Conformational fluxionality in a palladium(II) complex of flexible click chelator 4-phenyl-1-(2-picolyl)-1,2,3-triazole: A dynamic NMR and DFT study

An experimental and theoretical DFT study was carried out on the solution behavior in [D7]DMF for bis-chelate complex [Pd(L)2](BF4)2·2CH3CN (L = 4-phenyl-1-(2-picolyl)-1,2,3-triazole). In structure of [Pd(L)2]2+, the central square–planar palladium(II) cation is trans-chelated by two L substrates, each through the pyridine and the triazole N2 nitrogen atoms, forming two six-membered metallacycles. These can adopt boat-like conformations anti-trans-[Pd(L)2]2+ and syn-trans-[Pd(L)2]2+ in which the picolyl methylene carbons are anti or syn, respectively, relative to the palladium coordination plane. In solution, the boat-to-boat inversion at both metallacycles takes place. The conformers are in a dynamic equilibrium, which was monitored by variable-temperature (VT) 1H NMR spectroscopy in the temperature range of 223–353 K. The equilibrium lies on the side of the anti-trans-[Pd(L)2]2+ conformer and the corresponding reaction enthalpy and entropy is estimated to be 0.6 ± 0.5 kcal mol−1 and 0.8 ± 1 cal mol−1 K−1, respectively. From the full-line-shape analysis of resonances in the VT 1H NMR spectra, the activation enthalpy and activation entropy was determined to be 13.0 ± 0.4 kcal mol−1 and 2.7 ± 1.6 cal mol−1 K−1, respectively. The activation entropy close to zero suggests a nondissociative mechanism for the isomerisation. DFT investigation revealed that the isomerisation proceeds through a one step mechanism with a barrier of 11.40 kcal mol−1. The structures of the syn and anti conformers as well as that of the transition state were characterized. Energy decomposition analysis was carried out in order to explore the origins of the stability difference between the syn and anti isomers.