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(E,E)-1,1'-[1,2-Bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene) revisited: threefold configurational disorder of (S,S), (R,R) and (S,R) isomers, a detailed critique

Crystal structures described as concomitant triclinic (I) and monoclinic (II) polymorphs of meso-(E,E)-1,1'-[1,2-bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene) [Mohamed et al. (2016). Acta Cryst. C72, 57-62] have been re-investigated. The published model for II was distorted due to forcing the symmetry of space group C2/c on an incomplete structure model. It is shown here to be a likely three-component superposition of S,S and R,R enantiomers with a lesser amount of the meso form. A detailed analysis of how the improbable distortion in the published model aroused suspicion and the subsequent construction of undistorted chemically and crystallographically plausible alternatives having the symmetry of Cc and C2/c is presented. For the sake of completeness, an improved model for the triclinic P-1 structure of the meso isomer I, revised to include a minor disorder component, is also given.

Optimizing disordered crystal structures

A mixed-crystal full-mol­ecule disorder situation is revisited in Parkin et al. [Acta Cryst. (2023), C79, 77–82]. A new inter­pretation of the data leads to the conclusion that the crystal structure is more likely to be a three-component superposition of enantiomers and the meso isomer of an organic compound and the article is thus a good learning example for dealing with a highly disordered structure.

A mixed phosphine sulfide/selenide structure as an instructional example for how to evaluate the quality of a model

This paper compares variations on a structure model derived from an X-ray diffraction data set from a solid solution of chalcogenide derivatives of cis-1,2-bis-(di-phenyl-phosphan-yl)ethyl-ene, namely, 1,2-(ethene-1,2-di-yl)bis-(di-phenyl-phoshpine sulfide/selenide), C₂₆H₂₂P₂S_1.13Se_0.87. A sequence of processes are presented to ascertain the composition of the crystal, along with strategies for which aspects of the model to inspect to ensure a chemically and crystallographically realistic structure. Criteria include mis-matches between F _obs ² and F _calc ², plots of |F _obs| vs |F _calc|, residual electron density, checkCIF alerts, pitfalls of the OMIT command used to suppress ill-fitting data, comparative size of displacement ellipsoids, and critical inspection of inter-atomic distances. Since the structure is quite small, solves easily, and presents a number of readily expressible refinement concepts, we feel that it would make a straightforward and concise instructional piece for students learning how to determine if their model provides the best fit for the data and show students how to critically assess their structures.

Cyclic peptide structure prediction and design using AlphaFold

Deep learning networks offer considerable opportunities for accurate structure prediction and design of biomolecules. While cyclic peptides have gained significant traction as a therapeutic modality, developing deep learning methods for designing such peptides has been slow, mostly due to the small number of available structures for molecules in this size range. Here, we report approaches to modify the AlphaFold network for accurate structure prediction and design of cyclic peptides. Our results show this approach can accurately predict the structures of native cyclic peptides from a single sequence, with 36 out of 49 cases predicted with high confidence (pLDDT > 0.85) matching the native structure with root mean squared deviation (RMSD) less than 1.5 Å. Further extending our approach, we describe computational methods for designing sequences of peptide backbones generated by other backbone sampling methods and for de novo design of new macrocyclic peptides. We extensively sampled the structural diversity of cyclic peptides between 7-13 amino acids, and identified around 10,000 unique design candidates predicted to fold into the designed structures with high confidence. X-ray crystal structures for seven sequences with diverse sizes and structures designed by our approach match very closely with the design models (root mean squared deviation < 1.0 Å), highlighting the atomic level accuracy in our approach. The computational methods and scaffolds developed here provide the basis for custom-designing peptides for targeted therapeutic applications.

Crystal structure of poly[diaqua-bis(μ 2-1,4-diaminobutane-N:N′)cobalt(II)] dichloride, C8H28Cl2CoN4O2

C8H28Cl2CoN4O2, orthorhombic, Fddd (no. 70), a = 10.116(2) Å, b = 13.532(3) Å, c = 23.507(5) Å, V = 3217.8(11) Å3, Z = 8, R gt (F) = 0.0361, wR ref (F 2) = 0.1069, T = 100.5 K.

Anion and ether group influence in protic guanidinium ionic liquids

Ionic liquids are attractive liquid materials for many advanced applications. For targeted design, in-depth knowledge about their structure-property-relations is urgently needed. We prepared a set of novel protic ionic liquids (PILs) with a guanidinium cation with either an ether or alkyl side chain and different anions. While being a promising cation class, the available data is insufficient to guide design. We measured thermal and transport properties, nuclear magnetic resonance (NMR) spectra as well as liquid and crystalline structures supported by ab initio computations and were able to obtain a detailed insight into the influence of the anion and the ether substitution on the physical and spectroscopic properties. For the PILs, hydrogen bonding is the main interaction between cation and anion and the H-bond strength is inversely related to the proton affinity of the constituting acid and correlated to the increase of ¹H and ¹⁵N chemical shifts. Using anions from acids with lower proton affinity leads to proton localization on the cation as evident from NMR spectra and self-diffusion coefficients. In contrast, proton exchange was evident in ionic liquids with triflate and trifluoroacetate anions. Using imide-type anions and ether side groups decreases glass transitions as well as fragility, and accelerated dynamics significantly. In case of the ether guanidinium ionic liquids, the conformation of the side chain adopts a curled structure as the result of dispersion interactions, while the alkyl chains prefer a linear arrangement.

Cooperativity-Driven Reactivity of a Dinuclear Copper Dimethylglyoxime Complex

In this report, we present the dinuclear copper(II) dimethylglyoxime (H₂ dmg) complex [Cu₂ (H₂ dmg)(Hdmg)(dmg)]⁺ (1), which, in contrast to its mononuclear analogue [Cu(Hdmg)₂ ] (2), is subject to a cooperativity-driven hydrolysis. The combined Lewis acidity of both copper centers increases the electrophilicity of the carbon atom in the bridging μ₂ -O-N=C-group of H₂ dmg and thus, facilitates the nucleophilic attack of H₂ O. This hydrolysis yields butane-2,3-dione monoxime (3) and NH₂ OH that, depending on the solvent, is then either oxidized or reduced. In ethanol, NH₂ OH is reduced to NH₄ ⁺ , yielding acetaldehyde as the oxidation product. In contrast, in CH₃ CN, NH₂ OH is oxidized by Cu^II to form N₂ O and [Cu(CH₃ CN)₄ ]⁺ . Herein are presented the combined synthetic, theoretical, spectroscopic and spectrometric methods that indicate and establish the reaction pathway of this solvent-dependent reaction.

Fluorescence Sensing of Monosaccharides by Bis-boronic Acids Derived from Quinolinium Dicarboxamides: Structural and Spectroscopic Studies

Three new diboronic acid-substituted bisquinolinium salts were synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptors for six monosaccharides and two open-chain polyols in water at physiological pH. The dicationic pyridine-2,6-dicarboxamide-based receptors contain two N-quinolinium rings as the fluorescent units covalently linked to three different isomers of phenylboronic acid (ortho, 2; meta, 3; and para, 4) as chelating binding sites for polyols. Additions of glucose/fructose in the micromolar concentration range to receptors 2 and 3 induce significant fluorescence changes, but in the presence of arabinose, galactose, mannose, and xylose, only modest optical changes are observed. This optical change is attributed to a static photoinduced electron transfer mechanism. The meta-diboronic receptor 3 exhibited a high affinity/selectivity toward glucose (K = 3800 M^-1) over other monosaccharides including common interfering species such as fructose and mannitol. Based on multiple spectroscopic tools, electrospray ionization high-resolution mass spectrometry, crystal structures, and density functional theory calculations, the binding mode between 3 and glucose is proposed as a 1:1 complex with the glucofuranose form involving a cooperative chelating diboronate binding. These results demonstrate the usefulness of a new set of cationic fluorescent diboronic acid receptors with a strong ability for optical recognition of glucose in the sub-millimolar concentration range.

Compounds with Alternating Single and Double Bonds of Antimony and Silicon; Isoelectronic to Ethane-1,2-diimine

We present an approach for preparing chain-type unsaturated molecules with low oxidation state Si(I) and Sb(I) supported by amidinato ligands that exploit to generate heavy analogues of ethane 1,2-diimine. The reduction of antimony dihalide (R-SbCl₂) with KC₈ in the presence of silylene chloride afforded L─(Cl)Si═Sb─Tip (1) and L(Cl)Si═Sb─^TerPh (2), respectively. Compounds 1 and 2 further undergo reduction with KC₈ to produce Tip─Sb═LSi─LSi═Sb─Tip (3) and ^TerPh─Sb═LSi─LSi═Sb─^TerPh (4). The solid-state structures and DFT studies show that all compounds have σ-type lone pairs at each Sb atom. It forms a strong pseudo-π-bond with Si. The pseudo-π-bond is formed by the hyperconjugative donation of the π-type lone pair at Sb to the Si-N σ* MO. The quantum mechanical studies indicate that compounds 3 and 4 has delocalized pseudo-π-MOs arising from hyperconjugative interactions. Hence, 1 and 2 can be considered as isoelectronic to imine, while 3 and 4 are isoelectronic to ethane-1,2-diimine. The proton affinity studies indicate that the pseudo-π-bond resulting from the hyperconjugative interaction is more reactive than the σ-type lone pair.

Copper(II) complexes of cyclams with N-(2,2,2-trifluoroethyl)-aminoalkyl pendant arms as potential probes for 19F magnetic resonance imaging

A series of Cu(II) complexes with cyclam-based ligands containing two N-(2,2,2-trifluoroethyl)-aminoalkyl pendant arms in 1,8-positions (L1: 1,2-ethylene spacer, L2: 1,3-propylene spacer; L3: 1,4-butylene spacer) was studied in respect to potential use as contrast agents for ¹⁹F magnetic resonance imaging (MRI). A number of structures of the complexes as well as of several organic precursors were determined by single-crystal X-ray diffraction analysis. Geometric parameters (especially distances between fluorine atoms and the central metal ion) were determined for each complex and the identity of isomeric complex species present in solution was established. The NMR longitudinal relaxation times (T₁) of ¹⁹F nuclei in the ligands at clinically relevant fields and temperatures (1-2 s) were significantly shortened upon Cu(II) binding to 7-10 ms for [Cu(L1)]²⁺, 20-30 ms for [Cu(L2)]²⁺ and 20-50 ms for [Cu(L3)]²⁺. The trend of the relaxation time shortening is in accordance with the distance and number of chemical bonds between fluorine atoms and the Cu(II) ion. The signals show promising T₂*/T₁ ratios in the range 0.25-0.55, assuring their good applicability to ¹⁹F NMR/MRI. The results show that even the Cu(II) ion, with a small magnetic moment, causes significant relaxation enhancement with a long-range effect and can be considered as a highly suitable metal ion for efficient ¹⁹F MRI contrast agents.

DNA recognition site of anticancer tinidazole copper(II) complexes

This paper describes the recognition process of tetrahedral [Cu^II(tnz)₂X₂] (X = Cl, Br) complexes by a DNA chain, analyzing the specific interaction between the DNA bases and backbone with the metal and the tinidazole (tnz) ligand. We identified the coordination of the copper metal center with one or two phosphates as the first recognition site for the tinidazole copper(II) complexes, while the ligands present partial intercalation into the minor groove. Also, we discuss a novel trigonal copper(I) tnz bromide complex, obtained by reducing the previously reported [Cu(tnz)₂Br₂]. This complex sheds light on the mechanism of action of tnz metal complexes as one of the most stable DNA-complex adducts depicts a trigonal geometry around the copper ion.

Halogen Bonding Channels for Magnetic Exchange in Cu(II) Complexes with 2,5-Di(methylthio)-1,3,4-thiadiazole

Copper(II) complexes with 2,5-bis(methylthio)-1,3,4-thiadiazole (tda) formulated as [Cu(tda)_n X₂ ] (n=2, X=Cl^- , Br^- , C₂ N₃ ^- ; n= 1, X=C₂ N₃ ^- ) have been isolated and fully characterized. The crystal structures of all compounds have been determined using single-crystal X-ray diffraction (SCXRD). A study of the magnetic susceptibility in the range 1.77-300 K has shown that magnetic properties of the [Cu(tda)₂ Cl₂ ] and [Cu(tda)₂ Br₂ ] complexes match those of 1D chains of antiferromagnetically-coupled Cu²⁺ ions. The intrachain interaction J in [Cu(tda)₂ Cl₂ ] turns out to be ∼1.2 times weaker than in its bromide analogue. In its turn, [Cu(tda)₂ (C₂ N₃ )₂ ] exhibits J being an order of magnitude smaller and of the opposite ferromagnetic sign. Halogen bonding (HB) between adjacent complexes is much stronger than the H-bonds or π-π interactions between tda ligands according to the DFT calculations.

Crystal structure of sodium catena-poly[bis(thiourea-κ1 S)-tetrakis(μ 2-thiourea-κ2 S,S)tricopper(I)] difumarate, C14H29Cu3N12NaO8S6

C14H29Cu3N12NaO8S6, monoclinic, P2/n (no. 13), a = 15.357(2) Å, b = 5.441(2) Å, c = 19.885(2) Å, β = 106.386(3)°, V = 1594.1(6) Å3, Z = 2, R gt (F) = 0.0374, wR ref (F 2) = 0.1069, T = 100 K.

Crystal structure of bis(benzylamine-κ1 N)-bis((E)-2-methyl-3-phenylacrylato-κ1 O)copper(II), C34H36CuN2O4

C34H36CuN2O4, monoclinic, P21/n (no. 14), a = 15.4951(2) Å, b = 5.62730(10) Å, c = 17.3541(2) Å, β = 90.831(1)°, V = 1513.04(4) Å3, Z = 2, Rgt (F) = 0.0415, wRref (F 2) = 0.0986, T = 296 K.

Electrocatalytic CO2 Reduction with a Binuclear Bis-Terpyridine Pyrazole-Bridged Cobalt Complex

A pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO₂ ) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro-electrochemical studies under CO₂ and CO atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of CO₂ . In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single-site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at -1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO-containing species formed during the electrocatalytic process, which inhibit the reduction of CO₂ .

Bis(Vinylenedithio)-Tetrathiafulvalene-Based Coordination Networks

Novel coordination polymers embedding electroactive moieties present a high interest in the development of porous conducting materials. While tetrathiafulvalene (TTF) based metal-organic frameworks were reported to yield through-space conducting frameworks, the use of S-enriched scaffolds remains elusive in this field. Herein is reported the employment of bis(vinylenedithio)-tetrathiafulvalene (BVDT-TTF) functionalized with pyridine coordinating moieties in coordination polymers. Its combination with various transition metals yielded four isostructural networks, whose conductivity increased upon chemical oxidation with iodine. The oxidation was confirmed in a single-crystal to single-crystal X-ray diffraction experiment for the Cd(II) coordination polymer. Raman spectroscopy measurements and DFT calculations confirmed the oxidation state of the bulk materials, and band structure calculations assessed the ground state as an electronically localized antiferromagnetic state, while the conduction occurs in a 2D manner. These results are shedding light to comprehend how to improve through-space conductivity thanks to sulfur enriched ligands.

Supramolecular Host–Guest Assemblies of [M6Cl14]2–, M = Mo, W, Clusters with γ-Cyclodextrin for the Development of CLUSPOMs

Host–guest assemblies open up opportunities for developing novel functional CLUSPOM multicomponent systems based on transition metal clusters (CLUS), polyoxometalates (POMs) and macrocyclic organic ligands. In water–ethanol solution γ-cyclodextrin (γ-CD) interacts with halide metal clusters [M6Cl14]2– (M = Mo, W) to form sandwich-type structures. The supramolecular association between the clusters and CDs, however, remains weak in solution, and the interactions are not strong enough to prevent the hydrolysis of the inorganic guest. Although analysis of the resulting crystal structures reveals inclusion complexation, 1H NMR experiments in solution show no specific affinity between the two components. The luminescent properties of the host–guest compounds in comparison with the initial cluster complexes are also studied to evaluate the influence of CD.

Structure and Microbiological Activity of 1H-benzo[d]imidazole Derivatives

Three new crystal structures of 1H-benzo[d]imidazole derivatives were determined. In the structures of these compounds, an identical system of hydrogen bonds, C(4), was observed. Solid-state NMR was applied for testing the quality of the obtained samples. All of these compounds were tested for in vitro antibacterial activity against Gram-positive bacteria and Gram-negative bacteria, as well as antifungal activity, by checking their selectivity. ADME calculations indicate that the compounds can be tested as potential drugs.

Analysis of Cimetidine Crystal Polymorphs by X-ray Absorption Near-Edge Spectroscopy

Sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) measurements were performed to characterize the crystal polymorphs of the active pharmaceutical ingredients (APIs) containing sulfur atoms. Cimetidine (CIM) was used as a model API. Each crystal form of CIM has its own XANES spectrum, so we can discriminate the crystal form by its spectrum. The analysis of the crystal structure of CIM revealed that the difference in the shape of XANES spectra was ascribable to the difference in the C-S-C bond angle of CIM molecules and the intermolecular hydrogen bonds, such as C-H···S and N-H···S, and S-S interaction. It was found that the peak shape of the XANES spectrum is gentle when the C-S-C bond angle is large, while the peak shape can be steep when the C-S-C bond angle is small. Furthermore, it was found that the peak energy values varied depending on the hydrogen bonds and S-S interaction. By linear combination fitting using XANES spectra, it was possible to quantify the ratio of CIM form A crystal in mixed powders of form A and monohydrate crystals. These results indicate that XANES measurements can be a useful technique to evaluate the crystal polymorphism of APIs containing S atom in pharmaceutical formulation.

Approach to Di/Triorganotin(IV) Cations via Hydrolysis of Stannate Salts Bearing Alkanesulfonate Ligands

An in-depth study of the class of organotin cations bearing weakly coordinating trifluoromethanesulfonate/arylsulfonate has led to key insights into their stability, structural aspects, and role as catalysts. Related chemistry with alkanesulfonate ligands remains a missing link due to the strong Sn-O bond. The study reported herein describes the scope of diorganostannates, [n-Bu₄N][R₂Sn(OSO₂R¹)₃] (R = n-Bu, R¹ = Me(1), Et(2); R = Ph, R¹ = Me(3)), as reactive substrates in the presence of adventitious water to afford [n-Bu₂SnOH(OSO₂Me)] (4), [n-Bu₂Sn(H₂O)₄][n-Bu₄N][OSO₂Et]₃·H₂O (5), and [Ph₂Sn(H₂O)₄][n-Bu₄N]₂[OSO₂Me]₄ (6), respectively, the latter two being the first examples of salt cocrystals comprising tetra(aqua)diorganotin cations. Hydrolysis of 3 in the presence of 1,4-bis((1H-imidazol-1-yl)methyl)benzene (bix) as the N-donor ligand proceeds via disproportionation and yields [Ph₃Sn(bix)](OSO₂Me) (7) along with an insoluble solid, likely derived from the hydrolysis of PhSn(OSO₂Me)₃. Direct evidence of this phenomenon can be gleaned from ESI-MS of 3, which identifies mass clusters corresponding to [Ph₃Sn(OSO₂Me)₂]^- and [PhSn(OSO₂Me)₃-H⁺]^-. X-ray crystallographic studies of 1-7 are reported to establish their structural identity and the role of alkanesulfonate anions in the formation of supramolecular assemblies.

Valence Localization in Alkyne and Alkene Adducts of Synthetic [Fe4S4]+ Clusters

Reported herein are alkyne and alkene adducts of synthetic [Fe₄S₄]⁺ clusters that model intermediates and inhibitor-bound states in enzymes involved in isoprenoid biosynthesis. Treatment of the N-heterocyclic carbene-ligated cluster [(IMes)₃Fe₄S₄(OEt₂)][BAr^F₄] (IMes = 1,3-dimesitylimidazol-2-ylidene; [BAr^F₄]^- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) with phenylacetylene (PhCCH) or cis-cyclooctene (COE) results in displacement of the Et₂O ligand to yield the corresponding π complexes, [(IMes)₃Fe₄S₄(PhCCH)][BAr^F₄] and [(IMes)₃Fe₄S₄(COE)][BAr^F₄]. EPR spectroscopic analysis demonstrates that both clusters are doublets with g_iso > 2 and thus are spectroscopically faithful models of the analogous species characterized in the isoprenoid biosynthetic enzymes IspG and IspH. Structural and Mössbauer spectroscopic analysis reveals that both complexes are best described as [Fe₄S₄]⁺ clusters in which the unique Fe site engages in modest back-bonding to the π-acidic ligand. Paramagnetic NMR studies show that, even at room temperature, the alkyne/alkene-bound Fe centers harbor minority spin and therefore adopt an Fe²⁺ valence. We propose that such valence localization could likewise occur in Fe-S enzymes that interact with π-acidic molecules.

Synthesis of Water-Soluble Planar Cobalt(II), Nickel(II), and Copper(II) Hydroxo Clusters Using a (1,4,7-Triazacyclononane)cobalt(III) Complex as a Hydrolysis-Terminating Group

We report on a group of planar cobalt(II), nickel(II), and copper(II) hydroxo clusters that have a definite composition and are water-soluble: [{Co(tacn)(OH)₂}₆Co₇(OH)₁₂](NO₃)₂(CF₃SO₃)₆·10H₂O (1), [{Co(tacn)(OH)₂}₆Ni₇(OH)₁₂](NO₃)₂(CF₃SO₃)₆·10H₂O (2a), [{Co(tacn)(OH)₂}₆Ni₇(OH)₁₂](BNPP)₈·6CH₃NO₂·8H₂O [2b; BNPP = bis(p-nitrophenyl)phosphate], [{Co(tacn)(OH)₂}₁₂Ni₁₆(OH)₂₆(OH₂)₂](SO₄)₄(CF₃SO₃)₁₀·30H₂O (3a), [{Co(tacn)(OH)₂}₁₂Ni₁₆(OH)₂₆(OH₂)₂](SO₄)₈(CF₃SO₃)₂·44H₂O (3b), [{Co(tacn)(OH)₂}₂Co₂(OH)₂(OH₂)₄](SO₄)(CF₃SO₃)₂·4H₂O (4), [{Co(tacn)(OH)₂}₂Ni₂(OH)₂(OH₂)₄](SO₄)(CF₃SO₃)₂·4H₂O (5), and [{Co(tacn)(OH)₂}₄Cu₄(OH)₆](ClO₄)₆·5H₂O (6), where tacn is 1,4,7-triazacyclononane. The peripheral of each metal hydroxo cluster plane is chemically protected by the coordination of {Co^III(tacn)(OH)₂}⁺ groups to prevent further hydrolysis. These clusters were synthesized by the reaction of an equimolar amount of [Co(tacn)(OH₂)₃]³⁺ and cobalt, nickel, or copper salt at pH values in the range of 6.0-12.0. The structure of the cation in compounds 1, 2a or 2b, 4, and 5 is relevant to the surface structure of the cobalt phosphate and nickel borate oxygen-evolution catalysts; in particular, the Co₇(OH)₁₂ core in 1. Moreover, the arrangement of M₇(OH)₁₂ in 1 and 2a or 2b and Cu₄(OH)₆ in 6 represents the solid-state structures of the (111) face of the cubic CoO or NiO and the (002) plane of Cu(OH)₂, respectively. Extended X-ray absorption fine structure spectra of an aqueous solution of 1, 2a, 4, and 5 exhibit well-resolved peaks at the first and second coordination spheres due to the M-O and M···M distances, respectively; the solution-state bond distances were estimated, and they agreed well with the bond distances in the solid-state structures.

Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation

6,13-Bis(tri(isopropyl)silylethynyl)pentacene, a particularly stable acene derivative important for (opto)electronic materials, turns reactive upon electrochemical one-electron oxidation. One of the typically stabilizing tri(isopropyl)silylethynyl substituents becomes involved in a (4 + 2) cycloaddition after redox umpolung. The electrosynthetic dimerization of the title compound provides easy access under mild conditions to a complex scaffold, which includes an intact pentacene, an anthracene, and a phenylene unit, all electronically separated. The product's electrochemical redox properties are explained by superimposed cyclic voltammetric features of the pentacene and the anthracene moieties. The reaction path is analyzed on the basis of electroanalytical and ESR data, and an oxidation-cycloaddition-reduction sequence is elaborated. The contribution of homogeneous electron transfers (electron transfer chain reaction) is negligible, in accordance with the relative formal redox potentials of the starting compound and the product. Quantum chemical calculations indicate that the central cycloaddition should be described as a two-step process with a distonic radical cation intermediate. We suggest an extended notation to define the contribution of the components with respect to electron count in the two-step cycloaddition, [3 + 1, 1 + 1].

An atomically precise Ag18Cu8 nanocluster with rich alkynyl-metal coordination structures and unique SbF6- assembling modes

Elucidating the coordination structures and assembling modes of atomically precise metal nanoclusters (NCs) remains a hot topic as it gives answers to the underlying mechanism of nanomaterials and bulk materials in terms of structure-property relationships. Here we report a novel silver-copper alloy NC featuring rich alkynyl-metal coordination modes and unique SbF₆^- assembling structures. The NC, with the composition of [Ag₁₈Cu₈(dppp)₄(^tBu-C₆H₄CC)₂₂](SbF₆)₄ (dppp = 1,3-bis(diphenylphosphino)-propane), was prepared by a stepwise synthetic approach. Single-crystal X-ray diffraction analysis revealed that such a NC featured a staircase-like Ag₁₈Cu₈ kernel, which was protected by hybrid alkynyl and dppp ligands in diverse coordination structures and multiple environments. The structural analysis also revealed the unique function of SbF₆^- in inducing the assembly of cluster moieties, highlighting the importance of counterions in assembling nanomolecules. The diverse coordination structures of the protective ligands with metal ions and the indispensable roles of counterions in assembling the cluster moieties have also been supported by nuclear magnetic resonance (NMR) and electrospray ionization mass spectrometry (ESI-MS) studies, making it a model system to showcase the uniqueness of atomically precise metal NCs in illustrating the coordination chemistry of nanomaterials and bulk materials at the molecular level.

Interchangeability and Disorder in the Solid-State Structures of "Two Wall" Calix[4]pyrroles Equipped with Iodine and Ethynyl para-Substituents

Herein, the synthesis and X-ray structures of three α,β "two wall" aryl-extended calix[4]pyrroles having either identical (symmetrically substituted) or different (non-symmetrically substituted) meso-aryl substituents (aryl=4-ethynylphenyl and 4-iodophenyl) are reported. The X-ray structure of the co-crystal formed by the two symmetrically substituted calix[4]pyrroles is also described. In the solid state, all studied α,β-calix[4]pyrroles exhibit a 1,3-alternate conformation with two co-crystallized acetonitrile solvent molecules H-bonded to adjacent cis-pyrrole rings. Remarkably, the 1,3-conformer of the non-symmetrically substituted iodophenyl/ethynylphenyl compound is intrinsically chiral. The two enantiomers are present in the average asymmetric unit in a 65 : 35 occupancy ratio displaying a head-to-tail directional disorder. This is due to the functional complementarity and the isosteric and isoelectronic properties of the para-substituents: iodo and ethynyl. That is, the negative belt of iodine is similar to the negative π-system of the C≡C triple bond and the σ-hole in the iodine atom is similar to the positive proton at the C≡C-H group.

Structural insight into halide-coordinated [Fe4S4XnY4-n]2- clusters (X, Y = Cl, Br, I) by XRD and Mössbauer spectroscopy

Iron sulphur halide clusters [Fe₄S₄Br₄]^2- and [Fe₄S₄X₂Y₂]^2- (X, Y = Cl, Br, I) were obtained in excellent yields (77 to 78%) and purity from [Fe(CO)₅], elemental sulphur, I₂ and benzyltrimethylammonium (BTMA⁺) iodide, bromide and chloride. Single crystals of (BTMA)₂[Fe₄S₄Br₄] (1), (BTMA)₂[Fe₄S₄Br₂Cl₂] (2), (BTMA)₂[Fe₄S₄Cl₂I₂] (3), and (BTMA)₂[Fe₄S₄Br₂I₂] (4) were isostructural to the previously reported (BTMA)₂[Fe₄S₄I₄] (5) (monoclinic, Cc). Instead of the chloride cubane cluster [Fe₄S₄Cl₄]^2-, we found the prismane-shaped cluster (BTMA)₃[Fe₆S₆Cl₆] (6) (P1̄). ⁵⁷Fe Mössbauer spectroscopy indicates complete delocalisation with Fe^2.5+ oxidation states for all iron atoms. Magnetic measurements showed small χ_MT values at 298 K ranging from 1.12 to 1.54 cm³ K mol^-1, indicating the dominant antiferromagnetic exchange interactions. With decreasing temperature, the χ_MT values decreased to reach a plateau at around 100 K. From about 20 K, the values drop significantly. Fitting the data in the Heisenberg-Dirac-van Vleck (HDvV) as well as the Heisenberg Double Exchange (HDE) formalism confirmed the delocalisation and antiferromagnetic coupling assumed from Mössbauer spectroscopy.

An Inorganic Fluorescent Chemosensor: Rational Design and Selective Mg2+ Detection

A Zn²⁺ based complex, 3, displays greatly increased fluorescence emission in the presence of Mg²⁺. Fluorescent and computational studies suggest that 3 selectively interacts with Mg²⁺ due to optimal cavity size formation between two uncoordinated pyrazole side arms. This work thus represents a new approach to the development of fluorescent chemosensors.

Flux Growth of an Intermetallic with Interstitial Fluorides via Decomposition of a Fluorocarbon

La₁₅(FeC₆)₄F₂ was grown as large crystals by reacting iron in a La/Ni eutectic flux in the presence of decafluorobiphenyl (C₁₂F₁₀) which acts as both a carbon and fluoride source. This mild fluorinating technique enables the isolation of an intermetallic product containing fluoride interstitials, as opposed to forming ionic metal fluorides. The compound adopts a structure in the hexagonal crystal system with space group P6̅ which features FeC₆ units composed of a central iron atom coordinated by three ethylenide units in a trigonal planar configuration. The structure is related to the previously reported La₁₅(FeC₆)₄H, but with fluoride fully occupying the interstitial hydride positions, which induces partial occupancies and site splitting disorder in the adjacent layers of lanthanide ions. No supercell formation is observed.

Electrostatic CH-π Interactions Can Override Fluorine Gauche Effects To Exert Conformational Control

Fluorine gauche effects are conformational properties of 2-fluoroethanes often applied in modern molecular designs. However, the physical origins of fluorine gauche effects are not well understood, with the consensus favoring the established hyperconjugation theory over an emerging electrostatic model. Using a series of model systems, we show that a shift to fluorine gauche effects can be influenced by intramolecular CH⋅⋅⋅π aromatic interactions, a through-space event. Modulating the π-ring (forming the aromatic interaction) with substituent groups resulted in a linear Hammett relationship, thus indicating that the CH⋅⋅⋅π interaction has electrostatic features. For instance, attaching a nitro group (an electron-withdrawing substituent) to the π-ring weakened the CH⋅⋅⋅π interaction and led to a gauche preference, whereas an anti conformer is preferred with amine as substituent. The experimental results performed by using proton NMR spectroscopy are corroborated by gas-phase DFT calculations and solid-state X-ray crystallography.

Bridging the crystal and solution structure of a series of lipid-inspired ionic liquids

A series of 1,2-dimethylimidazolium ionic liquids bearing a hexadecyl alkyl chain are thoroughly examined via X-ray crystallography. The crystal structures reveal several key variations in the non-covalent interactions in the lipid-like salts. Specifically, distinct cation-cation π interactions are observed when comparing the bromide and iodide structures. Changing the anion to bis(trifluoromethane)sulfonimide (Tf₂N^-) changes these cation-cation π interactions with anion⋯π interactions. Additionally, several well-defined geometries of the cations are noted based on torsion and core-plane angles of the alkyl chains. Hirshfeld surface analysis is used to distinguish the interactions and geometries in the solid state, helping to reveal characteristic structural fingerprints for the compounds. The solid-state structures of the ionic liquids are correlated with the solution-state structures through UV-vis spectroscopic studies, further emphasizing the importance of the π interactions in the formation of aggregates. Finally, we investigated the thermal properties of the ionic liquids, revealing complex phase transitions for the iodide-containing species. These phase transitions are further rationalized via the analysis of the data gathered from the structures of the other crystallized salts.

Half-Sandwich Rhodium Complexes with Releasable N-Donor Monodentate Ligands: Solution Chemical Properties and the Possibility for Acidosis Activation

Cancer chemotherapeutics usually have serious side effects. Targeting the special properties of cancer and activation of the anticancer drug in the tumor microenvironment in situ may decrease the intensity of the side effects and improve the efficacy of therapy. In this study, half-sandwich Rh complexes are introduced, which may be activated at the acidic, extracellular pH of the tumor tissue. The synthesis and aqueous stability of mixed-ligand complexes with a general formula of [Rh(η⁵-Cp*)(N,N/O)(N)]^2+/+ are reported, where (N,N/O) indicates bidentate 8-quinolate, ethylenediamine and 1,10-phenanthroline and (N) represents the releasable monodentate ligand with a nitrogen donor atom. UV-visible spectrophotometry, ¹H NMR, and pH-potentiometry were used to determine the protonation constants of the monodentate ligands, the proton dissociation constants of the coordinated water molecules in the aqua complexes, and the formation constants of the mixed-ligand complexes. The obtained data were compared to those of the analogous Ru(η⁶-p-cymene) complexes. The developed mixed-ligand complexes were tested in drug-sensitive and resistant colon cancer cell lines (Colo205 and Colo320, respectively) and in four bacterial strains (Gram-positive and Gram-negative, drug-sensitive, and resistant) at different pH values (5-8). The mixed-ligand complexes with 1-methylimidazole displayed sufficient stability at pH 7.4, and their activation was found in cancer cells with decreasing pH; moreover, the mixed-ligand complexes demonstrated antimicrobial activity in Gram-positive and Gram-negative bacteria, including the resistant MRSA strain. This study proved the viability of incorporating releasable monodentate ligands into mixed-ligand half-sandwich complexes, which is supported by the biological assays.

Highly Cytotoxic Molybdenocenes with Strong Metabolic Effects Inhibit Tumour Growth in Mice

A series of six highly lipophilic Cp-substituted molybdenocenes bearing different bioactive chelating ligands was synthesized and characterized by NMR spectroscopy, mass spectrometry and X-ray crystallography. In vitro experiments showed a greatly increased cytotoxic potency when compared to the non-Cp-substituted counterparts. In vivo experiments performed with the dichlorido precursor, (Ph₂ C-Cp)₂ MoCl₂ and the in vitro most active complex, containing the thioflavone ligand, showed an inhibition of tumour growth. Proteomic studies on the same two compounds demonstrated a significant regulation of tubulin-associated and mitochondrial inner membrane proteins for both compounds and a strong metabolic effect of the thioflavone containing complex.

Straightforward and Efficient Protocol for the Synthesis of Pyrazolo [4,3-b]pyridines and Indazoles

An efficient method for the synthesis of pyrazolo [4,3-b]pyridines has been developed on the basis of readily available 2-chloro-3-nitropyridines via a sequence of SNAr and modified Japp-Klingemann reactions. The method offers a number of advantages including utilization of stable arenediazonium tosylates, operational simplicity as well as combining the azo-coupling, deacylation and pyrazole ring annulation steps in a one-pot manner. An unusual rearrangement (C-N-migration of the acetyl group) was observed and a plausible mechanism was proposed based on the isolated intermediates and NMR experiments. In addition, the developed protocol was successfully applied to the synthesis of 1-arylindazoles combining the Japp-Klingemann reaction and cyclization of the resulting hydrazone as a one-pot procedure.

Titanium(IV) Surface Complexes Bearing Chelating Catecholato Ligands for Enhanced Band-Gap Reduction

Protonolysis reactions between dimethylamido titanium(IV) catecholate [Ti(CAT)(NMe₂)₂]₂ and neopentanol or tris(tert-butoxy)silanol gave catecholato-bridged dimers [(Ti(CAT)(OCH₂tBu)₂)(HNMe₂)]₂ and [Ti(CAT){OSi(OtBu)₃}₂(HNMe₂)₂]₂, respectively. Analogous reactions using the dimeric dimethylamido titanium(IV) (3,6-di-tert-butyl)catecholate [Ti(CATtBu₂-3,6)(NMe₂)₂]₂ yielded the monomeric Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ and Ti(CATtBu₂-3,6)[OSi(OtBu)₃]₂(HNMe₂)₂. The neopentoxide complex Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ engaged in further protonolysis reactions with Si-OH groups and was consequentially used for grafting onto mesoporous silica KIT-6. Upon immobilization, the surface complex [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[KIT-6] retained the bidentate chelating geometry of the catecholato ligand. This convergent grafting strategy was compared with a sequential and an aqueous approach, which gave either a mixture of bidentate chelating species with a bipodally anchored Ti(IV) center along with other physisorbed surface species or not clearly identifiable surface species. Extension of the convergent and aqueous approaches to anatase mesoporous titania (m-TiO₂) enabled optical and electronic investigations of the corresponding surface species, revealing that the band-gap reduction is more pronounced for the bidentate chelating species (convergent approach) than for that obtained via the aqueous approach. The applied methods include X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and solid-state UV/vis spectroscopy. The energy-level alignment for the surface species from the aqueous approach, calculated from experimental data, accounts for the well-known type II excitation mechanism, whereas the findings indicate a distinct excitation mechanism for the bidentate chelating surface species of the material [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[m-TiO₂].

Chalcogen bonding in the solid-state structures of 1,3-bis(benzimidazoliumyl)benzene-based chalcogen-bonding donors

1,3-Bis(benzimidazoliumyl)benzene-based chalcogen-bonding catalysts were previously successfully applied in different benchmark reactions. In one of those examples, i.e. the activation of quinolines, sulfur- and selenium-based chalcogen-bonding catalysts showed comparable properties, which is unexpected, as the selenium-containing catalysts should show superior catalytic properties due to the increased polarizability of selenium compared to sulfur. Herein, we present four crystal structures of the respective 1,3-bis(benzimidazoliumyl)benzene-based chalcogen-bonding catalyst containing sulfur (3^S) and selenium (3^Se, three forms) as Lewis acidic centres. The sulfur-containing catalyst shows weaker chalcogen bonding compared to its selenium analogue, as well as anion-π interactions. The selenium-based analogues, on the other hand, show stronger chalcogen-bonding motifs compared to the sulfur equivalent, depending on the crystallization conditions, but in every case, the intermolecular interactions are comparable in strength. Other interactions, such as hydrogen bonding and anion-π, were also observed, but in the latter case, the interaction distances are longer compared to those of the sulfur-based equivalent. The solid-state structures could not further explain the high catalytic activity of the sulfur-containing catalysts. Therefore, a comparison of their σ-hole depths from density functional theory (DFT) gas-phase calculations was performed, which are again in line with the previously found properties in the solid-state structures.

3-Chloro-N,N-di-methyl-propan-1-aminium chloride

The organic cation in the title mol-ecular salt, C₅H₁₃NCl⁺·Cl^-, exhibits the gauche effect with a C-H bond of the C atom β to the chloro group donating electrons to the anti-bonding orbital of the C-Cl bond to stabilize the gauche conformation [Cl-C-C-C = -68.6 (6)°], as confirmed by DFT geometry optimizations that show a lengthening of the C-Cl bond relative to that of the anti conformation. Of further inter-est is the higher point group symmetry of the crystal (), compared that of the that of the mol-ecular cation, which arises from a supra-molecular head-to-tail square arrangement of four mol-ecular cations that circulate in a counterclockwise direction when viewed down the tetra-gonal c axis.

Synthesis and structures of dinuclear palladium complexes with 1,3-benzimidazolidine-2-thione and 1,3-imidazoline-2-thione

The synthesis and structures of dinuclear palladium complexes with 1,3-benz-imidazolidine-2-thione (bzimtH) and 1,3-imidazoline-2-thione (imtH) are reported, namely, bis-(μ-1H-benzimidazole-2-thiol-ato)-κ² N ³:S;κ² S:N ³-bis-[cyanido(tri-phenyl-phosphine-κP)palladium(II)], [Pd₂(C₇H₅N₂S)₂(CN)₂(C₁₈H₁₅P)₂] or [Pd₂(μ-N,S-bzimtH)₂(CN)₂(PPh₃)₂] (1), and bis-(μ-1H-imidazole-2-thiol-ato)-κ² N ³:S;κ² S:N ³-bis-[cyanido(tri-phenyl-phosphine-κP)palladium(II)] aceto-nitrile 0.58-solvate, [Pd₂(C₃H₃N₂S)₂(CN)₂(C₁₈H₁₅P)₂]·0.58C₂H₃N or [Pd₂(μ-N,S-imtH)₂(CN)₂(PPh₃)₂]·0.58C₂H₃N (2). The compound [Pd₂(μ-N,S-bzimtH)₂(CN)₂(PPh₃)₂] is located on a crystallographic twofold axis while [Pd₂(μ-N,S-imtH)₂(CN)₂(PPh₃)₂]. 0.58(C₂H₃N) contains two partially occupied aceto-nitrile solvent mol-ecules with occupancies of 0.25 and 0.33. In both of these compounds, the anionic bzimtH^- and imtH^- ligands coordinate through N,S-donor atoms in a bridging mode, covering four coordination sites of two metal centers and other two sites are occupied by two PPh₃ ligand mol-ecules. Finally, the remaining two sites of two metal centers are occupied by cyano groups, abstracted by the metals from the solvent during reaction. In the packing of the 1,3-benzimidazolidine- 2-thione and 1,3-imidazoline-2-thione complexes, there are intra-molecular π-π inter-actions involving the thione moiety as well as an N-H⋯N hydrogen bond linking the thione and cyano ligands. In addition, in 2, as well as the π-π inter-action involving the thione moieties, there is an additional π-π inter-action involving one of the thione moieties and an adjacent phenyl ring from the tri-phenyl-phosphine ligand. There are also C-H⋯N inter-actions between the imidazoline rings and the aceto-nitrile N atoms.

Ultrasmall PtAu2 nanoclusters activate endogenous anti-inflammatory and anti-oxidative systems to prevent inflammatory osteolysis

Rationale: Inflammatory osteolysis, characterized by abundant immune cell infiltration and osteoclast (OC) formation, is a common complication induced by bacterial products and/or wear particles at the bone-prosthesis interface that severely reduces long-term stability after implantation. Molecular nanoclusters are ultrasmall particles with unique physicochemical and biological properties that have great potential as theranostic agents for treating inflammatory diseases. Methods: In this study, heterometallic PtAu₂ nanoclusters with sensitive nitric oxide-responsive phosphorescence turn-on characteristics and strong binding interactions with cysteine were designed, making them desirable candidates for the treatment of inflammatory osteolysis. Results: PtAu₂ clusters exhibited satisfactory biocompatibility and cellular uptake behavior, with potent anti-inflammatory and anti-OC activities in vitro. In addition, PtAu₂ clusters alleviated lipopolysaccharide-induced calvarial osteolysis in vivo and activated nuclear factor erythroid 2-related factor 2 (Nrf2) expression by disrupting its association with Kelch-like ECH-associated protein 1 (Keap1), thereby upregulating the expression of endogenous anti-inflammatory and anti-oxidative products. Conclusion: Through the rational design of novel heterometallic nanoclusters that activate the endogenous anti-inflammatory system, this study provides new insights into the development of multifunctional molecular therapeutic agents for inflammatory osteolysis and other inflammatory diseases.