More Like this

1. Ligand field effects on the ground and excited states of reactive FeO ²⁺ species

   https://doi.org/10.1039/c8cp05372c  

   Kirkland, Justin K.; Khan, Shahriar N.; Casale, Bryan; Miliordos, Evangelos; Vogiatzis, Konstantinos D. (November 2018, Physical Chemistry Chemical Physics)

   High-valent Fe( iv )-oxo species have been found to be key oxidizing intermediates in the mechanisms of mononuclear iron heme and non-heme enzymes that can functionalize strong C–H bonds. Biomimetic Fe( iv )-oxo molecular complexes have been successfully synthesized and characterized, but their catalytic reactivity is typically lower than that of the enzymatic analogues. The C–H activation step proceeds through two competitive mechanisms, named σ- and π-channels. We have performed high-level wave function theory calculations on bare FeO 2+ and a series of non-heme Fe( iv )-oxo model complexes in order to elucidate the electronic properties and the ligand field effects on those channels. Our results suggest that a coordination environment formed by a weak field gives access to both competitive channels, yielding more reactive Fe( iv )-oxo sites. In contrast, a strong ligand environment stabilizes only the σ-channel. Our concluding remarks will aid the derivation of new structure–reactivity descriptors that can contribute to the development of the next generation of functional catalysts. 

   more » « less
2. 10 ⁶ -fold faster C–H bond hydroxylation by a Co ^III,IV ₂ (µ-O) ₂ complex [via a Co ^III ₂ (µ-O)(µ-OH) intermediate] versus its Fe ^III Fe ^IV analog

   https://doi.org/10.1073/pnas.2307950120  

   Li, Yan; Abelson, Chase; Que, Lawrence; Wang, Dong (December 2023, Proceedings of the National Academy of Sciences)

   The hydroxylation of C–H bonds can be carried out by the high-valent Co^III,IV₂(µ-O)₂complex2asupported by the tetradentate tris(2-pyridylmethyl)amine ligand via a Co^III₂(µ-O)(µ-OH) intermediate (3a). Complex3acan be independently generated either by H-atom transfer (HAT) in the reaction of2awith phenols as the H-atom donor or protonation of its conjugate base, the Co^III₂(µ-O)₂complex1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm⁻¹associated with the symmetric Co–O–Co stretching mode of the Co₂O₂diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for1aand2aby Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their “diamond core” structural assignments. The independent generation of3aallows us to investigate HAT reactions of2awith phenols in detail, measure the redox potential and pK_aof the system, and calculate the O–H bond strength (D_O–H) of3ato shed light on the C–H bond activation reactivity of2a. Complex3ais found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal2ato be 10⁶-fold more reactive in oxidizing hydrocarbon C–H bonds than corresponding Fe^III,IV₂(µ-O)₂and Mn^III,IV₂(µ-O)₂analogs, an unexpected outcome that raises the prospects for using Co^III,IV₂(µ-O)₂species to oxidize alkane C–H bonds. 

   more » « less
3. Facile Conversion of syn ‐[Fe ^IV (O)(TMC)] ²⁺ into the anti Isomer via Meunier's Oxo–Hydroxo Tautomerism Mechanism

   https://doi.org/10.1002/anie.201811454  

   Prakash, Jai; Sheng, Yuan; Draksharapu, Apparao; Klein, Johannes E. M. N.; Cramer, Christopher J.; Que, Jr., Lawrence (January 2019, Angewandte Chemie International Edition)

   Abstract Thesynandantiisomers of [Fe^IV(O)(TMC)]²⁺(TMC=tetramethylcyclam) represent the first isolated pair of synthetic non‐heme oxoiron(IV) complexes with identical ligand topology, differing only in the position of the oxo unit bound to the iron center. Both isomers have previously been characterized. Reported here is that thesynisomer [Fe^IV(O_syn)(TMC)(NCMe)]²⁺(2) converts into itsantiform [Fe^IV(O_anti)(TMC)(NCMe)]²⁺(1) in MeCN, an isomerization facilitated by water and monitored most readily by¹H NMR and Raman spectroscopy. Indeed, when H₂¹⁸O is introduced to2, the nascent1becomes¹⁸O‐labeled. These results provide compelling evidence for a mechanism involving direct binding of a water moleculetransto the oxo atom in2with subsequent oxo–hydroxo tautomerism for its incorporation as the oxo atom of1. The nonplanar nature of the TMC supporting ligand makes this isomerization an irreversible transformation, unlike for their planar heme counterparts. 

   more » « less
4. Concerted proton–electron transfer reactions of manganese–hydroxo and manganese–oxo complexes

   https://doi.org/10.1039/d0cc01201g  

   Mayfield, Jaycee R.; Grotemeyer, Elizabeth N.; Jackson, Timothy A. (January 2020, Chemical Communications)

   The enzymes manganese superoxide dismutase and manganese lipoxygenase use Mn III –hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C–H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn–oxo motifs in attacking substrate C–H bonds, presumably by a concerted proton–electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn–hydroxo and Mn–oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of Mn III –hydroxo and Mn IV –oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around Mn III –hydroxo and Mn IV –oxo motifs permit elucidation of structure–activity relationships. For Mn III –hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the Mn III/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the p K a of the Mn II –aqua product, which represents the proton-transfer component for CPET. For Mn IV –oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the Mn IV –oxo unit results in large rate enhancements for C–H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of Mn III –hydroxo and Mn IV –oxo adducts. 

   more » « less
5. Octahedral iron( iv )–tosylimido complexes exhibiting single electron-oxidation reactivity

   https://doi.org/10.1039/C9SC02526J  

   Sabenya, Gerard; Gamba, Ilaria; Gómez, Laura; Clémancey, Martin; Frisch, Jonathan R.; Klinker, Eric J.; Blondin, Geneviève; Torelli, Stéphane; Que, Lawrence; Martin-Diaconescu, Vlad; et al (October 2019, Chemical Science)

   High valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron( iv )–tosylimido complexes [Fe IV (NTs)(MePy 2 tacn)](OTf) 2 ( 1(IV)NTs ) and [Fe IV (NTs)(Me 2 (CHPy 2 )tacn)](OTf) 2 ( 2(IV)NTs ), (MePy 2 tacn = N -methyl- N , N -bis(2-picolyl)-1,4,7-triazacyclononane, and Me 2 (CHPy 2 )tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1(IV)NTs and 2(IV)NTs are rare examples of octahedral iron( iv )–imido complexes and are isoelectronic analogues of the recently described iron( iv )–oxo complexes [Fe IV (O)(L)] 2+ (L = MePy 2 tacn and Me 2 (CHPy 2 )tacn, respectively). 1(IV)NTs and 2(IV)NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, 1 H-NMR, resonance Raman, Mössbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [Fe III (HNTs)(L)] 2+ , 1(III)–NHTs (L = MePy 2 tacn) and 2(III)–NHTs (L = Me 2 (CHPy 2 )tacn) have been isolated after the decay of 1(IV)NTs and 2(IV)NTs in solution, spectroscopically characterized, and the molecular structure of [Fe III (HNTs)(MePy 2 tacn)](SbF 6 ) 2 determined by single crystal X-ray diffraction. Reaction of 1(IV)NTs and 2(IV)NTs with different p -substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1(IV)NTs and 2(IV)NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1(IV)NTs and 2(IV)NTs with hydrocarbons containing weak C–H bonds results in the formation of 1(III)–NHTs and 2(III)–NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction. 

   more » « less