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HOMOꢀ1.[20] The HOMOꢀ1 of 1opt is a lone pair orbital on
bent and twist angles[24] of 57.28 (Si1), 46.98 (Si6), and 64.58
(Figure 4). While a Wiberg bond index (WBI) analysis of the
the nitrogen atom [n(N)] with a minor contribution from the
3p(Si) orbital. The HOMO–LUMO gap in 1opt (4.40 eV) is
slightly larger than that in A (3.76 eV), but much smaller than
that of G (5.26 eV). TD-DFT calculations on 1opt at the
B3LYP/6-31 + G(d) level of theory were in good agreement
with the experimental absorption spectra (for further details,
see Figure S59). A comparison between the experimental and
theoretical spectra revealed that the longest absorption band
in 1 should be assigned to the n(Si)!3p(Si) transition, and
that the considerable hypsochromic shift of the absorption
band in 1 relative to A should be interpreted predominantly
in terms of an increased n(Si)–3p(Si) gap resulting from the
raised 3p(Si) orbital level on account of the n(N)!3p(Si)
donation. The shielded dSi of 1 compared to that of A should
also be explained by the increased n(Si)–3p(Si) gap, given that
dSi is approximately proportional to lmax [n(Si)!3p(Si)].[21]
According to a second-order perturbation theory analysis, the
stabilization energy of the n(N)!3p(Si) donation was esti-
mated to be + 571.7 kJmolꢀ1. The natural resonance theory
(NRT)[22] analysis of 1opt (Figure 3b) revealed a contribution
of zwitterionic resonance structure 1’’, resulting from the
n(N)!3p(Si) donation (14%), in addition to the major
contribution from silylene structure 1’ (84%).[23]
ꢀ
central Si Si bond afforded a value of 1.07, the NRT analysis
of 5opt showed that the major resonance structure contains
=
a Si Si bond (for further details, see Figure S63), thus
indicating that the central Si Si bond has a double-bond
ꢀ
character. The predicted absorption bands of 5opt, calculated
at the B3LYP/6-31 + G(d)//B3PW91-D3/6-31G(d) level of
theory, were in good agreement with the new bands observed
at low temperature (for details, see Figure S60). A similar
equilibrium between a monomeric silylene and its dimer in
solution was observed for C.[3b,25] The color of 1 did not
change in the crystalline state as a result of the head-to-tail
packing. The observed dimerization of 1 in solution suggests
a slight electrophilic nature of the silylene center in 1.
Similar to other isolable and persistent silylenes, 1 shows
high reactivity. Reactions of 1 with diphenylacetylene and
phenanthrenequinone furnished the corresponding [1+2]
cycloadduct 6 (63%) and [1+4] cycloadduct 7 (82%),
respectively (Figure 5). Although the reaction of 1 with
Silylene 1 shows interesting thermochromism in solution
at low temperatures. The color of a solution of 1 in
3-methylpentane is colorless at room temperature, while it
turns orange at 77 K. In the UV/Vis absorption spectrum,
a reversible spectral change was observed (for details, see
Figure S60). With decreasing temperature (< 150 K), intense
absorption bands with isosbestic points gradually emerge at
l = 485 and 315 nm, and indicates that a new species is in
equilibrium with 1 at low temperatures. Although we failed to
determine the thermodynamic parameters of the equilibrium
because of the considerable difference in the extinction
coefficients of 1 and the new species, it was tentatively
assigned to disilene 5, that is, the dimer of 1. At the B3PW91-
D3/6-31G(d) level of theory, 5 was located as a local minimum
(5opt; Figure 4). The distance between two two-coordinate
Figure 5. Products from the reaction of 1 with diphenylacetylene,
phenanthrenequinone, triethylsilane, and benzene (R=SiMe3).
triethylsilane did not proceed at room temperature, heating to
ꢀ
1008C afforded Si H insertion product 8 (73%). This
reaction clearly demonstrates that the reactivity of 1 is
between that of A and G: while A reacts with triethylsilane at
room temperature,[26] G does not react with triethylsilane,
even at 1108C.[9a] While no reaction was observed between
1 and benzene in the dark, silepin 9 (82%) was generated
upon irradiation at l = (350 ꢁ 10) nm, which corresponds to
the energy of the n!3p transition. We have previously
reported that the photochemical reaction of A with benzene,
which affords the corresponding silepin, proceeds by a singlet
excited state with 1,1-bissilyl radical nature.[6] The formation
of 9 from 1 thus suggests a 1,1-bissilyl radical nature for the
singlet excited state of 1 similar to that of A.
=
silicon atoms (2.487 ꢁ) in 5opt is far longer than typical Si Si
ꢀ
bonds (2.16–2.25 ꢁ) and still longer than typical Si Si bonds
(2.36 ꢁ). The geometry around the central Si atoms is
considerably distorted, and is reflected in estimated trans-
The high thermal stability of 1 also revealed interesting
reactivity that was not observed for A. In a sealed tube in
ꢀ
refluxing toluene, a benzylic C H insertion reaction between
1
and the solvent furnished hydridobenzylsilane 10
(Scheme 2). Although various examples of intramolecular
ꢀ
benzylic C H insertion reactions of silylenes have been
reported,[27,28] examples of intermolecular thermal benzylic
ꢀ
C H insertion reactions of silylenes still remain rare: only few
examples of transient acyclic diaminosilylenes, generated by
the reductive dehalogenation of the corresponding diamino-
dichlorosilanes, have been reported.[28] Although a reaction
between 1 and dihydrogen gas was not observed, even at
1508C,[29] 1 participated in the dehydrogenation of dihydro-
genated aromatic compounds. For example, the reaction
between 1 and 1,4-cyclohexadiene afforded dihydrosilane 11
Figure 4. Optimized structure of dimer 5 (5opt). Left: side view. Right:
viewed along the Si–Si axis. Hydrogen atoms omitted for clarity.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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