Cage Complexes of Carbenium and Silylium Cations with an Aromatic

Article pubs.acs.org/Organometallics

Cage Complexes of Carbenium and Silylium Cations with an Aromatic Base. Is the η6 Coordination Type Realizable? Valery F. Sidorkin,* Evgeniya P. Doronina, and Elena F. Belogolova A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, Favorsky, 1, Irkutsk 664033, Russian Federation S Supporting Information *

ABSTRACT: The cage cations [E((CH2)3)3C6R3]+ (E = C (1), Si (2); R = H (a), Li (b), F (c)) were studied theoretically using the MP2/6-311++G(d,p) approach. The migration of the electrophilic center E and a proton around the perimeter of the arene ring in the cations [E((CH2)3)3C6H3]+ was considered. Our results are indicative of the π type of E← arene coordination in the intramolecular complexes [E((CH2)3)3C6R3]+. The cation [Si((CH2)3)3C6H3]+ (2a), which corresponds to a peak (M − CH3)+ at m/z 229 in the mass spectrum of methylsilacyclophane, exists in a firmly established η1π form, as opposed to the case for the known complexes of the trivalent silicon atom with aromatic bases. The species [C((CH2)3)3C6F3]+ (1c), as well as 4a, obtained by connecting the equatorial carbon atoms in [Si((CH2)3)3C6H3]+ by a methylene bridge, are the first representatives of stable “face” complexes of carbenium and silylium cations with a benzene ring. The process of deprotonation of the complexes [C((CH2)3)3C6H3]+, [Si((CH2)3)3C6H3]+, and 4a was found to be energetically unfavorable, even in the presence of a strong base such as NEt3. The effect of the counterions BX4− (X = F, C6F5) and the polarity of solvents on the structure of the above cations was investigated using the examples of toluene and DMSO.

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INTRODUCTION The interactions of cations of different nature with π electronic systems play prominent roles in various fields of chemistry1 and biology,2 and they are also important for the design of new functional materials.3 The question concerning the structure of complexes, formed by carbenium (R3C+) and silylium (R3Si+) cations with aromatic bases, is also of fundamental importance4 in determining the mechanism of reactions of alkylation5 and silylation6 of arenes. Interest in the complexes of the trivalent silicon atom with arenes is also motivated by the problem of generation of the R3Si+ cation in condensed media in an isolated or π donor stabilized state7 and by potential advantages of using these strong Lewis acids in catalytic processes.1c,d,f According to the results of quantum chemical calculations, the methyl (H3C+)8 and isopropyl ((Me)2HC+)9 cations form with benzene (on the basis of X-ray data, H3C + also with hexamethylbenzene10) only stable σ associates (see Scheme 1). In studies of the interaction of the tert-butyl cation, Me3C+, with PhH8c,9 at the MP2 level of theory,9 the formation of the η1π structure preceding the σ complex was revealed.11 This structure was stabilized by a CH···π hydrogen bond. Recently, a suggestion has been made12 (supported by calculations at the M06/ccpVTZ level of theory) that the intramolecular interaction of the carbocationic center with an aromatic ring in the protonated benzobicyclo[3.2.1]octene, benzobicyclo[2.2.1]heptene, and benzobicyclo[4.2.1]nonene will belong, by steric reasons, solely to the η1π rather than to the σ type (see A in Scheme 2). It is © 2012 American Chemical Society

significant that the ECipsoCpara angle (α) is of key importance for assigning the complexes of electrophiles (in particular, R3C+ and R3Si+) with arenes to a certain type4a,13 (Scheme 1). The α angle is less than or equal to 90° in the ideal π structures, whereas in the ideal σ structures α = 125°. Complexes with 90° < α < 125° are assigned to a mixed σ/π type.4a,13b,14 The α angles can, in principle, take a continuum of values on the path of σ/π separation. On the basis of the value of the SiCipsoCpara(α) angle in the known intermolecular [R3Si·arene]+ (R = Et, arene = MeC6H5;7a R = Me, arene = R′nC6H6−n, R′ = H, Me, Et, Pr, Bu, n = 0−67j) and intramolecular (the derivatives of the 2,6-diarylphenyldimethylsilyl cation;7g,i see B in Scheme 2)15 complexes of silylium cations with arenes, the character of coordination in them should be assigned to a σ/π type. Indeed, according to X-ray data, the minimal α value, observed in the case of [Me3Si·1,3,5Me3C6H3]+,7j is 10° larger than the limiting value for the ideal π complexes, equal to 90°. From quantum chemical calculation results, a α value larger than 90° is also characteristic of the isolated states of [R3Si·arene]+.7j,8a−c,16 In the cations of bissilylated arenes13,17 (see C in Scheme 2), the α value amounts to about 128°; thus, these intramolecular complexes should be assigned to a σ type.18 Received: August 21, 2012 Published: November 1, 2012 7511

dx.doi.org/10.1021/om3008053 | Organometallics 2012, 31, 7511−7521

Organometallics

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Scheme 1. Structural Diversity of the Complexes of Carbenium and Silylium Cations with Benzene

Scheme 2. Intramolecular Complexes of Carbenium and Silylium Cations with Arenes

Scheme 3. Cage Carbenium and Silylium Cations

correspond to a second-order saddle point rather than to a minimum on the potential energy surface (PES).8a,9,16b In contrast, the η6 structure is typical for the associates of coordinatively saturated metal cations and ammonium with arenes.8a In these species, the electrostatic interaction between the cation charge and the quadrupole moment of the aromatic ring2a,22 plays an important role in the η6 cation π bonding. A principal question arises as to whether the obtained knowledge about the structural peculiarities of the complexes of coordinatively unsaturated σ electrophiles R3C+ and R3Si+ with arenes is also true for their complexes, which have a cage structure of the type D (Scheme 3). Indeed, the incorporation of the carbenium or silylium cationic center in D may result in steric forcing to its η6 or η1π rather than σ or σ/π coordination with the arene ring. To the best of our knowledge, no data on the structures of the cage complexes of trivalent carbon and silicon atoms with aromatic bases are available in the literature. In this connection we have performed a quantum chemical study of the structure of cations [C((CH2)3)3C6H3]+ (1a) and [Si((CH2)3)3C6H3]+ (2a) of the existing molecules of carba(HC((CH 2 ) 3 ) 3 C 6 H 3 ) 2 3 and silacyclophanes (RSi((CH2)3)3C6H3),24 as well as of the theoretically designed 1b,c, 2b,c, 3a, and 4a. The mass spectrum of MeSi-

Therefore, there are no data concerning the existence of the ideal (α ≤ 90°) stable η1π associates of the trivalent silicon atom with aromatic bases. It should be noted that in addition to the α angle also other factors, including the energy and the length of the coordination Si−Cipso contact and charge distribution,7a,e,13b,14,16 were used in solving the question of the character of the complexes of silylium cations with arenes. In spite of the structural variety of R3E+ complexes (E = C, Si) with arenes (Schemes 1 and 2), in essentially all the aforementioned experimentally obtained and theoretically studied compounds the cationic center is localized above one carbon atom of the benzene ring. The reason for this may be seen in the following considerations:1a,8a,19 upon the formation of adducts of the coordinatively unsaturated σ electrophiles R3C+ and R3Si+ with arenes an important role is played by the orbital interactions, whichdue to the structure of key orbitals (involving a doubly degenerate π HOMO of the donor ring and an unoccupied σ orbital of the acceptor cationic center) are effective only in the η1π andto a lesser extentη2 π structures and are maximized at the transition to the σ structure.20 Therefore, the formation of the “face”, i.e. η6, complexes [R3C·arene]+ or [R3Si·arene]+ is ruled out,21 and they 7512


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Figure 1. MP2/6-311++G(d,p) stable C1-unsymmetrical structures of the carba- (1a) and silacyclophane cations (2a,b) and their AIM molecular graphs. The bond critical points BCP(3,−1) are denoted by solid squares, and the ring critical points RCP(3,+1) are designated by open circles.

Table 1. Selected Geometrical Parameters,a Chemical Shifts δ(13C) and δ(29Si),b NICS(1) Values in the Benzene Ring, and the Relative Stabilitiesc of the C3-Symmetrical (η6) and C1-Unsymmetrical (η1) Forms of the Cage Cations 1−4 cation 1a(C1) 1a(C3) 1c(C1) 1c(C3) 2a(C1) 2a(C3)d 2b(C1) 2b(C3)d 2c(C1) 2c(C3)d 3a(C1) 3a(C3)d 4a(C3)

dEAr 2.609 2.620 2.375 2.071 2.455 2.086 1.805

dEC1 1.673 2.971 1.935 2.966 2.209 2.760 1.901 2.541 2.436 2.817 1.608 2.525 2.303

dEC2 2.953 2.972 2.762 2.496 2.837 2.508 2.297

∑∠(E)

α

∑∠(C1)

A

δ(13C)/δ(29Si)

NICS(1)

ΔE

ΔG

330.5 356.1 337.2 356.7 345.1 347.4 336.3 327.3 349.0 351.6 277.0 301.2 264.3

90.0

346.8

83.2

354.9

77.5

359.3

87.8

352.2

72.8

359.9

79.9

357.0

0.876 0.999 0.952 0.999 0.987 1.000 0.989 1.000 0.996 1.000 0.917 0.998 0.999

95.7 367.6 189.6 369.4 138.9 260.5 24.7 80.4 218.2 300.5 113.5 362.9 17.8

−3.2 −6.0 −5.7 −6.9 −6.2 −6.0 −6.4 −6.0 −7.2 −6.9 −3.5 −4.7 −5.4

0.00 9.69 1.99 0.00 0.00 3.17 0.0 17.12 0.00 0.75 0.0 20.0

0.00 9.58 1.96 0.00 0.00 4.44 0.0 18.99 0.00 2.50 0.0 20.45

a The distance from the cationic center E to the central point of the benzene ring (dEAr, Å) and to its carbon atoms C1 (dEC1, Å) and C2 (dEC2, Å), the sum of the bond angles at the central atom E (∑∠(E), deg), the angle between the EC1 bond and the vector C1C4 (α, deg), and the sum of the bond angles at the arene atom C1 (∑∠(C1), deg); A is a Julg parameter. bThe calculated and experimental chemical shifts δ(13C) and δ(29Si) for carbenium CMe3+ (δcalc(13C) 343.8 ppm; δexp(13C) 335.2 ppm34) and silylium SiMe3+ (δcalc(29Si) 402.9 ppm) cations. cThe difference in internal energies of the η1 and η6 forms of cations 1−3 corrected for the zero-point vibrational energies (ΔE, kcal/mol) and in Gibbs free energies (ΔG, kcal/ mol) calculated under the normal conditions (298.15 K, 1 atm). dThe structure has two imaginary frequencies.

values of ΔG and ΔG⧧ does not exceed 0.5 kcal/mol. For this reason only B3LYP/6-311++G(d,p) thermal corrections to ΔE were calculated for the processes of proton migration and deprotonation. The stationary points on the potential energy surface (PES) were identified by the number of the negative Hessian eigenvalues. Geometry optimization of the 1:1 complexes of cations 1, 2, and 4a with anions BX4− (X = F, C6F5) was carried out with MP2 and B3LYP methods using the 6-31+G(d,p) basis set. A search for associates 1·BF4−, 2·BF4−, and 4a·BF4−, which differ in the mutual arrangement of their components and which correspond to local minima on the PES, was performed by moving BF4− relative to the cationic moiety (the vibrational frequencies in this case were computed at the B3LYP/631+G(d,p) level of theory). The free energy of formation of 1·BF4−, 2·BF4−, 2a·B(C6F5)4−, and 4a·BF4−, ΔGc(298.15 K, 1 atm) was determined as the difference in the free energies of the complex and its initial components with the basis set superposition error (BSSE) correction, which was calculated using the counterpoise method.25

((CH2)3)3C6H324a shows a peak for (M − CH3)+ at m/z 229 corresponding to the cation 2a, but its character is unknown.

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COMPUTATIONAL DETAILS

The structure of the cations 1−4 was studied by the MP2/6-311+ +G(d,p) method. The energy difference between their C1-unsymmetrical and C3-symmetrical isomers was corrected for zero-point vibrations (ZPV), which were computed at the MP2/6-311++G(d,p) level of theory. When estimating the free energy ΔG (298.15 K, 1 atm) for the processes of migration of an electrophilic center and a proton around the perimeter of arene ring in the intramolecular complexes 1a(C1), 2a(C1), and 4a and their deprotonation, the geometry optimization of all ground and transition states was performed at the MP2/6-311++G(d,p) level of theory. In the case of migration of the electrophilic center a thermal correction to the internal energy E was calculated using the MP2/6-311++G(d,p) and less time-consuming B3LYP/6-311++G(d,p) methods. The difference in the corresponding 7513


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Figure 2. Diagram showing the relative stability for structures involved in the η1 ⇆ η1 transitions for the silacyclophane cation [Si((CH2)3)3C6H3]+ (2a).

The energy barrier of the transition 1a(C3) → 1a(C1) is low (0.5 kcal/mol). Therefore, the η6 form of 1a is not stable in both the thermodynamic and kinetic sense. In the case of the silacyclophane cation 2a the minimum was found only for the unsymmetrical structure 2a(C1) (Figure 1), whereas the “face” complex 2a(C3) (destabilized relative to 2a(C1) by 3.2 kcal/mol), as well as the intermolecular η6 associates [R3Si·PhH]+, correspond to a second-order saddle point.8a,9,16b The distances between contacted atoms Cc, C1 in 1a(C1) and Si, C1 in 2a(C1) are more than 1.5 Å shorter (Table 1) than the sum of the van der Waals radii of the corresponding atoms(rvdw(C) = 1.7 Å; rvdw(Si) = 2.1 Å).35 This fact, as well as the noticeable deviation of the sum of the bond angles at E (Table 1) from 360°, corresponding to the free plane cation R3E+, is convincing geometrical evidence for the presence of E← C1 bonding in the intermolecular complexes 1a(C1) and 2a(C1). The steric constraints on the mutual arrangement of the donor C6R3 and the acceptor EC3 fragments, which are characteristic for the cage structures 1a and 2a (“cage effect”), have not exerted a significant influence on the bond distance between the central atom E and the arene carbon C1 (i.e., Cipso). Indeed, the value of dSiCipso in 2a (2.209 Å) and in [Me3Si·PhH]+ (2.212 Å, according to the MP2/6-311++G(d,p) calculations, and 2.174−2.183 Å, according to X-ray data7j) are in good agreement with each other. The length of the Cc−Cipso contact in 1a is only 0.121 Å smaller than the bond length of the central carbon of the t-Bu cation and Cipso of the benzene ring in the σ complex [Me3C·PhH]+ (1.804 Å, MP2/6-311++G(d,p)).36 In contrast, the angles EC1C4(α) are dramatically influenced by the cage effect, which is characteristic of 1a and 2a. Indeed, the values of α in 1a (90.0°) and 2a (77.5°) do not exceed the limiting value for π complexes of 90° and differ strongly from ECipsoCpara(α) in the model compounds [Me3C·PhH]+ (111.6°, MP2/6-311++G(d,p)) and [Me3Si·PhH]+ (101.7°, MP2/6311++G(d,p); 103.5−104.7°, X-ray data7j). The hybridization of the arene carbon atom C1 in 1a and 2a is close to sp2 (see the value of ∑∠(C1) in Table 1). Thus, on the basis of the angle characteristics of the E←C1 bonding (i.e., α and ∑∠(C1)) and the aromatic character of the basal ring (i.e., taking into account the negative sign of NICS(1) and the value of the Julg parameter

The effect of the nonspecific solvation on the geometry of ionic pairs 1c·BF4− was estimated with the conductor-like polarizable continuum model (C-PCM).26 The values of the 13C and 29Si chemical shifts (δ(13C) and δ(29Si), respectively) were calculated within the GIAO B3LYP/6-311++G(2d,p) approximation (δ = σ(TMS) − σ(compound), where σ is the shielding constant, σ13C(TMS) = 183.2 ppm; σ29Si(TMS) = 329.4 ppm). The nucleus-independent chemical shifts (NICS) were estimated at the same level of theory at a distance of 1 Å from the center of the benzene ring (NICS(1)).27 All calculations were performed using the GAUSSIAN 0928 program package. At a structural level the degree of electron delocalization for the benzene ring in 1−4 was estimated using the value of the Julg parameter A:29 A = 1 − (225/n) ∑ (1 − ri /r )2 where n = 6 is the number of delocalized C−C bonds in the system, ri is the length of an individual C−C bond, and r is the mean length of six C− C bonds. In benzene, A = 1 (which corresponds to the absence of bond length alternation). In the Kekulé form of benzene, A = 0 (which corresponds to r(C−C) = 1.520 Å and r(CC) = 1.330 Å). For the AIM30 analysis of the MP2(Full)/6-311++G(d,p) electron distribution ρ(r) of molecules 1, 2, and 4a the Morphy 1.031 program was employed. The molecular orbital and natural bond orbital (NBO)32 analyses were carried out for the HF/6-31G(d) wave functions. It should be noted that some useful approaches cannot be used for studying the cation π bonding in the intramolecular complexes 1−4, as opposed to intermolecular complexes (which happens because of the problem of the choice of the model compounds). Examples are the energy decomposition analysis, EDA,33 or charge distribution analysis with an estimation of the extent of charge transfer from the aromatic fragment to a cationic center.

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RESULTS AND DISCUSSION η structures of Carba- (1) and Silacyclophane (2) Cations. The energy minima of the cyclophane cation 1a correspond to its C1-unsymmetrical (1a(C1)) and C3-symmetrical “face” (1a(C3)) structures, the latter lying 9.7 kcal/mol higher (Figure 1). The distances from the arene carbons C1 and C2 to the central carbon atom Cc of the essentially flattened equatorial fragment CcC3 (see the ∑∠(E) values in Table 1) differ by not more than 0.02 Å in isomer 1a(C3). Therefore, it may be assigned to the η6 type. 1

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Figure 3. Relative stability of isomers of the cations 1a and 2a, formed by proton migration from the ipso (C1) to the para (C4) position of the arene ring.

chemical shifts in the comparable model structures Me3C+ and Me3Si+ (see Table 1). In the cage cations 1a and 2a, the migration barriers (ΔG⧧) of the acceptor centers Cc and Si from the C1 to the C3 and C5 arene carbons (η1 ⇆ η1 transition) are equal to 10.1 and 3.2 kcal/mol, respectively (Figure 2, in the case of 2a). It is pertinent to note that the migration barrier of the methyl cation in [H3C·PhH]+ exceeds significantly that for the migration of H3Si+ in [H3Si·PhH]+.8a According to the ΔG⧧ value, the observed geometry of the C1-unsymmetrical complex 2a, as distinct from that of 1a, can be C3 symmetrical in a dynamic sense (on the NMR time scale). The relative stability of the isomers, which are formed upon proton transfer from the ipso to the para position of the aromatic ring in the acyclic systems [R3C·PhH]+ and [R3Si·PhH]+, depends on the nature of the cationic center.16b,41 Whereas for [R3C·PhH]+ the most stable structure is that with the position of the proton para to the alkyl group, for [R3Si·PhH]+ the most stable structure is the one with the position of the proton ipso to the silyl group. In contrast, in a plausible process of the proton migration around the benzene ring, the initial ipso isomers of structures 1a and 2a are found to be energetically more favorable (see Figure 3). This may be ascribed to the fact that the ortho (C2, C6) and para (C4) arene carbons in cations 1a and 2a are connected not with the hydrogen atoms, as in [R3C·PhH]+ and [R3Si·PhH]+, but with carbon atoms of the side chains. For this reason, C2, C4, and C6 in 1a and 2a have positive charges (see NBO charges in Table S1 in the Supporting Information), rather than negative charge, as in [R3C·PhH]+ and [R3Si·PhH]+.41b Such a charge distribution, as well as the steric effects that are characteristic of 1a and 2a, apparently lead to the differences between their proton migration energies and those of [R3C·PhH]+ and [R3Si·PhH]+.

A, Table 1), one can safely assign the intramolecular complexes 1a and (especially) 2a to the η1π type. The Cc←C1 bond distance in 1a is only ∼0.14 Å longer than a single C−C(aryl) bond (1.53 Å).37a In contrast, the distance between the cationic center Si and the arene carbon, contacted with Si, in 2a as well as in the associates [R3Si·arene]+,7a,j,8c,16 significantly (by ∼0.3 Å) exceeds the length of a “normal” Si− C(aryl) bond (1.88 Å).37b This difference is practically leveled out in the η1 complex [Si(CH2)3)3C6Li3]+ (2b; dSiC1 = 1.90 Å) having a more donating basal ring than 2a. The Si←C1 coordination in 2b (see the α, ∑∠(C1), A and NICS(1) values in Table 1), as well as in 2a, has π character. In the series of known Si complexes with arenes7a,g,i,j,8c,13,16,17 such short dative contacts as in the case of 2b were not observed earlier. The AIM analysis38 of the electron density distribution detected a BCP(3,−1) bond critical point in the internuclear E···C1 region of 1a and 2a,b (Figure 1). From the topological characteristics at this point, namely, the electron density (ρ(rc)), the Laplacian of the electron density (∇2ρ(rc)), the electron energy density (E(rc)) (1a, ρ(rc) = 1.142 e/Å3, ∇2ρ(rc) = −5.407 e/Å5, E(rc) = −0.74 hartree/Å3; 2a, ρ(rc) = 0.382 e/Å3, ∇2ρ(rc) = 0.301 e/Å5, E(rc) = −0.19 hartree/Å3; 2b, ρ(rc) = 0.660 e/Å3, ∇2ρ(rc) = 5.239 e/Å5, E(rc) = −0.38 hartree/Å3), the coordinative contact Cc←C1 in 1a may be assigned, as seen by the negative sign of ∇2ρ(rc), to a covalent type of interatomic interaction, whereas Si←C1 in 2a,b can be assigned to an intermediate type (from the positive sign of ∇2ρ(rc) the contacts can be characterized as ionic, but on the basis of the negative sign of E(rc)39 they should be assigned to a covalent type). The presence of Cc←C1 bonding in complex 1a and the Si← 1 C bonding in 2a,b is supported, along with the geometrical and AIM criteria, also by their calculated chemical shifts δ(13C) and δ(29Si). As expected,40 they are markedly upfield relative to the 7515


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Figure 4. MP2/6-311++G(d,p) optimized geometries of the stable “face” cations 1c(C3) and 4a(C3), an AIM molecular graph, and properties of the SiC2, SiC4, and SiC6 bond critical points for 4a(C3). BCP(3,−1) are denoted by solid squares, and RCP(3,+1) are designated by open circles.

∼20 kcal/mol more stable relative to the C3-symmetrical isomer. Moreover, the latter corresponds not to a minimum on the PES but to a second-order saddle point (Table 1). In contrast, we failed to fix the formation of the unsymmetrical form 4a(C1) on the PES of the cation 4a. A stationary point that corresponds to its “face” C3-symmetrical isomer was found (Figure 4). In 4a(C3) the distances from Si to the arene carbons C1 and C2 differ less than by 0.01 Å (Table 1). Thus, the Si← arene coordination in this species can be assigned to the η6 type. As follows from ΔG, the process of proton migration from the atom C1 to the atoms C2 (C6), C3 (C5), and C4 in 4a with the loss of C3 symmetry is energetically disadvantageous. Indeed, this process leads to formation of the unsymmetrical structures 4a′, 4a′′, and 4a′′′, which are less favorable than the initial species 4a by 70.2, 39.0, and 62.9 kcal/mol, respectively (see Figure S1 in the Supporting Information). Species 4a exhibits (Figure 4) significant displacement of the cationic center Si along the C3 symmetry axis from the equatorial plane toward the basal ring (ΔSi > 1 Å). Judging by the ∑∠(Si) value, the configuration of the bonds around Si in 4a (Table 1) corresponds to a trigonal bipyramid: i.e., the valence state of the silicon atom is close to the ideal p3 state, in which ∑∠(Si) = 270°. The distance from Si to a middle point of the practically planar benzene ring in 4a is ∼0.08 Å shorter than the length of the standard bond Si−C(aryl) (1.88 Å).37b Such a deep immersion of the trivalent silicon into the benzene ring π system of the η6 structure of 4a, according to the AIM analysis of the electron density, manifests itself (Figure 4) in the presence of the three bond paths SiC2, SiC4, and SiC6 (six bond paths were found with the MP2(full)/6-31G(d) wave function) and also in the absence of the ring critical point RCP(3,+1) in the donor ring and the cage critical point CCP(3,+3) inside the molecular cage. It is pertinent to note that similar AIM topological properties are exhibited by symmetrical intermolecular complexes of metal ions M (M = V+, Mn+, Co+, Cu+, etc.) with benzene.43 The multicenter contact Si←arene in 4a (Figure 4), as judged by the topological characteristics at the corresponding BCP(3,−1), should be assigned to an intermediate type of interatomic interaction.39 The calculated chemical shift δ(29Si) of the η6 complex of 4a (δ(29Si) 17.8 ppm) falls in the interval which is typical for tetraorganosilanes,44 and it is clearly significantly upfield relative to δ(29Si) in the model cation (Me)3Si+, and even in the related structure 2a (see Table 1). The polycyclic structure 4a45 does not seem to be among the anomalously strained ones. This can be seen from the following observations. (i) The value of the angle β (∼20°) in 4a, which is a measure of the deviation of the C−C bond from the benzene ring

“Face” η6 Structures of Carba- (1, 3) and Silacyclophane (2, 4) Cations. In spite of the steric assistance to the η6 rather than η1π coordination of the trivalent Cc and Si with an aromatic ring in complexes 1a and 2a, their C1-unsymmetrical isomers turned out to be energetically favored (Table 1) over the less constrained C3-symmetrical structures (for example, the isomer 1a(C1) according to MM3 estimations42 is 19.2 kcal/mol more strained than 1a(C3)). A possible reason for this can be the relative weakness of the multicenter bonding E←arene, which is characteristic of the “face” structures 1a(C3) and 2a(C3), as compared with the two-center coordination E←C1 in 1a(C1) and 2a(C1). The existence of 1a and 2a exclusively in the η6 form should be assisted by the following factors: (i) weakening of the E←C1 interaction in the η1π complexes 1a and 2a and (ii) creation of additional steric constraints on the way of the unsymmetrical deformation of their cage (transition of the η6 isomer to the η1 isomer). As mentioned above, the latter is induced by the orbital interaction of the arene HOMO and the unoccupied p orbital of the electrophilic center. The weakening of the E←C1 coordination in 1a(C1) and 2a(C1) can be attained by replacing the hydrogen atoms in the 1-, 3-, and 5-positions of their arene fragment by highly electronegative fluorine atoms: i.e., by decreasing the donor ability of the basal ring. It turns out that for the cation [C((CH2)3)3C6F3]+ (1c) the minima are located, which correspond both to the η6 form (Figure 4) and to the η1 form. According to the ΔG values, the former is ∼2 kcal/mol more stable than the latter (Table 1). As expected, the Cc←C1 contact in the η1π isomer of 1c (see the α, ∑∠(C1), A and NICS(1) values in Table 1) is essentially longer than that in the η1π isomer of 1a. In contrast with this, the structural characteristics of the η6 forms of 1a and 1c are in good agreement with each other. In these η6 forms, the multicenter coordination Cc←arene is very weak, which follows from the geometrical and NMR criteria (see the values of dCcAr, ∑∠(E), and chemical shifts δ(13C) in 1a(C3), 1c(C3), and CMe3+ in Table 1). For the cation [Si((CH2)3)3C6F3]+ (2c), as in the case of 2a, the minimum was found only for the unsymmetrical structure 2c(C1), while the “face” complex 2c(C3) (destabilized relative to 2c(C1) by 2.5 kcal/mol; see the ΔG values in Table 1) corresponds to a second-order saddle point. One of the possible ways for creation of the additional steric constraints against unsymmetrical deformation of the cages [E((CH2)3)3C6H3]+ is a linking of their equatorial carbon atoms by methylene bridges (transition from 1a and 2a to 3a and 4a, respectively). For 1a such a modification of its structure (cyclization of the equatorial fragment CcC3) did not give the expected result. Indeed, the C1-unsymmetrical isomer of 3a is 7516


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Figure 5. Orbital-correlation diagram for MO formation in the Si···Ar fragment of the “frozen” η6 cation 4a from the π orbitals of the benzene ring and the unoccupied 3pz orbital of the electrophilic center Si.

The cation 1a can be considered as an intermediate of the intramolecular alkylation reaction, while the cations 2a and 4a correspond to intermediates of the arene silylation reaction. The deprotonation process of 1a, 2a, and 4a, leading to the neutral products 5−7, respectively, turns out to be energetically unfavorable even in the presence of a strong base such as NEt3 (see eqs 1−3 in Figure 7). Indeed, the free energy change (ΔG) is equal to 12.5, 30.3, and 36.8 kcal/mol for eqs 1−3, respectively.48 The cage structures 5− 7 seem to be considerably more strained then the initial cations 1a, 2a, and 4a. Effect of Counterion and Solvent Polarity on the Hapticity of the Cations 1, 2, and 4a. The conclusions about the structure of the cations 1, 2, and 4a, as well as about the relative stability of their symmetrical and unsymmetrical forms, made on the basis of the MP2/6-311++G(d,p) calculations, also hold qualitatively at the MP2/6-31+G(d,p) and B3LYP/631+G(d,p) levels of theory (see Table S2 in the Supporting Information). This allowed us to consider the counterion effect on the gas-phase structures of 1, 2, and 4a by performing a MP2/ 6-31+G(d,p) and B3LYP/6-31+G(d,p) study of their associates with anions BX4− (X = F, C6F5), formed as simple 1:1 compositions. Moreover, using the example of 1c·BF 4 − complexes, which differ in the mutual arrangement of their component, we found that the much less time-consuming B3LYP method, as compared with MP2, can be used for the qualitative estimation of the anion effect (see Figure S2 in the Supporting Information). In the energetically favorable ionic pair 1c·BF4− with BF4− at the top of 1c (Figure 8), the cation 1c is in its C3-symmetrical “face” form. This seems to promote efficient attractive interaction of 1c and BF4− by means of three equivalent contacts BF···HC. In this connection it should be emphasized that the alternative associate 1c(C1)·BF4− is more than 20 kcal/mol less stable than 1c(C3)·BF4−, irrespective of its geometry and calculation method (Figure S2 in the Supporting Information). Surprisingly, it was found that complex 1a(C1)·BF4− is 5.3 kcal/mol destabilized with respect to the alternative 1a(C3)·BF4−, though the isolated cation 1a exists exclusively in the C1-unsymmetrical form (Figure S3 in the Supporting Information). The η1π structure of 2a (unlike that of 2b) is destroyed by the interaction with tetrafluoroborate BF4−, while it is retained in the

plane (Figure 4), falls inside the range 0−48°. According to the X-ray data, the latter interval is typical for cyclophane derivatives.46 (ii) The difference in the geometrical characteristics calculated for the four-membered cycles SiC3 in 4a and those obtained with electron diffraction in dimethylsilacyclobutane Me2SiC3H647 is comparatively small (bond lengths differ less than by 0.06 Å, and bond angles vary less than by 7°). In the NBO analysis, the arene ring of 4a is represented by six orbitals of the σ type and three orbitals of the π type corresponding to the CC bonds.24b The total energy of the donor−acceptor interaction of the three πC−C orbitals with the unoccupied orbital 3pz of Si in the η6 cation 4a is significant in magnitude. According to the estimate obtained using secondorder perturbation theory, ∑ΔE(2)[πC−C→3pz] amounts to 140.4 kcal/mol. As could be expected, it exceeds considerably the value of ∑ΔE(2)[πC−C→2pz] that characterizes the Cc←arene interaction in the η6 structures (18.4 kcal/mol in 1a and 15.6 kcal/mol in 1c). At the molecular orbital level a strong multicenter Si←arene coordination in the “frozen face” form 4a can be represented by the simple diagram given in Figure 5. For symmetry reasons, the unoccupied 3pz orbital of the electrophilic center Si can mix only with the a2u bonding MO of the arene ring with the formation of two MOs belonging to the pyramidal fragment Si···Ar: bonding Ψ1 and antibonding Ψ3. The MO Ψ2 comprises doubly degenerate orbitals of arene of the eg and eg′ type, which do not interact, in the case of the rigid backbone of 4a, with the 3pz orbital of Si (see Figure 5). Among the molecular orbitals of 4a one can recognize the composite orbitals Ψ1 and Ψ3, corresponding to the diagram in Figure 5 (Figure 6, Ψ1 as an example).

Figure 6. HF/6-31G(d)//MP2/6-311++G(d,p) bonding MO (HOMO-27) of the Si←arene fragment of the “face” cation 4a. 7517


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Figure 7. Deprotonation of the cations 1a, 2a, and 4a.

Figure 8. B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) (in boldface) optimized geometries of the most stable ionic pairs 1c(C3)·BF4− and 4a·BF4− in the isolated state and in solvents (toluene in parentheses, DMSO in brackets).

case of interaction with the weaker nucleophile B(C6F5)4− (see Figure S4 in the Supporting Information). In the former case fluoro anion transfer from BF4− to 2a is observed with the formation of a weak complex between trifluoroborate BF3 and the molecule of fluorosilacyclophane FSi((CH2)3)3C6H3 (see Figure S4 in the Supporting Information). This complex is unstable upon return to “normal” conditions (ΔEc = −5.6 kcal/

mol; ΔGc = 4.6 kcal/mol). In the latter the internuclear F···Si distance is close to the length of the standard bond F−Si (1.60 Å).37b In the 4a·BF4− complex of any structure the “face” gas-phase structure 4a is retained (Figure S5 in the Supporting Information). Among the structures of this complex, which differ in the mutual arrangement of its components, the most 7518


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The cations 1c, 2b, and 4a form ionic pairs with tetrafluoroborate BF4−, in which their gas-phase η1π (2b) or η6 (1c, 4a) structure is not destroyed. The η1π structure of 2a is retained only in the case of interaction with the relatively weak nucleophile B(C6F5)4−. In the complex 1a·BF4− the cation 1a has a “face” structure, rather than the expected η1 structure.

favorable one is characterized by a side relative position of 4a and BF4− (Figure 8) with ΔGc = −64.2 kcal/mol. The symmetry of 4a in this case does not correspond strictly to the C3 point group. Interestingly, the multicenter coordination Si←arene in the cation 4a is enforced when it enters into the composition of 4a·BF4 (see the Si···Ar distance in Figure 8). In contrast, in the cationic part of the stable complexes 1a(C3)·BF4− (ΔGc = −74.0 kcal/mol), 1c(C3)·BF4− (ΔGc = −77.5 kcal/mol), 2a(C1)·B(C6F5)4− (ΔGc = −47.9 kcal/mol), and 2b(C1)·BF4− (ΔGc = −105.1 kcal/mol) we observe an insignificant increase in the length of the contacts of electrophilic centers with the arene carbon C1 or with the arene (as in the case of 1a(C3)·BF4− and 1c(C3)·BF4−) as compared with those in free 1a,c and 2a,b (see examples in Figure 8). By the example of 1c·BF4− we have verified that in agreement with expectations49 the transition of the aforementioned ionic pairs to solution results in a weakening of the interaction between their components. The increase in the dielectric constant of the medium is accompanied by a decrease in the difference between the values of geometrical parameters of cations which are involved in associates and those in the isolated states (see Figure 8 and Figure S2 in the Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 with NBO charges on selected atoms of 1a(C1) and 2a(C1), Figure S1 with optimized geometries of 4a′, 4a′′, and 4a′′′, Table S2 with selected geometrical parameters and the relative stability of the C3-symmetrical and C1-unsymmetrical forms of 1, 2a, and 4a, Figures S2−S5 with optimized geometries of the possible structures of complexes 1c·BF4−, 1a·BF4−, 2a·BF4−, 2a·B(C6F5)4−, 2b·BF4−, and 4a·BF4−, and tables with Cartesian coordinates of calculated structures 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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*Tel: +7-3952-426545. Fax: +7-3952-419346. E-mail: svf@ irioch.irk.ru.

CONCLUSIONS On the basis of the MP2/6-311++G(d,p) calculation data, cage cations [E((CH2)3)3C6R3]+ (E = C (1), Si (2); R = H (a), Li (b), F (c)), which are characterized by the presence of the effect of steric constraints on the mutual arrangement of the electrophilic center E and the arene fragment, should be referred to as πdonor-stabilized structures. The energy minima for structure [C((CH2)3)3C6H3]+ (1a) correspond to η1π- and η6 “face” isomers, the latter lying 9.7 kcal/mol above the former. The cation [Si((CH2)3)3C6H3]+ (2a) which has the peak (M − CH3)+ at m/z 229 in the mass spectrum of methylsilacyclophane, exists in the firmly established η1π form, as opposed to the known complexes of the trivalent silicon atom with aromatic bases. In the η1π intramolecular complex [Si((CH2)3)3C6Li3]+ the length of the coordination contact (1.90 Å) is unprecedentedly short and practically coincides with the length of the standard bond Si−C (1.88 Å). The migration barriers (ΔG⧧) of the cationic centers Cc and Si from the C1 to the C3 and C5 arene carbons (η1 ⇆ η1 transition) are equal to 10.1 and 3.2 kcal/mol in 1a and 2a, respectively. Therefore, the observed geometry of the C1unsymmetrical complex 2a, in contrast to 1a, can belong to the C3-symmetrical configuration in a dynamic sense of the NMR time scale. The energetic advantage of the initial ipso structure is observed in a plausible process of the proton migration around the benzene ring in 1a(C1) and 2a(C1). The first representatives of the complexes of trivalent carbon and silicon atoms with arene, which exist exclusively in the η6 form, were found by means of (i) weakening of the Cc←Cipso interaction in [C((CH2)3)3C6H3]+, i.e., by the transition to the fluoro derivative [C((CH2)3)3C6F3]+ (1b) and (ii) creation of additional steric constraints in the path to the unsymmetrical deformation of their cage [Si((CH2)3)3C6H3]+, that is, by linking of their equatorial carbon atoms by methylene bridges, as in structure 4a. The distance from the cationic center Si to the middle point of the practically planar arene ring in 4a is shorter than the length of the single bond Si−C, suggesting a deep immersion of Si into the benzene ring π system. The deprotonation process of the intramolecular complexes 1a(C1), 2a(C1), and 4a is energetically unfavorable even in the presence of a strong base such as NEt3.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. P. L. A. Popelier for a copy of the MORPHY 1.0 program. Financial support from the Russian Foundation for Basic Research (Grant RFBR 07-03-00888) is gratefully acknowledged.

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REFERENCES

(1) See, for example: (a) Hubig, S. M.; Lindeman, S. V.; Kochi, J. K. Coord. Chem. Rev. 2000, 200−202, 831−873. (b) Schröder, D.; Wesendrup, R.; Hertwig, R. H.; Dargel, T. K.; Grauel, H.; Koch, W.; Bender, B. R.; Schwarz, H. Organometallics 2000, 19, 2608−2615. (c) Hara, K.; Akiyama, R.; Sawamura, M. Org. Lett. 2005, 7, 5621−5623. (d) Klare, H. F. T.; Bergander, K.; Oestreich, M. Angew. Chem., Int. Ed. 2009, 48, 9077−9079. (e) Yi, H.-B.; Lee, H. M.; Kim, K. S. J. Chem. Theory Comput. 2009, 5, 1709−1717. (f) Duttwyler, S.; Douvris, C.; Fackler, N. L. P.; Tham, F. S.; Reed, C. A.; Baldridge, K. K.; Siegel, J. S. Angew. Chem., Int. Ed. 2010, 49, 7519−7522. (2) See, for example: (a) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303−1324. (b) Choi, H. S.; Suh, S. B.; Cho, S. J.; Kim, K. S. Proc. Natl. Acad. Sci. U.S.A. 1998, 12094−12099. (c) McFail-Isom, L.; Shui, X.; Williams, L. D. Biochemistry 1998, 37, 17105−17111. (d) Zarić, S. D.; Popović, D. M.; Knapp, E.-W. Chem. Eur. J. 2000, 6, 3935−3941. (e) Gokel, G. W.; Barbour, L. J.; Ferdani, R.; Hu, J. Acc. Chem. Res. 2002, 35, 878−886. (3) See, for example: (a) Hong, B. H.; Bae, S. C.; Lee, C.-W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348−351. (b) Hong, B. H.; Lee, J. Y.; Lee, C.-W.; Kim, J. C.; Bae, S. C.; Kim, K. S. J. Am. Chem. Soc. 2001, 123, 10748−10749. (c) Singh, N. J.; Lee, H. M.; Suh, S. B.; Kim, K. S. Pure Appl. Chem. 2007, 79, 1057−1075. (4) (a) Hubig, S. M.; Kochi, J. K. J. Org. Chem. 2000, 65, 6807−6818. (b) Lenoir, D. Angew. Chem., Int. Ed. 2003, 42, 854−857. (5) (a) Banthorpe, D. V. Chem. Rev. 1970, 70, 295−322. (b) Olah, G. A. J. Chem. Res 1971, 4, 240−248. (6) (a) Allen, W. N.; Lampe, F. W. J. Chem. Phys. 1976, 65, 3378−3379. (b) Wojtyniak, A. C. M.; Stone, J. A. Int. J. Mass. Spectrom. Ion Processes 1986, 74, 59−79. (c) Cacace, F.; Crestoni, M.; De Petris, G.; Fornarini., S. Can. J. Chem. 1988, 66, 3099−3107. (7) (a) Lambert, J. B.; Zhang, S.; Stern, S. L.; Huffman, J. C. Science 1993, 260, 1917−1918. (b) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. 7519


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Article

Commun. 1993, 383−384. (c) Maerker, C.; Schleyer, P. v. R. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, pp 513−555. (d) Pauling, L. Science 1994, 263, 983−983. (e) Lambert, J. B.; Zhang, S.; Stern, S. L.; Ciro, S. M. Organometallics 1994, 13, 2430−2443. (f) Kochina, T. A.; Vrazhnov, D. V.; Sinotova, E. N.; Voronkov, M. G. Usp. Khim. 2006, 75, 107−124; Russ. Chem. Rev. (Engl. Transl.) 2006, 75, 95−110. (g) Duttwyler, S.; Do, Q.-Q.; Linden, A.; Baldridge, K. K.; Siegel, J. S. Angew. Chem., Int. Ed. 2008, 47, 1719−1722. (h) Romanato, P.; Duttwyler, S.; Linden, A.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 2010, 132, 7828−7829. (i) Romanato, P.; Duttwyler, S.; Linden, A.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 2011, 133, 11844−11846. (j) Ibad, M. F.; Langer, P.; Schulz, A.; Villinger, A. J. Am. Chem. Soc. 2011, 133, 21016−21027. (8) (a) Miklis, P. C.; Ditchfield, R.; Spencer, T. A. J. Am. Chem. Soc. 1998, 120, 10482−10489. (b) Smith, W. B. J. Phys. Org. Chem. 2002, 15, 347−352. (c) Cypryk, M. J. Organomet. Chem. 2003, 686, 164−174. (d) Ishikawa, Y.; Yilmaz, H.; Yanai, T.; Nakajima, T.; Hirao, K. Chem. Phys. Lett. 2004, 396, 16−20. (e) Zheng, F.; Sa, R.; Cheng, J.; Jiang, H.; Shen, J. Chem. Phys. Lett. 2007, 435, 24−28. (9) Heidrich, D. Angew. Chem., Int. Ed. 2002, 41, 3208−3210. (10) Borodkin, G. I.; Nagi, S. M.; Gatilov, Y. V.; Sharikov, M. M.; Rybalova, T. V.; Shubin, V. G. Zh. Org. Khim 1992, 28, 1806−1826. (11) The formation of the less stable η2π associate was detected9 along with the η1π associate. (12) Spencer, T. A.; Popovici-Muller, J.; Van Beusichem, B.; MacMillan, C. V.; Lavey, C. F.; Sin, J. M.; Ditchfield, R. Tetrahedron 2010, 66, 4441−4451. (13) (a) Panisch, R.; Bolte, M.; Muller, T. Organometallics 2007, 26, 3524−3529. (b) Reed, C. A. Acc. Chem. Res. 1998, 31, 325−332. (14) (a) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993, 262, 402−404. (b) Reed, C. A.; Xie, Z. Science 1994, 263, 985−986. (c) Reed, C. A.; Kim, K.-C.; Stoyanov, E. S.; Stasko, D.; Tham, F. S.; Mueller, L. J.; Boyd, P. D. W. J. Am. Chem. Soc. 2003, 125, 1796−1804. (15) The authors have been assigned the intramolecular complexes B (Scheme 2) to the η1π type by a collective body of X-ray data, though the angle α is equal to 102.8° in a7g and 109.4° in b.7i (16) (a) Olsson, L.; Cremer, D. Chem. Phys. Lett. 1993, 215, 433−443. (b) Schleyer, P. v. R.; Buzek, P.; Muller, T.; Apeloig, Y.; Siehl, H. -U. Angew. Chem., Int. Ed. Engl. 1993, 32, 1471−1473. (c) Olah, G. A.; Rasul, G.; Li, X.-Y.; Buchholz, H. A.; Sanford, G.; Prakash, G. K. S. Science 1994, 263, 983−984. (d) Olah, G. A.; Rasul, G.; Buchholz, H. A.; Li, X.-Y.; Prakash, G. K. S. Bull. Soc. Chim. Fr. 1995, 132, 569−574. (e) Olah, G. A.; Rasul, G.; Prakash, G. K. S. J. Organomet. Chem. 1996, 521, 271−277. (17) (a) Meyer, R.; Werner, K.; Muller, T. Chem. Eur. J. 2002, 8, 1163− 1169. (b) Choi, N.; Lickiss, P. D.; McPartlin, M.; Masangane, P. C.; Veneziani, G. L. Chem. Commun. 2005, 6023−6025. (18) According to the opinion of Müller et al.13a the arguments given in ref 17b that the cations C may be described not as arenium but as bissilylium with a Ph bridge are not quite convincing. (19) Pearson, R. G. Symmetry Rules for Chemical Reactions; Wiley: New York, 1976. (20) Upon the interaction of the unoccupied orbital of the cationic center R3C+ and R3Si+ with a doubly degenerate eg, eg′ π MO (HOMO) of arene, a σ electrophile may be connected either to one carbon atom, which possesses a maximal amplitude in the eg form, or to two carbon atoms in the eg′ form, i.e. with the formation of the η1 or η2 complexes, respectively (see the orbital correlation diagram in Figure 5). (21) In principle, the formation of the η6 complexes [R3C·arene]+ or [R3Si·arene]+ is possible at the relatively large distances (>3.5 Å)8a between reagents, i.e. with a predominance of electrostatic interactions, when the cation does not “feel” the orienting orbital influence of the π system of arene. (22) (a) Doughertu, D. A. Science 1996, 271, 163−168. (b) Garau, C.; Frontera, A.; Quiñonero, D.; Ballester, P.; Costa, A.; Deyà, P. M. J. Phys. Chem. A 2004, 108, 9423−9427. (23) Pascal, R. A., Jr.; Grossman, R. B.; Van Engen, D. J. Am. Chem. Soc. 1987, 109, 6878−6880.

(24) (a) Kwochka, W. R.; Damrauer, R.; Schmidt, M. W.; Gordon., M. S. Organometallics 1994, 13, 3728−3732. (b) Doronina, E. P.; Belogolova, E. F.; Sidorkin, V. F. Organometallics 2011, 30, 5595−5603. (25) Boys, S.; Bernardi, F. Mol. Phys. 1970, 19, 553−566. (26) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995−2001. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669−681. (27) (a) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317−6318. (b) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842−3888. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian, Inc., Wallingford, CT, 2010. (29) Julg, A.; François, P. Theoret. Chim. Acta (Berl.) 1967, 7, 249−259. (30) Bader, R. F. W. Atoms in Molecules: a Quantum Theory; Clarendon Press: Oxford, U.K., 1990. (31) (a) Popelier, P. L. A. Comput. Phys. Commun. 1996, 93, 212−240. (b) Popelier, P. L. A. Chem. Phys. Lett. 1994, 228, 160−164. (32) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926. (b) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1; Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, 1995. (33) (a) Morokuma, K. J. Chem. Phys. 1971, 55, 1236−1244. (b) Ziegler, T.; Rauk, A. Theor. Chim. Acta (Berlin) 1977, 46, 1−10. (34) Olah, G. A.; Donovan, D. J. J. Am. Chem. Soc. 1977, 99, 5026− 5039. (35) Bondi, A. J. Phys. Chem. 1964, 68, 441−451. (36) According to our calculations, the MP2/6-311++G(d,p) value dSiCipso = 2.212 Å in [Me3Si·PhH]+ is in good agreement with the earlier obtained values dSiCipso = 2.214 Å (HF/6-31G(d)),16d 2.196 Å (PBE1PBE/aug-cc-pVDZ),7j 2.229 Å (B3LYP/6-31G(d)).8c The dependence of dCcCipso in [Me3C·PhH]+ from the calculation method is more pronounced: dCcCipso = 1.604 Å (HF/6-31G(d)),16b 1.695 Å (B3LYP/6-31G(d)),8c 1.769 Å (MP2/6-31+G(d,p)),9 1.804 Å (MP2/ 6-311++G(d,p)). (37) (a) Allen, W. N.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1−S19. (b) Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, pp 181−265. (38) In the literature the applicability of the AIM analysis for the interpretation of unusual interatomic interactions (for example, cation− cation or anion−anion) is discussed. Up to now, this discussion is not closed. However, nobody puts in doubt the foundations of the AIM theory, which continues to be widely applied to study the electronic structure of various molecules. Matta, C. F.; Hernández-Trujillo, J.; Tang, T.-H.; Bader, R. F. W. Chem. Eur. J. 2003, 9, 1940−1951. Haaland, A.; Shorokhov, D. J.; Tverdova, N. V. Chem. Eur. J. 2004, 10, 4416− 4421. Poater, J.; Solà, M.; Bickelhaupt, F. M. Chem. Eur. J. 2006, 12, 2889−2895. Bader, R. F. W. Chem. Eur. J. 2006, 12, 2896−2901. Poater, J.; Solà, M.; Bickelhaupt, F. M. Chem. Eur. J. 2006, 12, 2902−2905. Farrugia, L. J.; Evans, C.; Tegel, M. J. Phys. Chem. A 2006, 110, 7952− 7961. Bader, R. F. W. J. Phys. Chem. A 2010, 114, 7431−7444. Dinda, S.; Samuelson, A. G. Chem. Eur. J. 2012, 18, 3032−3042. Jabłonśki, M. J. 7520


Organometallics

Article

Phys. Chem. A 2012, 116, 3753−3764. Dem’yanov, P.; Polestshuk, P. Chem. Eur. J. 2012, 18, 4982−4993. (39) The analogous AIM characteristics (∇2ρ(rc) > 0; E(rc) < 0) are typical for the X−Si bonds irrespective of the silicon atom coordination number. See, for example: (a) Anglada, J. M.; Bo, C.; Bofill, J. M.; Crehuet, R.; Poblet, J. M. Organometallics 1999, 18, 5584−5593. (b) Lyssenko, K. A.; Korlyukov, A. A.; Antipin, M. Yu.; Knyazev, S. P.; Kirin, V. N.; Alexeev, N. V.; Chernyshev, E. A. Mendeleev Commun. 2000, 88−90. (c) Kocher, N.; Henn, J.; Gostevskii, B.; Kost, D.; Kalikhman, I.; Engels, B.; Stalke, D. J. Am. Chem. Soc. 2004, 126, 5563− 5568. (d) Sidorkin, V. F.; Belogolova, E. F.; Pestunovich, V. A. Chem. Eur. J. 2006, 12, 2021−2031. (e) Sidorkin, V. F.; Doronina, E. P. Organometallics 2009, 28, 5305−5315. (f) Doronina, E. P.; Sidorkin, V. F.; Lazareva, N. F. Organometallics 2010, 29, 3327−3340. (40) (a) Harris, R. K., Mann, B. E., Eds. NMR and the Periodic Table; Academic Press: London, New York, San Francisco, 1978. (b) Olsson, L.; Ottosson, C.-H.; Cremer, D. J. Am. Chem. Soc. 1995, 117, 7460− 7479. (41) (a) Cacace, F.; Crestoni, M. E.; Fornarini., S. J. Am. Chem. Soc. 1992, 114, 6776−6784. (b) Ignat’ev, I. S.; Kochina, T. A. Zh. Obshch. Khim. 2005, 75, 1288−1292; Russ. J. Gen. Chem. (Engl. Transl.) 2005, 75, 1221−1224. (42) Ponder, J. TINKER Version 6.0; Washington University, St. Louis, MO, 2011. (43) Molina, J. M.; Dobado, J. A.; Melchor, S. J. Mol. Struct. (THEOCHEM) 2002, 589−590, 337−347. (44) Gupta, R. R.; Lechner, M. D.; Marsmann, H.; Mikhova, B.; Uhlig, F. In Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology-New Series; Group III Condensed Matter; Springer-Verlag: Berlin, 2008; Vol. 35F, p 463. (45) The top half of cation 4a is the norcubane moiety SiC6. Thus, one of the possible approaches to obtaining 4a might be, at the first stage, the synthesis of the corresponding sila-norcubane and its subsequent involvement in the catalyzed interaction with 1,3,5-substituted benzene and salt R3C+B− (B is a base; Corey reaction): Corey, J. Y.; Gust, D.; Mislow, K. J. Organomet. Chem. 1975, 101, C7. It should be noted that various norcubane systems are known, including those containing silicon: Gilardi, R.; Butcher, R. J.; Bashir-Hashemi, A. Acta Crystallogr., Sect. E: Struct. Rep. Online 2002, 58, o860. Baudler, M.; Scholz, G.; Tebbe, K. -F. Z. Anorg. Allg. Chem. 1990, 581, 111. Veith, M.; Spaniol, A.; Pohlmann, J.; Gross, F.; Huch, V. Chem. Ber. 1993, 126, 2625. Hemme, I.; Klingebiel, U.; Freitag, S.; Stalke, D. Z. Anorg. Allg. Chem. 1995, 621, 2093. (46) (a) Komarov, I. V. Usp. Khim. 2001, 70, 1123−1151; Russ. Chem. Rev. (Engl. Transl.) 70, 991−1016. (b) Es, D. S. v.; Egberts, A.; Nkrumah, S.; de Nijs, H.; de Wolf, W. H.; Bickelhaupt, F.; Veldman, N.; Spek, A. L. J. Am. Chem. Soc. 1997, 119, 615−616. (47) Novikov, V. P.; Tarasenko, S. A.; Samdal, S.; Shen, Q.; Vilkov, L. V. J. Mol. Struct. 1999, 477, 71−89. (48) It is pertinent to note that, in accordance with the experimental data,6c,41a the performed MP2/6-311++G(d,p) calculations demonstrate that upon the interaction of the cations [CH3C6H6]+ and [SiH3C6H6]+ with triethylamine, correspondingly, the formation of toluene, phenylsilane, and protonated amine is observed (ΔG = −59.4 and −38.4 kcal/mol, respectively): Fornarini, S. J. Org. Chem. 1988, 53, 1314. (49) (a) Milov, A. A.; Starikov, A. G.; Gridin, M. K.; Minyaev, R. M. Zh. Obshch. Khim. 2007, 77, 1294−1306; Russ. J. Gen. Chem. (Engl. Transl.) 2007, 77, 1373−1385. (b) Marshal, M. S.; Steele, R. P.; Thanthiriwatte, K. S.; Sherrill, C. D. J. Phys. Chem. A 2009, 113, 13628−13632.

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Cage Complexes of Carbenium and Silylium Cations with an Aromatic

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