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

Abstract

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)-kappa C-ortho,kappa(N)}](2)(mu-OAc)(2) (1). Compounds [Pd{C6H4CH=N(C6H4-4-CO2Me)-kappa C-ortho,kappa(N)}](2)(mu-Cl)(2) (2) and [Pd {C6H4CH] N(C6H4-4-CO2Me)-kappa C-ortho,kappa(N)}(py)(X)] [3 (X = OAc); 4 (X = Cl)] were also prepared and isolated in good yields by substitution reactions. H-1 and C-13{H-1} 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-d(5) to solutions of 1 and 2 in CDCl3 in a molar ratio pyridine-d(5)/1 or 2 approximate to 50-55 gave solutions A and B, respectively, which contained compounds 5 and 6 analogous to 3 and 4, but with pyridine-d(5) 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-d(5) 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(C6H4-4CO(2)Me)-kappa C-ortho,kappa(N)}(py-d(5))(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-d(5) in a molar ratio pyridine-d(5)/1 or 2 approximate to 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.