Structure and spin state of nonheme Fe^IVO complexes depending on temperature: Predictive insights from DFT calculations and experiments

The spin states (S = 1 and S = 2) of nonheme Fe^IVO 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 Fe^IVO species experimentally, since Fe^IVO models having the S = 1 and S = 2 spin states at the same time neither exist nor can be synthesized. However, recent synthesis of an Fe^IVO complex with an S = 1 spin state (triplet), [(Me₃NTB)Fe^IVO]²⁺ (1), and a structurally similar Fe^IVO complex but with an S = 2 spin state (quintet), [(TQA)Fe^IVO]²⁺ (2), 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 Fe^IVO complexes. While 1 and 2 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 Fe^IVO 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 (e.g., 233 K). In addition to 1 and 2, [(TPA)Fe^IVO]²⁺ (3) with an S = 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 S = 1 and S = 2 Fe^IVO complexes. The present results add another dimension to the discussion of the reactivites of nonheme Fe^IVO species, in which the structural preference and spin state of nonheme Fe^IVO species can vary depending on temperature.