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

The complexes [Pd(η 3 -allyl)(N-N‘)]ClO 4 [allyl = 2-butenyl or 3-methyl-2-butenyl, N-N‘ = C 5 H 3 (6-R)N-2-CH NR‘ (R = H, R‘ = Me, CMe 3, C 6 H 4 OMe-4; R = Me, R‘ = C 6 H 4 OMe-4) and C 5 H 4 N-2-CH 2 NMe 2 ] are present in solution with different isomers, the structures of which may be assigned by an 1 H NMR criterion based on chemical shift changes of the pyridine H(6) and/or of the allylic methyl protons, as confirmed also by 2D 1 H NMR spectra. The isomer distribution depends mainly on the steric requirements of both the allyl and N-N‘ ligands: for [Pd(η 3 -3-methyl-2-butenyl)(N-N‘)]ClO 4 the predominant isomer ( ca . 100%) has a structure with the allylic methyl groups cis to the coordinated pyridine nitrogen when N-N‘ = C 5 H 4 N-2-CH NCMe 3 and cis to the coordinated imino nitrogen when N-N‘ = C 5 H 3 (6-Me)N-2-CH NC 6 H 4 OMe-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 η 3 -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 (η 3 -2-methyl-2-propenyl)palladium(II) derivatives. For [Pd(η 3 -2-methyl-2-propenyl)(N-N‘)]ClO 4 [N-N‘ = C 5 H 3 (6-Me)N-2-CH NC 6 H 4 OMe-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 ClO 4 - . For a solution of [Pd(η 3 -3-methyl-2-butenyl)(N-N‘)]ClO 4 (N-N‘ = C 5 H 4 N-2-CH 2 NMe 2 ) 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−NMe 2 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.