Synthesis and properties of 2-(dimesitylboryl)benzylideneamines

Article abstract of DOI:10.1515/znb-2009-11-1219

Lithiation of 2-(2-bromophenyl)-dioxolane (1) followed by reaction with dimesitylboron fluoride afforded 2-(2-dimesitylborylphenyl)-dioxolane (2) which was deprotected to afford 2-dimesitylboryl-benzaldehyde (3). Compound 3 reacts with aliphatic amines such as n-butylamine and ethanolamine to afford the corresponding imines 2-(dimesitylboryl)benzylidenebutylamine (4) and 2-(dimesitylboryl)benzylideneethanolamine (5), respectively. Structural studies indicate coordination of the imine-nitrogen atom to the boron center. Imines 4 and 5 emit a green fluorescence near 510 nm with quantum yields approaching 10 %. TD-DFT calculations suggest that this emission arises from an intramolecular charge-transfer excited state.

Full text of DOI:10.1515/znb-2009-11-1219

Synthesis and Properties of 2-(Dimesitylboryl)benzylideneamines
Zureima Garc´ıa-Herna´ndeza,b and Franc¸ois P. Gabba¨ıa
a Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA
b Centro de Investigaciones Qu´ımicas, Universidad Auto´noma del Estado de Morelos,
Av. Universidad 1001. Col. Chamilpa, Cuernavaca, Morelos, 62209, Me´xico
Reprint requests to Prof. F. P. Gabba¨ı. E-mail: francois@tamu.edu
Z. Naturforsch. 2009, 64b, 1381 – 1386; received August 11, 2009
Dedicated to Professor Hubert Schmidbaur on the occasion of his 75th birthday
Lithiation of 2-(2-bromophenyl)-dioxolane (1) followed by reaction with dimesitylboron flu-
oride afforded 2-(2-dimesitylborylphenyl)-dioxolane (2) which was deprotected to afford 2-
dimesitylboryl-benzaldehyde (3). Compound 3 reacts with aliphatic amines such as n-butylamine
and ethanolamine to afford the corresponding imines 2-(dimesitylboryl)benzylidenebutylamine (4)
and 2-(dimesitylboryl)benzylideneethanolamine (5), respectively. Structural studies indicate coordi-
nation of the imine-nitrogen atom to the boron center. Imines 4 and 5 emit a green fluorescence
near 510 nm with quantum yields approaching 10 %. TD-DFT calculations suggest that this emission
arises from an intramolecular charge-transfer excited state.
Key words: Boron, Fluorescence, Imine, Aldehyde
Introduction
The chemistry of triarylboranes has experienced a
surge of interest because of the discovery of applica-
tions in the domain of optoelectronic and sensing [1 –
9]. The development of such applications often neces-
sitates the combination of a triarylboron unit with an-
other functionality. This point can be illustrated by re-
cent efforts in the chemistry of boron-nitrogen com-
pounds such as A, B and C [10 – 12] which have been
used as fluoride turn-on sensors in the case of A [10],
H2 activating catalysts in the case of B [11], or flu-
orescent dyes in the case of C [12]. These advances
have only been made possible by the discovery of high-
yield synthetic routes that facilitate the incorporation
of the triarylboron unit in the target derivatives. As part
of our contribution to this area of synthetic chemistry,
we have now decided to synthesize 2-dimesitylboryl-
benzaldehyde and investigate its reactivity with pri-
mary amines. These investigations were further mo-
tivated by the fact that 2-diarylboryl-benzaldehyde
derivatives have never been reported in the literature.
94 % yield (Scheme 1). Deprotection of the alde-
hyde functionality could be achieved by addition of
concentrated HClaq to a THF solution of 2 to af-
ford 2-dimesitylboryl-benzaldehyde (3) in 98 % yield
(Scheme 1). Compound 3 is stable and can be stored
at r. t. for extended periods of time. It has been char-
acterized by NMR spectroscopy and mass spectrome-
try which allowed detection of the molecular peak at
m/z = 355.8593 for [M+H]+. The 1H NMR spectrum
features all expected resonances including a signal at
δ = 9.69 ppm corresponding to the hydrogen atom of
the aldehyde functionality. The 11B NMR resonance
detected at 25 ppm is significantly upfield when com-
pared to that of 2 (75 ppm), thus suggesting that the
oxygen atom of the aldehyde functionality is coordi-
nated to the boron atom. A similar conclusion could
be derived from the IR spectrum of 3 which displays a
ν(co) at 1671 cm−1, significantly shifted to lower en-
ergy when compared to that of benzaldehyde (ν(co) =
Results and Discussion
Lithiation of 2-(2-bromophenyl)-dioxolane (1) fol-
lowed by reaction with dimesitylboron fluoride af-
forded 2-(2-dimesitylborylphenyl)-dioxolane (2) in
c
0932–0776 / 09 / 1100–1381 $ 06.00 ꢀ 2009 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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Z. Garc´ıa-Herna´ndez – F. P. Gabba¨ı · 2-(Dimesitylboryl)benzylideneamines
Scheme 1. a: n-BuLi, OEt2; b: Mes2BF; c: HCl,
THF, H2O; d: RNH2.
Table 1. Crystal data, data collections, and structure refine-
ments for 4 and 5.
4
5
Formula
Mr
C29H36NB
409.4
C27H32NBO
397.35
Crystal size, mm3
Crystal system
Space group
0.31×0.22×0.20 0.25×0.20×0.15
tetragonal
I41/a
monoclinic
P21
˚
a, A
17.7182(2)
17.7182(2)
31.3604(8)
90
9845.1(3)
16
1.11
0.1
3552
9.083(4)
8.684(3)
14.131(5)
95.108(6)
1110.2(7)
2
1.19
0.1
428
˚
b, A
˚
c, A
β, deg
3
˚
Fig. 1. DFT (B3LYP/6-31G(d)) optimized structure of 3.
H atoms (at the exception the aldehyde hydrogen) have been
omitted for clarity.
V, A
Z
ρ
calcd, g cm−3
µ, mm−1
F(000), e
Data collection
T, K
1700 cm−1) [13]. While we could not obtain single
crystals of this compound, a DFT geometry optimiza-
tion at the B3LYP/6-31G(d) level of theory corrobo-
rates these observations and indicates the presence of
173(2)
173(2)
Scan mode
hkl range
ω
ω
−25 ≤ h ≤ 23,
−25 ≤ k ≤ 24,
−27 ≤ l ≤ 44
−11 ≤ h ≤ 11,
−11 ≤ k ≤ 11,
−18 ≤ l ≤ 17
9325 / 4954
0.0353
˚
a B–O dative bond of 1.66 A length (Fig. 1). Inter-
estingly, formation of this intramolecular dative bond
does not restrict the reorientation of the mesityl groups,
as indicated by the detection of a single 1H NMR res-
onance for the ortho-methyl groups of the mesityl sub-
stituents.
Refl. measured / unique 67367 / 7549
Rint
0.0818
7549
Reflections used for
refinement
Parameters refined
R1a, wR2b all data
GoF (F2)
4954
281
271
0.1211/ 0.1733
1.030
0.344 / −0.321
0.0789/ 0.1564
1.010
Having confirmed the identity of 3, we decided
to survey its reaction chemistry. Compound 3
cleanly reacted with aliphatic primary amines
such as n-butylamine and ethanolamine to afford
imines 4 (96 % yield) and 5 (98 % yield), respectively
(Scheme 1). By contrast, reactions with aromatic
amines such as aniline failed to proceed cleanly.
Imines 4 and 5 have been fully characterized. The
−3
˚
ρfin (max / min), e A
0.347 / −0.305
R1 = ΣꢀFo| − |Fc||/Σ|Fo|;
wR2 = [(Σw(Fo − Fc2)2)
/
a
b
2
(Σw(Fo2)2)]1/2
.
the aldehyde functionality are responsible for this
difference.
The crystal structures of compounds 4 and 5 have
11B NMR resonances of these two compounds are been determined (Fig. 2, Table 1). In both cases, the
observed near 5 ppm, thus indicating coordination imine nitrogen atom is coordinated to the boron center
˚
˚
of the nitrogen to the boron atom. Formation of by N(1)–B(1) bond of 1.627(3) A for 4 and 1.630(4) A
this dative bond leads to the existence of a rigid for 5. These bond lengths, which can be compared
˚
structure, as reflected by the appearance of broad to the value of 1.625 A measured in C, suggest that
multiple 1H NMR resonances for the ortho-methyl the bulk and relative electron-richness of the Mes2B
and para-CH groups of the mesityl substituents. moiety when compared to the (C6F5)2B moiety does
These observations suggest that, unlike in 3, re- not weaken the strength of the B–N dative bond. For-
orientation of the mesityl substituents about the mation of this bond leads to a noticeable pyramidal-
C
ipso-boron bond of these derivatives is restricted. ization of the boron atom, as indicated by the sum
Presumably, the increased bulk and Lewis basic- of the C–B–C angles of 340.9◦ for 4 and 341.2◦
ity of the imine functionality when compared to for 5.
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Z. Garc´ıa-Herna´ndez – F. P. Gabba¨ı · 2-(Dimesitylboryl)benzylideneamines
1383
on the imine ligand (Fig. 4), this transition can be re-
garded as an intramolecular charge transfer (ICT) tran-
sition. This situation is reminiscent of that observed by
Wang in the [2-(2-pyridyl)phenyl-C,N]dimesitylboron
which features an ICT transition from the mesityl lig-
ands to the phenyl-pyridyl ligand [17]. Under a hand-
held UV lamp, both 4 and 5 display an intense green
emission. Accordingly, their emission spectra display
a broad band centered at 512 nm (λexcitation = 380 nm,
ΦF = 9.5%) in the case of 4 and 504 nm in the case
of 5 (λexcitation = 370 nm, ΦF = 9.3 %; Fig. 3). Since
simple benzalalkylimines are not fluorescent [18], the
emissive properties of 4 and 5 are noteworthy and can
most likely be assigned to the existence of this charge
transfer excited state as well as the molecular rigidity
arising from the chelate structure.
Experimental Section
2-(2-Bromophenyl)1,3-dioxolane (1) [19] was synthe-
sized by the published procedures. 2-Bromobenzaldehyde
(Alfa Aesar), ethanolamine (Alfa Aesar), ethylene glycol
(EMD), n-butyl amine (Aldrich), 2-bromoaniline (Alfa Ae-
sar), and a 1.9 M solution of n-butyllithium in hexane (Alfa
Aesar) were purchased and used as received. Et2O, THF and
hexane were dried by reflux under N2 over K. Air-sensitive
compounds were handled under an argon atmosphere us-
ing standard Schlenk and glovebox techniques. IR spectra
were recorded on an IR Affinity-1 FT IR spectrophotome-
ter (Shimadzu). UV/Vis spectra were recorded on a Cary
100 Bio UV/Vis spectrophotometer, and fluorescence spec-
tra were recorded on a Cary Eclipse spectrometer. Quantum
yields were determined in reference to either anthracene or
1,4-bis(5-phenyloxazol-2-yl) benzene and corrected for sol-
vent refractive index. Elemental analyses were performed at
Atlantic Microlab (Norcross, GA). Mass spectra were ob-
tained from the Mass Spectrometry Applications Laboratory
at Texas A&M University. NMR spectra were recorded at
ambient temperature on an Inova-400 FT NMR spectrometer
(399.59 MHz for 1H, 128.2 MHz for 11B, and 100.48 MHz
for 13C). Spectra are internally referenced to Me4Si (d1H
Fig. 2. Molecular structures of 4 (top) and 5 (bottom) in the
crystal. Ellipsoids have been set at 50 % probability. H atoms
are omitted for clarity. Pertinent metrical parameters can be
found in the text and in the deposited CIF files.
Since it is well known that rigid boron-containing
heterocycles possess interesting photophysical proper- (CDCl3) = 7.27; d13C (CDCl3) = 77.35) and externally ref-
erenced to BF3 · OEt2 for δ11B = 0. The melting points were
ties [8, 12, 14 – 16], we have measured the absorption
and emission spectra of 4 and 5. The low-energy por-
determined using a Melt II melting point apparatus equipped
with a digital thermometer Omega (ΩE).
tion of both spectra features a relatively sharp absorp-
tion band near 280 nm followed by a broader feature
centered around 350 nm (Fig. 3). TD-DFT calcula-
tions carried out at the B3LYP/6-31G(d) level of the-
ory on the model compound 6 indicate that the low en-
ergy edge of the absorption spectrum is dominated by
a HOMO-LUMO transition. Since the HOMO is local-
2-(2-Dimesitylborylphenyl)-dioxolane (2)
A 1.9 M solution of n-butyllithium in hexane (6.5 mL,
12 mmol) was added to a solution of 2-(2-bromophenyl)-1,3-
dioxolane (1) (2.82 g, 12.3 mmol) in diethyl ether (50 mL)
◦
◦
at 0 C. After 1 h of stirring at 0 C, the resulting white
◦
ized on one mesityl ring while the LUMO is localized suspension was cooled to −78 C. To this cooled suspen-
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1384
Z. Garc´ıa-Herna´ndez – F. P. Gabba¨ı · 2-(Dimesitylboryl)benzylideneamines
Fig. 3. UV/Vis absorption and
fluorescence spectra of
CHCl3.
4 in
Fig. 4. Optimized structure
of
6 (a), HOMO (b) and
LUMO (c). Surfaces shown with
an isodensity value of 0.03.
sion was added a solution of dimesitylboron fluoride (3.30 g, lution (0.6 mL). After 4 h of stirring, all solvents were re-
12.3 mmol) in diethyl ether (30 mL). The resulting mix- moved under reduced pressure affording 3 as a pale-yellow
◦
ture was allowed to warm to r. t. and stirred overnight. The solid in a crude yield of 98 % (0.20 g). M. p. 71 – 72 C. –
solvents were distilled off under reduced pressure leaving 1H NMR (399.59 MHz; CDCl3): δ = 2.00 (s, 12H, CH3-
a solid residue which was suspended in hexane (30 mL). Mes), 2.23 (s, 6H, CH3-Mes), 6.72 (s, 4H, H-Mes), 7.41
Filtration followed by evaporation of the solvent afforded (ddd, 1H, 3,4J = 9.8, 7.6, 1.6 Hz, CH), 7.63 (ddd, 1H,
compound 2 in a crude yield of 94 % (4.6 g). M. p. 112 – 3,4J = 9.8, 7.6, 1.6 Hz, CH), 7.97 (d, 1H, 3J = 9.8 Hz,
3
114 ◦C. – 1H NMR (399.59 MHz; CDCl3): δ = 1.97 (s, 6H, CH), 7.99 (d, 1H, J = 9.8 Hz, CH), 9.69 (s, 1H, HCO). –
1
CH3-Mes), 2.25 (s, 12H, CH3-Mes), 3.66 ( br s, 2H, OCH2), 11B{ H} NMR (128.2 MHz; CDCl3): δ = 25 (w1/2
=
554 Hz). – 13C NMR (100.48 MHz; CDCl3): δ = 21.2 (CH3-
Mes), 24.7 (CH3-Mes), 127.3, 128.6, 128.8, 129.7, 132.6 (all
CH), 128.5, 135.7, 136.6, 138.7, 139.2, 141.2 (all Cipso),
198.0 (CHO). – HRMS ((+)-ESI): m/z = 354.2434 (calcd.
377.2892 for C25H27BONa, [M+Na]+). – HRMS ((+)-ESI):
m/z = 354.2434 (calcd. 355.8593 for C25H27BO, [M+H]+).
3.82 (br s, 2H, OCH2), 5.49 (s, 1H, O2CH), 6.75 (s, 4H, H-
Mes), 7.27 (s(b), 2H, CH), 7.41 (br s, 1H, CH), 7.60 (br s,
1
1H, CH). – 11B{ H} NMR (128.2 MHz; CDCl3): δ = 73
(w1/2 = 1282 Hz). – 13C NMR (100.48 MHz; CDCl3): δ =
21.5, 22.6, 23.4 (all CH3-Mes), 64.9 (OCH2), 102.8 (CH-
O2), 125.1, 128.6, 128.7, 128.9, 130.6 (all CH), 128.4, 134.5,
139.2, 141.2, 141.5 (all Cipso). – HRMS ((+)-ESI): m/z =
398.2949 (calcd. 405.241 for C27H31BO2Li, [M+Li]+).
2-(Dimesitylboryl)benzylidenebutylamine (4)
To a stirred yellow solution of 3 (0.1 g, 0.28 mmol) in
toluene (10 mL) at r. t. was added n-butylamine (0.04 g,
2-(Dimesitylborane)benzaldehyde (3)
To a stirred solution of 2 (0.23 g, 0.58 mmol) in THF 0.56 mmol). After 3 h, the reaction mixture was evaporated
(10 mL)/distilled water (2 mL) was added a 10 % HCl so- to dryness to afford compound 4 in a crude yield of 98 %
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Z. Garc´ıa-Herna´ndez – F. P. Gabba¨ı · 2-(Dimesitylboryl)benzylideneamines
1385
(0.20 g). Further purification was achieved by recrystalliza- (calcd. 398.251 for C27H33BNO, [M+H]+). – C27H32BNO
◦
tion from CHCl3. M. p. 135 – 138 C. – UV/Vis (CHCl3): (397.4): calcd. C 81.60, H 8.11, N 3.52; found C 81.36,
λ
max(lgεmax) = 240 nm (4.91). – 1H NMR (399.59 MHz; H 8.23, N 3.61.
CDCl3): δ = 0.80 (t, 3H, 3J = 7.2 Hz), 1.42 (br s, 4H,
2CH2), 1.85 (br s, 3H, CH3-Mes), 2.00 (br s, 3H, CH3-
Mes), 2.19 (br s, 12H, CH3-Mes), 3.78, 3.86 (s, s, 2H,
CH2), 6.513 (s, 1H, H-Mes), 6.59 (s, 1H, H-Mes), 6.76 (s,
2H, H-Mes), 7.19 (ddd, 1H, 3,4,5J = 8.0, 7.6, 1.2 Hz, CH),
7.31 (ddd, 1H, 3,4,5J = 8.0, 7.6, 1.2 Hz, CH), 7.63 (d, 1H,
3J = 7.6 Hz, CH), 7.70 (d, 1H, 3J = 7.2 Hz, CH), 8.48 (s,
Crystallography
Details pertinent to the crystal structure determinations
are listed in Table 1. The measurements were carried out
using a Bruker Apex II-CCD area detector diffractome-
ter with a graphite-monochromated MoKα radiation (λ =
˚
0.71073 A). Crystals of appropriate size were selected and
1H, N=CH). – 11B{ H} NMR (128.2 MHz; CDCl3): δ =
1
mounted onto a nylon loop with Apiezon grease. Struc-
ture solution was accomplished using Direct Methods, which
successfully located most of the non-H atoms. Subsequent
refinement on F2 using the SHELXTL/PC package (ver-
sion 5.1) [20] allowed the location of the remaining non-
H atoms.
CCDC 741373 (4) and 741372 (5) contain the supple-
mentary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data request/
cif.
5 (w1/2 = 253 Hz). – 13C NMR (100.48 MHz; CDCl3): δ =
13.8 (CH3-Bu), 20.5, 32.4 (2CH2), 52.1 (NCH2), 21.0 (CH3-
Mes), 25.7 (CH3-Mes), 125.2, 125.7, 129.5, 130.5, 131.0,
132.4 (all CH), 128.7, 133.3, 134.7, 136.9, 137.1, 141.4,
141.7, 143.7 (all Cipso), 166.8 (C=N). – IR (film): ν = 3556,
2960, 2929, 2867, 1607, 1549, 1442, 1374 cm−1. – HRMS
((+)-ESI): m/z = 409.414 (calcd. 410.4019 for C29H37BN,
[M+H]+). – C29H36BN (409.4): calcd. C 85.08, H 8.86,
N 3.42; found C 84.83, H 8.85, N 3.48.
2-(Dimesitylboryl)benzylideneethanolamine (5)
Compound 5 was prepared in the same way as 4 in a crude
yield of 96 %. Further purification could also be achieved
by recrystallization from CHCl3. M. p. 190 – 192 ◦C. –
UV/Vis (CHCl3): λmax(lgεmax) = 240 nm (4.70). – 1H NMR
(399.59 MHz; CDCl3): δ = 1.92 (s, 3H, CH3-Mes), 1.98 (s,
3H, CH3-Mes), 2.19 (s, 12H, CH3 CH3-Mes), 2.75, 3.48 (s,
s, 2H, CH2), 4.05 (br s, 3H, CH2 and OH), 6.51 (s, 1H, H-
Mes), 6.61 (s, 1H, H-Mes), 6.73 (s, 1H, H-Mes), 6.77 (s,
1H, H-Mes), 7.20 (ddd, 1H, 3,4J = 7.6, 7.2, 1.2 Hz, CH),
7.33 (ddd, 1H, 3,4J = 7.6, 7.2, 1.2 Hz, CH), 7.66 (d, 1H,
3J = 7.6 Hz, CH), 7.72 (d, 1H, 3J = 7.6 Hz, CH), 8.67 (s,
Computational details
DFT geometry optimization and TD-DFT calculations
were carried out at the B3LYP/6-31G(d) level of theory using
the GAUSSIAN 03 suite of programs [21]. Frequency calcu-
lations carried out on the optimized structures confirmed the
absence of imaginary frequencies.
Acknowledgement
We gratefully acknowledge the following agencies for
funding: CONACyT (Grant 123605), the National Science
Foundation (CHE-0646916), and the Welch Foundation (A-
1423).
1
1H, N=CH). – 11B{ H} NMR (128.2 MHz; CDCl3): δ = 5
(w1/2 = 273 Hz). – 13C NMR (100.48 MHz; CDCl3): δ =
21.0 (CH3-Mes), 25.6, 25.7 (2CH3-Mes), 54.7 (NCH2), 61.7
(OCH2), 125.2, 126.2, 129.6, 130.5, 130.8, 132.6 (all CH),
129.3, 133.3, 135.0, 136.8, 136.9, 141.3, 141.7, 143.9 (all
Cipso), 170.0 (C=N). – IR (film): ν = 3551, 2916, 1547, 1609,
1454, 1438, 1374 cm−1. – HRMS ((+)-ESI): m/z = 397.414
Note added in proof
After this work was submitted, a paper describing the syn-
thesis of closely related 2-boryl-benzylideneamines has ap-
peared [22].
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