The 13C NMR of olefins complexed with a binuclear Ag(I)–Yb(III) chelate

The effect of silver ion complexation on the ¹³C NMR of several rigid olefin structures has been determined. The silver ion induced chemical shifts (AgIS) are not amenable to easy interpretation. Addition of Yb(fod)₃ forms binuclear Ag–Yb complexes with the olefin. Lanthanide induced shifts (LIS) fall off rationally with distance from the site of complexation and the averaged position of the lanthanide. The complexes may be used as probes of olefin stereochemistry.     

Selection of shift reagents for 17O NMR. Application to line assignments

Chemical shifts and line broadenings induced by a series of lanthanide dipivaloylmethanates Ln(dmp)₃ on the ¹⁷O NMR signal of methanol have been measured. The best high-field shift reagents are those containing the cations Tb⁺³ and Dy⁺³; very large shifts have been observed for even quite low concentrations of reagent. The shift reagents are able to discriminate between different oxygen functions and can, in principle, be used for line assignments.     

Use of silver trifluoroacetate together with lanthanide shift reagents for simplification of 1H NMR spectra of aromatic hydrocarbons

The equimolar mixtures of typical lanthanide shift reagents such as Eu(fod)₃, Pr(fod)₃ or Yb(fod)₃ with silver trifluoroacetate, previously used to induce paramagnetic shifts in the ¹H NMR spectra of alkenes, have been successfully applied to simple aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylenes. In benzene and p-xylene the signals of all the aromatic protons are shifted identically. In other substituted benzenes the magnitude of the induced shift depends on the distance between the proton and the substituents. In addition, the different behaviour of the signals of the methyl groups in meta-and para-xylene on the addition of the complex shift reagent allows the quantitative analysis of the two xylenes in their mixtures.     

Lanthanide(III) Complexes with Two Hexapeptides Incorporating Unnatural Chelating Amino Acids: Secondary Structure and Stability

Unnatural metal‐chelating amino acids bearing aminodiacetate side‐chains have been introduced into two hexapeptides to obtain efficient lanthanide‐binding peptides. The synthesis of the enantiopure Fmoc‐Ada_n(tBu)₂‐OH synthons is described with overall yields of 32 and 50 % for n=2 and n=3 side‐chain carbon atoms, respectively. The two peptides AcWAda_nPGAda_nGNH₂ ( P ⁿ ) were synthesized from the protected synthons by standard solid‐phase peptide synthesis. Studies of the lanthanide complexes of the two peptides P ⁿ by luminescence titrations, mass spectrometry, circular dichroism, and solution NMR spectroscopy demonstrate that the Ada_n chain length has a dramatic effect on the complexation properties. Indeed, the flexible compound P³ forms a mononuclear complex of moderate stability (β₁₁=10^9.9), which tends to transform into a binuclear species in the presence of excess of the metal ion. Interestingly, the more compact peptide P² provides stable Ln³⁺ complexes with the exclusive formation of the mononuclear Ln P² adduct. The stability constant of Tb P² is two orders of magnitude higher (β₁₁=10^12.1) than that measured for P³ . The 800 MHz NMR spectrum of the La³⁺ complex of P² evidences a well‐defined type II β‐turn as well as a hydrophobic Trp(indole)–Pro interaction. These interactions exemplify the non‐innocent character of the peptide spacer in the complex La P² as well as the role of a peptide secondary structure in the stabilization of metal complexes.     

N‐Heterocyclic Tridentate Aromatic Ligands Bound to [Ln(hexafluoroacetylacetonate)3] Units: Thermodynamic,Structural, and Luminescent Properties

Herein, we discuss how, why, and when cascade complexation reactions produce stable, mononuclear, luminescent ternary complexes, by considering the binding of hexafluoroacetylacetonate anions (hfac^?) and neutral, semi‐rigid, tridentate 2,6‐bis(benzimidazol‐2‐yl)pyridine ligands ( Lk ) to trivalent lanthanide atoms (Ln^III). The solid‐state structures of [Ln( Lk )(hfac)₃] (Ln=La, Eu, Lu) showed that [Ln(hfac)₃] behaved as a neutral six‐coordinate lanthanide carrier with remarkable properties: 1) the strong cohesion between the trivalent cation and the didentate hfac anions prevented salt dissociation; 2) the electron‐withdrawing trifluoromethyl substituents limited charge‐neutralization and favored cascade complexation with Lk ; 3) nine‐coordination was preserved for [Ln( Lk )(hfac)₃] for the complete lanthanide series, whilst a counterintuitive trend showed that the complexes formed with the smaller lanthanide elements were destabilized. Thermodynamic and NMR spectroscopic studies in solution confirmed that these characteristics were retained for solvated molecules, but the operation of concerted anion/ligand transfers with the larger cations induced subtle structural variations. Combined with the strong red photoluminescence of [Eu( Lk )(hfac)₃], the ternary system Ln^III/hfac^?/ Lk is a promising candidate for the planned metal‐loading of preformed multi‐tridentate polymers.     

Theoretical studies on the complexation of Eu(III) and Am(III) with HDEHP: structure,bonding nature and stability

Separation of trivalent lanthanides (Ln(III)) and actinides (An(III)) is a key issue in the advanced spent nuclear fuel reprocessing. In the well-known trivalent actinide lanthanide separation by phosphorus reagent extraction from aqueous komplexes (TALSPEAK) process, the organophosphorus ligand HDEHP (di-(2-ethylhexyl) phosphoric acid) has been used as an efficient reagent for the partitioning of Ln(III) from An(III) with the combination of a holdback reagent in aqueous lactate buffer solution. In this work, the structural and electronic properties of Eu³⁺ and Am³⁺ complexes with HDEHP in nitric acid solution have been systematically explored by using scalar-relativistic density functional theory (DFT). It was found that HDEHP can coordinate with M(III) (M=Eu, Am) cations in the form of hydrogen-bonded dimers HL₂^- (L=DEHP), and the metal ions prefer to coordinate with the phosphoryl oxygen atom of the ligand. For all the extraction complexes, the metal-ligand bonds are mainly ionic in nature. Although Eu(III) complexes have higher interaction energies, the HL₂^- dimer shows comparable affinity for Eu(III) and Am(III) according to thermodynamic analysis, which may be attributed to the higher stabilities of Eu(III) nonahydrate. It is expected that this work could provide insightful information on the complexation of An(III) and Ln(III) with HDEHP at the molecular level.     

Nuclear magnetic resonance of psychotherapeutic agents. IV—conformational analysis of 2,3-dihydro-1H-1,4-benzodiazepines

The lanthanide shift reagent (LSR)/¹H NMR study of some 7-chloro-5-phenyl-2,3-dihydro-1 H-1, 4-benzodiazepines shows that these compounds exist in CDCI₃ solution as two pseudo-boat conformers, rapidly interconverting at room temperature. Computer simulated lanthanide induced shifts (LIS) are consistent with LSR complexation by the imine nitrogen atom. The lanthanide shifts the conformational equilibrium towards conformer (a), with a pseudoequatorial 1-substituent: this effect may be the result of the greater basicity of 4-N when the 1-N lone pair is pseudoaxially directed, thus permitting an extended electron delocalization from 1- to 4-N through the fused benzene ring. The detection of H-9/1-Me through space spin-spin coupling in medazepam (7-chloro-1-methyl-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepine) supports the observed higher availability of conformer (a) to LSR. A LIS computer calculation to predict the population ratio results in reasonable agreement with the conformational changes based on the response of ³J(HH) to lanthanide addition. The electronic and steric effects which determine the greater stability of conformer (a) may be effective in the drug-receptor interaction.     

Lanthanide(III) Nitrate Complexes of Two 17 Membered N3O2-Donor Macrocycles

The interaction of lanthanide(III) ions with two N₃O₃-macrocycles, L¹ and L², derived from 2,6-bis(2-formylphenoxymethyl)pyridine and 1,2-diaminoethane has been investigated. Schiff-base macrocyclic lanthanide(III) complexes LnL¹(NO₃)₃ · xH₂O (Ln = Nd, Sm, Eu or Lu) have been prepared by direct reaction of L¹ and the appropriate hydrated lanthanide nitrate. The direct reaction between the diamine macrocycle L² and the hydrated lanthanide(III) nitrates yields complexes LnL²(NO₃)₃· H₂O only for Ln = Dy or Lu. The reduction of the Schiff-base macrocycle decreases the complexation capacity of the ligand towards the Ln(III) ions. The complexes have been characterised by elemental analysis, molar conductivity data, FAB mass spectrometry, IR and, in the case of the lutetium complexes, ¹H NMR spectroscopy.     

Synthesis and Relaxometric Properties of Gadolinium(III) Complexes of New Triazine‐Based Polydentate Ligands

Two new derivatives based on an s‐triazine structural motif were synthesized by attaching two 2,2′‐hydrazinylidenebis[acetic acid] moieties to the triazine ring to reach an overall heptadenticity for the complexation of lanthanide(III) cations. The remaining reactive site was exploited for the substitution with a functionizable amino group (see H₄ L1 ) and a lipophilic moiety (see H₄ L2 ). Luminescence‐lifetime determinations revealed the presence of a single H₂O molecule coordinated for [Eu( L1 )]. A complete ¹H‐NMR relaxometric study was carried out for the octacoordinated [Gd( L1 )] and [Gd( L2 )] complexes. A remarkably long H₂O residence lifetime (²⁹⁸τ_M =5.2 μs) was found by ¹⁷O‐NMR in the case of [Gd( L1 )]. Micelle formation of the lipophilic complex [Gd( L2 )] was evidenced, the critical micellization concentration (cmc) determined, and relaxometric properties of the system investigated.     

Complexation and DFT studies of lanthanide ions by (2-pyridylmethoxy)homooxacalixarene derivatives

The binding of lanthanide cations by 2-pyridylmethoxy derivatives of p-tert-butyldihomooxacalix[4]arene (1b), in the cone conformation, and p-tert-butylhexahomotrioxacalix[3]arene (2b), in both cone and partial cone conformations, was studied. These properties were assessed by extraction studies of the metal picrates from water into dichloromethane and stability constant measurements in methanol and acetonitrile, using spectrophotometric and microcalorimetric techniques. Proton NMR titrations with La³⁺ and Yb³⁺ cations were done in order to get information on the binding sites. Computational methods (density functional theory (DFT) calculations) were also used to complement the NMR data. The p-tert-butylcalix[4]arene analogue (3b) was also studied, and the results of the four ligands were compared. Partial cone-2b is the best extractant for lanthanide ions, showing some preference for the heavy lanthanides. In complexation, all four ligands show the same trend and a high selectivity for Yb³⁺ (ML, log β ≥ 7). Besides the formation of ML complexes, ML₂ species were also obtained. In most cases, these species were corroborated by the proton NMR studies. For partial cone-2b with Pr³⁺ the complexation process is enthalpically driven, whereas for 3b the formation of the ML₂ species with this cation is due to a favourable entropy term. DFT studies indicate that ligand 3b forms the most stable complex with La³⁺, followed by partial cone-2b.     

Complexation of rhodium(II) tetracarboxylates with aliphatic diamines in solution: 1H and 13C NMR and DFT investigations

The complexation of rhodium(II) tetraacetate, tetrakistrifluoroaceate and tetrakisoctanoate with a set of diamines (ethane‐1,diamine, propane‐1,3‐diamine and nonane‐1,9‐diamine) and their N,N′‐dimethyl and N,N,N′,N′‐tetramethyl derivatives in chloroform solution has been investigated by ¹H and ¹³C NMR spectroscopy and density functional theory (DFT) modelling. A combination of two bifunctional reagents, diamines and rhodium(II) tetracarboxylates, yielded insoluble coordination polymers as main products of complexation and various adducts in the solution, being in equilibrium with insoluble material. All diamines initially formed the 2 : 1 (blue), (1 : 1)_n oligomeric (red) and 1 : 2 (red) axial adducts in solution, depending on the reagents' molar ratio. Adducts of primary and secondary diamines decomposed in the presence of ligand excess, the former via unstable equatorial complexes. The complexation of secondary diamines slowed down the inversion at nitrogen atoms in NH(CH₃) functional groups and resulted in the formation of nitrogenous stereogenic centres, detectable by NMR. Axial adducts of tertiary diamines appeared to be relatively stable. The presence of long aliphatic chains in molecules (adducts of nonane‐1,9‐diamines or rhodium(II) tetrakisoctanoate) increased adduct solubility. Hypothetical structures of the equatorial adduct of rhodium(II) tetraacetate with ethane‐1,2‐diamine and their NMR parameters were explored by means of DFT calculations. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and characterisation of chelated (1-cyanoethyl)ruthenium(II) carbonyl complexes

The reaction between acrylonitrile and the RuH bond in HRu(CO)Cl(PPh₃)₃ results in the formation of a binuclear ruthenium(II) complex having chlorine bridges which are easily broken by sodio-derivatives of bidentate chelating ligands giving mononuclear hexacoordinated ruthenium(II) compounds. The RuC bond in these new complexes has been found to be stable towards nucleophilic reagents. The stereochemistry for these complexes has been suggested on the basis of IR, ¹H and ³¹P NMR spectra.     

Features of the Reaction of Heterocyclic Analogs of 2'-Alkoxychalcones with Lanthanide Shift Reagents

The reaction of lanthanide shift reagents with the heterocyclic analogs of substituted 2'-alkoxychalcones was studied. It was shown that the coordination of Eu(fod)³ and Yb(fod)³ with them takes place in different ways. The former forms mainly adducts of the chelate type with participation of the oxygen atoms of the carbonyl and alkoxyl groups in coordination, while from the latter only the adduct at the carbonyl group is obtained. For this reason it was concluded that there are some limitations to the use of the reagents for conformational analysis of organic compounds. It was shown that they can only be used to study the conformational movements of the molecules not affecting the complexation process.     

Effect of solvent on the reactions of coordination complexes: Part 16. Kinetics and mechanism of complexation of oxalatopentaammine-cobalt(III) with nickel(II) in methanol-water medium

The kinetics of reversible complexation of oxalatopentaammine cobalt(III) with Ni²⁺ has been investigated in MeOH + water media (0–50 (v/v) % MeOH) at 15.0–35.0°C and I = 0.10 mol dm^?3. Analysis of rate data indicates that the monobonded complex [(NH₃)₅ · CoOCOCO₂Ni]³⁺ in which Ni²⁺ is bound to the end carboxylate group is the possible reaction intermediate. The formation and dissociation rates of such a species are rate limiting for the overall formation and dissociation of the binuclear species, in which Ni²⁺ is chelated by the oxalate moiety. The rate and activation parameters for formation and dissociation of the binuclear species are moderately solvent sensitive and solvent structural effects are discernible in the nonlinear variations of ΔH^≠ and ΔS^≠ with solvent composition. The log k_r vs. Grunwald Winstein parameter (Y) plot for the dissociation of the binuclear species is markedly nonlinear.     

P NMR study of the stoichiometry, stability and thermodynamics of complex formation between palladium(II) acetate and bis(diphenylphosphino)ferrocene

The complexation reaction between palladium (II) acetate, and 1,1′-bis(diphenylphosphino)ferrocene, DPPF, was investigated in two different deuterated solvents CDCl₃ and DMSO at various temperatures using ³¹P NMR spectroscopy. The exchange between free and complexed DPPF is slow on the NMR time scale and consequently, two ³¹P NMR signals were observed. At metal ion-to-ligand mole ratio larger than 1, only one ³¹P NMR signal was observed, indicating the formation of a 1:1 Pd²⁺–DPPF complex in solution. The formation constant of the resulting 1:1 complexes was determined from the integration of two ³¹P signals. The values of the thermodynamic parameters (ΔH, ΔS and ΔG₂₉₈) for complexation were determined from the temperature dependence of stability constants. It was found that, in both solvents, the resulting complex is mainly entirely enthalpy stabilized and the ΔH compensates the TΔS contribution.     

Dependence of the enthalpies of formation of Ag<Superscript>+</Superscript> complexes with glycinate ion and the protonation of glycinate ion on the content of aqueous ethanol solvent

Enthalpy changes are determined by calorimetry for the reactions of glycinate ion (Gly⁻) proto- nation and its complexation with Ag⁺ ion at a temperature of 298 K and ionic strength 0.1 (NaClO₄) in an aqueous ethanol solvent containing 0.0–0.4 and 0.0–0.3 mole fraction of alcohol, respectively. An abnormal relationship of enthalpy changes is found for the processes of stepwise formation of mono- and bis-glycinates of silver(I) in water. It is shown that varying the ethanol content has virtually no effect on the exothermicity of Ag⁺ complexation reactions with glycinate ions at either coordination step and does not change the relationship of the step enthalpies. An analogy is observed in the relationship of solvation contributions from the reagents to the value of Δ_tr H° for the reactions of glycinate ion protonation and its complexation with silver(I) in aqueous ethanol solvents.     

Wasserstoff/Deuterium-Isotopeneffekte bei Europium-Verschiebungsexperimenten and Allencarbonsäureestern

Hydrogen/Deuterium Isotope Effects on Europium Induced Chemical Shifts of Allenic Esters It is shown by ¹H-NMR. experiments, that in the presence of europium shift reagents, mixtures of allenic esters and their partially deuterated counterparts give rise to α-, β-, γ, and δ-H/D isotope effects (cf. Table and Fig. as well as formula 6 for assignment). The measured effects (0.4 to 2.2%) are mainly attributable to C, H/C, D bond length differences which influence the complexation constant between the shift reagent and the substrate as well as the s-cis/s-trans conformational equilibrium of the allenic esters.     

Kinetics of acid- and iron(III)-catalysed aquation of cis-(salicylato)-(methyl/ethylamine) bis(ethylenediamine)cobalt(III)

Summary The kinetics of spontaneous, acid- and Fe^III-catalysed aquation of cis-[Co(en)₂(RNH₂)(SalH)]²⁺ complexes (R = Me, Et; SalH = C₆H₄(OH)CO _inf2 ^sup- ) were studied in acid perchlorate medium, I = 1.5 mol dm^–3 (NaClO₄) at 70–80°C. The Fe^III-catalysed aquation proceeds via formation of a binuclear species, the evidence of which follows from aquation, complexation and equilibrium studies. The spontaneous aquation rate shows steric acceleration with the increase of the nonlabile amine chain length, while that of acid- and Fe^III-catalysed aquation shows the opposite trend. An attempt is made to explain the discrepancy in the rate on the basis of solvent cosphere effects.     

In situ complexation of rhodium(II) tetracarboxylates with some derivatives of cysteine and related ligands studied by 1H and 13C nuclear magnetic resonance spectroscopy

The complexation of N-phthaloyl, N-formyl, and N,N-dimethyl derivatives of S-methylcysteine methyl ester (both racemic and optically pure) with three dimeric rhodium(II) salts, acetate Rh₂AcO₄, trifluoroacetate Rh₂TFA₄, and (R)-(+)-α-methoxy-α-trifluoromethylphenylacetate Rh₂Mosh₄ was investigated by nuclear magnetic resonance spectroscopy (NMR) at room and lower temperatures. The complexation was carried out in situ, in CDCl₃ solution using titration procedure; the results were examined by the analysis of ¹H and ¹³C NMR chemical shift change (Δδ). The complexation of free S-methyl cysteine and hydrochloride salt of its methyl ester was performed in D₂O solution. For comparison, complexation of some derivatives of leucine, phenylalanine, and proline was examined.

N-phthaloyl and N-formyl derivatives of cysteine formed 1 : 1 and 1 : 2 axial complexes with all dirhodium salts. Rhodium substrates were bonded via sulfur. In one case, the complexation of Rh₂TFA₄ by both sulfur and N-formyl oxygen was noted. Similar complexation of Rh₂TFA₄, via CHO group, was found for N-formyl derivatives of leucine, phenylalanine, and proline. For N,N-dimethyl derivative of cysteine, both N and S atoms were involved in bonding. At room temperature, in all cases, ligand exchange was fast on the NMR timescale.     

2H Solid-State NMR Spectroscopy Reveals the Dynamics of a Pyridine Probe Interacting with Coordinatively Unsaturated Metal Sites of MIL-100(Al) Metal–Organic Frameworks

Coordinatively unsaturated metal sites (CUS) play an important role in catalysis by metal-organic frameworks (MOF). Being an intrinsic part of the framework the CUS take the role of acidic sites active in industrially relevant processes such as condensation or oxidation reactions. The key step of such reactions represents the coordination of the reagents to CUS. In MOFs the mechanism of the reagent interaction with CUS is poorly understood. Herein, we characterize the interaction of a widely used acidity probe pyridine with CUS of MIL-100(Al) MOF by means of the ²H solid-state NMR spectroscopy. ²H NMR reveals that pyridine species, which are interacting with CUS and the ones which are coordinated to the Al−OH site, exhibit different motional behavior. ²H NMR line shape as well as T₁, T₂ relaxation analyses for [D₅]pyridine adsorbed in MIL-100(Al) allowed us to perform a detailed characterization of pyridine dynamics in both states including the kinetics of the exchange process between these adsorption states.