Selective Enhancement of SERS Spectral Bands of Salicylic Acid Adsorbate on 2D Ti3C2Tx-Based MXene Film

In this research, we have demonstrated that 2D Ti3C2Xn-based MXene (MXene) films are suitable for the design of surface-enhanced Raman spectroscopy (SERS)-based sensors. The enhanced SERS signal was observed for a salicylic acid molecule on Ti3C2Tx-based MXene film. Confirmation of the adsorption of the salicylic acid molecule and the formation of a salicylic acid–MXene complex were determined by experimental SERS-based spectral observations such as greatly enhanced out-of-plane bending modes of salicylic acid at 896 cm−1 and a band doublet at 681 cm−1 and 654 cm−1. Additionally, some other spectral features indicate the adsorption of salicylic acid on the MXene surface, namely, a redshift of vibrational modes and the disappearance of the carboxyl deformation spectral band at 771 cm−1. The determined enhancement factor indicates the value that can be expected for the chemical enhancement mechanism in SERS of 220 for out-of-plane vibrational modes. Theoretical modeling based on density functional theory (DFT) calculations using B3LYP/6311G++ functional were performed to assess the formation of the salicylic acid/MXene complex. Based on the calculations, salicylic acid displays affinity of forming a chemical bond with titanium atom of Ti3C2(OH)2 crystal via oxygen atom in hydroxyl group of salicylic acid. The electron density redistribution of the salicylic acid–MXene complex leads to a charge transfer effect with 2.2 eV (428 nm) and 2.9 eV (564 nm) excitations. The experimentally evaluated enhancement factor can vary from 220 to 60 when different excitation wavelengths are applied.

Redox Behavior of Chelate Complexes Based on 8-Oxyquinoline Promising in OLED Display Technology

The electrochemical behavior of 8-oxyquinoline and chelate complexes based on it (Sn (Oxin)Cl3, Ge (Oxin)Cl3, Ti (Oxin)Cl3, W(Oxin)2Cl4, Fe (Oxin)Br, Sb (Oxin)Cl2, Sb (Oxin)Cl4, Sn (Oxin)2Cl2, Ti (Oxin)2Cl2 have been studied by cyclic voltammetry in aprotic solvents in a three-electrode system on platinum and glass-graphite disk electrodes. It has been shown that in the case of metal oxyquinolinates, the ligand is 8-oxyquinoline reducing in two one-electron diffusion waves. The first wave is observed at less negative potentials than the first quinoline one, while the second waves have almost the same potentials. The first wave is related to the OH- proton discharge. The complexes under study are electrolytically oxidizable. A single two-electron peak is observed in the cyclic voltammogram in the anodic region for the Ti (Oxin)Cl3 chelate complex. This is probably associated with two irreversible sequential or parallel stages with close oxidation potentials. By analogy to the processes considered for 8-oxyquinoline, the rupture of the oxygen-metal bond is observed at the first stage. The resulting radical cation is unstable and decomposes into a radical and a cation with a positive charge in the titanium atom. Electrolytic oxidation of complexes Fe (Oxin)Br, Sb (Oxin)Cl2, Sb (Oxin)Cl4, and Sn (Oxin)2Cl2 is similar to that of Ti (Oxin)Cl3.

Electronic structure of Schiff-base peroxo{2,2′-[1,2-phenylenebis(nitrilomethanylylidene)]bis(6-methoxyphenolato)}titanium(IV) monohydrate: a possible model structure of the reaction center for the theoretical study of hemoglobin

An extensive characterization of [Ti(C22H18N2O6)]·H2O was performed by topological analysis according to Bader's quantum theory of atoms in molecules (QTAIM) from the experimentally (multipole model) and theoretically (DFT) determined electron density. To the best of our knowledge, this study is the first example of an experimental electronic structure of a coordination compound in which a peroxo anion is bonded to a 3d central atom. The titanium coordination polyhedron could be described as a deformed tetrahedral pyramid if the midpoint of the peroxide O—O bond (side-on mode) is considered to be in the quasi-apical position. According to the multipole model (MM) results, the titanium atom has a positive QTAIM charge of 2.05 e− which does not correspond to the formal Ti (IV) oxidation state. On the other hand, the peroxo oxygen atoms O(1) and O(2) have MM QTAIM charges of −0.27 and −0.12, respectively. This asymmetric charge density distribution on the peroxo oxygens is in agreement with the distorted orientation of the O2 moiety with respect to the titanium atom. Despite the fact that the overall MM charge of the O2 moiety is more remote from the formal −2 charge than from neutral O2, the O—O distance remains close to that in the peroxo O2 2− anion. In the case of DFT results, the titanium atom charge is also found to be close to +2, the O2 x− moiety charge is around −1, the optimized O—O distance is shorter by only ca 0.04 Å than the experimental value of 1.5005 (16) Å, and the DFT d-populations on titanium are found to be lower than the experimental MM value. This study is the first experimental electronic structure of a transition metal peroxo complex.

Effect of Titanium on FOX-7 - A DFT Treatment

FOX-7 is a push-pull type conjugated system which attracts attention as an insensitive high energy material. The present study considers its titanium composite within the restrictions of density functional theory at the levels of B3LYP/LANL2DZ and B3LYP/6-311++G(2df,2p). The results indicate that the titanium atom transfers some electron population to the organic component, thus forces one of the N-O bonds to rupture. Various structural, quantum chemical and UV-VIS spectral data are collected and discussed.

Synthesis and characterization of a tert-butyl ester-substituted titanocene dichloride: t-BuOOCCp2TiCl2

Bis[η5-(tert-butoxycarbonyl)cyclopentadienyl]dichloridotitanium(IV), [Ti(C10H13O2)2Cl2], was synthesized from LiCpCOOt-Bu using TiCl4, and was characterized by single-crystal X-ray diffraction and 1H NMR spectroscopy. The distorted tetrahedral geometry about the central titanium atom is relatively unchanged compared to Cp2TiCl2. The complex exhibits elongation of the titanium–cyclopentadienyl centroid distances [2.074 (3) and 2.070 (3) Å] and a contraction of the titanium–chlorine bond lengths [2.3222 (10) Å and 2.3423 (10) Å] relative to Cp2TiCl2. The dihedral angle formed by the planes of the Cp rings [52.56 (13)°] is smaller than seen in Cp2TiCl2. Both ester groups extend from the same side of the Cp rings, and occur on the same side of the complex as the chlorido ligands. The complex may serve as a convenient synthon for titanocene complexes with carboxylate anchoring groups for binding to metal oxide substrates.

INFLUENCE OF SUBSTITUTES ON THE RATE OF THE REACTION OF META- АND PARASUBSTITUTED BENZOIC ACIDS WTH ANILINE, CATALYZED BY POLYBUTOXYTITANATES

The polybutoxytitanates catalysis of acylation of anilines by meta- and parasubstituted benzoic acid results in substituted benzanilides. The rate constants of this second-order reaction (the first in terms of aniline and substituted benzoic acid; boiling ortho-xylene, 145 °С) correlates well according to the Hammett equation with two straight lines for individual groups of substituents with ρ = 1.76 (electron donors) and 0.12 (electron acceptors). Oxybenzoic and phthalic acids, that do not react with aniline and inhibit the interaction of the latter with benzoic acid, fall out of this dependence. Based on these data, as well as the results of a previous studies of the interaction of substituted anilines with a benzoic acid made under comparable conditions, a mechanism of bifunctional catalysis due to the formation of titanium polybutoxybenzoates in the first minutes of the reaction in situ — the true catalysts of the process, is proposed. The nucleophilic center of the catalyst can be represented by the carbonyl group of a substituted benzoate bound to a titanium atom, forming an H-bond with hydrogen atoms of the amino group of aniline, thus activated to react with a substituted benzoic acid. The titanium atoms of polytitanate (coordination catalysis) and their complexes with the resulting substituted benzanilides (acid catalysis) can act as the electrophilic center of a catalyst that activates the carbonyl group of a substituted benzoic acid to nucleophilic attack by aniline. A titanium atom bound to a substituted benzoate exhibits, depending on the nature of the substituent, various catalytic activity.

INFLUENCE OF SUBSTITUTES ON THE RATE OF REACTION OF SUBSTITUTED ANILINES WITH A BENZOIC ACID, CATALYZED BY POLYBUTOXYTITANATES

The catalytic production of benzanilide by the interaction of benzoic acid with aniline is an important model process recently intensively developed in the field of the concept of «green chemistry», direct catalytic amidation, and its study is an urgent scientific and practical task. To solve this problem, the effect of substituents in the aniline core on the catalysis by polybut-oxytitanates on acylation of substituted anilines by benzoic acid was studied. The rate constants of this the second-order reaction (the first with respect to substituted aniline and benzoic acid; boiling ortho-xylene, 145 °С, nitrogen flow) are well correlated according to the Hammett equation with three straight lines for individual groups of substituents with ρ = –0.86 (electron donors), 1.12 (weak electron acceptors), –2.83 (strong electron acceptors). To explain this dependence, a variant of the catalytic amidation mechanism is proposed, which takes into account the formation of titanium butoxybenzoates in the first minutes of the reaction — a true amidation catalyst; coordination (polarization of the carbonyl bond in the interaction with the catalyst titanium atom) and acid (polarization of the carbonyl bond in the interaction with the complex of the formed substitu- ted benzanilide with the titanium butoxyben-zoates) catalysis routes. Inhibition of the catalytic reaction is associated with the presence in the mass of water, relative excess of benzoic acid and a possible amide–imide tautomerism of substituted benzanilides, accompanied by the interaction of the imide form of the latter with titanium butoxybenzoates, which does not lead to the route of acid catalysis. The rate constants for catalytic acylation of substituted anilines, containing electron-donating substituents, with benzoic acid in the air are correlated according to the Hammett equation by a straight line segment with ρ = 0.99, which is associated with the predominant influence of the oxidation processes of the corresponding amines. Catalytic acylation under comparable conditions of substituted anilines, containing electron-withdrawing substituents, oxidation processes due to atmospheric oxygen have little effect on.

EFFECT OF SUBSTITUTED BENZANILIDES ADDITIVES ON THE SPEED OF CATALYZED BY TETRABUTOXYTIITANE AND POLYBUTOXYTIТANATE REACTION OF ANILINE WITH BENZOIC ACID

The catalytic synthesis of benzanilide by the interaction of benzoic acid with aniline is an important model reaction of direct catalytic amidation that has been intensively developed recently in the field of the concept of «green chemistry», and its study is an urgent task. In the framework of solving this problem the effect of the final product - benzanilide, and its substituted on catalysis by tetrabutoxytitanium and its partial hydrolysis products (polybutoxytitanates) was studied. With an increase in the concentration of the preliminary addition of benzanilide, the initial rate of catalytic interaction of benzoic acid with aniline decreases monotonically. Benzanilide itself does not catalyze the amidation process, does not hydrolyze under reaction conditions by water, and does not undergo other changes in the reaction mass. The kinetics of the formation of benzanylide in the presence of additives of a number of substituted benzanilides, containing electron-withdrawing substituents, showed higher values ​​of the reaction rate constants as compared to that for the formation of benzanilide in the absence of any additives. This suggests the presence of two routes of catalysis: the coordination route (polarization of the carbonyl group of the benzoic acid due to the interaction with the atom of titanium) and acid route. The latter can be associated with the formation of complexes due to the coordination of the titanium atom of the catalyst with the carbonyl group of the substituted benzanilide, the appearance of a relatively acidic NH=group and catalysis of conjugate acid: the titanium-containing catalyst + the corresponding substituted benzanilide. The formation of such a catalytic complex, by the example of a pair of benzanilide + tetrabutoxytitanium, was confirmed by the NMR 1H spectroscopy method. Catalysis of aniline acylation with benzoic acid in the presence of additions meta= and para=substituted benzanilides correlates well with the Hammett equation with two straight line segments with ρ=0.478 and ρ=-0.235, with a maximum, indicating a different effect of substituted benzanilides containing electron-donating and strong electron-withdrawing substitutes on the complexation with tetrabutoxytitanium and polybutoxytitanium and the change in their catalytic activity. The decrease in the rate of the catalytic formation of benzanilide is especially pronounced with the addition of ortho=substituted benzanilides, containing strong electron-withdrawing substituents, which at the same time have a high steric effect.

Conversion of dinitrogen to tris(trimethylsilyl)amine catalyzed by titanium triamido-amine complexes

Catalytic fixation of molecular dinitrogen into tris(trimethylsilyl)amine with TON up to 16.5 per titanium atom under ambient conditions was achieved using a well-defined titanium complex with a triamidoamine ligand.