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In enzymatic C-H activation by hydrogen tunneling, reduced barrier width is important for efficient hydrogen wave function overlap during catalysis. For native enzymes displaying nonadiabatic tunneling, the dominant reactive hydrogen donor-acceptor distance (DAD) is typically ca. 2.7 Å, considerably shorter than normal van der Waals distances. Without a ground state substrate-bound structure for the prototypical nonadiabatic tunneling system, soybean lipoxygenase (SLO), it has remained unclear whether the requisite close tunneling distance occurs through an unusual ground state active site arrangement or by thermally sampling conformational substates. Herein, we introduce Mn2+ as a spin-probe surrogate for the SLO Fe ion; X-ray diffraction shows Mn-SLO is structurally faithful to the native enzyme. 13C ENDOR then reveals the locations of 13C10 and reactive 13C11 of linoleic acid relative to the metal; 1H ENDOR and molecular dynamics simulations of the fully solvated SLO model using ENDOR-derived restraints give additional metrical information. The resulting three-dimensional representation of the SLO active site ground state contains a reactive (a) conformer with hydrogen DAD of ∼3.1 Å, approximately van der Waals contact, plus an inactive (b) conformer with even longer DAD, establishing that stochastic conformational sampling is required to achieve reactive tunneling geometries. Tunneling-impaired SLO variants show increased DADs and variations in substrate positioning and rigidity, confirming previous kinetic and theoretical predictions of such behavior. Overall, this investigation highlights the (i) predictive power of nonadiabatic quantum treatments of proton-coupled electron transfer in SLO and (ii) sensitivity of ENDOR probes to test, detect, and corroborate kinetically predicted trends in active site reactivity and to reveal unexpected features of active site architecture.

The ability of certain transition metals to mediate the reduction of N2 to NH3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N2 and mediate its protonation at one or more N atoms to furnish M(N(x)H(y)) species that can be characterized and, in turn, extrude NH3. By contrast, the direct protonation of Fe-N2 species to Fe(N(x)H(y)) products that can be characterized has been elusive. Herein, we show that addition of acid at low temperature to [(TPB)Fe(N2)][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-NH2](+), 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(N2)][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.

We report here an ENDOR study of an S = 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 (63,65)Cu nuclei exhibits strong and almost entirely isotropic coupling to the unpaired electron, show that this coupling atypically has a positive sign, aiso = +148 MHz, and indicate an apparently undetectably small quadrupolar coupling. When the intermediate is generated using (13)CO, coupling to the (13)C is observed, with aiso = +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.

This study investigates the influence of ligand charge on transition energies in a series of CuN2S2 complexes based on dithiocarbazate Schiff base ligands using Cu K-edge X-ray absorption spectroscopy (XAS) and Kβ valence-to-core (VtC) X-ray emission spectroscopy (XES). By comparing the formally Cu(II) complexes [CuII(HL1)] (HL12− = dimethyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and [CuII(HL2)] (HL22− = dibenzyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and the formally Cu(III) complex [CuIII(L2)], distinct changes in transition energies are observed, primarily attributed to the metal oxidation state. Density functional theory (DFT) calculations demonstrate how an increased negative charge on the deprotonated L23− ligand stabilizes the Cu(III) center through enhanced charge donation, modulating the core transition energies. Overall, significant shifts to higher energies are noted upon metal oxidation, emphasizing the importance of scrutinizing ligand structure in XAS/VtC XES analysis. The data further support the redox-innocent role of the Schiff base ligands and underscore the criticality of ligand protonation levels in future spectroscopic studies, particularly for catalytic intermediates. The combined XAS-VtC XES methodology validates the Cu(III) oxidation state assignment while offering insights into ligand protonation effects on core-level spectroscopic transitions. [ABSTRACT FROM AUTHOR]

Carbenes are known to stabilize main group element compounds with unusual electronic properties. Herein, we report the synthesis of carbene-stabilized group 13 metal radicals (cAAC)MX2(IPr) (M = Al, X = Br 3; M = Ga, X = Cl 4) and the corresponding cations [(cAAC)MX2(IPr)][B(C6F5)4] (M = Al, X = Br 5; M = Ga, X = Cl 6), which were characterized spectroscopically and by sc-XRD. Quantum chemical calculation gave insights into their electronic structures. [ABSTRACT FROM AUTHOR]

In this study, we combine in situ spectroelectrochemistry coupled with electron paramagnetic resonance (EPR) and X‐ray absorption spectroscopies (XAS) to investigate a molecular Ru‐based water oxidation catalyst bearing a polypyridinic backbone [RuII(OH2)(Py2Metacn)]2+. Although high valent key intermediate species arising in catalytic cycles of this family of compounds have remain elusive due to the lack of additional anionic ligands that could potentially stabilize them, mechanistic studies performed on this system proposed a water nucleophilic attack (WNA) mechanism for the O−O bond formation. Employing in situ experimental conditions and complementary spectroscopic techniques allowed to observe intermediates that provide support for a WNA mechanism, including for the first time a Ru(V) oxo intermediate based on the Py2Metacn ligand, in agreement with the previously proposed mechanism. [ABSTRACT FROM AUTHOR]

The dinickel(II) dihydride complex (1K) of a pyrazolate‐based compartmental ligand with β‐diketiminato (nacnac) chelate arms (L−), providing two pincer‐type {N3} binding pockets, has been reported to readily eliminate H2 and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2Na, 2K) of L− are now synthesized via a direct reduction route. They feature two adjacent T‐shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local dx2-y2 type magnetic orbitals are oriented into the bimetallic cleft, enabling metal–metal cooperative 2 e− substrate reductions as shown by the rapid reaction with H2 or O2. X‐ray crystallography reveals distinctly different positions of the K+ in 1K and 2K, suggesting a stabilizing interaction of K+ with the dihydride unit in 1K. H2 release from 1K is triggered by peripheral γ‐C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H2 elimination from the bimetallic cleft. [ABSTRACT FROM AUTHOR]

Oxidative addition of Cp*SbX2 (X=Cl, Br, I; Cp*=C5Me5) to group 13 diyls LM (M=Al, Ga, In; L=HC[C(Me)N (Dip)]2, Dip=2,6‐iPr2C6H3) yields elemental antimony (M=Al) or the corresponding stibanylgallanes [L(X)Ga]Sb(X)Cp* (X=Br 1, I 2) and ‐indanes [L(X)In]Sb(X)Cp* (X=Cl 5, Br 6, I 7). 1 and 2 react with a second equivalent of LGa to eliminate decamethyl‐1,1'‐dihydrofulvalene (Cp*2) and form stibanyl radicals [L(X)Ga]2Sb. (X=Br 3, I 4), whereas analogous reactions of 5 and 6 with LIn selectively yield stibanes [L(X)In]2SbH (X=Cl 8, Br 9) by elimination of 1,2,3,4‐tetramethylfulvene. The reactions are proposed to proceed via formation of [L(X)M]2SbCp* as reaction intermediate, which is supported by the isolation of [L(Cl)Ga]2SbCp (11, Cp=C5H5). The reaction mechanism was further studied by computational calculations using two different models. The energy values for the Ga‐ and the In‐substituted model systems showing methyl groups instead of the very bulky Dip units are very similar, and in both cases the same products are expected. Homolytic Sb−C bond cleavage yields van der Waals complexes from the as‐formed radicals ([L(Cl)M]2Sb. and Cp*.), which can be stabilized by hydrogen atom abstraction to give the corresponding hydrides, whereas the direct formation of Sb hydrides starting from [L(Cl)M]2SbCp* via concerted β‐H elimination is unlikely. The consideration of the bulky Dip units reveals that the amount of the steric overload in the intermediate I determines the product formation (radical vs. hydride). [ABSTRACT FROM AUTHOR]

Valence‐to‐Core (VtC) X‐ray emission spectroscopy (XES) was used to directly detect the presence of an O−O bond in a complex comprising the [CuII2(μ‐η2:η2‐O2)]2+ core relative to its isomer with a cleaved O−O bond having a [CuIII2(μ‐O)2]2+ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [CuII2(μ‐η2:η2‐O2)]2+ core corresponds to the copper stabilized in‐plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O−O bond activation in μ‐η2:η2‐O2 species and highlight the potential of this method for time‐resolved studies of reaction mechanisms. [ABSTRACT FROM AUTHOR]

Keywords: spectroelectrochemistry; water oxidation; ruthenium; catalyst; in situ electrochemistry EN spectroelectrochemistry water oxidation ruthenium catalyst in situ electrochemistry 1 1 1 02/21/22 20220211 NES 220211 B The Front Cover b shows a periodic table focused on the Ru element and the complementary X-ray absorption spectroscopies and electron paramagnetic resonance obtained by in-situ spectroelectrochemistry to confirm the participation of the Ru(V) oxo in the catalytic water oxidation by this Ru molecular catalyst. Spectroelectrochemistry, water oxidation, ruthenium, catalyst, in situ electrochemistry. [Extracted from the article]

Aquo and hydroxo ligands play an essential role in the chemistry of many copper enzymes and small molecule catalysts. The formation of a series of copper complexes with H 2 O and OH - ligands in various positions, including [Cu(bpy)(OAc)(H 2 O) 2,ax ] + (Cu-I), [Cu(bpy)(OH) 2,eq (H x O) 2,ax ] (Cu-III), [Cu(OH) 4,eq (H x O) 2,ax ] 2- (Cu-IV), [Cu(bpy)(H 2 O) 2,eq (H 2 O) 2,ax ] 2+ (Cu-V) and [Cu(bpy) 2 (H 2 O) ax ] 2+ (Cu-VI), were investigated through Electron Paramagnetic Resonance (EPR) and UV-Vis spectroscopy in aqueous copper bipyridine solutions in the dependence of the pH and the copper-to-bipyridine ratio (bpy = 2,2'-bipyridine). 2 H- and 17 O-enrichment of the copper complexes allowed us to determine the 1 H and 17 O nuclear hyperfine interactions of their H x O ligands via Q-band Electron Nuclear Double Resonance (ENDOR) spectroscopy. These techniques gave direct insight into the metal-ligand covalencies and geometries and were further supported by Density Functional Theory (DFT) calculations. It is shown that 1 H and 17 O ENDOR spectroscopy can aid in (1) determining the coordination position, thereby differentiating between equatorial and axial H x O ligands and (2) distinguishing equatorial aqua and hydroxo ligands, particularly through their anisotropic dipolar components. We further studied the influence of trans coordinating ligands on the hyperfine parameters of aquo and hydroxo ligands, enabled through contrasting the coordination environments in the examined complexes, supported by quantum chemical computations.