Diffusion NMR studies on neutral and cationic square planar palladium(II) complexes

Diffusion NMR investigations were carried out in CD₂Cl₂ for a series of neutral (1-7) and cationic (8-10) square planar palladium complexes. Diffusion data were elaborated through a modified Stokes-Einstein equation that takes into account the size and shape of molecules. The hydrodynamic volume at infinite dilution of all complexes was found to be similar to the crystallographic volume and always much larger than the van der Waals volume. The self-aggregation tendency of [Pd(N,C⁻)(N,N)][PF₆] ionic complexes [(N,C⁻) = (C₆H₄-(Ph)C(O)-CN-Et); 8, (N,N) = 2,2′-bipirydine; 9, (N,N) = (2,6-(iPr)₂-C₆H₃)NC(Me)-C(Me)N(2,6-(iPr)₂-C₆H₃); 10, (N,N) = (2,6-(iPr)₂-C₆H₃)NC(R′)-C(R′)N(2,6-(iPr)₂-C₆H₃), R′₂ = naphthalene-1,8-diyl] was investigated by performing ¹H and ¹⁹F diffusion experiments as a function of the concentration. Clear evidence for the formation of ion triples containing two cationic units was obtained for 8, most likely due to the establishment of a weak Pd?O interaction. The tendency to form ion triples was much reduced in 9 and 10, having an increased steric hindrance in the apical positions. While 9 showed the usual tendency to afford a mixture of free ions and ion pairs, solvated ions were the predominant species in the case of 10 even at high concentration values (approaching 100 mM).