ABSTRACT: A grand unified model (GUM) is developed to achieve fundamental understanding of rich structures of all 71 liganded gold clusters reported to date. Inspired by the quark model by which composite particles (for example, protons and neutrons) are formed by combining three quarks (or flavours), here gold atoms are assigned three 'flavours' (namely, bottom, middle and top) to represent three possible valence states. The 'composite particles' in GUM are categorized into two groups: variants of triangular elementary block Au3(2e) and tetrahedral elementary block Au4(2e), all satisfying the duet rule (2e) of the valence shell, akin to the octet rule in general chemistry. The elementary blocks, when packed together, form the cores of liganded gold clusters. With the GUM, structures of 71 liganded gold clusters and their growth mechanism can be deciphered altogether. Although GUM is a predictive heuristic and may not be necessarily reflective of the actual electronic structure, several highly stable liganded gold clusters are predicted, thereby offering GUM-guided synthesis of liganded gold clusters by design.
Project description:Auration of o-trimethylsilyl arylphosphines leads to the formation of gold and gold-silver clusters with ortho-metalated phosphines displaying 3c-2e Au-C-M bonds (M=Au/Ag). Hexagold clusters [Au6 L4 ](X)2 are obtained by reaction of (L-TMS)AuCl with AgX, whereas reaction with AgX and Ag2 O leads to gold-silver clusters [Au4 Ag2 L4 ](X)2 . Oxo-trigold(I) species [Au3 O]+ were identified as the intermediates in the formation of the silver-doped clusters. Other [Au5 ], [Au4 Ag], and [Au12 Ag4 ] clusters were also obtained. Clusters containing PAu-Au-AuP structural motif display good catalytic activity in the activation of alkynes under homogeneous conditions.
Project description:In order to increase the understanding of the recently synthesized Au70S20(PPh3)12 cluster, we used the divide and protect concept and superatom network model (SAN) to study the electronic and geometric of the cluster. According to the experimental coordinates of the cluster, the study of Au70S20(PPh3)12 cluster was carried out using density functional theory calculations. Based on the superatom complex (SAC) model, the number of the valence electrons of the cluster is 30. It is not the number of valence electrons satisfied for a magic cluster. According to the concept of divide and protect, Au70S20(PPh3)12 cluster can be viewed as Au-core protected by various staple motifs. On the basis of SAN model, the Au-core is composed of a union of 2e-superatoms, and 2e-superatoms can be Au3, Au4, Au5, or Au6. Au70S20(PPh3)12 cluster should contain fifteen 2e-superatoms on the basis of SAN model. On analyzing the chemical bonding features of Au70S20(PPh3)12, we showed that the electronic structure of it has a network of fifteen 2e-superatoms, abbreviated as 15 × 2e SAN. On the basis of the divide and protect concept, Au70S20(PPh3)12 cluster can be viewed as Au4616+[Au12(µ3-S)108-]2[PPh3]12. The Au4616+ core is composed of one Au2212+ innermost core and ten surrounding 2e-Au4 superatoms. The Au2212+ innermost core can either be viewed as a network of five 2e-Au6 superatoms, or be considered as a 10e-superatomic molecule. This new segmentation method can properly explain the structure and stability of Au70S20(PPh3)12 cluster. A novel extended staple motif [Au12(µ3-S)10]8- was discovered, which is a half-cage with ten µ3-S units and six teeth. The six teeth staple motif enriches the family of staple motifs in ligand-protected Au clusters. Au70S20(PPh3)12 cluster derives its stability from SAN model and aurophilic interactions. Inspired by the half-cage motif, we design three core-in-cage clusters with cage staple motifs, Cu6@Au12(μ3-S)8, Ag6@Au12(μ3-S)8 and Au6@Au12(μ3-S)8, which exhibit high thermostability and may be synthesized in future.
Project description:We report a computational study on the structures and bonding of a charged molecular alloy D <sub>2h</sub> [Pd<sub>2</sub>As<sub>14</sub>]<sup>4-</sup> (1), as well as a model D <sub>2h</sub> [Au<sub>2</sub>Sb<sub>14</sub>]<sup>4-</sup> (2) cluster. Our effort makes use of an array of quantum chemistry tools: canonical molecular orbital analysis, adaptive natural density partitioning, natural bond orbital analysis, orbital composition analysis, and nucleus independent chemical shift calculations. Both clusters consist of two X<sub>7</sub> (X?=?As, Sb) cages, which are interconnected via a M<sub>2</sub> (M?=?Pd, Au) dumbbell, featuring two distorted square-planar MX<sub>4</sub> units. Excluding the Pd/As or Au/Sb lone-pairs, clusters 1 and 2 are 50- and 44-electron systems, respectively, of which 32 electrons are for two-center two-electron (2c-2e) As-As or Sb-Sb ? bonds and an additional 16 electrons in 1 for 2c-2e Pd-As ? bonds. No covalent Pd-Pd or Au-Au bond is present in the systems. Cluster 1 is shown to possess two globally delocalized ? electrons, whereas 2 has two ? sextets (each associated with an AuSb<sub>4</sub> fragment). Thus, 1 and 2 conform to the (4n?+?2) Hückel rule, for n?=?0 and 1, respectively, rendering them ?-aromaticity.
Project description:The structures and stabilities of a series of endohedral gold clusters containing ten gold atoms M@Au10 (M = W, Mo, Ru, Co) have been determined using density functional theory. The gradient-corrected functional BP86, the Tao-Perdew-Staroverov-Scuseria TPSS meta-GGA functional, and the hybrid density functionals B3LYP and PBE1PBE were employed to calculate the structures, binding energies, adiabatic ionization potentials, and adiabatic electron affinities for these clusters. The LanL2DZ effective core potentials and the corresponding valence basis sets were employed. The M@Au10 (M = W, Mo, Ru, Co) clusters have higher binding energies than an empty Au10 cluster. In addition, the large HOMO-LUMO gaps suggest that the M@Au10 (M = W, Mo, Ru, Co) clusters are all likely to be stable chemically. The ionization potentials and electron affinities for these clusters are very high, and the W@Au10 and Mo@Au10 clusters have electron affinities similar to the super-halogen Al13.
Project description:Homeostasis is a biological principle for regulation of essential physiological parameters within a set range. Behavioural responses due to deviation from homeostasis are critical for survival, but motivational processes engaged by physiological need states are incompletely understood. We examined motivational characteristics of two separate neuron populations that regulate energy and fluid homeostasis by using cell-type-specific activity manipulations in mice. We found that starvation-sensitive AGRP neurons exhibit properties consistent with a negative-valence teaching signal. Mice avoided activation of AGRP neurons, indicating that AGRP neuron activity has negative valence. AGRP neuron inhibition conditioned preference for flavours and places. Correspondingly, deep-brain calcium imaging revealed that AGRP neuron activity rapidly reduced in response to food-related cues. Complementary experiments activating thirst-promoting neurons also conditioned avoidance. Therefore, these need-sensing neurons condition preference for environmental cues associated with nutrient or water ingestion, which is learned through reduction of negative-valence signals during restoration of homeostasis.
Project description:The reaction of 2,6-F2 C6 H3 SiMe3 with Ph2 PLi provided 2,6-(Ph2 P)2 C6 H3 SiMe3 (1), which can be regarded as precursor for the novel anionic tridentate ligand [2,6-(Ph2 P)2 C6 H3 ]- (PCP)- . The reaction of 1 with [AuCl(tht)] (tht=tetrahydrothiophene) afforded 2,6-(Ph2 PAuCl)2 C6 H3 SiMe3 (2). The subsequent reaction of 2 with CsF proceeded with elimination of Me3 SiF and yielded the neutral tetranuclear complex linear-[Au4 Cl2 (PCP)2 ] (3) comprising a string-like arrangement of four Au atoms. Upon chloride abstraction from 3 with NaBArF 4 (ArF =3,5-(CF3 )2 C6 H3 ) in the presence of tht, the formation of the dicationic tetranuclear complex linear-[Au4 (PCP)2 (tht)2 ](BArF 4 )2 (4) was observed, in which the string-like structural motif is retained. Irradiation of 4 with UV light triggered a facile rearrangement in solution giving rise to the dicationic tetranuclear complex cyclo-[Au4 (PCP)2 (tht)2 ](BArF 4 ) (5), which comprises a rhomboidal motif of four Au atoms. In 3-5, the Au atoms are associated by a number of significant aurophilic interactions. The atom-economic and selective reaction of 3 with HgCl2 yielded the neutral trinuclear bimetallic complex [HgAu2 Cl3 (PCP)] (6) comprising significant metallophilic interactions between the Au and Hg atoms. Therefore, 6 may be also regarded as a metallopincer complex [ClHg(AuCAu)] between HgII and the anionic tridentate ligand [2,6-(Ph2 PAuCl)2 C6 H3 ]- (AuCAu)- containing a central carbanionic binding site and two "gold-arms" contributing pincer-type chelation trough metallophilic interactions. Compounds?1-6 were characterized experimentally by multinuclear NMR spectroscopy and X-ray crystallography and computationally using a set of real-space bond indicators (RSBIs) derived from electron density (ED) methods including Atoms In Molecules (AIM), the Electron Localizability Indicator (ELI-D) as well as the Non-Covalent Interaction (NCI) Index.
Project description:At neutral pH, formate binds to the haem a3 component of cytochrome c oxidase to give a complex that reacts differently from the non-liganded enzyme with reducing agents. Addition of sodium dithionite to the formate complex leads directly to the formation of the fully reduced species, whereas reduction with ascorbate/tetramethylenephenylene-diamine can lead to the production of a mixed-valence species. The stability of this mixed-valence form was studied, and the species appears to represent a 'steady-state' situation that is stable only in the presence of an excess of O2 and reducing equivalents. Characterization of the mixed-valence complex by electron paramagnetic resonance and magnetic circular dichroism reveals the presence of reduced low-spin haem a together with reduced detectable copper and high-spin ferric haem a3.
Project description:Meso tetraarylporphyrinato gold(iii) cations bearing different substituents at the aryl substituents (COOMe, COOH, NO2, NH2, NHAc, H, O n Bu, CF3) were prepared and characterised. Their reversible one-electron reductions were studied by (spectro)electrochemical means as well as by selective chemical one-electron reduction using cobaltocene. The preferred location of the spin density, namely gold centred or porphyrin centred, was probed by electron paramagnetic resonance spectroscopy (g values, 197Au hyperfine coupling) as well as by density functional theory calculations (spin densities). In all cases studied experimentally and theoretically, the gold(ii) valence isomer (5d9 electron configuration) is preferred over the porphyrin ? radical anion. In the hexafluorophosphate salt of the nitro derivative a further nitro ? radical anion valence isomeric species is significantly populated. In the presence of chloride ions this nitro ? radical anion/AuII valence isomeric equilibrium evolves towards the porphyrin ? radical anion. The electronic structures of the nitro ? radical and the AuII ? radical valence isomers (5d x2-y2 orbital) could be calculated by DFT methods. The electron transfer pathway between the nitro ? radical anion and the AuII valence isomer is well described by the location of the hexfluorophosphate counterion, the Au-N distances (corresponding to the totally symmetric stretching vibration), the symmetric stretching mode of the NO2 substituent and a meso-nitrophenyl rotation. The specific geometric and electronic properties of the favoured gold(ii) ? radical valence isomer, namely counterion dislocation and ? symmetry of the redox orbital, might stabilise charge-shifted states [(gold(ii) porphyrin)-donor?+] by retarding the back electron transfer to give the ground state (gold(iii) porphyrin)-donor. This will guide the design of (photo-induced) electron transfer pathways with tetraarylporphyrinato gold(iii) complexes as electron acceptors.
Project description:Electrochemical synthesis of H2O2 through a selective two-electron (2e-) oxygen reduction reaction (ORR) is an attractive alternative to the industrial anthraquinone oxidation method, as it allows decentralized H2O2 production. Herein, we report that the synergistic interaction between partially oxidized palladium (Pd?+) and oxygen-functionalized carbon can promote 2e- ORR in acidic electrolytes. An electrocatalyst synthesized by solution deposition of amorphous Pd?+ clusters (Pd3?+ and Pd4?+) onto mildly oxidized carbon nanotubes (Pd?+-OCNT) shows nearly 100% selectivity toward H2O2 and a positive shift of ORR onset potential by ~320?mV compared with the OCNT substrate. A high mass activity (1.946?A?mg-1 at 0.45?V) of Pd?+-OCNT is achieved. Extended X-ray absorption fine structure characterization and density functional theory calculations suggest that the interaction between Pd clusters and the nearby oxygen-containing functional groups is key for the high selectivity and activity for 2e- ORR.
Project description:The reduction of cytochrome c oxidase by Cr2+, followed by means of stopped-flow spectrophotometry, exhibits two phases: the faster Cr2+-concentration-dependent reaction has an initial rate constant of 1.1 X 10(4)M-1-S-1, but reaches a rate limit at high concentration of reductant; the slower phase is concentration-independent with a rate of 0.3S-1. The activation energies of the fast and the slow processes are 35 and 71 kJ/mol respectively. The reduction kinetics of the mixed-valence CO complex and the cyanide-inhibited enzyme were compared with those of the fully oxidized forms: both the liganded species have a fast phase identical with that found in the oxidized oxidase. A comparison of the kinetic difference spectra obtained for the fast phase of reduction of oxidized oxidase with those obtained on reduction of the liganded species suggests that the rapid phase arises from the reduction ofhaem a, and the slow phase from the reduction of haem a3.