New air stable diarylamino substituted diborane(4) derivatives: Synthesis and structures

Article abstract of DOI:10.1080/15421406.2018.1549228

In recent years, the rekindled interest in B/N ring systems has led to a plethora of novel compounds with varying substitution patterns. Diaminodiboranes are the starting compounds for the preparation of many boron compounds such as diborane derivatives,

Full text of DOI:10.1080/15421406.2018.1549228

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
New air stable diarylamino substituted
diborane(4) derivatives: Synthesis and structures
Duygu Barut Celepci, Ezgi Sepin, Yusuf Gökçen, Muhittin Aygün & Yüksel
Şahin
To cite this article: Duygu Barut Celepci, Ezgi Sepin, Yusuf Gökçen, Muhittin Aygün
& Yüksel Şahin (2018) New air stable diarylamino substituted diborane(4) derivatives:
Synthesis and structures, Molecular Crystals and Liquid Crystals, 668:1, 91-104, DOI:
10.1080/15421406.2018.1549228
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 668, NO. 1, 91–104
https://doi.org/10.1080/15421406.2018.1549228
New air stable diarylamino substituted diborane(4)
derivatives: Synthesis and structures
a
b
b
a
€
€
Duygu Barut Celepci , Ezgi Sepin , Yusuf Gokc¸en , Muhittin Aygun , and
b
€
Yuksel S¸ahin
a
b
_
€
Department of Physics, Faculty of Science, Dokuz Eylul University, 35160 Izmir, Turkey; Department of
Chemistry, Faculty of Arts and Sciences, Adnan Menderes University, 09010 Aydın, Turkey
KEYWORDS
ABSTRACT
Diborane(4); diaminodi-
boran(4); air-stable; crystal
structure; NMR
In recent years, the rekindled interest in B/N ring systems has led to
a plethora of novel compounds with varying substitution patterns.
Diaminodiboranes are the starting compounds for the preparation of
many boron compounds such as diborane derivatives, used for
many addition and insertion reactions into multiple bonds in the lit-
erature, however there are few air-stable diborane(4) derivatives. In
this study, to synthesize new air-stable B,B’-diaryl and bis(dimethyla-
mino)-B,B’-diarylaminodiboranes(4), the bulky amines were selected.
These derivatives were prepared through the reaction of lithium ani-
lide derivatives and 1,2-dichlorodiboranes. The structures of the com-
pounds were characterized by using nuclear magnetic resonance
(NMR) spectroscopy. The crystal and molecular structures of 3ae,
3af, 3bb, 3be, 3ce, 3cd and 3cf were determined using the single-
crystal X-ray diffraction technique. X-ray diffraction studies have
showed that the diarylamino group in the solid state is nearly
orthogonal to the plane due to the boron-bound bulky groups.
1. Introduction
Diarylamino substituted diboranes are an important class of compounds in boron
chemistry for use in the synthesis of other boron-containing compounds. Heterocycles
consisting of boron, nitrogen and carbon ring systems have led to a plethora of novel
compounds with varying substitution patterns, thus leading the way to potential appli-
cations. Especially, diarylamino substituted diboranes are the starting compounds for
the synthesis of boron heterocycles, such as diazatriborolidines, diazadiborolidines and
diazadiborinane derivatives [1–4]. Many researchers have focused on aminodiborane(4)
compounds and the reported studies have emphasized that the stability of compounds
can be increased with electron releasing and bulky groups [5–7]. Also, the previous
studies have reported that aliphatic amines-substituted diaminodiboranes are less stable
[8]. We believe that the ability of the aryl group to bind to the N-atom forms a steric
hindrance and reduces the efficiency of the nucleophilic N atoms, thus, allowing the sta-
bility of molecules. In this study, we present the synthesis, characterization and struc-
tural analysis of a variety of diaryl- and a dialkyl amino-substituted diborane(4)
€
CONTACT Yuksel S¸ahin
ysahin@adu.edu.tr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2018 Taylor & Francis Group, LLC

92
D. B. CELEPCI ET AL.
derivatives. All prepared compounds were found to be stable in air. Furthermore, the
study was conducted to exhibit further information for air, thermal stabilities and con-
formations of such species.
2. Experimental Section
2.1. Syntheses
General considerations: All reactions were carried out under argon, using standard
schlenk techniques. Solvents were dried, distilled, and saturated with argon. Glassware
was dried using a heat gun under high vacuum. NMR spectra were recorded on a
Varian 400 spectrometer. The chemical shifts are given in ppm, and are referenced
against external Me4Si, BF3ꢀOEt2. 1,2-Bis(dimethylamino)diborane(4) dichloride [9], 1,2-
bismesityldiborane(4) dichloride [10] and 1,2-bisdiduryldiborane(4) dichloride [11] and
were synthesized according to literatures.
2.2. General procedure for synthesis of arylamino substituted diborane(4)
derivatives
Arylamine derivative (1, 6 mmol) was dissolved in THF/hexane mixture (1:4, 50 mL)
and n-BuLi (7.5 mL, 12 mmol, 1.6 M solution in hexane) was added dropwise at 0 ꢁC.
The solution was warmed to room temperature and stirred overnight. A hexane solution
of R1ClB-BClR1 (0.75 g, 6 mmol) was added dropwise, resulting in a suspension of 2 at
-20 ꢁC. The mixture was slowly warmed to room temperature and removed of volatile
components in vacuum (Scheme 1). The residue was extracted into hexane/CH2Cl2 mix-
ture (1:2, 50 mL). The concentrated solution was kept to -30 ꢁC and the crystals
were obtained.
2.2.1. 1,2-Di(tert-butyl)amino-1,2-bis(dimethylamino)diborane, 3ad
Yielding 88%, a colorless viscous liquid.
1H-NMR (400 MHz, CDCl3, 300 K): d ¼ 1.18 (s, 18H, Me, t-Bu), 2.47 (s, 6H, Me,
NMe), 2.56 (s, 2H, NH), 2.63 (s, 6H, Me, NMe); 13C-NMR (100 MHz, CDCl3, 300 K):
d ¼ 33.3 (6C, Me, t-Bu), 35.3 (2C, Me, NMe), 42.6 (2C, Me, NMe), 49.4 (2C, t-Bu); 11B-
NMR (128.32 MHz, CDCl3, 300 K): d ¼ 33 (2B).
Scheme 1. Synthesis of diborane(4) 3 derivatives.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
93
2.2.2. 1,2-Bis(2-methoxyphenyl)amino-1,2-bis(dimethylamino)diborane, 3af
Yielding 87%, a colorless crystal, m.p.: 138 ꢁC.
1H-NMR (400 MHz, CDCl3, 300 K): d ¼ 2.76, 2.82 (each s, each 6H, Me2N), 3.92 (s,
6H, MeO, Ph), 5.87 (br., s, 2H, NH), 6.76-6.79 (m, 4H, p-, and m-H, Ph), 6.83, 6.85
(m, 2H, o-H, Ph), 7.20-7.23 (m, 2H, m’-H, Ph); 13C-NMR (100 MHz, CDCl3, 300 K):
d ¼ 35.9 (2C, MeN), 41.7 (2C, MeN), 55.6 (2C, MeO-C, Ph), 109.9 (2C, m-C, Ph), 114.9
(2C, p-C, Ph), 118.3 (2C, m-C, Ph), 121.3 (2C, o-C, Ph), 136.4 (2C, o-C, Ph), 147.9 (2C,
i-C, Ph); 11B-NMR (128.32 MHz, CDCl3, 300 K): d ¼ 33 (2B).
2.2.3. 1,2-Dimesitylamino-1,2-dimesityldiborane, 3bb
Yielding 91%, a colorless crystal, m.p.: 169 ꢁC.
1H-NMR (400 MHz, CDCl3, 300 K): d ¼ 1.91 (s, 12H, o-Me, NMes), 2.16 (s, 6H, p-
Me, NMes), 2.17 (s, 12H, o-Me, BMes), 2.22 (s, 6H, p-Me, BMes), 6.26 (s, 2H, NH),
6.68 (s, 4H, m-H, MesN), 6.71 (s, 4H, m-H, BMes); 13C-NMR (100 MHz, CDCl3, 300
K): d ¼ 18.8 (4C, o-C, NMes), 20.0 (2C, p-C, NMes), 21.0 (2C, p-C, BMes), 22.2 (4C,
o-C, BMes), 127.6 (4C, p-C, NMes), 128.9 (2C, p-C, BMes), 131.2 (4C, m-C, NMes),
133.3 (4C, m-C, BMes), 136.2 (4C, o-C, BMes), 138.6 (4C, o-C, NMes), 139.1 (br.
2C, i-C, BMes), 139.3 (2C, i-C, NMes); 11B-NMR (128.32 MHz, CDCl3, 300 K):
d ¼ 48 (2B).
2.2.4. 1,2-Bis(2,6-diisopropylphenyl)amino-1,2-dimesityldiborane, 3be
Yielding 87%, a colorless crystal, m.p.: 179 ꢁC
3
1H-NMR (400 MHz, C6D6, 300 K): d ¼ 0.90 (d, 24H, JHH ¼ 8 MHz, i-Pr -Me, Ph),
2.13 (s, 6H, p-Me, Mes), 2.32 (s, 12H, o-Me, Mes), 3.33 (sept., 4H, iPr -CH), 6.72 (s,
4H, m-H, Mes), 6.84 (s, 2H, HN), 7.00 (m, 6H, m- and p-H, Ph); 13C-NMR (100 MHz,
C6D6, 300 K): d ¼ 21.1 (2C, p-Me, Mes), 22.2 (4C, o-Me, Mes), 23.2 (8C, i-Pr-C), 28.4
(4C, i-Pr-CH), 123.5 (6C, m- und p-C, Ph), 125.8 (4C, m-C, Mes), 136.7 (4C, o-C,
Mes), 138.5 (br. 2C, i-C, Mes), 139.2 (4C, o-C, Ph), 139.4 (2C, p-C, Mes), 142.2 (2C, i-
C, Ph); 11B-NMR (128.32 MHz, C6D6, 300 K): d ¼ 50 (2B).
2.2.5. 1,2-Di(tert-butyl)amino-1,2-diduryldiborane, 3cd
Yielding 90%, a colorless crystal, m.p.: >210 ꢁC.
1H-NMR (400 MHz, CDCl3, 300 K): d ¼ 0.92 (s, 18H, Me, t-Bu), 2.09 (s, 12H, m-
Me), 2.21 (s, 12H, o-Me), 4.68 (br. s, 2H, NH), 6.82 (s, 2H, p-Dur); 13C-NMR
(100 MHz, CDCl3, 300 K): d ¼ 19.7 (8C, o-, m-Me, Dur), 31.5 (6C, Me, t-Bu), 51.7 (2C,
t-Bu), 128.9 (2C, p-C, Dur), 130.8 (br., 2C, i-C, Dur), 132.4 (4C, m-C, Dur), 132.8(4C,
o-C, Dur); 11B-NMR (128.32 MHz, CDCl3, 300 K): d ¼ 46 (2B).
2.2.6. 1,2-Bis(2,6-diisopropylphenyl)amino-1,2-diduryldiborane, 3ce
Yielding 89%, a colorless crystal, m.p.: >210 ꢁC.

94
D. B. CELEPCI ET AL.
3
1H-NMR (400 MHz, CDCl3, 300 K): d ¼ 0.78 (d, 24H, JHH ¼ 4 MHz, i-Pr -Me, Ph),
2.06 (s, 12H, m-Me, Dur), 2.12 (s, 12H, o-Me, Dur), 3.08 (sept., 4H, iPr -CH), 6.52
3
(s, 2H, NH), 6.77 (s, 2H, p-H, Dur), 6.95 (d, 4H, JHH ¼ 8 MHz, m-H, Ph), 7.02 (t, 2H,
3JHH ¼ 8 Hz, p-H, Ph); 13C-NMR (100 MHz, CDCl3, 300 K): d ¼ 18.8 (4C, o-C, Dur),
19.6 (2C, p-C, Dur), 22.9 (8C, i-Pr -C), 27.8 (4C, i-Pr -CH), 122.9 (2C, p-C, Ph), 124.8
(2C, p-C, Dur), 130.3 (4C, m-C, Dur), 133.1 (4C, m-C, Ph), 134.0 (4C, o-C, Ph), 138.9
(4C, o-C, Dur), 141.7 (2C, i-C, Dur), 142.2 (2C, i-C, Ph); 11B-NMR (128.32 MHz,
CDCl3, 300 K): d ¼ 52 (2B).
2.2.7. 1,2-Bis(2-methoxyphenyl)amino-1,2-diduryldiborane, 3cf
Yielding 91%, a colorless crystal, m.p.: >210 ꢁC.
1H-NMR (400 MHz, CDCl3, 300 K): d ¼ 2.11 (s, 12H, m-Me, Dur), 2.27 (s, 12H,
3
o-Me, Dur), 3.68 (s, 6H, MeO, Ph), 6.57 (t, 2H, JHH ¼ 6 Hz, p-H, Ph), 6.75 (d, 2H,
3
3
3JHH ¼ 6 MHz, m-H, Ph), 6.81 (t, 2H, JHH ¼ 6 MHz, m’-H, Ph), 6.85 (d, 2H, JHH
¼
6 MHz, o-H, Ph), 6.95 (s, 2H, p-H, Dur), 7.50 (br. s, 2H, NH); 13C-NMR (100 MHz,
CDCl3, 300 K): d ¼ 18.7 (4C, o-C, Dur), 19.7 (4C, m-C, Dur), 55.8 (2C, MeO-C, Ph),
110.6 (2C, m-C, Ph), 118.9 (2C, p-C, Ph), 121.1 (2C, m’-C, Ph), 122.3 (2C, o-C, Ph),
129.9 (2C, p-C, Dur), 132.8 (4C, m-C, Dur), 133.0 (4C, o-C, Dur), 134.2 (2C, o’-C, Ph),
143.7 (br., 2C, i-C, Dur), 149.1 (2C, i-C, Ph); 11B-NMR (128.32 MHz, CDCl3, 300 K):
d ¼ 48 (2B).
2.3. X-ray Crystallography
The crystallographic data collections of the compounds were conducted for 3ae at
100 K, for 3af, 3bb, 3be and 3cf at 150 K and for the rest at 293 K, on a Rigaku
Oxford Xcalibur diffractometer with an Eos-CCD detector using graphite-monochro-
mated MoKa radiation (k ¼ 0.71073 Å). Data collections, cell refinements and data
reductions along with absorption correction were performed using CrysAlisPro soft-
ware package [12]. Crystal structures were solved with the SHELXT structure solu-
tion program using Intrinsic Phasing method [13] embedded in the Olex2 [14].
Refinement of coordinates and anisotropic thermal parameters of non-hydrogen
atoms were carried out by the full-matrix least-squares method in SHELXL [15]. All
non-hydrogen atoms were refined anisotropically. In 3ae, two methyl carbon atoms
are disordered with a 43:57 (C27a:C27b; C28a:C28b) population distribution in one
of the isopropyl groups. All H atoms of 3af, 3be, 3cd and 3ce; H atoms of the disor-
dered groups in 3ae; H atoms except those bound to the nitrogen atoms of 3bb and
hydrogen atoms of the methyl groups in 3cf were placed at idealized positions and
refined using the riding model. All other hydrogens were found in the difference
map and refined freely. The crystal structure of 3cd was determined as a non-mero-
hedral twin with the final ratio of the twin domains being 0.310(2):0.690(2). The
summary of crystallographic data, experimental details, and refinement results for
the compounds are given in Table 1.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
95
Table 1. Crystallographic data and structure refinement parameters for the compounds.
3ae
28H48B2N4
3bb
C36H46B2N2
3be
C42H58B2N2
3ce
C44H62B2N2
3cd
3af
3cf
Formula
C
C28H46B2N2 C18H28B2N4O2 C34H42B2N2O2
Formula weight
Temperature (K) 100.01(15)
Crystal system
Space group, Z
a, b, c (Å)
462.32
528.37
150.00(10)
Monoclinic
P21/n, 4
11.2786(10) 16.3381(10) 13.4122(10)
16.8065(14) 16.3381(10) 17.2939(11)
612.52
640.58
293(2)
Monoclinic
432.29
293(2)
Triclinic
354.06
150.04(10)
Monoclinic Orthorhombic
C2/c, 4
18.397(4)
7.3856(15)
13.999(3)
90
93.999(17)
90
532.31
150.01(10)
150.02(10)
Tetragonal
P41212, 4
Triclinic
P-1, 2
9.8556(7)
10.4511(7)
C2/c, 4
P-1, 1
P212121, 4
8.9381(10)
15.8334(19)
21.421(3)
90
90
90
3031.5(6)
1.166
7.964(2)
9.881(3)
10.278(2)
71.36(2)
78.47(2)
66.59(3)
700.7(3)
1.024
16.4926(12) 17.5921(15) 14.4331(12) 17.1595(10)
a, b, c (ꢁ)
81.397(6)
76.489(6)
62.820(6)
1467.56(19)
1.046
0.060
508
90
104.985(10)
90
3221.2(5)
1.089
0.061
1144
10376
90
90
90
90
92.312(6)
90
3976.9(5)
1.070
0.060
1592
Volume (Å3)
3852.7(5)
1.056
0.059
1520
7237
1897.5(7)
1.239
0.080
760
qcalc (mg mꢂ3
)
l (mmꢂ1
F (000)
)
0.058
238
4560
0.071
1144
11289
Reflections
7806
6567
3049
collected
Reflections
unique/Rint
Parameters
GOF on F2
R, wR2
5920/0.017
5653/0.048
3656/0.065
3770/0.027 4560/merged 1664/0.046
5689/0.056
494
1.025
R1 ¼ 0.047
381
1.008
R1 ¼ 0.064
219
1.002
R1 ¼ 0.066
225
1.047
117
0.938
121
1.000
412
0.990
R1 ¼ 0.061
R1 ¼ 0.053
R1 ¼ 0.077
R1 ¼ 0.063
[I > 2r(I)]
wR2 ¼ 0.109 wR2 ¼ 0.123 wR2 ¼ 0.130 wR2 ¼ 0.128 wR2 ¼ 0.219 wR2 ¼ 0.143 wR2 ¼ 0.081
3. Results and Discussion
The B,B0-diaryl and bis(dimethylamino)-B,B’-diarylaminodiboranes(4) derivatives 3 were
prepared from reaction of the lithium anilide salt derivative 2 with 1,2-bis(dimethylami-
no)diborane(4) dichloride and 1,2-diaryldiborane(4) dichloride in a mixture of THF/
hexane at -20 ꢁC. Then, the suspension was allowed to warm to room temperature
(Scheme 1).
l
The new diaminodiborane(4) compounds were identified by their H-, 11B- and 13C-
NMR spectra. The NMR spectra of 3 derivatives and their chemical shifts are highly
similar to each other. The 11B NMR spectra of 3bb, 3ce, 3be, 3cf and 3cd showed a
characteristic broad singlet for the 2B atoms between 46 and 49 ppm, except for 3ad,
this is consistent with a symmetrically substituted aminodiborane [8]. Probably the pres-
ence of boron-bound substituents, which have a large volume, showed a slow rotation.
3ad is liquid at room temperature and its 11B-signal appears sharp at 32 ppm and the
11B-signal of 3af is also 33 ppm. The 11B-signals of boron atoms in the compounds are
shifted upfield as expected for symmetrically substituted tetraaminodiborane com-
pounds, because the boron atoms are more shielded by the amino groups. Also, the dif-
ference of chemical shifts of boron atoms might be explained in that, the lone electrons
of the nitrogen atom contribute to the aromaticity of the aryl group when an aromatic
amine ligand is bound to the boron atoms. A similar situation occurs in the chemical H
shifts of arylamino (-NH of 3cf, 3be, 3ce, 3bb, 3af, 3cd) which are at 7.50, 6.84, 6.53,
6.26, 5.78 and 4.68 ppm respectively. In contrast, the chemical H shift of tert-
Butylamino of 3ad is at 2.56 ppm. Due to the fact that boron atoms are shielded by
four N atoms, NH proton of tetraaminodiborane(4) is shifted upfield. In 13C NMR
spectra of 3ad, 3af, and 3ae [16] a discrete signal is observed at room temperature for

96
D. B. CELEPCI ET AL.
Figure 1. View of the molecular structure of 3ae showing the atom numbering scheme.
Displacement ellipsoids are drawn at the 30% probability level. Selected bond distances (Å), bond
angles and torsion angles (ꢁ): B1–B2 1.722(2), B1–N1 1.435(2), B1–N3 1.415(2), B2–N2 1.428(2), B2–N4
1.417(2), N1–C5 1.431(2), N2–C17 1.431(2), N3–C1 1.457(2), N3–C2 1.453(2); N1–B1–B2 119.12(12),
N1–B1–N3 118.39(13), B1–B2–N2 113.02(12), B1–B2–N4 123.14(13), N3–B1–B2 122.24(12), B1–N1–C5
126.14(12), B1–N3–C2 124.50(14), B1–N3–C1 123.51(13), B2–N2–N4 123.77(13), B2–N4–C3 122.19(13),
B2–N4–C4 128.15(13), B2–N2–C17 133.12(12), C1–N3–C2 111.93(14), C3–N4–C4 109.66(13);
N1–B1–B2–N2 55.15(17), N1–B1–B2–N4 -127.75(15), N1–B1–N3–C1 -169.14(17), N1–B1–N3–C2 7.9(3),
N2–B2–N4–C3 -178.11(15), N2–B2–N4–C4 1.5(2), N3–B1–B2–N4 58.1(2), C5–N1–B1–B2 14.7(2),
C5–N1–B1–N3 -170.85(13), C17–N2–B2–B1 168.95(13), C17–N2–B2–N4 -8.1(2).
each C atom of the NMe2 ligand due to the hindered rotation. This shows that there is
a back bonding between B and N. To obtain information on preferred conformations
and further insights into steric and electronic effects of ligands, 1,2-diaryl- and 1,2- (di-
t-butyl) amino-1,2-diboranes(4) were synthesized. X-ray structure determinations of
some representative derivatives were carried out and found that the diarylamino group
in the solid state was nearly orthogonal to the plane due to the bulky groups bound to
the boron atoms. Also, p bond between N-B atoms was detected with X-ray structural
analysis by means of the N-B bond distance values.
4. Structural Determination
The structures of 3ae, 3af, 3bb, 3be, 3ce, 3cd and 3cf have been determined using sin-
gle-crystal X-ray diffraction. Their molecular structures are depicted in Fig. 1–7, and
packing diagrams are shown in Fig.S1-S7 (see Electronic Supplementary Information).
3ae had been synthesized by Patton et al. [16]. Its crystal structure has been presented
in this study. The compound 3ae crystallizes in the triclinic crystal system space group
P-1. X-ray structural analysis reveals that the molecule features two boron atoms of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
97
Figure 2. View of the molecular structure of 3bb showing the atom numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level. Selected bond distances (Å), bond
angles and torsion angles (ꢁ): B1–B2 1.716(5), B1–N1 1.404(4), B1–C1 1.582(4), B2–N2 1.402(4),
B2–C28 1.584(4), N1–C10 1.432(3), N2–C19 1.435(3); C1–B1–B2 117.9(2), N1–B1–C1 122.3(3),
N1–B1–B2 119.2(3), N2–B2–B1 112.5(2), N2–B2–C28 124.7(3), C28–B2–B1 122.0(2), B1–N1–C10
129.1(2), B2–N2–C19 136.3(3); N1–B1–B2–N2 -170.1(3), N1–B1–B2–C28 0.0(4), C1–B1–B2–C28 171.5(2),
C10–N1–B1–B2 161.1(3), C10–N1–B1–C1 -9.9(5), C28–B2–N2–C19 3.4(5).
diborane(4), which connect to the NMe2 and tert-butylamino moieties (Fig. 1). The
B–B bond distance [1.722(2) Å] is slightly longer with respect to those reported pre-
viously for mesityl [1.717(15) Å], phenyl [1.714(4) Å and 1.719(2) Å] and pyrrolyl
[1.716(2) Å] substituted 1,2-bis(dimethylamino)diborane(4) [17, 18]. On the other
hand, this bond length is smaller when comparing with the similar studies reported
by Fırıncı et al. [1.735(6) Å] and Braunschweig et al. [1.731(2) Å] [19, 20]. Boron
atoms are close to the trigonal planar geometry except the B1–B2–N2 bond angle of
113.02(12)ꢁ, while nitrogen atoms have nearly large deviations from the trigonal pla-
narity. There is a strong twist around the B–B bond. The molecule adopts a stag-
gered conformation around this B–B bond considering the torsion angles N1/B1/B2/
N2 at 55.15(14)ꢁ and N3/B1/B2/N4 at 58.06(17)ꢁ. These values are comparable with
the very similar compound studied by Braunschweig and Damme [60.4(2)ꢁ] [20]. It’s
most likely due to the steric repulsion between tert-butylamino groups around the
boron atoms. Me2N–B bond lengths [1.415(2)–1.417(2) Å] are longer than those in
corresponding studies in the literature, and the Me2B–N bonds possess better
p–bond character [18–21]. The crystal structure is stabilized by intramolecular
C–HꢀꢀꢀN type weak interactions and van der Waals forces. Stacking of the molecules
in a head-to-tail and head-to-head fashion in the unit cell are shown in the packing
diagram in Fig. S1.

98
D. B. CELEPCI ET AL.
Figure 3. View of the molecular structure of 3be showing the atom numbering scheme.
Displacement ellipsoids are drawn at the 30% probability level. Selected bond distances (Å), bond
angles and torsion angles (ꢁ): B1–B1i 1.713(8), B1–N1 1.404(5), B1–C1 1.580(5), N1–C10 1.580(5);
N1–B1–B1i 114.6(4), N1–B1–C1 122.9(3), C1–B1–B1i 121.5(4), B1–N1–C10 132.9(3); C10–N1–B1–C1
i
1
-13.7(6), C10–N1–B1–B1 155.6(3); symmetry code (i) 1-x, 1-y, = -z.
2
3bb crystallizes in the monoclinic space group P21/n. The molecule comprises of
dimesityl and dimesitylamino fragments, of which the same groups are in trans posi-
tions to each other (Fig. 2). At the central point of the B–B bond, a crystallographic
inversion center lies, and the B2N2C2 unit is almost planar with the r.m.s. deviation of
0.047 Å. The B–B bond is characteristic of single bond with the bond length of
1.716(5) Å. The C–B–B, N–B–B and N–B–C bond angles around the boron atoms
have a range from 112.5(3) to 124.7(3)ꢁ. From this point of view, the B1 and B2 atoms
have distorted trigonal planar geometry. Mesitylamino moieties are tilted around the
nitrogen atoms, so N1 and N2 atoms also have distorted trigonal planar geometry. The
dihedral angles between the ring planes of C1–C6 and C10–C15, C1–C6 and
C19–C24, C10–C15 and C28–C33, and C19–C24 and C28–C33 are 61.52(14)ꢁ,
82.89(15)ꢁ, 74.63(14)ꢁ and 78.94(15)ꢁ, respectively. Intramolecular C–HꢀꢀꢀN type weak
interactions and van der Waals forces are responsible for the stabilization of the crystal
structure. Stacking of the molecules viewed from the b axis are shown in the packing
diagram in Fig. S2.
The asymmetric unit of the compounds 3be and 3ce contain half of the molecule.
i
ii
Molecules are completed with the symmetry operations of 1-x, 1-y, 1/2-z and 1-x, y,
3/2-z. 3be crystallizes in the tetragonal crystal system, P41212 space group, and 3ce

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
99
Figure 4. View of the molecular structure of 3ce showing the atom numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level. Selected bond distances (Å), bond
angles and torsion angles (ꢁ): B1–B1ii 1.732(4), B1–N1 1.408(2), B1–C1 1.591(3), N1–C11 1.445(2);
N1–B1–B1ii 114.8(2), N1–B1–C1 123.51(15), C1–B1–B1ii 121.34(19), B1–N1–C11 133.10(16);
C11–N1–B1–C1 -11.7(3), C11–N1–B1–B1 161.57(15); symmetry code (ii) 1-x, y, 3/2-z.
crystallizes in the triclinic P-1. Compounds feature 2,6-diisopropylanilide groups con-
nect to the diborane unit in trans positions. Also dimesityl and duryl groups in 3be
and 3ce, bond to the diborane unit in trans positions to each other (Fig. 3-4). The
B–B bond lengths [1.713(8) Å for 3be, and 1.732(4) Å for 3ce] are strictly different
when comparing the compounds. This result may be due to the steric repulsion
between the duryl and 1,2-diisopropylanilide moieties in the 3ce molecule. The geo-
metries around the boron atoms adopt distorted trigonal planar for both compounds,
with the bond angles that span a range from 114.6(4) to 123.5(15)ꢁ. The dihedral
angle of diisopropylanilide fragment with the mesityl is 57.71(17)ꢁ in 3be, while its
dihedral angle with duryl group is 72.22(8)ꢁ in 3ce. Crystal structures of the com-
pounds are stabilized by intramolecular C–HꢀꢀꢀN type weak interactions and van der
Waals forces. Stacking of the molecules viewed from the c axis are shown in the
packing diagram in Fig. S3-S4.
3cd crystallizes in the monoclinic crystal system, C2/c space group. The asymmetric
unit has half of the molecule. It is completed by inversion symmetry iii1-x, 1-y, 1-z. The
B1–B1iii bond length is 1.711(7) Å. The compound comprises of tert-butylamino and
duryl fragments connect to the boron atoms (Fig. 5). The C–B–B, N–B–B and N–B–C
bond angles around the boron atoms have a range from 116.3(3) to 123.5(3)ꢁ. This
result is proof of the distorted trigonal planarity of B1 and B2 atoms. Crystal structure

100
D. B. CELEPCI ET AL.
Figure 5. View of the molecular structure of 3cd showing the atom numbering scheme.
Displacement ellipsoids are drawn at the 30% probability level. Selected bond distances (Å), bond
angles and torsion angles (ꢁ): B1–B1iii 1.711(7), B1–N1 1.381(4), B1–C1 1.595(5), N1–C11 1.472(4);
N1–B1–B1iii 116.3(3), N1–B1–C1 123.5(3), C1–B1–B1iii 119.9(4), B1–N1–C11 135.7(3); C11–N1–B1–C1
-2.3(6), C11–N1–B1–B1iii 178.5(4); symmetry code (iii) 1-x, 1-y, 1-z.
of 3cd is stabilized by van der Waals forces. Stacking molecules viewed from the b axis
are shown in the packing diagram in Fig. S5.
3af crystallizes in the monoclinic space group C2/c. The asymmetric unit comprises
one half of the molecule, which is completed with the two-fold rotational symmetry
[symmetry code: iv1-x, y, 3/2-z]. X-ray structural analysis shows that the molecular
structure of the compound features two boron atoms connect to the NMe2 and methox-
yanilide fragments, of which the similar groups are in cis conformations (Fig. 6). The
B1–B1iv bond length is 1.717(6) Å, is comparable to those found in related B2N4 studies
[17–20]. B atoms are close to the trigonal planar geometry considering the B–B–N and
N–B–N bond angles. There is a strong twist around the B–B bond. The molecule adopts
a staggered conformation around this B–B bond considering the torsion angles N1/B1/
B1iv/N1iv at 65.34(15)ꢁ and N2/B1/B1iv/N2iv at 65.47(15)ꢁ. These values are comparable
with 3ae and the very similar compound studied by Braunschweig and Damme [60.4
(2)ꢁ] [20]. Similarly in 3af, this result may also be due to the steric repulsion between
2-methoxyanilide groups around the boron atoms. The crystal structure is stabilized by
intramolecular N–HꢀꢀꢀO and C–HꢀꢀꢀN type weak interactions besides the van der Waals

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
101
Figure 6. View of the molecular structure of 3af showing the atom numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level. Selected bond distances (Å), bond angles and torsion
angles (ꢁ): B1–B1iv 1.720(6), B1–N1 1.441(4), B1–N2 1.410(4), N1–C1 1.389(3), O1–C2 1.374(3), O1–C7
1.426(3); N1–B1–B1iv 119.9(2), N2–B1–B1iv 123.5(2), N1–B1–N2 116.6(2), C1–N1–B1 131.7(2), C2–O1–C7
117.5(2); C1–N1–B1–N2 -159.3(3), C8–N2–B1–N1 9.4(4), C8–N2–B1–B1iv -170.4(3), C9–N2–B1–N1
-175.7(2), C9–N2–B1–B1iv 4.5(4), C7–O1–C2–C1 -169.7(2); symmetry code (iv) 1-x, y, 3/2-z.
forces. Stacking of the molecules viewed from the c axis are shown in the packing dia-
gram in Fig. S6.
3cf crystallizes in the orthorhombic crystal system, P212121 space group. Similar to
3ce, the molecule consists of two duryl groups and differently has two 2-methoxyanilide
fragments, which are in trans positions to each other (Fig. 7). At the central point of
the B–B bond, a crystallographic inversion center lies, and the B2N2C2 unit has
almost perfect planarity with the r.m.s. deviation of 0.011 Å. The B–B bond is char-
acteristic of single bond with the bond length of 1.711(7) Å. The C–B–B, N–B–B
and N–B–C bond angles around the boron atoms have a range from 114.4(4) to
126.6(5)ꢁ. From this point of the view, the B1 and B2 atoms have distorted trigonal
planar geometry. The bond angles of 118.3(4)ꢁ and 117.4(4)ꢁ around the O1 and O2
atoms confirm their sp3 character. N1/B1/N2/B2 plane and the 2-methoxyanilide
moieties are together coplanar with the r.m.s. deviation of 0.048 Å. Duryl groups are
nearly perpendicular to this plane with the dihedral angles of C1–C6 is 76.01(18)ꢁ
and C11–C16 is 75.85(18)ꢁ. The dihedral angle between the duryl groups is 7.3(2)ꢁ,
and between the methoxyanilide groups is 1.8(2)ꢁ. On the other hand, 2-methoxyani-
lide fragments and duryl groups have dihedral angles between C28–33 – C1–6 is
80.5(2)ꢁ; C28–33 – C11–16 is 81.4(2)ꢁ; C21–26 – C1–6 is 79.7(2)ꢁ; C21–26 – C11–16 is
80.4(2)ꢁ. The crystal structure is stabilized by intramolecular N–HꢀꢀꢀO type weak
interactions and van der Waals forces. Stacking of the molecules viewed from the a
axis are shown in the packing diagram in Fig. S7.

102
D. B. CELEPCI ET AL.
Figure 7. View of the molecular structure of 3cf showing the atom numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level. Selected bond distances (Å), bond angles and tor-
sion angles (ꢁ): B1–B2 1.711(7), B1–N1 1.414(6), B1–C1 1.577(7), B2–N2 1.410(6), B2–C11 1.564(7),
N1–C21 1.412(5), N2–C28 1.411(5), O1–C26 1.376(5), O1–C27 1.428(5), O2–C33 1.378(5), O2–C34
1.422(5); N1–B1–B2 114.4(4), N1–B1–C1 125.0(4), C1–B1–B2 120.6(0), B1–N1–C21 135.1(4), N2–B2–B1
114.7(4), N2–B2–C11 126.6(5), C11–B2–B1 118.7(4), B2–N2–C28 134.4(4), C26–O1–C27 118.3(4),
C33–O2–C34 117.4(4); C1–B1–B2–N2 -0.4(6), C1–B1–N1–C21 4.4(8), C1–B1–B2–C11 -178.9(4),
C11–B2–B1–N1 2.4(6), C11–B2–N2–C28 -5.3(8), B1–N1–C21–C26 -176.8(5), N1–C21–C26–O1 2.3(6),
C21–C26–O1–C27
-175.3(4),
B2–N2–C28–C33
175.3(5),
N2–C28–C33–O2
-3.0(6),
C28–C33–O2–C34 174.6(4).
4. Conclusion
In the present study, we have reported the synthesis of new air stable primary arylamido
substituted diborane(4) derivatives. The structures of these compounds were character-
ized by using their one- and two-dimensional NMR spectra. The molecular and crystal
structures of the compounds 3ae, 3af, 3bb, 3be, 3cd, 3ce and 3cf were confirmed by
single-crystal X-ray diffraction technique. Structural analysis showed that for all com-
plexes the diarylamino group in the solid state is nearly orthogonal to the plane due to
the boron-bound aryl groups. Apart from the 3ce, in all complexes, B–B bond lengths
have normal values and compare well with those in diborane(4) compounds. In 3ce, the
B–B bond distance [1.732(4) Å] is relatively large, which may be due to the steric

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
103
repulsion between the duryl and 1,2-diisopropylanilide moieties. In the stabilization of
the crystal structures, mainly intramolecular C–HꢀꢀꢀN and N–HꢀꢀꢀO type weak interac-
tions and van der Waals forces play role. Diarylamino substituted diborane(4) deriva-
tives are used as starting compounds for the synthesis of other air-stable boron
compounds, such as diazadiborolidines, diazatriborolidines and diazadiborinane deriva-
tives. Similarly, alkylamino-substituted di- and tri-borolidine heterocycles have already
being demonstrated in the literature.
Acknowledgements
€ _
This study was supported by TUBITAK project No. 112T888, by Adnan Menderes University
BAP project No. FEF-17004, FEF-16010 and FEF-18009. The authors also acknowledge Dokuz
Eylu€l University for the use of the Rigaku Oxford Xcalibur Eos diffractometer (purchased under
University Research Grant no.2010.KB.FEN.13).
Supplementary data
Crystallographic data as .cif files for the structures reported in this paper have been deposited at
the Cambridge Crystallographic Data Center with CCDC 1859490, 1859494, 1859493, 1859491,
1859711, 1859492 and 1859495 for 3ae, 3af, 3bb, 3be, 3cd, 3ce and 3cf, respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (þ44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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