Inorganics, Volume 9, Issue 2 (February 2021) – 9 articles

In aqueous media, V^IV- and V^V-ions and compounds undergo chemical changes such as hydrolysis, ligand exchange and redox reactions that depend on pH and concentration of the vanadium species, and on the nature of the several components present. In particular, the behaviour of vanadium compounds in biological fluids depends on their environment and on concentration of the many potential ligands present. However, when reporting the biological action of a particular complex, often the possibility of chemical changes occurring has been neglected, and the modifications of the complex added are not taken into account. In this work, we highlight that as soon as most vanadium(IV) and vanadium(V) compounds are dissolved in a biological media, they undergo several types of chemical transformations, and these changes are particularly extensive at the low concentrations normally used in biological experiments. We also emphasize that in case of a biochemical interaction or effect, to determine binding constants or the active species and/or propose mechanisms of action, it is essential to evaluate its speciation in the media where it is acting. This is because the vanadium complex no longer exists in its initial form. Full article

Fe^IFe^I Fe₂(S₂C₃H₆)(CO)₆(µ-CO) (¹a–CO) and its Fe^IFe^II cationic species (²a⁺–CO) are the simplest model of the CO-inhibited [FeFe] hydrogenase active site, which is known to undergo CO photolysis within a temperature-dependent process whose products and mechanism are still a matter of debate. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations, the ground state and low-lying excited-state potential energy surfaces (PESs) of ¹a–CO and ²a⁺–CO have been explored aimed at elucidating the dynamics of the CO photolysis yielding Fe₂(S₂C₃H₆)(CO)₆ (¹a) and [Fe₂(S₂C₃H₆)(CO)₆]⁺ (²a⁺), two simple models of the catalytic site of the enzyme. Two main results came out from these investigations. First, a–CO and ²a⁺–CO are both bound with respect to any CO dissociation with the lowest free energy barriers around 10 kcal mol⁻¹, suggesting that at least ²a⁺–CO may be synthesized. Second, focusing on the cationic form, we found at least two clear excited-state channels along the PESs of ²a⁺–CO that are unbound with respect to equatorial CO dissociation. Full article

The dramatic expansion of the earth’s population can be directly correlated with the Haber–Bosch process for nitrogen fixation becoming widely available after World War II. The ready availability of artificial fertilizer derived thereof dramatically improved food supplies world-wide. Recently, artificial nitrogen fixation surpassed the natural process. The Haber–Bosch process is extremely energy and green-house gas intensive due to its high-temperature and H₂ demands. Many low valent Ti(II) complexes of N₂ are known. We report herein a preliminary investigation of the low-valent chemistry of Ti with the CCC-NHC ligand architecture. These CCC-NHC pincer Ti(IV) complexes are readily reduced with KC₈ or Mg powder. Preliminary results indicate very different reactivity patterns with alkynes and phosphines for this ligand architecture versus prior ligands. Successful reduction to an intact low-valent (CCC-NHC)Ti complex was confirmed by re-oxidation with PhICl₂. Full article

Gold phosphine derivatives such as thiolates have been recently proposed as catalysts or catalyst precursors. The relevance of the supramolecular environment on the fine-tuning of the catalytical activity on these compounds incentivizes the use of tools that are convenient to characterize in detail the non-covalent landscape of the systems. Herein, we show the molecular and supramolecular diversity caused by the changes in the fluorination pattern in a family of new XPhos goldfluorothiolate derivatives. Furthermore, we studied the supramolecular interactions around the Au centers using quantum chemical topology tools, in particular the quantum theory of atoms in molecules (QTAIM) and the non-covalent interaction index. Our results give detailed insights into the fluorination effects on the strength of intramolecular and intermolecular interactions in these systems. We have also used QTAIM delocalization indexes to define a novel hapticity indicator. Finally, we assessed the trans influence of the fluorothiolates on the phosphine in terms of the change in the δ ³¹P-NMR. These results show the feasibility of the use of fluorination in the modulation of the electronic properties of Buchwald phosphine gold(I) compounds, and thereby its potential catalytic activity. Full article

The equilibrium geometry and two measures (the equilibrium dissociation energy in the complete basis set limit, D_e(CBS) and the intermolecular stretching force constant k_σ) of the strength of the non-covalent interaction of each of six Lewis acids M–X (M = Cu, Ag, Au) with each of nine simple Lewis bases B (B = N₂, CO, HCCH, CH₂CH₂, H₂S, PH₃, HCN, H₂O, and NH₃) have been calculated at the CCSD(T)/aug-cc-pVTZ level of theory in a systematic investigation of the coinage–metal bond. Unlike the corresponding series of hydrogen-bonded B⋯HX and halogen-bonded B⋯XY complexes (and other series involving non-covalent interactions), D_e is not directly proportional to k_σ. Nevertheless, as for the other series, it has been possible to express D_e in terms of the equation D_e = cN_B.E_MX, where N_B and E_MX are the nucleophilicities of the Lewis bases B and the electrophilicities of the Lewis acids M–X, respectively. The order of the E_MX is determined to be E_AuF > E_AuCl > E_CuF > E_CuCl > E_AgF ≈ E_AgCl. A reduced electrophilicity defined as (E_MX/σ_max) is introduced, where σ_max is the maximum positive value of the molecular electrostatic surface potential on the 0.001 e/bohr³ iso-surface. This quantity is, in good approximation, independent of whether F or Cl is attached to M. Full article

Five-coordinate Cu(II) complexes, [Cu(Lⁿ)X]ClO₄/PF₆, where Lⁿ = piperazine ligands bearing two pyridyl arms and X = ClO₄⁻ for Lⁿ = L¹ (1-ClO₄), L² (2-ClO₄), L³ (3-ClO₄), and L⁶ (6-ClO₄) as well as [Cu(Lⁿ)Cl]PF₆ for Lⁿ = L¹ (1-Cl), L⁴ (4-Cl), and L⁵ (5-Cl) have been synthesized and characterized by spectroscopic techniques. The molecular structures of the last two complexes were determined by X-ray crystallography. In aqueous acetonitrile solutions, molar conductivity measurements and UV-VIS spectrophotometric titrations of the complexes revealed the hydrolysis of the complexes to [Cu(Lⁿ)(H₂O)]²⁺ species. The biological activity of the Cu(II) complexes with respect to DNA cleavage and cytotoxicity was investigated. At micromolar concentration within 2 h and pH 7.4, DNA cleavage rate decreased in the order: 1-Cl ≈ 1-ClO₄ > 3-ClO₄ ≥ 2-ClO₄ with cleavage enhancements of up to 23 million. Complexes 4-Cl, 5-Cl, and 6-ClO₄ were inactive. In order to elucidate the cleavage mechanism, the cleavage of bis(4-nitrophenyl)phosphate (BNPP) and reactive oxygen species (ROS) quenching studies were conducted. The mechanistic pathway of DNA cleavage depends on the ligand’s skeleton: while an oxidative pathway was preferable for 1-Cl/1-ClO₄, DNA cleavage by 2-ClO₄ and 3-ClO₄ predominantly proceeds via a hydrolytic mechanism. Complexes 1-ClO₄, 3-ClO₄, and 5-Cl were found to be cytotoxic against A2780 cells (IC₅₀ 30–40 µM). In fibroblasts, the IC₅₀ value was much higher for 3-ClO₄ with no toxic effect. Full article

To develop donor–acceptor–donor (D–A–D) type new photo-electric conversion materials, new tetrathiafulvalene (TTF)-Mq₂-TTF complexes 1 and 2 were synthesized, where two bis(n-hexylthio)tetrathiafulvalene moieties were attached to the Mq₂ part (1: M = Zn, 2: M = Ni, q = 8-quinolinato) through amide bonds. UV-Vis absorption spectra of these complexes showed strong and sharp absorption maxima at 268 nm and small absorption maxima around 410 nm, corresponding to those of Znq₂ and Niq₂ parts. Furthermore, complexes 1 and 2 exhibited absorption tails up to a much longer wavelength region of ca. 700 nm, suggesting the appearance of charge transfer absorption from TTF to the Mq₂ parts. The photoelectrochemical measurements on the thin films of these complexes casted on ITO-coated glass substrates suggest that positive photocurrents can be generated by the photoinduced intramolecular electron transfer process between the TTF and Mq₂ parts. Full article

A new Cu(II) complex is synthetized by the reaction of copper nitrate and a N-acylhydrazone ligand obtained from the condensation of o-vanillin and 2-thiophecarbohydrazide (H₂L). The solid-state structure of [Cu(HL)(H₂O)](NO₃)·H₂O, or CuHL for simplicity, was determined by X-ray diffraction. In the cationic complex, the copper center is in a nearly squared planar environment with the nitrate interacting as a counterion. CuHL was characterized by spectroscopic techniques, including solid-state FTIR, Raman, electron paramagnetic resonance (EPR) and diffuse reflectance and solution UV-Vis electronic spectroscopy. Calculations based on the density functional theory (DFT) assisted the interpretation and assignment of the spectroscopic data. The complex does not show relevant antioxidant activity evaluated by the radical cation of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) method, being even less active than the free ligand as a radical quencher. Cytotoxicity assays of CuHL against three human tumor cell lines, namely MG-63, A549 and HT-29, revealed an important enhancement of the effectiveness as compared with both the ligand and the free metal ion. Moreover, its cytotoxic effect was remarkably stronger than that of the reference metallodrug cisplatin in all cancer cell lines tested, a promissory result in the search for new metallodrugs of essential transition metals. Full article

Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a direct coordination bond with a nickel(II) ion. Complex formation of nickel(II) salts and tert-butyl 2-pyridyl nitroxides afforded a discrete 2p–3d–2p triad. Ferromagnetic coupling is favored when the magnetic orbitals, nickel(II) dσ and radical π*, are arranged in a strictly orthogonal fashion, namely, a planar coordination structure is characterized. In contrast, a severe twist around the coordination bond gives an orbital overlap, resulting in antiferromagnetic coupling. Non-chelatable nitroxide ligands are available for highly twisted and practically diamagnetic complexes. Here, the Ni–O–N–C_sp² torsion (dihedral) angle is supposed to be a useful metric to describe the nickel ion dislocated out of the radical π* nodal plane. Spin-transition complexes exhibited a planar coordination structure in a high-temperature phase and a nonplanar structure in a low-temperature phase. The gradual spin transition is described as a spin equilibrium obeying the van’t Hoff law. Density functional theory calculation indicates that the energy level crossing of the high- and low-spin states. The optimized structures of diamagnetic and high-spin states well agreed with the experimental large and small torsions, respectively. The novel mechanism of the present spin transition lies in the ferro-/antiferromagnetic coupling switch. The entropy-driven mechanism is plausible after combining the results of the related copper(II)-nitroxide compounds. Attention must be paid to the coupling parameter J as a variable of temperature in the magnetic analysis of such spin-transition materials. For future work, the exchange coupling may be tuned by chemical modification and external stimulus, because it has been clarified that the parameter is sensitive to the coordination structure and actually varies from 2J/k_B = +400 K to −1400 K. Full article