In arid ecosystems, current-year precipitation often explains only a small proportion of annual aboveground net primary production (ANPP). We hypothesized that lags in the response of ecosystems to ...changes in water availability explain this low explanatory power, and that lags result from legacies from transitions from dry to wet years or the reverse. We explored five hypotheses regarding the magnitude of legacies, two possible mechanisms, and the differential effect of previous dry or wet years on the legacy magnitude. We used a three-year manipulative experiment with five levels of rainfall in the first two years (−80% and −50% reduced annual precipitation (PPT), ambient, +50% and +80% increased PPT), and reversed treatments in year 3. Legacies of previous two years, which were dry or wet, accounted for a large fraction (20%) of interannual variability in production on year 3. Legacies in ANPP were similar in absolute value for both types of precipitation transitions, and their magnitude was a function of the difference between previous and current-year precipitation. Tiller density accounted for 40% of legacy variability, while nitrogen and carry-over water availability showed no effect. Understanding responses to changes in interannual precipitation will assist in assessing ecosystem responses to climate change-induced increases in precipitation variability.
Despite renewed interest in carbon dioxide (CO2) reduction chemistry, examples of homogeneous iron catalysts that hydrogenate CO2 are limited compared to their noble-metal counterparts. Knowledge of ...the thermodynamic properties of iron hydride complexes, including M–H hydricities (ΔG H– ), could aid in the development of new iron-based catalysts. Here we present the experimentally determined hydricity of an iron hydride complex: (SiP iPr 3)Fe(H2)(H), ΔG H– = 54.3 ± 0.9 kcal/mol SiP iPr 3 = Si(o-C6H4PiPr2)3−. We also explore the CO2 hydrogenation chemistry of a series of triphosphinoiron complexes, each with a distinct apical unit on the ligand chelate (Si–, C–, PhB–, N, B). The silyliron (SiPR 3)Fe (R = iPr and Ph) and boratoiron (PhBP iPr 3)Fe (PhBP iPr 3 = PhB(CH2PiPr2)3−) systems, as well as the recently reported (CP iPr 3)Fe (CP iPr 3 = C(o-C6H4PiPr2)3−), are also catalysts for CO2 hydrogenation in methanol and in the presence of triethylamine, generating methylformate and triethylammonium formate at up to 200 TON using (SiPPh 3)FeCl as the precatalyst. Under stoichiometric conditions, the iron hydride complexes of this series react with CO2 to give formate complexes. Finally, the proposed mechanism of the (SiP iPr 3)-Fe system proceeds through a monohydride intermediate (SiP iPr 3)Fe(H2)(H), in contrast to that of the known and highly active tetraphosphinoiron, (tetraphos)Fe (tetraphos = P(o-C6H4PPh2)3), CO2 hydrogenation catalyst.
Despite a well-developed and growing body of work in copper catalysis, the potential of copper to serve as a photocatalyst remains underexplored. Here we describe a photoinduced copper-catalyzed ...method for coupling readily available racemic tertiary alkyl chloride electrophiles with amines to generate fully substituted stereocenters with high enantioselectivity. The reaction proceeds at −40°C under excitation by a blue light-emitting diode and benefits from the use of a single, Earth-abundant transition metal acting as both the photocatalyst and the source of asymmetric induction. An enantioconvergent mechanism transforms the racemic starting material into a single product enantiomer.
The goal of using ammonia as a solar fuel motivates the development of selective ammonia oxidation (AO) catalysts for fuel cell applications. Herein, we describe Fe-mediated AO electrocatalysis with ...(bpyPy2Me)Fe(MeCN)22+, exhibiting the highest turnover number (TON) reported to date for a molecular system. To improve on our recent report of a related iron AO electrocatalyst, (TPA)Fe(MeCN)22+ (TON of 16), the present (bpyPy2Me)Fe(MeCN)22+ system (TON of 149) features a stronger-field, more rigid auxiliary ligand that maintains cis-labile sites and a dominant low-spin population at the Fe(II) state. The latter is posited to mitigate demetalation and hence catalyst degradation by the presence of a large excess of ammonia under the catalytic conditions. Additionally, the (bpyPy2Me)Fe(MeCN)22+ system exhibits a substantially faster AO rate (ca. 50×) at significantly lower (∼250 mV) applied bias compared to (TPA)Fe(MeCN)22+. Electrochemical data are consistent with an initial E 1 net H-atom abstraction step that furnishes the cis amide/ammine complex (bpyPy2Me)Fe(NH2)(NH3)2+, followed by the onset of catalysis at E 2. Theoretical calculations suggest the possibility of N–N bond formation via multiple thermodynamically plausible pathways, including both reductive elimination and ammonia nucleophilic attack. In sum, this study underscores that Fe, an earth-abundant metal, is a promising metal for further development in metal-mediated AO catalysis by molecular systems.
We report here a series of four- and five-coordinate Fe model complexes that feature an axial tri(silyl)methyl ligand positioned trans to a substrate-binding site. This arrangement is used to crudely ...model a single-belt Fe site of the FeMo-cofactor that might bind N ₂ at a position trans to the interstitial C atom. Reduction of a trigonal pyramidal Fe(I) complex leads to uptake of N ₂ and subsequent functionalization furnishes an open-shell Fe–diazenido complex. A related series of five-coordinate Fe–CO complexes stable across three redox states is also described. Spectroscopic, crystallographic, and Density Functional Theory (DFT) studies of these complexes suggest that a decrease in the covalency of the Fe–C ₐₗₖyₗ interaction occurs upon reduction and substrate binding. This leads to unusually long Fe–C ₐₗₖyₗ bond distances that reflect an ionic Fe–C bond. The data presented are contextualized in support of a hypothesis wherein modulation of a belt Fe–C interaction in the FeMo-cofactor facilitates substrate binding and reduction.
Photoinduced, copper-catalyzed coupling reactions are emerging as a powerful method for generating Csp3–Y (Y = C or heteroatom) bonds from alkyl electrophiles and nucleophiles. Corresponding ...three-component couplings of alkyl electrophiles, olefins, and nucleophiles have the potential to generate an additional Csp3–Y bond and to efficiently add functional groups to both carbons of an olefin, which serves as a readily available linchpin. In this report, we establish that a variety of electrophiles and a trifluoromethylthiolate nucleophile can add across an array of olefins (including styrenes and electron-poor olefins) in the presence of CuI/binap and blue-LED irradiation, thereby generating trifluoromethyl thioethers in good yield. The process tolerates a wide range of functional groups, and an initial survey of other nucleophiles (i.e., bromide, cyanide, and azide) suggests that this three-component coupling strategy is versatile. Mechanistic studies are consistent with a photoexcited Cu(I)/binap/SCF3 complex serving as a reductant to generate an alkyl radical from the electrophile, which likely reacts in turn with the olefin and a Cu(II)/SCF3 complex to afford the coupling product.
Carbon-nitrogen (C–N) bond-forming reactions of amines with aryl halides to generate arylamines (anilines), mediated by a stoichiometric copper reagent at elevated temperature (>180°C), were first ...described by Ullmann in 1903. In the intervening century, this and related C–N bond-forming processes have emerged as powerful tools for organic synthesis. Here, we report that Ullmann C–N coupling can be photoinduced by using a stoichiometric or a catalytic amount of copper, which enables the reaction to proceed under unusually mild conditions (room temperature or even -40°C). An array of data are consistent with a single-electron transfer mechanism, representing the most substantial experimental support to date for the viability of this pathway for Ullmann C–N couplings.
Bridging homogeneous molecular systems with heterogeneous catalysts is a promising approach for the development of new electrodes, combining the advantages of both approaches. In the context of CO2 ...electroreduction, molecular enhancement of planar copper electrodes has enabled promising advancement towards high Faradaic efficiencies for multicarbon products. Besides, nanostructured copper electrodes have also demonstrated enhanced performance at comparatively low overpotentials. Herein, we report a novel and convenient method for nanostructuring copper electrodes using N,N′‐ethylene‐phenanthrolinium dibromide as molecular additive. Selectivities up to 70 % for C≥2 products are observed for more than 40 h without significant change in the surface morphology. Mechanistic studies reveal several roles for the organic additive, including: the formation of cube‐like nanostructures by corrosion of the copper surface, the stabilization of these nanostructures during electrocatalysis by formation of a protective organic layer, and the promotion of C≥2 products.
Catalytic cubes: An efficient method for nanostructuring copper electrodes using N,N′‐ethylene‐phenanthrolinium dibromide as a molecular additive is reported. Using such Cu electrodes in electrochemical CO2 reduction produces C≥2 products with selectivities up to 70 % for more than 40 h without significant change in the surface morphology.
Although the alkylation of an amine by an alkyl halide serves as a “textbook example” of a nucleophilic substitution reaction, the selective mono-alkylation of aliphatic amines by unactivated, ...hindered halides persists as a largely unsolved challenge in organic synthesis. We report herein that primary aliphatic amines can be cleanly mono-alkylated by unactivated secondary alkyl iodides in the presence of visible light and a copper catalyst. The method operates under mild conditions (–10 °C), displays good functional-group compatibility, and employs commercially available catalyst components. A trapping experiment with TEMPO is consistent with C–N bond formation via an alkyl radical in an out-of-cage process.
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