NMR studies of preferential solvation of sodium cation in mixtures of N-methylformamide with water and some nonaqueous solvents

Sodium-23 NMR chemical shifts and linewidths have been measured for 0.1M NaClO₄ in binary mixtures of N-methylformamide (NMF) with a series of other solvents, as a function of the solvent mole fraction. The relative solvent composition at the isosolvation point, the mid-value of the Na-23 chemical shift between those measured in the respective pure solvents, reveals preferential solvation of the sodium cation in many cases. The isosolvation composition correlates well with the relative solvating abilities of the two solvents-as characterized by their donicities-provided that the cation-solvent interactions are of the hard-hard type and that they are not complicated by interionic interactions. The variation in the electric field gradient around the sodium nucleus, as the composition of the solvent changes, results in broadening of the resonance line. Maximum broadening occurs close to the solvent mole fraction corresponding to the isosolvation point.     

Determination of stability constants of cesium [2]-cryptand complexes in nonaqueous solvents by cesium-133 NMR

Cesium complexes with four diazapolyoxamacrobicyclic ligands ([2]-cryptands), C211, C221, C222, and C222B, were investigated in pyridine, acetone, propylene carbonate, N,N-dimethylformamide, acetonitrile, and dimethyl sulfoxide solutions by cesium-133 NMR. The relative stabilities of the complexes are in the order Cs⁺·C221Cs⁺·C222>Cs⁺·C222BCs⁺·C211. The formation constants are strongly influenced by the solvating abilities of the solvents. The NMR data and stereochemical considerations indicate that in the C222–Cs⁺ system there is an equilibrium between exclusive and inclusive conformations of the complex. The other three ligands must form only the exclusive complex.     

Spectroscopic studies of ionic solvation. XIX. Fluorine-19 and sodium-23 NMR studies of ionic interactions in nonaqueous solutions of hexafluorophosphate salts

Fluorine-19 and sodium-23 NMR measurements were carried out on sodium hexafluorophosphate solutions in a number of solvents. In solvents of medium polarity and donicity (e.g., propylene carbonate, acetone, acetonitrile) the ¹⁹ F chemical shift moved upfield with increasing concentration of the salt. This behavior is indicative of anion-cation interactions which may be of long-range type, i.e., formation of solvent-separated ion pairs; the possibility of contact ion pair formation, however, cannot be excluded. In solvents of low polarity and donicity (acetic acid, tetrahydrofuran), the salt is essentially completely associated in the 0.1–1.0M concentration range. On the other hand, in solvating solvents with high dielectric constants, such as dimethyl-formamide, dimethylsulfoxide, and formamide, there is very little ionic association in the same concentration range. The above conclusions are supported by ²³ Na chemical shift measurements. Potassium hexafluorophosphate solutions do not show any concentration dependence of the ¹⁹ F chemical shifts, while for tetra-n-butylammonium solutions the ¹⁹ F resonance moves downfield with increasing concentration of the salt.To whom correspondence should be addressed.     

Spectroscopic studies of ionic solvation. XXI. A raman,infrared, and NMR study of sodium perchlorate solutions in nonaqueous solvents

Sodium perchlorate solutions in several nonaqueous solvents were examined by²³Na,³⁵Cl-NMR, infrared, and Raman spectroscopic techniques. The formation of contact ion pairs lowers the symmetry of ClO ₄ ^– ion from T_d to C_3v or C_2v provided that the interaction is fairly strong. This was manifested for NaClO₄ solutions in acetonitrile, tetrahydrofuran, and pyridine. Combination of the vibrational measurements with NMR shows that in the above three cases the anion-cation interactions are quite strong. Sodium-23 NMR studies confirm the above results and, being a more sensitive technique, also indicates weak cation-anion interaction in propylene carbonate, formic acid, acetone, methanol, ethanol, dimethyl sulfoxide, and water. In all solvents the²³Na resonance shifts upfield with increasing concentration of NaClO₄, indicating that the replacement of solvent by ClO ₄ ^– ion decreases electron density around the cation.     

Spectroscopic studies of ionic solvation. XX. Cesium-133 NMR study of cesium salts in different solvents

Cesium-133 chemical shifts were measured in a number of solvents as a function of salt concentration and of the counterion. Infinite-dilution chemical shifts (vs. aqueous Cs⁺ ion at infinite dilution) ranged from +59.8 ppm for nitromethane solutions to –29.4 ppm for pyridine. In general, the magnitude of the downfield chemical shift reflected the donor ability of the solvents. Ion-pair formation constants were calculated from the concentration dependences of¹³³Cs chemical shifts in several nonaqueous solvents.     

Chelation of Na+ cations in nonaqueous solvents. Structural requirements and methodological considerations

Intrinsic formation constants for complexes of Na⁺ ions with a series of polyamines have been determined from²³Na NMR measurements in binary mixtures of tetrahydrofuran (THF) and amines. The results show in a totally unambiguous manner a regular decrease in the magnitude of the chelate effect with the number of atoms intervening between the nitrogen ligators. The key assumption, tetracoordination of the cation, is critically examined, and experimental evidence is adduced in its support.     

Sodium-23, cesium-133 and thallium-205 NMR study of sodium,cesium and thallium complexes with large crown ethers in nonaqueous solutions

Formation constants for complexes of dibenzo-21-crown-7, dibenzo-24-crown-8 and dibenzo-27-crown-9 with the Na⁺, Cs⁺ and Tl⁺ ions have been determined by multinuclear NMR measurements in several nonaqueous solvents. The measurements of the cesium complexes were carried out at different temperatures and the enthalpy and entropy of complexation were determined from the temperature dependence of the formation constants.The stabilities of these complexes in solvents of low to medium donicity, —nitromethane, acetonitrile, acetone, methanol, and propylene carbonate, vary in the order Tl⁺>Cs⁺>Na⁺. Depending on the relative sizes of the cation and of the ligand cavity, either a three-dimensional wrap-around complex or a two-dimensional crown complex are formed. For the cesium complexes, the values of H _c ^o and S _c ^o are definitely solvent-dependent and in all cases the complexes are enthalpy stabilized but entropy destabilized. A compensating effect is observed in the variation of the enthalpy and entropy of complexation so that the overall influence of the above solvents on the stability of the complexes is rather limited.     

Zinc-67 NMR study of zinc ions in water and in some nonaqueous and mixed solvents

Solutions of zinc salts in water, methanol (MeOH), dimethylformamide (DMF) and binary mixtures of water with the two nonaqueous solvents were studied by zinc-67 NMR measurements. Anhydrous zinc nitrate solutions in DMF and MeOH show upfield, concentration independent, chemical shifts at –27 and –19 ppm, respectively, vs. the aqueous solution standard. Addition of DMF or MeOH to an aqueous solution of a zinc salt results in a diamagnetic shift but for the addition of acetonitrile a paramagnetic shift results. In all cases the signal was broadened very considerably, e.g., in ZnCl₂ solution the linewidth increased from 40 to 600 Hz in going from water to 35% aqueous MeOH. Both⁶⁷Zn and¹³C NMR failed to show any complexation of Zn²⁺ ion by crown ethers in aqueous solution. A gradual addition of EDTA, of diaza-18-crown-6 or of tetraazacyclotetradecane resulted in an immediate broadening of the⁶⁷Zn signal which became undetectable when one equivalent of a ligand was added.     

205Tl NMR studies: Ion pairing of Tl+ and (CH3)2Tl+ with NO 3 − and ClO 4 − in various solvents

The concentration dependence of the²⁰⁵Tl chemical shifts of Tl⁺ and of (CH₃)₂Tl⁺ ions was determined in several solvents with NO ₃ ^– and ClO ₄ ^– counterions. In general, increased ion pairing caused a low-frequency shift of the²⁰⁵Tl resonance, with the exceptions of (CH₃)₂TlNO₃ inn-butylamine and TlNO₃ in N,N-dimethylformamide (DMF) and in hexamethylphosphorotriamide (HMPA). In HMPA,²⁰⁵Tl linewidths of both Tl⁺ and (CH₃)₂Tl⁺ increased dramatically with dilution below 0.1M. Analysis of the data allowed ion-pair formation constants and²⁰⁵Tl chemical shifts for the ion-paired cation and for the free (solvated) cation to be estimated for some of the solvents.     

Lithium-7, sodium-23, and cesium-133 NMR and far infrared study of alkali complexes with C222-dilactam in various solvents

Nuclear magnetic resonances of alkali nuclei,⁷Li,²³Na, and¹³³Cs, as well as far infrared measurements are used to study alkali complexes of a bicyclic diazapolyoxa ligand—the dilactam of cryptand C222. Measurements were carried out in pyridine, tetrahydrofuran, acetonitrile, nitromethane, dimethylformamide, and aqueous solutions. The complexing ability of the dilactam is similar to, but weaker than, that of the cryptand C222. The limiting chemical shifts of the complexed cations were solvent-dependent, indicating incomplete enclosure of the cation by the ligand. Formation constants of Li⁺ and Cs⁺ complexes were calculated from the chemical-shift dependence on the ligand/metal ion mole ratio.     

Hydrogen bonding networks in gabapentin protic pharmaceutical salts: NMR and in silico studies

Hydrogen bonds (HBs) play a key role in the supramolecular arrangement of crystalline solids and, although they have been extensively studied, the influence of their strength and geometry on crystal packing remains poorly understood. Here we describe the crystal structures of two novel protic gabapentin (GBP) pharmaceutical salts prepared with the coformers methanesulfonic acid (GBP:METHA) and ethanesulfonic acid (GBP:ETHA). This study encompasses experimental and computational electronic structure analyses of ¹H NMR chemical shifts (CSs), upon in silico HB cleavage. GBP:METHA and GBP:ETHA crystal packing comprise two main structural domains: an ionic layer (characterized by the presence of charge-assisted ⁺NH_GBP⋯ O⁻_METHA/ETHA HB interactions) and a neutral layer generated in a different way for each salt, mainly due to the presence of bifurcated HB interactions. A comprehensive study of HB networks is presented for GBP:METHA, by isolating molecular fragments involved in distinct HB types (NH⋯ O, OH⋯ O, and CH⋯ O) obtained from in silico disassembling of an optimized three-dimensional packing structure. Formation of HB leads to calculated ¹H NMR CS changes from 0.4 to ~5.8 ppm. This study further attempts to assess how ¹H NMR CS of protons engaged in certain HB are affected when other nearby HB, involving bifurcated or geminal/vicinal hydrogen atoms, are removed.     

The solubility and solvation of salts in mixed nonaqueous solvents. 1. Potassium halides in mixed aprotic solvents

The solubilities of potassium fluoride, chloride, and bromide in acetonitrile, N,N-dimethylformamide, and dimethylsulfoxide and in binary mixtures of these solvents were determined at 25°C. The standard molar Gibbs free energies of solution, _solnG°, in the neat solvents were related to the polarizabilities and basicities of the anions and the dipole moments and acidities of the solvents. The values of _solnG° in the mixtures were fitted by expressions from the quasi-lattice quasi-chemical theory. The mean number of each kind of solvent in the nearest environment of the ions was obtained from these results.     

Stabilities in water and transfer activity coefficients from water to nonaqueous solvents of 15-crown-5 and 16-crown-5-metal ion complexes

Stability constants of 1 : 1 16-crown-5 (16C5)-metal ion complexes were determined in water at 25°C by conductometry and potentiometry with ion-selective electrodes. The selectivity sequences of 16C5 in water for univalent and bivalent metal ions are Ag⁺ > Na⁺ Tl⁺ > K⁺ and Sr²⁺ > Ba²⁺ Pb²⁺, respectively. The stability of a given 16C5-metal ion complex in water is much lower than might be expected on the basis of the solvation power (i.e. relative solubility of the metal ion) of water for the metal ion. The same tendency is observed for the cases of 15-crown-5 (15C5) -metal ion complexes. Transfer activity coefficients () of 15C5 and 16C5 for tetradecane (TD)/water, TD/methanol, TD/acetonitrile, and propylene carbonate/water systems were determined at 25°C. From these data, contributions of a methylene group and an ether oxygen atom to the log value of a crown ether were then obtained. The values from water to acetonitrile, propylene carbonate, and methanol of 15C5- and 16C5-univalent metal ion complexes were calculated, s, M⁺, and L being a solvent, a univalent metal ion, and a crown ether, respectively. The log value is greater than the corresponding log value. The log values are negative. This indicates that, although the M^- ions are more soluble in water than in the nonaqueous solvents, when the crown ether forms a complex with the M⁺ ion, the complex becomes more soluble in the nonaqueous solvents than in water, compared with the free crown ether. It was concluded from this finding that the unexpectedly low stability of the crown ether-M⁺ complex in water is attributed to strong hydrogen bonding between ether oxygen atoms of the free crown ether and water.     

Synthesis, molecular structures and solution NMR studies of N-heterocyclic carbene-amine silver complexes

The synthesis of the Boc-protected 1-(2-aminoethyl)-3-methylimidazolium salts [BocNHCH₂CH₂ImMe]X [2]X (X = I, PF₆) and their straightforward transformation into [NH₂CH₂CH₂ImMe]X [3]X is reported. The reaction between [2]X and Ag₂O leads to the formation in the solid state of three different bonding motifs: a biscarbene salt [(NHC-NHBoc)₂Ag]PF₆ ([4]PF₆, NHC-NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-ylidene), a tetranuclear complex [Ag(NHC-NHBoc)₂]₂[Ag₂I₄], (5), and a polymeric silver “staircase” [(NHC-NHBoc)₂-Ag₄-I₄]_n, (6) composed of Ag₄I₄ clusters. The same reaction carried out with [3]I showed that a primary silver mono-NHC-NH₂ carbene complex of the type [(NHC-NH₂)AgI] (7) is likely to form but it is unstable in solution. The solid state molecular structures of [4]PF₆, 5 and 6 were determined by X-ray diffraction analysis, whereas PGSE NMR experiments were employed to investigate the hydrodynamic dimension of the imidazolium salts and silver complexes and, consequently, to gain information on the level of aggregation in solution. PGSE NMR studies were complemented by NOE NMR investigations in order to obtain information on anion-cation relative orientation within aggregates.     

33S NMR spectra of sulfonium salts: calculated and experimental

33S NMR chemical shifts were calculated by the scaled DFT and EMPI approaches for the fluoride, chloride and bromide of trimethylsulfonium ion (1) and S-methyltetrahydrothiophenium ion (2), in addition to the free cations. Experimental values were obtained for the iodides of 1 (delta +48, CS2 = 0 ppm) and 2 (delta +95), and were found to agree with the calculated values well within the standard deviation of 35 ppm (3.5% of the shielding range) established in earlier work for a great variety of sulfur compounds. An earlier literature value of delta +750 for the iodide of 2 is therefore to be replaced. Calculations provide a shift of delta +68 for S-methylthianium ion with equatorial methyl, indicating that the reported value of delta +670 for the iodide is also incorrect.     

Lithium-7 nuclear magnetic resonance and calorimetric study of lithium crown complexes in various solvents

Lithium-7 NMR studies have been carried out on lithium ion complexes with crown ethers 12C4, 15C5, and 18C6 in water and in several nonaqueous solvents. In all cases the exchange between the free and complexed lithium ion was fast on the NMR time scale, and a single, population average, resonance was observed. Both 1:1 and 2:1 (sandwich) complexes were observed between lithium ion and 12C4 in nitromethane solution. The stability of the complexes varied very significantly with the solvent. With the exception of pyridine, the stability varies inversely with the Gutmann donor number of the solvent. In general, the stability order of the complexes was found to be 15C5·Li⁺>12C4·Li⁺>18C6·Li⁺. Calorimetric studies on these complexes show that, in most cases, the complexes are both enthalpy and entropy stabilized.     

Binuclear and polynuclear transition metal complexes with macrocyclic ligands. 5. Novel complexes of asymmetric polydentate macrocyclic Schiff bases. Step-by-step synthesis

Reactions of 2,6-bis(3-aminopropylaminocarbonyl)pyridine (1) with 4-tert-butyl-2,6-diformylphenol and 2,5-diformylpyrrole in the presence of Ba(ClO₄)₂ in EtOH afford barium complexes with asymmetric macrocyclic Schiff bases as soft and hard ligands. The reaction of compound 1 with Cu(OCOCMe₃)₂ involves closure of a tetrahydropyrimidine ring to give a mononuclear complex, which was structurally characterized by X-ray diffraction analysis.     

NMR studies of interionic interactions in N-methylformamide

The NMR chemical shifts of alkali and thallium(I) salts with various monovalent anions have been measured in N-methylformamide solution. Lithium-7 chemical shifts are virtually concentration and counter-ion independent, presumably due to an absence of direct cation-anion interactions. The sodium-23, potassium-39 and cesium-133 chemical shifts of the salts studied depend on the anion and vary linearly with the concentration. The observed behavior can be accounted for by the formation of collisional ion pairs. On the other hand, the thallium-205 chemical shifts of thallium(I) nitrate and perchlorate were anion-dependent and varied non-linearly with the salt concentration. These results are indicative of contact ion pair formation; formation constants were calculated to be 2.6±0.4 M ^–1 for TlNO ₃ and 1.7±0.5 M ^–1 for TlClO ₄ . The cesium-133 NMR spectra of several mixed electrolyte systems also have been measured in N-methylformamide solution. The¹³³Cs chemical shifts also change linearly with the concentrations of the salts added to 0.10 M CsI/NMF solutions. The influence of the anions on the chemical shifts is the same as that observed for cesium salts alone.     

Influence of copper salts, solvents, and ligands on the structures of precatalytic phosphoramidite copper complexes for conjugate addition reactions

For copper-catalyzed enantioselective conjugate addition reactions of organozinc reagents, the available knowledge about the mechanism and the structures involved is still insufficient to understand in detail the strong influences of solvent, salt, and ligand size, or to enable a rational control of this reaction. Screening with three phosphoramidite ligands and four copper(I) salts using NMR spectroscopy has revealed a binuclear copper complex with mixed trigonal/tetrahedral stereochemistry as the basic structural motif of the ground state of precatalysts with highly stereoselective ligands. Ligands with smaller amine moieties allow higher coordination numbers and higher aggregation levels, leading to reduced ee values. Since the ESI mass spectra of several precatalytic copper halide complexes show a striking correlation with the structures observed in solution, ESI-MS may be used as a fast tool to determine the maximum number of phosphoramidite ligands attached to copper.     

Conductance of tetraalkylammonium salts in propylene carbonate at 25°C

Conductance measurements of 12 quaternary ammonium salts in propylene carbonate (PC) have been made at 25°C. The cations were either tetramethylammonium, tetraethylammonium, or tetra-n-butylammonium, and the concentrations of salt varied from about 2×10^–4 to 5×10^–3 M. The data were analyzed by the equation of Pitts. The results showed that the benzoate, nitrobenzoate, and pentachlorophenolate salts are completely dissociated. The nitrophenolate, chlorophenolate, methylsulfonate, and nitrate salts are only slightly associated (K _A from 2.5 to 6.5), while the acetate, phenylacetate, and nitrophenolate salts display somewhat more extensive association, with ionpair association constants from 17 to 45. Limiting molar conductances for the anions were derived. The factors affecting ionic mobilities in this dipolar aprotic solvent are discussed.On leave 1973–1975 from the University of Gdask, Poland.