Two-electron reduction of alkyl(sulfanyl)porphyrazines: a route to free-base and peripherally metallated asymmetric porphyrazines

Abstract

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.