Synthesis, characterization, photophysical properties of new fluorescent boron Schiff bases (BOSCHIBAs) and their application as cytoplasm staining dyes in vitro

Article abstract of DOI:10.1016/j.jorganchem.2017.10.003

In this paper, we report a series of Schiff bases synthesized by the condensation reaction between 2-hydroxy-1-naphthaldehyde with aniline derivatives as well as their further coordination with diphenylborinic acid for giving the corresponding boron Schif

Full text of DOI:10.1016/j.jorganchem.2017.10.003

Accepted Manuscript
Synthesis, characterization, photophysical properties of new fluorescent boron Schiff
bases (BOSCHIBAs) and their application as cytoplasm staining dyes in vitro
María M. Corona-López, Víctor M. Jiménez Pérez, Rodrigo Chan-Navarro, Marisol
Ibarra-Rodríguez, H.V. Rasika Dias, Arturo Chávez-Reyes, Blanca M. Muñoz-Flores
PII:
S0022-328X(17)30579-X
10.1016/j.jorganchem.2017.10.003
JOM 20123
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 26 June 2017
Revised Date: 2 October 2017
Accepted Date: 5 October 2017
Please cite this article as: Marí.M. Corona-López, Ví.M. Jiménez Pérez, R. Chan-Navarro, M. Ibarra-
Rodríguez, H.V. Rasika Dias, A. Chávez-Reyes, B.M. Muñoz-Flores, Synthesis, characterization,
photophysical properties of new fluorescent boron Schiff bases (BOSCHIBAs) and their application
as cytoplasm staining dyes in vitro, Journal of Organometallic Chemistry (2017), doi: 10.1016/
j.jorganchem.2017.10.003.
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ACCEPTED MANUSCRIPT
Synthesis, characterization, photophysical properties of new fluorescent boron Schiff
bases (BOSCHIBAs) and their application as cytoplasm staining dyes in vitro.
María M. Corona-Lópeza, Víctor M. Jiménez Péreza, Rodrigo Chan-Navarroa, Marisol
Ibarra-Rodrígueza, H. V. Rasika Diasb, Arturo Chávez-Reyes,c and Blanca M. Muñoz-
Floresa,*
aUniversidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, Ciudad
Universitaria, Av. Universidad s/n. C. P. 66451, Nuevo León, México.
bDepartment of Chemistry and Biochemistry, The University of Texas at Arlington,
Arlington, Texas 76019-0065, United States.
cCentro de Investigación y de Estudios Avanzados del IPN, Unidad Monterrey, PIIT, C.P.
66600. Apodaca, Nuevo León, México.
*Corresponding author: Tel +52(81)83294000, ext. 3401; Fax: 52 (81)83760570.
E-mail: blanca.munozfl@uanl.edu.mx (B. M. Muñoz-Flores)

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Abstract
In this paper, we report a series of Schiff bases synthesized by the condensation
reaction between 2-hydroxy-1-naphthaldehyde with aniline derivatives as well as their
further coordination with diphenylborinic acid for giving the corresponding boron Schiff
bases (BOSCHIBAs) (E)-N-((2-((di-phenylboryl)oxy)naphthalen-1-yl)methylene)pyridin-
3-amine
(1a),
(E)-N-((2-((di-phenylboryl)oxy)naphthalen-1-yl)methylene)-2,6-di-
methylaniline (2a), and (E)-N-((2-((di-phenylboryl)oxy)naphthalen-1-yl)methylene)-2,6-di-
isopropylaniline (3a). The resulting BOSCHIBAs were characterized by NMR (1H, 13C,
and 11B), FT-IR, and atmospheric pressure ionization time-of-flight mass spectrometry.
BOSCHIBA1a was structurally characterized by single-crystal. Structure analysis indicates
that the boron atom adopts a tetrahedral molecular geometry into a six-membered ring with
a half-chair conformation. BOSCHIBAs 1a-3a and their ligands 1-3 exhibit relatively low
quantum yields in a range from 1 to 3 %. In a bioimaging study by using the BOSCHIBAs
2a, and 3a as fluorescent staining dyes, those materials showed enhanced features in terms
of low cytotoxicity, simple synthesis, photostability, hydrolytic stability and specific
staining for cytoplasm structures.
Keywords: BOSCHIBAs, cytoplasm staining, low cytotoxicity, high photostability,
fluorescent bioimaging.

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Introduction
Nowadays fluorescence bioimaging (FBI) based on small organic molecules have
gained tremendous attention as an indispensable tool in cellular biology and biomedical
research because it provides a unique approach for the visualization of cell internal
structures with morphological details in tissue at a subcellular resolution level. Hence, FBI
might advance knowledge of cellular biology and disease at the molecular level: both in
vitro and in vivo [1]. Nevertheless, this technique is not invasive, and it is revolutionizing
the biomedical research and clinical practice [2].
On the flip side, the cytoplasm is a fluid thicker than water which contains all the
cell organelles and it provides mechanical support to the internal cell structures. Moreover,
the most important activities into the cell occur in the cytoplasm which include metabolic
process, breakdown of waste, glycolysis, and cell division. In this sense, recently
fluorescent cytoplasm markers have gained much interest since they can associated with the
diseases and abnormal cellular functions [3]. As examples of fluorescence cytoplasm
markers in the literature, there are reported antibodies, nanoparticles [4], quantum dots
[5,6], lanthanide complexes [7], and small organic molecules such as organoboron
compounds [8]. All these fluorophores must cover some important features for practical
analyses which include solubility, low cytotoxicity, photostability, and degradability. In
particular, the cytotoxicity, and photostability are two important parameters to be
measurement before thinking about the practical purpose or application. It is remarkable
that in recent years there has been considerable curiosity in the synthesis of organoboron
compounds in areas such as supramolecular chemistry [9], medicinal chemistry [10],
materials chemistry [11], imaging materials [12], organic light-emitting diodes [13],

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organic field effect transistors [14], and photo-responsive materials [15,16]. Specially, the
responsiveness has increased to those boron Schiff bases (BOSCHIBAs) due to they
provide an interesting variety of molecular structural conformations and while electron
withdrawing imine group (C=N) interacts with metal ions giving rise to materials with
different optoelectronic properties [17] and several new applications [18]. Furthermore,
bidentate Schiff bases could act as fluorescent molecular sensors for detection of toxic
heavy metals in living cells by confocal microscopy [19,20]. In the literature there are many
reports of the synthesis of BODIPY type boron compounds which have shown to be useful
in FBI. These compounds display adequate permeability through cell membrane and it
diffuses within the cytoplasm [21,22]. Likewise, to get FBIs in vitro or in vivo for practical
situations, the photostability under UV-Vis light irradiation plays an important role. With
the purpose to avoid the photobleaching, some strategies have been reported by the addition
of antifading compounds, organic antioxidants, triplet state quenchers [23], and
micrometer-sized polydimethylsiloxane wells [24]. However, it has scarcely been studied to
use the bulky groups in organic dyes to reduce the photobleaching effect [25]. We have
recently reported that mononuclear and binuclear BOSCHIBAs (Figure 1) [26] exhibit a
low cytotoxicity level which is good for potential biomedical application but they have also
a poor capacity for staining cells with a low quantum yield ( ΦF 1%) attributed to the
boron atom is outside the ligand-plane.
Keeping this in mind, we designed new fluorescent BOSCHIBAs where the π-
system delocalization was increased by the addition of the naphthyl group [27] and the
photo-stability was also increased by the use of bulky groups (Scheme 1). All BOSCHIBAs
11
13
1a-3a and their free ligand1-3 were fully characterized by NMR (1H, B, and C), ATR-

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IR spectroscopy, and atmospheric pressure ionization time-of-flight mass spectrometry. In
the case of BOSCHIBA 1a, it was characterized by X-ray diffraction.
Figure 1. Here
Experimental Section
General marks
All starting materials were procured from Aldrich Chemical Company. Solvents
were used without further purification. The diphenylborinic acid (Ph2BOH) was prepared in
situ as previously reported in the literature [28]. Melting points were confirmed by using an
Electrothermal Mel-Temp apparatus. Infrared spectra were recorded using a Bruker Tensor
27 FT-IR spectrophotometer equipped with a Pike MiracleTM ATR accessory with single
1
13
11
reflection ZnSe ATR crystal. H, C, and B NMR spectra were recorded in CDCl3 on a
1
13
Bruker advance DPX 400. Chemical shifts (ppm) are relative to (CH3)4Si for H and C.
11B NMR spectra were referenced externally to BF3 OEt2. Atmospheric pressure ionization
time-of-flight mass spectrometry(APCI-TOF-MS) in positive ion mode was acquired on an
Agilent Technologies instrument.
X-ray crystallography
The X-ray crystallography data for 1a (CCDC: 1520512)were measured at 100(2) K
on a Bruker D8 Quest with a Photon 100 CMOS detector equipped with an Oxford
Cryosystems 700 series cooler, a Triumph monochromator, and a Mo Kα fine-focus sealed
tube (λ = 0.71073 Å). Intensity data were processed using the Bruker Apex II program

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suite. All the calculations for the structure determination were carried out using the
SHELXTL package (version 6.14). Initial atomic positions were located by direct methods
using XS, and the structures of the compounds were refined by the least-squares method
using SHELXL. Absorption corrections were applied by using SADABS. All the non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in idealized
positions and refined as riding atoms with relative isotropic displacement parameters.
Optical measurements
UV-Vis absorption spectra were measured on a UV spectra were obtained with a
Perkin Elmer Lambda 365 UV/Vis spectrophotometer. The emission spectra have been
recorded with a Fluorolog-3 research spectrometer, by exciting 10 nm below the longer
wavelength absorption band. Fluorescence quantum yields (ΦF) in solution were
determined according to the procedure reported in the literature [29] and using quinine
sulfate in H2SO4 0.1M (Φ= 0.54 at 310 nm) as the standard. Temperature was regulated at
25.0+0.5 °C with a water circulating bath. Three solutions with absorbance at the excitation
wavelength lower than 0.1 were analyzed for each sample and the ΦF was averaged.ΦF
measurements were measured by the relative method and the quantum yield of the
unknown, Φx, is calculated according to the following equation:
ꢈ
ꢅ
ꢆ
ꢇ
ꢇ
Φꢀ = Φꢁ. ꢂꢂ
.
.
.
ꢆ
ꢄ
(1)
ꢃ
ꢄ
ꢃ
ꢄ
ꢈ
ꢅ
ꢃ
ꢄ
ꢃ
where ΦR is the quantum yield of the standard, A is the absorbance of the solution, E is the
corrected emission intensity, I is the relative intensity of the exciting light and n is the
average refractive index of the solution. Subscripts R and X refer to the reference and
unknown compound, respectively.

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Photostability testing and stability in aqueous DMSO solutions
With the purpose to perform stability assay, BOSCHIBAs 1a-3a were adjusted to
give absorption of 0.5 u.a at the short wavelength absorption band. The samples were
illuminated with 1.2 mW/cm2 (365 nm) for 60 min with interval of 10 min at room
temperature with air atmosphere. Each absorption spectrum was measured by using a
Perkin Elmer Lambda 365 UV/Vis spectrophotometer in the region of 190-700 nm.
Likewise, the stability in aqueous solutions at 1% v/v of DMSO was evaluated under the
same experimental condition without photoirradiation. Both analyses are critical to assess
how stable the dye is during imaging applications discussed later.
Cytotoxicity Assays
B16F10 murine melanoma cells (ATCC CRL-6475, Manassas, VA) were used to
determine the cytotoxic effects of 1a-3a complexes. Cells were maintained in GIBCO-
DMEM/F12 culture media supplemented with 10% FBS and 1X antibiotic-antimitotic (all
from ThermoFisher Scientific, Waltham, MA), at 37°C in an atmosphere of 5% CO2. For
the experiments cells were plated in 96-wells plates at a cell density of 2000 cells per well
in 100 µL of media and let them undisturbed overnight before the treatments were added.
Compounds were added at concentrations of 0.1, 1, 2.5, 5, and 10 µg/mL. Forty-eight hours
later 10 µl of alamarBlue (Biosource Invitrogen Life Technologies, Carlsbad, CA) were
added to each well to determine cell viability following manufacturer’s instructions.

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Bioimaging Assays
To assess the BOSCHIBAs capabilities to stain cells in vitro, B16F10 cells were
plated at densities of 5x104 cells per well in 500 µL of media on coverslips in 12 wells
plates. After overnight incubation 10 µg/mL of each complex were added to each well and
2 hours later coverslips were mounted with Vectashield (Vector Laboratories, Inc.
Burlingame, CA) and analyzed by confocal microscopy in a Leica TCS SP5 Confocal
System at excitation wavelength of 405 nm and emission of 420-550 nm, or excitation of
488 nm and emission of 500-600 nm. DMSO treated cells were used as a control to
determine endogenous fluorescence.
General procedure of synthesis of BOSCHIBAs 1a-3a and Schiff bases 1-3.
(E)-1-((pyridin-3-ylimino)methyl)naphthalen-2-ol (1)
A homogeneous mixture of 2-hydroxy-1-naphthaldehyde (0.50 g, 2.91 mmol) with
3-aminopyridine (0.27 g, 2.91 mmol) in acetonitrile was heated under reflux for 48 h. The
reaction mixture was slowly cooled at room temperature and the precipitated product was
filtrated and washed with hexane to give 0.57 g (2.30 mmol, 79 % yield) of 1 as a yellow
solid. M.P.: 182° C; FT-IRυmax (cm-1): 3048, 1624 (C=N), 1562, 1484; UV/Vis (THF):
λabs/max (nm), [εmax*104 (M-1cm-1)]: 377 [0.10 ], 322; Fluorescence (CHCl3): λfluor (nm): 490;
3
1H NMR (400 MHz, CDCl3) δ (ppm): 7.16 (1H, d, , J = 8.0 Hz, H-3), 7.37 (2H, m, , H-7,
3
3
H-14), 7.54 (1H t, J = 16 Hz, H-8), 7.65 (1H, d, J = 8.0 Hz, H-13), 7.75 (1H, d, 3J = 8.0
3
3
3
Hz, H-6), 7.84 (1H, d, J = 12 Hz, H-4), 8.13 (1H, d, J = 8.0 Hz, H-9), 8.54 (1H, d, J =
3
8.0 Hz, H-15), 8.63 (1H, d, J = 4.0 Hz, H-17), 9.42 (1H, s, H-11), 14.93 (1H, s, OH); 13C
{1H} NMR (100 MHz, CDCl3) δ (ppm): 109.42 (C-1), 119.22 (C-9), 120.68 (C-3), 123.96

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(C-14), 124.09 (C-7), 127.80 (C-13), 128.35 (C-8), 129.53 (C-6), 132.94 (C-10), 136.52
1
(C-4), 142.99 (C-17), 143.75 (C-12), 147.76 (C-15), 158.70 (C-11), 166.20 (C-2); H/13C
HETCOR NMR
δ (ppm): 7.16/120.54 (H-3/C-3), 7.36/123.84 (H-14/C-14), 7.54/128.21
(H-8/C-8), 7.64/127.66 (H-13/C-13), 7.75/129.41 (H-6/C-6), 7.84/136.39 (H-4/C-4),
8.12/119.09 (H-9/C-9), 8.54/147.64 (H-15/C-15), 8.64/142.89 (H-17/C-17), 9.42/158.57
1
(H-11/C-11); H/1H COSY NMR
δ (ppm):7.89/7.15 (H-4/H-3), 7.55/7.38 (H-8/H-7),
7.64/7.38 (H-13/H-14), 7.73/7.38 (H-6/H-7), 8.12/7.54 (H-9/H-8), 8.54/7.38 (H-15/H-14).
(E)-1-(((2,6-dimethylphenyl)imino)methyl)naphthalen-2-ol (2)
Preparation of 2 was accomplished like that of 1 from 2-hydroxy-1-naphthaldehyde
(0.50 g, 2.91 mmol) with 2,6-dimethylaniline (0.35 g, 2.91 mmol). The product was
obtained as a yellow solid with yield of 75 % (0.60 g, 2.18 mmol). M.P.: 118° C. ATR-
IRυmax: 3074, 2990, 2948, 1613 (C=N), 1573, 1161 cm-1; UV/Vis (THF): λabs/max (nm),
[εmax*104 (M-1cm-1)]: 364 [0.58 ], 315; Fluorescence (CHCl3): λfluor (nm): 473;1H NMR
3
(400 MHz, CDCl3), δ (ppm): 7.13 (1H, d, J = 8.0 Hz, H-3), 7.20 (3H, m, H-14, H-15, H-
16), 7.36 (1H, t, 3J = 16.0 Hz, H-7), 7.51 (1H, t, 3J = 16.0 Hz, H-8), 7.77 (1H, d, 3J = 8 Hz,
3
3
H-6), 7.86 (1H, d, J = 8.0 Hz, H-4), 8.00 (1H, d, J = 8.0 Hz, H-9), 2.34 (6H, s, H-18, H-
13
19), 9.14 (1H, s, H-11), 15.24 (1H s, OH); C NMR {1H}(100 MHz, CDCl3) δ (ppm):
18.77 (C-18,C-19), 108.49 (C-1), 118.85 (C-9), 121. 61 (C-15), 123.50 (C-7), 125.70 (C-3),
127.41 (C-5), 128.14 (C-8), 128.70 (C-14, C-16), 129.39 (C-6), 129.70 (C-13, C-17),
133.27 (C-10), 136.06 (C-4), 145.86 (C-12), 161.32 (C-11), 168.02 (C-2); 1H/13C HETCOR
NMR
δ(ppm): 2.31/18.60 (H-18,19/C-18,19), 7.14/125.66 (H-3,/C-3), 7.20/128.69 (H-14,
16/C-14, 16), 7.23/121.56 (H-15/C-15), 7.37/123.46 (H-7/C-7), 7.52/128.10 (H-8/C-8),

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7.79/129.36 (H-6/C-6), 7.87/136.04 (H-4/C-4), 8.01/118.79 (H-9/C-9), 9.16/161.27 (H-
1
11/C-11); H/1H COSY NMR
δ
(ppm):7.86/7.13 (H-4/H-3), 7.77/7.38 (H-6/H-7), 7.51/7.38
(H-8/H-7), 7.99/7.51 (H-9/H-8).
(E)-1-(((2,6-diisopropylphenyl)imino)methyl)naphthalen-2-ol (3)
Preparation of 3 was accomplished like that of 1 from 2-hydroxy-1-naphthaldehyde
(0.5 g, 2.91 mmol) with 2,6-diisopropylaniline (0.51 g, 2.91 mmol). The product was
obtained as a yellow solid with yield of 73 % (0.70 g, 2.12 mmol). M.P.: 192° C. AT-IRυmax
cm-1: 2980, 2964, 2943, 1620 (C=N), 1575, 1172, UV/Vis (THF): λabs/max (nm), [εmax*104
1
(M-1cm-1)]: 362 [0.71 ], 315; Fluorescence (CHCl3): λfluor (nm): 477; H NMR (400 MHz,
CDCl3), δ (ppm): 1.13 (12H, d, 3J = 8.0 Hz, H-19, H-20, H-22, H-23), 3.02 (2H, septet, 3J =
3
28.0 Hz, H-18, H-21), 7.10 (1H , d, J = 8.0 Hz, H-3), 7.15 (3H, m, , H-14, H-15, H-16),
7.24 (t, 1H, 3J = 16 Hz, H-8), 7.39 (1H, t, 3J = 16 Hz, H-7), 7.66 (1H, d, 3J = 8.0 Hz, H-6),
3
3
7.76 (1H, d, J = 8.0 Hz, H-4), 7.89 (1H, d, J = 8.0 Hz, H-9), 8.98 (1H, s, H-11), 15.13
13
(1H, s, OH); C NMR {1H}(100 MHz, CDCl3) δ (ppm): 22.66 (C-19, C-20, C-22, C-23),
27.27 (C-18, C-21), 107.30 (C-1), 117.70 (C-9), 120.34 (C-3), 122.39 (C-8), 122.49 (C-14,
C-16), 125.21 (C-15), 126.38 (C-5), 127.07 (C-7), 128.30 (C-6), 132.15 (C-10), 134.87 (C-
1
4), 139.22 (C-13, C-17), 142.94 (C-12), 160.61 (C-11), 166.35 (C-2); H/13C HETCOR
NMR δ(ppm): 1.09/22.54 (H-19, 20, 22, 23/C-19, 20, 22, 23), 3.01/27.14 (H-18, 21/C-18,
21), 7.10/120.31 (H-3/C-3), 7.14/122.43 (H-14, 16/C-14, 16), 7.15/125.17 (H-15/C-15),
7.23/122.34 (H-8/C-8), 7.38/127.05 (H-7/C-7), 7.65/128.26 (H-6/C-6), 7.74/134.83 (H-4/C-
4), 7.88/117.65 (H-9/C-9) 8.98/160 (H-11/C-11); 1H/1H COSY NMR
δ (ppm):7.75/7.09 (H-
4/H-3), 7.38/7.25 (H-7/H-8), 7.38/7.65 (H-7/H-6), 7.88/7.25 (H-9/H-8);

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(E)-N-((2-((diphenylboryl)oxy)naphthalen-1-yl)methylene)pyridin-3-amine (1a)
A solution of (E)-1-((pyridin-3-ylimino)methyl)naphthalen-2-ol 1 (0.1 g, 0.40
mmol) with diphenylborinic acid (0.18 g, 1.0 mmol) in acetonitrile was heated under reflux
for 48 h. The reaction mixture was slowly cooled to room temperature, and the precipitated
product was filtrated and washed with hexane followed by recrystallization in a
chloroform/hexane mixture to give 0.16 g (0.39 mmol, 95%) of BOSCHIBA 3a as a yellow
crystalline solid. M.P.: 222° C; FT-IR
(cm-1): 2962, 1624, 1608(C=N), 1550, 1458,
υmax
1312, 1204, 1140, 892, 878, 825, 795, 745, 702 (O-B),623,597; UV/Vis (THF): λabs/max
1
(nm), [εmax*104 (M-1cm-1)]: 425 [0.51 ], 337; Fluorescence (CHCl3): λfluor (nm): 530; H
NMR (400 MHz, CDCl3) δ (ppm): 7.98 (1H, d, 3J = 4 Hz, H-3), 7.09 (7H, m, H-13, 4H-m,
3
3
2H-p), 7.27 (2H, m, H-7, H-8), 7.36 (4H, d, J = 8.0 Hz, H-8, 4H-o), 7.43 (1H, t, J = 16
3
3
Hz, H-14), 7.59 (1H, d, J = 8.0 Hz, H-6), 7.77 (1H, d, J = 8.0 Hz, H-4), 7.81(1H, d, 3J =
3
13
12 Hz, H-9), 8.33 (1H, d, J = 4.0 Hz, H-15), 8.37(1H, s, H-17), 8.86 (1H, s, H-11); C
NMR {1H}(100 MHz, CDCl3) δ (ppm): 111.56 (C-1), 119.27 (C-9), 121.52 (C-3), 123.04
(C-7), 124.70 (C-13), 126.79 (C-14), 127.27 (C-m, C-p), 127.75 (C-5), 129.42 (C-8),
129.72 (C-6), 132.31 (C-10), 132.60 (C-4), 133.73 (C-o), 141.29 (C-i), 142.56 (C-17),
144.99 (C-12), 148.82 (C-15), 157.75 (C-11), 165.96 (C-2); 11B NMR (128 MHz, CDCl3) δ
(ppm): 7.38; APCI-TOF-MS in positive ion mode calc. for [(C28H21N2O4B+H)+]: 413.1700
u.m.a; Exp.: 413.1822 u.m.a
(E)-N-((2-((diphenylboryl)oxy)naphthalen-1-yl)methylene)-2,6-dimethylaniline (2a)
Preparation of BOSCHIBA 2a was accomplished like that of 1a from Schiff base 2
(0.1 g, 0.36 mmol) with diphenylborinic acid (0.17 g, 0.93 mmol). The product was

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obtained as a yellow solid with a yield of 92 % (0.14 g, 0.32 mmol); M.P.: 212° C; FT-
IRυmax (cm-1): 2964, 1624, 1605(C=N), 1549, 1458, 1312, 1205, 1140, 1000, 892, 879, 825,
796, 745, 702 (O-B), 635; UV/Vis (THF): λabs/max (nm), [εmax*104 (M-1cm-1)]: 400 [0.39 ],
1
327; Fluorescence (CHCl3): λfluor (nm): 480; H NMR (400 MHz, CDCl3) δ (ppm): 6.82
(1H, d, 3J = 8.0 Hz, H-3), 6.97 (7H, m, H-15, 4H-m, 2H-p), 7.16 (2H, d, 3J = 12 Hz, H-14,
3
3
3
H-16), 7.27 (1H, t, J = 12 Hz, H-7), 7.32 (1H, d, J = 8.0 Hz, 4H-o), 7.45 (1H, t, J = 20
Hz, H-8), 7.67 (1H, d, 3J = 8.0 Hz, H-6), 7.77 (1H, d, 3J = 8.0 Hz, H-4), 7.86 (1H, d, 3J = 8
Hz, H-9), 1.79 (2H, s, H-18, H-19), 8.64 (1H, s, H-11); 13C NMR {1H}(100 MHz, CDCl3) δ
(ppm): 18.12 (C-18, C-19), 110.45 (C-1), 118.22 (C-9), 120.72 (C-15), 123.20 (C-7),
125.16 (C-13), 125.47 (C-m, C-p), 126.59 (C-5), 126.70 (C-8), 127.47 (C-14, C-16), 128.00
(C-13, C-17), 128.53 (C-6), 132.10 (C-o), 132.67 (C-i), 133.67 (C-10), 138.88 (C-4),
11
144.01 (C-12), 159.89 (C-11), 163.70 (C-2); B NMR (128 MHz, CDCl3) δ (ppm): 4.76;
APCI-TOF-MS in positive ion mode calc. for [(C31H26NO4B+H)+]: 440.2100 u.m.a; Exp.:
440.2182 u.m.a
(E)-N-((2-((diphenylboryl)oxy)naphthalen-1-yl)methylene)-2,6-diisopropylaniline (3a)
Preparation of BOSCHIBA 3a was accomplished like that of 1a from Schiff base 3
(0.1 g, 0.30 mmol) with diphenylborinic acid (0.13 g, 0.71 mmol). The product was
obtained as a green solid with a yield of 30 % (0.044 g, .091 mmol ); M.P.: 200° C; FT-
IRυmax (cm-1): 2967, 1623, 1607(C=N), 1547, 1456, 1343, 1312, 1206, 1141, 999, 894, 878,
825, 796, 745, 702 (O-B), 634; UV/Vis (THF): λabs/max (nm), [εmax*104 (M-1cm-1)]: 398
[0.81 ], 329; Fluorescence (CHCl3): λfluor (nm): 480;1H NMR (400 MHz, CDCl3), δ (ppm):
3
3
1.13 (6H, d, J = 8.0 Hz, H-22, H-23), 1.16 (6H, d, J = 8.0 Hz, H-19, H-20,), 3.57 (2H,

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septeto, 3J = 28 Hz, H-18, H-21), 7.20 (1H, d, 3J = 8.0 Hz, H-3), 7.29 (3H, m, , H-14, H-15,
3
H-16), 7.35 (6H, m, 4H-m, 2H-p), 7.49 (1H, t, J = 16 Hz, H-7), 7.54 (5H, m, H-8, 4H-o),
8.00 (1H, d, 3J = 12 Hz, H-6), 8.14 (1H, d, 3J = 12 Hz, H-4), 8.28 (1H, d, 3J = 8.0 Hz, H-9),
13
8.90 (1H, s, H-11); C NMR {1H}(100 MHz, CDCl3) δ (ppm): 23.40 (C-19, C-20, C-22,
C-23), 28.28 (C-18, C-21), 108.82 (C-1), 119.12 (C-9), 121.42 (C-3), 124.13 (C-14, C-16),
124.80 (C-7), 127.31 (C-15), 128.04 (C-m, C-p), 129.05 (C-5), 129.37 (C-8), 129.79 (C-6),
130.87 (C-10), 132.74 (C-4), 135.16 (C-13, C-17),135.69 (C-o), 140.2234 (C-i), 144.33 (C-
12), 161.06 (C-11), 163.21 (C-2); 11B NMR (128 MHz, CDCl3) δ (ppm): 4.42; APCI-TOF-
MS in positive ion mode calc. for [(C35H34NO4B+H)+]: 496.2700 u.m.a; Exp.: 496.2807
u.m.a
1-naphthylmethyllidenebenzoilhydrazone-phenyl-boron(4a).
A solution of (E)-N'-((2-hydroxynaphthalen-1-yl)methylene)benzohydrazide (0.29
g, 1 mmol) and phenylboronic acid (0.122 g, 1 mmol) in acetonitrile were heated under
reflux for 48 hours. The reaction mixture was slowly warmed to room temperature, the
solvent is evaporated and adding ethyl acetate, it was allowed to evaporate slowly at 10
o
1
days, giving a yellow solid. Yield of 0.28 g (74%). M. P.: 152 C. H NMR (400 MHz,
DMSO-d6) δ (ppm): 7.09 (1H m,H-3), 7.33 (1H, m,H-15), 7.39 (1H, m, H-7), 7.43 (1H, m,
H-m), 7.45 (2H,m,H-o, p), 7.52 (1H, t,H-14), 7.57 (1H, m, H-8), 7.67 (1H,m,H-16), 7.87
13
(2H, m, H4,H-6), 8.11 (1H,d,H9), 8.71 (1H,s,H-11); C NMR {1H}(100 MHz, DMSO-d6)
δ (ppm):110.43 (C1), 119.79 (Cp), 121.03 (C4), 123.74 (C7), 127.16 (C5), 127.68 (C15),
128.32 (C3), 128.42 (C10), 128.76 (C14), 128.97 (C8), 129.27 (C6), 131.45 (Cm), 131.53
(C13), 132.12 (C16), 137.47 (C9), 141.53 (C11), 155.61 (C2), 170.89 (C12).

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Results and Discussion
Synthesis
The BOSCHIBAs 1a-3a were obtained by condensation of diphenylborinic acid
(synthesized in situ) with the corresponding free ligand under reflux in acetonitrile (Scheme
1). In order to carry out a comparative study of the photophysical properties, the
corresponding Schiff bases 1-3 were also prepared by a condensation reaction between 2-
hydroxy-1-naphthaldehyde with the corresponding aniline. All the BOSCHIBAs and their
ligands were obtained in nearly quantitative yield (73 to 95%) with high atom economy
(1a: 92; 2a: 91, and 3a: 93%) and low environmental E factor (1a: 0.05; 2a: 0.09), except
3a, that showed a poor chemical yield (30%) after the final purification step as well as a
high E value (2.30) (See electronic supporting information). In general, the condensation
reactions demonstrated to be a cheap, simple, faster, and reproducible synthetic route with a
high atomic economy and low E values.
Scheme 1. Here
Spectroscopic and spectrometric characterization
The formation of the N→B coordinate bond for BOSCHIBAs 1a-3a was evidenced
in the first instance by Boron NMR spectroscopy (See electronic supporting information
Figures S1-S3). For example, one broad signal can be observed in a range from 7.38 to 4.42
13
ppm, corresponding to tetracoordinated boron atoms respectively (See Table 1). In the C
NMR spectra of BOSCHIBAs 1a-3a, the signals for C-11 are shifted to low frequencies
(157.7-161.0 ppm) with respect to the Schiff bases 1-3 (158.7-161.3 ppm) owing to the

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coordination to boron [30]. 1H NMR spectra confirmed the formation of the Schiff bases 1-
3 with simple signals in the range from 8.64 to 9.42 ppm which are typical for imine groups
(See electronic supporting information figures S4-S21) [31]. In the solid state, the FT-IR
spectral analysis showed the stretching vibration bands attributed to imine group (C=N) for
BOSCHIBAs 1a-3a which were shifted to lower wavenumbers in comparison with the free
ligands 1-3, demonstrating a decrease in strength when the new coordination bond is
formed (See electronic supplementary information Fig. S22-S27) [32]. Atmospheric
pressure ionization time-of-flight mass spectrometry(APCI-TOF-MS) analysis in positive
ion mode for BOSCHIBA 1a-3a confirm the molecular ion peak (1a: 413.1822, 2a:
440.2183, and 3a: 496.2807) which correspond to the expected molecular ion mass (See
electronic supplementary information Fig. S28-S30). As a characteristic for BOSCHIBAs
2a, and 3a, both molecules show a fragment due to the loss of one ring of benzene bonded
to boron atom as well as the formation of boratropylium ion is attributed to loss of a
fragment from 86 Da to 33 Da. In addition 3a shows a fragment at m/z 157 that is related to
loss of 2,6-diisopropylaniline fragment from 175 Da (Scheme 2).
Table 1. Here
Scheme 2.Here
X-ray analyses.
The crystal structure for BOSCHIBA 1a is represented in Figure 2. BOSCHIBA 1a
belongs to the monoclinic space group P21/c, and its crystal structure analysis reveals that
contains a tetra-coordinated boron atom and a three-ring-fused skeleton with a N→B

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coordination bond length of 1.632(1) Å where boron atom adopt typical tetrahedral
geometry. According to Höpfl formula, the boron atom has a strong dative bond from
nitrogen atom corroborated by a tetrahedral character equal to 94.35 % [34]. The bond
length of B-O is 1.505(1) Å, which is similar to those of the organoboron complexes
reported previously [35,36]. The imine bond length for 1a is 1.310 (2) Å, a value closer to
that single bonds due to the formation of new N-B coordination bond. Moreover, the imine
bond shows a configuration E with regard to the naphthalene group due to the quelate ring
formation. Figure 3 exhibits the formation of a dimer through [C-H(13) π-system(N-2)
2.5912 Å] as well as the formation of a 1D chain through [C-H(21) π-system(O-2)
intermolecular interactions. BOSCHIBA 1a has a non-planar structure which is deduced
from the dihedral angle (41.08°) calculated between naphthalene and pyridine rings. An
aspect of relevance of the structure 1a is the deviation (θ) of the boron atom from the
naphthalenimino-plane that has a value equal to 0.336 Å, which is less than those
previously reported by our group research. In this study, we found that boron compounds
derivatives from salicylidenebenzohydrazide have great θ from the salicylidenimino-plane
with values in a range from 0.488 to 0.749 Å [37]. Most likely this parameter could
increase the luminescence properties as it has been reported for fluorescent borinates
derived from benzoxazoles and benzothiazoles [38]. In those structures, the authors found
two phenyl groups attached to the boron atom affectively keep luminescent rigid-fused π-
conjugated skeletons apart, making these molecules highly emissive.
Figure 2. Here

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Figure 3. Here
Table 2. Here
Photophysical properties.
The optical properties of BOSCHIBAs 1a-3a and their free ligands 1-3 were
obtained in spectroscopic grade tetrahydrofuran (Table 3), as it is a common solvent that
dissolves all molecules reported here. Figure. 4 shows the electronic absorption spectra of
BOSCHIBAs 1a-3a, and (Inset) those corresponding ligands. The BOSCHIBAs 1a-3a
show a main absorption band in a range from 328 to 341 nm attributed to the π-π*
electronic transition through the molecule. Moreover by making comparison with their
corresponding free ligands, this π-π* intraligand electronic transition is observed in the
BOSCHIBAs at longer wavelengths relative likely because a less distortion of the naphtyl
imine system after complexation (torsion angle of C12-N1-C11-C1 is 171.91o (9)). When
the BOSCHIBAs are formed (Figure 4), the bands are red-shifted due to the larger
electronic delocalization because of the formation of the imine system, in according to the
displacement observed for H-11 protons in 1H NMR analysis (Table 1).
Figure. 4 Here
The fluorescence spectra of the BOSCHIBAs 1a-3a and their free ligands are shown
in the figure. 5. BOSCHIBAs 2a and 3a show a broad blue emission bands at 492, and 493
nm without an apparent change in the fluorescence emission maximum with respect to their

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free ligands. In fact, both emission wavelengths are practically identical which indicates
that the addition of bulky substituents can not tuned the emission wavelength in the visible
spectrum (See table 3).Conversely, BOSCHIBA 1a presents a broad band centered at 528
nm with a moderated red shift of 92 nm with respect its free ligand. This change implies
that the complexation with boron affects the geometry in the excited state of the
naphthalene fluorophore when it is substituted with a heterocyclic compound. An
interesting aspect is that the emission can be tuned from the blue (1) to the green (3a)
regions by changing from free ligand to complex. As a general remark, the fluorescence
quantum yield (ΦF) is very weak for all molecules, except 3a, that exhibits a moderate
increase of 3%. It is important to mention that 1a showed the highest ΦF might be the
coplanarity of heterocyclic system. In all cases, large Stoke's shift (∆ν) indicates that the
geometry of the molecule changes dramatically after excitation and non-radiative losses are
probably due to an internal conversion process, according to the previously reported studies
by our research group [36] and could also be related to the low ΦF measured for all
molecules under our experimental conditions.
Figure 5. Here
Table 3. Here
Photostability testing and stability in aqueous DMSO solutions
For bioimaging applications, it is desirable that the molecules demonstrate that the
light exposure does not change their chemical-optical properties. The photostability curves

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for the BOSCHIBAS 1a-3a are given in the electronic supplementary information (Fig.
S31-S33). Figure 6 shows the degradation plots of BOSCHIBAs at different exposure time
under UV light irradiation at 365 nm. Based on our strategy of bulky groups, the molecules
were irradiated at 365 nm under ambient temperature with air atmosphere by time intervals
from 10 to 60 min, and they were monitored by UV spectroscopy. BOSCHIBA 1a, without
bulky organic groups, shows a degradation of less than 15% at 20 min, affording the free
ligand 1 after the light exposure. In the case of BOSCHIBAs 2a and 3a, with bulky groups,
an insignificant degradation is observed after being exposed for 40 min at the same
wavelength. The spectral analysis revealed that there are not remarkable changes in the
main absorption bands for 2a and 3a which suggests that chemical modification using
bulky groups increase the photostability. In order to compare the effect of bulky groups on
the photostability of BOSCHIBAs 1a-3a, a BOSCHIBA identified as 4a with an
unsubstituted phenyl group was synthesized (Scheme 1). Photo-stability studies, performed
at 365 nm, revealed that 4a shows a slightly lower resistance to degradation ( 10% at 40
min) in comparison with BOSHIBAs 2a and 3a ( 5% at 40 min) when they are exposed at
this long wavelength. Therefore according with the behavior of experimental data
measured, the inclusion of a benzene ring and its substitution with bulky groups increase
the resistance to degradation compared with pyridyl group which is less stable and more
reactive than benzene (Figure 6). Likewise, the stability in aqueous solutions at 1% v/v of
DMSO for BOSCHIBAs were evaluated under the same experimental condition without
photoirradiation. All the BOSCHIBAs 1a-3a demonstrated to be highly stable in aqueous
DMSO solution after 60 min because there were not any important changes in the
absorption peaks (See electronic supporting information Fig. S35 and S36).

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Figure 6. Here
Cytotoxicity assay
The cytotoxicity of BOSCHIBAs 1a-3a were assessed by reassuring assay, using
DMSO as solvent at different concentrations (0.1 to 10 µg/mL) for 24 hours (Figure 7).
Following incubation of 0.1 µg/mL for 24 hours, it is observed that less than 9% of B16F10
cells died. The same happened when the concentration is increased to 2.5 µg/mL. When the
concentration of 1a was increased to 10 µg/mL, cell viability remained above 60 %, while
for 2a and 3a it is above 50%. Therefore, BOSCHIBAs 1a-3a showed relatively low
cytotoxicity in the concentration range and incubation times examined in this study. In
general, the cytotoxicity, cell-permeable characteristics of fluorescent materials are critical
to the bio-imaging application, since the analysis of the viability of cytotoxicity showed
BOSCHIBAs are not toxic to the corresponding cells.
Figure 7. Here.
Bioimaging cells.
The capacity of BOSCHIBAs 1a-3a to produce fluorescent staining in cells was
tested on B16F10 cells. Figure 8 shows on the top row that DMSO treated cells do not
fluoresce at either wavelength (Figure 8-B and 8-C). When cells were treated with 1a a
very weak blue cytoplasm fluorescent stain was observed (Figure 8-E), but a strong green
stain was observed throughout the whole cell including nucleus (Figure 8-F). 2a, and3a
produced a strong cytoplasm blue stain with a few brighter spots in each cell (Figure 9 a

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and b), which could suggest an endocytic internalization pathway. On the other hand, 2a
and 3a also produced a weak green stain in the cells; however, this stain is generalized to
the whole cell (Figure 8-I). Cells treated with 3a show a similar fluorescent stain pattern as
described for 2a, only the blue staining is significantly stronger when 2a is used (Figure 8-
K and 8-L). The fact that blue and green stains compartmentalize differently could mean
that once the original BOSCHIBAs reach the cell interior are being metabolized to at least
two different molecules, one with capacity to get all the way into the nucleus, staining the
whole cell; while the other can only reach the cytoplasm. By comparison, the BOSCHIBAs
2a and 3a with bulky aryl groups showed better fluorescent staining while 1a exhibits a
poor staining.
Figure 8. Here
Figure 9. Here
Conclusions
In summary, we have reported the design, synthesis and chemo-optical
characterization of three BOSCHIBAs with good chemical yields based on easy and cheap
condensation reactions. BOSCHIBAs 2a and 3a showed a low fluorescence emission
(ΦF 1%) while that inclusion of bulky groups induces a higher resistance to degradation
compared with unsubstituted cyclic and heterocyclic analogs. All the BOSCHIBAs 1a-3a
demonstrated to be high stability in aqueous solution. Bioimaging studies for 2a and 3a
revealed their potential application as enhanced cytoplasmic staining dyes with a low

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cytotoxicity, a better staining (from 0.1 to 2.5 µg/mL), a high stability against UV radiation
and hydrolytic stability.
Acknowledgments
Authors thank to CONACYT (grant: 240011) and PAICYT-UANL (CE336-15) for
financial support. M. M. C.-L. and M. I.-R. thank the Ph. D. scholarship.
Supplementary Information
Appendix A Supplementary Data
The CCDC 1520512 contains the supplementary crystallographic data. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif or from the
Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, CB2 1EZ, UK).
Appendix B Supplementary Data
Supplementary figures and calculation of green chemistry metrics of the
BOSCHIBAs 1a-3a.
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1
13
11
Table 1. Selected H, C, and B NMR (ppm), and IR (cm-1) spectroscopic data for
BOSCHIBAs 1a-3a, and their ligands 1-3.
1H
13C
FT-IR
C=N
Comp.
1
11B
H-11
C-2
C-13/C-17 C-11
C-12
9.42
166.20 127.80/143.00 158.70
143.70
-
1624
2
3
9.14
8.98
168.00
166.30
129.70
139.22
161.30
160.60
145.80
142.90
-
-
1613
1620
1a
8.86
165.90 124.70/142.56 157.70
144.90
7.38
1608
2a
3a
8.64
8.90
163.70
163.20
128.00
135.16
159.80
161.00
144.00
144.30
4.76
4.42
1605
1607

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Table 2. Crystal data for BOSCHIBA 1a.
Empirical formula
Formula weight
Temperature, K
Wavelength
Crystal system
Space group
a, [Å]
C28H21BN2O
412.29
100(2)
1.54056
Monoclinic
P2(1)/c
17.2063(14)
8.1494(7)
16.6279(13)
90.00
b, [Å]
c, [Å]
α, [o]
β, [o]
114.3280(10)
90.00
γ, [o]
V [Ǻ3]
1367.5(8)
4
Z
-3
1.289
0.078
ρcalc,mg.cm
µ, mm-1
2θ range for data collection 1.27 – 27.00°
No. of reflns collected
20996
No. of indep reflns
[Rint]
4322
[Rint]
Goodness of fit
R1, wR2 (I>2σ(I))
R1, wR2 (all data)
1.048
0.0366; 0.0993
0.0388; 0.1019

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Table 3. Photophysical properties of BOSCHIBAs 1a-3a and their ligands 1-3 in THF.
ε *104
[M-1cm-1]
λem
ΦF
λabs
∆ν
[cm-1]
Compound
[nm]
[nm]
[%]
324 (378)
316 (366)
316 (364)
341 (426)
328 (402)
0.10 (0.12)
0.58 (0.52)
0.71 (0.60)
0.51 (0.38)
0.39 (0.21)
490
461
475
528
492
6047
5630
6420
4535
4550
0.05
0.03
0.01
3.80
1.07
1
2
3
1a
2a
328 (402)
0.81 (0.44)
493
4592
1.01
3a

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