Synthetically Reversible, Proton-Mediated Nitrite N-O Bond Cleavage at a Dicopper Site.

A monocationic dicopper(I,I) nitrite complex [Cu(μ-κ:κ-ON)DPFN][NTf] () (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine, NTf = N(SOCF)), was synthesized by treatment of a dicopper acetonitrile complex, [Cu(μ-MeCN)DPFN][NTf] (), with tetrabutylammonium nitrite ([BuN][NO]). DFT calculations indicate that is one of three linkage isomers that are close in energy and presumably accessible in solution. Reaction of the μ-κ:κ-ON complex with -TolSH produces nitrous acid (HONO) and the corresponding dicopper thiolate species via an acid-base exchange reaction.

Copper-Catalyzed C(sp)-H α-Acetylation: Generation of Quaternary Centers.

α-substituted ketones are important chemical targets as synthetic intermediates as well as functionalities in natural products and pharmaceuticals. We report the α-acetylation of C(sp)-H substrates R-H with arylmethyl ketones ArC(O)Me to provide α-alkylated ketones ArC(O)CHR at RT with BuOOBu as oxidant via copper(I) -diketiminato catalysts. Proceeding via alkyl radicals R•, this method enables α-substitution with bulky substituents without competing elimination that occurs in more traditional alkylation reactions between enolates and alkyl electrophiles.

Direct O mediated oxidation of a Ni(II)NO structural model complex for the active site of nickel acireductone dioxygenase (Ni-ARD): characterization, biomimetic reactivity, and enzymatic implications.

A new biomimetic model complex of the active site of acireductone dioxygenase (ARD) was synthesized and crystallographically characterized (ii1). 1 displays carbon-carbon oxidative cleavage activity in the presence of O towards the substrate 2-hydroxyacetophenone. This reactivity was monitored UV-Visible and NMR spectroscopy.

C-H amination chemistry mediated by trinuclear Cu(I) sites supported by a ligand scaffold featuring an arene platform and tetramethylguanidinyl residues.

Tripodal ligands that can encapsulate single or multiple metal sites in -symmetric geometric configurations constitute valuable targets for novel catalysts. Of particular interest in ligand development are efforts toward incorporating apical elements that exhibit little if any electron donicity, to enhance the electrophilic nature of a positioned active oxidant (, metal-oxo, -nitrene). The tripodal ligand TMGtrphen-Arene has been synthesized, featuring an arene platform 1,3,5-substituted with phenylene arms possessing tetramethylguanidinyl (TMG) residues.

Electrocatalytic Dinitrogen Reduction to Ammonia Using Easily Reducible N-Fused Cobalt Porphyrins.

Single-site molecular electrocatalysts, especially those that perform catalytic conversion of N to NH under mild conditions, are highly desirable to derive fundamental structure-activity relations and as potential alternatives to the current energy-consuming Haber-Bosch ammonia production process. Combining theoretical calculations with experimental evidence, it has been shown that easily reducible cobalt porphyrins catalyze the six-electron, six-proton reduction of dinitrogen to NH at neutral pH and under ambient conditions. Two easily reducible N-fused cobalt porphyrins - CoNHF and CoNHF(Br) - reveal NRR activity with Faradic efficiencies between 6-7.

Integrated Study on Methane Activation: Exploring Main Group Frustrated Lewis Pairs through Density Functional Theory, Machine Learning, and Machine-Learned Force Fields.

Frustrated Lewis Pairs (FLP) are an important advance in metal-free catalysis due to their ability to activate a variety of small molecules. Many studies have focused on a very limited sample of Lewis acids and bases. Herein, we disclose an automated exploration algorithm using density functional methods, artificial neural networks (ANNs), and a molecule builder that incentivizes the exploration of favorable FLP space for the activation of methane two mechanisms: deprotonation and hydride abstraction.

Reduction of (pddi)Cr reveals redox noninnocence C-C bond formation amidst competing electrophilicity: [(cpta)CrMe] ( = 0, 1) and [(pta)Cr].

Reversible cyclopropane formation is probed as a means of redox noninnocence in diimine/diamide chelates reduction and complex anion formation. Competition from imine attack renders complications in the latter approach, and electrochemical measurements with calculational support provide the rationale.

Niobium Carbide and Tantalum Carbide as Nitrogen Reduction Electrocatalysts: Catalytic Activity, Carbophilicity, and the Importance of Intermediate Oxidation States.

Significant interest in the electrocatalytic reduction of molecular nitrogen to ammonia (the nitrogen reduction reaction: NRR) has focused attention on transition metal carbides as possible electrocatalysts. However, a fundamental understanding of carbide surface structure/NRR reactivity relationships is sparse. Herein, electrochemistry, DFT-based calculations, and photoemission studies demonstrate that NbC, deposited by magnetron sputter deposition, is active for NRR at pH 3.

Deciphering the Mechanism of Base-Triggered Conversion of Ammonia to Molecular Nitrogen and Methylamine to Cyanide.

We report an in-depth investigation into the ammonia oxidation mechanism by the catalyst [Ru(tpy)(dmabpy)NH] ([Ru(NH)]). Stoichiometric reactions of [Ru(NH)] were carried out with exogenous noncoordinating bases to trigger a proposed redox disproportionation reaction, which was followed using variable-temperature NMR spectroscopy. An intermediate species was identified as a dinitrogen-bridged complex using N NMR and Raman spectroscopy on isotopically labeled complexes.

Nitrene transfer from a sterically confined copper nitrenoid dipyrrin complex.

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds (, 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 Cu 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.

Benzimidazole-diamide (bida) Pincer Chromium Complexes: Structures and Reactivity.

Serendipitous discovery of bida (i.e., N-Ar-N-((1-Ar-1-benzo[]imidazol-2-yl)methyl)benzene-1,2-diamide; Ar = 2,6-Pr-CH), a potentially redox noninnocent, hemilabile pincer ligand with a methylene group that may facilitate proton/H atom reactivity, prompted its investigation.

Stability and activity of titanium oxynitride thin films for the electrocatalytic reduction of nitrogen to ammonia at different pH values.

The production of ammonia for agricultural and energy demands has accelerated research for more environmentally-friendly synthesis options, particularly the electrocatalytic reduction of molecular nitrogen (nitrogen reduction reaction, NRR). Catalyst activity for NRR, and selectivity for NRR over the competitive hydrogen evolution reaction (HER), are critical issues for which fundamental knowledge remains scarce. Herein, we present results regarding the NRR activity and selectivity of sputter-deposited titanium nitride and titanium oxynitride films for NRR and HER.

On the mechanism of intermolecular nitrogen-atom transfer from a lattice-isolated diruthenium nitride intermediate.

Catalyst confinement within microporous media provides the opportunity to site isolate reactive intermediates, enforce intermolecular functionalization chemistry by co-localizing reactive intermediates and substrates in molecular-scale interstices, and harness non-covalent host-guest interactions to achieve selectivities that are complementary to those accessible in solution. As part of an ongoing program to develop synthetically useful nitrogen-atom transfer (NAT) catalysts, we have demonstrated intermolecular benzylic amination of toluene at a Ru nitride intermediate confined within the interstices of a Ru-based metal-organic framework (MOF), Ru(btc)X (btc = 1,3,5-benzenetricarboxylate, , Ru-HKUST-1 for X = Cl). Nitride confinement within the extended MOF lattice enabled intermolecular C-H functionalization of benzylic C-H bonds in preference to nitride dimerization, which was encountered with soluble molecular analogues.

Vanadium oxide, vanadium oxynitride, and cobalt oxynitride as electrocatalysts for the nitrogen reduction reaction: a review of recent developments.

The electrocatalytic reduction of molecular nitrogen to ammonia-the nitrogen reduction reaction (NRR)-is of broad interest as an environmentally- and energy-friendly alternative to the Haber-Bosch process for agricultural and emerging energy applications. Herein, we review our recent findings from collaborative electrochemistry/surface science/theoretical studies that counter several commonly held assumptions regarding transition metal oxynitrides and oxides as NRR catalysts. Specifically, we find that for the vanadium oxide, vanadium oxynitride, and cobalt oxynitride systems, (a) there is no Mars-van Krevelen mechanism and that the reduction of lattice nitrogen and Nto NHoccurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice; and (b) that NRR and the hydrogen evolution reaction do occur in concert under the conditions studied for Co oxynitride, but not for V oxynitride.

Crystal structure and DFT calculations of CpNbH(SiIMe)(SiFMe): an asymmetric bis(silyl) niobocene hydride complex.

An asymmetric bis(silyl) niobocene hydride complex, namely, bis(η-cyclopentadienyl)(fluorodimethylsilyl)hydrido(iododimethylsilyl)niobium, [Nb(CH)(CHFSi)(CHISi)H] or CpNbH(SiIMe)(SiFMe), has been studied to determine the effect of the silyl ligand on the position of the hydride attached to the Nb atom. It has been shown that when a Group 17 atom is substituted onto one of the silyl ligands, there is a greater interaction between the hydride and this ligand, as demonstrated by a shorter Si..

Three Component Cascade Reaction of Cyclohexanones, Aryl Amines, and Benzoylmethylene Malonates: Cooperative Enamine-Brønsted Acid Approach to Tetrahydroindoles.

A three-component cascade reaction comprising cyclic ketones, arylamines, and benzoylmethylene malonates has been developed to access 4,5,6,7-tetrahydro-1-indoles. The reaction was achieved through cooperative enamine-Brønsted catalysis in high yields with wide substrate scopes. Mechanistic studies identified the role of the Brønsted acid catalyst and revealed the formation of an imine intermediate, which was confirmed by X-ray crystallography.

Mapping the Basicity of Selected 3d and 4d Metal Nitrides: A DFT Study.

Nitride complexes have been invoked as catalysts and intermediates in a wide variety of transformations and are noted for their tunable acid/base properties. A density functional theory study is reported herein that maps the basicity of 3d and 4d transition metals that routinely form nitride complexes: V, Cr, Mn, Nb, Mo, Tc, and Ru. Complexes were gathered from the Cambridge Structural Database, and from the free energy of protonation, the p(N) of the nitride group was calculated to quantify the impact of metal identity, oxidation state, coordination number, and supporting ligand type upon metal-nitride basicity.

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

The exposure of CrCl(THF) to 1 equiv. of TEMPO and 1 equiv. [TEMPO]Na afforded (η-O,N-TEMPO)CrCl (1, 67%); addition of [TEMPO]Na to 1 yielded (η-O,N-TEMPO)Cr(TEMPO) (2).

NO Coupling at Copper to -Hyponitrite: NO Formation via Protonation and H-Atom Transfer.

Copper nitrite reductases (CuNIRs) convert NO to NO as well as NO to NO under high NO flux at a mononuclear type 2 Cu center. While model complexes illustrate N-N coupling from NO that results in symmetric -hyponitrite [Cu]-ONNO-[Cu] complexes, we report NO assembly at a single Cu site in the presence of an external reductant Cp*M (M = Co, Fe) to give the first copper -hyponitrites [Cp*M]{[Cu](κ-ON)[Cu]}. Importantly, the κ-N-bound [Cu] fragment may be easily removed by the addition of mild Lewis bases such as CNAr or pyridine to form the spectroscopically similar anion {[Cu](κ-ON)}.

Metal-Ligand-Anion Cooperation in C-H Bond Formation at Platinum(II).

Thermolysis of [(BPI)Pt(CH)][OTf] (BPI = 1,3-bis(2-(4--butyl)pyridylimino)isoindole) to release methane and form (BPI)Pt(OTf) is reported. Kinetic, mechanistic, and computational studies point to an unusual anion-assisted pathway that obviates the need for a higher oxidation state intermediate to couple the metal-bound methyl group with the ligand-bound hydrogen. Leveraging this insight, a triflimide derivative of the (BPI)Pt complex was shown to activate benzene, highlighting the role of the counteranion in controlling the activity of these complexes.

Copper(III) Metallacyclopentadienes via Zirconocene Transfer and Reductive Elimination to an Isolable Phenanthrocyclobutadiene.

Despite the widespread use of copper catalysis for the formation of C-C bonds, debate about the mechanism persists. Reductive elimination from Cu(III) is often invoked as a key step, yet examples of its direct observation from isolable complexes remain limited to only a few examples. Here, we demonstrate that incorporation of bulky mesityl (Mes) groups into the α-positions of a phenanthrene-appended zirconacyclopentadiene, CpZr(2,5-Mes-phenanthro[9,10]C), enables efficient oxidative transmetalation to the corresponding, formal Cu(III) metallacyclopentadiene dimer.

Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents.

Recent research has highlighted the key role played by the electron affinity of the active metal-nitrene/imido oxidant as the driving force in nitrene additions to olefins to afford valuable aziridines. The present work showcases a library of Co(II) reagents that, unlike the previously examined Mn(II) and Fe(II) analogues, demonstrate reactivity trends in olefin aziridinations that cannot be solely explained by the electron affinity criterion. A family of Co(II) catalysts (17 members) has been synthesized with the assistance of a trisphenylamido-amine scaffold decorated by various alkyl, aryl, and acyl groups attached to the equatorial amidos.

Electrocatalytic Reduction of Nitrogen to Ammonia: the Roles of Lattice O and N in Reduction at Vanadium Oxynitride Surfaces.

Vanadium oxynitride and other earth-abundant oxynitrides are of growing interest for the electrocatalytic reduction of nitrogen to NH. A major unresolved issue, however, concerns the roles of lattice N and lattice O in this process. Electrochemistry and photoemission data reported here demonstrate that both lattice N and dissolved N are reduced to NH by cathodic polarization of vanadium oxynitride films at pH 7.

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)FeX species afforded disproportionation products such as (dmpPI)FeX (dmp = 2,6-Me-CH, X = Cl, Br; -X) and (dippPI)FeX (dipp = 2,6-Pr-CH, X = Cl, Br; -X), which were independently prepared by reductions of (PI)FeX in the presence of PI. The crystal structure of -Br possessed an asymmetric unit with two distinct electromers, species with different electronic GSs: a low-spin ( = 1/2) configuration derived from an intermediate-spin = 1 core antiferromagnetically (AF) coupled to an = 1/2 PI ligand, and an = 3/2 center resulting from a high-spin = 2 core AF-coupled to an = 1/2 PI ligand.

DFT and TDDFT Study of the Reaction Pathway for Double Intramolecular C-H Activation and Functionalization by Iron, Cobalt, and Nickel-Nitridyl Complexes.

Previous work was successful in synthesizing a nickel amine, [Cz(Pyr)(NH-Pyr)], by double C-H activation and functionalization via irradiating a disphenoidal Ni(II) azido complex, [Cz(Pyr)NiN]. The present work seeks to expand upon the earlier research and to substitute the metal with iron or cobalt. Density functional theory (DFT)-B3LYP/6-31+G(d') and APFD/Def2TZVP-was used to simulate the generation of an intermediate with significant nitridyl radical character after the loss of N from the starting azido complex.