Fluorine/phenyl chelated boron complexes: Synthesis, fluorescence properties and catalyst for transfer hydrogenation of aromatic ketones

Article abstract of DOI:10.1016/j.jfluchem.2014.03.004

The synthesis of salen/salan ligands (L₁ and L₂) and their fluorine/phenyl chelated boron complexes [L_(1,2)BF₂] or [L_(1,2)BPh₂] is described in this paper. The fluorine/phenyl chelated boron complexes were synthesized from the reaction of BF₃·OEt₂ or BPh₃ with the corresponding ligands in different solvent. The boron complexes display high stability and can be handled in air due to the presence of coordinative B N and covalent BO bonds in their structures. The salen/salan ligands (designated as salan, a saturated version of the corresponding salen ligands) and their fluorine/phenyl chelated boron complexes have been characterized by ¹H, ¹³C NMR and ¹⁹F NMR spectra, elemental analysis, FT-IR spectra, UV-vis spectra, LC-MS spectra, melting point and fluorescence spectroscopy. The fluorescence efficiencies of BF₂-chelate boron complexes are greatly improved compared to those of the BPh₂-chelate boron analogs based on the same salen/salan ligands, probably due to the enhanced conjugation degree of the diphenyl boron chelation, which can effectively prevent molecular aggregation. The boron complexes [L_(1,2)BF₂] or [L _(1,2)BPh₂] were also applied to the transfer hydrogenation of aromatic ketones to the corresponding alcohol derivatives in the presence of iso-PrOH as the hydrogen source. Catalytic studies showed that all complexes are good catalytic precursors for transfer hydrogenation of aryl alkyl ketones in 0.1 M iso-PrOH solution. This transfer hydrogenation is characterized by low reversibility under these conditions.

Full text of DOI:10.1016/j.jfluchem.2014.03.004

Journal of Fluorine Chemistry 162 (2014) 9–16
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Fluorine/phenyl chelated boron complexes: Synthesis, fluorescence
properties and catalyst for transfer hydrogenation of aromatic ketones
Ahmet Kilic a, , Murat Aydemir b,e, Mustafa Durgun a, Nermin Meric¸ b, Yusuf Selim Ocak c,e
,
*
Armagan Keles a, Hamdi Temel d,e
a Department of Chemistry, Faculty of Science, University of Harran, 63190 S¸ anlıurfa, Turkey
b Department of Chemistry, Faculty of Science, University of Dicle, 21280 Diyarbakir, Turkey
c Department of Science, Faculty of Education, University of Dicle, 21280 Diyarbakir, Turkey
d Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Dicle University, Diyarbakir 21280, Turkey
e Science and Technology, Application and Research Center (DUBTAM), Dicle University, Diyarbakir 21280, Turkey
A R T I C L E I N F O
A B S T R A C T
Article history:
The synthesis of salen/salan ligands (L1 and L2) and their fluorine/phenyl chelated boron complexes
[L(1,2)BF2] or [L(1,2)BPh2] is described in this paper. The fluorine/phenyl chelated boron complexes were
synthesized from the reaction of BF3ꢀOEt2 or BPh3 with the corresponding ligands in different solvent.
The boron complexes display high stability and can be handled in air due to the presence of coordinative
B N and covalent B–O bonds in their structures. The salen/salan ligands (designated as salan, a
saturated version of the corresponding salen ligands) and their fluorine/phenyl chelated boron
complexes have been characterized by 1H, 13C NMR and 19F NMR spectra, elemental analysis, FT-IR
spectra, UV–vis spectra, LC–MS spectra, melting point and fluorescence spectroscopy. The fluorescence
efficiencies of BF2–chelate boron complexes are greatly improved compared to those of the BPh2–chelate
boron analogs based on the same salen/salan ligands, probably due to the enhanced conjugation degree
of the diphenyl boron chelation, which can effectively prevent molecular aggregation. The boron
complexes [L(1,2)BF2] or [L(1,2)BPh2] were also applied to the transfer hydrogenation of aromatic ketones
to the corresponding alcohol derivatives in the presence of iso-PrOH as the hydrogen source. Catalytic
studies showed that all complexes are good catalytic precursors for transfer hydrogenation of aryl alkyl
ketones in 0.1 M iso-PrOH solution. This transfer hydrogenation is characterized by low reversibility
under these conditions.
Received 28 January 2014
Received in revised form 3 March 2014
Accepted 8 March 2014
Available online 16 March 2014
Keywords:
Fluorinated boron complexes
Fluorescence
Catalysts
Transfer hydrogenation
Ketone
ß 2014 Elsevier B.V. All rights reserved.
1. Introduction
organic light-emitting diodes (OLEDs) [11,12], electron-transport
materials [13,14], sensitizers in solar cells [15,16], nonlinear optics
The fluorine/phenyl chelated boron complexes, one of the most
important types of fluorescent dyes, have been extensively studied.
In recent years, fluorescent materials of organic fluorine or phenyl
boron complexes continue to be developed because of their utilities
in many various fields of current research, and the many important
applicationsinscienceandtechnology.Amongboroncompounds,as
the most well-known and having excellent fluorescent properties is
borondipyrromethenes(BODIPYs)withN,N-bidentateligands[1–8].
These types of compounds are widely used as chemosensors [9,10],
[17,18] as well as antibacterial properties [19] and hydrogen-
transfer catalysts [20–22]. Therefore, in recent years, design and
synthesis of novel boron compounds to explore their broad
applications have attracted much attention. Among boron com-
pounds, three main types of BF2/BPh2 chelated boron complexes are
classified as N,N-bidentate, N,O-bidentate and O,O-bidentate com-
pounds. Compared with three-coordinated boron complexes, which
require bulky substituents (such as mesityl or other groups) to
stabilizefour-coordinatedboroncomplexesareingeneralstableand
can be obtained readily [23]. Also, it is known that boron is strongly
electrophilic by virtue of its tendency to fill the vacant orbital and
complete the octet, so in contrast to organometallic compounds,
organoboron compounds are in general more stable due to the
increased covalency of B–O and B N bonds [24]
*
Corresponding author at: Department of Chemistry, Harran University,
TR-63190 S¸ anliurfa, Turkey. Tel.: +90 4143183587; fax: +90 4143183541.
E-mail address: kilica63@harran.edu.tr (A. Kilic).
http://dx.doi.org/10.1016/j.jfluchem.2014.03.004
0022-1139/ß 2014 Elsevier B.V. All rights reserved.

10
A. Kilic et al. / Journal of Fluorine Chemistry 162 (2014) 9–16
Catalytic transfer hydrogenation is an effective method for the
2. Results and discussion
reduction of ketones and aldehydes to alcohols under mild reaction
conditions [25–27]. Transfer hydrogenation with the aid of a stable
hydrogen donor is a useful alternative method for catalytic
hydrogenation by molecular hydrogen [28,29]. In transfer hydro-
genation, organic molecules such as primary and secondary
alcohols [30] or formic acid and its salts [31] have been employed
as the hydrogen source. The use of the hydrogen donor has some
advantages over the use of molecular hydrogen since it avoids risks
and constraint associated with hydrogen gas as well as the
necessity for pressure vessels and other equipments. A number of
transition metal complexes are known to be catalyzing hydrogen
transfer from an alcohol to a ketone [32]. In the mid 1920s,
Meerwein, Verley and Ponndorf (MPV-reaction), respectively,
reported the first examples of hydrogen transfer from an alcohol
to a ketone [33–35]. The first example of a transition metal-
catalyzed transfer hydrogenation, employing an Ir-DMSO-com-
plex, was presented in 1967 by Trocha-Grimshaw and Henbest
[36]. Recently, Ikariya and coworkers have reviewed bifunctional
transition metal-based molecular catalysts for transfer hydrogena-
tion of ketones under different experimental conditions [37].
Some catalyst systems are expensive because synthesis of
compounds requires many chemical synthetic steps. Hence, there
is a need for economic, relatively mild and environmentally
friendly catalyst system. So, we preferred the BF2/BPh2 chelated
boron complexes as catalyst for transfer hydrogenation of
aromatic ketones. As far as we know, there are few reports on
BF2/BPh2 chelated boron complexes containing salen/salan
ligands have been used as catalysts for transfer hydrogenation
of aromatic ketones. Compounds of this type are considerably
interesting for their simple synthesis, easily accessibility and good
luminescence and fluorescence properties. Interestingly, a few of
N,O-bidentate BF2/BPh2 chelated boron compounds are the
efficient hydrogen-transfer catalysts. In this work, salen/salan
ligands were selected because they can function as chelating
ligands upon treatment with a boron compounds such as BPh3 or
2.1. Synthesis of compounds
AssummarizedinScheme1, thesalenligand(L1)was prepared in
moderate yield by refluxing the 1:1 molar ratio of 3,5-di-tert-
butylsalicylaldehyde and 4-amino-2,2,6,6-tetramethylpiperidine in
absolute ethanol and in the presence of 3-4 drops of formic acide as
catalyst. The salan ligand (L2) has been prepared by reduction of the
corresponding salen ligand (L1) with NaBH4 and in EtOH solution.
Both salen ligand (L1) and salan ligand (L2) are obtained in good
yield, 86 and 78%, respectively, as yellow or white solid. The
satisfactory spectroscopic results were acquired for both salen
ligand (L1) and salan ligand (L2). As anticipated, treatment of the
salen ligand (L1) or salan ligand (L2), with an excess of boron reagent
BF3.OEt2 in anhydrous benzene and in the presence of Et3N or with
an excess of boron reagent BPh3 in anhydrous toluene under N2
atmosphere afforded the boron complexes [L(1,2)BF2] or [L(1,2)BPh2],
respectively as outlined in Scheme 2. Both BF2-chelated boron
complexes [L(1,2)BF2] and BPh2-chelated ones [L(1,2)BPh2] are
obtained in good yield (72–65%) as yellow or white solids after
purification by column chromatography. Again, the satisfactory
spectroscopic results were acquired for all boron complexes. We
chose to employ the the salen ligand (L1) and salan ligand (L2) and
their boron complexes [L(1,2)BF2] or [L(1,2)BPh2] for several reasons:
they are (1) easily prepared, (2) scarcely seen in literature, (3) to the
best of our knowledge, there are few reports that boron complexes
containing the salen/salan ligands used as catalyst for the transfer
hydrogenation of aromatic and (4) the salen/salan ligands bind
strongly to boron ions. The salen ligand (L1) and salan ligand (L2) and
their boron complexes [L(1,2)BF2] or [L(1,2)BPh2] are stable in
different solutions and in the solid state upon extended exposure to
air. This high chemical stability in this class of compounds is
attributed to the bidentate chelation of the salen ligand (L1) and
salanligand(L2)totheboroncenter. Althoughmucheffortwasmade
to obtain single crystals of the compounds, we failed to do so.
However, the 1H, 13C NMR and 19F NMR spectra, elemental analysis,
FT-IR spectra, UV–vis spectra, LC–MS spectra, melting points and
fluorescence spectra support to the proposed structures.
BF3ꢀOEt2 conjugated
p-systems that was good for studying the
catalytic transfer hydrogenation of aromatic ketones. As
a
continuation of our previously reported work, we have synthe-
sized BF2/BPh2 chelated boron complexes containing electron
withdrawing or electron-donating substituents on their ligands.
We show that salen/salan ligands (L1 and L2) and their boron
complexes [L(1,2)BF2] or [L(1,2)BPh2] display attractive fluores-
cence and catalytic activity.
2.2. Spectroscopic characterization
The 1H and 13C NMR of the salen (L1) and salan (L2) ligands and
their fluorine/phenyl boron complexes [L(1,2)BF2] and [L(1,2)BPh2]
Scheme 1. Synthesis of the salen/salan ligands (L1 and L2).

A. Kilic et al. / Journal of Fluorine Chemistry 162 (2014) 9–16
11
Scheme 2. Synthesis of the [L(1,2)BF2] and [L(1,2)BPh2] boron complexes.
have been recorded in CDCl3 at room temperature, and also, the 19
NMR of the fluorinated boron complexes [L1BF2] and [L2BF2] have
been recorded in DMSO-d6 at room temperature, and the
assignments are listed in the experimental section. The disappear-
F
ꢂ1620 cmꢁ1 showing a small frequency shift [38], which may be
attributed to the formation of B N dative bond in these
complexes. Since salen ligand (L1) is converted to salan ligand
(L2), this peak is not observed for other boron complexes, as
expected. Appearance of the strong stretching vibrations in the
region of 1060–1057 cmꢁ1 for [L(1,2)BF2] and in the region of 1028–
1017 cmꢁ1 for [L(1,2)BPh2] may be due to the B N vibration mode
and those in the region of 882–876 cmꢁ1 were attributed to the B–F
stretching vibrations for [L(1,2)BF2] complexes and in the region of
886–882 cmꢁ1 were attributed to the B–Ph stretching vibrations
for [L(1,2)BPh2] complexes [39,40].
The absorption spectral characteristics of the salen (L1) and
salan (L2) ligands and their fluorine/phenyl boron [L(1,2)BF2] and
[L(1,2)BPh2] complexes were studied in CH3OH or THF solutions at
room temperature, in the range 200–1100 nm, and the results are
given in the experimental section. To make a reliable comparison,
the UV–vis absorption spectral behaviors of the salen (L1) and salan
(L2) ligands and their fluorine/phenyl boron [L(1,2)BF2] and
[L(1,2)BPh2] complexes were studied under the same experimental
conditions. Upon complexation with boron ions, new absorption
bands appearing at 220–402 nm in CH3OH and 226–398 nm in THF
ance of the signal due to phenolic OH protons (
d = 13.91 ppm for
(L1) and = 13.82 ppm for (L2), which disappear on D2O addition)
d
in all boron complexes refers to their involvement in coordination
with the boron center which implies the formation of B–O bond in
these boron complexes, as expected. The resonance for the
azomethine –CH55N– proton of salen ligand (L1) (
shifts to highfield in complexes as 8.38 ppm in [L1BF2] and
8.31 ppm in [L1BPh2]. Furthermore, in the 13C NMR spectra, the
signals for carbon atoms of the same group were observed at
d = 8.49 ppm)
d
d
d
164.54, 163.32 and 160.93 ppm for (L1), [L1BF2] and [L1BPh2],
respectively. The resonances for the azomethine –CH55N proton
the 1H and those of carbon in the 13C NMR spectra shifts to lower
field compared to those of free ligand, indicating the formation of
B–N bond and also confirming the participation of the iminic
nitrogen atoms in coordination for boron complexes [3]. Distinc-
tive signals for salan ligand (L2) and its boron [L2BF2] and [L2BPh2]
complexes are observed in the range 4.50–4.03 ppm for CH2–NH
proton in their 1H NMR spectra (see experimental section) and in
the range 47.61–46.09 ppm for CH2–NH carbon in their 13C NMR
spectra, as expected. Additionally for [L1BF2] and [L2BF2] com-
plexes, in the 19F NMR spectrum of each complex, the resonances
were attributed to
p
!
p
* or n !
p*, while charge transfer (CT)
transitions were found to be shifted to lower or higher energy
regions compared to the those of free ligands. Furthermore,
appearance of new bands at longer wavelength may be assigned to
ligand to boron center charge transfer (LBCT) transitions. The
characteristic absorption bands below 292 nm in CH3OH or THF
are found at
d
= ꢁ148.35 for [L1BF2] and
d
= ꢁ148.44 ppm for
[L1BF2] as quartet signal, respectively. These resonances show that
another solid evidence for the chelation of the BF2 group with the
salen (L1) and salan (L2) ligands. These values and the other NMR
data are in good agreement with the proposed structures.
solutions are practically identical and can be attributed to
p
!
p*
transitions in the phenyl ring or azomethine (–CH55N–) groups.
The other absorption bands observed within the range of 328–
352 nm in CH3OH or THF solutions are most probably due to n ! p*
transition of azomethine (–CH55N–) moiety. In the lower-energy
region, the absorption spectra show new bands at 391–402 nm in
CH3OH or THF solutions are most probably assigned as charge-
transfer transitions.
The fluorescence data of the salen (L1) and salan (L2) ligands and
their fluorine/phenyl boron [L(1,2)BF2] and [L(1,2)BPh2] complexes
were studied in THF solvent and the results are given in Figs. 1–3.
As shown in Fig. 1, the salen (L1) and salan (L2) ligands exhibit
fluorescence emission band located at 388 nm when excited at
354 nm for (L1), the emission band is located 385 nm when excited
The main FT-IR frequencies of the salen (L1) and salan (L2)
ligands and their fluorine/phenyl boron complexes [L(1,2)BF2] and
[L(1,2)BPh2] along with their proposed assignments are given in the
experimental section. Intermolecular H-bond (O–Hꢀ ꢀ ꢀN) gives a
band at 3182–2640 cmꢁ1 in FT-IR spectra of free salen (L1) and
salan (L2) ligands, which disappears in those of boron complexes
[L(1,2)BF2] and [L(1,2)BPh2], indicating deprotonation of organic
ligand through coordination. On the other hand, the existence of an
y
(B–O) peak in the range 1190–1183 cmꢁ1 for the boron
complexes, provide conclusive evidence for the formation of
foregoing boron complexes [21]. band was observed at
1632 cmꢁ1 in the FT-IR spectra of salen ligand (L1) distinctive
from salan ligand (L2), owing to azomethine (HC55N) group. In the
FT-IR spectra of [L1BF2] and [L1BPh2], this band was observed at
A
at 340 nm for (L2), which can be assigned to
The emission spectra shift to longer wavelengths due to the
p
!
p* transition [41].
y
existence azomethine (–CH55N–) group in the salen (L1) ligand or

12
A. Kilic et al. / Journal of Fluorine Chemistry 162 (2014) 9–16
(L2) ligands and theirfluorine/phenyl boron [L(1,2)BF2] and
[L(1,2)BPh2] complexes may be caused by an increase in rigidity
of the salen/salan ligands skeleton of boron complexes due to the
chelation of the BF2 or BPh2 groups [42]. As shown in Figs. 2 and 3,
the fluorescence emission band was observed at 453–503 nm and
when excited at 350–385 nm for the complexes, which can be
assigned as
be due to enlargement of the
p
!
p
* transition, also this bathochromic shift should
system, as a result of p–
p
p
conjugation. The formation of the B N dative bond via the
donation of lone-pair of the N atom to the B atom lowers the energy
gap between p* and p of the free salen (L1) and salan (L2) ligands,
and the chelating ring makes the ligands more rigid, which can
reduce the loss of energy via vibrational motions and increase the
emission efficiency [43].
The LC–MS spectra of the free salen (L1) and salan (L2) ligands
and their fluorine/phenyl boron [L(1,2)BF2] and [L(1,2)BPh2]
complexes were taken as evidence for the formation of the
proposed structures. The molecular ion peak is in good agreement
with the suggested molecular formula found from elemental
analyses and the other spectroscopic methods. The molecular ion
peak at m/z = 373.30 (calculated mass: 373.59) amu [M]+ confirms
the proposed formula for ligand (L1), at m/z = 375.32 (calculated
mass: 375.60) amu [M]+ confirms the proposed formula for ligand
(L2), at m/z = 420.12 (calculated mass: 420.39) amu [M]+ confirms
the proposed formula for [L1BF2], at m/z = 422.32 (calculated mass:
422.40) amu [M]+ confirms the proposed formula for [L2BF2], at m/
z = 537.46 (calculated mass: 537.61) amu [M+H]+ confirms the
proposed formula for [L1BPh2], and at m/z = 538.43 (calculated
mass: 538.62) amu [M]+ confirms the proposed formula for
[L2BPh2].
Fig. 1. The fluorescence spectra of salen/salan ligands (L1 and L2) recorded in THF.
2.3. Catalytic studies
Here, we observe the catalytic activity of boron [L(1,2)BF2] and
[L(1,2)BPh2] complexes in the transfer hydrogenation of aromatic
ketones to the corresponding alcohols. In a typical experiment,
0.005 mmol of the complex and 0.5 mmol of ketone were added to
a solution of NaOH in iso-PrOH (0.025 mmol of NaOH in 5 mL iso-
PrOH) and refluxed at 82 8C, the reaction being monitored by GC
techniques. In all the reactions, fluorine/phenyl boron [L(1,2)BF2]
and [L(1,2)BPh2] complexes catalyzed the reduction of ketones to
the corresponding alcohols via hydrogen transfer from iso-PrOH.
Recently, we have reported that the fluorine/phenyl boron
complexes as active catalysts in the reduction of aromatic ketones
[21]. The observed activity of these fluorine/phenyl boron
complexes have encouraged us to investigated other analogous
ligands. These fluorine/phenyl boron [L(1,2)BF2] and [L(1,2)BPh2]
complexes as catalyst precursors for the transfer hydrogenation of
acetophenone have been tested and typical results are summa-
rized in Table 1. As seen Table 1, with a complex/NaOH ratio of 1/5,
the complexes are very active leading to a quantitative transfor-
mation of the acetophenone. At room temperature no noticeable
formation of 1-phenylethanol was observed (Table 1, entries 1–4).
Furthemore, as can be inferred from Table 1 (entries 5–8), the
precatalysts as well as the presence of NaOH are necessary to
observe appreciable conversions. The base facilitates the formation
of boron alkoxide by abstracting proton of the alcohol and
Fig. 2. The fluorescence spectra of [L(1,2)BF2] boron complexes recorded in THF.
Fig. 3. The fluorescence spectra of [L(1,2)BPh2] boron complexes recorded in THF.
due to energy loss via fast rotation of different groups in the salan
(L2) ligand. The emission spectra of boron [L(1,2)BF2] and
[L(1,2)BPh2] complexes are significantly red-shifted in comparison
with the free salen (L1) and salan (L2) ligands (Figs. 2 and 3). The
fluorescence efficiencies of BF2–chelate boron complexes are
greatly improved compared to those of the BPh2-chelate boron
analogs based on the same salen/salan ligands, probably due to the
enhanced conjugation degree and of the diphenyl boron chelation,
which can effectively prevent molecular aggregation. The differ-
ence of fluorescence properties between the salen (L1) and salan
subsequently alkoxide undergoes
b-elimination to give hydride,
which is an active species in this reaction. The role of the base is to
generate a more nucleophilic alkoxide ion which would rapidly
attack the boron complex responsible for dehydrogenation of iso-
PrOH. As Table 1 shows, high conversions can be achieved with the
[L(1–2)BF2] and [L(1–2)BPh2] catalytic systems. The results of
optimization studies showed clearly that the excellent conversions
were achieved in the reduction of acetophenone to 1-phenyletha-
nol when the complex [L2BPh2] was used as the catalytic

A. Kilic et al. / Journal of Fluorine Chemistry 162 (2014) 9–16
13
Table 1
Transfer hydrogenation of acetophenone with iso-PrOH catalyzed by [L1BF2], [L1BPh2], [L2BF2] and [L2BPh2].
O
OH
Cat. /
NaOH
iso-PrOH
Catalyst
acetone
S/C/NaOH
e
Entry
Time
Conversion (%)d
TOF (hꢁ1
)
1
2
[L1BF2]a
100:1:5
100:1:5
100:1:5
100:1:5
100:1
48 h
48 h
48 h
48 h
48 h
48 h
48 h
48 h
12 h
15 h
15 h
8 h
Trace
Trace
Trace
Trace
<3
–
[L1BPh2]a
[L2BF2]a
–
3
–
4
[L2BPh2]a
[L1BF2]b
[L1BPh2]b
[L2BF2]b
[L2BPh2]b
[L1BF2]c
[L1BPh2]c
[L2BF2]c
[L2BPh2]c
–
5
–
6
100:1
<3
–
7
100:1
<3
–
8
100:1
<3
–
9
100:1:5
100:1:5
100:1:5
100:1:5
98
<10
<10
<10
12
10
11
12
99
99
99
Reaction conditions:
a
At room temperature; acetophenone/Cat./NaOH, 100:1:5.
Refluxing in iso-PrOH; acetophenone/Cat, 100:1, in the absence of base.
Refluxing in iso-PrOH; acetophenone/Cat/NaOH, 100:1:5.
Determined by GC (three independent catalytic experiments).
Referred at the reaction time indicated in column; TOF = (mol product/mol Cat.) ꢃ hꢁ1
b
c
d
e
.
precursor, with a substrate-catalyst molar ratio (100:1) in iso-PrOH
at 82 8C (Table 1, entries 9–12).
attributed to purely electronic effects [44], substrates involving
para- and ortho-substituted acetophenone derivatives were
investigated. The results indicated that a strong electron with-
drawing substituents, such as F, Cl and Br were capable of higher
conversion (Table 2). Conversely, the most electron-donor sub-
stituents, (2-methoxy or 4-methoxy) led to lower conversion. It is
well-known that the presence of an electron withdrawing group
has generally been found to facilitate the hydrogen transfer
reaction [45,46], and this has been attributed to the hydridic
nature of the reducing species involved. As such, reactions
with fluoro proceeded to higher conversion owing to rapid hydride
transfer, while reactions with electron-donating substituents
Encouraged by the high catalytic activities gained in these
preliminary studies, we next extended our investigations to
include hydrogenation of acetophenone derivatives. A variety of
simple ketones (S/C = 100/1) can be transformed to the corre-
sponding secondary alcohols with high conversion, as exemplified
in Table 2. The complexes [L1BF2] and [L2BPh2] were also
extensively investigated with a variety of substrates. As expected,
electronic properties (the nature and position) of the substituents
on the phenyl ring of the ketone caused the changes in the
reduction degree. To ensure that the observed results could be
Table 2
Transfer hydrogenation results for substituted acetophenones with the catalyst system, [L1BF2] and [L2BPh2].a
O
OH
OH
O
Cat.
R
R
+
+
c
Entry
R
Time
Conversion (%)b
TOF (hꢁ1
)
Cat: [L1BF2]
1
2
3
4
5
6
4-F
9 h
10 h
12 h
8 h
99
98
99
98
99
98
10
10
4-Cl
4-Br
<10
12
4-NO2
4-Me
4-MeO
18 h
20 h
<10
<10
Cat: [L2BPh2]
1
2
3
4
5
6
4-F
6 h
8 h
99
98
99
98
99
98
17
12
4-Cl
4-Br
9 h
11
4-NO2
4-Me
4-MeO
5 h
20
12 h
15 h
<10
<10
Reaction conditions:
a
Catalyst (0.005 mmol), substrate (0.5 mmol), iso-PrOH (5 mL), NaOH (0.025 mmol %), 82 8C, respectively, the concentration of acetophenone derivatives
is 0.1 M.
b
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), yields are based on methyl aryl ketone.
c
TOF = (mol product/mol Cat.) ꢃ hꢁ1
.

14
A. Kilic et al. / Journal of Fluorine Chemistry 162 (2014) 9–16
(2-methoxy or 4-methoxy) proceeded in a slower and more
controlled manner [22,47]. The examination of the results
indicates clearly that with each of the tested complexes, the best
yield was achieved in the reduction of acetophenone derivatives
when [L2BPh2] was used as the catalyst precursor.
4.3. GC analysis
GC analyses were performed on a Shimadzu 2010 Plus HP
6890N Gas Chromatograph equipped with capillary column (5%
biphenyl, 95% dimethylsiloxane) (30 m ꢃ 0.32 mm ꢃ 0.25
mm).
The GC parameters were for transfer hydrogenation of ketones
as follows; initial temperature, 110 8C; initial time, 1 min; solvent
delay, 4.48 min; temperature ramp 80 8C/min; final temperature,
200 8C; final time, 21.13 min; injector port temperature, 200 8C;
3. Conclusion
In summary, we have isolated and structurally characterized
two ligands and their fluorine/phenyl boron [L(1,2)BF2] and
[L(1,2)BPh2] complexes. The absorption spectra, NMR spectra, FT-
IR spectra, LC–MS spectra and fluorescence properties of the boron
[L(1,2)BF2] and [L(1,2)BPh2] complexes have been thoroughly
investigated and compared with those of free salen (L1) and salan
(L2) ligands. The fluorescence efficiencies of BF2–chelate boron
complexes are greatly improved compared to those of the BPh2-
chelate boron analogs based on the same salen/salan ligands,
probably due to the enhanced conjugation degree and of the
diphenyl boron chelation, which can effectively prevent molecular
aggregation. Furhermore, these fluorine/phenyl boron [L(1,2)BF2]
and [L(1,2)BPh2] complexes catalyzed the reduction of acetophe-
none derivatives via hydrogen transfer from iso-PrOH. They have
also exhibited promising catalytic activity in the transfer
hydrogenation reaction of various ketones. The procedure is
simple and efficient towards various aryl ketones. Future
investigations are aiming at the development of an asymmetric
version of this process.
detector temperature, 200 8C, injection volume, 2.0
mL.
4.4. Synthesis of the ligands
4.4.1. Synthesis of the ligand (L1)
The salen ligand (L1) was synthesized by a reported procedure
with some modifications [48]. Color: yellow, yield: 86%, m.p:
115 8C. Anal. Calc. for [C24H40N2O] (mw: 372.6 g/mol): C, 77.37; H,
10.82; N, 7.52, found: C, 77.01; H, 10.96; N, 7.44; LC–MS (Scan ES+):
m/z (%) calculated mass [M+H]+: 373.59, found: 373.30 (1 0 0). FT-
IR (KBr pellets,
3060 (Ar–CH), 2956–2871
(C–O); 1H NMR (CDCl3, TMS, 400.1 MHz,
y
max/cmꢁ1): 3258
(Aliph-CH), 1632
ppm): 13.91 (s, 1H,
y
(NH), 3182–2643
y
(O–Hꢀ ꢀ ꢀN),
y
y
y
(C55N) and 1174
y
d
OH), 8.49 (s, 1H, HC55N), 7.39 (d, 1H, Ar–CH), 7.13 (d, 1H, Ar–CH),
4.01 (d, 1H, CH–CH2), 2.35 (d, 2H, CH–CH2), 2.11 (d, 2H, CH–CH2),
1.86 (s, 1H, NH) and 1.80–1.52 (m, 30H, C–CH3); 13C NMR (CDCl3,
TMS, 100.6 MHz,
d ppm): 164.54 (HC55N), 158.40, 140.33, 137.03,
127.11, 126.13, 118.24 (Ar–CH), 61.42 (C–CH3), 51.41 (CH–CH2),
46.38 (CH–CH2), 35.38 (C–CH3), 34.94 (C–CH3), 31.87 (C–CH3) and
4. Experimental
28.71 (C–CH3); UV–vis (
227, 262 and 328 (THF).
lmax, nm): 233, 259, and 329 (CH3OH);
4.1. Materials and measurements
4.4.2. Synthesis of the ligand (L2)
All reagents and solvents were of reagent-grade and obtained
from commercial suppliers. 1H and 13C NMR with 19F NMR spectra
were recorded at 25 8C using a Agilent-VNMRS-400 spectrometer
operating at 400.1 MHz, 100.6 MHz, and 376.3 MHz, respectively.
Infrared spectra were recorded on a Perkin Elmer Spectrum RXI FT-
To a stirred solution of ligand (L1) (1.50 g, 4.03 mmol) in
absolute ethanol (70 mL), sodium borohydride (0.98 g,
24.17 mmol) was slowly added at 0 8C temperature. When the
mixture was colorless at room temperature for 24 h, it was poured
over distilled water (70 mL) and extracted with dichloromethane.
The combined organic phases were dried and the solvents were
removed in vacuo. The white solid of salan ligand (L2) was obtained
from recrystallization in a mixture of chloroform (CHCl3) and
CH3OH. Color: white, yield: 78%, m.p: 182 8C. Anal. Calc. for
[C24H42N2O] (mw: 374.6 g/mol): C, 76.95; H, 11.30; N, 7.48; found:
C, 76.83; H, 11.24; N, 7.53. LC–MS (Scan ES+): m/z (%) calculated
IR spectrometer (4000–400 cmꢁ1
) as KBr pellets. Elemental
analyses were performed by using a LECO CHNS model 932
elemental analyzer. UV–vis spectra were obtained with a Perkin-
Elmer model Lambda 25 spectrophotometer in the wavelength
range from 200 to 1100 nm at room temperature. Fluorescence
spectra were obtained with Perkin-Elmer model LS55 spectrome-
ter. Melting points were measured in open capillary tubes with an
Electrothermal 9100 melting point apparatus and are uncorrected.
LC–MS results were recorded on an Agilent LC/MSD LC–MS/MS
spectrometer. GC analyses were performed on a Shimadzu 2010
Plus HP 6890N Gas Chromatograph.
mass [M+H]+: 375.60, found: 375.32 (100). FT-IR (KBr pellets, ymax
/
cmꢁ1): 3275
2955–2865
(CDCl3, TMS, 400.1 MHz,
y
(NH), 3117–2640
y
(O–Hꢀ ꢀ ꢀN), 3000
y
(Ar–CH),
(C–O); 1H NMR
ppm): 13.82 (s, 1H, OH), 7.17 (d, 1H, Ar–
y
(Aliph–CH), 1606
y
(NH) and 1166
y
d
CH), 6.83 (d, 1H, Ar–CH), 4.03 (s, 2H, NH–CH2), 3.94 (t, 1H, CH–
CH2), 2.98 (s, 2H, NH), 1.92 (d, 4H, CH–CH2) and 1.39–1.08 (m, 30H,
C–CH3); 13C NMR (CDCl3, TMS, 100.6 MHz,
d ppm): 154.92, 140.80,
4.2. General procedure for the transfer hydrogenation of ketones
136.36, 123.38, 123.21, 122.86 (Ar–CH), 51.36 (C–CH3), 50.70 (CH–
CH2), 49.90 (CH–CH2), 46.09 (CH2–NH), 35.38 (C–CH3), 35.21 (C–
CH3), 31.98 (C–CH3), 29.94 (C–CH3). and 29.00 (C–CH3); UV–vis
(lmax, nm, * = shoulder peak): 232* and 283 (CH3OH); 226, 282 and
335* (THF).
Typical procedure for the catalytic hydrogen-transfer reaction:
a solution of the boron complexes [L(1-2)BF2] as electron donors
and phenyl boron complexes [L(1-2)BPh2] as electron acceptors
(0.005 mmol), NaOH (0.025 mmol) and the corresponding ketone
(0.5 mmol) in degassed iso-PrOH (5 mL) were refluxed until the
reactions were completed. Then, an aliquot of the catalytic solution
(1 mL) was removed with a syringe and evaporated under reduced
pressure. The resultant oil was subjected to flash chromatography
(silica gel-60, Et2O) and subsequently evaporated under reduced
pressure to yield clear liquids in each case. After this time, a sample
of the reaction mixture is taken off, diluted with acetone and
analyzed immediately by GC. Conversions obtained are related to
the residual unreacted ketone. Furthermore, 1H NMR spectral data
for the resultant products were consistent with previously
reported results.
4.5. Synthesis of the BF2-chelated boron complexes [L(1,2)BF2]
In a two-necked, 100 mL round-bottom flask and equipped with
a blanket of nitrogen (N2) was placed 40 mL of anhydrous benzene
at room temperature. To this solution it was added 0.50 g,
1.34 mmol of ligand (L1) and 0.50 g, 1.33 mmol of ligand (L2) in
15 mL anhydrous benzene respectively, followed by triethylamine
(Et3N) (0.2 mL, 1.34 mmol) was added to the for each solution.
After the reaction mixture was stirred for 50 min, boron trifluoride
diethyl etherate (BF3ꢀEt2O) (0.50 mL, 4.02 mmol) was added

A. Kilic et al. / Journal of Fluorine Chemistry 162 (2014) 9–16
15
slowly. The mixture was stirred at 40 8C for 14 h and then
yellowish compound for [L1BF2] or white compound for [L2BF2]
was gradually precipitated from the solution. After cooling to room
temperature, the solution was filtrated and the product was
washed several times with hexane and diethyl ether, successively.
The solid product was then purified by recrystallization and
sublimation.
calculated mass [M+H]+: 537.61, found: 537.46 (30). FT-IR (KBr
pellets, (Ar–CH), 2956–2847 (Aliph-CH),
max/cmꢁ1): 3068
1620 (C55N), 1602 (N–H), 1190 (B–O), 1156 (C–O), 1028 (B–
N) and 886 ppm): 8.31
(B–Ph); 1H NMR (CDCl3, TMS, 400.1 MHz,
y
y
y
y
y
y
y
y
y
d
(s, 1H, HC55N), 7.79 (d, 4H, Ph–CH,), 7.73 (d, 1H, Ar–CH), 7.68 (d, 1H,
Ar–CH), 7.45–7.32 (m, 6H, Ph–CH), 3.78–3.363 (m, 1H, CH–CH2),
2.02 (s, 1H, NH), 1.62 (d, 4H, CH–CH2) 1.47 (s, 18H, C–CH3) and 1.34
(s, 12H, C–CH3); 13C NMR (DMSO-d6, TMS, 100.6 MHz,
d ppm):
4.5.1. Synthesis of the complex [L1BF2]
160.93 (HC55N), 144.72, 140.08, 139.91, 137.96, 137.63, 135.11,
134.12, 134.01, 132.77, 131.85, 131.42, 126.49, 125.70, 119.41 and
113.27 (Ar–CH), 54.08 (C–CH3), 51.94 (CH–CH2), 45.56 (CH2–NH),
37.85 (C–CH3), 34.91 (C–CH3), 31.55 (C–CH3), 29.80 (C–CH3). and
Color: yellow, yield: 72%, m.p: 162 8C. Anal. Calc. for
[C24H39N2OBF2] (mw: 420.4 g/mol): C, 68.57; H, 9.35; N, 6.66;
found: C, 68.49; H, 9.39; N, 6.59; LC–MS (Scan ES+): m/z (%)
calculated mass [M]+: 420.39, found: 420.12 (35). FT-IR (KBr
28.05 (C–CH3). UV–vis (
lmax, nm): 243, 292 and 392 (CH3OH); 248,
pellets,
(Aliph-CH), 1623
N) and 882
(B–F); 1H NMR (CDCl3, TMS, 400.1 MHz,
y
max/cmꢁ1): 3195
(C55N), 1183
y
(N–H), 3129
y
(Ar–CH), 2969–2847
(C–O), 1060 (B–
ppm): 8.38
267, 292 and 391 (THF).
y
y
y
(B–O), 1165
y
y
y
d
4.6.2. Synthesis of the complex [L2BPh2]
(s, 1H, HC55N), 7.79 (d, 1H, Ar–CH), 7.61 (d, 1H, Ar–CH), 4.40–4.36
(m, 1H, CH–CH2), 2.28 (d, 2H, CH–CH2), 2.14 (d, 2H, CH–CH2), 1.88
(s, 1H, NH) and 1.99–1.85 (m, 30H, C–CH3); 13C NMR (CDCl3, TMS,
Color: white, yield: 65%, m.p: 119 8C; Anal. Calc. for
[C24H41N2OBPh2] (mw: 538.6 g/mol): C, 80.28; H, 9.54; N, 5.20;
found: C, 80.23; H, 9.58; N, 5.18; LC–MS (Scan ES+): m/z (%)
calculated mass [M]+: 538.62, found: 538.43 (42). FT-IR (KBr
100.6 MHz,
d ppm): 163.32 (HC55N), 153.35, 135.98, 131.79,
122.64, 122.54, 113.78 (Ar–CH), 66.72 (C–CH3), 53.05 (CH–CH2),
52.39 (CH–CH2), 41.88 (C–CH3), 38.11 (C–CH3), 30.66 (C–CH3),
29.98 (C–CH3), 27.38 (C–CH3) and 26.16 (C–CH3); 19F NMR (DMSO-
pellets,
CH) 1608
(B–Ph); 1H NMR (CDCl3, TMS, 400.1 MHz,
y
max/cmꢁ1): 3051 and 3026
(N–H), 1183 (B–O), 1166
y
(Ar–CH), 2964–2871
(C–O), 1017 (B–N) and 882
ppm): 7.76 (d, 1H, Ar–
y(Aliph-
y
y
y
y
y
d
d6, 376.3 MHz,
d
ppm) ꢁ148.35 (q, J(B–F) = 31 Hz, BF2). UV–vis
CH), 7.68 (d, 1H, Ar–CH), 7.43–7.25 (m, 10H, Ph–CH), 4.26 (s, 2H,
NH–CH2), 3.24 (s, 1H, CH–CH2), 2.71 (s, 2H, NH), 1.89 (d, 4H, CH–
CH2) and 1.38–1.02 (m, 30H, C–CH3); 13C NMR (CDCl3, TMS,
(
l
max, nm, * = shoulder peak): 223, 262, 292* and 332 (CH3OH);
227, 261, 286* and 329 (THF).
100.6 MHz, d ppm): 154.68, 140.23, 140.01, 138.07, 137.61, 134.92,
4.5.2. Synthesis of the complex [L2BF2]
134.20, 133.86, 132.68, 132.15, 131.73, 126.32, 126.04, 119.30 and
114.38 (Ar–CH), 56.12 (C–CH3), 52.46 (CH–CH2), 46.73 (CH2–NH),
37.82 (C–CH3), 35.23 (C–CH3), 32.17 (C–CH3), 28.93 (C–CH3). and
28.12 (C–CH3); UV–vis (lmax, nm, * = shoulder peak): 236, 251, 337
and 402* (CH3OH); 248, 291, 332 and 398* (THF).
Color: white, yield: 68%, m.p: 145 8C. Anal. Calc. for
[C24H41N2OBF2] (mw: 422.4 g/mol): C, 68.24; H, 9.78; N, 6.63;
found: C, 68.18; H, 9.70; N, 6.67; LC–MS (Scan ES+): m/z (%)
calculated mass [M]+: 422.40, found: 422.32 (25). FT-IR (KBr
pellets,
(Aliph-CH) 1578
N) and 876
(B–F); 1H NMR (CDCl3, TMS, 400.1 MHz,
y
max/cmꢁ1): 3169
(N–H), 1186
y
(N–H), 3095
y
(Ar–CH), 2969–2878
(C–O), 1057 (B–
ppm): 7.70
y
y
y
(B–O), 1167
y
y
Acknowledgements
y
d
(d, 1H, Ar–CH,), 7.36 (d, 1H, Ar–CH,), 4.50 (d, 2H, NH–CH2,), 3.68 (s,
1H, CH–CH2), 1.99 (s, 2H, NH), 1.84 (d, 4H, CH–CH2) and 1.40–1.04
Partial support from Harran University (HUBAK Project No:
12159, Sanliurfa, Turkey), Dicle University (Project number: 2009
DPTK 120580) and Science and Technology Application and
(m, 30H, C–CH3); 13C NMR (CDCl3, TMS, 100.6 MHz,
d ppm):
154.64, 143.32, 133.78, 129.80, 124.21, 121.18, 114.46 (Ar–CH),
59.24 (C–CH3), 50.48 (CH–CH2), 47.66 (CH–CH2), 47.61 (CH2–NH),
41.33 (C–CH3), 35.46 (C–CH3), 32.22 (C–CH3), 30.75 (C–CH3). and
¨
Research Center (DUPTAM) is gratefully acknowledged.
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J(B–F) = 30 Hz, BF2). UV–vis (
d
ppm) ꢁ148.44 (q,
l
max, nm, * = shoulder peak): 220*, 281
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