S-Adenosyl methionine (SAM) is employed as a [4Fe-4S]-bound cofactor in the superfamily of radical SAM (rSAM) enzymes, in which one-electron reduction of the [4Fe-4S]-SAM moiety leads to homolytic cleavage of the S-adenosyl methionine to generate the 5′-deoxyadenosyl radical (5′dAdo•), a potent H-atom abstractor. HydG, a member of this rSAM family, uses the 5′dAdo• radical to lyse its substrate, tyrosine, producing CO and CN that bind to a unique Fe site of a second HydG Fe–S cluster, ultimately producing a mononuclear organometallic Fe-l-cysteine-(CO)2CN complex as an intermediate in the bioassembly of the catalytic H-cluster of [Fe–Fe] hydrogenase. Here we report the use of non-native tyrosine substrate analogues to further probe the initial radical chemistry of HydG. One such non-native substrate is 4-hydroxy phenyl propanoic acid (HPPA) which lacks the amino group of tyrosine, replacing the CαH-NH2 with a CH2 at the C2 position. Electron paramagnetic resonance (EPR) studies show the generation of a strong and relatively stable radical in the HydG reaction with natural abundance and 13C2-HPPA, with appreciable spin density localized at C2. These results led us to try parallel experiments with the more oxidized non-native substrate coumaric acid, which has a C2=C3 alkene substitution relative to HPPA’s single bond. Interestingly, the HydG reaction with the cis-p-coumaric acid isomer led to the trapping of a new radical EPR signal, and EPR studies using cis-p-coumaric acid along with isotopically labeled SAM reveal that we have for the first time trapped and characterized the 5′dAdo• radical in an actual rSAM enzyme reaction, here by using this specific non-native substrate cis-p-coumaric acid. Density functional theory energetics calculations show that the cis-p-coumaric acid has approximately the same C–H bond dissociation free energy as 5′dAdo•, providing a possible explanation for our ability to trap an appreciable fraction of 5′dAdo• in this specific rSAM reaction. The radical’s EPR line shape and its changes with SAM isotopic substitution are nearly identical to those of a 5′dAdo• radical recently generated by cryophotolysis of a prereduced [4Fe-4S]-SAM center in another rSAM enzyme, pyruvate formate-lyase activating enzyme, further supporting our assignment that we have indeed trapped and characterized the 5′dAdo• radical in a radical SAM enzymatic reaction by appropriate tuning of the relative radical free energies via the judicious selection of a non-native substrate.

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to inter-spin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spinspin broadening, iron and nickel atoms were doped into non-magnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites was solvent-exposed (i.e. at an edge, corner, or defect site). These solvent-exposed sites featured an axial ZFS of 0.21 cm -1 when hydrated. The ZFS increased dramatically upon dehydration (to -1.5 cm -1 ), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other ("inert") Fe(III) site maintained an axial ZFS of 0.19-0.20 cm -1 under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (0.30 cm -1 ) in hydrated material, whereas two distinct Ni(II) ZFS values (0.30 and 1.1 cm -1 ) were observed in the dehydrated samples. Although the magnetically-dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g. by simple vacuum drying) is an important step towards identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials.

Water-stable organic mixed valence (MV) compounds have been prepared by the reaction of reduced bis­(imino)­pyridine ligands (I2P) with the trichloride salts of Al, Ga, and In. The coordination of two tridentate ligands to each ion affords octahedral complexes that are accessible with five ligand charge states: [(I2P0)­(I2P–)­M]2+, [(I2P–)2M]+, (I2P–)­(I2P2–)­M, [(I2P2–)2M]−, and [(I2P2–)­(I2P3–)­M]2–, and for M = Al only, [(I2P3–)2M]3–. In solid-state structures, the anionic members of the redox series are stabilized by the intercalation of Na+ cations within the ligands. The MV members of the redox series, (I2P–)­(I2P2–)­M and [(I2P2–)­(I2P3–)­M]2–, show characteristic intervalence transitions, in the near-infrared regions between 6800–7400 and 7800–9000 cm–1, respectively. Cyclic voltammetry (CV), NIR spectroscopic, and X-ray structural studies support the assignment of class II for compounds [(I2P2–)­(I2P3–)­M]2– and class III for M = Al and Ga in (I2P–)­(I2P2–)­M. All compounds containing a singly reduced I2P– ligand exhibit a sharp, low-energy transition in the 5100–5600 cm–1 region that corresponds to a π*−π* transition. CV studies demonstrate that the electron-transfer events in each of the redox series, Al, Ga, and In, span 2.2, 1.4, and 1.2 V, respectively.

A straightforward synthesis of water-soluble silver nanoparticles stabilized by PTA (1,3,5-triaza-7-phosphaadamantane, a water-soluble phosphine ligand) ligands was developed. The nanoparticles were thoroughly characterized by ultraviolet–visible spectroscopy, 31P nuclear magnetic resonance spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The effectiveness of the Ag–PTA nanoparticles as catalysts for the hydration of nitriles to amides in water under mild conditions was explored using a series of substituted benzonitriles and cyanohydrins. In comparison to all previously investigated homogeneous catalysts, the Ag–PTA system excels at cyanohydrin hydration, including acetone cyanohydrin hydration. Cyanohydrins are in equilibrium with small amounts of cyanide, and experiments revealed that the Ag–PTA nanoparticles disassemble in the presence of cyanide. The catalyst solution, which is proposed to contain a soluble Ag­(CN) n 1–n complex (with n likely equal to 2), remained unpoisoned even in the presence of 10 equiv of cyanide. It is suggested that no cyanide poisoning occurs because the Ag­(I) complex is labile. Overall, the Ag–PTA catalyst system (a) is not poisoned by cyanide, (b) catalyzes hydration reactions under mild conditions (in air and at relatively low temperatures), (c) is easily synthesized from cheap starting materials, and (d) can hydrate heteroaromatics in good yields. The recognition of the importance of labile metal cyanide bonding represents an important step forward in catalyst design for improving the catalytic hydration of acetone cyanohydrin.

Electrochemical generation of ammonia (NH3) from nitrogen (N2) using renewable electricity is a desirable alternative to current NH3 production methods, which consume roughly 1 % of the world's total energy use. The use of catalysts to manipulate the required electron and proton transfer reactions with low energy input is also a chemical challenge that requires development of fundamental reaction pathways. This work presents an approach to the electrochemical reduction of N2 into NH3 using a coordination complex of aluminum(III), which facilitates NH3 production at −1.16 V vs. SCE. Reactions performed under 15N2 liberate 15NH3. Electron paramagnetic resonance spectroscopic characterization of a reduced intermediate and investigations of product inhibition, which limit the reaction to sub‐stoichiometric, are also presented.