AXIAL ALKYL LIGAND REACTIVITY IN 5-COORDINATE GALLIUM(III) PORPHYRIN COMPLEXES

Photochemical reactions of alkylgallium(III) complexes of tetraphenyl porphyrin dianion (TPP) have been monitored by H-1 NMR spectroscopy. Red-purple (TPP)Ga(III) (cyclopentylmethyl) . C6H5CH3, C57H47N4Ga, crystallizes in triclinic space group P1BAR with a = 10.877(3), b = 11.389(2), c = 18.786(3) angstrom, alpha = 81.02(2), beta = 11.389(2), gamma = 74.36(2)-degrees 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 angstrom out of the N4 plane. The Ga-C bond distance is 1.992(6) angstrom and the Ga-N distances span a narrow range from 2.095(4) to 2.110(3) angstrom. Photolysis of (TPP)Ga(III) ((CH2)4CH = CH2) in benzene solution results in isomerization to form (TPP)Ga(III) (CH2CH(CH2)4) as a result of homolysis of the Ga-C bond and cyclization of the 5-hexenyl radical. In the presence of dioxygen photolysis of (TPP)GaIIIC2H5 produces (TPP)Ga(III) (OOC2H5) and photolysis of (TPP)Ga(III) ((CH2)4CH = CH2) yields TPPGa(III) (OO(CH2)4CH = CH2). 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)4CH = CH2).