Axial alkyl ligand reactivity in five-coordinate gallium(III) porphyrin complexes

Photochemical reactions of alkylgallium(III) complexes of tetraphenyl porphyrin dianion (TPP) have been monitored by ¹H NMR spectroscopy. Red-purple (TPP)Ga^III(cyclopentylmethyl)·C₆H₅CH₃, C₅₇H₄₇N₄Ga, crystallizes in triclinic space group P1 with a=10.877(3), b=11.389(2), c=18.786(3) Å, α=81.02(2),β=11.389(2), γ=74.36(2)° at 130 K with Z=2. Refinement of 477 parameters and 4616 reflections yielded R=0.069, R_w=0.072. The structure shows that the gallium ion is five-coordinate with the gallium 0.58 Å out of the N₄ plane. The GaC bond distance is 1.992(6) Å and the GaN distances span a narrow range from 2.095(4) to 2.110(3) Å. Photolysis of (TPP)Ga^III((CH₂)₄CHCH₂) in benzene solution results in isomerization to form (TPP)Ga^III(CH₂CH(CH₂)₄) as a result of homolysis of the GaC bond and cyclization of the 5-hexenyl radical. In the presence of dioxygen photolysis of (TPP)Ga^IIIC₂H₅ produces (TPP)Ga^III(OOC₂H₅) and photolysis of (TPP)Ga^pIII((CH₂)₄CHCH₂) yields TPPGa^pIII(OO(CH₂)₄CHCH₂). In the latter case no isomerization (cyclization) to form TPPGa^III((CH₂CH(CH₂)₄) occurs because the 5-hexenyl radical is intercepted by dioxygen before it can cyclize. Nitrosobenzene (a radical trap) also inhibits isomerization (cyclization) of (TPP)Ga^III((CH₂)₄CHCH₂).