Kβ X-ray Emission Spectroscopy of Cu(I)-Lytic Polysaccharide Monooxygenase: Direct Observation of the Frontier Molecular Orbital for H2O2 Activation

Lytic polysaccharide monooxygenases (LPMOs) catalyze the degradation of recalcitrant carbohydrate polysaccharide substrates. These enzymes are characterized by a mononuclear Cu(I) active site with a three-coordinate T-shaped "His-brace" configuration including the N-terminal histidine and its amine group as ligands. This study explicitly investigates the electronic structure of the d10 Cu(I) active site in a LPMO using Kβ X-ray emission spectroscopy (XES). The lack of inversion symmetry in the His-brace site enables the 3d/p mixing required for intensity in the Kβ valence-to-core (VtC) XES spectrum of Cu(I)-LPMO. These Kβ XES data are correlated to density functional theory (DFT) calculations to define the bonding, and in particular, the frontier molecular orbital (FMO) of the Cu(I) site. These experimentally validated DFT calculations are used to evaluate the reaction coordinate for homolytic cleavage of the H2O2 O-O bond and understand the contribution of this FMO to the low barrier of this reaction and how the geometric and electronic structure of the Cu(I)-LPMO site is activated for rapid reactivity with H2O2.

Fenton-like Chemistry by a Copper(I) Complex and H2O2 Relevant to Enzyme Peroxygenase C-H Hydroxylation

Lytic polysaccharide monooxygenases have received significant attention as catalytic convertors of biomass to biofuel. Recent studies suggest that its peroxygenase activity (i.e., using H₂O₂ as an oxidant) is more important than its monooxygenase functionality. Here, we describe new insights into peroxygenase activity, with a copper(I) complex reacting with H₂O₂ leading to site-specific ligand-substrate C-H hydroxylation. [Cu^I(TMG₃tren)]⁺ (1) (TMG₃tren = 1,1,1-Tris{2-[N²-(1,1,3,3-tetramethylguanidino)]ethyl}amine) and a dry source of hydrogen peroxide, (o-Tol₃P═O·H₂O₂)₂ react in the stoichiometry, [Cu^I(TMG₃tren)]⁺ + H₂O₂ → [Cu^I(TMG₃tren-OH)]⁺ + H₂O, wherein a ligand N-methyl group undergoes hydroxylation giving TMG₃tren-OH. Furthermore, Fenton-type chemistry (Cu^I + H₂O₂ → Cu^II-OH + ·OH) is displayed, in which (i) a Cu(II)-OH complex could be detected during the reaction and it could be separately isolated and characterized crystallographically and (ii) hydroxyl radical (·OH) scavengers either quenched the ligand hydroxylation reaction and/or (iii) captured the ·OH produced.

Particle Swarm Fitting of Spin Hamiltonians: Magnetic Circular Dichroism of Reduced and NO-Bound Flavodiiron Protein

A particle swarm optimization (PSO) algorithm is described for the fitting of ground-state spin Hamiltonian parameters from variable-temperature/variable-field (VTVH) magnetic circular dichroism (MCD) data. This PSO algorithm is employed to define the ground state of two catalytic intermediates from a flavodiiron protein (FDP), a class of enzymes with nitric oxide reductase activity. The bimetallic iron active site of this enzyme proceeds through a biferrous intermediate and a mixed ferrous-{FeNO}⁷ intermediate during the catalytic cycle, and the MCD spectra of these intermediates are presented and analyzed. The fits of the spin Hamiltonians are shown to provide important geometric and electronic insight into these species that is compared and contrasted with previous reports.

The three-spin intermediate at the O-O cleavage and proton-pumping junction in heme-Cu oxidases

[Figure: see text].

31P NMR Chemical Shift Tensors: Windows into Ruthenium Phosphinidene Complex Electronic Structures

A series of octamethylcalix[4]pyrrole/ruthenium phosphinidene complexes (Na₂[1=PR]) can be accessed by phosphinidene transfer from the corresponding RPA (A = C₁₄H₁₀, anthracene) compounds (R = ^tBu, ⁱPr, OEt, NH₂, NMe₂, NEt₂, NⁱPr₂, NA, dimethylpiperidino). Isolation of the tert-butyl and dimethylamino derivatives allowed comparative studies of their ³¹P nuclear shielding tensors by magic-angle-spinning solid-state nuclear magnetic resonance spectroscopy. Density functional theory and natural chemical shielding analyses reveal the relationship between the ³¹P chemical shift tensor and the local ruthenium/phosphorus electronic structure. The general trend observed in the ³¹P isotropic chemical shifts for the ruthenium phosphinidene complexes was controlled by the degree of deshielding in the δ₁₁ principal tensor component, which can be linked to the σ_RuP/π_RuP* energy gap. A "δ₂₂-δ₃₃ crossover" effect for R = ^tBu was also observed, which was caused by different degrees of deshielding associated with polarizations of the σ_PR and σ_PR* natural bond orbitals.

An Azophosphine Synthetic Equivalent of Mesitylphosphaazide and Its 1,3-Dipolar Cycloaddition Reactions

Dibenzo-7-phosphanorbornadiene-substituted diazene MesN₂PA (1, where Mes = mesityl, A = anthracene, or C₁₄H₁₀), a synthetic equivalent of mesitylphosphaazide (MesN₂P) and anthracene, was synthesized by treatment of [Ph₃BPA][Na(OEt₂)₂] with [MesN₂]OTf (OTf = CF₃SO₃^-) in thawing tetrahydrofuran (14% isolated yield). Treatment of 1 with unsaturated molecules cyclooctyne, [Na(dioxane)_2.5][OCP] (phosphaethynolate), and Ad-C≡P (Ad = adamantyl) results in the corresponding [3 + 2] phosphaazide-(phospha)alkyne cycloadducts, with concomitant loss of anthracene in 65%, 49%, and 38% isolated yield, respectively. Structural data for the phosphaethynolate cycloadduct ([3][Na(12-crown-4)₂]) were obtained in a single-crystal X-ray diffraction study. A diazatriphosphole was generated by combining 1 with P₂A₂, a thermally activated anthracene-based molecular precursor to diphosphorus (P₂). Thermolysis (33-65 °C) of 1 in benzene-d₆ leads to anthracene extrusion. This process has a unimolecular kinetic profile and proceeds with activation parameters of ΔH^⧧ = 21.6 ± 0.3 kcal/mol and ΔS^⧧= -4.9 ± 0.8 cal/(mol K).

A Thioether-Ligated Cupric Superoxide Model with Hydrogen Atom Abstraction Reactivity

The central role of cupric superoxide intermediates proposed in hormone and neurotransmitter biosynthesis by noncoupled binuclear copper monooxygenases like dopamine-β-monooxygenase has drawn significant attention to the unusual methionine ligation of the Cu_M ("Cu_B") active site characteristic of this class of enzymes. The copper-sulfur interaction has proven critical for turnover, raising still-unresolved questions concerning Nature's selection of an oxidizable Met residue to facilitate C-H oxygenation. We describe herein a model for Cu_M, [(^TMGN₃S)Cu^I]⁺ ([1]⁺), and its O₂-bound analog [(^TMGN₃S)Cu^II(O₂^•-)]⁺ ([1·O₂]⁺). The latter is the first reported cupric superoxide with an experimentally proven Cu-S bond which also possesses demonstrated hydrogen atom abstraction (HAA) reactivity. Introduction of O₂ to a precooled solution of the cuprous precursor [1]B(C₆F₅)₄ (-135 °C, 2-methyltetrahydrofuran (2-MeTHF)) reversibly forms [1·O₂]B(C₆F₅)₄ (UV/vis spectroscopy: λ_max 442, 642, 742 nm). Resonance Raman studies (413 nm) using ¹⁶O₂ [¹⁸O₂] corroborated the identity of [1·O₂]⁺ by revealing Cu-O (446 [425] cm^-1) and O-O (1105 [1042] cm^-1) stretches, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed a Cu-S interatomic distance of 2.55 Å. HAA reactivity between [1·O₂]⁺ and TEMPO-H proceeds rapidly (1.28 × 10^-1 M^-1 s^-1, -135 °C, 2-MeTHF) with a primary kinetic isotope effect of k_H/k_D = 5.4. Comparisons of the O₂-binding behavior and redox activity of [1]⁺ vs [2]⁺, the latter a close analog of [1]⁺ but with all N atom ligation (i.e., N₃S vs N₄), are presented.

Isolation of an elusive phosphatetrahedrane

This exploratory synthesis investigation was undertaken to determine the viability of replacing a single carbon vertex with another p-block element in a highly strained tetrahedrane molecule. Phosphorus was selected for this purpose because the stable molecular form of elemental phosphorus is tetrahedral. Our synthetic strategy was to generate an unsaturated phosphorus center bonded to a substituted cyclopropenyl group, a situation that could lead to closure to provide the desired phosphatetrahedrane framework. This was accomplished by dehydrofluorination of the in situ generated fluorophosphine H(F)P(C ^t Bu)₃. Tri-tert-butyl phosphatetrahedrane, P(C ^t Bu)₃, was then isolated in 19% yield as a low-melting, volatile, colorless solid and characterized spectroscopically and by a single-crystal x-ray diffraction study, confirming the tetrahedral nature of the molecule's PC₃ core. The molecule exhibits unexpected thermal stability.

Organoiron- and Fluoride-Catalyzed Phosphinidene Transfer to Styrenic Olefins in a Stereoselective Synthesis of Unprotected Phosphiranes

Catalytic phosphiranation has been achieved, allowing preparation of trans-1-R-2-phenylphosphiranes (R = t-Bu: 1-t-Bu; i-Pr: 1-i-Pr) from the corresponding dibenzo-7-(R)-7-phospha-norbornadiene (RPA, A = C₁₄H₁₀, anthracene) and styrene in 73% and 57% isolated yields, respectively. The cocatalyst system requires tetramethylammonium fluoride (TMAF) and [Fp(THF)][BF₄] (Fp = Fe(η⁵-C₅H₅)(CO)₂). In the case of the t-Bu derivative, the reaction mechanism was probed using stoichiometric reaction studies, a Hammett analysis, and a deuterium labeling experiment. Together, these suggest the intermediacy of iron-phosphido FpP(F)(t-Bu) (2), generated independently from the stoichiometric reaction of [Fp(t-BuPA)][BF₄] with TMAF. Two other plausible reaction intermediates, [Fp(t-BuPA)][BF₄] and [Fp(1-t-Bu)][BF₄], were prepared independently and structurally characterized.

Orthophosphate and Sulfate Utilization for C-E (E = P, S) Bond Formation via Trichlorosilyl Phosphide and Sulfide Anions

Reduction of phosphoric acid (H₃PO₄) or tetra- n-butylammonium bisulfate ([TBA][HSO₄]) with trichlorosilane leads to the formation of the bis(trichlorosilyl)phosphide ([P(SiCl₃)₂]^-, 1) and trichlorosilylsulfide ([Cl₃SiS]^-, 2) anions, respectively. Balanced equations for the formation of the TBA salts of anions 1 and 2 were formulated based on the identification of hexachlorodisiloxane and hydrogen gas as byproducts arising from these reductive processes: i) [H₂PO₄]^- + 10HSiCl₃ → 1 + 4O(SiCl₃)₂ + 6H₂ for P and ii) [HSO₄]^- + 9HSiCl₃ → 2 + 4O(SiCl₃)₂ + 5H₂ for S. Hydrogen gas was identified by its subsequent use to hydrogenate an alkene ((-)-terpinen-4-ol) using Crabtree's catalyst ([(COD)Ir(py)(PCy₃)][PF₆], COD = 1,5-cyclooctadiene, py = pyridine, Cy = cyclohexyl). Phosphide 1 was generated in situ by the reaction of phosphoric acid and trichlorosilane and used to convert an alkyl chloride (1-chlorooctane) to the corresponding primary phosphine, which was isolated in 41% yield. Anion 1 was also prepared from [TBA][H₂PO₄] and isolated in 62% yield on a gram scale. Treatment of [TBA]1 with an excess of benzyl chloride leads to the formation of tetrabenzylphosphonium chloride, which was isolated in 61% yield. Sulfide 2 was used as a thionation reagent, converting benzophenone to thiobenzophenone in 62% yield. It also converted benzyl bromide to benzyl mercaptan in 55% yield. The TBA salt of trimetaphosphate ([TBA]₃[P₃O₉]·2H₂O), also a precursor to anion 1, was found to react with either trichlorosilane or silicon(IV) chloride to provide bis(trimetaphosphate)silicate, [TBA]₂[Si(P₃O₉)₂], characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. Trichlorosilane reduction of [TBA]₂[Si(P₃O₉)₂] also provided anion 1. The electronic structures of 1 and 2 were investigated using a suite of theoretical methods; the computational studies suggest that the trichlorosilyl ligand is a good π-acceptor and forms σ-bonds with a high degree of s character.

Diazomethane umpolung atop anthracene: an electrophilic methylene transfer reagent

Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H₂CN₂A (1, A = C₁₄H₁₀ or anthracene).

Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H₂CN₂A (1, A = C₁₄H₁₀ or anthracene). The synthesis of this hydrazone is reported from Carpino's hydrazine H₂N₂A through treatment with paraformaldehyde. Compound 1 has been found to be an easy-to-handle solid that does not exhibit dangerous heat or shock sensitivity. Effective umpolung of the diazomethane unit imbues 1 with electrophilicity at the methylene carbon center. Its reactivity with nucleophiles such as H₂CPPh₃ and N-heterocyclic carbenes is exploited for C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond formation with elimination of dinitrogen and anthracene. Similarly, 1 is demonstrated to deliver methylene to a nucleophilic singlet d² transition metal center, W(ODipp)₄ (2), to generate the robust methylidene complex [2 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH₂]. This behavior is contrasted with that of the Wittig reagent H₂CPPh₃, a more traditional and Brønsted basic methylene source that upon exposure to 2 contrastingly forms the methylidyne salt [MePPh₃][2 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH].

An exploding N-isocyanide reagent formally composed of anthracene, dinitrogen and a carbon atom

An anthracene-based N-isocyanide was synthesized and its reactivity studied. This sensitive compound was structurally characterized as a free species and as a ligand in a ruthenium complex, and underwent C-atom transfer upon treatment with an O-atom donor to evolve CO.

Terminal tungsten pnictide complex formation through pnictaethynolate decarbonylation

Treatment of a four-coordinate tungsten(iv) complex with pnictaethynolate ions installs terminal tungsten–nitrogen, –phosphorus, and –arsenic triple bondsviadecarbonylation.

Phosphinidene Reactivity of a Transient Vanadium P≡N Complex

Toward the preparation of a coordination complex of the heterodiatomic molecule PN, P≡N-V(N[^tBu]Ar)₃ (1, Ar = 3,5-Me₂C₆H₃), we report the use of ClPA (A = C₁₄H₁₀, anthracene) as a formal source of phosphorus(I) in its reaction with Na[NV(N[^tBu]Ar)₃] (Na[4]) to yield trimeric cyclo-triphosphane [PNV(N[^tBu]Ar)₃]₃ (3) with a core composed exclusively of phosphorus and nitrogen. In the presence of NapS₂ (peri-1,8-naphthalene disulfide), NapS₂P-NV(N[^tBu]Ar)₃ (6) is instead generated in 80% yield, suggesting trapping of transient 1. Upon mild heating, 3 readily fragments into dimeric [PNV(N[^tBu]Ar)₃]₂ (2), while in the presence of bis(trimethylsilyl)acetylene or cis-4-octene, the respective phosphirene (Ar[^tBu]N)₃VN-PC₂(SiMe₃)₂ (7) or phosphirane (Ar[^tBu]N)₃VN-P(C₈H₁₆) (8) compounds are generated. Kinetic data were found to be consistent with unimolecular decay of 3, and [2+1]-cycloaddition with radical clocks ruled out a triplet intermediate, consistent with intermediate 1 reacting as a singlet phosphinidene. In addition, both 7 and 8 were shown to reversibly exchange cis-4-octene and bis(trimethylsilyl)acetylene, serving as formal sources of 1, a reactivity manifold traditionally reserved for transition metals.

A Joint Experimental and Computational Study of the Negative Ion Photoelectron Spectroscopy of the 1-Phospha-2,3,4-triazolate Anion, HCPN3(.)

We report here the results of a combined experimental and computational study of the negative ion photoelectron spectroscopy (NIPES) of the recently synthesized, planar, aromatic, HCPN3(-) ion. The adiabatic electron detachment energy of HCPN3(-) (electron affinity of HCPN3(•)) was measured to be 3.555 ± 0.010 eV, a value that is intermediate between the electron detachment energies of the closely related (CH)2N3(-) and P2N3(-) ions. High level electronic structure calculations and Franck-Condon factor (FCF) simulations reveal that transitions from the ground state of the anion to two nearly degenerate, low-lying, electronic states, of the neutral HCPN3(•) radical are responsible for the congested peaks at low binding energies in the NIPE spectrum. The best fit of the simulated NIPE spectrum to the experimental spectrum indicates that the ground state of HCPN3(•) is a 5π-electron (2)A″ π radical state, with a 6π-electron, (2)A', σ radical state being at most 1.0 kcal/mol higher in energy.

Highly Fluorinated Ir(III)-2,2':6',2″-Terpyridine-Phenylpyridine-X Complexes via Selective C-F Activation: Robust Photocatalysts for Solar Fuel Generation and Photoredox Catalysis

A series of fluorinated Ir(III)-terpyridine-phenylpyridine-X (X = anionic monodentate ligand) complexes were synthesized by selective C-F activation, whereby perfluorinated phenylpyridines were readily complexed. The combination of fluorinated phenylpyridine ligands with an electron-rich tri-tert-butyl terpyridine ligand generates a "push-pull" force on the electrons upon excitation, imparting significant enhancements to the stability, electrochemical, and photophysical properties of the complexes. Application of the complexes as photosensitizers for photocatalytic generation of hydrogen from water and as redox photocatalysts for decarboxylative fluorination of several carboxylic acids showcases the performance of the complexes in highly coordinating solvents, in some cases exceeding that of the leading photosensitizers. Changes in the photophysical properties and the nature of the excited states are observed as the compounds increase in fluorination as well as upon exchange of the ancillary chloride ligand to a cyanide. These changes in the excited states have been corroborated using density functional theory modeling.

Negative ion photoelectron spectroscopy of P2N3-: electron affinity and electronic structures of P2N3˙

We report here a negative ion photoelectron spectroscopy (NIPES) and ab initio study of the recently synthesized planar aromatic inorganic ion P2N3-, to investigate the electronic structures of P2N3- and its neutral P2N3˙ radical. The adiabatic detachment energy of P2N3- (electron affinity of P2N3˙) was determined to be 3.765 ± 0.010 eV, indicating high stability for the P2N3- anion. Ab initio electronic structure calculations reveal the existence of five, low-lying, electronic states in the neutral P2N3˙ radical. Calculation of the Franck-Condon factors (FCFs) for each anion-to-neutral electronic transition and comparison of the resulting simulated NIPE spectrum with the vibrational structure in the observed spectrum allows the first four excited states of P2N3˙ to be determined to lie 6.2, 6.7, 11.5, and 22.8 kcal mol-1 above the ground state of the radical, which is found to be a 6π-electron, 2A1, σ state.

[Ir(N^N^N)(C^N)L]+: a new family of luminophores combining tunability and enhanced photostability

The relatively unexplored luminophore architecture [Ir(N^N^N)(C^N)L](+) (N^N^N = tridentate polypyridyl ligand, C^N = 2-phenylpyridine derivative, and L = monodentate anionic ligand) offers the stability of tridentate polypyridyl coordination along with the tunability of three independently variable ligands. Here, a new family of these luminophores has been prepared based on the previously reported compound [Ir(tpy)(ppy)Cl](+) (tpy = 2,2':6',2″-terpyridine and ppy = 2-phenylpyridine). Complexes are obtained as single stereoisomers, and ligand geometry is unambiguously assigned via X-ray crystallography. Electrochemical analysis of the materials reveals facile HOMO modulation through ppy functionalization and alteration of the monodentate ligand's field strength. Emission reflects similar modulation shifting from orange to greenish-blue upon replacement of chloride with cyanide. Many of the new compounds exhibit impressive room temperature phosphorescence with lifetimes near 3 μs and quantum yields reaching 28.6%. Application of the new luminophores as photosensitizers for photocatalytic hydrogen generation reveals that their photostability in coordinating solvent is enhanced as compared to popular [Ir(ppy)2(bpy)](+) (bpy = 2,2'-bipyridine) photosensitizers. Yet, the binding of their monodentate ligand emerges as a source of instability during the redox processes of cyclic voltammetry and mass spectrometry. DFT modeling of electronic structure is provided for all compounds to elucidate experimental properties.