Structural flexibility and role of vicinal 2-thienyl rings in 2,3-dicyano-5,6-di(2-thienyl)-1,4-pyrazine, [(CN)2Th2Pyz], Its palladium(II) complex [(CN)2Th2Pyz(PdCl 2)2], and the related pentametallic pyrazinoporphyrazines [(PdCl2)4Th8TPyzPzM] (M = Mg II(H2O), ZnII)

The solid state and solution structure of 2,3-dicyano-5,6-di(2-thienyl)-1, 4-pyrazine, [(CN)2Th2Pyz], and its PdII derivative, [(CN)2Th2Pyz(PdCl2) 2]•H2O, formed by reaction of [(CN) 2Th2Pyz] with [(C6H5CN) 2PdCl2] were characterized by X-ray, UV-visible, 1H and 13C NMR, and extended X-ray absorption fine structure (EXAFS) spectral measurements. The X-ray crystal structure of [(CN)2Th2Pyz] shows the presence of one thienyl ring positioned orthogonal to the rest of the molecule, with the two vicinal thienyl rings lying orthogonal to each other in a rare arrangement. NMR studies of [(CN)2Th2Pyz] in the solid state and in solutions of dimethylformamide or dimethyl sulfoxide confirm a nonequivalence of the thienyl rings in the solid state and also in solution. EXAFS results indicate that two distinct PdII coordination sites are formed at the di(2-thienyl)pyrazino moiety of [(CN)2Th2Pyz(PdCl 2)2]•H2O, with identical Pd-N pyz (2.03(3) Å) and Pd-Cl (2.36(3) Å) bond lengths but with different Pd-S1 (2.25(4) Å) and Pd-S2 (3.21(5) Å) bond distances in an overall asymmetric molecular framework. Density functional theory (DFT) and time-dependent DFT (TDDFT) theoretical studies also provide information about the structure and spectral behavior of the precursor and its metalated PdII derivative. 1H/13C NMR and UV-visible spectral measurements were also carried out on two heteropentametallic porphyrazine macrocycles which were prepared by a reaction of PdCl2 with [Th8TPyzPzM] where Th8TPyzPz = tetrakis-2,3-[5,6-di-(2-thienyl)-pyrazino]porphyrazinato dianion and M = Mg II(H2O) or ZnII. Spectroscopic data on the newly synthesized [(PdCl2)4Th8TPyzPzM] compounds suggest that the binding of PdCl2 involves coordination sites of the type S2(th)PdCl2 with the two thienyl rings of each di(2-thienyl)pyrazino fragment bound to PdII in an equivalent manner ("th-th" coordination). This is similar to what was found for the corresponding octapyridinated analogues ("py-py" coordination). © 2011 American Chemical Society.