Self-association of new mixed-ligand diimine–N-acyl-N′,N′-dialkyl thioureate complexes of platinum(II) in acetonitrile solution†

Abstract

The complexes [Pt(bipy)L]PF₆ and [Pt(phen)L]PF₆ (HL = N-acyl-N′,N′-di-n-butylthiourea) showed pronounced self-association in CD₃CN solution at 25 °C according to an equilibrium 2[Pt(diimine)(L-S,O)]⁺ ⇌ [{Pt(diimine)(L-S,O)}₂]²⁺ for which association constants, K_D, have been estimated from the concentration dependence of the ¹H NMR shifts of these complexes. The values of K_D range from 1.8 to 114 M^–1 depending on the structure of the diimine as well as the nature of the N-acyl group; the values of the 1,10-phenanthroline complexes are generally larger than those for the corresponding 2,2′-bipyridine analogues. In general, N-1-naphthoyl-N′,N′-dibutylthiourea complexes self-associate to a lesser extent than the corresponding N-benzoyl analogues. From the temperature dependence of the K_D values, the thermodynamic parameters ΔG, ΔH and ΔS have been estimated. An average Δ(ΔG) increment of 2.4 ± 0.4 kJ mol^–1 per aromatic ring of the diimine moiety co-ordinated to the complexes is consistent with energies generally associated with π–π stacking and cation π interactions observed for other systems.