Complexation Behavior of Pinene–Bipyridine Ligands towards Lanthanides: The Influence of the Carboxylic Arm

The complexation behavior of two novel, chiral pinene–bipyridine-type ligands ((–)-HL1 and (–)-HL2) containing a carboxylic arm towards lanthanide Ln(III) (Ln = La, Eu, Lu) ions was investigated through spectroscopic methods. The association constants of the mononuclear complexes determined from the UV-Vis titrations indicated that the ligand (–)-HL1 possessing a shorter carboxylic arm formed more stable complexes compared with (–)-HL2, whose carboxylic arm had one more methylene unit. This is due to the formation of more stable seven-member metal chelate rings in the first case as compared with the eight-member metal chelate rings in the second. IR and fluorescence spectroscopy provided additional information about the structure of these complexes.

The Potential Antimicrobial Action of Human Mucin 7 15-Mer Peptide and Its Metal Complexes

Mucin 7 (encoded byMUC7) is a human salivary protein that has a role in the natural immune system. Fragments of mucin 7 exhibit antimicrobial activity against bacteria and yeast. Although the antimicrobial properties of peptides have been known and studied for decades, the exact mechanism of action of antimicrobial peptides (AMPs) is still unclear. It is known that some AMPs require divalent metal ions to activate their activity. Herein, we investigated three 15-mer MUC7 peptides, one of which (mother peptide, sequence, L3) is a synthetic analog of a fragment naturally excised from MUC7 (with His3, His8, and His 14) and its two structural analogs, containing only two histidine residues, His3, His13 and His8, His13 (L2 and L1, respectively). Since there is a correlation between lipophilicity, the presence of metal ions (such as Cu(II) and Zn(II)) and antimicrobial activity of AMP, antimicrobial properties of the studied peptides, as well as their complexes with Cu(II) and Zn(II) ions, were tested for activity against Gram-positive (Enterococcus faecalis, Staphylococcus epidermidis) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria and fungi (Candida albicans). The results were correlated with their lipophilicity. Coordination and thermodynamic studies (potentiometry, UV-Vis, CD) revealed the formation of mainly mononuclear complexes in solution for all studied systems with different stability in the physiological pH range.

Alcohol Oxidation Assisted by Molybdenum Hydrazonato Catalysts Employing Hydroperoxide Oxidants

Molybdenum(VI) catalysts were obtained from methanol or acetonitrile by the reaction of [MoO2(C5H7O2)2] and isonicotinoyl- or nicotinoyl-based aroylhydrazones. Reactions in methanol resulted in the formation of the mononuclear complexes [MoO2(L1–4)(MeOH)] (1a–4a), while the ones in acetonitrile provided polynuclear complexes [MoO2(L1–4)]n (1–4). Crystals of polynuclear compound, [MoO2(L3)]n∙H2O (3∙H2O), suitable for X-ray diffraction analysis were obtained by the solvothermal procedure at 110 °C. Complexes were characterized by infrared spectroscopy (IR-ATR), nuclear magnetic resonance (NMR), elemental analysis (EA), and thermogravimetric analysis (TGA). The prepared catalysts were tested in alcohol oxidation reactions. Carveol, cyclohexanol, and butan-2-ol were investigated substrates. Because the alcohol oxidations are very challenging due to various possible pathways, the idea was to test different oxidants, H2O2, TBHP in water and decane, to optimize the researched catalytic system.

The Unusual Role of Pro in Cu(II) Binding by His2-Cyclopentapeptide

In this paper, we present findings from studying the interaction of copper(II) ions with the His2-cyclopentapeptide and the role of proline used for the purpose of potentiometric titration and UV-Vis, CD and EPR spectroscopic measurements. Experiments of two homodetic peptides differing by one amino acid residue were conducted for a ligand to metal ratio of 1:1 in the pH range 2.5–11.0. The presented studies reveal that peptides form only mononuclear complexes, and the CuH2L complex appears in the system first (for both L1 and L2). Study results show that the presence of Pro influences the structure of formed complexes and their stabilities and has a strong impact on the efficiency of copper(II) coordination.

Solvent-Induced Hysteresis Loop in Anionic Spin Crossover (SCO) Isomorph Complexes

Reactions of Fe(II) with the tris-(pyridin-2-yl)ethoxymethane (py3C-OEt) tripodal ligand in presence of the pseudohalide ancillary NCE- (E = S, Se, BH3) ligands led to a series of three mononuclear complexes formulated as [Fe(py3C-OEt)2][Fe(py3C-OEt)(NCE)3]2·2CH3CN, with E = S (1), BH3 (2) and Se (3). Single crystal characterizations (complexes 1-2) and X-ray powder diffraction (complexes 1-3) reveal monomeric isomorph structures formed by the spin crossover (SCO) anionic [Fe(py3C-OEt)(NCE)3] complex, associated with the low spin (LS) cationic [Fe(py3C-OEt)2]2+ complex and two solvent acetonitrile molecules. In the [Fe(py3C-OEt)2]2+ cation, the Fe(II) is coordinated by two py3C-OEt tridentate ligands, while the [Fe(py3C-OEt)(NCE)3] anion displays a hexacoordinated environment involving three N-donor atoms of one py3C-OEt ligand and three nitrogen atoms arising from of the three (NCE) coligands. The magnetic studies show the presence of gradual SCO behavior for the three complexes: a one-step transition around 205 K for 1 and two step-transitions for compounds 2 and 3, centred at 245 K and 380 K for 2, and at 170 K and 298 K for 3. The magnetic behaviors of complexes 1 and 2 remain unchanged when heating up to 500 K, while complex 3 shows significant changes which are caused by the crystallisation solvent loss above room temperature.

Mono- and Binuclear Copper(II) and Nickel(II) Complexes with the 3,6-Bis(picolylamino)-1,2,4,5-Tetrazine Ligand

Four new compounds of formulas [Cu(hfac)2(L)] (1), [Ni(hfac)2(L)] (2), [{Cu(hfac)2}2(µ-L)]·2CH3OH (3) and [{Ni(hfac)2}2(µ-L)]·2CH3CN (4) [Hhfac = hexafluoroacetylacetone and L = 3,6-bis(picolylamino)-1,2,4,5-tetrazine] have been prepared and their structures determined by X-ray diffraction on single crystals. Compounds 1 and 2 are isostructural mononuclear complexes where the metal ions [copper(II) (1) and nickel(II) (2)] are six-coordinated in distorted octahedral MN2O4 surroundings which are built by two bidentate hfac ligands plus another bidentate L molecule. This last ligand coordinates to the metal ions through the nitrogen atoms of the picolylamine fragment. Compounds 3 and 4 are centrosymmetric homodinuclear compounds where two bidentate hfac units are the bidentate capping ligands at each metal center and a bis-bidentate L molecule acts as a bridge. The values of the intramolecular metal···metal separation are 7.97 (3) and 7.82 Å (4). Static (dc) magnetic susceptibility measurements were carried out for polycrystalline samples 1–4 in the temperature range 1.9–300 K. Curie law behaviors were observed for 1 and 2, the downturn of χMT in the low temperature region for 2 being due to the zero-field splitting of the nickel(II) ion. Very weak [J = −0.247(2) cm−1] and relatively weak intramolecular antiferromagnetic interactions [J = −4.86(2) cm−1] occurred in 3 and 4, respectively (the spin Hamiltonian being defined as H = −JS1·S2). Simple symmetry considerations about the overlap between the magnetic orbitals across the extended bis-bidentate L bridge in 3 and 4 account for their magnetic properties.

Potentiometric Study of the Copper(II) Ions Complexation Process with Thiourea in Acidic Medium

Potentiometric titration method was used to study the copper(II) ions complexation process with thiourea in medium containing 1 mol/l of hydrochloric acid at a temperature of 298 K. It was found out that mononuclear (CuTMn) and binuclear (Cu2TMn) complexes were observed in the copper(II) --- thiourea system depending on the copper ions concentration. It was determined that in acid medium with copper(II) ions concentration less than 5 · 10--4 mol/l and mononuclear complexes of the composition are generated in the system, which constant is equal to lg β3 = 11.9. With an increase in the copper(II) ions concentration (CCu+2 > 5 · 10--4 mol/l), binuclear thiourea complex dominates in the system. Stability constant logarithm of the Cu2TM6 composition binuclear complex is equal to 27.5 and was calculated using the modified Leden's method. Based on the constants, complexes existence regions were determined depending on the ratio CTM/CCu+2. With the CTM/CCu+2 ≥ 5 relations, existence of a dominant complex significantly depends on the copper(II) ions concentration. It turned out that fractions of all complex particles were growing with an increase in the copper(II) ions concentration in the system. It was found that stability of thiourea complexes in the CuLi2+, AgLi+, AuLi+ rows was increasing