Tuning the π -backbonding and σ - trans effect of N^C^N coordinated Pt(II) complexes. Kinetic and computational study

Article abstract of DOI:10.1080/00958972.2014.1001752

The nucleophilic substitution reaction of cyclometallated complexes; [PtL²Cl] (L² = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL³Cl] (L³ = 2,4-di(2-pyridinyl)-fluorobenzene), and [PtL⁴Cl] (L⁴ = 3,5-di(2-pyridinyl)-toluene) with a series of neutral nucleophiles with different steric properties, thiourea (TU), N,N-dimethylthiourea (DMTU), and N,N,N′,N′-tetramethylthiourea (TMTU), was studied under pseudo-first-order conditions in methanol solution of an ionic strength of 0.1 M (0.09 M LiCF₃SO₃ and 0.01 M LiCl). The rate of substitution of the chloro ligand was studied as a function of nucleophile concentration and temperature using UV-visible and stopped-flow spectrophotometric techniques. The observed pseudo-first-order rate constants for the substitution reactions obey the rate law kob_s = k₂[Nu] + k-₂. The reactivity of the investigated complexes when [PtL¹Cl] is used as a reference follows the order [PtL²Cl] > [PtL³Cl] > [PtL⁴Cl] > [PtL¹Cl]. The lability of the chloro group is dependent on the extent of π-backbonding and the σ-trans effect of the ligand backbone. [PtL²Cl] and [PtL³Cl], which have a common electron-withdrawing fluoride on the ligand trans to the leaving group, have a higher reaction rate compared to [PtL⁴Cl], which has an electron-donating methyl group attached to the ligand backbone. The position of the substituent on the phenyl group trans to the leaving group also influences the overlap of frontier molecular orbitals which result in controlling the reactivity of the fluoro complexes. In general, the results show that the nature of the substituent, either electron withdrawing or electron donating, results in an increase in the rate of substitution. Second-order kinetics and large negative activation entropies (ΔS^#) support an associative substitution mechanism. The experimental data are supported by DFT calculations.

Full text of DOI:10.1080/00958972.2014.1001752

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Tuning the π-backbonding and σ-trans
effect of N^C^N coordinated Pt(II)
complexes. Kinetic and computational
study
Tshephiso R. Papoa & Deogratius Jaganyia
a School of Chemistry and Physics, University of KwaZulu-Natal,
Pietermaritzburg, South Africa
Accepted author version posted online: 22 Dec 2014.Published
online: 07 Jan 2015.
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To cite this article: Tshephiso R. Papo & Deogratius Jaganyi (2015) Tuning the π-backbonding and
σ-trans effect of N^C^N coordinated Pt(II) complexes. Kinetic and computational study, Journal of
Coordination Chemistry, 68:5, 794-807, DOI: 10.1080/00958972.2014.1001752
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Journal of Coordination Chemistry, 2015
Vol. 68, No. 5, 794–807, http://dx.doi.org/10.1080/00958972.2014.1001752
Tuning the π-backbonding and σ-trans effect of N^C^N
coordinated Pt(II) complexes. Kinetic and computational study
TSHEPHISO R. PAPO and DEOGRATIUS JAGANYI*
School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg, South Africa
(Received 9 August 2014; accepted 5 December 2014)
The mechanistic pathway for the substitution reaction of a strong trans labilizing phenyl of N^C^N
platinum(II) complexes by biorelevant thiourea nucleophiles go through the solvent associated
pathway.
The nucleophilic substitution reaction of cyclometallated complexes; [PtL2Cl] (L2 = 3,5-di(2-pyridi-
nyl)-fluorobenzene), [PtL3Cl] (L3 = 2,4-di(2-pyridinyl)-fluorobenzene), and [PtL4Cl] (L4 = 3,5-di(2-
pyridinyl)-toluene) with a series of neutral nucleophiles with different steric properties, thiourea
(TU), N,N-dimethylthiourea (DMTU), and N,N,N′,N′-tetramethylthiourea (TMTU), was studied
under pseudo-first-order conditions in methanol solution of an ionic strength of 0.1 M (0.09 M
LiCF3SO3 and 0.01 M LiCl). The rate of substitution of the chloro ligand was studied as a function
of nucleophile concentration and temperature using UV–visible and stopped-flow spectrophotometric
techniques. The observed pseudo-first-order rate constants for the substitution reactions obey the rate
law kobs = k2[Nu] + k−2. The reactivity of the investigated complexes when [PtL1Cl] is used as a ref-
erence follows the order [PtL2Cl] > [PtL3Cl] > [PtL4Cl] > [PtL1Cl]. The lability of the chloro group
is dependent on the extent of π-backbonding and the σ-trans effect of the ligand backbone. [PtL2Cl]
and [PtL3Cl], which have a common electron-withdrawing fluoride on the ligand trans to the leaving
*Corresponding author. Email: jaganyi@ukzn.ac.za
© 2015 Taylor & Francis

N^C^N coordinated Pt(II) complexes
795
group, have a higher reaction rate compared to [PtL4Cl], which has an electron-donating methyl
group attached to the ligand backbone. The position of the substituent on the phenyl group trans to
the leaving group also influences the overlap of frontier molecular orbitals which result in
controlling the reactivity of the fluoro complexes. In general, the results show that the nature of the
substituent, either electron withdrawing or electron donating, results in an increase in the rate of sub-
stitution. Second-order kinetics and large negative activation entropies (ΔS#) support an associative
substitution mechanism. The experimental data are supported by DFT calculations.
Keywords: Kinetics; Mechanistic; Platinum; Substitution
Introduction
Square planar platinum(II) complexes have been given much attention due to their
application in many areas such as organic light-emitting diodes [1], photovoltaic devices
[2], photocatalysts [3], chemosensors [4], phosphorescent emitters for displays, lighting
applications [5], and as anticancer drugs due to the slow ligand exchange kinetics [6].
Thousands of platinum complexes have been synthesized and tested as potential antitumor
drugs following the discovery of the antitumor properties of cis-diamminedichloro-platinum
(II) more commonly known as cisplatin in the late 1960s [7]. These new platinum(II)
complexes were synthesized to improve activity, reduce toxicity, and increase solubility [8].
This has led to interest in the substitution behavior of square planar d8 platinum(II)
complexes as the mechanism of action of cisplatin involves the formation of adducts with
DNA [9].
Kinetic and mechanistic studies involving square planar complexes with one or more
Pt–C bonds [10–14] have also been given much attention due to the ability of the Pt–C
bond to stabilize the square planar geometry through the kinetic trans effect [14, 15]. The
trans effect can accelerate the substitution rate by up to six orders of magnitude for
complexes with a single Pt–C bond trans to the leaving group [16]. In addition, two cis
Pt–C bonds have been reported to induce a mechanistic change over from the common
associative substitution pathway [10] to a dissociative mechanism [12, 17]. Two Pt–C bonds
have been found to weaken the leaving group through stabilization of the three coordinate
(14-electron) intermediate in which the leaving group is dissociated from the platinum(II)
[12, 14, 18]. A strong π-acceptor non-leaving ligand stabilizes the trigonal bipyramidal
transition state by accepting π-back donation of electron density from the platinum(II) [16].
Kinetic studies have shown that for cyclometallated complexes with aliphatic carbon
donors, the reaction rate is in minutes, but, in the case of a phenyl acting as a carbon donor,
the reaction rate is 102 times faster; this is due to the in-plane configuration of the phenyl
ring which enables effective π-backbonding as the electrophilicity of the metal center is
enhanced. The π-acceptor effects of in-plane pyridine donors showed that π-backbonding in
the cis position enhances the electrophilicity of the metal center more than in the trans posi-
tion [19], as observed in [Pt(bipy)(Ph)Cl], where the electrophilicity of the metal is influ-
enced by the in-plane phenyl ring when the 6-phenyl-2,2′-bipyridine ligand becomes part of
an extensive π-acceptor system and the strong π-electron withdrawal of the 2,2′-bipyridine
fragment [13].
In a study carried out to investigate the lability of chloride in Pt(II) complexes of the type
[Pt(N^N^N)Cl], [Pt(N^N^C)Cl], and [Pt(N^C^N)Cl], it was found that when a carbon was
introduced cis to the leaving group, the reactivity of the complex reduced due to

796
T.R. Papo and D. Jaganyi
accumulation of electron density around the metal center, while the introduction of carbon
trans to the leaving group increased the reactivity due to the kinetic trans effect [20].
In order to tune the reactivity of a strong trans labilizing phenyl of N^C^N platinum(II)
complexes with different substituents (electron-withdrawing and electron-donating groups)
on the phenyl moiety trans to the leaving group on the Pt(II) complexes [PtL2Cl], [PtL3Cl],
and [PtL4Cl] were synthesized, characterized and their ligand substitution reactions were
investigated. This study sheds light on the role of increasing π-backbonding through elec-
tron-withdrawing effect relative to increasing trans σ-effect. The substitution reactions were
monitored using sulfur containing nucleophiles of different steric effects, viz. thiourea (TU),
N,N-dimethylthiourea (DMTU), and N,N,N′,N′-tetramethylthiourea (TMTU). Literature data
for [PtL1Cl] were included for comparison purposes. The structures of the investigated
complexes are shown in scheme 1.
Experimental
All reactions involving the synthesis of ligands and coordination of ligands to platinum(II)
were carried out under nitrogen using standard Schlenk techniques. Organic solvents (tolu-
ene, methanol, hexane, dichloromethane, diethyl ether, acetic acid, and ethanol) used in the
synthesis were purchased from Merck. Toluene, methanol, hexane, and ethanol were
purified by standard distillation methods prior to use [21]. K2PtCl4 was obtained from
Strem. 1,3-Dibromobenzene, 1,3-dibromo-5-fluorobenzene, 1,3-dibromo-4-fluorobenzene,
3,5-dibromo-toluene, tri-n-butylstannylpyridine, bis(triphenylphosphine)-palladium dichlo-
ride, lithium chloride, potassium fluoride, sodium bicarbonate, and sodium sulfate were
obtained from Aldrich and used as received. The thiourea nucleophiles TU, DMTU, and
TMTU were also obtained from Aldrich and used as supplied.
Synthesis of ligands
The ligands 3,5-di(2-pyridinyl)-fluorobenzene, 2,4-di(2-pyridinyl)-fluorobenzene, and 3,5-di
(2-pyridinyl)-toluene were synthesized according to published methods [22], with minor
modifications.
CH3
F
F
N
N
Pt
N
N
N
N
Pt
Cl
N
Pt
N
Pt
Cl
PtL4Cl
Cl
PtL1Cl
Cl
PtL2Cl
PtL3Cl
Scheme 1. Structures of the investigated platinum(II) complexes. [PtL1Cl] was taken from the literature.

N^C^N coordinated Pt(II) complexes
3,5-Di(2-pyridinyl)-fluorobenzene, L2
797
To a mixture of 1,3-dibromo-5-fluorobenzene (1.39 g, 5.51 mM), 2-tri-n-butylstannylpyri-
dine (5.00 g, 13.6 mM), bis(triphenylphosphine)palladium dichloride (1.00 g, 1.43 mM),
and lithium chloride (0.37 g, 8.84 mM) in an oven dried Schlenk tube, distilled toluene
(40 mL) was added. The mixture was degassed via the freeze-pump-thaw method until no
gas bubbles were observed and refluxed at 110 °C under nitrogen for 48 h. After cooling
the reaction mixture to room temperature, an aqueous solution of potassium fluoride
(40 mL) was added and the insoluble black residue was filtered and washed with toluene.
The toluene was removed under reduced pressure. Dichloromethane (100 mL) and aqueous
sodium bicarbonate (100 mL) were added to the remaining residue. The organic phase was
separated, washed with sodium bicarbonate (2 × 100 mL), then dried over anhydrous
sodium sulfate and evaporated to obtain a yellow residue. This residue was then purified
over silica with a mixture of 90% hexane: 10% diethyl ether, whose mole fraction was mea-
sured to 50% hexane: 50% diethyl ether to obtain a pale yellow solid (0.8078 g, 58%).
Anal. Calcd for C16H11N2F: C, 76.83; H, 4.43; N, 11.25. Found: C, 76.84; H, 4.82; N,
1
10.86. H NMR (400 MHz, CDCl3) δ, 8.73 (ddd, 2H), 8.44 (t, 1H), 7.79–7.89 (m, 6H),
7.29 (ddd, 2H), TOF MS-ES+, m/z: 251.0989.
2,4-Di(2-pyridinyl)-fluorobenzene, L3
This ligand was prepared by a procedure similar to the one described above for synthesis of
3,5-di(2-pyridinyl)-fluorobenzene, starting from 2,4-dibromo-1-fluorobenzene (1.39 g,
5.5 mM) instead of 1,3-dibromo-5-fluorobenzene, together with 2-tri-n-butylstannylpyridine
(5.00 g, 14 mM), bis(triphenylphosphine)palladium dichloride (1.00 g, 1.43 mM), and lith-
ium chloride (0.38 g, 8.8 mM) in 40 mL toluene. After refluxing the reaction mixture for
48 h under nitrogen, the mixture was worked up as before and an oily yellow liquid which
later solidified was obtained (0.8647 g, 62%). Anal. Calcd for C16H11N2F: C, 76.83; H,
1
4.43; N, 11.25. Found: C, 76.33; H, 4.22; N, 11.01. H NMR (400 MHz, CDCl3) δ, 8.75
(dd, 1H), 8.68 (dd, 1H), 8.58 (dd, 1H), 8.09 (ddd, 1H), 7.20 – 7.82 (m, 4H), 7.18 – 7.29
(m, 3H), TOF MS-ES+, m/z: 273.0803 (M + Na+).
3,5-Di(2-pyridinyl)-toluene, L4
This ligand was also prepared by a procedure similar to the one described above for synthe-
sis of 3,5-di(2-pyridinyl)-fluorobenzene, starting from 3,5-dibromo-toluene (1.38 g,
5.5 mM) instead of 1,3-dibromo-5-fluorobenzene, together with 2-tri-n-butylstannylpyridine
(5.38 g, 15 mM), bis(triphenylphosphine)palladium dichloride (312 mg, 0.444 mM) and
lithium chloride (2.38 g, 55 mM) in 30 mL toluene. After refluxing the reaction mixture for
48 h under nitrogen, the mixture was worked up as before and an oily yellow liquid which
later solidified was obtained (0.7041 g, 51%). Anal. Calcd for C17H14N2: C, 82.89; H, 5.73;
1
N, 11.38. Found: C, 82.78; H, 5.61; N, 11.82. H NMR (400 MHz, CDCl3) δ, 8.74 (ddd,
2H), 8.41 (t, 1H), 7.96 (dd, 2H), 7.86 (dd, 2H), 7.78 (ddd, 2H), 7.24 (ddd, 2H), 2.53 (s,
3H), TOF MS-ES+, m/z: 243.3045.

798
T.R. Papo and D. Jaganyi
Synthesis of complexes
[PtL2Cl], [PtL3Cl], and [PtL4Cl] were prepared by reacting the corresponding ligand with
K2PtCl4 in acetic acid at reflux under nitrogen, as reported for [PtL1Cl] [23]. The purity of
1
the complexes was analyzed using CHN analysis, H NMR spectroscopy, and mass spectra.
[PtL2Cl]: Yield: (80.6 mg, 49.4%). Anal. Calcd for C16H10N2PtF: C, 39.97; H, 2.31; N,
1
5.83. Found: C, 39.72; H, 1.83; N, 5.58. H NMR (400 MHz, CDCl3) δ, 9.39 (d, 2H), 7.99
(t, 2H), 7.67 (d, 2H), 7.35 (t, 2H), 7.23 (s, 2H). TOF MS-ES+, m/z: 444.0482.
[PtL3Cl]: Yield: (84.9 mg, 51.4%). Anal. Calcd for C16H10N2PtF: C, 39.97; H, 2.31; N,
1
5.83. Found: C, 39.57; H, 1.96; N, 5.42. H NMR (400 MHz, CDCl3) δ, 9.34 (dd, 1H),
9.24 (dd, 1H), 7.92 (m, 3H), 7.58 (d, 1H), 7.40 (ddd, 1H), 7.29 (m, 1H), 7.22 (m, 1H),
6.84 (m, 1H). TOF MS-ES+, m/z: 444.0476.
[PtL4Cl]: Yield: (106.8 mg, 53.0%). Anal. Calcd for C17H13N2Pt: C, 42.91; H, 2.75; N,
1
5.89. Found: C, 42.88; H, 2.72; N, 5.83. H NMR (400 MHz, DMSO-d6) δ, 9.09 (d, 2H),
8.18 (t, 2H), 8.07 (d, 2H), 7.58 (s, 2H), 7.57 (t, 2H), 2.32 (s, 3H). TOF MS-ES+, m/z:
444.0476. TOF MS-ES+, m/z: 440.2834.
Physical measurements and instrumentation
NMR spectra for the ligands and the complexes were recorded on a Bruker Avance DPX
400 using a 5 mm BBOZ probe at 30 °C, with chemical shifts referenced to the relevant
solvent signal. Low resolution electron-spray ionization (ESI+) mass spectra of the samples
were recorded on a TOF Micromass LCT Premier spectrometer operated in positive ion
mode. Spectra for wavelength selection for the kinetic investigations were recorded on a
Cary 100 Bio UV–visible spectrophotometer with a cell compartment thermostated by a
Varian Peltier temperature controller having an accuracy of 0.05 °C. All kinetic measure-
ments were performed under pseudo-first-order conditions and were carried out on an
Applied Photophysics SX 20 stopped-flow analyzer coupled to an online data acquisition
system. The temperature was maintained within a range of 0.1 °C using a coupled temper-
ature control system. All kinetic data were analyzed using the Origin 7.5® graphical
analysis software package [24].
Preparation of complex and nucleophile solutions for kinetic analysis
Stock solutions of platinum(II) complexes and the nucleophiles used in the kinetic analysis
were prepared by dissolving known amount in methanol solution with an ionic strength of
0.1 M. The 0.1 M ionic strength solution of the methanol was made up by dissolving the
required amount of LiCF3SO3 (0.09 M) and LiCl (0.01 M) in methanol, as CF3SOꢀ3 does
not coordinate to Pt(II) [25]. Lithium chloride was added to suppress spontaneous solvolysis
reactions.
The complex concentrations were maintained at 0.05 mM and the solutions of the nucle-
ophiles viz. TU, DMTU, and TMTU were prepared fresh by dissolving a known amount of
the required nucleophile in methanol solution (0.1 M ionic strength) to afford a final

N^C^N coordinated Pt(II) complexes
799
concentration 50 times higher than that of the metal complex. Subsequent dilutions of the
stock nucleophile solution afforded solutions of 10, 20, 30, and 40 times the concentration
of the metal complex to assure pseudo-first order conditions.
Computational modeling
In an effort to understand the electronic differences of the complexes studied, calculations
were performed. Ground-state electronic structures of [PtL1Cl], [PtL2Cl], [PtL3Cl], and
[PtL4Cl] were optimized in the gas phase by density functional theoretical (DFT)
calculations to identify the energy-minimized structures based on B3LYP/LANL2Z [26]
(Los Alamos National Laboratory 2 double ζ) level theory, with inner core electrons of
platinum replaced by relativistic effective core potential. In addition, composite basis sets
Table 1. DFT calculated structures showing HOMO and LUMO frontier molecular orbitals for the
studied Pt(II) complexes.
Complex structure
HOMO
LUMO
PtL1Cl
PtL2Cl
PtL3Cl
PtL4Cl

800
T.R. Papo and D. Jaganyi
Table 2. Computational analysis of Pt(II) complexes showing bond lengths, bond
angles, and electronic data obtained from DFT calculations.
Parameter
[PtL1Cl]
[PtL2Cl]
[PtL3Cl]
[PtL4Cl]
Bond lengths (Å)
Pt–Cl
2.505
2.500
2.498
2.506
Bond angles (°)
C–Pt–Cl
N1–Pt–N2
179.99
161.51
180.00
161.46
179.96
161.69
179.98
161.29
NBO charges
Pt
Cl
C
0.426
−0.560
−0.101
−5.57
−2.17
3.40
0.429
−0.554
−0.114
−5.73
−2.45
3.28
0.430
−0.552
−0.090
−5.76
−2.32
3.44
0.425
−0.561
−0.106
−5.48
−2.14
3.35
HOMO (eV)
LUMO (eV)
ΔΕ (eV)
ω
4.410
5.096
4.742
4.335
Note: ω = Electrophilicity index.
[27] 6-31G*(C and H), 6-311+G (N, S, Cl), and LANL2DZ (Pt) were applied to [PtL1Cl]
to confirm minimized structure (zero imaginary frequency) and (one imaginary frequency)
transition state [28]. The synchronous transit-guided quasi-Newton method [29], in this
article for instance, QST2 was employed to locate the transition state. Singlet states were
used due to the low electronic spin of Pt(II) complexes. The Gaussian 09 set of programs
was used for all computational analysis [30]. Table 1 shows a summary of DFT calculated
HOMOs and LUMOs, and table 2 is a representation of the data calculated for the
complexes.
Results
Substitution of the coordinated chloride from the three Pt(II) complexes by thiourea
nucleophiles of different nucleophilicities and steric hindrance were investigated under
pseudo-first-order kinetics. The kinetic reactions of coordinated chloride were followed
spectrophotometrically by following the change in absorbance at suitable wavelengths as a
function of time using the stopped-flow technique and UV–visible spectrophotometry. The
selected wavelengths are indicated in table SI 1 (see online supplemental material at http://
dx.doi.org/10.1080/00958972.2014.1001752). The kinetic traces gave excellent fits to a
single exponential decay function to generate the observed pseudo-first-order rate constant,
k
obs. Figure 1 shows a typical kinetic trace obtained using the stopped-flow technique for
the reaction between [PtL3Cl] and TU nucleophile at an ionic strength of 0.1 M (CF3SOꢀ3 )
from which the kobs was calculated.
The kobs were calculated from the kinetic traces using the online non-linear least-squares
fit of exponential data [31] to equation (1) using Origin 7.5 graphical analysis software
[24],
At ¼ Ao þ ðAo ꢀ A Þ expðꢀkobstÞ
(1)
1

N^C^N coordinated Pt(II) complexes
801
0.036
0.034
0.032
0.030
0.028
0.026
0.00
0.05
0.10
0.15
0.20
0.25
Time / s
Figure 1. Kinetic trace obtained from the stopped-flow spectrophotometer showing a single exponential fit for the
reaction between [PtL3Cl] (0.049 mM) with TU (0.98 mM) in methanol monitored at 282 nm, I = 0.1 M at 298 K.
where Ao, At, and A∞ represent the absorbance of the reaction mixture initially, at time t
and at the end of the reaction, respectively. The obtained kobs were plotted against the con-
centration of the entering nucleophile. A linear dependence on the nucleophile concentra-
tion was observed for all reactions with a non-zero intercept. This indicates that the
addition of 10 mM LiCl was not sufficient to completely prevent the solvolysis pathway.
70
60
TU
DMTU
TMTU
50
40
30
20
10
0.0002
0.0003
0.0004
0.0005
0.0006
0.0007
0.0008
Nu / M
Figure 2. Dependence of kobs on the entering nucleophile concentration for the displacement of chloride on
[PtL2Cl] in methanol, I = 0.1 M (LiSO3CF3), T = 298 K.

802
T.R. Papo and D. Jaganyi
90
75
60
45
30
15
0
283
288
293
298
303
0.00000
0.00015
0.00030
0.00045
0.00060
0.00075
[TU] / M
Figure 3. Dependence of kobs on the entering TU concentration for the displacement of chloride on [PtL3Cl] in
methanol (I = 0.1 M) at various temperatures from 283 to 303 K.
Table 3. Summary of reaction rates at 25 °C and activation parameters for the substitution reactions of platinum
(II) complexes with nucleophiles (Nu) in methanol (ionic strength of 0.09 M LiSO3CF3 and 0.01 M LiCl).
Complex
Nu
k2 (M−1 −1
s
)
ΔH# (kJ M−1
)
ΔS# (JK−1 M−1
)
k−2 (s−1
)
TUa
44,500 2000
18,400 500
7400 300
21
16
10
1
1
1
−85
−109
−135
5
4
3
*
*
*
DMTUa
TMTUa
TU
DMTU
TMTU
89,203 1540
57,050 2661
46,176 699
20
10
14
1
1
1
−145
−150
−109
5
3
3
11.0 0.4
10
9
1
2
TU
DMTU
TMTU
73,985 2623
53,140 1541
26,552 966
12
12
16
1
1
1
−111
−172
−113
3
3
4
17
1
16.4 0.3
15.9 0.8
TU
DMTU
TMTU
51,858 1210
26,860 1024
23,158 1099
14
5.6 0.4
15
1
−109
−142
−93
5
1
4
6.9 0.1
6.9 0.5
3.8 0.6
1
aValues extracted from reference [20].
*Reported solvolytic path, with no calculated parameters.

N^C^N coordinated Pt(II) complexes
803
6.0
4.5
3.0
-1.6
-1.8
-2.0
-2.2
-2.4
TU
DMTU
TMTU
0.00330
0.00335
0.00340
0.00345
0.00350
0.00355
1/T / K-1
À Á
against for reaction of [PtL3Cl] with TU, DMTU, and TMTU at various temperatures
k2
T
1
T
Figure 4. Plot of In
from 283 to 303 K.
The parallel solvolysis reaction pathway is well known in Pt(II) chemistry [32]. Typical
plots for the concentration dependence of the observed first-order rate constants are shown
in figures 2 and 3, and the rate law can be expressed by equation (2). The second-order rate
constant k2 for the reaction of each metal complex with a particular nucleophile was
obtained from the slope of linear regression of plot of kobsversus nucleophile concentration
using the software Origin 7.5 package® [24] and are summarized in table 3.
Y
N
Pt
N
Y
N
Pt
N
k2
Cl
+
z
Nu
Nu
z
+
Cl
+Nu
fast
k-2
+S
Y
N
Pt
N
S
z
Y = H, Z = H [PtL1Cl]
Y = F, Z = H [PtL3Cl]
Y = H, Z = F [PtL2Cl]
Y = H, Z = CH3 [PtL4Cl]
Nu = TU, DMTU and TMTU, S = Solvent
Scheme 2. Proposed mechanism of substitution from the investigated complexes by a series of thiourea nucleo-
philes.

804
T.R. Papo and D. Jaganyi
kobs ¼ k2½Nuꢁ þ k
(2)
ꢀ2
The temperature dependence of the second-order rate constants was investigated over a
temperature range of 10–30 °C at an interval of 5 °C. Typical Eyring plots obtained for
[PtL3Cl] for the three nucleophiles are shown in figure 4. Using the Eyring equation [31],
the enthalpy of activation (ΔH#) and entropy of activation (ΔS#) were determined from
the slope and the y-intercept, respectively. These activation parameters are summarized in
table 3.
Experimental data in table 3 shows that the mechanism of substitution can be represented
by scheme 2. One pathway involves the direct attack by the nucleophile (k2) while the other
involves formation of a solvent-complex (k−2) followed by rapid substitution of the
coordinated solvent. The second step is independent of concentration of the nucleophile.
Discussion
Three platinum(II) complexes, [PtL2Cl], [PtL3Cl], and [PtL4Cl], were synthesized using
literature methods and characterized. The three complexes have a common coordinated
phenyl ring trans to the leaving group but differ by substituting electron-withdrawing or
electron-donating groups on the ligand framework. We describe the substitution behavior of
these Pt(II) complexes with thiourea-based nucleophiles. [PtL1Cl] was included for
comparison, and thus was taken from the literature [20].
The introduction of either electron-withdrawing or donating group has little or no signifi-
cant effect on the Pt–Cl bond length or NBO charge on the platinum based on the computa-
tional data summarized in table 2. However, the substituents impact the overall
electrophilicity of the whole complex showing an increase in case of electron-withdrawing
atoms and a decrease for electron-donating atom when [PtL1Cl] is used as reference. This
shows that the ability of the complexes with electron-withdrawing substituents to accept
electrons is enhanced through stabilization of HOMO–LUMO energy. This results in
enhanced reactivity as seen in table 3.
The general reactivity trend for chloride substitution by the thiourea nucleophiles follows
the order [PtL2Cl] > [PtL3Cl] > [PtL4Cl] > [PtL1Cl] for all nucleophiles. The trend in reac-
tivity can be explained by electronic effects. There is no specific trend for the solvolysis
path, k−2, and these values are small, contributing very little to the observed rate.
In previous studies, results have been reported for substitution reactions of Pt(II) com-
plexes containing metal–carbon bonds, with a single Pt–C bond located trans to the leaving
group. These studies generally reported high substitution reactivity due to the σ-bound car-
bon ligands, that have a large kinetic trans effect and back-bonding in the case of the in
plane aryl ligand [33].
Using [PtL1Cl] as the basis for comparison, it can be seen from table 3 that the reactivity
increases when an electron-withdrawing as well as electron-donating group is attached to
the phenyl trans to the leaving group. Using the rate constants for TU to compare the reac-
tivity of the complexes, [PtL2Cl] and [PtL3Cl] react about two times faster than [PtL1Cl].
The fluoro group in both [PtL2Cl] and [PtL3Cl] withdraws electrons from the phenyl ligand
trans to the leaving group which causes the ligand moiety to lose electron density to
fluorine, increasing the electrophilicity of the system as supported by DFT calculations. The

N^C^N coordinated Pt(II) complexes
805
withdrawal of electron density destabilizes the ground state but stabilizes the transition state
due to increase in π-backbonding in [PtL2Cl] and [PtL3Cl] when compared to the unsubsti-
tuted [PtL1Cl] complex. This leads to an increase in the reactivity of these complexes. The
overlap of electron density affects reactivity when comparing [PtL2Cl] and [PtL3Cl]. For
[PtL2Cl] there is uniform distribution of LUMO with the trans carbon having a more nega-
tive NBO charge (−0.114) in comparison to that of [PtL3Cl] whose LUMO is concentrated
on the side of the cyclometallated ligand to which fluorine is attached. This results in the
trans atom having a less negative NBO charge (−0.090). The orbital overlap influences the
overall electrophilicity of the complex with [PtL2Cl] (5.096) showing a better ability of
accepting electrons in the transition state through π-backbonding in comparison to [PtL3Cl]
(4.742). This difference accounts for the observed reactivity between [PtL2Cl] and [PtL3Cl].
The phenyl group can withdraw or donate electrons through resonance [34]. When it acts
as an electron withdrawer, one would have expected the σ-trans effect to decrease due to
the introduction of an electron-drawing atom such as fluoro, with a net decrease in reactiv-
ity. In contrast, the reactivity increases and the DFT calculations show no significant change
in Pt–NBO charges or the Pt–Cl bond lengths. The DFT calculations reveal the change in
the electrophilicity of the whole complex, upon introduction of electron-withdrawing fluoro
group on the phenyl ring. This observation can be understood by looking at the delocaliza-
tion of the phenyl’s π-electrons around the ring. This facilitates distribution of electron den-
sity from the attached group around the complex making the complex more electrophilic.
The electron-withdrawing fluoro group increases the electron-withdrawing capabilities of
the phenyl ring through π-backbonding, which enhances the formation of a new bond by
stabilizing the 18-electron, five-coordinate transition state (figure SI 13, Supplementary
Material). Previous studies involving π-backbonding [19, 21, 35] of a coordinated terpyri-
dine ligand reported similar results for electron-withdrawing group attached to the ancillary
position of terpyridine [20, 36].
In the case of the electron-donating methyl in [PtL4Cl] the ability of the phenyl ligand to
accept electrons from the metal center is reduced as compared to electron-withdrawing
groups in [PtL2Cl] and [PtL3Cl]. Earlier work carried out using terpyridine demonstrated
that electron-donating groups attached in the ancillary positions of the ligand backbone
decrease the substitution reaction [33, 34, 37], due to the decrease in electrophilicity of the
complex. This reduces the π-backbonding effect, destabilizing the five-coordinate transition
state resulting in a slower reaction rate [38]. This was also observed by Schmülling et al.
[15]. In the present study, the introduction of an electron-donating group did not slow the
rate of reaction when compared to the parent complex [PtL1Cl], in fact the introduction of a
methyl group on the phenyl ring in [PtL4Cl] resulted in an increase in the reaction rate by a
small margin with all the nucleophiles when compared to [PtL1Cl]. This result can be
explained to be due to a marginal increase in trans σ-effect. It is clear from looking at the
current results and what has been reported in literature that both π-backbonding and trans
σ-effect play roles in influencing the rate of reaction. In the current study, the slight acceler-
ation of the substitution process for electron-donating group is linked to the ground state
destabilization due to strong σ-electron donation along the C–Pt–Cl axis.
The results in table 3 shows that reactivity of complexes with electron-withdrawing sub-
stituents is much higher than that of complexes with electron-donating groups; [PtL2Cl] and
[PtL3Cl] react at a faster rate than [PtL4Cl]. This is because, for electron-withdrawing sub-
stituents, there is an increase in π-backbonding which together with a strong σ-trans effect
enhances the substitution reaction, while for electron-donating the π-backbonding is weak.
The results also show that the position of the substituent on the coordinated phenyl in the

806
T.R. Papo and D. Jaganyi
case of fluoro has no significant difference, as the rate constants are practically the same
within the experimental error.
The kinetic results shown in table 3 clearly indicate that substitution of chloride by the
nucleophiles TU, DMTU, and TMTU depends on the steric hindrance of the nucleophiles.
The reactivity decreases according to the increase in steric hindrance of the nucleophiles for
all the complexes, i.e. the most sterically hindered TMTU reacts significantly slower than
the other nucleophiles. Thus, the rate of substitution for the nucleophiles is as follows:
TU > DMTU > TMTU.
The values of the activation enthalpies are small and positive (ΔH#) while the activation
entropies (ΔS#) are large and negative (table 3). The sensitivity of the second-order rate con-
stants to different nucleophiles and the significantly negative intrinsic entropies of activation
(ΔS#) values are in line with the associative substitution mechanism and a net increase in
the bond order in the transition state which is common for d8 square-planar metal
complexes [10, 39, 40].
Conclusion
The results in this study have shown that the lability of coordinated chloride depends on the
electronic properties of the Pt(II) complexes, and the general reactivity trend for the
complexes is [PtL2Cl] > [PtL3Cl] > [PtL4Cl] > [PtL1Cl]. Changing the nature (electron-
withdrawing or electron-donating group) of the substituents on the phenyl ligand trans to
the leaving group in Pt(II) complexes affects the π-backbonding and the σ-trans effect of
the complex system and, thus, the reactivity of the complexes is influenced. Complexes
with fluoro substituents, [PtL2Cl] and [PtL3Cl], have higher reaction rates due to the
enhanced π-backbonding as compared to the parent complex, [PtL1Cl]. In [PtL4Cl], the
π-backbonding is weakened by the presence of an electron-donating group, but the σ-trans
effect is enhanced resulting in an increase in the reactivity. The results show that irrespec-
tive of the nature of the substituent on phenyl, the reactivity increases. It can therefore be
concluded that attaching a substituent (electron-donating or -withdrawing) trans to
the leaving group in a strong σ-trans system such as N^C^N results in an increase in the
substitution reaction. The reactivity of the nucleophiles is dependent on their steric effects.
The mode of activation remains associative for all the studied complexes.
Acknowledgements
The authors gratefully acknowledge financial support from the University of
KwaZulu-Natal and the National Research Funding. The authors are also indebted to Centre
for High Performance Computing (CHPC) cluster, Cape Town, South Africa for running
time consuming QST2 transition state DFT calculations and Dr I.M. Wekesa for doing the
calculation.
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