Covalent triflates as synthons for silolyl- and germolyl cations

Abstract

The synthesis of 1-silolyl and 1-germolyl triflates from the corresponding chlorides by salt metathesis reaction is reported. These covalent triflates are ideal starting materials for the preparation of ionic silolyl- and germolyl-imidazolium triflates by their reaction with N-heterocyclic carbenes. Similarily, ionic silolyl- and germolyl-oxophosphonium triflates are obtained by substitution of the triflate group by triethylphosphane oxide Et₃PO. The analysis of their ³¹P NMR chemical shifts according to the Gutmann–Beckett method reveal the high Lewis acidity of the underlying silolyl and germolyl cations. Further analysis of structural and NMR parameters of the silolyl- and germolyl-imidazolium and oxophosphonium triflates indicates that these compounds are covalently bonded silole and germole derivatives with insignificant contributions from silolyl- or germolyl cations. Silolyl and germolyl triflates are however synthetic equivalents of these cations and might serve as a source for electrophilic silolyl and germolyl units.

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Introduction

Sila- and germacyclopentadienes 1 (siloles and germoles) found widespread interest due to their favorable photophysical properties.^1–4 Cross hyperconjugation between the ER₂ group and the butadiene part of the heterocycle results in a substantial lowering of the LUMO and an increase of the electron affinity of the group 14 heteroles (tetroles).^5–8 In addition, the discovery of the aggregation induced emission (AIE) effect of aryl-substituted siloles by Tang and coworker provided a new impetus to this field and had significant impact on materials chemistry.^9–11 The interest of our group in silole and germole chemistry was directed to their use as starting materials for new silicon and germanium heterocycles with unprecedented structures and properties.¹² During these investigations, we studied the synthesis and properties of classical reactive intermediates such as the heterolyl radicals 2, heterolyl anions 3 and of more uncommon intermediates such as heterolyl dianions 4 (Fig. 1).^13–16 The missing link in these series of reactive tetrolyl derivates are the corresponding cations 5. Silolyl and germolyl cations 5 are isolobal to boroles^17–21 and alumoles^22–26 and therefore they are potentially antiaromatic species of high Lewis acidity. The Lewis acidity of these species should clearly exceed that of the neutral group 13 heterocyclopentadienes due to their extra positive charge and the expected lowering of their LUMO energy level. In view of the unprecedented reactivity of cationic¹⁶ or electron deficient boroles, such as Piers’ borole 6,^27–31 we were convinced that these tetrolyl cations are rewarding synthetic targets.

Fig. 1 Tetroles, tetrolyl radicals, anions, dianions, and cations and Piers’ borole.

We report here on our attempts to synthesize silolyl and germolyl cations and on our finding that covalent heterolyl triflates are ideal starting materials for the synthesis of ionic heterolyl triflates that allows gauging the Lewis acidity of the underlying cations.

Results and discussion

Initially, we investigated tetrolyl cations [7]⁺ and [8a]⁺ computationally and compared their structural and electronic properties with that of the isoelectronic borole 9 and alumole 10.³² We choose for this DFT study at the M06-2X/def2-TZVP-level a substitution pattern at the heterocyclopentadiene ring that is identical or close to that used in the experimental study.

Structure optimization of these four heterocyclopentadienes gave localized double and single bonds for the butadiene part of the heterole ring (Table 1). The inner-cyclic C–C bond lengths for all four compounds are remarkably similar and differ significantly from those calculated for the silole ([11]²⁻) and germole dianion ([12]²⁻), both with a delocalized 6π-electron system.¹⁵ In agreement with these structural parameters, nucleus independent chemical shift (NICS) calculations, an established computational aromaticity/antiaromaticity test protocol,^33,34 suggest for both cations [7]⁺ and [8a]⁺ as well as for the neutral borole 9 a marked antiaromatic character (NICS(1)^zz = +21 to +25). The alumole 10 is according to the NICS-criteria less antiaromatic. For the dianions [11]²⁻ und [12]²⁻ strongly negative NICS(1)^zz values indicate the aromatic ring current and serve here as example for aromatic heterole derivatives. The calculated shape of frontier orbitals for the heterocyclopentadienes [7]⁺, [8a]⁺, 9 and 10 are all very similar: the HOMO being the non-symmetric combination of the two π–CC bonds and the LUMO being localized mainly at the heteroatom (see Fig. 2 for a representative example). As a result of the positive charge of the silicon and germanium species the LUMO energy levels for the tetrolyl cations are much lower than calculated for the neutral boron and aluminum heterocycles (Table 1). This implies high Lewis acidity for the cations, which is corroborated by very high calculated fluoride ion competition (FIC) energies ([7]⁺: 453 kJ mol⁻¹; [8a]⁺, 353 kJ mol⁻¹, see Table 1).^35,36 The FIC energies were calculated using the pair [FBEt₃]⁻/BEt₃ according to Scheme 1 and as cornerstones serve the FIC values of antimony pentafluoride (248 kJ mol⁻¹), of tris-pentafluorophenylborane (BCF, 155 kJ mol⁻¹) and of trimethylsilylium (462 kJ mol⁻¹).

Fig. 2 Surface diagrams of the π-orbitals of germolyl cation [8a]⁺ (calculated at M062X/def2-TZVP for the isolated molecule, isodensity value 0.05).

Scheme 1 Fluoride ion competition [FIC] reaction to evaluate the acidity of different Lewis acids LA.^35,36

Table 1 Computed bond lengths [pm], LUMO energies (eV), nucleus independent chemical shifts (NICS) and fluoride ion competition (FIC) values [kJ mol⁻¹] of heterolyl cations and related species (M06-2X/def2-TZVP)

Cpd	a [pm]	b [pm]	c [pm]	NICS (1)^zz^a	E(LUMO) [eV]	FIC^b [kJ mol^−1]
a NICS(1)^zz: Eigenvector of the NICS calculated for a point 100 pm above the center of the five-membered ring that is orthogonal to the ring plane. b FIC values and LUMO energy levels are calculated using the SCIPCM method to include the influence of the solvent benzene.
7 ^+	182	136	155	+21	−4.36	435
8a ^+	192	136	155	+21	−4.38	353
9	158	135	153	+25	−1.65	77
10	194	135	154	+12	−1.29	152
11 ^2−	186	144	139	−18
12 ^2−	196	143	139	−19

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The computational evaluation of tetrolyl cations [7]⁺ and [8a]⁺ suggests that these cations are antiaromatic similar to the isoelectronic boroles. Due to their positive charge the LUMO energy level is however significantly lowered, which results in a very high predicted Lewis acidity.

In our hands, the established protocols for tetryl cation synthesis were unsuccessful for the preparation of silolyl and germolyl cations.³⁷ That is, the reaction of 1-hydrido-1H-germoles and 1H-siloles with trityl cation in the presence of a weakly coordinating anion (WCA) (Corey reaction, Scheme 2, path a)^38,39 failed in all investigated cases. Either the reaction was very slow (with large aryl groups) or it led to a mixture of non-identified compounds (with small aryl groups). Similarly, the method of Lambert and colleagues to apply an allyl unit as leaving group using triethylsilylbenzenium as electrophile failed (Scheme 2, path b).⁴⁰ The salt metathesis reaction using silver salts of WCAs is a viable route to germylium ions.^41,42 We observed, however, no reaction of a 1-H-germole chloride with silver salts of two different WCAs (Scheme 2, path c). Even in the presence of external donors with the potential to stabilize the incipient cation, the ionization of a 1-H-germole was not successful (Scheme 2, path d).

Scheme 2 Attempted synthesis of silolyl and germolyl cations.

In contrast to silver salts of WCAs (Scheme 2, path c), silver triflate reacts in toluene at room temperature with 1H-germolyl and 1H-silolyl chlorides, 13 and 14, to give the corresponding heterolyl triflates 15 and 16 in high isolated yields (74–92%, Scheme 3). While there is no reaction of chlorides 1 and 2 with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene (ImMe₄), the corresponding triflates 15 and 16 undergo a clean substitution reaction to give the germolyl- and silolylimidazolium triflates 17[OTf], 18[OTf] in almost quantitative yields (see Scheme 3).

Scheme 3 Synthesis of tetrolyl triflates 15, 16, [17]–[20]OTf.

The newly synthesized compounds were characterized by multinuclear NMR spectroscopy and X-ray diffraction (XRD) analysis. The ¹³C NMR chemical shifts of the ring carbon atoms C¹–C⁴ of triflates 15, 16, 17[OTf] and 18[OTf] are unremarkable and differ only slightly from those of the corresponding chlorides (see Table 2). It is worth mentioning that the calculated ¹³C NMR chemical shifts for DFT-optimized molecular structures of the investigated molecules and cations differ only slightly (Δδ¹³C^theo = 0 to −5, Table 2). Compared to other silyltriflates (e.g. Me₃SiOTf, δ²⁹Si = 43.7),³⁷ the ²⁹Si NMR resonance of the covalent silolyl triflate 16 is high-field shifted and matches almost that of silolyl chloride 14. The silicon nuclei of the ionic imidazolium triflate 18 is even stronger shielded (by Δδ²⁹Si = 22.8 compared to the triflate 16) and indicates tetracoordination for the silole-silicon atom (see Table 2). The strong coordination of the carbene to the tetrole ring finds additional support by the high-field ¹³C NMR resonance of the carbene carbon atom of the ImMe₄ substituent in triflates 17[OTf] and 18[OTf]. The ¹³C NMR signals are shifted from δ¹³C = 213.7 for ImMe₄ to a region that is typically found for silyl imidazolium ions (δ¹³C = 144.2–146.4 ([17]⁺, [18]⁺) vs. δ¹³C = 145 ([21]⁺).^43–47 The resonance frequencies of the ¹⁵N nuclei vary only slightly compared to those of the free carbene ([17]⁺, [18]⁺: δ¹⁵N = 180.9–184.9, ImMe₄: δ¹⁵N = 177.5).⁴⁸ Interestingly, the ¹³C NMR chemical shift of the CF₃ group of the covalently bound triflate group in 15 and 16 differs by Δδ¹³C = 3 from those of the same group in the ionic imidazolium triflates 17[OTf] and 18[OTf]. Although the difference is small, it is reproducible and significant (Table 2). Given that the salt-like composition of 17[OTf] and 18[OTf] is demonstrated by its solid state structure (see below), this ¹³C NMR chemical shift difference to the covalently bound sulfonylesters 15 and 16 indicate its ionic nature also in benzene solution. To summarize, our accumulated NMR data give no indication of a tetrolyl cation character of the investigated compounds. In particular, their experimental NMR data differs significantly from those calculated for germolyl cations [8]⁺ and silolyl cations [7]⁺ (see Table 2).

Table 2 Experimental NMR data of silole and germole derivatives 13–16 and triflates 17[OTf]–20[OTf]. Calculated NMR chemical shifts (in parenthesis, italic, at M06L/def2tzvp//M06-2X/def2-tzvp)

Cpd	δ ^13C(1/4)	δ ^13C(2/3)	δ ^29Si	Others^a
a ^19F NMR chemical shifts are given relative to δ^19F(CFCl3) = 0; ^15N NMR chemical shifts are given relative to δ^15N(H3CNO2) = 379.9.
13a	136.8	163.9
13b	142.7	159.9
14	140.0	171.6	17.9
15a	131.5 (128)	167.4 (165)		δ ^13C(CF3) = 119.8
15b	136.2 (133)	164.4 (162)		δ ^13C(CF3) = 119.6
16	135.4	175.5	16.5	δ ^13C(CF3) = 119.3
17a[OTf]	137.4 (130)	166.5 (167)		δ ^13C(NCN) = 144.1; δ^13C(CF3) = 122.1; δ^15N = 182.3
17b[OTf]	140.8 (135)	165.0 (165)		δ ^13C(NCN) = 148.3; δ^13C(CF3) = 122.6; δ^15N = 180.9
18[OTf]	140.0	175.1	−6.3	δ ^13C(NCN) = 141.9; δ^13C(CF3) = 122.2; δ^15N = 184.9
19a[OTf]	131.3 (128)	169.0 (168)		δ ^13C(CF3) = 122.3; δ^31P = 93.2
19b[OTf]	137.1 (133)	166.2 (164)		δ ^13C(CF3) n.d.; δ^31P = 92.7
20[OTf]	136.1	175.1	9.6 ^2J(SiP) = 15 Hz	δ ^13C(CF3) = 121.8; δ^31P = 98.9
[8a]^+	(119)	(181)
[7]^+	(112)	(188)	(228)

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The solid-state structures of compounds 15, 16 and 17[OTf], 18[OTf] could be resolved and their structural molecular parameters are given in the ESI.†Fig. 3 shows the molecular structures of the pentamethylphenyl substituted germolyl species 15b and [17b]⁺ and provides structural parameters that are representative for this class of compound. The central tetrolyl units of compounds 15, 16 and 17[OTf], 18[OTf] closely resembles each other. All are almost planar, and the butadiene unit shows the expected bond length alternation (C¹=C²/C³=C⁴: 135–136 pm; C²–C³: 151–153 pm). The inner cyclic Ge–C¹/C⁴ bond lengths in the germole derivatives 15 and [17]⁺ (193–196 pm) are at the shorter end of the typical range of Ge–C bonds (195–200 pm).⁴⁹ Similarly, the Si–C¹/C⁴ bonds in siloles 16 and [18]⁺ (185–187 pm) are slightly shorter than the standard value for Si–C(sp²) bonds of 187.8 pm.⁵⁰ The E–O bonds of the covalent triflates 15 and 16 are longer than typical Ge–O (175–185 pm) or Si–O (162.9 pm) bonds.^49,50 Finally, the E–C^carb are in the typical range for NHC–carbon–germanium or –silicon bonds (200–205 pm).^51–53 The coordination environment of the silicon and the germanium atoms in 15, 16, [17]⁺, and [18]⁺ is tetrahedral, even the pentamethylphenyl-substituted germanium derivatives 15b and [17b]⁺ show no pronounced trigonal flattening. Taking the sum of the bond angles between the germanium atom and its three carbon substituents ∑α(GeC₃) as an indicator,^54,55 it is obvious that neither the triflate 15b nor the germolylimidazolium cation [17b]⁺ shows significant germylium ion-like character. Both their ∑α(GeC₃) values are far from 360°, the ideal value for germylium ions and are in the typical range of tetracoordinated germanium compounds (Fig. 4). In summary, also the structural data of triflates 15, 16 and 17[OTf], 18[OTf] give no indication of tetrolyl cation character. This is in agreement with the substantial bond dissociation energy, BDE, that is calculated for the Si–C^carb and Ge–C^carb bonds in [17]⁺ and [18]⁺ (BDE = 327 ([17a]⁺); 282 ([17b]⁺); 350 ([18]⁺) kJ mol⁻¹) and which are in the same order than typical bond strength of Ge–C (255 kJ mol⁻¹) and Si–C (301 kJ mol⁻¹) single bonds.^56–58

Fig. 3 Molecular structures (a) of germole triflate 15b in the crystal; (b) of the cation [17b]⁺ in the crystal 17b[OTf]; (c) of the cation [19b]⁺ in the crystal of 19b[OTf]. Thermal ellipsoid presentation at 50% probability, hydrogen atoms and silyl methyl groups are omitted. Pertinent bond lengths [pm] and angles [°]. 15b: Ge–C¹ 193.60(12), Ge–C⁴ 193.92(12), C¹–C² 136.02(15), C²–C³ 151.76(17), C³–C⁴ 135.33(15), Ge–C^ipso 194.62(10), Ge–O 191.23(8), ∑α(GeC₃) = 341.9. [17b]⁺: Ge–C¹ 196.13(25), Ge–C⁴ 194.85(26), C¹–C² 135.31(37), C²–C³ 150.51(35) C³–C⁴ 135.29(37), Ge–C^ipso 196.16(25), Ge–C^carb 200.90(26), ∑α(GeC₃) = 334.8. [19b]⁺: Ge–C¹ 194.39(18), Ge–C⁴ 193.80(19), C¹–C² 135.48(29), C²–C³ 152.58(28) C³–C⁴ 135.47(28), Ge–C^ipso 195.35(18), Ge–O 185.13(15), O–P 154.47(16), α (Ge–O–P) 140.606(97), ∑α(GeC₃) = 337.5.

Fig. 4 Top: The sum of the bond angles of the GeR₃ group, ∑α(GeC₃), as a structural measure for the covalent character of the Ge–Do linkage (Do = donor).^54,55 Bottom: Sequence of ∑α(GeC₃) of representative examples for free germylium ions, donor-stabilized germylium ions and triaryl germanol Ar*₃GeOH (Ar* = 2,6-bis(tert-butoxyphenyl).^42,59,60 The data of the germolyl derivatives 15b, [17b]⁺ and [19b]⁺ are shown for comparison.

Although the synthesis of salts of heterolyl cations [7]⁺ and [8]⁺ was not successful, we were able to determine their Lewis acidity by applying the Gutmann–Beckett protocol.^61,62 The covalent triflates 15 and 16 react readily with triethylphosphane oxide, Et₃PO, at room temperature in benzene to give the germolyl- and silolyl-oxyphosphonium triflates 19a[OTf], 19b[OTf] and 20[OTf] in high isolated yields (68–98%; see Scheme 3). All three oxophosphonium triflates were fully characterized by NMR spectroscopy and XRD analysis. The obtained ³¹P NMR data characterize all three investigated cations as strong Lewis acids (see Table 2). The determined deshielding relative to Et₃PO, Δδ³¹P, indicates that the germolyl cations [8]⁺ (Δδ³¹P = 46.5–47.0) and the silolyl cation [7]⁺ (Δδ³¹P = 52.7) are stronger Lewis acids than the isoelectronic boroles and even more powerful acids than trisarylgermylium and silylium ions (see Fig. 5).^16,20,36 The solid-state structures of 19[OTf] and 20[OTf] confirm their ionic constitution with clearly separated oxophosphonium cations and triflate anions (see ESI†). Overall, the metrics of the heterolyl ring of the cations [19a]⁺, [19b]⁺ and [20]⁺ are very close to those of the covalent triflates 15 and 16 and the imidazolium cations [17]⁺ and [18]⁺. Fig. 3c shows as a representative example the molecular structure of [19b]⁺. Attempts to measure the actual Lewis acidity of the covalent triflates 15 and 16 using the weaker nucleophilic p-fluorobenzonitrile (FBN method) failed due to their missing reactivity against nitriles.⁶³ Although the attempts to dissolve a sample of 15a in THF resulted in the formation of a polymeric material, the covalent triflates 15 and 16 are only poor catalysts in the Diels Alder reaction (DAR) of 2,3-dimethylbutadiene with methylacrylate at room temperature. For example, 5 mol% of 15a as catalyst gives after 4 d at room temperature a conversion of only 13% (see ESI for details†). AlCl₃, the standard catalyst for this type of DAR gives under the same conditions a marginal turn-over of 4%.

Fig. 5 Lewis acidity of germolyl [8]⁺ and silolyl cations [7]⁺ according to the Gutmann–Beckett scale and comparison with related main group element based Lewis acids (Ar* = 3,5-di-t-butylphenyl).^20,36,64,65

Conclusions

Tetrolyl cations [7]⁺ and [8]⁺ are predicted to be antiaromatic compounds with Lewis acidities that are significantly higher than those determined for related boroles and predicted for alumoles. Established protocols for the synthesis of silylium and germylium ions failed in the case of tetrolyl cations [7]⁺ and [8]⁺. In contrast, the covalent tetrolyl triflates 15 and 16 are available in high yields from the corresponding chlorides by simple salt metathesis reactions. These are ideal starting materials for the preparation of ionic tetrolyl derivates. The reaction with the N-heterocylic carbene ImMe₄ gives access to tetrolylimidazolium ions [17]⁺ and [18]⁺. NMR spectroscopic and structural data of the ionic triflates [17]OTf and [18]OTf identifies them as typical imidazolium salts with no tetrolyl cation character. Consistently, the covalent triflates 15, 16 show little activity as Lewis acids in Diels Alder cyclizations. The triflate substituent in 15 and 16 is replaced by triethylphosphane oxide to give the corresponding oxophosphonium salts [19]OTf and [20]OTf. Comparison of their ³¹P NMR chemical shift with that of the free phosphane oxide confirms the high Lewis acidity of the underlying tetrolyl cations [7]⁺ and [8]⁺ (Gutmann–Beckett Lewis acidity scale). From a synthetic perspective, the covalent (15, 16) and the ionic triflates [17]OTf–[20]OTf are useful reagents for the transfer of electrophilic silolyl or germolyl units.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the DFG via the GRK 2226. Computations were done at the HPC Cluster, CARL, University of Oldenburg, funded by the DFG (INST 184/108-1 FUGG) and the Ministry of Science and Culture (MWK) of the Lower Saxony State.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, analytical data, complete XRD data and details of the computational work. CCDC 2170123–2170134. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt01446g

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