Oxoiron(IV) complexes as synthons for the assembly of heterobimetallic centers such as the Fe/Mn active site of Class Ic ribonucleotide reductases.

Nonheme oxoiron(IV) complexes can serve as synthons for generating heterobimetallic oxo-bridged dimetal complexes by reaction with divalent metal complexes. The formation of FeIII-O-CrIII and FeIII-O-MnIII complexes is described herein. The latter complexes may serve as models for the FeIII-X-MnIII active sites of an emerging class of Fe/Mn enzymes represented by the Class 1c ribonucleotide reductase from Chlamydia trachomatis and the R2-like ligand-binding oxidase (R2lox) found in Mycobacterium tuberculosis. These synthetic complexes have been characterized by UV-Vis, resonance Raman, and X-ray absorption spectroscopy, as well as electrospray mass spectrometry. The FeIII-O-CrIII complexes exhibit a three-band UV-Vis pattern that differs from the simpler features associated with FeIII-O-FeIII complexes. The positions of these features are modulated by the nature of the supporting polydentate ligand on the iron center, and their bands intensify dramatically in two examples upon the binding of an axial cyanate or thiocyanate ligand trans to the oxo bridge. In contrast, the FeIII-O-MnIII complexes resemble FeIII-O-FeIII complexes more closely. Resonance Raman characterization of the FeIII-O-MIII complexes reveals an 18O-sensitive vibration in the range of 760-890 cm-1. This feature has been assigned to the asymmetric FeIII-O-MIII stretching mode and correlates reasonably with the Fe-O bond distance determined by EXAFS analysis. The likely binding of an acetate as a bridging ligand to the FeIII-O-MnIII complex 12 lays the foundation for further efforts to model the heterobimetallic active sites of Fe/Mn enzymes.

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe-O-M Complexes Derived from [FeIV(Oanti/syn)(TMC)]2+ Isomers.

Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron(IV) moiety, as exemplified by the prototypical [(TMC)FeIV(Oanti)(NCCH3)](OTf)2, where anti indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported syn isomer [(TMC)FeIV(Osyn)(OTf)](OTf). The ability to access two isomers of [(TMC)FeIV(Oanti/syn)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH3CN)(TMC)FeIII-Oanti-CrIII(OTf)4(NCCH3)] (1) by inner-sphere electron transfer between Cr(OTf)2 and [(TMC)FeIV(Oanti)(NCCH3)](OTf)2. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)2 and [(TMC)FeIV(Osyn)(NCCH3)](OTf)2, which is formulated as [(TMC)FeIII-Osyn-CrIII(OTf)4(NCCH3)] based on its characterization by UV-vis, resonance Raman, Mössbauer, and X-ray absorption spectroscopic methods, as well as electrospray mass spectrometry. Its pre-edge area (30 units) and Fe-O distance (1.77 Å) determined by X-ray absorption spectroscopy are distinctly different from those of 1 (11-unit pre-edge area and 1.81 Å Fe-O distance) but more closely resemble the values reported for [(TMC)FeIII-Osyn-ScIII(OTf)4(NCCH3)] (3, 32-unit pre-edge area and 1.75 Å Fe-O distance). This comparison suggests that 2 has a square pyramidal iron center like 3, rather than the six-coordinate center deduced for 1. Density functional theory calculations further validate the structures for 1 and 2. The influence of the distinct TMC topologies on the coordination geometries is further confirmed by the crystal structures of [(Cl)(TMC)FeIII-Oanti-FeIIICl3] (4Cl) and [(TMC)FeIII-Osyn-FeIIICl3](OTf) (5). Complexes 1-5 thus constitute a set of complexes that shed light on ligand topology effects on the coordination chemistry of the oxoiron moiety.

Spectroscopic and Theoretical Study of Spin-Dependent Electron Transfer in an Iron(III) Superoxo Complex.

It was shown previously (J. Am. Chem. Soc. 2014, 136, 10846) that bubbling of O2 into a solution of Fe(II)(BDPP) (H2BDPP = 2,6-bis[[(S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl]pyridine) in tetrahydrofuran at -80 °C generates a high-spin (SFe = (5)/2) iron(III) superoxo adduct, 1. Mössbauer studies revealed that 1 is an exchange-coupled system, [Formula: see text], where SR = (1)/2 is the spin of the superoxo radical, of which the spectra were not well enough resolved to determine whether the coupling was ferromagnetic (S = 3 ground state) or antiferromagnetic (S = 2). The glass-forming 2-methyltetrahydrofuran solvent yields highly resolved Mössbauer spectra from which the following data have been extracted: (i) the ground state of 1 has S = 3 (J < 0); (ii) |J| > 15 cm(-1); (iii) the zero-field-splitting parameters are D = -1.1 cm(-1) and E/D = 0.02; (iv) the major component of the electric-field-gradient tensor is tilted ≈7° relative to the easy axis of magnetization determined by the MS = ±3 and ±2 doublets. The excited-state MS = ±2 doublet yields a narrow parallel-mode electron paramagnetic resonance signal at g = 8.03, which was used to probe the magnetic hyperfine splitting of (17)O-enriched O2. A theoretical model that considers spin-dependent electron transfer for the cases where the doubly occupied π* orbital of the superoxo ligand is either "in" or "out" of the plane defined by the bent Fe-OO moiety correctly predicts that 1 has an S = 3 ground state, in contrast to the density functional theory calculations for 1, which give a ground state with both the wrong spin and orbital configuration. This failure has been traced to a basis set superposition error in the interactions between the superoxo moiety and the adjacent five-membered rings of the BDPP ligand and signals a fundamental problem in the quantum chemistry of O2 activation.

C-H Bond Cleavage by Bioinspired Nonheme Oxoiron(IV) Complexes, Including Hydroxylation of n-Butane.

The development of efficient and selective hydrocarbon oxidation processes with low environmental impact remains a major challenge of the 21st century because of the strong and apolar nature of the C-H bond. Naturally occurring iron-containing metalloenzymes can, however, selectively functionalize strong C-H bonds on substrates under mild and environmentally benign conditions. The key oxidant in a number of these transformations is postulated to possess an S = 2 Fe(IV)═O unit in a nonheme ligand environment. This oxidant has been trapped and spectroscopically characterized and its reactivity toward C-H bonds demonstrated for several nonheme iron enzyme classes. In order to obtain insight into the structure-activity relationships of these reactive intermediates, over 60 synthetic nonheme Fe(IV)(O) complexes have been prepared in various laboratories and their reactivities investigated. This Forum Article summarizes the current status of efforts in the characterization of the C-H bond cleavage reactivity of synthetic Fe(IV)(O) complexes and provides a snapshot of the current understanding of factors that control this reactivity, such as the properties of the supporting ligands and the spin state of the iron center. In addition, new results on the oxidation of strong C-H bonds such as those of cyclohexane and n-butane by a putative S = 2 synthetic Fe(IV)(O) species that is generated in situ using dioxygen at ambient conditions are presented.

Characterization of a paramagnetic mononuclear nonheme iron-superoxo complex.

O2 bubbling into a THF solution of Fe(II)(BDPP) (1) at -80 °C generates a reversible bright yellow adduct 2. Characterization by resonance Raman and Mössbauer spectroscopy provides complementary insights into the nature of 2. The former shows a resonance-enhanced vibration at 1125 cm(-1), which can be assigned to the ν(O-O) of a bound superoxide, while the latter reveals the presence of a high-spin iron(III) center that is exchange-coupled to the superoxo ligand, like the Fe(III)-O2(-) pair found for the O2 adduct of 4-nitrocatechol-bound homoprotocatechuate 2,3-dioxygenase. Lastly, 2 oxidizes dihydroanthracene to anthracene, supporting the notion that Fe(III)-O2(-) species can carry out H atom abstraction from a C-H bond to initiate the 4-electron oxidation of substrates proposed for some nonheme iron enzymes.

Trigonal bi- and monopyramidal cobalt(II) complexes of a novel guanidine-based tripodal ligand.

The novel ligand DIG(3)tren has three N',N''-diisopropylguanidinyl (DIG) moieties. We report on the structures of two cobalt complexes that show how an isopropylamino group from each DIG acts as a flap that can either close over the metal or rotate away from the metal to open up a site for auxiliary ligand binding. Two of the -NH(iPr) flaps are open in pink [Co(DIG(3)tren)(OAc)]OAc (1), and each of these flaps provides a hydrogen bond to stabilize acetate binding to trigonal bipyrimidal cobalt. The flaps are closed in blue [Co(DIG(3)tren)][BPh(4)](2) (2), yielding a rare example of a trigonal (mono)pyramidal [ML](2+) ion.

Characterization of a heterobimetallic nonheme Fe(III)-O-Cr(III) species formed by O2 activation.

We report the generation and spectroscopic characterization of a heterobimetallic [(TMC)Fe(III)-O-Cr(III)(OTf)4] species (1) by bubbling O2 into a mixture of Fe(TMC)(OTf)2 and Cr(OTf)2 in NCCH3. Complex 1 also formed quantitatively by adding Cr(OTf)2 to [Fe(IV)(O)(TMC)(NCCH3)](2+). The proposed O2 activation mechanism involves the trapping of a Cr-O2 adduct by Fe(TMC)(OTf)2.

Evidence for an oxygen evolving iron-oxo-cerium intermediate in iron-catalysed water oxidation.

The non-haem iron complex α-[Fe(II)(CF3SO3)2(mcp)] (mcp=(N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-cis-diaminocyclohexane) reacts with Ce(IV) to oxidize water to O2, representing an iron-based functional model for the oxygen evolving complex of photosystem II. Here we trap an intermediate, characterized by cryospray ionization high resolution mass spectrometry and resonance Raman spectroscopy, and formulated as [(mcp)Fe(IV)(O)(µ-O)Ce(IV)(NO3)3](+), the first example of a well-characterized inner-sphere complex to be formed in cerium(IV)-mediated water oxidation. The identification of this reactive Fe(IV)-O-Ce(IV) adduct may open new pathways to validate mechanistic notions of an analogous Mn(V)-O-Ca(II) unit in the oxygen evolving complex that is responsible for carrying out the key O-O bond forming step.