Experimental and Theoretical Study of the Vibrational Spectra of 12-Crown-4−Alkali Metal Cation Complexes

Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5 (1), N(SiMe3)2 (2), η5-C5H4SiMe3 (3), and η5-C5H5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)6][{Ti(η5-C5Me5)}4(μ3-N)4] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)2, L = 18-crown-6 (8), crypt-222 (9); R = η5-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = η5-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}2(μ-η5:η5-C5H5)][{Ti4(η5-C5Me5)3(η5-C5H5)}(μ3-N)4] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(μ-η5:η5-C5H5)Ti4(η5-C5Me5)3(μ3-N)4]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η5-C5Me5)(μ-NH)}3(μ3-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(μ4-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}2] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(μ3-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)2][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (18) similar to the ion pair [K(crypt-222)][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (19) obtained in the treatment of 15 with cryptand-222.

Recent Advances in the Lithium Recovery from Water Resources: From Passive to Electrochemical Methods

The ever-increasing amount of batteries used in today's society has led to an increase in the demand of lithium in the last few decades. While mining resources of this element have been steadily exploited and are rapidly depleting, water resources constitute an interesting reservoir just out of reach of current technologies. Several techniques are being explored and novel materials engineered. While evaporation is very time-consuming and has large footprints, ion sieves and supramolecular systems can be suitably tailored and even integrated into membrane and electrochemical techniques. This review gives a comprehensive overview of the available solutions to recover lithium from water resources both by passive and electrically enhanced techniques. Accordingly, this work aims to provide in a single document a rational comparison of outstanding strategies to remove lithium from aqueous sources. To this end, practical figures of merit of both main groups of techniques are provided. An absence of a common experimental protocol and the resulting variability of data and experimental methods are identified. The need for a shared methodology and a common agreement to report performance metrics are underlined.

Crown-Ether Functionalized Graphene Oxide Membrane for Lithium Recovery from Water

The massive worldwide transition of the transport sector to electric vehicles has dramatically increased the demand for lithium. Lithium recovery by means of ion sieves or supramolecular chemistry has been extensively studied in recent years as a viable alternative approach to the most common extraction processes. Graphene oxide (GO) has also already been proven to be an excellent candidate for water treatment and other membrane related applications. Herein, a nanocomposite 12-crown-4-ether functionalized GO membrane for lithium recovery by means of pressure filtration is proposed. GO flakes were via carbodiimide esterification, then a polymeric binder was added to improve the mechanical properties. The membrane was then obtained and tested on a polymeric support in a dead-end pressure setup under nitrogen gas to speed up the lithium recovery. Morphological and physico-chemical characterizations were carried out using pristine GO and functionalized GO membranes for comparison with the nanocomposite. The lithium selectivity was proven by both the conductance and ICP mass measurements on different sets of feed and stripping solutions filtrated (LiCl/HCl and other chloride salts/HCl). The membrane proposed showed promising properties in low concentrated solutions (7 mgLi/L) with an average lithium uptake of 5 mgLi/g in under half an hour of filtration time.

Actinyl-cation interactions: experimental and theoretical assessment of [Np(vi)O2Cl4]2- and [U(vi)O2Cl4]2- systems

The interaction of the actinyl (AnO₂²⁺) oxo group with low-valent cations influences the chemical and physical properties of hexavalent actinides, but the impact of these intermolecular interactions on the actinyl bond and their occurrence in solution and solid state phases remain unclear. In this study, we explore the coordination of alkali cations (Li⁺, Na⁺, K⁺) with the [NpO₂Cl₄]^2- coordination complexes using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory (DFT) calculations and compare to the related uranyl system. Three solid-state coordination compounds ([Li(12-crown-4)]₂[NpO₂Cl₄] (LiNp), [Na(18-crown-6)H₂O]₂[NpO₂Cl₄] (NaNp), and [K(18-crown-6)]₂[NpO₂Cl₄] (KNp) have been synthesized and characterized using single-crystal X-ray diffraction and Raman spectroscopy. Only Li⁺ cations interact with the neptunyl oxo in the solid-state compounds and this results in a red-shift of the NpO₂²⁺ symmetric stretch (ν₁). Raman spectra of Np(vi) solutions containing lower Li⁺ concentrations display a single peak at ∼854 cm^-1 and increasing the amount of Li⁺ results in the ingrowth of a second band at 807 cm^-1. DFT calculations and vibrational analysis indicate the lower frequency vibrational band is the result of interactions between the Li⁺ cation and the neptunyl oxo. Comparison to the related uranyl system shows similar interactions occur in the solid state, but subtle differences in the actinyl-cation modes in solution phase.

Conformational Study of the Structure of dibenzo-18-crown-6. Comparison with 18-crown-6

We report for the first time the conformational analysis of dibenzo-18-crown-6, db18c6. The conformational search was carried out using the CONFLEX conformational search method of cyclic molecules. Energies were calculated for the low-lying predicted conformations at different levels of theory up to the G3MP2 level. At the G3MP2 level, the predicted ground state (GS) conformation was more stable than the experimental conformation by only 1.60kcal/mol. Strong similarity was found between the GS structure and experimental conformations of db18c6 and 18-crown-6, 18c6. The GS and experimental conformations of db18c6 are non-planar. This allows db18c6 to exist in optically active enantiomers. Similar to 18c6, it was concluded that the db18c6 structure is stabilized by intramolecular hydrogen bond. We also performed the computations for the water and chloroform solution phase, where the same conformation was predicted as the GS conformation.

Conformational analysis of 9-crown-3, 9-thiacrown-3 and 9-azacrown-3

Crown ethers are an important class of molecules with wide applications. Crown ethers are large ring flexible molecules with a large number of possible conformations. In the current manuscript, we report the conformational analysis results of one of the smallest crown ether systems, namely, 9-crown-3, 9c3, 9-thiacrown-3, 9t3, and 9-azacrown-3, 9a3. The conformational search is performed using the CONFLEX conformational search method utilizing the MMFF94s force field. 8, 11 and 62 conformations were predicted for 9c3, 9t3 and 9a3, respectively. The ab initio computations were performed at B3LYP and MP2 levels using the 6-311G** basis set. For the accurate prediction of the ground state conformation and the energy order of the low-lying energy conformations, the computations were performed at the G4 level for some selected conformations. Factors affecting the stability of different conformations of 9c3, 9t3 and 9a3 are discussed. These factors are also discussed with respect to larger crown ether systems. The ab initio computations were performed also for water and chloroform as solution phases. The same ground state predicted in the gas phase was also predicted in the solution water and chloroform phases.

Conformational and vibrational analysis of 18-crown-6-alkali metal cation complexes

Conformational analysis was performed for the 18-crown-6-alkali metal cation complexes, 18c6-AMCCs, using the CONFLEX method. The number of predicted conformations of the 18c6-Li+, Na+, K+, Rb+ and Cs+ complexes was 10, 24, 15, 9 and 4 conformations, respectively. Electronic and geometrical structures were calculated for the predicted conformations at the HF, B3LYP, CAM-B3LYP, M06 and MP2 levels. Binding energies and enthalpies of the ground state conformations were also calculated. Vibrational, IR and Raman, spectra of free 18c6 and 18c6-AMCCs were measured. Comparison between the calculated vibrational frequencies using multi-scale-factor scaling of the B3LYP force field and the experimental vibrational frequencies predicted that the 18c6-K+, Rb+ and Cs+ complexes exist in the D3d, C3v and C3v conformations, respectively. It was also predicted that the 18c6-Na+ complex exists in a D3d-like conformation. It was not possible to identify in what conformation the 18c6-Li+ complex exists.

Experimental and theoretical study of the vibrational spectra of 12-thiacrown-4 and 18-thiacrown-6, evaluation of the performance of the different anharmonic force fields compared to the scaled quantum mechanical force fields

We report, to the best of our knowledge, for the first time the vibrational, IR and Raman, spectra of 12-thiacrown-4 (12t4) and 18-thiacrown-6 (18t6). To predict in what conformation 12t4 and 18t6 exist, for the vibrational analysis of both molecules and to assess the performance of the different computational methods for the accurate prediction of the vibrational frequencies of relatively large molecules, the computations were done using the harmonic and anharmonic force fields using the 6-31G* and 6-311G** basis sets. The computations were performed at the HF, B3LYP, CAM-B3LYP, BLYP, BP86, G96LYP, PBE1PBE, TPSSH and MP2 levels. Comparison was made between the calculated and experimental vibrational frequencies as indicated by the root-mean-square (rms) deviations, using either the unscaled and scaled harmonic vibrational frequencies and, unscaled, anharmonic vibrational frequencies. For the harmonic vibrational frequencies two scaling schemes were used. One uses one-scale-factor (1SF) scaling and the other uses 8SF scaling. In terms of the vibrational analysis of 12t4 and 18t6, the report confirms the solid state X-ray structure of D4 of 12t4 and C2 of 18t6. It is concluded that a lower rms deviation is obtained using 1SF scaled harmonic vibrational frequencies at even the HF/6-31G* level than using anharmonic vibrational frequencies at the MP2/6-311G** level. The CAM-B3LYP method showed some improvement over the traditional B3LYP method.

Conformational study of the structure of 18-thiacrown-6

Conformational analysis was performed for 18-thiacrown-6 (18t6) using the CONFLEX method and the MMFF94s force field. Computations were performed for some of the low energy conformations at the HF, B3LYP, CAM-B3LYP, M06, M06L, M062x, M06HF and MP2 levels. The computations were also performed using the DFT-D3 method along with the TPSS and PBE functionals. The study predicted a new C2 conformation as the ground state conformation of 18t6. This new C2 conformation is more stable than the experimentally known solid state conformation by 4.7 kcal/mol at the MP2/6-311G** level. This conformation has all of the SCCS dihedral angles adopt exodentate structure. However, the experimentally known conformation of the solid phase has two of the SCCS dihedral angles violating this exodentate rule. It was concluded that for 18t6 stability a linear dihedral SCCS angle requirement is more important than a gauche CSCC dihedral angle requirement.

Complexation of syndiotactic polystyrene with 12-crown-4

Solid-state complexation of syndiotactic polystyrene (sPS) with a crown ether compound, 1,4,7,10-tetraoxa-cyclododecane (12-crown-4), took place when a film of sPS/chloroform clathrate was subjected to a guest exchange procedure assisted with a plasticizing agent. The new guest 12-crown-4 molecules were incorporated into the crystalline region of the sPS film, without causing a large conformational change of host sPS helices. X-ray diffraction and thermogravimetric investigations showed that sPS/12-crown-4 complex had a clathrate complex structure which contained four 12-crown-4 molecules per unit cell. IR and Raman data suggested that 12-crown-4 took a C(i) -type conformation in the sPS complex phase.