First Four-Coordinate Iron(III) Porphyrin

Chemistry professor Kenneth S. Suslick, graduate student Ming Fang, and crystallographer Scott R. Wilson of the University of Illinois, Urbana-Champaign, report the first four-coordinate Fe(III) porphyrin complex, a rare example of isolating an extremely reactive electrophilic metal cation (J. Am. Chem. Soc. 2008, 130, 1134). The researchers created the complex by reacting a hydroxy-substituted iron porphyrin with triisopropyl chlorosilane to form a sterically hindered bis-pocket siloxyl porphyrin. They further replaced an original chloro ligand with a series of weakly coordinating anions. Several of these anions formed four-coordinate Fe(III) porphyrins in solution, but only the very weakly coordinating hexabromocarborane anion formed the four-coordinate Fe(III) structure in both solution and crystalline forms. In the solid phase, other anions such as triflate enter the porphyrin pocket and coordinate the ion, but the bulky carborane remains fully outside the porphyrin pocket, leading to the isolated cation. Suslick suspects the approach to make electrophilic metal cations won't be limited to porphyrins. "The right combination of steric hindrance might work for other complexes," he says.