Qualitative and quantitative discrimination of fake and true alkene rotation processes in pd(η2-olefin) complexes. A new bimolecular mechanism

The fluxional behaviour of [Pd(η2-fn)(N-SMe)] (2) (fn = fumaronitrile, N-SMe = 2-methylthiomethylpyridine) and of [Pd(η2-tmetc)(N-N′-4-anisyl)] (3) (tmetc = tetramethylethylenetetracarboxylate, N-N′-4-anisyl = 2-(4-methoxyphenyliminomethane)pyridine) were monitored by 1H NMR spectroscopy and quantitatively determined by line-shape analyses (for 2) and selective inversion recovery experiments (for 3). The coalescence of the AB multiplet of fn hydrogens of 2 is concentration dependent and presents a strongly negative ΔS≠, suggesting the intermediacy of a dimeric complex and ruling out the hypothesis of olefin rotation. The accurate evaluation of all spectral features also allows determination of the approaching mode of the monomeric units. The inversion transfer between the tmetc methyls of 3 reveals a true propeller-like olefin rotation. The presence of a nucleophilic electron pair at sulfur in 2 triggers the formation of the dimeric intermediate.