Neutral and cationic germanium(IV) fluoride complexes with phosphine coordination – synthesis, spectroscopy and structures

Abstract

The neutral complexes trans-[GeF₄(PⁱPr₃)₂] and [GeF₄(κ²-L)] (L = CH₃C(CH₂PPh₂)₃ or P(CH₂CH₂PPh₂)₃) are obtained from [GeF₄(MeCN)₂] and the ligand in CH₂Cl₂. Treatment of [GeF₄(PMe₃)₂] with n equivalents of TMSOTf (Me₃SiO₃SCF₃) leads to formation of the series [GeF_4−n(PMe₃)₂(OTf)_n] (n = 1, 2, 3), each of which contains six-coordinate Ge(IV) with trans PMe₃ ligands and X-ray structural data confirm that the OTf groups interact with Ge(IV) to varying degrees. Unexpectedly, [GeF₃(PMe₃)₂(OTf)] undergoes reductive defluorination in solution, forming the Ge(II) complex, [Ge(PMe₃)₃][OTf]₂ (and [FPMe₃]⁺). The bulkier PⁱPr₃ leads to formation of the ionic [GeF₃(ⁱPr₃P)₂][OTf], containing a [GeF₃(ⁱPr₃P)₂]⁺ cation. [GeF₄{o-C₆H₄(PMe₂)₂}], containing the cis-chelating diphosphine, also reacts with n equivalents of TMSOTf to generate [GeF_4−n{o-C₆H₄(PMe₂)₂}(OTf)_n] (n = 1, 2, 3). As for the PMe₃ system, the trifluoride, [GeF₃{o-C₆H₄(PMe₂)₂}(OTf)], is unstable to reductive defluorination in solution, producing the pyramidal Ge(II) complex [Ge{(o-C₆H₄(PMe₂)₂}(OTf)][OTf], whose crystal structure has been determined. The [GeF₃{Ph₂P(CH₂)₂PPh₂}(OTf)] and [GeF₂{Ph₂P(CH₂)₂PPh₂}(OTf)₂], obtained similarly from the parent tetrafluoride complex, are poorly soluble, however their structures were confirmed crystallographically. The complexes in this work have been characterised via variable temperature ¹H, ¹⁹F{¹H} and ³¹P{¹H} NMR studies in solution, IR spectroscopy and microanalysis and through single crystal X-ray analysis of representative examples across each series. Trends in the NMR and structural parameters are also discussed.

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Introduction

P-block coordination and organometallic chemistry has seen a surge in research activity over recent years. There have been a variety of drivers for this, including, for example, our deeper understanding of the intrinsic chemical and structural diversity of the p-block acceptors compared to the d-block ions, the motivation to develop efficient metal-free catalysts, precursors for the growth of semiconductor materials for electronic and optical applications, as well as the range of radionuclides in the p-block that offer exciting prospects in medical imaging and therapy.^1,2a Fluoride derivatives of several p-block elements, e.g. B, Al, Ga and P, have drawn significant interest as prospective carriers for the positron-emitting fluorine-18 isotope in new positron emitting tomography (PET) imaging agents.³ However, the wider coordination chemistry of the p-block fluorides is still relatively limited in scope, with the majority of examples featuring the Group 13 with hard N- and O-donor ligands.^3–5 Recently we have reported several series of neutral and cationic Sn(IV) fluoride coordination complexes involving a range of hard and soft donor ligands.⁶

Germanium tetrafluoride is a molecular monomer with boiling point = −36.5 °C, which has been shown to form a range of six-coordinate complexes either by the direct reaction of GeF₄ with the neutral ligands, or by using an appropriate adduct, typically [GeF₄(MeCN)₂]. Other nitriles like NCCH₂X (X = F and Cl) also form [GeF₄(NCCH₂X)₂]; for X = F, the crystal structure shows that the nitrile ligands lie cis.⁷ Reacting germanium tetra-ethoxide, aqueous HPF₆ and pyridine under solvothermal conditions forms trans-[GeF₄(py)₂].⁸ The reaction of 2,2′-bipyridine, 1,10-phenanthroline, or Me₂NCH₂CH₂NMe₂ with [GeF₄(MeCN)₂] in CH₂Cl₂ leads exclusively to the formation of the cis-isomer with the neutral ligand chelating, as expected.^9,10 The tetra-aza macrocycle Me₄[14]aneN₄ (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) forms [GeF₄(κ²-Me₄[14]aneN₄)], where the ligand binds in an exocyclic bidentate fashion and the Ge(IV) retains the four bound fluorides.⁹

[GeF₄(MeCN)₂] also reacts with OPR₃ (R = Me, Et, Ph) to form [GeF₄(OPR₃)₂], which exist as a cis/trans mixture in solution,¹¹ while bidentate phosphine oxides form only the cis isomer.¹² While the heavier GeCl₄ and GeBr₄ analogues react with OPMe₃ to form cationic or dicationic complexes of the form [GeCl₃(OPMe₃)₃]₂[GeCl₆] and [GeX₂(OPMe₃)₄][X]₂ (X = Cl, Br), the latter through self-ionisation, this does not occur for GeF₄, consistent with the Ge–X bonds becoming weaker as the halide becomes heavier.¹¹

There are a few reports of germanium fluoride complexes with neutral, soft donor ligands.³ The reaction of [GeF₄(MeCN)₂] with two equivalents of PMe₃ forms trans-[GeF₄(PMe₃)₂], whereas using AsEt₃ does not produce an isolable complex, and while AsEt₃ reacts with GeCl₄ to form trans-[GeCl₄(AsEt₃)₂], this complex undergoes slow redox chemistry in solution, forming AsEt₃Cl₂ and GeCl₂.¹³ [GeCl₄(PMe₃)₂] can be isolated from the solvent-free reaction of GeCl₄ and PMe₃; although, if a solvent is employed, redox chemistry occurs and the product is [PMe₃Cl][GeCl₃] exclusively.¹³ A small number of diphosphine complexes, cis-[GeF₄(diphosphine)] (diphosphine = o-C₆H₄(PR₂)₂ (R = Me, Ph), R₂P(CH₂)₂PR₂; R = Me, Et, Cy, Ph),¹³ and dithioether complexes, cis-[GeF₄{RS(CH₂)₂SR}] (R = Me, Et, ⁱPr) have also been described, with crystal structures reported for representative examples.¹⁴

Examples of cationic Ge(IV) fluoride complexes are few. The reaction of [GeF₄(MeCN)₂] with Me₃[9]aneN₃ (1,4,7-trimethyl-1,4,7-triazacyclononane) in CH₂Cl₂ leads to [GeF₃(Me₃[9]aneN₃)]₂[GeF₆], with the strong preference for tridentate coordination of the macrocycle causing F⁻ displacement; this cation has been confirmed crystallographically.⁹ Dicationic germanium fluoride complexes can be obtained by the fluorination of Ge(II) complexes. The reaction of [Ge(BIMEt₃)][OTf]₂ (BIMEt₃ = tris(1-ethyl-benzoimidazol-2-ylmethyl)amine) with XeF₂ leads to the clean formation of [GeF₂(BIMEt₃)][OTf]₂, driven by the formation of the strong Ge–F bonds. The reaction of this difluoride complex with one equivalent of TMSOTf then leads to [GeF(OTf)(BIMEt₃)][OTf]₂. This reaction can be reversed by the addition of one equivalent of NaBF₄. The structures of both complexes have been reported.¹⁵

We describe here the coordination chemistry of GeF₄ with neutral mono-, bi-, tri- and tetra-phosphine ligands, together with the reactions of [GeF₄(PR₃)₂] (R = Me or ⁱPr), [GeF₄{o-C₆H₄(PMe₂)₂}], and [GeF₄{Ph₂P(CH₂)₂PPh₂}] with TMSOTf (Me₃SiO₃SCF₃), revealing that fluoride ligands can be readily abstracted, in some cases sequentially, generating a range species with OTf⁻ anions, which interact with the Ge(IV) centres to varying degrees. The new complexes have been characterised by variable temperature multinuclear NMR (¹H, ¹⁹F{¹H} and ³¹P{¹H}) and IR spectroscopy, together with microanalyses, and their identities confirmed by X-ray crystal structures of representative examples.

Results and discussion

Germanium tetrafluoride phosphine complexes

Scheme 1 summarises the reactions of [GeF₄(MeCN)₂] with phosphines in this work.

Scheme 1 Summary of the neurtal phosphine complexes of GeF₄ prepared in this work.

Reaction of [GeF₄(MeCN)₂] with 2 equiv. of PR₃ (R = Me, ⁱPr) in CH₂Cl₂ affords the complexes [GeF₄(PR₃)₂] in good yield as colourless solids. Spectroscopic data for [GeF₄(PMe₃)₂] are in accord with those reported,¹³ and slow evaporation of a CH₂Cl₂ solution deposited crystals suitable for single crystal X-ray analysis. The structure is centrosymmetric (Fig. 1), with a distorted octahedral geometry and confirming the trans isomer. This is the first structurally characterised monodentate phosphine complex of germanium fluoride.

Fig. 1 View of the crystal structure of [GeF₄(PMe₃)₂] showing the atom labelling scheme. The ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: −x, −y, −z. Selected bond lengths (Å) and angles (°) are: Ge1–P1 = 2.3717(6), Ge1–F1 = 1.8240(13), Ge1–F2 = 1.8158(13), F1–Ge1–F2 = 90.99(6).

The analogous complex [GeF₄(ⁱPr₃P)₂] was prepared to allow the effect of the increased steric requirement of the phosphine on the fluoride abstraction chemistry (vide infra) to be explored. Its room temperature ³¹P{¹H} and ¹⁹F{¹H} NMR spectra contain broad resonances at +24.6 ppm and −65.0 ppm, respectively, indicating fast exchange on the NMR time scale. Low temperature NMR data support this (see Fig. S2.3 and S2.5†).

To expand the range of multidentate phosphine complexes of GeF₄ and test the possibility of increasing the number of coordinated phosphine donors in this work, the tridentate ligand CH₃C(CH₂PPh₂)₃ was reacted with [GeF₄(MeCN)₂] in a 1 : 1 ratio to form [GeF₄(κ²-CH₃C{CH₂PPh₂}₃)], crystals of which were grown by layering a CH₂Cl₂ solution of the compound with hexane. The structure (Fig. 2) shows that the ligand coordinates in a cis-bidentate fashion, generating a six-membered chelate ring, with the third -PPh₂ group remaining uncoordinated, hence the Ge(IV) retains all four fluorides.

Fig. 2 View of the crystal structure of [GeF₄(κ²-CH₃C{CH₂PPh₂}₃)] showing the atom labelling scheme. The ellipsoids are drawn at the 50% probability level and H atoms and CH₂Cl₂ solvent molecule are omitted, and phenyl rings are displayed as wireframe for clarity. Selected bond lengths (Å) and angles (°) are: Ge1–P1 = 2.4731(4), Ge1–P2 = 2.5030(4), Ge1–F1 = 1.7872(9), Ge1–F2 = 1.7778(9), Ge1–F3 = 1.7976(9), Ge1–F4 = 1.7735(9), P1–Ge1–P2 = 92.886(13), F1–Ge1–F3 = 172.44(4), F2–Ge1–F4 = 92.47(4), P1–Ge1–F4 = 174.90(3), P2–Ge1–F2 = 178.75(3).

In CD₂Cl₂ the room temperature ¹H NMR spectrum from a freshly prepared and analytically pure sample of [GeF₄(κ²-CH₃C{CH₂PPh₂}₃)] shows three sets of methylene resonances, corresponding to the two different environments within the chelated portion, as well as the uncoordinated arms. In the ¹⁹F{¹H} spectrum (298 K) two broad resonances of equal intensity are seen at δ = −72.2 and −108.6, corresponding to the two environments in the cis isomer. Another sharper resonance at δ = −137.7 corresponds to GeF₄. Consistent with this, the ³¹P{¹H} NMR spectrum shows a broad resonance at δ = −4.0, as well as some uncoordinated triphosphine (−26.4 ppm). These data suggest that the complex is dynamic in solution and the phosphine is weakly bound, with both uncoordinated phosphine and GeF₄ being seen in the NMR spectra. Upon cooling to 183 K the ³¹P{¹H} NMR spectrum shows a tdd at −4.5 ppm and a singlet at −27.9 ppm in the 2 : 1 ratio expected for κ²-coordination.

This triphosphine complex has approximately C_2v symmetry at Ge, so group theory predicts four IR active Ge–F stretching vibrations. However, it is common for these to overlap and in the present case only two broad peaks are seen at 517 and 603 cm⁻¹.

The tripodal tetraphosphine, P(CH₂CH₂PPh₂)₃, was also reacted with [GeF₄(MeCN)₂] in a 1 : 1 molar ratio, leading to the formation of [GeF₄{κ²-P(CH₂CH₂PPh₂)₃}]. At 298 K the ¹H NMR spectrum shows two broad singlets in the methylene region at 2.03 and 2.28 ppm, and broad resonances in the aromatic region with a 1 : 1 : 5 integration, suggesting a dynamic process interchanging the terminal –PPh₂ groups in solution. This is consistent with the room temperature ¹⁹F{¹H} NMR data where a single broad resonance is seen at −94.8 ppm, while the ³¹P{¹H} NMR spectrum shows broad, ill-defined multiplets to high frequency of the tetraphosphine itself. However, cooling to 183 K causes these broad resonances in the ³¹P{¹H} NMR spectrum to sharpen into three resonances in a 1 : 2 : 1 ratio, shown in Fig. 3(a). The resonance corresponding to the pendent arms has an integral of two, consistent with bidentate coordination via the apical P donor and one –PPh₂ pendant arm. The ³¹P{¹H} NMR spectrum was also simulated using the SPINACH software package¹⁶ to confirm the coupling scheme (Fig. 3(b)).

Fig. 3 (a) ³¹P{¹H} NMR spectrum of [GeF₄{κ²-P(CH₂CH₂PPh₂)₃}] recorded at 183 K (CH₂Cl₂); (b) ³¹P{¹H} NMR spectrum of [GeF₄(κ²-P{CH₂CH₂PPh₂})] simulated using the SPINACH¹⁶ package; the inset shows the NMR active nuclei giving rise to the coupling scheme. Spin system: J_P1P3 = J_P12P3 = 35 Hz; J_P3P4 = 336 Hz; J_P4F1 = J_P4F2 = 133 Hz; J_P3F1 = J_P3F2 = 153 Hz; J_F1F3 = J_F1F4 = 54 Hz; J_F2F3 = J_F2F4 = 54 Hz; J_F3F4 = 54 Hz; J_P3F4 = 188 Hz; J_P4F3 = 207 Hz.

The ¹⁹F{¹H} NMR spectrum at 213 K supports this (Fig. 4), showing three distinct resonances, a doublet of doublet of triplets at −80.4 ppm and two sets of what appear to be doublets of quartets, but which are in fact doublets of doublets of triplets (Fig. S11.2.5†). Overall, the solution data are therefore consistent with the isomer illustrated in Fig. 5, in which the apical P atom and one –PPh₂ arm are coordinated.

Fig. 4 ¹⁹F{¹H} NMR spectrum of [GeF₄{κ²-P(CH₂CH₂PPh₂)₃}] at 213 K (CH₂Cl₂).

Fig. 5 The isomer of [GeF₄{κ²-P(CH₂CH₂PPh₂)₃}] present in CH₂Cl₂ at low temperature from the NMR data.

Reactions of [GeF₄(PMe₃)₂] with TMSOTf

Scheme 2 shows the products from reactions of [GeF₄(PMe₃)₂] with different ratios of TMSOTf.

Scheme 2 Summary of the GeF₄/PMe₃/TMSOTf chemistry in this work.

The reaction of [GeF₄(PMe₃)₂] with one equivalent of TMSOTf (TMSOTf = Me₃SiO₃SCF₃) in CH₂Cl₂ leads to the formation of [GeF₃(PMe₃)₂(OTf)], which was isolated as a white powder. The ¹H NMR spectrum shows that the doublet corresponding to the PMe₃ groups at δ = 1.68 ppm, i.e. Δδ = +0.22 ppm vs. [GeF₄(PMe₃)₂]. The room temperature ³¹P{¹H} NMR spectrum shows a broad singlet resonance at 3.1 ppm, with Δδ = +15.5 ppm vs. [GeF₄(PMe₃)₂]. At 183 K the singlet shifts to +5.2 ppm and splits into a triplet of doublets (²J_PF = 192, 134 Hz) as shown in Fig. 6, consistent with the triflate being coordinated to germanium at this temperature; a quartet would be expected if it were not (assuming a trigonal bipyramidal geometry, as seen in [SnCl₃(PMe₃)₂]⁺).¹⁷

Fig. 6 ³¹P{¹H} NMR spectrum of [GeF₃(PMe₃)₂(OTf)] at 183 K (CD₂Cl₂).

The ¹⁹F{¹H} NMR data at room temperature show only the triflate resonance, whereas at 183 K three resonances are observed; a triplet of triplets at −123.4 ppm (²J_PF = 134, ²J_FF = 41 Hz) [F], due to the fluorine trans to the triflate, a triplet of doublets at −85.9 ppm [2F] (²J_PF = 192, ²J_FF = 41 Hz), from the fluorines cis to the triflate and a singlet at −78.9 ppm [3F] from the triflate. The couplings are only evident in the ¹⁹F{¹H} spectrum at 183 K; around 210 K the couplings are lost and the peaks broaden significantly. Eventually, the peaks merge together, indicating fast exchange between the different fluorine environments. This is shown by the stacked variable temperature ¹⁹F{¹H} NMR spectra in Fig. 7. The exchange of the fluorine environments is probably due to reversible triflate dissociation in solution.

Fig. 7 Stacked ¹⁹F{¹H} NMR spectra of [GeF₃(PMe₃)₂(OTf)] in CD₂Cl₂ showing the temperature dependant behaviour.

Although the complex can be isolated as a white powder, it is relatively unstable even under inert atmosphere conditions. Over a period of 24 h the colour changes from white to dark brown, and this product is insoluble in CH₂Cl₂, hence all spectroscopic measurements on this complex were performed on a freshly synthesised sample. The complex is also unstable in solution and a CH₂Cl₂ solution of the compound left to evaporate overnight led to the deposition of crystals of the Ge(II) complex, [Ge(PMe₃)₃][OTf]₂, containing a Ge(II) dication, the direct synthesis (from GeCl₂(dioxane), PMe₃ and TMSOTf) and crystal structure of which we have reported recently.¹⁸ This indicates that the [GeF₃(PMe₃)₂(OTf)] is susceptible to redox chemistry and further analysis of its ¹⁹F{¹H} NMR spectrum over time shows the appearance of a doublet at −135.7 ppm (J_PF = 936 Hz) (Fig. S3.2.5†), consistent with the formation of [FPMe₃]⁺,¹⁹ indicating that the Ge(II) dication is a result of reductive defluorination in solution. This can be compared to the reaction of GeCl₄ with PMe₃ in CH₂Cl₂, which also undergoes a redox process forming [PMe₃Cl][GeCl₃]. Notably, the germanium tetrafluoride phosphine complexes do not appear to exhibit any tendency to undergo redox behaviour in solution.

A CH₂Cl₂ solution of the Ge(IV) complex layered with hexane and stored at −78 °C (dry ice), however, produced crystals of [GeF₃(PMe₃)₂(OTf)]. The structure shows (Fig. 8) that the triflate is coordinated in the solid state, with a distorted octahedral geometry at Ge, similar to the case for [SnX₃(PMe₃)₂(OTf)] (X = F or Cl).^6,17

Fig. 8 The structure of [GeF₃(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: Ge1–P1 = 2.3546(3), Ge1–P2 = 2.3655(3), Ge1–F1 = 1.8149(7), Ge1–F2 = 1.7753(7), Ge1–F3 = 1.8015(7), Ge1–O1 = 2.1878(9), P1–Ge1–P1 = 171.122(11), F1–Ge1–F3 = 170.48(3), O1–Ge1–F2 = 177.54(3).

A similar reaction with TMSOTf was undertaken using [GeF₄(ⁱPr₃P)₂]. PⁱPr₃ has a larger Tolman cone angle than PMe₃ (160° vs. 118°),²⁰ hence the increased steric requirement may reduce the likelihood of triflate coordination. The room temperature ³¹P{¹H} NMR spectrum of the resulting [GeF₃(ⁱPr₃P)₂][OTf] shows a quartet 46.8 ppm (Fig. 9(a), contrasting with the behaviour of the PMe₃ analogue, and the quartet remains at 183 K. The quartet coupling indicates that the three fluorides are in the same environment, therefore, the triflate is not bound in solution. In the ¹⁹F{¹H} NMR spectrum a triplet is seen at −56.7 ppm, together with a sharp singlet at −79.0 ppm (OTf), with a 1 : 1 integration ratio. The large positive shift of the fluorine resonance is also consistent with a large increase in positive charge on the GeF₃ unit, and hence supports the conclusion that the complex is ionic in solution, i.e. [GeF₃(PⁱPr₃)₂][OTf] (Fig. 9(b)).

Fig. 9 (a) ³¹P{¹H} NMR spectrum of [GeF₃(ⁱPr₃P)₂][OTf] (298 K, CH₂Cl₂) showing the quartet resonance due to the [GeF₃(ⁱPr₃P)₂]⁺ cation; (b) proposed solution structure of [GeF₃(ⁱPr₃P)₂][OTf] (the singlet at ca. 41 is due to a small amount of [HPⁱPr₃]⁺).

The addition of two equivalents of TMSOTf to [GeF₄(PMe₃)₂] in CH₂Cl₂ results in the formation of [GeF₂(PMe₃)₂(OTf)₂] as a white powder, which is stable for several weeks under inert atmosphere conditions. The room temperature ³¹P{¹H} NMR spectrum shows a broad resonance at +25.8 ppm, which corresponds to Δδ = +22.6 ppm vs. the trifluoride, [GeF₃(PMe₃)₂(OTf)]. At 233 K, this resonance shifts to +27.8 ppm and splits into a triplet (²J_PF = 83 Hz). In the room temperature ¹⁹F{¹H} NMR spectrum two resonances are observed, one at −78.3 ppm corresponding to triflate and a broad peak at −119.5 ppm due to the germanium bound fluorines. At 233 K this broad resonance sharpens into a triplet at −122.3 ppm (²J_PF = 83 Hz). These data are consistent with the formation of the bis-triflate complex.

The evaporation of a concentrated CH₂Cl₂ solution of [GeF₂(PMe₃)₂(OTf)₂] deposited crystals, and the X-ray structure of this complex is shown in Fig. 10(a). It has pseudo-octahedral coordination with two relatively long contacts to the oxygen atoms of the κ¹-triflates, Ge–OTf = 2.375(3) and 2.396(3) Å, which are ∼0.3 Å longer the Ge–OTf bond in [GeF₃(PMe₃)₂(OTf)]. The P–Ge–P bond angle is 150.29(5)°, a significant deviation from an ideal octahedral complex. This, together with the large cis F–Ge–F bond angle of 97.4(14)°, suggests that core the ‘GeF₂(PMe₃)₂’ unit could alternatively be described as a pseudo-tetrahedral dication, with the triflates interacting only weakly.

Fig. 10 The structures of (a) [GeF₂(PMe₃)₂(OTf)₂] showing the atom labelling scheme (there are three independent molecules in the asymmetric unit). The ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity and only the closest oxygen of the triflate is drawn as an ellipsoid for clarity. Selected bond lengths (Å) and angles (°) for the Ge1-centred moiety are: Ge1–P1 = 2.3100(12), Ge1–P2 = 2.3074(12), Ge1–F1 = 1.747(3), Ge1–F2 = 1.748(3), Ge1⋯O1 = 2.374(3), Ge1⋯O2 = 2.396(3) P1–Ge1–P2 = 150.29(5), F1–Ge1–F2 = 97.40(14); (b) [GeCl₂(AsEt₃)₂(OTf)₂] showing the atom labelling scheme. The ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity and only the closest oxygen of the triflate is drawn as an ellipsoid for clarity. Selected bond lengths (Å) and angles (°) are: Ge1–As1 = 2.4048(4), Ge1–As2 = 2.4169(4), Ge1–Cl1 = 2.1453(7), Ge1–Cl2 = 2.1602(7), Ge1⋯O1 = 2.6848(19), Ge1⋯O2 = 2.7436(19), As1–Ge1–As2 = 125.186(16), Cl1–Ge1–Cl2 = 101.36(3).

A concentrated solution containing GeCl₄:2TMSOTf:2AsEt₃ in CH₂Cl₂ left to evaporate slowly also afforded crystals, in this case of [GeCl₂(AsEt₃)₂(OTf)₂] (Fig. 10(b)). This complex is analogous to [GeF₂(PMe₃)₂(OTf)₂], although in the dichloro species Ge⋯OTf contacts are even longer, at 2.6848(19) and 2.7436(2) Å, indicating that the Ge–OTf interactions are even weaker. The As–Ge–As angle is 125.186(16)° and the Cl–Ge–Cl bond angle is 101.36(3)°, i.e. significantly closer to ideal angles for a tetrahedral species than an octahedron.

The addition of three equivalents of TMSOTf to [GeF₄(PMe₃)₂] in CH₂Cl₂ forms [GeF(PMe₃)₂(OTf)₃] as a stable white solid. In the ¹H NMR spectrum a doublet of doublets can be seen at 2.02 ppm, which continues the trend of increasing chemical shift going from the tetrafluoride to the monofluoride species, and consistent with an increasing positive charge along the series.

The room temperature ³¹P{¹H} spectrum shows a sharp doublet at 32.3 ppm (²J_PF = 75 Hz) as expected for the monofluoride complex, while the room temperature ¹⁹F{¹H} spectrum shows a sharp triplet at −107.2 ppm (²J_PF = 75 Hz) as well as two broad triflate resonances at −77.6 and −78.1 ppm. The broad triflate peaks suggest that there is a triflate exchange process occurring in solution at this temperature (Table 1).

Table 1 Selected multinuclear data for the complexes [GeF_4−n(PMe₃)₂(OTf)_n] for n = 0, 1, 2, 3

Complex	δ ^31P{^1H}/ppm	δ ^19F{^1H}^a/ppm	^2 J(^31P–^19F)/Hz	^2 J(^19F–^19F)/Hz
a Excluding the triflate resonances.
trans-[GeF4(PMe3)2]^13 (240 K)	−12.4 (quin)	−96.9 (t)	196	—
[GeF3(PMe3)2(OTf)] (183 K)	5.2 (dt)	−85.9 (td), −123.4 (tt)	192, 134	41
[GeF2(PMe3)2(OTf)2] (233 K)	27.4 (t)	−122.3 (t)	83	—
[GeF(PMe3)2(OTf)3] (298 K)	32.3 (d)	−107 (t)	75	—

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For the phosphine complexes in Table 2, a decrease in both the Ge–P and Ge–F bond distances is observed as the fluorides are replaced by triflates, consistent with an increase in positive charge and Lewis acidity at the germanium centre. This picture is also supported by the increase in the average C–P–C bond angle of the phosphine ligand going down the series, with the bond angle in the bis-triflate complex being almost 10° larger than in the phosphine itself and almost 3° larger than in [GeF₄(PMe₃)₂]. The same trends are seen with the germanium chloride arsine complexes. Scheme 1 summarises the chemistry of the monophosphine complexes with TMSOTf.

Table 2 Selected geometric parameters for complexes of the form [GeF_4−n(PMe₃)₂(OTf)_n] for n = 0, 1, 2 and [GeCl₄(AsEt₃)₂] and [GeCl₂(AsEt₃)₂][OTf]₂

Complex	d(Ge–E) (E = P or As)/Å	d(Ge–X) (X = F or Cl)/Å	E–Ge–E angle/°	Range of C–E–C angles^a/°
a C–P–C angle of free PMe3 = 99.46°, C–As–C angle of free AsEt3 = 98.50°.
[GeF4(PMe3)2]	2.3717(6)	1.8158(13), 1.8240(13)	180.0	105.83(13)–106.93(11)
[GeF3(PMe3)2(OTf)]	2.3655(3), 2.3546(3)	1.8015(7)(cis), 1.8149(7) (cis), 1.7753(7) (trans)	171.122(11)	106.27(6)–109.00(6)
[GeF2(PMe3)2(OTf)2]	2.3074(12), 2.3100(12)	1.747(3), 1.748(3)	150.29(5)	107.0(2)–110.9(3)
[GeCl4(AsEt3)2]^13	2.4904(9)	2.3233(19), 2.3296(19)	180.0	105.1(4)–106.4(4)
[GeCl2(AsEt3)2][OTf]2	2.4048(4), 2.4169(4)	2.1453(7), 2.1602(7)	125.186(16)	105.79(12)–113.81(12)

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Reactivity of GeF₄ complexes bearing bi- and multi-dentate phosphines with TMSOTf

Scheme 3 summarises the reaction chemistry involving [GeF₄(diphosphine)] with TMSOTf.

Scheme 3 Reactions involving [GeF₄(diphosphine)] with TMSOTf in this work.

The reaction of [GeF₄{o-C₆H₄(PMe₂)₂}] with one equivalent of TMSOTf in CH₂Cl₂ leads to the formation of [GeF₃{o-C₆H₄(PMe₂)₂}(OTf)], with a small high frequency shift for the Me resonance in the ¹H NMR spectrum. With one triflate bound, two isomers are possible, with mer or fac fluorines (Fig. 11).

Fig. 11 The two isomers possible for [GeF₃{o-C₆H₄(PMe₂)₂}(OTf)] with the triflate coordinated.

The ¹⁹F{¹H} NMR spectrum of this complex at 183 K (Fig. 12) has a resonance at −109.5 ppm, which appears as a doublet of doublet of doublets with two different ²J_PF couplings and one ²J_FF coupling, as well as a triplet of triplets at −123.7 ppm with one ²J_PF coupling and one ²J_FF. In the ³¹P{¹H} NMR spectrum (183 K) there is a resonance at −23.1 ppm, which is a doublet of doublet of doublets with three distinct ²J_PF couplings, indicating that there is only one phosphorus chemical environment. These data are consistent with the fac isomer being present at this temperature.

Fig. 12 ¹⁹F{¹H} NMR spectrum of [GeF₃{o-C₆H₄(PMe₂)₂}(OTf)] at 183 K (CH₂Cl₂).

A CH₂Cl₂ solution of this complex layered with hexane deposited some crystals identified as [Ge{o-C₆H₄(PMe₂)₂}(OTf)][OTf], which is clearly not representative of the bulk product. The reduction from Ge(IV) to Ge(II) with concomitant complete defluorination was also seen in the PMe₃ system (above). While in that case the doublet due to [FPMe₃]⁺ was observed, the corresponding fluorophosphorane derived from o-C₆H₄(PMe₃)₂ is not known, and no obvious by-product of this type was evident. However, the reductive defluorination is also assumed to proceed via fluorination of the diphosphine, which may then undergo further reaction. The structure of [Ge{o-C₆H₄(PMe₂)₂}(OTf)][OTf] shows (Fig. S1, ESI†) one of the triflates coordinated to the Ge(II) centre, d(Ge–OTf) = 2.0968(15) Å. There are two longer contacts to the weakly interacting triflates at 2.6438(17) Å and 2.971 Å, completing an approximately five-coordinate geometry around the germanium, the complex exists as weakly associated dimers in the solid state. The complex has a similar geometry to the previously reported complex [GeCl(Me₂PCH₂CH₂PMe₂)][OTf], however the structural data in this case are not available so more detailed comparisons cannot be made.²¹

Using two equiv. of TMSOTf to [GeF₄{o-C₆H₄(PMe₂)₂}] in CH₂Cl₂ led to the formation of [GeF₂{o-C₆H₄(PMe₂)₂}(OTf)₂]. Here the methyl ¹H NMR resonance at 2.03 ppm, is to high frequency of both the tetrafluoride and trifluoride derivatives. In this difluoride complex three possible isomers exist if the triflates are bound (Fig. 13).

Fig. 13 The three possible isomers of (six-coordinate) [GeF₂{o-C₆H₄(PMe₂)₂}(OTf)₂].

The room temperature ¹⁹F{¹H} NMR spectrum shows a resonance at −100.5 ppm as doublet of doublets, correspondingly in the room temperature ³¹P{¹H} NMR spectrum there is a doublet of doublets at −23.2 ppm, which both have the same coupling constants (²J_PF = 121, 74 Hz). This is consistent with the 3^rd isomer (rhs) being present in solution at room temperature.

Crystals from this product grown from a CH₂Cl₂ solution of the complex layered with hexane were indeed shown to be [GeF₂{o-C₆H₄(PMe₂)₂}(OTf)₂] and the structure is shown in Fig. 14(a). In the structure the triflates are trans to each other and the fluorines are cis, consistent with the NMR data.

Fig. 14 (a) The structure of [GeF₂{o-C₆H₄(PMe₂)₂}(OTf)₂] showing the atom labelling scheme. The ellipsoids are draw at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Ge1–P1 = 2.4033(16), Ge1–P2 = 2.3814(17) Ge1–F1 = 1.751(4), Ge1–F2 = 1.776(4), Ge1–O1 = 1.949(5), Ge1–O2 = 2.018(5), P1–Ge1–P2 = 86.33(6), O1–Ge1–O2 = 172.4(2), F1–Ge1–F2 = 92.7(2); (b) the structure of [GeF{o-C₆H₄(PMe₂)₂}(OTf)₃] showing the atom labelling scheme. There are two [GeF{o-C₆H₄(PMe₂)₂}(OTf)₃] in the asymmetric unit; only one is shown. The ellipsoids are drawn at the 50% probability level and H atoms and a CH₂Cl₂ solvent molecule are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Ge1–P1 = 2.3900(9), Ge1–P2 = 2.3840(9) Ge1–F1 = 1.7681(18), Ge1–O1 = 1.948(2), Ge1–O2 = 1.939(2), Ge1–O3 = 1.917(2), P1–Ge1–P2 = 86.54(3), O2–Ge1–O3 = 83.67(10), O1–Ge1–F1 = 176.27(9).

Addition of three equiv. of TMSOTf to [GeF₄{o-C₆H₄(PMe₂)₂}] in CH₂Cl₂ leads to the formation of [GeF{o-C₆H₄(PMe₂)₂}(OTf)₃]. There are two possible isomers in the case where the triflates are bound (Fig. 15).

Fig. 15 The two possible isomers of (six-coordinate) [GeF{o-C₆H₄(PMe₂)₂}(OTf)₃].

The room temperature ¹⁹F{¹H} NMR spectrum shows a triplet at −94.1 ppm, while in the ³¹P{¹H} NMR spectrum a doublet at −13.5 ppm with a coupling constant of ²J_PF = 73 Hz is evident, consistent with the fac isomer in solution. A CH₂Cl₂ solution of the complex layered with hexane deposited crystals and X-ray structure analysis (Fig. 14(b)) confirmed the fac geometry is maintained in the solid state.

The reaction of [GeF₄{Ph₂P(CH₂)₂PPh₂}] with TMSOTf in CH₂Cl₂ leads to [GeF₃{Ph₂P(CH₂)₂PPh₂}(OTf)]. Crystals of this compound were grown by layering a CH₂Cl₂ solution of the complex with hexane, and the structure is shown in Fig. 16(a).

Fig. 16 (a) The structure of [GeF₃{Ph₂P(CH₂)₂PPh₂}(OTf)] showing the atom labelling scheme. The ellipsoids are draw at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Ge1–P1 = 2.4294(10), Ge1–P2 = 2.4321(11), Ge1–F1 = 1.791(2), Ge1–F2 = 1.762(2), Ge1–F3 = 1.775(2), Ge1–O1 = 1.965(3), P1–Ge1–P2 = 85.49(3), F1–Ge1–F3 = 173.69(10), P1–Ge1–O1 = 177.73(8), P2–Ge1–F2 = 173.04(8); (b) the structure of [GeF₂{Ph₂P(CH₂)₂PPh₂}(OTf)₂] showing the atom labelling scheme. The ellipsoids are draw at the 50% probability level and H atoms and a CH₂Cl₂ solvent molecule are omitted for clarity. Selected bond lengths (Å) and angles (°) are: Ge1–P1 = 2.4217(6), Ge1–F1 = 1.7641(11), Ge1–O1 = 1.934(7), P1–Ge1–P1 = 86.25(2), F1–Ge1–F1 = 91.43(8), O1–Ge1–O1 = 172.3(5).

In the structure of [GeF₃{Ph₂P(CH₂)₂PPh₂}(OTf)] the diphosphine ligand is chelating and the fluorines are mer, this is in contrast to [GeF₃{o-C₆H₄(PMe₂)₂}(OTf)] where the NMR data suggests a fac arrangement. It is likely that the different electronic and steric properties of the ligands dictate the preferred isomer.

From the same batch of crystals, a crystal of [GeF₂{Ph₂P(CH₂)₂PPh₂}(OTf)₂] was also identified. The structure of this complex is shown in Fig. 16 (b). The geometry is analogous to the o-C₆H₄(PMe₂)₂ complex with both fluorides trans to the phosphine ligand and the triflates mutually trans.

As with monodentate phosphines, there is a decrease in d(Ge–P) and an increase in the P–Ge–P bond angles as fluoride is replaced by triflate, while the Ge–F distances remain largely unaffected (Table 3). The d(Ge–O) distances are much shorter for the diphosphine complexes compared to the monophosphine complexes.

Table 3 Selected structural parameters for the Ge(IV) diphosphine complexes

Complex	d(Ge–P)/Å	d(Ge–F)/Å	P–Ge–P angle/°	d(Ge–O)/Å
[GeF4{o-C6H4(PMe2)2}] ^13	2.4273(12)	1.815(2)	85.61(4)	—
	2.4273(11)	1.809(2) (cis)
		1.765(2)
		1.772(2) (trans)
[GeF2{o-C6H4(PMe2)2}(OTf)2]	2.4033(16)	1.751(4)	86.33(6)	1.949(5)
	2.3814(17)	1.776(4)		2.018(5)
[GeF{o-C6H4(PMe2)2}(OTf)3]	2.3900(9)	1.7681(18)	86.54(3)	1.948(2)
	2.3840(9)			1.939(2)
				1.917(2)
[GeF4{Ph2P(CH2)2PPh2}] ^13	2.4636(7)	1.7692(14) (trans)	84.08(2)	—
	2.4822(7)	1.7731(14)
		1.7829(13) (cis)
		1.7987(14)
[GeF3{Ph2P(CH2)2PPh2}(OTf)]	2.4294(10)	1.791(2)	85.49(3)	1.965(3)
	2.4321(11)	1.775(2) (cis)
		1.762(2) (trans)
[GeF2{Ph2P(CH2)2PPh2}(OTf)2]	2.4217(6)	1.7641(11)	86.25(2)	1.934(7)

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The complexes involving the Ph₂P(CH₂)₂PPh₂ ligand were very poorly soluble, hindering acquisition of solution NMR data. Finally, attempts to increase the number of phosphine donor groups at Ge(IV) through fluoride abstraction led to mixtures of species (from the NMR data) for both the triphosphine and tetraphosphine complexes, along with evidence of reduction to Ge(II). Hence this was not pursued further.

Experimental

The syntheses were carried out using standard Schlenk and vacuum line techniques, with samples 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. Germanium tetrafluoride was obtained from Fluorochem and GeCl₄ from Acros Organics and were used as received. Phosphine ligands and AsEt₃ were obtained from Sigma-Aldrich or Strem and used as received, except for o-C₆H₄(PMe₂)₂ which was made by the literature route.²² CH₂Cl₂ and MeCN were dried by distillation from CaH₂ and n-hexane from sodium wire. [GeF₄(PMe₃)₂], [GeF₄{o-C₆H₄(PMe₃)₂}] and [GeF₄{Ph₂P(CH₂)₂PPh₂}] were made as described.¹³

Infrared spectra were recorded as Nujol mulls between CsI plates using a PerkinElmer Spectrum 100 spectrometer over the range 4000–200 cm⁻¹. ¹H, ¹⁹F{¹H} and ³¹P{¹H} NMR spectra were recorded from CH₂Cl₂/CD₂Cl₂ solutions unless otherwise stated, using a Bruker AV400 spectrometer and are referenced to TMS via the residual solvent resonance, CFCl₃, and 85% H₃PO₄ respectively. Microanalyses were undertaken by London Metropolitan University or Medac.

[GeF₄(PⁱPr₃)₂]

To a suspension of [GeF₄(MeCN)₂] in CH₂Cl₂ (5 mL) two equiv. of PⁱPr₃ was added as a solution in CH₂Cl₂ (2 mL) and the reaction mixture stirred for 2 h to yield a clear colourless solution. The volatiles were removed in vacuo to leave a white solid which was washed with hexane (3 × 10 mL) and dried in vacuo. Yield: 0.941 g (76%). Satisfactory analytical data could not be obtained despite repeated attempts on different samples, with the elemental compositions varying from sample to sample. ¹H NMR (CD₂Cl₂, 298 K): δ = 1.33 (dd, ³J_HH = 15 Hz, ³J_PH = 7 Hz, [6H]), 2.38 (m, [1H]), ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = –65.0 (s); (183 K): δ = −62.9 (²J_PF = 162 Hz) ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 24.6 (s); (183 K): δ = 24.9 (quint, ²J_PF = 162 Hz). IR (Nujol/cm⁻¹): ν = 590s (Ge–F).

[GeF₃(PMe₃)₂(OTf)]

To a solution of [GeF₄(PMe₃)₂] (0.100 g, 0.33 mmol) in CH₂Cl₂ (2 mL), a solution of TMSOTf (0.074 g, 0.33 mmol) in CH₂Cl₂ (2 mL) was added dropwise to form a clear solution. The reaction was stirred for 2 h, when the volatiles were removed in vacuo leaving a solid that was washed with hexane (3 × 10 mL) and dried in vacuo to form a white powder. Yield: 0.080 g (56%). Required for C₇H₁₈F₆GeO₃P₂S (430.86): C, 19.5; H, 4.2. Found: C, 19.6; H, 4.3%. ¹H NMR (CD₂Cl₂, 298 K): δ = 1.68 (d, ²J_HP = 13.2 Hz). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −78.76 (s, [3F], OTf), −94.74 (br, [3F]); (183 K): δ = –78.9 (s, [3F], OTf), −85.9 (td, [2F], ²J_PF(cis-OTf) = 192 Hz, ²J_FF = 41 Hz, GeF), −123.4 (tt, [F], ²J_{PF(trans-OTf)} = 134 Hz, ²J_FF = 41 Hz, GeF). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 5.2 (br s); (183 K): δ = 5.2 (td, ²J_PF(cis-OTf) = 192 Hz, ²J_{PF(trans-OTf)} = 134 Hz). IR (Nujol/cm⁻¹): 572 (br, m), 515 (m) Ge–F.

[GeF₂(PMe₃)₂(OTf)₂]

Method as above using [GeF₄(PMe₃)₂] (0.100 g, 0.33 mmol) and TMSOTf (0.148 g, 0.66 mmol). White solid. Yield: 0.140 g (75%). Required for C₈H₁₈F₈GeO₆P₂S₂ (560.63): C, 17.1; H, 3.2. Found: C, 17.0; H, 3.3%. ¹H NMR (CD₂Cl₂, 298 K): 1.96 (m); (233 K): δ = 1.98 (d, ²J_HP = 12 Hz). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −78.3 (s, [6F], OTf), −119.5 (br s, [2F], GeF); (233 K): δ = –78.3 (br, [6F], OTf), −122.3 (t, [2F], ²J_PF = 83 Hz, GeF). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 25.8 (br); (233 K): δ = 27.8 (t, ²J_PF = 83 Hz). IR (Nujol/cm⁻¹): ν = 573 (br m) (Ge–F).

[GeF(PMe₃)₂(OTf)₃]

Method as above using [GeF₄(PMe₃)₂] (0.100 g, 0.33 mmol) and TMSOTf (0.222 g, 1.00 mmol). White solid. Yield: 0.175 g (76%). Required for C₉H₁₈F₁₀GeO₉P₂S₃ (691.00): C, 15.6; H, 2.6. Found: C, 15.8; H, 2.6%. ¹H NMR (CD₂Cl₂, 298 K): δ = 2.03 (dd, ²J_HP = 13.8 Hz, ⁴J_FH = 1 Hz). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = –77.6 (br s, OTf), −78.1 (br s, OTf, total OTf integral [9F]), −107.2 (t, ²J_PF = 75 Hz, [F], Ge–F). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 32.3 (d, ²J_PF = 75 Hz). IR (Nujol/cm⁻¹): 583 (br, m) Ge–F.

[GeF₃(PⁱPr₃)₂][OTf]

Method as above using [GeF₄(PⁱPr₃)₂] (0.100 g, 0.33 mmol) and TMSOTf (0.074 g, 0.33 mmol). White powder. Yield: 0.188 g (48%). Required for C₁₈H₄₂F₆GeO₃P₂S·1/2CH₂Cl₂ (629.63): C, 35.3; H, 6.9. Found: C, 35.3; H, 6.9%. ¹H NMR (CD₂Cl₂, 298 K): δ = 1.43 (dd, ³J_HH = 16 Hz, ³J_PH = 7 Hz, [6H]), 2.69 (m, [1H]). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = –79.0 (s, [3F], OTf), −56.7 (t, ²J_PF = 153 Hz, [3F], Ge–F). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = 46.8 (q, ²J_PF = 153 Hz). IR (Nujol/cm⁻¹): 572 m (br m) Ge–F.

[GeF₃{o-C₆H₄(PMe₃)₂}(OTf)]

Method as above using [GeF₄{o-C₆H₄(PMe₃)₂}] (0.100 g, 0.288 mmol) and TMSOTf (0.064 g, 0.288 mmol) and stirring for 30 min. White solid. Yield: 0.084 g (61%). Required for C₁₁H₁₆F₆GeO₃P₂S (476.86): C, 27.7; H, 3.4. Found: C, 27.5; H, 3.6%. ¹H NMR (CD₂Cl₂, 298 K): δ = 1.90 (m, [12H], Me), 7.84 (m, [4H], Ar–H). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): −78.3 (s, OTf), −113.3 (br s, GeF); (183 K): δ = −78.3 (s, [3F], [OTf]), −109.5 (ddd, [2F], ²J_PF = 135, 76 Hz; ²J_FF = 43 Hz), −123.7 (tt, [1F], ²J_PF = 94 Hz; ²J_FF = 43 Hz). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = –24.1 (br s); (183 K): δ = –23.1 (ddd, ²J_PF = 135, 94, 76 Hz). IR (Nujol/cm⁻¹): 518 (m), 579 (m), 602 (m) Ge–F.

[GeF₂{o-C₆H₄(PMe₃)₂}(OTf)₂]

Method as above using [GeF₄{o-C₆H₄(PMe₃)₂}] (0.100 g, 0.288 mmol) and TMSOTf (0.128 g, 0.576 mmol) and stirring for 30 min. White solid. Yield: 0.102 g (58%). Required for C₁₂H₁₆F₈GeO₆P₂S₂·CH₂Cl₂ (691.84): C, 22.6; H, 2.6. Found: C, 22.7; H, 3.1%. ¹H NMR (CD₂Cl₂, 298 K): δ = 2.03 (m, [12H], Me), 7.91 (m, [4H], Ar–H). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −77.5 (s, OTf), −78.2 (s, OTf), −100.5 (dd, ²J_PF = 121 Hz, 74 Hz); (183 K): δ = −77.7 (s, OTf), −78.0 (s, OTf), −78.3 (s, OTf), −79.1 (s, OTf), −101.1 (dd, ²J_PF = 126, 72 Hz), ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = –23.2 (dd, ²J_PF = 121 Hz, 74 Hz); (183 K): δ = −19.9 (dd, ²J_PF = 126, 72 Hz) IR (Nujol/cm⁻¹): 637 (m) Ge–F.

[GeF{o-C₆H₄(PMe₃)₂}(OTf)₃]

Method as above using [GeF₄{o-C₆H₄(PMe₃)₂}] (0.100 g, 0.288 mmol) and TMSOTf (0.192 g, 0.864 mmol) and stirring for 30 min., then the solution was layered with hexane (10 mL). After 3 days the product crystallised. Crystals were collected by filtration, washed with hexane (3 × 10 mL), and dried in vacuo. Yield: 0.086 g (40%). Required for C₁₃H₁₆F₁₀GeO₉P₂S₃ (736.98): C, 21.2; H, 2.2. Found: C, 22.1; H, 3.8%. ¹H NMR (CD₂Cl₂, 298 K): δ = 2.03 (m, Me), 2.16 (m, Me), 2.25 (m, Me) (sum of methyl resonances [12H]), 7.97 (m, [4H], Ar–H). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = −77.1 (s, OTf), −77.3 (s, OTf), −77.6 (s, OTf), −78.2 (br, OTf), −81.8 (t, ²J_PF = 82 Hz)⁺, −91.9 (t, ²J_PF = 69 Hz)⁺, −94.0 (t, ²J_PF = 73 Hz, GeF)*. ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = –13.5 (d, ²J_PF = 73 Hz)*, −8.53 (d, ²J_PF = 69 Hz); (* = major species; ⁺ = minor species). IR (Nujol/cm⁻¹): 633 (m) Ge–F.

[GeF₃{Ph₂P(CH₂)₂PPh₂}(OTf)]

Method as above using [GeF₄{Ph₂P(CH₂)₂PPh₂}] (0.100 g, 0.183 mmol) and TMSOTf (0.041 g, 0.184 mmol). White solid. Yield: 0.045 g (36%). Required for C₂₇H₂₄F₆GeO₃P₂S (677.12): C, 47.9; H, 3.6 Found: C, 47.7; H, 3.7%. The product was not sufficiently soluble in CH₂Cl₂ or CD₃NO₂ to obtain reliable solution NMR data. IR (Nujol/cm⁻¹): 519 (m), 526 (m), 573 (m) Ge–F.

[GeF₂{Ph₂P(CH₂)₂PPh₂}(OTf)₂]

Method as above using [GeF₄{Ph₂P(CH₂)₂PPh₂}] (0.100 g, 0.183 mmol) and TMSOTf (0.081 g, 0.364 mmol). White solid. Yield: 0.119 g (81%). Required for C₂₈H₂₄F₈GeO₆P₂S₂·1/2CH₂Cl₂ (849.60): C, 40.3; H, 3.0. Found: C, 39.9; H, 3.0%. The product was not sufficiently soluble in CH₂Cl₂ or CD₃NO₂ to obtain reliable solution NMR data. IR (Nujol/cm⁻¹): 502 (m), 527 (m) Ge–F.

[GeF₄{κ²-CH₃C(CH₂PPh₂)₃}]

To a suspension of [GeF₄(MeCN)₂] (0.200 g, 0.867 mmol) in CH₂Cl₂ (2 mL), CH₃C(CH₂PPh₂)₃ (0.541 g, 0.867 mmol) was added as a solid and the resulting solution is stirred for 2 h forming a clear colourless solution. Volatiles were removed in vacuo to yield a white solid, which was washed with hexane (3 × 10 mL) and dried in vacuo. Yield: 0.413 g (62%) Required for C₄₁H₃₉F₄GeP₃·1/2 CH₂Cl₂ (815.70): C, 61.1; H, 4.9. Found: C, 61.2; H, 5.1%. ¹H NMR (CD₂Cl₂, 298 K): δ = 0.87 (s, [3H] CH₃), 2.69 (br s, [6H], CH₂), 7.28–7.58 (m, ArH); (183 K): δ = 0.73 (s, [3H]), 2.01 (br m, [2H]), 2.79 (br s, [2H]), 2.90 (br s, [2H]), 7.28–7.58 (m, ArH). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = –108.6 (br s, GeF), −72.2 (br s, GeF); (243 K): δ = –74.4 (overlapping multiplets, [2F]), −109.3 (ddt, ²J_PF = 125, 67 Hz, ²J_FF = 62 Hz); (183 K): δ = –77.7 (br m, [2F]), −110.3 (br m, [2F]). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ = –4.0 (br s), −23.5(s); (183 K): δ = –4.5 (tdd, [2P],²J = 142, 125, 67 Hz), −27.9 (s, [1P]). IR (Nujol/cm⁻¹): 517 (br), 603 (br) Ge–F.

[GeF₄{κ²-P(CH₂CH₂PPh₂)₃}]

To a suspension of [GeF₄(MeCN)₂] (0.200 g, 0.867 mmol) in CH₂Cl₂ (2 mL), P(CH₂CH₂PPh₂)₃ (0.581 g, 0.867 mmol) was added as a solid and the resulting solution was stirred for 2 h to give a clear colourless solution, from which volatiles were removed in vacuo to yield a white solid. The resulting solid was washed with hexane (3 × 10 mL) and dried in vacuo. Yield 0.408 (57%). Required for C₄₂H₄₂F₄GeP₄ (819.25): C, 61.6; H, 5.2. Found: C, 62.1; H, 5.5%. ¹H NMR (CD₂Cl₂, 298 K): δ = 2.03 (br s, [6H], CH₂), 2.28 (br s, [6H], CH), 7.38–7.48 (br m, [30H], ArH). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ = –94.8 (br s), −114.2 (br s); (213 K): δ = –80.4 (ddt, ²J_PF = 153, 133 Hz, ²J_FF = 54 Hz, [2F]), −108.4 (ddt, ²J_PF = 205, ²J_FF = 54, 54 Hz, [F]), −114.1 (ddt, ²J_PF = 188 Hz, ²J_FF = 54, 54 Hz, [F]), ³¹P{¹H} NMR (CD₂Cl₂, 183 K): δ = –3.2 (ddtt, [P], ²J_PP = 336, ²J_PF = 188, 153,³J_PP = 35 Hz), −12.4 (d, [2P], ³J_PP = 35 Hz), −17.6 (ddt, [P], ²J_PF = 336, 207, 133 Hz). IR (Nujol/cm⁻¹): 508 (m), 517 (m), 589 (m), 608 (m) Ge–F.

Reaction of GeCl₄ two eq. of AsEt₃ and two eq. of TMSOTf

To a solution of GeCl₄ (0.134 g, 0.625 mmol) in CH₂Cl₂ (2 mL), AsEt₃ (0.200 g, 1.23 mmol) was added as a solution in CH₂Cl₂ (2 mL). To this mixture TMSOTf (0.274 g, 1.23 mmol) in CH₂Cl₂ (2 mL) was added and the reaction mixture was stirred for 1 h, during which the solution remained colourless. The solution was concentrated to 1 mL, layered with hexane (3 mL) and stored at −18 °C. After a few days a colourless crystalline material formed, which was collected by filtration and dried in vacuo to yield a white powder. Yield: 0.261 g. ¹H NMR (CD₂Cl₂, 298 K): indicates a complex mixture of species, which could not be identified with certainty.

X-Ray experimental

Crystals were grown as described above. 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 and OLEX.²³ H atoms bonded to C were placed in calculated positions using the default C–H distance and refined using a riding model. [GeF₂{Ph₂P(CH₂)₂PPh₂}(OTf)₂] contains some substitutional disorder between OTf and F on the axial positions (ratio 85 : 15), which was modelled satisfactorily. The structures of [GeF₂(PMe₃)₂(OTf)₂] and [Ge{o-C₆H₄(PMe₂)₂}(OTf)][OTf] contained disordered triflate which was modelled using split occupancies. Details of the crystallographic parameters are given in Table S1 (ESI†). CCDC reference numbers for the crystallographic information files in cif format are 2113230 [GeF₄(PMe₃)₂], 2113231 [GeF₃(PMe₃)₂(OTf)], 2113232 [GeF₂(PMe₃)₂(OTf)₂], 2113233 [GeF₂{o-C₆H₄(PMe₂)₂}(OTf)₂], 2113234 [GeF{o-C₆H₄(PMe₂)₂}(OTf)₃], 2113235 [GeCl₂(AsEt₃)₂)(OTf)₂], 2113236 [Ge{o-C₆H₄(PMe₂)₂}(OTf)][OTf], 2113237 [GeF₃{Ph₂P(CH₂)₂PPh₂}(OTf)], 2113415 [GeF₄(κ²-CH₃C(CH₂PPh₂)₃}], 2113416 [GeF₂{Ph₂P(CH₂)₂PPh₂}(OTf)₂].†

Conclusions

This work shows that neutral complexes of GeF₄ can be extended to the tri- and tetraphosphine ligands CH₃C(CH₂PPh₂)₂ and P(CH₂CH₂PPh₂)₃, both of which bond in a bidentate κ²-mode only. The treatment of [GeF₄(PMe₃)₂] with n equivalents of TMSOTf leads to formation of the series of complexes, [GeF_4−n(PMe₃)₂][OTf]_n (n = 1, 2, 3), each based on six-coordinate Ge(IV). [GeF₃(PMe₃)₂(OTf)] is unstable in solution, with the Ge(II) complex, [Ge(PMe₃)₃][OTf]₂, crystallising from the solution. The observation of [FPMe₃]⁺ in the NMR spectrum strongly suggests the occurrence of reductive defluorination in solution. Using the bulkier PⁱPr₃ allows formation of [GeF₃(ⁱPr₃P)₂][OTf], whose variable temperature NMR spectra strongly indicate is a triflate salt of the [GeF₃(ⁱPr₃P)₂]⁺ monocation.

[GeF₄{o-C₆H₄(PMe₂)₂}] reacts with n equivalents of TMSOTf to generate the [GeF_4−n{o-C₆H₄(PMe₂)₂}][OTf]_n (n = 1, 2, 3) series, and again the trifluoride, [GeF₃{o-C₆H₄(PMe₂)₂}(OTf)], was shown to be unstable to reductive defluorination in solution, producing [Ge{(o-C₆H₄(PMe₂)₂}(OTf)][OTf], which features a Ge(II) monocation.

The ¹⁹F and ³¹P NMR chemical shifts and couplings and the X-ray structural trends observed with sequential fluoride removal across the series are consistent with an increase in positive charge at germanium as fluoride is replaced with triflate.

Conflicts of interest

The authors have no conflicts to declare.

Acknowledgements

We thank EPSRC for funding via the ADEPT Programme Grant (EP/N035437/1) and a studentship to R. P. K. (EP/N509747/1). We also thank Dr M. E. Light for helpful discussions regarding aspects of the crystallographic data and the Crystallographic Reviewer for very helpful suggestions regarding treatment of some of the disorder observed.

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic parameters for the structures reported (Table S1), the crystal structure of [Ge{o-C₆H₄(PMe₂)₂}(OTf)][OTf] (Fig. S1), together with multinuclear NMR and IR spectra associated with each of the new compounds described (Fig. S2–S14). CCDC 2113230, 2113231, 2113232, 2113233, 2113234, 2113235, 2113236, 2113237, 2113415 and 2113416. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1dt03339e

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