Oxidation of Ammonia Catalyzed by a Molecular Iron Complex: Translating Chemical Catalysis to Mediated Electrocatalysis.

Ammonia is a promising candidate in the quest for sustainable, clean energy. With its capacity to serve as an energy carrier, the oxidation of ammonia opens avenues for carbon-neutral approaches to address worldwide growing energy needs. We report the catalytic chemical oxidation of ammonia by an Earth-abundant transition metal complex, trans-[LFe(MeCN)][PF], where L is a macrocyclic ligand bearing four N-heterocyclic carbene (NHC) donors.

Hydrogen Atom Abstraction from an Os(NH) Complex Generates an Os(NH) Complex: Experimental and Computational Analysis of the N-H Bond Dissociation Free Energies and Reactivity.

Double hydrogen atom abstraction from (TMP)Os(NH) (TMP = tetramesitylporphyrin) with phenoxyl or nitroxyl radicals leads to (TMP)Os(NH). This unusual bis(amide) complex is diamagnetic and displays an N-H resonance at 12.0 ppm in its H NMR spectrum.

Weakening the N-H Bonds of NH Ligands: Triple Hydrogen-Atom Abstraction to Form a Chromium(V) Nitride.

Weakening and cleaving N-H bonds is crucial for improving molecular ammonia (NH) oxidation catalysts. We report the synthesis and H-atom-abstraction reaction of bis(ammonia)chromium porphyrin complexes Cr(TPP)(NH) and Cr(TMP)(NH) (TPP = 5,10,15,20-tetraphenyl--porphyrin and TMP = 5,10,15,20-tetramesityl--porphyrin) using bulky aryloxyl radicals. The triple H-atom-abstraction reaction results in the formation of Cr(por)(≡N), with the nitride derived from NH, as indicated by UV-vis and IR and single-crystal structural determination of Cr(TPP)(≡N).

Protonation of Serine in Gas and Condensed and Microsolvated States in Aqueous Solution.

Identification of molecules and elucidation of their chemical structure are ubiquitous problems in chemistry. Mass spectrometry (MS) can be used due to its sensitivity and versatility. For detection to occur, analytes must be ionized and transferred to the gas phase.

Multiple N-H and C-H Hydrogen Atom Abstractions Through Coordination-Induced Bond Weakening at Fe-Amine Complexes.

We report the use of the reported Fe-phthalocyanine complex, PcFe (; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), to generate PcFe-amine complexes , , and . Treatment of or to an excess of the stable aryloxide radical, 2,4,6-tri-butylphenoxyl radical (ArO), under NH resulted in catalytic H atom abstraction (HAA) and C-N coupling to generate the product 4-amino-2,4,6-tri-butylcyclohexa-2,5-dien-1-one () and ArOH. Exposing to an excess of the trityl (CPh) variant, 2,6-di--butyl-4-tritylphenoxyl radical (ArO), under NH did not lead to catalytic ammonia oxidation as previously reported in a related Ru-porphyrin complex.

Design of robust 2,2'-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces.

The attachment of the 2,2'-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes was investigated using ab initio simulations performed on a new cluster model for methyl-terminated silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface impurity due to required surface processing chemistry, and thus could plausibly result in the observed surface instability of the linker.

Intramolecular Electrostatic Effects on O, CO, and Acetate Binding to a Cationic Iron Porphyrin.

Noncovalent electrostatic interactions are important in many biological and chemical reactions, especially those that involve charged intermediates. There has been a growing interest in using electrostatic ligand designs-placing charges in the second coordination sphere-to improve molecular reactivity, catalysis, and electrocatalysis. For instance, an iron porphyrin bearing four cationic -trimethylanilinium groups, Fe(-TMA), has been reported to be an exceptional electrocatalyst for both the carbon dioxide reduction reaction (CORR) and the oxygen reduction reaction (ORR).

Oxidation of Ammonia with Molecular Complexes.

Oxidation of ammonia by molecular complexes is a burgeoning area of research, with critical scientific challenges that must be addressed. A fundamental understanding of individual reaction steps is needed, particularly for cleavage of N-H bonds and formation of N-N bonds. This Perspective evaluates the challenges of designing molecular catalysts for oxidation of ammonia and highlights recent key contributions to realizing the goals of viable energy storage and retrieval based on the N-H bonds of ammonia in a carbon-free energy cycle.

Selectivity-Determining Steps in O Reduction Catalyzed by Iron(tetramesitylporphyrin).

The oxygen reduction reaction (ORR) is the cathode reaction in fuel cells and its selectivity for water over hydrogen peroxide production is important for these technologies. Iron porphyrin catalysts have long been studied for the ORR, but the origins of their selectivity are not well understood because the selectivity-determining step(s) usually occur after the rate-determining step. We report here the effects of acid concentration, as well as other solution conditions such as acid p, on the HO/HO selectivity in electrocatalytic ORR by iron(tetramesitylporphyrin) (Fe(TMP)) in DMF.

Diversion of Catalytic C-N Bond Formation to Catalytic Oxidation of NH through Modification of the Hydrogen Atom Abstractor.

We report that (TMPRu(NH) (TMP = tetramesitylporphryin) is a molecular catalyst for oxidation of ammonia to dinitrogen. An aryloxy radical, tri--butylphenoxyl (·), abstracts H atoms from a bound ammonia ligand of (TMP)Ru(NH), leading to the discovery of a new catalytic C-N coupling to the para position of · to form 4-amino-2,4,6-tri--butylcyclohexa-2,5-dien-1-one. Modification of the aryloxy radical to 2,6-di--butyl-4-tritylphenoxyl radical, which contains a trityl group at the para position, prevents C-N coupling and diverts the reaction to catalytic oxidation of NH to give N.

Triple hydrogen atom abstraction from Mn-NH complexes results in cyclophosphazenium cations.

All hydrogen atoms of the NH3 in [Mn(depe)2(CO)(NH3)]+ are abstracted by 2,4,6-tri-tert-butylphenoxyl radical, resulting in the isolation of a rare cyclophosphazenium cation, [(Et2P(CH2)2PEt2)N]+, in 76% yield. An analogous reaction is observed for [Mn(dppe)2(CO)(NH3)]+. Computations suggest insertion of NHx into a Mn-P bond provides the thermodynamic driving force.

Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction.

Catalysts for the oxidation of NH are critical for the utilization of NH as a large-scale energy carrier. Molecular catalysts capable of oxidizing NH to N are rare. This report describes the use of [Cp*Ru(P N )( NH )][BAr ], (P N =1,5-di(phenylaza)-3,7-di(tert-butylphospha)cyclooctane; Ar =3,5-(CF ) C H ), to catalytically oxidize NH to dinitrogen under ambient conditions.

Mechanism of Catalytic O Reduction by Iron Tetraphenylporphyrin.

The catalytic reduction of O to HO is important for energy transduction in both synthetic and natural systems. Herein, we report a kinetic and thermochemical study of the oxygen reduction reaction (ORR) catalyzed by iron tetraphenylporphyrin (Fe(TPP)) in N, N'-dimethylformamide using decamethylferrocene as a soluble reductant and para-toluenesulfonic acid ( pTsOH) as the proton source. This work identifies and characterizes catalytic intermediates and their thermochemistry, providing a detailed mechanistic understanding of the system.

Design and reactivity of pentapyridyl metal complexes for ammonia oxidation.

Oxidation of NH3 to N2 by pentapyridyl metal complexes via hydrogen atom abstraction was investigated computationally. Quantum chemical analysis reveals insights on orbital symmetry requirements for efficient NH3 oxidation. The most promising complex, [(PY5)Mo(NH3)]2+, was studied experimentally.

Evaluation of attractive interactions in the second coordination sphere of iron complexes containing pendant amines.

The interactions between pendant amines in the second coordination sphere and ligands in the first coordination sphere are important for understanding the structures and reactivity of complexes containing PR2NR'2 ligands, which have been shown to be highly active H2 oxidation/production catalysts. A series of [Fe(PPh2NBn2)2(X)(Y)]n+ complexes have been prepared and structurally characterized. These complexes have two different ligands with which the pendant amines of the diphosphine ligand can interact.

Anion control of tautomeric equilibria: Fe-H N-H influenced by NH···F hydrogen bonding.

Counterions can play an active role in chemical reactivity, modulating reaction pathways, energetics and selectivity. We investigated the tautomeric equilibrium resulting from protonation of Fe(PNP)(CO) (PNP = (EtPCH)NMe) at Fe or N. Protonation of Fe(PNP)(CO) by [(EtO)H][B(CF)] occurs at the metal to give the iron hydride [Fe(PNP)(CO)H][B(CF)].

Catalytic Silylation of N and Synthesis of NH and NH by Net Hydrogen Atom Transfer Reactions Using a Chromium P Macrocycle.

We report the first discrete molecular Cr-based catalysts for the reduction of N. This study is focused on the reactivity of the Cr-N complex, trans-[Cr(N)(PN)] (PCr(N)), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-PN ligand is critical to preserve the structural integrity of the catalyst.

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst.

Recent work has shown that Cp*Rh(bpy) [Cp* = pentamethylcyclopentadienyl, bpy = 2,2'- bipyridine] undergoes endo protonation at the [Cp*] ligand in the presence of weak acid (EtNH; pK = 18.8 in MeCN). Upon exposure to stronger acid (e.

Proton-hydride tautomerism in hydrogen evolution catalysis.

Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about the energy landscape for H2 production can be obtained by chemical characterization of catalytic intermediates, but few have been observed to date. We have isolated and characterized a key intermediate in 2e(-) + 2H(+) → H2 catalysis.

Reactivity of a series of isostructural cobalt pincer complexes with CO2, CO, and H(+).

The preparation and characterization of a series of isostructural cobalt complexes [Co(t-Bu)2P(E)Py(E)P(t-Bu)2(CH3CN)2][BF4]2 (Py = pyridine, E = CH2, NH, O, and X = BF4 (1a-c)) and the corresponding one-electron reduced analogues [Co(t-Bu)2P(E)Py(E)P(t-Bu)2(CH3CN)2][BF4]2 (2a-c) are reported. The reactivity of the reduced cobalt complexes with CO2, CO, and H(+) to generate intermediates in a CO2 to CO and H2O reduction cycle are described. The reduction of 1a-c and subsequent reactivity with CO2 was investigated by cyclic voltammetry, and for 1a also by infrared spectroelectrochemistry.

Atomic layer deposition of quantum-confined ZnO nanostructures.

The modulation of optoelectronic properties, such as the bandgap of a pure-component semiconductor material, is a useful ability that can be achieved by few techniques. Atomic layer deposition (ALD) was used here to experimentally demonstrate the ability to deposit films that exhibit quantum confinement on three-dimensional surfaces. Polycrystalline ZnO films ranging from approximately 1.