In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O-activating iron enzymes. This work details an electronic-structure investigation of [Fe(O)(L)(NCMe)] (L = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm in the E(d → d) excited state. Both values are similar to those measured for [Fe(O)(TMC)(NCMe)] (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm. We can unequivocally assign them to the ligand field transitions of d → d, d → d, and d → d, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe(O) (SR-TPA)(NCMe)] (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO) core of complex 1 have similar transition energies, whereas the excitation energy for d → d is significantly higher (∼12 000 cm for [Fe(O)(SR-TPA)(NCMe)]). Our results thus substantiate that the tetracarbene ligand (L) of complex 1 does not significantly affect the bonding in the (FeO) unit but strongly destabilizes the d orbital to eventually lift it above d. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.

We demonstrate the application of infrared photodissocation spectroscopy for determination of the Fe═O stretching frequencies of high-valent iron(IV)-oxo complexes [(L)Fe(O)(X)] (L = TMC, N4Py, PyTACN, and X = CHCN, CFSO, ClO, CFCOO, NO, N). We show that the values determined by resonance Raman spectroscopy in acetonitrile solutions are on average 9 cm red-shifted with respect to unbiased gas-phase values. Furthermore, we show the assignment of the spin state of the complexes based on the vibrational modes of a coordinated anion and compare reactivities of various iron(IV)-oxo complexes generated as dications or monocations (bearing an anionic ligand). The coordinated anions can drastically affect the reactivity of the complex and should be taken into account when comparing reactivities of complexes bearing different ligands. Comparison of reactivities of [(PyTACN)Fe(O)(X)] generated in different spin states and bearing different anionic ligands X revealed that the nature of anion influences the reactivity more than the spin state. The triflate and perchlorate ligands tend to stabilize the quintet state of [(PyTACN)Fe(O)(X)], whereas trifluoroacetate and nitrate stabilize the triplet state of the complex.

Terminal non-heme iron(IV)−oxo compounds are among the most powerful and best studied oxidants of strong C−H bonds. In contrast to the increasing number of such complexes (>80 thus far), corresponding one-electron-reduced derivatives are much rarer and presumably less stable, and only two iron(III)−oxo complexes have been characterized to date, both of which are stabilized by hydrogen-bonding interactions. Herein we have employed gas-phase techniques to generate and identify a series of terminal iron(III)−oxo complexes, all without built-in hydrogen bonding. Some of these complexes exhibit ∼70 cm −1 decrease in ν(Fe−O) frequencies expected for a half-order decrease in bond order upon one-electron reduction to an S = 5/2 center, while others have ν(Fe−O) frequencies essentially unchanged from those of their parent iron(IV)−oxo complexes. The latter result suggests that the added electron does not occupy a d orbital with FeO antibonding character, requiring an S = 3/2 spin assignment for the nascent Fe III −O − species. In the latter cases, water is found to hydrogen bond to the Fe III −O − unit, resulting in a change from quartet to sextet spin state. Reactivity studies also demonstrate the extraordinary basicity of these iron(III)−oxo complexes. Our observations show that metal−oxo species at the boundary of the "Oxo Wall" are accessible, and the data provide a lead to detect iron(III)−oxo intermediates in biological and biomimetic reactions.

FeV(O)(OH) species have long been proposed to play a key role in a wide range of biomimetic and enzymatic oxidations, including as intermediates in arene dihydroxylation catalyzed by Rieske oxygenases. However, the inability to accumulate these intermediates in solution has thus far prevented their spectroscopic and chemical characterization. Thus, we use gas-phase ion spectroscopy and reactivity analysis to characterize the highly reactive [FeV(O)(OH)(5tips3tpa)]2+ (32+) complex. The results show that 32+ hydroxylates C–H bonds via a rebound mechanism involving two different ligands at the Fe center and dihydroxylates olefins and arenes. Hence, this study provides a direct evidence of FeV(O)(OH) species in non-heme iron catalysis. Furthermore, the reactivity of 32+ accounts for the unique behavior of Rieske oxygenases. The use of gas-phase ion characterization allows us to address issues related to highly reactive intermediates that other methods are unable to solve in the context of catalysis and enzymology.

Terminal oxo complexes of late transition metals are frequently proposed reactive intermediates. However, they are scarcely known beyond Group 8. Using mass spectrometry, we prepared and characterized two such complexes: [(N4Py)Co III (O)] + ( 1 ) and [(N4Py)Co IV (O)] 2+ ( 2 ). Infrared photodissociation spectroscopy revealed that the Co−O bond in 1 is rather strong, in accordance with its lack of chemical reactivity. On the contrary, 2 has a very weak Co−O bond characterized by a stretching frequency of ≤659 cm −1 . Accordingly, 2 can abstract hydrogen atoms from non‐activated secondary alkanes. Previously, this reactivity has only been observed in the gas phase for small, coordinatively unsaturated metal complexes. Multireference ab‐initio calculations suggest that 2 , formally a cobalt(IV)‐oxo complex, is best described as cobalt(III)‐oxyl. Our results provide important data on changes to metal‐oxo bonding behind the oxo wall and show that cobalt‐oxo complexes are promising targets for developing highly active C−H oxidation catalysts.