Substitution reactions on cyclometallated Pt(IV) complexes; a kinetic survey of the bond strength of Pt-S, N, and P-based ligands

DOI: 10.1016/S0020-1693(03)00117-8

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Inorganica Chimica Acta p. 269 - 277 (2003)

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Authors:

Esteban, Julio

Font-Bardia, Mercè

Gallego, Carlos

González, Gabriel

Martínez, Manuel

Solans, Xavier

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Article abstract of DOI:10.1016/S0020-1693(03)00117-8

The substitution reactions of a series of sulfur, nitrogen and phosphorus-based ligands (L) on Pt(IV) complexes having a chloride ligand, the cyclometallated imine ligand CC₅Cl₄CHNCH₂C₆H₅, and two methyl groups and the leaving ligand in a mer-geometrical arrangement, [Pt(C^∧N)(CH₃)₂Cl(L)], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the leaving ligands. Although in all cases a limiting dissociative mechanism has been found to apply, where the dissociation of the leaving ligand corresponds to the rate-determining step, the relative values of the different rate constants involved in the process, i.e. dissociation/association of the leaving/entering ligands and the effect of the solvent, change dramatically in the systems studied. The results agree with the previously determined mechanism operating on these substitution reactions, and serve as a very good measure of the strength of the Pt-L bond, which is influenced by both electronic and steric characteristics. The possible solvent-assisted hydrogen bonding of the leaving ligand with the rest of the molecule is made also evident in the relevant situations. The equilibrium constants detected for some of the systems indicate that, although Pt(IV) complexes are expected to be hard acids and prefer nitrogen to sulfur or phosphorus donors, this is not so. This result is in line with the softening of the nature of the Pt(IV) centres of complexes containing several Pt-C bonds.

Full text of DOI:10.1016/S0020-1693(03)00117-8

Inorganica Chimica Acta 351 (2003) 269ꢀ277

www.elsevier.com/locate/ica

Substitution reactions on cyclometallated Pt(IV) complexes; a kinetic

S, N, and P-based ligands

survey of the bond strength of Ptꢀ

Julio Esteban ^a, Merce` Font-Bardia ^b, Carlos Gallego ^a, Gabriel Gonza´lez ^a,

Manuel Mart´ınez ^a,*, Xavier Solans ^b

^aDepartment de Qu´ımica Inorga`nica, Universitat de Barcelona, Mart´ı i Franque`s 1-11, E-08028 Barcelona, Spain

^bDepartament de Cristal lograﬁa, Mineralogia i Dipo`sits Minerals, Universitat de Barcelona, Mart´ı i Franque`s s/n, E-08028 Barcelona, Spain

Received 17 February 2003; accepted 22 February 2003

Abstract

The substitution reactions of a series of sulfur, nitrogen and phosphorus-based ligands (L) on Pt(IV) complexes having a chloride

ligand, the cyclometallated imine ligand CC₅Cl₄CHNCH₂C₆H₅, and two methyl groups and the leaving ligand in a mer-geometrical

arrangement, [Pt(C^ﬄN)(CH₃)₂Cl(L)], have been studied as a function of temperature, solvent, and electronic and steric

characteristics of the leaving ligands. Although in all cases a limiting dissociative mechanism has been found to apply, where the

dissociation of the leaving ligand corresponds to the rate-determining step, the relative values of the different rate constants involved

in the process, i.e. dissociation/association of the leaving/entering ligands and the effect of the solvent, change dramatically in the

systems studied. The results agree with the previously determined mechanism operating on these substitution reactions, and serve as

a very good measure of the strength of the Ptꢀ/L bond, which is influenced by both electronic and steric characteristics. The possible

solvent-assisted hydrogen bonding of the leaving ligand with the rest of the molecule is made also evident in the relevant situations.

The equilibrium constants detected for some of the systems indicate that, although Pt(IV) complexes are expected to be hard acids

and prefer nitrogen to sulfur or phosphorus donors, this is not so. This result is in line with the softening of the nature of the Pt(IV)

centres of complexes containing several Ptꢀ

/C bonds.

# 2003 Published by Elsevier Science B.V.

Keywords: Pt(IV) complexes; Solvent; Substitutions; Kinetics

1. Introduction

crucial, but poorly understood; redox assisted substitu-

tion has always prevented its proper study [9].

Although substitution processes taking place on inert

t⁶_2gmetal complexes are a clear candidate for mechan-

istic studies [1], only Co(III), Rh(III), and Fe(II)

complexes have produced an important database [2].

Pt(IV) complexes, have been seldom used for this

purpose in spite of the inherent importance of platinum

We have been involved lately in the mechanistic study

of the substitution reactions on a series of cyclometal-

lated complexes that allowed the clean presence of

Pt(IV) centres (Eq. 1) [10ꢀ12].

chemistry in oxidative additionꢀ

reactions [3,4], hydrido complexes [5], and the Pt(IV)ꢀ

Pt(II) reduction processes involved in the new genera-

tion of anticancer platinum pharmaceuticals [6ꢀ8]. In

any of these processes the initial substitution reaction is

reductive elimination

ð1Þ

In most of the cases, a sulfide dissociation reaction to

produce a pentacoordinated intermediate, that subse-

quently associates with the entering phosphine ligand

has been proposed [10,13]; a direct associative process

* Corresponding author. Tel.: ꢁ

725.

E-mail address: manel.martinz@qi.ub.es (M. Mart´ınez).

34-934-021 273; fax: ꢁ34-934-907

0020-1693/03/$ - see front matter # 2003 Published by Elsevier Science B.V.

doi:10.1016/S0020-1693(03)00117-8

270

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ277

for systems having both important hydrogen bonding

possibilities, and electron withdrawing ligands, has also

been seen to operate [11]. Following on this line, we

decided to study the substitution reactions of a series of

S, N, and P-based leaving ligands, the goal being the

study of possible enhanced hydrogen bonding in the

transition state [14], and the effect produced on the

introduction of harder ligands, with relatively small

spectra were obtained in the Unitat de RMN d’Alt

Camp de la Universitat de Barcelona.

2.2. Products

All the methylamine ligands used in this study were

obtained from distillation over NaOH from the aqueous

commercially available products, pyridine and propyla-

mine were used as purchased; solvents were distilled and

keep under N₂. Complexes 1 and 2 have been prepared

according to the established literature procedures

[10,11]. For the preparation of the amine substituted

derivatives the standard published procedure [18,19] has

been also applied; characterization of the complexes has

cone angles and/or high basicity, as leaving ligands [15ꢀ

17].

We present in this paper the study of the substitution

processes of the complexes indicated in Scheme 1 with

entering ligands (E) MeNH₂, PrNH₂, Me₂NH, Me₃N,

py and PPh₃, in acetone and toluene solutions at

different temperatures and in various solvents to pro-

duce the corresponding substituted compounds.

been achieved via H NMR spectra (Table S1, supple-

mentary material). Solid samples of complexes 4, 5, 6, 7,

8, and 9 have been prepared by reaction of the

stoichiometric amounts of 1 with the corresponding

amine in acetone or toluene solution, after 1 day, the

solutions were taken to dryness and the remaining solid

dissolved in the minimum amount of acetone for

recrystallization. For complex 3, the solvolysis in

DMSO of complex 1, followed by vacuum distillation

at 60 8C, produced a solid that, after workup, could not

be dissolved neither in acetone, nor in chloroform; its

suspension in acetone or chloroform-containing SMe₂

or PPh₃produced the clean spectra of compounds 1, 9.

By similitude with previous studies, and the poor ligand

characteristics of DMSO on Pt(IV) centres [12,20,21],

the solid was assumed to the dinuclear chloride-bridged

complex [{Pt(Me)₂(CC₅Cl₄CHNCH₂Ph)(m-Cl)}₂], its

solution in DMSO-containing acetone or chloroform

produced solution of the desired compound 3, the value

Scheme 1.

For this purpose all starting complexes involved, not

previously characterized (3, 4, 5, 6, 7, 8), have also been

prepared. The X-ray crystal structures of compounds 5,

and 7 have been determined to corroborate, both the

validity of the monitored substitution processes, and the

geometrical arrangement of the ligands around the

Pt(IV) centre in the final complexes. Although in all

cases the reaction studied takes place through the

formation of a pentacoordinated intermediate and

ulterior entry of the incoming ligand, the relative values

of the rate constants involved in the process vary

dramatically. Changes in equilibrium constants, discri-

mination ratio of the pentacoordinated intermediate,

and dissociation rate constants can be easily associated

with the relative steric and electronic characteristics of

the leaving/entering ligands and solvent polarity.

of 18 Hz for the Ptꢀ

indicates the S bonded nature of the ligand.

Yields were always in the 60ꢀ80% margin (Anal.

Calc. for: 4

¹₂CH₃COCH₃, C, 34.04; H, 3.40; N, 4.29%;

H(SOMe₂) coupling constant

Found: C, 33.6; H, 3.8; N, 4.8%. Calc. for: 5, C, 35.0; H,

3.50; N, 4.30%; Found: C, 35.2; H, 3.7; N, 4.2%. Calc.

for: 6, C, 33.90; H, 3.32; N, 4.39%; Found: C, 33.9; H,

5.7; N, 4.2%. Calc. for: 7, 0.18C₆Cl₆, C, 34.3; H, 3.30; N,

3.98%. Found: C, 34.7%; H, 3.7; N, 3.5%. Calc. for: 8

CH₃COCH₃, C, 39.5; H, 3.45; N, 3.84%. Found: C,

39.3; H, 3.4; N, 3.9%. Calc. for: 9, C, 47.77; H, 3.42; N,

1.64%. Found: C, 47.6; H, 3.4; N, 1.7%).

2.3. Crystallography

2. Experimental

Good quality X-ray crystals from compounds 5 and 7

were obtained by cooling concentrated acetone solutions

of the complexes in a refrigerator for extended periods.

Selected prismatic crystals were mounted on a MAR345

difractometer with a image plate detector and using

2.1. Instruments

¹H NMR spectra were recorded with a Varian XL-

200 and Bruker 250 DMX spectrometers. Chemical

shifts (in ppm) were measured relative to SiMe₄, the

solvent used was acetone(d₆) or chloroform(d₁). All

graphite monochromated Mo Ka radiation (l 0.71073

A) in the vꢂ

2u scan mode. Unit-cell parameters were

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ

/277

271

determined from automatic centering of 9822 (5) or 25

(7) reflections (3Bu B318) and refined by least-squares

method. Lorentz-polarization and absorption correc-

tions were made. The structure was solved by direct

methods, using the ^SHELXScomputer program [22] and

refined by full-matrix least-squares method with the

SHELX97 computer program. The function minimized

range where none of the solvents absorb. Runs with t_/ꢀ

^/2

170 s were recorded on an HP8452A instrument

equipped with a thermostated multicell transport; runs

within the 7ꢀ170 s margin were recorded on a HP8452A

or a J&M TIDAS instrument and using a High-Tech

SFA-11 Rapid Kinetics Accessory; for t_/B

7 s an

^/2

Applied-Photophysics stopped-flow instrument con-

nected to a J&M TIDAS spectrophotometer was used.

Observed rate constants were derived from absorbance

versus time traces at the wavelengths where a maximum

increase and/or decrease of absorbance was observed,

table S8 (supplementary material) collects the obtained

k_obsvalues for all the complexes studied, as a function of

the starting complex, entering amine ligand, and tem-

perature and solvent. No dependence of the observed

rate constant values on the selected wavelengths was

detected, as expected for reactions where a good

retention of isosbestic points is observed. The general

kinetic technique is that previously described

[10,11,13,18,19], pseudo-first order conditions were

2 2

was awjjF_oj ꢂ

jF_cj j where wꢃ

[s²(I)ꢁ

(0.0385P)²ꢁ

11.943P]^ꢂ1, and Pꢃ

(jF_oj ꢁ2jF_cj )/3, f, f? and fƒ were

taken from International Tables of X-Ray Crystal-

lography [23]. H atoms were computed and refined,

using a riding model, with an overall isotropic tempera-

ture factor equal to 1.2 time the equivalent temperature

factor of the atom which are linked. Tables S2ꢀS7

collect relevant crystallographic data.

2.4. Kinetics

For the substitution processes, the reactions were

followed by UVꢀVis spectroscopy in the 500ꢀ330 nm

Table 1

Crystal data, and relevant bond angles and distances for the crystal structure of the compounds 5 and 7

Length (A)

Compound

Crystal data

Bond

Bonds

Angle (8)

Ptꢀ

Cl1

2.062(6)

2.059(6)

2.015(6)

2.151(5)

2.189(6)

2.408(2)

C1ꢀ

C2ꢀ

C3ꢀ

N1ꢀ

N2ꢀ

Ptꢀ

83.8(3)

99.9(3)

172.2(3)

93.5(3)

87.1(2)

87.1(3)

88.4(2)

176.1(3)

90.6(2)

79.2(2)

96.2(2)

173.3(2)

94.3(2)

93.5(2)

86.5(2)

Space group

Formula

Monoclinic, P2₁/c

C₁₉H₂₃Cl₅N₂Pt

10.4530(10)

16.3740(10)

13.9310(10)

103.4710(10)

2318.8

/C3

C11

a (A)

b (A)

c (A)

b (8)

Cl1

V (A )

r_calc(g cm^ꢂ3

)

1.867

651.37

Temperature (K)

l (A)

293

0.10697

13935

Cl1

N₀(F_oꢀ

2u_max(8)

R(F_o), wR₂(F_o²)

¹₂C₆Cl₆

2s)

3821

0.0398, 0.0896

Ptꢀ

Cl1

2.063(11)

2.054(11)

2.037(9)

2.146(7)

2.308(9)

2.394(3)

C1ꢀ

C2ꢀ

C3ꢀ

N1ꢀ

N2ꢀ

Ptꢀ

Cl1

85.5(6)

85.6(4)

89.3(4)

178.7(4)

90.2(3)

100.1(5)

174.8(5)

93.3(5)

87.0(4)

80.1(4)

95.3(3)

171.4(3)

91.9(3)

92.4(2)

89.1(2)

Space group

Formula

Monoclinic, C2/c

C₂₂H₂₄Cl₈N₂Pt

22.7180(10)

13.5170(10)

18.3240(10)

105.48

a (A)

b (A)

c (A)

b (8)

V (A )

r_calc(g cm^ꢂ3

5422.8(6)

1.948

)

95.127

Temperature (K)

l (A)

293

0.10697

13261

6259

Cl1

N₀(F_oꢀ

2u_max(8)

R(F_o), wR₂(F_o²)

2s)

0.0457, 0.1252

272

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ277

maintained whenever possible, and the platinum con-

centration was kept at 2.5ꢄ

10^ꢂ4M to avoid undesired

decomposition reactions. No substitution reactions in-

volving NH₂Me in acetone solution could be carried out

given the undesired condensation reaction of the amine

with the acetone which produced a dramatic effective

decrease on the amine concentration.

3. Results

Complexes 1 and 2 have been prepared according to

the literature [10,11,18,19]. Reaction of the dimethylsul-

fide derivatives with the corresponding ligands in

acetone solution, produced the expected 6, 7, 8 and 9

complexes. For compounds 4 and 5 toluene solutions

were used instead, due to the condensation reaction of

the monoalkylamines with acetone occurring under

these conditions, while for compound 3 solvolysis in

DMSO of complex 1 was the method used. The

complexes that have been characterized by their ele-

mental analysis and NMR spectra (Table S1); the X-ray

crystal structure determination of compounds 5 and 7

has been carried out (Table 1, Fig. 1).

The structures (Tables S2ꢀS7, supplementary mate-

rial) indicate that the products have the same geome-

trical arrangement than that found for most of these

complexes, as expected [10ꢀ12,18,19]. In the case of 7

the crystals contain of a C₆Cl₆molecule, per molecule

of complex, which helps crystallization due to stacking

of the phenyl rings. In fact the starting

C₆Cl₆CHNCH₂C₆H₅imine [18] had only a 5ꢀ10% of

excess C₆Cl₆, concentration on the final obtained solid is

dramatically increased to 18% (see Section 2) and to the

above-mentioned 50% for the X-ray quality crystals; no

good crystals could be obtained with an analytically

pure sample of the imine starting material. No special

features are found in the structure, as a whole it seems to

be less constrained in angles and distances than the

previously known structures of similar complexes. The

Fig. 1. View of the structures of complexes 5 (a) and 7(b). Ellipsoids

indicate 20% probability.

the less favoured available arrangement. Some sort of

interaction should be established in solution that

produces this form, as already kinetically detected for

the SBzl₂by phosphine substitution reactions [11,14].

¹H NMR monitoring of the substitution processes on

the complexes under kinetic conditions indicated in all

cases that the process is clean. Table 2 collects all the

equilibrium constants derived from ¹H NMR experi-

ments for the reactions studied kinetically; it is clear that

the effect of size and donor capabilities of the leaving/

entering ligands plays a crucial role for the values

obtained.

Ptꢀ/Cl bonds are longer, as expected, given the coordi-

nation of a better donor (N versus S) that produces a

less acidic Pt(IV) centre. Nevertheless, it is interesting to

see that, even with a small ligand such as NH₂Pr, the

structure of compound 5 keeps having the trans^aromCꢀ

Cl structure found for much more crowded systems.

Anyway, the relative position of the substituents on the

octahedral Meꢀ

Clꢀ

N^amineface is revealing, while for

the 7 complex the Meꢀ

Clꢀ

N^amineface has the same L-

The substitution reactions studied have been mon-

like arrangement found for similar SMe₂and PPh₃

systems [10,11,18,19], for the 5 compound the arrange-

ment is D-like, as that found for the SBzl₂complex, 2

[11]. Furthermore, the position of the benzyl dangling

arm of the imine ‘faces’ both the NH₂Pr and the SBzl₂

ligands, in their respective complexes, which seems to be

itored via electronic spectroscopy in the full 500ꢀ330 nm

wavelength range. The presence of only one clean

process is confirmed by the retention of isosbestic points

in the UVꢀ/Vis spectra over time. In some cases, and

under the desired pseudo-first order conditions of the

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ

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273

reflects the behaviour observed for the substitution

processes indicated.

In view of this dependence, and our previously

published work [10ꢀ13], the reaction mechanism and

rate law depicted in Scheme 2 and Eq. 2 were used to fit

the data for the systems indicated.

Scheme 2.

Fig. 2. Plots of the dependence of the observed rate constants on the

concentration ratio of entering/leaving (k) and leaving/entering (I)

ligand concentrations for the reaction of 5 with PPh₃in toluene

solution at 35 8C.

concentration of the leaving ligand, no reaction was

detected, as expected from the small values of the

equilibrium constants measured. In these cases the

reactions were studied only in the presence of entering

ligand, as the absorbance versus time traces showed a

good first order behaviour [10ꢀ12]. Nevertheless, for

most of the reactions studied a retardation effect on the

rate constant is detected in the presence of leaving ligand

in the reaction mixture; in all these cases, a concomitant

direct dependence on the concentration of entering

ligand is observed, Fig. 2 shows the concentration

effects described.

Even so, for some of the substitution reactions

studied, where an equilibrium is established, the ob-

served reaction rate constant is found independent on

both entering and leaving ligand concentrations. Fig. 3

ð2Þ

For the substitution reactions showing the saturation

behaviour indicated in Fig. 2, the plots derived from the

dependencies indicated in Eq. 2a were applied to

calculate the values of k₁k_ꢂ1/k₂, and k_ꢂ2(when

measurable) from the asymptotic values and the fitted

equation; the values of k₂cannot be estimated sepa-

rately, as in similar circumstances [10,24]. The substitu-

tion reactions showing a lack of any concentration

dependence (Fig. 3) can only be compromised with one

of the facts indicated in Eq. 2b or Eq. 2c, in both cases

the value for k_obsbecomes that for k₁. Eq. 2c could only

be applied when no equilibrium was detected (see Table

2), while Eq. 2b was applied whenever an equilibrium

situation is observed with no concomitant entering or

leaving ligand concentration dependence of k_obs. Table 3

collects all the kinetic and thermal activation parameters

for the dissociation processes studied (k₁, and DH₁^"and

DS₁^"), while Table 4 collects the kinetic parameters

related with the reversibility of the whole process (k_ꢂ2

)

Fig. 3. Plots of the dependence of the observed rate constants on the

concentration of entering and leaving ligand concentrations for the

reaction of 1 with NMe₃in acetone solution at 15 8C.

and the pentacoordinated intermediate discrimination

factor (k_ꢂ1/k₂). For complexes 1 and 2, the values of k₁

274

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ277

determined in this study agree within the 95% margin of

the predicted values derived for the SMe₂by phosphine

substitution processes previously studied, consequently

their thermal activation parameters are those previously

described [10,11]. Fig. 4, clearly indicates the validity of

the reaction mechanism indicated in Scheme 2, the

Eyring plots of k₁and k_ꢂ2, for those systems where

these values correspond to the same process, agree

within error; the activation parameters collected in

Table 3 are derived from the global fitting.

Table 2

Values of the equilibrium constants measured for the substitution

processes studied

Complex

Leaving ligand (L)

Entering ligand (E)

K_eq

SMe₂

NH₂Me

NH₂Pr

NHMe₂

NMe₃

n.d.

n.d

0.67

n.d.

3.4

SBzl₂

NH₂Me

NHMe₂

NMe₃

PPh₃

SOMe₂

n.d.

3.0

4. Discussion

NH₂Me

NH₂Pr

NHMe₂

NMe₃

PPh₃

2.0

The trends obtained for the equilibrium constants

measured for the systems depicted in Table 2 could be

explained on the basis of the varying the stereo-

electronic characteristics of the entering and leaving

ligands. The degree of advance of the sulfide by NMe₃

substitution on 1, 2 and 3 observed is perfectly

consistent with the difference in the size of the SMe₂,

SBzl₂and SOMe₂ligands, despite of the important

differences in the inductive effects of the two sulfides.

The greater basicity and smaller cone angle of the

NH₂Me and NHMe₂entering ligands (NH₂Me, pK_a

PPh₃

n.d.

n.d

7.7

PPh₃

Measured at room temperature in CDCl₃solution; n.d., indicates

that no equilibrium was detected under the kinetic conditions used in

the study.

9.8, uꢃ

1328) [15ꢀ17] seems to the dominant factor for

the complete displacement of the equilibrium for the

formation of the amine complexes under the conditions

used. The substitution process of the amine ligands by

10.6, uꢃ

1068; NHMe₂, pK_a10.8, uꢃ1198; NMe₃, pK_a

Table 3

Summary of kinetic and thermal activation parameters for the leaving ligand reactions studied; phosphine data from references [18,19]

Complex

Leaving ligand (L)

Entering ligand (E)

Phosphines

Solvent

²⁹⁸k₁(s^ꢂ1

)

DH₁^"(kJ mol^ꢂ1

)

DS₁^"(kJ mol^ꢂ1

31910

)

Acetone

Chloroform

SMe₂

0.04

919

NH₂Me

NH₂Pr

NHMe₂

NMe₃

Acetone

Toluene

Phosphines

Acetone

Chloroform

0.53

579

ꢂ

6298

SBzl₂

NHMe₂

NMe₃

Phosphines

NH₂Me

NHMe₂

NMe₃

Toluene

0.91

789

1597

SOMe₂

PPh₃

Acetone

Toluene

Acetone

Toluene

Acetone

Toluene

629

569

1059

1119

94911

1079

ꢂ

ꢂ/

129

379

299

50920

4930

46913

NH₂Me

NH₂Pr

NHMe₂

PPh₃

0.00011

0.00018

0.00032

ꢂ/

PPh₃

^aNMe₃

0.81

1059

919

1129

84912

119

529

^bPPh₃

PPh₃

0.0031

0.0001

/14

/11

indicates

that the values agree with the phosphine substitution processes.

Fitting includes k_ꢂ2data from SMe₂and SBzl₂by NMe₃substitution, see Fig. 4^b.

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ

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275

Table 4

Summary of the kinetic parameters related with the reversibility of the process and the pentacoordinated intermediate discrimination factor

(s^ꢂ1

)

ꢂ2

298

Complex

Leaving ligand (L)

Entering ligand (E)

Phosphines

Solvent

k_ꢂ1/k₂

Acetone

Chloroform

SMe₂

1.1

n.d.

NH₂Me

NH₂Pr

Toluene

Acetone

Toluene

Acetone

Toluene

Acetone

Chloroform

Acetone

Toluene

Acetone

Toluene

Acetone

Toluene

Acetone

Toluene

0.5

0.8

n.d.

NHMe₂

NMe₃

1.1

n.d.

1.1

n.d.

Equals tok₁

SBzl₂

Phosphines

n.d.

NHMe₂

NMe₃

2.0

3.2

0.4

1.5

2.0

1.0

2.0

n.d.

0.8

6.5

2.2

n.d.

0.062

n.d.

NH₂Me

NHMe₂

NMe₃

0.062

n.d.

SOMe₂

PPh₃

NH₂Me

NH₂Pr

NHMe₂

PPh₃

Equals to k1

0.00010

0.000070

n.d.

NMe₃

0.7

n.d.

PPh₃

1.7

0.000080 ^b

Average for the conditions studied.

From the global fitting indicated in Table 3, see Fig. S 1.

PPh₃on complexes 4, 5, 6 and 7 follows the same trend

found for the sulfide leaving ligands, the cone angles of

the leaving ligand seem to be the dominant factor for the

equilibrium displacement position, despite the variation

in pK_avalues. In this respect substitution by pyridine

(pK_a5.19) on complex 1 does not follow the same trend

than that for the alkylamines; probably the important

electronic changes introduced on the nitrogen donor are

too important for the maintenance of the trend obtained

with the NH₂Me, NHMe₂and NMe₃sequence.

dissociation of the Ptꢀ

NMe₃bond under different

premises. The same assumption applies for the 4 plus

PPh₃system given the fact that the equilibrium is not

fully displaced (Figure S1). It is interesting to note that

only for systems having more positively charged proton

centres in the leaving ligand (SBzl₂, SOMe₂, and

NHMe₂) an important difference in the thermal activa-

tion parameters of the dissociation rate constants is

obtained in acetone and toluene. For the SBz1₂and

NHMe₂complexes (2, 6) the values for k₁are smaller in

acetone due to the difficulties in the dissociation, while

the values of the thermal activation parameters have a

tendency for a more ordered and less enthalpic demand-

ing transition state. It seems clear that the postulated

solvent-assisted interaction between the leaving ligand

and the Pt skeleton for these systems is operative [11].

Surprisingly, for the DMSO complex, 3, the opposite

trend is observed, indicating that these type of interac-

tion has to have different characteristics. The fact that

the oxygen at the pyramidal SOMe₂can cancel the

charge separation of the ligand itself could prevent the

above mentioned solvent-assisted electrostatic interac-

tion with the rest of the molecule. Even so, the thermal

activation parameters indicated for the reaction do not

seem to agree very well with this fact, the activation

entropy in toluene solution being clearly negative.

Taking into account that the amount of DMSO present

in the solutions is very high (see Table S8), due to the

The simple kinetic behaviour shown by the substitu-

tion processes studied on complexes 1 and 2 agrees well

with that found for the substitution processes with

phosphines [10ꢀ12]. Even the differences detected on

changing the solvent, when possible (see Section 2), due

to solvent-assisted interactions with the leaving SBzl₂

ligand on complex 2 substitutions are equivalent (Table

3) [11]. Only for the 1 plus NMe₃systems the kinetic

behaviour is too simple to evaluate the k_ꢂ1/k₂para-

meter (Table 4), the data are interpreted according to

Eq. 2b. Although having k₁ꢃ

coincidence, it is clear that if k_ꢂ1

k_ꢂ2seems too much of a

k₂(or k_ꢂ1k_ꢂ2

ꢁ

k₁k₂), according to Eq. 2c, is the responsible of the

observed concentration dependence, the approximation

should held even more strongly for complex 2, where

k_ꢂ1is expected to be smaller. This is not so, and

consequently it has to be k₁ꢃ/k_ꢂ2, as corroborated by

Fig. 4 which shows the Eyring plot corresponding to the

276

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ277

very labile character of the ligand (see Section 2), the

polarity of the solvents must be dramatically modified,

specially that of toluene. In this case the trend observed

in Table 3 for the substitution by PPh₃on compound 3

agree with the results for the substitution of complexes 2

and 6, and those for other complexes [11]. If this is the

case, the solvent assistance in toluene should be

reinforced by the high concentration of DMSO in the

solution medium, while for acetone the differences

different leaving ligands are evident. Furthermore,

when differences attributable to the presence of suitable

proton centres that allow solvent-assisted interactions

with the leaving ligand, are present, the trend in the

values found for ²⁹⁸k₁parallel the observed differences

in the values of DS^", indicating the ordering occurring

on such interactions. In this respect, all the values

determined for k_ꢂ1/k₂for compounds 2 and 6, and

collected in Table 4, indicate that the back entrance of

the leaving ligand with suitable proton centres is

favoured in acetone when compared with the same

value in toluene, as would be expected. The opposite

trend is observed for the substitutions on complex 3,

nevertheless the reasons indicated above (fairly concen-

trated toluene solutions of DMSO being de facto more

polar) point to the same type of process.

should be at a minimum. As a whole, the Ptꢀ

/SOMe₂

bond dissociation occurs both in toluene and acetone

solutions with a certain degree of ordering derived from

the acetone and/or DMSO assistance in the interactions

with the Pt complex inert skeleton. Consequently the

activation enthalpies are low as expected from this

ordering [11].

The trend indicated by the ²⁹⁸k₁data of Table 3 is a

clear indicative of the strength of the Pt-leaving ligand

bond that includes both enthalpic and entropic compo-

nents. That is, not only are the enthalpic demands

important for the dissociation but also the degree of

dissociation needed to reach the transition state. This is

very well shown in the complexes with NH₂Me and

NMe₃as leaving ligands, where, despite the fact that

similar enthalpic demands apply, the degree of dissocia-

tion on the amine in the transition state is larger for the

more bulky NMe₃. The trend indicates that the effective

The surprising lack of an important discrimination

factor found in the k_ꢂ1/k₂, ratio of Table 4 for this

intermediate has already been detected, but the same

behaviour has already been found for other processes

involving isolated pentacoordinate intermediates [10ꢀ

12]. It is interesting to note, though, that the trend

observed for the available k_ꢂ1/k₂data does not corre-

spond to that observed for k₁. That is, although PPh₃is

a worse leaving ligand than NHMe₂, according to the

dissociation rate constant, the proposed pentacoordi-

nated intermediate favours the entrance of NHMe₂in

front of PPh₃. Important solvent-assisted interactions

are bound to be operating, specially in polar solutions,

where the discrimination factors increase to larger

values. When either solvent (or solution polarity) or

leaving ligand (NMe₃) do not allow these type of

interactions, these ratios approach to unity as found in

most of the systems.

Finally, the existence of the k_ꢂ2term in Eq. 2 (Tables

2 and 4) is easily related with the sequence of substitu-

tion bond strength mentioned above. Only for systems

with entering and leaving ligands close in the sequence,

simultaneous observation of k₁and k_ꢂ2has been

possible.

substitution strength of the Ptꢀ

L bond is PPh₃ꢀ

pyꢀ

NH₂MeꢀNHMe₂ꢀNMe₃ꢀSMe₂ꢀ

SBzl₂ꢀSOMe₂,

which is not in line with the expected for the theoreti-

cally hard Pt(IV) centre; no doubt that the presence of

three Ptꢀ/C bonds in the complex produces important

changes in the hardness of the centre. Nevertheless, the

steric demands included in the cone angle of the

Acknowledgements

We acknowledge ﬁnancial support for the BQU2001-

3205 project from the Ministerio de Ciencia y Tecnolo-

g´ıa.

References

[1] M.L. Tobe, J. Burgess, Inorganic Reaction Mechanisms, Addi-

son-Wesley, Reading, MA, 1999.

Fig. 4. Combined Eyring plot for the substitution reactions of NMe₃

by PPh₃and py on the studied complex skeleton. Open symbols

indicates k₁for the process occurring on 7; solid symbols refer to k_ꢂ2

for the inverse process taking place on 1 and 2.

[2] M.V. Twigg, Mechanisms of Inorganic and Organometallic

Reactions, Plenum, New York, 1991.

[3] D.D. Wick, K.I. Goldberg, J. Am. Chem. Soc. 119 (1997) 10235.

J. Esteban et al. / Inorganica Chimica Acta 351 (2003) 269ꢀ

/277

277

[4] C.R. Baar, G.S. Hill, J.J. Vittal, R.J. Puddephatt, Organometal-

lics 17 (1998) 32.

[15] A.L. Seligson, W.C. Trogler, J. Am. Chem. Soc. 113 (1991) 2520.

[16] T.L. Brown, K.J. Lee, Coord. Chem. Rev. 128 (1993) 89.

[17] R.M. Smith, A.E. Martell, Critical Stability Constants, Plenum,

New York, 1989.

[5] J.R. Alger, J.H. Prestegord, J. Magn. Resonance 27 (1977) 137.

[6] T. Shi, J. Berglund, L.I. Elding, Inorg. Chem. 35 (1996) 3498.

[7] S. Choi, S. Mahalinggaiah, S. Delaney, N.R. Neale, S. Massod,

Inorg. Chem. 38 (1999) 1800.

[18] M. Crespo, M. Mart´ınez, E. de Pablo, J. Chem. Soc., Dalton

Trans. (1997) 1231.

[8] M.D. Hall, T.W. Hambley, Coordin. Chem. Rev. 232 (2002) 49.

[9] A. Peloso, Coordin. Chem. Rev. 10 (1973) 123.

[10] P.V. Bernhardt, C. Gallego, M. Martinez, Organometallics 19

(2000) 4862.

[19] M. Crespo, M. Mart´ınez, J. Sales, Organometallics 12 (1993)

4297.

[20] R. Romeo, M.R. Plutino, L.M. Scolaro, S. Stoccoro, G.

Minghetti, Inorg. Chem. 39 (2000) 4749.

[11] P.V. Bernhardt, C. Gallego, M. Martinez, T. Patella, Inorg.

Chem. 41 (2002) 1747.

[21] R. Romeo, Comments Inorg. Chem. 11 (1990) 21.

[22] G.M. Sheldrick, ^SHELXL97, Program for Crystal Structure

Determination, University of Gottingen, Gottingen, Germany,

¨ ¨

[12] M. Font-Bard´ıa, C. Gallego, G. Gonza´lez, M. Martinez, A.E.

Merbach, X. Solans, J. Chem. Soc., Dalton Trans., in press.

[13] M. Font-Bard´ıa, C. Gallego, M. Martinez, X. Solans, Organo-

metallics 21 (2002) 3305.

1997.

[23] International Tables of X-Ray Crystallography, Kynoch Press,

Birmingham, 1974.

[14] H. Kumita, T. Kato, K. Jitsukawa, H. Einaga, H. Masuda, Inorg.

Chem. 40 (2001) 3936.

[24] J.H. Espenson, Chemical Kinetics and Reaction Mechanisms,

McGraw-Hill, New York, 1995.

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