Inorganics, Volume 7, Issue 4 (April 2019) – 14 articles

The catalytic cycle of the oxygen-evolving complex (OEC) of photosystem II (PSII) comprises five intermediate states S_i (i = 0–4), from the most reduced S₀ state to the most oxidized S₄, which spontaneously evolves dioxygen. The precise geometric and electronic structure of the S_i states, and hence the mechanism of O–O bond formation in the OEC, remain under investigation, particularly for the final steps of the catalytic cycle. Recent advances in protein crystallography based on X-ray free-electron lasers (XFELs) have produced new structural models for the S₃ state, which indicate that two of the oxygen atoms of the inorganic Mn₄CaO₆ core of the OEC are in very close proximity. This has been interpreted as possible evidence for “early-onset” O–O bond formation in the S₃ state, as opposed to the more widely accepted view that the O–O bond is formed in the final state of the cycle, S₄. Peroxo or superoxo formation in S₃ has received partial support from computational studies. Here, a brief overview is provided of spectroscopic information, recent crystallographic results, and computational models for the S₃ state. Emphasis is placed on computational S₃ models that involve O–O formation, which are discussed with respect to their agreement with structural information, experimental evidence from various spectroscopic studies, and substrate exchange kinetics. Despite seemingly better agreement with some of the available crystallographic interpretations for the S₃ state, models that implicate early-onset O–O bond formation are hard to reconcile with the complete line of experimental evidence, especially with X-ray absorption, X-ray emission, and magnetic resonance spectroscopic observations. Specifically with respect to quantum chemical studies, the inconclusive energetics for the possible isoforms of S₃ is an acute problem that is probably beyond the capabilities of standard density functional theory. Full article

Wilson disease is an inherited disorder caused by mutations in the ATP7B gene resulting in copper metabolism disturbances. As a consequence, copper accumulates in different organs with most common presentation in liver and brain. Chelating agents that nonspecifically chelate copper, and promote its urinary excretion, or zinc salts interfering with the absorption of copper from the gastrointestinal tract, are current medications. Also gene therapy, restoring ATP7B gene function or trials with bis-choline tetrathiomolybdate (WTX101) removing excess copper from intracellular hepatic copper stores and increasing biliary copper excretion, is promising in reducing body’s copper content. Therapy efficacy is mostly evaluated by testing for evidence of liver disease and neurological symptoms, hepatic synthetic functions, indices of copper metabolisms, urinary copper excretions, or direct copper measurements. However, several studies conducted in patients or Wilson disease models have shown that not only the absolute concentration of copper, but also its spatial distribution within the diseased tissue is relevant for disease severity and outcome. Here we discuss laser ablation inductively coupled plasma spectrometry imaging as a novel method for accurate determination of trace element concentrations with high diagnostic sensitivity, spatial resolution, specificity, and quantification ability in experimental and clinical Wilson disease specimens. Full article

The pyridine-2,6-carboxylic esters pydicR₂ with R = Me or Ph form the unprecedented mononuclear Cu^II complexes [Cu(pydicR₂)Cl₃]⁻ in one-pot reactions starting from pyridine-2,6-carboxychloride pydicCl₂, Cu^II chloride, and NEt₃ in MeOH or PhOH solution under non-aqueous conditions. The triethylammonium salts (HNEt₃)[Cu(pydicR₂)Cl₃] were isolated. The methyl derivative could be crystallized to allow a XRD structure determination. Both structures were optimized using DFT calculations in various surroundings ranging from gas phase and the non-coordinating solvent CH₂Cl₂ to the weakly coordinating acetone and well-coordinating solvents acetonitrile (MeCN) or dimethylformamide (DMF), while detailed calculation showed the charge distribution, dipole moments, and HOMO–LUMO gap energies changing upon solvation. According to these calculations, the ion pairs and the anionic Cu^II complexes were stable, which shows only Cu–Cl bond elongation and weakening of the charge transfer between the anionic complex and the cation as solvents become polar. Synthesis attempts in the presence of water yielded the Cu^II complexes [Cu(pydic)(OH₂)₂]_n and [Cu(OH₂)₆][{Cu(pydic)}₂(µ-Cl)₂], which results from pydicCl₂ hydrolysis. Alternatively, the new pydic(IPh)₂ (IPh = 2-iodo-phenyl) ester ligand was synthesized and reacted with anhydrous CuCl₂, which yields the new binuclear complex [{Cu(pydic(IPh)₂)Cl}₂(µ-Cl)₂]. EPR spectroscopy of the solid compounds reveals typical axial spectra in line with the observed and DFT calculated geometries. Cyclic voltammetry and UV–vis absorption spectroscopy in solution are in line with un-dissociated complex species [Cu(pydicR₂)Cl₃]⁻. Full article

Density Functional Theory and Complete Active Space Self-Consistent Field (CASSCF) methodologies are used to explore the electronic structure of the cationic V–N clusters, [V₄N₄]⁺ and [V₆N₆]⁺, that have been identified in recent mass spectrometric experiments. Our calculations indicate that both clusters are based on cubane-like fragments of the rock-salt lattice. In the smaller [V₄N₄]⁺ cluster, the V–V bonding is delocalized over the tetrahedron, with net bond orders of 1/3 per V–V bond. In [V₆N₆]⁺, in contrast, the V–V bonding is strongly localized in the central V₂N₂ unit, which has a short V=V double bond. CASSCF calculations reveal that both localized and delocalized V–V bonds are highly multi-configurational. Full article

The lowest-energy isomer of C ${}_2$ Si ${}_2$ H ${}_4$ is determined by high-accuracy ab initio calculations to be the bridged four-membered ring 1,2-didehydro-1,3-disilabicyclo[1.1.0]butane (1), contrary to prior theoretical and experimental studies favoring the three-member ring silylsilacyclopropenylidene (2). These and eight other low-lying minima on the potential energy surface are characterized and ordered by energy using the CCSD(T) method with complete basis set extrapolation, and the resulting benchmark-quality set of relative isomer energies is used to evaluate the performance of several comparatively inexpensive approaches based on many-body perturbation theory and density functional theory (DFT). Double-hybrid DFT methods are found to provide an exceptional balance of accuracy and efficiency for energy-ordering isomers. Free energy profiles are developed to reason the relatively large abundance of isomer 2 observed in previous measurements. Infrared spectra and photolysis reaction mechanisms are modeled for isomers 1 and 2, providing additional insight about previously reported spectra and photoisomerization channels. Full article

The electrochemical reduction of the complex [Fe₂(CO)₆{µ-(SCH₂)₂GeMe₂}] (1) under N₂ and CO is reported applying cyclic voltammetry. Reduction of complex 1 in CO saturated solutions prevents the possible release of CO from the dianion 1²⁻, while the latter reacts with additional CO forming a spectroscopically uncharacterized product P1. This product undergoes a reversible redox process at E_1/2 = −0.70 V (0.2 V∙s⁻¹). In this report, the structure of the neutral complex 1, isomers of dianionic form of 1, and P1 are described applying DFT computations. Furthermore, we propose reaction pathways for H₂ production on the basis of the cyclic voltammetry of complex 1 in presence of the strong acid CF₃SO₃H. Full article

The pyrochlore structure (A₂B₂O₇) has been an object of consistent study by materials scientists largely due to the stability of the cubic lattice with respect to a wide variety of chemical species on the A or B sites. The criterion for stability under ambient conditions is controlled by the ratio of these cations, which is empirically 1.36 < R_A/R_B < 1.71. However, under applied pressure synthesis conditions, the pyrochlore lattice is stable up to R_A/R_B ∼ 2.30, opening up possibilities for new compounds. In this review, we will highlight recent work in exploring new rare-earth pyrochlores such as the germanates RE₂Ge₂O₇ and platinates RE₂Pt₂O₇. We highlight recent discoveries made in these pyrochlores such as highly correlated spin ice behavior, spin liquid ground states, and exotic magnetic ordering. Full article

Polyiodide chemistry is among the first historically reported examples of supramolecular forces at work. To date, owing to the increasingly recognized role of halogen bonding and the incorporation of iodine-based components in several devices, it remains an active field of theoretical and applied research. Herein we re-examine azacyclophanes as a class of ligands for the stabilization of iodine-dense three-dimensional networks, showing how we devised novel possible strategies starting from literature material. The new set of azacyclophane ligands affords novel crystal structures possessing intriguing properties, which develop on a double layer. At a macroscopic level, the obtained networks possess a very high iodine packing density (less than 2 times more diluted than crystalline I₂): a simple parameter, I_N, is also introduced to quickly measure and compare iodine packing density in different crystals. On the microscopic level, the present study provides evidence about the ability of one of the ligands to act as a three-dimensional supramolecular mold for the template synthesis of the rarely observed heptaiodide (I₇⁻) anion. Therefore, we believe our approach and strategy might be relevant for crystal engineering purposes. Full article

The intrinsic bonding nature of ${\lambda }^3$ -iodanes was investigated to determine where its hypervalent bonds fit along the spectrum between halogen bonding and covalent bonding. Density functional theory with an augmented Dunning valence triple zeta basis set ( $\omega$ B97X-D/aug-cc-pVTZ) coupled with vibrational spectroscopy was utilized to study a diverse set of 34 hypervalent iodine compounds. This level of theory was rationalized by comparing computational and experimental data for a small set of closely-related and well-studied iodine molecules and by a comparison with CCSD(T)/aug-cc-pVTZ results for a subset of the investigated iodine compounds. Axial bonds in ${\lambda }^3$ -iodanes fit between the three-center four-electron bond, as observed for the trihalide species IF ${}_2^{-}$ and the covalent FI molecule. The equatorial bonds in ${\lambda }^3$ -iodanes are of a covalent nature. We explored how the equatorial ligand and axial substituents affect the chemical properties of ${\lambda }^3$ -iodanes by analyzing natural bond orbital charges, local vibrational modes, the covalent/electrostatic character, and the three-center four-electron bonding character. In summary, our results show for the first time that there is a smooth transition between halogen bonding → 3c–4e bonding in trihalides → 3c–4e bonding in hypervalent iodine compounds → covalent bonding, opening a manifold of new avenues for the design of hypervalent iodine compounds with specific properties. Full article

9-Dimethyloxonium, 10-dimethyloxonium, 9-methoxy and 10-methoxy derivatives of nido-carborane (9-Me₂O-7,8-C₂B₉H₁₁, 10-Me₂O-7,8-C₂B₉H₁₁, [9-MeO-7,8-C₂B₉H₁₁]⁻, and [10-MeO-7,8-C₂B₉H₁₁]⁻, respectively) were prepared by the reaction of the parent nido-carborane [7,8-C₂B₉H₁₂]⁻ with mercury(II) chloride in a mixture of benzene and dimethoxymethane. Reactions of the 9 and 10-dimethyloxonium derivatives with triethylamine, pyridine, and 3-methyl-6-nitro-1H-indazole result in their N-methylation with the formation of the corresponding salts with 9 and 10-methoxy-nido-carborane anions. The reaction of the symmetrical methoxy derivative [10-MeO-7,8-C₂B₉H₁₁]⁻ with anhydrous FeCl₂ in tetrahydrofuran in the presence of t-BuOK results in the corresponding paramagnetic iron bis(dicarbollide) complex [8,8′-(MeO)₂-3,3′-Fe(1,2-C₂B₉H₁₀)₂]⁻, whereas the similar reactions of the asymmetrical methoxy derivative [9-MeO-7,8-C₂B₉H₁₁]⁻ with FeCl₂ and CoCl₂ presumably produce the 4,7′-isomers [4,7′-(MeO)₂-3,3′-M(1,2-C₂B₉H₁₀)₂]⁻ (M = Fe, Co) rather than a mixture of rac-4,7′- and meso-4,4′-isomers. Full article

The reaction of Bi, BiCl₃, and TlCl in the ionic liquid [BMIm]Cl·4AlCl₃ (BMIm = 1-n-butyl-3-methylimidazolium) at 180 °C yielded air-sensitive black crystals of (Bi₈)Tl[AlCl₄]₃. X-ray diffraction on single crystals at room temperature revealed a structure containing ${\left[\mathrm{Tl}\right(\mathrm{AlCl}}_4{)}_3]_{\infty }^1{}^{2-}$ strands separated by isolated Bi₈²⁺ square antiprisms. The thallium(I) ion is coordinated by twelve Cl⁻ ions of six [AlCl₄]⁻ groups, resulting in a chain of face-sharing [TlCl₁₂]¹¹⁻ icosahedra. The Bi₈²⁺ polycation is disordered, simulating a threefold axis through its center and overall hexagonal symmetry (space group P6₃/m). Slowly cooling the crystals to 170 K resulted in increased order in the Bi₈ cluster orientations. An ordered structure model in a supercell with a’ = 2a, b’ = 2b, c’ = 3c and the space group P6₅ was refined. The structure resembles a hexagonal perovskite, with complex groups in place of simple ions. Full article

Two zinc(II) hexaborate(2−) complexes, [Zn{κ³O-B₆O₇(OH)₆}(κ³N-dien)]·0.5H₂O (dien = NH(CH₂CH₂NH₂)₂) (1) and (NH₄)₂[Zn{κ²O-B₆O₇(OH)₆}₂(H₂O)₂]·2H₂O (2), and a zinc(II) dodecaborate(6−) complex, (1,3-pnH₂)₃[(κ¹N-H₃N{CH₂}₃NH₂)Zn{κ³O-B₁₂O₁₈(OH)₆}]₂·14H₂O (1,3-pn = 1,3-diaminopropane) (3), have been synthesized and characterized by single-crystal XRD studies. The complexes crystallized through self-assembly processes, from aqueous solutions containing 10:1 ratios of B(OH)₃ and appropriate Zn(II) amine complex: [Zn(dien)₂](OH)₂, [Zn(NH₃)₄](OH)₂, and [Zn(pn)₃](OH)₂. The hexaborate(2−) anions in 1 and 2 are coordinated to octahedral Zn(II) centres as tridentate (1) or bidentate ligands (2) and the dodecaborate(6−) ligand in 3 is tridentate to a tetrahedral Zn(II) centre. Full article

Three different 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetamide (DOTAM) derivatives bearing as amide N-substituents phenyl, p-methoxyphenyl and p-ethylbenzoate groups were synthesized and the ¹H and ¹⁷O NMR relaxometric behaviour of the Gd(III)-chelates and chemical exchange saturation transfer (CEST) effect of the Eu(III) complexes were evaluated. The electronic properties of the substituents were shown to strongly influence the coordinated water exchange rate (k_ex), resulting in five times faster k_ex for the electron donating phenylmethoxy group compared to the electron withdrawing ethylbenzoate group. Full article

Bimetallic structures of the general type [M₂(µ-S)₂] are omnipresent in nature, for biological function [M₂(µ-S)₂] sites interconvert between electronically distinct, but isostructural, forms. Different from structure-function relationships, the current understanding of the mechanism of formation and persistence of [M₂(µ-S)₂] sites is poorly developed. This work reports on bimetallic model compounds of nickel that interconvert between functional structures [Ni₂(µ-S)₂]^+/2+ and isomeric congeners [2{κ-S–Ni}]^2+/+, S = Aryl-S⁻, in which the nickel ions are geometrically independent. Interconversion of the two sets of structures was studied quantitatively by UV–VIS absorption spectroscopy and cyclic voltammetry. Assembly of the [Ni₂(µ-S)₂]⁺ core from [2{κ-S–Ni}]⁺ is thermodynamically and kinetically highly preferred over the disassembly of [Ni₂(µ-S)₂]²⁺ into [2{κ-S–Ni}]²⁺. Labile Ni-η^2/3-bonding to aromatic π-systems of the primary thiophenol ligand is critical for modeling (dis)assembly processes. A phosphine coligand mimics the role of anionic donors present in natural sites that saturate metal coordination. Three parameters have been identified as critical for structure formation and persistence. These are, first, the stereoelectronic properties of the metals ions, second, the steric demand of the coligand, and, third, the properties of the dative bond between nickel and coligand. The energies of transition states connecting functional and precursor forms have been found to depend on these parameters. Full article