Self-association of new mixed-ligand diimine–N-acyl-N′,N′-dialkyl thioureate complexes of platinum(II) in acetonitrile solution
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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, KD, have been estimated from the concentration dependence of the 1H NMR shifts of these complexes. The values of KD 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 KD 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.