Journal of the American Chemical Society
Communication
Figure 2. Frontier molecular orbitals (HOMO−1, HOMO, and LUMO, calculated at the DFT B3LYP level with the 6-31G(d) basis set [3-21G(d)
for Sb] for the model compounds Mes*ESi(SiMe3)2 (Mes* = 2,4,6-tBu3-C6H2) 1′ (E = P), 2′ (E = As) and 3′ (E = Sb): phosphasilene 1′ (left),
arsasilene 2′ (center), and stibasilene 3′ (right).
hypothesis well agrees with the experimentally observed
undistorted geometry (see above) around the ESi bonds:
planar skeleton with no signs of either trans-bending at Si or
twisting around the ESi bond.
AUTHOR INFORMATION
Corresponding Author
■
Notes
Inspection of the frontier molecular orbitals (FMO) revealed
that all model compounds, phosphasilene 1′, arsasilene 2′, and
stibasilene 3′, uniformly exhibited identical FMO patterns:
HOMO−1 is mostly represented by the pnictogen lone pair n-
orbital (with the admixture of the conjugated aryl group π-
orbital), whereas HOMO and LUMO are almost pure π- and
π*-orbitals of the ESi bond (Figure 2).13 Diagnostically, on
going from phosphasilene to arsasilene to stibasilene the
HOMO energy levels remarkably increase from −5.42 eV (in
1′) to −5.22 eV (in 2′) to −4.98 eV (in 3′), whereas the
HOMO−1 energy levels progressively decreased from −5.44
eV (in 1′) to −5.50 eV (in 2′) to −5.52 eV (in 3′). Such trend
can be well rationalized given the greater s-character of the lone
pair MO (62% for 1′, 69% for 2′ and 75% for 3′) and notably
weaker π-bonds descending group 15, resulting in the lowering
of nonbonding n-orbital and a substantial rise of the bonding π-
orbital energy levels on going from 1′ to 2′ to 3′. Again, this is a
clear manifestation of the general tendency of the reluctance of
the heavier main group elements to hybridize and form
multiple bonds.10,15 Given the progressive lowering of the
LUMO energy levels from −1.80 eV (in 1′) to −1.85 eV (in
2′) to −1.93 eV (in 3′), one can notice quite remarkable, albeit
expectable, drop in the HOMO−LUMO energy gap: from 3.62
eV (in 1′) to 3.37 eV (in 2′) to 3.05 eV (in 3′).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Grant-in-Aid for
Scientific Research program (Nos. 23655027, 24245007,
24550038, and 90143164) from the Ministry of Education,
Science, Sports, and Culture of Japan. We thank Prof. Jean
Escudie and Dr. Henri Ranaivonjatovo (Universite Paul
́ ́
Sabatier, Toulouse, France) for providing a sample of
Mes*AsF2 and for the helpful discussions.
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* Supporting Information
Experimental Section (including synthetic procedures, spectral
and crystallographic data for 2 and 3), computational data for
the model compounds 1′, 2′, and 3′, and crystal data (CIF).
This material is available free of charge via the Internet at
K.; Szilvas
́
i, T.; Blom, B.; Inoue, S.; Epping, J.; Driess, M. J. Am. Chem.
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dx.doi.org/10.1021/ja5026084 | J. Am. Chem. Soc. 2014, 136, 6243−6246