Isomer distribution and interconversion in cationic allylpalladium(II) complexes with 2-(iminomethyl)pyridine ligands

The complexes [Pd(eta(3)-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, CMe(3), C(6)H(4)OMe-4; R = Me, R' = C(6)H(4)OMe-4) and C5H4N-2-CH(2)NMe(2)] are present in solution with different isomers, the structures of which may be assigned by an H-1 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(eta(3)-3-methyl-2-butenyl)(N-N')]ClO4 the predominant isomer (ca. 100%) has a structure with the allylic methyl groups cts to the coordinated pyridine nitrogen when N-N' = C5H4N-2-CH=NCMe(3) and cis to the coordinated imino nitrogen when N-N' = C5H3(6-Me)N-2-CH=NC(6)H(4)OMe-4. Ln 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 eta(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 (eta(3)-2-methyl-2-propenyl)palladium(II) derivatives. For [Pd(eta(3)-2-methyl-2-propenyl)(N-N')]ClO4 [N-N' = C5H3(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 ClO4-. For a solution of [Pd(eta(3)-3-methyl-2-butenyl)(N-N')]ClO4 (N-N' = C5H4N-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 degrees C does not involve Pd-NMe(2) bond breaking. The rupture of this bond takes place when the temperature is raised to ca. 90 degrees C or when the complex is dissolved in dimethyl-d(6) sulfoxide at ambient temperature.