Current Inorganic Chemistry (Discontinued) - Volume 5, Issue 2, 2015

New bis-chelate oxime-and-amide (2E,2'E)-N,N'-(2-hydroxypropane-1,3-diyl)bis[2- (hydroxyimino)propanamide]) ligand has been synthesised and characterised. Protonation constants for it and structurally similar (2E,2'E)-N,N'-propane-1,3-diylbis[2-(hydroxyimino)propanamide] ligand were determined by potentiometric titrations at 25 °C in 0.100 M NMe4Cl aqueous medium and led to the conclusion of a tetraprotic (LH4) behaviour, contrary to previously reported in literature LH2 model. Protonation enthalpies for the two ligands were determined by calorimetric titrations and the tetraprotic behaviour in both cases was confirmed. Complete thermodynamic characterisation of the protonation process revealed peculiar sequence of the stepwise enthalpy and entropy values. The latter were interpreted in terms of structural rearrangement of the ligand forms at various stages of protonation. Results of the UV-Vis spectroscopic titrations for both ligands were analysed and compared with spectra computed at the DFT level. Experimental and calculated spectra were shown to be in satisfactory agreement, and support key assumptions about the nature of protonated species inferred from the thermodynamic studies. Observed spectroscopic transitions were designated as π→π* ones.

The preparation of 2-pyridyl cyanoxime (hereafter referred to as H(2PCO)), along with several metal complexes, is described in detail. The cyanoxime was investigated in solutions, and its two protonation constants were measured spectrophotometrically following a significant shift of the compound’s π→π* transitions at 208 and 280 nm. The first, with pK1=1.81(9), corresponds to proton release from the pyridine moiety, and the second, pK2=6.77(1), corresponds to a proton release from the hydroxyimino group. The H(2PCO) in solution easily deprotonates to form a yellow conjugated anion, which demonstrates significant solvatochromism in solutions: the bathochromic shift of n→π* band for 2PCO- in methanol (MeOH) and dimethylformamide (DFM) is 90 nm, which is one of the highest on record for cyanoximes. The crystal structures of the free ligand, H(2PCO); its ionic derivative, PPh4(2PCO); and its divalent complexes, [Ni(2PCO)2·2H2O], [Ni{2PCO}2(H2O)}2], mer-PPh4[Ni(2PCO)3]·2H2O, and mer-AsPh4[Fe(2PCO)3], were determined. The protonated cyanoxime H(2PCO) and as anion in transition metal complexes adopt cis-anti configuration and act as bidentate-chelate or tridentate (chelate + bridging mode) ligand. Data from the 57Fe Mössbauer spectroscopy indicated that the “AsPh4[Fe(2PCO)3]” sample comprised of 85-88% low-spin Fe(II) compound state with small value of the QS, and 12-15% Fe2+ compound being in high-spin state. The major component corresponds to the mer-isomer, while the high-spin component is suggested for the [Fe(2PCO)2·2H2O] complex. The formation constants in the H(2PCO) – Fe2+ system were measured using UV-visible spectroscopy and showed that the intensely colored [Fe(2PCO)3]- anionic complex (λmax=533 nm) has Kf= 11.09. This finding is the second highest established for colored iron(II) complexes after [Fe(phen)3]2+ cationic compound that is traditionally used in analytical chemistry.

Two new mononuclear complexes based on a hydrazine ligand (2E,N'Z)-2-(3,5-dimethyl- 1H-pyrazol-1-yl)-N'-(2-hydroxy-benzylidene)-2-(hydroxyimino)acetohydrazide (H3POAS): [Ni(H2POAS)2]· 2CH3OH·0.5H2O (1) and [Fe(H2POAS)2]·0.5CH3OH·0.5H2O (2) have been synthesized and characterized by IR-spectroscopy, elemental and X-ray single crystal analyses. The reported complexes have similar structures and reveal distorted octahedral N2O4 coordination arrangement of the central atoms formed by the phenolic, the azomethine and the carboxylic groups of the ligand. The molecules of the structures are united by multiple-branched systems of hydrogen bonds, π-stacking and the Van Der Waals forces. In addition, stability of the reported complexes in methanolic solution has been studied by ESI mass-spectrometry.

A detailed characterization by UV-visible, IR and NMR (1H, 13C, 15N) spectroscopies of two isomeric 1,2- and 2,1-nitrosonaphthols (1 and 2) has been carried out. The crystal structures of these iconic compounds, which were widely used in analytical chemistry in the past, were determined and evidenced their quinone-oxime nature in solid state. Compound 1 exists in the crystal as a syn- diastereomer in the closed form due to a strong intramolecular H-bond, while compound 2 is an anti- diastereomer forming a dimer with two intermolecular H-bonds. The oxime character of 1 and 2 in solutions was confirmed after careful studies of the NMR spectra of both compounds in solvents of different polarity and donor properties: CD2Cl2, acetone-d6 and DMSO-d6. An equilibrium state between oxime syn- and anti- isomers exists and can be modulated in a controlled way by changing media's polarity. Both compounds were also prepared labeled with 15N (50%) for recording their NMR nitrogen-15 spectra, and for the identification/assignment of vibrations with the participation of the >C=N-OH fragment. During synthesis of compound 1 a persistent, very polar, red-colored impurity 3 was detected, isolated and crystallographically characterized. It was found to be a rather unusual π-complex between the final compound 1 and its precursor 2-naphthol. The red color originates from the CT-band in the visible region of the UV/Vis-spectrum where the naphthol acts as a donor with 1 being an acceptor. Both are separated at 3.03 A.

Metallurgical chemistry is the most ancient science of chemistry. It is related to the recovery of metals from ores and their refining. In its modern form, it involves separation of finely ground minerals by flotation, melting of ores, aqueous processing of ores, and the application of electric current to separate and purify metals from aqueous solution or from a fused salt. Metallurgical chemistry is closely related to the chemical industry.

Using first principles plane-waves pseudopotential method, the thermodynamic stability, the elastic constants, and structural phase transition of technetium diboride (TcB2) under pressure are revealed, as well as the role of metallic bond on its hardness. Seven possible structures are chosen to probe, including marcasite-type orthorhombic, OsB2-type orthorhombic, simple tetragonal (ST-type), MoB2-type hexagonal, AlB2-type hexagonal, ReB2-type hexagonal, and diamond-type cubic lattices. The calculations demonstrate that the ReB2-TcB2 phase holds the most energetically stable structure in a larger range of pressure. According to the criteria of the lowest Gibbs energy, the phase transition point Pt = 147.5 GPa between the ReB2-TcB2 and AlB2-TcB2 phases is firstly determined. Furthermore, Mulliken overlap population analysis allows us to use a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bond. The superior performance and large hardness (34.0 GPa) of ReB2-TcB2 suggest that it is an incompressible, anisotropic, brittle and hard material.