Structural, UV-Visible, and Electrochemical Studies on 2,3-Dicyano-5,6-di-2-pyridylpyrazine, [(CN)2Py2Pyz], Related Species and Its Complexes [(CN)2Py2PyzMCl2] (M=PtII, PdII)

2,3-Dicyano-5,6-di-2-pyridylpyrazine, [(CN)2Py2Pyz], which autocyclotetramerizes to give the macrocycle tetrakis[5,6-di(2-pyridyl)-2,3-pyrazino]porphyrazine, [Py8TPyzPzH2], bearing externally four dipyridinopyrazine fragments, reacts with bis(benzonitrile) dichloroplatinum(II), [(C6H5CN)2PtCl2], in CH3CN, affording the monometalated species [(CN)2Py2PyzPtCl2]. Single-crystal X-ray work on this compound shows that PtII is bound to [(CN)2Py2Pyz] through the two pyridine N atoms (“py-py” coordination) in a way similar to that found for its monopalladium analogue, [(CN)2Py2PyzPdCl2]. Cyclic voltammetry of [(CN)2Py2PyzPtCl2] and [(CN)2Py2PyzPdCl2] in nonaqueous media (pyridine, DMSO, and DMF) indicates that the electron-withdrawing effect of the coordinated PtCl2 and PdCl2 units results in an initial one-electron reduction (E1/2 = -0.60 and -0.54 V vs SCE in DMSO, respectively), which is easier by 0.25-0.30 V than the unmetalated [(CN)2Py2Pyz] (first reduction: E1/2 = -0.87 V vs SCE). These electrochemical data are analyzed along with new results for a selected number of related pyrazine and 2,3-dicyanopyrazine molecules as well as earlier reported data on the mono- and bis-N-methylated derivatives [(CN)2Py(2- Mepy)Pyz]+ and [(CN)2-(2-Mepy)2Pyz]2+, with these latter species being formed by reaction of the precursor [(CN)2Py2Pyz] with methyl iodide or p-toluensulfonate. The data in this study are also compared to electrochemical data previously reported for a triad of palladium(II) porphyrazine macrocycles obtained from the precursor [(CN)2Py2Pyz], i.e., [Py8TPyzPzPd], the corresponding pentanuclear complex [(PdCl2)4Py8TPyzPzPd] (presenting “py-py” coordination at the dipyridinopyrazine fragments), and the octacation [(2-Mepy)8TPyzPzPd]8+ (N-methylated at the pyridine rings). Thin-layer UV-visible spectra of singly reduced [(CN)2Py2Pyz]- and its metalated analogues, [(CN)2Py2PyzPtCl2]- and [(CN)2Py2-PyzPdCl2]-, were measured in pyridine, DMF, and DMSO and show π-π* transitions, as well as unusually intense absorptions in the near-IR region (500-900 nm) of the spectrum.