Synthesis and crystal structure of the dinuclear cyclopalladated compounds of methyl (E)-4-(benzylideneamino)benzoate with acetato and chlorido bridge ligands: Study of their splitting reactions with pyridine

Reaction of methyl (E)-4-(benzylideneamino)benzoate C₆H₅CH=N(C₆H₄-4-CO₂Me) with Pd(OAc)₂ produced the dinuclear acetato bridge ortho-cyclopalladated compound [Pd{C₆H₄CH=N(C₆H₄-4-CO₂Me)-κC_ortho,κN}]₂(μ-OAc)₂ (1). Compounds [Pd{C₆H₄CH=N(C₆H₄-4-CO₂Me)-κC_ortho,κN}]₂(μ-Cl)₂ (2) and [Pd{C₆H₄CH=N(C₆H₄-4-CO₂Me)-κC_ortho,κN}(py)(X)] [3 (X = OAc); 4 (X = Cl)] were also prepared and isolated in good yields by substitution reactions. ¹H and ¹³C{¹H} NMR in CDCl₃ solution of compounds 3 and 4 revealed that they consisted of a mixture of trans- and cis-N,N isomers. Addition of pyridine-d₅ to solutions of 1 and 2 in CDCl₃ in a molar ratio pyridine-d₅/1 or 2 ≈50-55 gave solutions A and B, respectively, which contained compounds 5 and 6 analogous to 3 and 4, but with pyridine-d₅ rather than pyridine in their structural formula. In these solutions, the trans- and cis-N,N geometrical isomers of compounds 5 and 6 were interconverting between them in a dynamic equilibrium. In addition, an exchange between free and coordinated pyridine-d₅ was also taking place in solutions A and B. The NMR data for solution A showed that the dynamic equilibrium between the cis- and trans-N,N isomers of compound 5 was shifted to the trans-N,N isomer. However, the NMR data for solution B suggested that in this solution the equilibrium between the cis- and trans-N,N isomers of compound 6 was shifted to the cis-N,N isomer. Interconversion between the trans- and cis-N,N isomers of compounds 5 and 6 in solutions A and B plausibly proceeded through the intermediate ionic complexes [Pd{C₆H₄CH=N(C₆H₄-4-CO₂Me)-κC_ortho,κN}(py-d₅)₂]X [7 (X = OAc), 8 (X = Cl)]. Ionic complexes 7 and 8 were not observed in CDCl₃ solution but were the major species in D₂O solutions containing compounds 1 and 2 and pyridine-d₅ in a molar ratio pyridine-d₅/1 or 2 ≈50-55. The crystal structure of the adduct 1·2(CH₃COOH) and that of compound 2 were determined by single crystal X-ray diffraction. A theoretical study on the difference in free Gibbs energy in CHCl₃ solution between the cis- and trans-N,N isomers of compounds 3 and 4 is also included in this work.