An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

A new NMR -based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p-fluoro-benzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The 19F NMR chemical shifts and the 1J(CF) coupling constants of these nitrilium ions vary in a predictable manner with the donor capability of the stabilizing group. Both spectroscopic parameter are suitable probes for scaling the acidity of Lewis acids. These new probes allow to discriminate between very similar Lewis acids which is not possible with conventional NMR tests such as the well-established Gutmann-Beckett method.


Introduction
Lewis acid (LA) catalysts are of widespread use in synthetic chemistry and catalysis. [1] Consequently,t here is ac onstant quest for new Lewis acids of particular strengtha nd for Lewis acids with clearly defined acidity,w hichi sa djusted to the synthetic challenge. Silyl Lewis acids are particularly interesting because they span av ery broad spectrum of different acidity strengths, beginning with moderate LAs, such as trimethylsilyl chloride, to extremely stronge xamples, such as triarylsilylium ions or solvent complexeso ft rialkylsilylium ions. [2] Recently, cationics ilyl Lewis acidsi np articular have come into the focus of syntheticc hemistsd ue to their promising exceptional high Lewis acidity. [2b, 3, 4] The requirement for their beneficial use in preparative work is however ac lear controlo ver their reactivity. [5] In many cases, this was achieved by intramolecular coordination of weak Lewis bases (LB) to the cationic silicon center that results in tetra-coordination for the silicon center ( Figure 1). In saying that, the parallels to intramolecular Frustrated Lewis Pairs (FLPs)p opularized by Erker and Stephan become obvious. [1c, d, 3p, 6] The strength of the LA/LB interaction determines structure, spectroscopic properties and, in the context of this report most important, the Lewis acidity of these species. For syntheticp urposes, aq uantitative evaluation of the Lewis acidity is desirable and ac learr ankingo fs imilar Lewis acids is especially needed. [7] Severale xperimental methods have been established that allow for the scalingo fL ewis acidity. The most prominent ones are based on the change in NMR chemical shifts of ap robe Lewis base upon coordination to as eries of Lewis acids. The Gutmann-Beckett method uses the change of the 31 PNMR chemical shift of the probe Lewis base OPEt 3 upon complexation for scaling different Lewis acids. [8] In Childs method the base is crotonaldehyde and the 1 HNMR chemical shift of the g-protoni sp robed. [9] Related to these methods,H ilt and co-workersapplied the 2 HNMR chemical shifts of the g-deuterium of the Lewis acid complexes of perdeutero-pyridine to gauge their acidity. [10] Although Childs methodc ould not be applied to cationic silyl Lewisa cids, pyridine as well as phosphane oxides have been used to measure their Lewis acidity and these investigations revealed the high Lewis acidity of cationic silicon compounds. [10c, 11] In addition, it was shown that these methods fail to gauge correctly the Lewis acidity of intramolecularly stabilized silyl cations I.I n both cases,t he interaction between the externalp robe base and the silyl Lewis acid cancels the intramolecular interaction between the stabilizing donor and the silyl group. [10c, 11] Conse- quently,t he authors did not report the Lewis acidity of the internallys tabilized silyl Lewis acid I but, instead, that of an ot relevant donor-free species II.Acomparison highlights this issue:t riarylsilylium ion 1 [12] and silyl cation 2c, [13] which is stabilized by the remote selenylether donor,a re very different with respect to their electronic properties (as for example shown by their vastly different 29 Si NMR chemical shift) and their reactivity.N evertheless,t he Gutmann-Beckett method assigns to both nearly the same Lewis acidity,e xpressed by almosti dentical Dd 31 Pv alues ( Figure 1). Moreover,t he selenylstabilized silyl cation 2c is, accordingt ot he Gutmann-Beckett method, even slightly more Lewis acidic than silylium ion 1. Clearly,the stabilization of the silyl cation 2c by the selenylether substituent is pushed back by the externalb ase phosphine oxide and all information on the actual acidity of the stabilized cation 2c is lost. This example highlightst he need for al ess strong Lewis base as ap robe,w hich would allow for the classificationo fs ubtle distinctions in the Lewis acidity of intramolecularlys tabilized silyl cations. Here, we report our results by using p-fluorobenzonitrile (FBN) as an NMR probe for the scaling of intramolecularly stabilized silyl Lewis acids.
Silylium ions are commonly accepted to be stronger Lewis acids than tricoordinated boron compounds. [11] We prepared silylium ion 5 by the standard Corey protocol using [Ph 3 C] [B(C 6 F 5 ) 4 ]i nm ethylene chloride in the presence of exactly one equivalent of FBN which gave the silylated nitriliumb orate 6[B(C 6 F 5 ) 4 ]. [2c] Nitrilium ion 6 is characterized by the 29 Si NMR chemicalshift of d 29 Si = 23.0, in the typical spectralrange for silylatedn itriliumi ons( d 29 Si = 6-40). [16] The stronger deshielding of the p-fluorine atom (d 19 F = À86.6) and the large 1 J(CF) coupling constant[ 1 J(CF) = 272.8 Hz] indicate ah igher contribution of the quinoid resonance structure for the silylated nitrilium ion 6 compared to the nitriliumb orate ylide 4 and suggests a higher Lewis acidity for silylium ion 5.
Next, we tested the reactions of FBN with aseries of silyl cations stabilized by interaction with chalcogenyla nd halogen substituents based on the acenaphthene (2)a nd naphthalene (9)b ackbone. Recently,s ulfur-and oxygen-stabilized silyl cations have attracted specialinterestasp ossible Lewis acidic catalysts. [4,13] The selenyl-stabilized cation 2c was chosen as at est case, considering that the cationic silicon atom and the selenium-based donor group can be easily monitored by NMR spectroscopy.S ilyl borate 2c[B(C 6 F 5 ) 4 ]w as prepared according to Scheme 1b yu sing the standard Corey reaction [17] andw as fully characterizedb ym ultinuclear NMR spectroscopy.I ts identity was verifiedb yc omparison with literature data. [13] Finally, the molecular structure of cation 2c was unequivocally established by an XRD analysis of the salt [2c] 2 [B 12 Br 12 ]. The molecular structure of the acenaphthene-based cation 2c closely resembles that of the previously reported naphthalene-based cation 9c (Figure 4). [13] The silicon atom in 2c is tetracoordinated with as ignificant trigonal flattening of the tetrahedral coordination of the silicon atom [Sa(SiC 3 ) = 346.18]. The coordina-   (6)pm (2c)v s. 240.6 pm (9c)].T he attractive Lewis acid-basei nteraction between the two atoms in peri-position of the acenaphthene moiety does not lead to significant strain in cation 2c. [18] This is indicated by the sum of the bay angles Sb of 355.98 that is close to the ideal value of 3688 for the unsubstituted acenaphthene (see Figure 4). [19] The silicon atom is placed 39 pm above the plane spanned by the ten carbon atoms of the naphthalene subunit, while the seleniuma tom is essentially placed in this plane. Nitrilium ion 10 c was synthesized by addition of exactly an equimolar amount of FBN to as olution of 2c[B(C 6 F 5 ) 4 ]i nm ethylene dichloride (Scheme 1). Its formation is indicated by as ignificant high-field shift of the 29 Si NMR resonance (Dd 29 Si = À38) and by ad eshielding of the seleniuma tom (Dd 77 Se = 26) compared to cation 2c ( Figure 5). Interestingly,t he 77 Se NMR chemicals hift of nitrilium ion 10 c is still markedlys maller than reported for dicoordinated bisarylselenides such as selenide 7c and suggests fort he selenium atom at rigonal pyramidal coordination environment. In agreement, the substantial 1 J(SiSe) coupling constant of 33 Hz points to the presence of aS i ÀSe bond. [20] Thec oordinationo ft he nitrile to the silicon atom is shownb yt he low field shift of the 19 FNMR signal and an increaseo ft he 1 J(CF) coupling constant [Dd 19 F = 4.9, D 1 J(CF) = 4.5 Hz].U pon formationoft he nitrilium ion 10 c from selenonium ion 2c the diasterotopic methyl groups, syn/anti Me, at the silicon atom become magnetically equivalenta tr oom temperature in both 1 Ha nd 13 CNMR spectra ( Figure 5). This indicates a lower barrier for the inversion of the trigonal pyramidal configuration att he selenium atom. [13] All attempts to obtain definite structurali nformation for the pentacoordination of the silicon atom in nitrilium ion 10 c by growing suitable single crystals for XRD analysis from salts of weaklyc oordinating anions,s uch as perfluorinated tetraarylborates or brominated dodecahedral borates, failed. [21] Either slow decompositiono ccurredo rc rystals of the corresponding silyl cation 2c salts were obtained. For example, the salt [2c] 2 [B 12 Br 12 ]c rystallized from am ethylene chloride/hexane solutiono fn itrilium closo-borate [10 c] 2 [B 12 Br 12 ].
In summary,t he NMR spectroscopicd ata forn itrilium ion 10 c clearly indicatepentacoordination for the silicon atom and as ignificant bonding interaction between the silicon and selenium atoms, although the nitrile is coordinated to the silicon atom. Thefact that nitrile coordination does not cancelt he sili-Scheme1.Formation of silyl cations 2, 5 and 9 and subsequent transformation to nitriliumions 6, 10 and 11 (2, 5, 7 and 10:acenaphthene-based compounds; 8, 9, 11:n aphthalene-based compounds).  con-selenium interaction suggested to us that the spectroscopic data of nitriliumi on 10 c,s uch as 19 FNMR chemical shift and 1 J(CF) couplingc onstant,r eflect the Lewis acidity of the stabilized silyl cation. The modification of these two parameters clearly show the attenuation of the acidity of the silyl Lewis acid 2c by the seleniumd onor compared to silyl cation 5.T ot est if both NMR spectroscopic parameters are general measures of the acidity of intramolecularly stabilized Lewis acids, we analyzed the NMR spectroscopicd ata obtained for the series of nitriliumb orates 10[B(C 6 F 5 ) 4 ]a nd 11[B(C 6 F 5 ) 4 ] (Scheme 1). All investigated nitrilium borates 10[B(C 6 F 5 ) 4 ]a nd 11[B(C 6 F 5 ) 4 ]w eref ully characterized by multinuclear NMRs pectroscopya nd the complete data are summarized in the Supporting Information. Results that are pertinent for the discussion are shown in Ta ble 1. The 19 F{ 1 H} NMR spectraw ere collected at room temperature in CD 2 Cl 2 ,a nd were calibrated against the signal of the p-fluorine atoms of the [B(C 6 F 5 ) 4 ] À anion (d 19 F = À163.44), which was referenced against fluorobenzene [d 19 F(C 6 H 5 F) = À113.78]. [22] In this series of intramolecularly stabilized silyl cations,t he two NMR parameters varied from d 19 F = À102.1 and 1 J(CF) = 257.1 Hz for the telluryl-substituted nitrilium ion 10 d to d 19 F = À86.6 and 1 J(CF) = 272.8 Hz for the unsubstituted silyl cation 6 ( Figure 6a,b). The valuesf or the tellurium compound 10 d are close to those measured for the free FBN, which indicates for cation 2d the strongesti nteraction between the donor group and the silicon center.F or the other extreme, the bromo-substituted species 10 e,t he parameters are close to those for cation 5.H ence, bromo-substi-tuted silyl cation 2e is the strongest Lewis acid in this series, whereas the tellurium compound 2d is the weakest. As expected, we noticedaclear linear correlation between both NMR parameters, which indicates that both values can be used as measure for the Lewis acidityo fi ntramolecularly stabilized silyl cations (Figure7). Consequently,b oth scales predict the same  order of Lewis acidity for the tested silyl Lewis acids. We note, however, that the relative position of BCF varies somewhat on both scales (see Figure 6a,b). In addition, we note that both scales indicate al arger dispersion of the data for weak Lewis acids and as maller separation for stronger Lewis acids (Figure 6a,b). Clearly, this confers to the sequence of strong Lewis acids al arger degree of uncertainty.N evertheless, when acenaphthene-based silyl Lewis acids 2 are compared to the corresponding naphthalene systems 9,t he Lewis acidity is always larger for the acenaphthene-based cation. For example, for the donor OPh, cation 2a is more Lewis acidic than 9a.T his is in line with the weakerS i-Do (Do = donor) interaction previously found for acenaphthene-based cations 2 compared to the naphthalene systems 9 with the same donor. [13] Quantumm echanical calculations at the M06-2X/def2-TZVP level of theory wereu sed to gain furtheri nsights. This computationalm odel is justifiedb yt he close agreement between the theoretically predicted molecular structureo fc ation 2c and that determined experimentally by XRD.O ptimized structural parameters that are pertinent to the discussion, such as bond lengths and bond angles in the bay region of cation 2c,d iffer by less than 1% and provide as olid basis for the structuraldiscussion.T he calculated structural dataf or all investigated nitrilium ions are summarizedi nT able 2. As at ypical example, Figure 8c ompares the calculated molecular structure of cation 2c with that of nitrilium ion 10 c.I nn itrilium ion 10 c the silicon atom adopts at rigonal bipyramidal coordination sphere with the selenyl group and the nitrile at the axial positions and an almostp lanar trigonal basis spanned by the silicon atom and its three carbon substituents [Sa(SiC 3 ) = 3598]. The Si-Se distance is 275.6 pm, elongated by 30.9 pm compared to selenonium ion 2c, [13] and the coordination of the nitrile is indicated by the short SiÀNd istance of 199.7 pm. Both values are significantly smaller than the respective sum of the van der Waals radii [SvdW; 400 pm (SiSe) and 365 pm (SiN)]. [23] The sum of the bay angles (Sb)t aken as am easure for strain induced by the peri substitution indicates no significant hindrance in cation 2c and in nitriliumi on 10 c [Sb = 3578 (2c)a nd 3658 (10 c)]. [18] Very similar structural parameters are computedf or all nitriliumi ons 10 and 11 and are summarizedi nT able 2. A first idea about the Lewis acidity of the stabilized silyl cations is provided by the complexation energyb etween the cation and the coordinating nitrile, which is the bond dissociation energy of the SiÀN(FBN) bond, BDE (SiÀN) (   ]. Based on these calculated cation-nitrile interaction energies, the strongestL ewis acid among our test set is the bromo-stabilized silyl cation 2e andt he weakesti s the telluryl-stabilized 2d,i nq ualitative agreement with the scales based on 19 FNMR chemical shifts and the 1 J(CF) coupling constants ( Figure 6a,b). Ac ommonly accepted theoretical scale for Lewis acidity is based on calculated fluoride ion affinities. [7,24] We calculated the fluorine ion affinity (FIA)f or cations 2 and 9 versus BEt 3 at the M06-2X/Def2-TZVP level of theory with inclusion of solvent effectsu sing the SCIPM model with methylene chloride as solvent according to reaction(1) (Scheme 2). Ther esults are summarized in Ta ble 1a nd are graphically displayed in Figure 6c.T he FIAs for the stabilized silyl cations 2, 9 are clearly separated from that predicted for the non-stabilized cation 5 (303 kJ mol À1 )and fall in the relative narrow range (FIA = 171-224 kJ mol À1 ). The relative order of the Lewisa cidity for cations 2 and 9 on the FIA scale is close to that given by the experimental d 19 FNMR and 1 J(CF) NMR parameters, althought here is no linear correlation between the experimental scales and the FIA scale ( Figure 7b). It is worth noting that for the limiteds ubset of data of chalcogenyl-substituted cations all three scales provide the same sequenceo fi ncreasing Lewis acidity as they do for the subset of halo-substituted cations ( Figure 6). The missing correlation between the computed FIA scale and the two experimental nitrilium-based scales might be rationalized by ac loser inspection of the computed structures of cations 2/9,n itrilium ions 10/11 and the corresponding silylfluoride 12/13.F or the series of selenyl-substituted compounds, the computed structuresa re shown in Figure 8. By taking the sum of the bay angles Sb as an indicator for kind of interaction between the selenyl and the silyl group, it becomes evident that there is small attractive interaction in the stabilized silyl cation 2c (Sb8 = 3578). The interaction becomes slightly repulsive in the nitriliumi on 10 c with ap entacoordinated silicon atom (Sb8 = 3658)a nd finally the repulsion between the peri-substituents is significant in silyl fluoride 12 c (Sb8 = 3818). [25] This comparison indicates that the attractive donor-acceptor interaction originally present in cation 2c,i sp reserved to ac ertain extenti nn itrilium ion 10 c but it is significantly pushed backa nd is even repulsive in silyl fluoride 12 c.T his suggests that the scales based on the NMR parameters of the nitrilium ions 10 and 11 are better suited to gauge the modifications of the Lewis aciditybythe intramolecular donorthan the theoretical FIA scale is. Free triarylsilylium ions, such as tris-pentamethylphenyl silylium, 1 (Pemp 3 Si + ), are not stable in CH 2 Cl 2 solutions, [12b] therefore we quantified their Lewis acidity with nitrile 1 in chlorobenzene solution.T he borate 1[B(C 6 F 5 ) 4 ]w as synthesized by substituent-exchange reaction and identified by NMR spectroscopy. [12] An equimolar amount of FBN was added and the formationo ft he corresponding nitriliumi on 14 was indicated by the substantially high-field shifted 29 Si resonance [d 29 Si (1)   In this respect, the linear arrangemento ft he atoms in the nitrile donorisc ertainly beneficial. The relative order of Lewis acidity was tested experimentally for the hydride transfer reactiono fs ilane 7c with bromonium ion 2e.Abiphasic solution of 2e[B(C 6 F 5 ) 4 ]w as prepared by the standard Corey protocol from the silane 7e in [D 6 ]benzene (Scheme3). The cleanf ormation of cation 2e was indicated by 1 HNMR spectroscopy (see Supporting Information). Addition of one equivalent of selenyl silane 7c at room temperature gave the less Lewisa cidic selenyl-stabilized silyl cation 2c (d 29 Si = 64, d 77 Se = 254) [13] and the bromoacenaphthyl silane 7e was recovered [d 29 Si = À15, d 1 H = 5.46, 0.64 (SiHMe 2 )].

Conclusion
The reactions of the weak donor p-fluorobenzonitrile, FBN, with af amilyo fi nternallys tabilized silyl cations 2 and 9 were studied. The formed nitrilium ions 10 and 11 are stable in benzene solutions and in many cases, also in methylenec hloride. They were fully characterizedb yN MR spectroscopy.T wo spectroscopic parameters, the 19 FNMR chemical shift of the p-fluorine atom and the 1 J(CF) coupling constant between the fluorine atom and the p-carbona tom were found to be sensitive to the different Lewis acidity of the investigated intramolecularly stabilized silyl cations.T he NMR investigations that were supported by the results of density functional calculations indicate at rigonal bipyramidal coordination environment for the silicon atom in nitriliumi ons 10 and 11.T herefore, the coordination of the nitrile to the silicon centeri sn ot strong enough to cancelt he interaction with the internal donor.A saconsequence, the electronic situation of nitriliumi ons 10 and 11 as disclosedi nt heir NMR parameters reflect closely the original situation in the underlying silyl Lewis acids 2 and 9.T his suggests that FBN is aN MR probe well suited to distinguisha nd to gauge the Lewis acidity of silyl Lewis acids that are stabilized by relativelystrong donors. This is shown by the clear discrimination between very similar silyl Lewis acids, such as iodine compound 2f,t hiophenyl ether 2b,a nd phenylselenyl ether 2c (see Figure 6a, b). The sequence found with the FBN probe is supported by the DFT calculations for the complexation energies with FBN, BDE (SiÀN) ( Table 1) and by competition experiments. The Lewis acidity based on the FBN scale parallels that of the theoretically derived FIA scale ( Figure 6c); there is, however, no linear correlation.T he lack of proportionality between both scales can be rationalized by the stronger affinity of the fluoride ion used in the theoretical scale to silicon, which suppresses the small differences in the intramolecular donation by the different donorg roups. Therefore, we suggest that the FBN NMR probe is well suited for gauging the actual Lewis acidity of as tabilized silyl Lewis acid. The probe is applicable to strong donors (weak Lewis acids), although there is clearly al imitation for very strong donors, which do not allow for an additional coordination of the nitrile.F or weak donors (strong Lewis acids), their influence on the Lewis acidity of the donor-stabilized acidi sc anceled as soona st hey are replaced by the nitrile. The advantage of the FBN probe compared to established scales, such as the Gutmann-Beckett method, is demonstrated by the example quoted in the introduction, that is, the direct comparison between silyliumi on 1 and the internally stabilized silyl cation 2c.P ractically,t he same Lewis acidity of both ions is suggested on the basis of the Gutmann-Beckett method, whereas the FBN probe differentiatess ubstantially between both cations. This is ar esult that is expectedf rom the vastly different reactivity of silylium ion 1 and intramolecularly stabilized silyl cation 2c (Figure 9).
The FBN NMR probe in our hands is av aluablee xtension of the toolbox of methods for quantifying the actual acidity of stabilized Lewis acids, which do find more and more applications in synthesis. It allows for the discrimination between silyl cations stabilized by donors of very similar donor ability and it is therefore useful for the design and fine tuningo fs ilyl Lewis acids in particulara nd of Lewis acidsi ng eneral. Currently,w e are applying the FBN method to other intramolecularly stabilized cationic Lewis acids.