Dynamic NMR Study of Ethene Exchange in Cationic CNN-Type Platinum(II) Complexes

Cationic ethylene platinum(II) complexes of the type [Pt(CNN)(C2H4)]+, containing a methyl fragment and different diimines (NN), or terdentate (κC-κ2NN′) anionic ligands, were synthesized and fully characterized both as solids and in solution [NN = 2,2′-dipyridylamine, 1; 2,2′-dipyridylsulfide, 2; 1,10-phenanthroline, 3; 4,7-diphenyl-1,10-phenanthroline, 4; 3,4,7,8-tetramethyl-1,10-phenanthroline, 5; 2,2′-bipyridine, 6; HC-NN = 6-tert-butyl-2,2′-bipyridine, 7; 6-neo-pentyl-2,2′-bipyridine, 8; 6-phenyl-2,2′-bipyridine, 9; 6-(α-methyl)benzyl-2,2′-bipyridine, 10; 6-(α-ethyl)benzyl-2,2′-bipyridine, 11; 6-(α,α-dimethyl)benzyl-2,2′-bipyridine, 12]. Crystals suitable for X-ray analysis of complexes 5 and 7 were obtained. Ethene exchange at the cyclometalated platinum(II) complexes 7, 8, and 10−12 was studied by 1H NMR line-broadening experiments in chloroform-d, as a function of both temperature and olefin concentrations. For the other prepared complexes the process was too fast to be monitored on the NMR time scale even at the lowest temperature. The ethylene exchange rates show a linear dependence on the concentration of the free ligand, with a negligible k1 term indicating that either a solvolytic or a dissociative pathway to the products is absent or negligible. The values of the second-order rate constants kexc, as obtained by linear regression analysis of the experimental data at 298 K, are in a range of ca. 104−105 s−1 m−1. The activation entropies are negative, ranging between −129 and −112 J K−1 mol−1, as expected for associative processes. The activation process is largely entropy controlled: the TΔS⧧ contribution to the free energy of activation is extremely large, amounting to more than 80% for all complexes, with a smaller enthalpy contribution. All the experimental findings evidence that the mechanism takes place via an associative attack by the entering olefin, through a well-ordered, stable pentacoordinated transition state with the two ethene molecules on the trigonal plane. The reactivity of [Pt(CNN)(C2H4)]+ complexes is strongly dependent on the choice of coordinated 6-substituted-2,2′-bipyridines, especially when the terdentate anionic fragment is capable of generating steric crowding and congestion on the coordination plane.