Effect of the Entering Group

Substitution Reaction Kinetics

2.5 Factors Influencing the Reactivity of Square Planar Platinum(II) Complexes Complexes

2.5.1 Effect of the Entering Group

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Figure 2.10 Photograph of the Applied Photophysics SX 20 stopped-flow system coupled to an online data acquisition system setup used by the University of KwaZulu Natal, Pietermaritzburg campus kinetics research group.

2.5 Factors Influencing the Reactivity of Square Planar Platinum(II)

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The nucleophilicity of the ligand is often influenced by several factors:^7b

(a) Polarisability: Polarisability is an important aspect for the rates than for equilibria^14a which is often explained by using Peasons’s

“Hard Soft Acid Base” ^†(HSAB) theory.^7b,39 Increasing polarisability of the metal centre increases the effectiveness of the incoming nucleophile.

(b) Basicity: The basicity of a nucleophile depends on its pKa which often correlates with the nucleophilicity of the entering nucleophile towards the metal centre. The relationship between the logk2 and the basicity of the entering nucleophiles for the substitution reaction of Pt(II) terpyridine type complexes with azole nucleophiles has been reported recently.^21a

(c) Oxidability: Ligands which are easily oxidised are considered as good nucleophiles (strong reducing agents) and their strength can be identified based on their electrode reduction potential.

(d) Metal Centre: The nature of the reaction centre affects the rate of substitution of square planar complexes. Thus, the nature of metal centre limits the applicability of nucleophilicity scales in inorganic chemistry since reactions show dependence on the nature of the metal centre. Heavier elements are better polarised in the transition state following the order: Ni(II) > Pd(II) > Pt(II).

(e) Solvation Energy: Ligands which are easily solvated are weak nucleophiles because energy is required to remove the solvent shell before it gets coordinated with the metal centre.

†Hard acids (metal ions) are small and highly charged (eg: Li⁺ and Mg²⁺) and thieir valence electron shell is not easily distorted while soft acids (metal ions) are large and possess low charge having an easily distortable or removable valence electron shell. Polarisability refers to the “softness” of the nucleophile, i.e. “Hard”

nucleophiles prefers “hard” metal centres and “soft” nucleophiles prefers “soft” substrates or metal centres.

Pt(II) being a soft metal centre is more effective towards large and soft donors.

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Studies reported on Pt(II) complexes show that the nucleophilic reactivity order for commonly used nucleophiles follow the order:^14a,14c,40

R3P > Tu > I^ > SCN > ^ N^3 > NO₂^ > Br^ > py > aniline ~ NH3 ~Cl > H^ 2O >OH ^

It is also important to note that the nucleophilicity of the reagents is independent of their base strength. Based on earlier studies,^14c,40 the most detailed study on nucleophilicity was reported by Bellucco⁴¹ for trans-Pt(py)2Cl2 in methanol at 30 °C (Equation 2.38).²

Pt Cl Cl

py

py + Yⁿ Pt + Cl^-

Y Cl

py py

(n+1)

(2.35)

By using trans-Pt(py)2Cl2 as the standard, the nucleophilic reactivity constants, n_Pt was defined as:^9b

^o

o o

Pt log

S Y

k

n  k (2.36)

where kY = the measured second order rate constant for the entering nucleophile

o

ks = the second order rate constant for the attack of solvent in an associative

mechanism and is equal to ks / [Methanol]

kS = the observed rate constant for solvent attack (methanol) on the complex.

Plots of log kY for different Pt(II) complexes with various number of nucleophiles yielded linear free energy relationship (LFER).⁴² Such a linear free energy relationship is given by:^9b,14a

log kY = sn_Pt + log kS

or

log k2 = sn_Pt + log k-2 (2.37)

where the slope, s depends on the complex and is called the nucleophilic discrimination factor, a measure of the sensitivity of the metal centre to the nucleophilicity of the incoming ligand.^14a A larger value of s means that the reaction is very sensitive to the

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changes in nucleophilicity of the substituting ligands. Ligands that are capable of forming dative π-bonding with Pt(II) centre in the transition state have larger values of s (smaller kS)^14a which enhances the addition of the electrons from the nucleophile.

Generally, Pt(II) complexes with softer ligands have larger s values. The intercept log ks

is the intrinsic reactivity. A smaller value of intrinsic reactivity is accompanied with very fast reactions and hence, a greater nucleophilic discrimination factor. Thus, the nucleophilic reactivity constant is a measure of the reactivity of a nucleophile towards a Pt(II) metal centre.

After a number of investigations, it was also reported that the nucleophilicity of the incoming ligand also depends on other properties^14a such as biphilicity of the ligand and the charge effect^14c,43 while steric hindrance plays a minor role.⁴⁴ Some of the experimental results obtained are presented in Table 2.1. For many nucleophiles, a plot of kY against

n

_Pt is a straight line as represented in Figure 2.11.

Table 2.1 Some nucleophilic constants given for Pt(py)2Cl2 with different nucleophiles of donor atoms.^14a,41,45

Nucleophile n^o_Pt Nucleophile n^o_Pt Nucleophile n^o_Pt

O-Donor

CH3O^ˉ <2.4

N-Donor C6H5NH2

NH3

C5H5N H2N-NH2

3.02 3.06 3.13 3.85

Halogens

36Clˉ Brˉ Iˉ

3.04 4.18 5.42 S-Donor

(C6H5)2S S=C(NH2)2

(CH3)2S SCNˉ

4.38 7.17 4.73 6.65

Se-Donor (C6H5CH2)2Se (CH3)2Se

5.39 5.56 P-Donor

(C6H5)P (C2H5)3P

8.79 8.85 C-Donor

CNˉ 7.14

As-Donor

(C2H5)3As 7.54

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0 1 2 3 4 5 6 7

-3 -2 -1 0 1 2 3 4 5 6 7 8

5 + logk Y

_Pt N^-₃ Br^-

I^- SCN^-

MeOH

NO^-₂Cl^- H₂O

N^-₃ NH₃

Br^-

I^- SCN^- TU SeCN^-

Figure 2.11 Correlation of the rates of reaction of Pt(II) complexes with the standard trans- Pt(py)2Cl2 for different nucleophiles: •, trans-Pt(PEt3)2Cl2 in methanol at 30 °C; ∎, Pt(en)Cl2 in water at 35 °C, produced from references.^14a,41,46

In a study van Eldik et al.⁴⁷ reported the nucleophilic discrimination ability of Pt(II) complexes of [Pt(N-N-C)Cl] (N-N-CH = 6-phenyl-2,2'-bipyridine), [Pt(N-C-N)Cl] (N-CH- N = 1,3-di(2-pyridyl)benzene), and [Pt(N-N-N)Cl]Cl (N-N-N = 2,2':6',2''-terpyridine) where the rate constants for the substitution of Cl⁻ by MeOH, Br⁻ and I⁻ were measured as well as the corresponding logk2 verse nPt. In that study, it was found that due to the biphilic nature of thiourea, its nucleophilic discriminating factor was misleading from plot. The resulting values for s were reported to be 0.61 ± 0.04 (NCN), 0.9 ± 0.1 (NNC), 1.39 ± 0.02 (NNN) respectively.