Systematic differences in electrochemical reduction of the structurally characterized anti-cancer platinum( IV) complexes [Pt{((p-HC(6)F(4))NCH(2))(2)}-(pyridine)(2)Cl(2)], [Pt{((p-HC(6)F(4))NCH(2))(2)}(pyridine)(2)(OH)(2)], and [Pt{((p-HC(6)F(4))NCH(2))(2)}(pyridine)(2)(OH)Cl].

Article abstract of DOI:10.1016/j.jinorgbio.2012.07.016

The putative platinum(IV) anticancer drugs, [Pt{((R)NCH₂) ₂}(py)₂XY] (X,Y = Cl, R = p-HC₆F₄ (1a), C₆F₅ (1b); X,Y = OH, R = p-HC₆F ₄ (2); X = Cl, Y = OH, R = p-HC₆F₄ (3), py = pyridine) have been prepared by oxidation of the Pt^II anticancer drugs [Pt{((R)NCH₂)₂}(py)₂] (R = p-HC ₆F₄ (4a) or C₆F₅ (4b)) with PhICl₂ (1a,b), H₂O₂ (2) and PhICl ₂/Bu₄NOH (3). NMR spectroscopy and the X-ray crystal structures of 1b, 2 and 3 show that they have octahedral stereochemistry with the X,Y ligands in the trans-position. The net two electron electrochemical reduction of 1a, 2 and 3 has been studied by voltammetric, spectroelectrochemical and bulk electrolysis techniques in acetonitrile. NMR and other data reveal that reduction of 1a gives pure 4a via the elimination of both axial chloride ligands. In the case of 2, one end of the diamide ligand is protonated and the resulting -NH(p-HC₆F₄) group dissociated giving a [Pt{N(p-HC₆F₄)CH₂CH ₂NH(p-HC₆F₄)}] arrangement, one pyridine ligand is lost and a hydroxide ion retained in the coordination sphere. Intriguingly, in the case of reduction of 3, a 50% mixture of the reduction products of pure 1a and 2 is formed. The relative ease of reduction is 1 > 3 > 2. Testing of 1a, 2 and 3 against L1210 and L1210(DDP) (DDP = cis-diamine- dichloroplatinum(II)) mouse leukaemia cells shows all to be cytotoxic with IC₅₀ values of 1.0-3.5 μM. 2 and 3 are active in vivo against AHDJ/PC6 tumor line when delivered in peanut oil despite being hard to reduce electrochemically, and notably are more active than 4a delivered in this medium whilst comparable with 4a delivered in saline/Tween.

Full text of DOI:10.1016/j.jinorgbio.2012.07.016

Journal of Inorganic Biochemistry 115 (2012) 226–239
Contents lists available at SciVerse ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Systematic differences in electrochemical reduction of the structurally characterized
anti-cancer platinum(IV) complexes [Pt{((p-HC6F4)NCH2)2}-(pyridine)2Cl2],
[Pt{((p-HC6F4)NCH2)2}(pyridine)2(OH)2], and
☆
[Pt{((p-HC6F4)NCH2)2}(pyridine)2(OH)Cl]
Si-Xuan Guo a, Dayna N. Mason a, Susan A. Turland a, Eric T. Lawrenz a, Lance C. Kelly a, Gary D. Fallon a,
b
b
Bryan M. Gatehouse a, Alan M. Bond a, , Glen B. Deacon
, Andrew R. Battle , Trevor W. Hambley ,
⁎
a,⁎⁎
Silvina Rainone c, Lorraine K. Webster c, Carleen Cullinane c
a
School of Chemistry, Monash University, Clayton, Vic. 3800, Australia
Centre for Heavy Metals Research, School of Chemistry, University of Sydney, NSW 2006, Australia
Pharmacology and Developmental Therapeutics Unit, Peter MacCallum Cancer Institute, A'Beckett St., Melbourne, Vic. 8006, Australia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The putative platinum(IV) anticancer drugs, [Pt{((R)NCH2)2}(py)2XY] (X,Y=Cl, R=p-HC6F4 (1a), C6F5
(1b); X,Y=OH, R=p-HC6F4 (2); X=Cl, Y=OH, R=p-HC6F4 (3), py = pyridine) have been prepared by
oxidation of the PtII anticancer drugs [Pt{((R)NCH2)2}(py)2] (R=p-HC6F4 (4a) or C6F5 (4b)) with PhICl2
(1a,b), H2O2 (2) and PhICl2/Bu4NOH (3). NMR spectroscopy and the X-ray crystal structures of 1b, 2 and 3
show that they have octahedral stereochemistry with the X,Y ligands in the trans-position. The net two electron
electrochemical reduction of 1a, 2 and 3 has been studied by voltammetric, spectroelectrochemical and bulk
electrolysis techniques in acetonitrile. NMR and other data reveal that reduction of 1a gives pure 4a via the elim-
ination of both axial chloride ligands. In the case of 2, one end of the diamide ligand is protonated and the
resulting –NH(p-HC6F4) group dissociated giving a [Pt{N(p-HC6F4)CH2CH2NH(p-HC6F4)}] arrangement, one pyr-
idine ligand is lost and a hydroxide ion retained in the coordination sphere. Intriguingly, in the case of reduction
of 3, a 50% mixture of the reduction products of pure 1a and 2 is formed. The relative ease of reduction is 1>3>2.
Testing of 1a, 2 and 3 against L1210 and L1210(DDP) (DDP = cis-diamine-dichloroplatinum(II)) mouse leukaemia
cells shows all to be cytotoxic with IC50 values of 1.0–3.5 μM. 2 and 3 are active in vivo against AHDJ/PC6 tumor line
when delivered in peanut oil despite being hard to reduce electrochemically, and notably are more active than 4a
delivered in this medium whilst comparable with 4a delivered in saline/Tween.
Received 30 April 2012
Received in revised form 13 July 2012
Accepted 16 July 2012
Available online 27 July 2012
Keywords:
Platinum(IV) organoamide complexes
Crystal structure
Electrochemistry
Axial ligands
Mechanism
Anti-cancer activity
© 2012 Elsevier Inc. All rights reserved.
1. Introduction
direction is that platinum(IV) complexes are more inert to substitution
than PtII derivatives, and as a consequence they are expected to have
The biologically advantageous properties of platinum(II) complexes
in the treatment of cancer were first described by Rosenberg [1] in 1965.
Since that time, there has been great interest in the development of a
wide range of platinum(II)-based anticancer drugs as well as under-
standing their mode of action [2–9]. However, more recently, the use
of platinum(IV) complexes is being considered as a means of overcom-
ing the toxic effects and resistance now identified with use of the
platinum(II) analogues [2,6,10–21]. The rationale for this new research
fewer side-effects because of a more restricted range of interactions
with biomolecules than their more labile platinum(II) analogues
[21–24]. Some studies have reported that platinum(IV) complexes
may bind to DNA and hence could achieve anti-cancer activity without
prior reduction [11,12,14,21]. However, the significant anti-cancer ac-
tivity associated with the use of the platinum(IV) complexes is still
widely considered to be a result of their biotransformation into
platinum(II) species which display cytotoxic effects, since they are
able to be reduced by both extracellular and intracellular ligands
[21,25–27]. Furthermore, it has been demonstrated that the anti-
cancer activity of some platinum(IV) complexes is enhanced in the
presence of reducing agents, e.g. intracellular glutathione [18], ascorbic
acid and a protein sulfhydryl group [26,27].
☆
Dedication: This paper is dedicated to the memory of our friend and colleague Pro-
fessor Hans Freeman, a pioneer in structural bioinorganic chemistry.
Corresponding author. Tel.: +61 3 990 51338; fax: +61 3 9905 4597.
⁎
⁎⁎ Corresponding author. Tel.: +61 3 990 54568; fax: +61 3 9905 4597.
E-mail addresses: Alan.Bond@monash.edu (A.M. Bond), Glen.Deacon@monash.edu
(G.B. Deacon).
The above discussion implies that studies on the reduction of
anti-cancer drugs are important in understanding their biological
0162-0134/$ – see front matter © 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jinorgbio.2012.07.016

S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
227
activity. Typically, PtIV complexes are six co-ordinate, whereas PtII
complexes are only four co-ordinate. Consequently, overall two
electron reductive elimination reactions are likely to occur. The
identity of the leaving groups, the thermodynamics and the ki-
netics of the various steps are important in the reduction of PtIV
to PtII. The lability of the axial ligand bonds in six co-ordinate
platinum(IV) complexes is likely to influence significantly the
reduction process.
reduction potentials, two complexes 2 and 3 are anti-tumor active,
both in vitro and in vivo.
Despite attempts to develop biological activity - reduction peak
potential relationships, and the postulated importance of the identity
of the axial ligands, no study has yet been undertaken to identify the
products formed by electrochemical reduction of PtIV anti-cancer
drugs. In order to explicitly confirm the presumed existence of an
overall two electron reductive elimination reaction and establish in
more detail the role of axial ligands in the reduction mechanism of
anti-cancer platinum(IV) complexes, studies of products formed
after electrochemical reduction of three related platinum(IV) com-
plexes 1a, 2 and 3 (Scheme 1) have been undertaken. Structural rep-
resentations of the PtIV complexes are given in Scheme 1. The
techniques of transient forms of cyclic voltammetry at macrodisk
electrodes, near steady-state voltammetry with microdisk electrodes,
spectroelectrochemistry, bulk electrolysis and NMR spectroscopy
were employed in acetonitrile. Surprisingly, the original PtII complex
4a is formed as the sole Pt containing reduction product only when
the PtIV dichloro derivative 1a is reduced.
Reduction of platinum complexes can be achieved chemically or elec-
trochemically. In the case of chemical reduction, Siddik and co-workers
[15,18] have pointed out the possible correlation of the reduction rate
with the overall biological activity when hydroxo and chloro axial ligands
are present. Additionally, Choi et al. [28] have reported that chemical re-
duction rates with ascorbate correlate with electrochemically deter-
mined irreversible reduction peak potentials of a series of platinum(IV)
complexes under conditions of cyclic voltammetry. The complex with
more electronegative ligands or a bulkier ligand has a higher reduction
rate and a more positive reduction peak potential (more easily reduced)
[14,28]. On the other hand, PtIV derivatives of oxaliplatin do not show a
correlation between electrochemical reduction potentials and rate of re-
duction by glutathione [29]. Clearly, correlation of chemical and electro-
chemical data is not clearcut. A major problem is that the electrochemical
processes are completely irreversible hence neither the thermodynamics
nor electrode kinetics has been evaluated. In a cyclic voltammetric study
in dichloromethane or ethanol (0.1 M Bu4NClO4), Hambley et al. [30]
reported the effect of axial and equatorial ligand variation on the irre-
versible reduction peak potentials of a series of organoplatinum(IV)
complexes. These authors found that axial ligands generally have a stron-
ger influence on the peak potential than equatorial ones. Significantly,
these organoplatinum(IV) complexes show in vitro activities even
though they are not soluble in water, and some organoplatinum com-
plexes have in vitro activity even with very negative irreversible reduc-
tion peak potentials [30]. More recently, Hambley et al. [31] showed
that the extent of protein binding ability of platinum(IV) complexes in
the RPMI 1640 tissue culture medium supplemented with foetal calf
serum (RPMI/FCS) correlated with the voltammetric peak potentials,
with the most readily reduced species binding to the greatest extent,
again emphasizing the importance of axial and carrier ligands on
reduction peak potentials and the cytotoxic activity of platinum(IV) com-
plexes. This study also again emphasized that even though platinum(IV)
complexes can be very difficult to reduce to PtII electrochemically, they
still can have activities as high as those of the parent platinum(II) com-
plex from which they were derived by chemical oxidation.
The basis for design and development of PtIV drugs has recently
been reviewed [32]. Three examples have made progress towards
clinical use, tetrachloro(trans-1,2-cyclohexanediamine)platinum(IV)
(tetraplatin), cis, trans, cis-dichlorodihydroxobis(isopropylamine)
platinum(IV) (iproplatin), and trans, cis, cis-diacetatodichloro(ammine)
(cyclohexylamine)platinum(IV) (satraplatin, formerly JM216). The first
was discontinued following clinical trials due to severe irreversible
neuropathy side effects [14]. The second was abandoned through lack
of advantage over cisplatin [14], whilst satraplatin, though showing
some value for prostate cancer, has had dosage/response prob-
lems [20] and has not received FDA approval yet [14]. The problems
with the last compound may be related to the quite rapid reduction
to PtII species [32]. However, making complexes difficult to reduce
(either thermodynamically or kinetically) may make them biologically
ineffective and it requires a delicate balance to avoid premature ‘in
vivo’ reduction yet still reduce to its active PtII species in the vicinity of
the target.
We now report the synthesis, structures, detailed electrochemi-
cal reduction behavior, and biological activity of
a series of
platinum(IV) complexes, 1a, 1b, 2, 3 (Scheme 1) derived from its an-
ticancer active “rule breaker / nontraditional” [2,33,34] platinum(II)
complexes [Pt{((R)NCH2)2}(py)2] 4a and 4b (Scheme 1) (py=pyri-
dine) [35–38]. Despite having very negative voltammetric peak
Scheme 1. PtIV complexes [Pt{(RNCH2)2}(py)2XY] (1a, 1b, 2, 3) and the parent PtII
complexes [Pt{(RNCH2)2}(py)2] (4a, 4b).

228
S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
2. Experimental
2.1. Chemicals and reagents
with SHELXL-97 [41]. The program X-seed was used as a graphical in-
terface. Hydrogen atoms attached to carbon were placed in idealised
positions and allowed to ride on the atom to which they are attached.
Data collection and refinement parameters are compiled in
Table 1.
Full details of the structure determinations have been deposited
with the Cambridge Crystallographic Data Centre as CCDC 878812 for
1b, CCDC 202725 for 2, and CCDC 878811 for 3. Copies of this informa-
tion can be obtained free of charge from The Director, CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (Fax: +44 1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
[Pt{((p-HC6F4)NCH2)2}(py)2] (4a) was synthesized according to lit-
erature methods and was characterized by 1H and 19F NMR spectroscopy
[37,38]. Acetone (BDH) was laboratory reagent grade. Hydrogen perox-
ide (30%) (Aldrich) was stored at 4 °C. Acetonitrile (HPLC grade, Aldrich)
was dried over basic alumina prior to use. Tetrabutylammonium
hexafluorophosphate (Bu4NPF6) (GFS) and tetrabutylammonium
tetrafluoroborate (Bu4NBF4) (Aldrich) were purified according to
literature procedures [39], and used as the electrolytes in ace-
tonitrile at a concentration of 0.1 M. Tetrabutylammonium hydrox-
ide (Bu4NOH) (Sigma–Aldrich, 40% in H2O) was used as received.
Iodobenzene dichloride (PhICl2) was prepared by a reported method [40].
2.4. Synthesis
2.4.1. Synthesis of cis, trans, cis-[N,N’-bis(2,3,5,6-tetrafluorophenyl)
ethane-1,2-diaminato(2-)] dichlorodipyridineplatinum(IV) (1a)
[Pt{((p-HC6F4)NCH2)2}(py)2] (0.10 mmol) was dissolved in base
washed chloroform (40 ml) and a stoichiometric amount of iodobenzene
dichloride dissolved in base washed chloroform (10 ml) was added. The
mixture was allowed to stir for 0.5 h and then the solvent was removed
under reduced pressure. The dark red/black residue was dissolved in the
minimum amount of acetone, and water was added until the solution
was slightly cloudy. This mixture was cooled at −10 °C overnight. The
crystalline or powdery red/black precipitate was collected by filtration
and then air-dried. Yield 68%, m.p. 258 °C. (Anal. found: C, 36.9; H, 2.1;
N, 7.1%. C24H16Cl2F8N4Pt requires: C, 37.0; H, 2.1; N, 7.2%). 19F NMR spec-
trum: −141.0 (br m, 4 F, F(3,5)); −142.5 (br m, 4 F, F(2,6)). 1H NMR
spectrum: δ (CDCl3 at 25 °C): 8.86 (d with 195Pt satellites, 3J(PtH) 27 Hz,
2.2. Instrumentation
1H NMR and 19F NMR spectra were obtained with a Bruker DPX 300
spectrometer (XWIN-NMR, version 3.1 software) just prior to and imme-
diately after bulk electrolysis experiments. NMR spectra of complexes
were recorded with Bruker AM300 or DRX400 spectrometers with
(CD3)2CO as solvent at 30 °C unless indicated otherwise. The internal ref-
erences were tetramethylsilane for 1H NMR, and trichlorofluoromethane
for 19F NMR experiments. Infrared spectra in the range 4000-400 cm−1
were recorded with a Perkin-Elmer 1600 FTIR spectrophotometer as
Nujol and hexachlorobutadiene mulls between NaCl or KBr plates. Far
IR spectra were recorded as Vaseline mulls between polyethylene plates
with a Bruker IFS 120 HR instrument. Electrospray mass spectra were
recorded on a Micromass Platform benchtop QMS with an electrospray
source, or a Bruker BioApec 47e FTMS with 4.7 T superconducting
magnet and fitted with an Analytica Electrospray Source. Fast atom bom-
bardment mass spectra (FABMS) were recorded with a Kratos (AEI) MS
30 instrument at Brock University, Canada. For platinum containing ions,
only the most intense peak (195Pt, 231(PtCl), 266(PtCl2)) is given for each
cluster with the correct isotope pattern. Melting points were determined
using an Electrothermal IA6304 apparatus and were uncalibrated.
Microanalyses were performed by the Campbell Microanalytical
Laboratory, University of Otago, New Zealand. UV/Visible spectra
were recorded using a Cary 5 spectrophotometer, running on Varian
OS2 software. Molar (decadic) absorption coefficients are given units
3
3J(HH) 5.3 Hz, 4 H, H(2,6)(py)); 7.79 (t, J(HH) 7.5 Hz, 2 H, H(4)(py)),
3
3
4
7.20 (t, J(HH) 7.4 Hz, 4 H, H(3,5)(py)); 6.74 (tt, J(HF) 9.6 Hz, J(HF)
7.0 Hz, 2 H, p-HC6F4); 3.00 (br s, 4 H, CH2). IR: 2923 m, 2854 m,
1626 m, 1608 m, 1491 m, 1463 vs, 1223w, 1167 m, 1118 m, 1070 m,
1018 m, 936 m, ν(CF), 885w, 814vw, 763 m, 724 m, 692 m, 643w,
474w, 452 m, 436w, 417w, 396vw, 372 m, 328 m, ν(Pt-Cl), 316w,
264w, 244w, 223w, 205w, 189w, 143w, 107vw cm−1. Mass spectrum
(FAB): 777 (5%, [Pt{(p-HC6F4)N(CH2)}(py)2Cl2]+); 743 (20, [Pt
{(p-HC6F4)N(CH2)}(py)2Cl]+); 708 (100, [Pt{(p-HC6F4)N(CH2)}2
(py)2 +H]+); 665 (9, [Pt{(p-HC6F4)N(CH2)}2(py)Cl+H]+); 629
(20, [Pt{(p-HC6F4)N(CH2)}2(py)]+); 356 ([NH(p-HC6F4)(CH2)2]+). A
similar yield was obtained for a similar synthesis with acetone as solvent.
of M−1 cm−1
.
2.4.2. Synthesis of cis, trans, cis-[N,N’-bis(pentafluorophenyl)ethane-1,
2-diaminato(2-)] dichlorodipyridineplatinum(IV) (1b)
2.3. X-ray crystallography
An analogous synthesis to the preceding method was used. Yield 81%,
m.p. 119 °C (dec). (Anal. found: C, 35.1; H, 1.7; N, 6.8%. C24H14Cl2F10N4Pt
19
Crystals of compounds 1a and 3 were attached to thin glass fibers
and mounted onto a Bruker SMART 1000 CCD diffractometer employing
graphite-monochromated Mo Kα radiation (λ=0.71073 Å) generated
from a sealed tube for data collection at 294 K. Empirical absorption cor-
rections determined with SADABS [41] were applied to the data, and the
data integration and reduction were undertaken with SAINT and XPREP
[42]. The data reduction included the application of Lorentz and polari-
zation corrections. The structures were solved by direct methods with
SHELXS-86 [43], and extended and refined using teXsan [44]. The struc-
tures were refined on F2 by full-matrix least-squares with anisotropic
thermal parameters for all non-H atoms and calculated (riding model)
positions for H atoms with Uij set at 1.5 times that of the parent atom.
Crystals of 2 were mounted on a fine glass fibre using viscous
hydrocarbon oil and collections were maintained at 123 K using an
open-flow N2 Oxford Cryosystems cryostream. Data were collected
with an Enraf-Nonius Kappa-CCD diffractometer, equipped with
graphite monochromated Mo Kα radiation (λ=0.71013 Å). Data
were initially processed with the SAINT [45] program. For the
Kappa, data was processed using the DENZO-SMN [46] program.
Structures were solved using direct methods with SHELXS-97 [47]
and refined using conventional alternating least squares methods
requires: C, 35.4; H, 1.7; N, 6.8%).
F NMR spectrum: -140.9 (m, 4 F,
F(2,6)); −165.8 (m, 2 F, F(4)); -166.7 (m, 4 F, F(3,5)). 1H NMR spectrum:
Table 1
Crystallographic data for [Pt{((C6F5)NCH2)2}(py)2Cl2] (1b), [Pt{((p-HC6F4)NCH2)2}(py)2
(OH)2] (2) and [Pt{((p-HC6F4)NCH2)2}(py)2(OH)Cl] (3).
Formula
C
24H14Cl2F10N4Pt
C24H18F8N4O2Pt•
0.5 Me2CO (2)
C24H17ClF8N4OPt
(1b)
(3)
F.W.
Lattice
Space gp
a (Å)
b (Å)
c (Å)
α (º)
β (º)
γ (º)
V (Å3)
Z
813.01
Monoclinic
C2 / c
19.990(4)
9.536(2)
16.369(3)
90
126.91(2)
90
2495(1)
4
770.55
triclinic
P-1
759.98
triclinic
P-1
10.1830(1)
12.2616(1)
13.1563(1)
98.8205(4)
111.3508(4)
112.5891(4)
1327.93(2)
2
10.512(5)
12.358(4)
12.961(4)
100.71(3)
110.98(3)
109.52(3)
1389(1)
2
R
Rw
T (K)
0.0211
0.0185
273.2
0.036
0.093
123
0.054
0.044
273.2

S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
229
8.83 (m with 195Pt satellites 3J(PtH) 21 Hz, 4 H, H(2,6)(py)); 8.07 (tt, 3J(HH)
8 Hz, 4 J(HH) 3 Hz, 2 H, H4(py)); 7.51 (m, 4 H, H(3,5)(py)); 3.00 (overlaps
with H2O, CH2) (base washed CDCl3): 8.89 (m with 195Pt satellites as
shoulders, 2 H, H(2,2′)(py)); 8.89 (m with 195Pt satellites as shoulders,
2 H, H(6,6′)(py)); 7.98 (br t, J(H,H) 8 Hz, 2 H, H4(py)); 7.41 (m, 4 H,
3
H(3,5)(py)); 6.95 (m, 2 H, p-HC6F4); 2.95 (two overlapping m, 4 H,
3
4
3
shoulders, 4 H, H(2,6)(py)); 7.83 (tt, J(HH) 8 Hz, J(HH) 3 Hz, 2 H,
CH2); 2.20 (s with 195Pt satellites J(Pt,H) 23 Hz, 1 H, OH). (−90 °C) 9.18
H(4)(py)); 7.25 (overlaps with CHCl3, H(3,5)(py)); 2.91 (br s with 195Pt
(br s, 1 H, py(H2)), 8.98–8.65 (3 overlapping br s, 3 H, py{H(2′,6,6′)});
8.15 (br t, 2 H, py(H4)); 7.58 (br s, 4 H, py{H(3,5)}); 7.35 (br s, 2 H,
p-HC6F4); 4.24 (br s, 2 H, CH2); 2.28 (br s with Pt satellites, 1 H, OH);
1.90–1.50 ( 2 overlapping br s, 2 H, C'H2). (CDCl3): 9.07 (m with 195Pt
satellites as shoulders, 2 H, H(2,2′)(py)); 8.89 (m with 195Pt satellites as
3
satellites J(Pt,H) 15 Hz, 4 H, CH2). IR: 3124w, 3095w, 3058w, 2966w,
2934w, 2872w, 1610 m, 1506 s, 1488 s, 1420w, 1367w, 1307w, 1249w,
1220w, 1167 m, 1129w, 1070 s, 1046 m, 1032 s, 982 m, 884 m, 772w,
729w, 700w, 670w, 643 m, 580w, 368 s, 335 m, 316 s [v(Pt-Cl)], 229w,
203w cm−1
.
Mass spectrum (Electrospray+ve ion): 815 (100%,
shoulders, 2 H, H(6,6')(py)); 7.98 (br t, J(H,H) 8 Hz, 2 H, H4(py)); 7.41
3
[MH+]); 797 (76, [Pt{(C6F5)N(CH2)}2(py)2(OH2)Cl]+); 744 (20, [Pt
{(C6F5)N(CH2)}2(py)2 H]+). Single crystals were obtained by diffusion
of water into an ethyl acetate solution.
(m, 4 H, H(3,5)(py)); 6.95 (m, 2 H, p-HC6F4); 2.95 (m, 5 H, CH2 and
OH); 2.18 (s, 3 H, Me of acetone). IR: 3530w[ν(O\H)], 3101 m,
3038 m, 2932 m, 2864w, 1708 m (ν(CO) acetone), 1634 m, 1607 m,
1494 s, 1447 s, 1240w, 1222 m, 1212w, 1189w, 1166 m, 1118 s,
1069 s, 1050 m, 1037w, 1018 m, 959w, 938 s, 886 m, 811w, 799w,
774w, 766 m, 741w, 725w, 701 m, 694 m, 668w, 646 m, 593w, 561 m
[ν(Pt-O)], 553 s, 377w, 313 s [ν(Pt-Cl)], 252w cm−1. Mass spectrum
(Electrospray+ve ion): 775 (5%, [MH-OH+OCH3]+); 761 (100,
[MH+]); 664 (20, [Pt{(p-HC6F4)N(CH2)}]2(py)2Cl]+).
2.4.3. Synthesis of cis, trans, cis-[N,N′-bis(2,3,5,6-tetrafluorophenyl)
ethane-1,2-diaminato(2-)] dihydroxodipyridineplatinum(IV) (2)
[Pt{((p-HC6F4)NCH2)2}(py)2] (0.20 - 0.50 mmol) was dissolved in ac-
etone (20 ml). Excess 30% hydrogen peroxide (0.1–0.4 ml) was added
and the reaction mixture was stirred at 80 °C for 1 h. After cooling,
MnO2 was added to the flask to degrade catalytically any excess hydro-
gen peroxide, the solution was filtered through Celite and was carefully
evaporated to dryness. The residue was dissolved in a minimum of ace-
tone, and water was added until the solution was slightly cloudy. Crystal-
lization proceeded with cooling overnight at −20 °C. Yield 60–90%, m.p.
187–189 °C. (Anal. found: C, 38.9; H, 2.2; N, 7.5%. C24H18F8N4O2Pt
requires: C, 38.9; H, 2.5; N, 7.6%). 19F NMR spectrum: −143.3 (br m,
4F, p-HC6F4); -143.5 (m, 4F, p-HC6F4). 1H NMR spectrum: 9.08, (d with
2.4.4.2. Method 2. [Pt{((p-HC6F4)NCH2)2}(py)2Cl2] (0.070 mmol) was
dissolved in 50 ml acetone, and silver acetate (0.17 mmol) dissolved
in warm water was then added. This reaction mixture was stirred in
the dark overnight. The precipitate of silver chloride and silver ace-
tate was then filtered off through Celite. The red filtrate was then con-
centrated under reduced pressure. TLC showed two compounds
(Eluent 1:1, thf: petroleum ether). The more mobile band had the
same Rf value as the platinum(IV) material. Evaporation at 50 °C
under vacuum gave a dark red solid residue which was dissolved in
a minimum amount of thf and the two components were separated
by column chromatography on neutral alumina. A red band was col-
lected first and the solvent was allowed to slowly evaporate off giving
red crystals. The crystal chosen for X-ray diffraction analysis was
found to be the target complex. However, NMR spectroscopy of the
bulk crystals showed that they were a mixture of this complex and
the starting material.
3
195Pt satellites J(PtH) 21 Hz, 4 H, H(2,6)(py)); 7.90 (t, 2 H, H(4)(py));
7.38 (t, 4 H, H(3,5)(py)); 6.85 (tt, 2 H, p-HC6F4); 2.90 (s with 195Pt satel-
3
lites J(PtH) 21 Hz, 4 H, CH2); 0.75 (br s, 2 H, OH). IR: 3567 m, 3099w,
3082w, 3048vw, 3006vw, 2926 m, 2863 m, 1630 m, 1610 m, 1495vs,
1450vs, 1243w, 1214 m, 1172 s, 1114 s, 1099 s, 1067 s, 1020 s, 957w,
938vs, 880 m, 824 m, 768 m, 695 m, 648w, 597w, 551 m cm−1. Mass
spectrum (Electrospray+ve ion): m/z 723 (0.6%, [C24H16F8N4OPt]+);
707 (0.5, [Pt{(p-HC6F4)N(CH2)}2(py)2]+); 644 (0.8, [C19H11F8N3OPt]+);
352 (9.5, [C14H4F8N2]+); 79 (100, py+). FABMS: m/z 742 (10%, MH+);
707 (11, [Pt{(p-HC6F4)N(CH2)}2(py)2]+); 645 (100, [Pt{(p-HC6F4)
N(CH2)}2(py)(OH)]+); 628 (19, [Pt{(p-HC6F4)N(CH2)}2(py)]+); 546
(15, [C14H3F8N2Pt]+); 485 (37, [Pt{(p-HC6F4)N(CH2)}(py)(OH)2]+);
467 (56, [Pt{(p-HC6F4)N(CH2)}(py)(O)+]); 450 (46, [Pt{(p-HC6F4)
NCH}(py)]+); 370 (20, [Pt(py)2(OH)]+); 353 (49, [Pt(py)2]+); 274
(60, [Pt(py)]+). The higher yields were obtained by partial evaporation
of acetone to 5 ml, addition of water until this solution was cloudy and
cooling. Single crystals of a hemiacetone solvate were obtained by slow
evaporation from acetone.
2.5. Electrochemical Instrumentation and Procedures
As these complexes do not dissolve in water, acetonitrile, which is
highly polar with a high dielectric constant and has a wide negative
potential window, was chosen as the solvent in electrochemical studies.
Cyclic voltammetric, potential step chronoamperometric, and bulk elec-
trolysis measurements were carried out in nitrogen degassed acetoni-
trile (0.1 M electrolyte) solutions using Bioanalytical Systems BAS
Model 100B or 100A electrochemistry workstations (Bioanalytical
Systems, West Lafayette, IN, USA). A silver wire placed in acetonitrile
(0.1 M electrolyte), but separated from the analyte by a glass frit was
used as a quasi-reference electrode. The reference potential was cali-
brated against that of the ferrocene/ferricenium (Fc/Fc+) redox couple
as an internal reference from measurements made on the oxidation of
1 mM Fc present in the same solution. The diameters of the glassy
carbon macrodisc and microdisc electrodes were calibrated using the
one-electron reduction of 10 mM potassium ferricyanide in aqueous
0.5 M KCl and the theory relevant to cyclic voltammetry (macrodisk)
and steady-state voltammetry (microdisk) respectively [48] and the
2.4.4. Synthesis of cis,trans,cis-[N,N′-bis(2,3,5,6-tetrafluorophenyl)
ethane-1,2-diaminato(2-)] chlorohydroxodipyridineplatinum(IV) (3)
2.4.4.1. Method 1. [Pt{((p-HC6F4)NCH2)2}(py)2] (0.20–0.30 mmol)
was dissolved in 50–100 ml acetone and a stoichiometric amount of
tetrabutylammonium hydroxide in 5 ml of water was added. To oxi-
dize the platinum complex, a 20% excess of iodobenzene dichloride
in 20 ml acetone was added slowly to the above mixture, which
was allowed to stir for at least 0.5 h. The solvent was then removed
under reduced pressure. The red/orange residue was dissolved in
20 ml acetone and water added until the solution was cloudy. This
mixture was stored overnight at −10 °C and the resulting deep orange
microcrystalline precipitate was collected by filtration and air-dried.
Yield 62%, m.p. 130 °C (dec). (Anal. found: C, 38.4, 38.6; H, 1.8, 2.0; N,
6.9, 7.0%. C24H17ClF8N4OPt requires: C, 37.9; H, 2.3; N, 7.4%; a 0.5 Me2CO
solvate requires: C, 38.8; H, 2.6; N, 7.1%). 19F NMR spectrum: -140.6 (br
s, 2 F, F(2,2′)); -143.2 (m, 6 F, F(3,5,6,6′)). (−90 °C) −138.7 (br s, 1 F,
F2); −140.3 (br s, 1 F, F6'); −141.3 (br s, 1 F, F2′); −143.3 (br s, 2 F,
F3,5); −143.7 (br s, 2 F, F3′,5′); −144.2 (br s, 1 F, F6). 1H NMR spectrum:
(For compound numbering see Figure S1) 9.07 (m with 195Pt satellites as
known diffusion coefficient of 7.6×10−6 cm2 s−1 for [Fe(CN)6]3−
.
For cyclic voltammetric studies, a conventional three-electrode cell
was employed, with a 1 mm diameter glassy carbon (GC) or platinum
macrodisk electrodes or 11 μm diameter GC or Pt microdisk electrodes
as the working electrode, and a Pt wire as the auxiliary electrode. The
reference electrode was as described above. Prior to voltammetric
experiments, the working electrode was polished with 0.3 μm alumina
on a clean polishing cloth (Buehler, USA), rinsed with water, sonicated
in water thoroughly to remove alumina, rinsed successively with
water and acetone and finally dried under nitrogen gas.

230
S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
For bulk electrolysis experiments, both the working and auxiliary
[49–51]. Treatment of [Pt{(R)NCH2}2(py)2] (R=p-HC6F4 or C6F5)
complexes 4a or 4b with PhICl2 gave the PtIV complexes [Pt{(R)
NCH2}2(py)2Cl2] 1a, 1b in good yield (Scheme 2 (I)). An analogous
electrodes were constructed from cylindrical platinum gauze. The work-
ing and reference electrodes were placed in a 12 ml volume glass cylin-
der that contained a glass frit base, to provide separation from the
auxiliary electrode compartment. The glass cylinder containing the
working and reference electrode and 10 ml of 5 mM solution of plati-
num complex dissolved in acetonitrile (0.1 M Bu4NPF6) were placed in-
side a larger cylindrical glass cell which contained the auxiliary electrode
and approximately 15 ml of acetonitrile (0.1 M Bu4NPF6). The whole
cell was covered by a lid with 3 inlets for electrode connections and 2
for inlet and outlet nitrogen lines, and during the course of bulk electrol-
ysis, the solutions were stirred vigorously via use of a stirring bar and
purged with solvent saturated nitrogen. The electrolysis was stopped
when the ratio of the average current during the time interval just
passed to that of the first time interval, reached 1%. Cyclic voltammetric
measurements were undertaken on the solution present in the working
electrode compartment (before and after bulk electrolysis) using the 3
electrode arrangement described above.
–
oxidation of cis-[Pt(C6F5)2(PhN3Ph)] to the corresponding dichloride
has been reported [52]. Use of iodine(III) reagents to oxidize platinum
/ palladium(II) complexes to tetravalent derivatives is a currently active
synthetic method [53–59]. The trans-Cl2 arrangement in 1b was
established by X-ray crystallography (below). A similar structure is like-
ly for 1a, both complexes having similar single v(Pt-Cl) frequencies and
NMR spectra (Experimental section).
Complex 2 was obtained by oxidation of 4a with hydrogen per-
oxide (Scheme 2 (II)). Analogous oxidations of PtII ammine/amine
complexes form part of the synthetic route to PtIV drugs such as
satraplatin [60]. Closer analogues come from oxidation of bis
(polyfluorophenyl)platinum(II) complexes to the corresponding
dihydroxoplatinum(IV) complexes [30,61]. The complex showed a
single v(Pt\O) absorption consistent with a trans-(OH)2 array
which was confirmed by X-ray crystallography (below).
Spectroelectrochemical studies were carried out using a Cary 5
UV–vis-NIR Spectrophotometer (Varian OS/2 Warp software) to obtain
electronic spectra and a BAS Model 100A electrochemistry system to
apply the constant potential. A 1 mm thick quartz cuvette was used as
the electrochemical cell, with a platinum gauze working electrode, a
platinum wire contained in a salt bridge as the auxiliary electrode, and
the same reference electrode as employed in the voltammetric studies.
UV-visible spectra (800 nm to 200 nm) were recorded continuously at
a scan rate of 504 nm min−1 until the electrolysis was completed.
Unless otherwise stated, electrochemical studies were carried out
at ambient temperature of (20 1)°C.
Complex 3 was optimally obtained by oxidation of 4a with PhICl2
in the presence of an equimolar amount of tetrabutylammonium hy-
droxide in aqueous acetone (Scheme 2 (III)). A reaction of 1a with an
excess of silver acetate in aqueous acetone resulted in partial conver-
sion into 3 and single crystals were grown following attempted chro-
matographic separation of 3 from residual 1a. 3 has absorption bands
attributable to v(Pt\O) and v(Pt-Cl) in the far infrared spectra.
The composition of all bulk compounds was confirmed by micro-
analysis, NMR spectra and mass spectra. Single crystals of 2 contained
0.5 acetone/molecule but bulk samples were acetone-free as indicat-
ed by microanalysis and IR spectroscopy. Conversely, although single
crystals of 3 did not contain acetone, 1H NMR spectra of bulk samples
showed 0.5 molecule of acetone and the IR spectrum displayed signif-
icant v(CO) absorption, whilst the %C analysis was consistent with 0.5
acetone of crystallization.
2.6. Anticancer activity studies
Both in vitro [35] and in vivo [36] activity studies of the platinum
complexes were carried out using reported procedures.
A number of significant effects were noted in the 1H and 19F NMR
spectra compared with those of their PtII precursors. For example, the
o-H(py) chemical shifts are at higher frequencies (0.2 – 0.5 ppm) for
1a, 1b, 2 and 3 than for 4a and 4b, attributable to the greater electroneg-
ativity of the higher oxidation state. On the other hand, the 195Pt-H
coupling constants, where resolved, for o-H(py) and CH2 resonances,
were smaller. In many cases, these were visible only as unresolved shoul-
ders, indicating relatively small values. In the 19F NMR spectra, the most
definitive feature is the high frequency shift of F2,6 from ca. –151 ppm
3. Results and Discussion
3.1. Synthesis of PtIV Organoamidometallic Complexes
The PtIV complexes 1a, 1b, 2 and 3 were prepared by oxidation of
the PtII organoamides 4a or 4b. More commonly, PtIV organoamides
are prepared by deprotonation of amineplatinum(IV) derivatives
Scheme 2. Oxidation of PtII organoamides 4a, 4b. (I) R=p-HC6F4 or C6F5; (II)–(IV) R=p-HC6F4.

S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
231
for 4a, 4b to ca. –143 ppm for the PtIV complexes. In the case of the
p-HC6F4 derivatives 1a, 2 and 3, the F2,6 and F3,5 resonances are near
overlapping, and for 2, they could not be distinguished despite use of a
range of decoupling experiments.
(X, Y=Cl) (1a) at a 1 mm diameter glassy carbon electrode in acetoni-
trile (0.1 M Bu4NBF4) when the potential was scanned in the negative di-
rection showed three major irreversible reduction processes (labeled 1, 2
and 4 in Fig. 2A) with peak potentials of −964, -2539 and −3098 mV,
and a shoulder (labeled 3 in Fig. 2A) at around −2887 mV. When the po-
tential was first scanned from the open circuit value (−200 mV) to
+600 mV, no oxidation process was observed. However, if the potential
was switched after reduction process 1 or 2, two new oxidation process-
es (labeled 6' and 7' in Fig. 2A) at potentials around +94 and +308 mV
vs. Fc/Fc+ were observed, indicating that these two oxidation processes
are associated with the product of the first reduction process. When
the potential was switched after process 4, these two oxidation processes
remained unchanged and one extra oxidation process at −323 mV (5′)
was observed, which is associated with the reduction process 4. The
peak potential data obtained at a scan rate of 1 V s−1 are summarized
in Table 3.
The potential range available in the negative potential region is
smaller at Pt electrodes, so only the first irreversible reduction peak 1
is clearly observed at this surface, along with the associated oxidation
processes 6' and 7' (see Fig. 2B and data in Table 3). This observation is
consistent with the result obtained at the GC electrode, confirming that
the oxidation processes 6' and 7' are indeed associated with products
formed as a result of reduction process 1. It will emerge that process 1
corresponds to an overall 2-electron PtIV/PtII process. Consequently,
this process and processes 6' and 7', which occur due to the oxidation
of the PtII species formed by reduction process 1, are the only ones of bi-
ological relevance. Hence, only these processes are examined in detail in
this paper. For the same reason, ligand based oxidation process seen at
potentials more positive than 600 mV vs. Fc/Fc+ also are not discussed.
Cyclic voltammograms obtained with 1 mM [Pt{((p-HC6F4)NCH2)2}-
(py)2(OH)2] (2) exhibited three major irreversible reduction processes
at much more negative potentials than those observed for 1a (Fig. 3
and Table 3). During the reverse or oxidative potential scan, three oxida-
tion processes are seen (Fig. 3). Via use of experiments in which the po-
tential was switched between processes A and B, B and C, and C and the
solvent limit, it was found that peak F' is associated with peak A, peak E′
with peak B, and peak D′ with peak C. At a Pt working electrode, again
only the first irreversible reduction peak A is clearly observed with
only one oxidation process F' (Table 3) being coupled to this initial reduc-
tion reaction. This is consistent with the result obtained at a GC electrode,
but is different from that observed for 1a, where two oxidation processes
are found. In this case, process A and process F′, which arises due to the
oxidation of the PtII species formed by reduction process A, are the only
ones of biological relevance.
More complex spectra were observed for the hydroxo(chloride)
complex 3 owing to the unsymmetrical axial substitution coupled
with the inclination of the polyfluorophenyl groups and pyridine li-
gands to the coordination plane and the location of CH2 groups above
and below this plane. In the 1H NMR spectrum of 3 at room tempera-
ture, there are two distinct o-H(py) resonances and two overlapping
CH2 multiplets, but single resonances for m, p-H(py) and p-HC6F4. On
cooling, the o-H(py) signals partly coalesce at −70 to −80 °C before
partly resolving as four singlets at −90 °C correspondingly to four dis-
tinct o-H atoms. At −60 to −70 °C, the methylene resonances coalesce
before resolving into a broad singlet (2 H) widely separated (>2 ppm)
from two partly overlapping broad singlets (each 1 H) at −90 °C,
approaching separation into four distinct methylene H atoms. In the
19F NMR spectrum of 3 at room temperature, an o-F resonance is ex-
tremely broad and barely resolved (2 F), the remaining o-F resonance
overlapping with the m-F resonances. From 0 to −40 °C, four fluorine
resonances can be resolved, and at −50 to −70 °C, all but one signal
broaden into the baseline. However, at −90 °C, there are four separate
19F resonances corresponding to four distinct o-F atoms, and two sepa-
rate m-F resonances. Thus, by −90 °C, the spectra reflect the asymme-
try evident in the solid state structure of 3.
3.2. Structures of complexes 1b, 2 and 3
The molecular structures of 1b, 2 and 3 are displayed in Fig. 1 and
selected band distances are listed in Table 2. All complexes have octa-
hedral stereochemistry with trans-Cl2, (OH)2 or (OH)Cl ligands (XPtY
angles: 177.83(5), 179.73(9), 176.4(2)° respectively) in the axial po-
sition. The equatorial sites are occupied by a chelating {(R)NCH2}2−
(R=C6F5 or p-HC6F4) ligand and cis-pyridine donors, as found in
the structure of the square planar PtII precursor 4a [62,63].
Both the Pt-Cl and Pt-OH bond lengths are as expected for PtIV com-
plexes with trans-Cl2 or trans-(OH)2 linkages. Thus, the former are similar
to values for [Pt{((HOCH2CH2)NCH2)2}Cl4] (Pt-Cl 2.3094(13), 2.3202(13)
Ǻ) [64] and [Pt{κ1(N)-(Me2NCH2CH2OH)}{κ2(N,O)-(Me2NCH2CH2O)}Cl3]
(Pt-Cl 2.319(18), 2.315(19) Ǻ) [65], whilst Pt-OH lengths compare
well with those of cis, cis, trans-[Pt(NH3)2Cl2(OH)2] (2.008(5) Ǻ) [66]
and trans, cis, cis-[Pt(OH)2(C6F5)2(py)2] (2.012(3) Ǻ) [61]. However, in
the chloro(hydroxo) complex 3, both Pt-Cl and Pt-OH bond lengths
are significantly longer than in 1b and 2 respectively, suggesting higher
trans- influences with the unsymmetrical arrangement. Comparison of
the Pt-N bond lengths (Table 2) with those of the precursor complex
4a [62] indicates that with the exception of Pt-N(amide) of 3 (which
has larger esds), the values for the PtIV complexes are larger indicating
that the higher coordination number of 1b, 2, 3 and the increased
crowding outweigh the effect of the higher oxidation state. The length-
ening is most evident with Pt-N(py) bond lengths, and may be related
to the reduction in N(py)-Pt-N(py) bond angles of 4–6° from 4a to 1b,
2 and 3. Any apparent compensating increase in N(amide)-Pt-N(amide)
angles is not significant at the 3 esd level. Whereas the stereochemistry of
the amide nitrogen donors is triangular (ε 359°) for 4a, the PtIV complexes
show a more pyramidal arrangement (ε 343–344°). The N(amide)-C6F5 /
p-HC6F4 bond lengths (ca. 1.39 Ǻ) have some partial double bond char-
acter when compared with N(amide)-CH2 values, but the shortening is
not as marked as shown by N(py)-C(ortho) bond lengths (Table 2).
Cyclic voltammograms obtained for X=Cl, Y=OH (3) derivative
exhibit characteristics that are intermediate to those found for 1a
and 2. Thus, this complex is slightly easier to reduce than for 2 but
harder to reduce than for 1a. It exhibits four reduction peaks at a GC
electrode and four small oxidation peaks on the reverse scan, where
two of them are in the potential region where processes E' and F'
are found for 2 and the other two in the potential region where pro-
cesses 6' and 7' are found for 1a. At a Pt electrode, this complex also
exhibits only one reduction process similar to process A for 2. Howev-
er, three oxidation peaks were observed, with one in similar potential
region where process F' is found for 2, and two in similar potential re-
gions where processes 6' and 7' are found for 1a (Table 3). Data for all
complexes are summarized in Table 3 at a scan rate of 1 V s−1 and
concentration of 1.0 mM. For every complex, all processes of the
kind A, D', E', F' remain fully irreversible over the scan rate range of
10 mV s−1 to 10 V s−1
.
3.3. Cyclic voltammetry at macrodisk electrodes
Data in Table 3 reveal that the reduction potentials become more
negative in the order X, Y=OH>X=Cl, Y=OH >>X, Y=Cl. Thus,
reduction of 1a is most readily achieved whilst reduction of 2 and 3
are much more difficult. The order of reduction is consistent with
the results reported by Choi et al. [28] at a glassy carbon electrode
in aqueous 0.1 M KCl (pH 7.0) at a scan rate of 100 mV s−1. In
As these complexes are not soluble in water, the electrochemical
studies were carried out in acetonitrile, which has a large electrochemical
window and is highly polar with a high dielectric constant. Cyclic
voltammograms obtained with 1 mM [Pt{((p-HC6F4)NCH2)2}(py)2XY]

232
S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
Fig. 1. X-ray crystal structures of [Pt{((C6F5)NCH2)2}(py)2Cl2] (1b), [Pt{((p-HC6F4)NCH2)2}(py)2(OH)2] (2) and [Pt{((p-HC6F4)NCH2)2}(py)2(OH)Cl] (3).

S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
233
Table 2
Selected bond lengths of [Pt{((C6F5)NCH2)2}(py)2Cl2] (1b), [Pt{((p-HC6F4)NCH2)2}(py)2(OH)2] (2), [Pt{((p-HC6F4)NCH2)2}(py)2(OH)Cl] (3) and [Pt{((p-HC6F4)NCH2)2}(py)2] (4a).
Bond
C
24H14Cl2F10N4Pt
C24H18F8N4O2Pt•
0.5 Me2CO
(2) (Å)
C
24H17ClF8N4Opt
C24H16F8N4Pt
(4a) (Å)
(1b) (Å)
(3) (Å)
Pt\X
Pt\N(amide)
2.318(1) (X_Cl)
2.056(3)
2.017(3) (X_O) 2.008(3) (X_O)
2.033(3)
2.033(3)
2.090(3)
2.083(3)
1.470(5)
1.473(5)
1.395(5)
1.392(5)
1.348(5)
1.337(5)
1.353(5)
1.348(5)
2.347(3) (X_Cl) 2.066(7) (X_O)
2.032(10)
2.008(10)
2.106(10)
2.144(9)
1.50(2)
1.49(1)
1.39(2)
1.35(1)
1.34(1)
1.33(1)
1.35(1)
1.32(1)
1.993(4)
2.038(4)
1.457(6)
1.386(6)
1.349(6)
1.354(6)
Pt\N(py)
2.129(3)
1.475(6)
1.390(4)
1.333(5)
1.355(5)
N(amide)\CH2
N(amide)\C6F5/C6F4H
N(py)\C(ortho)
N(py)\C(ortho)
contrast, the initial reduction for each PtIV complexes at Pt electrodes
is observed at significantly more negative potentials than when GC
electrodes are used, but processes found on the reverse (oxidation)
scan are similar, so products formed do not appear to be dependent
on the electrode surface. In addition, examination of the Epox values
obtained following the first reduction steps indicate the formation
of different products for each PtIV complex.
organoamide, [Pt{((p-HC6F4)NCH2)2}(py)2] (4a), occurs at a potential
of about +38 mV vs. Fc/Fc+ with a second oxidation process at about
+573 mV [68]. These processes are in the same region as for processes
6' and 7' for 1a, and for two oxidation processes for 3, but not for pro-
cesses E' and F' for 2. Thus, on the basis of cyclic voltammetric data,
the reduction outlined in Eq. (1) is most likely responsible for the obser-
vation of process 1 for 1a.
As noted above, the initial reduction process and coupled oxidation
processes are of major interest of this study. For convenience, the first
reduction process is designated as process 1, and the oxidation process-
es of interest found in the reverse scan of cyclic voltammogram as
processes 6' and 7' (Fig. 2). The initial oxidation step for the PtII
½PtIVfððp ꢀ HC6F4ÞNCH2Þ gðpyÞ Cl2ꢁ
2
2
þ 2e−→½PtIIfððpꢀHC6F4ÞNCH2Þ gðpyÞ ꢁ þ 2Cl−
ð1Þ
2
2
However, the reduction of PtIV species containing two hydroxide
ligands do not appear to form the PtII organoamide, 4a, as the major
product on the timescale of transient cyclic voltammetry at GC or Pt
macrodisk electrodes. While for 3, 4a is likely to be one of the
products.
3.4. Steady-state voltammetry at microdisk electrodes
In order to quantitatively evaluate the number of electrons involved
in the first reduction process of the PtIV complexes, both near
steady-state voltammetric and potential step chronoamperometric
data were obtained for the irreversible process of each complex using
11 μm diameter GC and Pt microdisk electrodes.
The diffusion-controlled limiting current, id, obtained at a microdisc
electrode under steady-state conditions is given by [48],
id ¼ 4nFDac
ð2Þ
where n is the total number of electron transferred; F is Faraday's con-
stant; D and c are the diffusion coefficient and concentration of the
PtIV complexes respectively; and a is the radius of the electrode.
The time-dependent current, i, for a diffusion-controlled mul-
tiple electron transfer process, recorded by
a potential step
chronoamperometric measurements after the electrode potential
is stepped from a value where there is no Faradaic process to one
where the electron transfer process is diffusion-controlled, is
governed by the relationship [48,67],
−1=2
i
id
inorm
¼
¼ 0:7854 þ 0:8862τ−1=2 þ 0:2146e−0:7823τ
ð3Þ
where inorm (= i/id) and τ=4Dt/a2 are the dimensionless current
and time, respectively. Eq. (3) was initially derived by numerical
simulation of a one-electron transfer reaction. However, it is equally
applicable to an irreversible multi-electron transfer process when
the potential is stepped to a sufficiently negative value where the di-
mensionless transient current is governed solely by the diffusion of
the PtIV complexes [48]. Consequently, via use of Eq. (2), the value of
Fig. 2. Cyclic voltammograms obtained at 20 °C for reduction of 1.0 mM [Pt{((p-HC6F4)
NCH2)2}(py)2Cl2] (1a) in acetonitrile (0.1 M Bu4NBF4). Scan rate: 1 V s−1. Working
electrodes: (A) glassy carbon macrodisk, (B) platinum macrodisk.

234
S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
Table 3
Summary of peak potentialsa obtained at a scan rate of 1 V s−1 when cyclic voltammetric experiments are undertaken with 1 mM [Pt{((p-HC6F4)NCH2)2}(py)2XY] in acetonitrile
(0.1 M Bu4NPF6).
X
Y
GCb
Ptb
Epred
Epox
Epred
Epox
Cl
OH
Cl
Cl
OH
OH
−964
−2098
−1916
−2539
−2939
−2704
−2887
−3123
−2887
−3098
−3089
−323
−2769
−636
+94
−975
−275
+308
−285
+325
−1295
−2394
−2150
86
−265
−256
320
106
−511
+122
227
Reduction (Epred) and oxidation (Epox) peak potentials in mV vs. Fc/Fc+
Potential switched prior to onset of solvent reduction.
.
a
b
D for the PtIV complexes can be obtained without knowing n, or
indeed, even knowing the concentration of the PtIV complexes.
Once the value of D is known, n is then easily obtained via measure-
ment of id from a steady-state voltammetric experiment and use of
Eq. (2).
3.5. Spectroelectrochemistry
In order to elucidate the identity of the presumed PtII products
formed from the initial two-electron reduction processes of the three
PtIV complexes, 1a, 2 and 3, in situ spectroelectrochemical studies
were undertaken in a thin layer cell at a platinum gauze electrode.
Fig. 5 shows the UV–Visible spectra of 0.25 mM 1a in acetonitrile
(0.1 M Bu4NBF4) before and after exhaustive electrolysis in the thin
layer cell as well as the spectrum of 0.25 mM 4a in the same electrolyte
at 20 °C. It is clear from Fig. 5 that the product derived from reduction of
1a exhibits an identical spectrum to that of 4a (small decrease in con-
centration noted in electrochemical generation, which may be a result
of diffusion out of the thin layer part of the cell), indicating that the
main reduction product is 4a. Table 4 contains the UV-Visible absorp-
tion λmax data obtained both before and after exhaustive reductive
Examples of data obtained from near steady-state voltammetric and
chronoamperometric measurements for 2 on GC electrodes are shown
in Fig. 4. In order to minimize complications arising from the presence
of the background (charging) current in chronoamperometric experi-
ments at short times, a relatively high concentration of PtIV complexes
(5.0 mM) was used in these experiments. Use of data obtained in
Fig. 4 and Eqs. (2) and (3) gave a diffusion coefficient of (1.3
0.1)×10−5 cm2 s−1 for 2 and an n-value of 2.0 0.2 for the number
of electrons associated with the reduction process. Experiments on
the other two PtIV compounds gave diffusion coefficients of (1.3
0.1)×10−5 cm2 s−1 and (1.2 0.1)×10−5 cm2 s−1 for 3 and 1a
respectively, and n-values of 2.0 0.2 for both complexes. Similar re-
sults for all three complexes were obtained at an 11 μm Pt microdisk
electrode.
A
The above results support Eq. (1) above and imply that the reduc-
tion of the three PtIV complexes [Pt{((p-HC6F4)NCH2)2}(py)2XY] (X,
Y=Cl, OH) can be generalized to the reaction outlined in Eq. (4).
½PtIVfððpꢀHC6F4ÞNCH2Þ gðpyÞ XYꢁðsolutionÞ þ 2e−→PtIIproductðsÞ ð4Þ
2
2
Further details of the mechanism cannot be unraveled from
these data. However, combining these data with transient cyclic
voltammetric data (see above) imply that the reduction of PtIV
organoamides must involve a complex reaction scheme, with an
overall transfer of two electrons being accompanied by chemical
transformations, probably involving loss of ligands.
4
B
3
2
1
0
0
10
20
30
40
-1/2 -1/2
/s
t
Fig. 4. Electrochemical reduction of 5.0 mM [Pt{((p-HC6F4)NCH2)2}(py)2(OH)2] (2)
in acetonitrile (0.10 M Bu4NBF4) at an 11 μm diameter GC microdisk electrode.
(A) Near-steady-state voltammogram obtained at a scan rate of 10 mV s−1. (B) Potential
step chronoamperometry when the potential is stepped from −1.6 V to −2.5 V (vs.
Fig. 3. Cyclic voltammogram obtained at a glassy carbon macrodisk at 20 °C for reduc-
tion of 1.0 mM [Pt{((p-HC6F4) NCH2)2}(py)2(OH)2] (2) in acetonitrile (0.1 M Bu4NBF4).
Scan rate: 1 V s−1
.
Fc/Fc+). (—) experimental; (− −) theoretical, with D=1.3×10−5 cm2 s−1
.

S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
235
cyclic voltammograms were recorded before and after each bulk elec-
trolysis experiment.
3.6.1. Coulometric determination of n-value
The coulometric studies based on integration of the current-time
curve for bulk electrolysis gave n-values of 1.9 0.2, 1.8 0.2 and
2.2 0.2 for 1a, 3 and 2, respectively. These values are consistent with
the results obtained using much shorter time scale steady-state
voltammetry (see above) and confirm that the reduction of PtIV to PtII
occurs for each of the compounds over a wide range of time scales.
The current-time curve for 1a showed exponential current decay,
which suggests that the reduction of 1a is a straightforward reduction,
with one main product. However, the current-time curves for 3 and 2
did not exhibit simple exponential current decay, which indicates that
intermediates are formed during the course of these bulk electrolysis
experiments.
Fig. 5. Electronic spectra obtained at 20 °C in CH3CN (0.1 M Bu4NBF4) from 0.25 mM
[Pt{((p-HC6F4)NCH2)2}(py)2Cl2] (1a) before (black solid line) and after exhaustive
electrolysis at −1250 mV (red dashed line). The spectrum of 0.25 mM [Pt
{((p-HC6F4)NCH2)2}(py)2] (4a) is also provided (blue dotted line) for reference
purposes.
3.6.2. Monitoring bulk electrolysis reactions by cyclic voltammetry
Cyclic voltammetry was used to monitor the changes after con-
trolled potential bulk electrolysis. For all three complexes after the re-
ductive electrolysis, process 1 disappeared completely as expected
while the other reduction processes that occurred at more negative
potentials remained virtually unchanged. Processes 6′ and 7′ for 1a
in Fig. 2 were now observed when the potential was initially scanned
to the positive direction. Cyclic voltammograms obtained after bulk
electrolysis of 1a are indistinguishable from those of 4a. As concluded
from the spectroelectrochemistry results, the cyclic voltammetric
data also imply that 4a is the main reduction product of 1a.
Cyclic voltammograms obtained before and after bulk electrolysis
of a solution of 2 at both GC and Pt electrodes are shown in Fig. 6. For
voltammograms obtained in the reductive direction at GC electrodes,
process A disappears as expected, but processes in similar positions to
B and C remain. In scans that commence in the oxidative direction,
process F' is now detected as the first oxidative process and it has a
similar peak current value to process A obtained prior to bulk elec-
trolysis. However, some new oxidation processes are also observed
at potentials more positive than that for process F'. An analogous
situation is presented when voltammograms are obtained at Pt elec-
trodes, except that processes B and C are not detected and reduction
process A is simply replaced by oxidation process F' during the course
of bulk electrolysis. Again, it is clear that products formed at short
voltammetric timescale and longer bulk electrolysis timescale differ
when a hydroxy ligand is present.
electrolysis for all compounds. Spectroelectrochemical data obtained at
0 °C are similar to those obtained at 20 °C.
The UV-Visible spectra of 0.25 mM 2 in acetonitrile (0.1 M
Bu4NBF4) after exhaustive bulk electrolysis at −2450 mV in the
thin layer cell shows a different spectrum to that of 0.25 mM 4a in
the same electrolyte at 20 °C, indicating that exhaustive electrolysis
of 2 does not produce 4a on the tens of minutes time scale. Fast
scan cyclic voltammetric experiments also imply that this is not the
product even on millisecond time scales.
Examination of the electronic spectral data contained in Table 4
indicates that, the first reduction process for 3 may generate some
PtII complex, 4a as noted by λmax values at 369 and 289 nm, but at
least one other species is also present that absorbs in the 243 nm re-
gion. The broad absorption peak in the 243 nm region looks like the
sum of the absorptions at 263 and 234 nm for 4a and the absorption
at 250 nm for the reduction product of 2 (Figure S2). In contrast, the
reduction of 1a almost quantitatively forms 4a (Table 4, Fig. 5), indi-
cating that 1a loses both axial chloride ligands during the course of
electrolysis and is reduced back to the PtII compound. Obviously,
the PtIV complexes with axial hydroxy ligand(s) undergo different re-
action pathways to that identified for the dihalide complex.
3.6. Characterization of products of bulk electrolysis by cyclic
voltammetry and NMR
The fact that cyclic voltammograms obtained after bulk electroly-
sis differ greatly from those for 4a obtained under identical condi-
tions [68] confirms that the product of the first reduction process of
2 is not this compound. These data are therefore consistent with the
spectroelectrochemical results.
Following bulk electrolysis of 3, an oxidation process equivalent to
F' for 2 and two processes equivalent to 6' and 7' for 1a are detected, a
result which agrees with the cyclic voltammetric data. This is again
consistent with the spectroelectrochemical results for this complex.
In order to further probe the product(s) of the first reduction pro-
cess, and to ascertain if the number of electrons transferred on longer
timescales by coulometry remains at 2, bulk electrolysis experiments
were carried out with 5.0 mM concentrations of the PtIV complexes in
CH3CN (0.1 M Bu4NPF6) at platinum electrodes. The potentials used
for the bulk electrolysis experiments are the same as those used for
spectroelectrochemistry (see Table 4). 1H and 19F NMR spectra and
3.6.3. Monitoring bulk electrolysis reaction by 19F and 1H NMR
spectroscopy
Table 4
UV-visible spectra and voltammograms obtained following bulk
electrolysis imply that 4a is the major product of the reduction of
1a, but that other products are found for the reduction of 2 or 3.
NMR spectra provide more structurally rich data than produced by
these techniques and enable more details of the products of reduction
to be deduced.
As a reference point for these studies it needs to be noted that the
19F NMR spectrum of the parent PtII complex 4a exhibits two multi-
plets at −144.5 ppm and −150.5 ppm [37], whilst the protonated li-
gand [(p-HC6F4)NHCH2]2 gives rise to two multiplets at −142.3 ppm
and −159.9 ppm [35].
UV-Visible absorption data obtained for 0.25 mM solutions of [Pt{((p‐HC6F4)NCH2)2}
(py)2XY] before and after bulk electrolysis in CH3CN (0.1 M Bu4NBF4) under
spectroelectrochemical conditions in a thin layer cell.
X
Y
Ere (mV) (vs. Fc/Fc+
a
)
λ
max before
λ
max after reduction
reduction (nm)
(nm)
Cl
Cl
−1400
425 347 255 367 291 263 234
OH OH −2450
Cl
4a
386 298 234 328 280
408 295 250 369 289
250
243
OH −2380
N/Ab
N/Ab
367 292 263 234
a
Ere=reductive potential applied during the course of bulk electrolysis.
Not available.
b

236
S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
Fig. 6. Cyclic voltammograms obtained at GC (A and B) and Pt (C and D) electrodes, before (black solid line) and after (red dashed line) exhaustive electrolysis of 5.0 mM [Pt
{((p-HC6F4)NCH2)2}(py)2(OH)2] (2) in CH3CN (0.1 M Bu4NPF6). Scan rate: 1 V s−1
.
3.6.3.1. Reduction of [Pt{((p-HC6F4)NCH2)2}(py)2Cl2] (1a). The 19F NMR
spectrum of 1a in CH3CN (0.1 M Bu4NPF6) exhibits two multiplets at
−141.5 ppm and −142.8 ppm, whilst the bound pyridine resonances
occur in the range of 7.2 to 8.8 ppm in the 1H NMR spectrum (Table 5).
After bulk electrolysis, the two 19F multiplets have shifted to −144.5
and −150.5 ppm and now exactly match those found for the parent
CH3CN (0.1 M Bu4NPF6) includes three proton resonances associated
with the bound pyridines in the range of 7.2 to 8.8 ppm, and the pro-
ton peak of (p-HC6F4) at 6.70 ppm (Table 6).
The 19F spectrum of the reduction product of 2 shows four equal
intensity resonances, two close to those of 4a and two close to
those of the free ligand [(p-HC6F4)NHCH2]2. Accordingly, it is likely
that in its reduction process, one nitrogen of the diamide ligand has
been protonated and detached from platinum giving a [Pt-N(p-HC6-
F4)CH2CH2NH(p-HC6F4)] arrangement. The F2,6 chemical shift of the
-NH(p-HC6F4) unit does not correspond to that expected if this
group is coordinated to platinum [69]. The 1H NMR spectrum of the
reduction product shows two (p-HC6F4) resonances consistent with
the above proposal. There are also two sets of pyridine resonances,
one with H2,6 clearly displaced from the free ligand values, hence in-
dicative of coordination to platinum, and one corresponding to those
of free pyridine. Confirmation that the former is attached to platinum
comes from characterization of 195Pt 1H coupling, 3J(Pt,H) =36 Hz, and
the size of the coupling is consistent with the pyridine being trans to
an amide nitrogen [37] as in the reactant 2. The absence of satellites
on the second pyridine resonance suggests that one pyridine has
been displaced. The presence of –NH is confirmed by a resonance at
5.15 ppm consistent with 5.47 ppm for [(p-HC6F4)NHCH2]2 [35].
Charge balance for –NH formation suggests the presence of OH−. A
free OH− would partly react with the fluorocarbon ligand to give
1
PtII organoamide 4a. Similarly in the H NMR spectrum, the chemical
shifts of the electrochemically reduced species match those of the PtII
complex (see Table 5). The above results fully confirm that bulk elec-
trolysis of 1a leads to formation of 4a and is accompanied by the loss
of the two axial chloride ligands, while the equatorial ligands remain
bound. These NMR data confirm the spectroelectrochemical data that
a two-electron reductive elimination leads to formation of 4a as in
Eq. (5).
ð5Þ
Table 5
1H and 19F NMR spectra of [Pt{((p-HC6F4)NCH2)2}(py)2Cl2] (1a) obtained before and
after a two electron reduction at 20 °C at a platinum electrode in CH3CN (0.1 M
Bu4NPF6).
3.6.3.2. Reduction of [Pt{((p-HC6F4)NCH2)2}(py)2(OH)2] (2). The 19F
NMR spectra of 2 dissolved in CH3CN (0.1 M Bu4NPF6) exhibits two
resonances at −143.5 and −143.6 ppm (Table 6). This is attributed
to the presence of fluorines existing in two different (2×4 F) environ-
ments in the organoamide ligand, though the near overlap prevents
specific assignments to F3,5 or F2,6. The 1H NMR spectrum of 2 in
Before reduction
After reduction
1H NMR (ppm)
19F NMR (ppm)
8.71
−141.5
7.89
7.38
−142.8
6.55
8.43
−144.5
7.68
7.07
−150.5
6.21

S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
237
Table 6
1H and 19F NMR spectra of [Pt{((p-HC6F4)NCH2)2}(py)2(OH)2] (2) obtained before and after a two-electron reduction at 20 °C at a platinum electrode in CH3CN (0.1 M Bu4NPF6).
Before reduction
8.93
After reduction
1H NMR
(ppm)
7.73
7.22
6.70
9.15
7.24
−143.3
8.53
6.27
−145.2
7.83
6.01
−154.5
7.70
5.15
−159.8
7.30
19F NMR
(ppm)
−143.5
−143.6
additional fluorine resonances, as a spectrum of 4a plus Bu4NOH
shows. Thus, there is plausibly an OH− ligand cis to both py and the
loses both the axial chloride ligands while keeping the equatorial ligands
as in the PtIV complex. In contrast, 2 appears to lose axial hydroxide and
equatorial pyridine ligands after reduction and forms a new and partly
identified complex in which one –N(p-HC6F4) group is protonated and
displaced from platinum and the remaining coordinated amide nitrogen
is trans to py. The reduction process of 3 involves formation of an almost
equimolar mixture of the products formed by reduction of 2 and 1a.
These results reveal that 1a is the only PtIV complex in this study that
can be reduced solely to the parent PtII complex 4a.
remaining amide donor and
a CH3CN (solvent) ligand in the
remaining coordination position. Accordingly, the reduction product
is [Pt{(p-HC6F4)NCH2CH2NH(p-HC6F4)}(OH)(py)(CH3CN)], with py
and the amide donor trans to each other. Mechanistic details associat-
ed with the complex reduction process are not yet known, but this
process is clearly not a simple reductive elimination reaction of
OH−, as free hydroxide would lead to further reactions which were
not observed (see above), in contrast to the situation when chloride
is lost from 1a.
3.7. Anticancer activity studies
The complexes 1a, 2 and 3 were all found to show good growth in-
hibition against L1210 and cisplatin-resistant L1210 mouse leukae-
mia cell lines by in vitro studies (Table 8). The results for 1a, 2 and 3
approach values for the lead organoamidoplatinum(II) complex 4a.
This is not unexpected for 1a as the electrochemical results showed
that it is reduced to the platinum(II) derivative 4a, and that 50% of
4a is formed when 3 is reduced. Although no 4a is formed when 2
is reduced, a PtII amide complex is formed. Although 1a, 2 and 3 are
up to six times less effective than cisplatin against the normal line,
they are at least twice as effective against the cisplatin resistant line.
In vivo studies were also undertaken for 1a, 2 and 3 against ADJ/PC6, a
solid tumor model often used to assess platinum drugs [70]. Results for 2
and 3 were particularly encouraging (Table 9). When delivered in peanut
oil, a non-toxic medium for water-insoluble drugs with some relevance to
lipophilicity, the activity of low toxicity 2 and 3 was also similar to that of
cisplatin delivered in saline. The activity was similar to that of the PtII lead
drug 4a delivered in saline / tween, but was much more effective than 4a
delivered in peanut oil. Given the high reduction potentials of 2 and 3,
providing a basis for avoiding side effects from premature reduction
before reaching the target tumor, the substantial activity is very promis-
ing. Inconsistency in toxicity between batches of 1a prevented satisfac-
tory in vivo assessment. The ready conversion into 4a may be a factor.
3.6.3.3. Reduction of [Pt{((p-HC6F4)NCH2)2}(py)2(OH)Cl] (3). At 20 °C,
3 exhibits two resonances at −141.4 ppm and −143.3 ppm in the
19F NMR spectrum in CH3CN (Table 7), with the second being sharper
than the first. In contrast, at −30 °C, four resonances are detected at
−142.1 ppm, –143.4 ppm, -143.6 ppm and −144.4 ppm. This im-
plies that 4 fluorine environments can be distinguished in (p-HC6F4)
when the rotations of the fluorocarbon rings are slowed at low tem-
peratures. In the range 7.2 to 9.2 ppm the 1H NMR spectrum of 3 at
20 °C shows three broad singlets at 8.94 ppm, 8.75 ppm and 7.24 ppm
as well as one multiplet at 7.80 ppm arising from the protons of the
bound pyridines and one proton resonance of (p-HC6F4) at 6.81 ppm.
At −30 °C, the three broad resonances at 8.94 ppm, 8.75 ppm and
7.24 ppm at 20 °C all become multiplets.
After bulk electrolysis of 3 in CH3CN (0.1 M Bu4NPF6), both the 19F
and 1H NMR spectra (Table 7) are even more complicated than those
following exhaustive reduction of 2. The 19F NMR spectrum (at 20 °C)
shows six resonances of almost equal intensity at −143.1 ppm,
-144.5 ppm, -145.5 ppm, -150.5 ppm, –153.6 ppm and −159.7 ppm.
The resonances at −144.5 ppm and -150.5 ppm are coincidental with
those of the parent PtII complex, 4a, while the other 4 resonances are
within experimental error of those detected after reduction of 2.
These data indicate that two types of products are found after a 2 elec-
tron reduction of 3 on this long time scale experiment, with 4a being
one of the products. The 1H NMR spectra revealed a total of eight reso-
nances arising from the presence of both bound pyridine and free pyri-
dine in the range of 7.1 to 9.1 ppm. The intensities of the free pyridine
resonances were about half of those of coordinated pyridine as deter-
mined by integration of the 1H spectrum. Thus, all 1H NMR data are con-
sistent with the results obtained by 19F NMR spectroscopy.
4. Conclusions
The platinum(IV) organoamide complexes [Pt{((p-HC6F4)NCH2)2}
(py)2XY] 1a (X=Y=Cl), 2 (X=Y=OH), 3 (X=Cl, Y=OH) and [Pt
{((C6F5)NCH2)2}(py)2Cl2] 1b have been prepared by oxidation of the
platinum(II) derivatives 4a and 4b, and the structures of 1b, 2 and 3
established by X-ray crystallography as having the X, Y ligands in a
trans orientation.
The electrochemical reduction of the platinum(IV) complexes
[Pt{((p-HC6F4)NCH2)2}(py)2XY] (X, Y=Cl, OH) (1a, 2, 3) involves over-
all two electron reductive elimination reactions. In principle, it might
Bulk electrolysis experiments, with NMR examination of products,
demonstrates that the first reduction processes of the three PtIV com-
plexes [Pt{((p-HC6F4)NCH2)2}(py)2XY] are associated with an PtIV/PtII
couple and a reductive elimination reaction. However, the identity of
the axial ligands determines the reaction pathway. Thus, 1a simply
Table 7
1H and 19F NMR spectra of [Pt{((p-HC6F4)NCH2)2}(py)2(OH)Cl] (3) obtained before (at −30 °C) and after (at 20 °C) a two electron reduction at 20 °C at a platinum electrode in
CH3CN (0.1 M Bu4NPF6).
Before reduction (−30 °C)
After reduction (20 °C)
1H NMR
(ppm)
8.94
8.75
7.80
7.24
6.81
9.03
7.30
−143.1
−150.5
8.53
7.07
8.43
6.32
−144.5
−153.6
7.81
6.21
7.70
6.03
−145.5
−159.7
7.41
5.13
19F NMR
(ppm)
−142.1
−143.6
−143.4
−144.4

238
S.-X. Guo et al. / Journal of Inorganic Biochemistry 115 (2012) 226–239
Table 8
Appendix A. Supplementary data
Comparison of in vitro anticancer activities of the platinum complexes.
Supplementary data for this article can be found online at http://
dx.doi.org/10.1016/j.jinorgbio.2012.07.016.
Platinum complex
Solvent
L1210a
L1210/DDPb
c
c
(IC50
)
(IC50)
[PtII{((p‐HC6F4)NCH2)2}(py)2] (4a) [35]
[Pt{((p‐HC6F4)NCH2)2}(py)2Cl2] (1a)
EtOH
Acetone
1.2
1.7
1.6
3.2
2.1
2.3
0.5
1.0
2.9
2.8
3.3
2.6
2.4
6.9
References
[Pt{((p-HC6F4)NCH2)2}(py)2(OH)2] (2)
[Pt{((p-HC6F4)NCH2)2}(py)2(OH)Cl] (3)
EtOH
Acetone
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be expected that the PtII complex, [Pt{((p-HC6F4)NCH2)2}(py)2] (4a),
could be formed in each case via the reaction
½PtIVfððpꢀHC6F4ÞNCH2Þ gðpyÞ XYꢁ→½PtIIfððpꢀHC6F4ÞNCH2Þ gðpyÞ ꢁ
2
2
2
2
þ X− þ Y−
ð6Þ
However, the products of the reduction process differ in each
case, and the expected product predicted on the basis of Eq. (6) is
formed quantitatively only in the reduction of the complex where
X, Y=Cl (1a). In the two electron reduction of [Pt{((p-HC6F4)
NCH2)2}(py)2(OH)2] (2), 4a is not formed, rather [Pt{(p-HC6F4)
NCH2CH2NH(p-HC6F4)}(OH)(py)(CH3CN)] is formed with amide
and py nitrogens trans to each other. For [Pt{((p-HC6F4)NCH2)2}
(py)2(OH)Cl] (3) 50% of 4a is formed and 50% of product(s) formed
from 2. The ease of the PtIV/PtII reduction process increases in the
order 2b3b1a. The fact that the OH− containing species are reduced
at very negative potentials is probably indicative of strong bonding
of this ligand in oxidation state IV compounds. In these complexes,
the irreversible peak reduction potential is highly dependent on
the nature of the axial ligands. Despite the difficulty in reduction,
1a, 2 and 3 are active in vitro, whilst 2 and 3 show good in vivo activ-
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2 and 3 are more active than the PtII precursor 4a. Thus, either the PtIV
complexes 2 and 3 are intrinsically active or they can be reduced in vivo
near the target, if PtII species are responsible for the activity. The key ob-
servation is that good biological activity can be obtained despite large
negative voltammetric peak reduction potentials, which can obviate
premature reduction of drugs. These results are also consistent with
the recent report of a lack of correlation between reduction potential
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Table 9
Comparison of in vivo anticancer activities of the platinum complexes against the ADJ/
PC6 tumor.
Platinum complex
Vehicle
Dose
%T/Ca
43
[PtII{((p‐HC6F4)NCH2)2}(py)2] (4a) [36]
[PtII{((p‐HC6F4)NCH2)2}(py)2] (4a) [36]
[Pt{((p‐HC6F4)NCH2)2}(py)2Cl2] (1a)
[Pt{((p-HC6F4)NCH2)2}(py)2(OH)2] (2)
[Pt{((p-HC6F4)NCH2)2}(py)2(OH)Cl] (3)
Cisplatin
Peanut oil
Saline/tween
Peanut oil
Peanut oil
Peanut oil
Saline
80,100
120
800
600
800
6
1.5
b
2.2
1.65
3.2
a
Derived from the mean tumor weight of the treated animals divided by that of the
control animals.
b
Toxicity inconsistency between samples.

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