Boon K. Teo

Crystallographic Snapshots of the Interplay between Reactive Guest and Host Molecules in a Porous Coordination Polymer: Stereochemical Coupling and Feedback Mechanism of Three Photoactive Centers Triggered by UV-Induced Isomerization, Dimerization, and Polymerization Reactions

We carried out photopolymerization by [2 + 2] dimerization of a photoreactive guest molecule in the channels of a photoreactive porous coordination polymer. The photoreactions of the guest and two host ligands were monitored by single-crystal X-ray crystallography, providing snapshots of the interplay between the reactive centers. By correlating the structures of these three photocenters, a strong synergism was discovered among three reaction (quasi)equilibria and three types of photochemical reactions (isomerization, dimerization, and polymerization). This result indicates a strong coupling and feedback mechanism among the photocenters moderated by the coordination backbone.

Bulky Surface Ligands Promote Surface Reactivities of [Ag141X12(S-Adm)40]3+ (X = Cl, Br, I) Nanoclusters: Models for Multiple-Twinned Nanoparticles

Surface ligands play important roles in controlling the size and shape of metal nanoparticles and their surface properties. In this work, we demonstrate that the use of bulky thiolate ligands, along with halides, as the surface capping agent promotes the formation of plasmonic multiple-twinned Ag nanoparticles with high surface reactivities. The title nanocluster [Ag141X12(S-Adm)40]3+ (where X = Cl, Br, I; S-Adm = 1-adamantanethiolate) has a multiple-shell structure with an Ag71 core protected by a shell of Ag70X12(S-Adm)40. The Ag71 core can be considered as 20 frequency-two Ag10 tetrahedra fused together with a dislocation that resembles multiple-twinning in nanoparticles. The nanocluster has a strong plasmonic absorption band at 460 nm. Because of the bulkiness of S-Adm, the nanocluster has a low surface thiolate coverage and thus unusually high surface reactivities toward exchange reactions with different ligands, including halides, phenylacetylene and thiols. The cluster can be made water-soluble by metathesis with water-soluble thiols, thereby creating new functionalities for potential bioapplications.

Site Preference in Multimetallic Nanoclusters: Incorporation of Alkali Metal Ions or Copper Atoms into the Alkynyl-Protected Body-Centered Cubic Cluster [Au7Ag8(C≡CtBu)12]+

The synthesis, structure, substitution chemistry, and optical properties of the gold-centered cubic monocationic cluster [Au@Ag8 @Au6 (C≡Ct Bu)12 ]+ are reported. The metal framework of this cluster can be described as a fragment of a body-centered cubic (bcc) lattice with the silver and gold atoms occupying the vertices and the body center of the cube, respectively. The incorporation of alkali metal atoms gave rise to [Mn Ag8-n Au7 (C≡Ct Bu)12 ]+ clusters (n=1 for M=Na, K, Rb, Cs and n=2 for M=K, Rb), with the alkali metal ion(s) presumably occupying the vertex site(s), whereas the incorporation of copper atoms produced [Cun Ag8 Au7-n (C≡Ct Bu)12 ]+ clusters (n=1-6), with the Cu atom(s) presumably occupying the capping site(s). The parent cluster exhibited strong emission in the near-IR region (λmax =818 nm) with a quantum yield of 2 % upon excitation at λ=482 nm. Its photoluminescence was quenched upon substitution with a Na+ ion. DFT calculations confirmed the superatom characteristics of the title compound and the sodium-substituted derivatives.

From Racemic Metal Nanoparticles to Optically Pure Enantiomers in One Pot

A general strategy, using mixed ligands, is utilized to synthesize atomically precise, intrinsically chiral nanocluster [Ag78(DPPP)6(SR)42] (Ag78) where DPPP is the achiral 1,3-bis(diphenyphosphino)propane and SR = SPhCF3. Ag78 crystallizes as racemates in a centric space group. Using chiral diphosphines BDPP = 2,4-bis(diphenylphosphino)pentane, the enantiomeric pair [Ag78(R/S-BDPP)6(SR)42] can be prepared with 100% optical purity. The chiral diphosphines gives rise to, separately, two asymmetric surface coordination motifs composed of tetrahedral R3PAg(SR)3 moieties. The flexible nature of C-C-C angles between the two phosphorus atoms restricts the relative orientation of the tetrahedral R3PAg(SR)3 moieties, thereby resulting in the enantiomeric selection of the intrinsic chiral metal core. This proof-of-concept strategy raises the prospect of enantioselectively synthesizing optically pure, atomically precise chiral noble metal nanoclusters for specific applications.

From Symmetry Breaking to Unraveling the Origin of the Chirality of Ligated Au13Cu2 Nanoclusters

A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au13 Cu2 , into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au13 Cu2 (DPPP)3 (SPy)6 ]+ was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au13 Cu2 ((2r,4r)/(2s,4s)-BDPP)3 (SPy)6 ]+ was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images in the range of 250-500 nm with maximum anisotropy factors of 1.2×10-3 . DFT calculations provided good correlations with the observed CD spectra of the enantiomers and, more importantly, revealed the origin of the chirality. Racemization studies show high stability (no racemization at 70 °C) of these chiral nanoclusters, which hold great promise in applications such as asymmetry catalysis.

Co-crystallization of atomically precise metal nanoparticles driven by magic atomic and electronic shells

This paper reports co-crystallization of two atomically precise, different-size ligand-stabilized nanoclusters, a spherical (AuAg)267(SR)80 and a smaller trigonal-prismatic (AuAg)45(SR)27(PPh3)6 in 1:1 ratio, characterized fully by X-ray crystallographic analysis (SR = 2,4-SPhMe2). The larger cluster has a four concentric-shell icosahedral structure of Ag@M12@M42@M92@Ag120(SR)80 (M = Au or Ag) with the inner-core M147 icosahedron observed here for metal nanoparticles. The cluster has an open electron shell of 187 delocalized electrons, fully metallic, plasmonic behavior, and a zero HOMO-LUMO energy gap. The smaller cluster has an 18-electron shell closing, a notable HOMO-LUMO energy gap and a molecule-like optical spectrum. This is the first direct demonstration of the simultaneous presence of competing effects (closing of atom vs. electron shells) in nanocluster synthesis and growth, working together to form a co-crystal of different-sized clusters. This observation suggests a strategy that may be helpful in the design of other nanocluster systems via co-crystallization.Atomic shell closure and electronic shell closure are generally considered to be competing effects in stabilizing magic-sized metal nanoclusters. Here, the authors show, by co-crystallizing two differently-sized clusters, that both mechanisms can work concurrently during magic cluster synthesis.