Structure and spin state of nonheme FeO complexes depending on temperature: predictive insights from DFT calculations and experiments.

The spin states ( = 1 and = 2) of nonheme FeO intermediates are believed to play an important role in determining their chemical properties in enzymatic and biomimetic reactions. However, it is almost impossible to investigate the spin state effect of nonheme FeO species experimentally, since FeO models having the = 1 and = 2 spin states at the same time neither exist nor can be synthesized. However, recent synthesis of an FeO complex with an = 1 spin state (triplet), [(MeNTB)FeO] (), and a structurally similar FeO complex but with an = 2 spin state (quintet), [(TQA)FeO] (), has allowed us to compare their reactivities at 233 K. In the present study, we show that structural variants control the spin-state selectivity and reactivity of nonheme FeO complexes. While and were proposed to be in an octahedral geometry based on DFT calculations and spectroscopic characterization done at 4 K, further DFT calculations show that these species may well assume a trigonal bipyramidal structure by losing one coordinated solvent ligand at 233 K. Thus, the present study demonstrates that the structure and spin state of nonheme FeO complexes can be different at different temperatures; therefore, the structural and/or spin state information obtained at 4 K should be carefully used at a higher temperature (, 233 K). In addition to and , [(TPA)FeO] () with an = 1 spin state, whose spin state was determined spectroscopically and theoretically at 233 K, is included in this study to compare the chemical properties of = 1 and = 2 FeO complexes. The present results add another dimension to the discussion of the reactivites of nonheme FeO species, in which the structural preference and spin state of nonheme FeO species can vary depending on temperature.