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The novel quaternary compound Rb0.2Ba0.4Cr5Se8 was synthesized and characterized in both single crystal and polycrystalline forms. Crystallizing in the monoclinic crystal system (space group C2/m, cell parameters a = 18.7071(4) Å, b = 3.6030(1) Å, c = 8.9637(3) Å, ß = 104.494(2)°) and isostructural to pseudo-hollandite compounds, it features mixed Rb and Ba occupancy within its one-dimensional channels. High-temperature X-ray diffraction revealed no decomposition up to 973 K, and the thermal expansion coefficient at 300 K was determined to be 2.6(1)·10-5 K-1. Spin-polarized density functional theory (DFT) calculations showed that the density of states for Rb0.2Ba0.4Cr5Se8 is more polarized than that of Ba0.5Cr5Se8, resulting in a higher Seebeck coefficient, which was experimentally confirmed to reach a peak value of 400 µV·K-1 at 620 K. Resistivity measurements indicated a degenerate semiconducting behavior below 550 K, with a resistivity peak of 100 mΩ·cm at that temperature, leading to a maximum power factor of 0.21 mW·m-1·K-2. Thermal conductivity measurements indicated low values around 0.8 W·m-1·K-1 in the 300-900 K range, resulting in a thermoelectric figure of merit of 0.22 at 873 K. Decorrelated transport properties observed in this double-inserted pseudo-hollandite compound make Rb0.2Ba0.4Cr5Se8 a good example of beneficial synergistic effects for higher thermoelectric performance.
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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.
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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-κ2Sâ³:κ1Sâ´)] (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.
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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.
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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.
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The iconic W@Au12 icosahedral bare cluster reaches the favorable closed-shell superatomic electron configuration 1S2â¯1P6â¯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 1S2â¯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.
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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 (<4â 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 ≤10â µM concentrations. The anthracene-analogue exhibited intracellular accumulation within lysosomes, preventing its interaction with cellular DNA/RNA. However, cytotoxicity was evident at 1â 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.
Assuntos
Benzeno , Paraquat , Antracenos , Biologia , DNA/química , Humanos , Estrutura Molecular , RNARESUMO
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.
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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.
Assuntos
Eletrônica , Redes Neurais de ComputaçãoRESUMO
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.
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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.
Assuntos
Compostos Organometálicos , Selênio , Boratos , Compostos Organometálicos/química , Modelos Moleculares , Nióbio , Boro , Enxofre/químicaRESUMO
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 7200â cm-1 . The N-methylpyridinium compounds have a single reduction at ca. -1.5â 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.
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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 T1âS1 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.
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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.
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The electronic structure of the spherical Mackay icosahedral nanosized cluster Pd55 (PiPr3 )12 (µ3 -CO)20 is analyzed by using DFT calculations. Results reveal that it can be considered as a regular superatom with a "magic" electron count of 20, characterized by a 1S2 1P6 1D10 2S2 jellium configuration. Its open shell nature is associated with partial occupation of non-jellium, 4d-type, levels located on the interior of the Pd55 kernel. This shows that the superatom model can be used to rationalize the bonding and stability of spherical ligated groupâ 10 clusters, despite their apparent 0-electron count.
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Mixed-valence compounds can be used for the design of molecular quantum-dot cellular automata (QCA). Here, we investigate the QCA properties of a three-dot "Y"-shaped functionalized zwitterionic neutral closo-carborane model 1-(3,5-{Cp(dHpe)Fe-C≡C-}2(C6H3))-10-Cp(dHpe)Fe-C≡C-closo-1-CB9H8 (1) (Cp = cyclopentadienyl (η5-C5H5) and dHpe = 1,2-bis(phosphino)ethane (H2PCH2CH2PH2)) as a neutral clocked molecular half-cell. DFT results clearly demonstrate that 1 can display simultaneously the two most basic properties necessary for clocked QCA operation, i.e., bistable switching behavior and clocked control. This is possible due to the three stable states (two active and one null) of 1, corresponding to occupation of each of the three iron-ethynyl groups by the positive charge. In addition, the proximal electronic bias effects can be overcome by the zwitterionic nature of 1, which could be imposed by external counterions, rendering these effects more predictable.
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The structure and bonding of a series of selected phosphine-protected gold clusters (Aun-P) of nuclearity varying from n = 6 to 13 were investigated by density functional theory (DFT) calculations and compared to those of the hypothetical homologues in which phosphines were replaced by N-heterocyclic carbene (NHC) analogues (Aun-C). Both the Aun-P and Aun-C series exhibit similar stabilities and structural features, except for n = 6, where some differences are noted. The NHC ligands are found to be even slightly more strongly bonded to the gold core (by a few kilocalories per mole per ligand) than phosphines. Investigation of the optical properties of both series using time-dependent DFT calculations indicates similarities in the nature and energies of the UV-vis optical transitions and, consequently, relatively similar shapes of the simulated spectra, with a general blue-shift tendency when going from Aun-P to Aun-C. The fluorescence behavior observed experimentally for some of the Aun-P species is expected to occur also for their Aun-C analogues, which can be extended to other carbene-ligand-protected nanoclusters. Our results show that it should be possible to stabilize gold clusters with NHC ligands, in relation to the seminal Au13-ligand-protected core, offering novel building blocks for the design of nanostructured materials with various properties.
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Irradiation of [Re2(CO)10] in the presence of BH3·thf resulted in the formation of several rhenium diborane(6) species, for example, [{(OC)4Re}{Re(CO)3}2(µ3-η2:η2:η2-B2H6)(µ-H)], 1; [{(OC)4Re}2{Re(CO)3}(µ3-η2:η2:η1-B2H6)(µ-H)], 2; and [{(OC)4Re}2(µ-η2:η2-B2H6)], 3, comprising diverse coordination modes of the [B2H6]2- ligand. Compound 1 contains a tris(bidentate) [B2H6]2- unit, whereas 2 consists of an unsymmetrically bound [µ3-η2:η2:η1-B2H6]2- ligand. In contrast, the irradiation of [Mn2(CO)10] with BH3·thf yielded only the Mn analogue of 1, [{(OC)4Mn}{Mn(CO)3}2(µ3-η2:η2:η2-B2H6)(µ-H)], 1'. In an attempt to generate the bimetallic Mn-diborane(6), we have carried out the reaction of 1' with PCy3 under photolytic conditions. The reaction led to the formation of two single base stabilized unsymmetrical diborane(5) species, [{Mn(CO)3}{Mn(CO)2PCy3}(µ-η2:η2-B2H5·PCy3)(µ-H)], 4, and [{Mn(CO)2PCy3}(η3-B2H5·PCy3)], 5. As [B2H6]2- and [B2H5·PCy3]- are isoelectronic, the bondings in 4 and 5 are analogous to that of diborane(6) species 1-3. All the new species have been characterized spectroscopically, and their structures were further confirmed by single-crystal X-ray diffraction studies. DFT-type quantum chemical calculations were carried out that provided insight into the bonding interaction of [B2H6]2- and [B2H5·PCy3]- with the M(CO)n fragments.
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DFT calculations were carried out on bare Au32 and Au33 nanoclusters with various charges, in order to analyze their stability with respect to different cluster electron numbers. Results indicate that in addition to the neutral Au32 hollow species, significant HOMO-LUMO gaps are computed for [Au32]8+ (hollow) and [Au32]4+ (two-shell structure). Species with smaller HOMO-LUMO gaps can reach stability upon "passivation" by a ligand shell, as experimentally exemplified. Icosahedral frameworks of Ih or lower symmetry are favored for the cationic nanoclusters whereas different structures are computed for the anionic ones.
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A large number of metallaborane clusters and their derivatives with various structural arrangements are known. Among them, M2B5 clusters and derivatives constitute a significant class. Transition metals present in these species span from group 4 to group 7. Their structure can vary from oblatonido, oblatoarachno, to arachno type open structures. Many of these clusters appear to be hypoelectronic and are often considered as 'rule breakers' with respect to the classical Wade-Mingos electron counting rules. This is due to their unique highly oblate (flattened) deltahedral structures featuring a cross-cluster M-M interaction. Many theoretical calculations were performed to elucidate their electronic structure and chemical bonding properties. In this review, the synthesis, structure, and electronic aspects of the transition metal M2B5 clusters known in the literature are discussed. The chosen examples illustrate how, in synergy with experiments, computational results can provide additional valuable information to better understand the electronic properties and electronic requirements which govern their architecture and thermodynamic stability.