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The biomimetic diiron complex 4-(N2)2, featuring two terminally bound Fe-N2 centers bridged by two hydrides, serves as a model for two possible states along the pathway by which the enzyme nitrogenase reduces N2. One is the Janus intermediate E4(4H), which has accumulated 4[e-/H+], stored as two [Fe-H-Fe] bridging hydrides, and is activated to bind and reduce N2 through reductive elimination (RE) of the hydride ligands as H2. The second is a possible RE intermediate. 1H and 14N 35 GHz ENDOR measurements confirm that the formally Fe(II)/Fe(I) 4-(N2)2 complex exhibits a fully delocalized, Robin-Day type-III mixed valency. The two bridging hydrides exhibit a fully rhombic dipolar tensor form, T ≈ [- t, + t, 0]. The rhombic form is reproduced by a simple point-dipole model for dipolar interactions between a bridging hydride and its "anchor" Fe ions, confirming validity of this model and demonstrating that observation of a rhombic form is a convenient diagnostic signature for the identification of such core structures in biological centers such as nitrogenase. Furthermore, interpretation of the 1H measurements with the anchor model maps the g tensor onto the molecular frame, an important function of these equations for application to nitrogenase. Analysis of the hyperfine and quadrupole coupling to the bound 14N of N2 provides a reference for nitrogen-bound nitrogenase intermediates and is of chemical significance, as it gives a quantitative estimate of the amount of charge transferred between Fe and coordinated N, a key element in N2 activation for reduction.

The substitution of an alkyl electrophile by a nucleophile is a foundational reaction in organic chemistry that enables the efficient and convergent synthesis of organic molecules. Although there has been substantial recent progress in exploiting transition-metal catalysis to expand the scope of nucleophilic substitution reactions to include carbon nucleophiles.sup.1-4, there has been limited progress in corresponding reactions with nitrogen nucleophiles.sup.5-8. For many substitution reactions, the bond construction itself is not the only challenge, as there is a need to control stereochemistry at the same time. Here we describe a method for the enantioconvergent substitution of unactivated racemic alkyl electrophiles by a ubiquitous nitrogen-containing functional group, an amide. Our method uses a photoinduced catalyst system based on copper, an Earth-abundant metal. This process for asymmetric N-alkylation relies on three distinct ligands--a bisphosphine, a phenoxide and a chiral diamine. The ligands assemble in situ to form two distinct catalysts that act cooperatively: a copper/bisphosphine/phenoxide complex that serves as a photocatalyst, and a chiral copper/diamine complex that catalyses enantioselective C-N bond formation. Our study thus expands enantioselective N-substitution by alkyl electrophiles beyond activated electrophiles (those bearing at least one sp- or sp.sup.2-hybridized substituent on the carbon undergoing substitution).sup.8-13 to include unactivated electrophiles. In the presence of three ligands and light, two distinct copper catalysts combine to produce enantioenriched secondary amides from racemic alkyl electrophiles and primary amide nucleophiles., Author(s): Caiyou Chen [sup.1] , Jonas C. Peters [sup.1] , Gregory C. Fu [sup.1] Author Affiliations: (1) Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, USA Main [...]

The ability of certain transition metals to mediate the reduction of N2 to NH3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N2 and mediate its protonation at one or more N atoms to furnish M(N(x)H(y)) species that can be characterized and, in turn, extrude NH3. By contrast, the direct protonation of Fe-N2 species to Fe(N(x)H(y)) products that can be characterized has been elusive. Herein, we show that addition of acid at low temperature to [(TPB)Fe(N2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N-NH2](+), a doubly protonated hydrazido(2-) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N2)][Na(12-crown-4)] can proceed along a distal or "Chatt-type" pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N-N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided.

[((Ar) PMI)Mo(CO)4 ] complexes (PMI=pyridine monoimine; Ar=Ph, 2,6-di-iso-propylphenyl) were synthesized and their electrochemical properties were probed with cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC). The complexes undergo a reduction at more positive potentials than the related [(bipyridine)Mo(CO)4 ] complex, which is ligand based according to IR-SEC and DFT data. To probe the reaction product in more detail, stoichiometric chemical reduction and subsequent treatment with CO2 resulted in the formation of a new product that is assigned as a ligand-bound carboxylate, [( iPr 2PhPMI)Mo(CO)3 (CO2 )](2-) , by NMR spectroscopic methods. The CO2 adduct [( iPr 2PhPMI)Mo(CO)3 (CO2 )](2-) could not be isolated and fully characterized. However, the C-C coupling between the CO2 molecule and the PDI ligand was confirmed by X-ray crystallographic characterization of one of the decomposition products of [( iPr 2PhPMI)Mo(CO)3 (CO2 )](2-) .

In situ FT-IR measurements and electronic structure calculations are reported for the reduction of CO2 catalyzed by the macrocyclic complex [CoIIN4H]2+ (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene). Beginning from the [CoIIN4H]2+ resting state of the complex in wet acetonitrile solution, two different visible light sensitizers with substantially different reducing power are employed to access reduced states. Accessing reduced states of the complex with a [Ru(bpy)3]2+ sensitizer yields an infrared band at 1670 cm-1 attributed to carboxylate, which is also observed for an authentic sample of the one-electron reduced complex [CoN4H(MeCN)]+ in CO2 saturated acetonitrile solution. The results are interpreted based on calculations using the pure BP86 functional that correctly reproduces experimental geometries. Continuum solvation effects are also included. The calculations show that Co is reduced to CoI in the first reduction, which is consistent with experimental d-d spectra of square Co(I) macrocycle complexes. The energy of the CO2 adduct of the one-electron reduced catalyst complex is essentially the same as for [CoN4H(MeCN)]+, which implies that only a fraction of the latter forms an adduct with CO2. By contrast, the calculations indicate a crucial role for redox noninnocence of the macrocyclic ligand in the doubly reduced state, [CoI(N4H) -•], and show that [CoI(N4H) -•] binds partially reduced CO2 fairly strongly. Experimentally accessing [CoI(N4H) -•] with an Ir(bpy)3 sensitizer with greater reducing power closes the catalytic cycle as FT-IR spectroscopy shows CO production. Use of isotopically substituted C18O2 also shows clear evidence for 18O-substituted byproducts from CO2 reduction to CO.

Electrochemical CO2 reduction (CO2R) at low pH is desired for high CO2 utilization; the competing hydrogen evolution reaction (HER) remains a challenge. High alkali cation concentration at a high operating current density has recently been used to promote electrochemical CO2R at low pH. Herein we report an alternative approach to selective CO2R (>70 % Faradaic efficiency for C2+ products, FEC2+) at low pH (pH 2; H3PO4/KH2PO4) and low potassium concentration ([K+]=0.1 M) using organic film‐modified polycrystalline copper (Modified‐Cu). Such an electrode effectively mitigates HER due to attenuated proton transport. Modified‐Cu still achieves high FEC2+ (45 % with Cu foil /55 % with Cu GDE) under 1.0 M H3PO4 (pH≈1) at low [K+] (0.1 M), even at low operating current, conditions where HER can otherwise dominate. [ABSTRACT FROM AUTHOR]