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

ABSTRACT: 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.

SUBMITTER: Lee NY

PROVIDER: S-EPMC5609531 | biostudies-literature | 2017 Aug

REPOSITORIES: biostudies-literature

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

Lee Na Young NY Mandal Debasish D Bae Seong Hee SH Seo Mi Sook MS Lee Yong-Min YM Shaik Sason S Cho Kyung-Bin KB Nam Wonwoo W

Chemical science 20170530 8

The spin states (S = 1 and S = 2) of nonheme FeIVO 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 FeIVO species experimentally, since FeIVO 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 ...[more]

PMID: 28970926

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Project description:The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N(4) ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe(IV)(O)(L)(OTf)](+) (OTf = trifluoromethanesulfonate) and [Fe(IV) (O)(L)](2+) were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)Py(2)) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [Fe(IV)(O)(L)(OTf)](+) cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe(IV)(O)(L)](2+) cations. DFT calculations on [Fe(IV)(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C-H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright ? trajectory and prevent optimal ?(CH)-?*(z2) overlap, all the reactions still proceed preferentially on the quintet (S = 2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S = 2 reactions for both hexa- and pentacoordinate complexes.

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

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Structure and spin state of nonheme Fe<sup>IV</sup>O complexes depending on temperature: predictive insights from DFT calculations and experiments.

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