (t), −149.3 (t)	n.o.		52, 52		2030, 2020
trans		−149.0 (s)	−778.4 (quin)				2466
[SnF3(dmso)3][OTf] mer		−153.7 (t), −145.8 (d)	−744 (m)		59, 59		2063. 2181
fac		−155.2 (s)	−735 (q)				2063
[SnF4(pyNO)2] cis		−168.7 (t), −166.5 (t)	n.o.		51, 51		2030, 2081
trans		−164.2 (s)	−778.5 (quin)				1951
[SnF3(pyNO)3][OTf] mer		−171.6 (t), −169.9 (d)	∼−770		59, 59		2268, 2244
fac		−172.4 (s)	∼−770				2159
[SnF4(DMF)2] cis		−169.5 (t), −161.9 (t)	n.o.		50, 50		2015. 1907
trans		−161.8 (s)	n.o.				unclear
[SnF3(DMF)3][OTf] mer		−169.2 (t), −164.0 (d)	n.o.		55, 55		2143, 1985
fac		−168.8 (s)	n.o				2150
trans-[SnF4(py)2]^d		−163.8 (s)	−670.8 (quin)				1983
[SnF3(py)3][OTf] mer		−165.8 (t), −158.8 (d)	−771 (m)		40, 40		1717, 1737
fac		−157.2 (s)	−763 (q)				1588
[SnF4(OPPh3)2]^ecis	42.3	−159.8 (t), −146.2 (t)	−775.1 (ttt)		53, 53	22^f	1850, 1730
Trans	42.5	−149.8 (s)	−770.0 (quin)			20^f	1704
[SnF3(OPPh3)3][OTf] mer	46.0, 43.9	−141.2 (t), −134.5 (d)	n.o.		61, 61		1811, 2000
fac	41.7	−142.1 (s)					1878
[SnF2(OPPh3)4][OTf]2cis	47.1, 48.4	−122.4 (s)	n.o.			96^g, 78^g	2069
trans	50.6	−123.1 (s)	n.o.			78^g	1812

---

The reaction of trans-[SnF₄(PMe₃)₂] with two equivalents of TMSOTf forms the complex [SnF₂(PMe₃)₂(OTf)₂] as identified by the microanalysis. Here the low temperature ¹⁹F{¹H} and ³¹P{¹H} NMR spectra are triplets which suggests that the complex has trans phosphines, with the fluorides either cis or trans. At 183 K both the ¹⁹F{¹H} and ³¹P{¹H} NMR spectra (ESI S3.2 and S3.3†) show, in addition to resonances of [SnF₂(PMe₃)₂(OTf)₂], some minor species which may be [SnF₂(PMe₃)₂(OTf)_2−n][OTf]_n. Attempts to grow crystals for an X-ray study to confirm the geometry of [SnF₂(PMe₃)₂(OTf)₂] failed due to limited solution stability, and instead gave some crystals of the tin(II) complex, [Sn(PMe₃)₂(OTf)₂] (see below). The reaction of trans-[SnF₄(PMe₃)₂] with three equivalents of TMSOTf leads to the formation of the complex [SnF(PMe₃)₂(OTf)₃]. The ¹⁹F{¹H} and ³¹P{¹H} spectra of this complex are sharp at room temperature; the ³¹P{¹H} NMR spectrum shows a doublet resonance indicating equivalent phosphines (mutually trans), whilst the ¹⁹F{¹H} NMR spectrum has two resonances, a singlet corresponding to triflate and a triplet corresponding to the tin bound fluoride, in a 9 : 1 ratio. This complex is not stable in solution and over a number of hours these resonances decrease in intensity and new resonances consistent with the formation of [PMe₃F]⁺ appear.²³ A few crystals deposited from the solution were revealed by structure solution to be a phosphine free tin(II) polymer, Sn₃F₅(OTf) (see below).

The sequential abstraction of fluoride from [SnF₄(PMe₃)₂] leads to a high frequency shift in the phosphorus resonance and an increase in ¹J_¹¹⁹SnP and ¹J_¹¹⁹SnF, which is consistent with an increase in charge at the tin centre. There is a decrease in ²J_PF as the series is traversed. The triflate has a trans influence on the fluorides, best demonstrated in the monotriflate complex where both the ²J_PF and ¹J_¹¹⁹SnF couplings are smaller for the trans fluorine than for the cis. The ¹¹⁹Sn NMR spectra are expected to be complex, and in some the couplings are not all clearly resolved even at 183 K (Table 1 and ESI†).

In an attempt to generate cations, the reaction of TMSOTf with trans-[SnF₄(PⁱPr₃)₂] was explored. The PⁱPr₃ has a Tolman cone angle²⁴ of 160° which compares with 118° in PMe₃. Whilst it was possible to sequentially remove one or two fluorines from the tin, the resulting complexes had very limited stability and attempts to obtain X-ray crystallographic data were unsuccessful. However, based upon the microanalysis and multinuclear NMR data they were formulated as [SnF₃(PⁱPr₃)₂(OTf)] and [SnF₂(PⁱPr₃)₂(OTf)₂], with the NMR data suggesting the triflates are most likely coordinated to the tin in solution at low temperature, like the trimethylphosphine complexes, although dissociated at room temperature. The reaction of [SnF₄(PⁱPr₃)₂] with one equivalent of TMSOTf produces the complex [SnF₃(PⁱPr₃)₂(OTf)]. The room temperature ³¹P{¹H} NMR spectrum of which is a quartet, indicating the three Sn-bound F⁻ groups are equivalent, whilst the ¹⁹F{¹H} NMR spectrum has broad resonances flanked by tin satellites (as well as a triflate resonance), which sharpen as the temperature is lowered to 183 K. The room temperature data support the tentative assignment of the tin species as a trans-trigonal bipyramid, although cation-OTf interactions most likely occur at 183 K and in the solid state.

The reaction of [SnF₄(PⁱPr₃)₂] with two equivalents of TMSOTf leads to the formation of [SnF₂(PⁱPr₃)₂(OTf)₂]. The ³¹P{¹H} NMR spectrum of this complex is a triplet, whilst the ¹⁹F{¹H} spectrum is a broad resonance at δ = −57.0, which is flanked by tin satellites. It is likely that weak interactions with the triflate anions may be present in solution, and the solution decomposes quickly. Curiously, the reaction of TMSOTf with [SnCl₄(ER₃)₂] (E = P or As) only generated [SnCl₃(ER₃)₂OTf]; further halides were not removed even with the addition of excess TMSOTf.¹⁰

As described above, decomposition of some of the triflate complexes in solution produced crystals of tin(II) compounds. From [SnF₂(PMe₃)₂(OTf)₂] one decomposition product was [Sn(PMe₃)₂(OTf)₂] (Fig. 4), which has a four-coordinate primary coordination sphere around the Sn(II) centre, based upon a trigonal bipyramid with equatorial phosphines, axial triflates and a vacant equatorial vertex. There are weak long-range contacts between neighbouring molecules through a Sn-triflate interaction (3.044 Å) to yield a 1D polymer (ESI Fig. S8†). The Sn–P bond distances in this species are ca. 0.2 Å longer and the Sn-OTf ca. 0.3 Å shorter than those in the Sn(IV) complex, [SnF₃(PMe₃)₂(OTf)] (above).

Fig. 4 Crystal structure of [Sn(PMe₃)₂(OTf)₂] showing the atom labelling scheme. The ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–P1 = 2.7796(10), Sn1–P2 = 2.6610(8), Sn1–O1 = 2.523(3), Sn1–O4 = 2.342(3), P1–Sn1–P2 = 96.27(3), O1–Sn1–O4 = 154.39(10).

One product from the decomposition of [SnF(PMe₃)₂(OTf)₃] in solution is the Sn(II) phosphine-free compound, Sn₃F₅(OTf), which has an extended 2D sheet structure formed mainly through Sn–F bridges (see ESI. Fig. S9†).

Fluoro-tin(IV) cations with hard N- and O-donor ligands

The work described above shows that fluoride can be abstracted sequentially from tetrafluorotin(IV) phosphine complexes, to form tri-, di- and mono-fluoride complexes in which OTf coordination replaces the fluorides removed. However, these complexes mostly have limited stability, and attempts to introduce further phosphine into the coordination sphere, for example by reacting TMSOTf with [SnF₄(κ²-triphos)] (bearing a pendant –PPh₂ function) does not lead to formation of the intended [SnF₃(κ³-triphos)][OTf]. Similarly, adding PMe₃ to [SnF₃(PMe₃)₂(OTf)] did not lead to clean substitution of the OTf. Hence, in this section we explore fluoride abstraction from tetrafluorotin(IV) complexes containing hard, neutral nitrogen and oxygen donor ligands in the presence of further neutral ligand. The neutral complexes [SnF₄(L)₂] (L = dmso, pyNO, OPPh₃, py, DMF) have been reported in the earlier literature,^8,13,14,19 but most with limited data. The key spectroscopic data are given in Table 1 and the Experimental section. The structures of trans-[SnF₄(OPR₃)₂] (R = Me, Ph)^8,19 have been determined previously and those of trans-[SnF₄(pyNO)₂] and trans-[SnF₄(py)₂] are shown in Fig. 5. The NMR spectra of most of the [SnF₄(L)₂] complexes show both cis and trans isomers present in solution, with the relative amounts varying significantly with L (Table 1) (Scheme 2).

Fig. 5 Crystal structures of (a) [SnF₄(pyNO)₂]·CH₂Cl₂ and (b) the centrosymmetric [SnF₄(py)₂] showing the atom labelling schemes. The ellipsoids are drawn at the 50% probability level and H atoms and lattice CH₂Cl₂ are omitted for clarity. There are two crystallographically independent [SnF₄(pyNO)₂]·CH₂Cl₂ moieties in asymmetric unit and only one is shown. Selected bond lengths (Å) and angles (°) are: (a) Sn1–F1 = 1.945(3), Sn1–F2 = 1.945(4), Sn1–F3 = 1.943(4), Sn1–F4 = 1.953(3), Sn1–O1 = 2.081(3), Sn1–O2 = 2.081(3), N1–O1 = 1.364(5), N1–O2 = 1.359(5), O1–Sn1–O2 = 172.29(15), F1–Sn1–F3 = 177.59(16), F2–Sn1–F4 = 177.20(16); (b) Sn1–F1 = 1.954(7), Sn1–F2 = 1.956(7), Sn1–N1 = 2.175(10), F1–Sn1–F2 = 90.1(3), F1–Sn1–N1 = 90.0(3), F2–Sn1–N1 = 90.0(3).

Scheme 2

The complexes were mostly poorly soluble in CH₂Cl₂ and generally solution data were acquired from CH₃NO₂/CD₃NO₂ solutions, with stronger donor solvents being avoided since these may displace the neutral ligands from the tin. The high melting point of MeNO₂ (244 K) limited low temperature NMR studies. The isomers present are readily identified from the ¹⁹F{¹H} NMR spectra with the trans showing a sharp singlet resonance and the cis, two triplets, in both cases with accompanying ^117/119Sn satellites. The relative amounts of the two isomers are solvent dependent, although we have not explored this in any detail. The ¹J_SnF couplings are large and usually the separate couplings to the two tin isotopes were resolved, despite their similar values (the magnetogyric ratio ¹¹⁹Sn/¹¹⁷Sn is 1.046) (Table 1). A typical example is shown in Fig. 6.

Fig. 6 The ¹⁹F{¹H} NMR spectrum of [SnF₄(dmso)₂] (298 K) showing the resonances of the two isomers with ^117/119Sn satellites.

The reactions of the [SnF₄(L)₂] complexes with one equivalent of TMSOTf, followed by addition of one equivalent of L, afforded [SnF₃(L)₃][OTf] (L = dmso, py, pyNO, DMF, OPPh₃). The ¹⁹F{¹H} NMR spectra show that in solution both the mer- and fac-isomers are present, the former with doublet and triplet resonances due to ²J_FF, and the latter a singlet, again all with ^117/119Sn isotope satellites. A typical example is shown in Fig. 7.

Fig. 7 The ¹⁹F{¹H} NMR spectrum of [SnF₃(dmso)₃][OTf] (298 K) showing the resonances of the two isomers with ^117/119Sn satellites (the OTf resonance is not shown).

Observing the ¹¹⁹Sn spectra proved more challenging due to a combination of factors, including the presence of two isomers, the complex multiplet patterns which often overlapped, and the modest receptivity of ¹¹⁹Sn (D_c = 26). Typically, the central lines of the multiplets were identified, allowing δ(¹¹⁹Sn) be determined, but the weaker outer lines of the multiplets were often less clear. Several complexes, including [SnF₄(DMF)₂], [SnF₃(DMF)₃][OTf] and [SnF₃(OPPh₃)₃][OTf], failed to exhibit a ¹¹⁹Sn resonance (in CH₃NO₂ at 253 K), presumably due to reversible neutral ligand dissociation on the tin NMR timescale. Similar behaviour has been reported for [SnF₄(MeCN)₂].⁸ Attempts to isolate pure samples of difluorotin(IV) cations [SnF₂(L)₄][OTf]₂ from [SnF₄(L)₂], with two equivalents of TMSOTf and excess L, proved more challenging. Typically, mixtures of [SnF₂(L)₄][OTf]₂ and [SnF₃(L)₃]OTf were identified in situ by the ¹⁹F{¹H} NMR spectra. It did prove possible to isolate an analytically pure sample of [SnF₂(OPPh₃)₄][OTf]₂. The ¹⁹F{¹H} and ³¹P{¹H} NMR spectra of this complex confirmed that both cis and trans isomers of the phosphine oxide complex were present in CH₃NO₂ solution, with the cis form the more abundant. However, we did not observe a ¹¹⁹Sn NMR spectrum from this complex even at 253 K (the lower limit for CH₃NO₂). The X-ray crystal structure of [SnF₂(OPPh₃)₄][OTf]₂ showed it to be of the trans isomer (Fig. 8).

Fig. 8 Structure of the cation in [SnF₂(OPPh₃)₄][OTf]₂ showing the atom labelling scheme. The ellipsoids are drawn at the 50% probability level and H atoms and OTf anions are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–F2 = 1.9314(16), Sn1–F1 = 1.9164(17), Sn1–O3 = 2.040(2), Sn1–O4 = 2.0447(19), Sn1–O2 = 2.040(2), Sn1–O1 = 2.057(2), F1–Sn1–F2 = 178.78(7), O3–Sn1–O1 = 176.24(8), O–2Sn1–O4 = 178.85(8).

The tin environment is very close to a regular octahedron and the d(Sn–F) are rather shorter than in the neutral [SnF₄(L)₂] (L = pyNO, py) complexes above, although the d(Sn–O) are not significantly different to those in the neutral parent complex trans-[SnF₄(OPPh₃)₂].⁸

The [SnF₃(L)₃][OTf] complexes are less robust in solution than the neutral [SnF₄(L)₂], and upon standing, resonances corresponding to varying amounts of decomposition/disproportionation products, including [SnF₄(L)₂], were observed in some systems, especially for L = OPPh₃ and DMF. It is notable that the crystal used to determine the structure of [SnF₄(py)₂] (above) was grown from a sample of [SnF₃(py)₃][OTf], and our inability to isolate pure [SnF₂(L)₄][OTf]₂ with other N- or O-donor ligands suggest some ligand redistribution occurs in solution.

Conclusions

A series of new triflate complexes of tin(IV) fluoride with neutral donor co-ligands have been synthesised. Variable temperature multinuclear NMR spectroscopic data show that soft phosphine ligands lead to neutral tri- and (unstable) di- and mono-fluoride complexes with two trans phosphines and one, two or three κ¹-OTf groups, respectively. The triflate is not displaced by added phosphine (even using the potentially tridentate triphos ligand). In contrast, harder O- and N-donor ligands form the six-coordinate mer- and fac-[SnF₃L₃]⁺ (L = dmso, dmf, OPPh₃, pyNO and py) monocations, with further TMSOTf producing cis- and trans-[SnF₂(OPPh₃)₄]²⁺ dications, the latter being confirmed as the trans isomer in the solid state by X-ray analysis.

Conflicts of interest

The authors have no conflicts to declare.

Acknowledgements

We thank EPSRC for funding via the ADEPT Programme grant (EP/N035437/1), a studentship to R. P. K. (EP/N509747/1) and GE Healthcare and EPSRC for a CASE studentship to M. S. W. (EP/R513325/1).

References

1. (a) S. Aldridge and C. Jones, Chem. Soc. Rev., 2016, 45, 763 and references therein; (b) K. Chansaenpak, B. Vabre and F. P. Gabbaı, Chem. Soc. Rev., 2016, 45, 954; (c) J. Burt, W. Levason and G. Reid, Coord. Chem. Rev., 2014, 260, 65; (d) C. I. Rat, C. Silvestru and H. J. Breunig, Coord. Chem. Rev., 2013, 257, 818; (e) T. J. Hadlington, M. Hermann, G. Frenking and C. Jones, J. Am. Chem. Soc., 2014, 136, 3028; (f) R. D. Rittinghaus, J. Tremmel, A. Růžička, C. Conrads, P. Albrecht, A. Hoffman, A. N. Ksiazkiewicz, A. Pich, R. Jambor and S. Herres-Pawlis, Chem. – Eur. J., 2020, 26, 212; (g) C. H. de Groot, C. Gurnani, A. L. Hector, R. Huang, M. Jura, W. Levason and G. Reid, Chem. Mater., 2012, 24, 4442.
2. P. G. Harrison, in Comprehensive Coordination Chemistry, ed. G. Wilkinson, J. A. McCleverty and R. D. Gillard, Pergamon, Oxford, 1988, vol. 3, p. 183.
3. J. Parr, in Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, Oxford, 2004, vol. 3, p. 545.
4. The Chemistry of Tin, ed. P. J. Smith, Chapman & Hall, London, 1998.
5. P. G. Harrison, The Chemistry of Tin, Blackie, London, 1989.
6. G. R. Willey, T. J. Woodman, U. Somasundaram, D. R. Aris and W. Errington, J. Chem. Soc., Dalton Trans., 1998, 2575.
7. G. R. Willey, A. Jarvis, J. Palin and W. Errington, J. Chem. Soc., Dalton Trans., 1994, 255.
8. M. F. Davis, M. Clarke, W. Levason, G. Reid and M. Webster, Eur. J. Inorg. Chem., 2006, 2773.
9. E. MacDonald, L. Doyle, S. S. Chitnis, U. Werner-Zwanziger, N. Burford and A. Decken, Chem. Commun., 2012, 48, 7922.
10. V. K. Greenacre, R. P. King, W. Levason and G. Reid, Dalton Trans., 2019, 48, 17097.
11. M. Bork and R. Hoppe, Z. Anorg. Allg. Chem., 1996, 622, 1557.
12. (a) S. L. Benjamin, W. Levason and G. Reid, Chem. Soc. Rev., 2013, 42, 1460; (b) W. Levason, F. M. Monzittu and G. Reid, Coord. Chem. Rev., 2019, 391, 90.
13. C. E. Michelson, D. S. Dyer and R. O. Ragsdale, J. Inorg. Nucl. Chem., 1970, 32, 833.
14. C. J. Wilkins and H. M. Haendler, J. Chem. Soc., 1965, 3174.
15. S. H. Hunter, V. M. Langford, G. A. Rodley and C. J. Wilkins, J. Chem. Soc. A, 1968, 305.
16. A. D. Adley, P. H. Bird, A. R. Fraser and M. Onyszchuk, Inorg. Chem., 1972, 11, 1402.
17. J. P. Clark, V. M. Langford and C. J. Wilkins, J. Chem. Soc. A, 1967, 792.
18. (a) D. Tudela and F. Rey, Z. Anorg. Allg. Chem., 1989, 575, 202; (b) D. Tudela and F. Patron, Inorg. Synth., 1997, 31, 92.
19. M. F. Davis, W. Levason, G. Reid and M. Webster, Polyhedron, 2006, 25, 930.
20. R. Suter, A. Swidan, C. L. B. Macdonald and N. Burford, Chem. Commun., 2018, 54, 4140.
21. (a) G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112.
22. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339.
23. A. M. Forster and A. J. Downs, Polyhedron, 1985, 4, 1625.
24. C. A. Tolman, Chem. Rev., 1977, 77, 313.

---

Footnote

† Electronic supplementary information (ESI) available: Multinuclear NMR and IR spectra associated with each of the new compounds described, together with the crystal structure of Sn₃F₅(OTf), the X-ray crystallographic parameters. CCDC 2104490–2104493, 2104976, 2104984 and 2106812. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1dt02948g

---