Disilylfluoronium Ions—Synthesis, Structure, and Bonding - American

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Disilylfluoronium Ions—Synthesis, Structure, and Bonding Nicole L€uhmann,† Hajime Hirao,‡,§ Sason Shaik,‡ and Thomas M€uller*,† †

Institut f€ur Reine und Angewandte Chemie, Carl von Ossietzky University Oldenburg, Carl von Ossietzky Strasse 9-11, D-26129 Oldenburg, Federal Republic of Germany ‡ Department of Organic Chemistry and The Lise Meitner-Minerva Center For Computational Quantum Chemistry, The Hebrew University, 91904 Jerusalem, Israel

bS Supporting Information ABSTRACT: The synthesis of disilylfluoronium ions 4 with a naphthalene-1,8-diyl backbone via the corresponding arenium ions 3 is reported. The cations were isolated in the form of their [B(C6F5)4]
salts. The borates 3[B(C6F5)4] and 4[B(C6F5)4] are active in catalytic hydrodefluorination reactions using fluorodecane as substrate. Methylphenyl-substituted arenium ions 3c,d undergo an interconversion reaction via a formal 1,3methyl group migration at room temperature. This rearrangement was shown by DFT methods to proceed by a multiple-step sequence involving a methonium-like transition state. The NMR parameters for disilylfluoronium ions 4 indicate, in agreement with DFT calculations, the presence of a symmetric Si
F
Si linkage in these cations. QTAIM, NBO, and VB analyses reveal that the high ionic contributions to the bonding in the SiFSi moiety are responsible for the symmetric structures of these cations.

’ INTRODUCTION One decade after the successful synthesis and structural characterization of silylium ions, the chemistry of these extremely electrophilic compounds has now reached a turning point.1,2 While the taming of the high reactivity was the focus in the beginning,2 the target is now the ingenious use of exactly this high electrophilicity of silylium ions for novel, synthetically useful applications in synthesis.3 The first examples already demonstrated that the enormous Lewis acidity can be successfully applied in Diels
Alder cyclizations4 and in Mukaiyama aldol4c condensation reactions. Particularly successful was the use of silyl cation salts in the C
F activation reactions. Pioneering work by the group of Ozerov5 and by our group6 demonstrated that silyl arenium ions 1 and disilyl cations 2 can be used in highly efficient catalytic hydrodefluorination reactions of aliphatic organofluorides and in C
C cross-coupling reactions between silyl arenes and aromatic fluorides. Quite recently, Siegel and co-workers extended this concept and developed an intriguing catalytic Friedel
Crafts arylation chemistry.7 In our C
F activation work, we used disilyl cations of type 2 with a naphthalene-1,8-diyl backbone as catalysts.6 The electron deficiency of the silicon atoms in these disilyl cations can be modified and regulated by the bridging substituent X and by the substituents R1
R4, and thereby their reactivity becomes controllable. As an extension of our previous work on disilyl arenium ions 38,9 and fluoronium ions 4,6a we present here the synthesis of several new fluoronium ions via the corresponding arenium ions that can be applied in catalytic C
F activation reactions. In addition, we report on a surprisingly high tendency for substituent r 2011 American Chemical Society

Scheme 1. Synthesis of Precursor Disilanes (R1 = R2 = Ph, 5a; R1 = R2 = Tol, 5b; R1 = Me, R2 = Ph 5c)

scrambling in arenium ions 3 and give an account on the bonding in fluoronium ions 4, with particular emphasis on a comparison to its carbon congener.

Received: May 14, 2011 Published: June 28, 2011 4087

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Scheme 2. Synthesis of Arenium (3) and Fluoronium Ions (4)a

a

See Table 1 for the assignment of R1
R5.

Table 1. Significant NMR Parameters of Cations 3, 4, and 6 R4

δ19F (1JSiF (Hz))

δ1H(SiH) (1JHSi (Hz))

δ13C1; δ13C2,6; δ13C4

R2

R3

3aa

Ph

Ph

Ph

5.21 (247)

95.1; 164.7, 165.8; 153.1

3ba cis-3c

Tol Tol Tol Me Ph Me

CH3
3.3 (SiTol2);
11.4 (SiHTol, 249) H
10.2 (SiHMe); 4.8 (SiMePh)

5.34 (243) 4.82 (237)

89.9; 163.5, 164.5; 170.7 95.7; c

trans-3c

Me Ph

H


11.8 (SiHMe); 6.1 (SiMePh)

4.93 (229)

95.3; c

5.35 (233)

95.6; 165.4, 166.0; 153.3

H

Me

3d

Me Me Ph

4a

Ph

4b

Tol Tol Tol Tol

Ph

cis-/trans-4cd Me Ph

Ph

R5

δ29Si (1JSiH (Hz))

R1

compd

H Ph

Me Ph

4d

Me Me Ph

Ph

6b

Me Me Me Me


2.2 (SiPh2);
9.7 (SiHPh, 248)


13.5 (SiHPh); 12.9(SiMe2) 40.0 (253)


130.8 (253)

40.2 (253)


129.1 (253)

58.4 (250); 58.3 (250) SiMePh


134.1 (250);
133.2 (250)

76.8 (SiMe2, 245); 41.2 (SiPh2, 250)
137.1 (250)e 77.2 (242)


144.0

a

From ref 8b. b From ref 6a. c No clear assignment of the carbon atoms C2/6 and C4 could be made, due to overlapping signals from the second stereoisomer and from 3d; see the Experimental Section and Supporting Information. d Only obtained as a mixture with 4d. e The splitting of the 29Si satellites due to the coupling of the 19F nuclei with two different 29Si atoms could not be resolved.

’ RESULTS AND DISCUSSION The symmetric precursor disilanes 5a
c were prepared along a synthetic route reported previously from 1-bromonaphthalene via 1,8-dilithionaphthalene (Scheme 1) by a salt metathesis reaction.8b The disilanes 5 were obtained in low to moderate yields. The substitution pattern at the silicon atom in compound 5c generates two centers of chirality, and therefore it was obtained as a nonseparable mixture of two diastereomers. The unsymmetrical peri-silyl-substituted naphthalene 5d was obtained in a stepwise lithiation/silylation reaction sequence (Scheme 1). Disilanes 5 were transformed quantitatively into the corresponding arenium ions 3 by reaction with trityl tetrakis(pentafluorophenyl)borate, [Ph3C][B(C6F5)4], in benzene or toluene (Scheme 2).8 The borates 3[B(C6F5)4] were obtained after washing with benzene and pentane as a slightly yellow, glassy material. The newly obtained arenium borates 3c[B(C6F5)4] and 3d[B(C6F5)4] were fully characterized by NMR spectroscopy. The most significant data are summarized in Table 1, along with those reported previously for 3a,b. The reaction of arenium borates 3[B(C6F5)4] with trifluorotoluene in benzene yields quantitatively, according to NMR spectroscopy, the fluoronium salts 4[B(C6F5)4] (Scheme 2). The colorless amorphous salts were washed with pentane and characterized by NMR spectroscopy (Table 1). The 13C NMR spectra of a benzene-d6 solution of the salt 3d[B(C6F5)4] show a chemical shift pattern which is characteristic for arenium ions.8 That is, three signals at low field (δ13C2,6 166.0, 165.4; δ13C4 153.3) for the deshielded methine carbon atoms at the ortho, ortho0 , and para positions of the bridging aryl group and one signal at a relatively high field position (δ13C1

95.6) for the sp3-hybridized ipso carbon atom are detected. Two distinct signals in the 29Si{1H} NMR spectra (δ29Si
13.5, 12.9) indicate the asymmetric substitution of the arenium ion 3d. The NMR spectroscopic identification of the arenium ion 3c is hampered by the formation of both possible stereoisomers cisand trans-3c upon ionization. According to the integration of the SiH 1H NMR signals the stereoisomers are formed in a 3:2 ratio. The assignments of the 1H and 29Si NMR signals to the individual isomers (see Table 1 and the Supporting Information) was supported by two-dimensional NOESY, 1H/1H COSY, and 29 Si/1H HMBC spectroscopy. The NOESY spectrum was central to the assignment. It allowed distinguishing between the trans isomer, trans-3c, which features two methyl groups at the silicon atoms in 1,3-diaxial positions on the same side of the newly formed six-membered ring. In this configuration a NOE contact between both methyl groups is possible. In contrast, in stereoisomer cis-3c, with an anti orientation of these methyl groups, their spatial separation is too large for significant NOE transfer.

NMR spectra of arenium borates 3a[B(C6F5)4] and 3b[B(C6F5)4] showed no significant change upon standing at room temperature or slightly elevated temperatures.8b In contrast, upon prolonged standing of a benzene-d6 solution of arenium 4088

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Scheme 3. Independent Synthesis of Arenium Ions 3c,d and Their Interconversion

Figure 1. 500 MHz 1H NMR spectra of 3d[B(C6F5)4] in benzene at 300 K (a) shortly after preparation (only the region of the SiH resonances is shown) and (b) after 1 h at 300 K (spectrum resolution enhanced by gauss multiplication). (c) Calculated reaction coordinate for the isomerization reaction 3d f cis-3c (at the B3LYP/6-311G(d,p) level). Free enthalpies at 298 K, G298, are given relative to the conformer of arenium ion 3d with the phenyl substituent in the equatorial position (3d(Ph eq)).

Scheme 4. Solvolysis of Tris(trimethylsilyl)methylsilyl Halides 7 According to Eaborn and Co-workers10

borate 3d[B(C6F5)4] additional signals in the NMR spectra indicate the formation of both stereoisomers of arenium ion 3c (Scheme 3 and Figure 1). Similarly, warming of a benzene solution of a freshly prepared cis/trans mixture of 3c[B(C6F5)4] yields 3d in equilibrium with cis-/trans-3c. From the integration of the signals of the SiH hydrogen atoms at 300 K the relative equilibrium concentrations 3d:cis-3c:trans-3d are 1.5:1.6:1 (Figure 1b). Formally, the isomerization 3d f 3c requires a 1,3-shift of one methyl group across the six-membered ring. This alkyl group migration bears a close resemblance to seminal work by Eaborn and colleagues.10 They previously noted that silyl halides 7 underwent dissociative solvolysis reactions and gave in addition to the expected product 8 also rearranged silanes 9 (Scheme 4). These 1,3-methyl migrations are thought to proceed via methyl-bridged intermediates 10, although equilibrium

between trivalent silyl cations 11 and 12 could not be excluded on the basis of the experimental data at that time (Scheme 4). A related 1,3-methyl migration was noticed by Sekiguchi and co-workers in a polysilyl silylium ion,11 and similar methyl migrations are important steps in Lewis acid catalyzed rearrangements of polysilanes.12 In this connection the reversible intramolecular transformation 3c f 3d presented here is of principal interest, and therefore, we conducted in addition to the experimental investigations a DFT study.13 The results of the DFT calculations at the B3LYP/6-311G(d,p) level suggests that the intramolecular isomerization 3c f 3d is a multiple-step reaction involving the two high-lying intermediates 13 and 14 (see Scheme 5 and Figure 1c).14 The calculations predict an overall free enthalpy barrier of 90 kJ mol
1 at room temperature, with the highest barrier being that connected with the opening of the 4089

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Scheme 5. Suggested Mechanism for the Isomerization 3d f 3c Based on DFT Calculations at B3LYP/6-311G(d,p)

Figure 2. Calculated molecular structures of pertinent stationary points along the reaction coordinate 3d f cis-3c (B3LYP/6-311G(d,p) level, bond lengths and interatomic distances given in pm, bond angles and sum of bond angles (∑R) given in deg, hydrogen atoms bonded to aryl groups not shown).

aryl bridge in cation 3d and formation of the secondary silyl cation 13 (Figures 1c and 2). The relatively high calculated barrier is in qualitative agreement with the observed slow conversion at room temperature. The computed structure of silyl cations 13 and 14 indicate the stabilization of the positively charged silicon atom by an agostic interaction with one C
H bond of the methyl group of the second silyl group. This is suggested by the calculated molecular structure of cation 13, which features an elongated C
H bond (by 2.1 pm compared to CH bonds of the same methyl group; see Figure 2) in the methyl group directed toward the secondary silylium ion and a significantly pyramidalized tricoordinated silicon center (∑R = 345.9°). A methyl-bridged species with a pentacoordinated carbon atom analogous to neutral methyl-bridged dialuminium compounds15 or to those suggested by Eaborn et al.10 does not correspond to a minimum along the reaction coordinate for the transformation 3c/3d. Such a molecular arrangement was identified, however, as transition state TS(13/14), which interconverts the two silyl cations 13 and 14 (see Figure 2). Aryl-substituted disilylfluoronium ions 3 are identified by their typical 29Si and 19F NMR parameters (Table 1), which can be compared with data for the related cations 6, 15, and 16.6a,16,17 The silicon atoms of the SiFSi group in cations 4 are strongly deshielded (δ29Si 40.0
76.8), which indicates considerable charge accumulation at the tetracoordinated silicon atom. For a more detailed analysis this chemical shift region can be further subdivided into segments that are characteristic for diaryl-substituted (δ29Si 40
41), for arylmethyl-substituted (δ29Si 58), and for dimethyl-substituted silicon atoms (δ29Si 77
78). The 19 F NMR resonances of the bridging fluorine atoms in arylsubstituted disilylfluoronium ions 4 (δ19F
130.8 to
137.1) are deshielded compared to those of neutral triarylfluorosilanes (e.g., δ19F(Ph3SiF) =
170.4)18 and fall in the chemical shift region reported previously for alkyl-substituted disilylfluoronium ions 6 (δ19F
144.0)6a and 16 (δ19F
132.0).17 The 1JSiF constant between the central fluorine atom and the neighboring silicon atoms in cations 4 (1JSiF = 245
253 Hz; see Table 1) is only

Table 2. Catalytic Hydrodefluorination Reactions Using Arenium and Fluoronium Borates as Catalystsa entry

cat.

RF

RH

amt, mol %

TON

1

3a [B(C6F5)4]

n-C10H21F

n-C10H22

2.0

48

2

3b [B(C6F5)4]

n-C10H21F

n-C10H22

4.3

23

3

3d [B(C6F5)4]

n-C10H21F

n-C10H22

1.0

39

4

4a [B(C6F5)4]

n-C10H21F

n-C10H22

1.0

45

5

4d [B(C6F5)4]

n-C10H21F

n-C10H22

1.0

51

6b

6 [B(C6F5)4]

n-C10H21F

n-C10H22

2.9

35

7b

18 [B(C6F5)4]

n-C10H21F

n-C10H22

2.2

45

a

For exact reaction conditions, see the main text and the Experimental Section. The temperature in all cases was 25 °C. b From ref 6a.

slightly smaller than in neutral fluorosilanes (i.e., 1JSiF in Ph3SiF is 281 Hz and in PhMe2SiF is 278 Hz).18 In particular, it is significantly larger than the 1JSiF coupling constant found for the dynamic molecule 17 (1JSiF = 127 Hz).19 In this case, the JSiF coupling constant is considerably reduced due to a degenerate fluorine exchange which is fast on the NMR time scale even at 240 K. In addition, the very similar 1JSiF coupling constants in the asymmetrically substituted cation 4d (1JMe2Si
F = 245 Hz; 1JPh2Si
F = 250 Hz) suggest a similar bonding situation for both SiF linkages in this particular cation. Thus, the large 1JSiF coupling constants in cations 4 rule out the possibility of a fast equilibrium between unsymmetrically bridged cations and suggest for these aryl-substituted disilyl fluoronium ions a symmetrical Si
F
Si moiety, similar to those firmly established by X-ray diffraction experiments for their alkyl-substituted counterparts 6 and 16.6a,17

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Table 3. Significant Computed Structural Parameters for Fluoronium Ions 4 and 6 and Related Compoundsa entry

compd

point group

Si
F (pm)

F
Si
F (deg)

1

4a

C1

181.6, 182.7

129.2

2

4b

C1

181.5, 182.7

129.1

3

cis-4c

C1

181.8, 182.3

130.6

4

trans-4c

C2

182.0

131.0

5

4d

C1

178.9 (Me2SiF), 185.8 (Ph2SiF)

128.0

6

6

C2

182.0

128.7

7

6b

C2

180.7

127.9

8 9

6c,d 19

C1 C3

175.5(8), 176.3(5) 163.9

129.9(1)

10

20

C1

143.2,g 249.0h

112.7

11

20e,f

C1

142.4(2)g, 244.4(2)h

111.1(1) 163.0(3)

k

12

16

C2

175.3(9)

13

22

C3

142.7

NBO charge at F (e)//WBI (E-F)i
0.60//0.322, 0.312 (SiFSi)

F (e bohr
3)//r2F (e bohr
5)j 0.07, 0.06//0.429, 0.400


0.63//0.573 (SiF)

0.08//0.862


0.41//0.759, 0.029 (C
F/C+)

0.201, 0.018//0.233, 0.077


0.42//0.779

0.207//0.185

a

At the B3LYP/6-311G(d,p) level. b Calculated at the B3LYP/6-311+G(2d,p) level. c Experimental data from XRD of the [B(C6F5)4] salt. d From ref 6a. e Experimental data from XRD of the [BF4] salt. f From ref 20. g C
F bond length. h CF 3 3 3 C+ distance. i At the B3LYP/6-311+G(d,p) level. j Calculated using an HF/6-311G(d,p) wave function. k From ref 17.

The disilylarenium borates 3a[B(C6F5)4], 3b[B(C6F5)4], and 3d[B(C6F5)4] and disilylfluoronium borates 4a[B(C6F5)4] and 4d[B(C6F5)4] are active in catalytic C
F activation processes. Test reactions were performed at room temperature using a small quantity of the disilyl cation borate, n-fluorodecane as substrate, and triethylsilane as the solvent and hydride source. After 30
60 min the alkyl fluoride was consumed and the reaction was completed. The determined turnover numbers (TON) of 23
51 clearly indicated a catalytic reaction (Table 2). Arenium ions 3 and fluoronium ions 4 showed comparable activities which are practically identical with those reported previously for the related fluoronium and hydronium ions 6 and 18.6 In the absence of XRD data, reliable structural parameters for fluoronium ions 4 were obtained from density functional computations.13 The most significant computed molecular structural data for cations 4 are summarized in Table 3 and are compared with experimental data found previously for the tetramethylsubstituted fluoronium ion 6 and the bis(trimethylsilyl)fluoronium species 16.6a,17 The calculated structural data for cation 6 indicate an overall good agreement with the experimental data; however, the Si
F bonds are computed too long by Δδ = 5.7
6.5 pm (at the B3LYP/6-311G(d,p) level; see Table 3, entries 6 and 8). Basis set extension to a 6-311+G(2d,p) basis set improves the situation (Δδ = 4.4
5.2 pm; Table3, entry 7); however, the size of the studied cations and the extent of the study excluded the use of this large basis set throughout. The computed structure of the tetraphenyl-substituted fluoronium ion 4a is shown in Figure 3. All theoretical structures show in all significant structural aspects a close resemblance to the experimental molecular structure of cations 6 and 16. The most prominent feature of cations 4 is a nearly symmetric bent Si
F
Si linkage (bend angles R = 128.0
130.8°) with Si
F bond lengths that are clearly elongated compared to the computed Si
F bond length in triphenylsilyl fluoride (Ph3SiF, 19) (181.5
182.7 pm in cations 4a
c and 163.9 pm in 19; see Table 3 and Figure 3). Even in the unsymmetrically substituted cation 4d the two computed Si
F bond lengths differ only by 6.9 pm and the shortest Si
F bond is elongated by 9% compared to the SiF single bond in silyl fluoride 19. Therefore, the combined experimental NMR spectroscopic and theoretical data

Figure 3. Calculated molecular structure of fluoronium ion 4a (B3LYP/6-311G(d,p), bond lengths given (in pm) in boldface and bond angles in italics, hydrogen atoms omitted): (black) carbon; (gray) silicon; (white) fluorine.

Figure 4. (a) Contour plots of the calculated electron density of 4a in the Si(1)
F
Si(2) plane. The bond paths are indicated as bold lines and the corresponding bond critical points as red circles. (b) Contour plot of the Laplacian of the electron density, r2F(r), in the same plane. Red contours indicate regions of local charge accumulations (r2F (r) < 0) and blue contours regions of local charge depletion (r2F(r) > 0).

suggest that bis-silyl-substituted fluoronium ions 4 exist as single minima on their respective potential energy surfaces (PES). These findings are in contrast with the results reported by Gabbaï and co-workers for the closely related carbocation 20.20 4091

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Scheme 6. Bonding in Silyl-Substituted Fluoronium Ions 4 and in Carbocation 20

According to XRD measurements, NMR spectroscopic investigations, and quantum mechanical studies, carbocation 20 adopts a molecular structure with a nonsymmetrical C
F/C+ linkage, consisting of one covalent C
F bond and one dative F/C+ interaction. The symmetrical fluoronium ion 21 is a local maximum on this PES, separating two equivalent minimum structures corresponding to carbocation 20.20 This intriguing difference in particular between the silicon compound 4a and its carbon congener 20 aroused our interest and prompted us to compare both cations more closely by means of natural bond orbital (NBO) analysis and quantum theory of atoms in molecules (QTAIM) and by considering valence bond correlation mixing diagrams (VBCMD) for a closely related model. According to the results of the NBO analysis,21 the negative charge at the fluorine atom is virtually the same in fluoronium ion 4a as in the neutral model fluorosilane Ph3SiF (19) (
0.63e (19);
0.60e (4a)). This indicates only insignificant charge transfer from the fluorine atom to the silicon atoms in cation 4a. The calculated bond orders (as gauged by the Wiberg bond index (WBI))22 are nearly identical for both Si
F bonds in fluoronium ion 4a, and they are significantly smaller than those computed for the model compound 19 (see Table 3), in agreement with the expected multiple-center bonding in cation 4a. In the framework of the QTAIM23 analysis the Si
F bonds in both silyl fluoride 19 and fluoronium ion 4a are highly ionic, as indicated by their small electron densities F and by their substantial positive Laplacian r2F(r) at the bond critical points (see Figure 4 and Table 3). The properties of the electron densities for both bond critical points of the Si
F
Si linkage in 4a are nearly identical, as expected for a symmetrical structure. It is noteworthy, however, that the ionicity of the Si
F bonds in cation 4a is significantly smaller than that of the Si
F bond in the neutral fluorosilane 19, as indicated by the less positive Laplacian r2F(r) (see Table 3). In summary, the results of preceding analyses for 4a suggest that the ionic canonical structure 4(B) (Scheme 6) is important for the understanding of the bonding in fluoronium ions 4. The character of the C
F bonds in the model trityl fluoride 22 as well as in the carbocation 20 is clearly less ionic than in their silicon analogues cation 3a and silyl fluoride 19. This is indicated by the higher electron density F(r) and the less positive Laplacian r2F(r) at the bond critical points of the C
F bonds. The bonding characteristics of the C
F bond in cation 20 are very similar to those in the model compound 22 (see Table 3). In addition, only a relative small electron density F(r) is computed at the bond critical point for the F/C+ interaction. Therefore, the results of the QTAIM suggest that the C
F/C+ linkage in carbocation 20 consists of a covalent C
F bond and a dative F/C+ interaction, in agreement with the previous bonding analysis by Gabbaï, Hall, and co-workers.20 For a complementary understanding of the differences between the two obviously so closely related cations 4a and 20, we used the valence bond configuration mixing diagram (VBCMDs) model24 and analyzed the principal VB structures involved

during the fluorine migration between the cations H3E+ and the fluoride F
EH3 (E = C, Si). The energies of VB structures for the distorted and bond-symmetric states were computed at the VBSCF/6-31G(d) level using the XMVB program25 linked to GAMESS (see Figures S1 and S2 in the Supporting Information for details).26 The final (adiabatic) energy curve was inferred from the relative energies and mixing patterns of the VB structures. Figures 5 depicts the energy run of the covalent Heitler
London (HL) and the ionic structures for a fluoride ion transfer process between two EH3+ cations. It is seen that for both E = Si and E = C, the ionic structure lies below the covalent structures. Furthermore, at the symmetric geometry the electrostatic stabilization is maximized and this leads to a symmetric single-minimum structure with a triple-ion character, H3E+F
+EH3. The triple ionic structure marks the main difference between E = C and E = Si. Thus, the energy gap from the triple-ionic to the covalent structures is much larger for E = Si, and the corresponding curve is also much deeper at the symmetric geometry. These features of the triple-ionic structures in the case of silicon are a result of two factors: (i) the lower ionization potential (IP) of the silicon radical center, which lowers the triple-ionic curve relative to the HL curves (i.e. IP = 786 kJ mol
1 for SiH3• and IP = 951 kJ mol
1 for CH3•, at the B3LYP/6-311G(d,p) level) and (ii) the high positive charge localization at the silicon atom in SiH3+ with even negative charge on the corresponding hydrogen atoms (see Figure 6). In contrast, in the case of E = C, the positive charge is delocalized over all the atoms of the cation. As a consequence of this high positive charge, the ionic radii for the SiH3+ cation is smaller than for the CH3+ cation (e.g., r(CH3+) = 64 pm and r(SiH3+) = 35 pm),27a and this allows a closer contact between anion and the silicon cation, thereby maximizing the electrostatic interaction between the silicon cations and the fluoride anion. In contrast, with the chargedelocalized carbenium ions, the electrostatic interaction is weaker and the curve is less deep. In both cases in Figures 5, the covalent (HL) structures mix with the triple-ionic structure to shape the final (adiabatic) energy curve.24,27 Since the covalent
ionic energy separation at the reactant and product geometries for E = C are small, the mixing is significantly larger than in the symmetrical F- - -C- - -F structure. This, in turn, leads to a single transition state separating the two unsymmetrical fluorinebridged clusters (Figure 5a). The fluoride transfer process between two silylium ions (Figure 5b), however, clearly dominates along the complete reaction coordinate by the very favorable triple-ionic structure. This is because the energy gap between the ionic and the HL structures is much larger than in the case of methylium ions. With such large gaps between the ionic and covalent structures, their mixing is smaller and the ionic curve retains its shape with a minimum at the symmetric geometry. In conclusion, the silylium ions in our model behave more like protons having a concentrated positive charge and unlike carbocations which possess delocalized charges, a fact which underlines 4092

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Figure 5. Valence bond configuration mixing diagrams (VBCMDs) for the fluoride transfer (a) between two methyl cations and (b) between two silyl cations: (red) energy profiles of the covalent Heitler
London (HL) structures of reactant and product (ΦHLR, ΦHLP); (blue) energy profile of the ionic structures (ΦIon); (black) qualitative adiabatic energy curves.

Figure 6. Calculated NBO charges (in italics) for methylium and silylium cations (at the B3LYP/6-311G(d,p) level).

the close relationship of fluoronium ions such as 4 with protonated hydrogen fluoride [H
F
H]+.17

’ CONCLUSION The synthesis of bis-silylated fluoronium ion salts 4[B(C6F5)4] via the corresponding arenium borates 3[B(C6F5)4] is reported. The arenium ions 3 were formed by hydride transfer reaction between the peri-substituted naphthylsilanes 5 and trityl borate, Ph3C[B(C6F5)4]. Subsequent reaction of the so obtained arenium borates 3[B(C6F5)4] with trifluorotoluene yields the fluoronium ions 4. The disilyl cations 3 and 4 were characterized by multinuclear NMR spectroscopy. The bis(methylphenylsilyl)- and (dimethylsilyl)(diphenylsilyl)-substituted arenium ions 3c,d, respectively, undergo at room temperature a slow interconversion via 1,3-methyl group transfer. The results of DFT calculations suggest for this reaction a multiple-step mechanism via the high-lying unsymmetrical methyl-bridged intermediates 13 and 14 and a methonium-like transition state (TS13/14). For all fluoronium ions 4, the obtained NMR spectroscopic data, in particular the large determined 1JSiF coupling constants (1JSiF = 242
253 Hz), and the results of DFT calculations indicate a molecular structure with a symmetric Si 3 3 3 F 3 3 3 Si

linkage similar to those found for the closely related cations 66a and 1617 by XRD measurements. Arenium and fluoronium borates 3[B(C6F5)4] and 4[B(C6F5)4] are active catalysts in CF activation processes. As already noted for related silyl cation based catalysts, aliphatic C
F groups were exclusively reduced.5,6 The determined TONs show the catalytic course of the reaction. The intriguing difference in structure between the closely related bissilylfluoronium ion 4a and the carbocation 2020 was analyzed applying NBO theory, QTAIM, and valence bond schemes. The theoretical analysis indicates that the high ionicity of the Si
F bond is the main factor which determines the symmetrical Si 3 3 3 F 3 3 3 Si arrangement in cation 4a.

’ EXPERIMENTAL SECTION General Procedures. Diethyl ether and tetrahydrofuran were distilled from sodium/benzophenone. Benzene, benzene-d6, hexane, and pentane were distilled from sodium. TMEDA, Me2HSiCl, and Ph2HSiCl were distilled freshly from calcium hydride. 1-Fluorodecane, R,R,R-trifluorotoluene, and hexafluorobenzene were freshly destilled from P4O10. All reactions were carried out under inert conditions. NMR spectra were recorded on Bruker DPX-250, DPX 400, Avance 300 and Avance 500 spectrometers at 305 K, if not stated otherwise. 1H and 13C NMR spectra were calibrated using residual solvent signals δ1H(CHCl3) 7.24, δ13C(CDCl3) 77.0, δ1H(C6D5H) 7.20, and δ13C(C6D6) 128.0. 29Si NMR spectra were calibrated using external Me2HSiCl (δ29Si 11.1 vs TMS) and 19F NMR against external CFCl3 (δ19F(CFCl3) 0.0). 29Si NMR spectra were recorded using the INEPT pulse sequence. Mass spectra were recorded on a Finnigan MAT 212 instrument with electron ionization. 1,8-Dibromonaphthalene and [Ph3C][B(C6F5)4] were synthesized as described in the literature.28,29 Compound 5c. n-Butyllithium (12 mmol,1.6 M in hexane) was added slowly to a stirred solution of 10 mmol of 1-bromonaphthalene in 4093

dx.doi.org/10.1021/om2003128 |Organometallics 2011, 30, 4087–4096

Organometallics 30 mL of dry diethyl ether at
40 °C. After a further 30 min of stirring 15 mL of dry hexane was added at
75 °C and the supernatant clear solution was decanted off via a flexible Teflon tube. The precipitate was washed three times with 20 mL portions of dry hexane and dried under vacuum. Then 14 mmol of n-butyllithium (1.6 M in hexane) and 14 mmol of TMEDA were added to the resulting white precipitate. The reaction mixture was heated to reflux for 3 h. The supernatant brown solution was decanted off via a syringe. The precipitate was washed once with hexane and cooled to
30 °C. After addition of 10 mL of diethyl ether, 25 mmol of methylphenylchlorosilane was added. The suspension was warmed to room temperature and heated to reflux for 1 h. The solvent was removed under vacuum, and hexane was added. The suspension was filtered and the solvent removed under vacuum. The crude product was purified by column chromatography on silica using THF: Rf = 0.9. Yield: 8.7 mmol as a colorless oil, 87%. 1H NMR (499.87 MHz, C6D6): δ 0.59 (d, 6H, CH3, 3JHH = 3.6 Hz), 0.68 (d, 6H, CH3, 3JHH = 3.6 Hz), 5.89
5.93 (m, 4H, SiH, 1JSiH = 198.8 Hz), 7.13
7.25 (m), 7.49
7.51 (m, 5H), 7.54
7.56 (m, 6H), 7.69
7.77 (m, 9H), 7.83
7.85 (m, 4H). 13C NMR (125.77 MHz, C6D6): δ 0.0 (CH3), 0.3 (CH3), 126.8, 127.5, 127.9, 131.2, 131.3, 131.3, 131.7, 132.9, 134.0, 135.7, 136.5, 137.2, 137.3, 137.4, 137.6, 137.8, 137.9, 139.8, 139.9, 140.2. 29Si NMR (99.30 MHz, C6D6): δ
20.3 (1JSiH = 198.8 Hz). HRMS (m/z): calcd for 12C241H2428Si2 368.1217, found 368.1406, GC/ MS (m/z (%)): 368.2 [M]+ (2), 247.1 (100) [M
SiCH3C6H5], 197.1 (60), 169.1 (35), 121.1 (22), 105.0 (30). 1-Bromo-8-(dimethylsilyl)naphthalene. n-Butyllithium (10 mmol,1.6 M in hexane) was added to a solution of 10 mmol of 1,8-dibromonaphthalene in tetrahydrofuran at
78 °C, and the mixture was stirred for 90 min. After 10 mmol of Me2HSiCl was added via a syringe, the mixture was warmed to room temperature. After aqueous workup the crude product was purified by column chromatography on silica using hexane: Rf = 0.52. Yield: 9.5 mmol as a colorless oil, 95%. 1H NMR (499.87 MHz, C6D6): δ 0.61 (d, 6H, CH3, 3JHH = 3.7 Hz), 5.44 (sept, 1H, Si
H, 3JHH = 3.7 Hz, 1JSiH = 198.6 Hz), 6.88 (m, 1H), 7.17 (m, 1H), 7.45 (m, 1H), 7.53 (m, 1H), 7.67 (m, 1H), 7.96 (m, 1H). 13C NMR (125.77 MHz, C6D6): δ 1.1 (CH3), 123.5 (C
Br), 125.6 (CH), 125.9 (CH), 129.6 (CH), 131.5 (CH), 132.5 (CH), 136.4 (Cq), 137.0 (Cq), 137.1 (Cq), 138.9 (CH). 29 Si NMR (99.30 MHz, C6D6): δ
13.3. Mass spectrum (EI; m/z (%)): 265.9(20) [M + 2]+, 263.9 (27) [M]+, 265.0 (28) [M + 2
H]+, 262.9 (26) [M
H]+, 250.9 (100) [M + 2
Me]+, 248.8 (98) [M
Me]+ , 169.0 (85). HRMS (m/z) calcd for 12 C121H1379Br28Si 263.9970, found 263.9964. Compound 5d. n-Butyllithium (7 mmol,1.6 M in hexane) was added to a solution of 7 mmol of 1-bromo-8-(dimethylsilyl)naphthalene in diethyl ether at
78 °C, and the mixture was stirred for 90 min at this temperature. After 7 mmol of Ph2HSiCl was added via a syringe, the mixture was warmed to room temperature. The solvent was removed under vacuum, and hexane was added. The resulting suspension was filtered. The filtrate was concentrated at
20 °C until the product started to crystallize. Yield: 1.8 mmol of a white solid, 25%. 1H NMR (499.87 MHz, C6D6): δ 0.30 (d, 6H, CH3, 3JHH =3.4 Hz), 5.40 (sept, 1H, Si
H, 3JHH = 3.4 Hz, 1JSiH = 193.7 Hz), 6.44 (s, 1H, Si
H, 1JSiH = 203.2 Hz), 7.09
7.20 (m, 9H,), 7.27
7.30 (m, 1H), 7.59
7.61 (m, 4H), 7.72
7.73 (m, 1H), 7.94
7.96 (m, 1H). 13C NMR (125.77 MHz, C6D6): δ
0.8 (CH3), 124.7 (CH), 124.9 (CH), 129.6 (CH), 131.6 (CH), 132.4 (CH), 133.3 (Cq), 134.5 (Cq), 135.5 (CH), 136.2 (Cq), 136.3 (CH), 138.7 (Cq), 140.3 (CH), 142.7 (Cq). 29Si NMR (99.30 MHz, C6D6): δ
19.5 (SiH(CH3)2),
19.2 (SiHPh2). HRMS (m/z): calcd for 12 C241H2428Si2 368.1217, found 368.1425, GC/MS (EI; m/z (%)): 368 [M]+ (13), 309 (100), 275 (7), 247 (13), 231 (55), 182 (26), 135 (12), 105 (9), 78 (3). Arenium Borate cis-/trans-3c[B(C6F5)4]. A 0.5 mmol portion of 5c was slowly added to a stirred solution of 0.5 mmol (460 mg) of

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[Ph3C][B(C6F5)4] in 2 mL of benzene at 5
10 °C via a syringe. The mixture was stirred for 30 min at 20 °C. Stirring was stopped, and the two phases were allowed to separate. The lower phase was transferred into an NMR tube and investigated by NMR spectroscopy. The brown salt was purified by washing with 2 mL of pentane and then with 1 mL of benzene to give cis-/trans-3c[B(C6F5)4] in a 3:2 mixture as a glassy solid. Data for cis-3c[B(C6F5)4]: 1H NMR (499.87 MHz, C6D6) δ
0.60 (d, 3H, CH3, 3JHH = 3.2 Hz),
0.35 (s, 3H, CH3), 4.82 (m, 1H, SiH 1JSiH = 236.6 Hz); 13C NMR (125.77 MHz, C6D6) δ
5.5,
2.6, 95.7 (Cipso);30 29Si NMR (99.30 MHz, C6D6) δ
10.2 (SiH), 4.8 (SiMePh). Data for trans-3c[B(C6F5)4]: 1H NMR (499.87 MHz, C6D6) δ
0.42 (d, 3H, CH3, 3JHH = 3.2 Hz), 0.57 (s, 3H, CH3), 4.82 (m, 1H, SiH, 1JSiH =228.6 Hz); 13C NMR (125.77 MHz, C6D6) δ
5.1,
1.1, 95.3 (Cipso));30 29Si NMR (99.30 MHz, C6D6) δ
11.8 (SiH), 6.1 (SiMePh). Arenium Borate 3d[B(C6F5)4]. A 2 mL portion of benzene was added to a mixture of 0.5 mmol of solid 5d and 0.5 mmol of [Ph3C][B(C6F5)4] via vacuum transfer. The solution was warmed to room temperature for 30 min. Stirring was stopped, and the two phases were allowed to separate. The lower phase was transferred into an NMR tube and investigated by NMR spectroscopy. The brown salt was purified by washing with 2 mL of pentane and then with 1 mL of benzene to give 3d[B(C6F5)4] as a glassy solid. 1H NMR (499.87 MHz, C6D6): δ
0.55 (s, 3H, CH3), 0.37 (s, 3H, CH3), 5.35 (s, 1H, SiH, 1 JSiH = 233.4 Hz), 6.67
7.34 (m, ArH), 7.60
7.63 (m, ArH), 7.72
7.79 (m, ArH), 8.08
8.11 (m, ArH). 13C NMR (125.77 MHz, C6D6, T = 305 K): δ
2.0 (CH3), 1.4 (CH3), 95.6 (C1), 122.2 (Cq), 126.1 (Cq), 126.3, 126.4, 126.4, 126.1, 128.5, 128.6, 128.8 (Cq), 129.9, 132.8, 133.6 (Cq), 134.2, 134.2, 134.5, 134.8, 135.0, 136.6, 136.7 (d, 1JCF = 240.4 Hz, [B(C6F5)4]), 138.4, 138.5 (d, 1JCF = 240.4 Hz, [B(C6F5)4]), 139.6 (Cq), 148.8 (d, 1JCF = 240.4 Hz, [B(C6F5)4]), 153.3 (C4), 165.4, 166.0. 29Si NMR (99.30 MHz, C6D6): δ
13.5 (SiHPh), 12.9 (SiMe2). Synthesis of Fluoronium Borates 4[B(C6F5)4]. General Procedure. A solution of 0.5 mmol of 3[B(C6F5)4] in benzene was cooled to 8 °C. After 0.3 mmol of neat R, R, R-trifluortoluene was slowly added, the cooling was stopped. The reaction mixture was stirred at 50 °C for 3 h. After the stirring was stopped, a two-phase reaction mixture was obtained. The lower phase was transferred into an NMR tube and investigated by NMR spectroscopy. The dark red [B(C6F5)4] salts were purified by washing with pentane. 4a[B(C6F5)4]. 1H NMR (499.87 MHz, C6D6): δ 7.04
7.07 (m, 8H), 7.12
7.13 (m, 8H), 7.26
7.30 (m, 6H), 7.32
7.34 (m, 2H), 7.90
7.92 13 C NMR (125.77 MHz,0 C0 6D6): δ 121.9 (d, 4JCF = 5.6 Hz, (m,0 2H). 2 /70 C ), 123.4 (d, JCF = 19.1 Hz, C1 /8 ), 126.5, 129.4, 134.0, 134.8, 135.9, 136.1, 136.9 (d, 1JCF = 246.3 Hz, CF, [B(C6F5)4]
) 137.8 (d, JCF = 14.2 Hz, C1), 138.9 (d, 1JCF = 246.3 Hz, [B(C6F5)4]
), 149.1 (d, 1JCF = 242.6 Hz, [B(C6F5)4]
). 29Si NMR (99.30 MHz, C6D6): δ 40.0 (d, 1JSiF = 253.1 Hz). 19F NMR (470.27 MHz, C6D6): δ
166.4 (m, CF),
162.4 (t, 3JFF = 20 Hz),
132.2 (m, CF),
130.8 (s, 1JSiF = 253 Hz, SiF). 4b[B(C6F5)4]. 1H NMR (250.131 MHz, C6D6): δ 2.13 (s, 12H, CH3), 6.93
6.96 (m, 8H), 7.16
7.20 (m, 8H), 7.27
7.34 (m, 2H), 7.45
7.43 (m, 2H), 7.89
7.92 (m, 2H). 13C NMR (62.902 MHz, C6D6): C1), 122.7 (d, 2JCF = 14.4 Hz, δ 21.4 (CH3), 120.4 (d, 2JCF = 19.80 Hz, 0 10 /80 5 3 /60 C ), 126.4 (d, JCF = 1.8 Hz, C 0 ),0 130.2 (C3), 135.6 (C10 ), 136.9 (C2), 137.8 (d, 4JCF = 5.6 Hz, C2 /7 ), 136.9 (d, 1JCF = 248.8 Hz, [B(C6F5)4]
), 138.8 (d, 1JCF = 245.9 Hz, [B(C6F5)4]
), 146.1 (C4), 149.1 (d, 1JCF = 241.6 Hz, [B(C6F5)4]
). 29Si NMR (49.694 MHz, C6D6): δ 40.2 (d, SiF, 1JSiF = 253 Hz). 19F NMR (235.325 MHz, C6D6): δ
166.7 (t, 3JFF = 18 Hz, [B(C6F5)4]
),
162.7 (t, 3JFF = 20 Hz, [B(C6F5)4]
),
132.0 (m, [B(C6F5)4]
),
129.1 (d, SiF, 1JSiF = 253 Hz). 4d[B(C6F5)4]. 1H NMR (499.87 MHz, C6D6): δ 0.16 (d, 6H, 3JHF = 13.1 Hz, CH3), 7.11
7.12 (m, 1H), 7.23 (m, 2H), 7.25
7.28 (m, 1H), 7.29
7.31 (m, 5H), 7.35
7.38 (m, 4H). 13C NMR (125.77 MHz, C6D6): δ
0.8 (d, 2JCF = 14.2 Hz), 121.0 (d, JCF = 14.8 Hz), 123.2 4094

dx.doi.org/10.1021/om2003128 |Organometallics 2011, 30, 4087–4096

Organometallics (d, JCF = 19.6 Hz), 125.9 (d, JCF = 10.7 Hz), 126.4 (d, JCF = 2.0 Hz), 126.5 (d, JCF = 3.1 Hz), 129.7, 133.7, 134.4 (d, JCF = 7.6 Hz), 135.0, 135.2, 135.9, 136.1, 138.9 (d, 1JCF = 246.3 Hz, [B(C6F5)4]
), 142.1, 149.1 (d, 1JCF = 242.6 Hz, [B(C6F5)4]
). 29Si NMR (99.30 MHz, C6D6): δ 41.2 (d, SiPh2F, 1JSiF = 249.5 Hz), 76.8 (d, SiMe2F, 1JSiF = 245.9 Hz). 19F NMR (470.27 MHz, C6D6): δ
166.5 (m, 8F),
162.6 (m, 4F),
137.1 (s, 1F, SiF, 1JSiF = 245.9 Hz),
131.9 (m, 8F).

Typical Procedure for the Catalytic Hydrodefluorination of 1-Fluorodecane. A mixture of 5 mmol of 1-fluorodecane, 1 mL of

triethylsilane, and 100 μL of hexafluorobenzene as internal standard was added to 0.05 mmol of 3[B(C6F5)4] or 4[B(C6F5)4] at 0 °C. The mixture was warmed to room temperature and stirred overnight. The progress of the reaction was followed by 19F NMR spectroscopy. The product Et3SiF was detected by NMR spectroscopy and GC/MS. Decane was identified by GC/MS. Et3SiF: 19F NMR (235.334 MHz, C6D6) δ
175.2 (s, 1JSiF = 288.7 Hz); 29Si NMR (49.696 MHz, C6D6): δ 31.6 (d, 1JSiF = 288.7 Hz). Decane: GC/MS (EI; m/e (%)) 142 (5), 113 (3), 99 (4), 98 (4), 85 (15), 84 (6), 71 (30), 70 (13), 57 (88), 55 (27), 43 (100), 41 (82), 29 (50), 27 (30).

’ ASSOCIATED CONTENT

bS

Supporting Information. Text, tables, and figures giving a complete description of the computations, including optimized molecular structures in Cartesian coordinates and absolute energies for all calculated compounds, and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses

§ Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371.

’ ACKNOWLEDGMENT This work was supported by the DFG (No. Mu1440/6-1). The Center for Scientific Computation (CSC) is thanked for providing computational resources. ’ REFERENCES (1) (a) Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. 1997, 36, 400. (b) M€uller, T.; Lambert, J. B.; Zhao, Y. Organometallics 1998, 17, 298. (c) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J. Am. Chem. Soc. 1999, 121, 5001. (d) Kim, K.-C.; Reed, C. A.; Elliot, D. W.; Mueller, L. J.; Tham, F.; Lin, L. Science 2002, 297, 825. (2) Reviews: (a) Lee, V. Y.; Sekiguchi, A. Organometallic Compounds of Low-Coordinate Si, Ge, Sn and Pb; Wiley: Chichester, U.K., 2010. (b) Lee, V. Y.; Sekiguchi, A. Acc. Chem. Res. 2007, 40, 410. (c) M€uller, T. Adv. Organomet. Chem. 2005, 53, 155. (3) Klare, H. F. T.; Oestreich, M. Dalton Trans. 2010, 39, 9163. (4) (a) Johannsen, M.; Jørgensen, K. A.; Helmchen, G. J. Am. Chem. Soc. 1998, 120, 7637. (b) For a clarification of the nature of the catalyst used in ref 4a see: Olah, G. A.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 1999, 121, 9615. (c) Hara, K.; Akiyama, R.; Sawamura, M. Org. Lett. 2005, 5621. (d) Klare, H. F. T.; Bergander, K.; Oestreich, M. Angew. Chem., Int. Ed. 2009, 48, 9077. (5) (a) Douvris, C.; Ozerov, O. V. Science 2008, 321, 1188. (b) Scott, V. J.; C) elenligil-C) etin, R.; Ozerov, O. V. J. Am. Chem. Soc. 2005,

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