Neutral and cationic phosphine and arsine complexes of tin(IV) halides: synthesis, properties, structures and anion influence

Abstract

The reaction of trans-[SnCl₄(PR₃)₂] (R = Me or Et) with trimethylsilyltriflate (TMSOTf) in CH₂Cl₂ solution substitutes one chloride to form [SnCl₃(PR₃)₂(OTf)]; addition of excess TMSOTf does not substitute further chlorides. The complexes have been fully characterised by microanalysis, IR and multinuclear NMR (¹H, ¹³C{¹H}, ¹⁹F{¹H}, ³¹P{¹H}, ¹¹⁹Sn) spectroscopy. The crystal structure of [SnCl₃(PMe₃)₂(OTf)] revealed mer-chlorines and trans-phosphines. In contrast, trans-[SnBr₄(PR₃)₂], [SnCl₄{Et₂P(CH₂)₂PEt₂}], [SnCl₄{o-C₆H₄(PMe₂)₂}] and [SnCl₄{o-C₆H₄(AsMe₂)₂}] did not react with TMSOTf in CH₂Cl₂ solution even after 3 days. The arsine complexes, [SnX₄(AsEt₃)₂] (X = Cl, Br), were confirmed as trans-isomers by similar spectroscopic and structural studies, while attempts to isolate [SnI₄(AsEt₃)₂] were unsuccessful and reaction of SnX₄ with SbR₃ (R = Et, ⁱPr) resulted in reduction to SnX₂ and formation of R₃SbX₂. trans-[SnCl₄(AsEt₃)₂] is converted by TMSOTf into [SnCl₃(AsEt₃)₂(OTf)], whose X-ray structure reveals the same geometry found in the phosphine analogues, with the triflate coordinated. The salts, [SnCl₃(PEt₃)₂][AlCl₄] and [SnCl₂(PEt₃)₂][AlCl₄]₂ were made by treatment of [SnCl₄(PEt₃)₂] with one and two mol. equivalents, respectively, of AlCl₃ in anhydrous CH₂Cl₂, whereas reaction of [SnCl₄(AsEt₃)₂] with AlCl₃ produced a mixture including Et₃AsCl₂ and [Et₃AsCl][Sn(AsEt₃)Cl₅] (the latter identified crystallographically). In contrast, using Na[BAr^F] (BAr^F = [B{3,5-(CF₃)₂C₆H₃}₄]⁻) produced [SnCl₃(PEt₃)₂][BAr^F] and also allowed clean isolation of the arsine analogue, [SnCl₃(AsEt₃)₂][BAr^F]. [SnCl₄{o-C₆H₄(PMe₂)₂}] also reacts with AlCl₃ in CH₂Cl₂ to form [SnCl₃{o-C₆H₄(PMe₂)₂}][AlCl₄] and [SnCl₂{o-C₆H₄(PMe₂)₂}][AlCl₄]₂. Multinuclear NMR spectroscopy on the [AlCl₄]⁻ salts show that δ³¹P and δ¹¹⁹Sn move progressively to high frequency on conversion from the neutral complex to the mono- and the di-cations, whilst ¹J(¹¹⁹Sn–³¹P) follow the trend: [SnCl₃{o-C₆H₄(PMe₂)₂}]⁺ > [SnCl₄{o-C₆H₄(PMe₂)₂}] > [SnCl₂{o-C₆H₄(PMe₂)₂}]²⁺. DFT studies on selected complexes show only small changes in ligand geometries and bond lengths between the halide and triflate complexes, consistent with the X-ray crystallographic data reported and the HOMO and LUMO energies are relatively unperturbed upon the introduction of (coordinated) triflate, whereas the energies of both are ca. 4 eV lower in the cationic species and reveal significant hybridisation across the pnictine ligands.

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Introduction

Much of the coordination chemistry of the transition metal ions with neutral ligands is based around metal halide complexes.¹ In addition to neutral complexes [ML_aX_b] (M = transition metal, X = halide, L = neutral ligand), related cations [ML_cX_b−1]⁺ or [ML_dX_b−2]²⁺ (c, d ≥ a) are commonly known, formed either spontaneously in the presence of excess L, or upon the addition of large, weakly coordinating anions.¹ In some cases complexes of the type [ML_xX_y][MX_z] form spontaneously under appropriate conditions, a process sometimes labelled as ‘self-ionisation’. In contrast, complexes of p-block metal and metalloid halides are overwhelmingly neutral [M′L_aX_b] (M′ = p-block metal) with much stronger M–X than M–L bonding.¹ In general, p-block Lewis acids coordinate the neutral ligands relatively weakly, and these are often partially dissociated in solution or undergoing rapid dissociative exchange. The neutral ligands fill ‘free’ coordination sites, but do not compete with the halide ligands for the metal centre. The tin(IV) halides, SnX₄, typify this pattern in that the majority of complexes are octahedral with neutral ligands filling two coordination sites.² Rare exceptions are provided by 1,4,7-trimethyl-1,4,7-triazacyclonane, 1,3,5-trimethyl-1,3,5-triazacyclohexane and 1,4,7-trithiacyclonane (L³), which generate the ‘self-ionisation’ complexes, [SnX₃(L³)]₂[SnX₆] (X = Cl or Br),³ although more flexible acyclic polydentate ligands, such as MeC(CH₂AsMe₂)₃, behave as bidentate ligands, as in [SnX₄{κ²-MeC(CH₂AsMe₂)₃}].⁴

Complexes of the tin(IV) halides, SnX₄ (X = Cl or Br), with neutral phosphine and arsine ligands have been explored in considerable detail⁵ and almost all are of the octahedral trans-[SnX₄(ER₃)₂] or cis-[SnX₄(bidentate)] (bidentate = diphosphine or diarsine) types. SnI₄ is a weaker Lewis acid and forms similar, but less stable adducts,⁵ whilst complexes of SnF₄ have been much less studied.^4,5 Reported examples of stibine complexes of p-block halides are extremely rare.^5,6

The formation of cationic complexes would be expected to enhance the Lewis acidity of the tin centre, and examples of halo- or organo-tin cations with polydentate and pincer ligands have been described.^7–12 MacDonald et al.¹¹ obtained [SnCl₃(PMe₃)₂][AlCl₄] and [SnCl₂(PMe₃)₂][AlCl₄]₂ by treatment of trans-[SnCl₄(PMe₃)₂] with one or two mol. equivalents of AlCl₃, respectively. The former has a distorted trigonal bipyramidal geometry with trans axial PMe₃ ligands. The dication has trans PMe₃ groups, and two cis interactions from the [AlCl₄]⁻ anions completing a very distorted octahedron. The Sn–Cl distances decrease along the series [SnCl₄(PMe₃)₂] > [SnCl₃(PMe₃)₂]⁺ > [SnCl₂(PMe₃)₂]²⁺. In the present work we have explored the reaction of tin(IV) halides with a series of mono- and bi-dentate phosphine, arsine and stibine ligands and the abstraction/replacement of some of the halides from the resulting complexes by other weakly coordinating anions, including triflate ([CF₃SO₃]⁻) via treatment with trimethylsilyltriflate, [AlX₄]⁻via addition of aluminium trihalides and using the large diffuse [BAr^F]⁻ anion, via addition of its Na⁺ salt (BAr^F = [B{3,5-(CF₃)₂C₆H₃}₄]⁻).

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. Tin(IV) halides, TMSOTf, and aluminium halides were obtained from Sigma-Aldrich and used as received. Phosphine and arsine ligands were obtained from Sigma-Aldrich or Strem, except for o-C₆H₄(PMe₂)₂ and o-C₆H₄(AsMe₂)₂, which were prepared by literature methods.¹³ SbEt₃, SbⁱPr₃¹⁴ and Na[BAr^F]¹⁵ were made as described. CH₂Cl₂ was dried by distillation from CaH₂ and diethyl ether and n-hexane from Na.

Infrared spectra were recorded as Nujol mulls between CsI plates using a PerkinElmer Spectrum 100 spectrometer over the range 4000–200 cm⁻¹. ¹H, ¹³C{¹H}, ¹⁹F{¹H}, ³¹P{¹H}, ²⁷Al and ¹¹⁹Sn NMR spectra were recorded from CDCl₃ or CH₂Cl₂/CD₂Cl₂ solutions using a Bruker AV400 spectrometer and referenced to TMS via the residual solvent resonance, CFCl₃, 85% H₃PO₄, [Al(H₂O)₆]³⁺ in H₂O at pH = 1 or SnMe₄ as appropriate. Typically, [Cr(acac)₃] was added as a relaxation agent when recording the ¹¹⁹Sn spectra, except for those containing [AlCl₄]⁻, when it appeared to react with the complexes, turning green. In these cases a 2 seconds pulse delay was used. Microanalyses were undertaken by London Metropolitan University.

[SnCl₄(AsEt₃)₂]

A solution of AsEt₃ (0.058 g, 0.36 mmol) in CH₂Cl₂ (5 mL) was added to a solution of SnCl₄ (0.047 g, 0.18 mmol) in CH₂Cl₂ (5 mL). The colourless solution was left to stir for 1 h before the product was isolated by evaporation of the solvent under vacuum to afford a white solid. Crystals suitable for single crystal X-ray diffraction were grown by slow evaporation of a concentrated CH₂Cl₂ solution of the compound. Yield 0.093 g, 88%. Required for C₁₂H₃₀As₂Cl₄Sn (584.7): C, 24.6; H, 5.2. Found: C, 24.5; H, 5.0%. IR (Nujol/cm⁻¹): ν = 288s (Sn–Cl). ¹H NMR (CDCl₃, 295 K): δ = 2.34 (q, [12H], ³J_HH = 7.6 Hz, CH₂), 1.42 (t, [18H], ³J_HH = 7.6 Hz, CH₃). ¹³C{¹H} NMR (CDCl₃, 295 K): δ = 8.62 (CH₂), 15.04 (CH₃). ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): n.o.; (CD₂Cl₂, 193 K): δ = −657.4.

[SnBr₄(AsEt₃)₂]

A solution of AsEt₃ (0.144 g, 0.89 mmol) in CH₂Cl₂ (5 mL) was added to a solution of SnBr₄ (0.195 g, 0.45 mmol) in CH₂Cl₂ (5 mL). The pale yellow solution was left to stir for 1 h before the product was isolated by removal of the solvent to afford a yellow solid. Crystals suitable for single crystal X-ray diffraction were grown by slow evaporation of a concentrated CH₂Cl₂ solution of the compound. Yield 0.175 g, 50%. Required for C₁₂H₃₀As₂Br₄Sn (762.5): C, 18.9; H, 4.0. Found: C, 19.0; H, 4.0%. IR (Nujol/cm⁻¹): ν = 210s (Sn–Br). ¹H NMR (CD₂Cl₂, 295 K): δ = 2.35 (q, [12H], ³J_HH = 7.6 Hz, CH₂), 1.43 (t, [18H], ³J_HH = 7.6 Hz, CH₃), ¹³C{¹H} NMR (CD₂Cl₂, 295 K): δ = 8.5 (CH₂), 14.8 (CH₃). ¹¹⁹Sn NMR (CD₂Cl₂, 295 K): δ = −1173.4; (CD₂Cl₂, 193 K): δ = −1125.3.

[SnCl₃(AsEt₃)₂(OTf)]

[SnCl₄(AsEt₃)₂] (0.150 g, 0.26 mmol) was dissolved in CH₂Cl₂ (10 mL) to form a clear solution. To this TMSOTf (0.057 g, 0.26 mmol) was added in CH₂Cl₂ (5 mL) and the clear solution was stirred for 2 h. The solvent was removed and the white powder was dried in vacuo. Crystals suitable for single crystal X-ray diffraction were grown from slow evaporation of a solution of the compound in CH₂Cl₂. Yield 0.138 g, 77%. Required for C₁₃H₃₀As₂Cl₃F₃O₃SSn (698.1): C, 22.4, H, 4.3. Found C, 21.9; H, 4.2%. IR (Nujol/cm⁻¹): ν = 294s, 378w (Sn–Cl), 1156m (–OSO₂), 1231m, 1200m (CF₃). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.39 (t, [18H], ³J_HH = 7.8 Hz, CH₃), 2.38 (q, [12H], ³J_HH = 7.8 Hz, CH₂). ¹³C{¹H} NMR (CD₂Cl₂, 295 K): δ = 9.0 (CH₂), 16.0 (CH₃). ¹⁹F{¹H} NMR (CD₂Cl₂, 295 K): δ = −78.7 (s, CF₃). ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): n.o.; (CD₂Cl₂, 183 K): δ = −620 (s).

[SnCl₃(PMe₃)₂(OTf)]

[SnCl₄(PMe₃)₂]¹⁶ (0.300 g, 0.72 mmol) was suspended in CH₂Cl₂ (5 mL). To this TMSOTf (0.016 g, 0.72 mmol) was added in CH₂Cl₂ (5 mL). The solution was stirred for 16 h and the solvent and volatiles were removed and the resulting white powder dried in vacuo. The powder was then washed with diethyl ether (3 × 10 mL) and dried in vacuo. Crystals suitable for single crystal X-ray diffraction were grown by the slow evaporation of a saturated solution of the compound in CH₂Cl₂. Yield 0.215 g, 57%. Required for C₇H₁₈Cl₃F₃O₃P₂SSn (526.1): C, 16.0, H 3.5. Found C, 15.7; H, 3.4%. IR (Nujol/cm⁻¹): ν = 296m, 306m, 377w (Sn–Cl), 1173m (–OSO₂), 1200m, 1235m (CF₃). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.83 (m, CH₃). ¹³C{¹H} NMR (CD₂Cl₂, 295 K): δ = 10.09 (t, ¹J + ³J_31P–13C = 19.4 Hz, CH₃). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 8.74 (s, ¹J_31P–117Sn = 2814, ¹J_31P–119Sn = 2963 Hz). ¹⁹F{¹H} NMR (CD₂Cl₂, 295 K): δ −78.6 (s, CF₃). ¹¹⁹Sn (CD₂Cl₂, 298 K): δ = −516 (t, ¹J_P–119Sn = 2963 Hz).

[SnCl₃(PEt₃)₂(OTf)]

[SnCl₄(PEt₃)₂]¹⁶ (0.08 g, 0.16 mmol) is dissolved in CH₂Cl₂ (10 mL). To this TMSOTf (0.036 g, 0.16 mmol) was added in CH₂Cl₂ (5 mL), resulting in a clear colourless solution which was stirred for 2 h. Solvent and volatiles were removed in vacuo and the resultant white powder was washed with diethyl ether (3 × 10 mL) and then dried in vacuo. Yield 0.065 g, 66%. Required for C₁₃H₃₀Cl₃F₃O₃P₂SSn (610.2): C, 25.6; H, 5.0. Found, C, 25.4; H, 5.1%. IR (Nujol/cm⁻¹): ν = 289m, 301w, 374w (Sn–Cl), 1187m (–OSO₂), 1235m (CF₃). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.30 (m, [18H], CH₃), 2.33 (m, [12H], CH₂). ¹³C{¹H} NMR (CD₂Cl₂, 295 K): δ = 7.59 (t, ¹J + ³J_31P–13C = 2.9 Hz, CH₂), 13.95 (t, ¹J + ³J_31P–13C = 11 Hz, CH₃). ¹⁹F{¹H} NMR (CD₂Cl₂, 295 K): δ = −78.6 (s, CF₃). ³¹P{¹H} (CD₂Cl₂, 298 K): δ = 28.45. (CD₂Cl₂, 183 K): δ = 33.58 (s, ¹J_31P–117Sn = 2617 Hz, ¹J_31P–119Sn = 2734 Hz). ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): δ = −535; (CD₂Cl₂, 183 K): δ = −535 (t, ¹J_31P–119Sn = 2737 Hz).

[SbEt₃Cl(OTf)]

SnCl₄ (0.100 g, 0.38 mmol) was dissolved in 5 mL of CH₂Cl₂. To this, SbEt₃ (0.160 g, 0.77 mmol) was added in 5 mL of CH₂Cl₂, resulting in the precipitation of a large amount of white solid. TMSOTf (0.085 g, 0.38 mmol) was added in CH₂Cl₂ (5 mL), and the reaction stirred for 16 h. The liquid was filtered away from the unidentified solid and concentrated to dryness in vacuo, leading to a white solid. Crystals suitable for single crystal X-ray diffraction were grown by slow evaporation of a concentrated CH₂Cl₂ solution of the compound. Yield 0.064 g, 30%. Required for C₇H₁₅ClF₃O₃SSb (393.3): C, 21.4; H, 3.8 Found C, 21.7; H, 4.2%. IR (Nujol/cm⁻¹): ν = 329m (Sb–Cl), 1167w (–OSO₂), 1198w, 1235w (CF₃). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.62 (t, [9H], ³J_HH = 7.7 Hz, CH₃), 2.80 (q, [6H], ³J_HH = 7.7 Hz, CH₂). ¹³C{¹H} (CD₂Cl₂, 295 K): δ = 9.68 (CH₂), 28.55 (CH₃).

[SnCl₃{o-C₆H₄(PMe₂)₂}][AlCl₄]

[SnCl₄{o-C₆H₄(PMe₂)₂}]⁴ (0.150 g, 0.33 mmol) was suspended in CH₂Cl₂ (5 mL), and AlCl₃ (0.044 g, 0.33 mmol) was added. This resulted in a pale yellow clear solution which was stirred for a further 2 h. The solution was then concentrated to dryness in vacuo producing a white powder, which was washed with n-hexane (3 × 10 mL) and the solid separated and dried in vacuo to leave a white powder. Yield: 0.120 g, 61%. Required for C₁₀H₁₆AlCl₇P₂Sn (591.9): C, 20.3; H, 2.7, Found C, 20.4; H, 2.8%. IR (Nujol/cm⁻¹): ν = 322m, 342m (Sn–Cl), 451m, 497br ([AlCl₄]⁻). ¹H NMR (CD₂Cl₂, 295 K): δ = 2.06 (t, ²J + ⁵J_P–H = 4.5 Hz, [12H], CH₃), 7.86–7.92 (m, [4H], Ar–H). ¹³C{¹H} NMR (CD₂Cl₂, 295 K): δ = 11.52 (t, ¹J + ³J_31P–13C = 16.4 Hz, CH₃), 130.7–131.3, 134.2, 135.3 (Ar). ²⁷Al NMR (CD₂Cl₂, 295 K): δ = 102.8 (s, [AlCl₄]⁻). ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = −24.7 (s, ¹J_31p–117Sn = 930 Hz, ¹J_31P–119Sn = 972 Hz). ¹¹⁹Sn (CD₂Cl₂, 298 K): δ = −469 (t, ¹J_31–119Sn = 972 Hz).

[SnCl₂{o-C₆H₄(PMe₂)₂}][AlCl₄]₂

[SnCl₄{o-C₆H₄(PMe₂)₂}] (0.100 g, 0.22 mmol) was suspended in CH₂Cl₂ (5 mL), and to this AlCl₃ (0.058 g, 0.44 mmol) was added. This resulted in a pale yellow clear solution, which was stirred for a further 2 h. The solution was then concentrated to dryness in vacuo producing a white sticky solid, which was then washed with n-hexane (3 × 10 mL), the solid separated and dried in vacuo to leave a white sticky solid. Yield: 0.81 g 51%. Required for C₁₀H₁₆Al₂Cl₁₀P₂Sn (725.4): C, 16.7; H, 2.2. Found C, 17.5; H, 2.7%. IR (Nujol/cm⁻¹): ν = 294m, 310m (Sn–Cl), 452sh, 487s ([AlCl₄]⁻). ¹H NMR (CD₂Cl₂, 295 K): δ = 2.18 (t, ²J + ⁵J_PH = 4.5 Hz, [12H], CH₃), 7.94–8.30 (m, [4H], Ar–H. ¹³C{¹H} NMR (CD₂Cl₂, 295 K): δ = 12.28 (t, ¹J + ³J_31P–13C = 16.1 Hz, CH₃), 129.8–130.1, 134.4, 135.9 (Ar–H). ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = −23.7 (¹J_31P–117Sn = 815 Hz, ¹J_31P–119Sn = 853 Hz). ²⁷Al NMR (CD₂Cl₂, 295 K): δ = 102.9 (s, [AlCl₄]⁻). ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): δ = −429 (t, ¹J_31P–119Sn = 850 Hz).

[SnCl₃(PEt₃)₂][AlCl₄]

[SnCl₄(PEt₃)₂] (0.200 g, 0.40 mmol) was suspended in CH₂Cl₂ (5 mL) and AlCl₃ (0.054 g, 0.40 mmol) was added. The suspension was stirred for 2 h leaving a clear and colourless solution; the solution was concentrated to dryness leaving a fine white powder which was washed with Et₂O (3 × 10 mL) and dried in in vacuo to yield a white powder. Yield: 0.160 g, 63%. Required for C₁₀H₁₆AlCl₇P₂Sn (630.04): C, 22.9; H, 4.8. Found C, 23.1; H, 5.3%. IR (Nujol/cm⁻¹): ν = 280m, 289m (Sn–Cl), 484s, 503sh ([AlCl₄]⁻). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.36 (m, [18H], CH₃), 2.32 (m, [12H], CH₂). ¹³C{¹H} NMR (CD₂Cl₂, 295 K): δ = 7.34 (t, ²J + ⁴J_31P–13C = 2.93 Hz, –CH₃), 14.12 (t, ¹J + ³J_31P–13C = 22.45 Hz). ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = 35.2 (¹J_31P–117Sn = 2208 Hz, ¹J_31P–119Sn = 2317 Hz); (CD₂Cl₂, 183 K): δ = 27.0 (¹J_31P–117Sn = 2426 Hz, ¹J_31P–119Sn = 2520 Hz). ²⁷Al NMR (CD₂Cl₂, 295 K): δ = 103.2 (s, [AlCl₄]⁻). ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): δ = −350.2 (br t, ¹J_31P–119Sn = 2307 Hz).

[SnCl₂(PEt₃)₂][AlCl₄]₂

[SnCl₄(PEt₃)₂] (0.200 g, 0.40 mmol) was suspended in CH₂Cl₂ (5 ml), to this AlCl₃ (0.107 g, 0.80 mmol) was added, the suspension was stirred for 2 h leaving a clear and colourless solution; the solution was concentrated to dryness and washed with Et₂O (3 × 10 ml) then dried in vacuo leaving a sticky white solid. Yield 0.201 g (66%) Anal. Required for C₁₂H₃₀Al₂Cl₁₀P₂Sn (763.39) C, 18.9; H, 4.0. Found C, 19.0; H, 3.8, IR (Nujol/cm⁻¹) = 289m, (Sn–Cl), 484s, ([AlCl₄]⁻). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.39 (m, [18H], CH₃), 2.38 (m, [12H], CH₂), ¹³C{¹H}NMR (CD₂Cl₂, 295 K): δ = 7.50 (t, ²J + ⁴J_31P–13C = 2.93 Hz, –CH₃), 14.51 (t, ¹J + ³J_31P–13C = 13.2 Hz), ²⁷Al NMR (CD₂Cl₂, 295 K): δ = 103.0, ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = 40.8 (¹J_31P–117Sn = 2172 Hz, ¹J_31P–119Sn = 2274 Hz). ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): δ = −301.4 (br, t, ¹J_31P–119Sn = 2291 Hz).

Reaction of [SnCl₂(PEt₃)₂][AlCl₄]₂ + PEt₃

[SnCl₂(PEt₃)₂][AlCl₄]₂ (0.100 g, 0.13 mmol) was dissolved in CH₂Cl₂ (3 mL), to this PEt₃ (0.015 g, 0.22 mmol) was added in CH₂Cl₂ (2 mL). To this 10 drops of CD₂Cl₂ is added. A portion of the solution was used for in situ NMR studies.

²⁷Al NMR (CD₂Cl₂, 295 K): δ = 103.1 (s, [AlCl₄]⁻), 110.8 (d, ¹J_31P–27Al = 267 Hz, [AlCl₃(PEt₃)]). ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = −14.8 (sextet, ¹J_27Al–31P = 264 Hz, [AlCl₃(PEt₃)]), 32.2 (¹J_31P–117Sn = 2312 Hz, ¹J_31P–119Sn = 2394 Hz, new Sn-PEt₃ containing cation – see text). ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): δ = −449 (t, ¹J_31P–119Sn = 2394 Hz).

[SnCl₃(PEt₃)₂][BAr^F]

A Schlenk was charged with [SnCl₄(PEt₃)₂] (0.100 g, 0.20 mmol) and Na[BAr^F] (0.178 g, 0.20 mmol). To this CH₂Cl₂ (5 mL) was added the resulting cloudy solution was stirred for 2 h, the solution was then filtered and concentrated to dryness in vacuo yielding a white solid. Yield: 0.201 g, 75%. Required for C₄₄H₄₂BCl₃F₂₄P₂Sn (1324.3): C, 39.9, H, 3.2 Found C, 39.8, H, 3.3%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.35 (m, [18H], CH₃), 2.29 (m, [12H], CH₂), 7.57 (s, [4H], [Bar^F]), 7.72 (m, [8H], [Bar^F]); ¹⁹F{¹H} NMR (CD₂Cl₂, 295 K): δ = −63.0 (s, –CF₃) ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = 37.1 (¹J_31P–117Sn = 2208 Hz, ¹J_31P–119Sn = 2311 Hz); ¹¹⁹Sn NMR (CD₂Cl₂, 298 K): δ = −379 (t, ¹J_31P–119Sn = 2311 Hz).

[SnCl₃(AsEt₃)₂][BAr^F]

A Schlenk was charged with [SnCl₄(AsEt₃)₂] (0.100 g, 0.17 mmol) and Na[BAr^F] (0.152 g, 0.17 mmol). To this CH₂Cl₂ (5 mL) was added the resulting cloudy solution was stirred for 2 h and then filtered and concentrated to dryness in vacuo yielding a white solid. Yield: 0.162 g, 67%. Required for C₄₄H₄₂As₂BCl₃F₂₄Sn (1412.2): C, 37.4, H, 3.0 Found C, 37.3, H, 2.8%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.44 (t, ³J_HH = 7.7 Hz, [18H], CH₃), 2.42 (q, ³J_HH = 7.7 Hz, [12H], CH₂), 7.57 (s, [4H], [Bar^F]), 7.72 (m, [8H], [Bar^F]); ¹⁹F{¹H} NMR (CD₂Cl₂, 295 K): δ = −63.0 (s, –CF₃); ¹¹⁹Sn NMR (CD₂Cl₂, 183 K): δ = −388 (br s).

[SnCl₃{o-C₆H₄(PMe₂)₂}][BAr^F]

A Schlenk was charged with [SnCl₄{o-C₆H₄(PMe₂)₂}] (0.100 g, 0.17 mmol) and Na[BAr^F] (0.152 g, 0.17 mmol) and to this CH₂Cl₂ (5 mL) was added. The resulting cloudy solution was stirred for 2 h and then filtered and concentrated to dryness in vacuo yielding a white solid. Yield: 0.162 g, 67%. Required for C₄₂H₂₈BCl₃F₂₄P₂Sn (1286.2): C, 39.2, H, 2.2 Found C, 39.3, H, 2.0%. ¹H NMR (CD₂Cl₂, 295 K): δ = 2.01 (t, ²J + ⁵J_PH = 4.7 Hz, [12H], CH₃), 7.57 (s, [4H], [BAr^F]), 7.72 (m, [8H], [BAr^F]), 7.82 (m, [4H], [BAr^F]);¹⁹F{¹H} NMR (CD₂Cl₂, 295 K): δ = −63.0 (s, –CF₃); ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = −24.5 (¹J_31P–Sn = 1007 Hz); ¹¹⁹Sn NMR (CD₂Cl₂, 183/298 K): δ = n.o.

[SnBr₃(AsEt₃)₂][BAr^F]

A Schlenk was charged with [SnBr₄(AsEt₃)₂] (0.077 g, 0.1 mmol) and Na[BAr^F] (0.090 g, 0.1 mmol) to this CH₂Cl₂ (5 mL) was added; the resulting cloudy solution was stirred for 2 h, filtered and concentrated to dryness in vacuo yielding a light yellow gum, which was then dissolved in Et₂O (5 mL), then the solution was concentrated to dryness in vacuo yielding a light yellow solid. Yield: 0.091 g, 58%. Required for C₄₄H₄₂As₂BBr₃F₂₄Sn (1448.2): C, 34.2, H, 2.7. Found C, 34.3, H, 2.9%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.39 (t, ³J_HH = 7.6 Hz, [18H], CH₃), 2.42 (br, [12H], CH₂), 7.58 (s, [4H], [Bar^F]), 7.73 (m, [8H], [Bar^F]); ¹³C{¹H}NMR (CD₂Cl₂, 295 K): δ = 9.1 (s, –CH₃), 15.9 (s, –CH₂), 118.1(s), 123.8(s), 126.5(s), 129.3(m), 135.4(s), 162.3(q, ¹J_11B–13C = 49.9 Hz); ¹⁹F{¹H} NMR (CD₂Cl₂, 295 K): δ = −62.8 (s, –CF₃). ¹¹⁹Sn NMR (CD₂Cl₂, 183/298 K): δ = n.o.

X-Ray experimental

Crystals of the complexes were grown from CH₂Cl₂ solutions allowed to evaporate slowly 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-2014/7.41.¹⁸ H atoms bonded to C were placed in calculated positions using the default C–H distance and refined using a riding model. Details of the crystallographic parameters are given in Table S1 in the ESI.† CCDC reference numbers for the crystallographic information files in cif format are CCDC 1916557 [SnCl₄(AsEt₃)₂], CCDC 1916558 [SnBr₄(AsEt₃)₂], CCDC 1916559 [SnCl₃(PMe₃)₂(OTf)], CCDC 1916560 [SnCl₃(AsEt₃)₂(OTf)], CCDC 1916561 [SnCl₂(PEt₃)₃][AlCl₄]₂, CCDC 1916562 [Et₃AsCl][SnCl₅(AsEt₃)].†

Results and discussion

Neutral tin(IV) halide complexes

The neutral tin(IV) phosphine complexes used in this study were made by literature methods or minor modifications thereof, viz- [SnCl₄(PEt₃)₂],¹⁷ [SnCl₄(PMe₃)₂],¹⁶ [SnBr₄(PMe₃)₂],¹⁶ [SnCl₄{o-C₆H₄(PMe₂)₂}],⁴ [SnCl₄(Et₂PCH₂CH₂PEt₂)],⁴ as was [SnCl₄{o-C₆H₄(AsMe₂)₂}].¹⁶ Their identity and purity were confirmed by comparison with literature data; key spectroscopic data are given in Table 1 for comparison purposes. Tin complexes of triethylarsine were reported as long ago as 1949,¹⁹ but lacked any spectroscopic or structural data. The crystal structure of [SnCl₄(AsEt₃)₂] (Fig. 1(a)) reveals it to be the trans isomer and that the d(Sn–Cl) are not significantly different to those in trans-[SnCl₄(PEt₃)₂]¹⁷ or trans-[SnCl₄(PMe₃)₂],¹¹ but slightly longer than that in trans-[SnCl₄(AsPh₃)₂]²⁰ (2.415(1), 2.424(1) Å). The d(Sn–As) of 2.65964(16) Å is significantly shorter than that in the triphenylarsine complex (2.7623(3) Å), consistent with the stronger donor power of the trialkylarsine ligand. In trans-[SnBr₄(AsEt₃)₂] (Fig. 1(b)) the d(Sn–As) is 2.6773(3) Å, only very slightly longer than in the chloride.

Fig. 1 (a) The structure of [SnCl₄(AsEt₃)₂] showing the atom numbering scheme. Ellipsoids are drawn at the 50% level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–Cl1 = 2.4498(4), Sn1–Cl2 = 2.4544(4), Sn1–As1 = 2.65964(16), As1–Sn1–As1 = 180.0, Cl1–Sn1–Cl2 = 90.367(15), Cl1–Sn1–Cl1 = 180.0, Cl2–Sn1–Cl2 = 180.0; (b) the structure of [SnBr₄(AsEt₃)₂] showing the atom numbering scheme. Ellipsoids are drawn at the 50% level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–Br1 = 2.6286(3), Sn1–Br2 = 2.6173(3), Sn1–As1 = 2.6773(3), As1–Sn1–As1 = 180.0, Br1–Sn1–Br2 = 89.298(11), Br1–Sn1–Br1 = 180.0, Br2–Sn1–Br2 = 180.0.

Table 1 Selected spectroscopic data^a

Compound	δ(^31P{^1H})	δ(^119Sn)	^1 J(^119Sn–^31P)/Hz	ν(Sn–X)/cm^−1	Ref.
a NMR data from CH2Cl2 solution at 293 K, except. b 193 K.
[SnCl4(PMe3)2]	+1.6	−554	2635	321	11
[SnCl4(PEt3)2]	+20.9	−572	2391	—	17
[SnCl4{o-C6H4(PMe2)2}]	−28.1	−616	945	307, 296	4
[SnCl4(Et2PCH2CH2PEt2)]	−4.9	−615	1049	318, 307, 282	4
[SnCl4{o-C6H4(AsMe2)2}]	—	−675	—	323, 315, 304, 300	16
[SnCl4(AsEt3)2]	—	−657^b	—	288	This work
[SnBr4(AsEt3)2]	—	−1173^b	—	210	This work
[SnCl3(AsEt3)2(OTf)]	—	−620^b	—	294, 378	This work
[SnCl3(AsEt3)2][BAr^F]	—	−388	—	—	This work
[SnCl3(PMe3)2(OTf)]	+8.7	−516	2963	296, 306	This work
[SnCl3(PEt3)2(OTf)]	+33.4^b	−535^b	2737^b	289, 301	This work
[SnCl3(PEt3)2][AlCl4]	+35.5	−350	2307	280, 289	This work
[SnCl2(PEt3)2][AlCl4]2	+40.8	−301	2274	289	This work
[SnCl3(PEt3)2][BAr^F]	+37.1	−379	2311	286	This work
[SnCl3(PMe3)2][AlCl4]	+10.8	−456	2725	—	11
[SnCl2(PMe3)2][AlCl4]2	+16.9	−333	2591	—	11
[SnCl3{o-C6H4(PMe2)2}][AlCl4]	−24.7	−469	972	322, 342	This work
[SnCl2{o-C6H4(PMe2)2}][AlCl4]2	−23.7	−429	853	294, 305	This work

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Attempts to isolate a complex of AsEt₃ with SnI₄ were not successful, as noted previously by others.¹⁹

Addition of a CH₂Cl₂ solution of SbR₃ (R = Et or ⁱPr) to CH₂Cl₂ solutions of SnX₄ (X = Cl or Br) result in immediate precipitation of grey solids, which contain essentially no C/H (microanalysis), whilst the supernatant solutions contain R₃SbX₂, based upon comparisons of their ¹H NMR spectra with literature data and ESI.† ²¹ This suggests that any Sn(IV)–stibine complexes are too unstable to isolate, and redox chemistry occurs [SnX₄ + SbR₃ → SnX₂ + R₃SbX₂]. Cunningham et al.²² reported that reaction of SbPh₃ with SnX₄ initially gave yellow solids which became white on washing with pentane. The white solids had ¹¹⁹Sn Mössbauer spectra consistent with SnX₂, indicating similar, but slower, redox chemistry occurring with the triarylstibine. The redox chemistry in these tin systems contrasts with the reactions of Group 13 trihalides with trialkylstibines, which gave simple adducts.⁶

Reactions of [SnX₄(ER₃)₂] (E = P, As) and [SnCl₄(L–L)] (L–L = o-C₆H₄(PMe₂)₂, Et₂P(CH₂)₂PEt₂ or o-C₆H₄(AsMe₂)₂) with TMSOTf

The complexes [SnX₄(ER₃)₂] (X = Cl, ER₃ = PMe₃, PEt₃, AsEt₃; X = Br, ER₃ = PMe₃, AsEt₃) were treated with one mol. equivalent of TMSOTf in anhydrous CH₂Cl₂ solution with a view to removing one chloride ligand (as TMSCl) to create a tin cation. Triflate can behave either as an anion or coordinate to the metal; in practice we found that that it coordinates to the tin in the present study.

For the chloride systems the reactions proceeded to completion in ∼2 h to give good yields of moisture-sensitive white powders, identified by microanalysis as [SnCl₄(ER₃)₂(OTf)] (Scheme 1). Curiously, the corresponding reactions with the tin(IV) bromide complexes did not occur, and after 24 h the NMR spectra of these systems showed only unchanged starting materials. Similarly, no reaction occurred between [SnCl₄{Et₂P(CH₂)₂PEt₂}],⁴ [SnCl₄{o-C₆H₄(PMe₂)₂}]⁴ or [SnCl₄{o-C₆H₄(AsMe₂)₂}]¹⁶ with TMSOTf in CH₂Cl₂ solution even after 3 d. For the systems which produced mono triflate complexes, use of more TMSOTf failed to give any evidence that a second chloride could be removed.

Scheme 1 Reactions with TMSOTf.

Colourless crystals of [SnCl₃(PMe₃)₂(OTf)] and [SnCl₃(AsEt₃)₂(OTf)] were grown by slow evaporation of solutions in anhydrous CH₂Cl₂, and their X-ray structure analyses revealed the geometry shown in Fig. 2. The structures show six-coordinate tin centres with mer-chlorines, and trans pnictines with the triflate trans to chlorine. Comparison of the two structures shows that the d(Sn–Cl_transCl) are not significantly different and in both cases are longer than d(Sn–Cl_transOTf). The d(Sn–Cl_transCl) and d(Sn–As) are also similar to those in [SnCl₄(AsEt₃)₂]. The d(Sn–O) distances are ∼2.26 Å, which fall at the lower end of the range found in octahedral Sn(IV) triflates (∼2.2–2.6 Å)²³ and may be compared to the sum of the covalent radii of Sn and O (2.15 Å).²⁴

Fig. 2 (a) The structure of [SnCl₃(PMe₃)₂(OTf)] showing the atom numbering scheme. Ellipsoids are drawn at the 50% level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–Cl1 = 2.4457(9), Sn1–Cl2 = 2.4558(9), Sn1–Cl3 = 2.4085(10), Sn1–P1 = 2.5496(9), Sn1–P2 = 2.5506(9), Sn1–O1 = 2.266(3), P1–Sn1–P2 = 164.16(3), Cl1–Sn1–Cl2 = 174.66(3), Cl3–Sn1–Cl1 = 92.67(3), Cl3–Sn1–Cl2 = 92.68(4), O1–Sn1–Cl1 = 89.25(7), O1–Sn1–Cl2 = 85.41(7), O1–Sn1–P1 = 82.17(7), O1–Sn1–P2 = 82.51(7); (b) the structure of [SnCl₃(AsEt₃)₂(OTf)]: selected bond lengths (Å) and angles (°) are: Sn1–Cl1 = 2.4480(7), Sn1–Cl2 = 2.4168(8), Sn1–Cl3 = 2.4533(7), Sn1–As1 = 2.6451(4), Sn1–As2 = 2.6530(4), Sn1–O1 = 2.259(2), As1–Sn1–As2 = 172.239(12), Cl1–Sn1–Cl2 = 92.83(3), Cl3–Sn1–Cl1 = 173.76(3), Cl3–Sn1–Cl2 = 93.17(3), O1–Sn1–As2 = 86.28(6), O1–Sn1–As1 = 85.96(6), O1–Sn1–Cl1 = 88.46(6), O1–Sn1–Cl3 = 85.52(6).

Comparison of the NMR spectroscopic data (Table 1) shows that conversion of [SnCl₄(PR₃)₂] into [SnCl₃(PR₃)₂(OTf)] results in modest high frequency shifts of the ¹¹⁹Sn and ³¹P resonances and a more marked increase in ¹J_Sn–P. The ¹¹⁹Sn chemical shift also moves to high frequency in [SnCl₃(AsEt₃)₂(OTf)] compared to [SnCl₄(AsEt₃)₂].

The reaction of SnCl₄, SbEt₃ and TMSOTf in CH₂Cl₂ again resulted in redox chemistry, with [Et₃SbCl(OTf)] isolated as one product in ∼30% yield, along with much insoluble grey solid, which had no significant C/H content and is most likely SnCl₂ (cf.ref. 22). Crystals of the former were grown from CH₂Cl₂ solution and although the crystal quality was poor, confirmed them to be [Et₃SbCl(OTf)], which has a trigonal bipyramidal geometry with equatorial ethyl groups (see ESI†).

Reactions of [SnX₄(ER₃)₂] and [SnCl₄(L–L)] with AlX₃

No examples of ‘self-ionisation’, such as [SnX₃(ER₃)₂]₂[SnX₆] or [SnX₂(L–L)₂][SnX₆], have been reported in reaction of SnX₄ with pnictines,⁵i.e. SnX₄ is not a sufficiently strong Lewis acid to abstract a halide from a second tin centre in these systems. This is also consistent with ligands such as MeC(CH₂AsMe₂)₃ failing to displace a halide from a neutral [SnX₄{κ²-MeC(CH₂AsMe₂)₃}],⁴ to form [SnX₃{κ³-MeC(CH₂AsMe₂)₃}]⁺. However, using the stronger Lewis acid, AlCl₃, has been successful in removing a chloride in some cases.¹¹

The [SnCl₃(PEt₃)₂][AlCl₄] and [SnCl₂(PEt₃)₂][AlCl₄]₂ were made by treatment of [SnCl₄(PEt₃)₂] with one and two mol. equivalents, respectively, of AlCl₃ in anhydrous CH₂Cl₂ (Scheme 2), and behave similarly to the previously reported¹¹ [SnCl_x(PMe₃)₂][AlCl₄]_y (x = 3, y = 1; x = 2, y = 2).

Scheme 2 Reactions of the tin complexes with AlCl₃.

Attempts to grow crystals from a CH₂Cl₂ solution of [SnCl₂(PEt₃)₂][AlCl₄]₂ produced a few small colourless crystals, one of which was found on X-ray structure solution to be [SnCl₂(PEt₃)₃][AlCl₄]₂ containing a tris(phosphine) dication (Fig. 3), presumably as a result of rearrangement in solution during recrystallisation.

Fig. 3 The structure of [SnCl₂(PEt₃)₃][AlCl₄]₂ showing the atom numbering scheme. Note there are two crystallographically independent [SnCl₂(PEt₃)₃][AlCl₄]₂ units in the asymmetric unit. Ellipsoids are drawn at the 50% level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–Cl1 = 2.4531(13), Sn1–Cl2 = 2.4589(13), Sn1–P1 = 2.5679(14), Sn1–P2 = 2.5610(13), Sn1–P3 = 2.5525(13), Cl2–Sn1–P3 = 91.10(4), Cl2–Sn1–P2 = 92.07(5), Cl2–Sn1–P1 = 86.01(4), Cl1–Sn1–Cl2 = 178.28(4), Cl1–Sn1–P3 = 90.52(4), Cl1–Sn1–P2 = 87.51(4), Cl1–Sn1–P1 = 92.63(4), P3–Sn1–P2 = 123.83(4), P3–Sn1–P1 = 122.85(4), P2–Sn1–P1 = 113.31(5).

The structure contains a trigonal bipyramidal cation with axial chlorides, the bond angles showing only small deviations from the idealised values, and in contrast to [SnCl₂(PMe₃)₂][AlCl₄]₂, there is no interaction of the tin centre with the tetrachloroaluminate anions. This is likely due to the steric effect caused by the presence of the third PEt₃ ligand. The d(Sn–Cl) are very similar to the d(Sn–Cl_transCl) in the OTf complex in Fig. 3. There were insufficient crystals produced to obtain any spectroscopic data, so direct synthesis of a bulk sample was explored. However, addition of one mol. equivalent of PEt₃ to [SnCl₂(PEt₃)₂][AlCl₄]₂ in anhydrous CH₂Cl₂ solution does not form the new cation [SnCl₂(PEt₃)₃]²⁺. In situ NMR studies reveal a mixture of three main species (Experimental section); [AlCl₄]⁻, [AlCl₃(PEt₃)] (identified by its characteristic NMR spectra²⁵) and a new Sn-PEt₃ containing species. The ¹¹⁹Sn NMR spectrum shows a triplet ¹J(¹¹⁹Sn–³¹P) coupling and hence indicates only two phosphines coordinated to the tin centre, e.g. [SnCl_x(PEt₃)₂]ⁿ⁺. The ³¹P and ¹¹⁹Sn chemical shifts do not correspond to those of [SnCl₃(PEt₃)₂][AlCl₄].

The solution NMR data (Table 1) show that along the series [SnCl₄(PEt₃)₂], [SnCl₃(PEt₃)₂][AlCl₄], [SnCl₂(PEt₃)₂][AlCl₄]₂, the chemical shifts move to higher frequency in both the ³¹P and ¹¹⁹Sn spectra. The ³¹P{¹H} and ¹¹⁹Sn chemical shifts of the cations also vary significantly with temperature and the tin spectra have rather broad resonances, which probably reflects a variation in the interaction of the cations with the [AlCl₄]⁻ anions under different conditions.

The reaction of [SnCl₄(AsEt₃)₂] with AlCl₃ in CH₂Cl₂ did not proceed similarly to the reactions involving PEt₃. Monitoring the reaction by NMR spectroscopy identified a mixture of species present including starting materials and Et₃AsCl₂.²⁶ A few crystals grown from this mixture contained the chlorotriethylarsonium(V) cation and a pentachloro(triethylarsine)stannate(IV) anion, [Et₃AsCl][Sn(AsEt₃)Cl₅] (Fig. 4).

Fig. 4 The structure of [Et₃AsCl][SnCl₅(AsEt₃)] showing the atom numbering scheme. Ellipsoids are drawn at the 50% level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Sn1–As1 = 2.6696(5), Sn1–Cl3 = 2.4453(8), Sn1–Cl4 = 2.4304(9), Sn1–Cl2 = 2.4365(8), Sn1–Cl1 = 2.3930(9), Sn1–Cl5 = 2.4254(8), Cl3–Sn1–As1 = 87.09(2), Cl4–Sn1–As1 = 85.21(2), Cl4–Sn1–Cl3 = 91.59(3), Cl2–Sn1–As1 = 92.00(2, Cl2–Sn1–Cl3 = 88.20(3), Cl1–Sn1–Cl3 = 90.72(3, Cl1–Sn1–Cl4 = 90.90(3), Cl1–Sn1–Cl2 = 91.88(3), Cl1–Sn1–Cl5 = 93.86(3), Cl5–Sn1–As1 = 88.54(2), Cl5–Sn1–Cl4 = 91.32(3), Cl5–Sn1–Cl2 = 88.66(3).

Arsine substituted halostannate anions do not seem to have been reported previously,⁵ and the nearest literature example is the zwitterionic phosphine complex [SnCl₅(Ph₂PCH₂PPh₂H)].²⁷ Comparison of the key bond lengths with those in trans-[SnCl₄(AsEt₃)₂] shows that the d(Sn–As) and d(Sn–Cl_transCl) are little different, although d(Sn–Cl_transAs) is ∼0.05 Å shorter. The reaction of trans-[SnBr₄(AsEt₃)₂] with AlBr₃ also produced a mixture of products, including Et₃AsBr₂.²⁶

The reactions of [SnCl₄{o-C₆H₄(PMe₂)₂}] with one and two mol. equivalents of AlCl₃ in anhydrous CH₂Cl₂ were also investigated. The products were identified by microanalysis as [SnCl₃{o-C₆H₄(PMe₂)₂}][AlCl₄] and [SnCl₂{o-C₆H₄(PMe₂)₂}][AlCl₄]₂. These very moisture-sensitive complexes were poorly soluble in CH₂Cl₂ and attempts to grow crystals have been unsuccessful. Obtaining multinuclear NMR data was also hindered by their poor solubility in weakly coordinating organic solvents. However, the trends in chemical shifts and coupling constants within the series (Table 1) replicate those observed for [SnCl_x(PEt₃)₂][AlCl₄]_y (x = 4, y = 0; x = 3, y = 1; x = 2, y = 2), with δ¹¹⁹Sn and δ³¹P NMR moving to high frequency on going from the neutral complex to the mono- and the di-cations. The ¹J(¹¹⁹Sn–³¹P) also follow the same trend as the PMe₃ complexes, with ¹J_31P–119Sn [SnCl₃{o-C₆H₄(PMe₂)₂}]⁺ > [SnCl₄{o-C₆H₄(PMe₂)₂}] > [SnCl₂{o-C₆H₄(PMe₂)₂}]²⁺.

[SnCl₄{o-C₆H₄(AsMe₂)₂}] also reacted with AlCl₃ in anhydrous CH₂Cl₂ to form a poorly soluble white product, but the NMR spectra indicated a mixture of species present, which we were unable to identify.

Reactions of [SnX₄(EEt₃)₂] and [SnCl₄(L–L)] with Na[BAr^F]

Addition of Na[BAr^F] to a CH₂Cl₂ solution [SnX₄(EEt₃)₂] (E = P; X = Cl or Br; E = As; X = Cl) or [SnCl₄{o-C₆H₄(PMe₂)₂}] (Scheme 3) readily formed the [BAr^F]⁻ salts, [SnX₃(EEt₃)₂][BAr^F] and [SnCl₃{o-C₆H₄(PMe₂)₂}][BAr^F], with precipitation of NaX. The NMR spectroscopic data for the cations match those of the [AlCl₄]⁻ salts discussed above, and using Na[BAr^F] as the halide abstractor allowed generation of the [SnBr₃(AsEt₃)₂]⁺ cation that proved elusive with the other halide abstractors. Using excess Na[BAr^F] did not lead to further halide abstraction.

Scheme 3 Reactions of the tin complexes with Na[BAr^F].

DFT studies

Computational studies of the tin complexes were undertaken to supplement the experimental data. Ground state geometries of [SnCl₃(PMe₃)₂][AlCl₄], [SnCl₄(AsEt₃)₂], [SnCl₃(AsEt₃)₂(OTf)], [SnCl₄(PEt₃)₂],¹⁷ [SnCl₄(PMe₃)₂],¹¹ [SnCl₃(PMe₃)₂(OTf)], [SnCl₃(PMe₃)₂]⁺ were optimised alongside the uncoordinated ligands AsEt₃ and PEt₃ using DFT with the B3LYP functional and a basis set with a Stuttgart–Dresden effective core potential on Sn and As atoms and with an all-electron double-ζ basis set on all electrons in H, C, O, F, P, Al, S, and Cl atoms. Comparison of DFT results for the [SnCl₃(PMe₃)₂][AlCl₄] and [SnCl₄(PMe₃)₂] against those reported,¹¹ was used to test the suitability of the basis set – see ESI.†

The bond lengths from the calculated ligands are similar to those from available experimental data (Table S2†) and show that the average C–E–C angles from AsEt₃ and PEt₃ (98.5 and 100.4°, respectively) are slightly wider than those found in AsMe₃ (97.0°) and PMe₃ (98.8°). The average As–C bond distances from the DFT analysis (∼1.98 Å) are similar to those from experimental data (∼1.94 Å) for the tin complexes, showing a shortening in comparison to ‘free’ AsEt₃ (As–C = 2.01 Å). The P–C bond distances also follow this trend, with a shortening from the phosphine complexes (DFT: 1.87–1.89 Å; X-ray: ∼1.79 Å), compared to the free PMe₃ and PEt₃ (1.90–1.92 Å; X-ray: 1.84 Å). This, when combined with the widening of the C–As–C (DFT: 105.33–106.2°; X-ray: 105.9–106.0°) and C–P–C angles (DFT: 106.3–109.1°; X-ray: 107.1–109.1°) of the complexes, against the values for the ‘free’ ligands (AsEt₃: 98.5°; PMe₃: 98.8° and PEt₃: 100.4°), matches the trends seen for trialkylstibine ligands upon coordination to Group 13 metal halides,⁶ although the effect here is much smaller. The widening of the C–E–C angle corresponds to an increased s-character in the E–C bond and increased p-character in the lone pair. The comparable data from the halo tin-arsine and phosphine complexes shows the free ligands have s-character of AsEt₃: 12.20; PMe₃: 15.08; PEt₃: 15.57%, which doubles to 25.3–25.9% upon coordination.

Calculating the ground state structure of the [SnCl₄(AsEt₃)₂] and [SnCl₃(AsEt₃)₂(OTf)] complexes, alongside DFT calculations of the ionic [SnCl₃(AsEt₃)₂]⁺ complex, show that there is very little change in the bond lengths/angles and this is consistent with reported data for [SnCl₃(PMe₃)₂]⁺. Burford's complexes are also noted for a significant decrease in the P–Sn–P angle with increasing charge,¹¹ the neutral [SnCl₄(AsEt₃)₂] complex (X-ray: 180; DFT: 179.9°) and the cationic [SnCl₃(AsEt₃)₂]⁺ species show a similar trend (DFT: 174°).

Notably, while the HOMO and LUMO energies in the neutral tetrahalide and triflate complexes are not significantly different, those of the cationic species are some 4 eV lower in energy, consistent with the latter displaying higher Lewis acidity (Table S7†). Furthermore, using the AsEt₃ complexes as examples, the HOMO is concentrated on the SnCl₄ unit in the [SnCl₄(AsEt₃)₂], with some delocalisation onto the AsEt₃ ligands in [SnCl₃(AsEt₃)₂(OTf)], and this is even more obvious in [SnCl₃(AsEt₃)₂]⁺ (Fig. 5).

Fig. 5 Representation of the LUMO and HOMO of [SnCl₄(AsEt₃)₂], [SnCl₃(AsEt₃)₂(OTf)] and [SnCl₃(AsEt₃)₂]⁺ determined by DFT analysis, showing their relative energies.

Conclusions

This study has confirmed the high stability of tin tetrahalide complexes with neutral phosphine and arsine ligands and has demonstrated the formation of some complexes with Sn^IV : X ratio of 1 : 3 or 1 : 2.

In the case of SbEt₃, we found that SnCl₄ causes chlorination of the stibine, consistent with the greater stability of Sb(V) over As(V) (although P(V) is also more stable than As(V), the stronger basicity of the PR₃ ligands means that coordination is favoured over chlorination in this system).

Replacement of chloride by triflate to form [SnCl₃(ER₃)₂(OTf)] is possible for ER₃ = PMe₃, PEt₃, AsEt₃, but not with complexes of chelating bidentate ligands. This may indicate that reversible dissociation of the neutral ligand plays a role in the substitution. The failure of corresponding [SnBr₄(ER₃)₂] to react with TMSOTf was unexpected, although formation of TMSBr is less favoured energetically.

Aluminium trichloride is able to abstract chloride ligands from [SnCl₄(PEt₃)₂] and [SnCl₄{o-C₆H₄(PMe₂)₂}] to form mono- and di-cationic Sn(IV) complexes, but these reactions do not occur with arsine ligands, possibly due to the weaker donor arsines having less affinity for the hard tin(IV) cations. On the other hand, using the large, weakly coordinating [BAr^F]⁻ anion does lead smoothly to the formation of [SnCl₃(EEt₃)₂][BAr^F] for E = P and As, [SnBr₃(AsEt₃)₂][BAr^F], as well as affording [SnCl₃{o-C₆H₄(PMe₂)₂}][BAr^F].

In practice, the Na[BAr^F] works better as a halide abstractor to form the monocations (but does not form dications), whereas AlCl₃ can compete for coordination to the weakly bound arsine ligand, whilst it is clear that the triflate anion can coordinate readily to Sn(IV).

DFT studies have shown that bonding and orbital components in the triflate complexes are very similar to those for the tetrachlorides, while the HOMO and LUMO energies are somewhat lower (by ca. 4 eV) for the cations compared to the neutral species, indicating that the cationic species are stronger Lewis acids.

Conflicts of interest

The authors have no conflicts to declare.

Acknowledgements

We thank EPSRC for support via EP/P025137/1 and EP/N035437/1 and for a studentship to R. P. K. (EP/N509747/1).

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Footnote

† Electronic supplementary information (ESI) available: Multinuclear NMR and IR spectra associated with each of the new compounds described, together with the X-ray crystallographic parameters and full details of the DFT calculations. CCDC 1916557–1916562. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt03683k

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