Platinum pyridinecarboxaldimine complexes containing boronate esters

Article abstract of DOI:10.1139/v04-147

Condensation of 2-pyridinecarboxaldehydes with 2-, 3-, and 4-H ₂NC₆H₄Bpin (pin = 1,2-O₂C ₂Mc₄) gave the corresponding boron-containing pyridinecarboxaldimines (N-N_Bpin). Addition

Full text of DOI:10.1139/v04-147

1692
Platinum pyridinecarboxaldimine complexes
containing boronate esters
Hanni A. Darwish, Stephen J. Scales, Jennifer L. Horton, Liliya G. Nikolcheva,
Haiwen Zhang, Christopher M. Vogels, Mazen T. Saleh, Robert J. Ireland,
Andreas Decken, and Stephen A. Westcott
Abstract: Condensation of 2-pyridinecarboxaldehydes with 2-, 3-, and 4-H2NC6H4Bpin (pin = 1,2-O2C2Me4) gave the
corresponding boron-containing pyridinecarboxaldimines (N–NBpin). Addition of these ligands to [PtCl2(coe)]2 (coe =
cis-cyclooctene) gave complexes of the type cis-PtCl2(N–NBpin) in moderate yields. The platinum complexes have been
examined for their potential cytotoxicities against OV2008 (human ovarian carcinoma) and the analogous cisplatin-
resistant cell line C13.
Key words: boronate esters, pyridinecarboxaldimines, cytotoxicity, platinum, boron.
Résumé : La condensation des pyridine-2-carboxaldéhydes avec les 2-, 3- et 4-H2NC6H4Bpin (pin = 1,2-O2C2Me4)
conduit à la formation des pyridinecarboxaldimines contenant du bore (N–NBpin). L’addition de ces ligands au
[PtCl2(coe)]2 (coe = cis-cyclooctène) conduit à la formation de complexes du type cis-PtCl2(N–NBpin), avec des rende-
ments modérés. On a étudié les cytotoxicités potentielles des complexes du platine vis-à-vis du OV2008 (carcinome
ovarien humain) et la série de cellules C13 apparentées, qui est résistante au cisplatine.
Mots clés : esters de l’acide boronique, pyridinecarboxaldimines, cytotoxicité, platine, bore.
[Traduit par la Rédaction] Darwish et al. 1699
Introduction
cancer (14–16). BNCT is a bimodal form of therapy, which
depends on selectively depositing boron-10 atoms into the
cancerous tumour prior to irradiation by slow (thermal) neu-
trons. These known biological activities, along with their
ability to transport water insoluble reagents through mem-
branes (17, 18), prompted us to investigate the use of
boronate ester compounds as carrier ligands for biologically-
active metal complexes.
Cisplatin, cis-[PtCl2(NH3)2], and a few related platinum-
based complexes are currently used as anticancer agents
against testicular and ovarian malignancies (19–25). How-
ever, there are several limitations to platinum therapy such
as neural and kidney toxicity as well as intrinsic and ac-
quired resistance of tumor cells to the drugs (19). These
complications have provided incentive for further research in
the development of platinum-based complexes with in-
creased solubility and enhanced specificity towards cancer
cells. Recent studies have shown that cis-amminedichloro(2-
methylpyridine)platinum(II) (AMD473 or ZD0473, 1 in
Fig. 1) shows considerable cytotoxicity in cisplatin-resistant
cell lines (26, 27). Steric crowding from the methyl group is
Interest in compounds containing boronic acids
[RB(OH)2] or boronate esters [RB(OR′)2] arises from their
remarkable versatility in catalysed carbon–carbon bond for-
mation reactions, solid-phase synthesis, macrocyclic chemis-
try, organometallic and organic syntheses, and as glucose
sensors for diabetes therapy (1). Interest in these compounds
also arises from their potent biological activities (2–11). For
instance, α-aminoboronic acids are effective and reversible
inhibitors of serine proteases, a diverse group of proteolytic
enzymes whose numerous physiological functions include
digestion, growth, differentiation, and apoptosis. Proteases
are also vital in the generation of most disease processes
and, as a result, much effort has focused on the synthesis of
α-aminoboronic acids for possible applications as enzyme
inhibitors. Indeed, the boron compounds boromycin (12) and
aplasmomycin (13) are powerful antibiotics and analogous
amino acids containing boronic acids, such as L-4-borono-
phenylalanine, have also been investigated for their use in
boron neutron capture therapy (BNCT) for the treatment of
Received 5 June 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 14 January 2005.
H.A. Darwish, S.J. Scales, J.L. Horton, L.G. Nikolcheva, H. Zhang, C.M. Vogels, and S.A. Westcott.1 Department of
Chemistry, Mount Allison University, Sackville, NB E4L 1G8, Canada.
M.T. Saleh and R.J. Ireland. Department of Biology and Biochemistry, Mount Allison University, Sackville, NB E4L 1G7,
Canada.
A. Decken.2 Department of Chemistry, University of New Brunswick, Fredericton, NB E3B 5A3, Canada.
1Corresponding author (e-mail: swestcott@mta.ca).
2Corresponding author regarding X-ray crystallography (e-mail: adecken@unb.ca).
Can. J. Chem. 82: 1692–1699 (2004)
doi: 10.1139/V04-147
© 2004 NRC Canada

Darwish et al.
1693
Fig. 1. Structure of AMD473 (1) and complex 2.
apparatus. Microanalyses for C, H, and N were carried out at
Guelph Chemical Laboratories Ltd. (Guelph, Ont.).
Compound 3a
Under a nitrogen atmosphere, a THF (2 mL) solution
of pyridin-2-ylmethylene-[2-(4,4,5,5-tetramethyl-[1,3,2]-
dioxaborolan-2-yl)phenyl]amine (0.09 g, 0.29 mmol) was
added to [PtCl2(coe)]2 (0.10 g, 0.13 mmol) in THF (3 mL).
The reaction mixture was stirred for 18 h at which point a
precipitate was collected by suction filtration and washed in
turn with cold THF (3 × 1 mL) and Et2O (3 × 5 mL) to af-
ford 3a (0.06 g, 39%) as a yellow-orange solid; mp 270–
272 °C (decomposition). Spectroscopic NMR data (in
1
believed to decrease the rates of hydrolysis, and substitution
reactions of AMD473 thereby permit high selectivity in
binding with DNA (28). The primary mechanism of action
in these platinum drugs is believed to arise from the interac-
tion of the metal with DNA.
DMSO-d6): H δ: 9.50 (d, JH-H = 7 Hz, JH-Pt = 62 Hz, 1H,
Ar), 9.35 (s, JH-Pt = 92 Hz, 1H, C(H) = N), 8.47 (t, JH-H
7 Hz, 1H, Ar), 8.22 (d, JH-H = 7 Hz, 1H, Ar), 8.01 (t, JH-H
7 Hz, 1H, Ar), 7.74 (d, JH-H = 7 Hz, 1H, Ar), 7.60 (t, JH-H
7 Hz, 1H, Ar), 7.42 (t, JH-H = 7 Hz, 1H, Ar), 7.34 (d, JH-H
=
=
=
=
We have begun to develop AMD473 analogues by replac-
ing the NH3 group with a pendant imine group. Previous
studies have shown that the platinum complex derived from
aniline, 2 (Fig. 1), has shown considerable activity against
the hormone independent human mammary carcinoma cell
line MDA-MB 231 (29). Varying the aniline functionality al-
lows us to design compounds with a wide range of physical
and chemical properties that may provide steric congestion
around the platinum atom (29). As well, the use of bidentate
ligands prevents trans labilization and undesired displace-
ment of the ligands by sulfur and nitrogen donors in bio-
molecules, interactions believed responsible for some of the
adverse side effects associated with cisplatin (19). We report
herein our results on the synthesis and initial cytotoxic test-
ing of cis-dichloro(pyridin-2-ylcarboxaldimine)platinum(II)
compounds containing boronate esters.
7 Hz, 1H, Ar), 1.21 (s, 6H, O2C2(CH3)4), and 1.15 (s, 6H,
O2C2(CH3)4). 11B{1H} δ: 29.1 (br). 13C{1H} δ: 173.0, 157.7,
152.7, 149.7, 141.2, 135.4, 131.5, 130.1, 130 (br, C-B),
129.7, 128.3, 124.9, 84.5, 25.4, and 25.1. IR (cm-1) (Nujol):
2926, 2856, 1601 (C=N), 1462, 1377, 1300, 1240, 1200,
1144, 1076, 964, 860, 779, and 654. Anal. calcd. (%) for
C18H21N2BCl2O2Pt, C 37.64, H 3.69, N 4.88; found: C
37.94, H 3.33, N 4.84.
Compound 3b
Pyridin-2-ylmethylene-[3-(4,4,5,5-tetramethyl-[1,3,2]-dioxa-
borolan-2-yl)phenyl]amine (0.09 g, 0.29 mmol) in CH2Cl2
(5 mL) was added to [PtCl2(coe)]2 (0.10 g, 0.13 mmol) in
CH2Cl2 (5 mL) and the reaction mixture was stirred for 1 h
at which point solvent was removed under vacuum to give
an oil. Following trituration with hexane (3 × 10 mL) an or-
ange solid was recovered and recrystallized from THF
(5 mL) at room temperature to afford 3b (0.08 g, 54%) as a
Experimental
red-orange solid; mp 316 °C (decomposition). Spectroscopic
1
NMR data (in CDCl3): H δ: 9.76 (d, JH-H = 7 Hz, JH-Pt
=
60 Hz, 1H, Ar), 8.88 (s, JH-Pt = 84 Hz, 1H, C(H) = N), 8.19
(t, JH-H = 7 Hz, 1H, Ar), 7.97 (d, JH-H = 7 Hz, 1H, Ar), 7.83
(d, JH-H = 7 Hz, 1H, Ar), 7.81–7.72 (ov m, 2H, Ar), 7.66 (d,
JH-H = 7 Hz, 1H, Ar), 7.43 (t, JH-H = 7 Hz, 1H, Ar), and 1.27
(s, 12H, O2C2(CH3)4). 11B{1H} δ: 32.5 (br). 13C{1H} δ:
168.3, 156.5, 150.0, 146.1, 139.5, 135.6, 129.0, 129 (br, C-
B), 128.9, 128.4, 127.5, 127.4, 84.0, and 24.7. IR (cm-1)
(Nujol): 2895, 2856, 1610 (C=N), 1462, 1377, 1329, 1234,
1151, 933, 854, 816, 777, and 706. Anal. calcd. (%) for
C18H21N2BCl2O2Pt, C 37.64, H 3.69, N 4.88; found: C
37.25, H 3.62, N 4.73.
General
Reagents and solvents used were obtained from Aldrich
Chemicals. K2PtCl4 was purchased from Precious Metals
Online Ltd. (Melbourne, Australia) and [PtCl2(coe)]2 (coe =
cis-cyclooctene) was made following an established proce-
dure (30). 2-APBpin (pin = 1,2-O2C2Me4) was synthesized
by modification of a reported synthesis (31). Pyridinecar-
boxaldimine ligands were prepared as described elsewhere
(32) and (6-methyl-pyridin-2-ylmethylene)-[4-(4,4,5,5-
tetramethyl-[1,3,2]-dioxaborolan-2-yl)phenyl]amine was
synthesized under a nitrogen atmosphere and used in situ.
NMR spectra were recorded on a JEOL JNM-GSX270 FT or
1
Compound 3c
a Varian Mercury Plus 200 NMR spectrometer. H NMR
chemical shifts are reported in ppm and are referenced to re-
sidual protons in a deuterated solvent at 270 and 200 MHz.
11B NMR chemical shifts are referenced to external
BF3·OEt2 at 87 MHz. 13C NMR chemical shifts are refer-
enced to solvent carbon resonances as internal standards at
68 and 50 MHz. Multiplicities are reported as singlet (s),
doublet (d), triplet (t), multiplet (m), broad (br), and overlap-
ping (ov). Infrared spectra were obtained using a Mattson
Genesis II FT-IR spectrometer and are reported in cm–1.
Melting points were measured uncorrected with a Mel-Temp
Pyridin-2-ylmethylene-[4-(4,4,5,5-tetramethyl-[1,3,2]-
dioxaborolan-2-yl)phenyl]amine (0.18 g, 0.58 mmol) in
CH2Cl2 (5 mL) was added to [PtCl2(coe)]2 (0.20 g,
0.26 mmol) in CH2Cl2 (5 mL) and the reaction mixture was
stirred for 7 h at which point solvent was removed under
vacuum to give an oil. Following trituration with hexane
(3 × 10 mL) an orange solid was recovered and recry-
stallized from THF (5 mL) at room temperature to afford 3c
(0.19 g, 64%) as an orange solid; mp 304 °C (decomposi-
tion). Spectroscopic NMR data (in DMSO-d6): 1H δ: 9.48 (d,
© 2004 NRC Canada

1694
Can. J. Chem. Vol. 82, 2004
JH-H = 7 Hz, JH-Pt = 65 Hz, 1H, Ar), 9.33 (s, JH-Pt = 84 Hz,
in CH2Cl2 (5 mL) was added to [PtCl2(coe)]2 (0.08 g,
0.11 mmol) in CH2Cl2 (5 mL) and the reaction was heated at
reflux for 24 h. Removal of solvent under vacuum gave an
oil which was triturated with hexane (3 × 10 mL) to afford a
red-orange solid which was dissolved in a CH2Cl2:Et2O
(5:5 mL) solution and stored at 0 °C for 18 h. The resulting
precipitate was collected by suction filtration to afford 4c
(0.10 g, 77%) as an orange solid; mp 277 °C (decomposi-
1H, C(H) = N), 8.45 (t, JH-H = 7 Hz, 1H, Ar), 8.24 (d, JH-H
7 Hz, 1H, Ar), 8.00 (t, JH-H = 7 Hz, 1H, Ar), 7.77 (d, JH-H
=
=
7 Hz, 2H, Ar), 7.46 (d, JH-H = 7 Hz, 2H, Ar), and 1.32 (s,
12H, O2C2(CH3)4). 11B{1H} δ: 33.1 (br). 13C{1H} δ: 173.1,
157.6, 149.8, 149.6, 141.2, 134.8, 130.4, 130.2, 129.5 (br,
C-B), 124.4, 84.5, and 25.2. IR (cm-1) (Nujol): 2941, 2914,
2858, 1601 (C=N), 1460, 1377, 1356, 1315, 1263, 1144,
1084, 1018, 968, 858, 764, 723, and 656. Anal. calcd. (%)
for C18H21N2BCl2O2Pt, C 37.64, H 3.69, N 4.88; found: C
37.31, H 3.50, N 4.69.
1
tion). Spectroscopic NMR data (in CDCl3): H δ: 8.94 (s,
JH-Pt = 78 Hz, 1H, C(H) = N), 7.94 (t, JH-H = 7 Hz, 1H, Ar),
7.86 (d, JH-H = 7 Hz, 1H, Ar), 7.79 (d, JH-H = 7 Hz, 2H, Ar),
7.47 (d, JH-H = 7 Hz, 1H, Ar), 7.41 (d, JH-H = 7 Hz, 2H, Ar),
3.15 (s, 3H, CH3), and 1.33 (s, 12H, O2C2(CH3)4). 11B{1H}
δ: 31.2 (br). 13C{1H} δ: 169.4, 167.4, 157.6, 149.1, 138.9,
135.0, 131.7, 131 (br, C-B), 127.4, 123.3, 84.2, 28.0, and
25.0. IR (cm-1) (Nujol): 2925, 2856, 1581 (C=N), 1463,
1377, 1358, 1336, 1270, 1173, 1134, 1092, 1016, 935, 845,
787, 729, and 656. Anal. calcd. (%) for C19H23N2BCl2O2Pt,
C 38.79, H 3.95, N 4.76; found: C 38.70, H 3.99, N 4.76.
Compound 4a
Under an atmosphere of nitrogen, a THF (1 mL) solution
of (6-methyl-pyridin-2-ylmethylene)-[2-(4,4,5,5-tetramethyl-
[1,3,2]-dioxaborolan-2-yl)phenyl]amine (0.09 g, 0.28 mmol)
was added to [PtCl2(coe)]2 (0.09 g, 0.12 mmol) in THF
(3 mL). The reaction mixture was stirred for 8 h at which
point a precipitate was collected by suction filtration and
washed with Et2O (3 × 5 mL) to afford 4a (0.06 g, 42%) as
an orange solid; mp 285 °C (decomposition). Spectroscopic
X-ray crystallography
1
NMR data (in CDCl3): H δ: 8.77 (s, JH-Pt = 84 Hz, 1H,
Crystals of 3b and 4a were grown from saturated CH2Cl2
solutions at 5 °C. Single crystals were coated with Paratone-
N oil, mounted using a glass fibre, and frozen in the cold
stream of the goniometer. A hemisphere of data was col-
lected on a Bruker AXS P4/SMART 1000 diffractometer us-
ing ω and θ scans with a scan width of 0.3° and 30 s
exposure times. The detector distances were 4 (3b) and 5 cm
(4a). The data were reduced (33) and corrected for absorp-
tion (34). The structures were solved by direct methods and
refined by full-matrix least squares on F2 (35). All non-
hydrogen atoms were refined anisotropically. Hydrogen at-
oms were located in Fourier difference maps and refined
isotropically (3b) or included in calculated positions and re-
fined using a riding model (4a).
C(H) = N), 8.02 (t, JH-H = 7 Hz, 1H, Ar), 7.86 (d, JH-H
7 Hz, 1H, Ar), 7.68 (d, JH-H = 7 Hz, 1H, Ar), 7.56 (d, JH-H
7 Hz, 1H, Ar), 7.50 (t, JH-H = 7 Hz, 1H, Ar), 7.36 (t, JH-H
=
=
=
7 Hz, 1H, Ar), 7.29 (d, JH-H = 7 Hz, 1H, Ar), 3.25 (s, 3H,
CH3), and 1.25 (br s, 12H, O2C2(CH3)4). 11B{1H} δ: 29.3
(br). 13C{1H} δ: 161.9, 159.5, 155.2, 138.4, 136.8, 133.1,
129.1, 128.5, 126 (br, C-B), 119.6, 115.6, 114.8, 83.7, 25.2,
and 24.3. IR (cm-1) (Nujol): 2924, 2854, 1603 (C=N), 1462,
1377, 1348, 1144, 1072, 1018, 962, 858, 725, 654, and 596.
Anal. calcd. (%) for C19H23N2BCl2O2Pt, C 38.79, H 3.95, N
4.76; found: C 38.95, H 4.05, N 4.78.
Compound 4b
(6-Methyl-pyridin-2-ylmethylene)-[3-(4,4,5,5-tetramethyl-
[1,3,2]-dioxaborolan-2-yl)phenyl]amine (0.08 g, 0.25 mmol)
in CH2Cl2 (5 mL) was added to [PtCl2(coe)]2 (0.08 g,
0.11 mmol) in CH2Cl2 (5 mL) and the reaction was heated at
reflux for 24 h. Removal of solvent under vacuum gave an
oil which was triturated with hexane (3 × 10 mL) to afford a
red-orange solid which was dissolved in a CH2Cl2:Et2O
(5:5 mL) solution and stored at 0 °C for 18 h. The resulting
precipitate was collected by suction filtration to afford 4b
(0.08 g, 62%) as an orange solid; mp 264 °C (decomposi-
Cell culture
OV2008 (human ovarian carcinoma) and the analogous
cisplatin-resistant cell line C13 were a generous gift from
Dr. Barbara C. Vanderhyden of the Centre for Cancer Thera-
peutics, Ottawa Regional Cancer Centre, Ont. Both cell lines
were cultured in complete RPMI-1640 medium with L-
glutamine, supplemented with 5% fetal bovine serum, peni-
cillin (50 units/mL), and streptomycin (50 mg/mL). Cells
were incubated in a humidified atmosphere of 5% CO2 at
37 °C and were subcultured twice weekly using trypsin-
EDTA.
1
tion). Spectroscopic NMR data (in CDCl3): H δ: 8.91 (s,
JH-Pt = 80 Hz, 1H, C(H) = N), 7.97 (t, JH-H = 7 Hz, 1H, Ar),
7.81–7.77 (ov m, 2H, Ar), 7.70 (s, 1H, Ar), 7.59–7.51
(ov m, 2H, Ar), 7.40 (t, JH-H = 7 Hz, 1H, Ar), 3.21 (s, 3H,
CH3), and 1.33 (s, 12H, O2C2(CH3)4). 11B{1H} δ: 29.6 (br).
13C{1H} δ: 168.6, 167.9, 157.6, 146.5, 138.8, 136.0, 131.8,
129 (br, C-B), 128.7, 127.8, 127.7, 126.3, 84.3, 28.1, and
25.0. IR (cm-1) (Nujol): 2927, 2856, 1599 (C=N), 1568,
1462, 1428, 1377, 1319, 1252, 1228, 1166, 1144, 1101, 970,
924, 854, 798, 731, 704, and 606. Anal. calcd. (%) for
C19H23N2BCl2O2Pt, C 38.79, H 3.95, N 4.76; found: C
38.97, H 3.90, N 4.73.
Cell growth inhibition assay
Cells were seeded in 96-well plates at a concentration of
0.1–1.0 × 104 cells/well in 200 µL of complete media and
incubated for 24 h at 37 °C in a 5% CO2 atmosphere to al-
low for cell adhesion. Stock solutions (15 mmol/L) of the
compounds made in DMSO were filter sterilized, then di-
luted to 10 mmol/L in phosphate buffered saline (PBS,
0.02 mol/L phosphate, 0.11 mol/L NaCl, pH 7.0, sterile).
The 10 mmol/L solutions were diluted to 50 µmol/L and
1.4 mmol/L in complete media for treatment against
OV2008 and C13 cell lines, respectively, where 20 µL of
compound solutions were added to 180 µL of fresh medium
in wells to give final concentrations of 5 µmol/L against
Compound 4c
(6-Methyl-pyridin-2-ylmethylene)-[4-(4,4,5,5-tetramethyl-
[1,3,2]-dioxaborolan-2-yl)phenyl]amine (0.08 g, 0.25 mmol)
© 2004 NRC Canada

Darwish et al.
1695
Fig. 2. Synthesis of platinum complexes 3–4.
OV2008 and 140 µmol/L for C13. All assays were per-
formed in two independent sets of quadruplicate tests. Con-
trol groups containing no drug were run in each assay, along
with standards of cisplatin and a previously studied cis-
dichloro(pyridin-2-ylcarboxaldimine)platinum(II) complex (2).
Following 48 h of exposure, each well was carefully
rinsed with 200 µL of PBS solution. 3-(4,5-dimethylthiazol-
2-yl)-2,5-diphenyltetrazolium bromide (MTT) solution
(50 µL, 1 mg/mL doubly distilled (dd)H2O) along with
200 µL of fresh, complete media were added to each well,
and plates were incubated for 45 min. Following incubation,
the media was removed and the purple formazan precipitate
in each well was solubilized in 200 µL of DMSO.
Absorbances were measured using a VersaMax tunable
microplate reader (Molecular Devices, Sunnyvale, Calif.) at
570 nm. Reduction of cell proliferation was determined as a
fraction of the absorbance values for each drug treatment (T)
by the mean absorbance of the no-drug control (C), and cal-
culated using the expression 1 – T/C × 100%. Data from the
separate trials were combined using a t-test (P < 0.05).
Platinum complexes
We have found that platinum complexes containing
boronate ester appendages (PtCl2(N–NBpin), Fig. 2) could be
readily prepared by addition of pyridin-2-ylcarboxaldimine
ligands to CH2Cl2 solutions of [PtCl2(coe)]2 (43).
All new platinum complexes have been characterized by a
number of physical methods, including multinuclear NMR
spectroscopy. A significant downfield shift in the 1H NMR is
observed for the imine proton upon coordination of the
ligand to the metal center. For instance, the singlet at δ
8.36 ppm for the free 2-C5H4NCH=N[2-C6H4BO2C2Me4]
ligand shifts to 9.35 ppm in complex 3a. Platinum satellites
are also observed for this resonance (JH-Pt = 92 Hz) upon
complexation of the ligand to the metal. Similar trends are
observed for the pyridine hydrogen alpha to the nitrogen
atom as the chemical shift changes from δ 8.50 to 9.50 ppm
(JH-Pt = 62 Hz). Two distinct Bpin resonances are observed
1
in both the H (δ: 1.21 and 1.15 ppm) and 13C{1H} (δ: 25.4
and 25.1 ppm) NMR spectra for complex 3a, arising from
inequivalent methyl groups (Fig. 3). It is therefore possible
that two of the Bpin methyl groups are lying over the plane
of the platinum atom in a manner similar to that observed
for AMD473.
Results and discussion
Although we were unable to obtain single crystals of 3a to
verify if an appropriate conformational prerequisite for the
shielding effect was occurring, complex 3b has been charac-
terized by an X-ray diffraction study (Fig. 4). Crystallo-
graphic data are given in Table 1.3 The nitrogen–platinum
Ligands
Aniline derivatives containing boronate ester groups were
used, in place of the related boronic acids, in an attempt to
increase the solubility of the ligands and the subsequent
platinum complexes in organic solvents, and to prevent un-
wanted formation of anhydrides often seen in reactions with
the acids (36, 37). Addition of these primary amines to 2-
pyridinecarboxaldehydes gave the corresponding pyridine-
carboxaldimine ligands (N–NBpin) in high yield (38–41). All
new ligands were characterized by multinuclear NMR spec-
troscopy. For example, a singlet at ca. δ 8.4 ppm is observed
bonds of 2.023(2) Å (pyridine) and 2.023(3)
(imine) are
Å
similar to those reported in other platinum systems (37–39).
Slightly longer Pt—N bond distances are observed in related
five coordinate Pt(II) (37, 38) and Pt(IV) complexes (42).
The imine C(7)—N(8) distance of 1.298(3) Å is in the range
of accepted carbon–nitrogen double bonds (43). Interest-
ingly, the angle between the aromatic ring on the imine ni-
trogen and the N(1)-Pt-N(8) plane is 108.1°, a trend that is
observed in related diimine systems (44). Indeed, this struc-
tural feature has been used to create active and selective cat-
alyst precursors for the polymerization of alpha olefins,
where ortho substitution of the aniline group with bulky
groups is believed to shield and stabilize the metal center.
1
in the H NMR spectra for the imine methine peak and a
broad peak at ca. δ 30 ppm in the 11B NMR spectra for these
ligands is indicative of a three coordinate boron atom. This
latter result suggests that no appreciable inter- or intra-
molecular interaction between the Lewis-acidic boron and
the basic imine or pyridine functionality is occurring in solu-
tion (42).
3 Supplementary material may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Coun-
cil Canada, Ottawa, ON K1A 0S2, Canada. The material can also be obtained electronically at http://www.nrc.ca/cisti/irm/unpub_e.shtml.
Crystallographic information has also been deposited with the Cambridge Crystallographic Data Centre (CCDC 239618 and 239619).
Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2004 NRC Canada

1696
Can. J. Chem. Vol. 82, 2004
1
Fig. 3. Representative H NMR spectrum of compound 3a in DMSO-d6, showing an expansion of the aromatic (Ar) region.
Fig. 4. A view of complex 3b, with displacement ellipsoids drawn at the 30% probability level. H atoms have been omitted. Selected
bond distances and angles; Pt—N(8) 2.023(3), Pt—N(1) 2.023(2), Pt—Cl(2) 2.2844(8), Pt—Cl(1) 2.3011(7), C(7)—N(8) 1.298(3), B—
O(15) 1.374(4), B—O(18) 1.376(4), and C(11)—B 1.564(4) Å; N(8)-Pt-N(1) 80.17(9), N(8)-Pt-Cl(2) 95.36(7), N(1)-Pt-Cl(2) 175.43(7),
N(8)-Pt-Cl(1) 175.04(6), N(1)-Pt-Cl(1) 94.88(7), Cl(2)-Pt-Cl(1) 89.59(3), O(15)-B-O(18) 113.7(3), O(15)-B-C(11) 121.7(3), and O(18)-
B-C(11) 124.5(3)°.
© 2004 NRC Canada

Darwish et al.
Table 1. Crystallographic data collection parameters for 3b and 4a.
1697
Complex
3b
4a
Chemical formula
C18H21BCl2N2O2Pt
C19H23BCl2N2O2Pt
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
574.17
Monoclinic
P2(1)/c
14.8642(8)
9.8220(5)
15.5384(9)
109.936(1)
2132.6(2)
4
588.19
Orthorhombic
P2(1)2(1)2(1)
10.8778(5)
13.3995(7)
15.0437(8)
90
2192.73(19)
4
1.782
β (°)
V (Å3)
Z
ρcalcd. (mg m–3)
1.788
Crystal dimensions (mm)
Temperature (K)
Radiation
0.075 × 0.3 × 0.375
173(1)
Mo Kα (λ = 0.71073 Å)
6.844
22 717
7634
0.05 × 0.1 × 0.45
198(1)
Mo Kα (λ = 0.71073 Å)
6.658
14 427
4878
µ (mm–1)
Total reflections
Total unique reflections
Observed reflections
(Fo > 4σ(Fo))
No. of variables
Rint
6653
4772
249
0.0368
244
0.0179
Theta range (°)
2.50 to 32.50
2.04 to 27.50
Largest difference
peak/hole, e (Å–3)
S (Goodness-of-fit) on F2
4.777/–3.473
1.048
0.0330
0.769/–0.462
0.869
0.0132
a
R1 (l > 2σ(l))
b
wR2 (all data)
0.0903
0.0317
aR1 = Σ(| Fo| – |Fc|) / Σ| Fo|.
bwR2 = (Σ[w(Fo2 – Fc2)2] / Σ[Fo ])1/2, where w = 1/[σ2(Fo ) + (0.0637·P)2] (3b) and w = 1/[σ2(Fo )] (4a), where P = (max
4
2
2
2
(Fo , 0) + 2·Fc2)/3.
The boronate ester group in 3b is roughly coplanar with the
aromatic group in order to maximize the donation from the
ring pπ electrons to the empty p orbital on boron. No signifi-
cant interaction of the bulky pinacol group is observed, how-
ever, with the metal center.
Cytotoxic activity
We have tested the cytotoxic activity of all new platinum
complexes against both cisplatin-sensitive and cisplatin-
resistant human ovarian cells (Table 2), OV2008 and C13,
respectively, at concentrations corresponding to cisplatin’s
IC50 value against either cell line (45–49). While no activity
was observed against cisplatin-sensitive cells OV2008, com-
pounds 3a (with the Bpin group in the ortho position, Entry
3), 3b and 4b (with the Bpin group in the meta position, En-
tries 4 and 7, respectively) showed only minimal activity
against cisplatin-resistant cell line C13. Interestingly, the
analogous picoline complex 4a showed diminished activity
as compared with 3a. In general, the presence of the Bpin
groups in these complexes seems to greatly reduce their ac-
tivity, compared to the non-borylated analogue 2. It is possi-
ble that the Lewis-acidic boron atom in these complexes is
competing with, and concomitantly deactivating the plati-
num centre, by coordinating to the nitrogen donors in bio-
molecules. Further in vitro studies are therefore needed to
address the structure activity relationships in these complexes
in order to design a potent drug candidate.
We carried out an X-ray diffraction study on 4a to see if
the boronate ester group in the ortho position could effec-
tively crowd the platinum atom in a fashion similar to the
methyl group of AMD473. The molecular structure of 4a is
shown in Fig. 5 and its crystallographic data is summarized
in Table 1. While similar trends observed for 3b are found in
4a, inequivalent platinum nitrogen distances are observed,
Pt—N(9) 1.9936(19) Å (imine) and Pt—N(1) 2.057(2) Å
(pyridine). The longer Pt—N(1) bond reflects the large
strain due to the presence of the methyl group in the ortho
position to the binding atom in the pyridine ring. Bond an-
gles in 4a are also similar to those found in 3b and the twist
angle to the phenyl ring in 4a is 64.7°. Unfortunately, how-
ever, there appears to be no significant interaction of the
Bpin group with the platinum center. For instance, two of
the methyl groups of the Bpin moiety lie over the Pt with
long Pt—C(20) and Pt—C(21) distances of 5.133 and
6.023 Å, and the Pt—O and Pt—B distances are 4.158 and
4.483 Å, respectively, and are too far away for any signifi-
cant interaction. These results suggest that the ortho Bpin
groups in these complexes may not shield the platinum atom
as effectively as the methyl group in AMD473.
Conclusion
Pyridinecarboxaldimines derived from the condensation of
2-pyridinecaboxaldehydes and aminoboronate esters
H2NC6H4Bpin have been prepared in high yields and used as
© 2004 NRC Canada

1698
Can. J. Chem. Vol. 82, 2004
Fig. 5. A view of complex 4a, with displacement ellipsoids drawn at the 30% probability level. H atoms have been omitted. Selected
bond distances and angles; Pt—N(9) 1.9936(19), Pt—N(1) 2.057(2), Pt—Cl(2) 2.2830(7), Pt—Cl(1) 2.2981(6), C(8)—N(9) 1.291(3),
B—O(16) 1.3704, B—O(19) 1.366(4), and C(11)—B 1.555(4) Å; N(9)-Pt-N(1) 80.10(8), N(9)-Pt-Cl(2) 92.51(6), N(1)-Pt-Cl(2)
172.45(6), N(9)-Pt-Cl(1) 174.05(6), N(1)-Pt-Cl(1) 100.01(6), Cl(2)-Pt-Cl(1) 87.18(2), O(19)-B-O(16) 113.0(3), O(19)-B-C(11) 121.4(3),
and O(16)-B-C(11) 125.6(2)°.
trell College Science Award), the Natural Sciences and En-
gineering Research Council (NSERC), Canada Research
Chair/Canadian Foundation for Innovation/Atlantic Innova-
tion Fund programs, and Mount Allison University (MAU)
for financial support. We also thank Roger Smith and Dan
Durant (MAU) for expert technical assistance, Dr. Barbara
C. Vanderhyden of the Centre for Cancer Therapeutics (Ot-
tawa Regional Cancer Centre, Ont.) for providing us with
the OV2008 and C13 cell lines, Steve Geier for his help
with the manuscript, and anonymous reviewers for their
helpful comments.
Table 2. Percent reduction of cell prolifera-
tion of cisplatin-sensitive (OV2008) and
cisplatin-resistant (C13) cancer cells to com-
pounds at 5 and 140 µmol/L, respectively
(n = 8; SD).
Entry
Compound
OV2008
C13
1
2
3
4
5
6
7
8
Cisplatin
2
44 11
21 3
41 8
37 8
56 10
39 10
0
12 8
35 9
0
3a
3b
3c
4a
4b
4c
0
0
0
0
0
0
References
1. N. Miyaura and A. Suzuki. Chem. Rev. 95, 2457 (1995).
2. C. Morin. Tetrahedron, 50, 12521 (1994).
3. G.S. Weston, J. Blázquez, F. Baquero, and B.K. Shoichet. J.
Med. Chem. 41, 4577 (1998).
4. V.S. Stoll, B.T. Eger, R.C. Hynes, V. Martichonok, J.B. Jones,
and E.F. Pai. Biochemistry, 37, 451 (1998).
5. J. Adams, M. Behnke, S. Chen, A.A. Cruickshank, L.R.
Dick, L. Grenier, J.M. Klunder, Y. Ma, L. Plamondon, and
R.L. Stein. Bioorg. Med. Chem. Lett. 8, 333 (1998).
6. C. Gao, B.J. Lavey, C.L. Lo, A. Datta, P. Wentworth, Jr., and
K.D. Janda. J. Am. Chem. Soc. 120, 2211 (1998).
7. W. Han, J.C. Pelletier, L.J. Mersinger, C.A. Kettner, and C.N.
Hodge. Org. Lett. 1, 1875 (1999).
ligands for platinum. The resulting metal complexes have
shown negligible cytotoxicity against both cisplatin-sensitive
and cisplatin-resistant human ovarian cells, OV2008 and
C13, respectively, where the addition of a Lewis-acidic bo-
ron moiety appears to deactivate the metal complexes ability
to coordinate to DNA.
Acknowledgements
We thank Pfizer, Inc. for a Summer Undergraduate Re-
search Fellowship (HAD), the Research Corporation (Cot-
© 2004 NRC Canada

Darwish et al.
1699
8. J.D. Cox, N.N. Kim, A.M. Traish, and D.W. Christianson. Nat.
Struct. Biol. 6, 1043 (1999).
31. O. Baudoin, D. Guénard, and F. Guéritte. J. Org. Chem. 65,
9268 (2000).
9. W. Yang, X. Gao, and B. Wang. Med. Res. Rev. 23, 346
(2003).
10. T. Pandey and R.V. Singh. Synth. React. Inorg. Met.-Org.
32. L.G. Nikolcheva, C.M. Vogels, R.A. Stefan, H.A. Darwish,
S.J. Duffy, R.J. Ireland, A. Decken, R.H.E. Hudson, and S.A.
Westcott. Can. J. Chem. 81, 269 (2003).
Chem. 30, 855 (2000).
33. SAINT 6.02, Bruker AXS, Inc., Madison, Wisc., USA. 1997–
11. V.M. Dembitsky and M. Srebnik. Tetrahedron, 59, 579 (2003),
and refs. therein.
12. J.D. Dunitz, D.M. Hawley, D. Miklos, D.N.J. White, Y. Berlin,
R. Marusic, and V. Prelog. Helv. Chim. Acta, 54, 1709 (1971).
13. H. Nakamura, Y. Kitahara, T. Okasaki, and Y. Okami. J.
Antibiot. 30, 714 (1977).
1999.
34. G.M. Sheldrick, SADABS, Bruker AXS, Inc., Madison, Wisc.,
USA. 1999.
35. G.M. Sheldrick, SHELXTL 5.1, Bruker AXS, Inc., Madison,
Wisc., USA. 1997.
36. V. Martichonok and J.B. Jones. J. Am. Chem. Soc. 118, 950
(1996).
37. C.M. Vogels, L.G. Nikolcheva, D.W. Norman, H.A. Spinney,
A. Decken, M.O. Baerlocher, F.J. Baerlocher, and S.A.
Westcott. Can. J. Chem. 79, 1115 (2001).
14. G.W. Kabalka, B.C. Das, and S. Das. Tetrahedron Lett. 42,
7145 (2001).
15. A.H. Soloway, W. Tjarks, B.A. Barnum, F.-G. Rong, R.F. Barth,
I.M. Codogni, and J.G. Wilson. Chem. Rev. 98, 1515 (1998).
16. J. Thomas and M.F.Hawthorne. Chem. Commun. 1884 (2001).
17. P.R. Westmark and B.D. Smith. J. Am. Chem. Soc. 116, 9343
(1994).
38. C.R. Baar, L.P. Carbray, M.C. Jennings, and R.J. Puddephatt.
J. Am. Chem. Soc. 122, 176 (2000).
39. S.-J. Kim, N. Yang, D-H Kim, S.O. Kang, and J. Ko. Organo-
metallics, 19, 4036 (2000).
18. G.T. Morin, M.-F. Paugam, M.P. Hughes, and B.D. Smith. J.
Org. Chem. 59, 2724 (1994).
40. (a) A. Kundu and B.P. Buffin. Organometallics, 20, 3635
(2001); (b) B. P. Buffin, E.B. Fonger, and A. Kundu. Inorg.
Chim. Acta, 355, 340 (2003).
19. E.R. Jamieson and S.J. Lippard. Chem. Rev. 99, 2467 (1999).
20. J. Reedijk. Chem. Rev. 99, 2499 (1999).
21. E. Wong and C.M. Giandomenico. Chem. Rev. 99, 2451 (1999).
22. H. Rauter, R.D. Domencio, E. Menta, A. Oliva, Y. Qu, and N.
Farrell. Inorg. Chem. 36, 3919 (1997).
41. M. Crespo, X. Solans, and M. Font-Bardía. Organometallics,
14, 355 (1995).
42. H. Nöth and B. Wrackmeyer. In Nuclear magnetic resonance
spectroscopy of boron compounds. Springer, Berlin. 1978.
43. M.P. Shaver, C.M. Vogels, A.I. Wallbank, T.L. Hennigar, K.
Biradha, M.J. Zaworotko, and S.A. Westcott. Can. J. Chem.
78, 568 (2000).
44. S.D. Ittel, L.K. Johnson, and M. Brookhart. Chem. Rev. 100,
1169 (2000).
45. M.S. Robillard, M. Bacac, H. van den Elst, A. Flamigni, G.A.
van der Marel, J.H. van Boom, and J. Reedijk. J. Comb. Chem.
5, 821 (2003).
46. A. Trevisan, C. Marzano, P. Cristofori, M.B. Venturini, L.
Giovagini, and D. Fregona. Arch. Toxicol. 76, 262 (2002).
47. M.D. Todd, X. Lin, L.F. Stankowski, Jr., M. Desai, and G.H.
Wolfgang. J. Biomol. Screening, 5, 259 (1999).
48. S.M. Evans, A. Casartelli, E. Herreros, D.T. Minnick, C. Day,
E. George, and C. Westmoreland. Toxicol. in Vitro, 15, 579
(2001).
23. Y.-P. Ho, S.C.F. Au-Yeung, and K.K.W. To. Med. Res. Rev.
23, 633 (2003).
24. N. Poklar, D.S. Pilch, S.J. Lippard, E.A. Redding, S.U. Dun-
ham, and K.J. Breslauer. Proc. Natl. Acad. Sci. U.S.A. 93,
7606 (1996).
25. E.T. Martins, H. Baruah, J. Kramarczyk, G. Saluta, C.S. Day,
G.L. Kucera, and U. Bierbach. J. Med. Chem. 44, 4492
(2001), and refs. therein.
26. J.L. Butour and J.-P. Macquet. Eur. J. Biochem. 78, 455 (1977).
27. K. Neplechová, J. Kašpárková, O. Vrána, O. Nováková, A.
Habtemariam, B. Watchman, P.J. Sadler, and V. Brabec. Mol.
Pharmacol. 56, 20 (1999).
28. A.C.G. Hotze, Y. Chen, T.W. Hambley, S. Parsons, N.A.
Kratochwil, J.A. Parkinson, V.P. Munk, and P.J. Sadler. Eur. J.
Inorg. Chem. 1035 (2002).
29. H. Brunner, M. Schmidt, and H. Schönenberger. Inorg. Chim.
Acta, 123, 201 (1986).
30. S. Otto, A. Roodt, and L.I. Elding. J. Chem. Soc. Dalton
Trans. 2519 (2003).
49. R.A. Houghten, C. Pinilla, J.R. Appel, S.E. Blondelle, C.T.
Dooley, J. Eichler, A. Nefzi, and J.M. Ostresh. J. Med. Chem.
42, 3743 (1999).
© 2004 NRC Canada