General considerations. All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with argon gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. 2-MeTHF was degassed by three freeze-pump-thaw cycles, followed by drying over NaK to remove traces of water. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, filtered through an alumina plug, and dried over 3Å molecular sieves prior to use. All reagents were purchased from commercial vendors and used without further purification unless stated otherwise. P2P Ph FeBr2 (3) and P2P Ph57 FeCl2, 1 [H(OEt2)2][BAr F 4] (BAr F 4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), 2 Cp*2Co, 3 and KC8 4 were synthesized following literature procedures. Physical Methods. NMR spectra were recorded at room temperature unless otherwise noted. 1 H, 13 C and 29 Si chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent proton and 13 C resonances as internal standards. 29 Si NMR chemical shifts were determined from 29Si-HMBC two-dimensional spectra 15 N and 31 P and chemical shifts are reported relative to CH3NO2 and 85 % aqueous H3PO4 respectively. Solution phase magnetic measurement were performed by the method of Evans. 5 IR spectra were obtained using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N2 atmosphere. UV-Vis measurements were collected using a Cary 50 instrument with Cary WinUV software. X-band EPR spectra were obtained on a Bruker EMX spectrometer on 2-5 mM solutions prepared as frozen glasses in 2-MeTHF. Samples were collected at powers ranging from 20 µW to 2 mW and modulation amplitudes of 1-5 Gauss. Spectra were simulated using the Easyspin suite of programs with Matlab 2018. Mössbauer spectra were recorded on a spectrometer from SEE Co. operating in the constant acceleration mode in a transmission geometry. Spectra were recorded with the temperature of the sample maintained at 80 K. The sample was kept in an SVT-400 Dewar from Janis. The quoted isomer shifts are relative to the centroid of the spectrum of a metallic foil of α-Fe at room temperature. Data analysis was performed using the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes. Cyclic voltammetry measurements were carried out in a glovebox under an N2 atmosphere in a onecompartment cell using a CH Instruments 600B electrochemical analyzer. A glassy carbon electrode was used as the working electrode and a carbon rod was used as the auxiliary electrode. The reference electrode was AgOTf/Ag in THF isolated by a CoralPor™ frit (obtained from BASi). The ferrocenium/ferrocene couple (Fc + /Fc) was used as an external reference. THF solutions of electrolyte (0.1 M [NBu4][PF6]) and analyte were also prepared under an inert atmosphere. Hydrogen Analysis. The headspace of reaction f...

This study demonstrates the synergies and limits of multiple measurement types for the detection of smectite chemistry and oxidation state on planetary surfaces to infer past geochemical conditions. Smectite clay minerals are common products of water-rock interactions throughout the solar system, and their detection and characterization provides important clues about geochemical conditions and past environments if sufficient information about their composition can be discerned. Here, we synthesize and report on the spectroscopic properties of a suite of smectite samples that span the intermediate compositional range between Fe(II), Fe(III), Mg, and Al end-member species using bulk chemical analyses, X-ray diffraction, Vis/IR reflectance spectroscopy, UV and green-laser Raman spectroscopy, and Mössbauer spectroscopy. Our data show that smectite composition and the oxidation state of octahedral Fe can be reliably identified in the near infrared on the basis of combination and fundamental metal-OH stretching modes between 2.1–2.9 μm, which vary systematically with chemistry. Smectites dominated by Mg or Fe(III) have spectrally distinct fundamental and combination stretches, whereas Al-rich and Fe(II)-rich smectites have similar fundamental minima near 2.76 μm, but have distinct combination M-OH features between 2.24 and 2.36 μm. We show that with expanded spectral libraries that include intermediate composition smectites and both Fe(III) and Fe(II) oxidation states, more refined characterization of smectites from MIR data is now possible, as the position of the 450 cm–1 absorption shifts systematically with octahedral Fe content, although detailed analysis is best accomplished in concert with other characterization methods. Our data also provide the first Raman spectral libraries of smectite clays as a function of chemistry, and we demonstrate that Raman spectroscopy at multiple excitation wavelengths can qualitatively distinguish smectite clays of different structures and can enhance interpretation by other types of analyses. Our sample set demonstrates how X-ray diffraction can distinguish between dioctahedral and trioctahedral smectites using either the (02,11) or (06,33) peaks, but auxiliary information about chemistry and oxidation state aids in specific identifications. Finally, the temperature-dependent isomer shift and quadrupole splitting in Mössbauer data are insensitive to changes in Fe content but reliability differentiates Fe within the smectite mineral structure.

Highly reactive organometallic species that mediate reductive proton-coupled electron transfer (PCET) reactions are an exciting area for development in catalysis, where a key objective focuses on tuning the reactivity of such species. This work pursues ligand-induced activation of a stable organometallic complex toward PCET reactivity. This is studied via the conversion of a prototypical Cp*FeIII–H species, [FeIII(η5-Cp*)­(dppe)­H]+ (Cp* = C5Me5 –, dppe = 1,2-bis(diphenylphosphino)­ethane), to a highly reactive, S = 1/2 ring-protonated endo-Cp*H–Fe relative, triggered by the addition of CO. Our assignment of the latter ring-protonated species contrasts with its previous reported formulation, which instead assigned it as a hypervalent 19-electron hydride, [FeIII(η5-Cp*)­(dppe)­(CO)­H]+. Herein, pulse EPR spectroscopy (1,2H HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [FeI(endo-η4-Cp*H)­(dppe)­(CO)]+. A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe0(exo-η4-Cp*H)­(dppe)­(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFEC–H ≈ 29 kcal mol–1 and 25 kcal mol–1, respectively) cf. BDFEFe–H of 56 kcal mol–1 for [FeIII(η5-Cp*)­(dppe)­H]+. These weak C–H bonds are shown to undergo proton-coupled electron transfer (PCET) to azobenzene to generate diphenylhydrazine and the corresponding closed-shell [FeII(η5-Cp*)­(dppe)­CO]+ byproduct.

We report the characterization of an S = ½ iron π-complex, [Fe(η 6 -IndH)(depe)] + (Ind = Indenide (C9H7 -), depe = 1,2-bis(diethylphosphino)ethane), which results via C-H elimination from a transient Fe III hydride, [Fe(η 3 :η 2 -Ind)(depe)H] + . Owing to weak M-H/C-H bonds, these species undergo proton-coupled electron transfer (PCET) to release H2 through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open-shell πarene complex to have a BDFEC-H value of ≈ 50 kcal mol -1 , roughly equal to the BDFEFe-H of its Fe III -H precursor (ΔG o ≈ 0 between them). Markedly, this reactivity differs from related Fe(η 5 -Cp/Cp*) compounds, for which terminal Fe III -H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ringversus a metal-bound H-atom by way of continuouswave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.

We report the characterization of an S = 1 = 2 iron pcomplex, [Fe(h 6 -IndH)(depe)] + (Ind = Indenide (C 9 H 7 À ), depe = 1,2-bis(diethylphosphino)ethane), whichr esults via CÀH elimination from at ransient Fe III hydride,[ Fe(h 3 :h 2 -Ind)-(depe)H] + .O wing to weak M À H/C À Hb onds,t hese species appear to undergo proton-coupled electron transfer (PCET) to release H 2 through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies,r eveal the open-shell p-arene complex to have aB DFE C-H value of % 50 kcal mol À1 ,r oughly equal to the BDFE Fe-H of its Fe III À Hp recursor (DG8 8 % 0b etween them). Markedly,t his reactivity differs from related Fe(h 5 -Cp/Cp*) compounds,f or whicht erminal Fe III À Hc ations are isolable and have been structurally characterized,h ighlighting the effect of ab enzannulated ring (indene). Overall, this study provides as tructural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines avaluable approach for the differentiation of aring-versus ametal-bound H-atom by wayofcontinuouswave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.