Strain‐Modulated Reactivity: An Acidic Silane

Abstract Compounds of main‐group elements such as silicon are attractive candidates for green and inexpensive catalysts. For them to compete with state‐of‐the‐art transition‐metal complexes, new reactivity modes must be unlocked and controlled, which can be achieved through strain. Using a tris(2‐skatyl)methylphosphonium ([TSMPH3]+) scaffold, we prepared the strained cationic silane [TSMPSiH]+. In stark contrast with the generally hydridic Si−H bond character, it is acidic with an experimental pK a DMSO within 4.7–8.1, lower than in phenol, benzoic acid, and the few hydrosilanes with reported pK a values. We show that ring strain significantly contributes to this unusual acidity along with inductive and electrostatic effects. The conjugate base, TSMPSi, activates a THF molecule in the presence of CH‐acids to generate a highly fluxional alkoxysilane via trace amounts of [TSMPSiH]+ functioning as a strain‐release Lewis acid. This reaction involves a formal oxidation‐state change from SiII to SiIV, presenting intriguing similarities with transition‐metal‐mediated processes.


Introduction
Themain-group elements,sometimes also named s-and pblock elements,are adiverse part of the periodic table.T hey are the most prevalent components of the Earthsc rust and have enormous economic,i ndustrial and environmental significance.D espite all that, their catalytic applications are still scarce compared to those of transition metals, [1] although strategies aiming to bridge this gap have recently emerged. In particular,incorporating amain-group element into astrained ring system can unlock unusual reactivity.F or example, forcing non-trigonal geometries at ap hosphorus(III) centre can give it an electrophilic character in addition to the natural nucleophilicity arising from the lone pair. [2] Such biphilic compounds have been shown to engage in unusual bond activation pathways such as (reversible) oxidative addition of E À Hb onds (E = OR, NR 2 ,R u). [3] Both Lewis [4] and Brønsted [5,6] acid-base properties can also be manipulated using strain. In particular,D enmarks "strain-release Lewis acidity" [4] is based on the fact that the angle strain is partially relieved upon binding of anucleophile ( Figure 1A). It has been most extensively studied for silicon [7][8][9] resulting in an umber of highly-enantioselective synthetic protocols (most often C À Cb ond forming) that employ strained silanes as directing groups, [10] but also extends to other elements such as germanium [4] and aluminium. [11] Because of the generally hydridic character of SiÀH bonds,B rønsted SiH-acids are rare in general. Recent examples by Krempner and co-workers [12] and Beckmann and co-workers, [13] shown in Figure 1B,m ostly rely on electronic effects.I nt his paper, we show that ring strain can significantly contribute to the acidic character of aSiÀHbond.
Herein, we report the silanide-silane acid-base pair TSMPSi (1)/[TSMPSiH] + (2), with an unusually low solution pK a DMSO ( Figure 1B). It is experimentally shown to lie between 4.7 and 8.1, which is more acidic than phenol, benzoic acid (pK a DMSO of 18.0 [14] and 11.1, [15] respectively), and other silanes of which the pK a DMSO was reported. We analyse the physicochemical origins of this unusual acidity in terms of inductive and electrostatic effects and confirm its link to ring strain. In addition, the reactivity of both TSMPSi (1)a nd [TSMPSiH] + (2)i si nvestigated. Thei ncrease in strain that generally accompanies quaternization of the silicon atom in 1, together with charge separation effects,r ender it aw eaker nucleophile than typical silicon anions.Moreover, we provide B) Selected acidic silanes [12,13] and the silane discussedi nthis work (acidic protons are shown in bold). Experimental pK a values are given as projected onto the DMSO scale.
[*] S. Tretiakov,Dr. L. Witteman evidence that, under the influence of strain, quaternized 1 can transfer am ethyl group thus engaging into "strain-release methyl transfer". Finally, 2 shows strain-release Lewis acidity that manifests itself in the coordination and activation of aTHF molecule towards attack by weak nucleophiles,such as highly-delocalized aromatic anions.The product of THF ringopening exhibits ah igh degree of fluxionality,w hich is analysed using acombination of spectroscopic and computational tools.
Thestructure of 2 BARF features atetrahedral Si center;the Si-bound Hatom could be located from the difference Fourier maps.Interestingly,every cationic unit of 2 BARF is surrounded by three 1,4-dioxane molecules [18] with SiÀOdistances within 2.7807(13)-3.2260 (16) ,which is shorter than the sum of van der Waals radii (3.62 ), [19] suggesting weak Si···O interactions.The latter are best described as interactions between the O-centered lone pairs and the s-hole opposite the polar Si À N bonds. [20] Thes ilane hydrogen atom shows rather short contacts to the dioxane oxygens,b ut the Si^H^O angles of 82.7(10)-92.0(10)8 8 are unfavourable for hydrogen bonding. Thea bove suggests that the silicon centre has aL ewis acidic character,w hile the Si À Hb ond remains polarized towards hydrogen and is hence not prone to engage in hydrogen bonding.
It is worth pointing out that protonation causes marked geometrical changes around Si ( Figure 2B), wherein the SiÀN  Selected bond distances and angles are given in Section S3. [51] distances shorten from 1.840(2)-1.864 (2)  No other bond in the molecule changes by > 0.02 ,a nd no flat angle deforms by > 2.98 8.I nother words,s tructural perturbations due to proton addition are mostly confined to the SiN 3 fragment. Theobserved changes are consistent with an increased s-character of the Si À Nb onding orbitals as the Si-centred electron pair in 1 acquires higher p-character upon protonation, which is in line with Bentsr ule. [21] Experimental Solution pK a Determination Examining the solid-state structure of TSMPSi (1)aswell as k 3 complexes of Fe II ,N i II ,a nd Cu I with the TMSP scaffold, [16] it becomes apparent that the latter favors N^E^N angles that are close to 908 8.H ence,t he N^Si^N angles of about 1008 8 in [TSMPSiH] + (2)a re expected to generate strain within the cage structure.Inaddition, the 1 J Si,H coupling constant in 2 BARF (318.4 Hz) is significantly larger than that in the electronically similar yet unstrained tris-Npyrrolylsilane (284.5 Hz). [22] Similar increased 1 J C,H coupling constants in strained hydrocarbons [6] have been correlated to ah igh acidity of the corresponding CÀHb onds.T hese considerations,c ombined with the overall positive charge of [TSMPSiH] + (2), led us to anticipate its high SiH-acidity.
To test this,w eu ndertook experimental pK a measurements.O ft he several existing ways to measure pK a in nonaqueous media, [23] we chose abracketing approach:the basic form 1 was dissolved together with acids of known pK a ,a nd proton transfer was monitored using NMR. CH-acidic fluorenes 4 and 5 as well as 2,6-lutidinium-BAr F 4 salt 6 were used as reference acids ( Figure 3). Fluorenes have previously been used to measure the acidity of silanes, [12,24] and their conjugate bases are strongly delocalized anions which are not expected to coordinate to the protonated silicon centre.2 ,6-Lutidine is also unlikely to coordinate due to steric bulk. Because of the intrinsically low kinetic acidity of most CHacids,w ec hose [D 8 ]dioxane as as olvent, reasoning that ah ydrogen bond acceptor could promote fast and efficient proton transfer. It was given preference over the much more common [D 8 ]THF due to its lesser sensitivity to strong acids and lower susceptibility to (catalytic) ring-opening.T ocheck that an acid-base equilibrium is reached, the measurements were repeated starting from 2 BARF and the conjugate bases of the reference acids.
While the acidity of the reference acids that we used is known only in DMSO,c ationic silane 2 cannot exist in this solvent and immediately degrades.H owever,g iven al inear correlation between acidities in different solvents (i.e.t he relative acidity does not change from one solvent to another), [27] one can still perform an acid-base reaction in [D 8 ]dioxane and project the result onto the DMSO acidity scale. [28] Even though the projected pK a does not reflect solvent-specific effects (viz. degradation), it allows for comparison with pK a of other acids most of which are only known in DMSO. [27,30] Silanide 1 is not protonated by fluoradene (4)w hich has pK a DMSO of 10.5. With the stronger CH-acid 5 (pK a DMSO = 8.1), the protonation is also not observed directly.H owever, 5 undergoes base-catalysed isomerization in the presence of TSMPSi (1)(Section S5.1.2), suggesting comparable acidities. Additionally,t he solution becomes pale blue,i ndicating the presence of as mall concentration of the delocalized anion obtained by deprotonating 5.F inally,p rotonation of 1 with 2,6-lutidinium-BAr F 4 (6)i n[ D 8 ]dioxane is complete with release of free 2,6-lutidine.T hus,o ne can conclude that the projected pK a of silane 2 in DMSO lies between 4.7 and 8.1.
Thehigh acidity of cationic silane 2 is remarkable in view of the general hydridic character of silicon-bound hydrogen atoms.Infact, it is more acidic than phenol and benzoic acid (pK a DMSO of 18.0 [14] and 11.1, [15] respectively). It is also considerably more acidic than the few hydrosilanes whose pK a has been measured:t riphenylsilane (pK a THF % 35.1), [31] tris(trimethylsilyl)silane (pK a ether % 29.4), [24] as well as the cationic alkali metal-silane complexes shown in Figure 1B. [12] In the latter series,t he introduction of ac ationic centre into the molecule decreases the pK a in benzene by six to eleven orders of magnitude,suggesting that [TSMPSiH] + (2)owes at least apart of its acidity to the presence of apositive charge in the molecule.I ti sa dditionally worth mentioning that trichlorosilane, [32] tris(pentafluoroethyl)silane, [33] and tris(trichlorosilyl)silane [13] display acidic reactivity in solution, but no experimental pK a has been reported.

Computational Analysis of Acidity of [TSMPSiH] + (2)
I. Factors contributing to the acidity. Theh igh acidity of cationic silane [TSMPSiH] + (2)c an be ascribed to three cumulative effects:t he strong electron-withdrawing effect of indolyl nitrogens,ring strain, and the overall positive charge. Therespective contributions of these effects can, in principle, be roughly estimated by comparing solution acidity of aseries of silanes:S iH 4 ,t ris-N-skatylsilane (A in Figure 4A), the neutral isoelectronic Si-tethered analogue of 2 (B in Figure 4A;see also Section S5.1.3), and 2 itself.Thus,the acidity difference between SiH 4 and A would reflect the electron withdrawal, the difference between A and B would be reflective of strain, and the difference between B and 2 would give information about the influence of charge. Unfortunately,t he solution acidities of SiH 4 , A and B are experimentally unknown. Furthermore,the lack of reference experimental data hampers the development of reliable procedures for calculating these.T his problem can be partly circumvented by taking ad etour via gas-phase pK a s, which can be calculated with high precision by modern quantum Figure 3. Acids used in experimental studies. [25,26] Angewandte Chemie Forschungsartikel 9706 www.angewandte.de chemical methods. [34] There is an empirical linear dependence between gas-phase and solution pK a values within the same class of acids, [35,37,38] which is possible due to the fact that the free energy difference between an acid and ab ase is attenuated by differential solvation effects that are specific for the acid class,e lectric charge and solvent. Such correlations have been documented for phenols, [39] alcohols, [38,40] amines, [41] thiols, [41] and CH-acids. [42,43] In this way,t he gasphase pK a of 177.7 calculated for 2 can be compared to the gas-phase pK a of other cationic silanes described by Krempner and co-workers ( Figure 1B), [12] which we calculate as being 205.5 for M = Kand 197.8 for M = Li (Section S5.1.4). Knowing that pK a DMSO of the latter two silanes is 22.3 and 19.8, respectively, [12] and assuming al inear correlation, one arrives to aprojected pK a DMSO of 7.3 for 2,which falls within the experimental interval of 4.7-8.1. This gives us confidence in the ability of the computed gas-phase pK a st oa ccurately reflect trends in solution for silanes as well. Now,c omparing calculated gas-phase pK a MP2 of SiH 4 (271.6) to that of tris-N-skatyl silane A (241.6) shows that inductive effects reduce the pK a MP2 by roughly 30.0 units. Constraining N-skatyl moieties into the cage structure B (pK a MP2 = 234.5) decreases the sum of N^Si^N angles (S N^Si^N )f rom 324.78 8 in A to 311.68 8 B,w hile reducing the pK a MP2 by an additional 7.1 units,which is about one fourth of the electron withdrawal contribution. This trend is continued by the more constrained C-tethered analogue C,w hich has not been observed experimentally,but its conjugate base has been isolated. [17,44] With a S N^Si^N of 300.58 8,i ts gas-phase pK a MP2 equals 229.6. Theincreased acidity of B and C is also in line with ab uild-up of strain (9.0 and 16.5 kcal mol À1 , respectively) with reference to corresponding conjugate bases as analyzed by as eries of homodesmotic equations (Section S5.1.5). In other words,deprotonation is accompanied by strain release,which significantly contributes to the acidity of B, C,a nd, by extension, compound 2.
As for the influence of charge,t he calculated gas-phase pK a MP2 of 2 (177.7) is,asexpected, much lower than that of B (234.5) due to electrostatic effects.Because of adifference in solvation of neutral and charged species, [34] these effects are expected to remain significant but be strongly attenuated in solution relative to electron withdrawal and strain. Therefore, the influence of the positive charge on the solution acidity of 2 is difficult to quantify.
To qualitatively assess the role of the positive charge in the acidity of 2,w ep erformed Natural Bonding Orbital (NBO) analysis of the deprotonated form 1.I ts howed that the anionic lone pair on silicon is hosted in an orbital with predominant s-character (sp 0.41 hybridization). Thec orresponding Natural Localized Molecular Orbital (NLMO) is composed of Si atomic orbitals by 98.1 %, with only minor delocalization into s*(C À N) orbitals due to hyperconjugation (Section S5.1.6). In other words,the high acidity of 2 does not originate from increased delocalization of the negative charge in its conjugate base 1,a nd the interaction between the zwitterionic charges is almost entirely electrostatic.
II. On the role of strain. More insight into the influence of strain on acidity of silanes A-C ( Figure 4A)a nd 2 can be obtained by decomposing the deprotonation reaction into two formal stages ( Figure 4B). [45] In the first stage,aproton is abstracted from the silane with all other atomic coordinates frozen. Theenergy of this process (DE 1 )gauges the penalty of Figure 4. A) Comparison of MP2-calculated gas-phase acidities for neutral silanes, including tris(trichlorosilyl)silane, HSi(SiCl 3 ) 3 ,from Beckmann and co-workers. [13] The pK a MP2 of cationic silane 2 (177.7) is not shown on the scale along with neutral silanes since it cannot be directly compared (see text). B) Deprotonation energy decomposition Scheme at B3LYP-GD3BJ/6-311 ++G** level of theory. DE reflect changes in SCF energy.
charge separation in the course of heterolytic SiÀHb ond dissociation followed by electronic relaxation. In the second stage,t he deprotonated silane is allowed to relax to the geometry of the corresponding anion, which is described by the respective energy difference (DE 2 ). In essence, DE 2 characterizes how strongly the geometry of the silane resembles that of the corresponding silanide.T he larger the discrepancy between geometries,t he more energy is gained during relaxation, leading to am ore negative total energy balance and therefore higher acidity.I ti si mportant to note that upon relaxation of deprotonated B, C and 2,the N^C^X and C^X^C angles barely change while most of the relaxation occurs within the SiN 3 fragment, which is analogous to the differences between the X-ray crystal structures of 2 BARF and 1 ( Figure 2B). DE 1 energies become progressively more negative from A to C (total difference of À20.7 kcal mol À1 ), which correlates with decreasing N^Si^N valence angles in respective silanes. Consequently,a cute angles around silicon imposed by the ligand destabilize the silane allowing for easier proton abstraction, and thus higher acidity.B ecause of the different molecular charge,comparing DE 1 of cationic silane 2 with that of neutral silanes A-C would be meaningless.
During the second stage,relaxation, DE 2 energies become progressively less negative from A to C (total difference of + 4.0 kcal mol À1 ), which means that constraint makes the geometry of asilane closer to that of the corresponding anion, hence reducing the acidity.W hile there is as imilarity in valence angles between 2 and its isoelectronic analogue B, relaxation for the former is 3.4 kcal mol À1 less negative.W e connect this to the fact that the distance between positive phosphonium and negative silanide atoms increases upon relaxation. This should reduce electrostatic stabilization, hence slightly increasing DE 2 and lowering the acidity.
Even though DE 1 and DE 2 contribute to the gas-phase acidity in an opposite way,changes associated with DE 1 have larger magnitude,w hich leads to higher acidity with acuter N^Si^N angles.T he progression of DE 1 in Figure 4B is related to subtle differences in electronic structure as shown by aN atural Bonding Orbital (NBO) analysis of the respective silanes (Section S5.1.7). Thec omposition of the Si-based hybrid in the Si À HNBO of tris-N-skatyl silane (A)is sp 2.04 d 0.02 .T he s-character significantly increases in the cage compounds B (sp 2.01 d 0.02 ), C (sp 1.90 d 0.02 ), and in the cationic silane 2 (sp 1.82 d 0.02 ). This translates into asubtle shift in bond polarization:t he SiÀHb ond remains overall polarized towards hydrogen, as was inferred from the interaction of 2 with dioxane in the solid state (vide supra), but the Si contribution to the Si À Hbonding NBO increases from 41.3 % in A to 41.8 %inB,42.5 %inC,and 43.5 %in2.This relative shift reduces DE 1 and makes strained silanes more electronically similar to their respective anions with the silicon lone pair in the latter ranging in hybridization from sp 0.47 in B to sp 0.41 in TSMPSi (1). Overall, this makes heterolytic Si À H bond dissociation in strained silanes more favorable than in unstrained ones.

Reactivity of the Zwitterionic Silanide TSMPSi (1)
Theh igh acidity of [TSMPSiH] + (2)s uggests that the stabilized Si anion in TSMPSi (1)should behave as arelatively weak nucleophile,which was assessed by aseries of reactions (Scheme 2). Tr eatment with FeBr 2 in THF afforded the insoluble stable complex (TSMPSi)FeBr 2 (THF) (7), which was identified crystallographically.T his reaction is likely solubility-driven, since no complexation was observed with Fe(OTf) 2 and Fe(acac) 2 ,s uggesting only aw eak interaction with the Fe II centers.R eaction of 1 with Fe 2 (CO) 9 afforded (TSMPSi)Fe(CO) 4 (8)asastable complex, suggesting that the softer (in the HSAB sense) Fe(CO) 4 fragment is ab etter match for the soft Si À center in 1.F inally,r eaction of 1 with 1equiv of MeOTf resulted in methylation at silicon to form the cationic cage compound [TSMPSiMe] + OTf À (9). Quaternization of the silicon atom in TSMPSi (1)i se videnced by ac hange of the 29 Si NMR chemical shift from À48.0 ppm in 1 to 34.4 and À22.5 ppm in 8 and 9,respectively.Additionally, the 29 Si NMR signal in 9 shows as ap entet (J Si,H % J Si,P = 8.1 Hz) consistent with the assigned structure.
Relatively low donicity of the silicon center in TSMPSi (1) is also apparent from analysis of the CO-stretching frequencies of its complexes with metal carbonyls (Section S5.2.1), which places 1 in the vicinity of silylenes and P(NMe 2 ) 3 rather than other silicon anions.A no bvious explanation for this is the presence of ap ositive charge in the molecule,w hich electrostatically stabilizes the negative charge on silicon rendering it less available for bonding.T he second factor that contributes to weak nucleophilicity is abuild-up of strain that occurs upon quaternization. Thes train penalty can be separated from electrostatic effects and approximated by analyzing aseries of homodesmotic equations involving aSitethered analogue of 1 (Section S5.1.5). There,complexation with an FeBr 2 (THF) center leads to a4 .5 kcal mol À1 increase in strain, whereas methylation and protonation provide 8.3 and 9.0 kcal mol À1 of an increase,r espectively,c onfirming al ikely contribution of ring strain to the weakened nucleophilicity of 1.

Forschungsartikel
Thei solated zwitterion 1 is kinetically stable in solution despite the presence of an ucleophilic Si À site and an electrophilic P + -Me unit in the same molecule.Unexpectedly, exposing 1 to 0.1 equiv of MeOTf was found to result in nearly complete isomerization of 1 to the charge-neutral analogue iso-TSMPSi (10)( Section S5.2.2). This suggests that the cation [TSMPSiMe] + is subject to nucleophilic attack at the phosphonium methyl group by am olecule of 1,f orming 10 and generating an ew molecule of [TSMPSiMe] + (Scheme S7). This increased electrophilicity of the P-bound methyl group in [TSMPSiMe] + likely originates from ac ombination of its overall positive charge and increased ring strain. As amatter of fact, nearly complete isomerization also occurs in the presence of 0.25 equiv of HBAr F 4 ·2 Et 2 Otoform 2 BARF in situ (Section S5.2.2). Theloss of the P-bound methyl group from either [TSMPSiH] + (2)or[TSMPSiMe] + reduces the strain of the cage structure,which provides an additional driving force for this "strain-release methyl transfer" reaction.

Reactivity of the Cationic Silane [TSMPSiH] + (2)
Interestingly,r ecording NMR spectra of . We propose that this is due to coordination of aTHF molecule resulting in apentacoordinate silicon centre (2·THF in Scheme 3). NMR parameters calculated for 2·THF using DFT reproduce the experiment reasonably well, being À104.9 ppm, À392.7 and À7.3 Hz, respectively (Section S4.2). Importantly,t he 31 PNMR signal of 2 BARF only shifts from À7.6 ppm in [D 2 ]DCM to À7.0 ppm in [D 8 ]THF, ruling out THF interaction with the phosphonium center. Another point of note is that 1 Ha nd 13 Cs pectra of 2 BARF in [D 8 ]THF remain in line with C 3 symmetry,incontrast with the expected breaking of symmetry upon coordination of THF. This suggests af luxional process such as Berry-like pseudorotation or reversible dissociation.
THF coordination to silicon in [TSMPSiH] + (2)i sk ey to the observed reactivity of TSMPSi (1)w ith the CH-acidic fluoradene (4,pK a DMSO = 10.5) in [H 8 ]THF (Scheme 3). Next to am oderate amount of the isomerization product iso-TSMPSi (10; vide supra), the main product was spectroscopically identified as the THF ring-opening product 11 a,w hich exists in ad ynamic equilibrium with the open form 11 b. 1 HNMR spectra of the mixture (11 a/b)i n[ D 8 ]THF at 25 8 8C (Section S5.3.1) display an umber of broadened peaks corresponding to protons of the SiÀH, methyl phosphonium, and aromatic groups as well as four methylene signals between 0.75 and 3.25 ppm. Thel atter disappear if the reaction is conducted in [D 8 ]THF instead of [H 8 ]THF,w hich indicates that they belong to a À (CH 2 ) 4 À fragment from aT HF ringopening reaction. Moreover, 1 HC OSY spectrum shows that they form an isolated spin system (Section S5.3.2), while the NOE spectra (Section S5.3.3) indicate that the termini of the À(CH 2 ) 4 À fragment are located in spatial proximity to one aromatic doublet each, with the oxygen-bound terminus also being close to aSi À Hproton.
Thef ormation of 11 a/b can be explained by the generation of as mall amount of the Si IV cation [TMSPSiH] + (2) as the fluoradenide salt 2 Fl in an acid/base equilibrium with Si II compound TSMPSi (1)a nd 4. Thec ation then acts as as train-release Lewis acid for THF,g enerating the complex 2·THF characterized above (Scheme 3), which undergoes ring opening upon nucleophilic attack by the deprotonated fluoradene (4). These last steps can also be interpreted as the activation of THF by the transient frustrated Lewis pair 2 Fl . [46] Interestingly,products similar to 11 a/b form with other tested fluorenes of pK a DMSO within 8.1-11.6 (Section S5.3.4), while less acidic fluorenes exclusively lead to isomerisation to iso-TSMPSi (10). This can be understood from the fact that stronger acids will generate higher equilibrium concentration of the fluorenide anion, thus opening ak inetic pathway for Scheme 3. THF ring-opening with fluoradene (4). Suggestedr eaction mechanism is shown with dashed arrows.

Angewandte Chemie
Forschungsartikel the THF ring opening. This reaction sequence involves the insertion of both the Si center and aneutral molecule (THF) into aC À Hb ond coupled with af ormal oxidation state change from Si II in TSMPSi (1)t oS i IV in [TMSPSiH] + (2), which presents intriguing similarities with transition metalmediated processes.Inthe overall reaction, the silicon center sequentially acts as an ucleophile (base) and an electrophile (Lewis acid), demonstrating biphilic character that can be traced back to ring strain in [TSMPSiH] + (2).

Fluxionality of the THF Ring-Opening Product 11 a/b
TheT HF ring-opening product 11 a/b displays rich dynamic behavior in solution, which could be further elucidated by using variable-temperature NMR spectroscopy. Between À89 and 60 8 8C, 11 a/b displays only one 31 PNMR signal with at emperature-dependent chemical shift (Section S5.3.5), indicating ar apid temperature-dependent equilibrium between two forms 11 a and 11 b.Aregressive thermodynamic analysis shows that the transition starts above À60 8 8Ca nd is 78.5 %c omplete at 25 8 8Cw ith D r H = 10.4 kcal mol À1 and D r S = 37.5 cal mol À1 K À1 (Section S5.3.5). These thermodynamic parameters are consistent with ar ing-opening equilibrium between 11 a and 11 b involving breaking of aS i ÀNb ond (Scheme 3). Confirming this interpretation, the 29 Si NMR signal of 11 a, À120.2 ppm at À89 8 8Ci ndicative of ap enta-or hexacoordinate silane, [47] shifts to À74.9 ppm at 25 8 8C( Section S5.3.6). With consideration of the 78.5 % complete transition at 25 8 8C, this yields an extrapolated value of À62.5 ppm for pure 11 b,w hich is typical for tetrahedral silanes. [47] Our assignment of the temperature-dependent speciation in solution is supported by DFT-calculated 29 Si NMR chemical shifts and 1 J Si,H coupling constants for possible geometries on silicon using truncated molecular models (Section S5.3.6).
An NOE correlation between the oxygen-bound CH 2 group and the aromatic methyl group (Section S5.3.3) indicates that 11 b at least partially exists as the Z-stereoisomer with the free indolide and the alkoxy substituent on the same side of the central six-membered ring (11 b-Z in Scheme 4). This may appear surprising at first sight, considering that the ring-opening should proceed from the lowest-energy configuration of 11 a,w hich, according to DFT,h as at rigonal bipyramidal silicon centre with an axial alkoxy group.T here, dissociation of the most labile apical SiÀNbond should lead to the 11 b-E isomer (Scheme 4). Consequently, 11 b-Z likely forms via another mechanism that involves acertain degree of geometric fluxionality around silicon in 11 a,s ampling geometries leading to 11 b-Z upon reversible dissociation. Supporting the presence of an additional dynamic process in the system, 1 Hspectra of 11 a/b (Section S5.3.1) show only one aromatic methyl signal, whereas both the closed (11 a)and the open (11 b)isomers should display two signals in aratio of 2:1 as aresult of breaking of the C 3 symmetry.
To account for these observations,w eu ndertook ac omputational study on at runcated model of 11 a (3-methylindoles were reduced to pyrroles,R= HinScheme 4), supporting the overall exchange and isomerization mechanisms depicted in Scheme 4. Thea pical and equatorial N-atoms can swap in asingle step following the M2 positional exchange mechanism [48] with al ow barrier of 2.2 kcal mol À1 (Section S5.3.7), explaining the above spectroscopic observations. This exchange mechanism can be thought of as two consecutive Berry-type deformations, [49a] the product of the first one being the transition state for the overall process, 11 a-TS. While dissociation of the apical NÀSi bond from 11 a/11 a' ' readily yields 11 b-E/11 b-E' ' with ab arrier of 9.6 kcal mol À1 , as imilar process leading to 11 b-Z would have to start from the transition state structure 11 a-TS,i nw hich the hydride substituent occupies an apical position. This situation may indicate the presence of abifurcation on the potential energy surface, [50] which is supported by at wo-dimensional relaxed potential energy surface scan (Section S5.3.8). Theb ifurcation is best understood starting from the 4-coordinate structure 11 b-Z:a st he N-atom gradually approaches the Si center, the molecule first overcomes the transition state 11 b-Z-TS (12.8 kcal mol À1 )a nd then "avoids" the second transition state 11 a-TS to fall directly onto either of the degenerate energy minima 11 a or 11 a' '.
Overall, while the NOE spectra (Section S5.3.3) indicate the presence of 11 b-Z,t he comparable free energy of the isomer 11 b-E and low computed barriers for interconversion suggest that both stereoisomers likely coexist with 11 a in at hermal equilibrium. These observations demonstrate that, besides its ability to support facile interconversion of Si II and Si IV species,t he strained TSMP platform confers much conformational flexibility to the latter,a sit allows virtually all thermodynamically accessible 4-coordinate and 5coordinate geometries to be sampled at room temperature or below.

Conclusion
Theconstrained valence angles around the silicon atom at the bridgehead position of the bicyclic cationic silane [TSMPSiH] + (2)h ave aprofound effect on its reactivity.
First, at odds with the general hydridic character of silicon-bound hydrogen atoms,[ TSMPSiH] + (2)e xhibits an exceptionally low pK a DMSO within 4.7-8.1, which makes it more acidic than phenol, benzoic acid (pK a DMSO of 18.0 [14] and 11.1, [15] respectively) and the few silanes of which pK a DMSO was reported. [24,31] While this high acidity originates in part from the electron-withdrawing effect of the substituents on silicon and overall positive charge of 2,itissignificantly enhanced by the unusually acute N^Si^N angles imposed by the heterobicyclo[2.2.2]octane scaffold. Namely,D FT calculations suggest that strain increases the s-character of the Si À H bonding pair, polarizing the bond towards silicon and facilitating its heterolytic dissociation.
Then, protonation (or methylation) of the anionic Si atom in TSMPSi (1)i ncreases the reactivity of the opposing methylphosphonium unit, which can transfer am ethyl group with release of strain. This opens up ac harge neutralization pathway to form the phosphine/methylsilane isomer iso-TSMPSi (10)byi ntermolecular methyl transfer.
Finally,next to its high Brønsted acidity,[TSMPSiH] + (2) also behaves as as train-release Lewis acid, coordinating aT HF molecule so that the silicon atom assumes at rigonal bipyramidal geometry.T his is accompanied by activation of a-carbons of the THF ring, which makes it susceptible to such weak nucleophiles as highly-stabilized aromatic anions.I n particular,i ts conjugate base TSMPSi (1)r eacts with fluoradene (4)i nT HF to afford the linear product 11 a/b,w hich forms by nucleophilic ring opening of aT HF molecule coordinated to [TSMPSiH] + (2). In this process,the Si center sequentially acts as an ucleophile (base) and then as an electrophile (Lewis acid) to facilitate the activation of two relatively unreactive molecules,s uggesting the potential of such cage compounds as biphilic main-group centers.T his reaction sequence also illustrates the ability of the TSMP scaffold to support both the Si II and Si IV states along with the processes that interconvert them under mild conditions.I n addition, the trigonal bipyramidal silicon center in the addition product 11 a/b undergoes several facile fluxional processes (positional exchange and reversible SiÀNb ond dissociation), illustrating the flexibility of the TSMPSi (1) platform in terms of accessible geometries.
These observations collectively illustrate how acombination of charge,i nductive effects and strain can significantly manipulate the properties of asilicon atom leading to unusual reactivity.T he various reactive pathways accessible to the strained cage structure [TSMPSiH] + (2)s uggest that this or arelated platform may serve as an entry point into novel bond activation strategies based on the Si II /Si IV couple.