DFT and ONIOM Simulation of 1,3-Butadiene Polymerization Catalyzed by Neodymium-Based Ziegler-Natta System.

Using modern methods of quantum chemistry, a theoretical substantiation of the high cis-stereospecificity of 1,3-butadiene polymerization catalyzed by the neodymium-based Ziegler-Natta system was carried out. For DFT and ONIOM simulation, the most cis-stereospecific active site of the catalytic system was used. By analyzing the total energy, as well as the enthalpy and Gibbs free energy of the simulated catalytically active centers, it was found that the coordination of 1,3-butadiene in the trans-form was more favorable than in the cis-form by 11 kJ/mol. However, as a result of π-allylic insertion mechanism modeling, it was found that the activation energy of cis-1,3-butadiene insertion into the π-allylic neodymium-carbon bond of the terminal group on the reactive growing chain was 10-15 kJ/mol lower than the activation energy of trans-1,3-butadiene insertion. The activation energies did not change when both trans-1,4-butadiene and cis-1,4-butadiene were used for modeling. That is, 1,4-cis-regulation was due not to the primary coordination of 1,3-butadiene in its cis-configuration, but to its lower energy of attachment to the active site. The obtained results allowed us to clarify the mechanism of the high cis-stereospecificity of 1,3-butadiene polymerization by the neodymium-based Ziegler-Natta system.

Modulating the Inclusive and Coordinating Ability of Thiacalix[4]arene and Its Antenna Effect on Yb3-Luminescence via Upper-Rim Substitution.

The present work introduces the series of thiacalix[4]arenes (H4L) bearing different upper-rim substituents (R = H, Br, NO2) for rational design of ligands providing an antenna-effect on the NIR Yb3+-centered luminescence of their Yb3+ complexes. The unusual inclusive self-assembly of H3L- (Br) through Brπ interactions is revealed through single-crystal XRD analysis. Thermodynamically favorable formation of dimeric complexes [2Yb3+:2HL3-] leads to efficient sensitizing of the Yb3+ luminescence for H4L (Br, NO2), while poor sensitizing is observed for ligand H4L (H). X-ray analysis of the single crystal separated from the basified DMF solutions of YbCl3 and H4L(NO2) has revealed the transformation of the dimeric complexes into [4Yb3+:2L4-] ones with a cubane-like cluster structure. The luminescence characteristics of the complexes in the solutions reveal the peculiar antenna effect of H4L(R = NO2), where the triplet level at 567 nm (17,637 cm-1) arisen from ILCT provides efficient sensitizing of the Yb3+ luminescence.

Role of PSS-based assemblies in stabilization of Eu and Sm luminescent complexes and their thermoresponsive luminescence.

The present work introduces self-assembled polystyrenesulfonate (PSS) molecules as soft nanocapsules for incorporation of Eu3+-Sm3+ complexes by the solvent exchange procedure. The high levels of Eu3+- and Sm3+-luminescence of the complexes derives from the ligand-to-metal energy transfer, in turn, resulted from the complex formation of Eu3+and Sm3+ ions with the three recently synthesized cyclophanic 1,3-diketones. The structural features of the ligands are optimized for the high thermal sensitivity of Eu3+- luminescence in DMF solutions. The PSS-nanocapsules (∼100 nm) provide both colloid and chemical stabilization of the ultrasmall (3-5 nm) nanoprecipitates of the complexes, although their luminescence spectra patterns and excited state lifetimes differ from the values measured for the complexes in DMF solutions. The specific concentration ratio of the Eu3+-Sm3+ complexes in the DMF solutions allows to tune the intensity ratio of the luminescence bands at 612 and 650 nm in the heterometallic Eu3+-Sm3+ colloids. The thermal sensitivity of the Eu3+- and Sm3+-luminescence of the complexes derives from the static quenching both in PSS-colloids and in DMF solutions, while the thermo-induced dynamic quenching of the luminescence is significant only in DMF solutions. The reversibility of thermo-induced luminescence changes of the Eu3+-Sm3+ colloids is demonstrated by six heating-cooling cycles. The DLS measurements before and after the six cycles reveal the invariance of the PSS-based capsule as the prerequisite for the recyclability of the temperature monitoring through the ratio of Eu3+-to- Sm3+ luminescence.

Correction: A simple synthetic approach to enhance the thermal luminescence sensitivity of Tb3+ complexes with thiacalix[4]arene derivatives through upper-rim bromination.

Correction for 'A simple synthetic approach to enhance the thermal luminescence sensitivity of Tb3+ complexes with thiacalix[4]arene derivatives through upper-rim bromination' by Sergey N. Podyachev, et al., Dalton Trans., 2020, 49, 8298-8313, DOI: 10.1039/D0DT00709A.

A simple synthetic approach to enhance the thermal luminescence sensitivity of Tb3+ complexes with thiacalix[4]arene derivatives through upper-rim bromination.

The present work for the first time reports an application of the thiacalix[4]arene scaffold for the preparation of Tb3+ complexes possessing high thermal luminescence sensitivity in the physiological temperature range of 20-50 °C. Non-substituted thiacalix[4]arenes form luminescent complexes with Tb3+ ions, but they do not reveal any meaningful thermal sensitivity. To solve this problem, an upper-rim bromination of thiacalix[4]arenes, as well as distal bromination along with the embedding of two 1,3-diketone substituents are proposed as new simple synthetic approaches to enhance the thermal luminescence sensitivity of the Tb3+ complexes. A combination of mass spectrometry, NMR, UV-Vis and luminescence spectroscopy with quantum chemical calculations reveals a dimeric structure of the complexes formed by thiacalix[4]arenes with Tb3+ ions in DMF solutions. The steady-state luminescence of the Tb3+ complexes has demonstrated more than one order higher thermal sensitivity for the complexes of bromo-substituted ligands in comparison with the non-substituted thiacalix[4]arenes. The reasons for such behavior are discussed. The results highlight new opportunities for the thiacalix[4]arene platform for controlling ligand-to-metal energy transfer in terbium complexes and tuning their thermo-responsive luminescence properties.

Structural and photophysical properties of Tb3+-tetra-1,3-diketonate complexes controlled by calix[4]arene-tetrathiacalix[4]arene scaffolds.

The present work highlights the key aspects of the influence of calix[4]arene and tetrathiacalix[4]arene scaffolds on the structural and photophysical properties of Tb3+ complexes with tetra-1,3-diketone derivatives of the macrocycles in DMF solutions. The equilibrium forms of Tb3+ complexes with unsubstituted and functionalized by acetylacetonyl groups at the upper rim of calix[4]arenes and thiacalix[4]arenes are revealed from UV-, NMR, MALDI TOF mass spectroscopy, quantum-chemical calculations at the DFT level and luminescence spectroscopy data. In alkaline DMF solutions, the ligands form predominantly 1 : 1 complexes with Tb3+ ions. However, the replacement of a calix[4]arene-scaffold by a thiacalix[4]arene-scaffold in the tetra-1,3-diketone derivatives shifts the equilibrium forms of Tb3+ complexes from monomeric to the dimeric ones. DFT calculations in combination with experimental data reveal the most reliable structures of complexes. The quantitative analysis of the photophysical parameters in correlation with the structural features of the complexes highlights the specific inner-sphere environment of Tb3+ ions in the dimeric complexes with the thiacalix[4]arene derivatives as a reason for greater sensitization of Tb3+-centered luminescence.

Quantum-chemical simulations of the hydration of Pb(II) ion: structure, hydration energies, and pKa1 value.

Thermodynamic and structural aspects of the hydration of Pb(II) ions were explored based on DFT calculations combined with the supermolecular/continuum solvent model. Hydration of Pb(II) was considered as the formation of Pb(H2O)n2+ aqua complexes (n=6-9) from the gas phase Pb(II) ion. Hexa- and hepta-aqua Pb(II) complexes were shown to exhibit the hemidirected symmetry, while those containing eight and nine water molecules are characterized by the holodirected symmetry. The calculations showed that because Pb(H2O)n2+ complexes with six to nine water molecules have comparable thermodynamic stabilities, such complexes are likely to coexist in aqueous solutions. The deprotonation of Pb(H2O)n2+ complexes was shown to result in the formation of the mono-hydroxo complex [Pb(H2O)4OH]+. The pKa1 value determined for this reaction (7.58 for Pb(H2O)62+) was close to the experimental value of 7.61 used in recent models of aquatic equilibria. The density functional method ω-B97X(PCM-UAO) in combination with the atomic basis set 6-311++G(d,p) for O and H and the small-core electron effective pseudopotential (ECP) with the aug-cc-pvdz-PP basis set for Pb can be recommended for such calculations. Graphical abstract Structures of Pb(II) ions with varying numbers of water molecules in the inner hydration shell.

Chemical and biological properties of a supramolecular complex of tuftsin and cucurbit[7]uril.

Cucurbit[7]uril (CB7) is an uncharged and water-soluble macrocyclic host. CB7 binds to doubly protonated tuftsin, which is the tetrapeptide Thr-Lys-Pro-Arg, with moderate affinity (Ka=2.1×103M-1). In this study, the host-guest complexation was confirmed by fluorescence titration. This affinity would allow for easy release of the peptide under physiological conditions. According to density functional theory calculations, the structural binding motif involves hydrogen bonding. The most energetically stable form had the Arg side chain inside the CB7 cavity. The effects of the tuftsin-CB7 complex on the proliferation and cytokine activity of immune cells were studied. The complex had broader spectrum immunomodulation than free peptides, and caused statistically significant (p<0,05) changes in cytokine production (tumor necrosis factor-α, interleukin-2, interferon-γ, and interleukin-10) by mononuclear cells. By contrast, the free peptide only activated tumor necrosis factor-α production.

Formation of Pb(III) intermediates in the electrochemically controlled Pb(II)/PbO2 system.

The formation of lead dioxide PbO(2), an important corrosion product in drinking water distribution systems with lead-bearing plumbing materials, has been hypothesized to involve Pb(III) intermediates, but their nature and formation mechanisms remain unexplored. This study employed the electrochemical (EC) method of rotating ring disk electrode (RRDE) and quantum chemical (QC) simulations to examine the generation of intermediates produced during the oxidation of Pb(II) to PbO(2). RRDE data demonstrate that PbO(2) deposition and reduction involves at least two intermediates. One of them is a soluble Pb(III) species that undergoes further transformations to yield immobilized PbO(2) nanoparticles. The formation of this intermediate in EC system is mediated by hydroxyl radicals (OH(•)), as was evidenced by the suppression of intermediates formation in the presence of the OH(•) scavenger para-chlorobenzoic acid. QC simulations confirmed that the oxidation of Pb(II) by OH(•) proceeds via Pb(III) species. These results show that Pb(III) intermediates play an important role in the reactions determining transitions between Pb(II) and Pb(IV) species and could impact lead release in drinking water.

Formation thermodynamics of cucurbit[6]uril macrocycle molecules: a theory study.

Macrocyclical molecules are very important molecular cavitands for the supramolecular chemistry in view of host-guest complexation which is a basis for design of molecular devices. One striking example of macrocycles is the family of cucurbit[ n]uril molecules (CB[n]). For effective application of the cavitands of this family it is needed to elucidate the stabilization mechanism of different homologues CB[n]. In this study we have carried out a thermodynamical analysis of macromolecule cyclization from a monomer to a pentamer and CB6. It was found that water molecules, which are formed as one of the products of the reaction of glicoluril with formaldehyde, construct dimer and oligomer CB(m) so that an angle between adjacent building units corresponds to the stable homologue CB[6]. In the framework of the density functional theory (DFT), the structures of hydrated oligomers were optimized. Calculated thermodynamical functions were used for a description of cyclization mechanism of oligomers up to formation of CB[6]. It is assumed that the proposed model can be extended to the formation of higher homologues in diluted water-acid solutions.