Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 5. Synthesis, Physicochemical and Theoretical Studies of a Novel Pentanuclear Palladium(II) Complex and Related Mononuclear Species

New palladium(II) complexes of the free-base tetrakis[2,3-(5,6-di-2-pyridylpyrazino)porphyrazine], [Py8TPyzPzH2], have been prepared and their physicochemical properties examined. The investigated compounds are the pentanuclear species [(PdCl2)4Py8TPyzPzPd], the monopalladated complex [Py8TPyzPzPd], and its corresponding octaiodide salt [(2-Mepy)8TPyzPzPd](I)8. All three PdII complexes have a common central pyrazinoporphyrazine core and differ only at the periphery of the macrocycle, where the simple dipyridinopyrazine fragments present in [Py8TPyzPzPd] bear four PdCl2 units coordinated at the pyridine N atoms in the pentanuclear complex, [(PdCl2)4Py8TPyzPzPd], or carry pyridine−N(CH3)+ moieties in the iodide of the octacation [(2-Mepy)8TPyzPzPd]8+. The structural features of the pentanuclear complex [(PdCl2)4Py8TPyzPzPd], partly supported by X-ray data and solution 1H NMR spectra of the [(CN)2Py2PyzPdCl2] precursor, were elucidated through one- and two-dimensional 1H NMR spectra in solution and density functional theory (DFT) calculations. Structural information on the monopalladated complex [Py8TPyzPzPd] was also obtained from DFT calculations. It was found that in the complex [(PdCl2)4Py8TPyzPzPd] the peripheral PdCl2 units adopt a py−py coordination mode and the generated N2PdCl2 moieties are directed nearly perpendicular to the plane of the pyrazinoporphyrazine ring, strictly recalling the arrangement found for the palladated precursor [(CN)2Py2PyzPdCl2]. NMR and DFT results consistently indicate that of the four structural isomers predictable for [(PdCl2)4Py8TPyzPzPd], one having all four N2PdCl2 moieties pointing on the same side of the macrocyclic framework (i.e., isomer 4:0, plus the 3:1 and the 2:2-cis and 2:2-trans isomers), the 4:0 isomer (C4v symmetry) is the predominant form present. According to cyclic voltammetry and spectroelectrochemical results in pyridine, dimethyl sulfoxide (DMSO), and dimethylformamide (DMF), the monopalladated complex [Py8TPyzPzPd] undergoes four reversible or quasi-reversible one-electron ligand-centered reductions, similar to the behavior also observed for the pentanuclear complex [(PdCl2)4Py8TPyzPzPd], which shows an additional reduction peak attributable to the presence of PdCl2. Owing to the electron-withdrawing properties of the PdCl2 units, the pentanuclear complex is easier to reduce than the mononuclear complex [Py8TPyzPzPd], some related [Py8TPyzPzM] complexes, and their porphyrin or porphyrazine analogues, so much so that the corresponding monoanion radical is generated at potentials close to 0.0 V vs SCE in DMSO or DMF. In turn, the monoanion of [(2-Mepy)8TPyzPzPd](I)8 is also extremely easy to generate electrochemically. Indeed, because of the eight positively charged N-CH3+ groups in this complex the first reduction occurs at potentials close to +0.10 V in DMSO or DMF. The redox behavior of the mono- and pentapalladated complexes has been rationalized on the basis of a detailed DFT analysis of their ground-state electronic structure.