Two-electron Reduction of Alkyl(sulfanyl)porphyrazines: a Route to Free-base and Peripherally Metallated Asymmetric Porphyrazines

Treatment of octakis(octylthio)porphyrazine (H2OOSPz) with cis-bis(benzonitrile)dichloroplatinum(II) in a 1,2,4-trichlorobenzene (TCB)–n-BuOH mixture at 190 °C affords the asymmetric porphyrazine 2H-heptakis(octylthio)porphyrazine, H2HOSPz, in ca. 40% yield. In the key synthetic step the two-electron reduced diprotonated porphyrazine, [H2OOPz(4−)(H+)2] (hemiporphyrazinogen), forms in nearly quantitative yield. This species undergoes nucleophilic substitution of an octylsulfide group by H−, affording the asymmetric porphyrazine, this process being favoured by the high-temperature induced charge and structural intramolecular rearrangements. The resulting molecule shows mesomorphic behaviour. Interestingly, in the presence of sodium acetate, or in neat TCB, the asymmetric porphyrazine does not form at all. Under these conditions, a PtCl2 molecular fragment coordinates both to a porphyrazine aza bridge and to a thioether moiety leading to the formation of the [H2OOSPz]PtCl2 complex. The S,N-coordination of the PtCl2 unit was proved by 1H, 195Pt{1H}, and 13C NMR spectroscopy, and supported by DFT (B3LYP) calculations. The complex conjugates high asymmetry with strong solvatochromism and, therefore, it is potentially interesting for nonlinear optics.