Changes in aromaticity of spin-crossover complexes: a signature for non-innocent ligands

The influence of the spin state of the metal centre in spin crossover compounds on the aromaticity of the ligands has been investigated for iron(II)tris-bipyridine (Fe(bpy)32+), and Fe(II)(formazanate)2 (as a truncated model and the full phenyl substituted compound). It was found that the aromaticity of the bipyridine ligands is unaffected by changing the spin state of the central iron atom, but that of the formazanate ligands is reduced upon transition to the high-spin state. This change in aromaticity is rationalized using the symmetry selection rules for aromaticity in terms of virtual excitations from occupied to empty orbitals. A further consequence of this loss in aromaticity is a shift to higher energy in the ring vibrations of the formazanate compounds that can be observed in either its IR or Raman spectrum; this prediction has been confirmed here. This change in aromaticity as a consequence of change in spin state can be regarded as an indication for non-innocent ligands.

Personality in speed skaters with skater's cramp: A preliminary cross-sectional study

OBJECTIVE

Skater's cramp is a debilitating disorder in expert speedskaters and recent evidence from muscle and movement studies nominate it is a task-specific dystonia (TSD). Building on these studies we investigated clinical features and personality in skater's cramp, hypothesizing that similar to other TSDs, trait emotionality would be higher in affected skaters.

METHODS

In a cross-sectional study we employed the HEXACO inventory to examine the personality of a cohort of skaters with skater's cramp (n = 26) compared to age, sex, and experience-matched controls (n = 28). Affected skaters were selected based on relevant clinical features important to the diagnosis of TSD.

RESULTS

Sentimentality (a sub-factor of emotionality) was higher in affected skaters, but only in the male population. Extraversion was lower in skaters with skater's cramp. Clinical findings resembled other forms of TSD.

DISCUSSION

Higher sentimentality is in line with previous studies in TSD. Lower Extraversion in affected skaters was an unexpected finding that may be a new feature of skater's cramp and TSD. Due to our small sample size and cross-sectional design, these findings are preliminary, but offer tentative evidence of personality differences in skater's cramp in line with TSD.

Mechanistic studies of the palladium-catalyzed S,O-ligand promoted C-H olefination of aromatic compounds

Pd-catalyzed C-H functionalization reactions of non-directed substrates have recently emerged as an attractive alternative to the use of directing groups. Key to the success of these transformations has been the discovery of new ligands capable of increasing both the reactivity of the inert C-H bond and the selectivity of the process. Among them, a new type of S,O-ligand has been shown to be highly efficient in promoting a variety of Pd-catalyzed C-H olefination reactions of non-directed arenes. Despite the success of this type of S,O-ligand, its role in the C-H functionalization processes is unknown. Herein, we describe a detailed mechanistic study focused on elucidating the role of the S,O-ligand in the Pd-catalyzed C-H olefination of non-directed arenes. For this purpose, several mechanistic tools, including isolation and characterization of reactive intermediates, NMR and kinetic studies, isotope effects and DFT calculations have been employed. The data from these experiments suggest that the C-H activation is the rate-determining step in both cases with and without the S,O-ligand. Furthermore, the results indicate that the S,O-ligand triggers the formation of more reactive Pd cationic species, which explains the observed acceleration of the reaction. Together, these studies shed light on the role of the S,O-ligand in promoting Pd-catalyzed C-H functionalization reactions.

Bipolar Verdazyl Radicals for Symmetrical Batteries: Properties and Stability in All States of Charge

Redox flow batteries based on organic electrolytes are promising energy storage devices, but stable long-term cycling is often difficult to achieve. Bipolar organic charge-storage materials allow the construction of symmetrical flow batteries (i. e., with identical electrolyte composition on both sides), which is a strategy to mitigate crossover-induced degradation. One such class of bipolar compounds are verdazyl radicals, but little is known on their stability/reactivity either as the neutral radical, or in the charged states. Here, we study the chemical properties of a Kuhn-type verdazyl radical (1) and the oxidized/reduced form (1^+/- ). Chemical synthesis of the three redox-states provides spectroscopic characterization data, which are used as reference for evaluating the composition of the electrolyte solutions of an H-cell battery during/after cycling. Our data suggest that, rather than the charged states, the decomposition of the parent verdazyl radical is responsible for capacity fade. Kinetic experiments and DFT calculations provide insight in the decomposition mechanism, which is shown to occur by bimolecular disproportionation to form two closed-shell products (leuco-verdazyl 1H and triazole derivative 2).

Inter-muscular coherence in speed skaters with skater's cramp

INTRODUCTION

Skater's cramp is a career-ending movement disorder in expert speed skaters noted to be a likely task-specific dystonia. In other movement disorders, including task-specific dystonia, studies have found evidence of central dysregulation expressed as higher inter-muscular coherence. We looked at whether inter-muscular coherence was higher in affected skaters as a possible indicator that it is centrally driven, and by extension further evidence it is a task-specific dystonia.

METHODS

In 14 affected and 14 control skaters we calculated inter-muscular coherence in the theta-band in a stationary task where tonic muscle activation was measured at 10%, 20% and 50% of maximum voluntary contraction. Additionally, we calculated wavelet coherence while skating at key moments in the stroke cycle.

RESULTS

Coherence did not differ in the stationary activation task. While skating, coherence was higher in the impacted leg of affected skaters compared to their non-impacted leg, p = .05, η² = 0.031, and amplitude of electromyography correlated with coherence in the impacted leg, p = .009, R²_adjusted = 0.41. A sub-group of severely affected skaters (n = 6) had higher coherence in the impacted leg compared to the left and right leg of controls, p = .02, Cohen's d = 1.59 and p = .01, Cohen's d = 1.63 respectively. Results were less clear across the entire affected cohort probably due to a diverse case-mix.

CONCLUSION

Our results of higher coherence in certain severe cases of skater's cramp is preliminary evidence of a central dysregulation, making the likelihood it is a task-specific dystonia higher.

Neutral Formazan Ligands Bound to the fac-(CO)3Re(I) Fragment: Structural, Spectroscopic, and Computational Studies

Metal complexes with ligands that coordinate via the nitrogen atom of azo (N═N) or imino (C═N) groups are of interest due to their π-acceptor properties and redox-active nature, which leads to interesting (opto)electronic properties and reactivity. Here, we describe the synthesis and characterization of rhenium(I) tricarbonyl complexes with neutral N,N-bidentate formazans, which possess both N═N and C═N fragments within the ligand backbone (Ar¹-NH-N═C(R³)-N═N-Ar⁵). The compounds were synthesized by reacting equimolar amounts of [ReBr(CO)₅] and the corresponding neutral formazan. X-ray crystallographic and spectroscopic (IR, NMR) characterization confirmed the generation of formazan-type species with the structure fac-[ReBr(CO)₃(κ²-N²,N⁴(Ar¹-N¹H-N²═C(R³)-N³═N⁴-Ar⁵))]. The formazan ligand coordinates the metal center in the 'open' form, generating a five-membered chelate ring with a pendant NH arm. The electronic absorption and emission properties of these complexes are governed by the presence of low-lying π*-orbitals on the ligand as shown by DFT calculations. The high orbital mixing between the metal and ligand results in photophysical properties that contrast to those observed in fac-[ReBr(CO)₃(L,L)] species with α-diimine ligands.

Reversible On/Off Switching of Lactide Cyclopolymerization with a Redox-Active Formazanate Ligand

Redox-switching of a formazanate zinc catalyst in ring-opening polymerization (ROP) of lactide is described. Using a redox-active ligand bound to an inert metal ion (Zn²⁺) allows modulation of the catalytic activity by reversible reduction/oxidation chemistry at a purely organic fragment. A combination of kinetic and spectroscopic studies, together with mass spectrometry of the catalysis mixture, provides insight in the nature of the active species and the initiation of lactide ring-opening polymerization. The mechanistic data highlight the key role of the redox-active ligand and provide a rationale for the formation of cyclic polymer.

Blatter Radicals as Bipolar Materials for Symmetrical Redox-Flow Batteries

Redox-active organic molecules are promising charge-storage materials for redox-flow batteries (RFBs), but material crossover between the posolyte and negolyte and chemical degradation are limiting factors in the performance of all-organic RFBs. We demonstrate that the bipolar electrochemistry of 1,2,4-benzotriazin-4-yl (Blatter) radicals allows the construction of batteries with symmetrical electrolyte composition. Cyclic voltammetry shows that these radicals also retain reversible bipolar electrochemistry in the presence of water. The redox potentials of derivatives with a C(3)-CF₃ substituent are the least affected by water, and moreover, these compounds show >90% capacity retention after charge/discharge cycling in a static H-cell for 7 days (ca. 100 cycles). Testing these materials in a flow regime at a 0.1 M concentration of the active material confirmed the high cycling stability under conditions relevant for RFB operation and demonstrated that polarity inversion in a symmetrical flow battery may be used to rebalance the cell. Chemical synthesis provides insight in the nature of the charged species by spectroscopy and (for the oxidized state) X-ray crystallography. The stability of these compounds in all three states of charge highlights their potential for application in symmetrical organic redox-flow batteries.

Manganese(I)-Catalyzed H-P Bond Activation via Metal-Ligand Cooperation

Here we report that chiral Mn(I) complexes are capable of H–P bond activation. This activation mode enables a general method for the hydrophosphination of internal and terminal α,β-unsaturated nitriles. Metal−ligand cooperation, a strategy previously not considered for catalytic H–P bond activation, is at the base of the mechanistic action of the Mn(I)-based catalyst. Our computational studies support a stepwise mechanism for the hydrophosphination and provide insight into the origin of the enantioselectivity.

Three-State Switching of an Anthracene Extended Bis-thiaxanthylidene with a Highly Stable Diradical State

A multistable molecular switching system based on an anthracene-extended bis-thiaxanthylidene with three individually addressable states that can be interconverted by electrochemical, thermal, and photochemical reactions is reported. Besides reversible switching between an open-shell diradical- and a closed-shell electronic configuration, our findings include a third dicationic state and control by multiple actuators. This dicationic state with an orthogonal conformation can be switched electrochemically with the neutral open-shell triplet state with orthogonal conformation, which was characterized by EPR. The remarkably stable diradical shows kinetic stability as a result of a significant activation barrier for isomerization to a more stable neutral closed-shell folded geometry. We ascribe this activation barrier of ΔG^⧧(293 K) = 25.7 kcal mol^-1 to steric hindrance in the fjord region of the overcrowded alkene structure. The folded closed-shell state can be converted back to the diradical state by irradiation with 385 nm. The folded state can also be oxidized to the dicationic state. These types of molecules with multiple switchable states and in particular stable diradicals show great potential in the design of new functional materials such as memory devices, logic gates, and OFETs.

Foot placement variables of pedestrians in community setting during curve walking

BACKGROUND

There is no precise description of changes of gait during curve walking. Research in curve walking is exclusively performed in clinical settings.

RESEARCH QUESTION

Is there a difference in foot placement variables between the inner- and the outer leg during curve walking in a natural environment? And are these differences correlated with time or the curvature of the path?

METHOD

During this observational study, camera footage was shot on a crossing where pedestrians were not aware of being filmed. Participants (n = 21, male, 18-40 yrs) were selected from this video footage. Using the software package "Movieprocessing", the kinematic variables (time, curvature, stride length, step length, step width and relative foot angle (RFA)) were extracted from the collected data. A MANOVA and Pearson correlation test were performed to explore the data.

RESULTS

MANOVA showed no significant differences in stride length and step length between inner- and outer leg. In contrast, a significant difference between the inner (M = 0.06, SD = 0.05) and outer leg (M = 0.10, SD = 0.06, F(20,256) = 3.577, p < .001) for the step width, and the inner (M = 11.72, SD = 7.99) and outer leg (M = 11.30, SD = 8.07, F(20,256) = 4.542, p < .001) for RFA was found. Pearson correlation was significant for curvature and step width for both legs pooled (r = .28, p < .01) and the outer leg (r = .64, p < .01), as well for time and RFA in the inner (r= -.25, p < .01) and outer leg (r = .213, p < .01).

SIGNIFICANCE

This research funds further research in curve walking in natural conditions, since curve walking is found to be performed non-symmetrically and not determined by geometrics but by choice. Foot placement variables change gradually and differently for both legs during walking a curve.

Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions

Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (7). In total, six new compounds (2–7) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (T_1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin (S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex (6a, S = 2) and a rare square-planar structure with an intermediate-spin state (6b, S = 1). SQUID measurements, ⁵⁷Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a. The complex that contains additional OMe donors (7) results in a six-coordinate (NNO)₂Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (T_1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal–ligand π-covalency.

A series of Fe(II) complexes with formazanate ligands are reported, and ligand substituent effects on structure and spin-crossover properties are examined. These ligand modifications allow isolation of compounds with tetrahedral geometries in both low- and high-spin ground states as well as an intermediate-spin square-planar complex. Steric properties, π-stacking interactions, and additional donor substituents lead to a wide range of spin-crossover temperatures (T_1/2) in this class of compounds.

Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Δ_O) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T_1/2 above room temperature (300–368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an “inverted” ligand field, with an approximate “two-over-three” splitting of the d-orbitals and a high degree of metal–ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C–Ar or N–Ar rings, which is ascribed to the opposing effect on metal–ligand σ- and π-bonding.

Cation effects on dynamics of ligand-benzylated formazanate boron and aluminium complexes

The dynamic processes present in ligand-benzylated formazanate boron and aluminium complexes are investigated using variable temperature NMR experiments and lineshape analyses. The observed difference in activation parameters for complexes containing either organic countercations (NBu4+) or alkali cations is rationalized on the basis of a different degree of ion-pairing in the ground state, and the data are in all cases consistent with a mechanism that involves pyramidal inversion at the nitrogens in the heterocyclic ring rather than homolytic N-C(benzyl) bond cleavage.

Formazanate coordination compounds: synthesis, reactivity, and applications

Inorganic complexes of an emerging class of chelating N-donor ligands, formazanates, offer a unique combination of structurally tunable coordination modes, redox activity, and optoelectronic properties.

Improved Upper Limit on the Neutrino Mass from a Direct Kinematic Method by KATRIN

We report on the neutrino mass measurement result from the first four-week science run of the Karlsruhe Tritium Neutrino experiment KATRIN in spring 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are energy analyzed by a high-resolution MAC-E filter. A fit of the integrated electron spectrum over a narrow interval around the kinematic end point at 18.57 keV gives an effective neutrino mass square value of (-1.0_{-1.1}^{+0.9})  eV^{2}. From this, we derive an upper limit of 1.1 eV (90% confidence level) on the absolute mass scale of neutrinos. This value coincides with the KATRIN sensitivity. It improves upon previous mass limits from kinematic measurements by almost a factor of 2 and provides model-independent input to cosmological studies of structure formation.

A chiral self-sorting photoresponsive coordination cage based on overcrowded alkenes

In recent years, increasing efforts have been devoted to designing new functional stimuli-responsive supramolecular assemblies. Here, we present three isomeric supramolecular coordination complexes consisting of a Pd₂L₄ stoichiometry. As shown by NMR, CD and X-ray studies, as well as DFT calculations, these complexes form cage-like structures by chiral self-sorting. Photochromic ligands derived from first generation molecular motors enable light-driven interconversion between the three isomers. Two of the isomers were able to form host–guest complexes opening up new prospects toward stimuli-controlled substrate binding and release.

Hydration of nitriles using a metal-ligand cooperative ruthenium pincer catalyst

The hydration of a broad range of aliphatic and (hetero)aromatic nitriles is reported via catalysis by metal–ligand cooperative Ru pincer complexes under very mild conditions.

Nitrile hydration provides access to amides that are important structural elements in organic chemistry. Here we report catalytic nitrile hydration using ruthenium catalysts based on a pincer scaffold with a dearomatized pyridine backbone. These complexes catalyze the nucleophilic addition of H₂O to a wide variety of aliphatic and (hetero)aromatic nitriles in ^tBuOH as solvent. Reactions occur under mild conditions (room temperature) in the absence of additives. A mechanism for nitrile hydration is proposed that is initiated by metal–ligand cooperative binding of the nitrile.

Highly Selective Single-Component Formazanate Ferrate(II) Catalysts for the Conversion of CO2 into Cyclic Carbonates

The development of new families of active and selective single-component catalysts based on earth-abundant metal is of interest from a sustainable chemistry perspective. In this context, anionic mono(formazanate) iron(II) complexes bearing labile halide ligands, which possess both Lewis acidic and nucleophilic functionalities, have been developed as novel single-component homogeneous catalysts for the reaction of CO₂ with epoxides to produce cyclic carbonates. The influence of the halide ligand and the electronic properties of the formazanate ligand backbone on the catalytic activity are investigated by employing the iron(II) complexes with and without an additional nucleophile. Very high selectivity is achieved towards the formation of the cyclic carbonate products from various terminal and internal epoxides without the need of a cocatalyst.

Aluminum Complexes with Redox-Active Formazanate Ligand: Synthesis, Characterization, and Reduction Chemistry

The synthesis of aluminum complexes with redox-active formazanate ligands is described. Salt metathesis using AlCl₃ was shown to form a five-coordinate complex with two formazanate ligands, whereas organometallic aluminum starting materials yield tetrahedral mono(formazanate) aluminum compounds. The aluminum diphenyl derivative was successfully converted to the iodide complex (formazanate)AlI₂, and a comparison of spectroscopic/structural data for these new complexes is provided. Characterization by cyclic voltammetry is supplemented by chemical reduction to demonstrate that ligand-based redox reactions are accessible in these compounds. The possibility to obtain a formazanate aluminum(I) carbenoid species by two-electron reduction was examined by experimental and computational studies, which highlight the potential impact of the nitrogen-rich formazanate ligand on the electronic structure of compounds with this ligand.

The synthesis of a series of aluminum complexes with redox-active formazanate ligands is described and crystallographic, spectroscopic, and voltammetric characterization data are presented. The reduction chemistry of these newly synthesized complexes has been explored and the results are supported by a computational (DFT) study.

Structure and bonding in reduced boron and aluminium complexes with formazanate ligands

A comparison of structure and bonding in reduced formazanate B/Al complexes and their ligand-benzylated products is described. The kinetics of homolytic N–C(benzyl) bond cleavage in the latter compounds is studied.

Central-to-Helical-to-Axial-to-Central Transfer of Chirality with a Photoresponsive Catalyst

Recent advances in molecular design have displayed striking examples of dynamic chirality transfer between various elements of chirality, e.g., from central to either helical or axial chirality and vice versa. While considerable progress in atroposelective synthesis has been made, it is intriguing to design chiral molecular switches able to provide selective and dynamic control of axial chirality with an external stimulus to modulate stereochemical functions. Here, we report the synthesis and characterization of a photoresponsive bis(2-phenol)-substituted molecular switch 1. The unique design exhibits a dynamic hybrid central-helical-axial transfer of chirality. The change of preferential axial chirality in the biaryl motif is coupled to the reversible switching of helicity of the overcrowded alkene core, dictated by the fixed stereogenic center. The potential for dynamic control of axial chirality was demonstrated by using ( R)-1 as switchable catalyst to direct the stereochemical outcome of the catalytic enantioselective addition of diethylzinc to aromatic aldehydes, with successful reversal of enantioselectivity for several substrates.

Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N-H/N-C Bond Homolysis Due to the Formation of Stable Ligand Radicals

The reactivity of a boron complex with a redox-active formazanate ligand, LBPh₂ [L = PhNNC(p-tol)NNPh], was studied. Two-electron reduction of this main-group complex generates the stable, nucleophilic dianion [LBPh₂]^2–, which reacts with the electrophiles BnBr and H₂O to form products that derive from ligand benzylation and protonation, respectively. The resulting complexes are anionic boron analogues of leucoverdazyls. N–C and N–H bond homolysis of these compounds was studied by exchange NMR spectroscopy and kinetic experiments. The weak N–C and N–H bonds in these systems derive from the stability of the resulting borataverdazyl radical, in which the unpaired electron is delocalized over the four N atoms in the ligand backbone. We thus demonstrate the ability of this system to take up two electrons and an electrophile (E⁺ = Bn⁺, H⁺) in a process that takes place on the organic ligand. In addition, we show that the [2e^–/E⁺] stored on the ligand can be converted to E^• radicals, reactivity that has implications in energy storage applications such as hydrogen evolution.

A boron complex with a redox-active formazanate ligand in its two-electron-reduced state is shown to react with electrophiles (BnBr and H⁺). The resulting “borataleucoverdazyl” products have weak N−C and N−H bonds; homolytic cleavage reactions lead to stable ligand-based radicals. Thus, the accumulation of [2e⁻/E⁺] on the formazanate ligand and conversion to E^• radicals are demonstrated, and their potential relevance in energy-related electrocatalysis (e.g., proton reduction) is discussed.

Oxa-Michael Addition to α,β-Unsaturated Nitriles: An Expedient Route to γ-Amino Alcohols and Derivatives

Water addition to α,β-unsaturated nitriles would give facile access to the β-hydroxy-nitriles, which in turn can be hydrogenated to the γ-amino alcohols. We have previously shown that alcohols readily add in 1,4-fashion to these substrates using Milstein's Ru(PNN) pincer complex as catalyst. However, attempted water addition to α,β-unsaturated nitriles gave the 3-hydroxynitriles in mediocre yields. On the other hand, addition of benzyl alcohol proceeded in excellent yields for a variety of β-substituted unsaturated nitriles. Subsequent treatment of the benzyl alcohol addition products with TMSCl/FeCl3 resulted in the formation of 3-hydroxy-alkylnitriles. The 3-benzyloxy-alkylnitriles obtained from oxa-Michael addition also could be hydrogenated directly in the presence of acid to give the amino alcohols as their HCl salts in excellent yields. Hydrogenation under neutral conditions gave a mixture of the secondary and tertiary amines. Hydrogenation in the presence of base and Boc-anhydride gave the orthogonally bis-protected amino alcohols, in which the benzyl ether can subsequently be cleaved to yield Boc-protected amino alcohols. Thus, a variety of molecular scaffolds with a 1,3-relationship between O- and N-functional group is accessible starting from oxa-Michael addition of benzyl alcohol to α,β-unsaturated nitriles.

Ferrate(ii) complexes with redox-active formazanate ligands

The synthesis of mono(formazanate) iron complexes is described. In the presence of tetrabutylammonium halides, salt metathesis reactions afford the ferrate(ii) complexes [Bu4N][LFeX2] (L = PhNNC(p-tol)NNPh; X = Cl, Br) in good yield, and the products are characterized in detail. The high-spin ferrate(ii) complexes show cyclic voltammograms that are consistent with reversible, ligand-based one-electron reduction. The halides in these ferrate(ii) compounds are labile, and are displaced by 4-methoxyphenyl isocyanide (4 equiv.) as evidenced by formation of the low-spin, cationic octahedral complex [LFe(CNC6H4(p-OMe))4][Br]. Thus, a straightforward route to mono(formazanate) iron(ii) complexes is established.

Desymmetrization of meso-Dibromocycloalkenes through Copper(I)-Catalyzed Asymmetric Allylic Substitution with Organolithium Reagents

The highly regio- and enantioselective (up to >99:1 dr, up to 99:1 er) desymmetrization of meso-1,4-dibromocycloalk-2-enes using asymmetric allylic substitution with organolithium reagents to afford enantioenriched bromocycloalkenes (ring size of 5 to 7) has been achieved. The cycloheptene products undergo an unusual ring contraction. The synthetic versatility of this Cu(I)-catalyzed reaction is demonstrated by the concise stereocontrolled preparation of cyclic amino alcohols, which are privileged chiral structures in natural products and pharmaceuticals and widely used in synthesis and catalysis.

Lewis Acid Catalyzed Conversion of 5-Hydroxymethylfurfural to 1,2,4-Benzenetriol, an Overlooked Biobased Compound

5-Hydroxymethylfurfural (HMF) is a platform chemical that can be produced from renewable carbohydrate sources. HMF can be converted to 1,2,4-benzenetriol (BTO) which after catalytic hydrodeoxygenation provides a route to cyclohexanone and cyclohexanol. This mixture, known as KA oil, is an important feedstock for polymeric products such as nylons which use benzene as feedstock that is obtained from the BTX fraction produced in oil refineries. Therefore, the conversion of HMF to BTO provides a renewable, alternative route toward products such as nylons. However, BTO is usually considered an undesired byproduct in HMF synthesis and is only obtained in small amounts. Here, we show that Lewis acid catalysts can be utilized for the selective conversion of HMF to BTO in subsuper critical water. Overall, up to 54 mol % yield of BTO was achieved at 89% HMF conversion using ZnCl₂. ZnCl₂ and similarly effective Zn(OTf)₂ and Fe(OTf)₂ are known as relatively soft Lewis acids. Other Lewis acid like Hf(OTf)₄ and Sc(OTf)₃ gave increased selectivity toward levulinic acid (up to 33 mol %) instead of BTO, a well-known HMF derivative typically obtained by acid catalysis. Catalytic hydrodeoxygenation of BTO toward cyclohexanone in water was achieved in up to 45% yield using 5 wt % Pd on Al₂O₃ combined with AlCl₃ or Al(OTf)₃ as catalysts. Additionally, a mild selective oxygen induced dimerization pathway of BTO to 2,2',4,4',5,5'-hexahydroxybiphenyl (5,5'-BTO dimer) was identified.

Effects of high heeled shoes on gait. A review

BACKGROUND

Walking in high heels (HH) may alter gait in various ways, which may be of importance to designers and physicians.

RESEARCH QUESTION

How does walking in high heels alter gait and how can this be explained from a biomechanical and control point of view.

METHODS

Relevant literature has been collected in which high heeled walking was studied, after which the results were bundled and interpreted in a framework of biomechanics and control.

RESULTS

Major changes were found in the rollover function of the feet, the ankle and knee joints and the lower back, while step length and balance were compromised. An increase in heel height forces the foot in an increased plantar flexion, which in its turn increases knee flexion and lordosis of the lower back. All changes can be related with each other in a plausible pattern of movement and control.

Metal-ligand cooperative activation of nitriles by a ruthenium complex with a de-aromatized PNN pincer ligand

The pincer complex (PNN)RuH(CO), with a de-aromatized pyridine in the ligand backbone, is shown to react with nitriles in a metal-ligand cooperative manner. This leads to the formation of a series of complexes with new Ru-N(nitrile) and C(ligand)-C(nitrile) bonds. The initial nitrile cycloaddition products, the ketimido complexes 3, have a Brønsted basic (nitrile-derived) Ru-N fragment. This is able to deprotonate a CH₂ side-arm of the pincer ligand to give ketimine complexes (4) with a de-aromatized pyridine backbone. Alternatively, the presence of a CH₂ group adjacent to the nitrile functionality can lead to tautomerization to an enamido complex (5). Variable-temperature NMR studies and DFT calculations provide insight in the relative stability of these compounds and highlight the importance of their facile interconversion in the context of subsequent nitrile transformations.

Palladium alkyl complexes with a formazanate ligand: synthesis, structure and reactivity

The synthesis of formazanate palladium complexes is reported. Ligand-based redox-reactions and insertions of unsaturated substrates into the Pd–CH₃ bond are studied.

Third-Generation Light-Driven Symmetric Molecular Motors

Symmetric molecular motors based on two overcrowded alkenes with a notable absence of a stereogenic center show potential to function as novel mechanical systems in the development of more advanced nanomachines offering controlled motion over surfaces. Elucidation of the key parameters and limitations of these third-generation motors is essential for the design of optimized molecular machines based on light-driven rotary motion. Herein we demonstrate the thermal and photochemical rotational behavior of a series of third-generation light-driven molecular motors. The steric hindrance of the core unit exerted upon the rotors proved pivotal in controlling the speed of rotation, where a smaller size results in lower barriers. The presence of a pseudo-asymmetric carbon center provides the motor with unidirectionality. Tuning of the steric effects of the substituents at the bridgehead allows for the precise control of the direction of disrotary motion, illustrated by the design of two motors which show opposite rotation with respect to a methyl substituent. A third-generation molecular motor with the potential to be the fastest based on overcrowded alkenes to date was used to visualize the equal rate of rotation of both its rotor units. The autonomous rotational behavior perfectly followed the predicted model, setting the stage for more advanced motors for functional dynamic systems.

Asymmetric Synthesis of Second-Generation Light-Driven Molecular Motors

The enantiomeric homogeneity of light-driven molecular motors based on overcrowded alkenes is crucial in their application as either unidirectional rotors or as chiral multistate switches. It was challenging to obtain these compounds as single enantiomers via the established synthetic procedures due to loss of optical purity in the key step, i.e., the Barton–Kellogg olefination reaction. Searching for strategies to avoid racemization, a new class of light-driven molecular motors was designed, synthesized, and studied. The stereochemical integrity was fully preserved throughout the synthesis, and on the basis of photochemical and kinetic studies using UV/vis, CD, and ¹H NMR spectroscopy, it was established that they still function properly as unidirectional molecular motors.

Copper-Catalyzed Enantioselective Alkylation of Enolizable Ketimines with Organomagnesium Reagents

Inexpensive and readily available organomagnesium reagents were used for the catalytic enantioselective alkylation of enolizable N-sulfonyl ketimines. The low reactivity and competing enolization of the ketimines was overcome by the use of a copper-phosphine chiral catalyst, which also rendered the transformation highly chemoselective and enantioselective for a broad range of ketimine substrates.

Arylazoindazole Photoswitches: Facile Synthesis and Functionalization via SNAr Substitution

A straightforward synthetic route to arylazoindazoles via nucleophilic aromatic substitution is presented. Upon deprotonation of the NH group, a C₆F₅-substituted formazan undergoes facile cyclization as a result of intermolecular nucleophilic substitution (S_NAr). This new class of azo photoswitches containing an indazole five-membered heterocycle shows photochemical isomerization with high fatigue resistance. In addition, the Z-isomers have long thermal half-lives in the dark of up to several days at room temperature. The fluorinated indazole group offers a handle for further functionalization and tuning of its properties, as it is shown to be susceptible to a subsequent, highly selective nucleophilic displacement reaction.

Stable, crystalline boron complexes with mono-, di- and trianionic formazanate ligands

Boron complexes are isolated with formazanate ligands in mono-, di- and trianionic form, showing their distinctive ability to function as electron-reservoir.

Stereoselective Synthesis of 1-Tuberculosinyl Adenosine; a Virulence Factor of Mycobacterium tuberculosis

Despite its status as one of the world's most prevalent and deadly bacterial pathogens, Mycobacterium tuberculosis (Mtb) infection is not routinely diagnosed by rapid and highly reliable tests. A program to discover Mtb-specific biomarkers recently identified two natural compounds, 1-tuberculosinyl adenosine (1-TbAd) and N(6)-tuberculosinyl adenosine (N(6)-TbAd). Based on their association with virulence, the lack of similar compounds in nature, the presence of multiple stereocenters, and the need for abundant products to develop diagnostic tests, synthesis of these compounds was considered to be of high value but challenging. Here, a multigram-scale stereoselective synthesis of 1-TbAd and N(6)-TbAd is described. As a key-step, a chiral auxiliary-mediated Diels-Alder cycloaddition was developed, introducing the three stereocenters with a high exo endo ratio (10:1) and excellent enantioselectivity (>98% ee). This constitutes the first entry into the stereoselective synthesis of diterpenes with the halimane skeleton. Computational studies explain the observed stereochemical outcome.

Boron difluorides with formazanate ligands: redox-switchable fluorescent dyes with large stokes shifts

The synthesis of a series of (formazanate)boron difluorides and their 1-electron reduction products is described.

Folic acid supplementation influences the distribution of neural tube defect subtypes: A registry-based study

Periconceptional folic acid (FA) reduces neural tube defect (NTD) risk, but seems to have a varying effect per NTD subtype. We aimed to study the effect of FA supplementation on NTD subtype distribution using data from EUROCAT Northern Netherlands. We included all birth types with non-syndromal NTDs born in 1997-2012. By Fisher's exact test we analyzed possible differences in NTD subtype distribution between a correct FA supplementation group and incorrect FA supplementation group. We found proportionally fewer cervical/thoracic spina bifida cases and more lumbar/sacral spina bifida cases in the correct FA supplementation group, irrespective of the presence of the main NTD risk factors. The effect on NTD subtype distribution was only seen when FA supplementation was started before conception. We conclude that FA not only prevents the occurrence of a significant proportion of NTDs, but might also decrease the severity of NTDs, as long as supplementation is started before conception.