Spectroscopic and electrochemical properties of cyclopalladated complexes derived from 2,3-diphenylpyrazine

Abstract

A comparative study of complexes [Pd(dphpz)(N∧N)]PF6 [dphpz− is the deprotonated form of 2,3-diphenylpyrazine; (N∧N) is ethylenediamine (En), 2,2′-bipyridine (bpy), o-phenanthroline (phen), dipyrido[a,c]phenazine (dppz), 6,7-dicyanopyrido[f,h]quinoxaline (dicnq)] was made, using 1H NMR, electronic absorption, and emission spectroscopy, and also cyclic voltammetry. Steric interaction of the dphpz− phenyl rings leads to significant proton shielding in the carbanionic moiety of the cyclometallated ligand. Introduction of heterocyclic diimines instead of ethylenediamine decreases the desheilding of the dphpz− protons adjacent to the coordination center. Irrespective of the nature of the N∧N ligands, the cyclopalladated complexes are characterized by specific parameters of photo-and electrostimulated electron transfer processes involving the Pd(dphpz) orbitals, namely, by the long-wave absorption band with λmax 395±6 nm and e (2.2±1.2) × 103 1 mol−1 cm−1, the vibrationally structured low-temperature (77 K) luminescence resulting from the spinforbidden optical transfer from the excited to the ground state of the complex (energy E 00 19.27±0.07 kK, lifetime τ 160±30 μs), and the one-electron electroreduction wave with E 1−(2.0±0.1)V. For the [Pd(dphpz)·(N∧N)]+ complexes containing diazine derivatives of phenanthroline (dppz, dicnq), the degradation of the photoexcitation energy from two electronically excited states can occur as isolated process with successive transfer of electrons to the π orbitals localized on the remote moieties: [Pd(dphpz)] and diazine fragments of the N∧N ligands.