Synthesis, structures and luminescent behaviour of tridentate salicylaldiminato-type borate complexes

Article abstract of DOI:10.1016/j.ica.2009.11.006

Treatment of the ligands 3,5-tBu₂-2-(OH)C₆H₂CHNR [R = 2-(CO₂H)C₆H₄ (1a) and 2-(CO₂H)C₁₀H₆ (1b)] with trimethylborate, B(OMe)₃, in toluene yields,

Full text of DOI:10.1016/j.ica.2009.11.006

Inorganica Chimica Acta 363 (2010) 1173–1178
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structures and luminescent behaviour of tridentate
salicylaldiminato-type borate complexes
c
Victor Christou a, Scott E. Watkins a, Ruth McCann b, Carl Redshaw b, , Mark R.J. Elsegood
*
a Opsys Ltd., 8 Begbroke Business and Science Park, Sandy Lane, Yarnton, Oxford, OX5 1PF, UK
b Energy Materials Laboratory, School of Chemical Sciences and Pharmacy, The University of East Anglia, NR4 7TJ, UK
c Chemistry Department, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 April 2009
Received in revised form 9 October 2009
Accepted 8 November 2009
Available online 16 November 2009
Treatment of the ligands 3,5-tBu2-2-(OH)C6H2CHNR [R = 2-(CO2H)C6H4 (1a) and 2-(CO2H)C10H6 (1b)]
with trimethylborate, B(OMe)3, in toluene yields, after work-up, the yellow crystalline complexes
{[3,5-tBu2-2-(O)C6H2CHNR]B(OMe)} [R = 2-(CO2)C6H4 (2a) and 2-(CO2)C10H6 (2b)], respectively. Further
treatment of these complexes with trifluoromethanesulfonic (triflic) acid, CF3SO3H, followed by recrystal-
lisation from tetrahydrofuran (thf) afforded the triflate salts [{3,5-tBu2-2-(O)C6H2CHNR}B(thf)][CF3SO3]
[R = 2-(CO2)C6H4 (3a) and 2-(CO2)C10H6 (3b)]. An electroluminescent device was constructed using 2a,
which produced orange-green light with broad emission spectra (maximum brightness of 5 cd/m2 being
observed at 13 V). Compounds 1a and 2bꢀ2MeCN have been characterised by single crystal X-ray struc-
ture determinations.
Dedicated to Professor Jonathan Dilworth on
the occasion of his 65th birthday.
Keywords:
Borate compounds
Salicylaldiminato compounds
Luminescent
Ó 2009 Elsevier B.V. All rights reserved.
Crystal structures
1. Introduction
investigations to boron and describe herein the synthesis and
structures of the borate complexes {[3,5-tBu2-2-(O)C6H2CHNR]B-
In recent years there has been considerable interest in the use of
chelate ancillary ligands in complexes to be used for electrolumi-
nescent displays, in-part because these ligands afford high stability
and volatility [1]. This is particularly the case for boron(III)-based
organic light emitting devices (OLEDs), which generally exhibit in-
creased stability when compared with their aluminium counter-
parts. Of particular note is the use of ‘tunable’ nitrogen/oxygen
chelate donor ligands, such as quinolates and related species to
form monomeric complexes (see I–V, Chart 1), by Wang et al. [2]
and others. [3] More recent work by Ko et al. has utilized oxazolyl-
phenolate ligands, which on reaction with BPh3 afford complexes
of the form VI. An electroluminescence device was constructed
for Ar = NPh2, and possessed a maximum brightness of 2905 cd/
m2 at 13 V [4], whilst the use of Ar = 4-cyanophenyl, 2,4-difluoro-
phenyl, 4-chlorophenyl, phenyl, 4-methoxyphenyl and 4-dimeth-
ylaminophenyl allowed the system to be tuned from blue to
green. Other research groups have extended this methodology to
include organoboron polymers [5].
(O Me)} [R = 2-(CO2)C6H4 (2a) and 2-(CO2)C10H6 (2b)] together
with the synthesis of the triflate salts [{3,5-tBu2-2-
(O)C6H2CHNR}B(thf)] [CF3SO3] [R = 2-(CO2)C6H4 (3a) and 2-
(CO2)C10H6 (3b)].
The behaviour of the borates 2a and 2b towards UV light has
been examined and an electroluminescent device made utilizing
complex 2a. We note that the electroluminescent behaviour of
some Schiff-base born complexes (see Chart 3) have recently been
reported [7], whilst a series of mononuclear Schiff-base borates
and boron halides (Chart 4) have been reported, the latter being
used to dealkylate trimethyl phosphate [8].
2. Results and discussion
The ligands 1a and b were prepared via the condensation reac-
tion between 3,5-di-tert-butylsalicyladimine and anthranilic acid,
1,2-(NH2)(CO2H)C6H4, or 3-amino-2-napthoic acid, 2-(CO2H),3-
(NH2)C10H6, as pale yellow solids in good yield (typically ca. 75%
isolated yield). The 1H NMR spectra of these ligands exhibit reso-
nances in the regions d ꢁ11.6 for the OH (disappears on D2O addi-
tion) and ꢁ9.9 ppm for the CO2H, respectively (shifts being affected
by the extensive H-bonding discussed below). In the IR spectrum,
We have been investigating the chemistry of the Schiff-base li-
gand 3,5-tBu2-2-(OH)C6H2CHNC6H4-2-(CO2H) (1a, see Chart 2) to-
wards group 13 reagents [6]. We have now extended our
the characteristic
v(C@N) imine stretch is tentatively assigned to
* Corresponding author. Tel.: +44 1603 593137; fax: +44 1603 592003.
E-mail address: carl.redshaw@uea.ac.uk (C. Redshaw).
the weak absorption at 1612 cmꢂ1. A single crystal X-ray structure
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.11.006

1174
V. Christou et al. / Inorganica Chimica Acta 363 (2010) 1173–1178
Et
Et
O
B
B
N
N
N
N
O
O
B
N
I
II
III
N
O
N
N
N
R
O
B
R
B
N
B
N
N
R
O
B
O
O
F
IV (R = Et, Ph)
V
N
VI R =
Ph
Ph
NMe2
CN
F
Cl
OMe
Chart 1.
R
R
R
R
R
Ph
O
N
O
N
OH
OH
X
B
B
X
Bu
t
X
Bu
t
O
N
N
CO2H
t
Bu
CO2H
t
Bu
XI (X = Cl, Br, OMe;
R = o-Ph, (CH2)2, (CH2)3)
X (X = Cl, Br, OMe)
Chart 4.
1b
1a
Chart 2 (R = tert-butyl)
nolic ring, whilst the anthranilic acid derived ring is orientated
54.6° to the phenolic ring plane. Bond lengths and angles for 1a
are presented in Table 1; crystallographic data are given in Table 3.
Treatment of 1a and 1b with excess trimethylborate, B(OMe)3,
in refluxing toluene affords the corresponding borate complexes
{[3,5-tBu2-2-(O)C6H2CHNR]B(OMe)} [R = 2-(CO2)C6H4 (2a) and 2-
(CO2)C10H6 (2b)]. Both 2a and 2b can be obtained in good yields
(50–60%) as yellow crystalline solids on prolonged standing at
0 °C. In the 1H NMR spectrum, the imine resonances fall in the
8.8–8.9 ppm range, whilst in the 11B NMR spectrum of each com-
plex, the one resonance is found at d 5.53 (2a) and 5.73 ppm (2b)
Chart 2.
of 1a shows (Fig. 1) the molecule exhibits both inter- and
intra-molecular hydrogen bonding. The former involves the imine
nitrogen centre and the hydroxyl group such that H(1)ꢀꢀꢀN(1)
= 1.83 Å with angle O(1)–H(1)ꢀꢀꢀN(1) = 148°. The inter-molecular
H-bond is of similar strength and involves carboxylic acid groups
in neighbouring molecules: the H(3)ꢀꢀꢀO(2A) distance is 1.81 Å with
an angle O(3)–H(3)ꢀꢀꢀO(2A) of 174°. There is only a small (ca. 4.8°)
out of plane rotation of the C@N double bond relative to the phe-
N
F2B
O
BF2
N
N
N
N
N
BF2 F2B
O
BF2
O
O
VII
IX
VIII
Chart 3.

V. Christou et al. / Inorganica Chimica Acta 363 (2010) 1173–1178
1175
Fig. 1. Crystal structure of 1a emphasizing (a) intra-molecular H-bonding and (b) inter-molecular H-bonding.
Table 1
Selected bond lengths (Å) and angles (°) for 1a. Symmetry operator A = ꢂx, 1ꢂy, 1ꢂz.
Table 2
Selected bond lengths (Å) and angles (°) for 2b.
N(1)–C(15)
N(1)–C(16)
N(1)ꢀꢀꢀH(1)
O(2A)ꢀꢀꢀH(3)
O(1)–H(1)ꢀꢀꢀN(1)
O(3)–H(3) ꢀꢀO(2A)
1.2781(10)
1.4123(10)
1.83
B(1)–N(1)
B(1)–O(1)
B(1)–O(2)
B(1)–O(3)
N(1)–C(2)
1.566(3)
1.424(3)
1.459(2)
1.467(2)
1.298(2)
1.81
148
174
B(1)–O(1)–C(1)
B(1)–O(2)–C(4)
B(1)–O(3)–C(17)
B(1)–N(1)–C(2)
117.52(16)
125.12(15)
121.86(15)
123.33(14)
consistent with 4-coordinate boron [3,4]. In the IR spectrum, the
B–N absorption is found at 1019 (2a) and 1020 (2b) cmꢂ1
The X-ray crystal structure of 2b is shown in Fig. 2; bond
v
.
There is currently much interest in group 13 compounds incor-
porating salen-type ligands [10a], and cationic species thereof
[10b], primarily due to their application in catalysis. A facile entry
into cationic chemistry is via the addition of the strong acid
CF3SO3H (triflic acid) which is known to lead to cation–anion pair
formation [9]. Similarly, for 2a and 2b, reaction with one equiva-
lent of triflic acid in toluene followed by recrystallisation from
toluene/tetrahydrofuran (10:1) afforded the triflate salts [{3,5-
tBu2-2-(O)C6H2CHNR}B(thf)][CF3SO3] [R = 2-(CO2)C6H4 (3a) and 2-
(CO2)C10H6 (3b)]. Use of neat thf as solvent here is best avoided
as 3a and 3b act as Lewis acid catalysts for the polymerization of
thf. Such behaviour is unsurprising given that the related Atwood
systems (and [nBu2BOTf] were shown to polymerize propyleneox-
ide via a proposed anionic mechanism [9].
lengths and angles are in Table 2 (for crystallographic data see Ta-
ble 3). The boron centre has a tetrahedral geometry with near ideal
O–B–N angles of 107.45(16)° and 109.40(14)° for each of the six-
membered rings. The B–O(2) distance [1.459(2) Å] is similar to
those observed by Atwood for the related complex [LB(OSiPh3)]
[1.465(7) and 1.433(7) Å] (where L is the tridentate, doubly depro-
tonated ligand derived from N-salicylidene-o-aminophenol) [9].
The distance to the oxygen derived from the acid is 1.467(2) Å.
The B–N distance 1.566(3) Å is slightly shorter than that observed
by Atwood [1.5998(8) Å]. The C@N bond of 2b retains its double
bond character, being 1.298(2) cf 1.2781(10) Å in the free ligand 1a.
Electrochemically complex 2a undergoes three separate pro-
cesses within the solvent window (Fig. 3). At positive potentials
the complex undergoes an irreversible oxidation at 1.25 V. This is
followed by a (quasi) reversible oxidation at higher potential,
E1/2 = 1.48 V,
DEp = 0.12 V. In the reverse scan an irreversible
reduction occurs at ꢂ1.61 V. Variations in scan-rate, direction
and limits do not reveal any other details; the first oxidation and
the reduction remain completely irreversible.
Given the suitable physiochemical properties of 2a, an EL device
was fabricated. This device showed orange/green emission with an
EL spectrum (see Fig. 4) consisting of a single broad peak with a
maximum at 592 nm. The light output (not optimized) from the
device, with a maximum brightness of 5 cd/m2 being observed at
13 V, is poor and is far less impressive than that reported for
Fig. 2. Molecular structure of 2b.

1176
V. Christou et al. / Inorganica Chimica Acta 363 (2010) 1173–1178
Table 3
devices made from other boron Schiff-base complexes such as VII
(465 cd/m2), VIII (195 cd/m2) and IX (840 cd/m2). [7a,b] The spikes
in Fig. 4 represent artifacts from the measurements and are due to
small shorts in the device producing sparks of comparable bright-
ness to the EL emission.
Summary of crystallographic data.
Compound
1a
2bꢀ2MeCN
Formula
C22H27NO3
353.45
C27H30BNO4ꢀ2(C2H3N)
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
525.44
triclinic
monoclinic
P21/n
11.1089(5)
9.4740(5)
29.0748(14)
90
98.9395(9)
90
3022.8(3)
4
The PL spectrum of 2a in solution (excitation wave-
length = 350 nm) is broad with a maximum at 385 nm, has a smal-
ler peak at around 510 nm and a broad tail into the red. To the eye,
this complex appears to luminesce yellow in both solution and the
solid state. These features are at higher energy than in the EL spec-
trum (a single broad peak centred at around 592 nm, see Fig. 4).
The red-shift in emission is common in organic electrolumines-
cence, but a detailed explanation of the photophysics of this com-
plex remains as future work.
ꢀ
P1
6.8667(9)
10.8258(13)
13.3081(16)
81.213(2)
88.722(2)
84.492(2)
973.1(2)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
T (K)
150(2)
200(2)
In conclusion, the compounds described herein provide a useful
insight into the coordination environment afforded by the Schiff-
base ligands [3,5-tBu2-2-(O)C6H2CHNR], where R = 2-(CO2)C6H4
and 2-(CO2)C10H6. The borates {[3,5-tBu2-2-(O)C6H2CHNR]
B(OMe)}, upon irradiation by UV light, produce a bright yellow col-
our both in solution and in the solid-state, and an electrolumines-
cent device was constructed using 2a (R = 2-(CO2)C10H6), which
produced orange-green light with broad emission spectra (maxi-
mum brightness of 5 cd/m2).
Radiation, k (Å)
Crystal size (mm3)
hrange (°)
0.71073
0.71073
0.33 ꢃ 0.15 ꢃ 0.09 0.55 ꢃ 0.20 ꢃ 0.18
1.91–28.89
19 683
18 882
0.019
2.09–28.87
23 249
6993
0.022
4361
0.959, 0.986
6993/54/361
0.0528
Reflections measured
Unique reflections
Rint
Reflections with F2 > 2
Transmission factors
r
(F2)
12 723
Data/restraints/parameters
18 882/0/244
0.0442
0.1188
R1 [F2 > 2 (F2)]
r
wR2 (all data)
0.1657
Goodness-of-fit (GOF), S
Largest difference peak and
1.024
0.341, ꢂ0.237
1.034
0.263, ꢂ0.191
3. Experimental
hole (e Åꢂ3
)
3.1. General
All manipulations were carried out under an atmosphere of
nitrogen using standard Schlenk and cannula techniques or in a
conventional nitrogen-filled glove-box. Solvents were refluxed
over an appropriate drying agent, and distilled and degassed prior
to use. Elemental analyses were performed by the microanalytical
services of the School of Chemical Sciences at The University of
East Anglia. NMR spectra were recorded on a Varian VXR 400 S
spectrometer at 400 or a Gemini at 300 MHz (1H) at 96.3 MHz
(
11B) and 282.4 MHz (19F) at 298 K; chemical shifts are referenced
to the residual protio impurity of the deuterated solvent; IR spectra
(nujol mulls, KBr/CsI windows), Perkin–Elmer 577 and 457 grating
spectrophotometers.
Electrochemical measurements were carried in out in a stan-
dard, three electrode cell with a glassy carbon working electrode,
a Pt wire counter electrode and a silver wire pseudo reference
electrode. The dichloromethane solution contained 0.1 M tetrabu-
tylammonium hexafluorophosphate as the electrolyte. The concen-
tration of 2a was 1 ꢃ 10ꢂ3 M. The solution was sparged with
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Potential (V) vs Fc/Fc+
Fig. 3. CV of 2a in dichloromethane.
400
450
500
550
600
650
700
750
800
Fig. 4. EL spectrum of device ITO/
a-NPD (50 nm)/2a (50 nm)/LiF (0.5 nm)/Al (100 nm) at 13 V (vertical axis is luminescence; units are arbitrary).

V. Christou et al. / Inorganica Chimica Acta 363 (2010) 1173–1178
1177
nitrogen prior to each scan and the scans were run with a blanket
of nitrogen gas over the top. A scan of the blank solvent was carried
out prior to every run. At the completion of the scan, ferrocene was
added to a concentration of 1 ꢃ 10ꢂ3 M and the ferrocene/ferroce-
nium couple used as an internal reference. Under these conditions
48%. Anal. Calc. for C27H30BNO4ꢀ2MeCN: C, 70.9; H, 6.9; N, 8.0.
Found: C, 70.2; H, 6.7; N, 7.1%. Mass Spec. (EI): 443.3 ([M]+),
412.3 ([M]+ꢂOMe). IR: 2361w, 1700w, 1624w, 1610w, 1547w,
1305bw, 1260s, 1202bw, 1096bs, 1020bs, 880w, 794s, 723s,
695w, 666w. 1H NMR (CDCl3): d 1.34 (s, 9H, C(CH3)3), 1.51 (s, (H,
C(CH3)3), 2.00 (s, 3H, CH3CN), 3.24 (s, 3H, OCH3), 7.39 (d, 1H, arylH),
7.58 (m, 2H, arylH), 7.76 (d, 1H, arylH), 7.90 (m, 2H, arylH), 8.07 (s,
1H, arylH), 8.79 (s, 1H, arylH), 8.91 (s, 1H, CH@N). 11B NMR (CDCl3)
d 5.73s. UV–Vis (CH2Cl2) k/nm (relative absorbances): 230 (0.48),
273 (0.14), 300 (0.19), 334 (010), 419 (0.09). Emission max
(CH2Cl2): k 445 nm.
the ferrocene E1/2 = 284 mV,
DEp = 98 mV. All potentials in this re-
port are quoted relative to Fc/Fc+. All chemicals were obtained
commercially and used as received unless stated otherwise.
3.2. Preparation of 3,5-tBu2-2-(OH)C6H2CHN[2-(CO2H)C6H4] (1a)
Formic acid (4 drops) was added to a solution of 3,5-di-tert-bu-
tyl-2-hydroxybenzaldehyde (5.12 g, 21.85 mmol) and anthranilic
acid (3.00 g, 21.88 mmol) in ethanol (70 cm3). The solution was re-
fluxed for 12 h and concentrated to about 30 cm3 and allowed to
evaporate at ambient temperature for 2–3 days to afford 1a as a
yellow solid. Yield 5.30 g, 74%. Further recrystallisation from dime-
thoxyethane/diethylether (50:50) afforded 1a as a yellow crystal-
line solid. Anal. Calc. for C22H27NO3ꢀ0.75DME: C, 71.3; H, 8.2; N,
3.3. Found: C, 71.1; H, 8.0; N, 3.7%. Mass Spec. (Nano-electrospray
(positive mode), accurate mass measurement): found 354.2072
calculated 354.2064 ([M+H]+). IR: 1694w, 1655s, 1612w, 1415w,
1393m, 1364s, 1326w, 1273m, 1251m, 1223m, 1201w, 1169m,
1026w, 964w, 932w, 880w, 829w, 801w, 771m, 738m, 727s,
694w, 644w. 1H NMR (CDCl3): d 1.26 (s, 9H, C(CH3)3), 1.36 (s, (H,
C(CH3)3), 3.34 (s, 3H, OCH3, 1/2 dme), 3.49 (s, 2H, OCH2, 1/2
dme), 6.59–7.89 (several m, 7H, arylH+ CH@N), 9.81 (s, 1H,
CO2H), 11.58 (s, 1H, OH).
3.6. Preparation of [{3,5-tBu2-2-(O)C6H2CHN[2-
(CO2)C6H4]}B(thf)][CF3SO3]) (3a)
To a solution of 2a (1.00 g, 2.54 mmol) in toluene (20 cm3) was
added
a
solution of trifluoromethanesulfonic acid (0.23 cm3,
2.60 mmol) in toluene (10 cm3) at ambient temperature. After stir-
ring for 12 h, volatiles were removed in-vacuo and the yellow res-
idue was extracted into THF (30 cm3). Yield: 1.14 g, 77%. Anal. Calc.
for C27H32BSF3NO7: C, 55.7; H, 5.5; N, 2.4. Found: C, 55.7; H, 5.5; N,
2.4%. IR: 1610w, 1552w, 1288w, 1260m, 1225w, 1192bw, 1096bs,
1015bs, 960w, 805m, 723w, 662w. 1H NMR (C6D6) d 9.28 (s, 1H,
CH), 7.77, 7.11, 6.99 (3x bm, 6H, arylH), 3.35 (s, 4H, OCH2), 1.41
(s, 4H, CH2), 1.31 (s, 9H, C(CH3)3), 1.10 (s, 9H, C(CH3)3). 11B NMR
(C6D6) d 2.90. 19F NMR (C6D6) d ꢂ77.87. UV–Vis (CH2Cl2) k/nm (rel-
ative absorbances): 235 (0.42), 258 (0.09), 332 (0.38), 418 (0.11).
Emission max (CH2Cl2): k 385 nm.
3.3. Preparation of 3,5-tBu2-2-(OH)C6H2CHN[2-(CO2H)C10H6] (1b)
3.7. Preparation of [{3,5-tBu2-2-(O)C6H2CHN[2-
(CO2)C10H6]}B(thf)][CF3SO3]) (3b)
As for 1a, but using 3,5-di-tert-butyl-2-hydroxybenzaldehyde
(3.76 g, 16.05 mmol) and 3-amino-2-napthoic acid (3.00 g,
16.03 mmol) affording yellow 1b. Yield 4.90 g, 76%. Anal. Calc. for
C26H29NO3: C, 77.4; H, 7.2; N, 3.5. Found: C, 77.4; H, 7.1; N, 3.4%.
Mass Spec. (ES, positive mode): 404.5 ([M+H]+). IR: 1694w,
1655s, 1612w, 1495w, 1415m, 1364s, 1325w, 1273m, 1251m,
1223w, 1201w, 1169m, 1026w, 964w, 932w, 880w, 829w, 801w,
As for 3a, but using 2b (1.00 g, 2.26 mmol) and trifluorometh-
anesulfonic acid (0.20 cm3, 2.26 mmol). Yield: 0.87 g, 61%. Anal.
Calc. for C31H34BSF3NO7: C, 58.9; H, 5.4; N, 2.2. Found: C, 58.8; H,
5.5; N, 2.1%. IR: 1590w, 1280bm, 1255s, 1091bs, 1015bs, 866w,
794s, 723s, 690w. 1H NMR (C6D6) d 9.21 (s, 1H, CH), 8.98, 8.85,
7.78, 7.47 (4x bm, 8H, arylH), 3.35 (s, 4H, OCH2), 1.44 (s, 4H,
CH2), 1.27 (s, 9H, C(CH3)3), 1.12 (s, 9H, C(CH3)3). 11B NMR (C6D6)
d 3.23. 19F NMR (C6D6) d ꢂ77.82. UV–Vis (CH2Cl2) k/nm (relative
absorbances): 233 (0.52), 261 (0.38), 347 (0.10). Emission max
(CH2Cl2): k 383 nm.
771m, 727s, 694w, 644w. 1H NMR (CDCl3):
d 1.35 (s, 9H,
C(CH3)3), 1.45 (s, (H, C(CH3)3), 2.20 (s, 6H, 2MeCN), 7.28–7.61 (sev-
eral m, 9H, arylH+ CH@N), 9.89 (s, 1H, CO2H), 11.66 (s, 1H, OH).
3.4. Preparation of {3,5-tBu2-2-(O)C6H2CHN[2-(CO2)C6H4]}B(OMe)}
(2a)
3.8. Fabrication of electroluminescent device
To 1a (2.17 g, mmol) in toluene (30 cm3) was added via syringe
B(OMe)3 (1.6 cm3, mmol) and the system was refluxed for 12 h.
Following removal of volatiles in-vacuo, the residue was extracted
into warm MeCN. Prolonged standing at ambient temperature
afforded 2a as large yellow prisms. Yield 2.03 g, 59%. Anal. Calc.
for C23H28BNO4: C, 70.2; H, 7.2; N, 3.6. Found: C, 69.6; H, 7.1; N,
4.0%. 1H NMR (CDCl3): d 1.34 (s, 9H, C(CH3)3), 1.52 (s, (H,
C(CH3)3), 2.02 (s, 3H, CH3CN), 3.26 (s, 3H, OCH3), 7.28–8.33 (several
m, 6H, arylH), 8.80 (s, 1H, CH@N). 11B NMR (CDCl3) d 5.53 s. IR
The EL device using 2a as the emitting layer and electron trans-
port layer was fabricated on indium tin oxide (ITO). The ITO was
cleaned by successive 20 min sonications in neutral detergent,
1:1 NH3/H2O2 and then deionised water. Between each sonication
the substrates were rinsed with deionised water. The substrates
were dried in an oven at 120 °C for 1 h and then subjected to an
oxygen plasma clean. The device was prepared by evaporation
from molybdenum crucibles at pressures below 1 ꢃ 10ꢂ6 mbar.
The thickness of the layers was calibrated using a Dektak profilom-
(powdered sample)
m
/cmꢂ1: 2956 (m), 1739 (m), 1694 (m), 1611
eter. The structure of the device fabricated was: ITO/
a-NPD
(m), 1553 (w), 1506 (w), 1474 (s), 1392 (m), 1365 (vs), 1336 (m),
1307 (s), 1239 (m), 1202 (m), 1102 (s), 1071 (m), 1040 (m), 1019
(m), 878 (w), 796 (w), 758 (s), 750 (s), 693 (m), 681 (w). UV–Vis
(MeOH) k/nm (relative absorbances): 223 (0.78), 258 (0.23), 325
(0.32), 400 (0.15). Emission max (CH2Cl2): k 385 nm.
(50 nm)/2a (50 nm)/LiF (0.5 nm)/Al (100 nm), where a-NPD is
N,N0-diphenyl-N,N0-bis(1-naphthyl)-1-10-biphenyl-4,40-diamine.
The devices were tested under vacuum.
3.9. X-ray crystallography
3.5. Preparation of {[3,5-tBu2-2-(O)C6H2CHN[2-(CO2)C10H6]}]B(OMe)}
(2b)
Measurements for 1a and 2bꢀ2MeCN were made on a Bruker AXS
SMART 1000 CCD diffractometer equipped with graphite mono-
chromated Mo K
were employed. Cell parameters were refined from all strong reflec-
tions in each data set. Intensities were corrected semi-empirically
a radiation. Narrow-frame exposures (0.3° in x)
As for 2a, but using 1b (2.0 g, 4.96 mmol) and B(OMe)3 (1.2 cm3,
10.5 mmol) affording 2bꢀ2MeCN as yellow prisms. Yield 1.06 g,

1178
V. Christou et al. / Inorganica Chimica Acta 363 (2010) 1173–1178
(c) Q. Wu, M. Esteghamatian, N.-X. Hu, Z. Popovic, G. Enright, Y. Tao, M. D’Iorio,
S. Wang, Chem. Mater. 12 (2000) 79;
(d) S.-F. Liu, C. Seward, H. Aziz, N.-X. Hu, Z. Popovic´, S. Wang, Organometallics
for absorption, based on symmetry-equivalent and repeated reflec-
tions. The structures were solved by direct methods and refined
by full-matrix-least squares on F2 values for all unique data. All
non-hydrogen atoms were refined anisotropically. H atoms were
included in a riding model with Uiso set to be 1.2 times (1.5 times
for methyl-H) Ueq for the carrier atom. Further details are presented
in Table 3. In 1a the diffraction data were twinned via a rotation of
19 (2000) 5709;
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(2005) 143;
ꢀ
180° about [100] in direct space with a major:minor component
(i) Y. Cui, S. Wang, J. Org. Chem. 71 (2006) 6485.
ratio of 61.51:38.49(4)%. Modeling the twinning resulted in a sub-
stantial improvement in the structure refinement. In 2bꢀ2MeCN
one tBu group and both molecules of MeCN showed some signs of
disorder but were modeled as a single set of atomic positions,
refined with restraints on displacement parameters. Programs
used were Bruker AXS SHELXTL [11] for structure solution, refine-
ment, and molecular graphics, Bruker AXS SMART (control) and SAINT
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Acknowledgements
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The EPSRC are thanked for financial support. The EPSRC Mass
Spectrometry Service at Swansea is thanked for data.
Appendix A. Supplementary material
CCDC 705300 and 705301 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2009.11.006.
(e) Y. Tokoro, A. Nagai, K. Kokado, Y. Chujo, Macromolecules 42 (2009)
2988;
(f) H. Li, F. Jäkle, Macromolecules (2009), doi:10.1021/ma9001319.
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