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

• Synthesis of compounds [Pd{C6H4CH=N(C6H4-4-CO2Me)-κCortho,κN}](L)(X)] (L = py or py-d5 and X = OAc or Cl).
• Stereochemistry.
• Geometrical isomerization.
• Exchange between coordinated pyridine-d5 and free pyridine-d5.

Reaction of methyl (E)-4-(benzylideneamino)benzoate C6H5CHN(C6H4-4-CO2Me) with Pd(OAc)2 produced the dinuclear acetato bridge ortho-cyclopalladated compound [Pd{C6H4CHN(C6H4-4-CO2Me)-κCortho,κN}]2(μ-OAc)2 (1). Compounds [Pd{C6H4CHN(C6H4-4-CO2Me)-κCortho,κN}]2(μ-Cl)2 (2) and [Pd{C6H4CHN(C6H4-4-CO2Me)-κCortho,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{C6H4CHN(C6H4-4-CO2Me)-κCortho,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.

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