S. P. Babailov

NMR METHODS FOR MOLECULAR STRUCTURE STUDIES OF PARAMAGNETIC LANTHANIDE COMPLEXES IN SOLUTIONS. APPLICATIONS TO CROWN ETHER COMPLEXES

The literature data on NMR spectroscopic methods for investigation of the spatial structure and molecular dynamics of lanthanide complexes with macrocyclic potyethers in solutions are reviewed.

Lanthanides as NMR Probes of Fast Molecular Dynamics at High Magnetic Fields and Temperature Sensors: Conformational Interconversion of Erbium Ethylenediaminetetraacetate Complexes

(1)H NMR measurements are reported for D2O solutions of paramagnetic complex [Er(H2O)(EDTA(4-))](-) (I) for temperature interval 273-319 K. Diamagnetic complex [Lu(H2O)(EDTA(4-))](-) (II) was used as an NMR reference compound. The spectra obtained have been analyzed using a band-shape analysis technique in the framework of dynamic NMR (DNMR) taking into account the temperature dependence of lanthanide-induced shifts. Intramolecular dynamics in I was assigned to the interconversion of Δ-λE-δδδδ and Δ-δE-δδδδ conformers with estimated activation free energy ΔG(‡)(298 K) = 50 ± 4 kJ/mol. The methodology of paramagnetic 4f-element probe applications on the example of Er(3+) for the study of free-energy changes in chemical exchange processes, as well as the advantages of this method in comparison with DNMR studies of diamagnetic substances, is discussed. In accordance with the literature reviewed, the fulfilled experimental study is the first example of intramolecular dynamics determination for erbium complexes. An additional advantage of the investigation is in the approach proposed which extends the range of measurement of the NMR rate constants for paramagnetic 4f-element complexes compared to diamagnetic ones. Coordination compounds investigated represent a new type of thermometric NMR sensors and lanthanide paramagnetic probes for in situ temperature control in solutions.

NMR Studies of Mixed-Ligand Lanthanide (III) Complexes. Peculiarities of Molecular Structure, Dynamics and Paramagnetic Properties for Cerium Subgroup Chelates with Crown Ethers

The molecular structure, dynamics and paramagnetic properties of the complex cations [Ln(ptfa)2 (18-crown-6)]+ in deuterated toluene were studied for Ln = La, Ce, Pr and Nd. The activation enthalpy values of 68 ± 5, 55 ± 15 and 60 ± 13 kJ mol-1 for the 18-crown-6 conformationalinversion processes for the complexcations of Ce, Pr and Nd, respectively,were obtained. Quantitativeinvestigation of the lanthanide-induced chemical shifts shows a monotonic change of a spatial structure and magnetic susceptibility in comparison with the Bleaney predicted dependence. The free energy of molecular inversion activation for 18-crown-6 molecules in the complex cation [Ln(fod)2(18-crown-6)]+ is 74 ± 9 kJ mol-1 at 363 K, which is a little more than the value of the free energy of activation 64 ± 9 kJ mol-1 at 363 K in the complex cation [Ln(ptfa)2(18-crown-6)]+.

Azo-linked high-nitrogen energetic materials

In this review, we document the most recent developments in azo-linked heteroaromatic (imidazole, pyrazole, triazole, triazine, tetrazine and oxadiazole) high-nitrogen energetic materials and their salts. In this account, a wide range of azo materials such as coordination polymers and metal–organics are also included. The synthesis of these azo based compounds, their physical and explosive properties are presented. These energetic compounds generally exhibit desirable properties and performance, which in some cases are superior to those of the common energetic materials RDX, TNT, and TATB.

Protonation and acylation of the heterometallic complexes (μ-H)Os3(μ-O2CC5H4FeCp)(CO)10 and Fe{(μ-O2CC5H4)(μ-H)Os3(CO)10}2

The reactions of the heterometallic complexes (μ-H)Os3(μ-O2CC5H4FeCp)(CO)10 (1) and Fe{(μ-O2CC5H4)(μ-H)Os3(CO)10}2 (2) with CF3COOH, CF3SO3H, and AcCl were studied. The reaction of 1 with CF3COOH involves interaction with the Cp ligands, protonation of the O atom of the bridging carboxylate group, and oxidative degradation of the complex. At low concentrations, CF3SO3H protonates the O atom of the bridging carboxylate group, while at high concentrations, degradation of the complex takes place. The reaction of complex 2with either CF3COOH or low concentrations of CF3SO3H results in successive elimination of two [(μ-H)Os3(CO)10] cluster fragments due to protonation of the O atoms of the carboxylate groups. In the case of high CF3SO3H concentrations, the Os—Os bonds of both cluster fragments of 2 are also protonated to give the [Fe{(μ-O2CC5H4)(μ-H)2Os3(CO)10}2]2+ dication. The Friedel—Crafts acylation of 1 takes place only when a large excess of AcCl and AlCl3 is used to give two new complexes, (μ-H)Os3(μ-O2CC5H4FeC5H4C(O)CH3)(CO)10 and (μ-H)Os3(μ-O2CC5H3C(O)CH3FeCp)(CO)10 in a 2 : 1 ratio.

Photochemical reaction of the heterometallic complex (μ-H)Os3{μ-O2CC5H4Mn(CO)3}(CO)10 with triphenylphosphine

Photolysis of the heterometallic complex (μ-H)Os3{μ-O2CC5H4Mn(CO)3}(CO)10 together with PPh3 results in replacement of the CO groups by PPh3 both at the Mn atom and in the Os3 metallocycle to afford the complexes (μ-H)Os3{μ-O2CC5H4Mn(CO)2PPh3}(CO)10, (μ-H)Os3{μ-O2CC5H4Mn(CO)3}(CO)9}(CO)9PPh3, and (μ-H)Os3{μ-O2CC5H4Mn(CO)2PPh3}(CO)9PPh3 (two isomers). The reaction is also accompanied by the partial removal of the Mn(CO)3 group followed by the protonation of the cyclopentadienyl group and formation of triosmium clusters (μ-H)Os3(μ-O2CC5H4R}(CO)10 (R=H, Et).

NMR studies of hetero-ligand complexes of the lanthanides. Quantitative analysis of lanthanide-induced shifts

NMR (~H, 19F) and NMR relaxation spectroscopy has been used to investigate the structure of the ion pair complexes formed by the interaction of the Tris-pivaloyltrifluoroacetonates of lanthanides of the cerium subgroup with the macrocyclic ether 18-crown-6. It is suggested that the observed monotonic change in the structure of the cations of the ion pairs with increase in the atomic number of the lanthanide ion is due to the effect of the lanthanide contraction. The results obtained are compared with the results of x-ray analysis of related complex compounds.

NMR methods for molecular structure studies of paramagnetic lanthanide complexes in solutions. Applications to crown ether complexes

The literature data on NMR spectroscopic methods for investigation of the spatial structure and molecular dynamics of lanthanide complexes with macrocyclic polyethers in solutions are reviewed.

Dynamic NMR under nonstationary conditions: Theoretical model, numerical calculation, and potential of application

An expression has been derived for the time dependence of the NMR line shape for systems with multi-site chemical exchange in the absence of spin-spin coupling, in a zero saturation limit. The dynamics of variation of the NMR line shape with time is considered in detail for the case of two-site chemical exchange. Mathematical programs have been designed for numerical simulation of the NMR spectra of chemical exchange systems. The analytical expressions obtained are useful for NMR line shape simulations for systems with photoinduced chemical exchange.

Crystal structure of pentafluoroacetanilide

A new compound C6F5N(H)C(O)CH3 pentafluoroacetanilide, which is promising for the formation of coordination compounds with metal cations, is synthesized. Its crystal structure is determined (APEX DUO diffractometer, λMoKα, graphite monochromator, T = 150 K). Crystallographic data for C8H4F5NO are as follows: space group C2/c, a = 18.2460(7) Å, b = 10.5840(4) Å, c = 9.4489(4) Å, β = 116.198(1)°, V = 1637.28(11) Å3, Z = 8. The compound has a molecular structure formed of isolated pentafluoroacetanilide molecules. The average distances in the phenyl ring are 1.39 Å for C–C and 1.34 Å for C–F. The molecular structure of the studied compound is confirmed by 19F and 1H NMR in the solution CCl4/CDCl3 = 1:1 (v:v).