Inorganics, Volume 7, Issue 3 (March 2019) – 17 articles

Two new main group element carboranylamidinates were synthesized using a bottom-up approach starting from o-carborane, ortho-C₂B₁₀H₁₂ (1, = 1,2-dicarba-closo-dodecaborane). The first divalent germanium carboranylamidinate, GeCl[HL^Cy] (3, [HL^Cy]⁻ = [o-C₂B₁₀H₁₀C(NCy)(NHCy)]⁻, Cy = cyclohexyl), was synthesized by treatment of GeCl₂(dioxane) with 1 equiv. of in situ-prepared Li[HL^Cy] (2a) in THF and isolated in 47% yield. In a similar manner, the first antimony(III) carboranylamidinate, SbCl₂[HL^iPr] (4, [HL^iPr]⁻ = [o-C₂B₁₀H₁₀C(NⁱPr)(NHⁱPr)]⁻), was obtained from a reaction of SbCl₃ with 1 equiv. of Li[HL^iPr] in THF (56% yield). The title compounds were fully characterized by analytical and spectroscopic methods as well as single-crystal X-ray diffraction. Both compounds 3 and 4 are monomeric species in the solid state, and the molecular geometries are governed by a stereo-active lone pair at the metal centers. Full article

In addition to the underlying basic concepts and early recognition of halogen bonding, this paper reviews the conflicting views that consistently appear in the area of noncovalent interactions and the ability of covalently bonded halogen atoms in molecules to participate in noncovalent interactions that contribute to packing in the solid-state. It may be relatively straightforward to identify Type-II halogen bonding between atoms using the conceptual framework of σ-hole theory, especially when the interaction is linear and is formed between the axial positive region (σ-hole) on the halogen in one monomer and a negative site on a second interacting monomer. A σ-hole is an electron density deficient region on the halogen atom X opposite to the R–X covalent bond, where R is the remainder part of the molecule. However, it is not trivial to do so when secondary interactions are involved as the directionality of the interaction is significantly affected. We show, by providing some specific examples, that halogen bonds do not always follow the strict Type-II topology, and the occurrence of Type-I and -III halogen-centered contacts in crystals is very difficult to predict. In many instances, Type-I halogen-centered contacts appear simultaneously with Type-II halogen bonds. We employed the Independent Gradient Model, a recently proposed electron density approach for probing strong and weak interactions in molecular domains, to show that this is a very useful tool in unraveling the chemistry of halogen-assisted noncovalent interactions, especially in the weak bonding regime. Wherever possible, we have attempted to connect some of these results with those reported previously. Though useful for studying interactions of reasonable strength, IUPAC’s proposed “less than the sum of the van der Waals radii” criterion should not always be assumed as a necessary and sufficient feature to reveal weakly bound interactions, since in many crystals the attractive interaction happens to occur between the midpoint of a bond, or the junction region, and a positive or negative site. Full article

The influence of the environment on the functionality of the oxygen-evolving complex (OEC) of photosystem II has long been a subject of great interest. In particular, various water channels, which could serve as pathways for substrate water diffusion, or proton translocation, are thought to be critical to catalytic performance of the OEC. Here, we address the dynamical nature of hydrogen bonding along the water channels by performing molecular dynamics (MD) simulations of the OEC and its surrounding protein environment in the S₁ and S₂ states. Through the eigenvector centrality (EC) analysis, we are able to determine the characteristics of the water network and assign potential functions to the major channels, namely that the narrow and broad channels are likely candidates for proton/water transport, while the large channel may serve as a path for larger ions such as chloride and manganese thought to be essential during PSII assembly. Full article

The linear and second-order nonlinear optical (NLO) properties of two pyrene-pyridine chromophores, namely, 4-(pyren-1-yl)pyridine (L₁) and 4-(2-(pyren-1-yl)ethyl)pyridine (L₂), were investigated and modulated by performing protonation/deprotonation cycles or by complexation to d¹⁰ metal centers such as Zn(II) and Cu(I) to form the monomeric [Zn(CH₃CO₂)₂(L₁)₂] complex and the [CuI(L₂)]_n coordination polymer, respectively. The structures of L₁, L₂, [Zn(CH₃CO₂)₂(L₁)₂] and [CuI(L₂)]_n were determined by means of single-crystal X-ray diffraction studies. The NLO response, measured by the electric-field-induced second harmonic generation (EFISH) technique, was positive for both chromophores and showed an inversion of the sign after exposure to HCl vapors. This process was completely reversible and the original values were restored by simple exposure to NH₃ vapors. Coordination of L₁ to Zn(II) also resulted in a negative NLO response, although smaller in magnitude compared to the protonated form, due to the weak Lewis acidity of the “Zn(CH₃CO₂)₂” fragment. The results were also interpreted on the basis of DFT/TDDFT calculations. Full article

The catalytic potential of linked redox centres is exemplified by the catalytic site of [FeFe]-hydrogenases, which feature a diiron subsite linked by a cysteinyl S atom to a 4Fe4S cube. The investigation of systems possessing similarly-linked redox sites is important because it provides a context for understanding the biological system and the rational design of abiological catalysts. The structural, electrochemical and spectroscopic properties of Fe₂(CO)₅(CH₃C(CH₂S)₂CH₂SPhNO₂, I-bzNO₂ and the aniline analogue, I-bzNH₂, are described and IR spectroelectrochemical studies have allowed investigation of the reduction products and their reactions with CO and protons. These measurements have allowed identification of the nitrobenzenyl radical anion, quantification of the shifts of the (CO) bands on ligand-based reduction compared with NO₂/NH₂ exchange and protonation of the pendent ligand. The strength of thioether coordination is related to the electronic effects, where competitive binding studies with CO show that CO/thioether exchange can be initiated by redox processes of the pendent ligand. Stoichiometric multi electron/proton transfer reactions of I-bzNO₂ localised on nitrobenzene reductions occur at mild potentials and a metal-centred reduction in the presence of protons does not lead to significant electrocatalytic proton reduction. Full article

The structural design of nanosized electrocatalysts is extremely important for cathodic oxygen reduction reactions (ORR) and anodic oxidation reactions in small organic compounds in direct fuel cells. While Pt is still the most commonly used electrode material for ORR, the Pd electrocatalyst is a promising alternative to Pt, because it exhibits much higher electrocatalytic activity towards formic acid electrooxidation, and the electrocatalytic activity of ORR on the Pd electrode is the higher than that of all other precious metals, except for Pt. In addition, the mass activity of Pt in a core–shell structure for ORR can be improved significantly by using Pd and Pd-based materials as core materials. Herein, we review various nanoscale Pd-based bimetallic, trimetallic and core–shell electrocatalysts for formic acid oxidation and ORR of polymer electrolyte fuel cells (PEFCs). This review paper is separated into two major topics: the electrocatalytic activity towards formic acid oxidation over various Pd-based electrocatalysts, and the activity of ORR on Pd-based materials and Pd core–Pt shell structures. Full article

Geometries, equilibrium dissociation energies (D_e), intermolecular stretching, and quadratic force constants (k_σ) determined by ab initio calculations conducted at the CCSD(T)/aug-cc-pVTZ level of theory, with D_e obtained by using the complete basis set (CBS) extrapolation [CCSD(T)/CBS energy], are presented for the B···BeR₂ and B···MgR₂ complexes, where B is one of the following Lewis bases: CO, H₂S, PH₃, HCN, H₂O or NH₃, and R is H, F or CH₃. The BeR₂ and MgR₂ precursor molecules were shown to be linear and non-dipolar. The non-covalent intermolecular bond in the B···BeR₂ complexes is shown to result from the interaction of the electrophilic band around the Be atom of BeR₂ (as indicated by the molecular electrostatic potential surface) with non-bonding electron pairs of the base, B, and may be described as a beryllium bond by analogy with complexes such as B···CO₂, which contain a tetrel bond. The conclusions for the B···MgR₂ series are similar and a magnesium bond can be correspondingly invoked. The geometries established for B···BeR₂ and B···MgR₂ can be rationalized by a simple rule previously enunciated for tetrel-bonded complexes of the type B···CO₂. It is also shown that the dissociation energy, D_e, is directly proportional to the force constant, k_σ, in each B···MR₂ series, but with a constant of proportionality different from that established for many hydrogen-bonded B···HX complexes and halogen-bonded B···XY complexes. The values of the electrophilicity, E_A, determined from the D_e for B···BeR₂ complexes for the individual Lewis acids, A, reveal the order A = BeF₂ > BeH₂ > Be(CH₃)₂—a result that is consistent with the −I and +I effects of F and CH₃ relative to H. The conclusions for the MgR₂ series are similar but, for a given R, they have smaller electrophilicities than those of the BeR₂ series. A definition of alkaline-earth non-covalent bonds is presented. Full article

Gd₂O₃ nanoparticles doped with different amount of Yb³⁺ ions and coated with citrate molecules were prepared by a cheap and fast co-precipitation procedure and proposed as potential “positive” contrast agents in magnetic resonance imaging. The citrate was used to improve the aqueous suspension, limiting particles precipitation. The relaxometric properties of the samples were studied in aqueous solution as a function of the magnetic field strength in order to evaluate the interaction of the paramagnetic ions exposed on the surface with the water molecules in proximity. The nanoparticles showed high relaxivity values at a high magnetic field with respect to the clinically used Gd³⁺-chelates and comparable to those of similar nanosytems. Special attention was also addressed to the investigation of the chemical stability of the nanoparticles in biological fluid (reconstructed human serum) and in the presence of a chelating agent. Full article

The search for high relaxivities and increased specificity continues to be central to the development of paramagnetic contrast agents for magnetic resonance imaging (MRI). Ferritin, due to its unique surface properties, architecture, and biocompatibility, has emerged as a natural nanocage that can potentially help to reach both these goals. This review aims to highlight recent advances in the use of ferritin as a nanoplatform for the delivery of metal-based MRI contrast agents (containing Gd³⁺, Mn²⁺, or Fe₂O₃) alone or in combination with active molecules used for therapeutic purposes. The collected results unequivocally show that the use of ferritin for contrast agent delivery leads to more accurate imaging of cancer cells and a significantly improved targeted therapy. Full article

The presence of single-electron transfer (SET) steps in water oxidation processes catalyzed by first-row transition metal complexes has been recently recognized, but the computational characterization of this type of process is not trivial. We report a systematic theoretical study based on density functional theory (DFT) calculations on the reactivity of a specific copper complex active in water oxidation that reacts through two consecutive single-electron transfers. Both inner-sphere (through transition state location) and outer-sphere (through Marcus theory) mechanisms are analyzed. The first electron transfer is found to operate through outer-sphere, and the second one through inner-sphere. The current work proposes a scheme for the systematic study of single-electron transfer in water oxidation catalysis and beyond. Full article

After nearly 20 years of research on the use of ruthenium in the fight against cancer, only two Ru(III) coordination complexes have advanced to clinical trials. During this time, the field has produced excellent candidate drugs with outstanding in vivo and in vitro activity; however, we have yet to find a ruthenium complex that would be a viable alternative to platinum drugs currently used in the clinic. We aimed to explore what we have learned from the most prominent complexes in the area, and to challenge new concepts in chemical design. Particularly relevant are studies involving NKP1339, NAMI-A, RM175, and RAPTA-C, which have paved the way for current research. We explored the development of the ruthenium anticancer field considering that the mechanism of action of complexes no longer focuses solely on DNA interactions, but explores a diverse range of cellular targets involving multiple chemical strategies. Full article

Dye-sensitized solar cells (DSSCs) have attracted a substantial interest in the last 30 years for the conversion of solar power to electricity. An important component is the redox mediator effecting the transport of charge between the photoelectrode and the dark counter electrode (CE). Among the possible mediators, metal coordination complexes play a prominent role and at present are incorporated in several types of devices with a power conversion efficiency exceeding 10%. The present review, after a brief introduction to the operation of DSSCs, discusses at first the requirements for a successful mediator. Subsequently, the properties of various classes of inorganic coordination complexes functioning as mediators relevant to DSSC operation are presented and the operational characteristics of DSSC devices analyzed. Particular emphasis is paid to the two main classes of efficient redox mediators, the coordination complexes of cobalt and copper; however other less efficient but promising classes of mediators, notably complexes of iron, nickel, manganese and vanadium, are also presented. Full article

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area. Full article

The synthesis, reactivity, and potential of well-defined dinuclear gold complexes as precursors for dual-gold catalysis is explored. Using the preorganizing abilities of well-known wide bite angle diphosphine ligands, DBFPhos and DPEPhos, dinuclear Au(I)–Au(I) complexes 1 and 2 are used as precursors to form well-defined monocationic species with either a chlorido- or acetylido-ligand bridging the two gold centers. These compounds are active catalysts for the dual-gold heterocycloaddition of a urea-functionalized alkyne, and the preorganization of both Au-centers affords efficient σ,π-activation of the substrate, even at high dilution, significantly outperforming benchmark mononuclear catalysts. Full article

In an attempt to expand the library of M₂B₅ bicapped trigonal-bipyramidal clusters with different transition metals, we explored the chemistry of [Cp*WCl₄] with metal carbonyls that enabled us to isolate a series of mixed-metal tungstaboranes with an M₂{B₄M’} {M = W; M’ = Cr(CO)₄, Mo(CO)₄, W(CO)₄} core. The reaction of in situ generated intermediate, obtained from the low temperature reaction of [Cp*WCl₄] with an excess of [LiBH₄·thf], followed by thermolysis with [M(CO)₅·thf] (M = Cr, Mo and W) led to the isolation of the tungstaboranes [(Cp*W)₂B₄H₈M(CO)₄], 1–3 (1: M = Cr; 2: M = Mo; 3: M = W). In an attempt to replace one of the BH—vertices in M₂B₅ with other group metal carbonyls, we performed the reaction with [Fe₂(CO)₉] that led to the isolation of [(Cp*W)₂B₄H₈Fe(CO)₃], 4, where Fe(CO)₃ replaces a {BH} core unit instead of the {BH} capped vertex. Further, the reaction of [Cp*MoCl₄] and [Cr(CO)₅·thf] yielded the mixed-metal molybdaborane cluster [(Cp*Mo)₂B₄H₈Cr(CO)₄], 5, thereby completing the series with the missing chromium analogue. With 56 cluster valence electrons (cve), all the compounds obey the cluster electron counting rules. Compounds 1–5 are analogues to the parent [(Cp*M)₂B₅H₉] (M= Mo and W) that seem to have generated by the replacement of one {BH} vertex from [(Cp*W)₂B₅H₉] or [(Cp*Mo)₂B₅H₉] (in case of 5). All of the compounds have been characterized by various spectroscopic analyses and single crystal X-ray diffraction studies. Full article

Complexes of group III metals (rare earth and actinides) with 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (BTP) have been investigated by computational (DFT) and, in limited cases, by experimental (FT-IR, X-ray) techniques with the goal of determining the characteristics of metal–ligand interactions. The DFT calculations using the M062X exchange-correlation functional revealed that metal–ligand distances correlate with the ionic radii of the metals, in agreement with available X-ray diffraction results on the Sc, Y, La, U, and Pu complexes. A related blue-shift trend could be observed in seven characteristic bands in the IR spectra associated with metal–ligand vibrations. The computations uncovered considerable charge transfer interactions, particularly in the actinide complexes, as important covalent contributions to the metal–ligand bonding. The covalent character of the metal–ligand bonds decreases in the actinides, from U to Cm. Full article

The versatility of isocyanides (CNR) in organic chemistry has been tremendously enhanced by continuous advancement in transition metal catalysis. On the other hand, the urgent need for new and more sustainable synthetic strategies based on abundant and environmental-friendly metals are shifting the focus towards iron-assisted or iron-catalyzed reactions. Diiron complexes, taking advantage of peculiar activation modes and reaction profiles associated with multisite coordination, have the potential to compensate the lower activity of Fe compared to other transition metals, in order to activate CNR ligands. A number of reactions reported in the literature shows that diiron organometallic complexes can effectively assist and promote most of the “classic” isocyanide transformations, including CNR conversion into carbyne and carbene ligands, CNR insertion, and coupling reactions with other active molecular fragments in a cascade sequence. The aim is to evidence the potential offered by diiron coordination of isocyanides for the development of new and more sustainable synthetic strategies for the construction of complex molecular architectures. Full article