The Marriage of Peripherally Metallated and Directly Linked Porphyrins: Bromidobis(phosphine)platinum(II) as a Cation-Stabilizing Substituent on Directly Linked and Fused Triply Linked Diporphyrins

摘要

A meso-bromidoplatiniobis(-triphenylphosphine) η~1-organometallic porphyrin monomer was prepared by the oxidative addition of meso-bro-moZnDPP (DPP = dianion of 5,15-diphenylporphyrin) to a platinum(0) species. The meso-meso directly linked dimeric porphyrin (5) was prepared from this monomer by silver(I)-promoted oxidative coupling and planarized to give a triply linked dizinc(II) porphyrin dimer (8). Acidic demetallation of 8 afforded the bis(free base) 9. Dimer 5 was demetallated then remetallated with nickel(II) to give the dinickel(II) analogue 10, the X-ray crystal structure of which showed a twisted molecule with ruffled, orthogonal NiDPP rings, terminated by square-planar trans-[Pt-(PPh_3)_2Br] units. New compounds were fully characterized spectroscopically, and the fused diporphyrin exhibited a broad, low-energy, near-IR electronic absorption band near 1100 nm. Electro-chemical measurements of this series indicate that the organometallic fragment is a strong electron donor towards the porphyrin ring. The triply linked organometallic diporphyrin has a substantially lowered first one-electron oxidation potential (-0.35 V versus the ferrocene/ferrocenium couple (Fc/Fc~+)) and a narrow HOMO-LUMO gap of 0.96 V. Solutions prepared for NMR spectroscopy slowly decompose with degradation of the signals, which is attributed to partial oxidation to the cation radical. This paramagnetic species can be reduced in situ by hydrazine to restore the NMR spectrum to its former appearance. The combined influence of the two [Pt-(PPh_3)_2Br] electron-donating substituents is sufficient to make dimer 5 too aerobically unstable to allow further elaboration.