Uncovering the Activity of Alkaline Earth Metal Hydrogenation Catalysis Through Molecular Volcano Plots

Recent advances in alkaline earth (Ae) metal hydrogenation catalysis have broadened the spectrum of potential catalysts to include candidates from the main group, providing a sustainable alternative to the commonly used transition metals. Although Ae-amides have already been demonstrated to catalyze hydrogenation of imines and alkenes, a lucid understanding of how different metal/ligand combinations influence the catalytic activity is yet to be established. In this article, we use linear scaling relationships and molecular volcano plots to assess the potential of the Ae metal-based catalysts for the hydrogenation of alkenes. By analyzing combinations of eight metals (mono-, bi-, tri-, and tetravalent) and seven ligands, we delineate the impact of metal-ligand interplay on the hydrogenation activity. Our findings highlight that the catalytic activity is majorly determined by the charge and the size of the metal ions. While bivalent Ae metal cations delicately regulate the binding and the release of the reactants and the products, respectively, providing the right balance for this reaction, ligands play only a minor role in determining their catalytic activity. We show how volcano plots can be utilized for the rapid screening of prospective Ae catalysts to establish a guideline to achieve maximum activity in facilitating the hydrogenation process

Role of alkaline-earth metal in catalysed imine hydrogenations

Alkaline-earth metal (Ae) catalysts have been recently developed for challenging imine and alkene hydrogenation at moderate reaction conditions, providing a sustainable alternative to transition metal catalysis. The understanding of catalytic hydrogenations mediated by group 2 metals is underdeveloped and mechanistic studies are scarce from experimental and computational sides. Herein, we examine the role of the metal on the catalytic hydrogenation of imines by Ae[N(SiMe3)2]2 (Ae = Mg, Ca, Sr, Ba) using state-of-the-art computational techniques. Trends in energy barriers and turnover frequencies agree remarkably well with the experimentally observed increase in catalytic activity upon descending group 2 (Mg ≪ Ca &lt; Sr &lt; Ba). Structural and chemical bonding differences in the key intermediates were found to be the main driving force behind the enhanced reactivity of heavier Ae catalysts. More specifically, the N-Ae-H^ bond angle is drastically reduced in the Ca, Sr, and Ba catalytic species driven by the participation of the d-orbitals in the chemical bonding. The activation strain model reveals that these catalytic reactions are strain controlled and the higher activation barriers for the Mg catalyst originates from unfavourable bond angles in the Mg hydride species featuring linear structures and a more covalent metal-hydride bond. Further decomposition of the interaction energy reveals that stronger repulsive interactions destabilize the Mg species, indicating that the steric congestion due to the small Mg centre impedes reaction kinetics. Overall, the different aspects to be considered in the Ae catalyst design for imine hydrogenations are the strength and flexibility of the Ae-H bond, the bond ionicity, the N-Ae-H^ angle and the strength of the noncovalent interactions in the TOF-determining intermediate.</p

Staging Clavicular Development on MRI: Pitfalls and Suggestions for Age Estimation

BACKGROUND: MRI of the clavicle's sternal end has been studied for age estimation. Several pitfalls have been noted, but how they affect age estimation performance remains unclear. PURPOSE/HYPOTHESIS: To further study these pitfalls and to make suggestions for a proper use of clavicle MRI for forensic age estimation. Our hypotheses were that age estimation would benefit from 1) discarding stages 1 and 4/5; 2) including advanced substages 3aa, 3ab, and 3ac; 3) taking both clavicles into account; and 4) excluding morphological variants. STUDY TYPE: Prospective cross-sectional. POPULATION: Healthy Caucasian volunteers between 11 and 30 years old (524; 277 females, 247 males). FIELD STRENGTH/SEQUENCE: 3T, T1 -weighted gradient echo volumetric interpolated breath-hold examination (VIBE) MR-sequence. ASSESSMENT: Four observers applied the most elaborate staging technique for long bone development that has been described in the current literature (including stages, substages, and advanced substages). One of the observers repeated a random selection of the assessments in 110 participants after a 2-week interval. Furthermore, all observers documented morphological variants. STATISTICAL TESTS: Weighted kappa quantified reproducibility of staging. Bayes' rule was applied for age estimation with a continuation ratio model for the distribution of the stages. According to the hypotheses, different models were tested. Mean absolute error (MAE) differences between models were compared, as were MAEs between cases with and without morphological variants. RESULTS: Weighted kappa equaled 0.82 for intraobserver and ranged between 0.60 and 0.64 for interobserver agreement. Stages 1 and 4/5 were allocated interchangeably in 4.3% (54/1258). Age increased steadily in advanced substages of stage 3, but improvement in age estimation was not significant (right P = 0.596; left P = 0.313). The model that included both clavicles and discarded stages 1 and 4/5 yielded an MAE of 1.97 years, a root mean squared error of 2.60 years, and 69% correctly classified minors. Morphological variants rendered significantly higher MAEs (right 3.84 years, P = 0.015; left 2.93 years, P = 0.022). DATA CONCLUSION: Our results confirmed hypotheses 3) and 4), while hypotheses 1) and 2) remain to be investigated in larger studies. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2020;51:377-388.status: publishe

The influence of motion artefacts on magnetic resonance imaging of the clavicles for age estimation

PURPOSE: To determine how motion affects stage allocation to the clavicle's sternal end on MRI. MATERIALS AND METHODS: Eighteen volunteers (9 females, 9 males) between 14 and 30 years old were prospectively scanned with 3-T MRI. One resting-state scan was followed by five intentional motion scans. Additionally, a control group of 72 resting-state scans were selected from previous research. Firstly, six observers allocated developmental stages to the clavicles independently. Secondly, they re-assessed the images, allocating developmental statuses (immature, mature). Finally, the resting-state scans of the 18 volunteers were assessed in consensus to decide on the "correct" stage/status. Results were compared between groups (control, prospective resting state, prospective motion), and between staging techniques (stages/statuses). RESULTS: Inter-observer agreement was low (Krippendorff α 0.23-0.67). The proportion of correctly allocated stages (64%) was lower than correctly allocated statuses (83%). Overall, intentional motion resulted in fewer assessable images and less images of sufficient evidential value. The proportion of correctly allocated stages did not differ between resting-state (64%) and motion scans (65%), while correctly allocated statuses were more prevalent in resting-state scans (83% versus 77%). Remarkably, motion scans did not render a systematically higher or lower stage/status, compared to the consensus. CONCLUSION: Intentional motion impedes clavicle MRI for age estimation. Still, in case of obvious disturbances, the forensic expert will consider the MRI unsuitable as evidence. Thus, the development of the clavicle as such and the staging technique seem to play a more important role in allocating a faulty stage for age estimation.status: publishe