Structures and electronic properties of diisopropylaminoborane substituted with highly electron-rich π-Conjugated systems and their oxidized states

Article abstract of DOI:10.1246/bcsj.20190199

We have designed and synthesized three kinds of diisopropylaminoboranes substituted with two electron-donating π-conjugated systems, 2a: bis(phenothiazinyl) 2b: bis(N-phenyldihydrophenazinyl), and 2c: bis(phenoxazinyl) derivatives, and investigated their

Full text of DOI:10.1246/bcsj.20190199

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Structures and Electronic Properties of Diisopropylaminoborane Substituted with
Highly Electron-Rich -Conjugated Systems and Their Oxidized States
Kohei Yoshida, Shuichi Suzuki,* Masatoshi Kozaki and Keiji Okada
Advance Publication on the web August 23, 2019
doi:10.1246/bcsj.20190199
© 2019 The Chemical Society of Japan
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Structures and Electronic Properties of Diisopropylaminoborane Substituted with
pp
Highly Electron-Rich -Conjugated Systems and Their Oxidized States
Kohei Yoshida,1 Shuichi Suzuki,*1,2 Masatoshi Kozaki1,3 and Keiji Okada1,3
1Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka, Osaka 558-8585, Japan
2Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
3Osaka City University Advanced Research Institute for Natural Science and Technology (OCARINA), Sumiyoshi-ku, Osaka, Osaka 558-
8585, Japan
Shuichi Suzuki
Shuichi Suzuki graduated from Osaka University in 2001 and received his M.Sc. in 2003 and Ph.D. in 2006 under the
supervision of Prof. Kazuhiro Nakasuji and Prof. Yasushi Morita. After receiving his Ph.D. degree, he joined Prof. Okada’s
research group at Osaka City University as an Assistant Professor. In 2010, he was promoted to Lecturer. In 2015, he
moved to Osaka University as an Associate Professor. His current research interests focus on the developments of new
functional materials based on unique molecular assemblies.
Abstract
We have designed and synthesized three kinds of
diisopropylaminoboranes substituted with two electron-donating
containing such bonds have been reported to show the
fascinating photophysical and electronic properties derived from
the polar characteristics of B–N bonds and their unique intra-
and intermolecular interactions.2h,i,k–3a,c,e One of the interesting
aspects of the amine moieties on B–N bonds is that they can be
oxidized to form radical cationic bond which is a isoelectronic
structure of the C=C radical cation,4 the B–B radical anion,5 and
the C–B neutral radical.6 We have investigated the electronic
structure of the radical cationic species of tris(N-
phenothiazinyl)borane (1),7 and found (i) localization of the spin
and charge on one of the phenothiazine moieties and (ii)
elongation of the B–N bond directly bound to the phenothiazine
radical cation. Additionally, when two-electron oxidized species
of 1 has a planar structure, 2p orbitals of boron and three nitrogen
p-conjugated systems, 2a: bis(phenothiazinyl) 2b: bis(N-
phenyldihydrophenazinyl),
and
2c:
bis(phenoxazinyl)
derivatives, and investigated their structures and electronic
properties including those of oxidized species. The structural
analyses of 2a–c and 2a–c•+ indicated that the p-electronic
systems composed of boron and three nitrogen atoms showed
unique structural deformation by one-electron-oxidation. The
structures of the neutral compounds were almost C2-symmetrical,
indicating that the two donor p-electronic systems are almost
identical. In contrast, the radical cationic compounds were not
C2-symmetrical structures because the charges of the radical
cations were localized on one of the electron donors. In addition,
the B–N bonds directly bound to the radical cations were
elongated; also, perpendicular arrangements between the p-
orbital of boron and p-conjugated orbitals of the radical cations
were observed. We also succeeded in the observing the triplet
state of 2b2(•+), which was categorized as a heteroatom analogue
of the trimethylenemethane (TMM) p-electronic system, by
means of electron-spin resonance (ESR) spectroscopy.
atoms form
a
cross-conjugated system similar to
trimethylenemethane (TMM) analogues. However, its congested
structure prevents the planarization induced by the oxidation of
the compound and generation of two-electron oxidized species.
For further development of the present B–N p-electronic system
and investigations of the B–N bonding properties, we designed
a diisopropylaminoborane substituted with two phenothiazine
(PTZ) units, 2a, and its dihydrophenazine (DHP) and
phenoxazine (PXZ) analogues, 2b and 2c, respectively (Figure
1), in which the aliphatic amine will provide the double bond
characteristic for the B–N bond and stabilize the cationic species.
Herein, we have synthesized and isolated the neutral and radical
cationic species of 2a–c and investigated their electronic
structures. We have also succeeded in the generation of
di(radical cationic) species 2b2(•+) which was an isoelectronic
structure with the TMM diradical.
Keywords: Trimethylenemethane, Radical cation,
Main group elements
1. Introduction
p-Conjugated compounds incorporating p-block elements
other than carbon, nitrogen, and oxygen have been extensively
studied for the exploration of their synthetic interests, for the
elucidation of their electronic perturbations corresponding to
related hydrocarbons, and for the development of unique
electronic properties.1–3 Of these research avenues, compounds
possessing bonds composed of different kinds of typical
elements have recently received great interests because of their
unique molecular structures and electronic properties based on
the polar character of the bonds.3 Recently, we have focused on
the B–N bond as a strongly polar bond due to its combinations
of electron-deficient and electron-rich atoms. Compounds
S
+
N
B
N
N
B
–
B
N
N
N
N
N
N
X
X
X
X
S
S
2a: X = S, 2b: X = NPh, 2c: X = O
1
Figure 1. Chemical structures of 1 and 2a–c with their resonance
structures.

2. Experimental
were obtained by recrystallization from CH2Cl2–ethanol. 2a:
C30H30BN3S2; colorless solid; MW 507.52; mp 231–232 °C; 1H
NMR (400 MHz, Acetone-d6) d (ppm) 7.22 (d, J = 7.5 Hz, 4H),
7.10 (br, 4H), 7.05 (td, J = 7.5 and 1.3 Hz, 4H), 6.98 (td, J = 7.5
and 1.3 Hz, 4H), 3.66 (sept, J = 6.2 Hz, 2H), 0.95 (d, J = 6.2 Hz,
12H); 13C NMR (100 MHz, Acetone-d6) d (ppm) 145.4, 128.3,
128.2, 127.6, 124.7, 122.2, 48.2, 22.6; IR (KBr) 3058, 2977,
2958, 2929, 2868, 1589, 1573, 1477, 1460, 1365, 1317, 1299,
1228, 1188, 1172, 1157, 1118, 1074, 1039, 1002, 968, 929, 910,
893, 864, 850, 804, 746, 727, 707, 682, 659, 628, 588 cm–1; MS
(FAB+) m/z 507 [M+]; Anal. Calcd. for C30H30BN3S2: C, 71.00;
H, 5.96; N, 8.28, Found: C, 70.75; H, 6.04, N, 8.20.
Crystallographic data: CCDC 1936899.
General Information: Kanto Kagaku Silica gel 60
spherical was used for column chromatography. The progress of
reactions was monitored using thin-layer chromatography using
Merck TLC Aluminum oxide 60 F254 and Merck Silica gel 60
F254. Diisopropylaminodifluoroborane (iPr2NBF2)9 and
thianthrene radical cation tetrachlorogallate salt (TH+•GaCl4 )
– 10
were prepared according to the literatures. Tris(4-
bromophenyl)aminium hexafluoroantimonate (TBPA•+•SbF6 )
–
was prepared from tris(4-bromophenyl)amine and silver
hexafluoroantimonate according to literature with some
modification.11
10-(4'-tert-Butylphenyl)-10H-phenothiazine
(t-Bu-C6H4-PTZ) was prepared from 10H-phenothiazine and 1-
bromo-4-tert-butylbenzene by the cross-coupling reaction using
N-[Bis(10-phenylphenazin-5(10H)-yl)boranyl]-N-
(propan-2-yl)propan-2-amine (2b): Phenazine (435 mg, 2.41
mmol) was dissolved in a mixture of toluene (15 mL) and THF
(5 mL) under a nitrogen atmosphere. After cooling to 0 °C, a di-
n-butyl ether solution of phenyllithium (1.9 M, 1.4 mL, 2.64
mmol) was added to the solution. Then, a toluene solution (5
mL) of iPr2NBF2 (180 mg, 1.21 mmol) was immediately added
to the mixture. The mixture was stirred for 21 h at room
temperature, poured into water, and extracted with benzene. The
organic layer was washed with water and a saturated aqueous
NaCl solution, successively. The organic layer was dried over
Na2SO4, filtered, and then concentrated under reduced pressure.
Hexane was added to the residue and the precipitate was
collected by filtration, to give 2b as a brown solid (316 mg, 42%).
Single crystals suitable for X-ray crystal analysis were obtained
by recrystallization from CH2Cl2–hexane. 2b: C42H40BN5;
Pd2(dba)3–t-BuOK
dihydrophenazine radical cation perchlorate (DPP•+•ClO4 )
and 10-phenyl-10H-phenoxazine radical cation
in
toluene.
5,10-Diphenyl-5,10-
– 12
hexafluorophosphate (PXZ•+•PF6–)13 were prepared according to
the literatures. Toluene, tetrahydrofuran (THF), and diethyl ether
(Et2O) were dried and distilled over sodium. Dichloromethane
(CH2Cl2) and acetonitrile (CH3CN) were dried and distilled over
calcium hydride. The other chemicals were of commercial grade
1
and used without further purification. H and 13C NMR spectra
were recorded on a JEOL JNM-LA400 spectrometer. ESR
spectra were recorded on
a Bruker ELEXSYS E500
spectrometer. Melting points were measured using a Yanaco-
MP-J3 apparatus and were not corrected. Infrared spectra were
recorded using KBr pellets on a SHIMADZU FTIR-8700
spectrophotometer. UV/VIS/NIR absorption spectra were
measured on SHIMADZU UV-2500. FAB-MS spectra were
recorded on JMS-AX700 with NBA as a matrix. ESI-MS spectra
were recorded on a Bruker micrOTOF II. Cyclic voltammogram
measurements were carried out using ALS Electrochmical
Analyzer Model 610A and recorded with a glassy carbon as a
working electrode (WE), a Pt wire as a counter electrode (CE),
and a saturated calomel electrode (SCE) as a reference electrode
(RE) in dichloromethane containing 0.1 M n-butylammonium
1
reddish purple prism; MW 625.61; mp 214 °C (decomp.); H
NMR (400 MHz, Acetone-d6) d (ppm) 7.72 (d, J = 7.4 Hz, 4H),
7.56 (d, J = 7.4 Hz, 2H), 7.44 (d, J = 7.4 Hz, 4H), 6.97 (d-like, J
= 7.0 Hz, 4H), 6.69–6.62 (m, 8H), 6.07 (dd, J = 7.0 and 1.9 Hz,
4H), 3.85 (sept, J = 6.8 Hz, 2H), 1.05 (d, J = 6.8 Hz, 12H); 13
C
NMR (100 MHz, Acetone-d6) d (ppm) 142.1, 141.3, 136.7,
131.8, 131.4, 129.0, 123.5, 122.6, 119.9, 114.7, 48.3, 23.0; IR
(KBr) 3061, 2963, 2926, 1593, 1479, 1458, 1437, 1331, 1277,
1244, 1194, 1157, 1105, 1070, 1055, 740, 700, 669, 646, 631,
617 cm–1; MS (FAB+) m/z 625 [M+]; Anal. Calcd. for
C42H40BN5: C, 80.63; H, 6.44; N, 11.19, Found: C, 80.53; H,
6.60, N, 10.81. Crystallographic data: CCDC 1936903.
–
perchlorate (nBu4N+•ClO4 ). The redox potentials were finally
calibrated with the ferrocene/ferrocenium couple. UV/VIS/NIR
absorption spectra during electrochemical oxidation were
measured using an Ocean Optics HR4000 spectrometer, a 1-mm-
width cell equipped with a fine mesh platinum as a WE, a Pt wire
as a CE, and a SCE as a RE at suitable external potentials that
could generate the radical cation species in dichloromethane in
the presence of 0.1 M nBu4NClO4. X-ray data were collected by
a Rigaku Saturn CCD system with graphite monochromated
MoKa radiation. Elemental analyses were obtained from the
Analytical Center in Osaka City University (using Fisons
EA1108 and J-Science JM10).
N-(Di-10H-phenoxazin-10-ylboranyl)-N-(propan-2-
yl)propan-2-amine (2c): 10H-Phenoxazine (534 mg, 2.88
mmol) was dissolved in a mixture of toluene (20 mL) and THF
(10 mL) under a nitrogen atmosphere. After cooling to 0 °C, a
hexane solution of n-butyllithium (1.6 M, 1.9 mL, 3.02 mmol)
was added to the solution. Then, a toluene solution (5 mL) of
iPr2NBF2 (215 mg, 1.44 mmol) was immediately added to the
mixture. The mixture was stirred for 17 h at room temperature,
poured into water, and extracted with benzene. The organic layer
was washed with water and a saturated aqueous NaCl solution,
successively. The organic layer was dried over Na2SO4, filtered,
and then concentrated under reduced pressure. The residue was
purified by silica gel column chromatography using hexane–
CH2Cl2 (4:1 v/v) as an eluent, to afford 2c as a colorless solid
(457 mg, 67%). Single crystals suitable for X-ray crystal analysis
were obtained by recrystallization from CH2Cl2–ethanol. 2c:
C30H30BN3O2; colorless solid; MW 475.39; mp 206–207 °C; 1H
NMR (400 MHz, Acetone-d6) d (ppm) 7.03 (dd, J = 7.7 and 1.6
Hz, 4H), 6.96–6.84 (m, 12H), 3.67 (sept, J = 6.7 Hz, 2H), 1.01
(d, J = 6.7 Hz, 12H); 13C NMR (100 MHz, Acetone-d6) d (ppm)
149.5, 135.4, 125.0, 124.2, 119.9, 117.2, 48.5, 23.2; IR (KBr)
3034, 2968, 2930, 2872, 1618, 1591, 1479, 1443, 1387, 1369,
1313, 1254, 1205, 1186, 1150, 1130, 1103, 1042, 1005, 914, 853,
N-(Di-10H-phenothiazin-10-ylboranyl)-N-(propan-2-
yl)propan-2-amine (2a): 10H-Phenothiazine (3a) (1.40 g, 7.02
mmol) was dissolved in a mixture of toluene (60 mL) and THF
(30 mL) under a nitrogen atmosphere. After cooling to 0 °C, a
hexane solution of n-butyl lithium (1.65 M, 4.7 mL, 7.76 mmol)
was added to the solution. Then, a toluene solution (5 mL) of
iPr2NBF2 (524 mg, 3.52 mmol) was immediately added to the
mixture. The mixture was stirred 22 h at room temperature,
poured into water, and extracted with benzene. The organic layer
was washed with water and a saturated aqueous NaCl solution,
successively. The organic layer was dried over Na2SO4, filtered,
and then concentrated under reduced pressure. The residue was
purified by silica gel column chromatography using hexane–
CH2Cl2 (4:1 v/v) as an eluent, to afford 2a as a colorless solid
(662 mg, 37%). Single crystals suitable for X-ray crystal analysis

837, 762, 737 cm–1; MS (FAB+) m/z 475 [M+]; Anal. Calcd. for
C30H30BN3O2: C, 75.79; H, 6.36; N, 8.84, Found: C, 75.78; H,
6.52, N, 8.82. Crystallographic data: CCDC 1936908.
37% yield as a colorless solid. Phenazine 3b was converted to a
lithiated N-phenyl-9,10H-dihydrophenazine by substitution with
phenyllithium and subsequent addition of iPr2NBF2 to give 2b
in a 42% yield as a brown solid. Compound 2c was obtained
from phenoxazine 3c in a 67% yield as a colorless solid by the
same method as the preparation of 2a. Compounds 2a–c were
stable against air and moisture. In the solution state, four
aromatic NMR signals assigned to the donor moieties were
observed, which suggested that the donor molecules could be
freely rotated. We also obtained crystals of these compounds
suitable for X-ray crystal structure analysis by recrystallization
from suitable solvents.
–
Synthesis of 2a•+•GaCl4 : In a glove box filled with an
argon, 2a (38 mg, 0.0740 mmol) was dissolved in CH2Cl2 (25
mL). To this stirred solution was added a CH3CN solution (25
–
mL) of TH•+•GaCl4 (31 mg, 0.0735 mmol). After 30 min, the
solvent was evaporated under reduced pressure. The residue was
dissolved in CH2Cl2 (2 mL) and then Et2O (20 mL) was slowly
added to the CH2Cl2 solution. The generated precipitate was
collected by filtration, to give 2a•+•GaCl4– as a dark red solid (37
mg, 69%). The crystalline solid was obtained by recrystallization
1) 1.1 equiv nBuLi
Toluene–THF
–
0 °C
–
from CH2Cl2–benzene. 2a•+•GaCl4 : C30H30BN3S2•GaCl4; dark
H
TH+•GaCl4
N
red solid; MW 719.06; mp 189 °C (decomp.); MS (FAB+) m/z
–
2a•+•GaCl4
2a
2b
2c
+
–
507 [C30H30BN3S2 ], (FAB–) m/z 211 [GaCl4 ]; IR (KBr) 3068,
3038, 2968, 2931, 2873, 1531, 1481, 1454, 1386, 1371, 1352,
1294, 1276, 1249, 1220, 1186, 1163, 1136, 1112, 1097, 1070,
1058, 1031, 989, 908, 893, 850, 804, 754, 740, 717, 680, 659,
634, 609, 594 cm–1; Anal. Calcd. for C30H30BN3S2•GaCl4: C,
50.11; H, 4.21; N, 5.84. Found: C, 49.96; H, 4.25; N, 5.86; g =
2.0054. Crystallographic data: CCDC 1936901.
CH2Cl2–CH3CN
rt, 30 min
2) 0.5 equiv iPr2NBF2
0 °C to rt, 22 h
y. 37%
S
3a
y. 69%
1) 1.1 equiv PhLi
Toluene–THF
0 °C
N
–
TH+•GaCl4
–
2b•+•GaCl4
CH2Cl2–CH3CN
rt, 30 min
2) 0.5 equiv iPr2NBF2
N
0 °C to rt, 21 h
3b
y. 71%
y. 42%
–
Synthesis of 2b•+•GaCl4 : In a glove box filled with an
1) 1.1 equiv nBuLi
Toluene–THF
0 °C
H
N
–
TBPA+•SbF6
argon, 2b (61 mg, 0.0978 mmol) was dissolved in CH2Cl2 (20
–
2c•+•SbF6
mL). To this stirred solution was added a CH3CN solution (20
CH2Cl2
2) 0.5 equiv iPr2NBF2
O
3c
–
mL) of TH•+•GaCl4 (41 mg, 0.0962 mmol). After 30 min, the
rt, 10 min
0 °C to rt, 17 h
y. 75%
y. 67%
solvent was evaporated under reduced pressure. The residue was
dissolved in CH2Cl2 (2 mL) and then Et2O (20 mL) was slowly
added to the CH2Cl2 solution. The generated precipitate was
Scheme 1. Synthesis of 2a–c and their corresponding cationic
species.
–
collected by filtration, to give 2b•+•GaCl4 as a green solid (59
Br
Br
mg, 71%). The crystalline solid was obtained by recrystallization
–
from CH2Cl2–toluene. 2b•+•GaCl4 : C42H40BN5•GaCl4; green
•+
N
S
•+
S
solid; MW 837.15; mp 211 °C (decomp.); MS (FAB+) m/z 625
–
•SbF6
–
•GaCl4
[C42H40BN5 ], (FAB–) m/z 211 [GaCl4 ]; IR (KBr) 3063, 2978,
2930, 2878, 1591, 1549, 1507, 1489, 1472, 1447, 1387, 1373,
1352, 1340, 1317, 1269, 1188, 1161, 1138, 1119, 1092, 1072,
745, 700, 669, 646, 631, 611 cm–1; Anal. Calcd. for
C42H40BN5•GaCl4: C, 60.26; H, 4.82; N, 8.37. Found: C, 59.88;
H, 4.91; N, 8.05; g = 2.0031. Crystallographic data: CCDC
1936906.
+
–
Br
TBPA•+•SbF6
–
–
TH•+•GaCl4
Scheme 2. Chemical structures of oxidants used in the
preparation of the radical cations.
Electrochemical properties of 2a–c: The cyclic
voltammograms (CVs) of 2a–c were measured to gain insight on
their electrochemical properties (Figure 2, Table 1). Compounds
2a and 2b exhibited two reversible oxidation waves, while 1
exhibited one reversible and one irreversible oxidation waves at
+0.20 V (vs Fc/Fc+) and approximately +1.2 V,7 respectively.
The reversibility of the oxidation waves of 2a and 2b suggested
that both the one- and two-electron oxidized species are stable
within the time scale of the CV measurement; therefore, it would
be possible to prepare the two-electron oxidized species. The CV
of 2c showed two oxidation waves with a residual wave at +0.12
V after the second oxidation processes, suggesting that the
dication species are not stable (Figure S1). The large differences
between the first and second oxidation waves for 2a–c suggested
that the molecular structures were largely changed by one-
electron oxidation.
–
Synthesis of 2c•+•SbF6 : In a glove box filled with an argon,
2c (51 mg, 0.106 mmol) was dissolved in CH2Cl2 (15 mL). To
this stirred solution was added a CH2Cl2 solution (15 mL) of
TBPA•+•SbF6 (76 mg, 0.105 mmol). After 10 min, the solvent
–
was evaporated under reduced pressure. The residue was
dissolved in CH2Cl2 (2 mL) and then Et2O (20 mL) was slowly
added to the CH2Cl2 solution. The generated precipitate was
collected by filtration, to give 2c•+•SbF6 as a purple solid (56
–
mg, 75%). The crystalline solid was obtained by recrystallization
from CH2Cl2–hexane. 2c•+•SbF6 : C30H30BN3O2•SbF6; purple
–
solid; MW 711.140; mp 232 °C (decomp.); MS (ESI+) m/z 475
[C30H30BN3O2 ], (ESI–) m/z 235 [SbF6 ]; IR (KBr) 3275, 2967,
2836, 2722, 1637, 1591, 1569, 1515, 1492, 1476, 1407, 1288,
1269, 1197, 1139, 1082, 952, 835, 759, 663 cm–1; Anal. Cald. for
C30H30BN3O2•SbF6•(CH2Cl2)0.2: C, 49.82; H, 4.21; N, 5.77.
Found: C, 49.75; H, 4.26; N, 5.76. Crystallographic data: CCDC
1936909.
+
–
3. Results and Discussion
Syntheses of 2a–c: The syntheses of 2a–c are illustrated in
Scheme 1. Phenothiazine (3a) was treated with n-butyllithium in
toluene/THF
followed
by
the
addition
of
Figure 2. Cyclic voltammograms of (a) 2a, (b) 2b, and (c) 2c in
dichloromethane.
diisopropylaminodifluoroborane (iPr2NBF2)9 to give 2a in a

These results also indicated that the spin and charge are localized
on one of the DHP moieties.
Table 1. Oxidation potentials of 2a–c [a]
The UV/VIS/NIR spectrum of 2c•+ generated by an
electrochemical reaction showed absorption bands at lmax = 416,
535, and 722 nm (Figure 3c). Although the absorption maxima
E1/2ox1 / V
+0.24
–0.24
E1/2ox2 / V
+0.71
+0.25
E1/2ox2 – E1/2ox1 / V
2a
2b
2c
0.47
0.49
0.38
–
of the spectrum for 2c•+ were similar to that of PXZ•+•PF6
(Figure S4),13 the spectra shapes differed from one other. This
+0.34
+0.72
observation suggested the instability of the 2c•+ species.
[a] Conditions: 0.1 M n-BuNClO4 in dichloromethane; V vs
Fc/Fc+ = 0 V (+0.48 V vs SCE); working electrode: glassy
carbon; counter electrode: platinum; reference electrode: SCE;
scan rate: 100 mV s–1.
It is noted that degradation of the absorption bands of 2a–
c•+ which lacked isosbestic points were observed by further
electrochemical oxidation which was performed by applying the
potential enough to generate the two-electron oxidized species
of 2a–c, suggesting the instability of 2a–c2+ at room temperature.
In order to investigate the stabilities of the oxidized species
of 2a–c, we monitored their UV/VIS/NIR spectra using a thin-
layer cell during electrochemical oxidation through the
application of the appropriate external potentials in
Synthesis and characterization of the radical cations of
2a–c: The radical cations of 2a and 2b were obtained by
chemical oxidations using
a thianthrene radical cation
dichloromethane solution containing 0.1
M
tetra-n-
tetrachlorogallate salt (TH+•GaCl4– in Scheme 2) as an oxidizing
agent. The UV/VIS/NIR spectra of these radical cation salts in
dichloromethane are quite similar to those of the
electrochemically generated ones (Figures S5,S6). In addition,
degradation of the UV/VIS/NIR absorption spectra were not
observed under aerated conditions at room temperature, which
indicated that these radical ions were quite stable.
butylammonium perchlorate (Figure 3). The spectrum of 2a
showed no absorption in visible region before oxidation (Figure
3a). During the electrochemical oxidation (Eapplied = +0.44 V vs
Fc/Fc+), new broad absorption bands were observed at lmax
=
514, 770, and 861 nm accompanied by an isosbestic point at 345
nm. The spectrum shape and molar absorption coefficients were
similar to those of 1•+ 8 and the radical cation species of t-Bu-
C6H4-PTZ (Figure S2), indicating that the radical cation species
2a•+ was quantitatively generated by the electrochemical
oxidation. The spectrum also suggested that the spin and positive
charge were localized on only one of the PTZ moieties, as was
observed in the case for 1.7
To clarify the electronic spin structures of 2a and 2b, we
measured their electron spin resonance (ESR) spectra in
dichloromethane at room temperature (Figures 4,5). The ESR
spectra could be reproduced by simulation with the aid of
calculated hyperfine coupling constants (hfcs) using the
Gaussian 09 program (Figure S7).14 The presence of two non-
equivalent hfcs for the nitrogen nuclei bound to the boron atom
indicates that the electron spin in 2a/2b is localized on one of the
PTZ or DHP moieties, respectively, consistent with the findings
of the UV/VIS/NIR analysis. This localization was also
suggested by the spin density distributions calculated with
Gaussian 09.14
The tetrachlorogallate salt of 2c•+ could not be isolated.
Therefore, the product from chemical oxidation with tris(4-
bromophenyl)aminium hexafluoroantimonate (TBPA•+•SbF6– in
–
Scheme 2) was isolated as single crystals of 2c•+•SbF6 for
further analysis. In contrast to the high stability of 2a•+ and 2b•+,
2c•+ decomposed in solution state. The UV/VIS/NIR spectrum
–
of 2c•+•SbF6 in solution is different from that of the
electrochemically generated radical ion of 2c•+ (Figure 6).
Additionally, the shape of the spectrum changed depending on
the exposure time to aerated conditions. The ESR spectral
pattern was similar to that of 2a•+, although the spectrum of
Figure 3. UV/VIS/NIR spectral changes during electrochemical
oxidation of (a) 2a, (b) 2b, and (c) 2c: The applied voltages for
2a, 2b, and 2c were +0.44 V, –0.04 V, and +0.54 V (vs Fc/Fc+),
respectively. The dashed and solid lines show the spectra before
and after oxidation, respectively. The dotted lines show
intermediate states.
–
2c•+•SbF6 in dichloromethane could not be reproduced by
simulation.
The overview of the absorption spectral change observed
during the oxidation of 2b was similar to that of 2a. The
spectrum of 2b showed an absorption band at 323 nm along with
a quite small absorption band at approximately 450 nm (Figure
3b). The latter absorption was attributed to the one-electron
oxidation species of 2b which was generated at the low levels by
air oxidation due to its quite low oxidation potential. Absorption
bands at lmax = 466, 674, and 745 nm were increased with
isosbestic points at 298 and 371 nm by the application of an
external potential at –0.04 V, which was attributed to the
absorptions of the DHP radical cation on the basis of the
correlation to the spectrum shape of DPP•+•ClO4– (Figure S3).12
Figure 4. (a) Observed and simulated ESR spectra of
–
2a•+•GaCl4 in dichloromethane (g = 2.0054, n0 = 9.346556
MHz). (b) Top and side views of calculated spin density map of
2a•+. Red and yellow orbitals denoted the positive and negative
spin densities, respectively. The calculations were carried out by
the density functional theory with Gaussian 09 at the
UB3LYP/6-31G** level of theory using the coordinates
extracted from the X-ray crystal structure.

Crystal structures of 2a–c and their corresponding
radical cations: Single crystals of 2a–c and 2a–c•+•X– (X =
GaCl4 or SbF6) suitable for X-ray crystal structure analysis were
obtained by recrystallization from suitable solvents. The X-ray
crystal structure analyses of these single crystals demonstrated
the molecular conformational changes between the neutral and
radical cationic species. The molecular structures are shown in
Figure 7 (see also Figure 8 for discussions). Table S1 and S2 list
the crystallographic data for 2a–c and 2a–c•+•X– (X = GaCl4 or
SbF6), respectively. Table S3 lists the bond lengths and dihedral
angles surrounding the B–N p-electronic systems.
Figure 5. (a) Observed and simulated ESR spectra of
–
2b•+•GaCl4 in dichloromethane (g = 2.0031, n0 = 9.445195
MHz). (b) Top and side views of calculated spin density map of
2b•+. Red and yellow orbitals denoted the positive and negative
spin densities, respectively. The calculations were carried out by
the density functional theory with Gaussian 09 at the
UB3LYP/6-31G** level of theory using the coordinates
extracted from the X-ray crystal structure.
Figure 8. Specific names of atoms and plane for discussion of
the crystal structures (N3 = N2 for 2b).
–
Figure 6. (a) UV/VIS/NIR spectra of 2c•+•SbF6 in
The neutral compound 2a exhibited a pseudo-C2-
dichloromethane prepared from crystalline samples. The solid
and dotted lines showed spectra as just prepared and after 30 min,
symmetrical structure. The PTZ moieties in 2a possessed folded
structures along the N–S axis (butterfly structure).7,13 The folded
angles were found to be 139.6 and 138.2° for PTZ1 and PTZ2,
respectively. The summation of the bond angles around N1, N2,
and N3 were almost 360° (359.9, 359.6, and 359.9°,
respectively), which suggested sp2 hybridization of these atoms.
Interestingly, the B–N1 bond (1.404(3) Å) was shorter than B–
N2 and B–N3 (1.486(3), and 1.486(3) Å, respectively), which
suggested that the B–N1 bond showed a slightly stronger double
bond character. The dihedral angle between the plane defined by
the atoms around B (Plane-B) and those around N1 (Plane-N1)
was noticeably smaller than those of the corresponding dihedral
angles between Plane B and around N2 and N3 (Plane-B/Plane-
N1: 24.6°; Plane-B/Plane-N2: 56.1°; Plane-B/Plane-N3: 59.4°,
respectively), indicating the slight double bond character of the
B–N1 bond.
–
respectively. (b) Observed ESR spectra of 2c•+•SbF6 in
dichloromethane.
The overviews of the molecular structures of 2b and 2c in
the crystalline states were similar to that of 2a (Table S3).
Compounds 2b and 2c exhibited C2- and pseudo-C2-symmetrical
structures, respectively. The B–N1 bonds of both compounds
were shorter than the other B–N bonds of the compounds. The
dihedral angles defined atoms around B and those around N1
were quite small. These short bond lengths and small dihedral
angles indicated that the B–N1 bonds of 2b and 2c had slight
double bond character, based on the same reasoning as in the
case of 2a.
The structure of 2a•+ was quite different from that of the
corresponding neutral state. One of the phenothiazine moieties,
PTZ1, was found to exhibit a planar structure with a folded angle
of 176.4°, whereas the other (PTZ2) exhibited a folded structure
with a folded angle of 131.0°. This observation suggested that (i)
PTZ1 was in the radical cationic state and (ii) the radical cation
was localized on the PTZ1 moiety.13 The localization of the spin
was consistent with the results of the UV/VIS/NIR and ESR
spectra. Importantly, the bond length of B–N2 (1.547(4) Å) was
significantly longer than those of the other B–N bonds (B–N1:
1.393(4) and B–N3: 1.440(3) Å, respectively). The B–N2 bond
in the radical cationic state was longer than that in the neutral
–
Figure 7. ORTEP views of (a) 2a, (b) 2a•+•GaCl4 , (c) 2b, (d)
2b•+•GaCl4 , (e) 2c, and (f) 2c•+•SbF6 (50% probabilities):
Hydrogen atoms and counter ions are omitted for clarity. Carbon,
nitrogen, oxygen, sulphur, and boron atoms are shown as gray,
blue, red, orange, and green ellipsoids. For (c), the molecular
structures of the DHP moieties and the isopropyl groups of 2b
were crystallographically equal because of a C2-symmetric axis
along the B–N1 bond. For (f), one of the structures of 2c•+ is
shown although two kinds of crystallographically independent
units existed.
–
–

state, while the other B–N bonds were shorter. In addition, the
dihedral angle of Plane-B/Plane-N2 (86.0°) was much larger
than those of Plane-B/Plane-N1 (26.1°) and Plane-B/Plane-N3
(25.7°), which indicated that the vacant p-orbital on B was
almost perpendicular to the p-orbital on N2 but approximately
parallel to those on N1 and N3.
Structural deformations similar to those between 2a and
2a•+ were also observed in the case of 2b/2b•+ and 2c/2c•+ (Table
S3). The radical cationic molecules deformed from almost C2-
symmetric structures in their neutral states. The radical cationic
donor moieties had planar structures and were linked by longer
B–N2 bonds compared with lengths of the other B–N bonds of
the structures. The B–N2 bonds were also longer than those in
the neutral states. These observations suggested the localization
of charge and spin on one of the donor moieties (in which the N2
atoms were presented). The analysis of the results of the dihedral
angles related to the atoms around B and around N indicated that
the vacant p-orbitals on B were almost perpendicular to the p-
orbitals on N2 but approximately parallel to those on the other
N.
bonds compared to those in the neutral states and the higher
coplanarities of Plane-B/Plane-N1 and Plane-B/Plane-N3 to the
compensate for electron deficient environments on the B atoms
by effective conjugations with the neutral nitrogen atoms. These
electronic interactions were consistent with the Wiberg bond
indices. The large difference between the first and second
oxidation potentials of the donor moieties that were observed in
the CVs induced by the structural deformations between the
neutral and the radical cationic states. Notably, the Wiberg bond
indices of the B–N1 bonds suggested the significant double bond
character. The effective electron donating natures of N1 atoms
enhanced the stability in the cationic states, making it possible
to observe the two-electron oxidized species in the CVs. The B–
N2 bond length in 2c•+ was longer than the other B–N bonds,
which induced the smallest Wiberg bond index of the B–N bonds
of 2a–c•+. This tendency of weak interactions in the B–N2 bond
was presumed to be one of the reasons for the easy
decomposition of 2c•+.
Generation and detection of the di(radical cation)
species of 2b: The results of the CV for 2b prompted us to
observe di(radical cation) species using 2b. In an argon glove
Table 2. Wiberg bond indices of B–N bonds for 2a–c/2a–c•+ [a]
–
box, two equivalents of TBPA•+•SbF6 were added to the n-
B–N1
1.1026
1.1158
1.0367
1.1015
1.0824
1.1698
B–N2
0.7790
0.6446
0.8105
0.6579
0.7471
0.6389
B–N3
0.7796
0.9488
0.8105
0.9573
0.8462
0.9339
butyronitrile solution of 2b. The solution was transferred to an
ESR tube and its ESR spectrum was measured at 100 K (Figure
9a). Consequently, ESR signals attributed to randomly oriented
triplet species were observed in the outer regions of the central
broad signals for doublet impurities. Unfortunately, the spin-
forbidden signal was not observed in a half-field region due to
the low concentration of the di(radical cation) species. A spectral
simulation yielded the following spin Hamiltonian parameters
2a
2a•+
2b
2b•+
2c
2c•+ [b]
[a] The calculations were carried out by the density functional
theory with Gaussian 09 at the UB3LYP/6-31G** level of theory
using the coordinates extracted from the X-ray crystal structure.
[b] The calculated results of one of the crystallographically
independent molecules.
for the S = 1 state: |D/hc| = 0.0176 cm–1, |E/hc| » 0 cm–1, gxx
=
2.0038, gyy = 2.0038, gzz = 2.0051. The distance between the two
spin-centers was calculated to be ~5.3 Å from the D value using
the point-dipole approximation. This distance is comparable to
that between the average centers of two dihydrophenadine p-
electronic systems for the optimized structure from density
functional theory (Figure 9b,c). The calculation also suggested
that the di(radical cation) species of 2b was found as a triplet in
Neutral
B
Radical cation
N1
the ground state with
a weak intramolecular exchange
N1
interaction (2J/kB_calcd ~ +40 K) (Table S4),14,16 which is
expected from the TMM p-electronic system. In the optimized
structure of 2b2(•+), the p-planes of the dihydrophenazine radical
cation moieties were almost perpendicular to the vacant p-orbital
on B, which was correlated well with the case of the
mono(radical cation)s.
1e- oxidation
N3
B
N3
N2
N2
Scheme 2. Schematic representation of the electronic
interactions between the vacant p-orbital on the B atom and the
p-orbitals on the N atoms.
Wiberg bond indices of B–N bonds: Table 2 shows the
Wiberg bond indices which were calculated by Gaussian 09 on
the basis of the crystal structures.14,15 The indices of the B–N2
bonds in the oxidized states were smaller than those of the
corresponding bonds in the neutral states. In contrast, the indices
of the B–N1 and B–N3 bonds in the oxidized states were larger
than those in the neutral states. In the oxidized states, positive
charges were largely distributed on the N2, which induced the
elongations of the B–N2 bonds and the large dihedral angles of
the Plane-B/Plane-N2, attributed to unfavorable electronic
interactions between the vacant p-orbital of B and the cationic p-
orbital of N2 (Scheme 2). The B atoms in the radical cationic
species became much more electron deficient than those in the
neutral state because of the positively charged radical cation
moiety. These effects led to the shorter lengths of the other B–N
Figure 9. (a) Observed (solid line; in a frozen n-butyronitrile)
and simulated (broken line) ESR spectra. (b) The optimized
structure of 2b2(•+) in the triplet state and (c) spin densities
calculated using Gaussian 09 with UB3LYP/6-31G** level of
theory. Red and yellow orbitals denoted the positive and negative
spin densities, respectively.

4. Conclusion
3.
(a) J. Yoshino, N. Kano, T. Kawashima, Chem. Commun.
2007, 559. (b) A. Wakamiya, T. Taniguchi, S. Yamaguchi,
Angew. Chem. Int. Ed. 2006, 45, 3170. (c) X.-Y. Wang, H.-
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T. Kushida, A. Shuto, M. Yoshio, T. Kato, S. Yamaguchi,
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46, 1723. (b) C.-W. Chiu, F. P. Gabbaï, Angew. Chem. Int.
Ed. 2007, 46, 6878.
S. Suzuki, K. Yoshida, M. Kozaki, K. Okada, Angew.
Chem. Int. Ed. 2013, 52, 2499.
For example, (a) W. T. Borden, Diradicals, ed. by W. T.
Borden, John Wiley & Sons, New York, 1982, 1. (b) P. G.
Wenthold, J. Hu, R. R. Squires, W. C. Lineberger, J. Am.
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A. Meller, U. Seebold, W. Maringgele, M. Noltemeyer,
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We have designed and synthesized three kinds of
diisopropylaminoborane substituted with two electron-donating
p-conjugated moieties, 2a–c, to investigate the detailed
structures and electronic properties of unique B–N bond systems.
The desired neutral compounds were obtained by generating the
corresponding lithiated donor species followed by subsequent
additions of iPr2NBF2. The radical cationic compounds 2a–c•+
were also obtained by chemical oxidations using suitable
oxidants. The neutral and radical cationic species, excluding for
2c•+, had high stabilities under aerated conditions even in the
solution states. The X-ray crystal structure analyses revealed
unique structural deformations resulting from one-electron
oxidations. The neutral compounds had almost C2-symmetrical
structures, whereas those in the radical cationic species were not
C2-symmetrical because the radical cations were localized on
one of the electron donors. In the oxidized states, the nitrogen
atom with radical cationic character induced the elongation of
the B–N•+ bonds and the large dihedral angles of the Plane-
B/Plane-N•+, explained by the unfavorable electronic
interactions between the vacant p-orbital of the B atom and the
cationic p-orbital of the N atom. Finally, we observed triplet ESR
4.
5.
6.
7.
8.
signals attributed to the two-electron oxidized species 2b2(•+)
indicating that it has an isoelectronic structure with the TMM
diradical systems.
,
Acknowledgement
This work was supported by Grant-in-Aid for Scientific
Research (JSPS KAKENHI #Grant Numbers, JP15H00956
(K.O.) JP17K05790 (M.K. and K.O.) and JP26102005 (S.S),
JP17K05783 (S.S)). We thank to Dr. Rika Tanaka for X-ray
crystal structure analyses.
9.
10. M. Kuratsu, S. Suzuki, M. Kozaki, D. Shiomi, K. Sato, T.
Takui, K. Okada, Inorg. Chem. 2007, 46, 10153.
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