An EXAFS Study on Metal-Substitution Reaction of Homodinuclear Mercury(II) Porphyrin with Copper(II) in Aqueous Solution

The metal-substitution reaction of the homodinuclear mercury(II) porphyrin complex, [Hg2(tpps)]2− (H2tpps4− = tetrakis(4-sulfonatophenyl)porphyrin), with copper(II) has been studied by the EXAFS (extended X-ray absorption fine structure) method in an acetate buffer (pH 5.6). The tetrahedral mercury(II) acetato complex (rHg–O = 219 pm, rHg–O = 262 pm) reacts with the porphyrin to form the homodinuclear mercury(II) porphyrin, [Hg2(tpps)(CH3COO)2]4− (rHg–N = 222 pm, rHg–O = 265 pm), with a deformed structure, where two mercury(II) bind to two pyrrole nitrogens from upper and lower sides and pyrrole nitrogens are ruffled. The mercury(II) porphyrin was replaced with the distorted octahedral copper(II) acetato complex (rCu–O = 197 pm, rCu–O = 225 pm) to form the square planar copper(II) porphyrin, [Cu(tpps)]4− (rCu–N = 200 pm). The reaction mechanism of the large catalytic effect of mercury(II) on the copper(II) porphyrin formation is discussed from the structure of Hg2(tpps)2−, and the structure of the heterodinuclear intermediate, [Hg(tpps)Cu]2−, is proposed.