A balanced procedure for the treatment of cluster-ligand interactions on gold phosphine systems in catalysis

Synthesis and Stability of Mixed-Diphosphine Ligated Gold Clusters

Sub-nanometer gold clusters are promising size- and composition-tunable materials that may be used for advanced technological applications such as catalysis, energy generation, and microelectronics. Synthesis and characterization of phosphine ligated gold clusters containing different ligands provide insight into how steric and electronic effects resulting from changes in chemical functionality influence cluster size, stability, and formation in solution. Herein, we demonstrate that synthesizing gold clusters using two different diphosphines in solution at the same time results in a broad distribution of novel mixed-ligand clusters. In comparison, adding a second diphosphine to a solution of gold clusters presynthesized with another diphosphine does not result in extensive formation of mixed-ligand species. Utilizing high-mass resolution electrospray ionization mass spectrometry, we determined novel cluster compositions and observed size-dependent trends in gold clusters that undergo ligand exchange forming mixed diphosphine species. Adjacent peaks in the mass spectra, separated by characteristic mass-to-charge ratios, provide evidence for multiple 1,3-bis(diphenylphosphino)propane (L³) and 1,5-bis(diphenylphosphino)pentane (L⁵) ligands on cationic clusters containing 8, 10, 11, and 22 gold atoms. Energy-resolved collision-induced dissociation experiments provide qualitative insight into how different diphosphine ligands affect the relative stability of specific size gold clusters. Our results indicate that mixed-ligand clusters containing both L³ and L⁵ are generally more stable than their single ligand counterparts containing either L³ or L⁵. These molecular-level insights will facilitate the rational and scalable synthesis of gold clusters for targeted applications.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Unraveling the Nucleation Process from a Au(I)-SR Complex to Transition-Size Nanoclusters

Atomically precise noble metal nanoclusters provide a critical benchmark for the fundamental research of the origin of condensed matter because they retain the original state of the metal bonds. Also, knowledge about the transition from organometallic complexes to a nanoclusters is important for understanding the structural evolution of the nanoclusters, particularly their nucleation mechanism. Herein, three transition-size gold nanoclusters are prepared via a controlled diphosphine-mediated top-down routine. Starting from small-size nanoclusters, three new nanoclusters including Au₁₃(SAdm)₈(L⁴)₂(BPh₄) (Au₁₃), Au₁₄(S-c-C₆H₁₁)₁₀L⁴ (Au₁₄), and Au₁₆(S-c-C₆H₁₁)₁₁L^Ph* (Au₁₆) are obtained by controlled clipping on the surface and kernel of initial nanoclusters. Combining their atomically precise structures with DFT theoretical calculations, the overall atom-by-atom structural evolution process from Au₁₂(SR)₁₂ (0 e^-) to Au₁₈(SR)₁₄ (4 e^-) is mapped out. In addition, studies on their electronic structures show that the evolution from an organometallic complex to nanoclusters is accompanied by a dramatic decrease in the HOMO-LUMO gaps. Most importantly, the formation of the first Au-Au bond is captured in the "Au₄S₄ to Au₅" nucleation process from Au₁₂(SR)₁₂ complex to the Au₁₃ nanocluster. This work provides a deep insight into the origin of inner core in Au NCs and their structural transition relationship with metal complexes.

DFT Functionals for Modeling of Polyethylene Chains Cross-Linked by Metal Atoms. DLPNO-CCSD(T) Benchmark Calculations

Density functional theory (DFT) functionals for calculations of binding energies (BEs) of the polyethylene (PE) chains cross-linked by selected metal atoms (M) are benchmarked against DLPNO-CCSD(T) and DLPNO-CCSD(T1) data. PEX-M-PEX complexes as compared with plain parallel PEX···PEX chains with X = 3-9 carbon atoms are model species characterized by a cooperative effect of covalent C-M-C bonds and interchain dispersion interactions. The accuracy of DLPNO-CC methods was assessed by a comparison of BEs with the canonical CCSD(T) results for small PE3-M-PE3 complexes. Functionals for PEX···PEX and closed-shell PEX-M-PEX complexes (M = Be, Mg, Zn) were benchmarked against DLPNO-CCSD(T) BEs; open-shell complexes (M = Li, Ag, Au) were benchmarked against the DLPNO-CCSD(T1) method with iterative triples. Three dispersion corrections were combined with 25 DFT functionals for calculations of BEs with respect to PEX-M and PEX fragments employing def2-TZVPP and def2-QZVPP basis sets. Accuracy to within 5% for the closed-shell PEX-M-PEX complexes was achieved with five functionals. Less accurate are functionals for the open-shell PEX-M-PEX complexes; only two functionals deviate by less than 15% from DLPNO-CCSD(T1). Particularly problematic were PEX-Li-PEX complexes. A reasonable overall performance across all complexes in terms of the mean absolute percentage error is found for the range-separated hybrid functionals ωB97X-D3 and CAM-B3LYP/D3(BJ)-ABC.

Computing gold cluster energies with density functional theory: the importance of correlation

Energies calculated with density functional theory depend critically on the choice of the exchange-correlation functional. In this work, we use measured dissociation energies of Aun+ (n = 5-17) clusters as benchmark data to test two very different functionals for calculating total energies in these clusters; the simpler (and fast) PBE and the evolved (and expensive) B2PLYP double-hybrid functionals. PBE consistently gives poor agreement with the experimental results. In contrast, the B2PLYP functional, which implicitly includes electron correlation by performing a perturbative second-order correction, significantly improves the agreement of the calculations, at the cost of much more demanding computations. The better performance of the double-hybrid functional is ascribed to the longer range of the interatomic potential.

Uncertainty of exchange-correlation functionals in density functional theory calculations for lithium-based solid electrolytes on the case study of lithium phosphorus oxynitride

Amorphous lithium phosphorus oxynitride (LIPON) has emerged as a promising solid electrolyte for all-solid-state thin-film lithium batteries. In this context, the use of theoretical modeling to characterize, understand, or screen material properties is becoming increasingly important to complement experimental analysis or elucidate features at atomistic level that are difficult to obtain through experimental studies. Density functional theory (DFT) is the method of choice for quantum mechanical material modeling at the atomistic scale. The current state of the art represents DFT values, such as the formation or migration energies relevant for bulk phase of materials, as absolute numbers. Estimating the accuracy or fluctuation range of the different density functionals is challenging. In order to investigate the thermodynamic and kinetic properties of LIPON by DFT, an approach to describe the fluctuation range caused by the choice of the exchange-correlation (XC) functional is developed. Three different model systems were chosen to characterize various structural features of amorphous LIPON, which are distinguished by the cross-linking of the PO_u N_4-u -structural units. The uncertainty Ũ is introduced as a parameter describing the fluctuation range of energy values. The uncertainty approach does not determine the accuracy of DFT results, but rather a fluctuation range in the DFT results without the need for a reference value from a higher level of theory or experiment. The uncertainty was determined for both the thermodynamic Li-vacancy formation energies and the kinetic Li-vacancy migration energies in LIPON. We assume that the uncertainty approach can be applied to different material systems with different density functionals.

Effect of Charge and Phosphine Ligands on the Electronic Structure of the Au8 Cluster

In this work, we use density functional theory calculations with a hybrid exchange-correlation functional and effective core pseudopotentials to determine the geometry of bare and phosphine-protected Au₈ nanoclusters and characterize their electronic structure. Au₈ clusters were bonded to four and eight PH₃ ligands in order to evaluate the effect of ligand concentration on the electronic structure, while different positional configurations were also tried for four ligands attached to the cluster. We show that the neutral clusters become more nucleophilic as the ligands bind to the clusters at stable sites. The ground-state planar configuration of Au₈ is maintained depending on the concentration and position of ligands. The effect of ionizing to the +2 charge state results in disruption of planar geometry in some cases because of inoccupation of a molecular orbital with the Au-Au bonding character. Natural bond order charge analyses showed that Au atoms oxidize upon ionization, instead of phosphine. The net positive charge makes the clusters more electrophilic with a capacity to absorb electrons from nucleophiles depending on the concentration and position of ligands and on the concentration of low-coordinated gold atoms. Besides, ionization energies and electron affinities were calculated through different mechanisms, finding that both variables are much higher for charged systems and change inversely with the concentration of ligands.

Role of sterics in phosphine-ligated gold clusters

This study examined the solution-phase exchange reactions of triphenylphosphine (PPh₃) ligands on Au₈L₇²⁺ (L = PPh₃) gold clusters with three different tolyl ligands using electrospray ionization mass spectrometry to provide insight into how steric differences in the phosphines influence the extent of ligand exchange and the stability of the resulting mixed-phosphine clusters.

Observing the real time formation of phosphine-ligated gold clusters by electrospray ionization mass spectrometry

Real-time monitoring of the gold cluster synthesis by electrospray ionization mass spectrometry reveals distinct formation pathways for Au₈, Au₉ and Au₁₀ clusters.

Aun (n = 1,11) Clusters Interacting With Lone-Pair Ligands

We analyze the pattern of binding energies (BEs) of small Aun clusters (n = 1-7, 11) with lone-pair ligands (L = H2O, SH2, NH3, PH3, PF3, PCl3, and PMe3) employing the density functional theory. We use PBE0 functional with the dispersion correction and scalar relativistic effective core potential. This approach provides correct BEs when compared with benchmark CCSD(T) calculations for Au-L and Au2-L complexes. The pattern of BEs of Aun-L complexes is irregular with BE for Au3 ≈ Au4 > Au2 > Au7 > Au5 > Au11 > Au6 > Au1. Electron affinities (EAs) of Aun clusters exhibit oscillatory pattern with the cluster size. Binding energies of Aun-L complexes are oscillatory as well following EAs of Aun clusters. BEs of odd and even Aun-L complexes were analyzed separately. The bonding mechanism in odd Aun-L complexes is dominated by the lone pair → metal electron donation to the singly occupied valence Aun orbital accompanied by the back-donation. Even Aun clusters create covalent Aun-L bonds with BEs higher than those in odd Aun-L complexes. The BEs pattern and optimized geometries of Aun-L complexes correspond to the picture of creating the gold-ligand bond through the lone pair of a ligand interacting with the singly occupied molecular orbital in odd clusters or lowest unoccupied molecular orbital in even clusters of Aun. Ligands in both odd and even Aun-L complexes form three groups with binding energies that correlate with their ionization energies. The lowest BE is calculated for H2O as a ligand, followed by SH2 and NH3. PX3 ligands exhibit highest BEs.

Understanding ligand effects in gold clusters using mass spectrometry

This review summarizes recent research on the influence of phosphine ligands on the size, stability, and reactivity of gold clusters synthesized in solution. Sub-nanometer clusters exhibit size- and composition-dependent properties that are unique from those of larger nanoparticles. The highly tunable properties of clusters and their high surface-to-volume ratio make them promising candidates for a variety of technological applications. However, because "each-atom-counts" toward defining cluster properties it is critically important to develop robust synthesis methods to efficiently prepare clusters of predetermined size. For decades phosphines have been known to direct the size-selected synthesis of gold clusters. Despite the preparation of numerous species it is still not understood how different functional groups at phosphine centers affect the size and properties of gold clusters. Using electrospray ionization mass spectrometry (ESI-MS) it is possible to characterize the effect of ligand substitution on the distribution of clusters formed in solution at defined reaction conditions. In addition, ligand exchange reactions on preformed clusters may be monitored using ESI-MS. Collision induced dissociation (CID) may also be employed to obtain qualitative insight into the fragmentation of mixed ligand clusters and the relative binding energies of differently substituted phosphines. Quantitative ligand binding energies and cluster stability may be determined employing surface induced dissociation (SID) in a custom-built Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Rice-Ramsperger-Kassel-Marcus (RRKM) based modeling of the SID data allows dissociation energies and entropy values to be extracted. The charge reduction and reactivity of atomically precise gold clusters, including partially ligated species generated in the gas-phase by in source CID, on well-defined surfaces may be explored using ion soft landing (SL) in a custom-built instrument combined with in situ time of flight secondary ion mass spectrometry (TOF-SIMS). Jointly, this multipronged experimental approach allows characterization of the full spectrum of relevant phenomena including cluster synthesis, ligand exchange, thermochemistry, surface immobilization, and reactivity. The fundamental insights obtained from this work will facilitate the directed synthesis of gold clusters with predetermined size and properties for specific applications.

Water-Soluble Phosphine-Protected Au₁₁ Clusters: Synthesis, Electronic Structure, and Chiral Phase Transfer in a Synergistic Fashion

Synthesis of atomically precise, water-soluble phosphine-protected gold clusters is still currently limited probably due to a stability issue. We here present the synthesis, magic-number isolation, and exploration of the electronic structures as well as the asymmetric conversion of triphenylphosphine monosulfonate (TPPS)-protected gold clusters. Electrospray ionization mass spectrometry and elemental analysis result in the primary formation of Au11(TPPS)9Cl undecagold cluster compound. Magnetic circular dichroism (MCD) spectroscopy clarifies that extremely weak transitions are present in the low-energy region unresolved in the UV-vis absorption, which can be due to the Faraday B-terms based on the magnetically allowed transitions in the cluster. Asymmetric conversion without changing the nuclearity is remarkable by the chiral phase transfer in a synergistic fashion, which yields a rather small anisotropy factor (g-factor) of at most (2.5-7.0) × 10(-5). Quantum chemical calculations for model undecagold cluster compounds are then used to evaluate the optical and chiroptical responses induced by the chiral phase transfer. On this basis, we find that the Au core distortion is ignorable, and the chiral ion-pairing causes a slight increase in the CD response of the Au11 cluster.

Phosphine passivated gold clusters: how charge transfer affects electronic structure and stability

First principle calculations of small charged phosphine ligand-protected gold clusters have been performed in order to understand the major factors determining stability, including its size, shape, and charge dependence.

Cationic gold clusters ligated with differently substituted phosphines: effect of substitution on ligand reactivity and binding

Loss of substituted phosphine ligands is strongly correlated with the electron donating ability of the phosphorous lone pair. The results indicate that the relative ligand binding energies increase in the order PMe₃ < PPhMe₂ < PPh₂Me < PPh₃ < PPh₂Cy < PPhCy₂ < PCy₃.

Nucleobases tagged to gold nanoclusters cause a mechanistic crossover in the oxidation of CO

A mechanistic crossover is observed upon using nucleobase tagged gold clusters as catalysts favoring the Eley–Rideal mechanism, over the conventional Langmuir–Hinshelwood pathway followed using pristine gold clusters during CO oxidation.