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(PdCl2)2], and the Related Pentametallic Pyrazinoporphyrazines [(PdCl2)4Th8TPyzPzM] (M = MgII(H2O), ZnII)

The solid state and solution structure of 2,3-dicyano-5,6-di(2-thienyl)-1, 4-pyrazines [(CN)(2)Th(2)Pyz], and its pd(II) derivative, [(CN)(2)Th(2)Pyz(PdCl2)2]center dot H2O, formed by reaction of [(CN)(2)Th(2)Pyz] with [(C6H5CN)(2)Pdcl(2)] were characterized by X-ray, UV-visible, H-1 and C-13 NMR, and extended X-ray absorption fine structure (EXAFS) spectral measurements. The X-ray crystal structure of [(CN)(2)Th(2)Pyz] 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)(2)Th(2)Pyz] 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 Pd-II coordination sites are formed at the di(2-thienyl)pyrazino moiety of [(CN)(2)Th(2)Pyz(PdCl2)(2)]center dot H2O, with identical Pd-N-pyz (2.03(3) angstrom) and Pd-Cl (2.36(3) angstrom) bond lengths but with different Pd-Sl (2.25(4) angstrom) and Pd-S2 (3.21(5) angstrom) 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 Pd-II derivative. H-1/C-13 NMR and UV-visible spectral measurements were also carried out on two heteropentametallic porphyrazine macrocycles which were prepared by a reaction of PdCl2 with [Th(8)TPyzPzM] where Th(8)TPyzPz = tetrakis-2,345,6-di-(2-thienyl)-pyrazino]porphyrazinato dianion and M = Mg-II(H2O) or Zn-II. Spectroscopic data on the newly synthesized [(PdCl2)(4)Th(8)TPyzPzM] 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 Pd-II in an equivalent manner ("th-th" coordination). This is similar to what was found for the corresponding octapyridinated analogues ("py-py" coordination).