Synthesis, Structure, and Electrochemistry of Mononuclear and Face-to-Face Binuclear Orthometalated Complexes of Palladium(II) with N-Monodentate or N(1),N(3)-Bridging 1,3-Di-p-tolyltriazenido Ligands. Dependence on Geometrical Arrangement of the Electronic Communication between Two Equivalent Redox Sites

Abstract

The synthesis, electrochemistry, and structural characterization of the mononuclear complex [Pd{C6H4N(H)NC(CH3)C5H4N}(p-tolN−NNp-tol)] (1) containing the monodentate 1,3-di-p-tolyltriazenido ligand is described. Compound 1 is an example of a stable species containing a Pd−N amido bond cis to a Pd−C aryl bond. Kinetic parameters for the dynamic intramolecular N(1)−N(3) exchange of the monodentate ligand in complex 1 have been calculated. The cis and trans isomers of the orthometalated face-to-face complex [{Pd(C6H4NNC6H5)(μ-p-tolNNNp-tol)}2] (2) have also been prepared, and the crystal structure of the trans isomer is reported. There are noticeable differences in the electrochemical behavior of the mononuclear and binuclear species. From the electrochemical experiments on both isomers of 2 it is possible to recognize different redox sites, to calculate the electronic coupling between them, and to suggest where the reversible electron transfers occur. Each isomers of 2 undergoes two one-electron oxidations and two one-electron reductions. The electronic coupling (ΔE = 0.40 V) at oxidizing potentials is identical for both isomers of complex 2, suggesting that the oxidations occur on the Pd(μ-triazenido)2Pd framework which is common to both isomers. By contrast, the electronic coupling at reducing potentials is greater for cis-2 (ΔE = 0.33 V) than for trans-2 (ΔE = 0.25 V), suggesting that the reduction processes occur on the orthopalladated fragments, which are arranged differently on the two isomers. Thus, electronic communication between two equivalent redox centers in the same molecule depends not only on the nature of the bridging ligand but also on the geometrical arrangement of the redox centers.