A four-coordinate Fe(III) porphyrin cation.

Few families of compounds have been as intensely studied as iron porphyrin complexes. In spite of this, a truly four-coordinate Fe(III) porphyrin cation has never before been reported.1 The exceptional electrophilicity of the four-coordinate [FeIII(Porph)]+ cation will even coordinate arene solvent upon crystallization if the counteranion is extremely non-coordinating, as in the work of Reed with silver dihexabromocarborane,2,3 and in solution, a Br atom of the hexabromocarborane anion binds to the Fe.4 Here we present the first purely four-coordinate Fe(III) porphyrin both in the solid state and in solution. To do so, we have combined both steric and electronic factors by creating a very sterically hindered bis-pocket siloxyl porphyrin in conjunction with a bulky and very weakly coordinating anion. We have synthesized an extremely hindered bis-pocket siloxyl porphyrin, 5,10,15,20-tetrakis(2′,6′-bis(triisopropylsiloxy)phenyl)porphyrin (H2TipsiPP) by the reaction of the octahydroxyl iron porphyrin5 (5,10,15,20-tetrakis(2′,6′-dihydroxyphenyl)porphyrin with triisopropyl chlorosilane using imidazole as a catalyst (Scheme 1). In spite of the driving force of strong Si-O bond formation, high temperatures (250 °C) are required to overcome steric hindrance and fully silylate all eight sites. The resulting porphyrin is extremely sterically hindered with a pocket opening of only 2 Å. Figure 1 compares the space-filling models of the simple FeTPP(Cl) and the bis-pocket siloxylporphyrin FeIII(TipsiPP)(Cl). The steric hindrance prevents anion coordination by reducing the thermodynamic binding constant. To replace the sterically undemanding chloro ligand, a dichloromethane solution of FeIII(TipsiPP)(Cl) was mixed with 1 equiv of AgCB11H6Br6 or AgCF3SO3, which produces a color change from yellow to red (Figure 2). NMR, MALDI-TOF, and elemental analysis were used to confirm the purity of the final products. Both complexes react with water readily to form a five-coordinate spinadmixed Fe(III) complex, which has diagnostic UV-vis and NMR features (Figures S1 and S2 in Supporting Information (SI)). Proton NMR is a very sensitive probe for the spin state of iron.4,6,7 On the basis of the â-pyrrole proton NMR isomer shift, Reed has established a magnetochemical series for different anions.8 Surprisingly, solutions of [FeIII(TipsiPP)]+ with a variety of weakly coordinating anions (i.e., CB11H6Br6, SbF6, ClO4, and CF3SO3) all have exactly the same porphyrin 1H NMR chemical shifts with â-pyrrole proton chemical shift at -81 ppm in CD2Cl2 at 290 K (Figure S3 in SI), which corresponds to the Fe(III) intermediate spin state (S ) /2). Because these anions are very different both in nucleophilicity and size, the porphyrin NMR spectra would be different if the anions were bound to the iron. This is the case for FeIIITPP+: â-pyrrole proton chemical shifts for FeTPP(X) are -60 ppm for CB11H6Br6, -31.5 ppm for SbF6, 13 ppm for ClO4, and 39.3 ppm for CF3SO3. Possible assignments to either FeIII(TipsiPP•+) porphyrin cation radicals or FeII(TipsiPP) were excluded by independent synthesis of the pure complexes, which show very different chemical shifts and have different spin states. The solvent plays a central role in stabilization of the fourcoordinate Fe(III) cation. The four-coordinate species is only stable in halogenated solvents (i.e., CH2Cl2, CHCl3, CH2Br2). In aromatic solvents, such as toluene and benzene, it converts to an admixed spin state, probably due to arene coordination to the iron center.3 The â-pyrrole chemical shift of [FeIII(TipsiPP)]+ is the same in either CH2Cl2 or CH2Br2, which shows that the halocarbons do not perturb the iron center. The compound is not soluble in aliphatic solvents. Red-brown needle-shaped crystals of [Fe(TipsiPP)][CB11H6Br6] were grown by slow evaporation of a 1:1 mixture of dichloroScheme 1. Synthesis of a Bis-pocket Siloxylporphyrin, FeIII(TipsiPP)(Cl)