Axial Ligation Impedes Proton-Coupled Electron-Transfer Reactivity of a Synthetic Compound-I Analogue.

The nature of the axial ligand in high-valent iron-oxo heme enzyme intermediates and related synthetic catalysts is a critical structural element for controlling proton-coupled electron-transfer (PCET) reactivity of these species. Herein, we describe the generation and characterization of three new 6-coordinate, iron(IV)-oxo porphyrinoid-π-cation-radical complexes and report their PCET reactivity together with a previously published 5-coordinate analogue, FeIV(O)(TBP8Cz+•) (TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato3-) (2) (Cho, K. A high-valent iron-oxo corrolazine activates C-H bonds via hydrogen-atom transfer. J. Am. Chem. Soc. 2012, 134, 7392-7399). The new complexes FeIV(O)(TBP8Cz+•)(L) (L = 1-methyl imidazole (1-MeIm) (4a), 4-dimethylaminopyridine (DMAP) (4b), cyanide (CN-)(4c)) can be generated from either oxidation of the ferric precursors or by addition of L to the Compound-I (Cpd-I) analogue at low temperatures. These complexes were characterized by UV-vis, electron paramagnetic resonance (EPR), and Mössbauer spectroscopies, and cryospray ionization mass spectrometry (CSI-MS). Kinetic studies using 4-OMe-TEMPOH as a test substrate indicate that coordination of a sixth axial ligand dramatically lowers the PCET reactivity of the Cpd-I analogue (rates up to 7000 times slower). Extensive density functional theory (DFT) calculations together with the experimental data show that the trend in reactivity with the axial ligands does not correlate with the thermodynamic driving force for these reactions or the calculated strengths of the O-H bonds being formed in the FeIV(O-H) products, pointing to non-Bell-Evans-Polanyi behavior. However, the PCET reactivity does follow a trend with the bracketed reduction potential of Cpd-I analogues and calculated electron affinities. The combined data suggest a concerted mechanism (a concerted proton electron transfer (CPET)) and an asynchronous movement of the electron/proton pair in the transition state.

Electrophotocatalytic perfluoroalkylation by LMCT excitation of Ag(II) perfluoroalkyl carboxylates.

Molecular Ag(II) complexes are superoxidizing photoredox catalysts capable of generating radicals from redox-reticent substrates. In this work, we exploited the electrophilicity of Ag(II) centers in [Ag(bpy)2(TFA)][OTf] and Ag(bpy)(TFA)2 (bpy, 2,2'-bipyridine; OTf, CF3SO3-) complexes to activate trifluoroacetate (TFA) by visible light-induced homolysis. The resulting trifluoromethyl radicals may react with a variety of arenes to forge C(sp2)-CF3 bonds. This methodology is general and extends to other perfluoroalkyl carboxylates of higher chain length (RFCO2-; RF, CF2CF3 or CF2CF2CF3). The photoredox reaction may be rendered electrophotocatalytic by regenerating the Ag(II) complexes electrochemically during irradiation. Electrophotocatalytic perfluoroalkylation of arenes at turnover numbers exceeding 20 was accomplished by photoexciting the Ag(II)-TFA ligand-to-metal charge transfer (LMCT) state, followed by electrochemical reoxidation of the Ag(I) photoproduct back to the Ag(II) photoreactant.

Oxygen versus Sulfur Coordination in Cobalt Superoxo Complexes: Spectroscopic Properties, O2 Binding, and H-Atom Abstraction Reactivity.

A five-coordinate, disiloxide-ligated cobalt(II) (S = 3/2) complex (1) was prepared as an oxygen-ligated analogue to the previously reported silanedithiolate-ligated CoII(Me3TACN)(S2SiMe2) (J. Am. Chem. Soc., 2019, 141, 3641-3653). The structural and spectroscopic properties of 1 were analyzed by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR), and NMR spectroscopies. The reactivity of 1 with dioxygen was examined, and it was shown to bind O2 reversibly in a range of solvents at low temperatures. A cobalt(III)-superoxo complex, CoIII(O2·-)(Me3TACN)((OSi2Ph)2O) (2), was generated, and was analyzed by UV-vis, EPR, and resonance Raman spectroscopies. Unlike its sulfur-ligated analogue, complex 2 can thermally release O2 to regenerate 1. Vibrational assignments for selective 18O isotopic labeling of both O2 and disiloxide ligands in 2 are consistent with a 6-coordinate, Co(η1-O2·-)("end-on") complex. Complex 2 reacts with the O-H bond of 4-methoxy-2,2,6,6-tetramethylpiperidin-1-ol (4-MeO-TEMPOH) via H-atom abstraction with a rate of 0.58(2) M-1 s-1 at -105 °C, but it is unable to oxidize phenol substrates. This bracketed reactivity suggests that the O-H bond being formed in the putative CoIII(OOH) product has a relatively weak O-H bond strength (BDFE ∼66-74 kcal mol-1). These thermodynamic and kinetic parameters are similar to those seen for the sulfur-ligated Co(O2)(Me3TACN)(S2SiMe2), indicating that the differences in the electronic structure for O versus S ligation do not have a large impact on H-atom abstraction reactivity.

Iron Insertion at the Assembly Site of the ISCU Scaffold Protein Is a Conserved Process Initiating Fe-S Cluster Biosynthesis.

Iron-sulfur (Fe-S) clusters are prosthetic groups of proteins biosynthesized on scaffold proteins by highly conserved multi-protein machineries. Biosynthesis of Fe-S clusters into the ISCU scaffold protein is initiated by ferrous iron insertion, followed by sulfur acquisition, via a still elusive mechanism. Notably, whether iron initially binds to the ISCU cysteine-rich assembly site or to a cysteine-less auxiliary site via N/O ligands remains unclear. We show here by SEC, circular dichroism (CD), and Mössbauer spectroscopies that iron binds to the assembly site of the monomeric form of prokaryotic and eukaryotic ISCU proteins via either one or two cysteines, referred to the 1-Cys and 2-Cys forms, respectively. The latter predominated at pH 8.0 and correlated with the Fe-S cluster assembly activity, whereas the former increased at a more acidic pH, together with free iron, suggesting that it constitutes an intermediate of the iron insertion process. Iron not binding to the assembly site was non-specifically bound to the aggregated ISCU, ruling out the existence of a structurally defined auxiliary site in ISCU. Characterization of the 2-Cys form by site-directed mutagenesis, CD, NMR, X-ray absorption, Mössbauer, and electron paramagnetic resonance spectroscopies showed that the iron center is coordinated by four strictly conserved amino acids of the assembly site, Cys35, Asp37, Cys61, and His103, in a tetrahedral geometry. The sulfur receptor Cys104 was at a very close distance and apparently bound to the iron center when His103 was missing, which may enable iron-dependent sulfur acquisition. Altogether, these data provide the structural basis to elucidate the Fe-S cluster assembly process and establish that the initiation of Fe-S cluster biosynthesis by insertion of a ferrous iron in the assembly site of ISCU is a conserved mechanism.

A Reactive, Photogenerated High-Spin (S = 2) FeIV(O) Complex via O2 Activation.

Addition of dioxygen at low temperature to the non-heme ferrous complex FeII(Me3TACN)((OSiPh2)2O) (1) in 2-MeTHF produces a peroxo-bridged diferric complex Fe2III(µ-O2)(Me3TACN)2((OSiPh2)2O)2 (2), which was characterized by UV-vis, resonance Raman, and variable field Mössbauer spectroscopies. Illumination of a frozen solution of 2 in THF with white light leads to homolytic O-O bond cleavage and generation of a FeIV(O) complex 4 (ν(Fe=O) = 818 cm-1; δ = 0.22 mm s-1, ΔEQ = 0.23 mm s-1). Variable field Mössbauer spectroscopy measurements show that 4 is a rare example of a high-spin S = 2 FeIV(O) complex and the first synthetic example to be generated directly from O2. Complex 4 is highly reactive, as expected for a high-spin ferryl, and decays rapidly in fluid solution at cryogenic temperatures. This decay process in 2-MeTHF involves C-H cleavage of the solvent. However, the controlled photolysis of 2 in situ with visible light and excess phenol substrate leads to competitive phenol oxidation, via the proposed transient generation of 4 as the active oxidant.

Concerted Differential Changes of Helical Dynamics and Packing upon Ligand Occupancy in a Bacterial Chemoreceptor.

Transmembrane receptors are central components of the chemosensory systems by which motile bacteria detect and respond to chemical gradients. An attractant bound to the receptor periplasmic domain generates conformational signals that regulate a histidine kinase interacting with its cytoplasmic domain. Ligand-induced signaling through the periplasmic and transmembrane domains of the receptor involves a piston-like helical displacement, but the nature of this signaling through the >200 Å four-helix coiled coil of the cytoplasmic domain had not yet been identified. We performed single-molecule Förster resonance energy transfer measurements on Escherichia coli aspartate receptor homodimers inserted into native phospholipid bilayers enclosed in nanodiscs. The receptors were labeled with fluorophores at diagnostic positions near the middle of the cytoplasmic coiled coil. At these positions, we found that the two N-helices of the homodimer were more distant, that is, less tightly packed and more dynamic than the companion C-helix pair, consistent with previous deductions that the C-helices form a stable scaffold and the N-helices are dynamic. Upon ligand binding, the scaffold pair compacted further, while separation and dynamics of the dynamic pair increased. Thus, ligand binding had asymmetric effects on the two helical pairs, shifting mean distances in opposite directions and increasing the dynamics of one pair. We suggest that this reflects a conformational change in which differential alterations to the packing and dynamics of the two helical pairs are coupled. These coupled changes could represent a previously unappreciated mode of conformational signaling that may well occur in other coiled-coil signaling proteins.

A Nonheme Mononuclear {FeNO}7 Complex that Produces N2 O in the Absence of an Exogenous Reductant.

A new nonheme iron(II) complex, FeII (Me3 TACN)((OSiPh2 )2 O) (1), is reported. Reaction of 1 with NO(g) gives a stable mononitrosyl complex Fe(NO)(Me3 TACN)((OSiPh2 )2 O) (2), which was characterized by Mössbauer (δ=0.52 mm s-1 , |ΔEQ |=0.80 mm s-1 ), EPR (S=3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}7 complex with an S=3/2 spin ground state. The RR spectrum (λexc =458 nm) of 2 combined with isotopic labeling (15 N, 18 O) reveals ν(N-O)=1680 cm-1 , which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm-1 ). Complex 2 reacts rapidly with H2 O in THF to produce the N-N coupled product N2 O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N2 O in the absence of an exogenous reductant.

Proton-Coupled Electron-Transfer Reactivity Controls Iron versus Sulfur Oxidation in Nonheme Iron-Thiolate Complexes.

Reaction of the five-coordinate FeII(N4S) complexes, [FeII(iPr3TACN)(abtX)](OTf) (abt = aminobenzenethiolate, X = H, CF3), with a one-electron oxidant and an appropriate base leads to net H atom loss, generating new FeIII(iminobenzenethiolate) complexes that were characterized by single-crystal X-ray diffraction (XRD), as well as UV-vis, EPR, and Mössbauer spectroscopies. The spectroscopic data indicate that the iminobenzenethiolate complexes have S = 3/2 ground states. In the absence of a base, oxidation of the FeII(abt) complexes leads to disulfide formation instead of oxidation at the metal center. Bracketing studies with separated proton-coupled electron-transfer (PCET) reagents show that the FeII(aminobenzenethiolate) and FeIII(iminobenzenethiolate) forms are readily interconvertible by H+/e- transfer and provide a measure of the bond dissociation free energy (BDFE) for the coordinated N-H bond between 64 and 69 kcal mol-1. This work shows that coordination to the iron center causes a dramatic weakening of the N-H bond and that Fe- versus S-oxidation in a nonheme iron complex can be controlled by the protonation state of an ancillary amino donor.

Synthesis and Characterization of a Linear Triiron(II) Extended Metal Atom Chain Complex with Fe-Fe Bonds.

Extended metal atom chain (EMAC) complexes of first-row transition metals with metal-metal bonds have the potential to elicit unique magnetic properties and reactivities. Until now, the library of EMAC complexes with late-first-row transition metals was incomplete because of the omission of a triiron species with Fe-Fe bonding. Herein we report the synthesis and preliminary investigation of the first linear, triiron(II) complex containing close Fe-Fe interactions. The complex is supported by three dianionic 2,6-bis[(trimethylsilyl)amido]pyridine ligands (L), with an overall composition of Fe3L3, and pseudohelical ligand coordination stabilizing the local trigonal-planar geometry at each iron. Fe3L3 was characterized by X-ray diffraction, 1H NMR, cyclic voltammetry, electronic absorption, and Mössbauer spectroscopies. Evans method analysis indicated a large uncompensated spin and an S = 6 ground state, suggesting ferromagnetic coupling in the triiron chain, likely due to direct exchange.

Activation of Dioxygen by a Mononuclear Nonheme Iron Complex: Sequential Peroxo, Oxo, and Hydroxo Intermediates.

The activation of dioxygen by FeII(Me3TACN)(S2SiMe2) (1) is reported. Reaction of 1 with O2 at -135 °C in 2-MeTHF generates a thiolate-ligated (peroxo)diiron complex FeIII2(O2)(Me3TACN)2(S2SiMe2)2 (2) that was characterized by UV-vis (λmax = 300, 390, 530, 723 nm), Mössbauer (δ = 0.53, |ΔEQ| = 0.76 mm s-1), resonance Raman (RR) (ν(O-O) = 849 cm-1), and X-ray absorption (XAS) spectroscopies. Complex 2 is distinct from the outer-sphere oxidation product 1ox (UV-vis (λmax = 435, 520, 600 nm), Mössbauer (δ = 0.45, |ΔEQ| = 3.6 mm s-1), and EPR (S = 5/2, g = [6.38, 5.53, 1.99])), obtained by one-electron oxidation of 1. Cleavage of the peroxo O-O bond can be initiated either photochemically or thermally to produce a new species assigned as an FeIV(O) complex, FeIV(O)(Me3TACN)(S2SiMe2) (3), which was identified by UV-vis (λmax = 385, 460, 890 nm), Mössbauer (δ = 0.21, |ΔEQ| = 1.57 mm s-1), RR (ν(FeIV═O) = 735 cm-1), and X-ray absorption spectroscopies, as well as reactivity patterns. Reaction of 3 at low temperature with H atom donors gives a new species, FeIII(OH)(Me3TACN)(S2SiMe2) (4). Complex 4 was independently synthesized from 1 by the stoichiometric addition of a one-electron oxidant and a hydroxide source. This work provides a rare example of dioxygen activation at a mononuclear nonheme iron(II) complex that produces both FeIII-O-O-FeIII and FeIV(O) species in the same reaction with O2. It also demonstrates the feasibility of forming Fe/O2 intermediates with strongly donating sulfur ligands while avoiding immediate sulfur oxidation.

Dioxygen-Derived Nonheme Mononuclear FeIII(OH) Complex and Its Reactivity with Carbon Radicals.

A new tetradentate, monoanionic, mixed N/O donor ligand (BNPAPh2O-) with second coordination sphere H-bonding groups has been synthesized for stabilization of a terminal FeIII(OH) complex. The complex FeII(BNPAPh2O)(OTf) (1) reacts with O2 to give a mononuclear terminal FeIII(OH) complex, FeIII(OH)(BNPAPh2O)(OTf) (2), both of which were characterized by X-ray diffraction, electrospray ionization mass spectrometry, UV-vis, 1H and 19F nuclear magnetic resonance, 57Fe Mössbauer, and electron paramagnetic resonance spectroscopies. Treatment of 2 with carbon radicals (Ar3C·) gives Ar3COH and the FeII complex 1, in direct analogy with the elusive radical "rebound" process proposed for nonheme iron enzymes.

Microsecond and millisecond dynamics in the photosynthetic protein LHCSR1 observed by single-molecule correlation spectroscopy.

Biological systems are subjected to continuous environmental fluctuations, and therefore, flexibility in the structure and function of their protein building blocks is essential for survival. Protein dynamics are often local conformational changes, which allows multiple dynamical processes to occur simultaneously and rapidly in individual proteins. Experiments often average over these dynamics and their multiplicity, preventing identification of the molecular origin and impact on biological function. Green plants survive under high light by quenching excess energy, and Light-Harvesting Complex Stress Related 1 (LHCSR1) is the protein responsible for quenching in moss. Here, we expand an analysis of the correlation function of the fluorescence lifetime by improving the estimation of the lifetime states and by developing a multicomponent model correlation function, and we apply this analysis at the single-molecule level. Through these advances, we resolve previously hidden rapid dynamics, including multiple parallel processes. By applying this technique to LHCSR1, we identify and quantitate parallel dynamics on hundreds of microseconds and tens of milliseconds timescales, likely at two quenching sites within the protein. These sites are individually controlled in response to fluctuations in sunlight, which provides robust regulation of the light-harvesting machinery. Considering our results in combination with previous structural, spectroscopic, and computational data, we propose specific pigments that serve as the quenching sites. These findings, therefore, provide a mechanistic basis for quenching, illustrating the ability of this method to uncover protein function.

A Nonheme Thiolate-Ligated Cobalt Superoxo Complex: Synthesis and Spectroscopic Characterization, Computational Studies, and Hydrogen Atom Abstraction Reactivity.

The synthesis and characterization of a Co(II) dithiolato complex Co(Me3TACN)(S2SiMe2) (1) are reported. Reaction of 1 with O2 generates a rare thiolate-ligated cobalt-superoxo species Co(O2)(Me3TACN)(S2SiMe2) (2) that was characterized spectroscopically and structurally by resonance Raman, EPR, and X-ray absorption spectroscopies as well as density functional theory. Metal-superoxo species are proposed to S-oxygenate metal-bound thiolate donors in nonheme thiol dioxygenases, but 2 does not lead to S-oxygenation of the intramolecular thiolate donors and does not react with exogenous sulfur donors. However, complex 2 is capable of oxidizing the O-H bonds of 2,2,6,6-tetramethylpiperidin-1-ol derivatives via H atom abstraction. Complementary proton-coupled electron-transfer reactivity is seen for 2 with separated proton/reductant pairs. The reactivity studies indicate that 2 can abstract H atoms from weak X-H bonds with bond dissociation free energy (BDFE) ≤ 70 kcal mol-1. DFT calculations predict that the putative Co(OOH) product has an O-H BDFE = 67 kcal mol-1, which matches the observed pattern of reactivity seen for 2. These data provide new information regarding the selectivity of S-oxygenation versus H atom abstraction in thiolate-ligated nonheme metalloenzymes that react with O2.

Simultaneous Electrochemical and Emission Monitoring of Electrogenerated Chemiluminescence through Instrument Hyphenation.

One of the long-standing challenges to performing electrogenerated chemiluminescence (ECL) research is the need for dedicated instrumentation or highly customized cells to achieve reproducibility. This manuscript describes an approach to designing ECL systems through the hyphenation of existing laboratory instruments, which provide innate time correlation of electrochemical and emission data. This design methodology lowers the entry barrier required to obtaining reproducible ECL measurements and provides flexibility in the scope of applications. Uniquely, the simplicity of this system's experimental interface, a spectrochemical quartz cuvette, readily enables collaboration with finite element modeling that simulates ECL occurring in the cuvette-based cell. This combination of empirical and simulation data allowed for the investigation of the intertwined kinetics behind the coreactant ECL mechanism of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+) and tripropylamine (TPA). The complexity of the system measurable via the hyphenation methodology was further scaled though the addition of tris[2-(4,6-difluorophenyl)pyridinato-C2, N] iridium(III) (Ir(dFppy)3) and the observation of real time multiplexing.

Single-Molecule Fluorescence Detection of the Epidermal Growth Factor Receptor in Membrane Discs.

The epidermal growth factor receptor (EGFR) is critical to normal cellular signaling pathways. Moreover, it has been implicated in a range of pathologies, including cancer. As a result, it is the primary target of many anticancer drugs. One limitation to the design and development of these drugs has been the lack of molecular-level information about the interactions and conformational dynamics of EGFR. To overcome this limitation, this work reports the construction and characterization of functional, fluorescently labeled, and full-length EGFR in model membrane nanolipoprotein particles (NLPs) for in vitro fluorescence studies. To demonstrate the utility of the system, we investigate ATP-EGFR interactions. We observe that ATP binds at the catalytic site providing a means to measure a range of distances between the catalytic site and the C-terminus via Förster resonance energy transfer (FRET). These ATP-based experiments suggest a range of conformations of the C-terminus that may be a function of the phosphorylation state for EGFR. This work is a proof-of-principle demonstration of single-molecule studies as a noncrystallographic assay for EGFR interactions in real-time and under near-physiological conditions. The diverse nature of EGFR interactions means that new tools at the molecular level have the potential to significantly enhance our understanding of receptor pathology and are of utmost importance for cancer-related drug discovery.

Kinetics of Strand Displacement and Hybridization on Wireframe DNA Nanostructures: Dissecting the Roles of Size, Morphology, and Rigidity.

Dynamic wireframe DNA structures have gained significant attention in recent years, with research aimed toward using these architectures for sensing and encapsulation applications. For these assemblies to reach their full potential, however, knowledge of the rates of strand displacement and hybridization on these constructs is required. Herein, we report the use of single-molecule fluorescence methodologies to observe the reversible switching between double- and single-stranded forms of triangular wireframe DNA nanotubes. Specifically, by using fluorescently labeled DNA strands, we were able to monitor changes in intensity over time as we introduced different sequences. This allowed us to extract detailed kinetic information on the strand displacement and hybridization processes. Due to the polymeric nanotube structure, the ability to individually address each of the three sides, and the inherent polydispersity of our samples as a result of the step polymerization by which they are formed, a library of compounds could be studied independently yet simultaneously. Kinetic models relying on mono-exponential decays, multi-exponential decays, or sigmoidal behavior were adjusted to the different constructs to retrieve erasing and refilling kinetics. Correlations were made between the kinetic behavior observed, the site accessibility, the nanotube length, and the structural robustness of wireframe DNA nanostructures, including fully single-stranded analogs. Overall, our results reveal how the length, morphology, and rigidity of the DNA framework modulate the kinetics of strand displacement and hybridization as well as the overall addressability and structural stability of the structures under study.

Structures, Spectroscopic Properties, and Dioxygen Reactivity of 5- and 6-Coordinate Nonheme Iron(II) Complexes: A Combined Enzyme/Model Study of Thiol Dioxygenases.

The synthesis of four new FeII(N4S(thiolate)) complexes as models of the thiol dioxygenases are described. They are composed of derivatives of the neutral, tridentate ligand triazacyclononane (R3TACN; R = Me, iPr) and 2-aminobenzenethiolate (abtx; X = H, CF3), a non-native substrate for thiol dioxygenases. The coordination number of these complexes depends on the identity of the TACN derivative, giving 6-coordinate (6-coord) complexes for FeII(Me3TACN)(abtx)(OTf) (1: X = H; 2: X = CF3) and 5-coordinate (5-coord) complexes for [FeII(iPr3TACN)(abtx)](OTf) (3: X = H; 4: X = CF3). Complexes 1-4 were examined by UV-vis, 1H/19F NMR, and Mössbauer spectroscopies, and density functional theory (DFT) calculations were employed to support the data. Mössbauer spectroscopy reveals that the 6-coord 1-2 and 5-coord 3- 4 exhibit distinct spectra, and these data are compared with that for cysteine-bound CDO, helping to clarify the coordination environment of the cys-bound FeII active site. Reaction of 1 or 2 with O2 at -95 °C leads to S-oxygenation of the abt ligand, and in the case of 2, a rare di(sulfinato)-bridged complex, [Fe2III(µ-O)((2-NH2) p-CF3C6H3SO2)2](OTf)2 ( 5), was obtained. Parallel enzymatic studies on the CDO variant C93G were carried out with the abt substrate and show that reaction with O2 leads to disulfide formation, as opposed to S-oxygenation. The combined model and enzyme studies show that the thiol dioxygenases can operate via a 6-coord FeII center, in contrast to the accepted mechanism for nonheme iron dioxygenases, and that proper substrate chelation to Fe appears to be critical for S-oxygenation.

Observation of Radical Rebound in a Mononuclear Nonheme Iron Model Complex.

A nonheme iron(III) terminal methoxide complex, [FeIII(N3PyO2Ph)(OCH3)]ClO4, was synthesized. Reaction of this complex with the triphenylmethyl radical (Ph3C•) leads to formation of Ph3COCH3 and the one-electron-reduced iron(II) center, as seen by UV-vis, EPR, 1H NMR, and Mössbauer spectroscopy. These results indicate that homolytic Fe-O bond cleavage occurs together with C-O bond formation, providing a direct observation of the "radical rebound" process proposed for both biological and synthetic nonheme iron centers.

Nitride-bridged triiron complex and its relevance to dinitrogen activation.

Using a simple metathesis approach, the triiron(II) tribromide complex Fe3Br3L (1) reacts with tetrabutylammonium azide to afford the monoazide dibromide analogue Fe3(Br)2(N3)L (2) in high yield. The inclusion of azide was confirmed by IR spectroscopy with a ν(N3) = 2082 cm(-1) as well as combustion analysis and X-ray crystallography. Heating 2 in the solid state results in the complete loss of the azide vibration in the IR spectra and the isolation of the olive-green mononitride complex Fe3(Br)2(N)L (3). Solution magnetic susceptibility measurements support that the trimetallic core within 2 is oxidized upon generation of 3 (5.07 vs 3.09 µB). Absorption maxima in the UV-visible-near-IR (NIR) spectra of 2 and 3 support the azide-to-nitride conversion, and a broad NIR absorption centered at 1117 nm is similar to that previously reported for the intervalence charge-transfer band for a mixed-valent nitridodiiron cluster. The cyclic voltammograms recorded for 3 are comparable to those of 1 with no reductive waves observed between ∼0 and -2.5 V (vs Fc/Fc(+)), whereas a reversible one-electron redox process is observed for Fe3(NH2)3L (4). These results suggest that intercluster cooperativity is unlikely to predominate the dinitrogen reduction mechanism when 1 is treated with KC8 under N2.