This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe2(xdt)(CO)2(PMe3)4, where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2)aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near −950 mV vs Fc+/0. Crystallographic analyses confirm that 1 and 2 adopt C2-symmetric structures (Fe-Fe = 2.616, 2.625 Å, respectively). Low temperature protonation of 1 afforded exclusively [μ-H1]+, establishing the nonintermediacy of the terminal hydride ([t-H1]+). At higher temperatures, protonation afforded mainly [t-H1]+. The temperature dependence of the ratio [t-H1]+/[μ-H1]+ indicates that the barriers for the two protonation pathways differ by ~4 kcal/mol. Low temperature 31P{1H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe2(Hpdt)(CO)2(PMe3)4]+ ([S-H2]+). This intermediate converts to [t)H2]+ and [μ)H2]+ by a first order process (t1/2 ~ 2.5 h, 20 °C). Protonation of the 3 affords exclusively terminal hydrides, regardless of the acid or conditions to give [t-H3]+, which isomerizes to [t-H3′]+ wherein all PMe3 ligands are basal. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2]+) rearranges to the terminal hydride intramolecularly via a low energy pathway.
Direct electrochemical nitrogen reduction holds the promise of enabling the production of carbon emission-free ammonia, which is an important intermediate in the fertilizer industry and a potential green energy carrier. Here we show a strategy for ambient condition ammonia synthesis using a hydrogen permeable nickel membrane/electrode that spatially separates the electrolyte and hydrogen reduction side from the dinitrogen activation and hydrogenation sites. Gaseous ammonia is produced catalytically in the absence of electrolyte via hydrogenation of adsorbed nitrogen by electrochemically permeating atomic hydrogen from water reduction. Dinitrogen activation at the polycrystalline nickel surface is confirmed with 15 N 2 isotope labeling experiments, and it is attributed to a Mars−van Krevelen mechanism enabled by the formation of N-vacancies upon hydrogenation of surface nitrides. We further show that gaseous hydrogen does not hydrogenate the adsorbed nitrogen, strengthening the benefit of having an atomic hydrogen permeable electrode. The proposed approach opens new directions toward green ammonia.
Catalysis in confined spaces, such as provided by supramolecular cages, is quickly gaining momentum. It allows for second coordination sphere strategies to control the selectivity and activity of transition metal catalysts, beyond the classical methods like fine-tuning the steric and electronic properties of the coordinating ligands. Only a few electrocatalytic reactions within cages have been reported, and there is no information regarding the electron transfer kinetics and thermodynamics of redox-active species encapsulated into supramolecular assemblies. This contribution revolves around the preparation of M6L12 and larger M12L24 (M= Pd or Pt) nanospheres functionalized with different numbers of redox-active probes encapsulated within their cavity, either in a covalent fashion via different types of linkers (flexible, rigid and conjugated or rigid and non-conjugated) or by supramolecular hydrogen bonding interactions. The redox-probes can be addressed by electrochemical electron transfer across the rim of nanospheres and the thermodynamics and kinetics of this process are described. Our study identifies that the linker type and the number of redox probes within the cage are useful handles to fine-tune the electron transfer rates, paving the way for the encapsulation of electro-active catalysts and electrocatalytic applications of such supramolecular assemblies.
This paper reports on the protonation of phosphine-substituted diiron diphosphido carbonyls, analogues of diiron dithiolato centers at the active sites of hydrogenase enzymes. Reaction of the diphosphines (CH2) n (PPhH)2 (n = 2 (edpH2) and n = 3 (pdpH2)) with Fe3(CO)12 gave excellent yields of Fe2(edp)(CO)6 (1) and Fe2(pdp)(CO)6 (2). Substitution of Fe2(edp)(CO)6 with PMe3 afforded Fe2(edp)(CO)2(PMe3)4 (3; νCO 1855 and 1836 cm–1). Crystallographic analysis showed that 3 adopts an idealized C 2 symmetry, with pairs of phosphine ligands occupying apical–basal sites on each Fe center. Relative to that in the dithiolato complex, the Fe–Fe bond (2.7786(8) Å) is elongated by 0.15 Å. Treatment of 3 with H(OEt2)2BArF 4 (ArF = C6H3-3,5-(CF3)2) gave exclusively the C 2-symmetric μ-hydride complex [HFe2(edp)(CO)2(PMe3)4]+. This result contrasts with the behavior of the analogous ethanedithiolate Fe2(edt)(CO)2(PMe3)4 (edt = 1,2-C2H4S2), protonation of which gives both the bridging and terminal hydride complexes. This difference points to the participation of the sulfur centers in the formation of terminal hydrides. The absence of terminal hydride intermediates was also revealed in the protonation of the diphosphine diphosphido complexes Fe2(pdp)(CO)4(dppv) (4; dppv = cis-1,2-C2H2(PPh2)2) and Fe2(edp)(CO)4(dppbz) (5; dppbz = 1,2-C6H4(PPh2)2). Protonation of these diphosphine complexes afforded μ-hydrido cations with apical–basal diphosphine ligands, which convert to the isomer where the diphosphine is dibasal. In contrast, protonation of the dithiolato complex Fe2(pdt)(CO)4(dppv) gave terminal hydrides, which isomerize to μ-hydrides. In a competition experiment, 4 was shown to protonate faster than Fe2(pdt)(CO)4(dppv).
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