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) μ‐oxo bisporphyrin ( 1 ) in CH 2 Cl 2 initially leads to the formation of diiron(III) μ‐hydroxo bisporphyrin ( 2⋅ 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⋅ (TNP) 2 ). The progress of the reaction from μ‐oxo to μ‐hydroxo to axially ligated complex has been monitored in solution by using 1 H NMR spectroscopy because their signals appear in three different and distinct spectral regions. The X‐ray structure of 2⋅ TNP revealed that the nearly planar TNP counterion fits perfectly within the bisporphyrin cavity to form a strong hydrogen bond with the μ‐hydroxo group, which thus stabilizes the two equivalent iron centers. In contrast, such counterions as I 5 , I 3 , BF 4 , SbF 6 , and PF 6 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⋅ 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⋅ TNP and 2 + , and clearly supports the experimentally observed spin flip triggered by hydrogen‐bonding interactions. The counterion is shown to perturb the spin‐state 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 μ‐hydroxo bisporphyrins that have so far remained the most outstanding issue.