Hydrogen bonds to Au atoms in coordinated gold clusters

Accelerated Nanocomposite Hydrogel Gelation Times Independent of Gold Nanoparticle Ligand Functionality

The expansive use of hydrogels in healthcare relies on carefully tuned properties in dynamic environments with predictable behavior, including time sensitive biological systems and biomedical applications. To meet demands in these settings, nanomaterials are often introduced to a hydrogel matrix which simultaneously elevates potential applications while adding complexity to fundamental characteristics. With respect to drug delivery, gold nanoparticles have modifiable surfaces to carry an array of targeted drug treatments. However, different molecules acting as capping ligands possess different chemical structures that can impact gelation times. To understand the influence of capping ligand chemistry on polyacrylamide (PAM) based nanocomposite hydrogel radical gelation time, gold nanoparticle (Au NP) capping ligands were selected to encompass varying functional groups and molecular weights: citrate, cetyltrimethylammonium bromide, polyvinylpyrrolidone, and poly(acrylic acid). Gelation times were quantified as the storage-loss moduli crossover point in rheological time sweeps at constant strain and frequency. The dominating factor for gelation time was the presence of Au NPs, independent of a diverse range of capping ligand structures. The gelation times were also markedly faster than the same capping ligand structures used as stand-alone molecular additives. The accelerated Au NP gelation times, under 2 min, are attributed to the Au NPs acting as a cross-linker, promoting gelation. These results bolster the potential implementation of Au NP nanocomposite hydrogels in time-sensitive biomedical applications as robust drug carriers.

Thermodynamically and Dynamically Boosted Electrocatalytic Iodine Conversion with Hydroxyl Groups for High-Efficiency Zinc-Iodine Batteries

Rechargeable zinc-iodine (Zn-I2) batteries have shown immense potential for grid-scale energy storage applications, but there remain challenges of improving efficiency and cycling stability due to the sluggish iodine reduction reaction (IRR) kinetics and serious shuttle problem of polyiodides. We herein demonstrate an efficient metal-free hydroxyl (-OH)-functionalized carbon catalyst that effectively boosts the performance of Zn-I2 batteries. It has been found that the obtained electrocatalytic performance is strongly correlated with the surface oxygen chemical environment in the carbon matrix. Both theoretical calculations and experimental measurements have uncovered that the -OH group, rather than carbonyl (-C═O) and carboxyl (-COOH), provides the active electrocatalytic site for IRR, improves the iodine redox kinetics and the electrochemical reversibility, and facilitates I2 nucleation. As confirmed by a series of in situ and ex situ spectroscopy techniques, due to the favorable reaction thermodynamics and the lowered energy barrier for I3- dissociation, the O-H···I channels can effectively trigger the direct transformation of I2/I- and avoid the formation of stable polyiodides. As a result, the as-assembled battery of I2/oxygen-functionalized carbon cloth (I2/OCC-2)//Zn exhibits a high capacity of 2.27 mA h cm-2 at 1 mA cm-2, outstanding rate capability with 89.0% capacity retention at 20 mA cm-2, and long-term stability of 10,000 cycles.

Recent Trends in Group 11 Hydrogen Bonding

Conventional hydrogen bonding (H-bonding) has been extensively studied in organic and biological systems. However, its role in transition metal chemistry, particularly with Group 11 metals (i. e. Cu, Ag, Au) as hydrogen bond acceptors, remains relatively unexplored. Through a combination of experimental techniques, such as Nuclear Magnetic Resonance (NMR), Infrared spectroscopy (IR), X-Ray Diffraction (XRD), and computational calculations, several aspects of H-bonding interactions with Group 11 metals are examined, shedding light on its impact on structural motifs and reactivity. These include bond strengths, geometries, and effects on electronic structures. Understanding the intricacies of hydrogen bonding within transition metal chemistry holds promise for various applications, including catalytic transformations, the construction of molecular assemblies, synthesis of complexes displaying anticancer activities, or luminescence applications (e. g. Thermally Activated Delayed Fluorescence, TADF). This review encompasses the most significant recent advances, challenges, and future prospects in this emerging field.

Controlled nanocrystallization of gold nanoclusters within surfactant envelopes: enhancing aggregation-induced emission in solution

The nanocrystallization of functional molecules has been a subject of recent interest in the current development of nanotechnology. Herein, we report the unprecedented synthesis of single nanocrystals of a molecular gold nanocluster in a homogeneous solution by using surfactant-based nano-envelopes. The co-assembling of a Au8 nanocluster carrying lipophilic phosphine ligands with sodium dodecyl sulfate (SDS) in an aqueous solution results in the formation of sphere-shaped amorphous nano-aggregates coated with the surfactant. Upon sonication, the spherical amorphous aggregates are smoothly shape-shifted into discrete rhombic nanocrystals, which can be tracked by TEM and solution XRD. The transformation into single nanocrystals occurs exclusively without further growth or agglomeration, implying that the crystal growth is restricted within the surfactant nano-envelopes. The robust but flexible nature of the wrapped surfactant is likely responsible for the controlled crystallization. We also demonstrate that the amorphous-to-nanocrystalline transition in solution remarkably enhances the photoluminescence emission from the nanocluster, providing a clear example of crystallization-induced emission enhancement. Notably, the obtained nanocrystals showed high stability in solution and retained their shape, size, and PL intensity even after several months, owing to the densely packed surfactant shell. The present surfactant-directed nanocrystallization method may be applicable to other molecular species to contribute to the development of nanocluster science as well as the designed synthesis of nanomaterials.

Theoretical study of the saturation and nature of the hydrogen bonds to gold

Traditional hydrogen bonds are well-known to exhibit directionality and saturation. By contrast, gold involved hydrogen bonds (GHBs) have been extensively studied but remain lack of in-depth understanding towards the intrinsic nature and saturation property. This work exemplifies three series of complexes: [L-Au-L]-⋯(HF)n (L = H, CH3, (CH3)3; n = 1-8) containing GHBs to dig into the intrinsic nature with the aid of multiple theoretical analysis methods, finding that the formation of GHB is highly subject to orbital interactions along with steric hindrance. Moreover, the saturation level of GHBs largely depends on the ligand attached to the gold center, since different ligands typically possess varying electron-giving ability and steric volume. This work confirms the coexistence of as many as 6 GHBs for one Au atom and thoroughly studies the saturation level of GHBs, which will provide new insights into GHBs and facilitate future synthesis of more complicated gold complexes.

Recent Advances in Ligand Engineering for Gold Nanocluster Catalysis: Ligand Library, Ligand Effects and Strategies

Advances in new ligands in the last decade facilitated in-depth studies on the property-relationship of gold nanoclusters and promoted the rational synthesis and related applications of such materials. Currently, more and more new ligands are being explored; thus, the ligand library of AuNCs is being expanded fast, which also enables investigation of ligand effects of AuNCs via direct comparison of different ligating shell with the identical gold core. It is now widely accepted that ligands influence the properties of AuNCs enormously including stability, catalysis, photoluminescence among others. These studies inspired ligand engineering of AuNCs. One of the goals for ligand engineering is to develop ligated AuNC catalysts in which the ligands are able to exert big-enough influence on electronic and steric control over catalysis as in a transition-metal or an enzyme system. Although increasing attention is paid to the further expansion of ligand library, the investigation of design principles and strategies regarding ligands are still in their infant stage. This review summarizes the ligands for AuNC synthesis, the ligand effects on stability and catalysis, and recently developed strategies in promoting AuNC catalytic performance via ligand manipulation.

Photoluminescence control by atomically precise surface metallization of C-centered hexagold(i) clusters using N-heterocyclic carbenes

The properties of metal clusters are highly dependent on their molecular surface structure. The aim of this study is to precisely metallize and rationally control the photoluminescence properties of a carbon(C)-centered hexagold(i) cluster (CAu^I₆) using N-heterocyclic carbene (NHC) ligands with one pyridyl, or one or two picolyl pendants and a specific number of silver(i) ions at the cluster surface. The results suggest that the photoluminescence of the clusters depends highly on both the rigidity and coverage of the surface structure. In other words, the loss of structural rigidity significantly reduces the quantum yield (QY). The QY in CH₂Cl₂ is 0.04 for [(C)(Au^I-BIPc)₆Ag^I₃(CH₃CN)₃](BF₄)₅ (BIPc = N-isopropyl-N'-2-picolylbenzimidazolylidene), a significant decrease from 0.86 for [(C)(Au^I-BIPy)₆Ag^I₂](BF₄)₄ (BIPy = N-isopropyl-N'-2-pyridylbenzimidazolylidene). This is due to the lower structural rigidity of the ligand BIPc because it contains a methylene linker. Increasing the number of capping Ag^I ions, i.e., the coverage of the surface structure, increases the phosphorescence efficiency. The QY for [(C)(Au^I-BIPc²)₆Ag^I₄(CH₃CN)₂](BF₄)₆ (BIPc² = N,N'-di(2-pyridyl)benzimidazolylidene) recovers to 0.40, 10-times that of the cluster with BIPc. Further theoretical calculations confirm the roles of Ag^I and NHC in the electronic structures. This study reveals the atomic-level surface structure-property relationships of heterometallic clusters.

Why much of Chemistry may be indisputably non-bonded?

In this compendium, the wide scope of all intermolecular interactions ever known has been revisited, in particular giving emphasis the capability of much of the elements of the periodic table to form non-covalent contacts. Either hydrogen bonds, dihydrogen bonds, halogen bonds, pnictogen bonds, chalcogen bonds, triel bonds, tetrel bonds, regium bonds, spodium bonds or even the aerogen bond interactions may be cited. Obviously that experimental techniques have been used in some works, but it was through the theoretical methods that these interactions were validate, wherein the QTAIM integrations and SAPT energy partitions have been useful in this regard. Therefore, the great goal concerns to elucidate the interaction strength and if the intermolecular system shall be total, partial or non-covalently bonded, wherein this last one encompasses the most majority of the intermolecular interactions what leading to affirm that chemistry is debatably non-bonded.

Phosphine-Stabilized Hidden Ground States in Gold Clusters Investigated via a Aun(PH3)m Database

Nanoclusters are promising materials for catalysis and sensing due to their large surface areas and unique electronic structures which can be tailored through composition, geometry, and chemistry. However, relationships correlating synthesis parameters directly to outcomes are limited. While previous computational studies have mapped the potential energy surface of specific systems of bare nanoclusters by generating and calculating the energies of reasonable structures, it is known that environmental ions and ligands crucially impact the final shape and size. In this work, phosphine-stabilized gold is considered as a test system and DFT calculations are performed for clusters with and without ligands, producing a database containing >10000 structures for Aun(PH3)m (n ≤ 12). We find that the ligation of phosphines affects the thermodynamic stability, bonding, and electronic structure of Au nanoclusters, specifically such that "hidden" ground state cluster geometries are stabilized that are dynamically unstable in the pure gold system. Further, the addition of phosphine introduces steric effects that induce a transition from planar to nonplanar structures at 4-5 Au atoms rather than up to 13-14 Au atoms, as previously predicted for bare clusters. This work highlights the importance of considering the ligand environment in the prediction of nanocluster morphology and functionality, which adds complexity as well as a rich opportunity for tunability.

Structure and assembly of a hexanuclear AuNi bimetallic nanocluster

An Au₄Ni₂ nanocluster containing a square-planar [-PPh₂-Au-S-Au-]₂ ring and two nickel-pincer arms is reported here. Abundant intra- and inter-cluster noncovalent interactions promote the assembly of the nanocluster into a porous framework material. The assembly-dependent unique solubility and photoluminescence were also investigated.

Theoretical investigation of C1-C4 hydrocarbons adsorption and separation in a porous metallocavitand

The purification of light hydrocarbons is one of the most important chemical processes globally which consumes substantial energy. Porous materials are likely to improve the efficiency of the separation process by acting as regenerable solid adsorbents. To investigate such translational systems, the underlying mechanism of adsorption in the porous materials must be taken into account. Herein we report the adsorption and selective separation of C1-C4 hydrocarbons in the coinage metal-based macrocyclic metallocavitand Pillarplex, which exhibits excellent performance in the adsorption of CH4 at the ambient conditions with a binding energy of -17.9 kcal mol-1. In addition, the endohedral adsorption of C2-C4 hydrocarbon is impressive. The CH4, C2H4, C3H4, and 1,3-butadiene have potential uptake of 2.57, 4.26, 3.60, and 2.95 mmol g-1, respectively at ambient conditions are highest from their respective isomers. Selective separation of C1-C4 hydrocarbons is studied using ideal adsorption solution theory demonstrating its potential for one-step purification of C1-C3 hydrocarbons.

Tri(N-carbazolyl)phosphine Gold(I) Complexes: Structural and Catalytic Activity Studies

Twelve tri(N-carbazolyl)phosphine gold(I) complexes, bearing both protonated and deuterated aryl phosphorous triamide-type ligands, have been synthesized and characterized. An elusive Au-H(D) interaction between the H(D) atoms of the tri(N-carbazolyl)phosphine ligand at the H-1(D-1) position of the carbazolyl ring and the central gold atom was observed. Complexes 5(H)/5(D) bearing the dibrominated tri(N-carbazolyl)phosphine ligand exhibit isotopic polymorphism, in which two dramatically different crystal-packing modes between the protonated and deuterated forms occur. The catalytic potential of these complexes has been showcased in the gold(I)-catalyzed glycosylation with glycosyl o-alkynylbenzoates as donors, with TON being up to 27 000.

OH-···Au Hydrogen Bond and Its Effect on the Oxygen Reduction Reaction on Au(100) in Alkaline Media

Using ab initio molecular dynamics simulations with fully solvated ions, we demonstrate that solvated OH^- forms a stable hydrogen bond with Au(100). Unlike the hydrogen bond between H₂O and Au reported previously, which is more favorable for negatively charged Au, the OH^-···Au interaction is stabilized when a small positive charge is added to the metal slab. For electro-catalysis, this means that while OH₂···Au plays a significant role in the hydrogen evolution reaction, OH^-···Au could be a significant factor in the oxygen reduction reaction in alkaline media. It also points to a fundamental difference in the mechanism of oxygen reduction between gold and platinum electrodes.

Bicarbonate insertion triggered self-assembly of chiral octa-gold nanoclusters into helical superstructures in the crystalline state

Constructing atomically precise helical superstructures of high order is an extensively pursued subject for unique aesthetic features and underlying applications. However, the construction of cluster-based helixes of well-defined architectures comes with a huge challenge owing to their intrinsic complexity in geometric structures and synthetic processes. Herein, we report a pair of unique P- and M-single stranded helical superstructures spontaneously assembled from R- and S-Au8c individual nanoclusters, respectively, upon selecting chiral BINAP (2,2'-bis(diphenylphosphino)-1,1'-binaphthalene) and hydrophilic o-H2MBA (o-mercaptobenzoic acid) as protective ligands to induce chirality and facilitate the formation of helixes. Structural analysis reveals that the chirality of the Au8c individual nanoclusters is derived from the homochiral ligands and the inherently chiral Au8 metallic kernel, which was further corroborated by experimental and computational investigations. More importantly, driven by the O-H⋯O interactions between (HCO3 -)2 dimers and achiral o-HMBA- ligands, R/S-Au8c individual nanoclusters can assemble into helical superstructures in a highly ordered crystal packing. Electrospray ionization (ESI) and collision-induced dissociation (CID) mass spectrometry of Au8c confirm the hydrogen-bonded dimer of Au8c individual nanoclusters in solution, illustrating that the insertion of (HCO3 -)2 dimers plays a crucial role in the assembly of helical superstructures in the crystalline state. This work not only demonstrates an effective strategy to construct cluster-based helical superstructures at the atomic level, but also provides visual and reliable experimental evidence for understanding the formation mechanism of helical superstructures.

N-Heterocyclic carbene-based C-centered Au(I)-Ag(I) clusters with intense phosphorescence and organelle-selective translocation in cells

Photoluminescent gold clusters are functionally variable chemical modules by ligand design. Chemical modification of protective ligands and introduction of different metals into the gold clusters lead to discover unique chemical and physical properties based on their significantly perturbed electronic structures. Here we report the synthesis of carbon-centered Au(I)-Ag(I) clusters with high phosphorescence quantum yields using N-heterocyclic carbene ligands. Specifically, a heterometallic cluster [(C)(Au^I-L)₆Ag^I₂]⁴⁺, where L denotes benzimidazolylidene-based carbene ligands featuring N-pyridyl substituents, shows a significantly high phosphorescence quantum yield (Φ = 0.88). Theoretical calculations suggest that the carbene ligands accelerate the radiative decay by affecting the spin-orbit coupling, and the benzimidazolylidene ligands further suppress the non-radiative pathway. Furthermore, these clusters with carbene ligands are taken up into cells, emit phosphorescence and translocate to a particular organelle. Such well-defined, highly phosphorescent C-centered Au(I)-Ag(I) clusters will enable ligand-specific, organelle-selective phosphorescence imaging and dynamic analysis of molecular distribution and translocation pathways in cells.

Photoluminescent gold clusters have unique chemical and physical properties based on their perturbed electronic structures. Here, the authors report the synthesis of carbon-centered Au(I)-Ag(I) clusters with high phosphorescence quantum yields using N-heterocyclic carbene ligands.

Adenine oligomer directed synthesis of chiral gold nanoparticles

Precise control of morphology and optical response of 3-dimensional chiral nanoparticles remain as a significant challenge. This work demonstrates chiral gold nanoparticle synthesis using single-stranded oligonucleotide as a chiral shape modifier. The homo-oligonucleotide composed of Adenine nucleobase specifically show a distinct chirality development with a dissymmetric factor up to g ~ 0.04 at visible wavelength, whereas other nucleobases show no development of chirality. The synthesized nanoparticle shows a counter-clockwise rotation of generated chiral arms with approximately 200 nm edge length. The molecular dynamics and density functional theory simulations reveal that Adenine shows the highest enantioselective interaction with Au(321)R/S facet in terms of binding orientation and affinity. This is attributed to the formation of sequence-specific intra-strand hydrogen bonding between nucleobases. We also found that different sequence programming of Adenine-and Cytosine-based oligomers result in chiral gold nanoparticles' morphological and optical change. These results extend our understanding of the biomolecule-directed synthesis of chiral gold nanoparticles to sequence programmable deoxyribonucleic acid and provides a foundation for programmable synthesis of chiral gold nanoparticles.

Non-conventional hydrogen bonding and dispersion forces that support embedding mesitylgold into a tailored bis(amidine) framework

A bis(amidine) ligand operates as a molecular lock for two AuMes fragments. The resulting complex retains a flexible double macrocycle with two non-conventional N-H⋯C_ipso hydrogen bonds and distinct intramolecular dispersion forces. Instead of unfolding of the double-ring structure through bond rupture in solution, a conformational ring inversion is observed.

Hydrogen bonds and dispersion forces serving as molecular locks for tailored Group 11 bis(amidine) complexes

A flexible polydentate bis(amidine) ligand LH2 operates as a molecular lock for various coinage metal fragments and forms the dinuclear complexes [LH2(MCl)2], M = Cu (1), Au (2), the coordination...

Just Add the Gold: Aggregation-Induced-Emission Properties of Alkynylphosphinegold(I) Complexes Functionalized with Phenylene-Terpyridine Subunits

A series of organometallic complexes containing an alkynylphosphinegold(I) fragment and a phenylene-terpyridine moiety connected together by flexible linker have been prepared using the specially designed terpyridine ligands. The compounds were studied crystallographically to reveal that all of them contain a linearly coordinated Au(I) atom and a free terpyridine moiety. The different orientations of the molecules relative to each other in the solid state determine the multiple noncovalent interactions such as antiparallel ππ stacking, CH-π, and CH-Au, but no aurophilic interactions are realized. The organometallic Au(I) complexes obtained show fluorescence in the solution and dual singlet-triplet emission in the solid state. This means that their photophysical behavior is determined by both intermolecular lattice-defined interactions and Au(I) atom introduction. Density functional theory computational analysis supported the assignment of emission to intraligand electronic transitions only inside the phenylene-terpyridine part with no Au(I) involved. In addition, a study of the nature of the excited states for the "dimer" with an antiparallel orientation of the terpyridine fragment showed that this orientation leads to the generation of abstracted singlet and triplet states, lowering their energy in comparison with the monomer complex. Thus, the complexes obtained can be qualified as examples of Au(I)-containing organometallic aggregation-induced-emission luminogens.

A gold(I) 1,2,3-triazolylidene complex featuring the interaction between gold and methine hydrogen

A mesoionic N-heterocyclic carbene-gold(I) complex with a unique Au⋯H-C(methine) intramolecular hydrogen bonding interaction has been investigated in the solid state. The structure of this new neutral gold(I)-carbene was characterized by FT-IR and NMR spectroscopy, TGA, and X-ray diffraction techniques. Density functional theory (DFT) and atoms-in-molecule (AIM) analysis revealed that the gold-hydrogen bonding situation is more favored. Besides, the photophysical properties of the gold(I) complex were also investigated.

Group 11 metal complexes of the dinucleating triazole appended bisphosphine 1,4-bis(5-(diisopropylphosphaneyl)-1-phenyl-1H-1,2,3-triazol-4-yl)benzene

The synthesis of a triazole appended dinucleating bisphosphine 1,4-bis(5-(diisopropylphosphaneyl)-1-phenyl-1H-1,2,3-triazol-4-yl)benzene (2) and its coinage metal complexes are described. The dinucleating bisphosphine 2 was obtained by the temperature-controlled lithiation of 1,4-bis(1-phenyl-1H-1,2,3-triazol-4-yl)benzene (1a) and 1,4-bis(1-(2-bromophenyl)-1H-1,2,3-triazol-4-yl)benzene (1b) followed by the reaction with ⁱPr₂PCl. The reactions of 2 with copper(I) halides in 1 : 2 molar ratios yielded the [Cu(μ₂-X)]₂ dimeric complexes [{Cu(μ₂-X)}₂(PⁱPr₂N₃PhC₂)₂C₆H₄] (3, X = Cl; 4, X = Br; and 5, X = I), whereas the reaction of 2 with AgBr resulted in the formation of hetero-cubane complex [{Ag₄(μ₃-Br)₄}{(PⁱPr₂N₃PhC₂)₂C₆H₄}₂] (7). Similar reactions of 2 with AgX in 1 : 2 molar ratios yielded disilver complexes [{Ag(μ₂-X)}₂{(PⁱPr₂N₃PhC₂)₂C₆H₄}] (6, X = Cl and 8, X = I). Treatment of 2 with AgOAc in a 1 : 2 molar ratio afforded a dinuclear complex [Ag₂(μ₂-OAc)₂{(PⁱPr₂N₃PhC₂)₂(C₆H₄)}] (9) with one of the acetate ligands bridging the two metal centres in the side-on mode, whereas the other one adopting the end-on mode keeping the >CO group uncoordinated. The reaction of 2 with two equivalents of [AuCl(SMe₂)] afforded the digold complex [(AuClPⁱPr₂N₃PhC₂)₂C₆H₄] (10). The molecular structures of 2-5 and 7-10 were confirmed by single crystal X-ray analysis. Non-covalent interactions between Cu and C_arene were observed in the molecular structures of 3, 4 and 5. These weak interactions were also assessed by DFT calculations in terms of their non-covalent interaction plots (NCI) and QTAIM analyses.

Au⋅⋅⋅H-C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

Secondary ligand-metal interactions are decisive in many catalytic transformations. While arene-gold interactions have repeatedly been reported as critical structural feature in many high-performance gold catalysts, we herein report that these interactions can also be replaced by Au⋅⋅⋅H-C hydrogen bonds without suffering any reduction in catalytic performance. Systematic experimental and computational studies on a series of ylide-substituted phosphines featuring either a PPh3 (Ph YPhos) or PCy3 (Cy YPhos) moiety showed that the arene-gold interaction in the aryl-substituted compounds is efficiently compensated by the formation of Au⋅⋅⋅H-C hydrogen bonds. The strongest interaction is found with the C-H moiety next to the onium center, which due to the polarization results in remarkably strong interactions with the shortest Au⋅⋅⋅H-C hydrogen bonds reported to date. Calorimetric studies on the formation of the gold complexes further confirmed that the Ph YPhos and Cy YPhos ligands form similarly stable complexes. Consequently, both ligands showed the same catalytic performance in the hydroamination, hydrophenoxylation and hydrocarboxylation of alkynes, thus demonstrating that Au⋅⋅⋅H-C hydrogen bonds are equally suited for the generation of highly effective gold catalysts than gold-arene interactions. The generality of this observation was confirmed by a comparative study between a biaryl phosphine ligand and its cyclohexyl-substituted derivative, which again showed identical catalytic performance. These observations clearly support Au⋅⋅⋅H-C hydrogen bonds as fundamental secondary interactions in gold catalysts, thus further increasing the number of design elements that can be used for future catalyst construction.

The Elusive Au(I)···H-O Hydrogen Bond: Experimental Verification

Our long-standing interest in atypical bonding situations has recently led us to target complexes in which a metallobasic gold(I) center is hydrogen-bonded to a nearby OH functionality. Here, we report on the synthesis and characterization of two neutral gold(I) indazol-3-ylidene complexes bearing a carbinol or silanol group at the 4-position. As indicated by X-ray diffraction, ¹H NMR spectroscopy, IR spectroscopy, and extensive computational modeling, the OH group of these derivatives is engaged in a bona fide Au···H-O interaction. In addition to shedding light on an elusive bonding situation, these results also indicate that increasing the acidity of the OH functionality is not necessarily beneficial to the stability of the Au(I)···H-O interaction.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver

Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)_x L_y ]^z , have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)_x L_y ]^z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)_x L_y ]^z . This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)_x L_y ]^z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single-crystal X-ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand-protected Au/Ag clusters as new artificial elements on a nanometer scale.

Hydrogen bonding to metals as a probe for an inverted ligand field

Electron-rich, late transition metals are known to act as hydrogen-bonding (HBd) acceptors. In this regard, Pt(ii) centres in square-planar environments are particularly efficient. It is however puzzling that no convincing experimental evidence is currently available for the isoelectronic neighbour Au(iii) being involved in HBd interactions. We report now on the synthesis and characterisation of two series of isoleptic and isoelectronic (d8) compounds [(CF3)3Pt(L)]- and (CF3)3Au(L), where the L ligands are based on the quinoline frame and have been selected to favour HBd with the metal centre. Strong HBd interactions were actually found in the Pt(ii) compounds, based on structural and spectroscopic evidence, and they were further confirmed by theoretical calculations. In contrast, no evidence was obtained in the Au(iii) case. In order to find the reason underlying this general disparity, we undertook a detailed theoretical analysis of the model systems [(CF3)3Pt(py)]- and (CF3)3Au(py). This study revealed that the filled dz2 orbital is the HOMO in the case of Pt(ii), but is buried in the lower energy levels in the case of Au(iii). The sharply different electronic configurations involve ligand-field inversion on going from Pt to the next element Au. This is not a gradual but an abrupt change, which invalidates Au(iii) as a HBd-acceptor wherever ligand-field inversion occurs.

A diazene-bridged diruthenium complex with structural restraint defined by single meta-diphosphinobenzene

Creation of confined coordination spaces with controlled flexibility is of importance in mimicking enzymatic reactions. We found that a simple, non-chelating 1,3-bis(diphenylphosphino)benzene (DPPBz) assembled two Cp*Ru units to give a dinuclear complex, wherein only one DPPBz supports an open framework without metal-metal bonding. Subsequent treatment with an excess of hydrazine resulted in formal 2e^-/2H⁺ transfer from hydrazine to afford a diazene-bridged complex featuring intramolecular NHCl hydrogen bonds. In constrast, a monophosphine failed to stabilize the diazene-bridged dinuclear structure due to the lack of the enforcement of the conformation.

Unusual hydrogen implanted gold with lattice contraction at increased hydrogen content

The experimental evidence for the contraction of volume of gold implanted with hydrogen at low doses is presented. The contraction of lattice upon the addition of other elements is very rare and extraordinary in the solid-state, not only for gold but also for many other solids. To explain the underlying physics, the pure kinetic theory of absorption is not adequate and the detailed interaction of hydrogen in the lattice needs to be clarified. Our analysis points to the importance of the formation of hydride bonds in a dynamic manner and explains why these bonds become weak at higher doses, leading to the inverse process of volume expansion frequently seen in metallic hydrogen containers.

Photocatalytic C-H Thiocyanation of Corroles: Development of Near-Infrared (NIR)-Emissive Dyes

A new method of activating corrole macrocycles via an in situ generated SCN radical has been developed at very mild conditions at room temperature. This photoredox reaction resulted in the generation of tetrathiocyanatocorroles in good yields. The synthesis of tetrathiocyanatocorroles was never reported earlier. Single-crystal XRD analysis reveals that the insertion of four thiocyanate moieties at the four β-pyrrolic positions has imparted significant distortion to the corrole macrocycle. The generated tetrathiocyanatocorroles are different from the parent corroles in many ways. The photophysical properties of the newly synthesized tetrathiocyanatocorroles are dramatically altered from the parent corroles. The absorption feature of these modified corrole derivatives (both position and intensity) bears a nice similarity with the chlorophyll-a macrocycle. Thus, these newly synthesized molecules can be considered as spectroscopic model systems for chlorophyll-a pigments. The observed absorption and emission spectra of these tetrathiocyanatocorroles certainly point out that these newly developed ligand scaffolds and their various metal complexes will have immense potential as pigments in solar cells and also as NIR-emissive dyes. The observed C-H···Au weak interactions in a representative Au(III)-corrole complex point out that these complexes are capable of activating the unfunctionalized C-H groups and thus will have potential implications in C-H activation reactions.

Rare proximity enforced copper hydrogen interactions in copper(i)-chalcogenones

Homoleptic tetra-coordinated copper(i)-chalcogenone complexes have been reported with rare proximity-enforced intramolecular Cu⋯H–C(sp³) hydrogen bonding interactions.

Keep it tight: a crucial role of bridging phosphine ligands in the design and optical properties of multinuclear coinage metal complexes

Copper subgroup metal ions in the +1 oxidation state are classical candidates for aggregation via non-covalent metal-metal interactions, which are supported by a number of bridging ligands. The bridging phosphines, soft donors with a relatively labile coordination to coinage metals, serve as convenient and essential components of the ligand environment that allow for efficient self-assembly of discrete polynuclear aggregates. Simultaneously, accessible and rich modification of the organic spacer of such P-donors has been used to generate many fascinating structures with attractive photoluminescent behavior. In this work we consider the development of di- and polynuclear complexes of M(i) (M = Cu, Ag, Au) and their photophysical properties, focusing on the effect of phosphine bridging ligands, their flexibility and denticity.

From understanding the roles of tetraoctylammonium bromide in the two-phase Brust-Schiffrin method to tuning the size of gold nanoclusters

The two-phase Brust-Schiffrin (B-S) method has been widely used for synthesizing small-sized Au nanoparticles (NPs) of size 2-6 nm, as well as Au nanoclusters (NCs) of size <2 nm. However, size tuning of Au NCs at the atomic level by this method is challenging probably due to a lack of in-depth understanding of its mechanism. Herein, we report the identification of two roles of tetraoctylammonium bromide (TOAB) in the two-phase B-S method: TOAB not only transfers Au(iii) precursors but also transfers the reducing agent NaBH₄ from the aqueous to the organic phase. On this basis, we developed a novel two-phase synthetic strategy by decoupling the roles of the TOAB: (1) using the hydrophobic selenolate ligand to transfer Au(iii) precursors from the aqueous to the organic phase via the formation of selenolate-Au(i) complexes and (2) deploying a small amount of TOAB as "shuttles" to transfer NaBH₄ into the organic phase for controlled reduction of selenolate-Au(i) complexes in organic phase. Using this strategy, size tuning of Au NCs at the atomic level could be achieved by simply varying the amount of TOAB. The high yields of Au NCs (≥76%) together with the short synthetic time (≤3 h) and size-tuning capability further illustrate the attractiveness of this synthetic strategy. These advantages also present the classical B-S method with greater strength and flexibility towards NC synthesis.

Terahertz Raman Spectroscopy of Ligand-Protected Au8 Clusters

For ligand-protected gold clusters, geometrical differences of gold cores and/or the presence of secondary gold core-ligand interactions influence their unique optical and electronic properties and can, in principle, be detected by spectral changes of gold core vibrations (phonon modes) in ultralow-frequency Raman spectroscopy. We report experimental and theoretical Raman spectra of Au₈ clusters protected by phosphine ligands particularly in the "gold cluster fingerprint" region from 50 to 150 cm^-1 Raman shift (1.5 to 4.5 terahertz, THz). A characteristic core breathing mode observed at ca. 123 cm^-1 was sensitive to differences of core geometries. A new band was found at ca. 150 cm^-1, originating from a local strain on a polyhedral gold core caused by weak Au···π interactions. THz Raman spectroscopy can be utilized for metal nanoclusters to visualize core structural changes and Au···π interactions, which cannot be captured by single crystal X-ray analysis.

One-, Two-, and Three-Dimensional Self-Assembly of Atomically Precise Metal Nanoclusters

Metal nanoclusters (NCs), which consist of several, to about one hundred, metal atoms, have attracted much attention as functional nanomaterials for use in nanotechnology. Because of their fine particle size, metal NCs exhibit physical/chemical properties and functions different from those of the corresponding bulk metal. In recent years, many techniques to precisely synthesize metal NCs have been developed. However, to apply these metal NCs in devices and as next-generation materials, it is necessary to assemble metal NCs to a size that is easy to handle. Recently, multiple techniques have been developed to form one-, two-, and three-dimensional connected structures (CSs) of metal NCs through self-assembly. Further progress of these techniques will promote the development of nanomaterials that take advantage of the characteristics of metal NCs. This review summarizes previous research on the CSs of metal NCs. We hope that this review will allow readers to obtain a general understanding of the formation and functions of CSs and that the obtained knowledge will help to establish clear design guidelines for fabricating new CSs with desired functions in the future.

Ligand exchange on Au38(SR)24: substituent site effects of aromatic thiols

Understanding the critical roles of ligands (e.g. thiolates, SR) in the formation of metal nanoclusters of specific sizes has long been an intriguing task since the report of ligand exchange-induced transformation of Au38(SR)24 into Au36(SR')24. Herein, we conduct a systematic study of ligand exchange on Au38(SC2H4Ph)24 with 21 incoming thiols and reveal that the size/structure preference is dependent on the substituent site. Specifically, ortho-substituted benzenethiols preserve the structure of Au38(SR)24, while para- or non-substituted benzenethiols cause its transformation into Au36(SR)24. Strong electron-donating or -withdrawing groups do not make a difference, but they will inhibit full ligand exchange. Moreover, the crystal structure of Au38(SR)24 (SR = 2,4-dimethylbenzenethiolate) exhibits distinctive ππ stacking and "anagostic" interactions (indicated by substantially short AuH distances). Theoretical calculations reveal the increased energies of frontier orbitals for aromatic ligand-protected Au38, indicating decreased electronic stability. However, this adverse effect could be compensated for by the AuH-C interactions, which improve the geometric stability when ortho-substituted benzenethiols are used. Overall, this work reveals the substituent site effects based on the Au38 model, and highlights the long-neglected "anagostic" interactions on the surface of Au-SR NCs which improve the structural stability.

C-HAu interactions and optical properties of [(P,P)4Au6]2+ molecular gold nanoclusters

Two hexagold diphosphine-stabilized [(P,P)4Au6]2+ molecular nanoclusters with the same [core + exo] arrangement differing in the linker (phenylene or trimethylene) connecting the two P-donor sites have been subjected to theoretical studies with the aim of shedding light on two main questions. On one hand, from a previous study [J. Vícha, C. Foroutan-Nejadand M. Straka,Nat. Commun., 2019, 10, 1643], it is still unclear whether short C-H2Au contacts revealed in the corresponding crystal structures are just forced by bulky P(Ph)2 groups and, on the other hand, to what extent the linker affects the visible band position [M. A. Bakar, M. Sugiuchi, M. Iwasaki, Y. Shichibuand K. Konishi, Nat. Commun., 2017, 8, 576]. Here, it is demonstrated that even in simpler model systems in which bulky groups were replaced by PH2 groups, C-H2Au hydrogen bonding interactions are retained and show comparable values, as measured by NBO and QTAIM analyses, to those of 1 and 2. These analyses further confirmed a stronger HB in 1 than in 2. Also, the comparison of model systems without and with a linker connecting the phosphine groups showed a bathochromic shift of 47 and 60 nm, revealing the key role of the linker. The Δρ(r)EE-GS plots of 1 and 2 revealed electron density depletion in the inter-nuclear C-H2 region upon electronic transition unveiling its contribution to their optical properties.