Cyclic trinuclear complexes are coordination compounds featuring a ring structure with three metal ions bridged by organic ligands [
1-
3]. While these complexes show promise in chemical sensing, solid-state lighting, full-color displays, and catalysis [
4-
6], their limited air stability and typical bulk material form hinder their broader application, especially in catalytic processes. Strategies to address these limitations include utilizing CTCs as building blocks in covalent metal-organic frameworks (CMOFs)
via condensation reactions with organic linkers, which enhances catalytic activity while preserving CTCs integrity [
7-
9]. For instance, Wei
et al. have reported the preparation of Cu(Ⅰ) CMOFs nanosheets, which exhibited enhanced catalytic activity compared to their bulk-phase counterparts [
10]. Despite these advancements, a robust method to produce stable, nanostructured CTCs with high dispersibility and catalytic efficiency remains a significant challenge.
Diblock copolymers are polymer chains composed of two distinct blocks, each made up of different monomers that can have varying chemical or physical properties. The two blocks are typically linked by a covalent bond, and their properties, such as solubility, polarity, or crystallinity, can be tuned to create specific functional materials [
11-
14]. By designing diblock copolymers with specific functional groups, many studies have shown that double hydrophilic diblock copolymers, where both blocks are water-soluble, can act as crystal modifiers [
15-
18]. The presence of these functional groups can modify the surface properties of crystals, stabilize nanoparticles, or control the size and morphology of the crystalline structures [
19-
24]. This enables fine-tuning of material properties, such as their optical, electronic, or mechanical characteristics, by controlling the interface between the copolymer and the crystal surface [
25-
27]. More importantly, the chemical reactivity of one of the blocks in a diblock copolymer can drive the formation of copolymer aggregates, providing a versatile approach for the creation of copolymer nano-objects [
28-
30]. The ability to manipulate the self-assembly behavior
via chemical reactions allows for precise control over the morphology and functionality of these copolymer structures [
31,
32]. By leveraging the chemistry of diblock copolymers, this approach has enabled the development of a wide range of nanoscale materials with tailored properties for a variety of technological and industrial uses [
33,
34].
In the present study, we aim to achieve the particleization of CTCs using diblock copolymers as functional agents. Specifically, we synthesize a reactive diblock copolymer, poly(2-aminoethyl methacrylate hydrochloride)
50-
block-poly(glycerol monomethacrylate)
52,
via RAFT polymerization (
Fig. 1). For simplicity, this diblock copolymer is denoted as A
50-G
52, where “A” refers to the poly(2-aminoethyl methacrylate hydrochloride) block and “G” refers to the poly(glycerol monomethacrylate) block. The subscripts indicate the respective mean degrees of polymerization (DPs) of each block. We hypothesize that, upon interaction with CTCs, the diblock copolymer will self-assemble into aggregates driven by chemical reaction between the amine groups of the A block and the aldehyde groups present on the CTCs, leading to the formation of colloidally stable hybrid nanoparticles. To confirm the formation and structural characteristics of these nanoparticles, we employed a range of characterization techniques, including transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). These techniques allowed us to investigate the morphology, size distribution, and surface chemical composition of the resulting nanoparticles. Furthermore, the catalytic performance of the hybrid nanoparticles was assessed by evaluating their activity in the Sonogashira cross-coupling reaction, highlighting their potential for applications in catalysis.
The synthetic processes for the preparation of trinuclear copper complexes (tris(
µ2–4-carboxaldehyde-pyrazolato-
N,N')-tri-copper(Ⅰ) monohydrate, Cu
3(PyCA)
3·H
2O), diblock copolymers (A
50-G
52) and hybrid nanoparticles [A
50-G
52-Cu
3(PyCA)
3] were illustrated in
Fig. 1. Specifically, Cu
3(PyCA)
3·H
2O was prepared following a procedure reported by the Li research group (Step Ⅰ in
Fig. 1) [
35]. The detailed synthetic procedure was provided in Supporting information. As a classic trinuclear Cu(Ⅰ)-based triangular complex, this yellow crystal can be readily and reproducibly achieved by a solvothermal reaction. The crystal structure of the as-prepared Cu
3(PyCA)
3·H
2O is consistent with the literature, as determined by powder X-ray diffraction (PXRD) (Fig. S1 in Supporting information). TEM analysis revealed that Cu
3(PyCA)
3·H
2O are aggregates with irregular morphologies (
Fig. 2a).
RAFT polymerization, a controlled/living radical polymerization technique that allows for precise control over molecular weight, polymer architecture, and polymer composition [
36,
37], was utilized for the controlled synthesis of A
50-G
52 diblock copolymers. First, poly(2-aminoethyl methacrylate hydrochloride)
50 macromolecular chain transfer agent (A
50 macro-CTA) was prepared in a mixed solvent of water/ethanol (50/50, w/w, Fig. S2 in Supporting information); Subsequently, such a macro-CTA was further chain extended with glycerol monomethacrylate monomer, producing poly(2-aminoethyl methacrylate hydrochloride)
50-
block-poly(glycerol monomethacrylate)
52 diblock copolymers (A
50-G
52, Step Ⅱ in
Fig. 1)). The DP of such diblock copolymers was determined by proton nuclear magnetic resonance spectroscopy (
1H NMR). The A
50 macro-CTA is water-soluble. After chain-extension with poly(glycerol monomethacrylate) block, the resulting A
50-G
52 diblock copolymer can be dissolved in various solvents, including DMF, DMSO and H
2O (Fig. S3 in Supporting information). Gel permeation chromatography analysis showed that both A
50 macro-CTA and A
50-G
52 diblock copolymer exhibited relatively narrow molecular weight distributions (Fig. S4 in Supporting information), indicating the polymerizations were under good RAFT control [
38]. Each block of the A
50-G
52 diblock copolymer possesses an appropriate chain length, which is considered critical for the successful formation of A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles. Specifically, the A
50 block is sufficiently long to enable effective interaction with Cu
3(PyCA)
3·H
2O, while the G
52 block serves as a steric stabilizer, imparting good colloidal stability to the resulting hybrid structures.
No significant features were observed by TEM for A
50-G
52 diblock copolymers (
Fig. 2b). This observation indicated that no self-assembly of the A
50-G
52 diblock copolymers occurred during the solvent evaporation in the preparation of the TEM sample. However, by mixing Cu
3(PyCA)
3·H
2O with the A
50-G
52 diblock copolymers at 80 ℃ for 12 h (see Step Ⅲ in
Fig. 1), A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles were successfully formed, as confirmed by TEM imaging (
Fig. 2c). These hybrid nanoparticles exhibited unimodal size distribution, possessing a diameter of approximately 39 nm, as determined by DLS (
Fig. 2d). Notably, the diameter determined by DLS was larger than that observed by TEM, which can be attributed to two factors: (1) The A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles are in a swollen state in DMF media during the DLS measurement, and it is reasonable to expect that swollen nanoparticles are larger than their dry-state counterparts [
39], and (2) DLS measures the hydrodynamic diameter, which includes the stabilizer coronas surrounding the nanoparticles. Both of these factors could contribute to the observed discrepancy [
40,
41].
The process by which A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles form upon interaction between Cu
3(PyCA)
3·H
2O and A
50-G
52 diblock copolymers is of considerable interest. Each Cu
3(PyCA)
3·H
2O contains three aldehyde groups, while each A
50-G
52 diblock copolymer possesses fifty amine groups. These two types of chemical groups can react to form Schiff bases. From a polymer chemistry perspective, Cu
3(PyCA)
3 can function as a cross-linker for the A
50-G
52 diblock copolymers. Indeed, FT-IR spectra showed that an absorption band corresponding to the Schiff base C=N stretching vibration was observed for A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles (
Fig. 3). Accordingly, the characteristic stretching bands of the aldehyde (2787 and 2723 cm
-1) and out-of-plane bending peak of the primary amine were significantly attenuated (
Figs. 3b and c). These findings suggested the formation of Schiff base. Unfortunately, after the formation of the hybrid nanoparticles, Cu
3(PyCA)
3 lost its crystalline structure since the Cu
3(PyCA)
3 species are chemically anchored into the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles. Consequently, it was challenging to analyze Cu
3(PyCA)
3 within the hybrid nanoparticles by PXRD. Nevertheless, XPS study indicated the presence of Cu(Ⅰ) (
Fig. 3d), which was further confirmed by Auger peak (Figs. S5a and b in Supporting information). These findings suggested that the Cu
3(PyCA)
3 species remain intact during the formation of hybrid nanoparticles.
The formation of A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles is most likely driven by the cross-linking reaction between the aldehyde groups of Cu
3(PyCA)
3·H
2O and the primary amine groups of the A
50 block of the A
50-G
52 diblock copolymers. Self-assembly of diblock copolymers can be achieved through various methods, including chemical reactions, pH adjustments, or phase separation [
33,
42-
45]. In the present case, Cu
3(PyCA)
3·H
2O acts as a ternary cross-linking agent, facilitating the chemical cross-linking of the A
50-G
52 diblock copolymer. The cross-linked A
50 blocks then form the core of the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles, with the G
52 blocks serving as steric stabilizers. This hypothesis is supported by
1H NMR studies. Comparison of the
1H NMR spectra of various samples confirmed that the G
52 block acts as the stabilizer for the hybrid nanoparticles (
Fig. 4). This is supported by the fact that only the characteristic peaks of the G
52 block were observed in the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles. Prior to the completion of hybrid nanoparticle formation, signals from the A
50 block remain detectable (red arrows in
Fig. 4), which confirms the gradual formation of the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles.
It is well-known that the stability of Cu(Ⅰ)-based CTCs is relatively low due to their susceptibility to rapid oxidation, which readily converts Cu(Ⅰ) to Cu(Ⅱ) ions [
3,
46-
54]. Notably, A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles exhibited superior stability compared to the pristine Cu
3(PyCA)
3·H
2O species. XPS analysis revealed that copper remained its monovalent state in the A
50-G
52-Cu
3PyCA
3 hybrid nanoparticles even after storage for over six months (Fig. S6 in Supporting information). In contrast, Cu(Ⅱ) was observed in the pristine Cu
3(PyCA)
3·H
2O species after just one month, as indicated by the appearance of satellite peaks associated with Cu(Ⅱ) (Fig. S6a). In addition, both TEM study and DLS study indicated the morphology and size of A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles remain unchanged (Figs. S6b and c), further confirming their stability. Furthermore, thermogravimetric analysis (TGA) demonstrated that the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles were thermally stable up to 200 ℃ (Fig. S7 in Supporting information), providing a solid foundation for subsequent thermal catalysis studies.
The Sonogashira cross-coupling reaction, typically used to synthesize alkynylated aromatic compounds, is a widely used organic reaction that forms a carbon-carbon bond between an aryl or vinyl halide (such as a bromide or iodide) and an alkyne (acetylene or substituted alkynes) in the presence of a palladium catalyst and a copper co-catalyst, usually in the presence of a base [
55,
56]. To explore the catalytic potential of the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles, we conducted a model reaction between phenylacetylene and iodobenzene. As shown in
Table 1, the reaction was carried out in DMF solvent with K
2CO
3 as the base. As anticipated, no conversion was observed when A
50-G
52 or CuCl was used in control experiments. In the presence of Cu
3(PyCA)
3·H
2O, a conversion of approximately 70% was achieved, confirming that Cu
3(PyCA)
3·H
2O is capable of catalyzing the reaction. Remarkably, when A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles were used under identical conditions, the conversion increased to >99%.
Notably, no homo-coupled byproduct, diphenyldiacetylene, which typically forms
via a Glaser-type coupling reaction in air, was detected in the presence of A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles, highlighting the high selectivity of these catalysts. This represents a significant advantage over the Sonogashira cross-coupling reaction catalyzed by conventional copper catalysts (Table S1 in Supporting information) [
57,
58]. More importantly, these hybrid nanoparticles exhibited excellent stability and recyclability, maintaining their catalytic performance after five catalytic cycles (
Fig. 5a). This is because the steric G
52 block imparts colloidal stability to the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles and is unresponsive to pH variations. Therefore, changes in pH have no significant effect on the colloidal stability of the hybrid nanoparticles. Furthermore, we extended the use of these hybrid nanoparticles to catalyze reactions with various substituted aryl iodides (including methyl, methyl ester, ethyl ester, and amine derivatives), exhibiting high yields in each case (
Fig. 5b). The resulting products were characterized by NMR (Figs. S8-S17 in Supporting information). We attribute the enhanced catalytic performance to the chemical anchoring of Cu
3(PyCA)
3 within the A
50-G
52-Cu
3(PyCA)
3 hybrid nanoparticles. Under the catalytic conditions described above, the A
50-G
52-Cu
3(PyCA)
3 nanoparticles adopt a swollen state, which facilitates the easy accessibility of reagents to the Cu
3(PyCA)
3 species, which located at the swollen core. Additionally, the spatial separation of each Cu
3(PyCA)
3 unit within the nanoparticles allows them to function as a homogeneous catalyst, thereby promoting the rapid progression of the reaction.
In conclusion, we synthesized a unique diblock copolymer, A50-G52, in which the A50 block contains reactive amine groups, and the G52 block bears cis-diol groups, via RAFT polymerization. We demonstrate that A50-G52-Cu3(PyCA)3 hybrid nanoparticles can be readily prepared by the chemical reaction between Cu3(PyCA)3·H2O and A50-G52 diblock copolymers. Structural characterization using TEM, FT-IR, 1H NMR, and XPS confirmed that the Cu3(PyCA)3 species are anchored within the core of the hybrid nanoparticles, while the G52 block acts as a steric stabilizer, ensuring high dispersibility in various solvents. Remarkably, these hybrid nanoparticles exhibited enhanced catalytic efficiency and excellent recyclability in the Sonogashira cross-coupling reaction. This work highlights a novel strategy for particleizing CTCs through the rational design of reactive block copolymers, enabling the synthesis of CTC-based functional materials with improved stability and catalytic performance.
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