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. [CuI(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, [CuI(TMG3tren)]+ + H2O2 → [CuI(TMG3tren-OH)]+ + H2O, wherein a ligand N-methyl group undergoes hydroxylation giving TMG3tren-OH. Furthermore, Fenton-type chemistry (CuI + H2O2 → CuII-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.

Kß 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 Kß 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 Kß valence-to-core (VtC) XES spectrum of Cu(I)-LPMO. These Kß 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.

Kinetic analysis of amino acid radicals formed in H2O2-driven CuI LPMO reoxidation implicates dominant homolytic reactivity.

Lytic polysaccharide monooxygenases (LPMOs) have been proposed to react with both [Formula: see text] and [Formula: see text] as cosubstrates. In this study, the [Formula: see text] reaction with reduced Hypocrea jecorina LPMO9A (CuI-HjLPMO9A) is demonstrated to be 1,000-fold faster than the [Formula: see text] reaction while producing the same oxidized oligosaccharide products. Analysis of the reactivity in the absence of polysaccharide substrate by stopped-flow absorption and rapid freeze-quench (RFQ) electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) yields two intermediates corresponding to neutral tyrosyl and tryptophanyl radicals that are formed along minor reaction pathways. The dominant reaction pathway is characterized by RFQ EPR and kinetic modeling to directly produce CuII-HjLPMO9A and indicates homolytic O-O cleavage. Both optical intermediates exhibit magnetic exchange coupling with the CuII sites reflecting facile electron transfer (ET) pathways, which may be protective against uncoupled turnover or provide an ET pathway to the active site with substrate bound. The reactivities of nonnative organic peroxide cosubstrates effectively exclude the possibility of a ping-pong mechanism.

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.

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-ß-monooxygenase has drawn significant attention to the unusual methionine ligation of the CuM ("CuB") 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 CuM, [(TMGN3S)CuI]+ ([1]+), and its O2-bound analog [(TMGN3S)CuII(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: λmax 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 kH/kD = 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.

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 σRuP/πRuP* 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 σPR and σPR* natural bond orbitals.

Sulfur monoxide thermal release from an anthracene-based precursor, spectroscopic identification, and transfer reactivity.

Sulfur monoxide (SO) is a highly reactive molecule and thus, eludes bulk isolation. We report here on synthesis and reactivity of a molecular precursor for SO generation, namely 7-sulfinylamino-7-azadibenzonorbornadiene (1). This compound has been shown to fragment readily driven by dinitrogen expulsion and anthracene formation on heating in the solid state and in solution, releasing SO at mild temperatures (<100 °C). The generated SO was detected in the gas phase by MS and rotational spectroscopy. In solution, 1 allows for SO transfer to organic molecules as well as transition metal complexes.

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.

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.

Anthracene as a Launchpad for a Phosphinidene Sulfide and for Generation of a Phosphorus-Sulfur Material Having the Composition P2S, a Vulcanized Red Phosphorus That Is Yellow.

Thermolysis of a pair of dibenzo-7-phosphanorbornadiene compounds is shown to lead to differing behaviors: phosphinidene sulfide release and formation of amorphous P2S. These compounds, tBuP(S)A (1, A = C14H10 or anthracene; 59% isol. yield) and HP(S)A (2; 63%), are available through thionation of tBuPA and the new secondary phosphine HPA (5), prepared from Me2NPA and DIBAL-H in 50% yield. Phosphinidene sulfide [ tBuP═S] transfer is shown to proceed efficiently from 1 to 2,3-dimethyl-1,3-butadiene to form Diels-Alder product 3 with a zero-order dependence on diene. Platinum complex (Ph3P)2Pt(η2- tBuPS) (4, 47%) is also accessed from 1 and structurally characterized. In contrast, heating parent species 2 (3 h, 135 °C) under vacuum instead produces an insoluble, nonvolatile yellow residual material 6 of composition P2S that displays semiconductor properties with an optical band gap of 2.4 eV. Material 6 obtained in this manner from molecular precursor 2 is in a poorly characterized portion of the phosphorus-sulfur phase diagram and has therefore been subjected to a range of spectroscopic techniques to gain structural insight. X-ray spectroscopic and diffraction techniques, including Raman, XANES, EXAFS, and PDF, reveal 6 to have similarities with related compounds including P4S3, Hittorf's violet phosphorus. Various possible structures have been explored as well using quantum chemical calculations under the constraint that each phosphorus atom is trivalent with no terminal sulfide groups, and each sulfur atom is divalent. The structural conclusions are supported by data from phosphorus-31 magic angle spinning (MAS) solid state NMR spectroscopy, bolstering the structural comparisons to other phosphorus-sulfur systems while excluding the formulation of P2S as a simple mixture of P4S3 and phosphorus.

Synthetic and Spectroscopic Investigations Enabled by Modular Synthesis of Molecular Phosphaalkyne Precursors.

A series of dibenzo-7-phosphanorbornadiene compounds, Ph3PC(R)PA (1-R; A = C14H10, anthracene; R = Me, Et, iPr, sBu), are reported to be capable of thermal fragmentation to generate alkyl-substituted phosphaalkynes (RC≡P) concomitant with triphenylphosphine and anthracene. Facile preparation of these molecular precursors proceeds by treatment of ClPA with the appropriate ylide Ph3P═CHR (2 equiv). For methyl, ethyl, and isopropyl substituents, the phosphaalkyne conversions are measured to be 56-73% in solution by quantitative 31P NMR spectroscopy. In the case of compound 1-Me, the kinetic profile of its spontaneous unimolecular fragmentation is investigated by an Eyring analysis. The resulting 1-phosphapropyne is directly detected by solution NMR spectroscopy and gas phase rotational microwave spectroscopy. The latter technique allows for the first time measurement of the phosphorus-31 nuclear spin-rotation coupling tensor. The nuclear spin-rotation coupling provides a link between rotational and NMR spectroscopies, and is contextualized in relation to the chemical shift anisotropy.

Mechanism and Scope of Phosphinidene Transfer from Dibenzo-7-phosphanorbornadiene Compounds.

Dibenzo-7-phosphanorbornadiene compounds, RPA (A = C14H10 or anthracene), are investigated as phosphinidene sources upon thermally induced (70-90 °C) anthracene elimination. Analysis of substituent effects reveals that π-donating dialkylamide groups are paramount to successful phosphinidene transfer; poorer π-donors give reduced or no transfer. Substituent steric bulk is also implicated in successful transfer. Molecular beam mass spectrometry (MBMS) studies of each derivative reveal dialkylamide derivatives to be promising precursors for further gas-phase spectroscopic studies of phosphinidenes; in particular, we present evidence of direct detection of the dimethylamide derivative, [Me2N═P]. Kinetic investigations of iPr2NPA thermolysis in 1,3-cyclohexadiene and/or benzene-d6 are consistent with a model of unimolecular fragmentation to yield free phosphinidene [iPr2N═P] as a transient reactive intermediate. This conclusion is probed by density functional theory (DFT) calculations, which favored a mechanistic model featuring free singlet aminophosphinidenes. The breadth of phosphinidene acceptors is expanded to unsaturated substrates beyond 1,3-dienes to include olefins and alkynes; this provides a new synthetic route to valuable amino-substituted phosphiranes and phosphirenes, respectively. Stereoselective phosphinidene transfer to olefins is consistent with singlet phosphinidene reactivity by analogy with the Skell hypothesis for singlet carbene addition to olefins.

Spectroscopic Characterization, Computational Investigation, and Comparisons of ECX- (E = As, P, and N; X = S and O) Anions.

Three newly synthesized [Na+(221-Kryptofix)] salts containing AsCO-, PCO-, and PCS- anions were successfully electrosprayed into a vacuum, and these three ECX- anions were investigated by negative ion photoelectron spectroscopy (NIPES) along with high-resolution photoelectron imaging spectroscopy. For each ECX- anion, a well-resolved NIPE spectrum was obtained, in which every major peak is split into a doublet. The splittings are attributed to spin-orbit coupling (SOC) in the ECX• radicals. Vibrational progressions in the NIPE spectra of ECX- were assigned to the symmetric and the antisymmetric stretching modes in ECX• radicals. The electron affinities (EAs) and SO splittings of ECX• are determined from the NIPE spectra to be AsCO•: EA = 2.414 ± 0.002 eV, SO splitting = 988 cm-1; PCO•: EA = 2.670 ± 0.005 eV, SO splitting = 175 cm-1; PCS•: EA = 2.850 ± 0.005 eV, SO splitting = 300 cm-1. Calculations using the B3LYP, CASPT2, and CCSD(T) methods all predict linear geometries for both the anions and the neutral radicals. The calculated EAs and SO splittings for ECX• are in excellent agreement with the experimentally measured values. The simulated NIPE spectra, which are based on the calculated Franck-Condon factors, and the SO splittings nicely reproduce all of the observed spectral peaks, thus allowing unambiguous spectral assignments. The finding that PCS• has the greatest EA of the three triatomic molecules considered here is counterintuitive based upon simple electronegativity considerations, but this finding is understandable in terms of the movement of electron density from phosphorus in the HOMO of PCO- to sulfur in the HOMO of PCS-. Comparisons of the EAs of PCO• and PCS• with the previously measured EA values for NCO• and NCS• are made and discussed.

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.

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.

A Molecular Precursor to Phosphaethyne and Its Application in Synthesis of the Aromatic 1,2,3,4-Phosphatriazolate Anion.

Dibenzo-7-phosphanorbornadiene Ph3PC(H)PA (1, A = C14H10, anthracene) is reported here as a molecular precursor to phosphaethyne (HC≡P), produced together with anthracene and triphenylphosphine. HCP generated by thermolysis of 1 has been observed by molecular beam mass spectrometry, laser-induced fluorescence, microwave spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. In toluene, fragmentation of 1 has been found to proceed with activation parameters of ΔH(⧧) = 25.5 kcal/mol and ΔS(⧧) = -2.43 eu and is accompanied by formation of an orange insoluble precipitate. Results from computational studies of the mechanism of HCP generation are in good agreement with experimental data. This high-temperature method of HCP generation has pointed to new reaction chemistry with azide anion to produce the 1,2,3,4-phosphatriazolate anion, HCPN3(-), for which structural data have been obtained in a single-crystal X-ray diffraction study. Negative-ion photoelectron spectroscopy has shown the adiabatic detachment energy for this anion to be 3.555(10) eV. The aromaticity of HCPN3(-) has been assessed using nucleus-independent chemical shift, quantum theory of atoms in molecules, and natural bond orbital methods.

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.

[Ir(N

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.

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

Understanding the mechanistic coupling of molecular oxygen reduction and proton pumping for adenosine triphosphate synthesis during cellular respiration is the primary goal of research on heme-copper oxidases­the terminal complex in the membrane-bound electron transport chain. Cleavage of the oxygen-oxygen bond by the heme-copper oxidases forms the key intermediate PM, which initiates proton pumping. This intermediate is now experimentally defined by variable-temperature, variable-field magnetic circular dichroism spectroscopy on a previously unobserved excited state feature associated with its heme iron(IV)-oxo center. These data provide evidence that the iron(IV)-oxo in PM is magnetically coupled to both a copper(II) and a cross-linked tyrosyl radical in the active site. These results provide new insight into the oxygen-oxygen bond cleavage and proton-pumping mechanisms of heme-copper oxidases.