Surface-bound radicals represent an emerging frontier for controlling the physical and chemical properties of nanomaterials in applications across sensing, electrocatalysis, spintronics, and redox-switchable devices. However, air- and moisture-stable radicals are rare, and harnessing their chemical reactivity for nanomaterial function remains underexplored. Herein, we report the synthesis and characterization of a dianionic cobalt complex featuring a ligand-centered semiquinone radical and outer-sphere pyrene trimethylammonium cations ([Pyr]2[Co]), which can be noncovalently immobilized on single-walled carbon nanotubes (SWCNTs). The resulting hybrid material (SWCNT-[Pyr]2[Co]) features molecularly defined surface functionalization, as established by Raman and X-ray photoelectron spectroscopic characterization in combination with electron microscopy. Electron paramagnetic resonance (EPR) spectroscopy revealed an interaction between the paramagnetic cobalt complex and the SWCNT surface, with radical character persisting in the presence of air and moisture for several months. Electrochemical studies showed that the ligand-centered radical in [Pyr]2[Co] undergoes reversible oxidation at mild potentials, triggering selective chemical reactivity with exogenous cyanide ions (CN-). This feature, in combination with the environmental stability of the radical, was leveraged to demonstrate proof-of-concept electrochemical CN- detection using SWCNT-[Pyr]2[Co]. Overall, our findings establish a remarkably stable radical-nanotube interface and outline a general strategy for constructing hybrid materials with redox-switchable function.

The evolution of active sites in Cu-zeolites for the CH4-to-CH3OH conversion has been investigated during oxidative treatment in O2. Three samples with different frameworks but comparable Cu loadings and Si/Al ratios have been prepared to assess the influence of topology on material oxidizability and the nature of the generated Cu(II) species. Complementary spectroscopic studies highlight that isomeric Cu(II) centers hosted within different topologies are characterized by distinct formation rates. In turn, the framework-specific kinetics of Cu(II) site generation regulate the overall oxidation potential of the individual zeolites. Apart from the topology, the formation rate of different Cu(II) species is governed by their specific structure, with dimeric Cu(II) centers ([Cu2(µ-O)]2+) being generated faster than monomeric ([CuOH]+, Cu2+) ones. Elevated temperatures accelerate the evolution of Cu(II) monomers but cause [Cu2(µ-O)]2+ to undergo autoreduction. The reversibility of this process is framework-dependent. Consequently, even though two types of [Cu2(µ-O)]2+ form at low temperatures in each material, only specific ones remain after high-temperature treatment. The autoreduction of [Cu2(µ-O)]2+ is accompanied by its transient reduction by hydrocarbon residues, originating from the preceding treatment in CH4. The oxidative decomposition of these impurities yields H2O, which adsorbs on [Cu2(µ-O)]2+ masks their spectroscopic fingerprints, and renders them inactive.

Electron nuclear double resonance (ENDOR) spectroscopy is a powerful technique for probing the structure and function of paramagnetic centers via measuring the magnetic interactions of unpaired electrons with nearby nuclear spins. For systems with multiple magnetic nuclei, commonly encountered in transition metal complexes in catalysis or metalloproteins, ENDOR spectra often become very crowded due to the broad, anisotropic hyperfine (HF) and nuclear quadrupole (NQ) interactions in disordered systems. In this work, we substitute the Hahn echo in Davies ENDOR by a chirp echo of the Kunz-Böhlen-Bodenhausen scheme to resolve spectral overlap in a second dimension. Fourier transformation of the chirp echo directly yields an additional EPR dimension without increasing measurement time and reveals correlations between nuclear and electron transitions, thereby resolving spectral overlap, shown here for 1H, 14/15N and 63Cu in ENDOR spectra of the copper protein ScoI. Different influences of interactions along the two dimensions and the possibility for selective excitation to address specific spectral components are exploited to disentangle the small copper NQ-coupling from the large, anisotropic HF-coupling. Simple, efficient frequency-domain simulations reproduce the experimental 2D Chirp Echo Epr SpectroscopY (CHEESY) ENDOR spectra and provide a basis to extract spin Hamiltonian parameters. Limitations and benefits of CHEESY ENDOR are discussed in comparison to established ENDOR techniques, 2D Mims ENDOR and HYEND, which reveals a competitive signal-to-noise ratio for CHEESY ENDOR due to the inherent FT advantage and RF-chirp compatibility to enhance sensitivity. These features expand the scope and feasibility of ENDOR investigations to a wider range of applications.

Cubane-type iron-sulfur clusters (Fe4S4) are some of the most versatile metallocofactors and, as such, among multiple functions, primarily responsible for mediating challenging electron transfers (ETs). Their efficient ET chemistry is enabled by a conflated interplay of cofactor-protein interactions, which can be categorized into the covalent first (1°) sphere ones and the noncovalent second (2°) sphere ones. The latter have remained particularly elusive, as they are difficult to observe and assess directly and independently. Accordingly, our understanding of these effects is hampered by their entangled nature. To address this, we herein leverage a systematic series of synthetic Fe4S4 complexes, which allows spectroscopically investigating 2° sphere electrostatic interactions and covalent 1° sphere interactions separately from one another. We expand the study of 1° sphere interactions with a histidine-type ligand in [Fe4S4]1+ complexes to the [Fe4S4]2+ and [Fe4S4]3+ oxidation states, supporting the notion that 1° sphere interactions "fine-tune" the electronic/magnetic structure of these systems in a manner that persists at ambient temperatures. In contrast, scrutinizing the 2° sphere electric dipolar interactions in [Fe4S4]1+,2+,3+ complexes revealed that although similar effects are observable at extremely low temperatures, no significant alteration of the clusters' gross electronic/magnetic structure persists at the temperatures relevant to enzyme function. These results thus not only systematically catalogue the influence of 1° sphere covalent and 2° sphere electrostatic interactions on the observables and properties of Fe4S4 complexes, but also establish a clear energetic distinction between the two. As such, they will facilitate identifying the elusive 2° sphere interactions in biological systems, while also strengthening our biophysical understanding of structure-function relationships in Fe4S4 cofactors.

Organic redox systems that can undergo oxidative and reductive (ambipolar) electron transfer are elusive yet attractive for applications across synthetic chemistry and energy science. Specifically, the use of ambipolar redox systems in proton-coupled electron transfer (PCET) reactions is largely unexplored but could enable "switchable" reactivity wherein the uptake and release of hydrogen atoms are controlled using a redox stimulus. Here, we describe the synthesis and characterization of an ambipolar functionalized terthiophene (TTH) bearing methyl thioether and phosphine oxide groups that exhibits switchable PCET reactivity. Electrochemical studies established that the functionalized TTH can be reversibly oxidized and reduced, prompting the synthesis and characterization of cationic and anionic radicals on a single TTH platform. Combined structural, spectroscopic, and computational investigations revealed the influence of the methyl thioether and phosphine oxide moieties on the TTH electronic structure that results in the stabilization of both cationic and anionic radicals. Upon single-electron oxidation, the functionalized TTH serves as a hydrogen atom acceptor and undergoes PCET with 1,4-dihydroquinone to generate a TTH hydroxyphosphonium species. The process was found to be reversible upon single-electron reduction, with functionalized TTH acting as a hydrogen atom donor in a PCET reaction with 2,3-dimethylanthraquinone. The thermochemistry of the O-H bond formed and cleaved in functionalized TTH during the reaction sequence was investigated, revealing that a bond weakening of 30 kcal/mol underpins the switchable PCET reactivity. Overall, these studies provide an electrochemical, structural, spectroscopic, and thermochemical foundation for the use of ambipolarity to control PCET reactions in organic redox systems.

Regioselective hydroaminoalkylation of alkenes via α-C-H bond activation of alkylamines is an efficient process for the preparation of complex alkylamines minimizing stoichiometric waste. Herein, we report that a combination of Cp*TiMe3 and AlMe3 catalyzes the branch-selective hydroaminoalkylation of 1-alkenes, including styrene derivatives and 1,3-dienes, with N-methylaniline derivatives. Kinetic studies reveal that the active species are generated from in situ generated Cp*TiMe2(NMePh) and alkylaluminum. Continuous wave (CW) and pulse EPR spectroscopy show that multiple Ti(III) species, bearing amido and most probably alkyl ligands as well as an Al center, are formed, paralleling catalytic activity. Based on these findings complemented by DFT studies, we propose a reaction mechanism featuring d1 Ti(III) three-membered azatitanacycle species with amidoaluminate anions as key reaction intermediates, where alkene insertion into the Ti-C bond of the three-membered metallacycle intermediate drives selectivity. This step favors the branched over the linear product, which may stem from differences in the spin delocalization from the metal to the alkene antibonding orbital for the corresponding transition states.

Electron-nuclear double resonance (ENDOR) spectroscopy is an EPR technique to detect the nuclear frequency spectra of hyperfine coupled nuclei close to paramagnetic centers, which have interactions that are not resolved in continuous wave EPR spectra and may be fast relaxing on the timescale of NMR. For the common case of non-crystalline solids, such as powders or frozen solutions of transition metal complexes, the anisotropy of the hyperfine and nuclear quadrupole interactions renders ENDOR lines often several megahertz (MHz) broad, thus diminishing intensity. With commonly used ENDOR pulse sequences, only a small fraction of the NMR/ENDOR line is excited with a typical radiofrequency (RF) pulse length of several tens of microseconds ( µ s), and this limits the sensitivity in conventional ENDOR experiments. In this work, we show the benefit of chirped RF excitation in frequency-domain ENDOR as a simple yet effective way to significantly improve sensitivity. We demonstrate on a frozen solution of Cu(II)-tetraphenylporphyrin that the intensity of broad copper and nitrogen ENDOR lines increases up to 9-fold compared to single-frequency RF excitation, thus making the detection of metal ENDOR spectra more feasible. The tunable bandwidth of the chirp RF pulses allows the operator to optimize for sensitivity and choose a tradeoff with resolution, opening up options previously inaccessible in ENDOR spectroscopy. Also, chirp pulses help to reduce RF amplifier overtones, since lower RF powers suffice to achieve intensities matching conventional ENDOR. In 2D triple resonance experiments (TRIPLE), the signal increase exceeds 10 times for some lines, thus making chirped 2D TRIPLE experiments feasible even for broad peaks in manageable acquisition times.

Metal-organic frameworks (MOFs) offer a tunable platform for integrating functional spin centers into porous architectures with potential applications in catalysis and quantum sensing. Here, we identify a stable radical intrinsic to the aminoterephthalic acid linker in UiO-66-NH2. Combining multifrequency EPR and DFT, we unambiguously assign the long-misinterpreted EPR spectrum of UiO-66-NH2 to this linker-centered NH center dot spin center. The radical can pre-exist in the linker precursor and remains stabilized within the MOF. At the same time, we explore the possibility of introducing this spin center postsynthetically. We examine its electronic structure and coupling to the nuclear spin environment in as-synthesized UiO-66-NH2, underlining a new layer of quantum functionality in this widely used MOF. As a linker-based, chemically accessible species, this spin center could be introduced in other MOFs, offering a new platform for spin-defect engineering in catalysis, photophysics, and quantum sensing.

The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of its active sites. Based on a combination of IR, EPR spectroscopies, labelling experiments, and DFT modelling, we identified monomeric surface-supported Cr(III) hydrides, (≡SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involves a C-H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(III) alkyl species that insert ethylene through a Cossee-Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analogue, CpCr(CH(SiMe3)2)2 grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure as the UC catalyst, further supporting the proposed active site structure.

Cu- and Fe-exchanged zeolites have been widely investigated for their applicability in selective partial oxidation of CH4 and abatement of environmentally harmful nitrogen oxides. However, the differentiation between spectator and active sites is cumbersome due to their dynamic co-existence, which aggravates the elucidation of the redox dynamics of the latter. Therefore, correlated multi-spectroscopic approaches carried out under operando conditions are needed in order to disentangle the inherent reactivity of specific species. Here we describe the conceptualization of complementary operando spectroscopic methods for the investigation of ion-exchanged zeolites employed in redox reactions. The potential and versatility of this approach are demonstrated by means of two exemplary case studies. First, we present the insight generated by two custom operando ultraviolet-visible and electron paramagnetic resonance spectroscopy setups operating in batch mode into CH4-to-CH3OH conversion over Cu-exchanged mordenite. In addition to recording the method-dependent spectroscopic fingerprints of distinct Cu centers, the overall CH4 consumption is simultaneously monitored in both setups. The extracted apparent activation energies of the reactive centers obtained from the two systems are in good agreement with each other allowing to link the bulk reactivity to the intrinsic site-specific kinetics of all Cu centers. Second, the EPR setup is equipped with a flow reactor designed to carry out modulated excitation experiments with phase-sensitive detection in order to improve the signal-to-noise ratio and time resolution. The cell is employed to study the conversion of NO via selective catalytic reduction over Fe-exchanged ferrierite. In combination with operando X-ray absorption spectroscopy, a small portion of active Fe species consisting of monomers in gamma-positions and oligomeric structures located in the main channel is identified, which are kept in a redox active state under relevant reaction conditions and thus sustain the chemical transformation of NO.

Hybrid methylammonium (MA) lead halide perovskites have emerged as materials exhibiting excellent photovoltaic performance related to their rich structural and dynamic properties. Here, we use multifrequency (X-, Q-, and W-band) electron paramagnetic resonance (EPR) spectroscopy of Mn2+ impurities in MAPbCl(3) to probe the structural and dynamic properties of both the organic and inorganic sublattices of this compound. The temperature dependent continuous-wave (CW) EPR experiments reveal a sudden change of the Mn2+ spin Hamiltonian parameters at the phase transition to the ordered orthorhombic phase indicating its first-order character and significant slowing down of the MA cation reorientation. Pulsed EPR experiments are employed to measure the temperature dependences of the spin-lattice relaxation T-1 and decoherence T-2 times of the Mn2+ ions in the orthorhombic phase of MAPbCl(3) revealing a coupling between the spin center and vibrations of the inorganic framework. Low-temperature electron spin echo envelope modulation (ESEEM) experiments of the protonated and deuterated MAPbCl(3) analogues show the presence of quantum rotational tunneling of the ammonium groups, allowing to accurately probe their rotational energy landscape.

Thanks to recent advances in enzyme repurposing, hemoproteins have gained significant attention as versatile biocatalysts that catalyze a variety of transformations, ranging from oxidation to redox-neutral reactions. To complement these achievements, we report herein on our efforts to repurpose myoglobin (Mb) into an asymmetric ketoreductase, using PhSiH3 as reductant. Two rounds of mutagenesis afforded a double mutant capable of reducing with high enantioselectivity a broad range of prochiral aliphatic and aromatic ketones in the presence of whole cells. Additional rounds of directed evolution afforded a quintuple mutant with opposite enantioselectivity. Mechanistic investigations suggest that a fleeting Fe–H species undergoes heterolytic hydride transfer to afford enantiopure alcohols from the corresponding ketones. The excellent saturation kinetic profile, combined with the practicality of whole-cell biocatalysis under aerobic conditions, highlights the potential of repurposed Mb as an asymmetric ketoreductase with a broad substrate scope, thus expanding the reaction repertoire catalyzed by hemoproteins.

The CH4 oxidation performance of Cu-chabazite zeolites characterized by distinct Si/Al ratios and Cu loadings has been studied and the observed variations in reactivity have been correlated to the differences in the nature of the formed active centers. Plug flow reactor tests, in situ Fourier-transform infrared, and X-ray absorption spectroscopy demonstrate that a decrease in Cu loading shifts the reactivity/redox profile to higher temperatures and increases the CH3OH selectivity and Cu-efficiency. In situ electron paramagnetic resonance, Raman, ultraviolet-visible, Fourier-transform infrared, and photoluminescence spectroscopies reveal that this behavior is associated with the presence of monomeric Cu active sites, including bare Cu2+ and [CuOH]+ present at low Si/Al ratio and Cu loading. Formation of two distinct [Cu2(μ-O)]2+ moieties at higher Si/Al ratio or Cu loading forces these trends into the opposite direction. Operando electron paramagnetic resonance and ultraviolet-visible spectroscopy show that the apparent activation energy of monomeric Cu active species decreases with increasing Si/Al ratio, whereas the one of dimeric centers is unaffected.

The ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry (SOMC), EPR spectroscopy, and machine learning-supported XAS studies, we have studied the electronic structure of well-defined silica-supported Cr(III) alkyls, and identified the presence of several surface species from high to low spin Cr(III), associated with different coordination environments. Notably, low-spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre-two-electron Yb-γ-Me-β–Si secondary metal-ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb-carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.

Continuous-wave electron paramagnetic resonance (EPR) spectroscopy at 35 GHz is an essential cornerstone in multi-frequency EPR studies and is crucial for differentiating multiple species in complex systems due to the improved g -tensor resolution compared to lower microwave frequencies. Especially for unstable and highly sensitive paramagnetic centers, the reliability of the measurements can be improved upon through the use of a single sample for EPR experiments at all frequencies. Besides the advantages, the lack of common availability of oversized-sample resonators at 35 GHz often limits scientists to lower frequencies or smaller sample geometries, and the latter may be non-trivial for sensitive materials. In this work, we present the design and performance of an oversized-sample 35 GHz EPR resonator with a high loaded Q  value, Q L , of up to 2550, well-suited for continuous-wave EPR and pulsed single-microwave-frequency experiments. The design is driven by electromagnetic field simulations, and the microwave characteristics of manufactured prototypes were found to be in agreement with the predictions. The resonator is based on a cylindrical cavity with a TE 011 mode, allowing for 3 mm sample access. The design targets that we met include high sensitivity, robustness, and ease of manufacturing and maintenance. The resonator is compatible with commercial EPR spectrometers and with helium flow, as well as with cryogen-free cryostats, allowing for measurements at temperatures down to 1.8 K. To highlight the general applicability, the resonator was tested on metal centers, as well as on organic radicals featuring extremely narrow lines.

Higher magic angle spinning (MAS) frequencies than currently available are desirable to improve spectral resolution in NMR and EPR systems. While conventional strategies employ pneumatic spinning limited by fluid dynamics, this paper demonstrates the development of an optical spinning technique in which vacuum quality dictates the maximum achievable spinning frequency. Using optical traps, we levitated a range of micron-sized samples. Under vacuum we achieved optical rotation of a single 10 mu m diameter particle of vaterite at several mbar up to hundreds of Hz and of 20 mu m diameter SiO2 particles at <= 10(- 2) mbar at several kHz. At ambient conditions, we optically levitated gamma-irradiated alanine particles of 20-50 mu m diameter. Additionally, using a single chip EPR detector operating at 11 GHz, we measured the EPR spectrum for a 30 mu m gamma-irradiated alanine particle in contact with the chip surface (i.e., without optical levitation) in a single scan lasting 92 s. These observations suggest that a gamma-irradiated alanine particle having a diameter in the order of 30 mu m is a promising candidate for our aim of demonstrating the first magnetic resonance experiment on optically levitated samples. Furthermore, we discuss strategies, limitations, and the potential of implementing MAS with optical traps for NMR and EPR.

To characterize structure and molecular order in the nanometre range, distances between electron spins and their distributions can be measured via dipolar spin–spin interactions by different pulsed electron paramagnetic resonance experiments. Here, for the single-frequency technique for refocusing dipolar couplings (SIFTER), the buildup of dipolar modulation signal and intermolecular contributions is analysed for a uniform random distribution of monoradicals and biradicals in frozen glassy solvent by using the product operator formalism for electron spin S=1/2. A dipolar oscillation artefact appearing at both ends of the SIFTER time trace is predicted, which originates from the weak coherence transfer between biradicals. The relative intensity of this artefact is predicted to be temperature independent but to increase with the spin concentration in the sample. Different compositions of the intermolecular background are predicted in the case of biradicals and in the case of monoradicals. Our theoretical account suggests that the appropriate procedure of extracting the intramolecular dipolar contribution (form factor) requires fitting and subtracting the unmodulated part, followed by division by an intermolecular background function that is different in shape. This scheme differs from the previously used heuristic background division approach. We compare our theoretical derivations to experimental SIFTER traces for nitroxide and trityl monoradicals and biradicals. Our analysis demonstrates a good qualitative match with the proposed theoretical description. The resulting perspectives for a quantitative analysis of SIFTER data are discussed.

Cu-exchanged mordenite (MOR) is a promising material for partial CH4 oxidation. The structural diversity of Cu species within MOR makes it difficult to identify the active Cu sites and to determine their redox and kinetic properties. In this study, the Cu speciation in Cu-MOR materials with different Cu loadings has been determined using operando electron paramagnetic resonance (EPR) and operando ultraviolet-visible (UV/Vis) spectroscopy as well as in situ photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy. A novel pathway for CH4 oxidation involving paired [CuOH]+ and bare Cu2+ species has been identified. The reduction of bare Cu2+ ions facilitated by adjacent [CuOH]+ demonstrates that the frequently reported assumption of redox-inert Cu2+ centers does not generally apply. The measured site-specific reaction kinetics show that dimeric Cu species exhibit a faster reaction rate and a higher apparent activation energy than monomeric Cu2+ active sites highlighting their difference in the CH4 oxidation potential.