A kinetic and mechanistic study of heterodinuclear Ru(II)–Pt(II) complexes of flexible polyethyleneglycol ether linkers

Four heterometallic Ru(II)-Pt(II) complexes linked by polyethyleneglycol ether units viz; [(tpy)Ru(tpy)-O(CH₂CH₂O)n-(tpy)PtCl] (where tpy = 2,2′:6′,2″-terpyridine and <i>n</i> = 1 (RuPtdteg), 2 (RuPtdtdeg), 3 (RuPtdtteg) and 4 (RuPtdttteg)) were synthesized and characterised using ¹H and ¹⁹⁵Pt NMR spectroscopy, mass spectrometry and elemental analysis. The ligand substitution kinetics of the complexes with thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU) and iodide (I^-) were investigated under pseudo first-order conditions as a function of concentration and temperature using conventional Stopped-Flow techniques. The observed pseudo first-order rate constants followed the simple associative rate law <i>k</i>_obs = <i>k</i>₁[Nu]. Density Functional Theory (DFT) calculations demonstrate that the rate of substitution reactions of the heterometallic complexes is dependent on the geometry of the complexes more than it is on the electronic effects. An increase in the length of the polyethyleneglycol ether linker increases the entrapment effect of the nucleophiles due to the V-shape geometry of the complexes which in turn decreases the steric influence by the Ru(tpy)₂ moiety at the reactive Pt(II) metal centre and thus, the reactivity. The reactivity of the nucleophiles is sterically and electronically controlled. The activation parameters obtained support an associative mode of mechanism.