Reactivity of neutral nitrogen donors in square-planar d8 metal complexes: The system chloro(2,2′:6′,2″-terpyridine)platinum(II) cation with five-membered N-donor heterocycles in methanol

Article abstract of DOI:10.1016/j.poly.2006.03.017

The kinetics of the forward and reverse steps of the reaction [Pt(terpy)Cl]⁺ + nu ? [Pt(terpy)(nu)]²⁺ + Cl^- (terpy = 2,2′:6′,2″-terpyridine, nu = one of a number of thiazoles, oxazole, isoxazole, imidazole, pyrazole and 3,5-dimethylpyrazole, covering a wide range of basicities) have been studied in methanol at 25 °C. Both forward and reverse reactions obey the usual two-term rate law observed in square-planar substitution. The second-order rate constants for the forward reactions, k₂^f, show a slight dependence upon the basicity of the entering nu, while the steric hindrance due to the presence of one methyl group in the α position to the nitrogen markedly decreases the reactivity. The second-order rate constants for the reverse reactions, k₂^r, are very sensitive to the nature of the leaving group and a plot of log k₂^r against the pK_a of the conjugate acids of the unhindered five-membered N-donors is linear with a slope of -0.51. The results are compared with data from the literature regarding a series of pyridines reacting with the [Pt(terpy)Cl]⁺ cation under the same experimental conditions. Both in the forward and in the reverse reaction, the reactivity depends not only upon the ligand basicity but also upon the nature of the nucleophile in the order: (thiazoles, oxazole, isoxazole, imidazole, pyrazoles) > pyridines for the entry of N-donors and on the contrary for the displacement by Cl^-. Steric retardation, due to the presence of a methyl group in the α position to the nitrogen, is remarkably lower for five-membered N-donors if compared to pyridines both in the forward and in the reverse reaction.

Full text of DOI:10.1016/j.poly.2006.03.017

Polyhedron 25 (2006) 2698–2704
www.elsevier.com/locate/poly
Reactivity of neutral nitrogen donors in square-planar d8
metal complexes: The system
chloro(2,20:60,200-terpyridine)platinum(II) cation with
five-membered N-donor heterocycles in methanol
*
Bruno Pitteri , Marco Bortoluzzi
` `
Dipartimento di Chimica, Universita ‘‘Ca Foscari’’ di Venezia, Calle Larga S. Marta Dorsoduro 2137, 30123 Venezia, Italy
Received 25 January 2006; accepted 21 March 2006
Available online 5 April 2006
Abstract
The kinetics of the forward and reverse steps of the reaction [Pt(terpy)Cl]+ + nu ꢀ [Pt(terpy)(nu)]2+ + Clꢀ (terpy = 2,20:60,200-ter-
pyridine, nu = one of a number of thiazoles, oxazole, isoxazole, imidazole, pyrazole and 3,5-dimethylpyrazole, covering a wide range
of basicities) have been studied in methanol at 25 ꢀC. Both forward and reverse reactions obey the usual two-term rate law observed in
square-planar substitution. The second-order rate constants for the forward reactions, kf2, show a slight dependence upon the basicity
of the entering nu, while the steric hindrance due to the presence of one methyl group in the a position to the nitrogen markedly
decreases the reactivity. The second-order rate constants for the reverse reactions, kr2, are very sensitive to the nature of the leaving
group and a plot of log kr2 against the pKa of the conjugate acids of the unhindered five-membered N-donors is linear with a slope of
ꢀ0.51. The results are compared with data from the literature regarding a series of pyridines reacting with the [Pt(terpy)Cl]+ cation
under the same experimental conditions. Both in the forward and in the reverse reaction, the reactivity depends not only upon the
ligand basicity but also upon the nature of the nucleophile in the order: (thiazoles, oxazole, isoxazole, imidazole, pyrazoles) > pyri-
dines for the entry of N-donors and on the contrary for the displacement by Clꢀ. Steric retardation, due to the presence of a methyl
group in the a position to the nitrogen, is remarkably lower for five-membered N-donors if compared to pyridines both in the forward
and in the reverse reaction.
ꢁ 2006 Elsevier Ltd. All rights reserved.
Keywords: Platinum; Kinetics; Five-membered N-donors; Heterocycles; Nucleophilicity
1. Introduction
from mono-cationic and bis-cationic complexes [1–4]. In
all the cases, the kinetic behaviour is in accord with a reac-
tion profile involving a transition state in which the Pt–Cl
bond is almost completely formed, whereas the Pt–N bond
is significantly weakened. As a consequence, the reactivity
of entering pyridines and the other amines depends linearly
and only slightly upon their basicity according to a linear
free energy relationship of the type logk2 = a(pKa) + b,
where the parameters a and b take into account electronic
and steric effects. On the other hand, the reverse reaction,
the displacement by chloride of coordinated nitrogen
donors from Pt(II) complexes, involves that am moves
from a situation in which the Pt–N bond is completely
As a part of a systematic study about the nucleophilicity
and lability of neutral nitrogen donors towards square-
planar four-coordinate complexes and the relationship
between the reactivity and proton basicity, we have
reported kinetic and equilibrium data concerning the
reversible displacement of chloride by am (am = a number
of pyridines and some heterocyclic and aromatic amines)
*
Corresponding author. Tel.: +39 412348558/412348576; fax: +39
412348517.
E-mail address: pitteri@unive.it (B. Pitteri).
0277-5387/$ - see front matter ꢁ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.03.017

B. Pitteri, M. Bortoluzzi / Polyhedron 25 (2006) 2698–2704
2699
formed to one in which it is greatly weakened. In this case,
2.3. Preparation of complexes
even if the lability of the amines is still linearly related to
their basicity, the constant b for unhindered pyridines, that
is their lability, is smaller than the one of other sp3 aliphatic
N-donors bases and evidences have been presented for a
significant contribution of a p interaction between the filled
d orbitals of the metal and the empty antibonding orbitals
of the pyridine ring, that contributes to the stabilization of
the ground state of the substrates. This suggestion seems to
be in agreement with further deviations observed for
pyridines having a para-substituent able to contribute to
the p system [1,3]. In order to improve these experimental
observations and the range of applicability of the proposed
interpretation, we have now extended our study to five-
membered nitrogen donors (thiazoles and isostructural
species such as oxazole, isoxazole, imidazole and pyrazoles)
by measuring the kinetics of the forward and reverse steps
of the process (1).
[Pt(terpy)Cl]ClO4 was prepared starting from cis/trans-
[PtCl2(SMe2)2] as reported in the literature [9].
All other complexes of the type [Pt(terpy)(nu)](ClO4)2
(nu = thiazole, 4-methylthiazole, 3,5-dimethylthiazole, 5-
methylthiazole, 2NH2-thiazole, benzothiazole, oxazole,
isoxazole, imidazole, pyrazole and 3,5-dimethypyrazole)
were prepared as follows. AgClO4 (0.103 g, 0.5 mmol)
was added, under stirring in the dark, to a warm mixture
(10 ml, 50 ꢀC) of [Pt(terpy)Cl]ClO4 (0.281 g, 0.5 mmol) in
nitromethane. After 10 min, the AgCl formed was filtered
off and the solution was treated with 0.5 mmol of the nitro-
gen ligand, stirred for 15 min and finally diethylether
(50 ml) was added. The powdery product precipitated
was filtered off, washed twice with diethylether and dried
in vacuo. Yields >80% in all cases. Analytical and 1H
NMR data for the complexes are collected in Tables 1
and 2, respectively.
2þ
½PtðterpyÞClꢁþ þ nuꢀ½PtðterpyÞðnuÞꢁ þ Clꢀ
ð1Þ
1H NMR for coordinated five-membered N-donors are
given according to Scheme 1.
(nu = thiazole, 4-methylthiazole, 4,5-dimethylthiazole, 5-
methylthiazole, oxazole, isoxazole, imidazole, 3,5-di-
methylpyrazole, pyrazole and only for the reverse reaction
2-aminothiazole and benzothiazole). The kinetics were car-
ried out in methanol at 25 ꢀC in order to directly compare
the results with those obtained with other nitrogen donors
under the same experimental conditions [3] and to widen
and complete our precedent study regarding only two
unhindered five-membered N-donors [5]. The great impor-
tance of ligand substitution reactions on [Pt(terpy)L]n+
substrates is underlined by the recent publication of a num-
ber of papers regarding kinetics and mechanism of the
reactions between biologically relevant ligands and the
[PtCl(terpy)]+ or [Pt(H2O)(terpy)]2+ cations [6–8].
2.4. Kinetics
The reactions were initiated by adding a 0.05 mol dmꢀ3
dimethylformamide solution (2–20 ll) of the substrate com-
plex, [Pt(terpy)Cl]+ or [Pt(terpy)(nu)]2+ to a methanolic
solution (3 cmꢀ3) of the appropriate reagent, the nitrogen
donor or chloride ion respectively, previously brought to
the reaction temperature (25 ꢀC) in a thermostatted cell in
Table 1
Analytical data
Complex
Analysisa (%)
C
H
N
[Pt(terpy)Cl](ClO4)
31.7
(32.0)
30.5
2.13
(1.97)
1.84
7.41
(7.46)
7.75
2. Experimental
[Pt(terpy)(thiazole)](ClO4)2
2.1. Materials
(30.3)
31.3
(31.4)
32.2
(32.4)
31.56
(31.42)
30.0
(29.7)
35.6
(1.98)
2.07
(2.22)
2.38
(2.45)
2.02
(2.22)
2.14
(2.08)
1.98
(2.21)
2.24
(7.86)
7.68
(7.71)
7.39
(7.57)
7.83
(7.71)
9.51
(9.63)
7.12
(7.23)
8.12
[Pt(terpy)(4-methylthiazole)](ClO4)2
[Pt(terpy)(4,5-dimethylthiazole)](ClO4)2
[Pt(terpy)(5-methylthiazole)](ClO4)2
[Pt(terpy)(2-aminothiazole)](ClO4)2
[Pt(terpy)(benzothiazole)](ClO4)2
[Pt(terpy)(oxazole)](ClO4)2
2,20,6,200-Terpyridine was purchased from Aldrich. Pure
reagent-grade LiCl, CH3SO3Na, CH3SO3H, AgClO4
(Aldrich) were used without further purification. The thiaz-
oles, oxazole, isoxazole, imidazole and pyrazoles were
recrystallized or distilled before use when necessary. Anhy-
drous MeOH was obtained by distillation over Mg wires,
but traces of water did not appear to have any appreciable
effect upon the reactions.
(35.6)
31.2
(31.0)
31.1
(2.03)
2.17
(8.05)
7.91
2.2. Instruments
[Pt(terpy)(isoxazole)](ClO4)2
(31.0)
31.3
(31.1)
32.9
(33.2)
29.8
(31.1)
(2.03)
2.22
(2.17)
2.79
(2.65)
1.87
(2.17)
(8.05)
10.2
(10.1)
9.52
(9.68)
9.85
(10.1)
[Pt(terpy)(pyrazole)](ClO4)2
Electronic spectra and kinetic measurements were
obtained on a Perkin–Elmer Lambda 15 spectrophoto-
meter. H NMR spectra were taken on Bruker Avance 300
or Bruker AC 200 spectrometers. Elemental analyses were
performed by the Microanalytical Laboratory of the
University of Padua.
[Pt(terpy)(3,5-dimethylpyrazole)](ClO4)2
[Pt(terpy)(imidazole)](ClO4)2
1
a
Calculated values in parentheses.

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B. Pitteri, M. Bortoluzzi / Polyhedron 25 (2006) 2698–2704
Table 2
1H NMRa data of the complexes
Compound
1H NMR (d, ppm; J, Hz; CD3NO2, 298 K)
[Pt(terpy)(thiazole)](ClO4)2
9.85 (dd, 1H, 4JH–H = 2.2, 4JH–H = 1.0, 3JPt–H = 22, thiazole-H2); 8.70–7.30 (m, 11H,
terpy); 8.58 (dd, 1H, 3JH–H = 3.5, 4JH–H = 1.0, thiazole-H4); 8.33 (dd, 3JH–H = 3.5,
4JH–H = 2.2, thiazole-H5)
[Pt(terpy)(4-methylthiazole)](ClO4)2
9.75 (d, 1H, 4JH–H = 2.4, 3JPt–H = 22, thiazole-H2); 8.70–7.70 (m, 11H, terpy); 7.93 (dq,
4
4
4
4JH–H = 2.4, JH–H = 1.2, thiazole-H5); 2.89 (d, 3H, JH–H = 1.2, JPt–H = 6, thiazole-
Me4)
4
3
[Pt(terpy)(5-methylthiazole)](ClO4)2
[Pt(terpy)(2-aminothiazole)](ClO4)2
[Pt(terpy)(4,5-dimethylthiazole)](ClO4)2
9.60 (d, 1H, JH–H = 1.0, JPt–H = 22, thiazole-H2); 8.70–7.79 (m, 11H, terpy); 8.26 (qt,
4
4
4
1H, JH–H = 1.3, JH–H = 1.0, thiazole-H4); 2.75 (d, 3H, JH–H = 1.3, thiazole-Me5)
3
8.70–7.80 (m, 11H, terpy); 7.57, 7.12 (AB spin system, 2H, JAB = 4.3, thiazole-H4,5);
7.06 (s, br, 2H, thiazole-NH2)
9.50 (s, 1H, 3JPt–H = 22, thiazole-H2); 8.70–7.30 (m, 11H, terpy); 2.80 (s, 3H, 4JPt–H = 6,
thiazole-Me4); 2.65 (s, 3H, thiazole-Me5)
3
[Pt(terpy)(benzothiazole)](ClO4)2
[Pt(terpy)(oxazole)](ClO4)2
[Pt(terpy)(isoxazole)](ClO4)2
10.15 (s, 1H, JPt–H = 24, thiazole-H2); 8.86–7.34 (m, 15H, terpy + benzothiazole)
3
9.28 (s, 1H, JPt–H = 10, thiazole-H2); 8.80–7.20 (m, 13H, terpy+oxazole)
3
3
9.37 (d, 1H, JH–H = 2.2, isoxazole-H3); 9.30 (d, 1H, JH–H = 2.0, isoxazole-H5); 8.76–
3
3
7.76 (m, 11H, terpy); 7.28 (dd, 1H, JH–H = 2.2, JAB = 2.0 Hz, isoxazole-H4)
4
[Pt(terpy)(imidazole)](ClO4)2
1H NMR: 8.57 (t, 1H, JH–H = 1.8, imidazole-H2); 8.71–7.76 (m, 11H, terpy); 7.72 (dd,
3
3
1H, JH–H = 2.4, 4JH–H = 1.8, imidazole-H4); 7.65 (dd, 1H, JH–H = 2.4, 4JH–H = 1.8,
imidazole-H5). NH observed in d6-DMSO, 3.33 ppm, br.
3
[Pt(terpy)(pyrazole)](ClO4)2
8.70–7.75 (m, 11H, terpy); 8.37 (d, 1H, JH–H = 2.4, pyrazole-H3); 8.33 (d, 1H,
3JH–H = 2.4, pyrazole-H5); 7.46 (s, br, 1H, NH); 7.00 (t, 1H, JH–H = 2.4, pyrazole-H4)
8.72–7.75 (m, 12H, terpy + NH); 6.53 (s, 1H, pyrazole-H4); 2.62 (s, 3H, JPt–H = 6,
3
4
[Pt(terpy)(3,5-dimethylpyrazole)](ClO4)2
pyrazole-Me3); 2.56 (s, 3H, pyrazole-Me5)
a
Chemical shifts for 1H NMR are reported by reference to TMS.
to ascertain the site of binding to Pt(II) in its complexes,
but there is considerable evidence to indicate bonding
through nitrogen of which the following are probably
the most important: (i) thiazoles and oxazoles bind to
the analogous soft d8 Au(III) through nitrogen [11]; (ii)
thiazole ligands were found to be bonded to platinum
through the nitrogen atom from data in the literature
[12,13]; (iii) further proof of the Pt–N bond comes from
the crystal structure of solid trans-[Pt(dmso)(thiazole)Cl2]
determined by X-ray diffraction [14]; (iv) there is a general
similarity of the behaviour of these complexes with the
corresponding R-py (N-containing six-membered hetero-
cycles), that depends only upon the basicity of the
nitrogen.
Scheme 1. Numeration of the considered five-membered N-donor
heterocycles.
the spectrophotometer. The concentration of the entering
group was always large enough to provide pseudo-first-order
conditions. After preliminary repetitive scan experiments in
the range 240–360 nm to search for isosbestic points and spec-
tral changes, the kinetics were studied by measuring the
changing absorbance at suitable wavelengths as a function
of time. Pseudo first-order-rate constants (kobs/sꢀ1) were
obtained either from the gradients of plots of log(Dt ꢀ D )
3.1. Kinetics of displacement of chloride by nitrogen donors
from [Pt(terpy)Cl]+
1
vs. time or from a non-linear least-squares fit of experimental
data to Dt = D + (D0 ꢀ D )exp(ꢀkobst) with D0, D and
1
1
1
kobs as the parameters to be optimized (D0 = absorbance
The spectrophotometric changes observed in repetitive
scanning of the spectrum of the reaction mixture are
characteristic of a single chemical stage, with well main-
tained isosbestic points. Careful examination of the spec-
tral changes (subsequent changes have never been
observed) which occur after the reagents are mixed and
the close similarity of the spectra at the end of the reac-
tion with those of authentic samples of the expected
reaction products demonstrate that all the reactions that
have been studied kinetically involve the displacement of
coordinated chloride by the nitrogen base. The rate con-
stants were determined in the usual way from the change
after mixing the reactants, D = absorbance at completion
1
of the reaction). Rate constants were accurate to within 5%.
All the prepared complexes follow the Lambert–Beer
equation in the considered concentration range and exper-
imental conditions, so the presence of p-stacking effects [10]
can be reasonably excluded.
3. Results and discussion
Thiazoles, oxazole, isoxazole, imidazole and pyrazoles
are potentially ambidentate ligands and it is necessary

B. Pitteri, M. Bortoluzzi / Polyhedron 25 (2006) 2698–2704
2701
in absorbance as a function of time at a convenient
wavelength within the region 300–340 nm. All the reac-
tions were studied in the presence of a sufficient excess
of nucleophile over the substrate to provide pseudo-
first-order conditions and the observed rate constants,
kobs, collected in Table 3, obey to the general relation-
ship kobs ¼ kf1 þ kf2[nu], which is usual for nucleophilic
substitution at planar four coordinate d8 metal com-
plexes [15].
3.2. Kinetics of displacement of nitrogen donors by chloride
from [Pt(terpy)(nu)]2+
In all the reactions with an excess of chloride ion the
spectrophotometric changes are characteristic of a single
chemical stage, with the same isosbestic points observed
for the forward reactions. The spectra at the end of the
reactions coincide with those of authentic samples of
the expected products at the same concentration, so that
the displacement of the coordinated nitrogen donor by
chloride is assured. The reactions were studied in the
presence of 0.05 mol dmꢀ3 CH3SO3H at constant ionic
Table 3
a
First-order rate constants, kobs for the reaction [Pt(terpy)Cl]+ + nu !
strength (I = 0.1 mol dmꢀ3
,
CH3SO3Na). Preliminary
[Pt(terpy)(nu)]2+ + Clꢀ in methanol at 25 ꢀC
experiments showed that, at constant chloride concentra-
tion, the rate of reaction was independent of the concen-
tration of the acid over the range 0.02–0.2 mol dmꢀ3. The
acid simply serves to protonate the released nu and to
prevent the reverse reaction. For the weakest bases, oxa-
zole and isoxazole, the acid concentration is not sufficient
to ensure complete protonation of the released ligand.
However, because of their low basicity these species are
poor ligands towards Pt(II) complexes and also in these
cases a careful comparison of the spectra at the end of
the reactions with the spectra of an authentic sample of
[Pt(terpy)Cl]+ and nu at the same concentration and acid-
ity confirms the completion of the reactions. The
observed rate constants, kobs, collected in Table 4, obey
the general relationship kobs ¼ kr1 þ k2r ½Clꢀꢁ, where
kr1 and k2r have the usual meaning, and kr2 values at
I = 0.1 mol dmꢀ3 are reported in Table 5 together with
the corresponding values k02r extrapolated to zero ionic
strength. The extrapolation to zero ionic strength uses the
Nucleophile
Thiazole
[nu] (mol dmꢀ3
)
103 kobs (sꢀ1
)
0.05
0.08
0.10
0.12
0.15
21.9
32.7
44.1
54.1
68.0
5-Methylthiazole
Imidazole
0.02
0.04
0.06
0.08
0.10
12.1
26.4
39.1
53.2
63.1
0.003
0.005
0.008
0.010
0.015
21.0
34.0
56.8
67.1
101.7
4,5-Dimethylthiazole
3,5-Dimethylpyrazole
4-Methylthiazole
Oxazole
0.033
0.066
0.099
0.132
0.166
16.3
28.5
48.8
65.1
80.3
Debye–Huckel relationship, log k0r ¼ log k2r ꢀ 2aZaZbI1=2
=
¨
2
ð1 þ bI1=2Þ with Za = +2, Zb = ꢀ1, a = 1.90 and b =
1.55 (estimated from the relative permittivity of methanol
at 25 ꢀC). [16] Preliminary experiments showed that the rate
of the processes between the monocationic [Pt(terpy)Cl]+
complex and the neutral nitrogen donor is not influenced
by primary salt effects over the range 0.0–0.5 mol dmꢀ3
ionic strength, whereas that of the reverse reactions
between the bis-cationic species [Pt(terpy) (nu)]2+ and
chloride ion depends upon the ionic strength. As a conse-
quence the equilibrium constants, K, also depend upon
the ionic strength and in Table 5 the values K ¼ kf2=k2r
and K0 ¼ k2f =k02r, determined from the ratios of the second-
order rate constants for the forward and reverse reactions,
are reported.
0.01
0.02
0.03
0.04
0.05
7.43
14.7
23.2
29.0
37.4
0.05
0.10
0.15
0.20
0.25
17.4
34.1
52.9
66.9
89.8
0.15
0.20
0.25
0.30
0.35
27.9
36.2
47.2
55.5
65.4
The data obtained from the reaction [Pt(terpy)Cl]+ +
nu ! [Pt(terpy)(nu)]2+ + Clꢀ are compared to the entry
of pyridines and other nitrogen donors, under the same
experimental conditions [3], and are reported in Fig. 1,
where the values of log k2f are plotted against the pKa of
the entering nucleophiles. As far as the unhindered thia-
zole, 5-methylthiazole, oxazole, isoxazole, imidazole and
pyrazole are concerned, it is evident that the data fit quite
nicely the linear free-energy relationships log kf2 ¼ 0:23
pKa + constant already observed for the entry of unhin-
Isoxazole
0.05
0.10
0.15
0.20
0.25
9.79
21.1
31.2
42.3
49.5
Pyrazole
0.001
0.003
0.006
0.008
0.010
0.48
1.44
2.80
3.75
4.92
a
Errors on individual kobs determinations are in the 5% range.

2702
B. Pitteri, M. Bortoluzzi / Polyhedron 25 (2006) 2698–2704
Table 4
reasonably ascribed to the smaller C–N–X (X = C, O, N)
angle (@108ꢀ) of the five-membered N-donors with respect
to that of the pyridine (C–N–C @ 120ꢀ), which implies a
decrease of congestion in the trigonal bipyramidal transi-
tion state. Another difference between the five-membered
nucleophiles (unhindered thiazoles, oxazole, isoxazole,
imidazole and pyrazoles) and pyridines in the entry reac-
tion is the amount of steric retardation, as measured from
the parameter D1 attributable to the presence of a methyl
group in the a-position with respect to the N-donor atom.
In the case of a-methylpyridines the parameter Df1 is about
First-order rate constants, kobsa for the reaction [Pt(terpy) (nu)]2+ + Clꢀ
!
[Pt(terpy)Cl]+ + nu in methanol at 25 ꢀC (I = 0.1 mol dmꢀ3, CH3SO3Na)
nu
103[Clꢀ] (mol dmꢀ3
)
103 kobs (sꢀ1
)
Thiazole
0.25
0.37
0.75
1.50
8.20
12.3
25.7
44.5
5-Methylthiazole
4,5-Dimethylthiazole
2-Aminothiazole
0.40
0.60
1.00
1.50
8.41
11.8
16.5
25.7
1.5, corresponding to a difference of about 8.6 kJ molꢀ1
,
0.20
0.40
0.60
1.00
0.78
1.50
2.26
3.54
and it reduces to Df2 @ 0.25 (1.4 kJ molꢀ1) for the entry of
4-methylthiazole, 4,5-dimethylthiazole and 3,5-dimethylpy-
razole on the same complex.
The amount of steric retardation is related to the free
rotation of the ligand molecule around the Pt–N bond,
which leads to steric interference with the pyridine rings
of the terpy ligand placed in the axial position in the trigo-
nal bipyramidal transition state. For this reason, the differ-
ences in reactivity can be explained by considering the
different structures of the pyridines compared to that of
the five-membered N-donors (Scheme 2), which leads to a
decrease in steric congestion in the latter compounds if
compared to the former.
As far as the reverse reaction is concerned, the replace-
ment of the N-ligands by Clꢀ, the differences between the
five-membered heterocycles and the pyridines groups are
quite evident. In the replacement reactions of the N-ligand
from the bis-cationic Pt-terpy substrate by chloride, the six
five-membered N-donors are significantly less labile than
the unhindered pyridines (Fig. 2).
The dependence of the lability of the two kinds of
ligands upon basicity is almost the same and the data
fit nicely the free-energy relationship log kr2 ¼ ꢀ0:51
pKa + constant already reported [3] for the displacement
of unhindered pyridines, and the decrease in lability corre-
sponds to a difference of about 3 kJ molꢀ1 between the free
energy of activation of the two reacting systems. The labil-
ity of the five-membered N-donor heterocycles is consider-
ably lower than that of pyridines, strongly suggesting that,
in addition to possible but unlikely solvatation effects, the
Pt–N bond in the case of five-membered N-donors may
be reinforced by p bonding. It seems reasonable to con-
clude that the p system of five-membered N-donors can
interact in the ground state with the filled d orbitals of
Pt(II) better than pyridines. The fact that the thiazoles
are weaker r donors but fairly better p acceptors than pyri-
dines once coordinated to Pt(II) has been pointed out in a
work concerning hydrogen isotope exchange in Pt(II)-
thiazole systems [18]. Also in the present study, for the
pyridines system, the relatively large dependence of the
0.60
1.00
1.50
2.00
2.50
0.12
0.22
0.31
0.41
0.49
Pyrazole
0.20
0.40
0.60
0.80
1.00
7.80
14.9
24.3
31.6
38.7
3,5-Dimethylpyrazole
4-Methylthiazole
Oxazole
0.50
1.00
1.50
2.00
2.50
0.91
1.89
2.58
3.72
4.67
0.50
0.75
1.25
1.50
2.50
3.03
4.45
6.85
9.69
16.1
0.08
0.10
0.12
0.15
0.20
0.25
12.2
15.2
19.7
23.5
30.9
38.7
Isoxazole
0.20
0.30
0.40
0.50
0.60
20.3
30.5
41.0
50.8
58.7
Benzothiazole
0.20
0.40
0.60
1.00
1.50
9.65
14.9
22.8
34.2
50.0
Imidazole
5.0
10.0
30.0
50.0
0.29
0.57
1.71
2.95
a
Errors on individual kobs determinations are in the 5% range.
1
D can be regarded as the steric effect due to the introduction of a
dered pyridines on [Pt(terpy)Cl]+ [3], whereas the nucleo-
philicity is higher (D log k2f ffi 0:60, corresponding to a dif-
ference of about 3.4 kJ molꢀ1) and this fact can be
methyl group ortho to the nitrogen in the entering nucleophiles without
any interference from the basicity change that might arise from the
presence of the substituent.

B. Pitteri, M. Bortoluzzi / Polyhedron 25 (2006) 2698–2704
2703
Table 5
kf1þkf2½nuꢁ
kr1þkr2½Clꢀꢁ
2þ
Second-order rate constantsa and equilibrium constants for the reaction ½PtðterpyÞClꢁþ þ nu
ꢀ
½PtðterpyÞnuꢁ þ Clꢀ
Nucleophile
102kf2 (dm3 mol sꢀ1
)
102kr2 (dm3 mol sꢀ1
)
102k20r (dm3 mol sꢀ1
)
102Kðkf2=kr2Þ
102K0ðkf2=kr2Þ
pKa of nuH+
Thiazole
47
36
49
2
1
1
2800 200
660 40
114800
27060
14145
241.9
127920
631400
1.6 0.2
5.5 0.9
14.2 0.6
11300 300
0.041
0.133
0.346
2.769
2.55
3.26
3.73
7.65
1.2
4-Methylthiazole
4,5-Dimethylthiazole
Imidazole
Benzothiazole
Oxazole
2-Aminothiazole
5-Methylthiazole
Pyrazole
3,5-Dimethylpyrazole
Isoxazole
345
9
670 10
5.9 0.9
3120 60
15400 400
19.5 0.7
1500 100
3900 100
150 20
18.8 0.4
0.122 0.006
0.003
0.8
799.5
5.36
3.12
2.48
4.38
1.30
64
48
74
2
1
2
61500
159900
6150
4.3 0.4
1.23 0.06
0.104
0.030
1.203
0.005
49
8
20.1 0.1
9700 300
397700
0.207 0.007
pKa values were taken from Ref. [17].
a
Determined by weighted linear regression of kobs values vs. nucleophile concentration.
Fig. 1. log kf2 for the reaction [Pt(terpy)Cl]+ + nu ! [Pt(terpy)(nu)]2+
+
Fig. 2. log kr2 for the reaction [Pt(terpy)(nu)]2+ + Clꢀ ! [Pt(terpy)Cl]+
+
Clꢀ plotted against pKa of nuH+: d, oxazole, isoxazole, pyrazole,
thiazole, 5-methylthiazole, imidazole; s, 4-methylthiazole, 4,5-dimethy-
thiazole, 3,5-dimethylpyrazole; j, 4-cyanopyridine, 4-acetylpyridine,
methylisonicotinate, pyridine, 4-methylpyridine, 3-methylpyridine, 4-
aminopyridine; Ã, 2-methylpyridine, 2,4-dimethylpyridine; data for all
the pyridines were taken from Ref. [3].
nu plotted against pKa of nuH+: d, oxazole, isoxazole, pyrazole, thiazole,
5-methylthiazole, imidazole; s, 4-methylthiazole, 4,5-dimethythiazole,
3,5-dimethylpyrazole; j, pyridine, 4-methylpyridine, 3-methylpyridine, 4-
aminopyridine; Ã, 2-methylpyridine, 2,4-dimethylpyridine; data for all the
pyridines were taken from Ref. [3].
Scheme 2. Steric congestion in the trigonal bipyramidal transition state for substituted pyridines and thiazoles.

2704
B. Pitteri, M. Bortoluzzi / Polyhedron 25 (2006) 2698–2704
substitution lability upon ligand basicity suggests that the
formation of the transition state requires a considerable
change in the Pt–N bond, confirming therefore the obser-
vation already reported [1,4] that the transition state can
be described as a structure containing a well formed Pt–
Cl bond and a weak Pt–N bond. As far as the lability of
hindered five-membered N-donors (4-methylthiazole, 4,5-
dimethylthiazole and 3,5-dimethypyrazole) in the reverse
reactions of process (1) is concerned, a lower (as expected,
see above) steric effect for five-membered N-donors
(D @ 0.28, 1.4 kJ molꢀ1) than for pyridines (D @ 0.82,
4.6 kJ molꢀ1) has been observed (Fig. 2). Not only a-
methyl-substituted five-membered heterocycles, but also
other ligands such as 2-aminothiazole and benzothiazole
were considered. For these substrates the D value is higher
(0.70, 3.9 kJ molꢀ1) than that measured for 4-methylthiaz-
ole, 4,5-dimethylthiazole and 3,5-dimethylpyrazole. This
difference is due to the greater congestion on the metal cen-
tre caused by the NH2- and benzo-substituents, which dis-
favours the platinum–chloride bond formation. The ratio
between the second-order rate constants for the forward
and reverse reactions gives the corresponding equilibrium
constants for the systems:
logK0 versus pKa (Fig. 3) is linear for the group of five-
membered N-donors formed by thiazole, 5-methylthiazole,
oxazole, isoxazole, imidazole and pyrazole, and the free en-
ergy relationship logK0 = (0.74)pKa + constant can be
derived.
The dependence of the stability of the complexes upon
the basicity of the ligands is the same as that measured
for pyridines under the same experimental conditions [3].
However, the values of the equilibrium constants are
higher than those found in the previous case. Such a
difference may reflect a greater tendency of five-membered
N-donors, in comparison with pyridines, to give p back-
donation from the filled orbitals of the metal to the
antibonding orbitals of the ligand, leading to a relative
stabilization of the ground state of the products.
Acknowledgements
We thank the Italian Ministry of University for financial
support.
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