Paramagnetic iron (III) and diamagnetic gallium (III) porphyrin complexes with axial allyl and vinyl ligands

Article abstract of DOI:10.1016/s0020-1693(97)05656-9

Paramagnetic iron(III) and diamagnetic gallium(III) porphyrin complexes with axial allyl and vinyl ligands have been prepared by the addition of an organo-Grignard reagent to (porphyrin)M^IIICl in toluene-d₈ solution for in situ observation by ¹H NMR spectroscopy. The products are considerably less stable to chromatography and to warming to room temperature than are their alkyl counterparts. The spectral data indicate that the allyl and vinyl groups are coordinated in an η¹-fashion for the iron complexes at low temperatures (-80 to -40°C) and for the gallium complexes. Purple blocks of {(tetra(p-anisyl)porphyrin)Ga( vinyl)} · 2.5benzene crystallize in the space group P2₁/n with a = 11.654(3), b = 18.960(6), c = 23.916(6) A?, β = 97.69° at 130 K with Z = 4. Refinement of 9202 reflections, 668 parameters and 18 restraints yielded wK2 = 0.2041 and a conventional R1 = 0.071 for 6434 reflections with I>2σI. The structure shows that the gallium center is five-coordinate with a single, η¹-vinyl group as the axial ligand. Remarkably, in view of the tendency of related dioxygen ligands in porphyrin complexes to show four-fold disorder, the vinyl group is fully ordered. Dioxygen reacts with {(tetra(p-tolyl)porphyrin)Fe^III(ally1)} at -80°C to produce {(tetra(p-tolyl)porphyrin)Fe^III(OH) } and acrolein, O=CH-CH=CH₂, the expected products of insertion of dioxygen into an Fe-C σ-bond.