Role of chelate substituents and cis σ-effect on the rate of ligand substitution at Pt(N-N-N) and Pt(N-N-C) centres

Article abstract of DOI:10.1039/b311595j

Four complexes of the type [Pt(N-N-X)Cl] (X = N or C) were tailor synthesized for mechanistic studies in methanol. The terdentate ligands included terpy, 4′-Ph-terpy, 4′-(2?-CF₃-Ph)-terpy, and 4′-(2?-CF₃-Ph)-6-Ph-2,2′-bipy. The rate

Full text of DOI:10.1039/b311595j

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Role of chelate substituents and cis ꢀ-effect on the rate of ligand
substitution at Pt(N–N–N) and Pt(N–N–C) centres†
D. Jaganyi,*a D. Reddy,a J. A. Gertenbach,a A. Hofmannb and R. van Eldik*b
a School of Chemical and Physical Sciences, University of Natal, Pietermaritzburg 3209,
South Africa
b Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstr. 1,
91058 Erlangen, Germany
Received 22nd September 2003, Accepted 26th November 2003
First published as an Advance Article on the web 10th December 2003
Four complexes of the type [Pt(N–N–X)Cl] (X = N or C) were tailor synthesized for mechanistic studies in methanol.
The terdentate ligands included terpy, 4Ј-Ph-terpy, 4Ј-(2ٞ-CF3-Ph)-terpy, and 4Ј-(2ٞ-CF3-Ph)-6-Ph-2,2Ј-bipy. The rate
of substitution of the chloro ligand by thiourea, N,NЈ-dimethylthiourea, and N,N,NЈ,NЈ-tetramethylthiourea was
studied as a function of nucleophile concentration, temperature and pressure by using a stopped-flow technique. The
observed pseudo-first-order rate constants for the substitution reactions obeyed the simple rate law kobs = k2[Nu].
Second-order kinetics and negative activation entropies and volumes support an associative substitution mechanism.
At 298 K, the values of the second-order rate constant show a slight dependence on the nature of the moiety attached
to the terpy ligand. Changing from a nitrogen σ-donor to a carbon σ-donor in the cis position, results in a
deceleration of the substitution rate. The results suggest that the Pt–C bond in the cis position activates the metal
centre in a different way than in the trans position.
action in these reactions remains associative in nature.26–28
Introduction
These results demonstrate that the extent of electron density on
Kinetic and mechanistic studies involving the Pt–C bond
the metal centre plays a crucial role in dictating the nature of
remain to be of considerable interest. This is because of the
the substitution mechanism.
Whereas the influence of the trans-effect of a metal–carbon
ability of the metal–carbon bond to labilize square-planar d8
metal complexes through the kinetic trans effect.1–8 Interesting
bond on the rate of square-planar substitution reactions is very
results have been reported for substitution reactions of Pt()
significant, little is known about its cis-effect, and experimental
complexes containing metal–carbon bonds. All the studied
complexes with a single Pt–C bond had this bond located trans
to the leaving group. In general, these studies reported a very
results in the literature relating to this effect are contra-
dictory.29–35 This is due to the fact that the reactivity of Pt()
complexes is affected differently when σ-donor, π-acceptor and
steric properties of the cis ligand are varied. Despite these con-
high substitution reactivity, with an increase in rate of up to six
orders of magnitude.9–15 This labilization is ascribed to the
tradicting results, the general conclusion presented in most
σ-bound carbon ligands such as alkyl and aryl groups, that have
inorganic chemistry textbooks, is that the electronic effect of cis
a large kinetic trans effect, plus back-bonding in case of the
ligands is usually small when compared to that of the trans
in-plane aryl ligand.15–17 Mechanistically, the acceleration of the
ligands in Pt() complexes.36 Because of this, most of the
substitution process is linked to ground state destabilization
studies involving the Pt–C bond have centred on labilization by
due to the strong σ-electron donation along the C–Pt–X axis
ligands trans to the leaving group. A recent paper by Romeo
and stabilization of the transition state. Such tuning of the
and co-workers37 reported a study on the effect of cyclo-
reactivity of Pt() complexes through electronic effects has
metalation, where the metal–carbon bond was located in the cis
made it possible to bring about a “Pd()-type” of reactivity in
position. From a comparison with literature data, they were
able to conclude that the presence of a strong σ-donor carbon
group decelerated the rate of ligand substitution due to the
accumulation of electron density at the metal centre, making it
less electrophilic.
Apart from the introduction of Pt–C bonds, it has also been
these Pt() complexes.12,13,18–20 All in all, the reaction pathway
remains associative in nature.
Turning to substitution reactions of Pt() complexes that
have two carbon donors, these complexes mostly include two cis
σ-bound carbon and two sulfur donor atoms in non-aqueous
solvents. The most attractive property of these complexes is
that displacement of the sulfur ligands occurs via a dissoci-
shown that substituents on the terpy ligand also affect the
substitution rates of the corresponding Pt() complexes.38 It is
atively activated pathway. This is promoted by bond weakening
therefore our aim in the present paper to shed more light on the
of the leaving group due to the trans influence of the strong
role of the cis Pt–C bond and the influence that a substituted
Pt–C σ-donors and stabilization of the three-coordinate (14-
phenyl group, attached to the ancillary ligand (trans to the
electron) intermediate.8,21–23 The factors that favour a dissoci-
leaving group), may have on the metal centre and the leaving
ative pathway do not apply when carbon σ-donors that exhibit
group. To achieve this, terdentate Pt() complexes of the type
major π-acceptor properties, such as CO, are present. This is
evident from the substitution behaviour of [Pt(Ph)2(CO)(Et2S)],
[Pt(N–N–N)] or [Pt(N–N–C)] with varying substituents on the
central pyridine moiety were selected and their substitution
where the carbonyl group reduces the excess electron density on
behaviour was studied in detail.
the metal centre and forces the reaction to follow an associative
pathway.24,25 Substitution reactions of complexes involving four
metal–carbon bonds provided data for solvent exchange reac-
Experimental
tions of tetra-solvento square-planar complexes. The mode of
Materials and procedures
† Electronic supplementary information (ESI) available: A summary of
the selected wavelengths used in the kinetic measurements, all measured
rate constants and all the representative plots. See http://www.rsc.org/
suppdata/dt/b3/b311595j/
All reactions involving the synthesis of ligands were carried out
in air, whereas coordination of the ligands to Pt() was per-
formed under an inert atmosphere of nitrogen using standard
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 2 9 9 – 3 0 4
299

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Schlenk techniques. Absolute ethanol (Merck) and 2-acetyl-
pyridine (Aldrich) was used as supplied, whilst acetonitrile was
purified by the method of Carlson et al.39 Acetophenone
(Riedel de Haen), benzaldehyde and α,α,α-trifluoromethyl-
tolualdehyde(o) (Aldrich), were all purified by distillation prior
to use. Ammonium acetate was dried in a vacuum dessicator
over P2O5 for several days prior to use. Methanol was distilled
over magnesium before use. The metal salts K2PtCl4 (Strem),
[Pt(PhCN)Cl2] (Strem), lithium trifluoromethanesulfonate
(Aldrich), lithium perchlorate (BDH Chemicals) and AgSbF6
(Fluka AG) were used as supplied. The nucleophiles, thiourea
(TU), 1,3-dimethyl-2-thiourea (DMTU), and 1,1,3,3-tetra-
methyl-2-thiourea (TMTU), were obtained from Aldrich and
used as supplied. All other chemicals were of analytical reagent
quality.
[Pt{(CF3-Ph)Ph-bipy}Cl] (Pt4).
A
mixture of K2PtCl4
(100 mg; 0.24 mmol) and 4-(2ٞ-CF3-phenyl)-6-phenyl-2,2Ј-bi-
pyridine {(CF3-Ph)Ph-bipy} (90 mg; 0.24 mmol) was refluxed in
acetonitrile/water (10 ml; 1 : 1) for 48 h. The orange precipitate
was filtered and washed onto a glass frit with large amounts of
diethyl ether followed by smaller amounts of acetonitrile. The
resulting solid was extracted via the sohxlet method with di-
chloromethane and excess solvent removed under reduced pres-
sure. The precipitate was recrystallised by the slow diffusion of
diethyl ether into a dimethylformamide solution containing the
product. The solid product was recovered and dried in vacuo.
Yield 93 mg (64%). Molecular mass: 605.88 g molϪ1. Anal.
Calc. for C23H14ClF3N2Pt: C, 45.59; H, 2.33; N, 4.62%. Found:
C, 45.66; H, 2.03; N, 4.89%. UV/Vis (CH3CN): 305, 278sh, 259,
239 (π–π*). IR (KBr) (cmϪ1) 742 (m), 755 (m), 770 (m), 780 (m),
1036 (m), 1112 (s), 1128 (s), 1172 (s), 1313 (s), 1403 (m), 1417
(w), 1462 (w), 1478 (w), 1534 (m), 1584 (w), 1604 (m). 1H NMR‡
(DMSO-d6) 8.93(1H, d, H6Ј) 8.58(1H, d, H3Ј) 8.33(1H, t, H4Ј)
8.26(1H, s, H3) 7.98(1H, s, H5) 7.97(1H, d, H3ٞ) 7.94(1H, t, H5Ј)
7.88(1H, t, H5ЉЉ) 7.78(1H, t, H4ٞ) 7.70(1H, d, H6ٞ) 7.65(1H, d,
H3Љ) 7.52(1H, d, H6Љ) 7.17(1H, t, H5Љ) 7.06(1H, t, H4Љ).
The ligand 2,2Ј:6Ј,2Љ-terpyridine was obtained from Aldrich,
whereas 4Ј-phenyl-2,2Ј:6Ј,2Љ-terpyridine40–42 and 4Ј-(2ٞ-CF3-
phenyl)-2,2Ј:6Ј,2Љ-terpyridine43 were synthesized according
to published methods. This included the ligand precursors
[1-(2Ј-pyridyl)-3-(2Љ-CF3-phenyl)-prop-2-ene-1-one]43,44
and
N-{1-(2Ј-pyridyl)-1-oxo-2-ethyl}pyridinium iodide45 required
to synthesize 4Ј-(2ٞ-CF3-phenyl)-2,2Ј:6Ј,2Љ-terpyridine.
Physical measurements and instrumentation
1-phenyl-3-(2Љ-CF3-phenyl)-prop-2-en-1-one. A suspension of
α,α,α-trifluoromethyltolualdehyde (40 mmol) in absolute
ethanol (100 ml) was cooled to 0 ЊC. Acetophenone (4.8 g,
40 mmol) was diluted in ethanol (10 ml) and then added to the
cooled mixture. After stirring for 2 min at 0 ЊC, sodium
hydroxide (40 ml, 1.0 M) was added drop-wise and the mix-
ture was stirred for a further 3 h at 0 ЊC. Once stirring was
complete, the product was precipitated from the solution by
the addition of large quantities of ice water. The pale yellow
product was filtered and thoroughly washed on the frit with
50% aqueous ethanol. The 1-phenyl-3-(2Љ-CF3-phenyl)-prop-2-
en-1-one was isolated as a pale yellow powder and dried
in vacuo.
Elemental analyses were performed by the microanalytical
laboratory at the University of Natal (Pietermaritzburg) and by
Galbraith Laboratories Inc., Knoxville, TN, USA. Infrared
spectra were recorded as KBr discs on either Shimadzu FTIR-
1
4300 or Perkin Elmer Spectrum One spectrometers. H NMR
(200 MHz) were recorded on a Varian Gemini 200 spectrometer
at 25 ЊC with chemical shifts referenced to Si(CH3)4. Mass
spectra were obtained on a Hewlett Packard GC-MS using elec-
tron impact (EI) ionization. UV/Visible absorption spectra
were recorded using either Varian Cary 100 Bio or Varian Cary
50 Probe spectrophotometers.
Kinetics. All kinetic measurements were performed under
pseudo-first-order conditions using an Applied Photophysics
SX.18MV (v4.33) stopped-flow system coupled to an online
data acquisition system. High-pressure measurements were per-
formed on a laboratory-made high-pressure stopped-flow
instrument.48 These instruments were thermostatted to within
0.1 ЊC. The pseudo-first-order conditions were achieved by
having the nucleophile in at least 10-fold excess. The ionic
strength of the solution was maintained at 0.1 M using LiClO4
or LiCF3SO3.
Yield 9.13 g (94%), m.p. 53–55 ЊC. GC/MS: m/z 243 (Mϩ). IR
(KBr) ν(CO) 1664 cmϪ1. 1H NMR (CDCl3) δ 7.84(1H, AB, 3JHH
15.7 Hz, CHCO) 7.73(1H, AB, 3JHH 15.7 Hz, CHAr).
4-(2ٞ-CF3-phenyl)-6-phenyl-2,2Ј-bipyridine. The ligand pre-
cursors
1-phenyl-3-(2Љ-CF3-phenyl)-prop-2-en-1-one
(2.0
mmol) and N-{1-(2Ј-pyridyl)-1-oxo-2-ethyl}pyridinium iodide
(0.72 g, 2.2 mmol) were added to a 100 ml round-bottomed
flask. Ammonium acetate (10 g, excess) and absolute ethanol
(8.0 ml) were added and the mixture heated to reflux for 60 min
and thereafter allowed to cool. The ligand precipitated from the
ambient reaction mixture upon cooling as a dirty white solid,
which was filtered off and washed onto a glass frit with 50%
aqueous ethanol. The impure ligand was recrystallised from
ethanol (95%) to give a pale yellow powder.
Yield 0.52 g (70%), m.p. 129–130 ЊC. Anal. Calc. for
C26H18N2: C, 73.4; H, 4.0; N, 7.4%. Found: C, 73.4; H, 4.1; N,
7.4%. GC/MS.: m/z 376 (Mϩ). UV/Vis (CH3CN): 305, 278sh,
259, 239 (π–π*). IR (KBr) (cmϪ1): 626(w) 651(m) 688(m)
740(m) 774(m) 796(w) 1035(m) 1129(s) 1171(s) 1398(m)
1453(w) 1474(w) 1494(w) 1546(m) 1570(w) 1586(m) 1600(w).
1H NMR (CDCl3): 8.71(1H, d, H6Ј) 8.68(1H, d, H3Ј) 8.40(1H, s,
H3) 8.18(2H, m, H2Љ, 6Љ) 7.84(1H, t, H4Ј) 7.82(1H, m, H6ٞ)
7.78(1H, s, H5) 7.63(1H, d, H3ٞ) 7.61(2H, m, H4ٞ, 5ٞ) 7.50(2H, m,
H3Љ, 5Љ) 7.46(1H, m, H4Љ) 7.33(1H, m, H5Ј).
Results
Substitution of coordinated chloride (reaction 1) from each
of the four Pt() complexes (Chart 1) by three different
nucleophiles (Nu), i.e. thiourea (TU), 1,3-dimethyl-2-thiourea
(DMTU), and 1,1,3,3-tetramethyl-2-thiourea (TMTU), was
investigated under pseudo-first-order conditions using con-
ventional stopped-flow techniques. A typical kinetic trace
recorded by mixing solutions of Pt4 (5.285 × 10Ϫ5 M) and TU
(1.664 × 10Ϫ3 M) in the stopped-flow instrument at an ionic
strength of 0.1 M (LiClO4), is shown in Fig. 1.
The pseudo-first-order rate constants, kobs, calculated from
the kinetic traces were plotted against the concentration of the
Synthesis of complexes
The complex [Pt(terpy)Cl]Cl.2H2O (Pt1) was prepared accord-
(1)
ing to earlier literature methods.46 [Pt(4Ј-phenyl-2,2Ј:6Ј,2Љ-
terpyridine)Cl]ϩ
(Pt2)41,42
and
[Pt{4Ј-(2ٞ-CF3-phenyl)-
2,2Ј:6Ј,2Љ-terpyridine}Cl]ϩ (Pt3)43,44 were also synthesized fol-
lowing literature procedures. The method of Büchner et al.47
was used to coordinate the ligands to Pt().
1
‡ The H NMR spectrum is assumed to be that of the chloro complex
since the substitution of ClϪ by DMSO was found to be slow.
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Table 1 Summary of rate constants for the substitution of chloride by
TU, DMTU and TMTU in methanol, I=0.1 M (Pt1, Pt2 – LiCF3SO3;
Pt3, Pt4 – LiClO4), T =298 K.
k2, MϪ1sϪ1
Nucleophile
Pt1
Pt2
Pt3
Pt4
TU
DMTU
TMTU
1494 10
448 10
1258 13
1933 32
123.0 0.3
72.0 0.6
29.0 0.5
377
100
7
3
662
144
3
1
82
4
Fig. 2 Dependence of the pseudo-first-order rate constants (kobs) on
the entering nucleophile concentration for chloride substitution on Pt4
(5.28 × 10Ϫ5 M) in methanol, I=0.1 M (LiClO4), T =298.15 K.
Pressure effects were studied for Pt1, Pt2 and Pt4 using TU
and DMTU as the entering nucleophiles. Kinetic data in the
pressure range 10–130 MPa was used to plot ln kobs against
pressure. Since the concentration dependence plots were linear
without any intercept, the pressure dependence of kobs is that of
k2. Good linear relationships, as shown in a representative plot
in Fig. 3, were obtained, showing that the reactions are acceler-
ated by pressure, which is typical for an associative substitution
mechanism. This is in agreement with the negative activation
entropy values obtained from the temperature dependence
studies. The volumes of activation, ∆V≠, calculated from the
data in Fig. 3 are summarized along with the activation
enthalpy and entropy values in Table 2.
Chart 1 Structural formulas of the investigated complexes.
Fig. 1 Fit of a single exponential and residuals (lower part) for the
reaction of Pt4 (5.285 × 10Ϫ5 M) with thiourea (1.664 × 10Ϫ3 M)
followed at 348 nm, I=0.1 M (LiClO4), T =298.15 K.
Fig.
3
Determination of the activation volume, ∆V≠, for the
substitution of chloride in Pt4 by TU and DMTU in methanol.
[Pt4]=ca. 0.10 mM, [TU]=3.3 mM, [DMTU]=3.4 mM, I=0.1 M
(LiClO4), T =298 K.
incoming nucleophiles. The values used represented an average
of eight to twelve independent runs. Straight lines with zero
intercepts were obtained for each of the nucleophiles, suggest-
ing that the mechanism of the substitution can be represented
by reaction (1)and the corresponding rate law given by eqn. (2)
Discussion
Four complexes, [Pt(terpy)Cl]Clؒ2H2O (Pt1), [Pt(4Ј-Ph-terpy)-
Cl](SbF6) (Pt2), [Pt{4Ј-(2ٞ-CF3-Ph)-terpy}Cl](SbF6) (Pt3) and
[Pt{(4Ј-(2ٞ-CF3-Ph)-6-Ph-2,2Ј-bipy}Cl] (Pt4) were synthesized
by established procedures. The method of Kröhnke45 was used
to synthesize the ligands Ph-terpy, (CF3-Ph)-terpy and (CF3-
Ph)Ph-bipy, details for the first two were published before.41,43,44
Details of the synthesis and characterisation of the (CF3-
Ph)Ph-bipy ligand are given in the Experimental section. These
complexes were fully characterized analytically, chemically, and
spectroscopically by IR, GC-MS and 1H NMR. The 1H NMR
spectrum was recorded in deuterated DMSO to confirm the loss
of proton signal brought about by deprotonation upon ortho-
metallation to the Pt() metal. The spectral assignments for
[Pt{(4Ј-(2ٞ-CF3-Ph)-6-Ph-2,2Ј-bipy}Cl] (Pt4) were made with
kobs = k2[Nu]
(2)
Representative plots shown in Fig. 2 clearly indicate that the
substitution reactions were first-order in the incoming nucleo-
phile. The values of the second-order rate constants k2, which
results from the direct attack of the nucleophile as shown in
reaction (1), were obtained from the slopes of these plots at
25 ЊC and are summarized in Table 1.
The temperature dependence of k2, was studied over the
range 15 to 35 ЊC. The activation parameters, ∆H≠ and ∆S≠,
were calculated using the Eyring equation and are tabulated in
Table 2.
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Table 2 Summary of activation parameters for the substitution of chloride by TU, DMTU and TMTU in methanol, I=0.1 M (Pt1, Pt2 –
LiCF3SO3; Pt3, Pt4 – LiClO4).
Complex
Nucleophile
∆H≠/kJ molϪ1
∆S≠/J KϪ1 molϪ1
∆V≠/cm3 molϪ1
Pt1
TU
DMTU
TMTU
28.7 1.5
36.4 1.1
Ϫ88
Ϫ73
Ϫ91
5
4
8
Ϫ6.2 1.1
Ϫ5.9 0.2
35
2
Pt2
Pt3
Pt4
TU
DMTU
TMTU
30.3 0.2
38.3 1.4
44.4 1.5
Ϫ84.4 0.6
Ϫ7.7 0.8
Ϫ5.7 0.2
Ϫ68
Ϫ58
4
5
TU
DMTU
TMTU
30.0 0.6
Ϫ82
Ϫ74
Ϫ98
2
8
3
35
2
31.5 0.8
TU
DMTU
TMTU
34.6 0.5
38.1 1.2
Ϫ89
Ϫ82
2
4
Ϫ9.5 0.3
Ϫ8.2 0.1
29
5
Ϫ119 18
Table 3 Summary of selected bond lengths and angles relevant to the investigated complexes as obtained from the literature
a
b
c
d
[Pt1] Pt-
[Pt2] [Pt{4Ј-Ph--
[Pt3] [Pt{4Ј-(CF3-Ph)-
[Pt4] [Pt{4-(CF3-Ph)-
Complexes
(terpy)Cl]ϩ
terpy}Cl]ϩ
terpy}Cl]ϩ
6-C6H4-bipy}Cl]
Bond lengths/Å
Pt–Cl
Pt–N1
Pt–N3
Pt–N2
2.307(2)
2.018(5)
2.030(5)
1.930(4)
—
2.313(2)
2.039(9)
2.013(10)
1.936(9)
—
2.286(3)
2.004(10)
2.014(10)
1.931(8)
—
2.305(2)
2.038(6)
—
1.948(5)
2.057(5)
Pt–C (trans to N1)
Bond angles/Њ
N1–Pt–N3
N1–Pt–C (trans to N1)
N2–Pt–Cl
N1–Pt–N2
N2–Pt–N3
161.8(2)
—
178.9(1)
81.1(2)
80.8(2)
—
161.7(4)
—
179.8(2)
81.0(4)
80.7(4)
—
162.3(3)
—
179.9(3)
81.1(4)
81.2(4)
—
—
161.4(2)
177.4(1)
81.3(2)
—
N2–Pt–C (trans to N1)
80.1(2)
Ϫ
a [Pt(terpy)Cl]ϩ CF3SO3Ϫ; ref. 50. b [Pt{4Ј-Ph-terpy}Cl]ϩ BF4 ؒCH3CN; two cations per unit cell: data given are for the second cation only, as per
ref. 41. c [Pt{4Ј-(CF3-Ph)-terpy}Cl]ϩ SbF6Ϫ; ref. 43. d Ref. 44.
the aid of absolute value correlation spectroscopy (COSY)
experiments and by comparison with the spectrum recorded for
the very similar [Pt{6-(2-phenyl)-2,2Ј-bipyridine}Cl] reported
by Constable and Cargill Thompson.49 All analyses are in
agreement with the structural formulas shown in Chart 1.
The synthesized complexes made it possible to quantify the
effect of a σ-donor in the cis position relative to the leaving
group by performing a comparative study on Pt3 and Pt4.
Complexes Pt1 and Pt2 were selected to establish the role (both
electronic and steric) of the phenyl substituent in the 4Ј position
of the terpyridyl fragment in Pt2. This was extended by the
introduction of a CF3 group in the ortho position of the
4Ј-substituted phenyl ring of Pt3, and by the insertion of the
carbon atom in the cis position of Pt4.
The substitution reactions of the investigated complexes can
be interpreted in terms of the usual two-term rate-law in keep-
ing with the associative mode of activation generally found for
square planar complexes. This is supported by the activation
parameters: the activation entropies are all large and negative,
the activation enthalpies are relatively low, and the activation
volumes are all negative. The moderate difference in the rate
constants reflects the electronic role played by the nature of the
ancillary ligands.
lengths and angles) extracted from the literature and summar-
ized in Table 3.
The molecular structures of these complexes show similarity
in terms of planarity, bond lengths and angles. The co-
ordination geometry of the Pt() centre in all the complexes is
slightly distorted square planar with an angle of 161Њ deviating
from linearity. The 4Ј-phenyl group lies out-of-plane with
respect to the rest of the central pyridyl moiety of the ligand.
There is a twist about the interannular bond such that the plane
of the phenyl ring is at an angle of 33.3Њ with respect to the
plane of the central ring for Pt2. In the cases of Pt3 and Pt4 the
angles are 66.5 and 65.4Њ, respectively. This out-of-plane distor-
tion rules out any possible π interaction between the 4Ј-phenyl
ring and the terpyridyl or bipyridyl fragment and any extension
of π back-bonding, i.e., π electronic communication between
the two is not possible. The proximity of the phenyl ring to the
metal centre also eliminates any possible steric hindrance for
the entering nucleophile. The effectiveness of this would have
been larger if there was direct interaction between the phenyl
ring and the metal atom as in the case of substitution reactions
involving [Pt(bipy)(Ph)Cl]37 and trans-[Pt(PEt3)2(R)Cl].51
In the present case the moderate difference in the rate of
substitution on all the complexes is purely an electronic effect.
The phenyl system in Pt2 is a better σ-donor than that of Pt3
due the presence of the CF3-moiety, which withdraws electrons
from the ring. Therefore, the electron donating phenyl ring
reduces the π-acceptor abilities of the terpy ligand in the case of
Pt2, whereas the electron withdrawing CF3 substituted phenyl
ring increases them in the case of Pt3. The net effect is that π
back-bonding of the metal orbitals with the terpy ligand is
In comparing the reactivity of the complexes, it is clear from
the data in Table 1 that the reactivity of Pt1 is higher (factor of
1.2) than for Pt2, but lower (factor of 1.3) than for Pt3. The
differences are only moderate and can be accounted for in terms
of steric hindrance and σ-donicity of the phenyl substituent on
the terpyridyl fragment. To separate these two factors, it is
necessary to analyse the selected crystallographic data (bond
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slightly weaker in Pt2 than in Pt1, whereas it is more effective in
Pt3. Here the interactions between dxz and dyz and the empty/
anti-bonding π orbitals play a minor role; the main interaction
responsible for the enhanced reactivity is between the empty
pz orbital, which is filled with electrons in the transition state,
and the π-acceptor orbitals on the ligand.30 This means that
stabilization of the transition state through π back-bonding and
creation of a more electrophilic metal centre, promotes nucleo-
philic approach to the reaction centre and enhances the form-
ation of a new bond in Pt3. The retardation effect observed
between Pt1 and Pt2 has previously been noted when a
methoxy (electron donating) group was attached to the ancil-
lary ligand.5
compared for the four complexes (for TU : DMTU : TMTU:
Pt1 18 : 5.5 : 1; Pt2 13 : 3.8 : 1; Pt3 13 : 4.6 : 1; Pt4 4.2 : 2.5 : 1), it
can clearly be seen that the reactions with the Pt4 complex are
less sensitive to steric hindrance of the incoming ligand. This
can easily be explained by the relatively large Pt–C distance
(2.057(5) Å for Pt4) that is due to the large trans influence of the
carbon atom in comparison with the smaller Pt–N3 distance
(2.030(5), 2.013(10), and 2.014(10) Å, for Pt1, Pt2 and Pt3,
respectively) in the other complexes. This reduces the steric
interactions of the entering nucleophile with the cis groups of
the terpy moiety, resulting in a complex that has a reduced
steric discrimination in comparison with Pt1, Pt2 and Pt3.
To pin down the role of the cis σ-effect, the reactivities of Pt3
and Pt4 need to be compared. The data in Table 2 suggest that
the introduction of the Pt–C bond in the cis position to the
chloride atom reduces its lability by a factor of 16 when the
entering nucleophile is TU, with similar retardation on the rates
of substitution being observed for the other entering nucleo-
philes. This decrease in reactivity is due to the accumulation of
electron density at the metal centre, making it less electrophilic
than the analogous Pt3 complex. In doing so, the rate of substi-
tution decreases, since the incoming nucleophile is repelled by
the increase in electron density around the metal centre. The
cis effect of the carbon atom reported in this study has been
shown to decrease the lability of complexes whose reactivity is
enhanced by the presence of strong π-electron withdrawing
backbone.37,52
It is worth noting that these findings indicate that the cis and
trans carbon σ-donors influence the lability of the leaving group
in a different way. When positioned trans to the X group (C–Pt–
X), the well-known trans effect53 takes control of the reaction
as a result of the ground state labilization (trans influence) and
transition state stabilization (trans effect).5,13,19 This usually
involves the elongation of the Pt–X bond13,54,55 and corre-
sponding shortening of the aryl-metal bond,56 which if present
in the same plane, promotes π back-bonding. The net effect of
these two factors is an acceleration of the rate of substitution.
The retardation observed when Pt–C is in the cis position is due
to the accumulation of electron density around the metal centre
as already mentioned. The fact that this seems to have a less
significant effect on the Pt–X bond length is probably due to
lack of direct overlap of the orbitals of the two atoms cis to
each other, unlike in the case when they are trans to each other.
This means that while the Pt–C trans effect increases the ground
state destabilization of Pt() complexes, the Pt–C cis effect has
little if any of this character.52
Conclusions
The lability of coordinated chloride in the studied complexes is
controlled by the electrophilicity of the metal centre. This in
turn depends on the properties of the terpyridyl fragment. It is
clear that the introduction of substituents on the ancillary
terpyridyl ligand dictates the extent of π-electron withdrawal
through back-bonding effects. The presence of electron
donating substituents (phenyl ring in Pt2) on the terpy frag-
ment decreases the rate of substitution, whereas the addition of
an electron withdrawing (o-CF3-phenyl in Pt3) entity has the
opposite effect. The most significant feature of this work is the
elucidation of the role the cis Pt–C bond has on the lability of
the leaving group. The cis and trans σ-effects have an opposite
net effect. While the latter enhances the rate of substitution
through ground-state destabilization, the former decelerates
the process through the accumulation of electron density at the
metal centre. This does not only prevent the approach of the
nucleophile, but also suppresses the stabilization of the transi-
tion state. The mode of activation remains associative in nature
throughout the studied systems.
Acknowledgements
The authors gratefully acknowledge financial support from the
University of Natal, the South African National Research
Foundation and the Deutsche Forschungsgemeinschaft. We are
also grateful to the Alexander von Humboldt Foundation
for the donation of the UV-Vis spectrophotometer to the
University of Natal.
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