Tri-tert-butylsilylsilylenes with alkali metal substituents ( tBu3Si)SiM (M = Li, K): Electronically and sterically accessible triplet ground states

Article abstract of DOI:10.1021/ja077628v

Lithium-, sodium-, and potassium-substituted silirenes 2a-c have been prepared by the reaction of bis(tri-tert-butylsilyl)silirene 1 with the corresponding alkali metals and characterized by NMR spectroscopy and chemical trapping (2a and 2b). Lithio- and potassiosilylenes, Li(^tBu₃Si)₂Si: (4a) and K(^tBu₃Si)₂Si: (4b), were generated by low-temperature irradiation of 2a and 2b, respectively, and their EPR spectra revealed the characteristic broad EPR signals at 790 mT for 4a and 780 mT for 4b. The Curie law dependence observed for 4a was in accord with a triplet ground state, also assigned to 4b. Computations on model lithiosilylenes support these assignments. Copyright

Full text of DOI:10.1021/ja077628v

Published on Web 12/18/2007
Tri-tert-butylsilylsilylenes with Alkali Metal Substituents (tBu3Si)SiM (M ) Li,
K): Electronically and Sterically Accessible Triplet Ground States
Akira Sekiguchi,*,† Takashi Tanaka,† Masaaki Ichinohe,† Kimio Akiyama,*,§ and Peter P. Gaspar*,‡
Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki
305-8571, Japan, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Sendai,
Miyagi 980-8577, Japan, and Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4899
Received October 3, 2007; E-mail: sekiguch@chem.tsukuba.ac.jp; akiyama@tagen.tohoku.ac.jp; gaspar@wustl.edu
The preparation of a silylene with a triplet ground state and the
exploration of its chemistry has been described as “one of the most
important challenges in contemporary silicon chemistry.”1,2
A
strategy employing the steric bulk and low electronegativity of
trialkylsilyl substituents to reduce the one-electron promotion energy
from a σ2 to a σp configuration by increasing the p-character of
the nonbonding σ orbital3 ultimately led to success.4 The EPR
spectrum of (tBu3Si)2Si: provided unambiguous evidence for its
triplet ground state.4 Unfortunately, steric shielding of the dicoor-
dinate silicon of (tBu3Si)2Si: and of (iPr3Si)(tBu3Si)Si:, to which
an intramolecular reaction from a triplet state had been assigned
on the basis of product studies,5 suggests that exploration of the
chemistry of triplet silylenes requires molecules with sterically
accessible triplet ground states.
Figure 1. The X-band EPR spectrum of 4a generated by photolysis of 2a
in a 2-Me-THF glassy matrix at 14 K.
Scheme 1
Theoretical calculations have long predicted that extremely
electropositive substituents such as Li will lead to triplet ground
state silylenes with no need for the expansion of the bond angle at
the divalent silicon due to two bulky substituents.6 Here, the
synthesis of precursors (tBu3Si)SiC2(Et)2M (M ) Li, Na, K) is
described (Scheme 1) and the generation of triplet silylenes from
two of them (M ) Li, K) reported.
Scheme 2
Scheme 3
Formation of the lithium (2a)7 and potassium (2b) derivatives
by the reduction of 1,1-bis(tri-tert-butylsilyl)-2,3-diethylsilirene (1)
was verified by trapping with triisopropylsilyltriflate (Scheme 2).
3 was isolated in yields of 36% from 2a and 30% from 2b.5,8
Lithium- and potassium-substituted silylenes 4a and 4b, respec-
tively, were generated by photoextrusion, employing a low-pressure
mercury lamp, from metal-substituted silirenes 2a and 2b (Scheme
3).8 When a THF solution of 2a in a quartz NMR tube was irradiated
at λ ) 254 nm for 6 h at room temperature, 3-hexyne was detected
by NMR spectroscopy. The generation of silylenes was confirmed
by EPR experiments in the temperature range from 14 to 50 K.
Upon irradiation with 254 nm light (a low-pressure mercury lamp)
in a frozen 2-methyl-THF glassy matrix of 2a in a quartz EPR
tube for 1 h at 14 K, the characteristic X-band EPR spectrum of
randomly oriented triplet species, which has large zero-field splitting
(ZFS) parameters, was observed as a broad signal at around 790
mT (Figure 1) and assigned to silylene 4a.
In addition to the 790 mT signal of 4a, a strong one due to a
doublet state species (g ) 2.0054) also appeared at around 340
mT.9 The EPR signals persisted at 14 K for several hours after
light irradiation.
The zero-field splitting parameters were estimated from the
spectrum of 4a as previously described4 under the assumptions that
there are no unobserved weak lines below 1400 mT, and that the
free electron g value and E equal zero. The D value can then be
evaluated as 1.507 cm-1 from the resonance field of the perpen-
dicular (X,Y) orientation.4,10
To confirm that the observed EPR signal was associated with
the ground electronic state of silylene 4a, the temperature depen-
dence of the 790 mT signal intensity was examined as shown in
Figure 2. The changes were reversible between 15 and 50 K and
corresponded to strict adherence to the Curie law, IT ) constant,
where I is the EPR signal intensity and T is absolute temperature.
Thus, the ground state of 4a is a triplet, and the singlet state is not
populated to an appreciable extent at the maximum temperature
examined. An alternative explanation that the ground state is a
singlet with such a small S-T energy gap that thermal equilibrium
with the triplet state is maintained throughout the experiment may
be possible but is considered unlikely on the basis of the results
from DFT calculations that follow.
Potassium-substituted silylene 4b was generated similarly. Upon
irradiation of a frozen glass containing 2b in 2-methyl-THF at 254
nm for 1 h at 15 K, the X-band EPR spectrum of 4b was observed
as a broad signal at 780 mT (Figure 3). A D value of 1.510 cm-1
was estimated as outlined above.
† University of Tsukuba.
§ Tohoku University.
‡ Washington University.
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10.1021/ja077628v CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Si-Si-Li Angles (°) and Relative Energies (kcal/mol) of
S and T (Me3Si)SiLi(OMe2)n (n ) 0 - 3) Calculated at
(U)B3LYP/6-31G(d)
n
)
0
1
2
3
singlet S 66.6, 0
88.6, -21.3
99.8, -35.2
109.8, -44.4
triplet T
155.8, -8.0 165.8, -32.6 169.6, -49.5 170.8, -60.8
∆ES-T
8.0
11.3
14.3
16.4
respectively, and the small bond angle of S is due to agostic
interactions between Li and two H atoms leading to mean Li-H
distances of 2.02 Å.
Because silylenes 4a and 4b were generated in ether glasses, it
was suspected that coordination of the ether solvent to the metal
ion would strongly influence their structures and energetics. To
study this effect, calculations were carried out on a model system,
(Me3Si)SiLi(OMe2)n (n ) 0-3). Results are presented in Table 1.
The increase in ∆ES-T with increasing degree of solvation can
be explained by an increase in the effective electropositivity of the
lithium. The agostic interactions in S and T disappear with solvation
(see the Supporting Information). Coordination of an ether to the
divalent silicon atom selectively stabilizes S,12 but T remains the
predicted ground state.13 It is clear that coordination of ether solvent
to both the metal and the silylene center strongly influences the
geometry, ground state multiplicity, and singlet-triplet splitting of
4a and 4b.
Figure 2. Temperature dependence of the intensity of the EPR signal at
790 mT attributed to silylene 4a.
Acknowledgment. P.P.G. thanks the National Science Founda-
tion for support under Grant CHE-0316124. A.S. is grateful for
Grants-in-Aid for Scientific Research (Nos. 19105001, 19020012,
19022004, 19029006) from the Ministry of Education, Science,
Sports, and Culture of Japan.
Supporting Information Available: Experimental procedures,
spectroscopic data, computational results. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
Figure 3. The X-band EPR spectrum of 4b generated by photolysis of 2b
in a 2-Me-THF glassy matrix at 15 K.
(1) For reviews, see: (a) Apeloig, Y. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons Ltd.: New
York, 1989; Part 1, Chapter 2. (b) Gaspar, P. P.; West, R. In The Chemistry
of Organic Silicon Compounds II; Rappoport, Z., Apeloig, Y., Eds.; John
Wiley & Sons Ltd.: New York, 1998; Vol. 2, Part 3, Chapter 43. (c)
Gaspar, P. P.; Xiao, M.; Pae, D. H.; Berger, D. J.; Haile, T.; Chen, T.;
Lei, D.; Winchester, W. R.; Jiang, P. J. Organomet. Chem. 2002, 646,
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(3) Grev, R. S.; Schaefer, H. F., III; Gaspar, P. P. J. Am. Chem. Soc. 1991,
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(4) Sekiguchi, A.; Tanaka, T.; Ichinohe, M.; Akiyama, K.; Tero-Kubota, S.
Figure 4. B3LYP/6-31G* optimized structures for Li(tBu3Si)Si: 4a (left,
singlet; right, triplet).
J. Am. Chem. Soc. 2003, 125, 4962.
(5) Jiang, P.; Gaspar. P. P. J. Am. Chem. Soc. 2001, 123, 8622. This
interpretation is under question on the basis of quantum mechanical
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DFT (U)B3LYP/6-31G(d) calculations were carried out to
examine the nature of lithiosilylene 4a and to predict its singlet-
triplet splitting, ∆ES-T. As a benchmark, ES and ET versus Si-
Si-Si or Si-Si-Li were computed for (Me3Si)2Si: and Li(Me3-
Si)Si:, respectively.11 The potential curves for (Me3Si)2Si: crossed
at ca. 120° with minima for the singlet (S) and triplet (T) states at
ca. 100 and 130°, respectively, with S being favored by ca. 2 kcal/
mol. Previously, 97.3 and 123.4° were reported from a calculation
in which the singlet was treated at the TCSCF and the triplet at the
RHF level with a DZ(d) basis set.3 For Li(Me3Si)Si:, no crossover
angle was found, and the triplet was found to be the ground state
at all angles.11
(6) (a) Harrison, J. F.; Liedtke, R. C.; Liebman, J. F. J. Am. Chem. Soc. 1979,
101, 7162. (b) Colvin, M. E.; Breulet, J.; Schaefer, H. F., III. Tetrahedron
1985, 41, 1429. (c) Jespersen, K. K. J. Am. Chem. Soc. 1985, 107, 537.
(7) The silacyclopropenyllithium 2a was independently prepared by the
reaction of 1-bromo-2,3-diethyl-1-(tri-tert-butylsilyl)-1-silacycloprop-2-
ene with lithium in THF. Tanaka, T.; Ichinohe, M.; Sekiguchi, A. Chem.
Lett. 2004, 33, 1420.
(8) For the experimental procedure and spectral data, see Supporting
Information.
(9) The EPR signal observed at 340 mT corresponds to a typical silyl radical,
but its structure is unclear at this moment.
(10) Li, X.; Weissman, S. I.; Lin, T.-S.; Gaspar, P. P.; Cowley, A. H.; Smirnov,
A. I. J. Am. Chem. Soc. 1994, 116, 7899.
(11) For the computational results, see Supporting Information.
(12) For the experimental studies on the interaction of singlet silylenes with
Lewis bases, see: (a) Gillette, G. R.; Noren, G. H.; West, R. Organo-
metallics 1987, 6, 2617. (b) Ando, W.; Sekiguchi, A.; Hagiwara, K.;
Sakakibara, A.; Yoshida, H. Organometallics 1988, 7, 558.
(13) The ∆ES-T values for [Me2O)nLi](Me3Si)Si: r OMe2 (n ) 0-3) are
calculated to be -3.2, -1.0, 6.4, and 8.9 kcal/mol, respectively.
The ground state of Li(tBu3Si)Si: was found to be a triplet with
∆ES-T ) 9.4 kcal/mol. The structures of S and T Li(tBu3Si)Si:,
optimized at the (U)B3LYP/6-31G* level, are shown in Figure 4.
The Si-Si-Li angles calculated for S and T are 81.3 and 164.9°,
JA077628V
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