Modular Cocrystallization of Customized Carboranylthiolate-Protected Copper Nanoclusters via Host-Guest Interactions

Carborane Meets Metal Nanocluster: New Opportunities in Nanomaterials

ConspectusMetal nanoclusters, distinguished by their atom-precise structures and quantum size effect, are regarded as a crucial bridge between organometallic complexes and plasmonic metal nanoparticles. These nanoclusters are primarily composed of a metallic core enveloped by protective ligands, wherein the ligands play a vital role in determining the nanoclusters' synthesis, structural integrity, and physicochemical properties. Considerable efforts in ligand engineering have concentrated on exploring novel coordinating functional groups to advance nanocluster research, particularly in the precise and controlled synthesis of superatomic nanoclusters, fine-tuning their intrinsic properties, and subsequent assembly and application. However, the backbone of these ligands seems equally important but attracts less attention. It is reasonable that if the utility of the two moieties (coordinating functional group and backbone) provokes a profound synergistic effect, their contributions to the structures and properties of the resultant metal nanoclusters are extremely inestimable. In this context, carborane, with its spherical shape and three-dimensional aromaticity (electronic effect), has emerged as a promising candidate for ligand backbone design. Over the past two decades, the incorporation of carborane moieties into ligands has enabled the construction of various metal nanoclusters exhibiting distinct architectures, enhanced stability, and unique reactivity. Therefore, it is important to present the current status and challenges associated with carboranyl ligand-protected metal nanoclusters to guide their future development. This Account provides a comprehensive summary of the recent advances in carboranyl ligand-stabilized metal nanoclusters, with a primary focus on the contributions from our laboratory. We begin by discussing the unique advantages of introducing carborane-based ligands in metal nanocluster preparation, with particular emphasis on their virtues for the synthesis of superatomic nanoclusters, heterometal-doped nanoclusters, and isostructural nanoclusters. Subsequently, we summarize the carborane-based ligand engineering strategies for precise modification and hierarchical assembly of metal nanoclusters, elucidating how the incorporation of carborane facilitates the modulation of specific properties and promotes supramolecular and covalent assembly. Furthermore, we discuss the cooperativity achieved by carboranyl ligands and the metal nanocluster framework to broaden the scope of applications for these nanoclusters in versatile fields, including hypergolic fuels, a previously unexplored area. Finally, we discuss the challenges facing future research on carboranyl ligand-protected metal nanoclusters, including the incorporation of nido-carborane or metallocarborane, a fundamental understanding of structure-property relationships, and potential applications such as boron neutron capture therapy and radionuclide extraction. This Account aims to stimulate interest in the unique attributes of carborane-based ligands and their corresponding metal nanoclusters among students and researchers across diverse disciplines, including chemistry, crystal engineering, and materials science.

Significantly enhanced NIR emission of solid-state clusters based on Cu4Pt2 triggered with volatile organic compounds

Near-infrared (NIR) emitting metal clusters, recognized for their low toxicity, large Stokes shift, and exceptional photostability, hold considerable promise as stimuli-responsive luminescent materials for applications including organic vapor sensing, pollutant detection, and photoluminescent thermometers. However, their limited quantum yield (QY) for NIR emission poses a challenge, highlighting the need for developing light-up sensors with NIR emitting metal clusters to broaden the scope of applications. Herein, the carbazole-alkyne ligand-incorporated novel bimetallic cluster, Cu4Pt2(CZ-PrA)4(dppy)4(PF6)2 (CZ-Cu4Pt2, CZ-PrAH = 9-(Prop-2-yn-1-yl)-9H-carbazole; dppy = diphenyl-2-pyridylphosphine), was synthesized, which exhibits NIR emission centered at 740 nm in the solid state and shows a significant blue shift compared to the previously reported analogues. Temperature-dependent luminescence tests demonstrated an increase in emission intensity with decreasing temperature and a blue shift in the emission peak. Remarkably, its photoluminescence (PL) intensity increased significantly upon exposure to solvents like ethyl acetate, formaldehyde (HCHO), and chlorobenzene, raising the QY from an initial 16.1% to a range of 46.7%-70.3%. HCHO, in particular, boosted the emission intensity by over 200 times. The emission enhancement mechanism, elucidated through UV-vis diffuse reflectance spectroscopy, powder X-ray diffraction, femtosecond transient absorption spectroscopy, and single-crystal X-ray diffraction, reveals that weak intermolecular interactions, particularly hydrogen bonding between solvent molecules and ligands, restrict intramolecular rotations and vibrations, thus promoting radiative transitions. The CZ-Cu4Pt2 cluster shows potential applications in non-contact fluorescence thermometry and organic vapor detection.

Analogous copper nanoclusters (Cu16/17) with two electron superatomic and mixed valence copper(II)/copper(I) and copper(I)/copper(0) characters

The reported copper nanoclusters (Cu NCs) of either CuII or CuI or mixed valence (MV) CuII/CuI or CuI/Cu0 characters are found to be stabilized with a discrete set of ligand donors; hence, analogous Cu NCs with a common architecture supported by the same or nearly the same donor set that exhibit different MV states of Cu, such as CuII/CuI and CuI/Cu0, are unknown. Such a series of highest nuclearity copper clusters supported by aromatic thiol-S donor ligands, namely [(L4)12CuI15CuII(μ4-S)](PF6)3 (1), [(MeL4)12CuI15Cu0(μ4-S)]ClO4·8C7H8 (2) and [(L4)12CuI15Cu02(DMF)](PF6)3·C2H5OH·2C7H8 (3), where XL4 = 2-((3-X-thiophen)-(2-yl-methylene)amino)-4-(trifluoromethyl)benzenethiol (X = H/Me), have been synthesized and their electronic structural properties have been examined and reported herein. The Cu16 NCs, 1 and 2, feature a central sulfido-S (Ss) bridged tetracopper SsCu4 core inside a sphere-shaped Cu12S12 truncated octahedron. As 1 and 2 have a non-metal (chalcogen or halogen) central atom (here Ss) instead of a metallic Cu core inside the Cu12S12 shell, they are of the inverse coordination complex (ICC) category, rather than superatomic with a core-shell (the core is a metal and the shell is a metal-ligand framework) structure. The NC 1, in the presence of polar solvents, converts to a two electron superatomic Cu17 NC, 3. The NC 3 features a trigonal pyramidal-shaped Cu4 core inside a modified Cu12S12, i.e. Cu13S12 shell. The transformation of 1 to 3 may be visualized as the replacement of the central sulfido-S by an extra Cu atom (generated from decomposed molecules of 1) and the shifting of a Cu atom of the SsCu4 unit to the Cu12S12 shell, resulting in a Cu13S12 shell. The present work offers the first example of (i) an ICC that has Cu0 character (i.e.2), (ii) a superatomic Cu NC (i.e.3) stabilized by an aromatic thiol-S donor ligand and (iii) spontaneous ICC (i.e.1) → superatomic NC (i.e.3) conversion that does not require any reducing agent, but rather occurs in the presence of a dioxygen oxidant. The probable mechanisms for the reversible 1 ↔ 3 conversions have been discussed. The presence of Ss in 1 and 2 unveils the first evidence of benzene thiol C-S bond cleavage, to the best of our knowledge. The spectroelectrochemical studies shed light on the choice of CuII/CuI and CuI/Cu0 character of 1 and 2, respectively, which is supported by high resolution XPS and Cu LMM Auger spectroscopy.

Multicolor photoluminescence of Cu14 clusters modulated using surface ligands

Copper nanoclusters exhibit unique structural features and their molecular assembly results in diverse photoluminescence properties. In this study, we present ligand-dependent multicolor luminescence observed in a Cu14 cluster, primarily protected by ortho-carborane-9,12-dithiol (o-CBDT), featuring an octahedral Cu6 inner kernel enveloped by eight isolated copper atoms. The outer layer of the metal kernel consists of six bidentate o-CBDT ligands, in which carborane backbones are connected through μ3-sulphide linkages. The initially prepared Cu14 cluster, solely protected by six o-CBDT ligands, did not crystallize in its native form. However, in the presence of N,N-dimethylformamide (DMF), the cluster crystallized along with six DMF molecules. Single-crystal X-ray diffraction (SCXRD) revealed that the DMF molecules were directly coordinated to six of the eight capping Cu atoms, while oxygen atoms were bound to the two remaining Cu apices in antipodal positions. Efficient tailoring of the cluster surface with DMF shifted its luminescence from yellow to bright red. Luminescence decay profiles showed fluorescence emission for these clusters, originating from the singlet states. Additionally, we synthesized microcrystalline fibers with a one-dimensional assembly of DMF-appended Cu14 clusters and bidentate DPPE linkers. These fibers exhibited bright greenish-yellow phosphorescence emission, originating from the triplet state, indicating the drastic surface tailoring effect of secondary ligands. Theoretical calculations provided insights into the electronic energy levels and associated electronic transitions for these clusters. This work demonstrated dynamic tuning of the emissive excited states of copper nanoclusters through the efficient engineering of ligands.

Guest-Induced Helical Superstructure from a Gold Nanocluster-Based Supramolecular Organic Framework Enables Efficient Catalysis

Mimicking hierarchical assembly in nature to exploit atomically precise artificial systems with complex structures and versatile functions remains a long-standing challenge. Herein, we report two single-crystal supramolecular organic frameworks (MSOF-4 and MSOF-5) based on custom-designed atomically precise gold nanoclusters Au11(4-Mpy)3(PPh3)7, showing distinct and intriguing host-guest adaptation behaviors toward 1-/2-bromopropane (BPR) isomers. MSOF-4 exhibits sev topology and cylindrical channels with 4-mercaptopyridine (4-Mpy) ligands matching well with guest 1-BPR. Due to the confinement effect, solid MSOF-4 undergoes significant structural change upon selective adsorption of 1-BPR vapor over 2-BPR, resulting in strong near-infrared fluorescence. Single-crystal X-ray diffraction reveals that Au11(4-Mpy)3(PPh3)7 in MSOF-4 transforms into Au11Br3(PPh3)7 upon ligand exchange with 1-BPR, resulting in 1-BPR@MSOF-6 single crystals with a rarely reported helical assembly structure. Significantly, the double-helical structure of MSOF-6 facilitates efficient catalysis of the electron transfer (ET) reaction, resulting in a nearly 6 times increase of catalytic rates compared with MSOF-4. In sharp contrast, solid MSOF-5 possesses chb topology and cage-type channels with narrow windows, showing excellent selective physical adsorption toward 1-BPR vapor but a nonfluorescent feature upon guest adsorption. Our results demonstrate a powerful strategy for developing advanced assemblies with high-order complexity and engineering their functions in atomic precision.

Fluorescence resonance energy transfer in atomically precise metal nanoclusters by cocrystallization-induced spatial confinement

Understanding the fluorescence resonance energy transfer (FRET) of metal nanoparticles at the atomic level has long been a challenge due to the lack of accurate systems with definite distance and orientation of molecules. Here we present the realization of achieving FRET between two atomically precise copper nanoclusters through cocrystallization-induced spatial confinement. In this study, we demonstrate the establishment of FRET in a cocrystallized Cu8(p-MBT)8(PPh3)4@Cu10(p-MBT)10(PPh3)4 system by exploiting the overlapping spectra between the excitation of the Cu10(p-MBT)10(PPh3)4 cluster and the emission of the Cu8(p-MBT)8(PPh3)4 cluster, combined with accurate control over the confined space between the two nanoclusters. Density functional theory is employed to provide deeper insights into the role of the distance and dipole orientations of molecules to illustrate the FRET procedure between two cluster molecules at the electronic structure level.

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

Carborane-Cluster-Wrapped Copper Cluster with Cyclodextrin-like Cavities for Chiral Recognition

Chiral atomically precise metal clusters, known for their remarkable chiroptical properties, hold great potential for applications in chirality recognition. However, advancements in this field have been constrained by the limited exploration of host-guest chemistry, involving metal clusters. This study reports the synthesis of a chiral Cu16(C2B10H10S2)8 (denoted as Cu16@CB8, where C2B10H12S2H2 = 9,12-(HS)2-1,2-closo-carborane) cluster by an achiral carboranylthiolate ligand. The chiral R-/S-Cu16@CB8 cluster features chiral cavities reminiscent of cyclodextrins, which are surrounded by carborane clusters, yet they crystallize in a racemate. These cyclodextrin-like cavities demonstrated the specific recognition of amino acids, as indicated by the responsive output of circular dichroism and circularly polarized luminescence signals of Cu16 moieties of the Cu16@CB8 cluster. Notably, a quantitative chiroptical analysis of amino acids in a short time and a concomitant deracemization of Cu16@CB8 were achieved. Density functional tight-binding molecular dynamics simulation and noncovalent interaction analysis further unraveled the great importance of the cavities and binding sites for chiral recognition. Dipeptide, tripeptide, and polypeptide containing the corresponding amino acids (Cys, Arg, or His residues) display the same chiral recognition, showing the generality of this approach. The functional synergy of dual clusters, comprising carborane and metal clusters, is for the first time demonstrated in the Cu16@CB8 cluster, resulting in the valuable quantification of the enantiomeric excess (ee) value of amino acids. This work opens a new avenue for chirality sensors based on chiral metal clusters with unique chiroptical properties and inspires the development of carborane clusters in host-guest chemistry.

Highly intense NIR emissive Cu4Pt2 bimetallic clusters featuring Pt(i)-Cu4-Pt(i) sandwich kernel

Metal nanoclusters (NCs) capable of near-infrared (NIR) photoluminescence (PL) are gaining increasing interest for their potential applications in bioimaging, cell labelling, and phototherapy. However, the limited quantum yield (QY) of NIR emission in metal NCs, especially those emitting beyond 800 nm, hinders their widespread applications. Herein, we present a bright NIR luminescence (PLQY up to 36.7%, ∼830 nm) bimetallic Cu4Pt2 NC, [Cu4Pt2(MeO-C6H5-C[triple bond, length as m-dash]C)4(dppy)4]2+ (dppy = diphenyl-2-pyridylphosphine), with a high yield (up to 67%). Furthermore, by modifying the electronic effects of R in RC[triple bond, length as m-dash]C- (R = MeO-C6H5, F-C6H5, CF3-C6H5, Nap, and Biph), we can effectively modulate phosphorescence properties, including the PLQY, emission wavelength, and excited state decay lifetime. Experimental and computational studies both demonstrate that in addition to the electron effects of substituents, ligand modification enhances luminescence intensity by suppressing non-radiation transitions through intramolecular interactions. Simultaneously, it allows the adjustment of emitting wavelengths by tuning the energy gaps and first excited triplet states through intermolecular interactions of ligand substituents. This study provides a foundation for rational design of the atomic-structures of alloy metal NCs to enhance their PLQY and tailor the PL wavelength of NIR emission.

Atom-Precise Ligated Copper and Copper-Rich Nanoclusters with Mixed-Valent Cu(I)/Cu(0) Character: Structure-Electron Count Relationships

Copper homometallic and copper-rich heterometallic nanoclusters with some Cu(0) character are reviewed. Their structure and stability are discussed in terms of their number of "free" electrons. In many aspects, this structural chemistry differs from that of their silver or copper homologs. Whereas the two-electron species are by far the most numerous, only one eight-electron species is known, but more electron-rich nanoclusters have also been reported. Owing to the relatively recent development of this chemistry, it is likely that more electron-rich species will be reported in the future.

Parasitism in Metal Nanoclusters: A Case Study of (AuAg)25·(AuAg)27

Studying the interactions of atomically precise metal nanoclusters in their assembly systems is of great significance in the nanomaterial research field, which has attracted increasing interest in the last few decades. Herein, we report the cocrystallization of two oppositely charged atomically precise metal nanoclusters in one unit cell: [Au1Ag24(SR)18]- ((AuAg)25 for short) and [AuxAg27-x(Dppf)4(SR)9]2+ (x = 10-12; (AuAg)27 for short) with a 1:1 ratio. (AuAg)27 could maintain its structure in the presence of (AuAg)25, whether in the crystalline and the solution state, while the metastable (AuAg)27 component underwent a spontaneous transformation to (AuAg)16(Dppf)2(SR)8 after dissociating the (AuAg)25 component from this cocrystal, demonstrating the "parasitism" relationship of the (AuAg)27 component over (AuAg)25 in this dual-cluster system. This work enriches the family of cluster-based assemblies and elucidates the delicate relationship between nanoparticles of cocrystals.

Cocrystallization of Two Negatively Charged Dimercaptomaleonitrile-Stabilized Silver Nanoclusters

Studies on the assembly of atomically precise metal nanoclusters (NCs) are of great significance in the nanomaterial field, which has attracted increasing interest in the last few decades. Herein, we report the cocrystallization of two negatively charged atom-precise silver nanoclusters, the octahedral [Ag₆₂(MNT)₂₄(TPP)₆]^8- (Ag₆₂) and the truncated-tetrahedral [Ag₂₂(MNT)₁₂(TPP)₄]^4- (Ag₂₂) in a 1:2 ratio (MNT^2- = dimercaptomaleonitrile, TPP = triphenylphosphine). As far as we know, a cocrystal containing two negatively charged NCs has seldom been reported. Single-crystal structure determinations reveal that the component Ag₂₂ and Ag₆₂ NCs both adopt core-shell structures. In addition, the component NCs were separately obtained by adjusting the synthetic conditions. This work enriches the structural diversity of silver NCs and extends the family of cluster-based cocrystals.