A Four-Coordinate Fe(III) Porphyrin Cation

The free four-coordinate Fe(III) porphyrin cation has not been previously observed due to its extreme electrophilicity. In order to isolate this very reactive species, we have synthesized a highly sterically hindered bis-pocket siloxyl porphyrin which has a top opening of only 2 Å. NMR studies of this Fe(III) porphyrin cation with different weakly coordinating anions all give the same porphyrin proton chemical shifts in CD₂Cl₂, suggesting that no anion coordination to the iron occurs in solution. In the solid state, the four-coordinate Fe(III) bis-pocket porphyrin can be isolated with the bulky and weakly coordinating hexabromocarborane as the counterion:  the single-crystal X-ray structure confirms the absence of axial ligation to the iron. In contrast, with smaller anions (e.g., triflate), the Fe(III) bis-pocket porphyrin complexes are five-coordinate in the solid state, due probably to the smaller size and loss in the solid state of stabilization from anion solvation. Full characterization (EPR, Mössbauer, SSNMR, Evans Method, and SQUID magnetometry) shows that the four-coordinate Fe(III) bis-pocket porphyrin cation has a ³/₂ spin state, in agreement with theoretical predictions.