Alkaline earth metal-assisted dinitrogen activation at nickel.

Rare examples of trinuclear [Ni-N2-M-N2-Ni] core (M = Ca, Mg) with linear bridged dinitrogen ligands are reported in this work. The reduction of [iPr2NN]Ni(µ-Br)2Li(thf)2 (1) (iPr2NN = 2,4-bis-(2,6-diisopropylphenylimido)pentyl) with elemental Mg or Ca in THF under an atmosphere of dinitrogen yields the complex {iPr2NNNi(µ-N2)}2M (thf)4 (M = Mg, complex 2 and M = Ca, complex 3). The bridging end-on (µ-N2)2M(thf)4 moiety connects the two [iPr2NNNi]- nickelate fragments. A combination of X-ray crystallography, solution and solid-state spectroscopy have been applied to characterize complexes 2 and 3, and DFT studies have been used to help explain the bonding and electronic structure in these unique Ni-N2-Mg and Ni-N2-Ca complexes.

Tellurolate: an effective Te-atom transfer reagent to prepare the triad of group 5 metal bis(tellurides).

We show in this work how lithium tellurolate Li(X)nTeCH2SiMe3 (X = THF, n = 1, 1; X = 12-crown-4, n = 2, 2), can serve as an effective Te-atom transfer reagent to all group 5 transition metal halide precursors irrespective of the oxidation state. Mononuclear and bis(telluride) complexes, namely (PNP)M(Te)2 (M = V; Nb, 3; Ta, 4; PNP- = N[2-PiPr2-4-methylphenyl]2), are reported herein including structural and spectroscopic data. Whereas the known complex (PNP)V(Te)2 can be readily prepared from the trivalent precursor (PNP)VCl2, two equiv. of tellurolate, and elemental Te partially solubilized with PMe3, complex 3 can also be similarly obtained following the same procedure but with or without a reductant, Na/NaCl. Complex 4 on the other hand is formed from the addition of four equiv. of tellurolate to (PNP)TaF4. Having access to a triad of (PNP)M(Te)2 systems for group 5 metals has allowed us to compare them using a combination of theory and spectroscopy including Te-L1 edge XANES data.

Accelerating σ-Bond Metathesis at Sn(II) Centers.

Molecular main-group hydride catalysts are attractive as cheap and Earth-abundant alternatives to transition-metal analogues. In the case of the latter, specific steric and electronic tuning of the metal center through ligand choice has enabled the iterative and rational development of superior catalysts. Analogously, a deeper understanding of electronic structure-activity relationships for molecular main-group hydrides should facilitate the development of superior main-group hydride catalysts. Herein, we report a modular Sn-Ni bimetallic system in which we systematically vary the ancillary ligand on Ni, which, in turn, tunes the Sn center. This tuning is probed using Sn L1 XAS as a measure of electron density at the Sn center. We demonstrate that increased electron density at Sn centers accelerates the rate of σ-bond metathesis, and we employ this understanding to develop a highly active Sn-based catalyst for the hydroboration of CO2 using pinacolborane. Additionally, we demonstrate that engineering London dispersion interactions within the secondary coordination sphere of Sn allows for further rate acceleration. These results show that the electronics of main-group catalysts can be controlled without the competing effects of geometry perturbations and that this manifests in substantial reactivity differences.

Nitrene transfer from a sterically confined copper nitrenoid dipyrrin complex.

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds (e.g., amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin CuI complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction. The Cu nitrenoid, with an electronic structure most consistent with a triplet nitrene adduct of CuI, is accessed following geometric rearrangement of the azide adduct from κ1-N terminal ligation to κ1-N internal ligation with subsequent expulsion of N2. For perfluorinated arylazides, stoichiometric and catalytic C-H amination and aziridination was observed. Mechanistic analysis employing substrate competition reveals an enthalpically-controlled, electrophilic nitrene transfer for primary and secondary C-H bonds. Kinetic analyses for catalytic amination using tetrahydrofuran as a model substrate reveal pseudo-first order kinetics under relevant amination conditions with a first-order dependence on both Cu and organoazide. Activation parameters determined from Eyring analysis (ΔH‡ = 9.2(2) kcal mol-1, ΔS‡ = -42(2) cal mol-1 K-1, ΔG‡298K = 21.7(2) kcal mol-1) and parallel kinetic isotope effect measurements (1.10(2)) are consistent with rate-limiting Cu nitrenoid formation, followed by a proposed stepwise hydrogen-atom abstraction and rapid radical recombination to furnish the resulting C-N bond. The proposed mechanism and experimental analysis are further corroborated by density functional theory calculations. Multiconfigurational calculations provide insight into the electronic structure of the catalytically relevant Cu nitrene intermediates. The findings presented herein will assist in the development of future methodology for Cu-mediated C-N bond forming catalysis.

Outer coordination sphere influences on cofactor maturation and substrate oxidation by cytochrome P460.

Product selectivity of ammonia oxidation by ammonia-oxidizing bacteria (AOB) is tightly controlled by metalloenzymes. Hydroxylamine oxidoreductase (HAO) is responsible for the oxidation of hydroxylamine (NH2OH) to nitric oxide (NO). The non-metabolic enzyme cytochrome (cyt) P460 also oxidizes NH2OH, but instead produces nitrous oxide (N2O). While both enzymes use a heme P460 cofactor, they selectively oxidize NH2OH to different products. Previously reported structures of Nitrosomonas sp. AL212 cyt P460 show that a capping phenylalanine residue rotates upon ligand binding, suggesting that this Phe may influence substrate and/or product binding. Here, we show via substitutions of the capping Phe in Nitrosomonas europaea cyt P460 that the bulky phenyl side-chain promotes the heme-lysine cross-link forming reaction operative in maturing the cofactor. Additionally, the Phe side-chain plays an important role in modulating product selectivity between N2O and NO during NH2OH oxidation under aerobic conditions. A picture emerges where the sterics and electrostatics of the side-chain in this capping position control the kinetics of N2O formation and NO binding affinity. This demonstrates how the outer coordination sphere of cyt P460 is tuned not only for selective NH2OH oxidation, but also for the autocatalytic cross-link forming reaction that imbues activity to an otherwise inactive protein.

Scrutinizing formally NiIV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory.

Nickel K- and L2,3-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L2,3-edge XAS reveals that the physical d-counts of the formally NiIV compounds measured lie well above the d6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF62- is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d6 NiIV center. The reactivity of NiIV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.

Cytochrome P460 Cofactor Maturation Proceeds via Peroxide-Dependent Post-translational Modification.

Cytochrome P460s are heme enzymes that oxidize hydroxylamine to nitrous oxide. They bear specialized "heme P460" cofactors that are cross-linked to their host polypeptides by a post-translationally modified lysine residue. Wild-type N. europaea cytochrome P460 may be isolated as a cross-link-deficient proenzyme following anaerobic overexpression in E. coli. When treated with peroxide, this proenzyme undergoes maturation to active enzyme with spectroscopic and catalytic properties that match wild-type cyt P460. This maturation reactivity requires no chaperones─it is intrinsic to the protein. This behavior extends to the broader cytochrome c'ß superfamily. Accumulated data reveal key contributions from the secondary coordination sphere that enable selective, complete maturation. Spectroscopic data support the intermediacy of a ferryl species along the maturation pathway.

Ionothermal Synthesis of Metal-Organic Frameworks Using Low-Melting Metal Salt Precursors.

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with myriad potential applications in chemical separations, catalysis, and drug delivery. A major barrier to the application of MOFs is their poor scalability, as most frameworks are prepared under highly dilute solvothermal conditions using toxic organic solvents. Herein, we demonstrate that combining a range of linkers with low-melting metal halide (hydrate) salts leads directly to high-quality MOFs without added solvent. Frameworks prepared under these ionothermal conditions possess porosities comparable to those prepared under traditional solvothermal conditions. In addition, we report the ionothermal syntheses of two frameworks that cannot be prepared directly under solvothermal conditions. Overall, the user-friendly method reported herein should be broadly applicable to the discovery and synthesis of stable metal-organic materials.

Bonding and the role of electrostatics in driving C-C bond formation in high valent organocopper compounds.

The electronic structures and contrasting reactivity of [Cu(CF3)4]- and [Cu(CF3)3(CH3)]- were probed using coupled cluster and ab initio valence bond calculations. The Cu-C bonds in these complexes were found to be charge shift bonds. A key finding is that electrostatics likely prevent [Cu(CF3)4]- from accessing a productive transition state for C-C bond formation while promote one for [Cu(CF3)3(CH3)]-. These results therefore highlight essential design criteria for Cu-mediated C-C/C-heteroatom bond formation.

Lewis acid-assisted reduction of nitrite to nitric and nitrous oxides via the elusive nitrite radical dianion.

Reduction of nitrite anions (NO2-) to nitric oxide (NO), nitrous oxide (N2O) and ultimately dinitrogen (N2) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (-0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO22-), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N2O) and nitrite (NO2-). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO2- to NO.

Direct Reduction of NO to N2O by a Mononuclear Nonheme Thiolate Ligated Iron(II) Complex via Formation of a Metastable {FeNO}7 Complex.

Addition of NO to a nonheme dithiolate-ligated iron(II) complex, FeII(Me3TACN)(S2SiMe2) (1), results in the generation of N2O. Low-temperature spectroscopic studies reveal a metastable six-coordinate {FeNO}7 intermediate (S = 3/2) that was trapped at -135 °C and was characterized by low-temperature UV-vis, resonance Raman, EPR, Mössbauer, XAS, and DFT studies. Thermal decay of the {FeNO}7 species leads to the evolution of N2O, providing a rare example of a mononuclear thiolate-ligated {FeNO}7 that mediates NO reduction to N2O without the requirement of any exogenous electron or proton sources.

TEMPO coordination and reactivity in group 6; pseudo-pentagonal planar (η2-TEMPO)2CrX (X = Cl, TEMPO).

The exposure of CrCl2(THF)2 to 1 equiv. of TEMPO and 1 equiv. [TEMPO]Na afforded (η2-O,N-TEMPO)2CrCl (1, 67%); addition of [TEMPO]Na to 1 yielded (η2-O,N-TEMPO)2Cr(TEMPO) (2). Both 1 and 2 exhibit pseudo-pentagonal planar (PPP) geometry, instead of myriad alternatives. Calculations and spectral studies suggest the solid-state geometry persists in solution.

Electronic Structure of Ru26+ Complexes with Electron-Rich Anilinopyridinate Ligands.

Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru2(Xap)4Cl]+ species in the Ru26+ oxidation state. Despite paramagnetism of the compounds, 1H NMR spectroscopy proved highly informative for determining the isomerism of the Ru25+ and Ru26+ compounds. While most compounds are found to have the polar (4,0) geometry, with all four Xap ligands in the same orientation, some synthetic procedures resulted in a mixture of (4,0) and (3,1) isomers, most notably in the case of the parent compound Ru2(ap)4Cl. The isomerism of this compound has been overlooked in previous reports. Electrochemical studies demonstrate that oxidation potentials can be tuned by the installation of electron donating groups to the ligands, increasing accessibility of the Ru26+ oxidation state. The resulting Ru26+ monocations were found to have the expected (π*)2 ground state, and an in-depth study of the electronic transitions by Vis/NIR absorption and MCD spectroscopies with the aid of TD-DFT allowed for the assignment of the electronic spectra. The empty δ* orbital is the major acceptor orbital for the most prominent electronic transitions. Both Ru25+ and Ru26+ compounds were studied by Ru K-edge X-ray absorption spectroscopy; however, the rising edge energy is insensitive to redox changes in the compounds due to the broad line shape observed for 4d transition metal K-edges. DFT calculations indicate the presence of ligand orbitals at the frontier level, suggesting that further oxidation beyond Ru26+ will be ligand-centered rather than metal-centered.

Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure.

Sulfur/carbon/sulfur pincer ligands have an interesting combination of strong-field and weak-field donors, a coordination environment that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a proton-responsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N-H bond dissociation free energy, derived from the Bordwell equation, is 56 ± 2 kcal/mol. Electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with a large zero field splitting, and X-ray absorption spectra show a high Fe-S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and density functional theory calculations explain this phenomenon through a change between having the doubly occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.

Reversible C-C Bond Formation, Halide Abstraction, and Electromers in Complexes of Iron Containing Redox-Noninnocent Pyridine-imine Ligands.

The exploration of pyridine-imine (PI) iron complexes that exhibit redox noninnocence (RNI) led to several interesting discoveries. The reduction of (PI)FeX2 species afforded disproportionation products such as (dmpPI)2FeX (dmp = 2,6-Me2-C6H3, X = Cl, Br; 8-X) and (dippPI)2FeX (dipp = 2,6-iPr2-C6H3, X = Cl, Br; 9-X), which were independently prepared by reductions of (PI)FeX2 in the presence of PI. The crystal structure of 8-Br possessed an asymmetric unit with two distinct electromers, species with different electronic GSs: a low-spin (S = 1/2) configuration derived from an intermediate-spin S = 1 core antiferromagnetically (AF) coupled to an S = 1/2 PI ligand, and an S = 3/2 center resulting from a high-spin S = 2 core AF-coupled to an S = 1/2 PI ligand. Calculations were used to energetically compare plausible ground states. Polydentate diazepane-PI (DHPI) ligands were applied to the synthesis of monomeric dihalides (DHPI)FeX2 (X = Cl, 1-Cl2; X = Br, 1-Br2); reduction generated the highly distorted bioctahedral dimers (DHPA)2Fe2X2 ((3-X)2) containing a C-C bond formed from imine coupling; the monomers 1-X2 could be regenerated upon Ph3CX oxidation. Dihalides and their reduced counterparts were subjected to various alkyl halides and methyl methacrylate (MMA), generating polymers with little to no molecular weight control, indicative of simple radical-initiated polymerization.

Lithium superoxide encapsulated in a benzoquinone anion matrix.

Lithium peroxide is the crucial storage material in lithium-air batteries. Understanding the redox properties of this salt is paramount toward improving the performance of this class of batteries. Lithium peroxide, upon exposure to p-benzoquinone (p-C6H4O2) vapor, develops a deep blue color. This blue powder can be formally described as [Li2O2][Formula: see text] [LiO2][Formula: see text] {Li[p-C6H4O2]}0.7, though spectroscopic characterization indicates a more nuanced structural speciation. Infrared, Raman, electron paramagnetic resonance, diffuse-reflectance ultraviolet-visible and X-ray absorption spectroscopy reveal that the lithium salt of the benzoquinone radical anion forms on the surface of the lithium peroxide, indicating the occurrence of electron and lithium ion transfer in the solid state. As a result, obligate lithium superoxide is formed and encapsulated in a shell of Li[p-C6H4O2] with a core of Li2O2 Lithium superoxide has been proposed as a critical intermediate in the charge/discharge cycle of Li-air batteries, but has yet to be isolated, owing to instability. The results reported herein provide a snapshot of lithium peroxide/superoxide chemistry in the solid state with redox mediation.

A Nonheme Mononuclear {FeNO}7 Complex that Produces N2 O in the Absence of an Exogenous Reductant.

A new nonheme iron(II) complex, FeII (Me3 TACN)((OSiPh2 )2 O) (1), is reported. Reaction of 1 with NO(g) gives a stable mononitrosyl complex Fe(NO)(Me3 TACN)((OSiPh2 )2 O) (2), which was characterized by Mössbauer (δ=0.52 mm s-1 , |ΔEQ |=0.80 mm s-1 ), EPR (S=3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}7 complex with an S=3/2 spin ground state. The RR spectrum (λexc =458 nm) of 2 combined with isotopic labeling (15 N, 18 O) reveals ν(N-O)=1680 cm-1 , which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm-1 ). Complex 2 reacts rapidly with H2 O in THF to produce the N-N coupled product N2 O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N2 O in the absence of an exogenous reductant.

An Isolable Mononuclear Palladium(I) Amido Complex.

Mononuclear Pd(I) species are putative intermediates in Pd-catalyzed reactions, but our knowledge about them is limited due to difficulties in accessing them. Herein, we report the isolation of a Pd(I) amido complex, [(BINAP)Pd(NHArTrip)] (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthalene, ArTrip = 2,6-bis(2',4',6'-triisopropylphenyl)phenyl), from the reaction of (BINAP)PdCl2 with LiNHArTrip. This Pd(I) amido species has been characterized by X-ray crystallography, electron paramagnetic resonance, and multiedge Pd X-ray absorption spectroscopy. Theoretical study revealed that, while the three-electron-two-center π-interaction between Pd and N in the Pd(I) complex imposes severe Pauli repulsion in its Pd-N bond, pronounced attractive interligand dispersion force aids its stabilization. In accord with its electronic features, reactions of homolytic Pd-N bond cleavage and deprotonation of primary amines are observed on the Pd(I) amido complex.

Dph3 Enables Aerobic Diphthamide Biosynthesis by Donating One Iron Atom to Transform a [3Fe-4S] to a [4Fe-4S] Cluster in Dph1-Dph2.

All radical S-adenosylmethionine (radical-SAM) enzymes, including the noncanonical radical-SAM enzyme diphthamide biosynthetic enzyme Dph1-Dph2, require at least one [4Fe-4S](Cys)3 cluster for activity. It is well-known in the radical-SAM enzyme community that the [4Fe-4S](Cys)3 cluster is extremely air-sensitive and requires strict anaerobic conditions to reconstitute activity in vitro. Thus, how such enzymes function in vivo in the presence of oxygen in aerobic organisms is an interesting question. Working on yeast Dph1-Dph2, we found that consistent with the known oxygen sensitivity, the [4Fe-4S] cluster is easily degraded into a [3Fe-4S] cluster. Remarkably, the small iron-containing protein Dph3 donates one Fe atom to convert the [3Fe-4S] cluster in Dph1-Dph2 to a functional [4Fe-4S] cluster during the radical-SAM enzyme catalytic cycle. This mechanism to maintain radical-SAM enzyme activity in aerobic environments is likely general, and Dph3-like proteins may exist to keep other radical-SAM enzymes functional in aerobic environments.