Spin-State Ordering in Hydroxo-Bridged Diiron(III)bisporphyrin Complexes

Abstract

We report the synthesis, structure, and spectroscopic characterization of 1,2-bis[μ-hydroxo iron(III) 5-(2,3,7,8,12,13,17,18-octaethylporphyrinyl)]ethane with PF6(–) and SbF6(–) counteranions. The two iron centers are nonequivalent with admixed intermediate spin state (S = 3/2 with a minor contribution of S = 5/2) on each metal both in the solid and in solution. The molecules are compared with previously known μ-hydroxo complexes with other counterions, such as I3(–), BF4(–), and ClO4(–), which demonstrates that the nature of the counterion can affect the spin-state ordering dramatically. To understand how the spin-state ordering is affected by external perturbations, we also have done a comprehensive computational study. The calculations show that subtle environmental perturbations affect the spin-state ordering and relative energies and are likely to be the root cause of the variation in spin-state ordering observed experimentally.

Singlet versus Triplet Reactivity in an Mn(V)–Oxo Species: Testing Theoretical Predictions Against Experimental Evidence

“…15 Our model uses a corrolazine macrocycle (Scheme 1) with the peripheral aryl substituents replaced with hydrogen atoms (H 8 Cz), as previous work showed that the peripheral groups on porphyrin scaffolds have little influence on the spin state ordering and relative energies. 16 Reactivities with para-Z-substituted thioanisoles were calculated for Z = N(CH 3 ) 2 , NH 2 , OCH 3 , CH 3 , H, Br, CN, and NO 2 . The work was aimed at establishing whether the reaction mechanisms are electrophilic or nucleophilic and how the intrinsic chemical properties of oxidant and substrate affected these reactivity differences.…”

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

“…This also results in an equal interaction with the porphyrin rings and thus stabilizes the two equivalent iron centers in the molecule (see below). In sharp contrast, other counterions for ethane‐/ethene‐bridged μ‐hydroxo bisporphyrins (Scheme ) have been found to be directed towards one of the porphyrin rings, which makes that center different from the other one –…”

“…The most extreme cases were found with I 3 /I 5 counterions, with which one iron(III) center is in the high‐spin ( S =5/2) state whereas the other is in the admixed‐intermediate spin state ( S =3/2 with a minor contribution from S =5/2) . When BF 4 , PF 6 , and SbF 6 counterions were used, the two iron(III) centers are also found to be inequivalent with two different admixed‐intermediate spin states –. It has also been found that both ethane‐ and ethene‐bridged μ‐hydroxo complexes produce identical spin‐state behavior with the same counterion in both solid and solution phases.…”

“…Recently, our group has reported a series of ethane‐, ethene‐, and pyrrole‐bridged diiron(III) μ‐hydroxo bisporphyrins, which are prepared by simple protonation of the corresponding oxo‐bridged dimer by using strong Brönsted acids with weakly coordinating counterions, such as HI, HBF 4 , HClO 4 , HPF 6 , and HSbF 6 . Spectroscopic investigations have revealed that two iron(III) centers are mostly inequivalent in the μ‐hydroxo complexes, in which two different spin states of iron are stabilized in a single molecular framework even though both cores have identical molecular structures –. However, both iron centers were found to be equivalent when the μ‐hydroxo was replaced by an isoelectronic μ‐fluoro group .…”

“…An extensive computational study on ethane‐bridged diiron(III) μ‐hydroxo bisporphyrin has revealed that subtle environmental perturbation can change the spin‐state ordering in the complex . Counterions, however, have been shown to have a small and virtually insignificant effect on the optimized geometries.…”