Palladium-TCNQ-based charge-transfer complexes (TCNQ = 7,7′-8,8-tetracyanoquinodimethane)

The reaction between the palladium(0) complex [Pd(dba)2] (dba = dibenzylidenacetone) with TCNQ proceeds via an electron-transfer process leading to a solid having the empirical formula [Pd(TCNQ)2(CH3CN)x] (x < 1). The same reaction performed in the presence of a variety of ligands leads to compounds whose nature is dependent on the geometrical and electronic properties of the ligands. [Pd(dba)2] reacts with TCNQ in the presence of 2,2′-bipyridine (bipy) affording, depending on the experimental conditions, two charge-transfer (CT) solids of general formula [Pd( bipy)2(TCNQ)2], one of which exhibits spectral features of conducting TCNQ complexes. When the reaction is performed in the presence of the more steric demanding bis-chelating ligand, 2,9-dimethylphenanthroline (dmphen), the palladium complex [Pd(dmphen)(π-TCNQ)] is obtained. A complex with a similar stoichiometry and spectroscopic properties is obtained by reacting [Pd(dba)2] with TCNQ in the presence of PPh3, while by using the bis-chelating 1,2-bis(diphenylphosphino)ethane (dppe) ligand the CT salt [Pd(dppe)2] [TCQM]2 is obtained. The CT complex [Pd(terpy)(η1-TCNQ)] [TCNQ] is the product of the reaction of [Pd(dba)2] and TCNQ in the presence of the ter-chelating 2,2′:6′,2″-terpyridine ligand (terpy). All compounds have been characterized by UV-Vis/ near-IR and IR spectroscopic techniques, X-ray photoelectron (XP) and Auger electron (XAE) spectroscopy have been employed to determine the formal charge on the palladium metal.