Enter MnIV-NHC: A Dark Photooxidant with a Long-Lived Charge-Transfer Excited State.

Detailed photophysical investigation of a Mn(IV)-carbene complex has revealed that excitation into its lowest-energy absorption band (∼500 nm) results in the formation of an energetic ligand-to-metal charge-transfer (LMCT) state with a lifetime of 15 ns. To the best of our knowledge, this is the longest lifetime reported for charge-transfer states of first-row-based transition metal complexes in solution, barring those based on Cu, with a d10 configuration. A so-called superoxidant, Mn(IV)-carbene exhibits an excited state potential typically only harnessed via excited states of reactive organic radical species. Furthermore, the long-lived excited state in this case is found to be a dark doublet, with its transition to the quartet ground state being spin-forbidden, a contrast to most first-row literature examples, and a possible cause of the long lifetime. Showcasing excited state properties which in some cases exceed those of complexes based on precious metals, these findings not only advance the library of earth-abundant photosensitizers but also shed general insight into the photophysics of d3 and related Mn complexes.

Reactivity Differences of Trigonal Pyramidal Nonheme Iron(IV)-Oxo and Iron(III)-Oxo Complexes: Experiment and Theory.

High-valent metal-oxo species play critical roles in enzymatic catalysis yet their properties are still poorly understood. In this work we report a combined experimental and computational study into biomimetic iron(IV)-oxo and iron(III)-oxo complexes with tight second-coordination sphere environments that restrict substrate access. The work shows that the second-coordination sphere slows the hydrogen atom abstraction step from toluene dramatically and the kinetics is zeroth order in substrate. However, the iron(II)-hydroxo that is formed has a low reduction potential and hence cannot do OH rebound favorably. The tolyl radical in solution then reacts further with alternative reaction partners. By contrast, the iron(IV)-oxo species reacts predominantly through OH rebound to form alcohol products. Our studies show that the oxidation state of the metal influences reactivities and selectivities with substrate dramatically and that enzymes will likely need an iron(IV) center to catalyze C-H hydroxylation reactions.

A Spectroscopically Observed Iron Nitrosyl Intermediate in the Reduction of Nitrate by a Surface-Conjugated Electrocatalyst.

We report an iron-based graphite-conjugated electrocatalyst (GCC-FeDIM) that combines the well-defined nature of homogeneous molecular electrocatalysts with the robustness of a heterogeneous electrode. A suite of spectroscopic methods, supported by the results of DFT calculations, reveals that the electrode surface is functionalized by high spin (S = 5/2) Fe(III) ions in an FeN4Cl2 coordination environment. The chloride ions are hydrolyzed in aqueous solution, with the resulting cyclic voltammogram revealing a Gaussian-shaped wave assigned to 1H+/1e- reduction of surface Fe(III)-OH surface. A catalytic wave is observed in the presence of NO3-, with an onset potential of -1.1 V vs SCE. At pH 6.0, GCC-FeDIM rapidly reduces NO3- to ammonium and nitrite with 88 and 6% Faradaic efficiency, respectively. Mechanistic studies, including in situ X-ray absorption spectroscopy, suggest that electrocatalytic NO3- reduction involves an iron nitrosyl intermediate. The Fe-N bond length (1.65 Å) is similar to that observed in {Fe(NO)}6 complexes, which is supported by the results of DFT calculations.

Direct Aerobic Generation of a Ferric Hydroperoxo Intermediate Via a Preorganized Secondary Coordination Sphere.

Enzymes exert control over the reactivity of metal centers with precise tuning of the secondary coordination sphere of active sites. One particularly elegant illustration of this principle is in the controlled delivery of proton and electron equivalents in order to activate abundant but kinetically inert oxidants such as O2 for oxidative chemistry. Chemists have drawn inspiration from biology in designing molecular systems where the secondary coordination sphere can shuttle protons or electrons to substrates. However, a biomimetic activation of O2 requires the transfer of both protons and electrons, and molecular systems where ancillary ligands are designed to provide both of these equivalents are comparatively rare. Here, we report the use of a dihydrazonopyrrole (DHP) ligand complexed to Fe to perform exactly such a biomimetic activation of O2. In the presence of O2, this complex directly generates a high spin Fe(III)-hydroperoxo intermediate which features a DHP• ligand radical via ligand-based transfer of an H atom. This system displays oxidative reactivity and ultimately releases hydrogen peroxide, providing insight on how secondary coordination sphere interactions influence the evolution of oxidizing intermediates in Fe-mediated aerobic oxidations.

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover.

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well-characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt2-) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt2-. UV-vis-near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt2- π-manifold from the Fe(II)-centered spin-crossover. The TTFtt2--centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character.

Hydrosilylation of an Iron(IV) Nitride Complex.

The nitride ligand in iron(IV) complex PhB(MesIm)3Fe≡N reacts with excess H3SiPh to afford PhB(MesIm)3Fe(µ-H)3(SiHPh) as the major product, which has been structurally and spectroscopically characterized. Bulkier silane HaSiPh2 provides iron(II) amido complex PhB(MesIm)3FeN(H)(SiHPh2) as the initial product of the reaction, with excess H2SiPh2 affording diamagnetic PhB(MesIm)3Fe(µ-H)3(SiPh2) as the major product. Unobserved iron(II) hydride PhB(MesIm)3Fe-H is implicated as an intermediate in this reaction, as suggested by the results of the reaction between iron(II) amido PhB(MesIm)3FeN(H)tBu and H3SiPh, which provides PhB(MesIm)3Fe(H)(µ-H)2(Si(NHtBu)Ph) as the sole product.

Strong π-Backbonding Enables Record Magnetic Exchange Coupling Through Cyanide.

The paramagnetic cyano-bridged complex PhB(tBuIm)3Fe-NC-Mo(NtBuAr)3 (Ar = 3,5-Me2C6H3) is readily assembled from a new four-coordinate, high-spin (S = 2) iron(II) monocyanide complex and the three-coordinate molybdenum(III) complex Mo(NtBuAr)3. X-ray diffraction and IR spectroscopy reveal that delocalization of unpaired electron density into the cyanide π* orbitals leads to a reduction of the C-N bond order. Direct current (dc) magnetic susceptibility measurements, supported by electronic structure calculations, demonstrate the presence of strong antiferromagnetic exchange between spin centers, with a coupling constant of J = -122(2) cm-1. To our knowledge, this value represents the strongest magnetic exchange coupling ever to be observed through cyanide. These results demonstrate the ability of low-coordinate metal fragments to engender extremely strong magnetic exchange coupling through cyanide by virtue of significant π-backbonding into the cyanide ligand.

A Dimeric Hydride-Bridged Complex with Geometrically Distinct Iron Centers Giving Rise to an S = 3 Ground State.

Structural and spectroscopic characterization of the dimeric iron hydride complex [Ph2B(tBuIm)2FeH]2 reveals an unusual structure in which a tetrahedral iron(II) site (S = 2) is connected to a square planar iron(II) site (S = 1) by two bridging hydride ligands. Magnetic susceptibility reveals strong ferromagnetic coupling between iron centers, with a coupling constant of J = +110(12) cm-1, to give an S = 3 ground state. High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy confirms this model. A qualitative molecular orbital analysis of the electronic structure, as supported by electronic structure calculations, reveals that the observed spin configuration results from the orthogonal alignment of two geometrically distinct four-coordinate iron fragments held together by highly covalent hydride ligands.

Hydrogen atom abstraction by a high spin [FeIII=S] complex.

Iron sulfur clusters are critical to a plethora of biological processes; however, little is known about the elementary unit of these clusters, namely the [Fe=S]n+ fragment. Here, we report the synthesis and characterization of a terminal iron sulfido complex. Despite its high spin (S = 5/2) ground state, structural, spectroscopic, and computational characterization provide evidence for iron sulfur multiple bond character. Intriguingly, the complex reacts with additional sulfur to afford an S = 3/2 iron(III) disulfido (S2 2-) complex. Preliminary studies reveal that the sulfido complex reacts with dihydroanthracene to afford an iron(II) hydrosulfido complex, akin to the reactivity of iron oxo complexes. By contrast, there is no reaction with the disulfido complex. These results provide important insight into the nature of the iron sulfide unit.

Hydrocarbon Oxidation by an Exposed, Multiply Bonded Iron(III) Oxo Complex.

The iron oxo unit, [Fe=O] n+ is a critical intermediate in biological oxidation reactions. While its higher oxidation states are well studied, relatively little is known about the least-oxidized form [FeIII=O]+. Here, the thermally stable complex PhB(AdIm)3Fe=O has been structurally, spectroscopically, and computationally characterized as a bona fide iron(III) oxo. An unusually short Fe-O bond length is consistent with iron-oxygen multiple bond character and is supported by electronic structure calculations. The complex is thermally stable yet is able to perform hydrocarbon oxidations, facilitating both C-O bond formation and dehydrogenation reactions.