Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)^+. Fe^IV(O)(p‐Y‐PyO)]⁺ ( 1 ‐Y) (tmp=meso‐tetramesitylporphyrin, p‐Y‐PyO=para‐substituted pyridine N‐oxides, and Y=OCH₃, CH₃, H, Cl). In all of the oxidation reactions, the reactivities of 1 ‐Y are found to follow the order 1 ‐OCH₃ > 1 ‐CH₃ > 1 ‐H > 1 ‐Cl; negative Hammett ρ values of ?1.4 to ?2.7 were obtained by plotting the reaction rates against the σ_p values of the substituents of p‐Y‐PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)‐oxo porphyrin π‐cation radicals bearing electron‐donating axial ligands are more reactive in oxo‐transfer and hydrogen‐atom abstraction reactions. These results are counterintuitive since iron(IV)‐oxo porphyrin π‐cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO? H bond and thereby enhances its H‐abstraction activity. In addition, it weakens the Fe?O bond and encourages oxo‐transfer reactivity. Both are Bell–Evans–Polanyi effects, however, in a series of neutral ligands like p‐Y‐PyO, there is a relatively weak trend that appears to originate in two‐state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.     

Stable All‐Organic Radicals with Ambipolar Charge Transport

A series of neutral long‐lived purely organic radicals based on the stable [4‐(N‐carbazolyl)‐2,6‐dichlorophenyl]bis(2,4,6‐trichlorophenyl)methyl radical adduct (Cbz‐TTM) is reported herein. All compounds exhibit ambipolar charge‐transport properties under ambient conditions owing to their radical character. High electron and hole mobilities up to 10^?2 and 10^?3 cm² V^?1 s^?1, respectively, were achieved. Xerographic single‐layered photoreceptors were fabricated from the radicals studied herein, exhibiting good xerographic photosensitivity across the visible spectrum.     

Synthesis and properties of novel σ–π alternating polymers with triphenylamine and organosilicon units

We have synthesized novel σ–π conjugated polymers with N,N‐bis(p‐ethynylphenyl)‐N‐(p‐tolyl)amine as the π‐unit. The electroluminescent devices, with a double‐layer system composed of Alq and the present polymers as the emitting‐electron‐transporting and hole‐transporting layers respectively, emit green electroluminescence with a maximum intensity of 760 cd m^?2. Copyright © 2001 John Wiley & Sons, Ltd.     

A novel two‐dimensional metal–organic supramolecular framework including π–π stacking and hydrogen‐bond interactions

[μ‐N,N′‐Bis(pyridin‐3‐yl)benzene‐1,4‐dicarboxamide‐<!?show [forcelb]><!?tlsb=0.12pt>1:2κ²N:N′]bis{[N,N′‐bis(pyridin‐3‐yl)benzene‐1,4‐dicarboxamide‐κN]diiodidomercury(II)}, [Hg₂I₄(C₁₈H₁₄N₄O₂)₃], is an S‐shaped dinuclear molecule, composed of two HgI₂ units and three N,N′‐bis(pyridin‐3‐yl)benzene‐1,4‐dicarboxamide (L) ligands. The central L ligand is centrosymmetric and coordinated to two Hg^II cations via two pyridine N atoms, in a syn–syn conformation. The two terminal L ligands are monodentate, with one uncoordinated pyridine N atom, and each adopts a syn–anti conformation. The HgI₂ units show highly distorted tetrahedral (sawhorse) geometry, as the Hg^II centres lie only 0.34 (2) or 0.32 (2) Å from the planes defined by the I and pyridine N atoms. Supramolecular interactions, thermal stability and solid‐state luminescence properties were also measured.     

π–π stacking motifs in dialkylbis{5‐[(E)‐2‐aryldiazen‐1‐yl]‐2‐hydroxybenzoato}tin(IV) complexes

The diorganotin(IV) complexes of 5‐[(E)‐2‐aryldiazen‐1‐yl]‐2‐hydroxybenzoic acid are of interest because of their structural diversity in the crystalline state and their interesting biological activity. The structures of dimethylbis{2‐hydroxy‐5‐[(E)‐2‐(4‐methylphenyl)diazen‐1‐yl]benzoato}tin(IV), [Sn(CH₃)₂(C₁₄H₁₁N₂O₃)₂], and di‐n‐butylbis{2‐hydroxy‐5‐[(E)‐2‐(4‐methylphenyl)diazen‐1‐yl]benzoato}tin(IV) benzene hemisolvate, [Sn(C₄H₉)₂(C₁₄H₁₁N₂O₃)₂]·0.5C₆H₆, exhibit the usual skew‐trapezoidal bipyramidal coordination geometry observed for related complexes of this class. Each structure has two independent molecules of the Sn^IV complex in the asymmetric unit. In the dimethyltin structure, intermolecular O—H…O hydrogen bonds and a very weak Sn…O interaction link the independent molecules into dimers. The planar carboxylate ligands lend themselves to π–π stacking interactions and the diversity of supramolecular structural motifs formed by these interactions has been examined in detail for these two structures and four closely related analogues. While there are some recurring basic motifs amongst the observed stacking arrangements, such as dimers and step‐like chains, variations through longitudinal slipping and inversion of the direction of the overlay add complexity. The π–π stacking motifs in the two title complexes are combinations of some of those observed in the other structures and are the most complex of the structures examined.     

Structure determination of three furan‐substituted benzimidazoles and calculation of π–π and C—H...π interaction energies

The synthesis and structural characterization of 2‐(furan‐2‐yl)‐1‐(furan‐2‐ylmethyl)‐1H‐benzimidazole [C₁₆H₁₂N₂O₂, (I)], 2‐(furan‐2‐yl)‐1‐(furan‐2‐ylmethyl)‐1H‐benzimidazol‐3‐ium chloride monohydrate [C₁₆H₁₃N₂O₂⁺·Cl⁻·H₂O, (II)] and the hydrobromide salt 5,6‐dimethyl‐2‐(furan‐2‐yl)‐1‐(furan‐2‐ylmethyl)‐1H‐benzimidazol‐3‐ium bromide [C₁₈H₁₇N₂O₂⁺·Br⁻, (III)] are described. Benzimidazole (I) displays two sets of aromatic interactions, each of which involves pairs of molecules in a head‐to‐tail arrangement. The first, denoted set (Ia), exhibits both intermolecular C—H...π interactions between the 2‐(furan‐2‐yl) (abbreviated as Fn) and 1‐(furan‐2‐ylmethyl) (abbreviated as MeFn) substituents, and π–π interactions involving the Fn substituents between inversion‐center‐related molecules. The second, denoted set (Ib), involves π–π interactions involving both the benzene ring (Bz) and the imidazole ring (Im) of benzimidazole. Hydrated salt (II) exhibits N—H...OH₂...Cl hydrogen bonding that results in chains of molecules parallel to the a axis. There is also a head‐to‐head aromatic stacking of the protonated benzimidazole cations in which the Bz and Im rings of one molecule interact with the Im and Fn rings of adjacent molecules in the chain. Salt (III) displays N—H...Br hydrogen bonding and π–π interactions involving inversion‐center‐related benzimidazole rings in a head‐to‐tail arrangement. In all of the π–π interactions observed, the interacting moieties are shifted with respect to each other along the major molecular axis. Basis set superposition energy‐corrected (counterpoise method) interaction energies were calculated for each interaction [DFT, M06‐2X/6‐31+G(d)] employing atomic coordinates obtained in the crystallographic analyses for heavy atoms and optimized H‐atom coordinates. The calculated interaction energies are −43.0, −39.8, −48.5, and −55.0 kJ mol⁻¹ for (Ia), (Ib), (II), and (III), respectively. For (Ia), the analysis was used to partition the interaction energies into the C—H...π and π–π components, which are 9.4 and 24.1 kJ mol⁻¹, respectively. Energy‐minimized structures were used to determine the optimal interplanar spacing, the slip distance along the major molecular axis, and the slip distance along the minor molecular axis for 2‐(furan‐2‐yl)‐1H‐benzimidazole.     

Synthesis and antioxidant activities of novel N‐aryl (and N‐alkyl) γ‐ and δ‐imino esters and ketimines

Novel N‐aryl (and N‐alkyl) γ‐ and δ‐imino esters 2a–g ( 3a–g ) and N‐aryl (and N‐alkyl) ketimines 2h–j ( 3h–j ) were synthesized in high yields (80–99%) from their corresponding γ‐ and δ‐keto esters and ketones in this study. The structures of the synthesized compounds were clarified by Fourier transform infrared (FT‐IR), NMR (¹H and ¹³C), mass spectrometry, and elemental analyses. Isomerizations [E/Z] were also determined by their ¹H NMR spectra. The free‐radical scavenging activity of imines was evaluated using the 1,1‐diphenyl‐2‐picryl‐hydrazyl (DPPH) method. The relationships between the structure and antioxidant activity of these compounds are discussed. Among these compounds, 2a–c (at the concentration 1000 μg/mL) exhibit high antioxidant activity similar to those of the standards (butylated hydroxyanisole [ BHA], butylated hydroxytoluene [ BHT], and ascorbic acid).     

Controllable assembly of a three‐dimensional metal–organic supramolecular framework displaying hydrogen‐bonding and π–π stacking interactions

The complex poly[[aqua(μ₂‐phthalato‐κ²O¹:O²){μ₃‐2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetato‐κ⁴N²,N³:O:O′}{μ₂‐2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetato‐κ³N²,N³:O}dizinc(II)] dihydrate], {[Zn₂(C₁₀H₈N₃O₂)₂(C₈H₄O₄)(H₂O)]·2H₂O}_n, has been prepared by solvothermal reaction of 2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetonitrile (PPAN) with zinc(II). Under hydrothermal conditions, PPAN is hydrolyzed to 2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetate (PPAA⁻). The structure determination reveals that the complex is a one‐dimensional double chain containing cationic [Zn₄(PPAA)₄]⁴⁺ structural units, which are further extended by bridging phthalate ligands. The one‐dimensional chains are extended into a three‐dimensional supramolecular architecture via hydrogen‐bonding and π–π stacking interactions.     

Clusters of Ammonium Cation—Hydrogen Bond versus σ‐Hole Bond

MP2/6‐311++G(d,p) calculations were performed on the NH₄⁺ ??? (HCN)_n and NH₄⁺ ??? (N₂)_n clusters (n=1–8), and interactions within them were analyzed. It was found that for molecules of N₂ and HCN, the N centers play the role of the Lewis bases, whereas the ammonium cation acts as the Lewis acid, as it is characterized by sites of positive electrostatic potential, that is, H atoms and the sites located at the N atom in the extension of the H?N bonds. Hence, the coordination number for the ammonium cation is eight, and two types of interactions of this cation with the Lewis base centers are possible: N?H ??? N hydrogen bonds and H?N ??? N interactions that are classified as σ‐hole bonds. Redistribution of the electronic charge resulting from complexation of the ammonium cation was analyzed. On the one hand, the interactions are similar, as they lead to electronic charge transfer from the Lewis base (HCN or N₂ in this study) to NH₄⁺. On the other hand, the hydrogen bond results in the accumulation of electronic charge on the N atom of the NH₄⁺ ion, whereas the σ‐hole bond results in the depletion of the electronic charge on this atom. Quantum theory of “atoms in molecules” and the natural bond orbital method were applied to deepen the understanding of the nature of the interactions analyzed. Density functional theory/natural energy decomposition analysis was used to analyze the interactions of the ammonium ion with various types of Lewis bases. Different correlations between the geometrical, energetic, and topological parameters were found and discussed.     

The Stability of α‐Hydroperoxyalkyl Radicals

High‐level ab initio and Born–Oppenheimer molecular dynamic calculations have been carried out on a series of hydroperoxyalkyl (α‐QOOH) radicals with the aim of investigating the stability and unimolecular decomposition mechanism into QO+OH of these species. Dissociation was shown to take place through rotation of the C?O(OH) bond rather than through elongation of the CO?OH bond. Through the C?O(OH) rotation, the unpaired electron of the radical overlaps with the electron density on the O?OH bond, and from this overlap the C=O π bond forms and the O?OH bond breaks spontaneously. The CH₂OOH, CH(CH₃)OOH, CH(OH)OOH, and α‐hydroperoxycycloheptadienyl radical were found to decompose spontaneously, but the CH(CHO)OOH has a decomposition energy barrier of 5.95 kcal mol^?1 owing to its steric and electronic features. The systems studied in this work provide the first insights into how structural and electronic effects govern the stabilizing influence on elusive α‐QOOH radicals.     

N,N′‐Dimethylpyrazinediium bis(tetrafluoroborate) and N,N′‐diethylpyrazinediium bis(tetrafluoroborate): new examples of anion–π triads

Crystallization of N,N′‐dimethylpyrazinediium bis(tetrafluoroborate), C₆H₁₀N₂²⁺·2BF₄⁻, (I), and N,N′‐diethylpyrazinediium bis(tetrafluoroborate), C₈H₁₄N₂²⁺·2BF₄⁻, (II), from dried acetonitrile under argon protection has permitted their single‐crystal studies. In both crystal structures, the pyrazinediium dications are located about an inversion center (located at the ring center) and each pyrazinediium aromatic ring is π‐bonded to two centrosymmetrically related BF₄⁻ anions. Strong anion–π interactions, as well as weak C—H...F hydrogen bonds, between BF₄⁻ and pyrazinediium ions are present in both salts.     

Visible light‐induced thiol‐ene reaction: A new strategy to prepare Α,ω‐dithiol and Α,ω‐divinyl telechelic polythiolether oligomers

A new strategy is developed to prepare both α,ω‐dithiol and α,ω‐divinyl linear telechelic polythiolether oligomers by visible light induced thiol‐ene chemistry in the presence of a fac‐Ir(ppy)₃ photoredox catalyst. Polythiolether oligomers of well‐defined end groups and controlled molecular weights have been successfully synthesized at varying monomer molar ratios of 1,4‐benzenedimethanethiol (BDMT) to diethylene glycol divinyl ether (DEGVE). ¹H NMR and MALDI‐TOF MS analyses demonstrate that as‐prepared polythiolethers possess high end‐group fidelity, which is further supported by the successful polyaddition of polythiolethers bearing α,ω‐dithiol and α,ω‐divinyl groups. For example, with the α,ω‐dithiol‐ (M_n = 1900 g mol^?1, PDI = 1.25) and α,ω‐divinyl‐terminated (M_n = 2000 g mol^?1, PDI = 1.29) polythiolethers as macromonomers, the molecular weight of resulting polythiolether is up to 7700 g mol^?1 with PDI as 1.67. The reactivity of the terminal thiol group is further confirmed by the addition reaction with N‐(1‐pyrenyl)maleimide. UV‐vis spectra and fluorescene measurements suggest that fac‐Ir(ppy)₃ undergo a redox quenching process reacted with BDMT to generate thiyl free radicals. With these results, the mechanism of the thiol‐ene reaction catalyzed by photoredox catalyst is proposed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 740–749     

Luminescent Benzoquinolate‐Isocyanide Platinum(II) Complexes: Effect of Pt⋅⋅⋅Pt and π⋅⋅⋅π Interactions on their Photophysical Properties

The neutral compounds [Pt(bzq)(CN)(CNR)] (R=tBu ( 1 ), Xyl ( 2 ), 2‐Np ( 3 ); bzq= benzoquinolate, Xyl=2,6‐dimethylphenyl, 2‐Np=2‐napthyl) were isolated as the pure isomers with a trans‐C_bzq,CNR configuration, as confirmed by ¹³C{¹H} NMR spectroscopy in the isotopically marked [Pt(bzq)(¹³CN)(CNR)] (R=tBu ( 1′ ), Xyl ( 2′ ), 2‐Np ( 3′ )) derivatives (δ¹³C_CN≈110 ppm; ¹J(Pt,¹³C)≈1425 Hz]. By contrast, complex [Pt(bzq)(C≡CPh)(CNXyl)] ( 4 ) with a trans‐N_bzq,CNR configuration, has been selectively isolated from [Pt(bzq)Cl(CNXyl)] (trans‐N_bzq,CNR) using Sonogashira conditions. X‐ray diffraction studies reveal that while 1 adopts a columnar‐stacked chain structure with Pt–Pt distances of 3.371(1) Å and significant π???π interactions (3.262 Å), complex 2 forms dimers supported only by short Pt???Pt (3.370(1) Å) interactions. In complex 4 the packing is directed by weak bzq???Xyl and bzq???C≡E (C, N) interactions. In solid state at room temperature, compounds 1 and 2 both show a bright red emission (?=42.1 % 1 , 57.6 % 2 ). Luminescence properties in the solid state at 77 K and concentration‐dependent emission studies in CH₂Cl₂ at 298 K and at 77 K are also reported for 1 , 1·CHCl3 , 2 , 2' , 2·CHCl3 , 3 , 4 .     

Variation of the ℏω with the particle number and the appearance of “kinks” for atomic clusters

The dependence of the harmonic oscillator (HO) energy level spacing ?ω on the particle number N is studied analytically for atomic (metal) clusters on the basis of their electronic densities, parametrizing Ekardt's results (for sodium clusters) by means of a Fermi distribution. An interesting feature of such an approach is that it leads, under the assumptions made, to “kinks,” that is, to “marked discontinuities in the slope” of ?ω at the closed shells. These discontinuities diminish as N increases. For large N, ?ω becomes simply: ?ω?c₁N^?1/3+c₂N^?1. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Antiferromagnetic Interactions in 1D Heisenberg Linear Chains of 7‐(4‐Fluorophenyl) and 7‐Phenyl‐Substituted 1,3‐Diphenyl‐1,4‐dihydro‐ 1,2,4‐benzotriazin‐4‐yl Radicals

7‐(4‐Fluorophenyl) and 7‐phenyl‐substituted 1,3‐diphenyl‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl radicals were characterized by X‐ray diffraction analysis and variable‐temperature magnetic susceptibility studies. The radicals pack in 1D π stacks of equally spaced slipped radicals with interplanar distances of 3.59 and 3.67 Å and longitudinal angles of 40.97 and 43.47°, respectively. Magnetic‐susceptibility studies showed that both radicals exhibit antiferromagnetic interactions. Fitting the magnetic data revealed that the behavior is consistent with 1D regular linear antiferromagnetic chain with J=?12.9 cm^?1, zJ′=?0.4 cm^?1, g=2.0069 and J=?11.8 cm^?1, zJ′=?6.5 cm^?1, g=2.0071, respectively. Magnetic‐exchange interactions in benzotriazinyl radicals are sensitive to the degree of slippage, and inter‐radical separation and subtle changes in structure alter the fine balance between ferro‐ and antiferromagnetic interactions.     

Phosphoryl group differentiating α‐amino acids from β‐ and γ‐amino acids in prebiotic peptide formation

In recent years β‐amino acids have increased their importance enormously in defining secondary structures of β‐peptides. Interest in β‐amino acids raises the question: Why and how did nature choose α‐amino acids for the central role in life? In this article we present experimental results of MS and ³¹P NMR methods on the chemical behavior of N‐phosphorylated α‐alanine, β‐alanine, and γ‐amino butyric acid in different solvents. N‐Phosphoryl α‐alanine can self‐assemble to N‐phosphopeptides either in water or in organic solvents, while no assembly was observed for β‐ or γ‐amino acids. An intramolecular carboxylic–phosphoric mixed anhydride (IMCPA) is the key structure responsible for their chemical behaviors. Relative energies and solvent effects of three isomers of IMCPA derived from α‐alanine (2a–c), with five‐membered ring, and five isomers of IMCPA derived from β‐alanine (4a–e), with six‐membered ring, were calculated with density functional theory at the B3LYP/6‐31G** level. The lower relative energy (3.2 kcal/mol in water) of 2b and lower energy barrier for its formation (16.7 kcal/mol in water) are responsible for the peptide formation from N‐phosphoryl α‐alanine. Both experimental and theoretical studies indicate that the structural difference among α‐, β‐, and γ‐amino acids can be recognized by formation of IMCPA after N‐phosphorylation. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 232–241, 2003     

Synthesis,Crystal Structure,and Antibacterial Activity of 1,2,4‐Triazoles and 1,2,4‐Triazol‐3‐one

A straightforward method has been developed for the synthesis of 1,2,4‐triazol‐3‐one 3 and 1,2,4‐triazoles 6a , 6b , 6c , 6d starting from N¹‐substituted‐N¹‐tosylhydrazonates 2 and hydrazine monohydrate. This methodology affords a number of 1,2,4‐triazol‐3‐one 3 and 1,2,4‐triazoles 6a , 6b , 6c , 6d in reasonable yields. The structures of all new compounds were elucidated using infrared, ¹H and ¹³C NMR, high‐resolution mass spectrometry, elemental analysis, and the X‐ray crystallography (for compounds 3 and 6a ). Some of the newly synthesized compounds were screened for their antibacterial activity.     

π–π Dimerization of a mono(pyridyl–imine) platinum(II) chelate

The cation of the title complex salt, chlorido{2,2‐dimethyl‐N‐[(E)‐1‐(pyridin‐2‐yl)ethylidene]propane‐1,3‐diamine}platinum(II) tetrafluoridoborate, [PtCl(C₁₂H₁₉N₃)]BF₄, exhibits a nominally square‐planar Pt^II ion coordinated to a chloride ion [Pt—Cl = 2.3046 (9) Å] and three unique N‐atom types, viz. pyridine, imine and amine, of the tridentate Schiff base ligand formed by the 1:1 condensation of 1‐(pyridin‐2‐yl)ethanone and 2,2‐dimethylpropane‐1,3‐diamine. The cations are π‐stacked in inversion‐related pairs (dimers), with a mean plane separation of 3.426 Å, an intradimer Pt...Pt separation of 5.0785 (6) Å and a lateral shift of 3.676 Å. The centroid (Cg) of the pyridine ring is positioned approximately over the Pt^II ion of the neighbouring cation (Pt...Cg = 3.503 Å).     

First heterobimetallic AgI–CoIII coordination compound with both bridging and terminal –NO2 coordination modes: synthesis,characterization, structural and computational studies of (PPh3)2AgI–(μ‐κ2O,O′:κN‐NO2)–CoIII(DMGH)2(κN‐NO2)

An unusual heterobimetallic bis(triphenylphosphane)(NO₂)Ag^I–Co^III(dimethylglyoximate)(NO₂) coordination compound with both bridging and terminal –NO₂ (nitro) coordination modes has been isolated and characterized from the reaction of [CoCl(DMGH)₂(PPh₃)] (DMGH₂ is dimethylglyoxime or N,N′‐dihydroxybutane‐2,3‐diimine) with excess AgNO₂. In the title compound, namely bis(dimethylglyoximato‐1κ²O,O′)(μ‐nitro‐1κN:2κ²O,O′)(nitro‐1κN)bis(triphenylphosphane‐2κP)cobalt(III)silver(I), [AgCo(C₄H₇N₂O₂)₂(NO₂)₂(C₁₈H₁₅P)₂], one of the ambidentate –NO₂ ligands, in a bridging mode, chelates the Ag^I atom in an isobidentate κ²O,O′‐manner and its N atom is coordinated to the Co^III atom. The other –NO₂ ligand is terminally κN‐coordinated to the Co^III atom. The structure has been fully characterized by X‐ray crystallography and spectroscopic methods. Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) have been used to study the ground‐state electronic structure and elucidate the origin of the electronic transitions, respectively.     

Weak C—H...O and C—H...π hydrogen bonds and π–π stacking interactions in a series of four N′‐[(E)‐(aryl)methylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazides

Oxazolidin‐2‐ones are widely used as protective groups for 1,2‐amino alcohols and chiral derivatives are employed as chiral auxiliaries. The crystal structures of four differently substituted oxazolidinecarbohydrazides, namely N′‐[(E)‐benzylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₂H₁₂N₃O₃, (I), N′‐[(E)‐2‐chlorobenzylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₂H₁₂ClN₃O₃, (II), (4S)‐N′‐[(E)‐4‐chlorobenzylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₂H₁₂ClN₃O₃, (III), and (4S)‐N′‐[(E)‐2,6‐dichlorobenzylidene]‐N,3‐dimethyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₃H₁₃Cl₂N₃O₃, (IV), show that an unexpected mild‐condition racemization from the chiral starting materials has occurred in (I) and (II). In the extended structures, the centrosymmetric phases, which each crystallize with two molecules (A and B) in the asymmetric unit, form A+B dimers linked by pairs of N—H...O hydrogen bonds, albeit with different O‐atom acceptors. One dimer is composed of one molecule with an S configuration for its stereogenic centre and the other with an R configuration, and possesses approximate local inversion symmetry. The other dimer consists of either R,R or S,S pairs and possesses approximate local twofold symmetry. In the chiral structure, N—H...O hydrogen bonds link the molecules into C(5) chains, with adjacent molecules related by a 2₁ screw axis. A wide variety of weak interactions, including C—H...O, C—H...Cl, C—H...π and π–π stacking interactions, occur in these structures, but there is little conformity between them.