ARTICLE pubs.acs.org/Organometallics
Molecular Design of Hypercoordinated Silacyclophanes Evgeniya P. Doronina, Elena F. Belogolova, and Valery F. Sidorkin* A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, Favorsky, 1, Irkutsk 664033, Russian Federation
bS Supporting Information ABSTRACT: MP2 and DFT (B3LYP, M06-2X) methods using the 6-31G(d) and 6-311++G(d,p) basis sets are applied to the investigation of the structure of a wide series of silacyclophanes XSi[Y(CH2)n]3C6R3 19 (X = t-Bu, Me, NH2, F, OTf; Y = O, NH, CH2; R = H, SiH3, Li; n = 1, 2). The molecules studied exist exclusively in the out-C3-symmetric form (for X = NH2, OTf the symmetry is not strict). Identification of the presence and estimation of the extent of the hitherto unknown Si 3 3 3 Ar multicenter intramolecular interaction of the tetracoordinate silicon atom with the πsystem of the benzene ring in 19 was performed by analysis of geometric parameters, using the methods of Δδ 29Si coordination shift, NICS(1) values, and the results of the AIM, NBO, and MO analyses. In molecules XSi[YCH2CH2]3C6R3 14, irrespective of the nature of X, Y, and R, the multicenter interaction Si 3 3 3 Ar is weak. This interaction is strongly enhanced in compounds XSi[YCH2]3C6R3 59, with side chains shortened by one methylene group. The variation of geometric (dSiAr, ΔSi, ηα), electronic (∑δ(Si,C)), orbital (∑ΔE(2)[πCCfσ*Si‑X]), and NMR (Δδ 29Si) characteristics of the coordination center XSiY3Ar in 59, caused by varying the environments of the silicon atom and of the arene fragment, is typical for pentacoordinate silicon compounds.
’ INTRODUCTION The forced vicinity of certain atoms and functional groups to aromatic rings in cyclophanes is responsible for their unusual physicochemical properties and reactivity.1 It is therefore not surprising that special attention has been given to the synthesis of the in-isomers rather than the alternative out-isomers of silacyclophanes XSi[-L-]3C6H3 of the type A (in the literature the compounds with X = H, Me, t-Bu; L = Y(CH2)n; Y = O, CH2, n = 2, 3, 9;2 X = H, Me, F; L = C6H4Z; Z = SCH2, CH2SCH2, CH2S(CH2)23 are known).
the benzene ring in XSi[-L-]3C6H3 is substantially lower than that of the nitrogen atom in XSi[-L-]3N, and, second, apparently neither the location of the silicon atom and that of the basal ring nor the size of the side chains L is optimal for formation of the hitherto practically unknown dative multicenter contact XSi 3 3 3 Ar. Therefore, the question of the possibility of the existence of hypercoordinated cyclophanes remains open. In this connection, we have performed a quantum chemical study of the structure of theoretically designed molecules 19, including the two experimentally synthesized compounds t-BuSi[OCH2CH2]3C6H32b and MeSi[CH2CH2CH2]3C6H3.2c
The intramolecular interaction of the XSi with the benzene ring in the out-isomers, as distinct from that in the in-isomers, may increase the coordination number of the silicon atom to five.4 This is possible if interaction XSi 3 3 3 Ar from effective mixing of the orbitals of the acceptor XSi and π-donor arene fragments is strong enough.5 From the data of X-ray, multinuclear NMR spectroscopy and quantum chemical calculations, the currently known out-silacyclophanes,2,3d unlike their structural analogues silatranes XSi[-L-]3N (L = YCH2CH2; Y = O, NR, CH2),6 contain a tetrahedral rather than a pentacoordinate silicon atom. This is not surprising since the donating power of
’ COMPUTATIONAL DETAILS
r 2011 American Chemical Society
Computations of silacyclophanes 19 have been performed with full geometry optimization at the MP2 and DFT (B3LYP, M06-2X) levels of theory using the 6-31G(d) and 6-311++G(d,p) basis sets. The stationary points on the potential energy surface (PES) were identified by the number of negative Hessian eigenvalues. The 29Si chemical shifts relative to TMS [δ29Si = σ(TMS) σ(compound), where σ is the shielding constant; σ(TMS) = 328.1], the coordination chemical shifts (Δδ29Si) Received: April 21, 2011 Published: October 05, 2011 5595
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Table 1. Comparison of Some Geometrical Parameters for Silacyclophane t-BuSi[OCH2CH2]3C6H3, Which Were Determined in Crystals by X-ray Diffraction and Calculated in the Gas Phase (Isolated Molecule, B3LYP, M06-2X, MP2 with the 6-31G(d) and 6-311++G(d,p) Basis Sets) and in Polar Solvents medium
method
dSiAr, Å
d SiCt‑Bu, Å
dSiO, Å
— Ct‑BuSiO, deg
— OSiO, deg
β,a deg
crystal
X-rayb
3.25
108.9
109.9
H2O solution
IEF-PCMc B3LYP/6-31G(d) (ε = 78.4)
3.300
1.898
1.665
107.9
111.0
24.4
CHCl3 solution
IEF-PCM B3LYP/6-31G(d) (ε = 4.9)
3.298
1.898
1.665
107.9
111.0
24.4
gas
B3LYP/6-31G(d)
3.302
1.896
1.664
108.1
110.8
24.5
gas gas
B3LYP/6-311++G(d,p) MP2/6-31G(d)
3.313 3.224
1.894 1.877
1.663 1.670
108.6 108.0
110.3 110.9
24.2 24.3
gas
MP2/6-311++G(d,p)
3.240
1.873
1.661
108.7
110.3
22.7
gas
M06-2X/6-31G(d)
3.219
1.873
1.657
107.4
111.5
24.3
24
β is the angle between the CH2CAr and the plane of the three nearest carbon atoms. b Mean structural parameters are given.2b,c c The polar solvent effect was studied using the integral equation formalism version of the polarized continuum model (IEF-PCM).18 a
relative to corresponding model silanes XSi(OMe)3, XSi(NH2)3, and XSiMe3 [Δδ29Si = δ29Si(compound) δ29Si(silane)], and the nucleusindependent chemical shifts (NICS)7 were calculated with the GIAO B3LYP/6-311++G(2d,p) approximation. Most of calculations were performed using the GAUSSIAN 038 program package, except the geometry optimization and vibrational analysis with the M06-2X functional of a new family of DFT methods10 as well as the performing of the resource-consuming MP2 harmonic frequency calculations, which were made using the GAMESS9 program. For the latter calculations we took the same exponents as had been used for the geometry optimizations in the GAUSSIAN 03 program package. For the AIM analysis11 of the MP2(Full)/6-311++G(2d,p) electron distribution F(r) of the molecules under study the MORPHY 1.012 program was involved. Delocalization indices δ(A,B)13 were computed using the PROAIMV14 and the AIMDELOC15 programs at the HF/ 6-311++G(2d,2p) level of theory. The molecular orbital (MO) and NBO16 analyses were carried out on the HF/6-31G(d) wave functions. The degree of pentacoordination of the silicon atom, ηα, in the species 19 was calculated by the formula17 ηa = ((109.5 1/3∑3n = 1θn)/ (109.5 90)) 100%, where θn is a bond angle between the axial SiX and equatorial SiY bonds.
’ RESULTS AND DISCUSSION When choosing basic methods for the geometry optimization of silacyclophanes, we relied upon X-ray data, which is available only for t-BuSi[OCH2CH2]3C6H3.2b As can be seen from Table 1, the bond distances and angular characteristics of t-BuSi[OCH2CH2]3C6H3 have a low sensitivity to solvent effects, level of theory, and the size of the basis set. The only exception is the distance between the silicon atom and the center point of the benzene ring (dSiAr), which determines the Si 3 3 3 Ar interaction and which is substantially overestimated in the B3LYP method as compared to MP2 and M06-2X. The same trend is typical also for other types of coordination contacts.19 In general, it seems reasonable to use relatively economical MP2 and DFT methods with the 6-31G(d) basis set for comparative investigation of the structure of a series of silacyclophanes 19. It should be mentioned that the available experimental value of the 29 Si chemical shift for t-BuSi(OCH2CH2)3C6H3 in chloroform (58.4 ppm) is fairly well reproduced at the GIAO-B3LYP/ 6-311++G(2d,p)//B3LYP/6-31G(d) level of theory (62.4 ppm for the isolated molecule and 62.7 ppm in low polar solvent, IEF-PCM, ε = 4.9). The global minimum on the PES of cyclophanes 19 corresponds to the out-C3-symmetrical (for X = NH2, OTf the
symmetry is not strict) structures with the third-order axis passing through the XSi bond and the center point of the practically planar benzene ring (deviation of the C1 and C4 carbon atoms from the C2C3C5C6 plane does not exceed 0.06 Å).20 For convenience, when analyzing the structure of molecules 19, we will characterize the multicenter contact of the acceptor silicon fragment XSiY3 with the donor π-system of the arene fragment by the value of dSiAr. Silacyclophanes XSi[YCH2CH2]3C6R3 14. Typical for the two-centered noncovalent interactions Si 3 3 3 D (D is a donor center) observed in (OSi) monochelates, D = O,21 or in silatranes, D = N,6a,b is a shortening Si 3 3 3 D distance with respect to the sum of the van der Waals (vdW) radii of the Si and D atoms by at least 1 Å. Therefore, the SiO and SiN coordination in these compounds is clearly manifested, for example, in the 29Si NMR spectra or electron distribution. In contrast, for the series of silacyclophanes 14, irrespective of the nature of X, Y, or R, the difference between the values of dSiAr and the sum of the vdW radii of Si and C is less than 0.7 Å (Table 2, Figure 1). This is an unequivocal indication of the weakness of the Si 3 3 3 Ar interaction in XSi[YCH2CH2]3C6R3. Nevertheless, with the increase of the σ-acceptor strength of the axial substituent X at the silicon atom [Me < NH2 < F < OTf] in 14 for the given R and Y not only shortening of the Si 3 3 3 Ar contact and decrease of the deviation of the silicon atom from the plane of three equatorial atoms Y (ΔSi) but also a change of configuration of the bonds of the coordination center XSiY3Ar toward a trigonal-bipyramid geometry are observed (see the value of ηa in Table 2). With the increase of electronegativity of the equatorial atom Y [C < N < O] (and, thus, the corresponding increase of acceptor ability of Si) for the given axial substituents X in 14, one could anticipate the increase of intensity of the Si 3 3 3 Ar interaction. However, the values of dSiAr in the carba-derivatives 3 are notably lower than in the corresponding aza-derivatives 2, presumably due to steric reasons. The increase of the donating ability of the basal ring in 13 by replacement of hydrogen atoms in the 2, 4, and 6 positions of the arene fragment by lithium results in a substantial decrease of the dSiAr values, that is, strengthening the Si 3 3 3 Ar contact. This is demonstrated by comparing the values of dSiAr for the molecules of series 3 and 4 (Table 2).22 As a criterion of the coordination state (hypervalency) of Si in organosilicon derivatives and for estimation of the relative strength of additional SiD bonds, in the 29Si NMR spectroscopy 5596
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Table 2. Bond Lengths (dSiX, Å), Distances from Si to the Center Point of the Benzene Ring (dSiAr, Å) and to the Arene Carbon Atom C1 (dSiC1, Å), Bond Angles ( — XSiY and — YSiY, deg), Angle between the CH2CAr and the Plane of the Three Nearest Carbon Atoms in It (β, deg), Degree of Pentacoordination of the Silicon Atom (ηa, %), and Its Deviation from the Plane of the Equatorial Atoms (ΔSi, Å) for Silacyclophanes XSi[YCH2CH2]3C6R3 Calculated at the B3LYP/6-31G(d) and MP2/6-31G(d) (italics) Levels of Theorya X
R
dSiAr
d SiC1
dSiX
— XSiY
— YSiY
ΔSi
ηa
β
1a
Me
O
H
3.299
3.565
1.868
108.1
110.8
0.517
7
21.9
1b 1c
NH2 F
O O
H H
3.286 3.227
3.553 3.499
1.722 1.607
108.0 106.2
110.9 112.5
0.512 0.459
8 17
22.1 22.0
3.168
3.453
1.614
106.6
112.2
0.470
15
22.7
1d
OTf
O
H
3.170
3.449
1.726
104.3
114.1
0.404
27
21.5
2a
Me
NH
H
3.426
3.684
1.894
107.4
111.4
0.524
11
20.9
3.360
3.631
1.888
108.0
110.8
0.541
8
21.7
2b
NH2
NH
H
3.399
3.659
1.745
107.1
111.9
0.506
12
20.8
2c
F
NH
H
3.349
3.614
1.633
105.6
113.1
0.464
20
20.9
NH
H
3.290 3.271
3.567 3.541
1.641 1.786
106.2 102.9
112.5 115.1
0.481 0.383
17 34
21.7 20.5
2d
a
Y
OTf
3a
Me
CH2
H
3.333
3.599
1.910
102.4
115.5
0.419
36
16.6
3b
NH2
CH2
H
3.322
3.590
1.772
102.4
115.6
0.414
36
16.5
3c
F
CH2
H
3.251
3.525
1.648
100.3
116.9
0.345
47
16.6
3.159
3.448
1.656
100.1
117.0
0.337
48
17.1
3d
OTf
CH2
H
3.199
3.478
1.800
98.8
117.7
0.294
55
16.7
4a
Me
CH2
Li
3.318
3.583
1.917
102.0
115.8
0.404
39
17.7
4b 4c
NH2 F
CH2 CH2
Li Li
3.304 3.227
3.572 3.500
1.782 1.657
101.7 99.6
116.0 117.3
0.393 0.322
40 51
17.7 17.5
3.138
3.419
1.665
99.5
117.4
0.314
51
16.9
4d
OTf
CH2
Li
3.143
3.423
1.832
97.2
118.4
0.243
63
17.4
3.011
3.305
1.853
96.1
118.9
0.205
69
16.6
For unsymmetrical structures the geometric parameters given are averaged for all three chains.
Figure 1. B3LYP/6-31G(d) and MP2/6-31G(d) (italics) optimized geometries of silacyclophanes with the longest (MeSi[NHCH2CH2]3C6H3 (2a)) and shortest (TfOSi[CH2CH2CH2]3C6Li3 (4d)) Si 3 3 3 Ar distances.
the sign and the value of coordination shift Δδ29Si are used.23 For silacyclophanes XSi[YCH2CH2]3C6R3, depending on the surrounding of the silicon atom, examples of both negative and positive values of Δδ29Si exist (1d: Δδ29Si = 8.1 ppm; 2d: Δδ29Si = 6.6 ppm; 3a: Δδ29Si = 3.7 ppm; 4a: Δδ29Si = 1.0 ppm).
The small absolute value of Δδ29Si in molecules 14 (|Δδ29Si| < 10 ppm), taken together with the structural data given above, is indicative of a weakness of the Si 3 3 3 Ar dative contact.24 Silacyclophanes XSi[YCH2]3C6R3 59. Shortening of the side chains in molecules 14 by one methylene group gives rise to silacyclophanes 59 (see, for example, structures 6c, 7a, and 7c in Figure 2), which demonstrate much stronger coordination interaction Si 3 3 3 Ar than the original compounds 14. Indeed, the values of dSiAr in 59 are ∼0.40.7 Å less as compared to those in 14 (Tables 2, 3). For this reason, the coordination shifts Δδ29Si in 59 for any valence surrounding of Si, in contrast to 14, are negative in sign and of rather large value (4 ppm < |Δδ29Si| < 24 ppm) (see Table S1 in the Supporting Information).25 Only for carba-derivatives 79 is a tendency to a linear relationship (R = 0.86) observed between the coordination shifts Δδ29Si and the length dSiAr of the Si 3 3 3 Ar dative contact. The use of the cage silacyclophanes XSi[CH2CH2CH2]3C6R3 with very weak Si 3 3 3 Ar interaction, instead of acyclic silanes, as the reference compounds for calculation of Δδ29Si of molecules XSi[CH2CH2]3C6R3 results in substantial (R = 0.95) improvement of quality of the linear regression Δδ29Si = f(dSiAr) (see Figure 3). Similar relationships between the Δδ29Si and the length of the SiD bond were observed also for “normal” compounds of pentacoordinate silicon having the two-center (not multicenter) dative contact SiD (D = O,19g,21a N).6b The arene ring in silacyclophanes 19 is characterized by negative values of NICS7 (e.g., in 3c NICS(1) = 7.27, and in 7c 5597
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Figure 2. B3LYP/6-31G(d) and MP2/6-31G(d) (italics) optimized geometries of some silacyclophanes XSi[YCH2]3C6H3.
Table 3. Bond Lengths (dSiX, Å), Distances from Si to the Center Point of the Benzene Ring (dSiAr, Å) and to the Arene Carbon Atom C1 (dSiC1, Å), Bond Angles ( — XSiY and — YSiY, deg), Angle between the CH2CAr and the Plane of the Three Nearest Carbon Atoms in It (β, deg), Degree of Pentacoordination of the Silicon Atom (ηa, %), and Its Deviation from the Plane of the Equatorial Atoms (ΔSi, Å) for Silacyclophanes XSi[YCH2]3C6R3 Calculated at the B3LYP, M06-2X (bold), and MP2 (italics) Levels of Theory with the 6-31G(d) and 6-311++G(d,p) (in parentheses) Basis Setsa X
R
dSiAr
d SiC1
dSiX
— XSiY
— YSiY
ΔSi
ηa
β
5a 5b
Me NH2
O O
H H
2.783 2.775
3.074 3.067
1.858 1.713
103.7 103.5
114.5 114.7
0.402 0.394
30 31
40.1 40.2
5c
F
O
H
2.728
3.024
1.592
101.9
115.8
0.345
39
40.3
2.698
3.006
1.598
101.6
116.1
0.336
41
41.2
5d
OTf
O
H
2.697
2.998
1.696
100.9
116.5
0.315
44
40.2
6a
Me
NH
H
2.745
3.048
1.890
102.6
115.4
0.388
35
37.3
6b
NH2
NH
H
2.737
3.040
1.742
102.1
115.7
0.373
38
37.3
6c
F
NH
H
2.676
2.986
1.620
100.5
116.7
0.322
46
37.5
6d
OTf
NH
H
2.627 2.638
2.952 2.954
1.627 1.746
100.0 99.0
117.0 117.6
0.307 0.277
49 54
38.4 37.4
7a
Me
CH2
H
2.837
3.121
1.907
101.4
116.2
0.388
42
32.0
2.781
3.082
1.899
101.0
116.5
0.373
44
33.9 31.9
7b
NH2
CH2
H
2.829
3.113
1.764
101.1
116.4
0.378
43
7c
F
CH2
H
2.768
3.060
1.642
99.4
117.4
0.319
52
(2.762)
(3.055)
(1.657)
(99.3)
(117.5)
(0.313)
2.723
3.029
1.648
99.3
117.5
0.312
(2.730) 2.739
(3.038) 3.035
(1.652) 1.632
(99.4) 99.3
(117.4) 117.4
(0.316) 0.313
(52) 53 (52) 52
32.1 (32.0) 33.9 (34.1) 32.6
7d
OTf
CH2
H
2.720
3.017
1.781
97.9
118.1
0.269
59
32.0
8a
Me
CH2
SiH3
2.833
3.112
1.905
101.2
116.3
0.384
43
30.2
8b
NH2
CH2
SiH3
2.826
3.106
1.760
101.1
116.4
0.377
43
30.2
8c
F
CH2
SiH3
2.765
3.052
1.640
99.4
117.4
0.318
52
30.4
2.710
3.016
1.648
98.7
117.8
0.292
55
32.9
8d
OTf
CH2
SiH3
2.723
3.016
1.775
98.1
118.0
0.275
58
30.5
9a 9b
Me NH2
CH2 CH2
Li Li
2.805 2.795
3.105 3.095
1.917 1.778
100.2 99.9
116.9 117.1
0.347 0.338
48 49
35.9 35.6
9c
F
CH2
Li
2.726
3.034
1.653
98.1
117.4
0.276
58
35.7
2.631
2.950
1.668
97.0
118.5
0.237
64
34.3
2.622
2.944
1.828
95.7
119.1
0.187
72
35.3
9d a
Y
OTf
CH2
Li
For unsymmetrical structures the geometric parameters given are averaged for all three chains.
NICS(1) = 4.06) and practically by the absence of effect of the CArCAr bond alternation, which unambiguously points to its aromatic character. Note that the absolute NICS value in 14 is substantially larger than in 59. This is indicative of a larger shift of π-electron density from the basal ring toward the acceptor
fragment XSiY3, and, hence, of a stronger Si 3 3 3 Ar interaction in 59 as compared to that in 14. Variation of geometric parameters of the silicon polyhedron XSiY3Ar in 59 (see Table 3 and Figure 2) with increasing σ-acceptor strength of the axial substituent X [Me < NH2 < F < OTf] 5598
dx.doi.org/10.1021/om200340k |Organometallics 2011, 30, 5595–5603
Organometallics and donor strength of R [H < SiH3 < Li] for a given Y is consistent (as is the case for “normal” compounds of pentacoordinate silicon with the coordination center XSiY3D (D = O,21 N)).6 Indeed, the strengthening of the coordination contact Si 3 3 3 Ar (decrease of dSiAr) occurs along with planarization of the silicon atom surrounding (decrease of ΔSi) and the increase of the degree of its pentacoordination (ηa). The values of angle β determining the deviation of the CC bond of the side chain from the plane of the arene fragment and characterizing its out-of-plane deformation in silacyclophanes 59 are 1519 larger than in 14 (cf. Tables 2 and 3). This is indicative of a larger strain in the former cycles than in the latter. Nevertheless, the values of angle β in 59 do not go beyond the interval 048 typical for the experimentally studied derivatives
Figure 3. Dependence of the calculated values of dSiAr and coordination shifts Δδ29Si in structures 79.
ARTICLE
of cyclophanes.26 Therefore, molecules XSi[YCH2]3C6R3 are not abnormally strained. The order of weakening the Si 3 3 3 Ar contact with the decrease of electronegativity of the equatorial atoms Y (O > N > C) for given X in 57 (dSiAr (Y = CH2) > dSiAr (Y = O) > dSiAr (Y = NH)) is different from that in 13 (dSiAr (Y = NH) > dSiAr (Y = CH2) > dSiAr (Y = O)). An apparent reason for this could be a notably different strain (judged from the values of angle β) of the cage structures 57 and 13. Variation of the strength of the SiD dative bonds, which is inconsistent with the variation of the nature of Y, is observed for silatranes XSi[Y(CH2)2]3N27 and silaphosphanes XSi[Y(CH2)3]3P19f having the cage structure and containing pentacoordinate silicon. As distinct from the intramolecular complexes 19, in which the Si 3 3 3 Ar coordination can be sterically assisted, the intermolecular complexes of silanes XSiMe3, XSi(NH2)3, and XSi(OMe)3 with benzene, according to the MP2 and B3LYP calculations, are stabilized by the vdW contacts and/or weak hydrogen bonds of different type. For example, the MP2/6-311+ +G(d,p) absolute values of the energy of formation of associates FSiMe3 3 C6H6 and FSi(NH2)3 3 C6H6 (see Figure 4), ΔE, with the ZPV correction are less than 4.5 kcal/mol. Judged from the positive values of ΔG, these complexes are unstable under normal conditions (298.15 K, 1 atm). AIM, NBO, and Molecular Orbital Analyses of Silacyclophosphanes 19. The AIM analysis of the electron distribution F(r) in silacyclophanes 59, and more so in 14, irrespective of the method of optimization of their geometry, did not reveal bond critical points, BCP, (3, 1) in the internuclear region between the silicon atom and the arene carbon atoms. Even the
Figure 4. MP2/6-311++G(d,p) optimized geometries of complexes FSiMe3 3 C6H6 and FSi(NH2)3 3 C6H6. 5599
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Organometallics
Figure 5. B3LYP/6-31G(d) and MP2/6-31G(d) (italics) optimized geometry of cation H3N+Si[CH2CH2]3C6Li3.
Figure 6. Map of the Laplacian of the electron density for the FSiCCCAr moiety of silacyclophane 7c. Critical points (3, 1) are designated by solid squares. Maximum of the charge concentration in the region of arene carbon atom CAr is denoted by a cross. Geometric path of the SiCAr bond is shown by a dotted line. Dashed lines correspond to r2F(r) > 0 (regions of charge depletion) and solid lines to r2F(r) < 0 (regions of charge concentration). The contour values in e/a05 are (0.002, ( 0.004, and (0.008.
AIM molecular graph for the structure H3N+Si[CH2CH2]3C6Li3 (Figure 5) obtained by protonation of the amino group in 9b, in which the acceptor strength of the silicon atom and the donor strength of the basal ring are more favorable for realization of the Si 3 3 3 Ar interaction as compared to neutral silacyclophanes 7, does not contain BCP (SiCAr). Note that the question of the use of bonding paths as a reliable criterion of weak or unusual (including multicenter) interactions in molecular systems is debatable.28 Such interactions, for different reasons, can be even unconnected with bonding paths. As one of such reasons for molecules XSi[YCH2]3C6R3 and cation H3N+Si[CH2CH2]3C6Li3 we can point to a substantial shift of the maximum of the charge concentration in the region of their arene carbon atoms CAr with respect to the geometric path of the SiCAr bond (Figure 6).29 In such cases, to identify the attractive interaction, the AIM delocalization index δ(A,B) is often used.30 By definition, it is a quantitative measure of the number of shared electron pairs in the internuclear region AB. Ordinary nonpolar bonds are characterized by the value of δ(A,B) = 1, whereas for polar bonds the value of δ(A,B) is substantially lower.13,31 For example, in the molecule of methylsilane H3SiCH3 δ(Si,H) = 0.57 and δ(Si,C) = 0.44, respectively.
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Figure 7. Dependence of the length of the coordination contact dSiAr and its strength characteristics ∑δ(Si,C) (open squares) and ∑ΔE(2)[πCCfσ*SiX] (solid triangles) in structures 79.
It seems, therefore, evident that coordination interaction Si 3 3 3 Ar in silacyclophanes 19 must be described by six twocenter contacts SiCAr, that is, ∑δ(Si,C), rather than only one contact, δ(Si,C). The value of ∑δ(Si,C) for molecules 59 is quite significant (0.1 > ∑δ(Si,C) > 0.02). It seems appropriate to mention that the experimentally measured6 two-center dative contact Si 3 3 3 N in fluorosilatrane FSi[OCH2CH2]3N is characterized by the value of δ(Si,N) = 0.08. As should be expected, ∑δ(Si,C) for silacyclophanes 14 are about 1 order of magnitude less (0.03 > ∑δ(Si,C) > 0.003) than those for 59 (see Table S1 in the Supporting Information). In the NBO analysis of 19, the arene ring is represented by six orbitals of the σ-type and three orbitals of the π-type of CC bonds. The total energies of the DA interaction of the three πCC-orbitals with the antibonding orbital σ*SiX of the SiX bond (∑ΔE(2)[πCCfσ*SiX]) for 59 lie within the range 28 kcal/mol. Note that for silatranes XSi[OCH2CH2]3N with the coordination center XSiO3N, the quantitative NBO B3LYP/ 6-311G(d,p) estimation of the nNfσ*SiX conjugation gives the value of ΔE(2)[nNfσ*SiX] of ca. 39 kcal/mol.32 For the series of molecules with rather long side chains, 1, 2, and 3a,b, the πCCfσ*SiX interaction is negligible, unlike in 59. In the case of 3c,d and 4 it is substantially less than in the corresponding silacyclophanes 7c,d and 9 (see Table S2 in the Supporting Information). Upon variation of the nature of substituents X and R in 59 with fixed Y, the values dSiAr, ∑δ(Si,C), and ∑ΔE(2)[πCCfσ*SiX], which characterize the strength of the Si 3 3 3 Ar coordination, vary in a consistent manner.33 The decrease of dSiAr caused by the increased acceptor strength of Si and donor strength of the basal ring is followed by the increase of ∑δ(Si,C) and ∑ΔE(2)[πCCfσ*SiX] (Figure 7, molecules 79).34 It should be stressed that for quantitative estimation of the delocalization indices ∑δ(Si,C) and NBO energies ∑ΔE(2)[πCCfσ*SiX] the B3LYP/6-31G(d) optimized geometries of 19 were used. As was mentioned above, the B3LYP method underestimates coordination interactions relative to the MP2 method. Therefore, on going to the MP2 geometries, the values of ∑δ(Si,C) and ∑ΔE(2)[πCCfσ*SiX] in 19 may increase. For example, for structure 7c one has ∑δ(Si,C) = 0.045 (B3LYP) and 0.051 (MP2); ∑ΔE(2)[πCCfσ*SiX] = 3.2 (B3LYP) and 3.6 kcal/mol (MP2). At the molecular-orbital level the XSi 3 3 3 Ar interaction in silacyclophanes 19 can be represented by the following simple diagram (Figure 8). According to symmetry constraints, the MO of the SiX bond can mix only with the a2u bonding MO of the arene ring with the formation of three MO belonging to the axial fragment 5600
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Figure 8. Orbital correlation diagram for MO formation in the XSi 3 3 3 Ar fragment of silacyclophanes 19 from the π-orbitals of arene and σ-orbitals of the SiX bond.
Figure 9. HF/6-31G(d)//MP2/6-31G(d) bonding Ψ1 (HOMO20) of the XSi 3 3 3 Ar fragment of silacyclophane 9c.
XSi 3 3 3 Ar: bonding Ψ1 (having no nodal plane), Ψ2 (with one nodal plane), and antibonding Ψ4. MOs Ψ3 are doubly degenerate orbitals of arene of the e1g type, which do not interact with the MO of the SiX bond (see Figure 8). The composite orbitals Ψ1, Ψ2, and Ψ4 corresponding to the diagram in Figure 8 can be isolated (Figure 9, Ψ1 of molecule 9c as an example) in the system of MOs of silacyclophanes 6d, 7c,d, 8c,d, and 9c,d with the relatively strong interaction Si 3 3 3 Ar (dSiAr < 2.73 Å) and with a high degree of pentacoordination of the silicon atom (ηa > 50%). In other structures 19, the orbital interaction Si 3 3 3 Ar is practically not observed. In these molecules, MOs Ψ1 and Ψ2 are virtually “pure” orbitals a2u of the arene fragment and σSiX of the SiX bond, respectively.
’ CONCLUSION The structure of a wide series of silacyclophanes of general formula XSi[Y(CH2)n]3C6R3 19 (X = t-Bu, Me, NH2, F, OTf; Y = O, NH, CH2; R = H, SiH3, Li; n = 1, 2) has been studied at the MP2 and DFT (B3LYP, M06-2X) level of theory using the 6-31G(d) and 6-311++G(d,p) basis sets. Compounds 19 exist exclusively in the out-C3-symmetric form (for X = NH2, OTf the symmetry is not strict) with the third-order symmetry axis passing through the XSi bond and the center point of the benzene ring. In molecules XSi[YCH2CH2]3C6R3 14, irrespective of the nature of X, Y, and R, the multicenter interaction Si 3 3 3 Ar is
weak. This is demonstrated by the length of the dative contact (dSiAr), the value of the coordination shift (Δδ29Si), and the results of the AIM, NBO, and MO analysis. Strong enhancement of the interaction of the tetracoordinate silicon atom with the π-system of the benzene ring is observed in compounds XSi[YCH2]3C6R3 59 with side chains shortened by one methylene group. Along with the geometry parameters of 59, this is confirmed by the value and the sign of the coordination shift Δδ29Si and NICS(1), and the values of the total delocalization index (Σδ(Si,C)) and of the energy of interaction of the π-orbitals of the arene fragment with the SiX bond (∑ΔE(2)[πCCfσ*SiX]). The increase of the σ-acceptor power of the axial substituent X and donating power of R in 59 is followed by strengthening of the dative contact Si 3 3 3 Ar. A high degree of pentacoordination of the silicon atom, that is, ηa >50%, is found for structures 6d, 7c,d, 8c,d, and 9c,d. Strengthening of the Si 3 3 3 Ar interaction caused by variation of X and R in XSi[YCH2]3C6R3 with the surrounding in the equatorial plane being retained is accompanied by consistent changes of geometric (dSiAr, ΔSi, ηa), electronic (∑δ(Si,C)), orbital (∑ΔE(2)[πCCfσ*SiX]), and NMR (Δδ29Si) characteristics of the coordination center XSiY3Ar. The revealed trends are typical for pentacoordinate silicon compounds.
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables with selected geometrical, electronic, and magnetic characteristics of compound 19. Cartesian coordinates and total energies of all investigated structures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +7-3952-426545. Fax: +7-3952-419346. E-mail: svf@ irioch.irk.ru.
’ ACKNOWLEDGMENT The authors are indebted to Professor T. J. Barton for careful reading of the manuscript and valuable suggestions. We are grateful to Dr. P. L. A. Popelier for a copy of the MORPHY 1.0 5601
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Organometallics program. The financial support from the Russian Foundation for Basic Research (Grant RFBR 07-03-00888) is gratefully acknowledged.
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