Titanium and Vanadium Complexes of Tridentate Phenoxy-Imine and Phenoxy-Amine Ligands and Their Application in the Ring-Opening Polymerization of Cyclic Esters

Phenoxy-imine and phenoxy-amine proligands, with the additional OH donor groups 2,4-tBu2-6-(2-CH2(OH)-C6H4N=CH)C6H3OH (L1H2), 6-(2-CH2(OH)-C6H4N=CH)C6H3OH (L2H2), and 2,4-tBu2-6-(2-CH2(OH)-C6H4NH-CH)C6H3OH (L3H2), were synthesized and their titanium (Ti-L1-Ti-L3) and vanadium (V-L1-V-L2) complexes were prepared in reactions with Ti(OiPr)4 and VO(OiPr)3, respectively. All new compounds were characterized with the use of FTIR, 1H, and 13C NMR spectroscopy; X-ray crystallography was also used to study proligands. All the complexes proved to be active catalysts in the ring-opening polymerization (ROP) of ε-caprolactone, rac-lactide, and L-lactide in the melt. The effects of the complex structure (transition metal type, presence of tBu substituents, and type of nitrogen donor group), as well as the polymerization time and temperature, on the monomer conversion and polymer properties were investigated in detail.

IPr*Oxa - a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands

N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.

Insight into the Structure of Victorin, the Host-Selective Toxin from the Oat Pathogen Cochliobolus victoriae. Studies of the Unique Dehydroamino Acid β-Chlorodehydroalanine

Victorins, a family of peptide toxins, produced by the fungal pathogen Cochliobolus victoriae and responsible for disease of some oat varieties, contain a β-chlorodehydroalanine residue, ΔAla(βCl). To determine the conformational properties of this unique dehydroamino acid, a series of model compounds was studied using X-ray, NMR, and FT-IR methods, supported by theoretical calculations. The ΔAla(βCl) geometrical isomers differ in conformational profile. The isomer Z prefers the helical conformation α (φ, ψ = -61°, -24°), PPII type conformation β (φ, ψ = -47°, 136°), and semiextended conformation β2 (φ, ψ = -116°, 9°) in weakly and more polar solutions. The isomer E prefers mainly the extended conformation C5 (φ, ψ = -177°, 160°), but with an increase of the environment polarity also conformations β (φ, ψ = -44°, 132°) and α (φ, ψ = -53°, -39°). In the most stable conformations the N-H···Cl hydrogen bond (5^γ) occurs, created between the chlorine atom of the side chain and the N-H donor of the flanking amide group. The method of synthesis of the β-chlorodehydroalanine residue is proposed, by chlorination of dehydroalanine and then the photoisomerization from the isomer Z to E. The presented results indicate that the assignment of the geometrical isomer of the ΔAla(βCl) residue in naturally occurring victorins still remains an open question, despite being crucial for biological activity.

ItOct (ItOctyl) - pushing the limits of ItBu: highly hindered electron-rich N-aliphatic N-heterocyclic carbenes

ItBu (ItBu = 1,3-di-tert-butylimidazol-2-ylidene) represents the most important and most versatile N-alkyl N-heterocyclic carbene available in organic synthesis and catalysis. Herein, we report the synthesis, structural characterization and catalytic activity of ItOct (ItOctyl), C₂-symmetric, higher homologues of ItBu. The new ligand class, including saturated imidazolin-2-ylidene analogues has been commercialized in collaboration with MilliporeSigma: ItOct, 929 298; SItOct, 929 492 to enable broad access of the academic and industrial researchers within the field of organic and inorganic synthesis. We demonstrate that replacement of the t-Bu side chain with t-Oct results in the highest steric volume of N-alkyl N-heterocyclic carbenes reported to date, while retaining the electronic properties inherent to N-aliphatic ligands, such as extremely strong σ-donation crucial to the reactivity of N-alkyl N-heterocyclic carbenes. An efficient large-scale synthesis of imidazolium ItOct and imidazolinium SItOct carbene precursors is presented. Coordination chemistry to Au(i), Cu(i), Ag(i) and Pd(ii) as well as beneficial effects on catalysis using Au(i), Cu(i), Ag(i) and Pd(ii) complexes are described. Considering the tremendous importance of ItBu in catalysis, synthesis and metal stabilization, we anticipate that the new class of ItOct ligands will find wide application in pushing the boundaries of new and existing approaches in organic and inorganic synthesis.

One-Pot Phosphonylation of Heteroaromatic Lithium Reagents: The Scope and Limitations of Its Use for the Synthesis of Heteroaromatic Phosphonates

A one-pot lithiation–phosphonylation procedure was elaborated as a method to prepare heteroaromatic phosphonic acids. It relied on the direct lithiation of heteroaromatics followed by phosphonylation with diethyl chlorophosphite and then oxidation with hydrogen peroxide. This protocol provided the desired phosphonates with satisfactory yields. This procedure also had some limitations in its dependence on the accessibility and stability of the lithiated heterocyclic compounds. The same procedure could be applied to phosphonylation of aromatic compounds, which do not undergo direct lithiation and thus require the use of their bromides as substrates. The obtained compounds showed weak antiproliferative activity when tested on three cancer cell lines.

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI /AuIII Catalysis

In the last decade, major advances have been made in homogeneous gold catalysis. However, Au^I /Au^III catalytic cycle remains much less explored due to the reluctance of Au^I to undergo oxidative addition and the stability of the Au^III intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for Au^I /Au^III redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

CAAC-IPr*: easily accessible, highly sterically-hindered cyclic (alkyl)(amino)carbenes

IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) has emerged as a powerful highly hindered and sterically-flexible ligand platform for transition-metal catalysis. CAACs (CAAC = cyclic (al-kyl)(amino)carbenes) have gained major attention as strongly electron-rich carbon analogues of NHCs (NHC = N-heterocyclic carbene) with broad applications in both industry and academia. Herein, we report a merger of CAAC ligands with highly-hindered IPr*. The efficient synthesis, electronic characterization and application in model Cu-catalyzed hydroboration of alkynes is described. The ligands are strongly electron-rich, bulky and flexible around the N-Ar wingtip. The availability of various IPr* and CAAC templates offers a significant potential to expand the existing arsenal of NHC ligands to electron-rich bulky architectures with critical applications in metal stabilization and catalysis.

Well-Defined, Air- and Moisture-Stable Palladium-Imidazo[1,5-a]pyridin-3-ylidene Complexes: A Versatile Catalyst Platform for Cross-Coupling Reactions by L-Shaped NHC Ligands

We describe the development of [(NHC)Pd(cinnamyl)Cl] complexes of ImPy (ImPy = imidazo[1,5-a]pyridin-3-ylidene) as a versatile class of precatalysts for cross-coupling reactions. These precatalysts feature fast activation to monoligated Pd(0) with 1:1 Pd to ligand ratio in a rigid imidazo[1,5-a]pyridin-3-ylidene template. Steric matching of the C5-substituent and N2-wingtip in the catalytic pocket of the catalyst framework led to the discovery of ImPyMesDipp as a highly reactive imidazo[1,5-a]pyridin-3-ylidene ligand for Pd-catalyzed cross-coupling of nitroarenes by challenging C-NO2 activation. Kinetic studies demonstrate fast activation and high reactivity of this class of well-defined ImPy-Pd catalysts. Structural studies provide full characteristics of this new class of imidazo[1,5-a]pyridin-3-ylidene ligands. Computational studies establish electronic properties of sterically-restricted imidazo[1,5-a]pyridin-3-ylidene ligands. Finally, a scalable synthesis of C5-substituted imidazo[1,5-a]pyridin-3-ylidene ligands through Ni-catalyzed Kumada cross-coupling is disclosed. The method obviates chromatographic purification at any of the steps, resulting in a facile and modular access to ImPy ligands. We anticipate that well-defined [Pd-ImPy] complexes will find broad utility in organic synthesis and catalysis for activation of unreactive bonds.

Pd-PEPPSI N-Heterocyclic Carbene Complexes from Caffeine: Application in Suzuki, Heck, and Sonogashira Reactions

The first synthesis of Pd-PEPPSI N-heterocyclic carbene complexes derived from the abundant and renewable natural product caffeine is reported. The catalysts bearing 3-chloro-pyridine, pyridine and N-methylimidazole ancillary ligands were readily prepared from the corresponding N9-Me caffeine imidazolium salt by direct deprotonation and coordination to PdX2 in the presence of N-heterocycles or by ligand displacement of PdX2(Het)2. The model Pd-PEPPSI-caffeine complex has been characterized by x-ray crystallography. The complexes were successfully employed in the Suzuki cross-coupling of aryl bromides, Suzuki cross-coupling of amides, Heck cross-coupling and Sonogashira cross-coupling. Computational studies were employed to determine frontier molecular orbitals and bond order analysis of caffeine derived Pd-PEPPSI complexes. This class of catalysts offers an entry to utilize benign and sustainable biomass-derived Xanthine NHC ligands in the popular Pd-PEPPSI systems in organic synthesis and catalysis.

N-Heterocyclic Carbene Complexes of Nickel(II) from Caffeine and Theophylline: Sustainable Alternative to Imidazol-2-ylidenes

Xanthines, such as caffeine and theophylline, are abundant natural products that are often present in foods. Leveraging renewable and benign resources for ligand design in organometallic chemistry and catalysis is one of the major missions of green and sustainable chemistry. In this Special Issue on Sustainable Organometallic Chemistry, we report the first nickel-N-heterocyclic carbene complexes derived from Xanthines. Well-defined, air- and moisture-stable, half-sandwich, cyclopentadienyl [CpNi(NHC)I] nickel-NHC complexes are prepared from the natural products caffeine and theophylline. The model complex has been characterized by x-ray crystallography. The evaluation of steric, electron-donating and π-accepting properties is presented. High activity in the model Suzuki-Miyaura cross-coupling is demonstrated. The data show that nickel-N-heterocyclic carbenes derived from both Earth abundant 3d transition metal and renewable natural products represent a sustainable alternative to the classical imidazol-2-ylidenes.

Effect of conjugated system extension on structural features and electron-density distribution in charge-transfer difluoroborates

A comparative structural study of two related donor-acceptor pyridine-based BF₂ complexes, namely, 3-(dimethylamino)-1,1-difluoro-1H-pyrido[1,2-c][1,3,5,2]oxadiazaborinin-9-ium-1-uide, C₈H₁₀BF₂N₃O (1), and 3-{(1E,3E)-4-[4-(dimethylamino)phenyl]buta-1,3-dien-1-yl}-1,1-difluoro-1H-pyrido[1,2-c][1,3,5,2]oxadiazaborinin-9-ium-1-uide, C₁₈H₁₈BF₂N₃O (2), containing a dimethylamino group and either the shortest (in 1) or the longest (in 2) charge-transfer path known until now in this family of compounds, is presented. Single-crystal X-ray diffraction analysis supported by computational investigations shed more light on these systems, indicating, among other aspects, the predominance of C-H...F contacts in 1, the formation of antiparallel dimers held together by π-π interactions in both compounds, and the involvement of fused BF₂-bearing rings in the charge-transfer process.

Evaluation of Cyclic Amides as Activating Groups in N-C Bond Cross-Coupling: Discovery of N-Acyl-δ-valerolactams as Effective Twisted Amide Precursors for Cross-Coupling Reactions

The development of efficient methods for facilitating N-C(O) bond activation in amides is an important objective in organic synthesis that permits the manipulation of the traditionally unreactive amide bonds. Herein, we report a comparative evaluation of a series of cyclic amides as activating groups in amide N-C(O) bond cross-coupling. Evaluation of N-acyl-imides, N-acyl-lactams, and N-acyl-oxazolidinones bearing five- and six-membered rings using Pd(II)-NHC and Pd-phosphine systems reveals the relative reactivity order of N-activating groups in Suzuki-Miyaura cross-coupling. The reactivity of activated phenolic esters and thioesters is evaluated for comparison in O-C(O) and S-C(O) cross-coupling under the same reaction conditions. Most notably, the study reveals N-acyl-δ-valerolactams as a highly effective class of mono-N-acyl-activated amide precursors in cross-coupling. The X-ray structure of the model N-acyl-δ-valerolactam is characterized by an additive Winkler-Dunitz distortion parameter Σ(τ+χ_N) of 54.0°, placing this amide in a medium distortion range of twisted amides. Computational studies provide insight into the structural and energetic parameters of the amide bond, including amidic resonance, N/O-protonation aptitude, and the rotational barrier around the N-C(O) axis. This class of N-acyl-lactams will be a valuable addition to the growing portfolio of amide electrophiles for cross-coupling reactions by acyl-metal intermediates.

Ring opening polymerization of ε-caprolactone initiated by titanium and vanadium complexes of ONO-type schiff base ligand

A phenoxy-imine proligand with the additional OH donor group, 4,6-tBu2-2-(2-CH2(OH)-C6H4N = CH)C6H3OH (LH2), was synthesized and used to prepare group 4 and 5 complexes by reacting with Ti(OiPr)4 (LTi) and VO(OiPr)3 (LV). All new compounds were characterized by the FTIR, 1H and 13C NMR spectroscopy and LTi by the single-crystal X-ray diffraction analysis. The complexes were used as catalysts in the ring opening polymerization of ε-caprolactone. The influence of monomer/transition metal molar ratio, reaction time, polymerization temperature as well as complex type was investigated in detail. The complexes showed high (LTi) and moderate (LV) activity in ε-caprolactone polymerization and the resultant polycaprolactones exhibited Mn and Mw/Mn values ranging from 4.0 · 103 to 18.7 · 103 g/mol and from 1.4 to 2.5, respectively.

Synthesis and Inhibitory Studies of Phosphonic Acid Analogues of Homophenylalanine and Phenylalanine towards Alanyl Aminopeptidases

A library of novel phosphonic acid analogues of homophenylalanine and phenylalanine, containing fluorine and bromine atoms in the phenyl ring, have been synthesized. Their inhibitory properties against two important alanine aminopeptidases, of human (hAPN, CD13) and porcine (pAPN) origin, were evaluated. Enzymatic studies and comparison with literature data indicated the higher inhibitory potential of the homophenylalanine over phenylalanine derivatives towards both enzymes. Their inhibition constants were in the submicromolar range for hAPN and the micromolar range for pAPN, with 1-amino-3-(3-fluorophenyl) propylphosphonic acid (compound 15c) being one of the best low-molecular inhibitors of both enzymes. To the best of our knowledge, P1 homophenylalanine analogues are the most active inhibitors of the APN among phosphonic and phosphinic derivatives described in the literature. Therefore, they constitute interesting building blocks for the further design of chemically more complex inhibitors. Based on molecular modeling simulations and SAR (structure-activity relationship) analysis, the optimal architecture of enzyme-inhibitor complexes for hAPN and pAPN were determined.

The substituent effect of π-electron delocalization in N-methylamino-nitropyridine derivatives: crystal structure and DFT calculations

The crystal and molecular structures of 3-(N-methylamino)-2-nitropyridine, 5-(N-methylamino)-2-nitropyridine and 2-(N-methylamino)-5-nitropyridine have been characterized by X-ray diffraction. To perform conformational analysis, the geometries of the compounds as well as their conformers and rotamers were optimized at the B3LYP/6-311++G(3df,3pd) level. The resulting data were used to analyze the π-electron delocalization effect in relation to the methylamino group rotation in ortho-, meta- and para-substitution positions. Quantitative aromaticity indices were calculated based on which we estimated the electronic structures of the analyzed compounds. The substituent effect of the methylamino and nitro groups was also characterized by related descriptors, i.e., charge of the Substituent Active Region (cSAR(X)) calculated based on the Hirshfeld charges and Substituent Effect Stabilization Energy (SESE). It has been shown that all the used parameters were found to be mutually interrelated with much better correlations for the meta- and para- than the ortho-derivatives. The rotation of the methylamino group relative to the aromatic fragment has an effect on the change in delocalization of the π-electrons of the pyridine ring. Relations between cSAR(NHCH3) and cSAR(NO2) show almost identical sensitivity of the substituent effect in meta- and para-substituted derivatives, whereas in ortho-substituted analogs, different kinds of intramolecular interactions have been revealed. Furthermore, the analyzed dependency of SESE values on the torsional angle of the methylamino group indicates that an increase in the electron-attracting power of the substituent leads to a decrease in energy.

Structural phase transitions coupled with prominent dielectric anomalies and dielectric relaxation in [(CH3)3NH]2[KCo(CN)6] and mixed [(CH3)3NH]2[KFexCo1-x(CN)6] double perovskite hybrids

The crystals of pure [(CH₃)₃NH]₂[KFe(CN)₆] (TrMAFe) and [(CH₃)₃NH]₂[KCo(CN)₆] (TrMACo) as well as their mixed crystals (TrMAFe_xCo_1-x), with different ratios of x = 0, 0.12, 0.18, 0.49, 0.56, 0.73, 0.81, 1.0, have been grown from aqueous solutions. The structure of TrMACo has been determined at 360 K to be cubic (Fm3[combining macron]m). In phase II (100 K), the TrMACo crystal is monoclinic (C2/c). The thermal stability of the pure and mixed crystals has been determined by using both DTA and TGA. Based on the DSC results, we have found a single phase transition (PT) for both pure and mixed crystals. The Fe(iii) concentration was estimated by using the SEM technique. We have found a linear relationship between the PT temperature (T_c) and the molar concentration of Fe(iii). Based on the obtained results, a phase diagram has been constructed. The mechanism of the structural PT has been discussed based on the results of dielectric relaxation and ¹H NMR and X-ray spectroscopy measurements. The results confirmed that the PT mechanism of both pure and mixed crystals is related to the change in the dynamic state of the TrMA⁺ cations. The dielectric activation energy changes linearly with the mole fraction ranking from 35 to 38 kJ mol^-1, for the mixed crystals.

A novel approach for obtaining α,β-diaminophosphonates bearing structurally diverse side chains and their interactions with transition metal ions studied by ITC

Aminophosphonates are an important group of building blocks in medicinal and pharmaceutical chemistry. Novel representatives of this class of compounds containing nontypical side chains are still needed. The aza-Michael-type addition of amines to phosphonodehydroalanine derivatives provides a simple and effective approach for synthesizing N′-substituted α,β-diaminoethylphosphonates and thus affords general access to aminophosphonates bearing structurally diverse side chains. Thermodynamic analysis of the chosen aminophosphonates at physiological pH proves that they serve as potent chelators for copper(ii) ions and moderate chelators for nickel(ii) ions.

A convenient and general reaction is presented for the preparation of diaminophosphonates further evaluated as chelators of metal ions.

Annular Tautomerism of 3(5)-Disubstituted-1H-pyrazoles with Ester and Amide Groups

A series of disubstituted 1H-pyrazoles with methyl (1), amino (2), and nitro (3) groups, as well as ester (a) or amide (b) groups in positions 3 and 5 was synthesized, and annular tautomerism was investigated using X-ray, theoretical calculations, NMR, and FT-IR methods. The X-ray experiment in the crystal state showed for the compounds with methyl (1a, 1b) and amino (2b) groups the tautomer with ester or amide groups at position 3 (tautomer 3), but for those with a nitro group (3b, 4), tautomer 5. Similar results were obtained in solution by NMR NOE experiments in CDCl3, DMSO-d6, and CD3OD solvents. However, tautomer equilibrium was observed for 2b in DMSO. The FT-IR spectra in chloroform and acetonitrile showed equilibria, which can be ascribed to conformational changes of the cis/trans arrangement of the ester/amide group and pyrazole ring. Theoretical analysis using the M06-2X/6-311++G(d,p) method (in vacuo, chloroform, acetonitrile, and water) and measurement of aromaticity (NICS) showed dependence on internal hydrogen bonds, the influence of the environment, and the effect of the substituent. These factors, pyrazole aromaticity and intra- and inter-molecular interactions, seem to have a considerable influence on the choice of tautomer.

Reactions of Piperazin-2-one, Morpholin-3-one, and Thiomorpholin-3-one with Triethyl Phosphite Prompted by Phosphoryl Chloride: Scope and Limitations

The reaction of the title lactams with triethyl phosphite prompted by phosphoryl chloride provided six-membered ring heterocyclic phosphonates or bisphosphonates. These novel scaffolds might be of interest as building blocks in medicinal chemistry. The course of the reaction was dependent on the structure of the used substrate. Thus, morpholin-3-one and thiomorpholin-3-one readily provided the corresponding 1,1-bisphosphonates (compounds 1, 2, 7, 14 and 16), whereas the protection of their nitrogen atom resulted in the formation of dehydrophosphonates (compounds 5, 6, and 8). Piperazin-2-one reacted differently yielding mixture of cis- and trans- piperazine-2,3-diyl-bisphosphonates (compounds 10 and 11). Since cytosine could be considered as an analogue of piperin-2-one, its ditosyl derivative 18 was used as a substrate affording compound 19 being a product of phosphite addition to double bond. In dependence of their structures, hydrolysis of the obtained diethyl phosphonates resulted either in corresponding cyclic phosphonic acids or in the degradation of carbon-to-phosphorus bond.

Isostructural phase transition, quasielastic neutron scattering and magnetic resonance studies of a bistable dielectric ion-pair crystal [(CH3)2NH2]2KCr(CN)6

We have synthesised and characterised a novel organic-inorganic hybrid crystal, [(CH3)2NH2]2KCr(CN)6. The thermal DSC, TMA, DTG and DTA analyses indicate two solid-to-solid structural phase transitions (PTs). According to the X-ray diffraction experiments, the first PT at 220 K is isostructural, since it does not involve a change of the space group. This transition occurs between the states, where the (CH3)2NH2+ cations are orientationally disordered and ordered (frozen). The other reversible PT at 481 K leads to a melt-like phase similar to the one observed in plastic crystals or polar liquids. Dielectric spectroscopy has been used to characterise the switching properties of the dipole moments in the vicinity of the PTs. Continuous-wave electron paramagnetic resonance spectroscopy was employed to investigate the effect of ordering on the local environment of the Cr3+ ions. We have also applied the quasielastic neutron scattering (QENS) technique as well as 1H NMR spectroscopy to measure the dynamics of the (CH3)2NH2+ cations residing in the inorganic framework.

The structural, phonon and optical properties of [CH3NH3]M0.5CrxAl0.5−x(HCOO)3 (M = Na, K; x = 0, 0.025, 0.5) metal–organic framework perovskites for luminescence thermometry

We report the structural, phonon and optical properties of perovskite-type heterometallic formates templated by methylammonium cations.

Structures and energetic properties of 4-halobenzamides

The amide bond represents one of the most fundamental functional groups in chemistry. The properties of amides are defined by amidic resonance (n_N→π*_C=O conjugation), which enforces planarity of the six atoms comprising the amide bond. Despite the importance of 4-halo-substituted benzamides in organic synthesis, molecular interactions and medicinal chemistry, the effect of 4-halo-substitution on the properties of the amide bond in N,N-disubstituted benzamides has not been studied. Herein, we report the crystal structures and energetic properties of a full series of 4-halobenzamides. The structures of four 4-halobenzamides (halo = iodo, bromo, chloro and fluoro) in the N-morpholinyl series have been determined, namely 4-[(4-halophenyl)carbonyl]morpholine, C₁₁H₁₂XNO₂, for halo = iodo (X = I), bromo (X = Br), chloro (X = Cl) and fluoro (X = F). Computations have been used to determine the effect of halogen substitution on the structures and resonance energies. 4-Iodo-N-morpholinylbenzamide crystallized with a significant distortion of the amide bond (τ + χ_N = 33°). The present study supports the correlation between the Ar-C(O) axis twist angle and the twist angle of the amide N-C(O) bond. Comparison of resonance energies in synthetically valuable N-morpholinyl and N-piperidinyl amides demonstrates that the O atom of the morpholinyl ring has a negligible effect on amidic resonance in the series.

Preparation and molecular structures of N′-(2-heteroarylmethylidene)-3-(3-pyridyl)acrylohydrazides

The crystal and molecular structures of N′-(2-furylmethylidene)-3-(3-pyridyl)acrylohydrazide and N′-(2-thienylmethylidene)-3-(3-pyridyl)acrylohydrazide are reported, and the influence of the type of the heteroatom on the aromaticity of the aromatic rings is discussed. Both molecules are nearly planar. The geometry of the acrylohydrazide arrangement is comparable to that of homologous compounds. Density functional theory (DFT) calculations were performed in order to analyze the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecule. The most significant changes were observed in the values of the N–N and C–N bond lengths. The harmonic oscillator model of aromaticity index, calculated for the furan and thiophene rings, demonstrated a noticeable increase in aromaticity in comparison to isolated rings and their DFT-calculated structures. By contrast, the nucleus independent chemical shift index indicated a decrease in aromatic character of the rings containing heteroatoms.

2,2′-Bipyridin-1-ium hemioxalate oxalic acid monohydrate

The asymmetric unit of the title compound, C10H9N2 +·0.5C2O4 2−·C2H2O4·H2O, consists of a 2,2′-bipyridinium cation, half an oxalate dianion, one oxalic acid and one water molecule. One N atom in 2,2′-bipyridine is unprotonated, while the second is protonated and forms an N—H...O hydrogen bond. In the crystal, the anions are connected with surrounding acid molecules and water molecules by strong near-linear O—H...O hydrogen bonds. The water molecules are located between the anions and oxalic acids; their O atoms participate as donors and acceptors, respectively, in O—H...O hydrogen bonds, which form sheets arranged parallel to the ac plane.

π-Electron delocalization in 2-benzoyl-5-phenylpyrazolidin-3-one

The crystal and molecular structures of 2-benzoyl-5-phenylpyrazolidin-3-one have been characterized by X-ray diffraction along with density functional theory studies. Cinnamic acid chloride was reacted with benzhydrazide, yielding 2-benzoyl-5-phenylpyrazolidin-3-one. This product was formed in the transformation comprising the nucleophilic addition of benzhydrazide to the styryl fragment of the α,β-unsaturated arrangement and subsequent cyclization. The molecule contains two benzene rings and one five-membered heterocyclic ring with an N–N single bond. The five-membered ring is composed of three atoms of sp 3 hybridization and two atoms of sp 2 hybridization, which cause the flattening of the heterocyclic ring. The Harmonic Oscillator Model of Aromaticity and Nucleus-Independent Chemical Shift indexes, calculated for the benzene rings, demonstrate that there are no substantial interactions between the regions of π-electron delocalization in the molecule. In the crystal structure, there are N–H···O hydrogen bonds that link the molecules along the crystallographic c axis and weak intermolecular C–H···O hydrogen bonds.

Triclinic conformational polymorph of N,N,N′,N′-tetrakis(2-cyanoethyl)-1,2-ethylenediamine (TCED)

The crystal and molecular structures of two polymorphs of N,N,N′,N′-tetrakis(2-cyanoethyl)-1,2-ethylenediamine have been characterized by X-ray diffraction along with density functional theory (DFT) studies. The molecules differ from each other by conformation. N,N,N′,N′-tetrakis(2-cyanoethyl)-1,2-ethylenediamine has been synthesized by cyanoethylation of ethylenediamine. Cyanoethylation of vicinal diamines is important for the synthesis of hyperbranched polymeric materials applied as catalysts, surfactants and encapsulating agents in drug delivery systems. The molecular geometry of N,N,N′,N′-tetracyanoethyl-1,2-ethylenediamine is similar to that of homologous compounds. DFT calculations were performed to analyze the differences in the molecular geometry of the studied compounds in a crystalline state and for an isolated molecule.

Acetylhydroxamic acid

There is one independent molecule in the asymmetric unit of the title compound (alternatively namedN-hydroxyacetamide), C2H5NO2. It crystallizes in the noncentrosymmetric space groupP43. The structure is an anhydrous form of acetylhydroxamic acid with typical geometry that corresponds well with the hydrated structure described by Bracher & Small [Acta Cryst.(1970), B26, 1705–1709]. In the crystal, N—H...O and O—H...O hydrogen bonds connect the molecules into chains in thec-axis direction.

2,2-Difluoro-3-(4-fluorophenyl)-2H-benzo[e][1,3,2]oxazaborinin-3-ium-2-uide

There is one independent molecule in the asymmetric unit of the title compound, C13H9BF3NO, which crystallizes in the non-centrosymmetric space groupCc. In the molecular structure, the BF2-carrying ring is distorted from planarity and its mean plane makes a dihedral angle of 42.3 (1)° with the 4-fluorophenyl ring. F atoms are involved in all of the short intermolecular contacts of the crystal structure, which link molecules to form chains along [001] and [010].

2-[4-(Dimethylamino)phenyl]-3,3-difluoro-3H-naphtho[1,2-e][1,3,2]oxazaborinin-2-ium-3-uide

In the title compound, C19H17BF2N2O, a twist about the N—C single bond is observed, making the cross conjugation not as efficient as in the case of a planar structure. The borone complex has tetrahedral geometry. In the crystal, molecules are conected by weak C—H...F hydrogen bonds.

N,N-Dicyclohexylnitramine

Molecules of the title compound, C12H22N2O2, are composed of an nitramine group substituted by two cyclohexane rings. The cyclohexane rings have chair conformations, with the exocyclic C—N bonds in axial orientations. In the crystal, C—H...O hydrogen bonds connect the molecules intoC(6) [-101] zigzag chains.

4-Fluoro-N-methyl-N-nitroaniline

Molecules of the title compound, C7H7FN2O2, are composed of a nitramine group which is twisted with the respect to the aromatic ring, with an N—N—C—C torsion angle of −117.38 (12)°. In the molecule, the N—N bond length [1.3510 (15) Å] indicates some double-bond character, while the angle between the aromatic ring and the nitramine group rules out further delocalization in the molecule. In the crystal, C—H...F hydrogen bonds connect the molecules intoC11(6) chains along theaaxis. C—H...O hydrogen bonds form, which featureR22(12) loops and further connect these chains.

2-Methyl-N-(pyrazin-2-yl)propanamide–1,2,4,5-tetrafluoro-3,6-diiodobenzene (2/1)

In the title compound, C8H11N3O·0.5C6F4I2, molecules ofiPr-substituted pyrazine are co-crystallized with 1,4-diiodo-2,3,5,6-tetrafluorobenzene. The complete molecule of 1,4-diiodo-2,3,5,6-tetrafluorobenzene is generated by an inversion centre at the middle of the aromatic ring. Both molecules have normal geometry and theiPr acylamine group is disordered over two sets of sites with an occupancy ratio of 0.51:0.49. In the crystal, the components are linked by I...N halogen bonds [2.830 (2) Å] and C—H...F interactions are observed.

N,N′-Bis(pyridin-2-yl)octanediamide

The complete molecule of the title compound, C18H22N4O2, is generated by crystallographic inversion symmetry. In the crystal, N—H...N hydrogen bonds connect the molecules into [010] chains, which featureR22(8) loops. The packing is consolidated by C—H...O interactions.

N-(Pyrazin-2-yl)adamantane-1-carboxamide

Molecules of the title compound, C15H19N3O, are composed of an adamantine unit and a pyrazine ring connected to each other through an amide bond. The H—N—C=O moiety is close to planar [C—N—C—O and C—N—C—C torsion angles of 4.7 (2) and −173.8 (1)°, respectively]. The N3—C5 bond has partial double-bond character [1.370 (1) Å]. The geometries of the pyrazine ring and the adamantane substituent are normal and in good agreement with closely related structures. In the crystal, molecules are connected by N—H...O hydrogen bonds, forming zigzag chains in the [001] direction and are arranged in a herringbone fashion.

Crystal structure of bis-(allyl-ammonium) oxalate

The title salt, 2C₃H₈N⁺·C₂O₄²⁻, crystallized with six independent allyl­ammonium cations and three independent oxalate dianions in the asymmetric unit. One of the oxalate dianions is nearly planar [dihedral angle between CO₂ planes = 1.91 (19)°], while the other two are twisted with angles of 11.3 (3) and 26.09 (13)°. One cation has a synperiplanar (cis) conformation with an N—C—C—C torsion angle of 0.9 (3)°, whereas the five remaining cations are characterized by gauche arrangements, with the N—C—C—C torsion angles ranging from 115.9 (12) to 128.8 (3)°. One of the allyl­ammonium cations is positionally disordered (fixed occupancy ratio = 0.45:0.55). In the crystal, the cations and anions are connected by a number of strong N—H⋯O and N—H⋯(O,O) hydrogen bonds, forming layers parallel to (001), with the vinyl groups protruding into the space between the layers.

Crystal structure of iso-butyl-ammonium hydrogen oxalate hemihydrate

In the title hydrated mol­ecular salt, C₄H₁₂N⁺·C₂HO₄⁻·0.5H₂O, the O atom of the water mol­ecule lies on a crystallographic twofold axis. The dihedral angle between the CO₂ and CO₂H planes of the anion is 18.47 (8)°. In the crystal, the anions are connected to each other by strong near-linear O—H⋯O hydrogen bonds. The water mol­ecules are located between the chains of anions and iso­butyl­amine cations; their O atoms participate as donors and acceptors, respectively, in O—H⋯O and N—H⋯O hydrogen bonds, which form channels (dimensions = 4.615 and 3.387 Å) arranged parallel to [010].

Crystal structure of allyl-ammonium hydrogen succinate at 100 K

The asymmetric unit of the title compound, C₂H₈N⁺·C₄H₅O₄⁻, consists of two allyl­ammonium cations and two hydrogen succinate anions (Z′ = 2). One of the cations has a near-perfect syn-periplanar (cis) conformation with an N—C—C—C torsion angle of 0.4 (3)°, while the other is characterized by a gauche conformation and a torsion angle of 102.5 (3)°. Regarding the anions, three out of four carboxilic groups are twisted with respect to the central C–CH₂–CH₂–C group [dihedral angles = 24.4 (2), 31.2 (2) and 40.4 (2)°], the remaining one being instead almost coplanar, with a dihedral angle of 4.0 (2)°. In the crystal, there are two very short, near linear O—H⋯O hydrogen bonds between anions, with the H atoms shifted notably from the donor O towards the O⋯O midpoint. These O—H⋯O hydrogen bonds form helical chains along the [011] which are further linked to each other through N—H⋯O hydrogen bonds (involving all the available NH groups), forming layers lying parallel to (100).

Allyl-ammonium hydrogen oxalate hemihydrate

In the title hydrated mol-ecular salt, C3H8N(+)·C2HO4 (-)·0.5H2O, the water O atom lies on a crystallographic twofold axis. The C=C-C-N torsion angle in the cation is 2.8 (3)° and the dihedral angle between the CO2 and CO2H planes in the anion is 1.0 (4)°. In the crystal, the hydrogen oxalate ions are linked by O-H⋯O hydrogen bonds, generating [010] chains. The allyl-ammonium cations bond to the chains through N-H⋯O and N-H⋯(O,O) hydrogen bonds. The water mol-ecule accepts two N-H⋯O hydrogen bonds and makes two O-H⋯O hydrogen bonds. Together, the hydrogen bonds generate (100) sheets.