Monomeric half-sandwich rhodium hydride complexes are often proposed as intermediates in catalytic cycles, but relatively few such compounds have been isolated and studied, limiting understanding of their properties. Here, we report preparation and isolation of a monomeric rhodium(III) hydride complex bearing the pentamethylcyclopentadienyl (Cp*) and bis(diphenylphosphino)benzene (dppb) ligands. The hydride complex is formed rapidly upon addition of weak acid to a reduced precursor complex, Cp*Rh(dppb). Single-crystal X-ray diffraction data for the [Cp*Rh] hydride, which were previously unavailable for this class of compounds, provide evidence of the direct Rh-H interaction. Complementary infrared spectra show the Rh-H stretching frequency at 1986 cm −1 . In contrast to results with other [Cp*Rh] complexes bearing diimine ligands, treatment of the isolated hydride with strong acid does not result in H 2 evolution. Electrochemical studies reveal that the hydride complex can be reduced only at very negative potentials (ca. −2.5 V vs. ferrocenium/ferrocene), resulting in Rh-H bond cleavage and H 2 generation. These results are discussed in the context of catalytic H 2 generation, and development of design rules for improved catalysts bearing the [Cp*] ligand.

Coordination of alcohols to the single-electron reductant samarium diiodide (SmI 2 ) results in substantial O−H bond weakening, affording potent proton-coupled electron transfer (PCET) reagents. However, poorly defined speciation of SmI 2 in tetrahydrofuran (THF)/alcohol mixtures limits reliable thermodynamic analyses of such systems. Rigorous determination of bond dissociation free energy (BDFE) values in such Sm systems, important to evaluating their reactivity profiles, motivates studies of model Sm systems where contributing factors can be teased apart. Here, a bulky and strongly chelating macrocyclic ligand (( tBu2 ArOH) 2 Me 2 cyclam) maintains solubility, eliminates dimerization pathways, and facilitates clean electrochemical behavior in a well-defined functional model for the PCET reactivity of Sm II with coordinating proton sources. Direct measurement of thermodynamic parameters enables reliable experimental estimation of the BDFEs in 2-pyrrolidone and MeOH complexes of (( tBu2 ArO) 2 Me 2 cyclam)Sm II , thereby revealing exceptionally weak N−H and O−H BDFEs of 27.2 and <24.1 kcal mol −1 , respectively. Expanded thermochemical cycles reveal that this bond weakening stems from the very strongly reducing Sm II center and the formation of strong Sm III −alkoxide (and −pyrrolidonate) interactions in the PCET products. We provide a detailed analysis comparing these BDFE values with those that have been put forward for SmI 2 in THF in the presence of related proton donors. We suggest that BDFE values for the latter systems may in fact be appreciably higher than the system described herein. Finally, protonation and electrochemical reduction steps necessary for the regeneration of the PCET donors from Sm III −alkoxides are demonstrated, pointing to future strategies aimed at achieving (electro)catalytic turnover using Sm II -based PCET reagents.

Inspired by momentum in applications of reductive photoredox catalysis to organic synthesis, photodriven transfer hydrogenations toward deep (>2 e − ) reductions of small molecules are attractive compared to using harsh chemical reagents. Noteworthy in this context is the nitrogen reduction reaction (N 2 RR), where a synthetic photocatalyst system had yet to be developed. Noting that a reduced Hantzsch ester (HEH 2 ) and related organic structures can behave as 2 e − /2 H + photoreductants, we show here that, when partnered with a suitable catalyst (Mo) under blue light irradiation, HEH 2 facilitates delivery of successive H 2 equivalents for the 6 e − /6 H + catalytic reduction of N 2 to NH 3 ; this catalysis is enhanced by addition of a photoredox catalyst (Ir). Reductions of additional substrates (nitrate and acetylene) are also described.

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N 2 reduction reaction (N 2 RR) in particular, parameters that dictate selectivity for either the 6H + /6e − product ammonia (NH 3 ) or the 4H + /4e − product hydrazine (N 2 H 4 ) are poorly understood. To probe this issue, we have developed conditions to invert the selectivity of a tris(phosphino)borane iron catalyst (Fe), with which NH 3 is typically the major product of N 2 R, to instead favor N 2 H 4 as the sole observed fixed-N product (>99:1). This dramatic shift is achieved by replacing moderate reductants and strong acids with a very strongly reducing but weakly acidic Sm II −(2-pyrrolidone) core supported by a hexadentate dianionic macrocyclic ligand (Sm II −PH) as the net hydrogen-atom donor. The activity and efficiency of the catalyst with this reagent remain high (up to 69 equiv of N 2 H 4 per Fe and 67% fixed-N yield per H + ). However, by generating N 2 H 4 as the kinetic product, the overpotential of this Sm-driven reaction is 700 mV lower than that of the mildest reported set of NH 3 -selective conditions with Fe. Mechanistic data support assignment of iron hydrazido(2−) species FeNNH 2 as selectivity-determining: we infer that protonation of FeNNH 2 at N β , favored by strong acids, releases NH 3 , whereas oneelectron reduction to FeNNH 2 − , favored by strong reductants such as Sm II −PH, produces N 2 H 4 via reactivity initiated at N α . Spectroscopic data also implicate a role for Sm III -binding to anionic FeN 2 − (via an Fe−N 2 --Sm III species) with respect to catalytic efficacy.

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Here, the κ 2bis-diphenylphosphinoferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e -. Chemical and electrochemical studies showed that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible Rh II/I process at -0.96 V vs. ferrocenium/ferrocene (Fc +/0 ). This redox manifold was confirmed by isolation of an uncommon Rh(II) species that was characterized by EPR spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs Fc +/0 that corresponds to iron-centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf-centered oxidation drives a dramatic increase in acidity of the Rh-H moiety by 23 pK a units, a reactivity pattern confirmed by in situ 1 H NMR studies. Taken together, these results show that remote oxidation can effectively induce M-H activation and suggest that ligand-centered redox activity could be an attractive feature for design of new systems relying on hydride intermediates.