The transformation of white phosphorus via early transition metals is discussed. The modern multi-step synthesis of organophosphorus compounds is explored in detail.
A pseudotetrahedral cobalt(II) complex with a positive axial zero-field splitting parameter of D = 12.7 cm(-1), as determined by high-field EPR spectroscopy, is shown to exhibit slow magnetic ...relaxation under an applied dc field.
•The potential of the cerium(III/IV) redox couple spans >3V and is highly sensitive to its ligand environment.•Ligand properties that stabilize high oxidation-state cerium are different than for ...transition metals.•Redox transformations can be used synthetically to access cerium(IV) compounds.•Sluggish electron transfer kinetics plays a major role in the electrochemistry of cerium complexes.
A key characteristic of the element cerium is its reversible redox chemistry between trivalent and tetravalent forms, which is central to the application of cerium in synthetic and materials chemistry. Herein we survey the general thermodynamic and kinetic characteristics and reported potentials for molecular cerium redox chemistry. The collected data illustrate that the local electronic environment provided by the coordination sphere around a cerium ion has a great effect on the oxidizing ability of the ion. The survey also illustrates the ligand types that most effectively stabilize each oxidation state. We expect the collection and comparison of these data will facilitate the development of new cerium(IV) chemistry and applications in oxidation and reduction chemistry.
A set of robust molecular cobalt catalysts for the generation of hydrogen from water is reported. The cobalt complex supported by the parent pentadentate polypyridyl ligand PY5Me2 features high ...stability and activity and 100% Faradaic efficiency for the electrocatalytic production of hydrogen from neutral water, with a turnover number reaching 5.5 × 104 mol of H2 per mole of catalyst with no loss in activity over 60 h. Control experiments establish that simple Co(II) salts, the free PY5Me2 ligand, and an isostructural PY5Me2 complex containing redox-inactive Zn(II) are all ineffective for this reaction. Further experiments demonstrate that the overpotential for H2 evolution can be tuned by systematic substitutions on the ancillary PY5Me2 scaffold, presaging opportunities to further optimize this first-generation platform by molecular design.
We report ammonia oxidation by homolytic cleavage of all three H atoms from a MoNH3+ complex using the 2,4,6-tri-tert-butylphenoxyl radical to yield a Mo-alkylimido (MoNR+) complex (R = ...2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one). Chemical reduction of MoNR+ generates a terminal MoN nitride complex upon NC bond cleavage, and a MoNH+ complex is formed by protonation of the nitride. Computational analysis describes the energetic profile for the stepwise removal of three H atoms from MoNH3+ and formation of MoNR+.
The geometric constraints imposed by a tetradentate P4N2 ligand play an essential role in stabilizing square planar Fe complexes with changes in metal oxidation state. The square pyramidal ...Fe0(N2)(P4N2) complex catalyzes the conversion of N2 to N(SiR3)3 (R = Me, Et) at room temperature, representing the highest turnover number of any Fe-based N2 silylation catalyst to date (up to 65 equiv N(SiMe3)3 per Fe center). Elevated N2 pressures (>1 atm) have a dramatic effect on catalysis, increasing N2 solubility and the thermodynamic N2 binding affinity at Fe0(N2)(P4N2). A combination of high-pressure electrochemistry and variable-temperature UV–vis spectroscopy were used to obtain thermodynamic measurements of N2 binding. In addition, X-ray crystallography, 57Fe Mössbauer spectroscopy, and EPR spectroscopy were used to fully characterize these new compounds. Analysis of Fe0, FeI, and FeII complexes reveals that the free energy of N2 binding across three oxidation states spans more than 37 kcal mol–1.
The pentapyridine cobalt complex Co(PY5Me sub(2)) super(2+) and its congeners have been shown to catalyze proton reduction to hydrogen in aqueous solution over a wide pH range using electrical or ...solar energy input. Here, we employ electrochemical and spectroscopic studies to examine the mechanisms of proton reduction by this parent complex under soluble, diffusion-limited conditions in acetonitrile with acetic acid as the proton donor. Two pathways for proton reduction are identified viacyclic voltammetry: one pathway occurring from an acetonitrile-bound Co super(II/I) couple and the other pathway operating from an acetate-bound Co super(II/I) couple. Kinetics studies support protonation of a Co super(I) species as the rate-determining step for both processes, and additional electrochemical measurements further suggest that the onset of catalysis from the acetonitrile-bound Co super(II/I) couple is highly affected by catalyst electronics. Taken together, this work not only establishes the CoPY5Me sub(2) unit as a unique molecular platform that catalyzes the reduction of protons under soluble, diffusion-limited conditions in both aqueous and organic media, but also highlights the participation of anation processes that are likely relevant for a wide range of hydrogen-producing and related catalytic systems.
The geometric constraints imposed by a tetradentate P
N
ligand play an essential role in stabilizing square planar Fe complexes with changes in metal oxidation state. The square pyramidal Fe
(N
)(P
N
...) complex catalyzes the conversion of N
to N(SiR
)
(R = Me, Et) at room temperature, representing the highest turnover number of any Fe-based N
silylation catalyst to date (up to 65 equiv N(SiMe
)
per Fe center). Elevated N
pressures (>1 atm) have a dramatic effect on catalysis, increasing N
solubility and the thermodynamic N
binding affinity at Fe
(N
)(P
N
). A combination of high-pressure electrochemistry and variable-temperature UV-vis spectroscopy were used to obtain thermodynamic measurements of N
binding. In addition, X-ray crystallography,
Fe Mössbauer spectroscopy, and EPR spectroscopy were used to fully characterize these new compounds. Analysis of Fe
, Fe
, and Fe
complexes reveals that the free energy of N
binding across three oxidation states spans more than 37 kcal mol
.
A series of cerium complexes containing a 2,2′-methylenebis(6-tert-butyl-4-methylphenolate) (MBP2–) ligand framework is described. Electrochemical studies of the compound Li(THF)2Ce(MBP)2(THF)2 (1) ...reveal that the metal based oxidation wave occurs at −0.93 V vs Fc/Fc+. This potential demonstrates significant stabilization of the cerium(IV) ion in the MBP2– framework with a shift of ∼2.25 V from the typically reported value for the cerium(III/IV) couple of E°′ = +1.30 V vs Fc/Fc+ for Ce(ClO4)3 in HClO4 solutions. Compound 1 undergoes oxidation to form stable cerium(IV) species in the presence of a variety of common oxidants. The coordination of the redox-active ligands 2,2′-bipyridine and benzophenone to 1 result in complexes in which no apparent metal-to-ligand charge transfer occurs and the cerium ion remains in the +3 oxidation state.
Triple-Bond Reactivity of Diphosphorus Molecules Piro, Nicholas A; Figueroa, Joshua S; McKellar, Jessica T ...
Science (American Association for the Advancement of Science),
09/2006, Letnik:
313, Številka:
5791
Journal Article
Recenzirano
We report a mild method for generating the diphosphorus molecule or its synthetic equivalent in homogeneous solution; the P₂ allotrope of the element phosphorus is normally obtained only under ...extreme conditions (for example, from P₄ at 1100 kelvin). Diphosphorus is extruded from a niobium complex designed for this purpose and can be trapped efficiently by two equivalents of an organic diene to produce an organodiphosphorus compound. Diphosphorus stabilized by coordination to tungsten pentacarbonyl can be generated similarly at 25°C, and in this stabilized form it still efficiently consumes two organic diene molecules for every diphosphorus unit.
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