Synthesis, crystal structure, and intrinsic reactivity descriptors of coordination complexes of [(cis-PdCl2·L-proline) L-proline] and [trans-PdCl2·(glycine-OMe)2

Abstract

This work describes the synthesis and structural characterization of two palladium (II) complexes elucidated by infrared spectroscopy, dispersive energy analysis (EDS) with Scanning Electron Microscopy (SEM) and X-ray single-crystal diffraction analysis. The first chiral complex is derivatized from l-proline amino acid, obtaining [Cis-PdCl2·L-proline(L-proline)] (1), in which the structure does not have symmetry elements and crystallized in a triclinic crystal system on a non-centrosymmetric space group P1. The comparison with the structural complex of [trans-PdCl2·(glycine-OMe)2] (3) becomes necessary due to the anticancer activity of its analog, cis-platinum, and the catalytic activity caused by Pd as a metallic center. Complex 1 is therefore made up of a free proline molecule linked by a single hydrogen bond to a PdCl2 molecule forming a 5-member chelate ring of the COO···Pd···NH type with another proline molecule and displaying distorted angles and bond sizes to the square planar geometry of the palladium (Pd) center. Ring formation generates a new stereogenic center chemically correlated with the induction caused by the absolute configuration S of the initial amino acid. The crystal results strongly indicate the confirmed configuration by the Flack parameter and its crystal packing stabilization by an intermolecular H-bond network, highlighting as a secondary mimicking structure the β sheets formed by hydrogen bonding interaction of the N—H···Cl type responsible for the disorder moldered on pyrrolidine ring.Furthermore, the Pd(II) complex obtained by the glycine was [trans-PdCl2·(glycine-OMe)2] (3). Regarding its chemical structure, this complex is constituted by the union of two glycine methyl ester molecules binding to a Pd in a trans disposition concerning the almost perfect square plane, which also contains a C2 axis of rotation. Complex 3 crystallized in a monoclinic system and centrosymmetric space group P21/c, the packing is favored by four strong hydrogen bond type interactions presenting helices with 90° rotations between each layer.Owing to their great interest and relevance in Pd—N amino acid in catalytic and medicinal chemistry, the electronic and global reactivity properties of complexes 1 and 3 were further explored by computational methods in the framework of Density Functional Theory, finding 1 as the most reactive complex due to the narrower E-LUMO/E-HOMO energy gap, having this band gap in agreement with the lowest hardness value compared to those shown for 3, in which band gap and hardness are higher with a tendency to kinetic stability and controlled reactivity.