Shinjiro Takano

Light-induced spin-crossover magnet

The light-induced phase transition between the low-spin (LS) and high-spin (HS) states of some transition-metal ions has been extensively studied in the fields of chemistry and materials science. In a crystalline extended system, magnetically ordering the HS sites of such transition-metal ions by irradiation should lead to spontaneous magnetization. Previous examples of light-induced ordering have typically occurred by means of an intermetallic charge transfer mechanism, inducing a change of valence of the metal centres. Here, we describe the long-range magnetic ordering of the extended Fe(II)(HS) sites in a metal-organic framework caused instead by a light-induced excited spin-state trapping effect. The Fe-Nb-based material behaves as a spin-crossover magnet, in which a strong superexchange interaction (magnetic coupling through non-magnetic elements) between photo-produced Fe(II)(HS) and neighbouring Nb(IV) atoms operates through CN bridges. The magnetic phase transition is observed at 20 K with a coercive field of 240 Oe.

A Critical Size for Emergence of Nonbulk Electronic and Geometric Structures in Dodecanethiolate-Protected Au Clusters

We report on how the transition from the bulk structure to the cluster-specific structure occurs in n-dodecanethiolate-protected gold clusters, Au(n)(SC12)m. To elucidate this transition, we isolated a series of Au(n)(SC12)m in the n range from 38 to ∼520, containing five newly identified or newly isolated clusters, Au104(SC12)45, Au(∼226)(SC12)(∼76), Au(∼253)(SC12)(∼90), Au(∼356)(SC12)(∼112), and Au(∼520)(SC12)(∼130), using reverse-phase high-performance liquid chromatography. Low-temperature optical absorption spectroscopy, powder X-ray diffractometry, and density functional theory (DFT) calculations revealed that the Au cores of Au144(SC12)60 and smaller clusters have molecular-like electronic structures and non-fcc geometric structures, whereas the structures of the Au cores of larger clusters resemble those of the bulk gold. A new structure model is proposed for Au104(SC12)45 based on combined approach between experiments and DFT calculations.

Binding motif of terminal alkynes on gold clusters.

Gold clusters protected by terminal alkynes (1-octyne (OC-H), phenylacetylene (PA-H) and 9-ethynyl-phenanthrene (EPT-H)) were prepared by the ligand exchange of small (diameter <2 nm) Au clusters stabilized by polyvinylpyrrolidone. The bonding motif of these alkynes on Au clusters was investigated using various spectroscopic methods. FTIR and Raman spectroscopy revealed that terminal hydrogen is lost during the ligand exchange and that the C≡C bond of the alkynyl group is weakened upon attachment to the Au clusters. Acidification of the water phase after the ligand exchange indicated that the ligation of alkynyl groups to the Au clusters proceeds via deprotonation of the alkynes. A series of precisely defined Au clusters, Au34(PA)16, Au54(PA)26, Au30(EPT)13, Au35(EPT)18, and Au(41-43)(EPT)(21-23), were synthesized and characterized in detail to obtain further insight into the interfacial structures. Careful mass analysis confirmed the ligation of the alkynes in the dehydrogenated form. An upright configuration of the alkynes on Au clusters was suggested from the Au to alkyne ratios and photoluminescence from the excimer of the EPT ligands. EXAFS analysis implied that the alkynyl carbon is bound to bridged or hollow sites on the cluster surface.

Hierarchy of bond stiffnesses within icosahedral-based gold clusters protected by thiolates

Unique thermal properties of metal clusters are believed to originate from the hierarchy of the bonding. However, an atomic-level understanding of how the bond stiffnesses are affected by the atomic packing of a metal cluster and the interfacial structure with the surrounding environment has not been attained to date. Here we elucidate the hierarchy in the bond stiffness in thiolate-protected, icosahedral-based gold clusters Au25(SC2H4Ph)18, Au38(SC2H4Ph)24 and Au144(SC2H4Ph)60 by analysing Au L3-edge extended X-ray absorption fine structure data. The Au–Au bonds have different stiffnesses depending on their lengths. The long Au–Au bonds, which are more flexible than those in the bulk metal, are located at the icosahedral-based gold core surface. The short Au–Au bonds, which are stiffer than those in the bulk metal, are mainly distributed along the radial direction and form a cyclic structural backbone with the rigid Au–SR oligomers.

Slow-Reduction Synthesis of a Thiolate-Protected One-Dimensional Gold Cluster Showing an Intense Near-Infrared Absorption

Slow reduction of Au ions in the presence of 4-(2-mercaptoethyl)benzoic acid (4-MEBA) gave Au76(4-MEBA)44 clusters that exhibited a strong (3 × 10(5) M(-1) cm(-1)) near-infrared absorption band at 1340 nm. Powder X-ray diffraction studies indicated that the Au core has a one-dimensional fcc structure that is elongated along the {100} direction.

Controlled Synthesis of Carbon‐Supported Gold Clusters for Rational Catalyst Design

The development of novel catalysts based on metal clusters requires a rational design principle as well as atomically precise synthetic methods. Toward this goal, we have developed a method to precisely and independently control the size, composition, and surface modification of heterogeneous gold clusters by calcination of the ligand-protected Au clusters on carbon supports. We studied the effects of these structural parameters using benzyl alcohol oxidation as a test reaction. Unexpectedly, Au144 and Au∼330 on hierarchically porous carbon exhibited significantly higher turnover frequency than Au25 and Au38 . This size dependence is ascribed to the difference in the geometric structures of the Au clusters; Au144 and Au∼330 have an icosahedral-based structure whereas Au25 and Au38 have a face-centered cubic structure. Doping of a single Pd atom into Au25 supported on carbon nanotubes remarkably enhanced the catalytic activity. The doping effect is explained in terms of the accelerated formation of the carbocation intermediate due to electron transfer from Pd to Au, since the doped Pd is buried within the Au clusters and is located at the interface between the supports. Residual thiolates on Au25 affected both the activity and selectivity; selective oxidation to benzaldehyde was achieved at optimized coverage. Non-formation of benzoic acid is due to the suppression of oxidation activity by electron withdrawal by thiolates and non-formation of benzyl benzoate is due to the site-isolation effect by thiolates. These results will provide useful information for the rational design of gold-cluster-based catalysts with desired performance.

Controlled Synthesis: Size Control

Abstract This chapter focuses on the state-of-the-art methods of atomically precise synthesis of Au and Ag clusters whose surfaces are protected by monolayers of organic ligands such as thiols, phosphines, and terminal alkynes. Chemical compositions of ligand-protected Au and Ag clusters that had been isolated by the end of 2014 are surveyed. High stability of the isolated Au and Ag clusters is explained in terms of geometrical and electronic structures. Manifestation of molecular-like optical properties and non-face-centered cubic atomic packing below a critical size is demonstrated by taking alkanethiolate-protected Au clusters as an example.

Hydride-Doped Gold Superatom (Au9H)2+: Synthesis, Structure, and Transformation

Doping of a hydride (H-) into an oblate-shaped gold cluster [Au9(PPh3)8]3+ was observed for the first time by mass spectrometry and NMR spectroscopy. Density functional theory calculations for the product [Au9H(PPh3)8]2+ demonstrated that the (Au9H)2+ core can be viewed as a nearly spherical superatom with a closed electronic shell. The hydride-doped superatom (Au9H)2+ was successfully converted to the well-known superatom Au113+, providing a new atomically precise synthesis of Au clusters via a bottom-up approach.

A gold superatom with 10 electrons in Au13(PPh3)8(p-SC6H4CO2H)3

The title compound Au13(PPh3)8(p-MBA)3 (1) was synthesized by chemical reduction of the neutral complex Au(PPh3)(p-MBA)(p-MBA = p-SC6H4CO2H). Single-crystal X-ray diffraction analysis of 1 showed that the Au11 core is protected by seven PPh3 ligands and an Au2(p-MBA)3(PPh3)1 assembled ligand. Optical spectroscopy indicated that the electronic structure of the Au11 core of 1 is significantly different from that of the conventional Au11 superatom with an electron configuration of (1S)2(1P)6. Density-functional theory calculations demonstrated that the Au11 core can be viewed as a non-rare-gas-like superatom with an electron configuration of (1S)2(1P)6(1D)2.

Hydride-Mediated Controlled Growth of a Bimetallic (Pd@Au8)2+ Superatom to a Hydride-Doped (HPd@Au10)3+ Superatom

A hydride (H-)-doped bimetallic superatom (HPdAu8)+ was produced by reacting BH4- with an oblate (PdAu8)2+ superatom protected by PPh3. The H atom in (HPdAu8)+ survived during the sequential addition of Au(I)Cl to form an (HPdAu10)3+ superatom, in sharp contrast to the proton release from a H--doped pure gold superatom (HAu9)2+ in the growth process to (Au11)3+. Single-crystal X-ray diffraction analysis and density functional theory calculations on (HPdAu10)3+ showed that the interstitially doped H atom induced a notable deformation of the core.