Metal complexes of π-expanded ligands (2): Synthesis and characterizations of bis[2-[(octylimino)methyl]-1-pyrenolato-N,O] palladium(II) and the stabilized vacant [formula omitted] orbital

Abstract

A palladium(II) complex of our π-expanded ligand, i.e., a salicylaldimine-style ligand based on pyrene, was synthesized and characterized. A treatment of this ligand (L) with palladium(II) acetate afforded the desired metal complex, Pd(II)L2, 3(Pd). Single crystal diffraction study revealed that this complex is a square planar four-coordinate palladium(II) complex. The structure of 3(Pd), i.e., shapes of the complexes, bond distances and angles, packing motif, is nearly identical with those of the previously reported the platinum(II) complexes, 2(Pt), as well as in the range of the values reported for structurally similar salicylaldiminato palladium(II) complexes. Although the IR spectrum of 3(Pd) is similar to that of 2(Pt), 1H NMR of 3(Pd) reflects the electronegative character of palladium, i.e., higher field shift was observed comparing with the spectrum of 2(Pt). The incorporated palladium induces the stabilization of the Kohn–Sham orbitals containing d-orbital coefficients of the metal. The significant difference is the following: the LUMO of 2(Pt) is the π-orbitals of the ligand, contrary, that of 3(Pd) is the vacant 4dx2-y2 of Pd. Therefore, excitations from occupied π-orbitals to this LUMO are mostly forbidden and the allowed transition to LUMO+1 and the higher π-orbitals were shifted to blue region. The lowest excitation energy of 3(Pd) was found at 497nm.