Synthesis and Conformational Dynamics of Selenanthrene (Oxides): Establishing an Energetic Flexibility Index for Scaffolds.

The use of new dynamic scaffolds for constructing inorganic and organometallic complexes with enhanced reactivities is an important new research direction. Toward this fundamental aim, an improved synthesis of the dynamic scaffold selenanthrene, along with its monoxide, trans-dioxide and the previously unknown trioxide, is reported. A discussion of the potential reaction mechanism for selenanthrene is provided, and all products were characterized using 1H, 13C, and 77Se nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray crystallography. The dynamic ring inversion processes (i.e., "butterfly motion") for selenanthrene and its oxides were investigated using variable-temperature 1H NMR and density functional theory calculations. The findings suggest that selenanthrene possesses a roughly equal barrier to inversion as its sulfur analogue, thianthrene. However, selenanthrene oxides evidently possess larger inversion barriers as compared to their sulfur analogues due to the enhanced electrostatic intramolecular interactions inherent between the highly polar selenium-oxygen bond and adjacent C-H moieties. Finally, we propose a quantitative "flexibility index" in deg/(kcal/mol) for various tricyclic scaffolds to provide researchers with a comparative scale of dynamic motion across many different systems.

Semiconductor Band Structure, Symmetry, and Molecular Interface Hybridization for the Chemist.

Merging molecular bonding concepts with semiconductor- and materials-based concepts of band structure is challenging due to the mutually exclusive historical development and notations used in those respective fields: symmetry adapted linear combinations (SALCs) and Mulliken terms for molecules, versus k space and Bloch sums for materials. This lack of commonality brings the issue of hybridization (aka electronic coupling) between molecules and materials to the forefront in many aspects of modern chemical research─including nanocrystal properties, solar energy conversion, and molecular computing. It is thus critical to establish a holistic approach to hybridizing orbital (molecule) and plane-wave (semiconductor/material) systems to better describe symmetry-based molecule|material bonding and the corresponding symmetry-adapted molecular orbital (MO) diagrams. Such a unified approach would enable the construction of testable hypotheses about the role of symmetry and electronic structure in determining the extent of electronic coupling between molecular orbitals and semiconductor band structure. This Perspective provides an analysis and compendium of "translations" between the physics and chemistry language of group theory. In this vein, this approach describes the symmetries─and corresponding point groups─that occur in k space along the available descent in symmetry pathways (k space vectors). As a result, chemists may arrive at a more intuitive understanding of the band symmetries of semiconductors, as well as insights into the corresponding algebraic formulations. This analysis can ultimately generate MO diagrams for hybrid molecule|material systems. Lastly, an Outlook provides some context to the application of this analysis to modern problems at the interface of molecular and materials chemistry.

Methylation of Si(111) Modulates Molecular Orientation in Perylenediimide Thin Films.

Singlet fission produces a pair of low-energy spin-triplet excitons from a single high-energy spin-singlet exciton. While this process offers the potential to enhance the efficiency of silicon solar cells by ∼30%, meeting this goal requires overlayer materials that can efficiently transport triplet excitons to an underlying silicon substrate. Herein, we demonstrate that the chemical functionalization of silicon surfaces controls the structure of vapor-deposited thin films of perylenediimide (PDI) dyes, which are prototypical singlet fission materials. Using a combination of atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), we find terminating Si(111) with either a thin, polar oxide layer (SiOx) or with hydrophobic methyl groups (Si-CH3) alters the structures of the resulting PDI films. While PDI films grown on SiOx are comprised of small crystalline grains that largely adopt an "edge-on" orientation with respect to the silicon surface, films grown on Si-CH3 contain large grains that prefer to align in a "face-on" manner with respect to the substrate. This "face-on" orientation is expected to enhance exciton transport to silicon. Interestingly, we find that the preferred mode of growth for different PDIs correlates with the space group associated with bulk crystals of these compounds. While PDIs that inhabit a monoclinic (P21/c) space group nucleate films by forming tall and sparse crystalline columns, PDIs that inhabit triclinic (P1̅) space groups afford films comprised of uniform, lamellar PDI domains. The results highlight that silicon surface functionalization profoundly impacts PDI thin film growth, and rational selection of a hydrophobic surface that promotes "face-on" adsorption may improve energy transfer to silicon.

Distal scaffold flexibility accelerates ligand substitution kinetics in manganese(I) tricarbonyls: flexible thianthrene versus rigid anthracene scaffolds.

This work investigates the effect of molecular flexibility on fundamental ligand substitution kinetics in a pair of manganese(I) carbonyls supported by scaffold-based ligands. In previous work, we reported that the planar and rigid, anthracene-based scaffold with two pyridine 'arms' (Anth-py2, 2) serves as a bidentate, cis donor set, akin to a strained bipyridine (bpy). In the present work, we have installed a more flexible and dynamic scaffold in the form of thianthrene (Thianth-py2, 1), wherein the scaffold in the free ligand exhibits a ∼130° dihedral angle in the solid state. Thianth-py2 also exhibits greater flexibility (molecular motion) in solution compared with Anth-py2, as evidenced by longer 1H NMR T1 times Thianthy-py2 (T1 = 2.97 s) versusAnth-py2 (T1 = 1.91 s). Despite the exchange of rigid Anth-py2 for flexible Thianth-py2 in the complexes [(Anth-py2)Mn(CO)3Br] (4) and [(Thianth-py2)Mn(CO)3Br] (3), respectively, nearly identical electronic structures and electron densities were observed at the Mn center: the IR of 3 exhibits features at 2026, 1938 and 1900 cm-1, nearly identical to the features of the anthracene-based congener (4) at 2027, 1936 and 1888 cm-1. Most importantly, we assessed the effect of ligand-scaffold flexibility on reactivity and measured the rates of an elementary ligand substitution reaction. For ease of IR study, the corresponding halide-abstracted, nitrile-bound (PhCN) cations [(Thianth-py2)Mn(CO)3(PhCN)](BF4) (6) and [(Anth-py2)Mn(CO)3(PhCN)](BF4) (8) were generated in situ, and the PhCN → Br- back-reaction was monitored. The more flexible 3 (thianth-based) exhibited ∼3-4× faster ligand substitution kinetics (k25 C = 22 × 10-2 min-1, k0 C = 43 × 10-3 min-1) than the rigid analogue 4 (anth-based: (k25 C = 6.0 × 10-2 min-1, k0 C = 9.0 × 10-3 min-1) on all counts. Constrained angle DFT calculations revealed that despite large changes in the thianthrene scaffold dihedral angle, the bond metrics of 3 about the metal center remain unchanged; i.e. the 'flapping' motion is strictly a second coordination sphere effect. These results suggest that the local environment of molecular flexibility plays a key role in determining reactivity at the metal center, which has essential implications for understanding the reactivity of organometallic catalysts and metalloenzyme active sites. We propose that this molecular flexibility component of reactivity can be considered a thematic 'third coordination sphere' that dictates metal structure and function.

Game-Day Pitch and Throw Count Feasibility Using a Single Sensor to Quantify Workload in Youth Baseball Players.

Background: Pitch count recommendations are used to reduce injury risk in youth baseball pitchers and are based chiefly on expert opinion, with limited scientific support. Furthermore, they only account for pitches thrown against a hitter and do not include the total number of throws on the day a player pitched. Currently, counts are recorded manually. Purpose: To provide a method using a wearable sensor to quantify total throws per game that is compliant with Little League Baseball rules and regulations. Study Design: Descriptive laboratory study. Methods: Eleven male baseball players (age, 10-11 years) from an 11U (players 11 years and younger) competitive travel team were evaluated over a single summer season. An inertial sensor was placed above the midhumerus of the throwing arm and was worn during baseball games across the season. A throw identification algorithm capturing all throws and reporting linear acceleration and peak linear acceleration was used to quantify throwing intensity. Pitching charts were collected and used to verify actual pitches thrown against a hitter in a game versus all other throws identified. Results: A total of 2748 pitches and 13,429 throws were captured. On the day a player pitched, he averaged 36 ± 18 pitches (23%) and 158 ± 106 total throws (pitches in game as well as all warm-up pitches and other throws during game). In comparison, on a day a player did not pitch, he averaged 119 ± 102 throws. Across all pitchers, 32% of all throws were low intensity, 54% were medium intensity, and 15% were high intensity. The player with one of the highest percentages of high-intensity throws did not pitch as their primary position, while the 2 players who pitched most often had the lowest percentages. Conclusion: Total throw count can be successfully quantified using a single inertial sensor. Total throws tended to be higher on days a player pitched compared with regular game days without pitching. Clinical Relevance: This study provides a fast, feasible, and reliable method to obtain pitch and throw counts so that more rigorous research on contributing factors to arm injury in the youth athlete can be achieved.

Reliability of Wearable Sensors for Assessing Gait and Chair Stand Function at Home in People With Knee Osteoarthritis.

OBJECTIVE: To assess the reliability of wearable sensors for at-home assessment of walking and chair stand activities in people with knee osteoarthritis (OA). METHODS: Baseline data from participants with knee OA (n = 20) enrolled in a clinical trial of an exercise intervention were used. Participants completed an in-person laboratory visit and a video conference-enabled at-home visit. In both visits, participants performed walking and chair stand tasks while fitted with 3 inertial sensors. During the at-home visit, participants self-donned the sensors and completed 2 sets of acquisitions separated by a 15-minute break, when they removed and redonned the sensors. Participants completed a survey on their experience with the at-home visit. During the laboratory visit, researchers placed the sensors on the participants. Spatiotemporal metrics of walking gait and chair stand duration were extracted from the sensor data. We used intraclass correlation coefficients (ICCs) and the Bland-Altman plot for statistical analyses. RESULTS: For test-retest reliability during the at-home visit, all ICCs were good to excellent (0.85-0.95). For agreement between at-home and laboratory visits, ICCs were moderate to good (0.59-0.87). Systematic differences were noted between at-home and laboratory data due to faster task speed during the laboratory visits. Participants reported a favorable experience during the at-home visit. CONCLUSION: Our method of estimating spatiotemporal gait measures and chair stand duration function remotely was reliable, feasible, and acceptable in people with knee OA. Wearable sensors could be used to remotely assess walking and chair stand in participant's natural environments in future studies.

Threefold reactivity of a COF-embedded rhenium catalyst: reductive etherification, oxidative esterification or transfer hydrogenation.

The reactivity of the novel Re(I) catalyst [Re(C12Anth-py2)(CO)3Br] is modulated by its interactions with the covalent organic framework (COF) TFB-BD. The complex catalyzes either reductive etherification, oxidative esterification, or transfer hydrogenation depending on its local environment (embedded in TFB-BD, in homogeneous solution or co-incubated with TFB-BD, respectively). The results highlight that COFs can drastically modulate the reactivity of homogeneous catalysts.

Atomic layer deposited Al2O3 as a protective overlayer for focused ion beam preparation of plan-view STEM samples.

Preservation of analyte integrity during focused ion beam (FIB) sample preparation is a significant challenge in the scanning transmission electron microscopy (STEM) characterization of plan-view samples with sensitive surface chemistries. This can preclude the characterization of atomic arrangements, nanoscale surface coverages, and distributions and morphologies of functional molecular materials composed of surface-immobilized metal nanoparticles, clusters or coordination complexes. This work demonstrates effective protection of Pt nanoparticle (NP) morphology through a plan-view FIB lift-out and thinning procedure by encapsulating the sample surface in an Al2O3 overlayer grown by atomic layer deposition (ALD). High-angle annular dark field (HAADF)-STEM analysis was used in concert with energy dispersive X-ray spectroscopy (EDS) to identify and image sub-10 nm features attributed to Pt and to evaluate the distribution of implanted Ga+ (derived from the FIB milling beam). ALD is a mild chemical vapor deposition (CVD) technique that has the capability to generate dense, pinhole-free films with tunable compositions and properties, making this ALD-FIB procedure applicable to many sample architectures for plan-view lamella preparation and STEM analysis.

Inertial Measurement Units and Application for Remote Health Care in Hip and Knee Osteoarthritis: Narrative Review.

BACKGROUND: Measuring and modifying movement-related joint loading is integral to the management of lower extremity osteoarthritis (OA). Although traditional approaches rely on measurements made within the laboratory or clinical environments, inertial sensors provide an opportunity to quantify these outcomes in patients' natural environments, providing greater ecological validity and opportunities to develop large data sets of movement data for the development of OA interventions. OBJECTIVE: This narrative review aimed to discuss and summarize recent developments in the use of inertial sensors for assessing movement during daily activities in individuals with hip and knee OA and to identify how this may translate to improved remote health care for this population. METHODS: A literature search was performed in November 2018 and repeated in July 2019 and March 2021 using the PubMed and Embase databases for publications on inertial sensors in hip and knee OA published in English within the previous 5 years. The search terms encompassed both OA and wearable sensors. Duplicate studies, systematic reviews, conference abstracts, and study protocols were also excluded. One reviewer screened the search result titles by removing irrelevant studies, and 2 reviewers screened study abstracts to identify studies using inertial sensors as the main sensing technology and a primary outcome related to movement quality. In addition, after the March 2021 search, 2 reviewers rescreened all previously included studies to confirm their relevance to this review. RESULTS: From the search process, 43 studies were determined to be relevant and subsequently included in this review. Inertial sensors have been successfully implemented for assessing the presence and severity of OA (n=11), assessing disease progression risk and providing feedback for gait retraining (n=7), and remotely monitoring intervention outcomes and identifying potential responders and nonresponders to interventions (n=14). In addition, studies have validated the use of inertial sensors for these applications (n=8) and analyzed the optimal sensor placement combinations and data input analysis for measuring different metrics of interest (n=3). These studies show promise for remote health care monitoring and intervention delivery in hip and knee OA, but many studies have focused on walking rather than a range of activities of daily living and have been performed in small samples (<100 participants) and in a laboratory rather than in a real-world environment. CONCLUSIONS: Inertial sensors show promise for remote monitoring, risk assessment, and intervention delivery in individuals with hip and knee OA. Future opportunities remain to validate these sensors in real-world settings across a range of activities of daily living and to optimize sensor placement and data analysis approaches.

Synthesis and Magnetic Properties of Antimony-Ligated Co(II) Complexes: Stibines versus Phosphines.

Herein, we test the hypothesis that neutral, heavy-atom stibine donors can increase the extent of spin-orbit coupling on light, 3d transition metal. To this end, we developed a novel synthetic route toward coordinating a paramagnetic 3d metal ion─cobalt(II)─with neutral stibine ligands. Such complexes have not been reported in the literature due to the weak σ donor strength of stibines and the hard-soft mismatch between a 3d metal and a 5p ligand─which herein has been overcome using alkylated Sb donors. Magnetometry of [(SbiPr2Ph)2Co(I)2] (1) reveals that the stibine complex 1 exhibits a higher magnitude D value (D = |24.96| cm-1) than the spectroscopically derived value for the corresponding phosphine complex 3 (D = -13.13 cm-1), indicative of large zero-field splitting. CASSCF/NEVPT2 calculations corroborate the experimental D values for 1 and 3, predicting D = -31.9 and -8.9 cm-1, respectively. A re-examination of magnetic parameters across the entire series [(ER3)2Co(X)2] (E = P → Sb; X = Cl → I) reveals that (i) increasingly heavy pnictogens lead to an increased X-Co-X bond angle, which is correlated with larger magnitude D values, and (ii) for a given X-Co-X bond angle, the D value is always higher in the presence of a heavy pnictogen as compared with a heavy halide. Ab initio ligand field theory calculations for 1 (stibine complex) and 3 (phosphine complex) reveal no substantial differences in spin-orbit coupling (ζ = 479.2, 480.2 cm-1) or Racah parameter (B = 947.5, 943.9 cm-1), an indicator of covalency. Thus, some "heavy atom effect" on the D value beyond geometric perturbation is operative, but its precise mechanism(s) of action remains obscure.

Emerging artificial metalloenzymes for asymmetric hydrogenation reactions.

Artificial metalloenzymes (ArMs) utilize the best properties of homogenous transition metal catalysts and naturally occurring proteins. While synthetic metal complexes offer high tunability and broad-scope reactivity with a variety of substrates, enzymes further endow these complexes with enhanced aqueous stability and stereoselectivity. For these reasons, dozens of ArMs have been designed to perform catalytic asymmetric hydrogenation reactions, and hydrogenase ArMs are, in fact, the oldest class of ArMs. Herein, we report recent advances in the design of hydrogenase ArMs, including (i) the modification of natural [Fe]-hydrogenase by insertion of artificial metallocofactors, (ii) design of a novel ArM system from the tractable and inexpensive protein ß-lactoglobulin to afford a high-performing transfer hydrogenase, and (iii) the design of chimeric streptavidin scaffolds that drastically alter the secondary coordination sphere of previously reported streptavidin/biotin transfer hydrogenase ArMs.

Impact of COVID-19 Pandemic on Physical Activity, Pain, Mood, and Sleep in Adults with Knee Osteoarthritis.

Objective: To examine changes in physical activity, sleep, pain and mood in people with knee osteoarthritis (OA) during the ongoing COVID-19 pandemic by leveraging an ongoing randomized clinical trial (RCT). Methods: Participants enrolled in a 12-month parallel two-arm RCT (NCT03064139) interrupted by the COVID-19 pandemic wore an activity monitor (Fitbit Charge 3) and filled out custom weekly surveys rating knee pain, mood, and sleep as part of the study. Data from 30 weeks of the parent study were used for this analysis. Daily step count and sleep duration were extracted from activity monitor data, and participants self-reported knee pain, positive mood, and negative mood via surveys. Metrics were averaged within each participant and then across all participants for pre-pandemic, stay-at-home, and reopening periods, reflecting the phased re-opening in the state of Massachusetts. Results: Data from 28 participants showed small changes with inconclusive clinical significance during the stay-at-home and reopening periods compared to pre-pandemic for all outcomes. Summary statistics suggested substantial variability across participants with some participants showing persistent declines in physical activity during the observation period. Conclusion: Effects of the COVID-19 pandemic on physical activity, sleep, pain, and mood were variable across individuals with OA. Specific reasons for this variability could not be determined. Identifying factors that could affect individuals with knee OA who may exhibit reduced physical activity and/or worse symptoms during major lifestyle changes (such as the ongoing pandemic) is important for providing targeted healthcare services and management advice towards those that could benefit from it the most.

Scaffold-based [Fe]-hydrogenase model: H2 activation initiates Fe(0)-hydride extrusion and non-biomimetic hydride transfer.

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor. In comparison to other non-scaffolded acyl-containing complexes, the complex described herein retains molecularly well-defined chemistry upon addition of multiple equivalents of exogenous base. Clean deprotonation of the acyl(methylene) C-H bond with a phenolate base results in the formation of a dimeric motif that contains a new Fe-C(methine) bond resulting from coordination of the deprotonated methylene unit to an adjacent iron center. This effective second carbanion in the ligand framework was demonstrated to drive heterolytic H2 activation across the Fe(ii) center. However, this process results in reductive elimination and liberation of the ligand to extrude a lower-valent Fe-carbonyl complex. Through a series of isotopic labelling experiments, structural characterization (XRD, XAS), and spectroscopic characterization (IR, NMR, EXAFS), a mechanistic pathway is presented for H2/hydride-induced loss of the organometallic acyl unit (i.e. pyCH2-C[double bond, length as m-dash]O → pyCH3+C[triple bond, length as m-dash]O). The known reduced hydride species [HFe(CO)4]- and [HFe3(CO)11]- have been observed as products by 1H/2H NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO)4]. The former species (i.e. [HFe(CO)4]-) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate ([TolIm](BArF) = fluorinated imidazolium as hydride acceptor). This work provides mechanistic insight into the reasons for lack of functional biomimetic behavior (hydride transfer) in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase.

Magnetic Alignment of Electrospun Fiber Segments Within a Hydrogel Composite Guides Cell Spreading and Migration Phenotype Switching.

Fibrous extracellular matrix (ECM) proteins provide mechanical structure and adhesive scaffolding to resident cells within stromal tissues. Aligned ECM fibers play an important role in directing morphogenetic processes, supporting mechanical loads, and facilitating cell migration. Various methods have been developed to align matrix fibers in purified biopolymer hydrogels, such as type I collagen, including flow-induced alignment, uniaxial tensile deformation, and magnetic particles. However, purified biopolymers have limited orthogonal tunability of biophysical cues including stiffness, fiber density, and fiber alignment. Here, we generate synthetic, cell-adhesive fiber segments of the same length-scale as stromal fibrous proteins through electrospinning. Superparamagnetic iron oxide nanoparticles (SPIONs) embedded in synthetic fiber segments enable magnetic field induced alignment of fibers within an amorphous bulk hydrogel. We find that SPION density and magnetic field strength jointly influence fiber alignment and identify conditions to control the degree of alignment. Tuning fiber length allowed the alignment of dense fibrous hydrogel composites without fiber entanglement or regional variation in the degree of alignment. Functionalization of fiber segments with cell adhesive peptides induced tendon fibroblasts to adopt a uniaxial morphology akin to within native tendon. Furthermore, we demonstrate the utility of this hydrogel composite to direct multicellular migration from MCF10A spheroids and find that fiber alignment prompts invading multicellular strands to separate into disconnected single cells and multicellular clusters. These magnetic fiber segments can be readily incorporated into other natural and synthetic hydrogels and aligned with inexpensive and easily accessible rare earth magnets, without the need for specialized equipment. 3D hydrogel composites where stiffness/crosslinking, fiber density, and fiber alignment can be orthogonally tuned may provide insights into morphogenetic and pathogenic processes that involve matrix fiber alignment and can enable systematic investigation of the individual contribution of each biophysical cue to cell behavior.

Tuning p-Si(111) Photovoltage via Molecule|Semiconductor Electronic Coupling.

Photoelectrochemical (PEC) device efficiency depends heavily on the energetics and band alignment of the semiconductor|overlayer junction. Exerting energetic control over these junctions via molecular functionalization is an extremely attractive strategy. Herein we report a study of the structure-function relationship between chemically functionalized pSi(111) and the resulting solar fuels performance. Specifically, we highlight the interplay of chemical structure and electronic coupling between the attached molecule and the underlying semiconductor. Covalent attachment of aryl surface modifiers (phenyl, Ph; nitrophenyl, PhNO2; anthracene, Anth; and nitroanthracene, AnthNO2) resulted in high-fidelity surfaces with low defect densities (S < 50 cm/s). Electrochemical characterization of these surfaces in contact with methyl viologen resulted in systematically shifted band edges (up to 0.99 V barrier height) and correspondingly high photoelectrochemical performance (Voc up to 0.43 V vs MV2+) consistent with the introduction of a positive interfacial dipole. We extend this functionalization to HER conditions and demonstrate systematic tuning of the HER Voc using pSi(111)-R|TiO2|Pt architecture. Correlation of the shifts in barrier height with the photovoltage provides evidence for nonideality despite low surface recombination. Critically, DFT calculations of the electronic structure of the organic-functionalized interfaces show that the molecule-based electronic states effectively hybridized with the silicon band edges. A comparison of these interfacial states with their isolated molecular analogues further confirms electronic coupling between the attached molecule and the underlying semiconductor, providing an induced density of interfacial states (IDIS) which decreases the potential drop across the semiconductor. These results demonstrate the delicate interplay between interfacial chemical structure, interfacial dipole, and electronic structure.

Quantifying Throw Counts and Intensities Throughout a Season in Youth Baseball Players: A Pilot Study.

Overuse injuries in youth baseball players due to throwing are at an all-time high. Traditional methods of tracking player throwing load only count in-game pitches and therefore leave many throws unaccounted for. Miniature wearable inertial sensors can be used to capture motion data outside of the lab in a field setting. The objective of this study was to develop a protocol and algorithms to detect throws and classify throw intensity in youth baseball athletes using a single, upper arm-mounted inertial sensor. Eleven participants from a youth baseball team were recruited to participate in the study. Each participant was given an inertial measurement unit (IMU) and was instructed to wear the sensor during any baseball activity for the duration of a summer season of baseball. A throw identification algorithm was developed using data from a controlled data collection trial. In this report, we present the throw identification algorithm used to identify over 17,000 throws during the 2-month duration of the study. Data from a second controlled experiment were used to build a support vector machine model to classify throw intensity. Using this classification algorithm, throws from all participants were classified as being "low," "medium," or "high" intensity. The results demonstrate that there is value in using sensors to count every throw an athlete makes when assessing throwing load, not just in-game pitches.

Single-Step Sulfur Insertions into Iron Carbide Carbonyl Clusters: Unlocking the Synthetic Door to FeMoco Analogues.

The one-step syntheses, X-ray structures, and spectroscopic characterization of synthetic iron clusters, bearing either inorganic sulfides or thiolate with interstitial carbide motifs, are reported. Treatment of iron carbide carbonyl clusters [Fen (µn -C)(CO)m ]x (n=5,6; m=15,16; x=0,-2) with electrophilic sulfur sources (S2 Cl2 , S8 ) results in the formation of several µ4 -S dimers of clusters, and moreover, iron-sulfide-(sulfocarbide) clusters. The core sulfocarbide unit {C-S}4- serves as a structural model for a proposed intermediate in the radical S-adenosyl-L-methionine biogenesis of the M-cluster. Furthermore, the electrophilic sulfur strategy has been extended to provide the first ever thiolato-iron-carbide complex: an analogous reaction with toluylsulfenyl chloride affords the cluster [Fe5 (µ5 -C)(SC7 H7 )(CO)13 ]- . The strategy described herein provides a breakthrough towards developing syntheses of biomimetic iron-sulfur-carbide clusters like FeMoco.

Scaffold-Based Functional Models of [Fe]-Hydrogenase (Hmd): Building the Bridge between Biological Structure and Molecular Function.

The well-known dinuclear [FeFe] and [NiFe] hydrogenase enzymes are redox-based proton reduction and H2 oxidation catalysts. In comparison, the structural and functional aspects of the mononuclear nonredox hydrogenase, known as [Fe]-hydrogenase or Hmd, have been less explored because of the relatively recent crystallographic elucidation of the enzyme active site. Additionally, the synthetic challenges posed by the highly substituted and asymmetric coordination environment of the iron guanylylpyridinol (FeGP) cofactor have hampered functional biomimetic modeling studies to a large extent. The active site contains an octahedral low-spin Fe(II) center with the following coordination motifs: a bidentate acyl-pyridone moiety (C,N) and cysteinyl-S in a facial arrangement; two cis carbonyl ligands; and a H2O/H2 binding site. In [Fe]-hydrogenase, heterolytic H2 activation putatively by the pendant pyridone/pyridonate-O base serving as a proton acceptor. Following H2 cleavage, an intermediate Fe-H species is thought to stereoselectively transfer a hydride to the substrate methenyl-H4MPT+, thus forming methylene-H4MPT. In the past decade, chemists, inspired by the elegant organometallic chemistry inherent to the FeGP cofactor, have synthesized a number of faithful structural models. However, functional systems are still relatively limited and often rely on abiological ligands or metal centers that obfuscate a direct correlation to nature's design.Our group has developed a bioinspired suite of synthetic analogues of Hmd to better understand the effects of structure on the stability and functionality of the Hmd active site, with a special emphasis on using a scaffold-based ligand design. This systematic approach has contributed to a deeper understanding of the unique ligand array of [Fe]-hydrogenase in nature and has ultimately resulted in the first functional synthetic models without the aid of abiological ligands. This Account reviews the reactivity of the functional anthracene-scaffolded synthetic models developed by our group in the context of current mechanistic understanding drawn from both protein crystallography and computational studies. Furthermore, we introduce a novel thermodynamic framework to place the reactivity of our model systems in context and provide an outlook on the future study of [Fe]-hydrogenase synthetic models through both a structural and functional lens.

Non-Catalytic Benefits of Ni(II) Binding to an Si(111)-PNP Construct for Photoelectrochemical Hydrogen Evolution Reaction: Metal Ion Induced Flat Band Potential Modulation.

We report here the remarkable and non-catalytic beneficial effects of a Ni(II) ion binding to a Si|PNP type surface as a result of significant thermodynamic band bending induced by ligand attachment and Ni(II) binding. We unambiguously deconvolute the thermodynamic flat band potentials (VFB) from the kinetic onset potentials (Von) by synthesizing a specialized bis-PNP macrochelate that enables one-step Ni(II) binding to a p-Si(111) substrate. XPS analysis and rigorous control experiments confirm covalent attachment of the designed ligand and its resulting Ni(II) complex. Illuminated J-V measurements under catalytic conditions show that the Si|BisPNP-Ni substrate exhibits the most positive onset potential for the hydrogen evolution reaction (HER) (-0.55 V vs Fc/Fc+) compared to other substrates herein. Thermodynamic flat band potential measurements in the dark reveal that Si|BisPNP-Ni also exhibits the most positive VFB value (-0.02 V vs Fc/Fc+) by a wide margin. Electrochemical impedance spectroscopy data generated under illuminated, catalytic conditions demonstrate a surprising lack of correlation evident between Von and equivalent circuit element parameters commonly associated with HER. Overall, the resulting paradigm comprises a system wherein the extent of band bending induced by metal ion binding is the primary driver of photoelectrochemical (PEC)-HER benefits, while the kinetic (catalytic) effects of the PNP-Ni(II) are minimal. This suggests that dipole and band-edge engineering must be a primary design consideration (not secondary to catalyst) in semiconductor|catalyst hybrids for PEC-HER.

Bioinspired CNP Iron(II) Pincers Relevant to [Fe]-Hydrogenase (Hmd): Effect of Dicarbonyl versus Monocarbonyl Motifs in H2 Activation and Transfer Hydrogenation.

A set of bioinspired carbamoyl CNP pincer complexes are reported that are relevant to [Fe]-hydrogenase (Hmd). The dicarbonyl species [(CNHNNHPR2)Fe(CO)2I] [R = Ph, 1; R = iPr, 2] undergoes ligand deprotonation, resulting in the dearomatized complexes of formulas [(CNHNN=PR2)Fe(CO)2] (5 and 6). The crystal structure and 1H{31P} NMR spectroscopy of the iodide-bound dearomatized species [Na(18-crown-6)][(CNHNN=PPh2)Fe(CO)2I] (7) showed that the deprotonated moiety was the phosphoramine N(H) linkage. Separately, the monocarbonyl complexes [(CNHNNHPR2)Fe(CO)(MeCN)2](BF4) (8 and 9) synthesized, as well as deprotonated and dearomatized in similar fashion. Reactivity studies revealed that the parent dicarbonyl complexes require more forceful conditions for H2 activation, compared with the monocarbonyl complexes. The ligand backbone was not found to participate in H2 activation and H2 → hydride transfer to an organic substrate was not observed in either case. Density functional theory calculations revealed that the higher reactivity of the monocarbonyl complex in H2 splitting could be attributed to its higher affinity for H2. This behavior is attributed to two key points related to the requisite dπ(Fe) → σ*(H2) back-bonding interaction in a conventional M-H2 Kubas interaction: (i) generally, the weaker π donor capacity of the dicarbonyls, and (ii) specifically, the detrimental effect of a strongly π acidic CO ligand (versus weakly π acidic MeCN ligand) trans to the H2 activation site. The higher reactivity of the monocarbonyl complex is also evidenced by the catalytic transfer hydrogenation by monocarbonyl 8, whereas dicarbonyl 1 was ineffective. Overall, the results suggest that Nature uses the dicarbonyl motif in [Fe]-hydrogenase to diminish the interaction between the Fe center and dihydrogen, thereby preventing premature H2 activation prior to substrate (H4MPT+) binding and any resulting nonspecific hydride transfer reactivity.