305 research outputs found
Evidence for Intramolecular N-H...O Resonance-Asssisted Hydrogen Bonding in Beta-Enaminones and Related Heterodienes. A Combined Crystal-Structures, IR and NMR Spectroscopic, and Quantum-Mechanical Investigation
No full textThe resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between pi -delocalization and hydrogen-bond (H-bond) strengthening originally introduced (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Sec. 1989, 111, 1023; Bertolasi, V.; Gilli, P.; Ferretti, V.1 Gilli, G. J. Aln. Chem. Sec. 1991, 113, 4917) for explaining the abnormally strong intramolecular O-H...O bonds formed by the ...O=C-C=C-OH... beta -enolone fragment I which are typical of B-diketone enols. The applicability of this model to the intramolecular N-H...O hydrogen bonds formed by a number of heteroconjugated systems (...O=C-C=C-NH..., beta -enaminones II; O=C-C=N-NH..., ketohydrazones III; and ...O=N-C=C-NH..., nitrosoenamines IV) is investigated. The X-ray crystal structures of five molecules which close a six-membered ring by an intramolecular N-H...O bond through the resonant ...O=X-C=X-NH... (X = C, N) fragments II-IV are compared to those of two other molecules closing the same ring through the nonresonant ...O=C-C-C-NH... beta -aminone moiety V. Experimental findings are complemented by a CSD (Cambridge Structural Database) search of all compounds forming intramolecular N-H...O bonds through the molecular fragments II-V and by a comprehensive analysis of the IR v(NH) stretching frequencies and H-1 NMR delta (NH) chemical shifts available for compounds of these classes of known crystal structure. It is shown that all the descriptors of H-bond strength [d(N...O) shorthening, decrease of v(NH), increase of delta (NH), and increase of pi -delocalization within the heteroconjugated fragment] are mutually intercorrelated according to RAHB rules, which can then account for the strength of heteronuclear N-H...O bonds in II-IV as well as for that of the homonuclear O-H...O bonds in I. Heteronuclear N-H...O bonds appear, however, to have distinctive features. In particular, their strength turns out to be partially hampered by the proton affinity difference (BPA) between the N and O atoms, so that very strong H-bonds (2.65 greater than or equal to d(N...O) greater than or equal to 2.48 Angstrom, 3200 greater than or equal to v(NH) greater than or equal to 2340 cm(-1), 13 less than or equal to delta (NH) less than or equal to 18 ppm) can occur only when the pi -delocalization of the heterodienic moiety is associated with proper electron-attracting substituents which are able to decrease this Delta PA by increasing the NH acidity. Moreover, at variance with strong O-H...O RAHBs, whose protons are mostly found in nearly symmetrical positions, even the strongest N-H...O RAHBs are highly dissymmetric, despite the very similar changes undergone by both IR and H-1 NMR spectra in O-H...O and N-H...O H-bonded systems. Specificities of heteronuclear H-bonds are shown to be interpretable by the electrostatic-covalent H-bond model (ECHBM) which was previously developed for the homonuclear case (Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, C. J. Am. Chem. Sec. 1994, 116, 909). The conclusions drawn are corroborated by extended DFT quantum-mechanical calculations at the B3LYP/6-31+G(d,p)B3LYP/6-31+G(d,p) level of theory and by full geometry optimization carried out on 27 variously substituted heterodienes II-IV and nonresonant beta -aminones v. calculations allow the estimation of H-bond energies that are found to be approximately 2.75 kcal mol(-1) for nonresonant V and 5.22, 6.12, and 7.03 kcal mol(-1) for unsubstituted resonant II, III, and IV, respectively
An X-ray Crystallographic Study of 1,4-Di[(E)-2-(p-tolyl)-1- diazenyl]piperazine
No full textThe crystal structure of methyl 1,4-di[(E)-2-
(p-tolyl)-1-diazenyl]piperazine (4) has been determined by
single crystal X-ray diffraction analysis. The bis-triazene (4)
adopts a normal chair conformation in the piperazine
ring, with puckering parameters : u2 =-29(5)°,QT =
0.534(2) A and theta2 = 177.0(2)°. The crystal structure of 4 is
compared with the structure of the triazene (2a) and the
closely related bis-triazenes (3 and 5a). The piperazine ring
of 2a and 4 adopt a typical chair conformation, whereas the
piperazine ring of 3 adopts an unusual pseudo-boat confor-
mation. Crystal data: 4 C18H22N6, triclinic, space group
P1, a = 6.8925(2) A , b = 7.8574(3) A , c = 16.8856(8) A ,
a = 103.103(2)°, b = 90.528(2)°, c = 101.776(2)° and
V = 870.44 (6) A^3 , for Z = 2
Out-of-plane deformation pathways of the R(X=)C-NR2 fragment present in amides, thioamides, amidines, enamines, and anilines. A concerted study making use of structural data, molecular mechanics, and ab initio calculations
No full textThe R(X=)C-NR(1)R(2) (R(1), R(2) = alkyl groups) fragment is present in many classes of molecules and assumes, usually, a planar conformation owing to the C-N partial double-bond character. It can undergo,however, a cis-trans isomerization process by rotation around the C-N bond and concomitant nitrogen pyramidalization. In a previous paper (Gilli, G.; Bertolasi, V.; Bellucci, F.; Ferretti, V. J. Am. Chem. Sec. 1986, 108, 2420) the isomerization pathway was mapped by the use of some 90 crystal structures containing the fragment of interest, and a semiempirical potential giving the total energy of the fragment during its deformation was proposed. In the present work the previous sample of crystal structures is updated to the current state of the crystallographic databases; the observed geometries are compared with the out-of-plane deformation energy maps obtained by ab initio SCF calculations at the 4-31G level for sample molecules (thioformamide, formamide, formamidine, vinylamine, and aniline) representative of the five chemical classes investigated. It is shown that the originally proposed potential is validated by this analysis and that the values of the energetic barriers involved in the reaction, evaluated from the ab initio energy maps, are in good agreement with the available experimental data. It is found, moreover, that there is a specific class of compounds (o- and p-nitroanilines and polyconjugated enamines and amidines) which deviate from the general behavior. The crystal structures of two of these compounds are reported, and possible reasons for the discrepancies discussed
Ansa-dicyclopentadienyl Uranium (IV) Complexes Containing the potentially Chelating and/or Metal-Bridging Ligand [(CH2C5H4)2-2,6-C5H3N]2–.
No full textEvidence for Resonance-Assisted Hydrogen Bonding. 2.1Intercorrelation between Crystal Structure and Spectroscopic Parameters in Eight Intramolecularly Hydrogen Bonded 1,3-Diary 1–1,3-propanedione Enols
No full textCrystal structure analysis of eight 1,3-diaryl-1,3-propanedione enols has been accomplished with the aim of investigating in more detail the intramolecular hydrogen bond formed by the H-C=C-C=O fragment characterizing beta-diketone enols; structural data were correlated with spectroscopic parameters, that is IR nu(OH) stretching frequencies and H-1 NMR chemical shifts of the enolic proton. Present experimental data show that this hydrogen bond is characterized by the following interrelated features: (i) very short O- -O distances (2.432-2.554 angstrom); (ii) strong delocalization in the heteroconjugated fragment; (iii) lengthening of the O-H bond (to 1.20 angstrom); (iv) lowering of the nu(OH) frequencies (2566-2675 cm-1) and downfield shift of the enolic proton resonance (15.3-17.0 ppm). The data can bc interpreted by (and are in support of) the RAHB (resonance assisted hydrogen bond) model previously suggested (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023) for explaining the unusual features the hydrogen bond displays in these compounds. It is shown that the model is able to interpret fine structural details such as the general weakening of the intramolecular hydrogen bond caused by electron-donating 1,3-substituents or additional hydrogen bonds accepted by the carbonyl and the preference displayed by the proton for dwelling on the carbonyl oxygen having the smaller negative charge induced both by the nature of 1,3-substituents or by intermolecular hydrogen bonds or short contacts
Chelate polypyridine ligand rearrangement in Au(III) complexes
No full textThe reaction of gold(III) neutral complexes
AuBr(CN)2(N–N) {N–N = 2,20-bipyridine (bpy), 5,50-
dimethyl-2,20-bipyridine (Me2bpy), 1,10-phenanthroline
(phen)} with a stoichiometric amount of K[AuCl4] 2H2O
in nitromethane at room temperature led to the formation
of 1:1 electrolytes which were characterized by NMR and
IR spectroscopy, conductivity measurements, elemental
analyses and X-ray diffraction. Both the anions and the
cations of these salts are singly charged square-planar
Au(III) complexes and the cations have general formula
[AuCl2(N–N)]+. A hypothesis on the possible reaction
mechanisms is presented to give an explanation for the
formation of the reaction products
Reactivity of Yttrium Quinoline-Imine-Phenoxide Complexes Towards Inter- and Intramolecular Alkyl Nucleophilic Attacks
No full textYttrium(III) chloride complexes [YCl2(NNOH)] (1H) and
[YCl2(NNOMe)] (1Me) {NNOH = 2-tert-butyl-6-(quinolin-8-yliminomethyl)
phenoxide; NNOMe = 2-tert-butyl-6-[(2-methylquinolin-
8-ylimino)methyl]phenoxide} were synthesized by
treating a thf solution of YCl3 at room temperature with the
NNOH-H and NNOMe-H ligands {NNOH-H = 2-tert-butyl-6-
(quinolin-8-yliminomethyl)phenol; NNOMe-H = 2-tert-butyl-
6-[(2-methylquinolin-8-ylimino)methyl]phenol}, which were
previously deprotonated with potassium tert-butoxide or
thallium ethoxide. Surprisingly, reaction of 1H with the Grignard
reagent MeMgBr to prepare the corresponding dialkyl
derivative led to the formation of the tetranuclear magnesium dimer [Mg2BrCl{NN(Me)OH}(thf)]2 {NN(Me)OH =
2-tert-butyl-6-[1-(quinolin-8-ylamido)ethyl]phenoxide} (2),
whose structure was determined by single-crystal X-ray diffraction.
By reacting the neutral ligand NNOH-H with
Y(CH2SiMe3)3·2thf in toluene at low temperature the alkyl
complex [Y(CH2SiMe3){NN(CH2SiMe3)O}(thf)] {NN(CH2Si-
Me3)O = 2-tert-butyl-6-[1-(quinolin-8-ylamido)-2-trimethylsilanylethyl] phenoxide} (3) was isolated. The supposed reaction mechanism leading to the formation of 3 was simulated
by using the PM6 Hamiltonian
Application of the Free Wilson Model to the analysis of three different pharmacological activities tests in benzodiazepines
No full text---The Free-Wilson model has been applied to the analysis of activity data of 55 benzodiazepines derived from Footshock, Inclined Screen and Pentylenetetrazole tests. The comparison of the individual group contributions shows a similar trend for Footshock and Inclined Screen tests, while some differences are observed with respect to the Pentylenetetrazole test. The most relevant differences seem to be associated with the presence of highly electron-withdrawing substituents in positions 7 and 2'
Palladium(II) and platinum(II) pyrrolate-quinoline-imine chlorocomplexes by metal-assisted condensation reactions
No full textTautomerism and hydrogen bonding in reaction products from 4-aminopent-3-en-2-one and diazonium ions
No full textThe azo-coupling products have been prepared from 4-aminopent-3-en-2-one and 4-chloro- or 4-nitrobenzenediazonium ion.
Structure of the products obtained was studied by means of multinuclear NMR spectroscopy in CDCl3 solutions and X-ray
diffraction in solid state. Both products are predominantly present as azo forms in both the solution and the solid state, the azo
form content being higher in solid state. The substituent effect on azo-hydrazone tautomeric equilibrium of the substances is
more significant in solution than in solid state
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