Multinuclear NMR study and crystal structures of complexes of the types cis- and trans-Pt(Ypy)2X2, where Ypy=pyridine derivative and X=Cl and I

Article abstract of DOI:10.1016/S0020-1693(99)00303-5

Complexes of the types cis- and trans-Pt(Ypy)₂X₂ where Ypy is a methyl derivative of pyridine and X=Cl or I were studied by spectroscopic methods, especially by multinuclear NMR spectroscopy. In ¹⁹⁵Pt MNR, the cis-dichloro compounds were observed between -1998 and -2021 ppm in CDCl₃, while the trans compounds were found at slightly lower field, between -1948 and -1973 ppm. The diiodo species were observed at much higher field, between -3199 and -3288 ppm for the cis isomers and between -3122 and -3264 ppm for the trans isomers. In ¹H NMR, the ³J(¹⁹⁵Pt-¹H) coupling constants are larger for the cis compounds (about 42 ppm) than for the trans isomers (about 33 ppm). In ¹³C NMR, the values of ³J(¹⁹⁵Pt-¹³C) were also found to be larger for the cis complexes. There seems to be a slight dependence of the pK_a values of the protonated ligands on the δ(Pt) chemical shifts. The presence of π-bonding between Pt and the pyridine ligands do not seem very important. The crystal structures of three dichloro and five diiodo complexes were determined, in an attempt to obtain information on the trans influence of the three ligands. The results have shown that the iodo ligand has the greatest trans influence. Chloro and pyridine ligands seem to have similar trans influence, although, the chloro ligand seems to have a slightly larger influence than pyridine derivatives. One structure, trans-Pt(2-pic)₂I₂, has shown a syn conformation of the two 2-picoline ligands.

Full text of DOI:10.1016/S0020-1693(99)00303-5

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Inorganica Chimica Acta 295 (1999) 25–38
Multinuclear NMR study and crystal structures of complexes of
the types cis- and trans-Pt(Ypy)2X2, where Ypy=pyridine
derivative and X=Cl and I
Christian Tessier, Fernande D. Rochon *
De´partement de Chimie, Uni6ersite´ du Que´bec a` Montre´al, C.P. 8888, Succ. Centre-6ille, Montre´al, Que´bec, Canada H3C 3P8
Received 4 March 1999; accepted 28 June 1999
Abstract
Complexes of the types cis- and trans-Pt(Ypy)2X2 where Ypy is a methyl derivative of pyridine and X=Cl or I were studied
by spectroscopic methods, especially by multinuclear NMR spectroscopy. In 195Pt MNR, the cis-dichloro compounds were
observed between −1998 and −2021 ppm in CDCl3, while the trans compounds were found at slightly lower field, between
−1948 and −1973 ppm. The diiodo species were observed at much higher field, between −3199 and −3288 ppm for the cis
1
3
isomers and between −3122 and −3264 ppm for the trans isomers. In H NMR, the J(195Ptꢀ1H) coupling constants are larger
for the cis compounds (about 42 ppm) than for the trans isomers (about 33 ppm). In 13C NMR, the values of J(195Ptꢀ13C) were
3
also found to be larger for the cis complexes. There seems to be a slight dependence of the pKa values of the protonated ligands
on the l(Pt) chemical shifts. The presence of p-bonding between Pt and the pyridine ligands do not seem very important. The
crystal structures of three dichloro and five diiodo complexes were determined, in an attempt to obtain information on the trans
influence of the three ligands. The results have shown that the iodo ligand has the greatest trans influence. Chloro and pyridine
ligands seem to have similar trans influence, although, the chloro ligand seems to have a slightly larger influence than pyridine
derivatives. One structure, trans-Pt(2-pic)2I2, has shown a syn conformation of the two 2-picoline ligands. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: Crystal structures; Platinum complexes; Pyridine complexes
was later suggested that the complex cis-Pt(py)2Cl2 may
be too sterically hindered to have useful anti-tumor
properties [3]. The fact that the trans isomer is the most
active might lead to the development of a new class of
anti-tumor agents with pyridine derivatives. More re-
cently, the mixed-ligand species cis-Pt(NH3)(2-picol-
ine)Cl2 was found to be a very active compound and it
is currently in clinical trials [4].
Several earlier studies on the mechanism of action of
cisplatin have been reported [5]. There is an important
controversy regarding the ratio of the hydrolyzed spe-
cies inside the cells. Most of the early work postulated
that the main species present in the cells were the
(aqua)(hydroxo) species. More recently, House and co-
workers [6] have published a series of papers on the
hydrolysis of cisplatin in different pH conditions. They
calculated, with various rate constants, that the propor-
tion of the (aqua)(hydroxo) species inside the cells is
1. Introduction
The complex cisplatin, cis-Pt(NH3)2Cl2 is one of the
most widely used anti-cancer drugs. A good review of
the influence of structure on the activity of Pt drugs has
been recently published [1]. It was observed in the early
1990s, that when NH3 is replaced by pyridine or 4-pico-
line, the cytotoxicity of the complex trans-Pt(py)2Cl2 is
comparable to the one of cisplatin in leukemia L1210
cells, while the cis isomer is inactive [2]. The first
aquation rate of the complex cis-Pt(py)2Cl2 is of the
same order of magnitude as the one of cisplatin, sug-
gesting that solubility differences can be excluded to
explain the inactivity of the cis pyridine compound. It
* Corresponding author. Tel.: +1-514-987 3000 extn. 4896; fax:
+1-514-987 4054.
E-mail address: rochon.fernande@uqam.ca (F.D. Rochon)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00303-5

26
C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
about 7%, the (aqua)(chloro) species is 28% and the (hy-
droxo)(chloro) compound 32%. The method used by
these authors to detect the different species, did not per-
mit the observation of oligomeric species, which are
known to exist at the pH of the cells. Indeed, it has been
shown, mainly by 195Pt NMR and crystallographic
methods, that at neutral pH, several hydroxo-bridged
oligomers are formed [7,8]. Some of these species have
been found to be toxic to humans and may be partly re-
sponsible for the toxicity of cisplatin [9]. Therefore, it
seems important to study the hydrolysis reactions of the
Pt complexes at neutral pH, prior to the development of
new anti-tumor agents.
complexes in an attempt to gain information on the na-
ture of the PtꢀYpy bond and on the relative trans influ-
ence of the ligands.
2. Experimental
K2[PtCl4] was obtained from Johnson–Matthey and
was recrystallized in water before use. The pyridine
derivatives were obtained from Aldrich. CDCl3 was pur-
chased from CDN Isotopes and CD2Cl2 from MSD Iso-
topes.
The IR spectra were recorded as KBr pellets using a
Perkin–Elmer 783 grating spectrometer from 4000 to
260 cm−1. The decomposition points were measured on
a Fisher–Johns instrument and are not corrected. NMR
spectra were measured on a Varian Gemini 300BB in
CDCl3. The fields were 300.075, 75.462 and 64.4 MHz or
The objective of this project was to study the hydro-
lysis reactions of the complexes cis- and trans-
Pt(Ypy)2X2 (Ypy=pyridine derivative, X=halide),
which will be the subject of a subsequent paper. The
dichloro and diiodo pyridine (py) complexes have been
known for a long time and were characterized by IR
spectroscopy. The crystal structures of the complexes
cis- and trans-Pt(py)2Cl2 [10] and trans-Pt(py)2I2 [11]
have been reported, while the 195Pt and 1H NMR spectra
of these complexes were reported in DMF or DMSO
[2,12], solvents which are known to react with these com-
plexes. Less information is available for complexes with
a substituted pyridine (Ypy). No crystal structures were
reported. 1H NMR spectra of the complexes trans-
Pt(Ypy)2Cl2 (Ypy=4-pic, 3-pic and 2-pic) were pub-
lished [13] while the ones of cis-Pt(2-pic)2X2 (X=Cl−,
I−) were only partially reported [14]. In each case the sol-
vent used was DMSO. The only complexes measured in
a more inert solvent were cis- and trans-Pt(4-pic)2I2
which were reported in deuterated acetone [15]. A few
1
64.335 MHz, respectively for H, 13C and 195Pt. The
chloroform peak was used as an internal standard for 1H
(7.27 ppm) and 13C (77.0 ppm). Two external references
were used for 195Pt: K2[PtCl4] (−1628 ppm in D2O) and
K[Pt(DMSO)Cl3] (−2998 ppm in D2O), respectively for
the dichloro and diiodo complexes. The low field proton
NMR spectra were measured on a Varian EM-360L at
60 MHz. A V-2048 Signal Averager was used for the ac-
cumulations.
2.1. Crystallographic measurements and structure
resolution
The crystals were selected after examination under a
polarizing microscope for homogeneity. The unit cell
parameters were obtained by least-square refinement of
the angles 2q, ꢀ and  for 23 to 52 well-centered reflec-
tions on a Siemens P4 diffractometer using graphite-
3
coupling constants J(195Ptꢀ1H) were measured [16]. Fi-
nally, no 13C NMR study on these compounds has been
reported to date.
,
In this paper, we report a systematic and detailed
study of the multinuclear (1H, 13C and 195Pt) NMR spec-
tra of the complexes cis- and trans-Pt(Ypy)2X2 in chlo-
roform-d (methylene chloride-d2, for complexes
insoluble in CDCl3), a solvent which is relatively inert.
Complexes (cis and trans) with six different pyridine
derivatives were studied. The Dl values (l of complex−
l of ligand) were compared, in order to obtain informa-
tion on the nature of the PtꢀYpy bond. Pyridine has
empty antibonding orbitals, which could accept electron
density from the metal. The s bond should cause a
deshielding effect on the ligand (a positive Dl) and a
shielding effect on the metal, while the metal“ligand
back-donation (p bond) will produce the opposite effect.
The strength of the s bond should be related to the pKa
value of the protonated ligand. An aqueous study on the
complexes [Pt(Ypy)Cl3]− was recently published [17],
but no correlation between the pKa of the protonated lig-
ands and the chemical shifts was observed. We also re-
port in this paper, the crystal structures of eight
monochromatized Mo Ka radiation (u=0.71073 A).
The data collections were done at room temperature us-
ing the XSCANS program [18]. The background time to
scan time ratio was 0.5. The crystal data are listed in Ta-
bles 1 and 2. All the calculations were done using the
SHELXTL system [18]. The coordinates of the Pt atoms
were determined from Patterson map calculations, and
the positions of all the other non-hydrogen atoms were
found by the usual Fourier methods. The refinement of
2
the structures was done on Fo using all the reflections.
The H atoms were fixed at their calculated positions (rid-
ing model) with Ueq=1.2 (or 1.5 for methyl groups)×
Ueq of the C atom to which they are bonded. The
intensity data were corrected for absorption (Gaussian
integration) and for the effects of Lorentz and polariza-
tion. The refinement of the scale factor, coordinates and
anisotropic temperature factors of all the non-hydrogen
atoms converged to the R1 and wR2 values shown in Ta-
bles 1 and 2. The most intense residual peaks were found
in the close environment of the Pt atom.

C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
27
2.2. Syntheses
(l (ppm)): 8.765d Ho, 7.333dd Hm, 7.838t Hp. 13C NMR
(l (ppm)): 152.90 Co, 126.31 Cm, 138.52 Cp. 4-pic.
Yield: 77%. H NMR (l (ppm)): 8.654d Ho, 7.119d Hm,
1
2.2.1. cis-Pt(Ypy)2Cl2: (Ypy=py, 4-pic, 3-pic, 3,5-lut,
2,4-lut and 2-pic (crystal I))
2.361 4-CH3. 13C NMR (l (ppm)): 152.09 Co, 127.11
Cm, 150.86 Cp, 21,10 4-CH3. 3-pic. Yield: 69%. 1H
NMR (l (ppm)): first series (60%) 8.668 H2, 7.570d H4,
7.176dd H5, 8.560d H6, 2.294 3-CH3; second series
(30%, [Pt(3-pic)3Cl]+) 8.979/8.985 H2, 7.503d H4,
7.264dd H5, 8.920d H6; third series (10%, [Pt(3-
pic)Cl3]−) 9.044 H2, 7.371d H4, 6.999dd H5, 9.094d H6.
13C NMR (l (ppm)): first series (60%) 152.98 C2, 136.69
These complexes were prepared by slight variations
of the method of Kauffman [19] with 1 mmol of
starting material. The stirring time depended on the
steric hindrance of the ligand and varied from 1 day for
py, 4-pic, 3-pic and 3,5-lut to 5 days for 2-pic and
2,4-lut. The complexes were dried in vacuo instead of
1
heating at 100°C for 1 h. For py. Yield: 70%. H NMR
Table 1
Crystallographic data for the dichloro complexes
Crystal
I
II
III
Name
cis-Pt(2-pic)2Cl2
C12H14Cl2N2Pt
452.24
P21/c (No. 14)
10.067(3)
trans-Pt(3-pic)2Cl2
C12H14Cl2N2Pt
452.24
Trans-Pt(4-pic)2Cl2
C12H14Cl2N2Pt
452.24
P21/c (No. 14)
5.837(2)
Empirical formula
Formula weight
Space group
P1 (No. 1)
5.8080(10)
7.4680(10)
8.5670(10)
68.530(10)
79.33(2)
82.74(2)
339.12(8)
1
2.214
10.716
212
0.0246
,
a (A)
,
b (A)
9.706(3)
17.131(4)
8.247(3)
14.540(8)
,
c (A)
h (°)
i (°)
k (°)
120.230(10)
99.06(5)
3
,
V (A )
Z
1446.2(7)
4
2.077
10.051
848
691.2(5)
2
2.173
10.516
424
Dcalc (g cm−3
)
v (Mo Ka) (cm−1
)
F(000)
a R1
0.0435
0.0978
0.0311
0.0741
b wR2
0.0609
a R1=S(ꢀFo−Fcꢀ)/SꢀFoꢀ.
b wR2=[S(w(Fo 2−Fc 2)2)/S(w(Fo 2)2)]1/2
.
Table 2
Crystallographic data for the diiodo complexes
Crystal
IV
V
VI
VII
VIII
Name
cis-Pt(4-pic)2I2
C12H14I2N2Pt
635.14
P21/c (No. 14)
10.627(7)
cis-Pt(py)2I2
C10H10I2N2Pt
607.09
C2/c (No. 15)
17.723(6)
trans-Pt(4-pic)2I2
C12H14I2N2Pt
635.14
P21/n (No. 14)
4.755(2)
trans-Pt(3-pic)2I2
C12H14I2N2Pt
635.14
C2/c (No. 15)
14.218(2)
trans-Pt(2-pic)2I2
C12H14I2N2Pt
635.14
Empirical formula
Formula weight
Space group
¯
P1 (No. 2)
,
a (A)
8.210(4)
8.809(5)
12.797(7)
105.10(4)
95.35(4)
115.68(3)
782.4(7)
2
2.696
12.896
568
0.0696
0.1573
,
b (A)
11.082(8)
13.881(9)
9.893(3)
17.106(4)
18.924(7)
8.981(3)
11.5160(10)
9.8450(10)
,
c (A)
h (°)
i (°)
k (°)
103.10(3)
105.68(2)
102.59(3)
94.600(10)
3
,
V (A )
Z
1592(2)
4
2.650
12.673
1136
2888(2)
8
2.793
13.969
2144
788.8(5)
2
2.674
12.790
568
1606.8(3)
4
2.626
12.558
1136
Dcalc (g cm−3
)
v (Mo Ka) (cm−1
)
F(000)
a R1
0.0596
0.1398
0.0576
0.1167
0.0468
0.1083
0.0362
0.0893
b wR2
a R1=S(ꢀFo−Fcꢀ)/SꢀFoꢀ.
b wR2=[S(w(Fo 2−Fc 2)2)/S(w(Fo 2)2)]1/2
.

28
C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
C3, 139.21 C4, 125.67 C5, 150.05 C6, 18.40 3-CH3;
second series (20%/10%, [Pt(3-pic)3Cl]+) 153.70/152.50
C2, 137.00/135.43 C3, 138.93/140.04 C4, 124.58/124.26
C5, 150.86/150.20 C6, 18.33/18.19 3-CH3; third series
(10%, [Pt(3-pic)Cl3]−) 154.28 C2, 137.60 C3, 140.55 C4,
127.17 C5, 151.48 C6, 18.74 3-CH3. 3,5-lut. Yield: 59%.
IR (cm−1): 1600s, 1460s, 1450s, 1385s, 1275m, 1249s,
1153s, 1048m, 870s, 822m, 800m, 699s, 512w, 499w,
2.2.4. trans-Pt(2,4-lut)2Cl2 and trans-Pt(2-pic)2Cl2
An excess of ligand (approximately 20 mmol) was
added to a solution of 1 mmol purified K2[PtCl4] in 50
ml of water. The solution was stirred and heated (80°C)
for 2 h. Yellow crystals of trans-Pt(2,4-lut)2Cl2 and
trans-Pt(2-pic)2Cl2 were obtained, which were filtered,
washed with water, ethanol and ether and dried in
vacuo. 2,4-lut. Yield: 33%. IR (cm−1): 1621s, 1615s,
1490m, 1480m, 1450m, 1370m, 1300m, 1295m, 1235w,
1044m, 926w, 821s, 548w, 459s, 348s w(PtꢀCl), 320m
1
338s and 330s w(PtꢀCl). H NMR (l (ppm)): 8.412 Ho,
7.366 Hp, 2.252 3,5-(CH3)2. 13C NMR (l (ppm)): 150.09
Co, 135.82 Cm, 140.01 Cp, 18.21 3,5-(CH3)2. 2,4-lut.
Yield: 44%. IR (cm−1): 1625s, 1620s, 1492m, 1482m,
1455m, 1450m, 1383m, 1373m, 1300m, 1238s, 1043s,
1038s, 921w, 830s, 822s, 545w, 462m, 458m, 340s and
w(PtꢀN). 1H NMR (l (ppm)): 7.096 H3, 6.972d H5,
4
8.744d H6, 3.273 2-CH3, 2.387 4-CH3, J(195Ptꢀ1Hmethyl
)
11 Hz. 13C NMR (l (ppm)): first series (60%) 160.55 C2,
126.91 C3, 149.80 C4, 123.65 C5, 152.91 C6, 25.99 2-CH3,
20.74 4-CH3; second series (40%) 123.50 C5, 152.66 C6,
26.32 2-CH3. 2-pic. Yield: 31%. M.p. (dec.) 219–273°C.
1H NMR (l (ppm)): first series (60%) 7.246d H3, 7.602td
H4, 7.151t H5, 8.918d H6, 3.322 2-CH3,
4J(195Ptꢀ1Hmethyl) 14 Hz; second series (40%) 7.110t H5,
8.731d H6, 3.277 2-CH3. 13C NMR (l (ppm)) first series
(60%) 161.63 C2, 126.17 C3, 137.93 C4, 122.67 C5, 153.73
C6, 26.39 2-CH3; second series (40%) 162.70 C2, 126.08
C3, 137.88 C4, 122.47 C5, 153.59 C6, 26.74 2-CH3.
1
330s w(PtꢀCl). H NMR (l (ppm)): first series (80%)
7.078 H3, 7.028d H5, 8.871d H6, 3.203 2-CH3, 2.307
4-CH3; second series (20%) 8.888d H6, 3.263 2-CH3. 13
C
NMR (l (ppm)): 160.41 C2, 127.35 C3, 150.24 C4,
124.57 C5, 153.69 C6, 26.31 2-CH3, 20.53 4-CH3. 2-pic.
Yield: 20%. 1H NMR (l (ppm)): first series (70%)
7.264d H3, 7.619td H4, 7.195t H5, 9.048d H6, 3.237
2-CH3; second series (10%) 7.724td H4, 7.385t H5,
9.195d H6, 3.327 2-CH3; third series (10%) 7.479td H4,
7.006t H5, 8.916d H6, 3.100 2-CH3; fourth series (10%)
7.385td H4, 6.886t H5, 8.863d H6, 3.024 2-CH3. 13C
NMR (l (ppm)): 161.77 C2, 126.82 C3, 138.09 C4,
123.25 C5, 154.61 C6, 29.73 2-CH3.
2.2.5. cis-Pt(Ypy)2I2 (Ypy=py (crystal V), 4-pic
(crystal IV), 3-pic, 3,5-lut, 2-pic and 2,4-lut)
These complexes were synthesized by a modified
version of Dhara’s method [15,21]. For py. Yield: 90%.
1H NMR (l (ppm, in CD2Cl2)): 8.842d Ho, 7.337dd Hm,
7.807t Hp. 13C NMR (l (ppm, in CD2Cl2): 152.92 Co,
2.2.2. trans-Pt(Ypy)2Cl2 (Ypy=py, 4-pic (crystal III)
and 3,5-lut)
1
126.68 Cm, 138.98 Cp. 4-pic. Yield: 86%. H NMR (l
A slightly modified version of the literature method
[19] was used. Purified K2[PtCl4] (1 mmol in 10 ml of
water) was added to a solution of 2 ml of ligand in 20
ml of water. The solution was stirred and heated (1 h for
py and 4-pic and 5 h for 3,5-lut) until it became
colorless. With 3,5-lut, it was necessary to filter an
impurity (probably a Magnus-type salt). The remaining
manipulations were done according to Kauffman’s
method [19]. py. Yield: 73%. 1H NMR (l (ppm)): 8.889d
Ho, 7.304dd Hm, 7.771tt Hp. 13C NMR (l (ppm)): 153.74
(ppm)): 8.718d Ho, 7.117d Hm, 2.338 4-CH3. 13C NMR
(l (ppm)): 151.81 Co, 127.04 Cm, 150.84 Cp, 21.16
4-CH3. 3-pic. Yield: 94%. 1H NMR (l (ppm) in
CD2Cl2): 8.705 H2, 7.591d H4, 7.220dd H5, 8.648d H6,
2.305 3-CH3. 13C NMR (l (ppm) in CD2Cl2): 152.87 C2,
137.11 C3, 139.61 C4, 125.92 C5, 149.99 C6, 18.56 3-CH3.
3,5-lut. Yield: 80%. IR (cm−1): 1591s, 1438s, 1381s,
1246m, 1245s, 1152sh, 1148s, 1039m, 870s, 865s, 792m,
781m, 696s, 510m, 485m. 1H NMR (l (ppm)): 8.496 Ho,
7.349 Hp, 2.267 3,5-(CH3)2. 13C NMR (l (ppm)): 149.72
Co, 135.63 Cm, 139.90 Cp, 18.25 3,5-(CH3)2. 2-pic. Yield:
1
Co, 125.17 Cm, 138.28 Cp. 4-pic. Yield: 70%. H NMR
(l (ppm)): 8.711d Ho, 7.112d Hm, 2.407 4-CH3. 13C
NMR (l (ppm)): 152.83 Co, 126.00 Cm, 150.48 Cp, 21.00
4-CH3. 3,5-lut. Yield: 14%. IR (cm−1): 1591s, 1452s,
1380sh, 1370s, 1251s, 1148s, 1040m, 859s, 790m, 695s,
1
86%. H NMR (l (ppm)): first series (70%) 7.275d H3,
7.615td H4, 7.165dd H5, 9.071dd H6, 3.284 2-CH3;
second series (30%) 7.571td H4, 8.938dd H6, 3.242
2-CH3. 13C NMR (l (ppm)): 163.79 C2, 127.07 C3,
138.07 C4, 123.11 C5, 153.35 C6, 28.03 2-CH3. 2,4-lut.
Yield: 82%. IR (cm−1): 1620s, 1480m, 1450m, 1381m,
1372m, 1292m, 1231w, 1031m, 921w, 831s, 812s, 556w,
540w, 455m, 441m. 1H NMR (l (ppm)): first series
(55%) 7.047 H3, 6.969d H5, 8.882d H6, 3.205 2-CH3,
2.305 4-CH3; second series (45%) 7.064 H3, 8.901d H6,
3.169 2-CH3. 13C NMR: first series (55%) 159.84 C2,
127.82 C3, 149.94 C4, 124.20 C5, 152.53 C6, 26.78 2-CH3,
20.76 4-CH3; second series (45%) 127.73 C3, 124.33 C5,
150.72 C6, 27.81 2-CH3.
1
515m, 335s w(PtꢀCl). H NMR (l (ppm)): 8.489 Ho,
7.344 Hp, 2.301 3,5-(CH3)2. 13C NMR (l (ppm)): 150.94
Co, 134.71 Cm, 139.54 Cp, 18.14 3,5-(CH3)2.
2.2.3. trans-Pt(3-pic)2Cl2 (crystal II)
This complex was prepared according to the method
1
published by Kong and Rochon [20]. Yield: 10%. H
NMR (l (ppm)): 8.705 H2, 7.552d H4, 7.179dd H5,
8.673d H6, 2.351 3-CH3. 13C NMR (l (ppm)): 153.70 C2,
135.39 C3, 138.92 C4, 124.56 C5, 150.87 C6, 18.33 3-CH3.

C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
29
2.2.6. trans-Pt(Ypy)2I2 (Ypy=py, 4-pic (crystal VI),
3-pic (crystal VII) and 3,5-lut)
of the cis complex [20] with an excess of ligand in DMF
for the former and in water for the two others. A
slightly modified [15] version of the method of Dhara
[21] was used for the synthesis of the cis-diiodo com-
pounds. The trans-diiodo complexes with no ortho sub-
stituants were prepared according to the method of
Souchard et al. [15], while the trans-diiodo complexes
with 2-pic and 2,4-lut were synthesized from the iso-
merization of the cis complexes in DMSO with an
excess of ligand. This isomerization process required 3
weeks of stirring at room temperature.
The mechanism of action of the isomerization pro-
cess involves the solvent molecules acting as ligands. It
has been clearly shown that DMSO is not an inert
solvent for these types of complexes. The formation of
trans-Pt(Xpy)(DMSO)I2, was observed when cis-
Pt(Xpy)2I2 was dissolved in DMSO without the addi-
tion of Ypy [22]. The crystal structure of the complex
trans-Pt(DMSO)(3,5-lut)I2 was published previously
[22]. Marzilli et al. have also studied such cis and trans
complexes in DMSO [23].
These compounds were prepared according to the
published method [15]. For 3-pic and 3,5-lut, an insolu-
ble impurity (probably a Magnus-type salt) was filtered
1
prior to the addition of KI. py. Yield: 92%. H NMR
(l (ppm)): 8.967d Ho, 7.252dd Hm, 7.695tt Hp. 13C
NMR (l (ppm)): 155.68 Co, 125.22 Cm, 137.78 Cp.
4-pic. Yield: 85%. 1H NMR (l (ppm)): 8.762d Ho,
7.044d Hm, 2.387 4-CH3. 13C NMR (l (ppm)): 154.48
Co, 126.06 Cm, 149.93 Cp, 20.99 4-CH3. 3-pic. Yield:
1
15%. H NMR (l (ppm)): 8.776 H2, 7.465d H4, 7.122dd
H5, 8.739d H6, 2.326 3-CH3. 13C NMR (l (ppm)):
155.52 C2, 135.33 C3, 138.50 C4, 124.50 C5, 152.79 C6,
18.29 3-CH3. 3,5-lut. Yield: 67%. IR (cm−1): 1592s,
1452s, 1380s, 1376sh, 1251m, 1148s, 1040m, 851s, 692s,
1
518s. H NMR (l (ppm)): 8.547 Ho, 7.223 Hp, 2.262
3,5-(CH3)2. 13C NMR (l (ppm)): 152.80 Co, 134.68 Cm,
139.54 Cp, 18.14 3,5-(CH3)2.
2.2.7. trans-Pt(2-pic)2I2 (crystal VIII) and
The decomposition points of all the complexes were
measured. The cis complexes start to decompose at a
lower temperature than their trans analogues, because
the first step of the thermal decomposition of the cis
complexes is a cis“trans isomerization [12]. An
exothermic effect can be seen on the DSC diagrams of
the cis compounds, which are not shown in this paper
due to the uncertainty of the DH values. The tempera-
tures of the isomerizations are listed in Table 3, along
with the decomposition points of the trans compounds.
The trans-diiodo complexes decompose at lower tem-
peratures than the trans-dichloro compounds except for
the ligands which are substituted in ortho positions
(2-pic and 2,4-lut). The thermal isomerization of the cis
complexes might prove an interesting alternative
method to synthesize trans compounds, especially those
containing bulky ligands. However, the temperatures
should be very carefully controlled.
trans-Pt(2,4-lut)2I2
These complexes were synthesized by slightly modify-
ing the published method [12] where non-ortho substi-
tuted ligands were used. With the ligands studied by
these authors, the yellow precipitate of the trans com-
plexes appeared after 15 min. In our study with 2-pic
and 2,4-lut, 3 weeks of stirring were required. 2-pic.
Yield: 32%. 1H NMR (l (ppm)): first series (60%)
7.264d H3, 7.563td H4, 7.110dd H5, 8.990dd H6, 3.198
2-CH3; second series (40%) 8.913dd H6, 3.237 2-CH3.
13C NMR (l (ppm)): first series (60%) 165.27 C2, 126.62
C3, 137.93 C4, 122.55 C5, 155.20 C6, 28.26 2-CH3;
second series (40%) 122.49 C5, 154.59 C6, 28.89 2-CH3.
2,4-lut. Yield: 35%. IR (cm−1): 1620s, 1480m, 1450m,
1371m, 1291m, 1036m, 925w, 825s, 535w, 451m. 1H
NMR (l (ppm)): first series (70%) 7.098 H3, 6.903 H5,
8.786 H6, 3.167 2-CH3, 2.397 4-CH3; second series
(30%) 7.070 H3, 8.716d H6, 3.132 2-CH3. 13C NMR (l
(ppm)): first series (70%) 160.44 C2, 127.07 C3, 149.37
C4, 123.19 C5, 153.24 C6, 28.06 2-CH3, 20.67 4-CH3;
second series (30%) 163.82 C2, 126.03 C3, 123.50 C5,
154.72 C6, 27.44 2-CH3.
Table 3
Temperature of isomerization cis“trans and decomposing points of
the trans complexes
Ypy
X
Temperature (°C)
cis“trans
Temperature
(dec. pt) (°C)
3. Results and discussion
2,4-lut
3,5-lut
4-pic
2-pic
3-pic
py
Cl
I
Cl
I
Cl
I
Cl
I
Cl
I
Cl
I
177–200
165–185
not observed
not observed
218–238
160–180
203–211
160–182
197–219
268–300
269–300
282–\300
232–285
265–292
240–300
219–273
261–300
254–268
226–264
258–278
230–285
Complexes of the type cis- and trans-Pt(Ypy)2X2
with Ypy=2,4-lutidine (2,4-lut), 3,5-lutidine (3,5-lut),
4-picoline (4-pic), 2-picoline (2-pic), 3-picoline (3-pic)
and pyridine (py) and X=Cl and I were synthesized
and studied by different spectroscopic methods. Most
dichloro complexes were prepared according to the
method described by Kauffman [19], except for trans-
Pt(3-pic)2Cl2, trans-Pt(2-pic)2Cl2 and trans-Pt(2,4-
lut)2Cl2, which were synthesized from the isomerization
143–181
227–245
141–179

30
C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
The infrared spectra of the complexes agree well with
basicity of the pyridine ligands. The pKa of the proto-
nated Ypy varies between 5.22 and 6.63 [27]. The extent
of the electron donation in the formation of the s bond
is smaller than in the amine complexes. The electron
density on the Pt nucleus is thus reduced, resulting in a
deshielding of the signal in the 195Pt NMR spectrum. If
the l(195Pt) of the cis-Pt(Ypy)2Cl2 complexes are plot-
ted against the pKa of the protonated ligands, there
seems to be an increase in the shielding of the Pt atom
as the pKa increases, as expected. But the dispersion
around the straight line is fairly large, probably because
the values of l(195Pt) do not vary much with the
different ligands (−1998 to −2021 ppm) and the error
of each measurement is close to 5 ppm.
For cis-Pt(3-pic)2Cl2, two other peaks were observed
at −1785 and −2177 ppm. These two resonances are
believed to be caused by the presence of a Magnus-type
salt complex [Pt(3-pic)3Cl]+[Pt(3-pic)Cl3]−. The signal
at −1785 ppm was assigned to the anion [Pt(3-pic)Cl3].
The resonance of K[Pt(3-pic)Cl3] was reported in D2O
at −1816 ppm [17]. For ionic complexes, the 195Pt
signals are usually observed at slightly higher field in
D2O compared to CDCl3 solution. The higher field
signal arises from the resonance of the [Pt(3-pic)3Cl]+
ion. This salt is insoluble in water and the relative
intensities of the two 195Pt signals are identical.
The trans-dichloro complexes were observed between
−1948 and −1973 ppm, at lower field than their cis
analogues (about 45 ppm). A few values of trans com-
pounds are available in the literature (in DMF):
−2225 ppm (cyclobutylamine [25]), 2193 ppm (2-
adamantanamine [25]), 2181 ppm (Me2NH [25]), 2141
ppm (1-adamantanamine [25]), 2101 ppm (NH3 [28])
and −1958 ppm (py [2]). A plot of l(195Pt) versus pKa
of the protonated ligand is linear (r2=0.993) for the
literature complexes but the pKa of only four of them
are known. For our trans complexes, the results are
quite similar to those observed for the cis compounds.
There seems to be a slight increase in the Pt shielding,
as the pKa values of the protonated ligands increase.
But the dispersion is fairly large, because the values do
not vary very much (max. 25 ppm) and the error on
each measurement is about 5 ppm.
those published in Ref. [24]. The IR spectra of the
complexes with Ypy=3,5-lut and 2,4-lut have not been
reported to date. The main bands are listed in Section
2. Two w(PtꢀCl) bands are observed for the cis com-
plexes at 337 and 328 cm−1 (average values), while only
one band was observed at 342 cm−1 (average) for the
trans complexes. This is in agreement with group the-
ory, when idealized C26 and D2h symmetries are consid-
ered for the cis and trans complexes, respectively. Most
of the absorption bands of the ligands are not very
affected by the geometry of the complex or the coordi-
nated halide, with the exception of a few bands of the
B1 class, which are split in the cis geometry. The w(PtꢀI)
stretching bands could not be measured on the spec-
trophotometer used.
3.1. 195Pt NMR
The 195Pt NMR spectra of all the complexes were
measured in chloroform-d, except cis-Pt(py)2I2 and cis-
Pt(3-pic)2I2, which were measured in methylene chlo-
ride-d2 due to their low solubility in CDCl3. The l(Pt)
chemical shifts of the compounds are listed in Table 4.
The signals of the cis-dichloro complexes were observed
between −1998 and −2021 ppm. These values are at
lower field than the ones reported in the literature for
complexes containing amines, which were observed in
DMF at (l (amine, pKa)): −2188 (Me2NH, 10.73 [25]),
2224 (i-PrNH2, 10.63 [25]), 2222 (MeNH2, 10.62 [26]),
2184 (1-adamantanamine, 9.92 [25]) and −2104 ppm
(NH3, 9.25 [8]). Other values in the literature (with no
pKa available) are −2235 ppm (cyclobutylamine [25])
and −2230 ppm (2-adamantanamine [25]). If these
reported values of l(195Pt) in DMF are plotted against
the pKa of the protonated ligands, a straight line (r2=
0.943) is obtained.
The l(195Pt) of cis-Pt(py)2Cl2 was reported at −1965
ppm in DMF [2], slightly different from our value of
−1998 ppm in CDCl3. The resonances of the com-
plexes with pyridine derivatives are observed at lower
fields than the amine analogues because of the weaker
Table 4
195Pt chemical shifts (ppm) for the complexes Pt(Ypy)2X2
Ypy
pKa YpyH+
X=Cl−
X=I−
cis
trans
cis
trans
2,4-Lutidine
3,5-Lutidine
4-Picoline
2-Picoline
3-Picoline
Pyridine
6.63
6.15
5.98
5.96
5.63
5.22
−2014
−2011
−2003
−2021
−2008
−1998
−1960
−1958
−1948
−1973
−1955
−1948
−3261 and −3288
−3228
−3205
−3246
−3135
−3122
−3264
−3136
−3133
−3281 and −3312
−3199 a
−3199 a
a Measured in CD2Cl2.

C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
31
These data on the l(195Pt) chemical shifts for the
neutral cis- and trans-Pt(Ypy)2Cl2 complexes with the
different pyridine derivatives were unexpected. This
behavior is quite different from the results obtained on
a series of K[Pt(Ypy)Cl3] type complexes measured in
D2O [17]. The l(195Pt) of the latter compounds were
found to vary with the position of the substituent on
the pyridine ring. Ligands with no substituent in posi-
tion 2 (e.g. py, 3-pic and 4-pic) were found to form
complexes whose resonances were observed around
−1816 ppm. The signal of the compound containing
2-pic was found at −1796 ppm, while the 2,6-lut
complex was more deshielded (−1754 ppm) [17]. No
correlation between the l(195Pt) and the pKa values of
the protonated ligands was found. The differences in
chemical shifts were interpreted as a solvent effect, also
named by a number of authors as the ortho effect. The
195Pt NMR spectra of these ionic complexes were mea-
sured in a coordinating solvent, water. For complexes
containing ligands with no ortho substituent like
[Pt(py)Cl3]−, the molecules of water can easily ap-
proach the Pt atom on the two sides of the Pt plane,
causing a shielding effect on the Pt atom in NMR
spectroscopy. A subtituent in para or meta positions
does not affect the hydration process around the Pt
nucleus. But hydrophobic substituents in the ortho po-
sition (e.g. 2-pic or 2,6-lut) will change the solvation
sphere around the Pt atom. The pyridine ring will tend
to be in a perpendicular position relative to the Pt
plane, which will bring the hydrophobic methyl groups
directly on the Pt atom, preventing the approach of the
molecules of solvent. The presence of such groups will
reduce the electron density on the Pt atom and the
NMR signal is deshielded.
Pt(Ypy)(Me2SO)Cl2 (in DMSO). They have studied 26
different pyridine derivatives with substituents in vari-
ous positions. Attempts to correlate the 195Pt NMR
signals of the complexes with the basicity of the ligands
were made without success. They found that there was
a small correlation only with complexes substituted in
the para position. For cis complexes, there was an
upfield shift as the basicity of the ligand increased,
while they claimed that for the trans complexes, the
opposite trend was observed. We have looked at the
data reported by these authors and have found that the
maximum Dl(Pt) for the cis compounds is 25 ppm
while it was only 6 ppm for the trans isomers. There-
fore, the difference is significant for the cis isomers, but
certainly not for the trans compounds. We can con-
clude that these results are in agreement with ours. The
same authors also studied ionic complexes of the types
cis-[Pt(Ypy)(DMSO)2Cl]+ and cis-[Pt(Ypy)2(DMSO)-
Cl]+ in DMSO, a coordinating solvent [23]. Again, no
correlation was found with the pKa values except for
those complexes substituted in the para position. They
observed an upfield shift as the basicity of the ligands
increased. However, for these complexes, the Dl(Pt)
values are more significant, 40 and 52 ppm, respec-
tively. Therefore, we can conclude that the basicity of
the Ypy ligand has a greater influence on the l(Pt)
values for ionic complexes than for neutral compounds.
Furthermore, the correlation with the basicity of the
ligand is only seen with ligands which have no steric
hindrance close to the binding site.
Many factors other than the basicity of the ligands
can affect the 195Pt chemical shifts. One of these factors
is the presence of p bonding. Contrary to amine com-
plexes, pyridine has empty p* orbitals which can accept
electron density from the Pt atom. This back-donation
will reduce the electron density on the metallic center,
resulting in a deshielding in the 195Pt chemical shifts.
Another important factor which will influence the
195Pt chemical shifts is DE, an electronic energy differ-
ence in the Ramsey equation. The shielding of a heavy
atom like the 195Pt nucleus can be expressed as follows:
For the complexes Pt(Ypy)2Cl2, there seems to be a
light dependence of the l(Pt) on the pKa of the proto-
nated ligands as mentioned above, but no such relation
was found in the K[Pt(Ypy)Cl3] compounds measured
in D2O [17]. The solvent effect of the latter compounds
is probably too strong to observe other relations.
The difference in behavior between the ionic com-
plexes [Pt(Ypy)Cl3]− and the neutral compounds
Pt(Ypy)2Cl2 studied in this work, seems to be caused by
the different solvents. For [Pt(Ypy)Cl3]−, the com-
pounds with non-ortho-substituted ligands were ob-
served at higher field (in D2O), while we have observed
the reverse for the neutral disubstituted compounds (in
CDCl3). It has already been observed that the l(195Pt)
are dependent on the solvent. These values are usually
more shielded in water than in organic solvents. Fur-
thermore, the dependence of the chemical shift on the
solvation sphere around the Pt atom is more important
in a coordinating solvent like water. Another factor
might be the charge on the complex.
−e2h2
2m2c2
8C
DEA
4Ceg
2
2
ꢃ
n
a2g
|ꢀ
ꢁr−3ꢂC2alg
+
DEE
The terms DE refer to the electronic excitation energy,
but they have not been clearly defined for Pt complexes.
In the complex cis-Pt(py)2Cl2, experimental results seem
to indicate that the LUMO is a platinum d orbital but
Hu¨ckel calculations place a p* pyridine molecular or-
bital as the LUMO [29].
Because of the negative sign and the presence of DE
at the denominator, an increase in DE will result in a
greater shielding of the signal. The difference in the DE
values could explain the greater shielding of the cis
complexes, when compared to their trans analogues. In
order to verify this hypothesis, the electronic spectra of
Marzilli et al. [23] have studied the 195Pt NMR
spectra of complexes of the types cis- and trans-

32
C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
the dichloro complexes were measured in chloroform.
The lowest energy band (probably a d“p* transition,
because of the intensity, as suggested in the literature
[29]) observed for the cis complexes is at 33 800 cm−1
(average), while an energy of 32 900 cm−1 (average) is
obtained for the trans complexes. Assuming that these
transitions can be related to the DE values in the
Ramsey equation, the deshielding of the trans com-
plexes compared to their cis analogues can be at-
tributed to the smaller DE observed in the electronic
spectra of the trans complexes.
As observed for the cis- and trans-dichloro com-
pounds, there seems to be an increase in the shielding
of the Pt atom as the pKa of the protonated ligand
increases, but the dispersion is quite large. The 195Pt
signals of the diiodo complexes with 2,4-lutidine and
2-picoline were observed at much higher fields than the
other ligands used in this study. The dichloro com-
pounds with these ligands were also observed at higher
field but the difference with the other ligands is almost
not significant (average −2018 vs. −2005 ppm for the
cis and −1967 vs. −1952 ppm for the trans). For the
diiodo compounds, an important shielding of the 195Pt
nucleus is observed for ligands substituted in the ortho
positions (average −3286 vs. −3208 ppm for the cis
and −3255 vs. −3132 ppm for the trans). This ortho
shielding effect is contrary to previous findings reported
for ionic compounds of the type K[Pt(Ypy)Cl3] [17] in
D2O, where the ortho ꢀCH3 groups reduce the hydra-
tion of the Pt nucleus as discussed previously. In
CDCl3, a non-coordinating solvent, the solvation pro-
cess around the Pt atom is much less important. ortho-
Shielding effects were observed in D2O [17] and DMSO
[23] with pyridine derivatives containing substituents
like NH2, NHCH3 or CH2OH in the ortho position.
These substituents increase the electron density above
or below the coordinating plane, resulting in an in-
creased shielding on the Pt nucleus. But this is not the
case for our diiodo complexes, since the substituent is
hydrophobic. The difference in solvents is probably the
most important factor.
The 195Pt chemical shifts of the diiodo complexes are
listed in Table 4. They are observed at higher fields
than their dichloro analogues. The iodo ligands are
more polarizable than the chloro ligands. This will
result in a more covalent bond for the diiodo com-
plexes. The C terms in the Ramsey equation represent
the metallic coefficients of the platinum d orbitals. A
more covalent bond will result in a decrease in the C
terms and hence a shielding of the 195Pt NMR signals.
The cis-diiodo complexes were observed between
−3199 and −3312 ppm, at slightly lower fields than
complexes with amine ligands (in DMF): −3330
(EtNH2 [25]), 3211 (Me2NH [25]), 3327 (MeNH2 [25]),
3364 (1-adamantanamine [25]), 3333 (2-adaman-
tanamine [25]), 3264 (NH3 [30]), 3346 (cyclobutylamine
[25]) and −3302 ppm (cyclopentylamine [25]). Two
195Pt resonances of equal intensity were observed for
cis-Pt(2-pic)2I2 and cis-Pt(2,4-lut)2I2 which contain or-
tho-substituted ligands. These are believed to be caused
by the presence of two rotamers. Free rotation around
the PtꢀN is partially hindered by the presence of a
methyl substituent in the ortho position. Rotamers have
been observed in 195Pt NMR for bulky ligands like in
cis-Pt(en)(5%dAMP-N7)2 [31]. They are usually more
1
3.2. H and 13C NMR
1
The H and 13C NMR signals are listed in Section 2
with the proposed assignments. In the complexes cis-
Pt(2-pic)2Cl2, cis-Pt(2-pic)2I2 and cis-Pt(2,4-lut)2I2, a
small quantity of free ligand was detected. Some com-
plexes show more than one set of peaks. For the
complex cis-Pt(3-pic)2Cl2, they are due to the impurity
[Pt(3-pic)3Cl] [Pt(3-pic)Cl3], as seen by 195Pt NMR spec-
troscopy discussed above. The highest intensity signals
were assigned to the dichloro complex as observed by
195Pt NMR. The assignment for [Pt(3-pic)Cl3]− was
made by comparison with the literature 13C signals
reported for the potassium salt [17]. The trichloro spe-
cies is the most deshielded in 13C NMR and it was
1
commonly seen in H NMR and these results will be
discussed later in the text (see Section 3.2).
The trans-diiodo complexes were observed between
−3122 and −3264 ppm, at lower field (about 65 ppm)
than their cis analogues, as observed for the dichloro
complexes and ascribed to the higher DE for the cis-
dichloro complexes. The electronic spectra of the diiodo
complexes were also measured in chloroform, in order
to verify that the 195Pt signal is related to DE. Contrary
to dichloro complexes, the lowest-energy band which
we were able to observe is of greater energy for the
trans complexes. So the higher field observed for the
resonance of the cis complexes cannot be explained by
the electronic spectra. McFarlane [32] proposed that
the difference between the two isomers could be due to
differences in DE values for the pair of ligands present.
When the two kinds of ligands, L and X are very
different in trans influences, the difference in DE be-
tween the two isomers would tend to be larger. The
trans influence of the different ligands will be discussed
in more details in Section 3.5.
1
assumed that this is also the case in H NMR. For
[Pt(3-pic)3Cl]+, two sets of resonances should be ob-
served with a ratio of 2:1, since the three 3-pic ligands
are not equivalent. This is observed in 13C, but not in
1H NMR, probably because of overlapping of the
signals. Complexes with 2,4-lutidine and 2-picoline
showed several series of signals in proton and carbon
NMR, which were not observed in 195Pt NMR. We
believe that these signals can be attributed to the pres-
ence of rotamers. Rotation of the ligands around the

C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
33
Table 5
Dl(1H) (ppm) and 3J(195Ptꢀ1H) (Hz) for the complexes Pt(Ypy)2X2
Ypy
X
H2
H3
H4
H5
0.119
0.063
0.060
H6
S(Dl)
Dlavg
3J(195Ptꢀ1H)
2,4-lut
Cl− cis
trans
0.103
0.534
0.404
0.551
0.425
0.170
0.247
0.254
0.305
0.197
0.254
0.261
0.305
0.547
0.359
0.547
0.475
0.147
0.260
0.235
0.326
0.152
0.276
0.229
0.354
0.756
0.588
0.691
0.534
0.410
0.542
0.561
0.537
0.426
0.526
0.550
0.492
0.817
0.592
0.839
0.662
0.457
0.592
0.646
0.585
0.591
0.714
0.722
0.690
0.252
0.196
0.230
0.178
0.137
0.181
0.187
0.179
0.107
0.132
0.138
0.123
0.204
0.148
0.210
0.166
0.114
0.148
0.162
0.146
0.118
0.143
0.144
0.138
42
31
40
34
44
34
40
31
43
33
43
34
40
32
41
35
41
35
40
34
43
31
41
35
0.121
0.080
0.115
I− cis
trans
−0.006
3,5-lut
4-pic
2-pic
3-pic
py
Cl− cis
trans
0.170
0.247
0.254
0.305
0.197
0.254
0.261
0.305
0.070
0.048
0.053
I− cis
trans
−0.073
Cl− cis
trans
0.016
0.009
0.014
−0.059
0.132
0.114
0.016
0.009
0.014
−0.059
0.095
0.066
0.096
0.041
−0.001
0.002
0.043
−0.05
0.058
0.029
0.062
−0.023
I− cis
trans
Cl− cis
trans
0.043
0.053
0.053
0.014
0.087
0.069
0.108
−0.018
0.171
0.104
0.140
0.028
I− cis
trans
0.143
0.132
Cl− cis
trans
0.224
0.261
0.261
0.332
0.152
0.276
0.229
0.354
I− cis
trans
Cl− cis
trans
0.058
0.029
0.062
I− cis
trans
−0.023
meta carbons (3J(195Ptꢀ13C)). Coupling constants be-
tween ortho carbons and the 195Pt nucleus could not be
observed, probably because of inadequate orientation
PtꢀN bond occurs in the complexes but it is usually too
fast on the NMR time scale. When the pyridine deriva-
tive is substituted in ortho position, the rotation may be
slow enough to detect the different rotamers. For trans
complexes, two conformations are believed to exist. The
ortho methyl groups are either on opposite sides of the
coordination plane (anti ) or on the same side (syn).
The anti conformation is believed to be the most impor-
tant in solution. Therefore, for the trans complexes, the
most intense series was assigned to the anti rotamer,
while the second series is believed to be the syn rotamer.
A recent temperature proton NMR study has been
made with trans-Pt(2-pic)2Cl2 and the energy barrier
was assumed to be greater than 71 kJ mol−1 [17]. For
the cis isomers, the different signals are more difficult to
assign, since rotation is not believed to occur as freely
as in the trans complexes. The rotamers may be called
conformers and, as seen in Section 2, several different
signals are observed and cannot be assigned with cer-
tainty. The two 195Pt resonances observed for cis-Pt(2-
pic)2I2 and cis-Pt(2,4-lut)2I2 have already been assigned
as the two different conformations (syn and anti ) of
these complexes.
4
of the orbitals. A few J(195Ptꢀ1H) coupling constants
between the ortho methyl hydrogens and the 195Pt nu-
cleus were observed and are listed in Section 2. They
vary between 11 and 14 Hz.
The 195Pt coupling constants with the hydrogen nu-
cleus were measured on a low field NMR spectrometer
since the satellites are broader and less intense on a
higher field instrument, because of chemical shift an-
3
isotropy relaxation. The J(195Ptꢀ1H) values are listed
in Table 5. They agree well with the ones published for
the ligands pyridine, 4-picoline and 3,5-lutidine in
DMSO [12,13,16]. In one of these publications [13], a
signal observed between 3.38 and 3.49 ppm was as-
signed to the methyl groups. We believed that this
signal is caused by the presence of water (HDO) in the
DMSO-d6 used [33]. The 3J(195Ptꢀ1H) coupling con-
stants are geometry dependent. Values between 40 and
44 Hz were observed for cis complexes, while lower
values (31 to 35 Hz) were calculated for the trans
3
complexes. The J(195Ptꢀ13C) coupling constants for the
dichloro and diiodo complexes are listed in Table 6. No
values have been previously reported for disubstituted
complexes with pyridine derivatives. Similarly to the
3J(PtꢀH), the 3J(PtꢀC) coupling constants also seem
geometry dependent. The values obtained are 40–48
and 30–38 Hz for the cis and trans complexes, respec-
tively. In asymmetric ligands, the 3J(PtꢀC) could be
3.3. Coupling constants with the 195Pt nucleus
In proton and 13C NMR spectroscopies, satellites
arising from the coupling with the 195Pt nucleus are
observed for the ortho hydrogens (3J(195Ptꢀ1H)) and the

34
C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
Table 6
Dl(13C) (ppm) and 3J(195Ptꢀ1H) (Hz) for the complexes Pt(Ypy)2X2
Ypy
X
C2
C3
C4
C5
C6
CH3
2.06, −0.38
1.87, −0.17
2.99, −0.15
3.62, −0.24
0.03
S(Dl)
Dlavg
3J(195Ptꢀ13C)
2,4-lut
Cl− cis
trans
2.29
2.43
1.72
3.33
2.74
3.59
2.37
5.45
2.53
3.27
2.25
4.92
3.40
3.69
5.42
6.90
2.82
3.54
2.71
5.36
3.15
3.99
3.17
5.93
3.18
2.74
3.61
2.59
3.38
2.27
3.19
2.24
2.45
1.34
2.38
1.40
3.56
2.87
3.81
3.36
3.68
2.38
4.10
2.32
2.72
1.58
3.09
1.63
2.97
2.53
2.67
2.10
2.93
2.46
2.82
2.46
3.88
3.5
3.86
2.95
1.86
1.68
1.84
1.70
2.86
2.57
3.26
2.15
2.74
2.50
3.20
2.00
2.83
1.85
2.52
1.54
3.38
2.27
3.19
2.24
2.45
1.34
2.38
1.40
2.57
1.91
2.43
1.85
2.88
1.52
2.88
1.46
2.72
1.58
3.09
1.63
3.80
2.92
1.83
3.79
2.74
3.59
2.37
5.45
2.53
3.27
2.25
4.92
5.48
4.54
4.22
5.83
3.16
4.04
3.16
5.96
3.15
3.99
3.17
5.93
16.8
14.2
15.2
16.7
15.2
14.1
14.0
17.8
13.9
12.7
13.2
15.5
22.2
16.8
21.3
23.7
15.4
14.0
16.3
17.1
14.5
13.6
15.7
17.1
3.27
2.50
2.65
2.64
2.80
3.21
2.52
4.45
2.49
2.31
2.31
3.16
3.36
2.80
3.07
3.19
2.93
2.92
3.00
3.73
2.90
2.73
3.14
3.42
41 (C3)
31 (C3)
44 (C5)
32 (C5)
41
33
42
35
48
I− cis
trans
3,5-lut
4-pic
2-pic
3-pic
Py
Cl− cis
trans
−0.04
I− cis
trans
0.07
−0.04
0.06
−0.04
0.12
−0.05
5.29
2.09
3.59
4.07
0.04
−0.03
0.20
Cl− cis
trans
36
40
38
I− cis
trans
Cl− cis
trans
45 (C3)
30 (C3)
42 (C3)
31 (C3)
44 (C5)
38 (C5)
45 (C5)
38 (C5)
n. o.
31
I− cis
trans
Cl− cis
trans
I− cis
trans
−0.07
Cl− cis
trans
I− cis
trans
41
35
measured for C3 or C5. For the complexes with 2-pico-
line and 2,4-lutidine, only one coupling with these
carbons could be measured since the second one was
hidden by other resonances. For the ligand 3-picoline,
the coupling constant could not be seen because of the
low intensity of C3.
to bottom; 2,4-lut is the stronger base while pyridine is
the weakest. The relation Dl(1H) versus pKa of the
protonated ligand seems almost linear (r2=0.801 and
0.861, respectively for the dichloro and the diiodo
complexes) if the ligand 4-picoline is excluded. This
seems to indicate that the s(Ypy“Pt) bond is impor-
tant in our complexes and that the p(Pt“Ypy) bond is
almost either absent, or of the same order of magnitude
in all the complexes. For the cis-dichloro and cis-diiodo
complexes, no such relation is observed, since the
Dlavg(1H) seems to depend on the steric hindrance. The
values are larger for the 2,4-lut and 2-pic ligands. For
the diiodo complexes (Table 5) Dlavg(1H) is greater for
the cis complexes compared to their trans analogues,
which is in agreement with the higher shielding of the
195Pt nucleus observed for the cis complexes. This is not
the case for the dichloro complexes. Dlavg(1H) is greater
for the trans complexes, except when the ligand is
substituted in the ortho position. The presence of a
methyl group in the ortho position has a large influence
on the chemical shifts since the rotation around the
PtꢀN bond is more limited (especially in the cis iso-
mers) which changes the average environment around
the nuclei under study.
3.4. Chemical shift 6ariations upon Pt coordination
The chemical shift variation upon coordination
1
(Dl=lcomplex−lfree ligand) observed in H NMR were
calculated and are listed in Table 5. When rotamers
were observed, a weighted average value was calcu-
lated. Since a few of the lower intensity peaks were
hidden by the more intense ones, some of these values
are not very precise. Most of the values are positive,
indicating a decrease in electronic density on the
pyridine derivative with coordination to the metal. In
an attempt to study the nature of the PtꢀYpy bond, a
Dlaverage was calculated as follows: Dlaverage=S(Dl)/
number of aromatic hydrogens. We cannot choose one
particular hydrogen atom as a probe, since the only one
present in all ligands is H6 and its Dl is greatly influ-
enced by the presence of ortho substituents. The sum
S(Dl) cannot be used, since the number of aromatic
hydrogens is not constant. For the trans-dichloro and
trans-diiodo complexes, if the ligand 4-picoline is ex-
cluded, a decrease in Dlavg(1H) is observed as the
basicity of the ligand decreases. The ligands in Tables
4–6 are listed in decreasing order of basicity from top
The Dl values for 13C NMR are listed in Table 6.
These values seem larger for carbon atoms which are
substituted. Dl(13C) is close to zero for the methyl
carbons, which are not located in the ortho positions.
No correlation between Dl(13C) and the pKa of the
protonated ligands could be found, when a single car-

C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
35
form multiple bonding with pyridine. A Dl of −3.2
ppm was reported for C4, indicating that the electron
density on the ligand increased considerably. When M
is Rh(III) and Co(III) (which are harder metals), the p
bonding is assumed to be reduced and the Dl values on
C4 were evaluated as 4.1 and 4.9 ppm, respectively [34].
These high positive values were assumed to indicate the
absence of p bonding. Our Dl values on C4 lie between
1.70 and 3.88 ppm. If the Dl(C4) criterion is valid p
bonding in our complexes is probably not very impor-
tant. Furthermore, the linear relation between
Dlavg(1H) versus the pKa of the protonated ligands
obtained for the trans complexes also seems to support
this notion.
3.5. Crystal structures and the trans influence
The difference in l(195Pt) between the cis and trans
isomers (l(195Pt) is greater for the diiodo complexes
(average 65 ppm) than for the dichloro complexes (45
ppm). For the cis and trans isomers of PtL2X2, McFar-
lane [32] proposed that Dl(195Pt) is greater when the
ligands L and X are far from each other in the trans
influence series, since the difference in energy (DE in the
Ramsey equation) between the two isomers would be
larger. Contrary to the trans effect series, which is
relatively well known, the trans influence series is not
known with any certainty. In order to determine the
relative trans influence of the ligands used in this paper,
we have measured the crystal structures of eight differ-
ent complexes.
Fig. 1. Labeled diagram of cis-Pt(2-pic)2Cl2 (I).
bon atom was used as a probe. This is not surprising
since there are important local effects when a carbon
atom is substituted. An attempt to find a correlation
was made with S(Dl) without success. In order to
reduce the local effects on the substituted carbon
atoms, an attempt was made with Dlavg(13C), which
was calculated using only the non-substituted carbon
atoms. This attempt also failed. The Dl(13C) values
seem to be affected by factors other than the local
effects or the basicity of the ligands. For the dichloro
complexes, the Dl(13C) values seem to be greater for cis
complexes compared to their trans analogues (Table 6),
which is in agreement with the 195Pt NMR chemical
shifts. The contrary is observed for the diiodo
complexes.
The crystal structures of the dichloro complexes cis-
Pt(2-pic)2Cl2 (I) trans-Pt(3-pic)2Cl2 (II) and trans-Pt(4-
pic)2Cl2 (III) were determined. A labeled diagram of I is
shown in Fig. 1. For crystal III, the Pt atom is located
on an inversion center. In crystals I and II, the
molecules have the anti conformation, where the methyl
groups are on opposite sides of the platinum plane. The
PtꢀCl and PtꢀN bond distances are listed in Table 7.
For the three crystals, the average PtꢀCl bond in the
13C NMR can sometimes, provide information on
p-bonding in metal complexes. In a study of complexes
of the type [M(NH3)5(pyridine)]+m where m is the
oxidation state of the metal M (Ru(II), Rh(III) and
Co(III)), the Dl of C4 was used to provide information
on the extent of p bonding [34]. Ru(II) is known to
,
trans position to the chloro ligands is 2.301(4) A and it
Table 7
,
Selected bond distances (A) and dihedral angles (°) for the complexes Pt(Ypy)2X2
Complex
PtꢀN
PtꢀX
Dihedral angle
trans-Pt(py)2Cl2 [10]
trans-Pt(4-pic)2Cl2 (III)
trans-Pt(3-pic)2Cl2 (II)
cis-Pt(py)2Cl2 [10]
cis-Pt(2-pic)2Cl2 (I)
trans-Pt(py)2I2 [11]
trans-Pt(4-pic)2I2 (VI)
trans-Pt(3-pic)2I2 (VII)
trans-Pt(2-pic)2I2 (VIII)
cis-Pt(py)2I2 (V)
1.98(1)
2.024(5)
2.003(8), 2.031(11)
2.04(1), 2.01(1)
2.027(6), 2.039(7)
2.03(2)
2.018(7)
2.020(6)
2.024(17), 2.055(16)
2.037(10), 2.052(10)
2.056(13), 2.060(14)
2.308(5)
2.304(2)
2.299(3), 2.297(3)
2.300(5), 2.291(5)
2.304(2); 2.306(2)
2.597(1)
2.6034(10)
2.6033(6)
2.614(2), 2.617(2)
2.5783(12), 2.6023(12)
2.590(2), 2.598(2)
56.2
67.4(2)
69.6(2), 66.2(3)
62.0, 55.8
80.7(3), 71.7(3)
74.7
70.4(2)
63.5(3)
89.9(4), 81.9(5)
66.2(3), 60.4(5)
86.6(5), 75.3(4)
cis-Pt(4-pic)2I2 (IV)

36
C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
,
2.296(5) A, respectively [10]. The average PtꢀN bonds
in crystals I, II and III are 2.033(7) (trans to Cl−) and
,
2.021(7) A (trans to N). In cis- and trans-Pt(py)2Cl2,
,
these values are 2.025(10) and 1.98(1) A, respectively
[10]. The distances are not significantly different, but
the PtꢀN bond seems slightly longer when it is in the
trans position to chloro ligands. These results seem to
indicate that the trans influence of chloro and pyridine
ligands are very similar and that it might be greater for
chloro ligands. However, more data is needed to deter-
mine with certainty, that the trans influence of chloro
ligands is larger than the influence of pyridine ligands.
Five diiodo compounds were also studied by X-ray
diffraction methods. They are cis-Pt(4-pic)2I2 (IV), cis-
Pt(py)2I2 (V), trans-Pt(4-pic)2I2 (VI), trans-Pt(3-pic)2I2
(VII) and trans-Pt(2-pic)2I2 (VIII). Labeled diagrams of
IV, VI and VIII are shown in Figs. 2–4. For crystals VI
and VII, the Pt atom is located on an inversion center.
In crystal VII, the molecule again has the anti confor-
mation, but in trans-Pt(2-pic)2I2 (VIII), surprisingly, the
methyl groups are on the same side of the coordination
plane. In solution, the anti conformation is believed to
be predominant, but crystal VIII contained molecules
in the syn conformation. Packing forces are usually an
important factor in the conformation of such molecules
in the solid state. The distance C(6)···C(12) is approxi-
Fig. 2. Labeled diagram of cis-Pt(4-pic)2I2 (IV).
,
mately 4.0 A, which is the sum of the van der Waals
radii.
The PtꢀN and the PtꢀI bond distances are listed in
Table 7. The average PtꢀI bond located in trans posi-
,
tion to the iodo ligand is slightly longer (2.6076(12) A)
than the average value for bonds in trans position to
,
pyridine ligands (2.5922(16) A). This is also true for the
Fig. 3. Labeled diagram of trans-Pt(4-pic)2I2 (VI).
PtꢀN bonds. The average value for the trans complexes
,
is 2.028(10) A, while for cis complexes it is slightly
,
longer, 2.052(12) A.
In an attempt to confirm the relative order of iodo,
chloro and pyridine ligands in the trans influence series,
we have calculated the average bond distances PtꢀX
and PtꢀN for crystals I–VIII and have included the
three published structures (cis-Pt(py)2Cl2 [10] trans-
Pt(py)2Cl2 [10] and trans-Pt(py)2I2 [11]) which are
shown in Table 8. The data in the three latter published
structures have high standard deviations. Those indi-
cated in Table 8 are average values determined for the
11 crystal structures. These results confirm that the
Table 8
The average bond distances PtꢀX and PtꢀN for crystals I–VIII and
including the three published structures
Complex
PtꢀI
PtꢀCl
PtꢀN
Fig. 4. Labeled diagram of trans-Pt(2-pic)2I2 (VIII) showing the syn
conformation of the 2-picoline ligands.
trans to I Pt(Ypy)2I2
trans to Cl Pt(Ypy)2Cl2
trans to N Pt(Ypy)2I2
trans to N Pt(Ypy)2Cl2
2.605(1)
2.592(1)
2.052(12)
2.028(9)
2.020(12)
2.007(8)
2.303(4)
2.300(4)
,
is 2.305(2) A for bonds trans to the pyridine ligand. In
cis- and trans-Pt(py)2Cl2, these values are 2.308(5) and

C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
37
trans influence of the iodide ligand is greater than the
one for the pyridine ligands. If we compare the PtꢀN
distances found in the dichloro and the diiodo com-
plexes, the trans influence of the iodide ligands is
greater than the one of the chloro ligands. These data
seem also to indicate that the trans influence of chloro
ligands is greater than those of the pyridine ligands,
although the difference might be small. The order of
the trans influence series is then:
picoline, 3,5-lutidine and 2,4-lutidine, X=Cl− and I−)
were characterized by multinuclear NMR in a relatively
inert solvent. In 195Pt NMR, the diiodo complexes were
observed at much higher fields than the dichloro ana-
logues. As observed with complexes of the type
Pt(amine)2X2, the 195Pt signals of the cis complexes are
found at higher field than their trans analogues. For the
dichloro compounds, this was explained by the higher
DE for the cis complexes observed in the electronic
spectra. The difference in l(Pt) chemical shifts for the
cis and trans isomers is much larger for the diiodo
compounds. This difference was related to the relative
order of iodo, chloro and pyridine ligands in the trans
influence series. There seems to be a dependence of the
l(Pt) on the pKa of the protonated ligand, but the
dispersion about the linear relation is quite large. The
pKa values of the protonated pyridine derivatives were
measured in water or alcohol and they are influenced
by the presence of molecules of solvent close to the
protonated site (N atom). The intrinsic basicity of the
pyridine ligands can be evaluated from the gas phase
protonic affinities or by the Hammett constants, but
these values are not easily accessible. These values,
when they become more available may provide further
information on the nature of the metal–pyridine bond.
The 3J(195Ptꢀ1H) coupling constants in the
Pt(Ypy)2X2 complexes are larger for the cis isomers (41
Hz) than for the trans compounds (33 Hz), which might
be a relatively simple method to differentiate between
the two different isomers. The 3J(195Ptꢀ13C) are also
geometry-dependent. The average values are 43 and 34
Hz for the cis and trans compounds, respectively.
The crystal structures of eight complexes were deter-
mined and the results have shown that the iodido
ligand has a greater trans influence than chloride and
pyridine derivatives. This fact explained the greater
difference in l(195Pt) between the cis and trans isomers
for the diiodo complexes when compared to the
dichloro compounds. For complexes with 2-picoline
I−ꢁCl−\Ypy
These results seem to confirm the previous discussion
on the 195Pt chemical shifts observed in NMR. The
Dl(195Pt) (between the cis and trans isomers) is greater
for the diiodo complexes compared to the dichloro
complexes, because the iodide ligand has a larger trans
influence than the one of chloride ligands. Therefore
the difference between iodides and pyridine is greater
than the one between chloride and pyridine ligands in
the trans influence series. The larger difference between
Ypy and I− in the series leads to a larger difference in
the electronic excitation energy DE for the cis and trans
isomers. The trans isomers are therefore expected to
produce resonances at lower fields, since DE would be
smaller for these isomers.
The dihedral angles between the coordination plane
and the pyridine derivatives are listed in Table 7. In
general, the angles are larger for cis isomers, where
steric hindrance is more important. For the 2-picoline
complex (I), the angles are closer to 90° because of
steric hindrance of the ortho methyl groups.
The complexes trans-Pt(3-pic)2Cl2 (II) and trans-
Pt(4-pic)2Cl2 (III) show similar dihedral angles, greater
than the one in the trans-dichloro complex with
pyridine. Packing forces seem important because the
ligands are not sterically hindered near the coordination
plane. The iodo complex, trans-Pt(2-pic)2I2 (VIII)
shows dihedral angles which are almost perpendicular
due to steric hindrance of the ortho methyl groups by
the iodo ligands. Surprisingly, the dihedral angles in IV
are similar to those in I even if the 4-picoline ligand is
not sterically hindered near the binding site. The ligand
distances and bond angles are as expected.
It is interesting to observe that two trans crystals (II
and VIII) were synthesized from the isomerization of
the cis isomers in DMF or in DMSO, which are not
inert solvents for Pt complexes. The multinuclear NMR
spectra of II and VIII have shown the presence of only
one species. Therefore this method is an effective alter-
nate method to synthesize trans compounds with steri-
cally demanding ligands.
1
and 2,4-lutidine, rotamers were observed in H and 13C
NMR and in 195Pt NMR for the cis-diiodo complexes.
For these ligands, the rotation around the PtꢀN bond is
limited because of the presence of the ortho methyl
substituent. A linear relationship between the mean
1
chemical shift variation in H NMR and the pKa of the
protonated ligands was found for the trans-dichloro
and trans-diiodo complexes when the ligand 4-picoline
is omitted. This could indicate the importance of the
s(Pt“Ypy) bond. According to 13C NMR, the
p(Ypy“Pt) bond does not seem important in these
complexes if Dl(C4) is taken as a valid probe. Finally,
the crystal structure analyses of trans-Pt(3-pic)2Cl2 and
trans-Pt(2-pic)2I2, which were synthesized from the iso-
merization of the cis analogues in DMF and DMSO
respectively, confirmed the validity of the synthetic
method used to prepare these complexes.
4. Conclusions
Four series of complexes of the type cis- and trans-
Pt(Ypy)2X2 (Ypy=pyridine, 4-picoline, 3-picoline, 2-

38
C. Tessier, F.D. Rochon / Inorganica Chimica Acta 295 (1999) 25–38
16 (1977) 1192. (d) R. Faggiani, B. Lippert, C.J.L. Lock, B.
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Hollis, J.A. Schreifels, J.D. Hoeschele, J. Clin. Hematol. Oncol.
7 (1977) 138.
The following tables have been deposited to the
Cambridge Data File for the eight crystals: refined
atomic coordinates, Ueq factors and bond distances and
angles. Copies of this information may be obtained free
of charge from: The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (Fax: +44-1223-336-033;
email: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk). Other lists are available from the
authors on request. (CCDC numbers: 135013 and
135020).
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