Characterization of an Fe≡N−NH_2 Intermediate Relevant to Catalytic N_2 Reduction to NH_3

The ability of certain transition metals to mediate the reduction of N_2 to NH_3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N_2 and mediate its protonation at one or more N atoms to furnish M(N_xH_y) species that can be characterized and, in turn, extrude NH_3. By contrast, the direct protonation of Fe–N_2 species to Fe(N_xH_y) products that can be characterized has been elusive. Herein, we show that addition of acid at low temperature to [(TPB)Fe(N_2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N–NH_2]^+, a doubly protonated hydrazido(2−) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N_2)][Na(12-crown-4)] can proceed along a distal or “Chatt-type” pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N–N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided

Free H_2 Rotation vs Jahn−Teller Constraints in the Nonclassical Trigonal (TPB)Co−H_2 Complex

Proton exchange within the M–H_2 moiety of (TPB)Co(H_2) (Co–H_2; TPB = B(o-C_6H_4PiPr_2)_3) by 2-fold rotation about the M–H_2 axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP^(iPr)_3)Fe(H_2) (Fe–H_2) (SiP^(iPr)_3 = [Si(o-C_6H_4PiPr_2)_3]). The g-values for Co–H_2 and Fe–H_2 show that both have the Jahn–Teller (JT)-active ^2E ground state (idealized C_3 symmetry) with doubly degenerate frontier orbitals, (e)^3 = [|m_L ± 2>]^3 = [x^2 – y^2, xy]^3, but with stronger linear vibronic coupling for Co–H_2. The observation of ^1H ENDOR signals from the Co–HD complex, ^2H signals from the Co–D_2/HD complexes, but no ^1H signals from the Co–H_2 complex establishes that H_2 undergoes proton exchange at 2 K through rotation around the Co–H_2 axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (I = 1/2; Fermions), symmetric for identical deuterons (I = 1; Bosons). Analysis of the 1-D rotor problem indicates that Co–H_2 exhibits rotor-like behavior in solution because the underlying C_3 molecular symmetry combined with H_2 exchange creates a dominant 6-fold barrier to H_2 rotation. Fe–H_2 instead shows H_2 localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)→ H_2(σ^*) π-backbonding that becomes dependent on the H_2 orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L_2–M–E axis (E = Si for Fe–H_2; E = B for Co–H_2). Notably, the isotropic ^1H/^2H hyperfine coupling to the diatomic of Co–H_2 is nearly 4-fold smaller than for Fe–H_2

Spectroscopic characterization of the Co-substituted C-terminal domain of rubredoxin-2

Pseudomonas putida rubredoxin-2 (Rxn2) is an essential member of the alkane hydroxylation pathway and transfers electrons from a reductase to the membrane-bound hydroxylase. The regioselective hydroxylation of linear alkanes is a challenging chemical transformation of great interest for the chemical industry. Herein, we report the preparation and spectroscopic characterization of cobalt-substituted P. putida Rxn2 and a truncated version of the protein consisting of the C-terminal domain of the protein. Our spectroscopic data on the Co-substituted C-terminal domain supports a high-spin Co(II) with a distorted tetrahedral coordination environment. Investigation of the two-domain protein Rxn2 provides insights into the metal-binding properties of the N-terminal domain, the role of which is not well understood so far. Circular dichroism, electron paramagnetic resonance and X-ray absorption spectroscopies support an alternative Co-binding site within the N-terminal domain, which appears to not be relevant in nature. We have shown that chemical reconstitution in the presence of Co leads to incorporation of Co(II) into the active site of the C-terminal domain, but not the N-terminal domain of Rxn2 indicating distinct roles for the two rubredoxin domain

EPR, ENDOR, and Electronic Structure Studies of the Jahn–Teller Distortion in an Fe^V Nitride

The recently synthesized and isolated low-coordinate Fe^V nitride complex has numerous implications as a model for high-oxidation states in biological and industrial systems. The trigonal [PhB­(^<i>t</i>BuIm)₃Fe^VN]⁺ (where (PhB­(^<i>t</i>BuIm)₃^– = phenyltris­(3-<i>tert</i>-butylimidazol-2-ylidene)), (<b>1</b>) low-spin <i>d</i>³ (<i>S</i> = 1/2) coordination compound is subject to a Jahn–Teller (JT) distortion of its doubly degenerate ²E ground state. The electronic structure of this complex is analyzed by a combination of extended versions of the formal two-orbital pseudo Jahn–Teller (PJT) treatment and of quantum chemical computations of the PJT effect. The formal treatment is extended to incorporate mixing of the two <i>e</i> orbital doublets (30%) that results from a lowering of the idealized molecular symmetry from <i>D</i>_3<i>h</i> to <i>C</i>_3<i>v</i> through strong “doming” of the Fe–C₃ core. Correspondingly we introduce novel DFT/CASSCF computational methods in the computation of electronic structure, which reveal a quadratic JT distortion and significant <i>e</i>–<i>e</i> mixing, thus reaching a new level of synergism between computational and formal treatments. Hyperfine and quadrupole tensors are obtained by pulsed 35 GHz ENDOR measurements for the ^14/15N-nitride and the ¹¹B axial ligands, and spectra are obtained from the imidazole-2-ylidene ¹³C atoms that are not bound to Fe. Analysis of the nitride ENDOR tensors surprisingly reveals an essentially spherical nitride trianion bound to Fe, with negative spin density and minimal charge density anisotropy. The four-coordinate ¹¹B, as expected, exhibits negligible bonding to Fe. A detailed analysis of the frontier orbitals provided by the electronic structure calculations provides insight into the reactivity of <b>1</b>: JT-induced symmetry lowering provides an orbital selection mechanism for proton or H atom transfer reactivity

¹³C and ^63,65Cu ENDOR studies of CO Dehydrogenase from <i>Oligotropha carboxidovorans</i>. Experimental Evidence in Support of a Copper–Carbonyl Intermediate

We report here an ENDOR study of an <i>S</i> = 1/2 intermediate state trapped during reduction of the binuclear Mo/Cu enzyme CO dehydrogenase by CO. ENDOR spectra of this state confirm that the ^63,65Cu nuclei exhibits strong and almost entirely isotropic coupling to the unpaired electron, show that this coupling atypically has a positive sign, <i>a</i>_iso = +148 MHz, and indicate an apparently undetectably small quadrupolar coupling. When the intermediate is generated using ¹³CO, coupling to the ¹³C is observed, with <i>a</i>_iso = +17.3 MHz. A comparison with the couplings seen in related, structurally assigned Mo­(V) species from xanthine oxidase, in conjunction with complementary computational studies, leads us to conclude that the intermediate contains a partially reduced Mo­(V)/Cu­(I) center with CO bound at the copper. Our results provide strong experimental support for a reaction mechanism that proceeds from a comparable complex of CO with fully oxidized Mo­(VI)/Cu­(I) enzyme

One Electron Changes Everything. A Multispecies Copper Redox Shuttle for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are an established alternative photovoltaic technology that offers numerous potential advantages in solar energy applications. However, this technology has been limited by the availability of molecular redox couples that are both noncorrosive/nontoxic and do not diminish the performance of the device. In an effort to overcome these shortcomings, a copper-containing redox shuttle derived from 1,8-bis­(2′-pyridyl)-3,6-dithiaoctane (PDTO) ligand and the common DSC additive 4-<i>tert</i>-butylpyridine (TBP) was investigated. Electrochemical measurements, single-crystal X-ray diffraction, and absorption and electron paramagnetic resonance spectroscopies reveal that, upon removal of one metal-centered electron, PDTO-enshrouded copper ions completely shed the tetradentate PDTO ligand and replace it with four or more TBP ligands. Thus, the Cu­(I) and Cu­(II) forms of the electron shuttle have completely different coordination spheres and are characterized by widely differing Cu­(II/I) formal potentials and reactivities for forward versus reverse electron transfer. Notably, the coordination-sphere replacement process is fully reversed upon converting Cu­(II) back to Cu­(I). In cells featuring an adsorbed organic dye and a nano- and mesoparticulate, TiO₂-based, photoelectrode, the dual species redox shuttle system engenders performance superior to that obtained with shuttles based on the (II/I) forms of either of the coordination complexes in isolation

Conformational coupling of redox-driven Na+ -translocation in Vibrio cholerae NADH:quinone oxidoreductase

In the respiratory chain, NADH oxidation is coupled to ion translocation across the membrane to build up an electrochemical gradient. In the human pathogen Vibrio cholerae, the sodium-pumping NADH:quinone oxidoreductase (Na+-NQR) generates a sodium gradient by a so far unknown mechanism. Here we show that ion pumping in Na+-NQR is driven by large conformational changes coupling electron transfer to ion translocation. We have determined a series of cryo-EM and X-ray structures of the Na+-NQR that represent snapshots of the catalytic cycle. The six subunits NqrA, B, C, D, E, and F of Na+-NQR harbor a unique set of cofactors that shuttle the electrons from NADH twice across the membrane to quinone. The redox state of a unique intramembranous [2Fe-2S] cluster orchestrates the movements of subunit NqrC, which acts as an electron transfer switch. We propose that this switching movement controls the release of Na+ from a binding site localized in subunit NqrB.ISSN:1545-9993ISSN:1545-998

Free H₂ Rotation vs Jahn–Teller Constraints in the Nonclassical Trigonal (TPB)Co–H₂ Complex

Proton exchange within the M–H₂ moiety of (TPB)­Co­(H₂) (Co–H₂; TPB = B­(<i>o</i>-C₆H₄P^<i>i</i>Pr₂)₃) by 2-fold rotation about the M–H₂ axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP^<i>i</i>Pr₃)­Fe­(H₂) (Fe–H₂) (SiP^<i>i</i>Pr₃ = [Si­(<i>o</i>-C₆H₄P^<i>i</i>Pr₂)₃]). The <i>g</i>-values for Co–H₂ and Fe–H₂ show that both have the Jahn–Teller (JT)-active ²<i>E</i> ground state (idealized <i>C</i>₃ symmetry) with doubly degenerate frontier orbitals, (e)³ = [|<i>m</i>_<i>L</i> ± 2<i>></i>]³ = [<i>x</i>² – <i>y</i>², <i>xy</i>]³, but with stronger linear vibronic coupling for Co–H₂. The observation of ¹H ENDOR signals from the Co–HD complex, ²H signals from the Co–D₂/HD complexes, but <i>no</i> ¹H signals from the Co–H₂ complex establishes that H₂ undergoes proton exchange at 2 K through rotation around the Co–H₂ axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (<i>I</i> = 1/2; Fermions), symmetric for identical deuterons (<i>I</i> = 1; Bosons). Analysis of the 1-D rotor problem indicates that Co–H₂ exhibits rotor-like behavior in solution because the underlying <i>C</i>₃ molecular symmetry combined with H₂ exchange creates a dominant 6-fold barrier to H₂ rotation. Fe–H₂ instead shows H₂ localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe­(3d)→ H₂(σ*) π-backbonding that becomes dependent on the H₂ orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L₂–M–E axis (E = Si for Fe–H₂; E = B for Co–H₂). Notably, the isotropic ¹H/²H hyperfine coupling to the diatomic of Co–H₂ is nearly 4-fold smaller than for Fe–H₂

Free H₂ Rotation vs Jahn–Teller Constraints in the Nonclassical Trigonal (TPB)Co–H₂ Complex

Proton exchange within the M–H₂ moiety of (TPB)­Co­(H₂) (Co–H₂; TPB = B­(<i>o</i>-C₆H₄P^<i>i</i>Pr₂)₃) by 2-fold rotation about the M–H₂ axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP^<i>i</i>Pr₃)­Fe­(H₂) (Fe–H₂) (SiP^<i>i</i>Pr₃ = [Si­(<i>o</i>-C₆H₄P^<i>i</i>Pr₂)₃]). The <i>g</i>-values for Co–H₂ and Fe–H₂ show that both have the Jahn–Teller (JT)-active ²<i>E</i> ground state (idealized <i>C</i>₃ symmetry) with doubly degenerate frontier orbitals, (e)³ = [|<i>m</i>_<i>L</i> ± 2<i>></i>]³ = [<i>x</i>² – <i>y</i>², <i>xy</i>]³, but with stronger linear vibronic coupling for Co–H₂. The observation of ¹H ENDOR signals from the Co–HD complex, ²H signals from the Co–D₂/HD complexes, but <i>no</i> ¹H signals from the Co–H₂ complex establishes that H₂ undergoes proton exchange at 2 K through rotation around the Co–H₂ axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (<i>I</i> = 1/2; Fermions), symmetric for identical deuterons (<i>I</i> = 1; Bosons). Analysis of the 1-D rotor problem indicates that Co–H₂ exhibits rotor-like behavior in solution because the underlying <i>C</i>₃ molecular symmetry combined with H₂ exchange creates a dominant 6-fold barrier to H₂ rotation. Fe–H₂ instead shows H₂ localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe­(3d)→ H₂(σ*) π-backbonding that becomes dependent on the H₂ orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L₂–M–E axis (E = Si for Fe–H₂; E = B for Co–H₂). Notably, the isotropic ¹H/²H hyperfine coupling to the diatomic of Co–H₂ is nearly 4-fold smaller than for Fe–H₂