Metallkomplexe von Heteroarenen, VII: Bis(2,3,5,6-tetramethyl-η6-pyrazin)vanadium, der erste Bis(aren)metall-Komplex mit vier N-Atomen in der Koordinationssphäre / Metal Complexes of Heteroarenes, VII: Bis(2,3,5,6-tetramethyl-η6-pyrazine)vanadium, the First Bis(arene)metal Complex with Four N Atoms in the Coordination Sphere

Cocondensation of 2,3,5,6-tetramethyl-pyrazine with vanadium atoms affords [2,3,5,6-(CH3)4-η6-1,4-C4N2]2V (6) which has been studied by X-ray crystallography, EPR-spectro-scopy and cyclic voltammetry. As shown by unequal metal ring-atom distances (V—C = 219.6, V—N = 210.9 pm) the ligand in 6 is folded along the N—N axis (dihedral angle 13.6°). The conformation adopted by the cyclic ligands in the crystal has a torsional angle of 44.2° which comes close to the angle of 45° expected for maximum back-bonding, the filled vanadium orbitals 3 dx2-γ2and 3 dxy interacting separately with unoccupied b3u(π*) orbitals of the individual pyrazine rings. The hyperfine coupling constant a(51V) = 4.45 mT attests to comparatively small spin density at the metal nucleus. It is proposed, that substantial transfer of spin density is effected by V 3dz2→ag(σn+) overlap, the latter function representing the in-phase combination of the nitrogen “lone pair” orbitals. Contrary to bis(η6-phosphinine)vanadium 8, for the pyrazine complex 6 only the anion 6⁻ is formed reversibly in cyclovoltammetry, oxidation to 6+ being irreversible. However, in common with 8, the redox potentials for 6 show pronounced anodic shifts. Attempts towards η6-coordination of 2,4,6-trimethyl-1,3,5-triazine failed. Apparently, the 1:1 ratio of CH3/N is insufficient to furnish the triazine ring with the appropriate combination of π-donor and δ-acceptor properties essential for synergetic bond formation.

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Structure and Stability of Aromatic Nitrogen Heterocycles Used in the Field of Energetic Materials

Molecules ◽

10.3390/molecules25143232 ◽

2020 ◽

Vol 25 (14) ◽

pp. 3232

Author(s):

He-Hou Zong ◽

Chuang Yao ◽

Chang Q Sun ◽

Jian-Guo Zhang ◽

Lei Zhang

Keyword(s):

Energetic Materials ◽

Nitrogen Heterocycles ◽

Lone Pair ◽

Molecular Architecture ◽

Lone Pair Electron ◽

Nitrogen Lone Pair ◽

The Individual ◽

First Time ◽

Broad Interest ◽

Chemistry Community

Understanding the stabilization of nitrogen heterocycles is critical in the field of energetic materials and calls for innovative knowledge of nitrogen aromatics. Herewith, we report for the first time that nitrogen lone pair electron (NLPE) delocalization in five-membered nitrogen heterocycles creates a second σ-aromaticity in addition to the prototypical π-aromaticity. The NLPE delocalization and the attendant dual-aromaticity are enhanced as more carbon atoms in the ring are substituted by unsaturated nitrogen atoms. The presence of adjacent nitrogen atoms in the ring can enhance the aromaticity of the nitrogen heterocycles and improve in-crystal intermolecular binding strength but will decrease the firmness of the individual molecular architecture. Notably, such σ-aromaticity is not present in six-membered nitrogen heterocycles, probably due to the longer bonds and broader regions of their rings; therefore, six-membered heterocycles present overall lower aromaticity than five-membered heterocycles. This work brings new knowledge to nitrogen aromatics and is expected to inspire broad interest in the chemistry community.

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Structure, Charge and Spin Density in Na2Ni(CN)4.3H2O at 295 and 30 K

Acta Crystallographica Section B Structural Science ◽

10.1107/s0108768198002286 ◽

1998 ◽

Vol 54 (5) ◽

pp. 600-612 ◽

Cited By ~ 4

Author(s):

H. Ptasiewicz-Bak ◽

I. Olovsson ◽

G. J. McIntyre

Keyword(s):

Neutron Diffraction ◽

Crystal Field ◽

Spin Density ◽

Lone Pair ◽

Water Molecules ◽

Sodium Ions ◽

X Ray Diffraction ◽

X Ray ◽

Deformation Density ◽

The Individual

The earlier reported structure of the title compound, disodium tetracyanonickelate(II) trihydrate, Na2Ni(CN)4.3H2O, has been found to be incorrect and has now been redetermined. The charge distribution has been determined by multipole refinements against single-crystal X-ray diffraction data. In the refinement based on 30 K data a comparison was made between the results obtained using hydrogen positions and displacement parameters from X-ray diffraction with those using the values determined by neutron diffraction. The spin density was investigated by polarized neutron diffraction at 1.6 K. Crystal data: at T = 30 K: a = 7.278 (4), b = 8.856 (5), c = 15.131 (8) Å, α = 89.32 (5), β = 87.39 (4), γ = 83.61 (4)°; at 295 K: a = 7.392 (4), b = 8.895 (4), c = 15.115 (8) Å, α = 89.12 (2), β = 87.46 (2), γ = 84.54 (2)°. The structure contains practically square planar Ni(CN)_4^{2-} ions, which are stacked on top of each other in almost linear chains along the a direction. The separation between the Ni(CN)_4^{2-} planes is rather large, with Ni—Ni distances around 3.7 Å. The six crystallographically independent water molecules are each coordinated to two sodium ions, approximately in the tetrahedral (lone-pair) directions, and the polarizing influence of these sodium ions also appears to be reflected in the deformation density in the lone-pair plane. The charge density based on the deformation functions of all atoms in the structure is compared with the individual densities calculated from the deformation functions of only nickel or the separate water molecules. In this way the effects of simple superposition of the individual densities have been studied. In the planar Ni(CN)_4^{2-} ion the individual deformation density of nickel is in qualitative agreement with that expected from crystal-field theory. As the repulsion from the electrons is much weaker perpendicular to the Ni(CN)_4^{2-} plane than within this plane, the deformation density is considerably larger in the perpendicular direction. However, the largest maxima in the individual deformation density around nickel are not found precisely in the planes defined by nickel and the four cyanide ligands or in the perpendicular direction just mentioned, which illustrates that it is necessary to consider the crystal field due to the whole crystalline environment.

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Structure and bonding in cyclic phosphoramidates as determined by carbon-13 magnetic resonance

Canadian Journal of Chemistry ◽

10.1139/v79-004 ◽

1979 ◽

Vol 57 (1) ◽

pp. 21-26 ◽

Cited By ~ 5

Author(s):

Gerald W. Buchanan ◽

Frederick G. Morin

Keyword(s):

Magnetic Resonance ◽

Nitrogen Atom ◽

Electron Pair ◽

Chemical Shifts ◽

Lone Pair ◽

Ring Conformation ◽

Delocalized Electron ◽

Nitrogen Lone Pair ◽

Structure And Bonding

13C chemical shifts and 13C–31P couplings are reported for 11 cyclic phosphoramidates of ring sizes from four to nine. Vicinal couplings are compared with those of carbocyclic analogs and provide insight regarding the degree of nitrogen lone pair derealization into the N—P bond. For six-membered and larger rings, there appears to be nearly complete lone pair delocalization, i.e., a trigonal planar nitrogen atom. In azetidine derivatives the nitrogen lone pair remains localized, giving rise to a highly puckered ring conformation. Pyrrolidine derivatives are viewed as having a nitrogen with a partially delocalized electron pair.

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Interaction between an Amide Nitrogen Lone Pair and a Noncontiguous Carbonyl Carbon and its Effect on Amide Rotation

Nature ◽

10.1038/230457a0 ◽

1971 ◽

Vol 230 (5294) ◽

pp. 457-459 ◽

Cited By ~ 5

Author(s):

P. S. PORTOGHESE ◽

J. G. TURCOTTE

Keyword(s):

Lone Pair ◽

Amide Nitrogen ◽

Carbonyl Carbon ◽

Nitrogen Lone Pair ◽

Amide Rotation

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Characterization and Catalytic Activity of Poly(4-Vinylpyridine-Co-Divinylbenzene)-Co2+ Complex

Materials Science Forum ◽

10.4028/www.scientific.net/msf.494.363 ◽

2005 ◽

Vol 494 ◽

pp. 363-368 ◽

Cited By ~ 7

Author(s):

D. Lončarević ◽

Ž. Čupić

Keyword(s):

Catalytic Activity ◽

Molecular Level ◽

Supported Catalysts ◽

Lone Pair ◽

Polymer Support ◽

Polymeric Complex ◽

Reaction Conditions ◽

Pyridine Nitrogen ◽

Nitrogen Lone Pair ◽

Thermal Stability Of

Poly(4-vinylpyridine-co-divinylbenzene)-Co2+ was characterized using infrared spectroscopy (IR), thermogravimetric analysis (TG-DTA), N2-physisorption and polarography. Thermal analysis suggests sufficient thermal stability of the polymer support, under reaction conditions. From polarography measurements, the Co2+ content on polymer-supported catalysts is estimated and it was proved that no significant leaching occurred during the activity tests. At the molecular level, FTIR of P4VP-DVB-Co2+ reveals that the pyridine nitrogen lone pair coordinates to the metal center in the polymeric complex. The obtained P4VP-DVB-Co2+ catalysts performed interesting catalytic activity in reaction of the cyclohexane oxidation with air, indicating that increasing Co2+ content lowers the initiation temperature and raises the decomposition of cyclohexylhydroperoxide.

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The N-atom in [N(PR3)2]+ cations (R = Ph, Me) can act as electron donor for (pseudo) anti-electrostatic interactions

CrystEngComm ◽

10.1039/c5ce00352k ◽

2015 ◽

Vol 17 (20) ◽

pp. 3768-3771 ◽

Cited By ~ 11

Author(s):

Antonio Bauzá ◽

Antonio Frontera ◽

Tiddo J. Mooibroek ◽

Jan Reedijk

Keyword(s):

Hydrogen Bonding ◽

Electron Donor ◽

Molecular Hydrogen ◽

Electrostatic Interactions ◽

Lone Pair ◽

Dft Study ◽

Nitrogen Lone Pair

A CSD analysis and DFT study reveal that the nitrogen lone-pair in [N(PPh3)2]+ is partially intact and involved in intramolecular hydrogen bonding.

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Ab initio study of the nitrogen lone-pair proximity effect on a methyl C-H bond in 2-methylpyridine

Journal of Molecular Structure THEOCHEM ◽

10.1016/0166-1280(92)80071-s ◽

1992 ◽

Vol 254 ◽

pp. 271-277 ◽

Cited By ~ 5

Author(s):

M.F. Tufró ◽

R.H. Contreras ◽

J.C. Facelli

Keyword(s):

Ab Initio ◽

Proximity Effect ◽

Lone Pair ◽

Ab Initio Study ◽

Nitrogen Lone Pair

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A Discussion on photoelectron spectroscopy - Orderings of π and σ ionization potentials in carbocyclic and heterocyclic aromatic compounds

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1970.0066 ◽

1970 ◽

Vol 268 (1184) ◽

pp. 131-140 ◽

Cited By ~ 27

Keyword(s):

Photoelectron Spectroscopy ◽

Aromatic Compounds ◽

Lone Pair ◽

Ionization Potentials ◽

Photoelectron Spectra ◽

Light Sources ◽

Orbital Energies ◽

Substituted Pyridines ◽

Nitrogen Lone Pair ◽

Electron Correlation Effects

Data on calculated orbital energies and experimentally measured ionization potentials of carbocyclic and heterocyclic aromatic compounds are compared and contrasted. The ordering or orbital energies and ionization potentials do not always seem to parallel one another, probably owing to either electron correlation effects, or to deviations from Koopman’s theorem. The effects on photoelectron spectra of using different light sources and analysers are discussed in relation to their bearing on the orbital orderings of aromatic compounds. The high resolution He 584 A. photoelectron spectrum of pyridine is shown to be open to two interpretations regarding the ordering of the ionization potentials of the π orbitals and the ‘nitrogen lone pair’ (n). One of the interpretations involves the three lowest pyridine ionization potentials being π (9.2 eV), π L (9.5 eV) and n (10.5 eV) whilst the other has the first three ionization potentials being the order π , n, π . The photoelectron spectra of substituted pyridines and diazines are discussed in the light of the two possible explanations for the pyridine spectrum.

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Finding the best-fit background function for whole-powder-pattern fitting using LASSO combined with tree search

Journal of Applied Crystallography ◽

10.1107/s1600576720016751 ◽

2021 ◽

Vol 54 (2) ◽

Author(s):

Hideo Toraya

Keyword(s):

Linear Regression Analysis ◽

Powder Pattern ◽

Tree Search ◽

Data Set ◽

Tree Structures ◽

Root Node ◽

X Ray Crystallography ◽

Pattern Decomposition ◽

The Individual ◽

Best Fit

A new linear function for modelling the background in whole-powder-pattern fitting has been derived by applying LASSO (least absolute shrinkage and selection operator) and the technique of tree search. The background function (BGF) consists of terms b n L(2θ/180)−n/2 and b n H(1 − 2θ/180)−n/2 for the low- and high-angle sides, respectively. Some variable parameters of the BGF should be fixed at zero while others should be varied in order to find the best fit for a given data set without inducing overfitting. The LASSO algorithm can automatically select the variables in linear regression analysis. However, it finds the best-fit BGF with a set of adjustable parameters for a given data set while it derives a different set of parameters for a different data set. Thus, LASSO derives multiple solutions depending on the data set used. By regarding the individual solutions from LASSO as nodes of trees, tree structures were constructed from these solutions. The root node has the maximum number of adjustable parameters, P. P decreases with descending levels of the tree one by one, and leaf nodes have just one parameter. By evaluating individual solutions (nodes) by their χ2 index, the best-fit single path from a root node to a leaf node was found. The present BGF can be used simply by varying P in the range 1–10. The BGF thus derived as a final single solution was incorporated into computer programs for Pawley-based whole-powder-pattern decomposition and Rietveld refinement, and the performance of the BGF was tested in comparison with the polynomials currently widely used as the BGF. The present BGF has been demonstrated to be stable and to give an excellent fit, comparable to polynomials but with a smaller number of adjustable parameters and without introducing undulation into the calculated background curve. Basic algorithms used in statistics and machine learning have been demonstrated to be useful in developing an analytical model in X-ray crystallography.

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