The mechanisms of the few known molecular nitrogen-fixing systems, including nitrogenase enzymes, are of much interest but are not fully understood. We recently reported that Fe-N2 complexes of tetradentate P3E ligands (E = B, C) generate catalytic yields of NH3 under an atmosphere of N2 with acid and reductant at low temperatures. Here we show that these Fe catalysts are unexpectedly robust and retain activity after multiple reloadings. Nearly an order of magnitude improvement in yield of NH3 for each Fe catalyst has been realized (up to 64 equiv NH3 produced per Fe for P3B and up to 47 equiv for P3C) by increasing acid/reductant loading with highly purified acid. Cyclic voltammetry shows the apparent onset of catalysis at the P3BFe-N2/P3BFe-N2− couple and controlled-potential electrolysis of P3BFe+ at −45 °C demonstrates that electrolytic N2 reduction to NH3 is feasible. Kinetic studies reveal first-order rate dependence on Fe catalyst concentration (P3B), consistent with a single-site catalyst model. An isostructural system (P3Si) is shown to be appreciably more selective for hydrogen evolution. In situ freeze-quench Mössbauer spectroscopy during turnover reveals an iron-borohydrido-hydride complex as a likely resting state of the P3BFe-catalyst system. We postulate that HER activity may prevent iron hydride formation from poisoning the P3BFe-system. This idea may be important to consider in the design of synthetic nitrogenases and may also have broader significance given that intermediate metal-hydrides and hydrogen evolution may play a key role in biological nitrogen fixation.

We have recently reported on several Fe catalysts for N2-to-NH3 conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC8) and acid ([H(OEt2)2][BArF4]). Here we show that our original catalyst system, P3BFe, achieves both significantly improved efficiency for NH3 formation (up to 72% for e– delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH3 per Fe site), when employing a significantly weaker combination of reductant (Cp*2Co) and acid ([Ph2NH2][OTf] or [PhNH3][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido–hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system.

Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N, CO, and O is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, PFe, for catalyzing the nitrogen reduction reaction (NRR, N-to-NH conversion) versus HER changes as a function of acid p K. We find that there is a strong correlation between p K and NRR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N reduction (e.g., Fe(NNH) or Fe(NNH)) in the presence of the metallocene reductant Cp*Co. This suggests that the interaction of acid and reductant is playing a critical role in N-H bond-forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N-H bonds critical for NRR. Furthermore, DFT studies also suggest that the observed p K effect on NRR efficiency is attributable to the rate and thermodynamics of Cp*Co protonation by the different anilinium acids. Inclusion of Cp*Co as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv of NH per Fe at -2.1 V vs Fc).

Well-defined molecular catalysts for the reduction of N2 to NH3 with protons and electrons remain very rare despite decades of interest, and are currently limited to systems featuring Mo or Fe. This report details the synthesis of a molecular Co complex that generates superstoichiometric yields of NH3 (>200% NH3 per Co-N2 precursor) via the direct reduction of N2 with protons and electrons. While the NH3 yields reported herein are modest by comparison to previously described Fe and Mo systems, they intimate that other metals are likely to be viable as molecular N2 reduction catalysts. Additionally, comparison of the featured tris(phosphine)borane Co-N2 complex with structurally related Co-N2 and Fe-N2 species shows how remarkably sensitive the N2 reduction performance of potential pre-catalysts are. These studies enable consideration of structural and electronic effects that are likely relevant to N2 conversion activity, including π-basicity, charge state, and geometric flexibility.

This report describes the synthesis and characterization of 1,5-bis-triphenylphosphinegold(I) 1,2,3-triazolate (3((1,5))). The synthesis of the dinuclear complex 3((1,5)) is achieved via an unprecedented inorganic click (iClick) reaction between the metal-azide PPh(3)AuN(3) (1) and the metal-acetylide PPh(3)Au-C≡CPh (2). Characterization of 3((1,5)) includes multinuclear NMR spectroscopy, combustion analysis, and single crystal X-ray crystallography. Experimental characterization is complemented with density-functional-theory (DFT) calculations which indicate the 1,4-isomer 3((1,4)) is less stable by 3.3 kcal mol(-1). The energetic difference lies primarily in the ability of the phenyl group in the 4-position of 3((1,5)) to lie coplanar with the triazolate to create a delocalized π-bonding HOMO orbital.

Metal-azide-metal-acetylide cycloaddition (iClick) reactions to synthesize heterotrimetallics and an unexpected novel tetranuclear gold(I) complex, are described. In addition, a discussion regarding the connection between traditional azide-alkyne cycloaddition reactions and iClick is presented focusing on applications towards linking multiple metal ions.

Synthetic and kinetic experiments designed to probe the mechanism of O(2) activation by the trianionic pincer chromium(III) complex [(t)BuOCO]Cr(III)(THF)(3) (1) (where (t)BuOCO = [2,6-((t)BuC(6)H(3)O)(2)C(6)H(3)](3-), THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward O(2) activation upon dimerization (forming a μ-oxo species) or product inhibition, complex 1 becomes more active toward O(2) activation when dimerized. The product from O(2) activation, [(t)BuOCO]Cr(V)(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3). Complex 3 exists in equilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5, 35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a Cr(IV)-O-Cr(IV) dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first conclusively assigned example of a complex containing a Cr(IV) dimer; its spin Hamiltonian parameters are g(iso) = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O(2) was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O(2). Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh(3) accelerates O(2) activation by forming the five-coordinate complex trans-[(t)BuOCO]Cr(III)(OPPh(3))(2) (4).

The oxygen-atom-transfer (OAT) from [(t)BuOCO]Cr(V)(O)(THF) (2) (where (t)BuOCO = [2,6-C(6)H(3)(6-(t)BuC(6)H(3)O)(2)](3-), THF = tetrahydrofuran) to triphenylphosphine (PPh(3)) in THF produces [(t)BuOCO]Cr(III)(THF)(3) (1) and triphenylphosphine oxide (OPPh(3)) at a rate of 69.5(±1.9) M(-1) s(-1) (22 °C). Identical rate constants were attained when acetonitrile (MeCN) and dichloromethane/THF (CH(2)Cl(2)/THF) were used as solvents. Electron paramagnetic resonance (EPR) data shows that the six-coordinate complex, [(t)BuOCO]Cr(V)(O)(THF)(2) (2a) forms upon addition of THF to 2, suggesting 2a as the active OAT species in THF. Similarly, addition of OPPh(3) has no influence on the rate of OAT, but the addition of triphenylphosphorus ylide (CH(2)PPh(3)) to form [(t)BuOCO]Cr(V)(O)(CH(2)PPh(3)) (4) prevents OAT to PPh(3). In CH(2)Cl(2), a [Cr(IV)](2)(μ-O) intermediate forms during the OAT from 2 to PPh(3). The OAT from {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3) to PPh(3) reveals a zero-order dependence in PPh(3) indicating the dimer must first dissociate prior to OAT. The decay of 3versus time does not follow first-order kinetics due to the formation of a [(t)BuOCO]Cr(III)(THF) species (5) that inhibits the dissociation of 3. The change in concentration of 3versus time during OAT was simulated to obtain approximate rate constants.