Weekly Report (20150831-20150906)
Ke An
For the past week, I mainly focused on the calculations of the stability of 1,3,2-diazaborole and
1,4,2-diazaborole. Two different substituents (F and SiMe3) are employed to adjust the relative
stability of the two isomers. We firstly examined the stability of monofluoronated 1,3,2-diazaboroles
and 1,4,2-diazaboroles.
Figure 1. Relative electronic energies (in kcal/mol) and NICS(1)zz values (in parentheses) of (a)
1,3,2-diazaborole and 1,4,2-diazaborole; (b) monofluoronated 1,3,2-diazaboroles; (c)
monofluoronated 1,4,2-diazaboroles. (d) Mulliken and NBO charges of 1,3,2-diazaborole and 1,4,2diazaborole.
We can see in Figure 1a that 1,3,2-diazaborole is -30.4 lower in energy than that of 1,4,2-diazaborole.
(Yet the NICS(1)zz of 1,3,2-diazaborole is smaller than that of 1,4,2-diazaborole, because of
separated charge? Need to be discussed.) When introducing the fluorine at different positions, a
significant energy difference occurs in 1,3,2-diazaborole, as well as 1,4,2-diazaborole, which can
be as large as -93.5 kcal/mol (Figure 1b). NICS(1)zz of the three monofluoronated 1,3,2-diazaboroles
are -20.6, -16.8 and -18.7, respectively. It indicates that 1-fluoro-1,3,2-diazaborole is the most
aromatic and the 2-fluoro-1,3,2-diazaborole is the least aromatic. However, the stability of three
disagree with the aromaticity. The most aromatic 1-fluoro-1,3,2-diazaborle is the least stable. After
checking the Mulliken and NBO charges (Figure 1d) of 1,3,2-diazaborole we can see that the boron
atom has the most positive charge while the nitrogen atoms have the most negative ones. It is
understandable that fluorine is an electron-withdrawing substituent and prefers a positive-charged
position, which demonstrate that 2-fluoro-1,3,2-diazaborole could be the most stable. As regard for
the monofluoronated 1,4,2-diazaborole, the NICS(1)zz values of the five isomers are -24.8, -23.1, 25.3, -24.1 and -22.3, respectively. According to the Mulliken and NBO charges of 1,4,2diazaborole, the boron atom has the most positive charge, resulting in the lowest energy among the
five isomers. Though C3 has a negative charge, the NICS(1)zz is the most negative, thus the second
lowest energy (-46.9 kcal/mol) is a combined resulted of aromaticity and atom charge. In addition,
the energy difference between the most stable 2-fluoro-1,3,2-diazaboroles and the most stable 2fluoro-1,4,2-diazaborole is 31.8 kcal/mol.
Figure 2. Relative electronic energies and NICS(1)zz values of pentafluoronated 1,3,2-diazaborole
and 1,4,2-diazaborole.
The 1,2,3,4,5-fluoro-1,3,2-diazaborole is 34.8 kcal/mol lower in energy than that of 1,2,3,4,5fluoro-1,4,2-diazaborole (Figure 2). And the NICS(1)zz value still keeps less negative. The
aromaticity of both two compounds decrease, which can be due to the dihedral angle (NBNC = 16.0 and NBCN = 6.9). Meanwhile, the larger dihedral angle of 1,2,3,4,5-fluoro-1,3,2diazaborole indicates not only the less aromaticity, but also a lower energy.
Moreover, the energy differences between 1,3,2-diazaborole and 1,4,2-diazaborole for the most
stable bi-, tri-, and tetrafluoronated compounds are 33.7, 34.5 and 34.7 kcal/mol, respectively.
Figure 3 gives out all the relative electronic energies for the bi, tri, tetrafluoronated 1,3,2diazaboroles and 1,4,2-diazaboroles. Generally, the various energy differences can be attributed to
the atom charge and the dihedral angle. For example, the 1,3-fluoro-1,3,2-diazaborole is 92.5
kcal/mol higher in energy that that of 1,2-fluoro-1,3,2-diazaborole because of the negative charges
on the nitrogen atoms. While the -98.7 kcal/mol for 2,4,5-fluoro-1,3,2-diazaborole is a result of the
negative charge on boron and near neutral carbon atoms.
Figure 3. Relative electronic energies of (a) bi-, tri-, tetrafluoronated 1,3,2-diazaboroles and (b) bi-,
tri-, tetrafluoronated 1,4,2-diazaboroles.
We also employed the electron-donating group SiMe3 as the substituent to examine the stability of
the two isomers. As shown in Figure 4a, the 1-trimethylsilyl-1,3,2-diazaborle is the most stable
monotrimethylsilyl-1,3,2-diazaborole, which could be understandable because SiMe3 prefers the
negative-charged atoms. Similarly, the 1-trimethylsilyl-1,4,2-diazaborole is also the most stable one.
In addition, the energy difference decreases to 26.6 kcal/mol when the two diazaboroles are
substituted by five trimethylsilyl groups (why?). In addition, the energy difference between the most
stable 1-trimethylsilyl-1,3,2-diazaboroles and the most stable 1-trimethylsilyl-1,4,2-diazaborole is
30.0 kcal/mol.
Figure 4. Relative electronic energies (in kcal/mol) and NICS(1)zz values (in parentheses) of (a)
monotrimethylsilyl-1,3,2-diazaboroles;
(b)
monotrimethylsilyl-1,4,2-diazaboroles.
(c)
pentatrimethylsilyl-1,3,2-diazaborole and 1,4,2-diazaborole.
Moreover, the energy differences between 1,3,2-diazaborole and 1,4,2-diazaborole for the most
stable bi-, tri-, and tetratrimethylsilyl compounds are 30.1, 33.0 and 26.2 kcal/mol, respectively.
Figure 5 gives out all the relative electronic energies for the bi, tri, tetratrimethylsilyl 1,3,2diazaboroles and 1,4,2-diazaboroles. Generally, the various energy differences can be also mainly
attributed to the atom charge. For example, the 1,3-trimethylsilyl-1,3,2-diazaborole is the most
stable structure (-19.4 kcal/mol) because the two nitrogen atoms are the most negative charged. The
most stable 1,4-trimethylsilyl-1,4,2-diazaborole among the bitrimethylsilyl-1,4,2-diazaborole is
also a result of the negative charges on those atoms.
Figure 5. Relative electronic energies of (a) bi-, tri-, tetratrimethylsilyl-1,3,2-diazaboroles and (b)
bi-, tri-, tetratrimethylsilyl-1,4,2-diazaboroles.
In addition, one student in Mr. Xia’s group asked me to help calculate the aromaticity of the
compounds 3-6 and 3-7 in Figure 6.
Figure 6. The syntheses of osmapentalene derivatives incorporating with a pyridine ring.
Figure 7. NICS(0)zz, NICS(0), NICS(1)zz and NICS(1) values of each ring in complexes 3-6 and
3-7.
I have only calculated the NICS values of the two complexes. As shown in Figure 7 and according
to the NICS(1)zz values, the aromaticity of rings a and rings b in both complexes are not very
significant. While the aromatictity in rings c are large and ring d is antiaromatic.