Controlling the extent of π-backbonding in platinum(ii) terpyridyl systems: A detailed kinetic, mechanistic and computational approach

Article abstract of DOI:10.1039/b809697j

The rate of substitution of the chloride ligand from [Pt(terpy)Cl] ⁺ (Pt1) (where terpy = 2,2′:6′,2″-terpyridine) and its corresponding analogue [Pt(^tBu₃terpy)Cl]⁺ (Pt2) (where ^tBu₃terpy = 4,4′,4″-tri-tert- butyl-2,2′:6′,2″-terpyridine) by a series of neutral and anionic nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I^-) and thiocyanate (SCN^-), was determined under pseudo first-order conditions as a function of concentration and temperature using standard stopped-flow spectrophotometric techniques. The observed pseudo first-order rate constants for the substitution reactions obeyed the simple rate law k_obs = k₂[nucleophile]. Second-order kinetics and negative activation entropies support an associative mode of activation. The rate of substitution of chloride is observed to decrease with an increase in the steric bulk of the neutral nucleophiles, whilst rate of substitution by I^- was observed to be faster than that by SCN^-, in correlation with their polarizability and the softness of the metal centre. A comparison of the second-order rate constants, k₂, at 298 K, obtained for the substitution reactions of Pt1 and Pt2 shows that the introduction of strong σ-donating groups on the periphery of the terpyridyl backbone in Pt2 results in a corresponding decrease in the reactivity. DFT calculations at the B3LYP/LACVP** level of theory for the two complexes, Pt1 and Pt2, and a series of similar analogues containing either electron-donating or electron-withdrawing groups in the periphery positions demonstrate that the introduction of electron-donating groups decreases the positive charge on the metal centre and increases energy separation of the frontier molecular orbitals (E_HOMO-E_LUMO) of the ground state platinum(ii) complexes leading to a less reactive metal centre whilst the introduction of electron-withdrawing groups has an opposite effect leading to increased reactivity of the metal centre.

Full text of DOI:10.1039/b809697j

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PAPER
www.rsc.org/dalton | Dalton Transactions
Controlling the extent of p-backbonding in platinum(II) terpyridyl systems:
a detailed kinetic, mechanistic and computational approach†
D. Reddy and D. Jaganyi*
Received 9th June 2008, Accepted 18th August 2008
First published as an Advance Article on the web 15th October 2008
DOI: 10.1039/b809697j
The rate of substitution of the chloride ligand from [Pt(terpy)Cl]+ (Pt1) (where terpy =
2,2¢:6¢,2¢¢-terpyridine) and its corresponding analogue [Pt(tBu3terpy)Cl]+ (Pt2) (where tBu3terpy =
4,4¢,4¢¢-tri-tert-butyl-2,2¢:6¢,2¢¢-terpyridine) by a series of neutral and anionic nucleophiles, viz. thiourea
(TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I-) and
thiocyanate (SCN-), was determined under pseudo first-order conditions as a function of concentration
and temperature using standard stopped-flow spectrophotometric techniques. The observed pseudo
first-order rate constants for the substitution reactions obeyed the simple rate law kobs = k2[nucleophile].
Second-order kinetics and negative activation entropies support an associative mode of activation. The
rate of substitution of chloride is observed to decrease with an increase in the steric bulk of the neutral
nucleophiles, whilst rate of substitution by I- was observed to be faster than that by SCN-, in
correlation with their polarizability and the softness of the metal centre. A comparison of the
second-order rate constants, k2, at 298 K, obtained for the substitution reactions of Pt1 and Pt2 shows
that the introduction of strong s-donating groups on the periphery of the terpyridyl backbone in Pt2
results in a corresponding decrease in the reactivity. DFT calculations at the B3LYP/LACVP** level of
theory for the two complexes, Pt1 and Pt2, and a series of similar analogues containing either
electron-donating or electron-withdrawing groups in the periphery positions demonstrate that the
introduction of electron-donating groups decreases the positive charge on the metal centre and
increases energy separation of the frontier molecular orbitals (EHOMO - ELUMO ) of the ground state
platinum(II) complexes leading to a less reactive metal centre whilst the introduction of electron-
withdrawing groups has an opposite effect leading to increased reactivity of the metal centre.
In recent years, mechanistic investigations into the substitution
processes at square-planar Pt(II) centres have focused on tuning
Introduction
the reactivity of the metal centre.15–18 These studies have all
demonstrated the influence of the ligand structure about the metal
centre on the substitution behaviour of platinum complexes.
Van Eldik et al.17,18 systematically studied the effect of pro-
gressively adding p-acceptor ligands (viz. pyridine ligands) to the
chelating ligands in a series of platinum(II)-aqua complexes of
the type [Pt(L)(OH2)]2+ (where L = a tridentate N-donor chelate
or NH3/bipyridine). The authors found that the reactivity of the
associative substitution reaction on Pt(II) can be increased in a
stepwise manner by the addition of the p-acceptor ligands, with re-
activity ratio of the complexes with increasing p-acceptor ligands,
viz. [Pt(aaa)(OH2)]2+: [Pt(apa)(OH2)]2+: [Pt(ppp)(OH2)]2+ (where
aaa = diethylenetriamine; apa = 2,6-bis-aminomethylpyridine and
ppp = 2,2¢:6¢,2¢¢-terpyridine) being approximately 1 : 3.5 : 5600, in
the case of substitution of the aqua moiety by thiourea. The experi-
mental data also suggested that the p-cis effect was greater than the
p-trans effect. The authors concluded that the reactivity of these
Pt(II) complexes resulted from the electrophilicity of the metal
centre, as a direct result of electron back-donation of incoming
electron density to the ligand orbitals, which is further enhanced
by electronic communication between p-acceptor ligands.
Mechanistic investigations into ligand substitutions at square-
planar centres of d8 transition metal ions such as platinum(II)
and palladium(II), have been extensively reviewed and remain
among the most investigated and understood dynamic processes in
chemistry.1–7 The bulk of information relating to four-coordinate
planar substitution mechanisms comes from the reactions of Pt(II)
substrates, where a combination of redox stability and relatively
low reactivity allow for the synthesis of specifically designed
substrates and subsequent kinetic studies of these.2–5 In addition,
substitution reactions of square-planar Pt(II) complexes continue
to attract attention due to their applications in homogenous
catalysis8 and anti-tumour treatment.9,10
The substantial amount of kinetic and thermodynamic data
collected in the last few decades clearly demonstrate that the
reaction mechanisms of square-planar complexes is associative
in nearly all cases,2–5 with only a limited number of complexes
exhibiting a mechanistic changeover under specific conditions.11–14
School of Chemistry, University of KwaZulu-Natal, Private Bag X01,
Scottsville, Pietermaritzburg, 3209, South Africa
† Electronic supplementary information (ESI) available: Structures of
complexes and comparative kinetic data extracted from the literature as
described in the introduction; summary of selected wavelengths used in the
kinetic investigations; all measured rate constants and all representative
plots pertaining to this study. See DOI: 10.1039/b809697j
Density functional theory (DFT) calculations showed that the
addition of p-acceptor ligands resulted in an increase in the
positive charge on the metal and a corresponding decrease in
6724 | Dalton Trans., 2008, 6724–6731
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the energy separation of the frontier molecular orbitals (ELUMO
HOMO) of the ground state Pt(II) complexes.17
Lowe et al.19 demonstrated that the presence of a substituent
-
sodium iodide (NaI, 99%) was obtained from Saarchem and used
as supplied. Ultrapure water (Modulab Systems) was used for all
aqueous reactions. All other chemicals were of analytical reagent
quality.
The platinum(II) precursor, dichloro(1,5-cyclooctadiene)plati-
num(II) (1)21 and the corresponding platinum(II) terpyridyl com-
plexes, [Pt(terpy)Cl]Cl·2H2O (Pt1)22 and its corresponding ana-
logue [Pt(tBu3terpy)Cl]ClO4 (Pt2),23 were synthesized via known
literature procedures.
E
at the 4¢-position of the terpyridyl system alters the electronic
properties of the terpyridyl ligand. By studying the rates of hy-
drolysis of five (4¢-substituted-2,2¢:6¢,2¢¢-terpyridine)platinum(II)
(n-propylamine) complexes and subsequent analysis using the
Hammett equation, the authors found that in the case of electron-
donating groups they obtain a positive r-value. This led them to
conclude that the presence of the substituent at the 4¢-position
leads to a build-up of electron density in the transition state thus
rendering the metal centre less electrophilic. The authors also
suggested that this effect can be reduced by the introduction of
electron-withdrawing groups.
Indeed, work in our laboratories has also demonstrated that the
introduction of different ortho-substituted phenyl groups at the 4¢-
position of the terpyridyl backbone can moderately influence the
reactivity of the platinum(II) centre.15 In the case of substitution
of chloride by the neutral nucleophiles thiourea, 1,3-dimethyl-
2-thiourea and 1,1,3,3-tetramethyl-2-thiourea, we find that the
reactivity of [Pt{4¢-(phenyl)terpy}Cl]+ is slightly less than that
of the parent [Pt(terpy)Cl]+ due to their presence of the weak,
electron-donating phenyl group. However, the reactivity of [Pt{4¢-
(2¢¢¢-CF3-phenyl)terpy}Cl]+ is slightly greater than both [Pt{4¢-
(phenyl)terpy}Cl]+ and [Pt(terpy)Cl]+ due to the presence and
electron-withdrawing nature of the ortho-CF3-phenyl substituent
in the 4¢-position.
Synthesis of the platinum(II) terpyridyl complexes
Dichloro(1,5-cyclooctadiene)platinum(II) (1). To
a filtered,
aqueous solution of potassium tetrachloroplatinate (K2PtCl4)
(2.18 mg, 5.23 mmol) in water (55 ml), was added glacial acetic
acid (35 ml) and 1,5-cyclooctadiene (COD) (2.1 ml, 17 mmol).
The resultant mixture was heated, with vigorous stirring, at 90 ◦C
for approximately 30 min. During this period a colour change
from the initial deep-red solution to a pale-yellow solution was
observed. The product, dichloro(1,5-cyclooctadiene)platinum(II)
(1), precipitated as fine pale-yellow crystals before the 30 min had
elapsed. After 30 min, further precipitation of the product was
induced by concentrating the solution under reduced pressure
to a final volume of approximately 10 ml. The pale-yellow,
crystalline product was collected by filtration and washed with
successive 50 ml portions of water, ethanol and diethyl ether and
thereafter oven-dried at 100 ◦C for 1 h to afford the title complex,
dichloro(1,5-cyclooctadiene)platinum(II) (1) of sufficiently high
purity.
In keeping with this trend, this work describes, by means
of a kinetic and computational investigation, the possibility of
controlling the extent of p-backbonding by the simple introduction
of either electron-donating or electron-withdrawing groups at the
4, 4¢ and 4¢¢ positions of the terpyridyl backbone.
Yield: 1.75 g (88%). Anal. calc. for PtC8H12Cl2: C, 25.7; H, 3.2;
N, 0.0. Found: C, 26.2; H, 3.5; N, 0.0.
[Pt(terpy)Cl]Cl·2H2O (Pt1). To
a stirred suspension of
[Pt(COD)Cl2] (1) (500 mg, 1.34 mmol) in water (30 ml), was added
2,2¢:6¢,2¢¢-terpyridine (313 mg, 1.34 mmol). The resultant mixture
was warmed to ca. 45–50 ◦C with stirring. After approximately
15 min, the suspended metal complex completely dissolved to
afford a clear, orange–red solution which was cooled to room
temperature and filtered to remove any unreacted [Pt(COD)Cl2].
The filtrate was then evaporated under reduced pressure to
afford an orange–red solid, [Pt(terpy)Cl]Cl·2H2O (Pt1), which was
collected by filtration, washed thoroughly with diethyl ether and
air-dried.
Yield: 0.65 g (90%). Anal. calc. for PtCl2O2C15H15N3: C, 33.7; H,
2.8; N, 7.9. Found: C, 33.5; H, 2.7; N, 7.7. IR (KBr) cm-1: 465(m),
519(m), 722(m), 781(vs), 1029(m), 1091(w), 1245(sh), 1247(mb),
1284(m), 1316(ms), 1400(s), 1453(s), 1481(s), 1605(s), 3047(mb),
3409(sb).
Experimental
Materials & methodology
Reactions used in the synthesis of the platinum(II) terpyridyl
complexes were all carried out in air unless otherwise stated.
Potassium tetrachloroplatinate (K2PtCl4, 99.99%), 2,2¢:6¢,2¢¢-ter-
pyridine (terpy, 98%), 4,4¢,4¢¢-tri-tert-butyl-2,2¢:6¢,2¢¢-terpyridine
(tBu3terpy, 97%), sodium perchlorate monohydrate‡ (NaClO4·
H2O, 98%), lithium trifluoromethanesulfonate (LiCF3SO3, 96%),
1,5-cyclooctadiene (COD, 99%) and acetonitrile (CH3CN,
≥99.9%) were obtained from Aldrich and were used without fur-
ther purification. Silver perchlorate‡ (AgClO4, 99.998%, Aldrich)
was stored under nitrogen and used as supplied. Perchloric acid
(HClO4, 70% solution) and hydrochloric acid (HCl, 32% solution)
was obtained from Saarchem and used as supplied. Methanol
(Saarchem) was distilled over magnesium before use.20
[Pt(tBu3terpy)Cl]ClO4 (Pt2). K2PtCl4 (415 mg, 1.00 mmol)
was dissolved in a solution of 50% aqueous acetonitrile (30 ml)
and the resulting solution filtered to remove any metallic platinum.
To the stirred platinate solution was added 4,4¢,4¢¢-tri-tert-butyl-
2,2¢:6¢,2¢¢-terpyridine (402 mg, 1.00 mmol). The resulting mixture
was heated to reflux for 24 h. The resulting yellow solution was
filtered into 100 ml of a 0.50 M aqueous solution of NaClO4.
The filtrate was slowly cooled and the resulting yellow precipitate
was collected by filtration and washed with dilute hydrochloric
acid (0.1 M). The bright-yellow solid was recrystallized by vapour
diffusion of diethyl ether into an acetonitrile solution of the
The nucleophiles, thiourea (TU, 99%), 1,3-dimethyl-2-thiourea
(DMTU, 99%), 1,1,3,3-tetramethyl-2-thiourea (TMTU, 98%)§
and sodium thiocyanate (NaSCN, 98%) were obtained from
Aldrich and used without further purification. The nucleophile,
‡ Perchlorate salts are potentially explosive and should be handled with
caution and in small amounts.
§ Thiourea and its substituted derivatives are suspected carcinogens and
the necessary safety precautions must be adhered to when handling these
reagents.
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crude product to afford yellow needle-like crystals of the required
platinum(II) complex.
order of 10, 20, 30, 40 and 50 times the concentration of the metal
complex.
Yield: 0.53 g (71%). Anal. calc. for PtCl2O4C27H35N3: C, 44.3; H,
4.8; N, 5.7. Found: C, 44.4; H, 4.8; N, 5.7. IR (KBr) cm-1: 464(m),
487(w), 552(sh), 609(s), 623(s), 739(m), 825(sh), 870(s), 889(s),
907(m), 918(m), 1027(m), 1091(vs), 1179(m), 1258(s), 1297(w),
1374(m), 1405(sh), 1416(s), 1430(s), 1467(s), 1480(s), 1560(m),
1615(vs), 2014(s), 2872(m), 2909(m), 2959(vs), 3096(s), 3203(s),
Solutions of the anionic nucleophiles, I- and SCN-, were
prepared by dissolving known amounts of the appropriate sodium
salts and either LiCF3SO3 or NaClO4·H2O in 100 ml of distilled
methanol to afford a solution of final concentration that was ca.
50 times greater than that of the metal complex and an ionic
strength equal to 0.1 M. Subsequent dilutions with the solution
of fixed ionic strength (i.e. the 0.1 M methanolic solution of either
LiCF3SO3 or NaClO4) afforded a series of concentrations, in the
order of 10, 20, 30, 40 and 50 times the concentration of the metal
complex.
1
t
3424(wb). H NMR (CD3CN): 1.44 (s, 18H, Bu), 1.53 (s, 9H,
tBu), 7.73 (dd, 2H, JHH = 6.05 Hz, JHH = 2.10 Hz, H5¢) 8.65
3
4
(d {broad 195Pt satellites}, 2H, JHH = 6.05 Hz, H6). 13C NMR
3
(CD3CN): 30.28; 30.54 (CMe3), 37.11; 38.08 (CMe3), 122.6, 124.3,
126.4, 151.6, 155.4, 159.3, 168.4, 168.5. 195Pt NMR (CD3CN) =
-734.
Instrumentation & physical measurements
Elemental analyses were performed at the Institute for Inorganic
Chemistry, University of Erlangen-Nu¨rnberg, Egerlandstr. 1, Er-
langen, Germany. NMR spectra (1H, 13C and 195Pt) were recorded
on a Varian Gemini 500 spectrometer at 25 ◦C, with chemical
shifts referenced to Si(CH3)4. Infrared spectra were recorded using
KBr discs on a Perkin Elmer Spectrum One FTIR spectrometer.
UV/Visible absorption spectra were recorded using a Varian Cary
100 Bio spectrophotometer.
Computational modelling
Preliminary modelling of the complexes was performed using
the computational software package Spartan 04 for Windows,24,25
using the B3LYP26 density functional method (DFT)27,28 and the
LACVP+** (Los Alamos core valence potentials)29 pseudopoten-
tial basis set. B3LYP relates to the hybrid functional Becke’s three-
parameter formulation,26 which has been proven to be superior to
traditional functionals. The LACVP basis set employs effective
core potentials for K-Cu, Pb-Ag, Cs-La and Hf-Au while Pople’s
6–31G** basis set describes second and third row s- and p-block
elements.30,31 The performance of the DFT calculations at the
chosen level of theory was gauged by comparison of the calculated
structures to X-ray structure of [Pt(tBu3terpy)Cl]+ and of similar
terpy analogues of platinum(II), viz. [Pt(terpy)(OMe)]+.23,32
All kinetic measurements were performed under pseudo first-
order conditions using, initially, UV/Visible absorption spectra to
determine the suitable wavelength at which kinetic investigations
could be performed. All substitutions were observed to reach com-
pletion in less than sixteen minutes and subsequently an Applied
Photophysics SX.18MV (v4.33) stopped-flow system coupled to
an online data acquisition system was used to monitor the kinetic
process. All measurements were performed in a thermostatted
environment to within 0.1 ◦C. All data were graphically analysed
Preparation of complex and nucleophile solutions for
kinetic analysis
R
using the software package, Origin 5.0ꢀ.34
All ligand substitutions were followed for at least eight half-lives.
All kinetic measurements were best described by a single expo-
nential equation. The observed pseudo first-order rate constants,
The ‘solvent’ used in the preparation of complex and neutral
nucleophile solutions was prepared by dissolving the required
quantity of either LiCF3SO3¶ or NaClO4 in freshly distilled
methanol to afford a solution of concentration 0.10 M. Since
neither the triflate ion, CF3SO3 , nor the perchlorate ion, ClO4 ,
is known to coordinate to Pt(II),33 the substitution reactions
of the platinum complexes (Pt1–LiCF3SO3, Pt2–NaClO4) were
performed in these media.
k
obs, were calculated using the online non-linear least-squares fit
of exponential data to eqn (1):3
-
-
At = Ao + (Ao - A•)exp(-kobst)
(1)
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 dependence of the rate constant on the incoming nu-
cleophile concentration was analysed in this manner for all
nucleophiles whilst maintaining the temperature at 25 ◦C. The
second-order rate constant, k2, for the reaction of each metal
complex with a particular nucleophile was obtained from the slope
Solutions of the metal complex were prepared by dissolving the
required amount of the metal salt in 50 ml of the constant ionic
strength solution prepared above.
Solutions of the neutral nucleophiles, viz. TU, DMTU and
TMTU, were prepared by dissolving a known amount of the
required nucleophile in 100 ml of a solution having fixed ionic
strength (i.e. the 0.1 M methanolic solution of either LiCF3SO3
or NaClO4) to afford a final concentration that was ca. 50 times
greater than that of the metal complex, whilst maintaining an ionic
strength of 0.1 M. Subsequent dilutions with the same solution
of fixed ionic strength afforded a series of concentrations, in the
of a linear regression of a plot of the observed rate constants, kobs
,
R
versus the nucleophile concentration using Origin 5.0ꢀ.34 All plots
can be described by eqn (2):3
kobs = k2[Nu]
(2)
The dependence of the rate constant on temperature was
conducted in a similar manner, with the nucleophile concentration
being held constant at 30 times the concentration of the metal
co◦mplex whilst varying the temperature from 15 to 35 ◦C in
¶ The choice of the lithium salt was found to be dependent upon
the interaction of the metal complex when reacted with solutions of
the nucleophile (when prepared using the sodium perchlorate “solvent”
medium). When the complex Pt1 was reacted with nucleophile solutions
containing sodium perchlorate, precipitation of the metal complex was
observed. It was for this reason that lithium triflate was chosen for the
reactions with Pt1.
5
C intervals. The activation parameters, DHπ and DSπ, were
determined by applying the values of the observed rate constants,
k
obs, to the Eyring equation.3
6726 | Dalton Trans., 2008, 6724–6731
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Results
Substitution of coordinated chloride from each of the Pt(II)
complexes (Scheme 1) by both neutral, viz. thiourea (TU), 1,3-
dimethyl-2-thiourea (DMTU), and 1,1,3,3-tetramethyl-2-thiourea
(TMTU), and ionic nucleophiles, viz. iodide (I-) and thiocyanate
(SCN-), was investigated under pseudo-first-order conditions
using conventional stopped-flow techniques.
Fig. 1 Dependence of the pseudo first-order rate constants (kobs) on
the entering nucleophile concentration for chloride substitution on Pt2
(0.170 mM) in methanol, I = 0.1 M (NaClO4), T = 298.15 K.
The activation parameters, DHπ and DSπ, were calculated using
the Eyring equation3 and are tabulated in Table 1. A typical plot
is shown in Fig. 2.
Scheme 1
All kinetic traces were described by perfect single exponentials
(Fig. S1, ESI).† The pseudo first-order rate constants, kobs, calcu-
lated from the kinetic traces were plotted against the concentration
of the 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, suggesting
that any reverse reaction or influence or the solvent was negligible
and that the mechanism of the substitution can be represented by
Scheme 1 and the corresponding rate law given by Equation 2.
Representative plots shown in Fig. 1 clearly indicate that the
substitution reactions were first-order with respect to the incoming
nucleophile. The values of the second-order rate constants k2,
which results from the direct attack of the nucleophile as shown
in Scheme 1 were obtained from the slopes of these plots at 25 ◦C
and are summarized in Table 1.
Discussion
In the current study, two platinum(II) complexes, viz. [Pt(terpy)Cl]+
(Pt1) and its corresponding analogue [Pt(tBu3terpy)Cl]+ (Pt2),
were synthesized and their substitution behaviour studied in
order to investigate the possibility of controlling the extent of
p-backbonding in platinum(II) terpyridyl systems.
The presence of the tert-butyl groups at the 4, 4¢ and 4¢¢ positions
of the terpyridyl fragment in Pt2 made it possible to investigate the
role of strong electron-donating groups in controlling the extent
of p-backdonation from the metal centre.
The substitution reactions of the investigated complexes can
be interpreted in terms of the usual rate law in keeping with the
associative mode of activation generally found for square-planar
Table 1 Summary of rate constants and activation parameters and the corresponding standard deviationsa for the substitution of chloride by neutral
(TU, DMTU and TMTU) and ionic (I- and SCN-) nucleophiles in methanol, I = 0.1 M (NaClO4)
Complex
Nucleophile
k2/M-1s-1
DHπ/kJ mol-1
DSπ/J K-1 mol-1
TU
1494 10
448 10
28.7 1.5
36.4 1.1
35.0 2.0
47.4 3.2
48.0 2.0
-88
-73
-91
5
4
8
DMTU
TMTU
I-
82
243
4
4
-42 11
SCN-
17 0.2
-61
1
TU
452
152
38 0.2
60 0.4
5
1
33.6 0.2
39.9 1.0
35.5 0.2
49.0 0.2
57.2 0.2
-82
-70
-96
-47
-43
1
4
1
1
1
DMTU
TMTU
I-
SCN-
3
0.02
a Standard deviations obtained for k2 are those of the slope of the plot of kobs versus nucleophile concentration. Standard deviations for activation
parameters obtained are those from the slope (DHπ) and y-intercept (DSπ) obtained from a plot of ln(k2/T) versus 1/T.
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terpy) and 4,4¢,4¢¢-trichloro-2,2¢:6¢,2¢¢-terpyridine (Cl3-terpy), i.e.
terpyridyl ligands containing electron-withdrawing substituents
in the ancillary positions. Their studies indicated that the metal-
to-ligand charge transfer (MLCT) state energy, which is the
lowest energy excited state for these complexes, is lowered upon
chlorination of the terpyridyl ligand.
By performing simple PM3 semi-empirical calculations
(Table S6, ESI)† on the free ligands, the authors found that the
energies of the lowest-unoccupied molecular orbitals, LUMO and
LUMO+1 for Cl-terpy and Cl3-terpy are substantially lowered
relative to those for terpy, despite their being little or no
orbital contribution from the chlorine atoms. Contrastingly, the
LUMO and LUMO + 1 energies of 4,4¢,4¢¢-tri-tert-butyl-2,2¢:6¢,2¢¢-
terpyridine, are raised due to their presence of electron-donating
tert-butyl groups.
In an effort to gain further insight into how the ancillary
substituents on the terpyridyl fragment can influence the HOMO
and LUMO energies of the platinum(II) complexes, it was therefore
necessary to assess the molecular structures and electronic prop-
erties of a series of platinum(II) terpyridyl complexes containing
both electron-donating and electron-withdrawing substituents in
the 4, 4¢ and 4¢¢-positions of the terpyridyl backbone.
Minimum energy structures (full geometry optimizations) were
calculated for [Pt(terpy)Cl]+ (Pt1) and a series of [Pt(4,4¢,4¢¢-
R3-terpy)Cl]+ analogues (where R is either an electron-donating
(EDG) or electron-withdrawing (EWG) group present in the
ancillary positions) at the B3LYP/LACVP** level of theory. The
calculated geometries are acceptable considering the degree of
similarity between X-ray and DFT-calculated structures of the Pt-
terpy analogues, viz. [Pt(terpy)(OMe)]+ and [Pt(tBu3terpy)Cl]+.23,32
The calculated and observed bond distances and angles were in
Fig. 2 Plots of ln(k2/T) versus 1/T for the reaction of Pt2 with a series of
neutral and ionic nucleophiles at various temperatures in the temperature
range 15 to 35 ◦C.
complexes. This is well supported by the determined activation
parameters: the activation entropies are all large and negative
whilst the activation enthalpies are relatively low. The marked
difference in the rate constants of the two complexes reflects the
role played by the introduction of strong electron-donating groups
at the ancillary positions of the terpyridyl backbone.
From Table 1, it can be seen that the reactivity of Pt1 is greater
than that of Pt2 in all cases. In the case of the neutral nucleophiles;
TU, DMTU and TMTU, the increase in steric hindrance of the
nucleophiles is reflected by the corresponding decrease in the rate
constant, since the bulkier nucleophiles find it increasingly difficult
to closely approach the metal centre. This trend is accompanied
by relatively constant values of activation enthalpy, DHπ. The
mechanism remains associative in all cases.
˚
agreement to within 0.07 A (for correlation see Table S7, ESI).†
Table 2 summarizes the calculated bond distances, viz. Pt–
Cl, Pt–trans-N and Pt–cis-N, energies of the frontier molecular
orbitals and electron densities surrounding the Pt(II) and Cl centres
for each of the investigated derivatives, including Pt1 and Pt2.
From Table 2it can be seen that, in terms of structural differ-
ences, the introduction of either electron-donating or electron-
withdrawing groups in the ancillary positions of the terpyridyl
fragment has a negligible influence. The cis and trans Pt–N bonds
remain relatively constant, irrespective of the ancillary substituent,
whilst the Pt–Cl bond, relative to that of [Pt(terpy)Cl]+, is seen to
lengthen upon the introduction of electron-donating groups; in
contrast the introduction of electron-withdrawing groups leads to
a slight shortening of the Pt–Cl bond. In all instances, the Pt–Cl
In the case of the anionic nucleophiles, the rate of substitution
of the chloride species in Pt1 and Pt2 was lower than that
observed for substitution by the neutral nucleophiles, ca. 6 times
slower when comparing substitution of chloride from Pt1 by TU
and I- respectively. This reactivity trend is expected, based on
the nucleophilicities (n◦Pt) found for monocationic complexes of
platinum(II),3,35–37 varying in the order amine < Cl- < NO2
<
-
-
N3 < Br- < SCN- < I- < TU < SeCN- and correlates well with
previous studies.17,38 Again, the determined activation parameters
support an associative mode of substitution.
˚
Thus, the experimental data obtained in the current study
supports the conclusions drawn by both Lowe et al.19 and Jaganyi
et al.,15 in that again we notice that electron-donating groups
attached in the ancillary positions of the terpyridyl backbone do in
fact result in a deceleration in the rate of ligand substitution. Also,
unlike the above-mentioned studies, the substituents in the ancil-
lary positions are not limited to the trans position, where a combi-
nation of the well-known trans-effect2–5 and the electron-donating
properties of the substituent might work anti to each other.
The question therefore remains as to how the presence of
tert-butyl groups, i.e. the electron-donating groups, controls
the extent of p-backbonding in these terpyridyl systems. In a
study by Cummings et al.39 on the photophysical properties
on platinum(II) complexes of 4¢-chloro-2,2¢:6¢,2¢¢-terpyridine (Cl-
bond lengths do not deviate by more than 0.01 A from that of
[Pt(terpy)Cl]+.
This observed deviation in the Pt–Cl bond length, due to
the presence of either electron-donating or electron-withdrawing
groups in the ancillary positions respectively, despite being so
small, can be attributed to the trans-influence, which is the
influence of a coordinated ligand on the strength of the metal–
ligand bond trans to itself and is a static ground-state effect.
Thus, in the case of electron-donating groups being present in the
ancillary positions of the terpyridyl fragment, electron-donation
by the group in the 4¢-position into the pyridyl ring, will lead
to increased electron-density along the trans axis towards the
platinum(II) centre thus weakening the Pt–Cl bond as reflected
by the lengthening of this bond in the ground state. In the case
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t
Table 2 Computational analysis of [PtII(4,4¢,4¢¢-R3-terpy)Cl]+ analogues containing electron-donating (R = Bu–, H2N–, MeO–or HO–) and electron-
withdrawing (R = Cl–, NO2–, CF3–or CN–) groups in the ancillary positions of the terpy fragment
Electron donating groups (EDG)
Ref.
H
Electron withdrawing groups (EWG)
H2N–
MeO–
tBu–
HO–
Cl–
CF3–
CN–
NO2–
˚
Bond Lengths/A
Pt–Cl
2.358
1.967
2.050
2.353
1.969
2.050
2.352
1.963
2.050
2.352
1.966
2.048
2.347
1.966
2.052
2.344
1.966
2.051
2.341
1.964
2.051
2.338
1.965
2.050
2.336
1.964
2.051
Pt–trans-N
Pt–cis-N
MO Energies/eV
EHOMO
ELUMO
DE
-8.48
-5.05
3.43
-8.99
-5.54
3.45
-8.96
-5.55
3.41
-9.01
-5.61
3.40
-9.45
-6.16
3.29
-9.64
-6.43
3.21
-9.91
-6.77
3.14
-10.18
-7.23
2.95
-10.34
-7.47
2.87
NBO Charges
Pt
0.7007
0.7168
0.7155
0.7131
0.7300
0.7368
0.7478
0.7569
0.7620
Cl
-0.2241
-0.2083
-0.2055
-0.2024
-0.1873
-0.1791
-0.1683
-0.1573
-0.1504
of an electron-withdrawing groups being present, we find that
the opposite occurs as reflected by the shortening of the Pt–Cl
bond in the ground state. Since this effect is not mirrored by the
reactivity of the metal centre, we can assume that in this instance,
that this is a result of ground-state destabilization by the trans-
influence as opposed to transition-state stabilization by the trans-
effect.
We also find that the positive charge on the metal centre
decreases upon the introduction of electron-donating groups
to the terpy fragment but increases upon the introduction of
electron-withdrawing groups. Comparable effects have been noted
previously in similar systems17 and this is reflected by the reactivity
of these systems.
It is immediately noticeable from Table 2, that significant
differences result in the frontier molecular orbital energies and
the NBO charges of Pt and Cl upon the introduction of either
electron-donating or electron-withdrawing groups at the 4, 4¢
and 4¢¢ positions. When considering the HOMO and LUMO
energies, we see that compared to those of [Pt(terpy)Cl]+, the MO
energies of the analogues containing electron-donating groups are
higher, whilst those containing electron-withdrawing groups are
comparably lower.
the introduction of electron-donating groups and a corresponding
decrease in DE in the case of electron-withdrawing groups being
introduced. This is in correlation with what has been observed
earlier for the free ligands by Cummings et al.39
From the DFT-calculated ground state orbitals illustrated in
Table 3, we find that in the case of electron-donating groups
being present in the ancillary positions, the electron density of
the HOMO is centred above and below the metal centre and
towards the ancillary substituent, primarily along the trans axis,
leading to a less electrophilic metal centre. In the case of electron-
withdrawing groups being present, the electron density of the
HOMO about the metal centre is significantly reduced, or absent
as observed in the case of the substituent being the nitro (–NO2)
group, and is located more on the ligand rendering the metal centre
more electrophilic.
Indeed, based on the HOMO electron-densities, it would appear
that the overall p cis-effect is greater than the overall p trans-effect
for these systems. This correlates well with the work by Van Eldik
et al.17,18 who found that p cis-effect is greater than the p trans-effect
when dealing with platinum(II) complexes containing ligands with
significant p-acceptor properties.
It can also be observed from the data in Table 2 that
the magnitude of the values of EHOMO, ELUMO , DE and the
NBO charges, is dependent upon the relative strength of the
Subsequently, we also observe an increase in the energy separa-
tion of frontier molecular orbitals, (DE = EHOMO - ELUMO ), upon
Table 3 Some DFT-calculated (B3LYP/LACVP**) molecular orbitals, i.e. HOMOs and LUMOs, for [Pt(terpy)Cl]+ (Pt1), [Pt(tBu3terpy)Cl]+ (Pt2),
[Pt{(NH2)3terpy}Cl]+, [Pt{(NO2)3terpy}Cl]+ and [Pt(Cl3terpy)Cl]+
[Pt{(NH2)3terpy}Cl]+
[Pt(tBu3terpy)Cl]+
[Pt(terpy)Cl]+
[Pt(Cl3terpy)Cl]+
[Pt{(NO2)3terpy}Cl]+
HOMO
LUMO
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electron-donating or -withdrawing groups and also the manner
in which they function.
Conclusions
From the experimental data obtained, we find that the rate of
substitution of the chloride moiety by the selected nucleophiles
from Pt2 proceeds at a slower rate than that from Pt1. This
retardation in the rate of substitution has been attributed to the
presence of the electron-donating substituents in the ancillary
positions of the terpyridyl ligand in Pt2 which leads to a reduction
in the p-acceptor properties of the terpyridyl ring and subsequently
a less electropositive metal centre.
The experimental and computational results clearly illustrate
that the moderate decrease in reactivity of the metal centre upon
introduction of electron-donating groups is as a result of a decrease
in the positive charge of the metal centre and a corresponding
increase in the energy separation of the frontier molecular orbitals
(DE).
The introduction of electron-withdrawing groups leads to an
increase in the positive charge of the metal centre and correspond-
ing decrease in DE and should result in increased reactivity of the
metal centre. By placing these ancillary substituents in both the
cis and trans positions, we find that (i) the overall trans-influence
is greater than the overall trans-effect; and (ii) the overall p cis-
effect is greater than the overall p trans-effect. The strength of
the electron-donating or -withdrawing groups controls the overall
extent of p backdonation from the metal centre into the empty
ligand orbitals.
The manner in which some atoms or groups can withdraw or
donate electrons into a system can be through either inductive
(movement of electron density through s-bonds) or resonance
(movement of electron density through p-bonds) effects or a
combination of the two.40,41
From these, there are six possible combinations in which
electron-density distribution in a system can be achieved:40
(1) Groups that withdraw electrons by inductive effects only, e.g.
+
–CF3, –NH3 .
(2) Groups that donate electrons by inductive effects only, e.g.
–CH3, –CH2CH3, –CH(CH3)2.
(3) Groups that withdraw electrons through resonance only, e.g.
–CN, –COH, –CO2R (where R is any alkyl group).
(4) Groups that withdraw electrons both inductively and through
resonance, e.g. –NO2.
(5) Groups that withdraw electrons inductively but donate
electrons through resonance e.g. –OH, –NH2, –X (where X is a
halide moiety).
(6) Groups that can withdraw or donate electrons through
resonance, e.g. –Ph, –CHCH2.
From the combinations given, it can be clearly seen how the
ancillary groups chosen for the computational study function and
allows us to explain in detail the trends observed.
In the case of the electron-withdrawing groups, the trend
observed from the computed data correlates well with those in
literature. Groups such as the cyano (–CN) or nitro (–NO2) groups,
are known to strongly withdraw electron-density40,41 and the pres-
ence of these in the ancillary positions of the terpyridyl fragment
leads to greatest decrease in DE and also the greater positive charge
on the metal centre when compared to [Pt(terpy)Cl]+.
The fact that these effects are slightly more pronounced for
the nitro group than for the cyano group can be explained by
considering the combinations give above, in which the nitro groups
withdraws electron-density by both inductive and resonance
effects, whereas the cyano group is only electron-withdrawing
through resonance effects.
The current study thus consolidates the role of electron-
donating and electron-withdrawing groups in controlling the
reactivity of platinum(II) terpyridyl complexes.
Acknowledgements
The authors gratefully acknowledge financial support from the
University of KwaZulu-Natal and the South African National
Research Foundation (NRF). We are also grateful to the Alexan-
der von Humboldt Foundation for the donation of a UV-Vis
spectrophotometer to the University of KwaZulu-Natal.
In the case of the electron-withdrawing substituent being
the chloro (–Cl) moiety, the values obtained are slightly less
than those for cyano or nitro, since halide ions are consid-
ered to be weaker electron-withdrawing groups as a result of
them withdrawing electrons inductively but donating electrons
through resonance. However the electron-withdrawing ability,
as a result of the electronegativity of chlorine, is greater than
its electron-donating ability, due to poorer overlap of the 3p
orbital of chlorine with the 2p orbital of the carbon it is
attached to.
In the case of the amino (–NH2), methoxy (–OMe) and hydroxy
(–OH) groups, we find that these operate anti to the chloride
group in that they donate more electron-density through resonance
effects than they accept electron-density through inductive effects.
This is anticipated as a result of the lone pairs of electrons
present in these groups being in close proximity to the extended
p-conjugation of the terpyridyl fragment allowing for greater
delocalization into the pyridyl rings. The tert-butyl group also
functions as an electron-donating group, but primarily through
inductive effects, with increased branching in the alkyl group
leading to better electron-donating properties.
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