Pinastric acid revisited: a complete NMR and X-ray structure assignment

Chemical investigation of a terrestrial lichen has yielded the pulvinic acid derivative pinastric acid (4). The structure of 4 was secured by detailed spectroscopic analysis as well as via a single X-ray diffraction study. This is the first report of the X-ray structure and 2D NMR assignment of pinastric acid (4). Pinastric acid (4) displayed antitumour, antiviral and antimicrobial (both antibacterial and antifungal) activities. Whilst the antiviral and antimicrobial activities are consistent with previous findings of 4 this is the first report of the antitumour properties for the compound.     

Synthesis and structural characterization of lanthanide picrate complexes with a new binaphthylamide

Solid complexes of lanthanide picrates with N-Ethyl-2-{2′-[(ethyl-phenyl-carbamoyl)-methoxy]-[1,1′]binaphthalenyl-2-yloxy}-N-phenyl-acetamide (L), [Ln(pic)₃L] (Ln=La, Tb, Y), have been prepared and characterized by elemental analysis, IR and ¹H NMR spectra. The molecular structure of [Tb(pic)₃L] shows that the Tb(III) ion is nine-coordinated by four oxygen atoms from the L and five from two bidentate and one unidentate picrates. The complex forms a 1D supramolecular structure along z-axis.     

Synthesis, mechanism and NMR spectra of lanthanide complexes with a novel unsymmetrical Schiff base

Owing to its two unsymmetrical-NH_2 groups sited on different terminals, 2, 6-diaminocaproic acid (lysine) was used as a reactant for synthesizing a novel unsymmetrical Schiff base with salicylaldehyde on one side and ovanillin on the other for the first time. It is a new way to synthesize such a special unsymmetrical Schiff base. It is named "hetero bis-Schiff base" for distinguishing it from others. The synthesis method, formation mechanism as well as twelve new lanthanide complexes with the above ligand are reported and discussed herein. They were characterized by elementary analysis, molar conductivity, IR-spectra and especially by ~1H and ~(13)C NMR spectra. The results obtained may provide a new promising method for synthesizing similar unsymmetrical Schiff bases and their complexes.     

异双Schiff碱及其稀土配合物的合成、机理与波谱

利用2,6-二氨基己酸(赖氨酸)具有两端不对称结构的－NH~2基,合成了一端与水杨醛,另一端与2,4-二羟基苯甲醛缩合形成空间结构不对称的异双Schiff碱及其与稀土元素配合物。以元素分析、热分析、摩尔电导、红外光谱、EPR以及^1HNMR,^1^3C-^1HCOSY谱、固体高分辨^1^3C谱等表征,研究了这类不对称Schiff碱的形成机理和配位方式。     

4f-4f hypersensitivity in the absorption spectra and NMR studies on paramagnetic lanthanide chloride complexes with 1,10-phenanthroline in non-aqueous solutions

The optical absorption and NMR studies of trivalent lanthanide chloride complexes with 1,10-phenanthroline (phen) are presented and discussed. The 1H NMR spectra of the complexes of La, Pr, Nd, Eu and Yb have been studied in methanol-d(4). The resonances of phen in the NMR spectra of the paramagnetic complexes have been shifted to lower as well as higher fields, which is a manifestation of dipolar interaction. The H (2) protons of the heterocyclic amine display broad resonances. The degree of broadening in Pr, Nd, and Yb complexes follows the order Pr < Nd < Yb. The inter- and intra-molecular shift ratios show that the paramagnetic shift is predominantly due to dipolar interaction. The electronic spectra of Pr, Nd, Ho and Er complexes have been investigated in methanol, pyridine, DMSO and DMF, which reveal that the hypersensitive transitions exhibit larger variation in oscillator strength values and band shapes. The change in the coordination geometry of the complexes and relative basicity of ligand are found responsible for oscillator strength and band shape variation. The interelectronic repulsion and covalency parameters show covalent nature of bonding between the metal and the ligand.     

H-bonding effects on the IR and NMR spectra of N-tosyl-amino acid 2,6-bishydroxylmethyl pyridine monoesters

H-bonding in the title monoesters affects the most utilized types of spectrometries-infrared (IR) and nuclear magnetic resonance (NMR). The shifts of the three kinds of A-H stretching bands to lower wavenumbers in the vibrational spectra, and the 1H chemical shifts, which are to the low field, are observed and reasonably explained in this paper.     

Synthesis, infrared and fluorescence spectra of lanthanide complexes with a new amide-based 1,3,4-oxadiazole derivative

A new amide-based 1,3,4-oxadiazole derivative ligand 2,5-bis[2-(N,N-diethyl-1'-oxopropylamide)phenyl]-1,3,4-oxadiazole (L) and its complexes, Ln(NO3)3L (Ln=La, Eu, Gd, Tb, Er), were synthesized. The complexes were characterized by elemental analysis, infrared spectra and conductivity. The lanthanide ions were coordinated by O atoms from CO. The fluorescence properties of Eu(NO3)3L and Tb(NO3)3L in the solid state and in different solvents were investigated. Under the excitation of UV light, these complexes exhibit characteristic fluorescence of europium and terbium ions. The solvent factors influencing the fluorescent intensity were discussed.     

Thermal and spectroscopic investigations of new lanthanide complexes with 1,2,4-benzenetricarboxylic acid

The new 1,2,4-benzenetricarboxylates of lanthanide(III) of the formula Ln(btc)·nH₂O, where btc is 1,2,4-benzenetricarboxylate; Ln is La-Lu, and n=2 for Ce; n=3 for La, Yb, Lu; and n=4 for Pr-Tm were prepared and characterized by elemental analysis, infrared spectra and X-ray diffraction patterns. Polycrystalline complexes are isotructural in the two groups: La-Tm and Yb, Lu. IR spectra of the complexes show that all carboxylate groups from 1,2,4-benzentricarboxylate ligands are engaged in coordination of lanthanide atoms. The thermal analysis of the investigated complexes in air atmosphere was carried out by means of simultaneous TG-DTA technique. The complexes are stable up to about 30°C but further heating leads to stepwise dehydration. Next, anhydrous complexes decompose to corresponding oxides. The combined TG-FTIR technique was employed to study of decomposition pathway of the investigated complexes.     

Complete assignment of 1H and 13C NMR spectra of new pentacyclic triterpene acid benzyl esters

Complete assignments of ¹H and ¹³C NMR chemical shifts for newly synthesized benzyl esters of oleanolic, ursolic and crataegolic acid based on DEPT, HSQC, HMBC, COSY, TOCSY, NOE and ROESY experiments are reported. Copyright © 2002 John Wiley & Sons, Ltd.     

Solution-state NMR spectroscopy of famotidine revisited: spectral assignment, protonation sites, and their structural consequences

Multinuclear one (1D-) and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopic investigations of famotidine, the most potent and widely used histamine H₂-receptor antagonist, were carried out in dimethyl sulfoxide-d₆ (DMSO-d₆) and water. Previous NMR assignments were either incomplete or full assignment was based only on 1D spectra and quantum-chemical calculations. Our work revealed several literature misassignments of the ¹H, ¹³C, and ¹⁵N NMR signals and clarified the acid–base properties of the compound at the site-specific level. The erroneous assignment of Baranska et al. (J. Mol. Struct. 2001, 563) probably originates from an incorrect hypothesis about the major conformation of famotidine in DMSO-d₆. A folded conformation similar to that observed in the solid-state was also assumed in solution, stabilized by an intramolecular hydrogen bond involving one of the sulphonamide NH₂ protons and the thiazole nitrogen. Our detailed 1D and 2D NMR experiments enabled complete ab initio ¹H, ¹³C, and ¹⁵N assignments and disproved the existence of the sulphonamide NH hydrogen bond in the major conformer. Rather, the molecule is predominantly present in an extended conformation in DMSO-d₆. The aqueous acid–base properties of famotidine were studied by 1D ¹H- and 2D ¹H/¹³C heteronuclear multiple-bond correlation (HMBC) NMR-pH titrations. The experiments identified its basic centers including a new protonation step at highly acidic conditions, which was also confirmed by titrations and quantum-chemical calculations on a model compound, 2-[4-(sulfanylmethyl)-1,3-thiazol-2-yl]guanidine. Famotidine is now proved to have four protonation steps in the following basicity order: the sulfonamidate anion protonates at pH = 11.3, followed by the protonation of the guanidine group at pH = 6.8, whereas, in strong acidic solutions, two overlapping protonation processes occur involving the amidine and thiazole moieties.     

Structural and spectral assignment by 2D NMR of a new prenylated benzopyrancarboxylic acid and structural reassignment of a related compound

A new prenylated benzopyrancarboxylic acid, 1a (3,4‐dihydro‐5‐hydroxy‐2,7‐dimethyl‐8‐(2‐methyl‐2‐butenyl)‐ 2‐(4‐methyl‐1, 3‐pentadienyl)‐2H‐1‐benzopyran‐6‐carboxylic acid) was isolated from Peperomia amplexicaulis and fully characterized by 1D and 2D NMR and high‐resolution mass spectrometry. In the course of this investigation, the structure of a related compound (minus the carboxylic acid group) which was previously assigned as 2b was corrected to structure 1b . Copyright © 2003 John Wiley & Sons, Ltd.     

Temperature effects in x-ray photoelectron spectra of paramagnetic cupric and iron complexes with anomalous magnetic properties

The temperature dependence of the XPS spectra of paramagnetic cupric and iron complexes with anomalous magnetic properties is studied. It was found that the XPS spectra of polynuclear complexes with antiferromagnetic interaction, such as cupric acetate, do not change with temperature, although their magnetic moments diminish essentially. The Fe 2p spectra of mononuclear iron (III) complexes with the spin multiplicity transitions S = $\text{1}\text{2}$ ? S = $\text{5}\text{2}$ exhibit temperature-dependent reversible alterations of shake-up satellite intensity which correlate with the spin state of the paramagnetic Fe(III) ion. The results obtained prove the mechanism of appearance of intensive shake-up satellites in XPS spectra of paramagnetic 3d element complexes, which relates shake-up excitations with the interaction of the photoelectron with unpaired valence 3d electrons during photoionization.     

The NMR spectra of porphyrins-25: Meso-substituent effects in neutral and protonated porphyrins

The meso (methine) substituent chemical shifts (SCS) of a range of common functional groups have been obtained both for the neutral porphyrin molecule, and for the corresponding dications, in substituted octaethylporphyrin (OEP), etioporphyrin-I, and pyrroetioporphyrin-XV derivatives. The SCS are discussed in terms of both ring current variations and specific effects at the neighboring betasubstituents and the meso-proton opposite the perturbing substituent, using a ring current model to quantify the former. In the neutral molecules, meso substitution in OEP (Me, NO₂, CN, CHO) causes a 10% decrease in the macrocyclic ring current, and marked anisotropic shifts at the beta-positions flanking the meso function. The meso-NH₂ group introduces a much larger decrease (ca 35%) in the main ring current, due to conjugation of the amino group with the porphyrin π-system. In the porphyrin dications, SCS are much larger and there is some evidence of a concomitant decrease in the ring current of the adjacent pyrrole subunits. The meso-NMe₂; substituent at the γ-meso-position in pyrroetioporphyrin-XV has only small SCS in the neutral molecule, but a large shift (similar to that of NH₂) in the dication, due to the different orientation of the substituent with respect to the porphyrin plane in each case. The meso-OH substituent in the oxophlorin from etioporphyrin-I behaves as a conjugated OH group in the dication. The anomalous position of the meso-proton opposite to the perturbing substituent is noteworthy, and this could be due to electronic (resonance) effects, or to some protonation at this position.     

Two new Ni(II) Schiff base complexes: X-ray absolute structure determination,synthesis of a N-labelled complex and full assignment of its H NMR and C NMR spectra

The Ni(II) complex of the Schiff base of (S)-N-(2-benzoyl-4-chlorophenyl)-1-benzylpyrrolidine-2-carboxamide and glycine (1) [GKCl] and the hemihydrate of the Ni(II) complex of the Schiff base of (S)-N-(2-benzoylphenyl)-1-benzylpyrrolidine-2-carboxamide and 2-aminoisobutiric acid (2) [Me₂GK] were prepared and their absolute structures determined. The conformations of the complexes and their hydrogen bonding are discussed in detail. In complex 2, the repulsion between the benzyl group and an equatorial methyl group should affect the conformation of the benzyl group, distribution of the π-electron density in this group and distortion of the internal coordination sphere, while for complex 1, only minor conformational changes were expected. ¹H and ¹³C NMR data for the ¹⁵N-labelled complex 2 were acquired and fully assigned in order to study the influence of π-electron density of the benzyl group to the long-range ¹³C–¹⁵N and ¹³C–¹³C spin–spin interactions.     

Preparation of new mixed ligand complexes of lanthanide acetylacetonate with dibenzo-18-crown-6 and their IR spectra

Fourteen new mixed ligand complexes of lanthanide acetylacetonate and dibenzo-18- crown-6 (DB18C6) were prepared by three different methods. The composition of the complexes was shown to be Ln(acac)· DB18C6 by elemental analysis and X-ray powder diffraction. Infrared spectra of the complexes were investigated, and the results indicate that their absorption bands in the region of 400—4000 cm^?1 are complicated and without much difference from each other, but in the far infrared region various absorption bands of Ln—O can be observed and they changes regularly with the atomic number of lanthanides.     

Studies on 1H NMR, infrared and fluorescence spectra of rare earth complexes with a new binaphthylamide

Solid complexes of lanthanide picrates with N,N-dibenzyl-2-[2'-[(dibenzylcarbamoyl)-methoxy]-[1,1']binaphthalenyl-2-yloxy]-acetamide (L), [Ln(pic)(3)L] (Ln = La, Nd, Eu, Gd, Tb, Dy, Y), have been prepared and characterized by elemental analysis, IR, 1H NMR, UV-Vis spectra and conductivity measurements. The fluorescence property of the europium complex in solid state and in CHCl(3), acetone, AcOEt and DMF was studied.     

Synthesis and structural characterization of diorganotin(IV) complexes with 2,6‐pyridinedicarboxylic acid

Two new diorganotin(IV) derivatives of 2,6‐pyridinedicarboxylic acid, {[Ph₂Sn(2,6‐C₅H₃N)(COO)₂][Na(2,6‐C₅H₃N)(COOH) (COO)(CH₃OH)₂]} ( 1 ) and [Me₂Sn(2,6‐C₅H₃N)(COO)₂(H₂O)]^•H₂O ( 2 ) were synthesized by the reaction of Ph₃SnCl and PhMe₂SnI with 2,6‐pyridinedicarboxylic acid, respectively in the presence of sodium methoxide or potassium iso‐propoxide. The prepared compounds were characterized by mass spectrometry, IR, ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopies. The molecular structures of both complexes were determined by a single‐crystal X‐ray analysis. The X‐ray structure revealed pentagonal bipyramidal geometry around the tin atom for compound 1, which is incorporated with a hexacoordinated monosodium derivative of 2,6‐pyridinedicarboxylic acid. Complex 2 adopts a monomeric structure with two carboxylate oxygen atoms coordinated to tin in monodenate form from equatorial positions, and the coordination number is raised to six as the oxygen of water and pyridine nitrogen occupies the other equatorial positions of octahedron. Copyright © 2007 John Wiley & Sons, Ltd.     

Detailed 1H and 13C NMR structural assignment of ent-polyalthic acid,a biologically active labdane diterpene

In this paper, a complete ¹H and ¹³C NMR data assignment of ent-polyalthic acid, a biologically active labdane-type diterpene, is presented. The assignments were carried on the basis of spectroscopic data from ¹H NMR, ¹³C{¹H} NMR, gCOSY, gHMQC, and gHMBC experiments. Furthermore, a software-assisted methodology, using FOMSC3_rm_NB and NMR_MultSim programs, supported the detailed and unequivocal assignment of ¹H and ¹³C signals, allowing all hydrogen coupling constants to be determined and thus clarifying all hydrogen signal multiplicities.     

Relaxometric and solution NMR structural studies on ditopic lanthanide(III) complexes of a phosphinate analogue of DOTA with a fast rate of water exchange

A novel "ditopic" ligand containing two monophosphinate triacetate DOTA-like units linked by a thiourea bridge has been synthesized and its complexes with Ln3+ ions (Ln = Y, Eu, Gd, Dy) investigated by NMR spectroscopy and relaxometry. The presence of one water molecule in the first coordination sphere has been determined by the measurement of the dysprosium(III)-induced 17O NMR shifts. The 1H and 31P NMR spectra of the Eu(III) derivative indicate a higher abundance of the fast-exchanging twisted square antiprismatic (m) isomer than the isomeric square antiprismatic (M; m/M = 3:2) complex. The analysis of the 89Y and 13C T1 NMR relaxation times in the Gd(III)/Y(III) mixed complex have provided useful structural information. Values of ca. 6.3 and 8.2 A for the Gd...Y and Gd...C distances, respectively, have been estimated which indicate a rather compact solution structure. This result finds support in the value of the relaxivity whose increase (at 20 MHz and 298 K) on passing from the monomeric (5.7 s(-1) mM(-1)) to the ditopic complex (8.2 s(-1) mM(-1)) can be attributed to the doubling of the inner-sphere term following the doubling of the molecular size. The structural and dynamic relaxivity-controlling parameters were assessed by a simultaneous fitting of the variable temperature 17O NMR and 1H NMRD relaxometric data. The mean water residence lifetime (298tauM) has been found to be 53 ns, one of the shortest values reported for ditopic complexes. The reorientational correlation time is two times longer (298tauR = 183 ps) than the corresponding value of the parent monomeric Gd(III) complex, thus supporting the view of a limited degree of internal rotation. The possible influence of magnetic Gd-Gd coupling has been excluded by a comparison of the 1H NMRD profiles of the homodinuclear Gd(III)/Gd(III) and the heterodinuclear Gd(III)/Y(III) complexes.     

The solution structure of rhombic lanthanide complexes analyzed with a Model-free and crystal-field independent paramagnetic NMR method: application to nonaxial trimetallic complexes [LnxLu(3-x)(TACI-3H)2(H2O)6]3+ (x = 1-3)

The model-free approach has been extended with the derivation of a novel three-nuclei crystal-field independent method for investigating isostructurality in nonaxial (i.e., rhombic) complexes along the lanthanide series. Application of this technique to the heterotrimetallic sandwich complexes [LnLu2(TACI-3H)2(H2O)6]3+, which possess a single C2v-symmetrical paramagnetic center, unambiguously evidences isostructurality for Ln = Pr-Yb, while the variation of the second-rank crystal-field parameters and along the series prevents reliable structural analyses with the classical one-nucleus equation. Extension toward polymetallic magnetically noncoupled rhombic lanthanide complexes in [Ln2Lu(TACI-3H)2(H2O)6]3+ (two paramagnetic centers with Cs microsymmetry) and [Ln3(TACI-3H)2(H2O)6]3+ (three paramagnetic centers with C2v microsymmetry) requires only minor modifications of the original three-nuclei equation. Isostructurality characterizes [Ln2Lu(TACI-3H)2(H2O)6]3+ (Ln = Pr-Yb), while [Ln3(TACI-3H)2(H2O)6]3+ exhibit a structural change between Eu and Tb which results from the concomitant contraction of the three metallic centers. Particular attention has been focused on (i) the stepwise increase of contact (i.e., through-bond) and pseudocontact (i.e., through-space) contributions when the number of paramagnetic centers increases, (ii) the assignment of 13C resonances in the strongly paramagnetic complexes [Ln3(TACI-3H)2(H2O)6]3+ (Ln = Tb-Yb) for which reliable T1 measurements and [1H-13C] correlation spectra are not accessible, and (iii) the combination of crystal-field dependent and independent methods for analyzing the paramagnetic NMR spectra of axial and nonaxial lanthanide complexes.