Evidence of the Lewis‐Amphoteric Character of Tris(pentafluoroethyl)silanide, [Si(C2F5)3]−

Abstract According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non‐fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C2F5)3]− ion. With these findings and a HOMO–LUMO gap of 4.80 eV similar to N‐heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis‐amphoteric character similar to silylenes. Deprotonation of Si(C2F5)3H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C2F5)3]− anion, which add to benzaldehyde, benzophenone, CS2, and CO2 in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.


Introduction
In keeping with the Lewis definition, amphoteric species possess electron-deficient (Lewis acidic) and electron-rich (Lewis basic) centers.These reactive positions may be located on two different well separated atoms within one molecule.In as pecial class of Lewis-amphoteric systems,t he so-called frustrated Lewis pairs (FLPs), an adduct formation is effectively prevented by sterically demanding substituents, which leads to their inherent capability to activate or incorporate al arge variety of small molecules such as H 2 , CO 2 ,CS 2 ,SO 2 ,NO 2 ,NO, CO,orunsaturated hydrocarbons. [1] Yeta nother class of Lewis-amphoteric species derives from main group IV elements (C,S i, Ge,S n, and Pb) in the oxidation state + II. Here,t he Lewis acidic as well as the Lewis basic position are located at the same center,n amely the divalent tetrel atom. TheHOMO of these species displays asignificant s-orbital character,whereas the LUMO is usually represented by an empty p-orbital. [2] It is noteworthy that stable germylenes,s tannylenes,a nd plumbylenes were long known [3] before reports on stable divalent silicon compounds as Si(h 5 -C 5 Me 5 ) 2 (Jutzi,1986) [4] or the first stable N-heterocyclic silylene [HC-N t Bu] 2 Si (NHSi) ( Denk &W est, 1994). [5] Thel atter species are the higher homologues of N-heterocyclic carbenes (NHCs), the first of which has been presented by Arduengo III in 1991. [6] Denkss eminal discovery stimulated further intense research on silylene chemistry. [7][8][9][10][11] Due to their unique Lewis amphoteric properties,silylenes proved to be versatile and valuable building blocks in organosilicon chemistry,r eacting readily with multiple bonds as present in alkenes,a lkynes,k etones,i mines,a zides,a sw ell as heteroallenes X = C = Y( X, Y = O, S, NR with R = alkyl). Many of these transformations are initiated by side-on additions. [8][9][10][11][12] It is considered that silylenes with larger bite angles and small HOMO-LUMO gaps are more reactive than those with contrary properties. [13] Thus,f or example,a cyclic silylenes (HOMO-LUMO gap ca. 2eV) can even activate H 2 affording silanes, [14] whereas NHSis cannot (HOMO-LUMO gap ca. 5eV). [15] Quantum chemical calculations on the tris(trifluoromethyl)silanide anion, [Si(CF 3 ) 3 ] À ,d isclose aH OMO-LUMO gap of 4.80 eV as well as lower HOMO and LUMO energy levels (À7.06 eV; À2.26 eV) in comparison to the nonfluorinated analogue,[Si(CH 3 ) 3 ] À (À6.88 eV; À2.10 eV) (Figure 1). [16] This is clearly due to the strong electron withdrawing effect of the perfluoroalkyl groups. [17] These computations point to ar educed Lewis basicity and in turn an increased Lewis acidity of the perfluorinated species.

Results and Discussion
While the precursor for the [Si(C 2 F 5 ) 3 ] À ion, the tris(pentafluoroethyl)silane,S i(C 2 F 5 ) 3 H, has been originally synthesized over four steps, [18] an improved and more efficient synthesis is based upon the treatment of SiCl 3 Hw ith three equivalents of in situ generated pentafluoroethyllithium, LiC 2 F 5 ,i nn-dibutyl ether and subsequent isothermic distillation (Scheme 1).
Analogously to Lewis amphoteric silylenes, [26][27][28] the [Si-(C 2 F 5 ) 3 ] À ion undergoes as ide-on addition with carbonyl compounds like benzaldehyde and benzophenone to afford ap hosphazenium salt of the corresponding oxasiliranide anion 1a and 1b (Scheme 3). 29 Si NMR chemical shifts of pentafluoroethyl-substituted silicon compounds nicely correlate with the coordination number of the silicon atom and are thus useful in deducing the coordination number of the silicon atom. Thus,t etracoordinated silanes exhibit chemical shifts in the range of + 10 to À90 ppm. Penta-and hexacoordinated pentafluoroethyl silicon compounds show chemical shifts of À95 to À150 ppm ([SiR 5 ] À )a nd À150 to À200 ppm ([SiR 6 ] 2À ], respectively. [19,20,29] In keeping with this, 1a and 1b feature a 29 Si NMR chemical shift of À127.5 ppm and À122.8 ppm, respectively.T he 13 3 (h 2 -CPh 2 O)] À (1b)crystallizes in the triclinic space group P1 with Z = 4; one of the anions is disordered. 1b displays the geometry of ah ighly distorted square pyramid (t = 0.18 in the non-disordered anion and t = 0.12 in the disordered one) [31] with aC 2 F 5 substituent at the apex ( Figure 3). Thes tructural motif of at hree-membered SiÀOÀ Ch eterocycle is quite familiar from the side-on addition of silylenes to carbonyl compounds. [26][27][28] TheS i1ÀO1 and the C26ÀO1 bond in the three-membered ring of 1b are comparable to those of reported penta-and tetracoordinated oxasiliranes.The Si1ÀC26 bond of 1b (192.6(3) pm) is slightly longer than in other penta-(185.0-189.2 pm) [26][27][28] and tetracoordinated (184.9 pm) [32] oxasiliranes.T ot he best of our knowledge all pentacoordinated three-membered Si À O À Cheterocycles are neutral compounds.C onsequently, 1b represents the first example of as tructurally characterized negatively charged pentacoordinated oxasiliranide of type We further investigated the reaction of the [Si(C 2 F 5 ) 3 ] À ion with CS 2 .W hen [tmgP 1 H][Si(C 2 F 5 ) 3 ]i se xposed to carbon disulfide at low temperatures,aside-on addition to CS 2 takes place.This affords the phosphazenium salt of thiasiliranide 2, which is only stable at temperatures below À20 8 8C (Scheme 4).
A 19 FNMR spectrum of 2 at À20 8 8Cr eveals signals at À79.3 ppm and À118.1 ppm for the corresponding CF 3 and the CF 2 groups.C onsistent with ap entacoordinated silicon atom, 2 features a 29 Si NMR chemical shift of À102.6 ppm.

Research Articles
Since CO 2 is one of the main greenhouse gases known, enormous efforts have been made to capture,s tore,a nd activate CO 2 for the synthesis of value-added products.Apart from transition metal complexes, [39] some silylenes are known to successfully activate CO 2 . [40] Most silylenes form dimeric species like,f or example, IV, [41] whereas only af ew of them lead to compounds like V-VIII with chelating carbonate ligands ( Figure 6). [42] When a[Si(C 2 F 5 ) 3 ] À salt is treated with an excess of CO 2 , the corresponding salt of silicon carbonate 3 is formed (Scheme 5). Thef ormation of CO was monitored by IR spectroscopy of the gas phase.
The 19 FNMR spectrum of 3 reveals signals at À79.6 ppm and À121.0 ppm resulting from the CF 3 and CF 2 groups.I n accordance with apentacoordinated silicon center, 3 features ar esonance at À119.0 ppm in the 29 Si NMR spectrum. The carbonyl carbon atom resonates in the 13 (Figure 7). Silicon carbonate 3 exhibits ah ighly distorted geometry with aslight tendency to atrigonal bipyramid (t = 0.53). TheS i1ÀO2 bond (181.5(2) pm) is about 11 pm longer than the Si1ÀO1 bond (170.1(2) pm). Both SiÀObond lengths are in the range of known silicon carbonates (171.5-180.4 pm). [40,41] TheC 41 = O3 bond length is comparable to the ones in other silicon carbonates,u nderlining its doublebond character.T he sum of the angles about C41 of 359.98 8 confirms ap lanar coordination sphere.T hough some silicon carbonates are known, to the best of our knowledge 3 represents the first structurally characterized negatively charged silicon carbonate of the type [SiR 3 (h 2 -CO 3 )] À with exclusively organic substituents R = aryl, alkyl.

Computational Studies
In order to support our experimental findings,w e performed computational studies at B3LYP/6-31 + G(3d,p) level of theory [16] concerning the mechanism of the side-on addition of the tris(trifluoromethyl)silanide ion, [Si(CF 3 ) 3 ] À , to formaldehyde,b enzaldehyde,b enzophenone,a nd CS 2 as well as of the reaction with CO 2 .T or educe computational cost, electronically similar trifluoromethyl groups instead of pentafluoroethyl groups were used. Fort he same reason we only investigated the addition of formaldehyde in more detail. Fore ach depicted transition state an intrinsic reaction coordinate calculation was performed to ensure that they indeed connect the correct minima. Further details are given in the Supporting Information.
Theaddition of formaldehyde to [Si(CF 3 ) 3 ] À is aconcerted process leading to the product [Si(CF 3 ) 3 (h 2 -CH 2 O)] À . Figure 8 shows adistortion/interaction diagram [43] for the reaction path in relation to the Si-C formaldehyde distance.U nsurprisingly,t he energy required for distortion of the reactants increases with decreasing distance.F or values above 265 pm the interaction energy between the fragments outweighs the energy required for distortion due to long-range interactions of the negatively charged silanide and the dipole of formaldehyde.W elocated aweakly bound complex as aminimum with aSi-C distance of 367 pm and ac hange in electronic energy (DE,zero-point corrected) of À21.3 kJ mol À1 relative to the starting com-  pounds.H owever,t he change in Gibbs free energy (DG, Figure 9) is + 6.0 kJ mol À1 ,i ndicating that this complex has very little influence on the outcome of the reaction. Furthermore,w ep erformed an NBO analysis [44] of the silanide as well as of the transition state.I n[ Si(CF 3 ) 3 ] À the highest Lewis-NBO is the lone pair at the silicon atom (occupancy: 1.92) and the lowest non-Lewis-NBOs are antibonding and located at the SiÀCb onds (occupancy: 0.05 each). Thet ransition state is best described as an alcoholate with aS i À C formaldehyde bond. However,t he occupancyo ft he three lone pairs at the oxygen atom is relatively low (1.62, 1.88, and 1.98), whereas the antibonding Si À C formaldehyde (0.33) is relatively highly occupied due to an extremely large stabilization value from delocalization of one lone pair into this antibonding SiÀCNBO (256.6 kJ mol À1 ). Theantibonding Si À C trifluoromethyl NBOs have higher occupancies than the corresponding ones in [Si(CF 3 ) 3 ] À (anti-periplanar to O: 0.15, synclinal to O: 0.08 each). Delocalization of the lone pairs at the oxygen atom into the SiÀCa ntibond in trans position has stabilization values of 7.1 kJ mol À1 and 3.1 kJ mol À1 .
[Si(CH 3 ) 3 (h 2 -CH 2 O)] À is 82.1 kJ mol À1 higher in energy and therefore immaterial to the reaction. Since no transition state between the starting compounds and [Si(CH 3 ) 3 (h 1 -CH 2 O)] À was found, we performed ar elaxed potential energy scan along the SiÀC formaldehyde bond to gain some insight into the reaction path ( Figure 11). Apparently,i ti sahighly exothermic reaction with no activation barrier.
Thea ddition of [Si(CF 3 ) 3 ] À to benzaldehyde,b enzophenone,a nd CS 2 proceeds analogously to the reaction with formaldehyde.T he reaction parameters are given in Table 1. Thea ctivation barrier of the addition to the bulkier benzaldehyde is higher than of the addition to formaldehyde.T he addition to benzophenone has the highest activation barrier. Since the reaction time was by far the longest, this is consistent with the experimental results.T he DG value is positive,w hich contradicts the experiment, but those values are known to be error-prone.
Contrary to the other reactions,CO 2 does not add side-on in ac oncerted mechanism to [Si(CF 3 ) 3 ] À .S tationary points are shown in Figure 12. Although we located two local energy minima along the reaction pathway,[ Si(CF 3 ) 3 (h 1 -CO 2 )] À and     [a] Note that the reaction was carried out at temperatures below À20 8 8C due to decomposition of the product, but DG is given for room temperature.
[Si(CF 3 ) 3 (h 2 -CO 2 )] À , DG is positive in both cases,s uggesting that these are not the favored products.B ased on the experimental results we studied potential consecutive reactions.One possible reaction starts with the elimination of CO from [Si(CF 3 ) 3 (h 2 -CO 2 )] À followed by the addition of asecond equivalent of CO 2 ( Figure 13)

Conclusion
In this Research Article we reported on high yielding (up to 94 %) syntheses of room-temperature stable [Si(C 2 F 5 ) 3 ] À salts utilizing phosphazenium cations,w hich as weakly coordinating cations stabilize the reactive anion. This allows the structural characterization of the anion under so-called pseudo-gas phase conditions.Most importantly,while exploring the reactivity of the [Si(C 2 F 5 ) 3 ] À ion we disclosed its formal Lewis amphoteric behavior.J ust like silylenes,t he anion displays Lewis basic and Lewis acidic character.This is evident by the side-on additions to benzaldehyde,benzophenone,and CS 2 ,aswell as by the activation of CO 2 .Weisolated and structurally characterized novel negatively charged species like oxasiliranide 1b,t hiasiliranide 2,a nd silicon carbonate 3.T othe best of our knowledge, 1b and 3 represent the first examples of structurally characterized negatively charged hypervalent three-and four-membered heterocycles [R 3 Si(h 2 -CR' 2 O)] À and [R 3 Si(h 2 -CO 3 )] À (R, R ' = alkyl, aryl, H) with organic substituents.Preliminary investigations show that the tris(pentafluoroethyl)silanide catalyzes hydrosilylation of carbonyl compounds like benzaldehyde with triethylsilane via the herein isolated oxasiliranide 1a (Scheme 6). Detailed studies concerning this hydrosilylation and the activation of other small molecules by [Si(C 2 F 5 ) 3 ] À are in progress.

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