Isomer Distribution and Interconversion in Cationic Allylpalladium(II) Complexes with 2-(Iminomethyl)pyridine Ligands

The complexes [Pd(η3-allyl)(N-N‘)]ClO4 [allyl = 2-butenyl or 3-methyl-2-butenyl, N-N‘ = C5H3(6-R)N-2-CHNR‘ (R = H, R‘ = Me, CMe3, C6H4OMe-4; R = Me, R‘ = C6H4OMe-4) and C5H4N-2-CH2NMe2] are present in solution with different isomers, the structures of which may be assigned by an 1H NMR criterion based on chemical shift changes of the pyridine H(6) and/or of the allylic methyl protons, as confirmed also by 2D 1H NMR spectra. The isomer distribution depends mainly on the steric requirements of both the allyl and N-N‘ ligands: for [Pd(η3-3-methyl-2-butenyl)(N-N‘)]ClO4 the predominant isomer (ca. 100%) has a structure with the allylic methyl groups cis to the coordinated pyridine nitrogen when N-N‘ = C5H4N-2-CHNCMe3 and cis to the coordinated imino nitrogen when N-N‘ = C5H3(6-Me)N-2-CHNC6H4OMe-4. In chlorinated solvents (dichloromethane-d 2 or 1,1,2,2-tetrachloroethane-d 2) the isomers undergo mutual interconversion through a mechanism involving an apparent rotation of the η3-allyl ligand around its bond axis to the metal. The interconversion rates depend on the nature of allyl and N-N‘ ligands and increase considerably when the compounds are dissolved in dimethyl-d 6 sulfoxide. The apparent allyl rotation also occurs for the analogous (η3-2-methyl-2-propenyl)palladium(II) derivatives. For [Pd(η3-2-methyl-2-propenyl)(N-N‘)]ClO4 [N-N‘ = C5H3(6-Me)N-2-CHNC6H4OMe-4] the allyl rotation rate increases with increasing concentration up to a limiting constant value. This behavior is interpreted on the basis of a mechanism involving a fast and reversible association of the cationic complex with the perchlorate anion to form a loose ion pair which undergoes a rate-determining molecular geometry rearrangement upon coordination of ClO4 -. For a solution of [Pd(η3-3-methyl-2-butenyl)(N-N‘)]ClO4 (N-N‘ = C5H4N-2-CH2NMe2) in 1,1,2,2-tetrachloroethane-d 2, the 2-D ROESY spectrum suggests that the apparent allyl rotation at 28 °C does not involve Pd−NMe2 bond breaking. The rupture of this bond takes place when the temperature is raised to ca. 90 °C or when the complex is dissolved in dimethyl-d 6 sulfoxide at ambient temperature.