On the ambiguity of 1,3,2-benzodiazaboroles as donor/acceptor functionalities in luminescent molecules

Article abstract of DOI:10.1039/c3dt52836g

A series of 1,3-bis(perfluoroaryl)-2-(hetero)aryl-1,3,2-benzodiazaboroles, 1,3-^FAr₂-2-Ar-1,3,2-N₂BC₆H ₄ (Ar = Ph, ^FAr = C₆F₅5; Ar = Ph, ^FAr = 4-C₅

Full text of DOI:10.1039/c3dt52836g

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DOI: 10.1039/C3DT52836G
On the Ambiguity of 1,3,2ꢀBenzodiazaboroles as Donor/Acceptor
Functionalities in Luminescent Molecules
Lothar Weber*[a], Johannes Halama[a], Kenny Hanke[a], Lena Böhling[a], Andreas
Brockhinke*[a], HansꢀGeorg Stammler[a], Beate Neumann[a] and Mark A. Fox[b]
Keywords: Boron / Diazaboroles / Perfluoroaryls / Photophysics
Summary:
A series of 1,3ꢀbis(perfluoroaryl)ꢀ2ꢀ(hetero)arylꢀ1,3,2ꢀbenzodiazaboroles, 1,3ꢀFAr2ꢀ2ꢀArꢀ1,3ꢀ
F
F
F
2ꢀN2BC6H4 (Ar = Ph, Ar = C6F5 5; Ar = Ph, Ar = 4ꢀC5F4N 6; Ar = Ph, Ar = 4ꢀNCC6F4 7;
Ar = 2ꢀC4H3S, FAr = C6F5 8; Ar = 2ꢀC4H3S, FAr = 4ꢀC5F4N 9; Ar = 2ꢀC4H3S, FAr = 4ꢀNCC6F4
10), were synthesised by cyclocondensation of the adducts PhBBr2⋅PPh3 or 2ꢀthienyl
BBr2⋅PPh3 with N,N´ꢀbis(perfluoroaryl)ꢀoꢀphenylenediamines in the presence of 2,2,6,6ꢀ
tetramethylpiperidine. Similar treatments of the PPh3 adducts of 4ꢀ(1´,3´ꢀdiethylꢀ1´,3´,2´ꢀ
benzodiazaborolyl)ꢀphenyldibromoborane with the corresponding diamines gave rise to the
pushꢀpull compounds, C6H4(NEt)2Bꢀ1,4ꢀC6H4ꢀB(NFAr)2C6H4 (FAr = C6F5 11; 4ꢀC5F4N 12)
and C6H4(NEt)2Bꢀ2,5ꢀC4H2SꢀB(NFAr)2C6H4 (FAr = C6F5 13; 4ꢀC5F4N 14). The Xꢀray
structures of 8, 11, 12 and 13 were determined. Electronic structure calculations reveal that
the LUMOs are located at the perfluoroaryl groups in 5ꢀ14 thus the fluorinated
benzodiazaborolyl groups are considered as electronꢀwithdrawing moieties. These moieties
differ from alkylated benzodiazaborolyl groups which are regarded as donors. The emission
spectra for 5ꢀ14 show charge transfer bands with significant solvatochromism and large
Stokes shifts (6100ꢀ12500 cmꢀ1 in cyclohexane and 8900ꢀ15900 cmꢀ1 in CH2Cl2). The
emissions of the benzodiazaboroles, 5ꢀ10, arise from a different charge transfer (CT) process
to the local charge transfer (LCT) process typically found in many fluorescent
benzodiazaboroles. This novel remote charge transfer (RCT) process involving the
perfluoroaryl groups is supported by CAMꢀB3LYP computations. The pushꢀpull systems 11ꢀ
14 here give fluorescent emissions with moderate to high fluorescence quantum yields (65ꢀ
97%) that arise from the usual LCT process only.
[a] Fakultät für Chemie der Universität Bielefeld,
33615 Bielefeld, Germany
Email: lothar.weber@uniꢀbielefeld.de, andreas.brockhinke@uniꢀbielefeld.de
[b] Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, UK
1

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Electronic supplementary information (ESI) available: LippertꢀMataga analyses on 11ꢀ14,
table comparing experimental and computed geometrical parameters for 11ꢀ13, cartesian
coordinates for optimised geometries of 5, 11 and 20 and figures of MOs involved in the
transitions of parent benzodiazaborole and model of the push pull system 22.
Introduction:
The chemistry of fluorescent organoboranes for potential application in electrooptical
devices has attracted considerable interest. The most prominent threeꢀcoordinate boron
substituent in these systems is the dimesitylboryl group (BMes2, Mes = 2,4,6ꢀMe3C6H2)
which functions as a πꢀelectron acceptor due its vacant pꢀorbital at the boron centre.1 These
compounds can display sizable second and thirdꢀorder nonlinear optical (NLO)
properties, 2 wherein the BMes2 acceptor strength is usually somewhere between that
of NO2 and CN. They can also exhibit large twoꢀphoton absorption (TPA) crossꢀ
3
sections and strong twoꢀphoton excited fluorescence (TPEF).
Such electron
deficient compounds have low LUMO energies and have thus been shown to be
efficient electronꢀtransporting and/or emitting layers in organic light emitting diodes
4
(OLEDs).
Compounds with BMes2 groups are often strongly colored and/or
luminescent,5 and thus have potential for use as colorimetric or luminescent sensors
for anions, particularly fluoride ions. 6 Conjugated molecules with boryl side groups
have recently been shown to display very large Stokes shifts and high quantum
yields both in solution and the solid state, properties which were attributed to the lack
7
of close packing caused by the bulky mesityl groups. Besides those with BMes2
groups, optically interesting materials based on threeꢀcoordinate boron compounds
have been reported recently including vinyleneboranes,8 diboraanthracenes,9
borataanthracene
aminoboranes.13
anions,10
boracyclophanes,11
fluoreneboranes12
and
In the past decade, the chemistry of 1,3,2ꢀdiazaboroles rapidly developed14ꢀ16 and
some of these threeꢀcoordinate boron compounds are strongly luminescent.17ꢀ24
Calculations on 2ꢀarylethynylꢀ1,3ꢀdiethylꢀ1,3,2ꢀbenzodiazaboroles disclosed that the
diazaborolyl unit did not function as a πꢀacceptor as anticipated, but instead behaved as a πꢀ
donor substituent.19 It was experimentally confirmed that the πꢀdonor capacity of the
1,3ꢀdiethylꢀ1,3,2ꢀbenzodiazaborolyl group towards the BMes2ꢀacceptor is between those of
2

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methoxyꢀ and dimethylaminoꢀgroups.20 Benzodiazaboroles are also efficient donors when
linked to the carbon atoms of carborane clusters.25,26
In a recent paper, we reported on a series of carbazolylphenylꢀ and carbazolylthienylꢀ
derivatives of benzodiazaboroles, 1ꢀ4, (Chart 1) where both Nꢀatoms were substituted by
electronꢀwithdrawing pentafluorophenyl or tetrafluoropyridyl groups.27 Here, the
benzodiazaboroles served as πꢀacceptors towards the πꢀdonating Nꢀcarbazolyl unit. These
compounds are luminescent with Stokes shifts between 1200 and 5000 cmꢀ1 in cyclohexane
and between 4800 and 6300 cmꢀ1 in dichloromethane solutions. To understand the interaction
of the fluoroarylated diazaborolyl units with phenylꢀ or thienyl groups, the parent derivatives,
5ꢀ10, were targeted. The next logical step in the quest for novel pushꢀpull molecules with
tuneable photophysical properties was the construction of πꢀconducting scaffolds, 11ꢀ14, with
both the novel electronꢀwithdrawing fluoroarylꢀfunctionalised benzodiazaborolyl groups and
the familiar 1,3ꢀdiethylꢀ1,3,2ꢀbenzodiazaborolyl donor, as terminal substituents.
3

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Chart 1. Fluoroaryl benzodiazaboroles 1ꢀ14 discussed in this study.
4

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Results and Discussion
The reported synthetic method leading to the diamine precursors, N,N´ꢀbis(perfluoroaryl)ꢀ
orthoꢀphenylenediamines 15 and 16, was also applied here in the synthesis of the new
diamine 17 from pentafluorobenzonitrile and lithiated orthoꢀphenylenediamine (Scheme 1).27
These diamines were reacted with appropriate aryldibromoboraneꢀtriphenylphosphine adducts
to generate the fluoroarylꢀfunctionalised benzodiazaboroles, 1ꢀ10 here and elsewhere.27
F
CN
F
F
F
F
F
F
N
F
F
F
F
F
F
HN
HN
HN
HN
HN
HN
F
F
F
F
F
F
F
F
F
F
F
F
N
F
CN
15
16
17
PPh3
BBr2
i) BBr3
CH2Cl2
15ꢀ17
SiMe3
5ꢀ7
ii) PPh3
2 TMP
toluene, 100°C
PPh3
BBr2
i) BBr3
CH2Cl2
15ꢀ17
S
S
SiMe3
8ꢀ10
ii) PPh3
2 TMP
toluene, 100°C
Scheme 1. Syntheses of fluorinated benzodiazaboroles 5ꢀ10.
Aryldibromoboranes were produced by the treatment of trimethylsilylaryls with boron
tribromide and combined with an excess of triphenylphosphine in dichloromethane to afford
the corresponding adducts. These mixtures were subsequently freed from solvent in vacuo to
give colourless solid residues. Toluene solutions of the adducts were reacted with
phenylenediamines 15ꢀ17 in the presence of 2,2,6,6ꢀtetramethylpiperidine (TMP) at 100 °C.
5

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Under these conditions, the cyclocondensations were completed in 24ꢀ60 h. After removal of
the piperidinium salts, the filtrates were evaporated to dryness. Sublimation at ca. 100°C was
applied to remove excess PPh3 from 5, 6, 8 and 9. The crude products were crystallised from
nꢀhexane (5, 9), toluene (6) or nꢀhexane/CH2Cl2 mixtures (8) to give colourless crystals in
55ꢀ69% yields. In the case of crude compounds 7 and 10, featuring nitrile substituents,
purification was effected by crystallisation from nꢀhexane (7) or a nꢀhexane/CH2Cl2 2:1
mixture (10) to give yellow crystals in 14 and 36% yield respectively.
In contrast to other nonꢀfluorinated 2ꢀarylꢀ1,3ꢀbis(alkyl/aryl)benzodiazaboroles compounds
5ꢀ10 are not affected by contact with air and moisture, and thus may be stored in air for
months. The pentafluorophenyl derivatives 5 and 8 can be worked up in the presence of water
whereas compounds 6, 7, 9 and 10 are hydrolysed to phenylene diamines. Diazaboroles 5, 6, 8
and 9 are well soluble in the common aprotic organic solvents, and may be recrystallised from
alkanes at low temperatures. Derivatives 7 and 10 are poorly soluble in alkanes.
The reaction of freshly prepared 4ꢀtrimethylsilylphenyllithium with an equimolar amount of
2ꢀbromoꢀ1,3ꢀdiethylꢀ1,3,2ꢀbenzodiazaborole28 in diethyl ether in the temperature range
between ꢀ78 and 20°C led to the formation of 1ꢀdiazaborolylꢀ4ꢀtrimethylsilyl benzene 18 as
colourless needles in 50% yield (Scheme 2).
6

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Et
N
B
Br
N
Et
tBuLi
Et
Et2O
Et2O
N
Br
SiMe3
Li
SiMe3
B
SiMe3
ꢀ78°C to 0°C
N
Et
ꢀ78°C to r.t.
18
BBr3, CH2Cl2
r.t., 16h
PPh3
BBr2
Et
Et
PPh3
N
15, 16
N
B
BBr2
B
11, 12
CH2Cl2
2h
N
N
2 TMP
toluene, 100°C
Et
Et
Et
N
B
Br
N
Et
nBuLi
Et2O
N
Et
S
S
S
SiMe3
Li
B
SiMe3
SiMe3
N
r.t.
Et2O
r.t.
Et
19
BBr3, CH2Cl2
r.t., 1h
PPh3
BBr2
Et
Et
15, 16
N
N
PPh3
S
S
13, 14
B
B
BBr2
N
Et
N
CH2Cl2
2h
2 TMP
toluene, 100°C
Et
Scheme 2. Syntheses of pushꢀpull systems, 11ꢀ14.
The solution of 18 in CH2Cl2 was treated at room temperature with 1 equiv. of boron
tribromide over a period of 16 h and then combined with 1 equiv. of triphenylphosphine. The
obtained adduct was allowed to react with the orthoꢀphenylenediamine derivatives 15 or 16 in
hot toluene (100°C) in the presence of 2 equiv. of 2,2,6,6ꢀtetramethylpiperidine (TMP) to
afford products 11 and 12 as colourless solids in 32% and 23% yield respectively.
Similar protocols were applied to the preparation of the thiopheneꢀ2,5ꢀdiylꢀbridged
benzodiazaboroles 13 and 14 (Scheme 3). Thus, orthoꢀlithiated 2ꢀtrimethylsilylthiophene was
7

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coupled with 1 equiv. of the bromoborole in diethyl ether at 20°C to give 2ꢀthienyldiazaborole
19 as a pale yellow oil after distillation (yield 81%). The reaction of 19 with boron tribromide
was completed after 1 h, and the product was subsequently converted into its
triphenylphosphine adduct. Baseꢀassisted cyclocondensation of the adduct with phenylene
diamines 15 and 16 gave rise to the formation of colourless solids 13 and 14, in 46% and 21%
yield, respectively.
Unlike 5ꢀ10, compounds 11ꢀ14 are air and moisture sensitive. Under an argon atmosphere
they can be stored for months without decomposition. The pentafluorophenyl derivatives, 11
and 13, are well soluble in the common aprotic organic solvents, whereas the
tetrafluoropyridyl functionalised systems 12 and 14 are poorly soluble in alkanes.
To compare with the fluorinated derivative 5, the nonꢀfluorinated analogue 20 was
made in 80% yield using N,N´-diphenylꢀorthoꢀphenylenediamine as the diamine
precursor (Scheme 3).
CaH2
HN
HN
N
N
B
BBr2
+
toluene
20
Scheme 3. Synthesis of 20.
The 11B{1H} NMR spectra of 5ꢀ7 show singlets at δ = 30.4ꢀ30.9 ppm, which in comparison to
derivative 20 (δ = 28.3 ppm) are slightly deshielded reflecting the electronꢀwithdrawing
character of the heterocycle. Similarly, the 11B{1H} NMR spectra of 8ꢀ10 (δ = 27.4ꢀ28.5 ppm)
are downfield shifted with respect to 19 (δ = 26.7 ppm). The 11B{1H} NMR spectra of the 1,4ꢀ
phenyleneꢀbridged products 11 and 12 are characterised by broad singlets at δ = 29.3 and
28.7 ppm, respectively. The 2,5ꢀthiopheneꢀbridged derivatives 13 and 14 show broad
resonances at δ = 26.2 and 26.4 ppm, respectively. Thus, the 11B NMR spectra of the novel
compounds are well comparable with other 2ꢀphenylꢀ or 2ꢀthienylꢀ1,3,2ꢀbenzodiazaboroles
regardless of the substituents at the ring nitrogen atoms.20ꢀ23
8

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Xꢀray crystallography
Single crystals of 8 were grown from a 5:1 mixture of nꢀhexane and CH2Cl2 at 4°C
(Figure 1). The compound crystallised as meroedric twins in the orthorhombic space group
P212121. Due to the poor quality of the crystals and disordering of the thiophene rings, a
detailed discussion of the bonding parameters is meaningless. Here it should be mentioned,
however, that the thiophene ring and the BN heterocycle are close to coplanar (φ = 11.9°),
whereas the C6F5ꢀrings and the benzodiazaborole core enclose interplanar angles of 69.1 and
69.3°. For the related compound 3, the angle φ between the central thiophene ring and the
diazaborole plane was determined to 10.1°, whereas the C6F5 rings and the BN heterocycle
enclose interplanar angles of 65.2 and 87.6°.27
Figure 1. Molecular structure of 8.
Molecular structures were determined for the two phenyleneꢀbridged
bis(benzodiazaboroles) 11 and 12 as well as for the thiopheneꢀ2,5ꢀdiyl derivative 13 (Figure
2). Single crystals of 11 and 13 were grown from nꢀhexane whereas crystallisation of 12 was
achieved from a 10:1 mixture of nꢀhexane and dichloromethane at ꢀ20°C. Derivatives 11 and
9

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13 featuring pentafluorophenyl substituents crystallise in the triclinic space group P1, whereas
the monoclinic space group P21 was determined for the tetrafluoropyridyl derivative 12. The
asymmetric units of 12 and 13 contain two independent molecules with virtually identical
bond lengths and endocyclic bond angles.
10

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Figure 2. Molecular structures of 11 (top), 12 (middle) and 13 (bottom). Hydrogen atoms are
omitted for clarity.
The bond lengths within both benzodiazaborolyl parts of the molecules in 11ꢀ13 are
unexceptional with BꢀN, NꢀCring, and CꢀC bonds of 1.44, 1.40 and 1.40 Å. As expected the
exocyclic bonds N(1)ꢀC(7) and N(2)ꢀC(9) [ca. 1.46 Å] are significantly longer as compared
with bonds N(3)ꢀC(23) and N(4)ꢀC(28) [ca. 1.40 Å] in 11 and 12 or N(3)ꢀC(21) and N(4)ꢀ
C(27) [ca. 1.41 Å] in 13 to the differing hybridisation at the carbon atoms.
Significant differences are found for the CꢀB bond lengths and the mutual orientations
of the three ring systems as well as for the fluoroaryl substituents at the nitrogen atoms in 11ꢀ
13 (Figure 2). These parameters, summarised in Table 1, are of particular interest as a better
overlap between the πꢀorbitals on boron and on the carbon atom attached to boron would
occur if the arene moiety and the diazaborolyl functions are coplanar. The coplanarity
between thiophene and the B/N heterocycle is more favourable (4.9ꢀ45.8°, av. 24.6°) in 13
than for the diazaboroleꢀbenzene combination (41.4ꢀ45.3°, av. 43.3°) in 11 and 12 in the solid
state as found in related systems elsewhere.20,21,23 The BꢀC bond lengths involving the
11

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alkylated heterocycle are longer than those involving the fluoroarylated benzodiazaborolyl
groups which may be attributed to the electronꢀwithdrawing character of the aryl groups as
observed in similar systems.25,26
It is noted that the paraꢀphenylene ring in compound 11 contains two CꢀC bonds at
1.386(1) and 1.388(1) Å where the C atoms are unsubstituted and four CꢀC bonds at 1.401(1)ꢀ
1.404(1) Å indicating a small degree of quinoid character. This degree of quinoid character is
also found in the closely related structure of 11 with ethyl groups in place of fluoroaryl
groups.21 However, in derivative 12 changes in the CꢀC bond lengths in the bridge are not
significant within 3 esds.
12

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Table 1. Selected geometric parameters of 11ꢀ13. Two independent molecules are present in the crystal structures of 12 and 13. Et2BDB = 1,3ꢀ
diethylꢀ1,3,2ꢀbenzodiazaborolyl group, FAr2BDB = 1,3ꢀdifluoroarylꢀ1,3,2ꢀbenzodiazaborolyl group.
11
12A
12B
13A
13B
BꢀC bond length
At Et2BDB group
At FAr2BDB group
Ring orientation
Et2BDB – bridge
1.565(1) Å
1.551(1) Å
1.556(2) Å
1.542(2) Å
1.553(2) Å
1.537(2) Å
1.548(2) Å
1.537(2) Å
1.551(2) Å
1.540(2) Å
N(1)ꢀB(1)ꢀC(11)ꢀC(12)
ꢀ45.3(1)°
N(1)ꢀB(1)ꢀC(11)ꢀC(12)
44.1(2)°
N(7)ꢀB(3)ꢀC(43)ꢀC(44)
ꢀ42.5(2)°
N(1)ꢀB(1)ꢀC(11)ꢀS(1)
26.3(2)°
N(5)ꢀB(3)ꢀC(43)ꢀS(2)
ꢀ45.8(2)°
FAr2BDB – bridge
FArꢀBDB
N(3)ꢀB(2)ꢀC(14)ꢀC(13)
43.5(1)°
N(3)ꢀB(2)ꢀC(14)ꢀC(13)
ꢀ41.4(2)°
N(9)ꢀB(4)ꢀC(46)ꢀC(45)
42.9(2)°
N(3)ꢀB(2)ꢀC(14)ꢀS(1)
21.1(2)°
N(7)ꢀB(4)ꢀC(46)ꢀS(2)
4.9(2)°
B(2)ꢀN(3)ꢀC(23)ꢀC(24)
56.0(1)°
B(2)ꢀN(3)ꢀC(23)ꢀC(24)
60.1(2)°
B(4)ꢀN(9)ꢀC(55)ꢀC(56)
57.6(2)°
B(2)ꢀN(3)ꢀC(21)ꢀC(22)
67.9(2)°
B(4)ꢀN(7)ꢀC(53)ꢀC(54)
58.2(2)°
B(2)ꢀN(4)ꢀC(29)ꢀC(34)
56.1(1)°
B(2)ꢀN(4)ꢀC(28)ꢀC(32)
54.4(2)°
B(4)ꢀN(10)ꢀC(60)ꢀC(64)
57.2(2)°
B(2)ꢀN(4)ꢀC(27)ꢀC(32)
ꢀ69.0(2)°
B(4)ꢀN(8)ꢀC(59)ꢀC(64)
ꢀ60.5(2)°
13

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Photophysics
Photophysical data for compounds 1ꢀ14 and 20 in cyclohexane and CH2Cl2 solutions are
listed in Table 2. Absorption and emission spectra are shown in Figures S1ꢀ3. As no
emissions from solids of 1ꢀ14 under UV radiation were visible to the naked eye, no
solidꢀstate photophysical measurements were carried out. The absorption maxima for
the reported27 compounds 1ꢀ4 containing the carbazolyl groups are at λmax = 340ꢀ344 nm
whereas for compounds 5ꢀ12 and 20 they occur at λmax = 271ꢀ295 nm with little
solvatochromism. While the lowest energy bands in 1ꢀ4 arise from π > π* transitions
involving the πꢀbridge and carbazolyl groups,27 the π > π* transitions for compounds 5ꢀ12 and
20 are likely to involve the benzodiazaborolyl groups. The absorption maxima for the
thienylene ‘pushꢀpull’ systems 13 and 14 are at 322ꢀ330 nm suggesting that the thiophene unit
contributes significantly to the π > π* transitions.
Emission maxima of the new compounds 5ꢀ12 showed very large Stokes shifts of
between 6100 and 15900 cmꢀ1 in contrast to the carbazolyl boroles 1ꢀ4 with values between
1200 and 6100 cmꢀ1. A large Stokes shift indicates that the solvated species goes through a
significant geometry rearrangement under excitation.
Many known organic πꢀsystems containing 1,3ꢀdiethylꢀ and 1,3ꢀdiphenylꢀ1,3,2ꢀ
benzodiazaborolyl groups have Stokes shifts in the region of 5000ꢀ10000 cmꢀ1 such as the
nonꢀfluorinated analogue 20 here at 5200ꢀ6400 cmꢀ1 and high quantum yields.18ꢀ24 Such
emissions from these reported systems arise from the S0 < S1 transitions involving the
π*(organic πꢀsystem) and π(benzodiazaborolyl) orbitals (vide infra). In 20, the π*(organic πꢀ
system) orbital would be at the phenyl group attached to the boron atom (vide infra). These
emissions are considered to be charge transfer (CT) due to evidence of significant
solvatochromism as reflected in the solvatochromic shifts (Table 2). However, the boron atom
of the diazaborolyl unit is involved in both orbitals according to computations (vide infra)
so there is a degree of local excitation (LE) involved which may explain the remarkable
fluorescence properties of benzodiazaborolyl groups in organic πꢀsystems.18ꢀ24
As compounds 5 and 20 differ only in the fluorines instead of hydrogens at the phenyl
groups attached to the nitrogens of the benzodiazaborole moiety, it is logical to compare their
photophysical data. Compound 20 has a Stokes shift of 5200 cmꢀ1 and a quantum yield of
41% whereas compound 5 has the corresponding values of 10600 cmꢀ1 and 2% respectively.
In addition, the emission band for compound 5 has a more significant solvatochromic
shift of 4300 cmꢀ1 on going from cyclohexane to dichloromethane compared to 20
14

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DOI: 10.1039/C3DT52836G
with a solvatochromic shift of 1200 cmꢀ1. Thus, compounds with 1,3ꢀdifluoroarylꢀ1,3,2ꢀ
benzodiazaborolyl groups have more significant excited state geometry rearrangements than
compounds with nonꢀfluorinated benzodiazaborolyl groups suggesting a different charge
transfer (CT) process. This process involves the fluorinated aryl groups and, since the
fluorinated aryl orbitals (vide infra) involved in this process do not have contributions from
the boron atom in the benzodiazaborolyl unit, are considered to be genuine charge transfer.
To distinguish these two CT processes on the basis of the Stokes shifts, the
transition involved in the emission of 20 is denoted local charge transfer (LCT)
whereas the transition in the emission of 5 is remote charge transfer (RCT). If this
assumption is applied to compounds 6ꢀ10 then all emissions with large Stokes shifts
in cyclohexane or dichloromethane are RCT except for emissions with smaller
Stokes shifts of compounds 8 and 9 in cyclohexane which are LCT.(Table 2) These
CT assignments are supported by the larger solvatochromic shifts found for RCT.
The emission spectrum in dichloromethane for compound 8 contains two distinct
bands at 362 nm and 503 nm which are assigned LCT and RCT respectively. The
dual emission25,26,29 in 8 has lifetimes of ca 0.2 ns at 362 nm and 1.05 ns at 503 nm
which shows that two independent excited states are formed.
The fluorescence quantum yields, ΦF, for the compounds 1ꢀ14 clearly depend on the
nature of the substituent at the boron atom of the fluorinated benzodiazaborole. Low ΦF
values are observed with phenyl or thienyl groups (5ꢀ10 with ΦF up to 8%) and generally high
ΦF values with the 2ꢀ(1,3ꢀdiethylꢀ1,3,2ꢀbenzodiazaborolyl)aryl groups (11ꢀ14 with ΦF up to
97%).
The quantum yields differ considerably in the case of compounds with the
tetrafluoropyridyl groups (2, 4, 12, 14) where the values are lower in dichloromethane
than in cyclohexane. Nonꢀradiative processes are thus dominant in more polar
solvents where the excited states involve the tetrafluoropyridyl groups. By contrast,
the effect of solvent on the quantum yields is not obvious for compounds containing
the pentafluorophenyl and cyanotetrafluorophenyl groups.
LippertꢀMataga analyses carried out on 11ꢀ14 with four different solvents reveal that
the transition dipole moments ꢁꢂ are 15.8 D for 11, 23.0 D for 13 and 18.3 D for 14 which
support the charge transfer nature of their emissions.(Figures S4ꢀS7, Tables S2ꢀS5) Values
of 14.8 – 19.7 D have been determined for similar benzodiazaboroles with LCT emissions.22
The ꢁꢂ value for 12 could not be determined as there is no linear fit in the LippertꢀMataga
15

Dalton Transactions
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DOI: 10.1039/C3DT52836G
plot for 12. It is possible that either LCT or RCT emission takes place in different solvents for
12.
The pushꢀpull systems, 21 and 22 (Chart 2),20 which differ from 11ꢀ14 by virtue of
the electronꢀwithdrawing BMes2 group instead of the fluoroaryl diazaborolyl group, have
similar emission data to 11ꢀ14. In cyclohexane solutions, the Stokes shifts are 8800 cmꢀ1 and
6070 cmꢀ1 and the quantum yields are 90% and 99% for 11 and 21 respectively. Similarly, the
thiophene analogues have values of 6300 cmꢀ1 and 6210 cmꢀ1 and 59% and 81% for 13 and 22
respectively. It may be concluded here that the emissions of the pushꢀpull systems 11ꢀ14 are
LCT like for many other reported organic πꢀsystems with benzodiazaborolyl groups.18ꢀ24
Chart 2. Pushꢀpull systems, 21 and 22.
16

Page 17 of 42
Dalton Transactions
Table 2. Photophysical data for the benzodiazaboroles 2ꢀArꢀ1,3ꢀAr2ꢀBN2C6H4, 1ꢀ14 and 20. Cz = 4,7ꢀtBu2ꢀcarbazolyl group; BDB = 1,3ꢀEt2ꢀ
benzodiazaborolyl group.
~
~
2ꢀAr
1/3ꢀAr
Solvent
Stokes shift
Solvatochromic shift
[cmꢀ1]
λmax,Abs
λmax,Em
Φfl
ε
ν
ν
max,Em
max,Abs
[nm]
[Lmolꢀ1cmꢀ1]
[nm]
[%]
[cmꢀ1]
29100
29100
29100
29100
29400
29400
29400
29300
35300
35300
36400
36900
34800
35600
34400
34200
[cmꢀ1]
27900
24600
26200
23800
25400
24300
24400
23200
24270
20500
23900
21000
24000
21200
28300
27600
19900
27400
21600
24500
20700
25300
23400
24500
22400
24800
22400
23900
21600
28700
27500
[cmꢀ1]
1200
4500
2900
5300
4000
5100
5000
6100
10600
14900
12500
15900
10800
14400
6100
6600
14300
8100
14100
10100
13800
8800
11300
10500
12200
6300
CzC6H4
CzC6H4
CzC4H2S
CzC4H2S
C6H5
C6F5
4ꢀC5F4N
C6F5
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
344
344
344
344
340
340
340
341
283
283
275
271
287
281
291
292
17700
10000
11600
10400
15600
12000
11600
15900
11350
6960
18420
20400
7800
18370
10430
16340
358
407
377
414
394
411
410
431
405
489
419
477
417
472
353
362
503
363
464
408
483
395
428
408
447
404
446
418
464
349
364
79
26
47
1
2
3
4
5
6
7
8
3300
2400
1100
1100
4300
3400
3600
14
18
16
2
3
<1
1
<1
3
5
8
<1
4ꢀC5F4N
C6F5
C6H5
4ꢀC5F4N
4ꢀNCC6F4
C6F5
C6H5
C4H3S
500
8200
C4H3S
C4H3S
4ꢀC5F4N
4ꢀNCC6F4
C6F5
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
cꢀC6H12
CH2Cl2
281
280
289
290
293
295
286
289
322
320
330
325
295
295
35600
35700
34600
34500
34100
33900
35000
34600
31100
31300
30300
30800
33900
33900
12260
2130
7110
1
<1
1
9
6000
3700
2500
1700
2600
2800
1200
10
11
12
13
14
20
24110
15080
22150
26040
21930
18780
21710
10030
9990
2
BDBC6H4
BDBC6H4
BDBC4H2S
BDBC4H2S
C6H5
90
97
95
2
59
65
78
46
41
38
4ꢀC5F4N
C6F5
8900
6400
9200
5200
4ꢀC5F4N
C6H5
6670
6440
6400
17

Dalton Transactions
Page 18 of 42
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DOI: 10.1039/C3DT52836G
Computations
Ground state geometry optimisations of
level of theory. A very good agreement was found between the optimised S0 geometries and
the Xꢀray data of 11 13 Table S6). However, the relative orientations between the
5ꢀ14 and 20 were computed at the B3LYP/6ꢀ31G*
ꢀ
(
benzodiazaborolyl plane and the planes of the bridge and fluoroaryl groups are
somewhat more twisted in the computed geometries than in the experimental
geometries. It is not possible to rotate the fluoroaryl groups attached to the nitrogen atoms in
the benzodiazaboroles about the NꢀC bond. Geometries where both the
benzodiazaborole unit and one of the fluoroaryl groups are coplanar could not be
located due to the obvious steric barriers between these moieties. The sterics can be
visualised by adjusting the fluoroaryl group containing F1 for 8 in Figure 1 to be
coplanar with the benzodiazaborole unit resulting in the F1 atom occupying the space
where the H6 atom is. However, the aryl or thienyl group attached to the boron atom can be
rotated about the BꢀC bond with estimated energy barriers of only 3.8 and 1.1 kcal molꢀ1 for
5
and , respectively.
8
Molecular Orbitals
The frontier orbitals in the parent benzodiazaborole are the π and π* orbitals on the 1,3,2ꢀ
N2BC6H4 system (Figure S8). When a π–organic substituent is attached to boron, the HOMO
is generally on the benzodiazaborolyl group whereas the LUMO is typically on the π–system
attached to boron with some boron atom character. A π–organic substituent at nitrogen would
also result in a HOMO on the benzodiazaborolyl group and in a LUMO on the substituent
itself. Thus, if two π–organic substituents are attached to N and B like the systems discussed
in this study, the LUMO may be located at either the substituent at N or at B, whereas the
HOMO would be located on the benzodiazaborolyl moiety.
For the nonꢀfluorinated system, 1,2,3ꢀPh3ꢀ1,3,2ꢀN2BC6H4 20, the LUMO is at the phenyl
group attached to B. For the fluorinated system, 1,3ꢀ(C6F5)2ꢀ2ꢀPhꢀ1,3,2ꢀN2BC6H4
5
, however,
10
the LUMO resides at the C6F5 group (Figure 7). The same picture exists for compounds
6
ꢀ
where the LUMOs are also located on the aryl groups attached to the nitrogen atoms of the
diazaborole. The molecules with a thiophene unit have considerable contributions from the
thiophene in both frontier orbitals. Each pushꢀpull system (11
the ꢀdiethyl benzodiazaborolyl group and the LUMO at the aryl groups in the
benzodiazaborolyl group.
ꢀ
14) has the HOMO located at
N,
N
N,N
ꢀdiaryl
18

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Dalton Transactions
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DOI: 10.1039/C3DT52836G
Figure 7. Molecular orbitals in the optimised ground state (S0) geometries for 1,2,3ꢀPh3ꢀ
1,3,2ꢀN2BC6H4 20 (left column) and 1,3ꢀ(C6F5)2ꢀ2ꢀPhꢀ1,3,2ꢀN2BC6H4 (right column). The
5
MO contribution ratio of the three groups, Ph (at B) : N2BC6H4 : Ar (at N), are given.
19

Dalton Transactions
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TDꢀDFT computations
The hybrid functional CAMꢀB3LYP is used to compute charge transfer transitions as it has
the necessary physics to model charge contributions more correctly than the widely used
B3LYP functional.20 The data summarised in Table 3 show very good agreement between the
observed absorption maxima of the strong bands and the computed maxima with predicted
strong bands. However, bands with lower oscillator strengths are predicted at lower
energies for 6, 7, 9 and 10. These weak bands are from transitions from the
fluorinated rings to the benzodiazaborole unit (i.e. RCT). While the predicted weak
bands for the tetrafluoropyridyl compounds 6 and 9 are not observed due to the
strong bands, the weak bands for the cyano compounds 7 and 10 are present as
shoulder bands. Based on Gaussian deconvolution analyses, these bands are 314
nm (7) and 318 nm (10) in cyclohexane and 316 nm (7) and 322 nm (10) in
dichloromethane. This has implications in determining the Stokes shifts – a Stokes shift is
generally regarded as the energy difference between the lowest energy absorption band (S0 >
S1) and the highest energy emission band (S0 < S1). Nevertheless, using the computed S0 > S1
absorption data and observed emission data, the Stokes shifts remain large with values
between 4300 and 10600 cmꢀ1.
All the strong absorption bands for
5ꢀ14 and 20 are assigned as arising from πꢀ
benzodiazaborolyl > π*ꢀBꢀaryl transitions. As described earlier, they are defined as local
charge transfers (LCTs), these are not strictly local π > π* or charge transfer (CT) transitions
as both orbitals contain some boron atom character. Compounds with thiophene units have
considerable thiophene character in both frontier orbitals and therefore have thiophene
character in the important transitions.20 Transitions at lower energies are predicted for
compounds 6, 7, 9 and 10 on going from the benzodiazaborolyl HOMO to the fluorinated aryl
group LUMO and defined as remote charge transfers (RCTs).
20

Page 21 of 42
Dalton Transactions
Table 3. Comparison between observed and computed absorption data for the benzodiazaboroles, 5ꢀ14 and 20.
Osc.
Str.c
Calc.
S0 > S1
Abs.
Osc.
Str.
Calc.
Observed
Calculated
λmax
,
λmax,
Transition
Type
Transition
Type
Stokes shift Stokes shifte
Abs.
Obs.a
[nm]
283
Abs.
Calc.b
[nm]
283
Calc.d
[nm]
[cmꢀ1]
10600
12500
10800
6100
λ
max [cmꢀ1]
10600
11500
11600
5200
0.2206
0.1934
0.1778
0.2941
0.2730
0.2985
0.6368
0.4535
0.7578
0.6116
HOMO > LUMO+2
πꢀborolyl > π*ꢀBPh
HOMO > LUMO+2
πꢀborolyl > π*ꢀBPh
mixed
5
275
287
291
281
289
293
286
322
330
283
281
298
290
293
306
308
335
341
307
333
0.0236
0.0369
HOMO > LUMO
πꢀborolyl > π*ꢀ FAr
HOMO > LUMO
πꢀborolyl > π*ꢀ FAr
6
7
HOMO > LUMO
πꢀborolyl /Th > π*ꢀBTh/FAr
HOMO > LUMO+2
8
314
337
0.0293
0.0027
HOMO > LUMO
πꢀborolyl/Th > π*ꢀ FAr
HOMO > LUMO
8100
6900
9
πꢀborolyl /Th > π*ꢀBTh/FAr
HOMO > LUMO+2
10100
8800
9600
10
11
12
13
14
πꢀborolyl /Th > π*ꢀBTh/FAr
HOMO > LUMO+1
πꢀborolyl > π*ꢀ FAr
7400
πꢀEt2borolyl > π*ꢀBC6H4B
HOMO > LUMO+2
πꢀEt2borolyl > π*ꢀBC6H4B
HOMO > LUMO
πꢀEt2borolyl/Th > π*ꢀBThB
HOMO > LUMO,
10500
6300
8000
5100
6400
5400
HOMO > LUMO+2
πꢀEt2borolyl/Th > π*ꢀBThB/FAr
HOMO > LUMO
295
295
0.2667
5200
5200
20
πꢀborolyl > π*ꢀBPh
a in cyclohexane
b maxima with highest oscillator strength.
c oscillator strength.
d for calculated S0 > S1 transitions that are not also maxima with highest oscillator strengths.
e energy difference between computed absorption maxima and observed emission data.
21

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Excited state calculations
The parent benzodiazaborole where only local π > π* and π < π* transitions within the fused
heterocycle are possible has a computed Stokes shift of 2000 cmꢀ1 based on calculated
transitions from its S0 and S1 optimised geometries (Figure S8). This value agrees well with
the observed Stokes shift of ca 2000 cmꢀ1 for the related alkyl derivative, 2ꢀ
N2BC6H4. 24,26 For the benzodiazaboroles
14 and 20 here, the Stokes shifts are in the region
tBuꢀ1,3ꢀEt2ꢀ1,3,2ꢀ
5
ꢀ
of 5200ꢀ13400 cmꢀ1 as listed in Table 3. As the observed Stokes shifts are large, the S1
excited state geometries must be significantly different to the S0 ground state geometries.
Two distinct excited state optimised geometries were located for 5, 8 and 9 and are denoted
LCT and RCT. The LCT geometry involves changes within the diazaborolyl and Bꢀaryl
groups as reported elsewhere,20 whereas the RCT geometry involves a highly distorted aryl
group at N as shown in Figure 8 for 5. The Stokes shifts based on TDꢀDFT data from S0 and
S1(RCT) are generally larger than those from S0 and LCT as shown in Table 4.
Figure 8. Molecular orbitals involved in the emissions for the two distinct excited state
geometries of
5,
LCT and RCT with calculated values
22

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From Table 4, a comparison between the observed emission data in cyclohexane solutions and
the computed emission data indicates that RCT emissions take place for compounds
and 10 and LCT emissions take place for the other systems. RCT emissions are likely to take
place for compounds and in the more polar dichloromethane solutions as solvatochromic
shifts of 8200 and 6000 cmꢀ1 on going from cyclohexane to CH2Cl2 are too large for LCT
emissions to take place in both solvents. Two emission bands are observed in CH2Cl2 for
5, 6, 7
8
9
8
which may be assigned as LCT and RCT emissions for the high and low energy bands
respectively.
,
For compounds 11ꢀ14, the calculated and observed emission maxima based on LCT are in
good agreement (Table 4). The orbitals involved in the LCT emission for 11 are shown in
Figure 9. They closely resemble the orbitals involved in the LCT emission of the related pushꢀ
pull system, 21 (Figure S9) reported previously.20
Figure 9. Molecular orbitals involved in the emission for the excited state LCT geometry
of 11
.
Emission and Stokes shift values are calculated.
23

Dalton Transactions
Page 24 of 42
Table 4. Comparison between observed and computed emission data for the benzodiazaboroles, 5ꢀ14 and 20. The optimised LCT and RCT excited
state geometries are used to compute the emission data.
Stokes shifta
(LCT)
S0 < S1
Transition
Typeb
Stokes shifta
(RCT)
S0 < S1
Transition
Typeb
λ
max, Em
Obs,
λ
max, Em
λmax, Em
(RCT)
Calc
Emission
Assignment,
cyclohexane
(LCT)
Calc
Calc
Calc
cyclohexane
[nm]
[cmꢀ1]
[cmꢀ1]
[nm]
[nm]
405
419
417
353
363
408
395
408
404
418
349
345
ꢀc
6100
πꢀborolyl < π*ꢀBPh
461
486
484
473
505
511
ꢀd
13400
12000
9300
12500
12000
10100
πꢀborolyl < π*ꢀ FAr
πꢀborolyl < π*ꢀ FAr
RCT
RCT
RCT
LCT
LCT
RCT
LCT
LCT
LCT
LCT
LCT
5
6
7
8
ꢀc
πꢀborolyl < π*ꢀ FAr
395
406
ꢀc
395
418
436
448
372
8300
7200
πꢀborolyl/Th < π*ꢀBTh
πꢀborolyl/Th < π*ꢀBTh
πꢀborolyl/Th < π*ꢀ FAr
πꢀborolyl/Th < π*ꢀ FAr
πꢀborolyl/Th < π*ꢀ FAr
9
10
11
12
13
14
20
7400
8600
7000
7000
7000
πꢀborolyl < π*ꢀBC6H4B
πꢀborolyl < π*ꢀBC6H4B
πꢀborolyl/Th < π*ꢀBThB
πꢀborolyl/Th < π*ꢀBThB
πꢀborolyl < π*ꢀBPh
487
11900
πꢀborolyl/Th < π*ꢀ FAr
ꢀd
ꢀd
ꢀd
a Energy difference between computed absorption maxima (Table 3) and computed emission data.
b From molecular orbital calculations on the optimised excited state geometries where the LUMO is assumed to be the highest occupied spin orbital
and the HOMO is assumed as the second highest occupied spin orbital.
c Optimised LCT geometry could not be located.
d Optimised RCT geometry could not be located.
24

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DOI: 10.1039/C3DT52836G
Conclusions
Ten new
compounds,
5
ꢀ14
,
containing
the
electronꢀwithdrawing
bis(perfluoroaryl)benzodiazaborolyl groups were synthesised to examine the influence of the
fluorinated rings on the photophysical properties of benzodiazaboroles. The
benzodiazaboroles 5ꢀ10 exhibit weak fluorescence emissions with pronounced Stokes shifts
up to 15900 cmꢀ1 which arise from charge transfer of the excited states involving the
perfluoroaryl groups. This unusual CT process is supported by excited state geometries
located computationally and differs from the emission process involving many highly
fluorescent organic systems with benzodiazaborolyl groups. The pushꢀpull systems 11ꢀ14,
containing fluorinated benzodiazaborolyl groups as acceptors and alkylated
benzodiazaborolyl groups as donors, did not result in pure charge transfer emissions between
the two groups. The photophysical and computational data for these pushꢀpull systems
resemble other pushꢀpull benzodiazaboroles where the BMes2 group is used as the acceptor.
Thus, the πꢀdonor or πꢀacceptor character of a benzodiazaborole unit may easily be tailored
by the choice of the substituents at the nitrogens of the heterocycle.
25

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Experimental Section
All manipulations were performed under an atmosphere of dry, oxygenꢀfree argon by using
Schlenk techniques. All solvents were dried by standard methods and freshly distilled prior to
use. The compounds 1,2ꢀ(HNꢀ4´ꢀpyridyl)2C6H4,27 1,2ꢀ(HNPh)2C6H4,27 1,2ꢀ(HNC6F5)2C6H4,30
1ꢀBrꢀ4ꢀMe3SiꢀC6H4,31 2ꢀMe3SiꢀC4H3S32 and 2ꢀbromoꢀ1,3ꢀdiethylꢀ1,3,2ꢀbenzodiazaborole28
were prepared according to literature methods. Boron tribromide, 2,2,6,6ꢀ
tetramethylpiperidine, triphenylphosphine,
commercially.
n
ꢀ
and tertꢀbutyllithium were purchased
NMR spectra were recorded at room temperature in C6D6 or CDCl3 on a Bruker Avance III
300 (1H: 300, 11B: 96, 13C: 75, 19F: 282 MHz) or a Bruker Avance III 500 spectrometer (1H:
500, 11B: 160, 13C: 125, 19F: 470, 31P: 202 MHz) with SiMe4 (1H, 13C), BF3ꢃOEt2 (11B), CFCl3
(19F) and 85% H3PO4 (31P) as external standards. Some expected 13C peaks
corresponding to the boronꢀbound carbon atoms were not detected above the noise
F
level while some 13C peaks corresponding to carbon atoms of the Ar groups were
obscured by other peaks in the same region. Mass spectra were obtained with a VG
autospec sector field mass spectrometer (Micromass). In some cases, the Cꢀmass
fractions in the elemental analyses were too low, which may be rationalized by
incomplete combustion of boron carbides formed in this process.
2ꢀPhenylꢀ1,3ꢀbis(pentafluorophenyl)ꢀ1,3,2ꢀbenzodiazaborole (5)
A sample of boron tribromide (0.526 g, 2.1 mmol) was added at 20°C to a solution of
phenyltrimethylsilane (0.330 g, 2.2 mmol) in dichloromethane (30 mL). The mixture was
stirred for 16 h before a solution of triphenylphosphine (0.655 g, 2.5 mmol) in
dichloromethane (10 mL) was added. After 2 h of stirring at room temperature volatiles were
removed in vacuo, and NMR spectra were taken from the colourless solid residue
(
δ11B (C6D6) = ꢀ4.7
(s),
δ31P (C6D6) = ꢀ3.9
(s)
ppm).
This
adduct
and
N,N´ꢀbis(pentafluorophenyl)phenylenediamine 15 (0.924 g, 2.1 mmol) were dissolved in
toluene (70 mL) and 2,2,6,6ꢀtetramethylpiperidine was added (0.636 g, 4.5 mmol). The
mixture was heated at 100°C for 24 h and then cooled to 20°C. It was filtered and the filtrate
was evaporated to dryness. The residue was freed from triphenylphosphine by sublimation in
vacuo at 90ꢀ100°C. The residue was crystallised from
55%) of
nꢀhexane to yield 0.640 g (1.16 mmol,
5.
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1H NMR (C6D6): δ= 6.66 (m, 2H, CH=CHCH=CH), 7.00 (m, 3H,
m, pꢀPhꢀH), 7.05 (m, 2H,
CH=CHCH=CH), 7.34 (m, 2H,
o
ꢀPhꢀH) ppm. 13C{1H} NMR (C6D6): δ=110.8 (s,
CH=CHCH=CH), 122.3 (s, CH=CHCH=CH), 128.5, 130.5, 132.4 (3s, BCꢀCHꢀCHꢀCH),
136.1 (s, N2C2), 138.0 (dm, 1JC,F=255 Hz, C6F5), 140.6 (dm, 1JC,F=255 Hz, C6F5), 144.2 (dm,
1JC,F=255 Hz, C6F5) ppm. 11B{1H} NMR (C6D6): δ= 30.7 (s) ppm. 19F{1H} NMR (C6D6): δ= ꢀ
161.3 (m, 4F,
m pꢀF), ꢀ146.1 (m, 4F, oꢀF) ppm. MS/EI (m/z):
ꢀF), ꢀ154.3 (t, 1JF,F=22 Hz, 2F,
526.1 (100%) [M+]. C24H9BF10N2 (526.14): calcd.: C: 54.79, H: 1.72, N: 5.32; found: C:
54.62, H: 1.85, N: 5.23.
2ꢀPhenylꢀ1,3ꢀbis(2´,3´,5´,6´ꢀtetrafluoropyridyl)ꢀ1,3,2ꢀbenzodiazaborole (
6)
According to the protocol for 2.1 mmol of adduct PhBBr2 PPh3 was reacted with an
5
⋅
equimolar amount of N,N´ꢀbis(tetrafluoropyridyl)ꢀorthoꢀphenylenediamine 16 (0.853 g) and
4.5 mmol (0.636 g) of 2,2,6,6ꢀtetramethylpiperidine in toluene at 100°C for 24 h. After
filtration and evaporation of the filtrate to dryness triphenylphosphine was sublimed off at
90°C in vacuo. The residue was crystallised from toluene at ꢀ20°C to give product
6 (0.710 g,
69%).
1H NMR (C6D6): δ= 6.52 (m, 2H, CH=CHCH=CH), 6.90 (t, 2H, 3JH,H=7.1 Hz,
(m, 3H, CH=CHCH=CH and
ꢀPhꢀH), 7.10 (d, 2H, 3JH,H=7.0 Hz, ꢀPhꢀH) ppm. 13C{1H}
mꢀPhꢀH), 6.99
p
o
NMR (C6D6): δ= 111.2 (s, CH=CHCH=CH), 122.7 (s, CH=CHCH=CH), 128.5, 130.7, 131.9
(3s, BCꢀCHꢀCHꢀCH), 134.4 (s, N2C2), 138.6 (dm, 1JC,F=263 Hz, C5F4N), 144.0 (dm,
1JC,F=245 Hz, C5F4N) ppm. 11B{1H} NMR (C6D6): δ= 30.4 (s) ppm. 19F{1H} NMR (C6D6):
δ= ꢀ145.31 (m, 4F,
(10%) [M+ –HF]. HRMSꢀEI (C22H9BF8N4): calcd.: 492.07870; found: 492.07953.
oꢀF), ꢀ88.01 (m, 4F, m
ꢀF) ppm. MS/EI (m/z): 492.0 (100%) [M+], 472.0
N,N´ꢀorthoꢀBis(2,3,5,6ꢀtetrafluoroꢀ4ꢀcyanophenyl)ꢀphenylenediamine (17
)
A chilled solution (ꢀ78°C) of ꢀbutyllithium in ꢀhexane (116 mL, 186 mmol) was combined
n
n
portionwise with hexamethyldisilazane (30.0 g, 186 mmol). The mixture was transferred into
chilled (ꢀ78°C) dropping funnel and then added during 60ꢀ70 min to a cold (ꢀ78°C) solution
of orthoꢀphenylenediamine (5.0 g, 46.0 mmol) in THF (150 mL). The dark violet reaction
mixture was warmed to 0°C, and then reꢀcooled to ꢀ78°C before a sample of pentafluoroꢀ
benzonitrile (17.8 mL, 92.2 mmol) was added. The mixture was warmed to ambient
temperature within 16 h, and subsequently combined with water (80 mL) and THF (50 mL).
The organic phase was separated and washed with brine (2x50 mL). The combined aqueous
phases were extracted with THF (3x50 mL). Then the combined THF phases were dried with
27

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DOI: 10.1039/C3DT52836G
Na2SO4 and filtered. The filtrate was evaporated to dryness. The brown solid reaction residue
was suspended in toluene (250 mL). The obtained solid which contained the product was
filtered off and dried. This extraction procedure was repeated three times. Purification of the
crude product was effected by crystallisation from dichloromethane (350 mL). N,N´ꢀortho
ꢀ
Bis(2,3,5,6ꢀtetrafluoroꢀ4ꢀcyanophenyl)ꢀphenylenediamine 17 was obtained as pale pink
crystals (12.5 g, 27.5 mmol, 60%).1H NMR (CDCl3): δ= 6.08 (s, br, 2H, NH), 7.05 (m, 2H,
CH=CHCH=CH), 7.23 (m, 2H, CH=CHCH=CH) ppm. 13Cꢀ{1H}ꢀNMR (CD2Cl2): δ= 83.2 (m,
C5F4CCN), 108.7 (s, C6F4CN) 122.4 (s, CH=CHCH=CH), 126.4 (s, CH=CHCH=CH), 132.9
1
1
(s, N2C2), 138.6 (dm, JC,F = 246Hz, C6F4CN), 148.5 (dm, JC,F = 261Hz, C6F4CN) ppm.
19F{1H}ꢀNMR (CDCl3): δ= ꢀ150.6 (m, 4F,
ꢀF), ꢀ133.1 (m, 4F, ꢀF) ppm. MS/EI (m/z):
o
m
454.0 (100 %) [M+], 434.0 (52%) [M+ ꢀHF].C20H6F8N4 (454.28): calcd.: C: 52.88, H: 1.33, N:
12.33; found: C: 52.91, H: 2.70, N: 12.31.
2ꢀPhenylꢀ1,3ꢀbis(2´,3´,5´,6´ꢀtetrafluoroꢀ4´ꢀcyanophenyl)ꢀ1,3,2ꢀbenzodiazaborole (
7)
Analogously to the synthesis of 5 the adduct of PhBBr2 PPh3 (2.1 mmol) was reacted with
⋅
2.1 mmol (0.954 g) of N,N´ꢀorthoꢀbis(2,3,5,6ꢀtetrafluoroꢀ4ꢀcyanophenyl)ꢀphenylenediamine
17 and 2,2,6,6ꢀtetramethylꢀpiperidine (0.636 g, 4.5 mmol) in toluene at 100°C for 60 h. The
solid residue from the filtrated reaction mixture was dissolved in a mixture of CH2Cl2
(10 mL) and n-hexane (25 mL) and stored at ꢀ20°C, whereby PPh3 and unreacted
ortho-phenylenediamine precipitated. This mixture was filtered and the filtrate was
freed from solvent. The residue was dissolved in nꢀhexane and stored at ꢀ20°C for a few
1
days. By this large yellow orange crystals of
7
(162 mg, 0.30 mmol, 14%) were obtained. H
NMR (C6D6): δ= 6.63 (m, 2H, CH=CHCH=CH), 6.99 (m, 3H, m,pꢀPhꢀH), 7.03 (m, 2H,
CH=CHCH=CH), 7.22 (m, 2H,
ꢀPhꢀH) ppm. 13Cꢀ{1H} NMR (C6D6): δ=111.0 (s,
o
CH=CHCH=CH), 122.7 (s, CH=CHCH=CH), 128.5, 130.7, 131.9, (3s, BCꢀCHꢀCHꢀCH),
135.0 (s, N2C2), 143.2 (dm, 1JC,F=260 Hz, C6F4CN), 147.1 (dm, 1JC,F=264 Hz, C6F4CN) ppm.
11Bꢀ{1H} NMR (C6D6): δ= 30.9 (s) ppm. 19Fꢀ{1H} NMR ( C6D6): δ= ꢀ142.7 (m, 4F,
131.7 (m, 4F,
ꢀF) ppm. MS/EI (m/z): 540.1 (100%) [M+], 520.1 (5%) [M+ ꢀHF]. HRMSꢀEI
oꢀF), ꢀ
m
(C26H9BF8N4): calcd.: 540.07870; found: 540.07957.
2ꢀThienylꢀ1,3ꢀbis(pentafluorophenyl)ꢀ1,3,2ꢀbenzodiazaborole (8)
Following the same protocol, 2ꢀtrimethylsilylthiophene (0.343 g, 2.2 mmol) was reacted with
0.526 g (2.1 mmol) of BBr3 in 30 mL of CH2Cl2 for 2 h. After the addition of PPh3 (0.655 g,
2.5 mmol) the adduct was isolated from the solution (δ11B (C6D6) = ꢀ7.1 (s), δ31P (C6D6) = ꢀ
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4.5 (s) ppm) and dissolved in toluene (70 mL) together with N,N´ꢀbis(pentafluorophenyl)ꢀ
orthoꢀphenylenediamine 15 (0.924 g, 2.1 mmol) and 0.636 g (4.5 mmol) of 2,2,6,6ꢀ
tetramethylꢀpiperidine. The mixture was heated at 100°C for 16 h, cooled to room
temperature and filtered. After evaporation of the filtrate, the dry residue was first sublimed in
vacuo at 90ꢀ100°C to remove the phosphine and crystallised from an
nꢀhexane/CH2Cl2
mixture (25/5 mL) at 4°C to afford compound (0.750 g, 1.41 mmol, 67%) as colourless
8
crystals. 1H NMR (C6D6): δ= 6.75 (m, 2H, CH=CHCH=CH), 6.81 (t, 3JH,H =4.1 Hz, 1H, 4ꢀHꢀ
thiophene), 7.09 (d, 3JH,H=4.0 Hz, 1H, 3ꢀHꢀthiophene), 7.15 (m, 2H, CH=CHCH=CH), 7.64
(d, 3JH,H=4.1 Hz ,1H, 5ꢀHꢀthiophene) ppm. 13C{1H} NMR (C6D6): δ= 110.4 (s,
CH=CHCH=CH), 122.1 (s, CH=CHCH=CH), 128.5, 131.9, 134.8 (3s, BCCHCHCHS), 136.1
(s, N2C2), 138.0 (dm, 1JC,F=252 Hz, C6F5), 140.8 (dm, 1JC,F=255 Hz, C6F5), 144.5 (dm,
1JC,F=252 Hz, C6F5) ppm. 11B{1H} NMR (C6D6): δ= 27.8 (s) ppm. 19F{1H} NMR (C6D6): δ= ꢀ
160.9 (m, 4F,
m pꢀF), ꢀ145.2 (m, 4F, oꢀF) ppm. MS/EI (m/z):
ꢀF), ꢀ153.8 (t, 1JF,F=22 Hz, 2F,
531.9 (100%) [M+], 511.9 (6%) [M+ ꢀHF]. C22H7BF10N2S (532.16): calcd.: C: 49.65, H: 1.33,
N: 5.26; found: C: 48.88, H: 1.13, N: 5.15.
2ꢀThienylꢀ1,3ꢀbis(2´,3´,5´,6´ꢀtetrafluoropyridyl)ꢀ1,3,2ꢀbenzodiazaborole (9)
The adduct of 2ꢀBr2BꢀC4H3S and PPh3 (1.084 g, 2.1 mmol), was prepared as described before
and treated with 1 equiv. of N,N´ꢀbis(tetrafluoropyridyl)ꢀorthoꢀphenylenediamine 16 (0.853 g)
and 0.636 g (4.5 mmol) of 2,2,6,6ꢀtetramethylpiperidine in toluene at 100°C for 16 h. After an
analogous work up the reaction residue was sublimed in vacuo at 90°C to remove PPh3 and
then at 140°C, whereby product
9
separated as a colourless solid. The latter fraction was
1
purified by crystallisation from nꢀhexane to yield 0.650 g of product (1.30 mmol, 62%). H
NMR (CDCl3): δ= 6.94 (m, 2H, CH=CHCH=CH), 7.10 (d, 3JH,H=3.4 Hz, 1H, 3ꢀHꢀthiophene),
7.17 (t, 3JH,H=3.6 Hz, 1H, 4ꢀHꢀthiophene), 7.24 (m, 2H, CH=CHCH=CH), 7.64 (d,
3JH,H=4.0 Hz, 1H, 5ꢀHꢀthiophene) ppm. 13C{1H} NMR (CDCl3): δ= 110.9 (s,
CH=CHCH=CH), 119.9 (s, CH=CHCH=CH), 128.9, 132.5, 135.3 (3s, BCꢀCHꢀCHꢀCHꢀS),
134.7 (s, N2C2), 138.6 (dm, 1JC,F=265 Hz, C5F4N), 144.1 (dm, 1JC,F=248 Hz, C5F4N) ppm.
11B{1H} NMR (CDCl3): δ= 27.4 (s) ppm. 19F{1H} NMR (CDCl3): δ= ꢀ144.3 (m, 4F,
87.4 (m, 4F,
ꢀF) ppm. MS/EI (m/z): 498.1 (100%) [M+], 478.1 (10%) [M+ ꢀHF].
oꢀF), ꢀ
m
C20H7BF8N4S (498.16): calcd.: C: 48.22, H: 1.42, N: 11.25; found: C: 47.10, H: 1.55, N:
10.80.
29