Bimetallic Perthiocarbonate Complexes of Cobalt: Synthesis, Structure and Bonding.

The syntheses and structural elucidation of bimetallic thiolate complexes of early and late transition metals are described. Thermolysis of the bimetallic hydridoborate species [{Cp*CoPh}{µ-TePh}{µ-TeBH3-ĸ2Te,H}{Cp*Co}] (Cp* = ɳ5-C5Me5) (1) in the presence of CS2 afforded the bimetallic perthiocarbonate complex [(Cp*Co)2(µ-CS4-κ1S:κ2S')(µ-S2-κ2S″:κ1S‴)] (2) and the dithiolene complex [(Cp*Co)(µ-C3S5-κ1S,S'] (3). Complex 2 contains a four-membered metallaheterocycle (Co2S2) comprising a perthiocarbonate [CS4]2- unit and a disulfide [S2]2- unit, attached opposite to each other. Complex 2 was characterized by employing different multinuclear NMR, infrared spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. Preliminary studies show that [Cp*VCl2]3 (4) with an intermediate generated from CS2 and [LiBH4·THF] yielded thiolate species, albeit different from the cobalt system. Furthermore, a computational analysis was performed to provide insight into the bonding of this bimetallic perthiocarbonate complex.

Protic Processes in an Extended Pyrazinacene: The Case of Dihydrotetradecaazaheptacene.

Pyrazinacenes are linearly fused heteroaromatic rings, with N atoms replacing all apical CH moieties. Component rings may exist in a reduced state, having NH groups instead of N, causing cross-conjugation. These compounds have interesting optical and electronic properties, including strong fluorescence in the near-infrared region and photocatalytic properties, leading to diverse possible applications in bio-imaging and organic synthesis, as well as obvious molecular electronic uses. In this study, we investigated the behavior of seven-ring pyrazinacene 2,3,11,12-tetraphenyl-7,16-dihydro-1,4,5,6,7,8,9,12,13,14,15,16,17,18-tetradecaazaheptacene (Ph4H2N14HEPT), with an emphasis on protic processes, including oxidation, tautomerism, deprotonation, and protonation, and the species resulting from those processes. We used computational methods to optimize the structures of the different species and generate/compare molecular orbital structures. The aromaticity of the species generated by the different processes was assessed using the nucleus-independent chemical shifts, and trends in the values were associated with the different transformations of the pyrazinacene core. The computational data were compared with experimental data obtained from synthetic samples of the molecule tBu8Ph4H2N14HEPT.

Theoretical and DFT Study of Atypical Pentanuclear [(iPr3P)Ni]5Hn (n = 4, 6, 8) Clusters: What are the Rules?

The structure, bonding, and properties of a series of atypical pentanuclear nickel hydride clusters supported by electron-rich iPr3P of the type [(iPr3P)Ni]5Hn (n = 4, 6, 8; H4, H6, H8) and their anionic models where iPr3P are substituted by H- (H4', H6', H8') were investigated by density functional theory (DFT) calculations. All clusters were calculated to adopt a similar square pyramidal core geometry. Calculations indicate singlet ground states with small singlet-triplet gaps for H4 and H6, similar to previously reported experimental values. Molecular orbital theory description clusters were investigated using the simplified model complexes [HNi]5Hn5- (n = 4, 6, 8; H4', H6', H8'). The results show that there are three skeletal electron pairs (SEPs) in H4'. The addition of two molecules of H2 to form H6' and H8' results in the partial or full occupation of two degenerate MOs (e* set) that give two SEPs and one SEP, respectively. Indeed, the occupation of these low-lying weakly antibonding orbitals governs the multielectron chemistry available for these clusters and plays a role in their unique reactivity.

Synthesis, Crystal and Electronic Structure, and Thermal Conductivity Investigation of the Hollandite-like CsxCr5Te8 Phases (0.73 < x < 1).

This article presents a comprehensive study on the synthesis and structural and thermal conductivity properties of cesium-inserted chromium tellurides of formula CsxCr5Te8. Single crystals of three different compositions (x = 0.73, 0.91, and 0.97) were successfully synthesized and suggested the existence of a solid solution in the range 0.73 < x < 1. Through a detailed single-crystal characterization, the complete structure of these compounds is determined, revealing a distinct B-type hollandite-like structural form derived from the hollandite structure, in contrast to the more commonly observed A-type pseudo-hollandite in AM5X8-type chalcogenides (A = cation, M = transition metal, and X = chalcogen). Periodic density functional theory calculations predict the Cs0.73Cr5Te8 composition as the most stable, with a metallic conductive behavior. The thermal conductivity of bulk CsxCr5Te8 samples is measured to be 1.4 W m-1 K-1 at 300 K and increases with temperature up to 2 W m-1 K-1 at 673 K.

16-Vertex oblato-hypho-titanaborane [(Cp*Ti)2B14H18].

Although Lipscomb predicted in 1977 that supra-icosahedral boron clusters would be viable, their synthesis has been impeded by the unavailability of appropriate synthetic methodologies. Herein, we report the first examples of the open 16-vertex oblato-hypho-titanaborane clusters [(Cp*Ti)2B14H17R] (1: R = H; 2: R = Me) having a non-Wadean 19-skeletal-electron-pair count. Interestingly, these clusters show a six-membered [Ti2B4] open face, which could lead to closo-19-vertex clusters.

Monocarboxylate-protected two-electron superatomic silver nanoclusters with high photothermal conversion performance.

The first series of monocarboxylate-protected superatomic silver nanoclusters was synthesized and fully characterized by X-ray diffraction, fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and electrospray ionization mass spectrometry (ESI-MS). Specifically, compounds [Ag16(L)8(9-AnCO2)12]2+ (L = Ph3P (I), (4-ClPh)3P (II), (2-furyl)3P (III), and Ph3As (IV)) were prepared by a solvent-thermal method under alkaline conditions. These clusters exhibit a similar unprecedented structure containing a [Ag8@Ag8]6+ metal kernel, of which the 2-electron superatomic [Ag8]6+ inner core shows a flattened and puckered hexagonal bipyramid of S6 symmetry. Density functional theory calculations provide a rationalization of the structure and stability of these 2-electron superatoms. Results indicate that the 2 superatomic electrons occupy a superatomic molecular orbital 1S that has a substantial localization on the top and bottom vertices of the bipyramid. The π systems of the anthracenyl groups, as well as the 1S HOMO, are significantly involved in the optical and photothermal behavior of the clusters. The four characterized nanoclusters show high photothermal conversion performance in sunlight. These results show that the unprecedented use of mono-carboxylates in the stabilization of Ag nanoclusters is possible, opening the door for the introduction of various functional groups on their cluster surface.

From 8- to 18-Cluster Electrons Superatoms: Evaluation via DFT Calculations of the Ligand-Protected W@Au12(dppm)6 Cluster Displaying Distinctive Electronic and Optical Properties.

The iconic W@Au12 icosahedral bare cluster reaches the favorable closed-shell superatomic electron configuration 1S2 1P6 1D10, making it an 18-cluster electron (18-ce) superatom. Here, we pursue the evaluation of a ligand-protected counterpart based on the construction of a fully phosphine-protected [W@Au12(dppm)6] cluster strongly related to the characterized [Au13(dppm)6]5+ homometallic counterpart. The later cluster has the same total number of valence electrons as the former but is considered an 8-ce superatom with 1S2 1P6 configuration. The fundamental differences between 8- and 18-ce species are investigated. The character of the frontier orbitals varies from 1P/1D in the 8-ce case to a 1D/ligand for 18-ce species, enabling an efficient charge transfer toward the ligands upon irradiation, being interesting for electron injection in optoelectronic devices and black absorbers applications. Excited-state properties are also revisited, showing different geometrical and electronic structure variations between 8- and 18-ce species. Moreover, the continuum between the 8- and 18-ce limits has been explored by varying the nature of the encapsulated dopant between group 6 and group 11. The transition between the 8- and 18-ce counts can be formally situated between Pt (8-ce) and Ir (18-ce). Thus, 18-ce derivatives obtained as doped counterparts of homometallic gold clusters can introduce useful alternatives to achieve different properties in related structural motifs, which can be further explored owing to their extension of the well-established versatility of current gold nanoclusters.

Enhanced NH3 Sensing Performance of Mo Cluster-MoS2 Nanocomposite Thin Films via the Sulfurization of Mo6 Cluster Iodides Precursor.

The high-performance defect-rich MoS2 dominated by sulfur vacancies as well as Mo-rich environments have been extensively studied in many fields, such as nitrogen reduction reactions, hydrogen evolution reactions, as well as sensing devices for NH3, which are attributed to the under-coordinated Mo atoms playing a significant role as catalytic sites in the defect area. In this study, the Mo cluster-MoS2 composite was creatively synthesized through a one-step sulfurization process via H2/H2S gas flow. The Mo6 cluster iodides (MIs) coated on the fluorine-doped tin oxide (FTO) glass substrate via the electrophoretic deposition method (i.e., MI@FTO) were used as a precursor to form a thin-film nanocomposite. Investigations into the structure, reaction mechanism, and NH3 gas sensing performance were carried out in detail. The results indicated that during the gas flowing, the decomposed Mo6 cluster iodides played the role of template and precursor, forming complicated Mo cluster compounds and eventually producing MoS2. These Mo cluster-MoS2 thin-film nanocomposites were fabricated and applied as gas sensors for the first time. It turns out that after the sulfurization process, the response of MI@FTO for NH3 gas increased three times while showing conversion from p-type to n-type semiconductor, which enhances their possibilities for future device applications.

Eu- and Tb-adsorbed Si3N4 and Ge3N4: tuning the colours with one luminescent host.

Phosphor-converted white light emitting diodes (pc-LEDs) are efficient light sources for applications in lighting and electronic devices. Nitrides, with their wide-ranging applicability due to their intriguing structural diversity, and their auspicious chemical and physical properties, represent an essential component in industrial and materials applications. Here, we present the successful adsorption of Eu and Tb at the grain boundaries of bulk ß-Si3N4 and ß-Ge3N4 by a successful combustion synthesis. The adsorption of europium and terbium, and the synergic combination of both, resulted in intriguing luminescence properties of all compounds (red, green, orange and yellow). In particular, the fact that one host can deliver different colours renders Eu,Tb-ß-M3N4 (M = Si, Ge) a prospective chief component for future light emitting diodes (LEDs). For the elucidation of the electronic properties and structure of ß-Si3N4 and ß-Ge3N4, Mott-Schottky (MS) measurements and density functional theory (DFT) computations were conducted for the bare and RE adsorbed samples.

Trimetallic Chalcogenide Species: Synthesis, Structures, and Bonding.

In an attempt to isolate boron-containing tri-niobium polychalcogenide species, we have carried out prolonged thermolysis reactions of [Cp*NbCl4] (Cp* = ɳ5-C5Me5) with four equivalents of Li[BH2E3] (E = Se or S). In the case of the heavier chalcogen (Se), the reaction led to the isolation of the tri-niobium cubane-like cluster [(NbCp*)3(µ3-Se)3(BH)(µ-Se)3] (1) and the homocubane-like cluster [(NbCp*)3(µ3-Se)3(µ-Se)3(BH)(µ-Se)] (2). Interestingly, the tri-niobium framework of 1 stabilizes a selenaborate {Se3BH}- ligand. A selenium atom is further introduced between boron and one of the selenium atoms of 1 to yield cluster 2. On the other hand, the reaction with the sulfur-containing borate adduct [LiBH2S3] afforded the trimetallic clusters [(NbCp*)3(µ-S)4{µ-S2(BH)}] (3) and [(NbCp*)3(µ-S)4{µ-S2(S)}] (4). Both clusters 3 and 4 have an Nb3S6 core, which further stabilizes {BH} and mono-sulfur units, respectively, through bi-chalcogen coordination. All of these species were characterized by 11B{1H}, 1H, and 13C{1H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray crystallography. Moreover, theoretical investigations revealed that the triangular Nb3 framework is aromatic in nature and plays a vital role in the stabilization of the borate, borane, and chalcogen units.

Electronic Configurations of 3d Transition-Metal Compounds Using Local Structure and Neural Networks.

Machine learning (ML) methods extract statistical relationships between inputs and results. When the inputs are solid-state crystal structures, structure-property relationships can be obtained. In this work, we investigate whether a simple neural network is able to learn the 3d orbital occupations for the transition-metal (TM) centers in crystalline inorganic solid-state compounds using only the local structure around the transition-metal centers described by rotationally invariant fingerprints based on spherical harmonics and one-hot elemental encoding. A multilayer neural network trained on density functional theory (DFT) results of about 1800 samples was developed and showed good performance in predicting the TM orbital occupations (for both spin channels). We study in detail how the local structure affects the predictions of the local properties and how they provide physical insights for the design of a future machine learning model for materials chemistry. The proposed ML method is illustrated in practical application by predicting local magnetic moments of the transition-metal atoms in a full set of inorganic structures with large unit cells. Although less accurate compared to the experimental data, the ML results compared well with the DFT results, suggesting the feasibility of electronic property prediction based only on structure input.

Ligand-Induced Cuboctahedral versus Icosahedral Core Isomerism within Eight-Electron Heterocyclic-Carbene-Protected Gold Nanoclusters.

The controlled structural modification of ligand-protected gold clusters is evaluated by a proper variation of the size and shape of N-heterocyclic carbene (NHC) ligands. Density functional theory calculations show that the Au13 core of [Au13(NHC)8Br4]+ can be shaped into an icosahedron and/or a so far unexpected cuboctahedron depending on the sterical effect inferred by the NHC ligand side arms. As a result, the cluster properties can be modified, encouraging further exploration on controlled core isomerization in ligated gold cluster chemistry.

Methyl Viologens of Bis-(4'-Pyridylethynyl)Arenes - Structures, Photophysical and Electrochemical Studies, and their Potential Application in Biology.

A series of bis-(4'-pyridylethynyl)arenes (arene=benzene, tetrafluorobenzene, and anthracene) were synthesized and their bis-N-methylpyridinium compounds were investigated as a class of π-extended methyl viologens. Their structures were determined by single crystal X-ray diffraction, and their photophysical and electrochemical properties (cyclic voltammetry), as well as their interactions with DNA/RNA were investigated. The dications showed bathochromic shifts in emission compared to the neutral compounds. The neutral compounds showed very small Stokes shifts, which are a little larger for the dications. All of the compounds showed very short fluorescence lifetimes (<4 ns). The neutral compound with an anthracene core has a quantum yield of almost unity. With stronger acceptors, the analogous bis-N-methylpyridinium compound showed a larger two-photon absorption cross-section than its neutral precursor. All of the dicationic compounds interact with DNA/RNA; while the compounds with benzene and tetrafluorobenzene cores bind in the grooves, the one with an anthracene core intercalates as a consequence of its large, condensed aromatic linker moiety, and it aggregates within the polynucleotide when in excess over DNA/RNA. Moreover, all cationic compounds showed highly specific CD spectra upon binding to ds-DNA/RNA, attributed to the rare case of forcing the planar, achiral molecule into a chiral rotamer, and negligible toxicity toward human cell lines at ≤10 µM concentrations. The anthracene-analogue exhibited intracellular accumulation within lysosomes, preventing its interaction with cellular DNA/RNA. However, cytotoxicity was evident at 1 µM concentration upon exposure to light, due to singlet oxygen generation within cells. These multi-faceted features, in combination with its two-photon absorption properties, suggest it to be a promising lead compound for development of novel light-activated theranostic agents.

Looking at platinum carbonyl nanoclusters as superatoms.

Although the chemistry of carbonyl-protected platinum nanoclusters is well established, their bonding mode remains poorly understood. In most of them, the average Pt oxidation state is zero or slightly negative, leading to the apparent average configuration 5d10 6sε (ε = 0 or very small) and the apparent conclusion that metal-metal bonding cannot arise from the completely filled 5d shell nor from the empty (or almost empty) 6s orbitals. However, DFT calculations show in fact that in these species the actual average configuration is 5d10-x 6sx, which provides to the whole cluster a significant total number of 6s electrons that ensures metal-metal bonding. This ("excited") average configuration is to be related to that of coinage metals in ligated group 11 nanoclusters (nd10 (n + 1)sx). Calculations show that metal-metal bonding in most of these platinum nanoclusters can be rationalized within the concepts of superatoms and supermolecules, in a similar way as for group 11 nanoclusters. The "excited" 5d10-x 6sx configuration results from a level crossing between 5d combinations and 6s combinations, the former transferring their electrons to the latter. This level crossing, which does not exist in the bare Ptn clusters, is induced by the ligand shell, the role of which being thus not innocent with respect to metal-metal bonding.

Insight Into the Stability and Electronic and Optical Properties of N-Heterocyclic Carbene Analogues of Halogen/Phosphine-Protected Au13 Superatomic Clusters.

Atomically precise gold nanoclusters (AuNCs) belong to a relevant area offering useful templates with tunable properties toward functional nanostructures. In this work, we explored the feasible incorporation of N-heterocyclic carbenes (NHCs), as part of the protecting-ligand shell in AuNCs. Our results, which are based on the substitution of phosphine ligands in experimentally characterized AuNCs by NHCs in various eight-electron superatoms Au13 and M4Au9 (M = Cu, Ag), indicate similar electronic structure and stability but somewhat different optical properties. These findings support the feasible obtention of novel targets for explorative synthetic efforts featuring NHC ligands on medium-sized species based on the recurrent Au13 icosahedral core. The hypothetical species appear to be interesting templates for building blocks in nanostructured materials with tuned properties, which encourage experimental exploration of ligand versatility in homo- and heterometallic superatomic clusters.

Two-photon absorption properties of multipolar triarylamino/tosylamido 1,1,4,4-tetracyanobutadienes.

The synthesis and characterization of four new tetracyanobutadiene (TCBD) derivatives (1, 3c and 4b-c) incorporating tosylamido and 4-triphenylamino moieties are reported. Along with those of five closely related or differently branched TCBDs derivatives (2, 3a-b, 4c and 5), their linear and (third-order) nonlinear optical properties were investigated by electronic absorption spectroscopy and Z-scan measurements. Among these compounds, the tri-branched compounds 3c and 5 are the most active two-photon absorbers, with effective cross-sections of 275 and 350 GM at 900 nm, respectively. These properties are briefly discussed with the help of DFT calculations, focussing on structural and electronic factors, and contextualized with results obtained previously for related compounds.

On Heteronuclear Isoelectronic Alternatives to [Au13(dppe)5Cl2]3+: Electronic and Optical Properties of the 18-Electron Os@[Au12(dppe)5Cl2] Cluster from Relativistic Density Functional Theory Computations.

The development of well-defined atomically precise heteronuclear nanoclusters passivated by protecting ligands is presently a booming area, owing to the fact that doping well-known homonuclear nanostructures allows fine-tuning of their properties. Here, we explore by means of density functional theory calculations the possibility of doping the central gold atom in the classical [Au13(dppe)5Cl2]3+cluster (1) by Os. Although both [Au13(dppe)5Cl2]3+ and [Os@Au12(dppe)5Cl2] have the same total number of electrons, we show that they are not isoelectronic within the formalism of the superatom model, being respectively an 8- and an 18-electron species. It results that they exhibit similar structures but present significantly different optical behaviors (ultraviolet/visible and circular dichroism). Similar results are obtained for the Ru and Fe relatives. Emission properties indicate some redshift of the T1→S1 decay with respect to [Au13(dppe)5Cl2]3+, involving an equatorial distortion of the Au12Cl2 core in the T1 state, rather than the axial distortion afforded by 1. The sizable highest occupied molecular orbital-lowest unoccupied molecular orbital gaps found for the three doped species suggest that further experimental exploration of different stable doped species derived from the ligand-protected Au12Cl2 core should be encouraged.

Crystal, electronic and magnetic structures of a novel series of intergrowth carbometalates R4Co2C3 (R = Y, Gd, Tb).

A series of new ternary isostructural R4Co2C3 (R = Y, Gd, Tb) carbides was synthesized by annealing of arc-melted stoichiometric samples. The crystal structure of Tb4Co2C3 [space group P2/m, Pearson symbol mP18, a = 12.754(2) Å, b = 3.6251(4) Å, c = 7.0731(9) Å, ß = 105.601(6)°] was solved by direct methods from neutron powder diffraction data collected at 100 K. The room temperature unit cell parameters of the new phases were determined by X-ray powder diffraction technique. The crystal structure of Tb4Co2C3 is characterized as an intergrowth structure resulting from the stacking of alternating TbCoC (YCoC-type) and Tb2C (anti-CdCl2 type) fragments with a 2 : 1 ratio. Tb4Co2C3 orders ferromagnetically at TC = 35(1) K, whereas the isostructural Gd4Co2C3 reveals two magnetic transitions at TC1 = 82(3) K and TC2 = 13(2) K. Density functional theory (DFT) calculations confirm that the magnetic moments of the R4Co2C3 (R = Gd, Tb) carbides are exclusively due to the rare-earth elements. Y4Co2C3 is shown to be a Pauli-paramagnet by experimental and theoretical studies.

Chemistry of group 5 metallaboranes with heterocyclic thiol ligands: a combined experimental and theoretical study.

Thermolysis of [(Cp*Nb)2(B2H6)2], 1b (Cp* = η5-C5Me5), with 2-mercaptobenzothiazole, C6H4NSCSH (MBT), and 2-mercaptobenzoxazole, C6H4NOCSH (MBO), yielded hydrogen substituted compounds 2 and 3 with a general formula [(Cp*Nb)2(B2H6)(B2H5L)] (2: L = C6H4NSCS and 3: L = C6H4NOCS). A similar reaction of 1b with Ph2Se2 yielded the monosubstituted derivative [(Cp*Nb)2(B2H6){B2H5(PhSe)}], 4. All further efforts towards persubstitution of 1b under various drastic conditions were unfruitful. In parallel, in an effort to find a better synthetic route to the known Ta-aziridine complex [Cp*TaBH(C7H4NS2)CH2S2NC6H4], Cp*TaCl4 was treated with a 2-mercaptobenzothiazolyl-based borate ligand Na[H2B(C6H4NSCS)2]. Surprisingly, the reaction led to the formation of the half-sandwich trichloroaryltantalum(v) complex [Cp*TaCl3{κ2-N,S-C6H4NSCS}], 5, containing a heterocyclic thiol ligand. Using an alternative method complex 5 was isolated in good yield when Cp*TaCl4 was treated with the potassium salt of 2-mercaptobenzothiazole K[C6H4NSCS]. All the compounds were characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy, and their structures were unequivocally established by crystallographic analysis.

2- and 2,7-Substituted para-N-Methylpyridinium Pyrenes: Syntheses, Molecular and Electronic Structures, Photophysical, Electrochemical, and Spectroelectrochemical Properties and Binding to Double-Stranded (ds) DNA.

Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm-1 . The N-methylpyridinium compounds have a single reduction at ca. -1.5 V vs. Fc/Fc+ in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.