Remarkable Anion-Dependent Spin-State Switching in Diiron(III) μ-Hydroxo Bisporphyrins: What Role Do Counterions Play?

Addition of 2,4,6-trinitrophenol (HTNP) to an ethene-bridged diiron(III) mu-oxo bisporphyrin (1) in CH2Cl2 initially leads to the formation of diiron(III) mu-hydroxo bisporphyrin (2 center dot TNP) with a phenolate counterion that, after further addition of HTNP or dissolution in a nonpolar solvent, converts to a diiron(III) complex with axial phenoxide coordination (3 center dot(TNP)(2)). The progress of the reaction from m-oxo to m-hydroxo to axially ligated complex has been monitored in solution by using H-1 NMR spectroscopy because their signals appear in three different and distinct spectral regions. The X-ray structure of 2 center dot TNP revealed that the nearly planar TNP counterion fits perfectly within the bisporphyrin cavity to form a strong hydrogen bond with the mu-hydroxo group, which thus stabilizes the two equivalent iron centers. In contrast, such counterions as I-5, I-3, BF4, SbF6, and PF6 are found to be tightly associated with one of the porphyrin rings and, therefore, stabilize two different spin states of iron in one molecule. A spectroscopic investigation of 2 center dot TNP has revealed the presence of two equivalent iron centers with a high-spin state (S = 5/2) in the solid state that converts to intermediate spin (S = 3/2) in solution. An extensive computational study by using a range of DFT methods was performed on 2 center dot TNP and 2(+), and clearly supports the experimentally observed spin flip triggered by hydrogen-bonding interactions. The counterion is shown to perturb the spinstate ordering through, for example, hydrogen-bonding interactions, switched positions between counterion and axial ligand, ion-pair interactions, and charge polarization. The present investigation thus provides a clear rationalization of the unusual counterion-specific spin states observed in the mu-hydroxo bisporphyrins that have so far remained the most outstanding issue.