Preparation and characterization of nonclassical tetraazaporphyrin, bis(4-methylpyridine)[1,3,5,7,9,11,13,15-octaphenyltetra(3,4-thieno) tetraazaporphyrinato]ruthenium(II)

The tetramerization reaction of 2,5-diphenyl-3,4-dicyanothiophene (2) proceeded on treatment with ruthenium(III) trichloride, DBU, and 4-methylpyridine in 2-ethoxyethanol at 135 °C to give bis(4-methylpyridine) [1,3,5,7,9,11,13,15-octaphenyltetra(3,4-thieno)tetraazaporphyrinato] ruthenium(II)(3). Because the structure of this product cannot be represented by a usual bonding system, this molecule has to contain an unusual tetravalent sulfur atom in one of the four thiophene rings. In the ¹H NMR spectrum of the product, signals from 4-methylpyridine coordinated to the central ruthenium atom showed an upfield shift. The structure of 3 was determined by X-ray crystallography, which revealed that 3 has four thiophene units linked at their 3,4-positions to the tetraazaporphyrin scaffold. Four pairs of phenyl groups are in close proximity and are sterically congested, which causes the four thiophene rings to deviate from the mean plane of the central four pyrrole nitrogen atoms. The UV/Vis spectrum of 3 shows a Q-band absorption at λ_max = 758 nm. In the MCD spectrum of 3, dispersion-type absorptions (Faraday A term) are observed at 746 and 776 nm. The MCD spectra suggest that the two LUMOs of 3 could be degenerate even though its structure deviates from D_4h symmetry. The electrochemical properties of 3 were examined by cyclic voltammetry with Ag/AgNO₃ as the reference electrode. The optimized structure and the NMR shielding constants of a simplified model molecule were calculated by using density functional theory.