A kinetic and mechanistic study of dinuclear Pt(II) 2,2′:6′,2″-terpyridine compounds bridged with polyethyleneglycol ether flexible linkers

Abstract

A series of dinuclear Pt(II) complexes bridged with polyethyleneglycol ether of the type [ClPt(tpy)O(CH₂CH₂O)_n(tpy)PtCl]Cl₂ where n = 1 (Ptdteg), 2 (Ptdtdeg), 3 (Ptdtteg), 4 (Ptdttteg), and linker-free complex, (Ptdt) (where tpy = 2,2′:6′,2″-terpyridine), were synthesized and characterized to investigate the role of bridging polyethyleneglycol ether linker on the substitution reactivity of dinuclear Pt(II) complexes. Substitution reactions were studied using thiourea nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU) under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow reaction analyzer. The reactions gave single exponential fits following the rate law k_obs = k₂[Nu]. Introduction of polyethyleneglycol ether linker decreases the electrophilicity of the platinum center and the whole complex. The results obtained indicate that the rate of substitution is controlled by both electronic and steric hindrance which increases with the length of the linker. Experimental results are supported by density functional theory calculations and structures obtained at B3LYP/LANL2DZ level. The order of the reactivity of the nucleophiles is TU > DMTU > TMTU. The magnitude and the size of the enthalpy of activation and entropy of activation support an associative mode of mechanism, where bond formation in the transition state is favored.

Graphical Abstract

Introduction of ethyleneglycol ether linker decreases the electrophilicity of the platinum center and the whole complex. The rate of substitution reactions is controlled by both electronic and steric hindrance which increases with the length of the linker

Keywords:

• Substitution kinetics
• Mechanistic study
• Dinuclear platinum
• Polyethyleneglycol ether
• Terpyridine

Acknowledgements

The authors greatly acknowledge the financial support to Aishath Shaira from the University of KwaZulu-Natal, South Africa. We also thank Mr Craig Grimmer for his support with NMR measurements and Mrs Janse van Resenburg for her help with mass spectra and elemental analysis.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

^† s = singlet, d = doublet, t = triplet, dt = doublet of a triplet. The same representation is used for the other complexes.