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

摘要

New palladium(II) complexes of the free-base tetrakis[2,3-(5,6-di-2-pyridylpyrazino)porphyrazine], [Py(8)TPyzPzH(2)], have been prepared and their physicochemical properties examined. The investigated compounds are the pentanuclear species [(PdCl2)(4)Py(8)TPyzPzPd], the monopalladated complex [Py(8)TPyzPzPd], and its corresponding octaiodide salt [(2-Mepy)(8)TPyzPzPd)(I)(8). All three Pd-II complexes have a common central pyrazinoporphyrazine core and differ only at the periphery of the macrocycle, where the simple dipyridinopyrazine fragments present in [Py(8)TPyzPzPd] bear four PdCl2 units coordinated at the pyridine N atoms in the pentanuclear complex, [(PdCl2)(4)Py(8)TPyzPzPd), or carry pyridine-N(CH3)(+) moieties in the iodide of the octacation [(2-Mepy)(8)TPyzPzPd](8+). The structural features of the pentanuclear complex [(PdCl2)(4)Py(8)TPyzPzPd], partly supported by X-ray data and solution H-1 NMR spectra of the [(CN)(2)Py(2)PyzPdCl(2)] precursor, were elucidated through one- and two-dimensional H-1 NMR spectra in solution and density functional theory (DFT) calculations. Structural information on the monopalladated complex [Py(8)TPyzPzPd] was also obtained from DFT calculations. It was found that in the complex [(PdCl2)(4)Py(8)TPyzPzPd] 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)(2)Py(2)PyzPdCl(2)]. NMR and DFT results consistently indicate that of the four structural isomers predictable for [(PdCl2)(4)Py(8)TPyzPzPd], 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 (C-4v symmetry) is the predominant form present. According to cyclic voltammetry and spectroelectrochemical results in pyridine, dimethyl sulfoxide (DMSO), and dimethylformamide (DMF), the monopalladated complex [Py(8)TPyzPzPd] undergoes four reversible or quasi-reversible one-electron ligand-centered reductions, similar to the behavior also observed for the pentanuclear complex [(PdCl2)(4)Py(8)TPyzPzPd], 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 [Py(8)TPyzPzPd], some related [Py(8)TPyzPzM] 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)(8)TPyzPzPd](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 gound-state electronic structure.