Persistent silylium ions stabilized by polyagostic Si-H...Si interactions

Article abstract of DOI:10.1002/anie.200604258

Diagnostics for diagostics: The silylium ion 1 (see picture; C orange, Si red, H gray) is highly fluxional at room temperature but at -80°C exhibits a symmetric structure with a three-center two-electron Si-H-Si bond supported by two additional Si←H-Si ag

Full text of DOI:10.1002/anie.200604258

Communications
DOI: 10.1002/anie.200604258
Agostic Interactions
ꢀ
Persistent Silylium Ions Stabilized by Polyagostic Si H···Si
Interactions**
Andrey Y. Khalimon, Zi Hua Lin, R. Simionescu, Sergei F. Vyboishchikov,* and
Georgii I. Nikonov*
Dedicated to Professor J. Lorberth on the occasion of his 70th birthday
Silylium ions are much more reactive than corresponding
carbenium ions owing to the larger size of the silicon atom
and their higher electrophilicity.[1,2] In the absence of suffi-
cient steric protection, silylium ions readily interact even with
weakly nucleophilic molecules such as the perchlorate
anion,[3] halocarbons,[4] alkenes,[5] alkynes,[6] arenes,[7] and
even hydrocarbons and noble gases.[8,9] As a result, a genuine
tricoordinate (“naked”) silylium ion has been prepared only
recently.[10,11] More recently, Müller et al. have reported that
formation of a 3c–2e bond (R3Si-H-SiR3)+ can stabilize a
cationic silicon center (1 and 2) even in the absence of steric
protection.[12,13] A similar R3B···H-SiR3 interaction (3), with a
by Wrackmeyer et al.[14] Herein we report the case of
polyagostic Si-H-Si interactions in a series of silylium ions
derived from polysilyl-substituted benzenes.
Hydride abstraction[10a] from C6(SiHMe2)6 by [Ph3C][B-
(C6F5)4] generates the silylium ion 4 quantitatively according
to NMR spectra [Eq. (1)]. At ꢀ808C, the 1H NMR spectrum
ꢀ
stronger H Si bond and weaker B···H bonding, was described
of 4 in CD2Cl2 exhibits an effective C2v structure with three
broad SiH signals at d = 4.64 (g-SiH), 4.41 (b-SiH), and
4.26 ppm (a-SiH) with relative intensities of 2:2:1 and three
Me signals of equal intensity, at d = 0.88 (a-SiMe), 0.67 (b-
SiMe), and 0.49 ppm (g-SiMe).[15] The upfield shift of the a-
SiH and b-SiH resonances is a characteristic signature of
agostic bonding,[16] whereas the downfield shift of the a-SiMe
signal reflects the electron deficiency of cationic silylium
centers. The 29Si NMR spectrum at ꢀ858C, selectively decou-
pled from Me groups, reveals three signals, at d = 24.9 (d,
J
J
H,Si = 46.3 Hz), 15.3 (d, JH,Si = 118.9 Hz), and ꢀ5.3 ppm (d,
H,Si = 170.7 Hz)[17], assigned to the Si atoms in the a, b, and
[*] Dipl.-Chem. A. Y. Khalimon, Z. H. Lin, Dipl.-Ing. R. Simionescu,
Dr. G. I. Nikonov
g positions, respectively. The agostically stabilized silylium
center gives rise to a downfield 29Si NMR signal (d =
24.9 ppm) which is close to the value found for the H-bridged
cation 2 (d = 54.4 ppm).[12b] The cation 4 is stable in CD2Cl2
and CDCl3 solutions at least for several days.
Chemistry Department, Brock University
500 Glenridge Ave., St. Catharines, ON L2S3A1 (Canada)
Fax: (+1)905-6829020
E-mail: gnikonov@brocku.ca
ꢀ
The H Si coupling constants in 4 provide important
Dr. S. F. Vyboishchikov
information about the bonding situation in this compound.
Thus, whereas the coupling constant associated with the g-Si
atom (JH,Sig) of 170.7 Hz is close to the value expected for a
free SiHMe2 group, the highly decreased JH,Sia value of
46.3 Hz is indicative of a H-bridged silylium ion, a structural
motif similar to that one reported for 1 (JH,Si = 39 Hz) and 2
(JH,Si = 45.7 Hz).[12] The new prominent feature of 4 lies in the
fact that the two half-charged cationic centers in the
a positions induce waning agostic interactions with the
hydrides bound to the b-Si atoms, resulting in a noticeable
decrease of the JH,Sib value compared to the JH,Sig value (118.9
Institut de Química Computacional, Campus de Montilivi
Universitat de Girona, 17071 Girona, Catalonia (Spain)
Fax: (+34)97241-8356
E-mail: vybo@stark.udg.es
[**] This work was supported by Brock University, the Research
Corporation, and the Spanish Ministry of Education and Science
(Ramón y Cajal Program, grant CTQ2005-02698, and grant
PCI2005A7-0167). We thank Prof. Dr. V. I. Bakhmutov for many
valuable discussions and Dr. K. Y. Dorogov for assistance in
isolating the compound C6(SiHMe2)6.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
!
vs. 170.7 Hz). As a result of this additional Sia H-Sib
reflective of the weakness of agostic interaction. In fact, the
bonding, each equivalent silylium center adopts a distorted
TBP (trigonal-bipyramidal) geometry with the hydride atoms
in the apical positions (Figure 1).
ArSiaMe2 fragment is nearly planar, so that bonding to the
hydride is primarily provided through the Si p orbital,
resulting in severe reduction of the Fermi contact term.[12a,b]
As noted above, the coupling constant between the b-Si and
b-H atoms is decreased, (118.9 Hz (exptl) and ꢀ103.6 Hz
ꢀ
(calcd)), which is the result of elongation of the Si H bond
and distortion of the C-Sib-H bond angle to 94.68 owing to
agostic bonding (compare: 101.28 for the C-Sig-H angle).
Again, such a distortion leads to increased b-Si p character in
b
b
ꢀ
the Si H bond.
As expected, the 3c–2e Sia-Ha-Sia bond is characterized by
ꢀ
larger Mayer bond orders (Si H MBO = 0.445) and Wiberg
!
bond orders (WBO = 0.426) than the aSi HSib agostic
bonding (MBO = 0.242, WBO = 0.182; Table 1).[20] The nat-
ural bond orbital (NBO)[21] occupation of the H Si bond
b
b
ꢀ
g
g
ꢀ
(1.806) is reduced in comparison with the H Si single bond
Figure 1. a) DFT-calculated structure of agostically stabilized silylium
ion 4. b) DFT-calculated structure of agostically stabilized silylium ion
5. Interatomic separations are given in Š.
ꢀ
Table 1: Mayer bond orders, Wiberg bond orders, occupation of the H Si
natural bond orbital, and Mayer diatomic energies (MDE, in kcalmolꢀ1).
ꢀ
ꢀ
Cmpd Bond
d(Si H) MBO WBO H Si NBO
[Š]
MDE
Finally, the presence of agostic Ha and Hb hydrides is
occupation[a]
ꢀ
clearly seen from the remarkable red shift of their Si H bands
4
5
Ha-Sia 1.686
Hb-Sia 1.980
0.445 0.426
0.242 0.182
–
–
ꢀ94.4
ꢀ68.4
in the IR spectrum (1725 cmꢀ1 (exptl) vs. 1655 cmꢀ1 (calcd)
a
for Si H and 1978 cmꢀ1 (exptl) vs. 1898 cmꢀ1 (calcd) for the
ꢀ
Hb-Sib
Hg-Sib
Hg-Sig
1.558
2.504
1.508
0.664 0.692 1.806
0.104 0.034
0.828 0.880 1.938
0.325 0.286
0.582 0.584 1.722
0.114 0.041
0.821 0.871 1.934
ꢀ116.0
ꢀ35.0
symmetric stretch of two b-Si H bonds[18]). In contrast, the g-
ꢀ
–
ꢀ
Si H bonds give rise to a very broad band at about 2140
ꢀ126.1
ꢀ78.2
Hb-Sia 1.820
–
(exptl) versus 2134 and 2131 cmꢀ1 (calcd), in a region typical
of hydrosilanes.
Hb-Sib
Hg-Sib
Hg-Sig
1.595
2.515
1.514
ꢀ105.9
ꢀ36.8
–
The room-temperature 1H NMR spectrum of 4 corre-
sponds to an effective D6h symmetry, giving rise to a SiH
singlet at d = 4.60 ppm and a SiMe singlet at d = 0.78 ppm and
indicative of fast exchange. This exchange most likely occurs
through hydride transfer between the a silicon centers,
ꢀ122.2
[a] Missing data means that no NBO has been found.
ꢀ
assisted by the agostic bonding with the b-Si H bond.
(1.938) owing to electron-density transfer to the cationic Sia
centers. Large attractive (that is, negative) values of Mayer
The suggested H-bridged silylium ion structure supported
!
by two Sia H-Sib agostic interactions was further elucidated
diatomic energies[22] for the Si H bond in 4 (right-hand
ꢀ
by DFT calculations (Figure 1a).[19] The optimized structure
column of Table 1) present strong evidence of significant
Si···H interactions. Finally, bond critical points with negative
energy density were found by using the AIM approach[23] both
of 4 exhibits C2 symmetry. The bridging hydride forms two
ꢀ
equivalent elongated Si H bonds to the a-Si atoms with
!
lengths of 1.686 Š and a Si-H-Si bond angle of 130.28. If the
for Sia-Ha-Sia and Sia H-Sib interactions (see TableSI1 in the
ꢀ
Si H bonding is neglected, the a-silylium ion is almost planar
Supporting Information).
(the sum of bond angles is 358.98). A similar geometry
(1.607 Š and 134.78) has been calculated for 2 at the MP2/6-
311G** level.[12b] The agostic b-hydride forms an elongated
bond to the b-Si center (1.558 Š vs. 1.4–1.5 Š in hydro-
silanes), and a longer agostic bond to the a-silylium ion
Can we cut the “hydride current” in 4? To do this, we
introduced a “methyl insulator” in 4 by reacting the persil-
ylated toluene derivative MeC6(SiHMe2)5 with [Ph3C][B-
(C6F5)4] [Eq. (2)]. Contrarily to 4, NMR spectra of the
product of this reaction, the silylium ion [MeC6(SiHMe2)4-
(SiMe2)]+ (5), are temperature-independent down to ꢀ808C.
The room-temperature 1H NMR spectrum of 5 shows two SiH
ꢀ
(1.980 Š). For comparison, the “unperturbed” g-Si H bond
was calculated to be 1.507 Š and the Sib···Hg separation
(2.504 Š) is too long for a significant interaction. Calculation
of the 29Si NMR parameters using the GIAO method[19] gives
d = 22.1, 28.2, and ꢀ3.8 ppm for the a-, b-, and g-Si atoms,
respectively. A small JSi,H value of ꢀ38.2 Hz was calculated for
the bridging hydride, in fair agreement with the experimental
value (j J j = 46.3 Hz). The a-Si center is only weakly coupled
with the agostic b-hydrogen atom (JSi,H = ꢀ3.5 Hz), but the
negative sign of JSi,H suggests direct bonding.[16] It is important
to stress that this small absolute value of JSi,H is merely the
result of the geometry of the ArSiaMe2 group rather than
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Communications
signals, a septet at d = 4.53 (2JH,H = 3.6 Hz) for the free
an alternative view is adopted, the structures 4 and 5 exhibit
multiple agostic interactions induced by the silylium ion.
SigMe2H group and an upfield-shifted singlet at d =
4.28 ppm for Hb, indicative of agostic bonding. The methyl
groups give rise to three signals at d = 1.02 (silylium ion
Received: October 17, 2006
Revised: March 8, 2007
Published online: May 4, 2007
SiMea ), 0.84 (d, 2JH,H = 2.1 Hz, Meb), and 0.54 ppm (d, 2JH,H
=
2
3.6 Hz, Meg), with relative intensities of 1:2:2. Like 4, the
cation 5 is stable in chlorinated solvents at least for two days.
The 29Si NMR spectrum of 5 contains three signals. The
SigMe2H group, which is not involved in significant agostic
bonding, gives rise to a doublet of septets at d = ꢀ4.5 ppm
Keywords: agostic interactions · density functional calculations ·
.
silicon · silylium ions
1
(2JSi,H = 7.1 Hz, JSi,H = 166.2 Hz). The two SibMe2H groups,
ꢀ
complexing the silylium ion through two Si H agostic
interactions, have a downfield-shifted (compared to the
[1] Reviews: a) T. Müller, Adv. Organomet. Chem. 2005, 53, 155 –
216; b) C. A. Reed, Acc. Chem. Res. 1998, 31, 325 – 332; c) J. B.
Lambert, L. Kania, S. Zhang, Chem. Rev. 1995, 95, 1191; d) J.
Chojnowski, W. Stanczyk, Main Group Chem. News 1993, 1,
1371; e) P. D. Lickiss, J. Chem. Soc. Dalton Trans. 1992, 1333;
f) C. Eaborn, J. Organomet. Chem. 1991, 405, 173; g) J. B.
Lambert, W. J. Schulz, Jr., The Chemistry of Organic Silicon
Compounds, Part 2 (Eds.: S. Patai, Z. Rappoport), Wiley,
Chichester, 1989, p. 1007.
values expected for a PhSiMe2H moiety) signal at d =
ꢀ
33.5 ppm with a noticeably decreased direct Si H coupling
(1JSi,H = 87.2 Hz, 2JSi,H = 6.9 Hz). Finally, the silylium ion gives
rise to a multiplet at d = 34.3 ppm, which upon selective
decoupling from the Me protons resolves into a triplet with
JSia,Hb = 16.1 Hz, suggesting a diagostic bonding with the b-H
1
ꢀ
Si bonds. Cooling down the sample to ꢀ808C does not change
these spectral patterns, apart from the expected temperature-
induced shifts of resonances, consistent with the absence of
fluctionality in this system. The IR spectrum of 5 exhibits two
red-shifted bands[16] (1747 cmꢀ1 (exptl) vs. 1738 cmꢀ1 (calcd)
and 1810 cmꢀ1 (exptl) vs. 1817 cmꢀ1 (calcd)) for the agostic
[2] a) P. von R. Schleyer, Science 1997, 275, 39 – 40; b) P. P. Gaspar,
Science 2002, 297, 785 – 786.
[3] a) J. B. Lambert, J. A. McConnell, W. J. Schulz, Jr., J. Am. Chem.
Soc. 1986, 108, 2482 – 2484; b) J. B. Lambert, J. A. McConnell,
W. Schilf, W. J. Schulz, Jr., J. Chem. Soc. Chem. Commun. 1988,
455 – 456; c) G. K. S. Prakash, S. Keyaniyan, R. Aniszfeld, L.
Heiliger, G. A. Olah, R. C. Stevens, H. K. Choi, R. Bau, J. Am.
Chem. Soc. 1987, 109, 5123 – 5126; d) Z. Xie, D. J. Liston, T.
Jelinek, V. Mitro, R. Bau, C. A. Reed, J. Chem. Soc. Chem.
Commun. 1993, 384 – 386; e) M. Kira, T. Hino, H. Sakurai,
Chem. Lett. 1993, 153 – 156; f) S. R. Bahr, P. Boudjouk, J. Am.
Chem. Soc. 1993, 115, 4514 – 4519; g) D. Cremer, L. Olsson, C.-
H. Ottosson, J. Mol. Struct. (THEOCHEM) 1994, 313, 91 – 109.
[4] a) J. B. Lambert, Y. Zhao, S. M. Zhang, J. Phys. Org. Chem. 2001,
14, 370; b) C. A. Reed, Z. Xie, R. Bau, A. Benesi, Science 1993,
262, 402; c) Z. Xie, J. Manning, R. W. Reed, R. Mathur, P. D. W.
Boyd, A. Benesi, C. A. Reed, J. Am. Chem. Soc. 1996, 118, 2922 –
2928; d) M. Kira, T. Hino, H. Sakurai, J. Am. Chem. Soc. 1992,
114, 6697 – 6700.
[5] a) P. Jutzi, A. E. Bunte, Angew. Chem. 1992, 104, 1636; Angew.
Chem. Int. Ed. Engl. 1992, 31, 1605; b) H.-U. Steinberger, T.
Müller, N. Auner, C. Maerker, P. von R. Schleyer, Angew. Chem.
1997, 109, 667; Angew. Chem. Int. Ed. Engl. 1997, 36, 626; c) J.
Schuppan, B. Herrschaft, T. Müller, Organometallics 2001, 20,
4584 – 4592; d) T. Müller, C. Bauch, M. Ostermeier, M. Bolte, N.
Auner, J. Am. Chem. Soc. 2003, 125, 2158 – 2168.
a
b
ꢀ
Si -H-Si bonds and one merged band for the g-Si H bonds at
2107 cmꢀ1 (2094–2098 cmꢀ1 (calcd)). To the best of our
knowledge, such a diagostic bonding is unprecedented in
the chemistry of silicon cations. The closest analogy exists
ꢀ
only with the weakB H···Sn interactions reported by Izod
et al. for a stannylene compound.[24]
The DFT-calculated structure of 5 completely supports
the presence of diagostic bonding (Figure 1b). A structure
with approximate Cs symmetry was obtained as a result of
!
ꢀ
optimization. The Sia Hb-Sib bonds (1.818 and 1.822 Š) in 5
a
a
are longer than the Si
H bond in 4 but shorter than the
!
b
b
b b
Sia H -Si agostic interaction in 4. The H Si bond in 5
(1.594/1.596 Š) is elongated as a result of electron-density
transfer to the silylium ion. The Sib center exhibits the same
distorted C-Si-H angle of 93.38 as the agostic Sib center in 4.
The calculated JSi,H values are in fair accord with the absolute
experimental coupling constants (j J j in parentheses):
ꢀ
b
a
b
ꢀ
ꢀ
ꢀ14.8 Hz (16.1 Hz) for H Si , ꢀ76.3 Hz (87.2 Hz) for H
[6] T. Müller, R. Meyer, D. Lennatz, H.-U. Siehl, Angew. Chem.
2000, 112, 3203; Angew. Chem. Int. Ed. 2000, 39, 3074.
b
g
g
[25]
ꢀ
Si , and ꢀ166.2 Hz (156.2 Hz) for H Si. As in 4, the low
values of JSia,Hb and JSib,Hb are the result of a higher silicon
[7] a) J. B. Lambert, S. Zhang, C. Stern, J. C. Huffman, Science 1993,
260, 1917; b) J. B. Lambert, S. Zhang, J. Chem. Soc. Chem.
Commun. 1993, 383; c) P. von R. Schleyer, P. Buzek, T. Müller,
Y. Apeloig, H.-U. Siehl, Angew. Chem. 1993, 105, 1558; Angew.
Chem. Int. Ed. Engl. 1993, 32, 1471; d) L. Olsson, D. Cremer,
Chem. Phys. Lett. 1993, 215, 433; e) J. B. Lambert, S. Zhang,
Science 1994, 263, 984; f) L. Pauling, Science 1994, 263, 983;
g) G. A. Olah, G. Rasul, H. A. Buchholz, X.-Y. Li, G. Sandford,
G. K. S. Prakash, Science 1994, 263, 983; h) C. A. Reed, Z. Xie,
Science 1994, 263, 985; i) G. A. Olah, G. Rasul, H. A. Buchholz,
X.-Y. Li, G. K. S. Prakash, Bull. Soc. Chim. Fr. 1995, 132, 569;
j) M. Arshadi, D. Johnels, U. Edlund, C.-H. Ottosson, D. Cremer,
J. Am. Chem. Soc. 1996, 118, 5120; k) K. Werner, R. Meyer, T.
Müller, Book of Abstracts, 34th Organosilicon Symposium,
White Plains, New York, 2001, p. PS2 – 9; l) P. D. Lickiss, P. C.
Masangane, W. Sohal, Book of Abstracts, 34th Organosilicon
Symposium, White Plains, New York, 2001, p. C-16.
ꢀ
p contribution to the Si H bonding (NBO occupation of the
“free” Si p orbital is 0.484, Table 1). The diagostic Sib-H!
!
Sia H-Sib interaction in 5 (MBO = 0.325) is weaker than the
3c–2e Sia-H-Sia bond in 4 (MBO = 0.445) but stronger than its
!
ꢀ
Sia H-Sib agostic bonding (MBO = 0.242). Correspondingly,
b
b
b
b
ꢀ
the H Si bond in 5 is weaker than the H Si bond in 4
(MBO = 0.582 and 0.664, respectively). Finally, the AIM
!
study of 5 revealed bond critical points for both Sia H-Sib
interactions.
Although a pentacoordinate silicon anion with two apical
hydride atoms has been recently characterized,[26] 4 and 5
ꢀ
present the first examples of a compound in which two Si H
bonds serve as ligands to a hypervalent silicon center.[27] Or, if
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Chemie
[8] a) C. Maerker, J. Kapp, P. von R. Schleyer in Organosilicon
Chemistry: From Molecules to Materials, Vol. II (Eds.: N. Auner,
J. Weis), VCH, Weinheim, 1995, p. 329; b) C. Maerker, J. Kapp,
P. von R. Schleyer in The Chemistry of Organic Silicon Com-
pounds, Vol. 2 (Eds.: Z. Rappoport, Y Apeloig), Wiley, Chi-
chester, 1998, pp. 513 – 556; c) C.-H. Ottosson, D. Cremer,
Organometallics 1996, 15, 5309; d) Y. Apeloig, O. Merin-
Aharoni, D. Danovich, A. Ioffe, S. S. Shaik, Isr. J. Chem. 1993,
33, 387 – 402.
[17] Corresponds well to the value of 171 Hz measured from the left-
hand part of the g-HSi doublet in the 1H NMR spectrum. A
29Si INEPT+ NMR experiment at ꢀ808C afforded the values
116.9 Hz and 171.2 Hz for JH,Sib and JH,Sig, respectively (see the
Supporting Information).
[18] The asymmetric stretch at 1899 cmꢀ1 was calculated to be an
ꢀ
order of magnitude weaker. The observed b-Si H band at
1978 cmꢀ1 is broad and has a shoulder at about 1909 cmꢀ1 (see
the Supporting Information).
[9] For silylium ions stabilized by conjugation with multiple bonds,
see: a) M. Ichinohe, M. Igarashi, K. Sanuki, A. Sekiguchi, J. Am.
Chem. Soc. 2005, 127, 9978; b) A. Sekiguchi, T. Matsuno, M.
Ichinohe, J. Am. Chem. Soc. 2000, 122, 11250.
[10] a) J. B. Lambert, Y. Zhao, Angew. Chem. 1997, 109, 389; Angew.
Chem. Int. Ed. Engl. 1997, 36, 400; b) K.-C. Kim, C. A. Reed,
D. W. Elliott, L. J. Mueller, F. Tham, L. Lin, J. B. Lambert,
Science 2002, 297, 825; c) T. Müller, Y. Zhao, J. B. Lambert,
Organometallics 1998, 17, 278; d) J. Belzner, Angew. Chem. 1997,
109, 1331 – 1334; Angew. Chem. Int. Ed. Engl. 1997, 36, 1277 –
1280.
[11] For a two-coordinate cation, see: M. Driess, S. Yao, M. Brym, C.
van Wüllen, Angew. Chem. 2006, 118, 6882 – 6885; Angew.
Chem. Int. Ed. 2006, 45, 6730 – 6733.
[12] a) T. Müller, Angew. Chem. 2001, 113, 3123 – 3126; b) R.
Panisch, M. Bolte, T. Müller, J. Am. Chem. Soc. 2006, 128,
9676 – 9682; c) a similar five-membered ring structure was
characterized by NMR: A. Sekiguchi, A. Muratami, N.
Fukaya, Y. Kabe, Chem. Lett. 2004, 33, 530 – 531.
[13] X-ray structures of some related H-bridged silylium ions have
recently become available: a) P. D. Lickiss, P. C. Masangane, W.
Sohal, G. L. Veneziani, in Organosilicon Chemistry, Vol. V (Eds.:
N. Auner, J. Weis), Wiley-VCH, Weinheim, 2003, p. 45; b) S. P.
Hoffmann, T. Kato, F. S. Tham, C. A. Reed, Chem. Commun.
2006, 767 – 769; c) for a F-bridged cation, see reference [12b].
[14] B. Wrackmeyer, O. L. Tok, Y. N. Bubnov, Angew. Chem. 1999,
111, 214 – 217; Angew. Chem. Int. Ed. 1999, 38, 124 – 126.
[15] NMR spectra for the silylium ions reported herein are given in
the Supporting Information.
[19] All geometry optimizations were carried out with the Gaus-
sian03 program package by applying the PBE–PBE exchange
and correlation functionals. The spin–spin coupling constants
were calculated within the gauge-including atomic orbitals
(GIAO) method using the B3LYP hybrid functional. See the
Supporting Information for more details and references.
[20] a) I. Mayer, Chem. Phys. Lett. 1983, 97, 270; b) K. B. Wiberg,
Tetrahedron 1968, 24, 1083.
[21] E. D. Glendening, A. E. Reed, J. E. Carpenter, F. Weinhold,
NBO Version 3.1.
[22] a) I. Mayer, Chem. Phys. Lett. 2003, 382, 265; b) DFT-based
formalism: S. F. Vyboishchikov, P. Salvador, M. Duran, J. Chem.
Phys. 2005, 122, 244110; c) correlated formalism: S. F. Vyboish-
chikov, P. Salvador, Chem. Phys. Lett. 2006, 430, 204 – 209.
[23] a) R. F. Bader, W. Atoms in Molecules: A Quantum Theory,
Clarendon, New York, 1990; b) D. Cremer, E. Kraka, Angew.
Chem. 1984, 96, 612; Angew. Chem. Int. Ed. Engl. 1984, 23, 627;
c) D. Cremer, E. Kraka, Croat. Chem. Acta 1984, 57, 1259.
[24] K. Izod, W. McFarlane, B. V. Tyson, I. Carr, W. Clegg, R. W.
Harrington, Organometallics 2006, 25, 1135 – 1143.
[25] The calculated chemical shifts for Sia, Sib, and Sig are in
reasonable agreement with experimental values (in parenthe-
ses): d = 24.3 (34.3), 44.5 (33.6), ꢀ2.2 ppm (ꢀ4.5 ppm), respec-
tively.
[26] M. J. Bearpark, G. S. McGrady, P. D. Prince, J. W. Steed, J. Am.
Chem. Soc. 2001, 123, 7736.
ꢀ
[27] Complexation of a M H bond to a cis-positioned silyl ligand is
known in the form of interligand hypervalent interactions
MH···SiR2X (reference [16]).
[16] G. I. Nikonov, Adv. Organomet. Chem. 2005, 53, 217 – 309.
Angew. Chem. Int. Ed. 2007, 46, 4530 –4533
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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