Fluoro-Germanium (IV) Cations with Neutral Co-Ligands—Synthesis, Properties and Comparison with Neutral GeF 4 Adducts

: The reaction of [GeF 4 L 2 ], L = dmso (Me 2 SO), dmf (Me 2 NCHO), py (pyridine), pyNO (pyridine-N-oxide), OPPh 3 , OPMe 3 , with Me 3 SiO 3 SCF 3 (TMSOTf) and monodentate ligands, L, in a 1:1:1 molar ratio in anhydrous CH 2 Cl 2 formed the monocations [GeF 3 L 3 ][OTf]. These rare triﬂuoro-germanium (IV) cations were characterised by microanalysis, IR, 1 H, 19 F{ 1 H} and, where appropriate, 31 P{ 1 H} NMR spectroscopy. The 19 F{ 1 H} NMR data show that in CH 3 NO 2 solution the complexes exist as a mixture of mer and fac isomers, with the mer isomer invariably having the higher abundance. The X-ray structure of mer -[GeF 3 (OPPh 3 ) 3 ][OTf] is also reported. The attempts to remove a second ﬂuoride using a further equivalent of TMSOTf and L were mostly unsuccessful, although a mixture of [GeF 2 (OAsPh 3 ) 4 ][OTf] 2 and [GeF 3 (OAsPh 3 ) 3 ][OTf] was obtained using excess TMSOTf and OAsPh 3 . The reaction of [GeF 4 (MeCN) 2 ] with TMSOTf in CH 2 Cl 2 solution, followed by the addition of 2,2 (cid:48) :6 (cid:48) ,2”-terpyridine (terpy) formed mer -[GeF 3 (terpy)][OTf], whilst a similar reaction with 1,4,7-trimethyl-1,4,7-triazacyclononane (Me 3 -tacn) in MeCN solution produced fac -[GeF 3 (Me 3 -tacn)][OTf]. Dicationic complexes 22+ fragment (Me 4 -cyclen) and 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane (Me 4 -cyclam), which reacted with [GeF 4 (MeCN) 2 ] and two equivalents of TMSOTf to cleanly form the dicationic diﬂuoride salts, cis -[GeF 2 (Me 4 -cyclen)][OTf] 2 and trans -[GeF 2 (Me 4 -cyclam)][OTf] 2 . The 19 F{ 1 H} NMR spectroscopy shows that in CH 3 NO 2 solution there are four stereoisomers present for trans -[GeF 2 (Me 4 -cyclam)][OTf] 2 , whereas the smaller ring-size of Me 4 -cyclen accounts for the formation of only cis -[GeF 2 (Me 4 -cyclen)][OTf], and is conﬁrmed crystallographically. New spectroscopic data are also reported for [GeF 4 (L) 2 ] (L = dmso, dmf and pyNO). Density functional theory calculations were used to probe the effect on the bonding as ﬂuoride ligands were sequentially removed from the germanium centre in the OPMe 3 complexes.


Introduction
Transition-metal halides, especially those from the 3d series, are very frequently used as the metal source for the introduction of the transition-metal ion to a wide range of ligand types, from monodentates to chelates, to form coordination complexes, with easy and often spontaneous displacement of the halides [1]. In contrast, the p-block halides often retain the halides, and formation of cationic species with p-block Lewis acids derived from p-block halides is much less common.
The coordination chemistry of p-block elements containing the heavier halide coligands (Cl, Br or I) has been studied in great detail for over 50 years [1][2][3][4][5]. In marked In the corresponding tin fluoride systems, AlF 3 did not behave as a fluoride abstractor as it is an inert polymer [6], while reactions with Na[BAr F ] did not go to completion [21]. Therefore, we explored using TMSOTf. This has previously proved to be an efficient halide abstractor in group 14 halide chemistry, predominantly with the group 14 tetrachlorides [20,22], and in tin (IV) and germanium (IV) fluoride phosphine systems [16,21]. In the majority of the phosphine cases, the halide abstraction resulted in complexes with weakly coordinated triflate, rather than salts containing genuine cationic species. Examples included [SnF 4-n (PMe 3 ) 2 (OTf) n ] (n = 1-3) [21], [GeF 4−n (PMe 3 ) 2 (OTf) n ] (n = 1-3), and GeF 4−n {o-C 6 H 4 (PMe 2 ) 2 }(OTf) n ] (n = 1-3) [16]. In the case of Sn (IV), the reactions of [SnF 4 L 2 ] (L = dmso, py, pyNO, dmf, OPPh 3 ) with one equivalent each of TMSOTf and L produced [SnF 3 L 3 ]OTf cations, shown by NMR studies to be a mixture of mer and fac isomers in solution [21]. The attempts to remove a further fluoride using a second equivalent of TMSOTf and more L in most cases resulted in a mixture of [SnF 3 L 3 ]OTf and [SnF 2 L 4 ][OTf] 2 , although [SnF 2 (OPPh 3 ) 4 ][OTf] 2 was isolated and shown by an X-ray structure to be the trans isomer in the solid state. The NMR studies showed a mixture of the cis and trans form of this dication present in solution [21].
Here we report attempts to isolate fluoro-germanium (IV) cations with a range of neutral N-and O-donor ligands, including the N 3 -and N 4 -donor macrocyclic ligands. A comparison of the key spectroscopic data for [GeF 4 L 2 ] and [GeF 3 L 3 ] + types with the tin analogues and an exploration of the bonding via DFT calculations are reported, together with the promotion of endocyclic coordination of the tetra-aza macrocycles, yielding germanium difluoride dications.

Results
[GeF 4 L 2 ]: [GeF 4 L 2 ] (L = dmso, dmf, py, pyNO, OPPh 3 , OPMe 3 , OAsPh 3 ) were prepared by the direct reaction of [GeF 4 (MeCN) 2 ] with the ligands. The complexes with py, OPPh 3 , OPMe 3 , OAsPh 3 have been previously described [13,14] and the characterisation data in the present study were consistent with the published data. The X-ray crystal structures of trans-[GeF 4 L 2 ] (L = py, OPPh 3 , OPMe 3 ) and cis-[GeF 4 (FCH 2 CN) 2 ] have also been previously reported [13,14,23]. Their 19 F NMR spectroscopic data are given in Table 1. In solution at low temperatures, the 19 F{ 1 H} NMR data typically show two 1:1 triplets and a singlet indicating the presence of both cis and trans isomers ( Figure 1) [13,14], although the ambient temperature spectra of some are consistent with exchanging systems, (Table 1) and the relative amounts of the isomers vary with the solvent. Full details of the spectra of the new complexes are given in the Experimental section and the SI. The 19  [GeF3L3][OTf]: The general approach to the synthesis of the trifluoro-germanium cations utilised the reaction of [GeF4L2] with one equivalent of TMSOTf in anhydrous CH2Cl2, followed by the addition of a further equivalent of L (Scheme 1). Scheme 1. Synthesis of the complexes produced in this work containing monodentate ligands L or L′.
The structure reveals a near regular octahedral geometry with the d(Ge-F) and d(Ge-O) showing no significant effect of the trans ligands. A comparison with the structure of trans-[GeF4(OPPh3)2] [13] shows that the d(Ge-F) are identical, but the d(Ge-O) is slightly longer in the latter.
The cations were generally poorly soluble in chlorocarbons, and data were mostly obtained from the CH3NO2/CD3NO2 solutions, which have the limitation of a high M.P. (245 K), hence precluding lower temperature studies, but stronger donor solvents were avoided since they tend to displace the neutral ligands. The 19 F{ 1 H} NMR spectra show the presence of both mer and fac isomers with the former producing a doublet [2F] and a triplet [F] and the latter a singlet; usually the mer isomer is the more abundant. Figure 3 shows a typical example. The 19 [13] shows that the d(Ge-F) are identical, but the d(Ge-O) is slightly longer in the latter.
The cations were generally poorly soluble in chlorocarbons, and data were mostly obtained from the CH3NO2/CD3NO2 solutions, which have the limitation of a high M.P. (245 K), hence precluding lower temperature studies, but stronger donor solvents were avoided since they tend to displace the neutral ligands. The 19 F{ 1 H} NMR spectra show the presence of both mer and fac isomers with the former producing a doublet [2F] and a triplet [F] and the latter a singlet; usually the mer isomer is the more abundant. Figure 3 shows a typical example. The 19  [OTf] were obtained from CH 2 Cl 2 solution by slow evaporation and the X-ray structure analysis revealed them to be the mer isomer ( Figure 2).
The structure reveals a near regular octahedral geometry with the d(Ge-F) and d(Ge-O) showing no significant effect of the trans ligands. A comparison with the structure of trans-[GeF 4 (OPPh 3 ) 2 ] [13] shows that the d(Ge-F) are identical, but the d(Ge-O) is slightly longer in the latter.
The cations were generally poorly soluble in chlorocarbons, and data were mostly obtained from the CH 3 NO 2 /CD 3 NO 2 solutions, which have the limitation of a high M.P. (245 K), hence precluding lower temperature studies, but stronger donor solvents were avoided since they tend to displace the neutral ligands. The 19 F{ 1 H} NMR spectra show the presence of both mer and fac isomers with the former producing a doublet [2F] and a triplet [F] and the latter a singlet; usually the mer isomer is the more abundant. Figure 3 shows a typical example. The 19 F{ 1 H} NMR resonances (Table 1) occur in the range of δ = −80 to −155 depending upon the isomer and the neutral ligand present, and overlap with those of [GeF 4 L 2 ], although the δ(F) trans F are always at a higher frequency than the δ(F) trans N/O for a particular complex.  [GeF3(pyNO)3][Otf] appears to be somewhat unstable in CD3NO2 solution, decomposing slowly at room temperature over the period of the NMR acquisition, and the 19 F{ 1 H} NMR spectra usually show some [GeF4(pyNO)2] present (SI Figure S7.2). The [GeF3(OPR3)3][Otf] (R = Me, Ph) complexes were obtained in good yield and were stable in CD3NO2 solution; the mer isomer is the major form in both ( Figure 3). In addition to the 2 JFF coupling, the 19   [GeF3(pyNO)3] [Otf] appears to be somewhat unstable in CD3NO2 solution, decomposing slowly at room temperature over the period of the NMR acquisition, and the 19 F{ 1 H} NMR spectra usually show some [GeF4(pyNO)2] present (SI Figure S7.2). The [GeF3(OPR3)3][Otf] (R = Me, Ph) complexes were obtained in good yield and were stable in CD3NO2 solution; the mer isomer is the major form in both ( Figure 3). In addition to the 2 JFF coupling, the 19   [GeF 3 (pyNO) 3 ] [Otf] appears to be somewhat unstable in CD 3 NO 2 solution, decomposing slowly at room temperature over the period of the NMR acquisition, and the 19 F{ 1 H} NMR spectra usually show some [GeF 4 (pyNO) 2 ] present (SI Figure S7.2). The [GeF 3 (OPR 3 ) 3 ][Otf] (R = Me, Ph) complexes were obtained in good yield and were stable in CD 3 NO 2 solution; the mer isomer is the major form in both ( Figure 3). In addition to the 2 J FF coupling, the 19   The ability of OasPh3 to form [GeF2(OasPh3)4][Otf]2 contrasts with the other ligands (including OPR3, R = Me or Ph) and would indicate that the arsine oxide is a stronger donor towards the fluoro-germanium (IV) centre. A comparison of X-ray crystallographic data on several isostructural transition-metal pnictine oxide complexes showed that M-Oas was shorter than M-OP, which is evidence for the stronger binding of the OasPh3 towards hard acceptors [24][25][26].
[GeF3(L′)][OTf] (L′ = Me3-tacn, terpy): As indicated in the Introduction, the triaza macrocycle Me3-tacn was the only ligand able to directly form a cation upon reaction with GeF4 in anhydrous CH2Cl2, in the "self ionisation" complex [GeF3(Me3-tacn)]2[GeF6] [14]. This complex was insoluble in common solvents, but a crystal fortuitously obtained from the solid after extraction with CH2Cl2 was shown by X-ray structure determination to be [GeF3(Me3-tacn)]Cl, which is the chloride arising from the attack on the solvent by the displaced fluoride ion. The reaction of [GeF4(MeCN)2] with TMSOTf in anhydrous MeCN, followed by the addition of Me3-tacn formed fac-[GeF3(Me3-tacn)]OTf (Scheme 2), which was much more soluble and allowed the solution NMR data for the cation to be obtained. The 19 F{ 1 H} spectrum, which shows a singlet at −151.7 ppm, is consistent with the expected fac geometry. In the earlier study [14], the direct reaction of GeF4 with terpy in CH2Cl2 yielded an insoluble product of the composition [(GeF4)3(terpy)2], and since the IR spectrum of this complex did not show features characteristic of [GeF6] 2− [27], it was suggested to be oligomeric with both bridging and chelating terpy ligands. Here we found that the sequential reaction of [GeF4(MeCN)2] with TMSOTf and terpy (terpy = terpyridine) in CH2Cl2 solution (Scheme 2), afforded white [GeF3(terpy)]OTf, whose 19 2 contrasts with the other ligands (including OPR 3 , R = Me or Ph) and would indicate that the arsine oxide is a stronger donor towards the fluoro-germanium (IV) centre. A comparison of X-ray crystallographic data on several isostructural transition-metal pnictine oxide complexes showed that M-Oas was shorter than M-OP, which is evidence for the stronger binding of the OasPh 3 towards hard acceptors [24][25][26].
[GeF 3 (L )][OTf] (L = Me 3 -tacn, terpy): As indicated in the Introduction, the triaza macrocycle Me 3 -tacn was the only ligand able to directly form a cation upon reaction with GeF 4 in anhydrous CH 2 Cl 2 , in the "self ionisation" complex [GeF 3 (Me 3 -tacn)] 2 [GeF 6 ] [14]. This complex was insoluble in common solvents, but a crystal fortuitously obtained from the solid after extraction with CH 2 Cl 2 was shown by X-ray structure determination to be [GeF 3 (Me 3 -tacn)]Cl, which is the chloride arising from the attack on the solvent by the displaced fluoride ion. The reaction of [GeF 4 (MeCN) 2 ] with TMSOTf in anhydrous MeCN, followed by the addition of Me 3 -tacn formed fac-[GeF 3 (Me 3 -tacn)]OTf (Scheme 2), which was much more soluble and allowed the solution NMR data for the cation to be obtained. The 19 F{ 1 H} spectrum, which shows a singlet at −151.7 ppm, is consistent with the expected fac geometry. In the earlier study [14], the direct reaction of GeF 4 with terpy in CH 2 Cl 2 yielded an insoluble product of the composition [(GeF 4 ) 3 (terpy) 2 ], and since the IR spectrum of this complex did not show features characteristic of [GeF 6 ] 2− [27], it was suggested to be oligomeric with both bridging and chelating terpy ligands. Here we found that the sequential reaction of [GeF 4 (MeCN) 2 ] with TMSOTf and terpy (terpy = terpyridine) in CH 2 Cl 2 solution (Scheme 2), afforded white [GeF 3 (terpy)]OTf, whose 19
Comparing the geometric isomers of [GeF 4 (OPMe 3 ) 2 ], the cis isomer is only very slightly lower in energy by 1.31 kJ/mol (compared to RT = 2.48 kJ/mol) in the gas phase (Table S2). This is consistent with the experimental observation that both isomers are seen in solution, although in this case the position of the equilibrium will be affected by the chosen solvent. For the trifluoro monocations, the mer isomer is slightly more stable than the fac by 3.19 kJ/mol, which is also consistent with the solution state data, which indicate that mer-[GeF 3 (OPMe 3 ) 3 ] + is the more abundant isomer. For the dications cis/trans-[GeF 2 (OPMe 3 ) 4 ] 2+ , the calculations show that the cis isomer is much more stable than the trans isomer (18.50 kJ/mol lower in energy) (although we were unable to isolate these dications). For both isomers of [GeF 4 (OPMe 3 ) 2 ] the HOMO, HOMO-1 and HOMO-2 are combinations of lone pairs based on the fluorine ligands and the oxygens of the OPMe 3 ligand ( Figure S7). The LUMO and LUMO+2 have a Ge-F σ* character with the LUMO+1 being entirely based on the OPMe 3 ligand.
The HOMO, HOMO-1, and HOMO-2 of the geometric isomers of [GeF 3 (OPMe 3 ) 3 ] + are also based on combinations of lone pairs on the F and O atoms. For these complexes the LUMO is mostly Ge-F antibonding and LUMO+1/+2 are mostly ligand-based. For both isomers of [GeF 2 (OPMe 3 ) 4 ] 2+ the HOMO and HOMO-1/-2 are based on the lone pairs of the O and F atoms with the LUMO being mostly Ge-F antibonding and the LUMO+1/+2 mostly ligand-based (Figure 7 and SI).

Materials and Methods
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. TMSOTf was obtained from Sigma-Aldrich and distilled before use. Germanium tetrafluoride was obtained from Fluorochem and used as received. CH2Cl2 and MeCN were dried by distillation from CaH2 and n-hexane from sodium wire. Neutral ligands were obtained

Materials and Methods
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.
Infrared spectra were recorded as Nujol mulls between CsI plates using a PerkinElmer Spectrum 100 spectrometer over the range 4000-200 cm −1 . The 1 H 19 F{ 1 H} and 31 P{ 1 H} NMR spectra were recorded from CH 3 NO 2 /CD 3 NO 2 or CH 2 Cl 2 /CD 2 Cl 2 solutions unless otherwise stated, using a Bruker AV400 spectrometer and are referenced to Me 4 Si (via the residual solvent resonance), CFCl 3 , and 85% H 3 PO 4 , respectively. ESI + mass spectra were obtained in MeCN solution using a Waters Acquity Platform. Microanalyses were undertaken by Medac.

Crystals of mer-[GeF 3 (OPPh 3 ) 3 ][OTf] and cis-[GeF 2 (Me 4 -cyclen)]
[OTf] 2 xCH 3 NO 2 were grown from CH 2 Cl 2 solution and CH 3 NO 2 solution, respectively. Data collection 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 [30][31][32]. H atoms bonded to C were placed in calculated positions using the default C-H distance and refined using a riding model. Analysis of the data for [GeF 2 (Me 4 -cyclen)][OTf] 2 xCH 3 NO 2 revealed an inversion twin with a surprisingly large unit cell in space group P2 1 , with the asymmetric unit containing 14 cations, 28 anions and four CH 3 NO 2 solvent molecules that were resolved, and a further 5.5 CH 3 NO 2 solvent molecules per asymmetric unit were accounted for by solvent masking. There appeared to be no plausible higher symmetry space group and no missed symmetry. While the [GeF 2 (Me 4 -cyclen)] 2+ cations were generally well-defined, some of the OTf groups showed evidence of some rotational disorder, most of which were modelled satisfactorily. Given the very large cell and the inversion twin, while the identity of the complex and the cis octahedral coordination geometry at Ge are not in doubt, detailed comparisons of the geometric parameters are not justified. Details of the crystallographic parameters are given in Table S1. CCDC reference numbers for the crystallographic information files in cif format are 2174295 ([GeF 3 (OPPh 3

[GeF 3 (OAsPh 3 ) 3 )][OTf] and [GeF 2 (OAsPh 3 ) 4 )][OTf] 2 :
TMSOTf (0.089 g, 0.40 mmol) was added to a solution of GeF 4 (MeCN) 2 ] (0.092 g, 0.40 mmol) in CH 2 Cl 2 and the reaction mixture was allowed to stir for 2 h. To this, OAsPh 3 (0.32 g, 1.20 mmol) was then added, and the solution was stirred for 15 h, affording a white precipitate which was separated by filtration, washed in hexane and dried in vacuo. Yield 0.21 g. 1  [GeF 3 (terpy)][OTf]: TMSOTf (0.115 g, 0.52 mmol) was added to a solution of [GeF 4 (MeCN) 2 ] (0.119 g, 0.52 mmol) in CH 2 Cl 2 (10 mL) at room temperature. After stirring for 2 h, terpy (0.120 g, 0.52 mmol) was added, and the reaction mixture was stirred for 15 h. A white solid precipitated, which was separated by filtration, washed with n-hexane (3 × 5 mL) and dried in vacuo. Yield 0.150 g. Despite attempts on different batches both before and after attempted recrystallisation, satisfactory elemental analyses for this compound could not be obtained, most likely due to the very poor solubility of the complex and co-precipitation of inorganic materials with the complex. However, the spectroscopic data are consistent with that expected for the formulation, mer-[GeF 3 19

Conclusions
A series of fluoro-germanium (IV) cations [GeF 3 L 3 ][OTf] with neutral N-and Odonor co-ligands (L = dmso, dmf, pyNO, OPPh 3 , OPMe 3 , py) was prepared and fully characterised. In solution they exist as mixtures of mer and fac isomers, with the mer dominating. The attempts to prepare dications by removal of a further fluoride were only successful for L = OAsPh 3 (partially) and for the two tetra-aza macrocycles.  2 was isolated in a pure form), the similar removal of a second fluoride in the germanium systems did not occur for most of the L investigated. Standard sources [36] suggest that Sn-F and Ge-F bonds differ little in energy (456 and 464 kJ/mol, respectively), and there may be a significant kinetic factor in the germanium case. Such a situation is much more common in transition-metal rather than main-group chemistry, where partially filled d-orbitals and significant interactions of the ligands with the transition-metal d-orbitals can give rise to very significant kinetic barriers. Notably, in the present study the tetra-aza macrocyclic complex syntheses, [GeF 2 L"][OTf] 2 , required some 3 days to go to completion. Further, the ability of OAsPh 3 to form [GeF 2 (OAsPh 3 ) 4 ][OTf] 2 contrasts with OPR 3 , suggesting that OAsPh 3 is a stronger donor towards the fluoro-germanium (IV) centre. This is consistent with crystallographic data on isostructural early transition-metal pnictine oxide complexes, which showed that the M-OAs bond length was shorter than that for M-OP, suggesting stronger binding of the OAsPh 3 towards hard acceptors.
The DFT calculations provided evidence for trends in the stability of the isomers, although it should be remembered that the calculations are for gas-phase ions, and cation/anion interactions, packing effects in the solids, and solvation in solution will significantly affect the stabilities.  Table S1. X-ray crystallographic data; Figure S15. Frontier orbitals of cis/trans-[GeF 4 (OPMe 3 ) 2 ]; Figure S16. Frontier orbitals of fac/mer-[GeF 3 (OPMe 3 ) 3 ] + ; Figure S17. Frontier orbitals of cis/trans-[GeF 2 (OPMe 3 ) 4 ] 2+ . Table S2. The x,y,z coordinates used in the DFT calculations are also included.