NMR Crystallography Enhanced by Quantum Chemical Calculations and Liquid State NMR Spectroscopy for the Investigation of Se-NHC Adducts**

N-heterocyclic carbene ligands (NHC) are widely utilized in catalysis and material science. They are characterized by their steric and electronic properties. Steric properties are usually quantified on the basis of their static structure, which can be determined by X-ray diffraction. The electronic properties are estimated in the liquid state; for example, via the ⁷⁷Se liquid state NMR of Se-NHC adducts. We demonstrate that ⁷⁷Se NMR crystallography can contribute to the characterization of the structural and electronic properties of NHC in solid and liquid states. Selected Se-NHC adducts are investigated via ⁷⁷Se solid state NMR and X-ray crystallography, supported by quantum chemical calculations. This investigation reveals a correlation between the molecular structure of adducts and NMR parameters, including not only isotropic chemical shifts but also the other chemical shift tensor components. Afterwards, the liquid state ⁷⁷Se NMR data is presented and interpreted in terms of the quantum chemistry modelling. The discrepancy between the structural and electronic properties, and in particular the π-accepting abilities of adducts in the solid and liquid states is discussed. Finally, the ¹³C isotropic chemical shift from the liquid state NMR and the ¹³C tensor components are also discussed, and compared with their ⁷⁷Se counterparts. ⁷⁷Se NMR crystallography can deliver valuable information about NHC ligands, and together with liquid state ⁷⁷Se NMR can provide an in-depth outlook on the properties of NHC ligands.     

The chemistry of polyfluoroaromatic derivatives of groups va and via elements and their electronic structure

A review is given of recent syntheses of polyfluoroaromatic derivatives of group VA and VIA elements and their reactivity. The reactivity of N,N-dichloropolyfluoroanilines, polyfluorinated azoxybenzenes and other compounds is analysed. A new approach to oxidation of group VA elements in strong oxygen-containing acids is discussed. UV photoelectron (He I), X-ray fluorescent and Raman spectroscopic data are reported. Influence of the electronic structure of fluorine on polyfluorinated aromatic system is discussed on the basis of FKα -emission spectra. A relationship has been obtained between the electronic structure and the reactivity of polyfluoroaromatic derivatives of group VA and VIA elements.     

NMR structural aspects of the chemistry of V, Mo, W polyoxometalates

The results of structural investigations of vanadium, molybdenum, and tungsten polyoxoanions (PA) by ¹⁷O, ⁵¹V, ⁹⁵Mo, ¹⁸³W, and heteroatom NMR are generalized in this review. NMR spectroscopy possibilities to determine the structure of PA are discussed. NMR data on PA of different structures compositions nuclei are demonstrated.     

Effect of conformational control of chiral oxazaborolidine by π-π stacking interaction of a pentafluorophenyl group toward asymmetric borane reduction

A pentafluorophenyl group can act as a stereo-controlling group in oxazaborolidine-catalyzed asymmetric borane reduction through intramolecular π-π stacking interaction with a phenyl group. The intramolecular π-π interaction in oxazaborolidine bearing pentafluorophenyl group is confirmed by calculations and ¹H NMR study. The interaction affects the enantioselectivity of the asymmetric reduction of acetophenone while the extent is small.     

The through-space transmission of 77Se — 77Se coupling constants

The IPPP-INDO method is applied to study different interactions as well as transmission mechanisms of ⁷⁷Se — ⁷⁷Se coupling constants in cis- and trans-1,2-bis(methylseleno)ethene. Preferential conformations were determined using a modified version of the PCILO-CNDO method. The preferential conformation of the cis compound is such that nonbonding electron pairs of both Se atoms overlap to some extent. In this conformation a large and positive through-space transmission of the Fermi contact term of the ⁷⁷Se — ⁷⁷Se coupling takes place, which strongly depends on the Se — Se distances. The similarity between this coupling and others involving different nuclei is discussed. The total calculated coupling is in fair agreement with experimental values for similar compounds. The ⁷⁷Se — ⁷⁷Se coupling in the trans-compound is largely dominated by the orbital interaction, which is mainly transmitted through the vinyl π-electron system.     

The Influence of C(sp3)H–Selenium Interactions on the 77Se NMR Quantification of the π-Accepting Properties of Carbenes

Selenium NMR has become a standard tool for scaling the π-accepting character of carbenes. Herein, we highlight that non-classical hydrogen bonding (NCHB), likely resulting from hyperconjugation, can play a significant role in the carbene–selenium ⁷⁷Se NMR chemical shift, thus triggering a non-linear behavior of the Se-Scale.     

Etude par resonance magnetique nucleaire de composes organiques contenant des chalcogenes: Partie IV. Le disulfure de diphényle et ses analogues sélénié et telluré

NMR measurements (¹H, ¹³C, ⁷⁷Se) of diphenyl dichalcogenides are reported. They confirm the molecular conformation deduced from other physical experiments. Steric inhibitions are responsible for the conformation in the Te compound, while, in the S and Se compounds, the heteroatom lone pairs interact to some extent with the π systems of the rings.More fundamental discussions of the measured parameters are made, including the somewhat controversial existence of the title compounds in solution.     

Selenium-77 nuclear magnetic resonance in mono- and disubstituted benzo[b]selenophenes

The ⁷⁷Se chemical shifts of 79 mono- or disubstituted benzo[b]selenophenes are reported. The results in the 2- and 3-substituted derivatives closely parallel those previously obtained in the corresponding selenophenes. The measured parameters also correlate well with those measured in benzo[b]tellurophenes. The ¹²⁵Te and ⁷⁷Se chalcogen nuclei appear to be much more sensitive to substituent effects than the more classical nuclei. The effects of Cr(acac)₃, Eu(dpm)₃ and concentration on the ⁷⁷Se NMR spectra are briefly discussed.     

High-Resolution 77Se NMR of Ammonium Pyroselenite (NH4)2Se2O5 Crystals

The high-resolution ⁷⁷Se NMR spectra of ammonium pyroselenite crystals were recorded in the cross-polarization mode. The angular dependences of the chemical shifts of resonance lines with respect to liquid H₂SeO₄ were used to determine the chemical shift tensor parameters for the ⁷⁷Se nuclei of the Se₂O₅ pyroselenite ion.     

Galectin–Glycan Interactions: Guidelines for Monitoring by 77Se NMR Spectroscopy,and Solvent (H2O/D2O) Impact on Binding

Functional pairing between cellular glycoconjugates and tissue lectins like galectins has wide (patho)physiological significance. Their study is facilitated by nonhydrolysable derivatives of the natural O-glycans, such as S- and Se-glycosides. The latter enable extensive analyses by specific ⁷⁷Se NMR spectroscopy, but still remain underexplored. By using the example of selenodigalactoside (SeDG) and the human galectin-1 and -3, we have evaluated diverse ⁷⁷Se NMR detection methods and propose selective ¹H,⁷⁷Se heteronuclear Hartmann–Hahn transfer for efficient use in competitive NMR screening against a selenoglycoside spy ligand. By fluorescence anisotropy, circular dichroism, and isothermal titration calorimetry (ITC), we show that the affinity and thermodynamics of SeDG binding by galectins are similar to thiodigalactoside (TDG) and N-acetyllactosamine (LacNAc), confirming that Se substitution has no major impact. ITC data in D₂O versus H₂O are similar for TDG and LacNAc binding by both galectins, but a solvent effect, indicating solvent rearrangement at the binding site, is hinted at for SeDG and clearly observed for LacNAc dimers with extended chain length.     

17O NMR spectroscopy in the study of group 14 organometallic compounds

A study of Group 14 organometallic compounds using ¹⁷O NMR spectroscopy carried out at the Latvian Institute of Organic Synthesis has been reviewed. The main areas of ¹⁷O NMR application—studies of electronic effects, conformational analysis and coordinative interactions—have been characterized.     

NMR Spectra of Cyclic Nitrones. 7. The Influence of Substituents and a Hydrogen Bond on <Superscript>14</Superscript>N and <Superscript>17</Superscript>O Chemical Shifts in Derivatives of 3-Imidoazoline 3-Oxide

Derivatives of 3-imidazoline 3-oxide have been studied by ¹⁴N and ¹⁷O NMR methods. Regularities of the influence of substituents and of a hydrogen bond on chemical shifts have been made apparent. The range of changes of the chemical shifts of the nitrogen and oxygen nuclei of the nitrone group has been determined. Both in the ¹⁷O and in the ¹⁴N NMR spectra the signals of the amino derivatives are the highest field signals for the nitrone group, and the lowest field signals are the signals of the cyano derivatives in the series of derivatives investigated. Depending on the substituent (from amino to cyano group) the ¹⁷O chemical shifts varied over a range ∼155 ppm, but the interval of change of the ¹⁴N chemical shifts for the same substituents was ∼110 ppm. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1336–1341. September, 2005.     

NMR Spectra and Structure of Phosphorus-Containing Cycles with Acyclic P=Se and Cyclic P-C Bonds

Abstract

Using ¹H, ¹³C, ³¹P, ⁷⁷Se NMR it has been established that six-membered rings of 5-seleno-5-phenyl-2,2-dimetiiyl-l,3,5-dioxaphosphorinanes, 5-seleno-1,3,5-diazaphosphorinanes and 5-seleno-5-phenyl (alkyl)-2,2-dimethyl-1,3,2,5-dioxa-silaphosphorinanes in the solution are in chair conformation with the equatorial PSe group, whereas the eight-membered ring of 6-seleno-2,2,6-trimethyl-1,3,2,6-dioxa-silaphosphocane is mainly in the chair-chair conformation with the symmetry plane passing through the silicon atom and the axially oriented P=Se group. The selenium nuclear chemical shifts δ77_se -266–416 ppm in these compounds depend on the size of the cycle, and on the effect of the substituent of the cycle. The phosphorus nuclear chemical shifts δ31_pO-50 ppm depend on the phosphorus valence angle at the cycle and the electron infulence of the closest phosphorus substituents. For the stereoisomer pairs of the six-membered cycles the ³¹P nuclei in the isomer with the equatorial P=Se group have been shown to be magnetically screened to a greater degree. The spin-spin coupling of the directly bonded nuclei ¹J_PSe?708–752, ¹J_PC45–48, ¹J_CH128–152 Hz depends both on the size of the cycle and the character of substituents in it. The values of ¹J_PSeare affected by the position of substituents in the bicycles. The overall values of ¹J_PSe are greater for the axial orientation of the P=Se group than for the equatorial.     

Solution and solid state NMR studies of some selenium analogues of auranofin (an anti-arthritic gold drug)

Three mixed ligand complexes of gold(I) with phosphines and selenones, [Et₃PAuSe=C<]Br as analogues of auranofin (Et₃PAuSR) have been prepared and characterized by elemental analysis, IR and NMR methods. A decrease in the IR frequency of the C=Se mode of selenones upon complexation is indicative of selenone binding to gold(I) via a selenone group. An upfield shift in ¹³C NMR for the C=Se resonance of the selenones and downfield shifts in ³¹P NMR for the R₃P moiety are consistent with the selenium coordination to gold(I). ¹³C solid state NMR shows the chemical shift difference between free and bound selenone to gold(I) for ImSe and DiazSe to be ca 10 and 17?ppm respectively. Large ⁷⁷Se NMR chemical shifts (55?ppm) upon complexation in the solid state for [Et₃PAuDiazSe]Br compared to [Et₃PAuImSe]Br (10?ppm) indicates the former to be more stable and the Au–Se bond to be stronger than in the latter complex.     

Effect of fluorine replacement of hydrogen atoms in aromatic compounds on magnetic shielding of nuclei of heavy atoms at a distance from position of substitution

Conclusions From¹³C,¹⁵N,¹⁷O,¹⁹F,²⁹Si,³¹P,⁷³Ge,⁷⁷Se,¹¹⁹Sn, and¹²⁵Te NMR data, it follows that complete replacement of phenyl hydrogen atoms by fluorine in compounds C₆H₅X leads to a systematic increase in shielding of nuclei in the ring carbon atom bonded to the X group and in shielding of the nucleus of the -atom of X, along with deshielding of the nuclei of the - and -atoms of X. The effect evidently has a long-range character and can be described by the extended theory of Dewar and Kelemen.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 829–832, April, 1989.     

Electron affinities of double bond π* orbitals determined by means of electron transmission spectroscopy

The electron transmission spectra of small molecules containing C=C, C=N, C=O, C=S and N=N double bonds are reported. The electron affinities of these functional groups, associated with electron capture into their empty π^* orbitals, are discussed in terms of heteroatom electronegativities, geometrical variations and localization properties of the π^* orbitals. The largest electron acceptor properties were observed in the thioketone derivative, which generates a stable π anion state. The ionization energy values relating to the heteroatom lone pair and the filled π orbitals are also reported.     

Mesoionic compounds — structure and NMR parameters

It has been shown that¹H,¹³C,¹⁴N,¹⁵N,¹⁷O, and⁷⁷Se NMR are very successful techniques for the identification and investigation of potential valence tautomerism in all types of mesoionic compounds. Especially the combined use of¹⁴N and¹⁵N NMR studies has been shown to be very effective in solving structural problems. From¹⁵N NMR it is possible to obtain accurate chemical shifts and spin-spin couplings, whereas the¹⁴N NMR spectra provide us with nuclear relaxation and some chemical shift data. In particular¹⁴N NMR is very useful for the identification of charged nitrogen atoms in molecules containing non equivalent nitrogen positions. This information is available from the relative¹⁴N signal widths, which depend upon the rates of¹⁴N relaxation, positively charged nitrogen atoms usually have relatively slow¹⁴N relaxation rates thus giving rather sharp NMR signals. Some solid state, CP MAS,¹³C and¹⁵N NMR results are available for comparison with the solution NMR data.Published in Khimiya Geterotsiklischeskikh Soedinenii, No. 9, pp. 1180–1199. September 1995.     

Physical- and Bio-Organic Chemstry of Nonbonded Selenium· · ·Oxygen Interactions

Weak nonbonded interaction between a divalent selenium and an oxygen atom (i.e., Se···O interaction) frequently plays important roles in chemical and biological functions of selenium compounds. To establish that ⁷⁷ Se NMR is an easy experimental probe to diagnose the strength of an Se···O interaction, 3 series of 2-substituted benzeneselenenyl derivatives, which have an intramolecular Se···O interaction in solution, were employed. By comparing the ⁷⁷ Se NMR chemical shifts (δ _Se ) with those observed for other series of selenium compounds, which have an intramolecular Se···Y (Y = N, O, F, Cl, or Br) interaction, approximate linear correlation was found between the δ _Se values and the strengths of the nonbonded Se···Y interactions evaluated by natural bond orbital analysis at the B3LYP level. The correlation will be useful for estimating the strength of an Se···O interaction simply from the ⁷⁷ Se NMR chemical shift. By extending the chemistry of nonbonded Se···O interactions to structural biology, analogous S···O interactions have been discovered in protein architecture. The directional features were, however, different from those of Se···O and S···O interactions of small organic compounds.     

Complexation of Zn(II), Cd(II) and Hg(II) with thiourea and selenourea: A 1H, 13C, 15N, 77Se and 113Cd solution and solid-state NMR study

Zinc(II), cadmium(II) and mercury(II) complexes of thiourea (TU) and selenourea (SeU) of general formula M(TU)₂Cl₂ or M(SeU)₂Cl₂ have been prepared. The complexes were characterized by elemental analysis and NMR (¹H, ¹³C, ¹⁵N, ⁷⁷Se and ¹¹³Cd) spectroscopy. A low-frequency shift of the C=S resonance of thiones in ¹³C NMR and high-frequency shifts of N–H resonances in ¹H and ¹⁵N NMR are consistent with sulfur or selenium coordination to the metal ions. The Se nucleus in Cd(SeU)₂Cl₂ in ⁷⁷Se NMR is deshielded by 87?ppm on coordination, relative to the free ligand. In comparison, the analogous Zn(II) and Hg(II) complexes show deshielding by 33 and 50?ppm, respectively, indicating that the orbital overlap of Se with Cd is better. Principal components of ⁷⁷Se and ¹¹³Cd shielding tensors were determined from solid-state NMR data.     

Long-range nuclear spin–spin coupling between selenium-77 and phosphorus-31 in biphosphorus compounds

The coupling constants ⁿJ(⁷⁷Se³¹P), n = 1–4, have been measured in the proton-decoupled ³¹P NMR spectra of a range of diphosphorus selenides and diselenides. ³¹P–{¹H, ³¹P} and ³¹P–{¹H, ⁷⁷Se} triple resonance experiments have been used to establish the signs of the coupling constants, and it is found that both the magnitudes and signs depend upon the stereochemical relationship of the coupled nuclei.