Pd Nanoparticles Capped with [CpPd(II)Cl]2 Complexes for

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Pd Nanoparticles Capped with [CpPd(II)Cl]2 Complexes for Hydrogenation and Acid-Free Acetalization of α,β-Unsaturated Aldehydes Sheng-Jie Zhao,§,† Xiao Zhou,§,† Hong-Bao Li,*,†,‡ Kuang Liang,† Liu-Bo Ma,† Xiao-Xiang Fang,† Tan Zhao,† Cong Ling,† and An-Wu Xu*,†

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Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ Institutes of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, China S Supporting Information *

ABSTRACT: Hierarchical bottom-up self-assembly of nanoparticles by wet chemistry methods is a promising approach to the preparation of engineered metal−ligand networks with advanced catalytic properties. Here, we developed a facile strategy to synthesize self-organized metal−ligand Pd(0)− [CpPd(II)Cl]2 nanoreactor through a redox and in situ transmetalation reaction between palladium ions and titanium ions. The obtained integrated metallic Pd(0) nanoparticles with organic [CpPd(II)Cl]2 capping ligands have been systematically characterized. The self-assemblies can not only serve as a support providing physical barriers against aggregation of ultrasmall Pd nanoparticles with good stability while keeping Pd nanoparticle core highly accessible but can also act as an efficient bifunctional nanoreactor for the first realization of selective hydrogenation and acid-free acetalization cascade reaction. Because of the combination of the two functions of metallic Pd(0) nanoparticles and Lewis acid Pd(II) ions in the [CpPd(II)Cl]2 ligands, the obtained integrated Pd(0)−[CpPd(II)Cl]2 nanoreactor exhibits excellent hydrogenation selectivity and activity (up to 66%−98% yield) in the synthesis of the acetal derivatives from broad substrates of α,β-unsaturated aldehydes and alcohols under mild conditions. Our work has successfully engineered a metal−ligand ensemble to obtain an attractive bifunctional heterogeneous catalyst for cascade organic transformations, presenting a significant step toward the attainment of green and sustainable chemical reactions. KEYWORDS: metal−ligand, self-assembly, nanoreactor, hydrogenation, acetalization, cascade reaction, bifunctional

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INTRODUCTION The carbonyl group is of great versatility in organic transformations.1,2 However, carbonyl functional groups in aldehydes and ketones encounter acid, base and other redox reagents in the process of organic synthesis. Therefore, in a multistep synthesis the protection of carbonyl compounds by acetalization transformation with alcohols is a general and available approach. Acetals derived from carbonyl compounds and alcohols find widespread applications such as protective groups and synthetic components in commodity chemicals.3,4 Studies on acetalization transformation have been well investigated, yet there still exist some problems such as utilization of corrosive acids, long reaction time, high temperature, some byproducts, and poor chemoselectivity.5,6 Hence, developing an environmental friendly acetalization protocol has attracted much attention in atom-efficient functional group transformation. Obviously, the development of an efficient and highly selective heterogeneous metal catalyst is of great importance © XXXX American Chemical Society

for the production of valuable chemicals, pharmaceuticals, cosmetics and fragrances.7−10 For heterogeneous catalysis, both inorganic supports and organic modifiers could improve their catalytic activity and selectivity to some extent due to their significant electronic and steric effects.11−14 There have been some reports on metal catalysts modified by organic ligands to enhance the activity. However, the use of organic adsorbates to optimize the electronic structure of metal nanoparticles (NPs) to improve the selectivity is yet limited.15−17 Ernst et al. reported that thanks to the Nheterocyclic carbene (NHC) being coordinated to Pd NPs and changing their electronic structure, Pd/Al2O3 modified by NHC presents enhanced catalytic performance for the amination of aryl halides.18 Pd NPs modified by triphenylphosphine (PPh3) in the porous nanochannels of FDU-12 Received: June 19, 2019 Accepted: August 26, 2019

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DOI: 10.1021/acsanm.9b01169 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials

Figure 1. Schematic diagram for the synthesis of the colloidal metal−organic ligand networks of Pd(0)−[CpPd(II)Cl]2 NPs and photographs of the reaction solutions at different reaction stages.

formation of metal−ligand networks of an integrated Pd(0)− [CpPd(II)Cl]2 nanoreactor. These colloidal networks are employed as an ensemble of the nanoreactor for an atomeconomic and renewable synthesis of the acetal derivatives with excellent selectivity and activity from different α,βunsaturated aldehydes with various alcohols. In this cascade reaction, hydrogenation of olefinic groups yields saturated aldehydes, and subsequently acid-free acetalization of carbonyl groups produces the desired acetals. That is to say, that should kill two birds with one stone. Furthermore, we demonstrate that organic ligands can serve as supports providing physical barriers against aggregation of ultrasmall Pd NPs while keeping the Pd NP core highly accessible; the obtained nanocatalyst works as a bifunctional nanoreactor for the first realization of selective hydrogenation and acid-free acetalization reaction cascade in the combination of metallic Pd(0) NPs and Lewis acid Pd(II) in the [CpPd(II)Cl]2 redox ligands.

display excellent selectivity to alcohols in that PPh3 ligands possess the electron-donating ability.19 Somorjai’s group demonstrated supported Au NPs with NHC ligand work as an active and stable catalyst for lactonization transformation of allene-carboxylic acids.20 Unluckily, most of the previous works firstly synthesize metal NPs, followed by adding organic modifiers to tailor the surface microenvironment of these metal NPs.21−25 This weak attachment between organic adsorbates and metal NPs suffers from easy detachment of organic ligands and the structural damage of heterogeneous metal catalysts in the reaction process. Moreover, the added organic adsorbates could unevenly aggregate and block certain surface active sites, which lead to a low usage efficiency of metal catalysts.26−28 Thus, synthesizing ideal catalysts with stable structure and highly active sites via a one-step method is still challenging. The bottom-up nanofabrication of self-assembled NPs into porous networks in which self-assembly of NP building blocks mimics nature by employing weak and specific interactions is essential to achieve the desired structures and multifunctional catalysis.29−32 Functional modulation of metal NPs by the spontaneous patchiness of redox ligands into stable selfstanding assemblies is highly desirable. Deeper understanding of the functional modification by organic additives is still a challenge, yet this could develop a promising approach to boost catalytic property of metal nanocatalysts.33−35 In this context, we propose the use of titanocene monochloride Cp2Ti(III)Cl reductant obtained from nonhazardous materials such as titanocene dichloride Cp2Ti(IV)Cl2 and Zn, which reduces Pd(II) and Ti(IV) is simultaneously substituted by Pd(II), ensuring the fast in situ

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RESULTS AND DISCUSSION Pd ultrafine nanoparticles (NPs) with a capping ligand palladium(II) complex [CpPd(II)Cl]2 (named as Pd(0)− [CpPd(II)Cl]2) and their assemblies were synthesized by a facile redox and transmetalation reaction in a simple, direct, and environmentally benign manner. The preparation strategy for this nanoreactor is briefly illustrated in Figure 1. First, Cp2Ti(IV)Cl2 was reduced to Cp2Ti(III)Cl by zinc powder in tetrahydrofuran (THF) under the protection of nitrogen atmosphere after which the obtained trivalent titanium further reduced Pd(II) to Pd(0) NPs, and Ti(IV) was simultaneously substituted by Pd(II). The stoichiometry is 1.5 equiv of Pd B


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Figure 2. HRTEM images of the metal−organic ligand nanoreactor of Pd(0)−[CpPd(II)Cl]2 NPs (a,b) with the inset showing size distribution of Pd NPs buried in the assemblies. (c−f) STEM-EELS elemental mappings of Pd(0)−[CpPd(II)Cl]2 for Pd (d), Cl (e), C (f).

50 nm and nanospheres aggregate into chainlike networks. Figure S2 displays the typical transmission electron microscopy (TEM) images of as-prepared material. The lower-magnification TEM image confirms the formation of uniform freestanding pomegranate-like nanospheres. The higher magnification image shows well-dispersed ultrasmall Pd NPs are immobilized in the pomegranate-like particles. In the colloidal particles the highly dispersed Pd NPs are the “core” and the [CpPd(II)Cl]2 ligands are the “shell”, which form the hierarchical and promising nanoreactor. As depicted in Figure S3a, the surface area of the nanoreactor characterized by nitrogen (N2) adsorption−desorption isotherms is 34 m2/g, which implies the existence of nanopores in the nanoreactor Pd(0)−[CpPd(II)Cl]2. From the pore diameter distribution presented in Figure S3b, its main pore size distribution is about 2−20 nm, which can host hydrogen and methanol preferably on their cavities. This nanoreactor integrated metallic Pd(0) NPs with [CpPd(II)Cl]2 organic ligands potentially gives rise to superior catalytic reactivity. The generation of highly dispersed Pd NPs in [CpPd(II)Cl]2 ligand matrixes was further proved by high-resolution TEM (HRTEM). Figure 2b displays the lattice fringes of metallic Pd NPs with an interplanar distance of 2.25 Å, suggesting Pd NPs expose the (111) plane. X-ray diffraction pattern (XRD, Figure S4) exhibits that the diffraction peak at 40.1° is assigned to the characteristic (111) plane of Pd. Owing to the confinement of the organic ligand networks, Pd NPs are uniformly distributed in the ligand networks at the size of 2.7 ± 0.5 nm (the inset of Figure 2a). The theoretical surface area of Pd NPs was calculated as 39 m2/g according to the size of Pd NPs. The actual surface area of Pd NPs was measured as 28

atom per titanocene dichloride (see Experimental Section). Figure 1 presents photographs of the reaction solutions at different reaction stages. The color of solutions became red, turned into light green, and finally into black, suggesting that metal NPs were formed. The resultant black precipitate was filtered and washed by ethanol to remove any soluble metal ions (such as Zn2+ or Ti4+) and dried under vacuum. The content of titanium in the supernatant obtained by centrifugation was measured by inductively coupled plasmamass spectrometry (ICP-MS) analysis, which shows that all titanium ions remained in the supernatant and were not found in the precipitates, suggesting Ti(IV) was transmetalated by Pd(II) to form the [CpPd(II)Cl]2 ligand36 on Pd(0) NPs. The reduced Cp2Ti(III)Cl plays dual roles in the present strategy for the formation of colloidal assemblies of Pd(0)−[CpPd(II)Cl]2 NPs. First, it served as a reducing agent37,38 in the formation of Pd NPs without adding additional reductant. Meanwhile, the in situ transmetalated-surface ligand [CpPd(II)Cl]2 acted as “sticky bonds” for Pd(0) NPs that spontaneously self-assembled into colloidal metal−organic ligand networks. Compared with the traditional synthetic methods such as impregnation, deposition or precipitation, such a redox and transmetalation reaction not only leads to intimate contact and strong interaction between Pd NPs and support but also makes Pd NPs highly dispersed in the organic ligand matrixes. In addition, the simple preparation process and unique structures provide the possibility to enhance catalytic reactivity. The as-made Pd(0)−[CpPd(II)Cl]2 NPs were characterized by electron microscopy measurements. Figure S1 depicts the average size of pomegranate-like spherical particles is around C


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Figure 3. High-resolution XPS spectra of obtained Pd(0)−[CpPd(II)Cl]2 sample: (a) the survey spectrum; (b) C 1s spectrum; (c) Pd 3d spectrum; and (d) Cl 2p spectrum.

m2/g by static chemical adsorption analysis, as shown in Figure S5. By comparing these two values, the exposure of the Pd surface was evaluated to reach 71.8%, which helps to carry out a catalytic reaction. The colloidal assemblies with Pd NPs in [CpPd(II)Cl]2 ligand matrixes were further confirmed by scanning TEM (Figure 2c−f), exhibiting that C and Cl elements are homogeneously distributed in the shell layer whereas Pd NPs are dispersed in the core section of the assemblies. Specifically, we used Pd NPs as building blocks to fabricate an integrated nanoreactor of Pd(0)−[CpPd(II)Cl]2 held together by a network of capping ligands. Notably, no titanium element was observed by STEM-EELS elemental mapping, indicating titanium ions were in situ substituted by Pd(II) ions to form [CpPd(II)Cl)]2 capping ligand on the surfaces of Pd(0) NPs, which is confirmed by ICP-MS result and thermogravimetric analysis (TGA) (see Figure S6). The structure of the Pd(0)−[CpPd(II)Cl]2 sample was characterized by Fourier transform infrared spectroscopy (FTIR). Figure S7 depicts the FTIR spectrum (red curve) of Pd(0)−[CpPd(II)Cl]2, along with that of Cp2Ti(IV)Cl2 monomers (black curve). For Cp2TiCl2 monomers, the peaks at 3106 and 1442 cm−1 resulted from the cyclopentadienyl (Cp) ring CH stretching as well as CH bending vibration, respectively. Additionally, the fingerprint spectrum presents bending vibrations in the molecule ranging from 1150 to 700 cm−1. After [CpPd(II)Cl]2 ligands were bound onto Pd(0) NPs surfaces, the peak at 3106 cm−1 and bending vibrations in the fingerprint region became weakened, and the CH bending vibration at 1442 cm−1 was broadened due to the formation of semisandwich [CpPd(II)Cl]2. Figure S8 shows a typical Raman spectra of Pd(0)−[CpPd(II)Cl]2 (red

curve), together with that of Cp2Ti(IV)Cl2 monomers (black curve). The peaks at 1382.3 and 1576.5 cm−1 belonged to the D and G band, respectively, which could result from the sp2 carbon of the cyclopentadienyl (Cp) ring. The composition and chemical state of the integrated Pd(0)−[CpPd(II)Cl]2 nanoreactors were further examined by X-ray photoelectron spectroscopy (XPS) measurements. The C 1s, O 1s, Pd 3d, and Cl 2p elements can be readily identified, and no Ti 2p signal was observed (Figure 3a). As displayed in Figure 3b, the peak centered at 284.8 eV could result from the Cp carbons.39 Figure 3c exhibits a doublet peak, which corresponds to the Pd 3d5/2 and Pd 3d3/2. The two stronger peaks at 337.2 and 342.7 eV could belong to Pd(II) ions. The other two weaker peaks at 335.8 and 341.1 eV could belong to metallic Pd(0).40 Furthermore, the metallic Pd(0) and Pd(II) cations accounted for 32.8% and 67.2% of the whole Pd species of samples based on peak areas of Pd 3d, which is consistent with the composition of Pd(0)−[CpPd(II)Cl]2 with a molar ratio of Pd(0)/Pd(II) close to one-half. These results further demonstrate that in the fabrication process of Pd(0)− [CpPd(II)Cl]2 NPs the reduced Cp2Ti(III)Cl served as the electron donor for reducing the Pd salt precursor to Pd NPs and simultaneously the oxidized tetravalent titanium ion was transmetalated by divalent palladium to generate [CpPd(II)Cl]2 capping ligand. The actual total Pd content in the Pd(0)− [CpPd(II)Cl]2 sample was detected to be 58.4 wt % by ICPMS, and the loading amount of the Pd(II) on the organic support was calculated to be 39.2 wt %. Figure 3d shows that the peaks at 198.2 and 199.8 eV result from Cl − , demonstrating the presence of Cl− in the sample.41 The molar ratio of Pd(II) to Cl− was estimated as about 1:1 from D


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ACS Applied Nano Materials Table 1. Standard Reactions Conditions and Control Experiments

entry

variations from the standard conditionsa

conversion (%)b

yield (%)b

1 2 3 4 5 6c

standard conditions without H2 without catalyst commercial Pd/C catalyst 5 mL of MeOH 0.4 mmol HALD, instead of 0.4 mmol CAL

>99 trace trace >90 >99 >99

88 trace trace trace 87 98

a

Reaction conditions: 0.5 mg of catalyst, 0.4 mmol CAL, 3 mL methanol (MeOH), 5 bar H2, room temperature, 600 rpm, 25 min. bThe conversion of cinnamaldehyde and selectivity were measured by GC-MS and GC. cReaction conditions: 0.5 mg of catalyst, 0.4 mmol HALD, 3 mL MeOH, room temperature, 600 rpm, 25 min.

Hydrocinnamaldehyde (HALD) as the possible reaction intermediate can be proved by further studies. Notably, when HALD was used instead of CAL as the substrate for reaction while other condition were kept constant but without H2, the 3,3-dimethoxypropylbenzene was obtained in up to 98% yield (Scheme S1b, Table 1, entry 6), which further verifies that HALD acted as the reaction intermediate, suggesting that this catalytic system affords excellent hydrogenation selectivity. Thus, it can be seen that palladiumcatalyzed α,β-unsaturated aldehydes in this catalytic system follows a two-step cascade process consisting of selective hydrogenation and acetalization transformation with the corresponding HALD as a key intermediate. In comparison with the activity of other support platforms consisting of both Pd(0) and Lewis acid sites (Table S1), it can be found that in such catalyst systems, using alcohols as solvent, HALD of hydrogenated olefinic group or COL of hydrogenated carbonyl group is the main product. Therefore, developing a direct hydrogenation/acetalization tandem protocol, which is mild, environment friendly, and of high-performance, is very attractive. To verify the formation of the intermediate product of HALD, the hydrogenation selectivity of the Pd(0)−[CpPd(II)Cl]2 nanocatalyst has been further investigated theoretically by using simplified metal nanoparticle models (Figure S23). From the energy point of view, it is more favorable for selective hydrogenation of CAL to HALD rather than to COL. Therefore, this may qualitatively confirm the experimental observations that the surface atoms of Pd(0)−[CpPd(II)Cl]2 nanoreactor preferentially activate the CC group of CAL, thereby boosting hydrogenation selectivity for the generation of the HALD intermediate. On the basis of the experiment results and theoretical calculations for the first step of selective hydrogenation, it is confirmed that HALD acted as the intermediate product, and the presence of Pd(II) acidic sites provides the possibility for the second step of acetalization reaction. A tentative reaction mechanism is proposed (Figure 4). First, when hydrogen is introduced into the reaction system, molecular hydrogen is adsorbed and dissociated on metallic Pd(0) atom surface (Step 1). The cyclopentadienyl ligand with electron-donating property promotes the cleavage of HH bond12,14 and thus accelerates the hydrogenation reaction. Second, HALD is produced after the hydrogenation of the CC group on CAL (Step 2) and simultaneously the vacant coordination site of

XPS data, further confirming the composition of the capping ligand is [CpPd(II)Cl]2. Taken together, this novel strategy based on redox reactions and in situ metal-ion replacement leads to integrating metallic Pd(0) and [CpPd(II)Cl]2 capping ligands into an ensemble nanoreactor for efficient hydrogenation and subsequent acid-free acetalization, thus realizing the cascade reaction. The precise and effective synthesis for available raw material to versatile chemicals is of significance in organic transformation. Among the various acetalization procedures,3−6 direct hydrogenation and acid-free acetalization cascade transformation of broad α,β-unsaturated carbonyl substrates is attractive. The as-made products are a vital part of many commercial chemicals and play a key role in organic conversion synthesis, moreover, not only selective hydrogenation but also environmentally friendly acetalization is unexpected for achieving highly efficient conversion of unsaturated aldehydes to acetals. In this context, we discover a novel procedure for the direct hydrogenation/acetalization tandem reactions of α,β-unsaturated aldehydes over Pd(0)− [CpPd(II)Cl]2 nanoreactor. Using alcohols as a green solvent, acetals with good yields were prepared from different unsaturated aldehyde substrates. Initially, the reaction conditions were investigated by using cinnamaldehyde (CAL) as substrate. Figure S9 shows the kinetic curves of hydrogenation/acetalization transformation of CAL using Pd(0)−[CpPd(II)Cl]2 nanoreactor. After in-depth research, it was found that a yield of 88% 3,3−dimethoxypropylbenzene was produced with Pd(0)−[CpPd(II)Cl]2 as the catalyst under standard conditions (Table 1, entry 1). When H2 or nanocatalyst are lacking, no reaction occurred (Table 1, entries 2 and 3). By comparison, when we used commercial metallic Pd/C (10%) as the catalyst for the transformation of CAL, while other conditions were kept constant, no desired acetals could be detected and only the saturated aldehyde of the hydrogenated olefinic group was produced with a yield of 72% (Table 1, entry 4), indicating Pd(II) cations in the [CpPd(II)Cl]2 ligand play a key role in subsequent acid-free acetalization. The desired acetals can still be produced in 5 mL of methanol, which implies that the amount of solvent has little effect on the reaction (Table 1, entry 5). Additionally, in order to thoroughly understand the pathway of the cascade reactions further control experiments were done. We found that unsaturated acetals could not be detected in the absence of H2 (Scheme S1a, Table 1, entry 2). E


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To study the versatility of the methodology, tests of broad substrates of unsaturated carbonyl compounds (cinnamaldehyde, 3-methyl-2-butenal, and furfural) with various alcohols (methanol, ethanol, and ethylene glycol) were carried out under the same experimental conditions (Table 2). When methanol was replaced with ethanol or ethylene glycol for the reaction of CAL, the corresponding acetals were successfully produced with a yield of up to 87% and 90% respectively. The linear α,β-unsaturated carbonyl compound (3-methyl-2butenal) also proved to be suitable for this reaction under standard conditions; methanol, ethanol, and ethylene glycol were used for the reaction with 3-methyl-2-butenal to give rise to above a 99% conversion, which proceeded smoothly to obtain the desired acetals with excellent yields (98%, 97%, and 92%, respectively). Heterocyclic aldehydes (biomass-derived furfural) also joined in the reaction to produce the desired acetals with good yields (72%, 75%, and 66%, respectively). It is worth mentioning that all reactions were completed within 25 min except the ethylene glycol system (40 min), which demonstrates the superior activity of our obtained bifunctional nanocatalyst. Overall, selective hydrogenation/acetalization cascade transformations of both aromatic (CAL) and aliphatic (3-methyl-2-butenal) as well as heterocyclic (furfural) substrates by using aliphatic alcohols and diols (Figures S13−21) have been realized by an integrated Pd(0)− [CpPd(II)Cl]2 nanoreactor with excellent selectivity and activity at ambient conditions. Besides the ease of separation of heterogeneous metal catalysts, their recycling stability is also an important parameter to evaluate their performance. The stability of the Pd(0)− [CpPd(II)Cl]2 nanocatalyst was investigated using hydrogenation/acetalization reactions of cinnamaldehyde as an example (Figure S10). The catalytic activity of the Pd(0)−

Figure 4. Proposed reaction mechanism of direct selective hydrogenation/acetalization cascade reaction of α,β-unsaturated carbonyl substrate using an integrated of Pd(0)−[CpPd(II)Cl]2 nanoreactor.

Pd(II) activates methanol solvent. The existing Pd(II) Lewis acid sites in the [CpPd(II)Cl]2 ligands promote the subsequent acetalization reaction without the use of any acid.42 Afterward, aldehyde intermediate is thought to initially bind to Pd(II) through the interaction between positive charged Pd(II) and carbonyl group (Step 3). Then the attached −OCH3 group in Pd(II) migrates to carbonyl carbon while the Pd(II) site activates another methanol (Step 4).43 Finally, the desired acetal is produced through the oxocarbenium ion of the hemiacetal, and at the same time palladium sites are regenerated for the following catalytic reaction (Step 5).

Table 2. Palladium-Catalyzed Hydrogenation/Acetalization Reaction Cascade of Different α,β-Unsaturated Aldehydes with a Variety of Alcoholsa,b,c

α,β-Unsaturated aldehydes (cinnamaldehyde, 3-methyl-2-butenal, and furfural); alcohols (methanol, ethanol, and ethylene glycol). bReaction condition: 0.5 mg of catalyst, 0.4 mmol aldehydes, 3 mL alcohols, 5 bar H2, room temperature, 600 rpm, 25 min. For ethylene glycol system, the reaction time was increased to 40 min. cIsolated yield of product (%) was determined by GC-MS and GC. a

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titanium. Under the protection of nitrogen atmosphere, the solutions were filtered to remove unreacted excess zinc powder, then the Pd(C2H3O2)2 solution was added to the above supernatant. The resulting black solids were centrifuged and washed with ethanol to remove any residual metal ions (such as Zn2+ or Ti4+) and dried overnight for further use. The reactions are shown as the following

[CpPd(II)Cl]2 nanocatalyst showed no obvious loss during five catalytic cycles for hydrogenation/acetalization cascade reactions of cinnamaldehyde. ICP-MS analysis of the supernatants for the cycling test of the nanoreactor has been performed and it was found that the amount of the Pd leaching is less than 2 wt % during the catalytic process. After the fifth cycle, the Pd(0)−[CpPd(II)Cl]2 nanocatalyst was measured by TEM. It was observed from TEM images (Figure S11) at different magnifications that Pd NPs did not accumulate obviously and were still distributed in the ligand networks with the size of 2.75 ± 0.5 nm (the inset of Figure S11b). Moreover, the elemental composition and valence state of the Pd(0)− [CpPd(II)Cl]2 nanocatalyst after the fifth cycle was further measured by XPS analysis. Figure S12b exhibits a doublet peak, which corresponds to the Pd 3d5/2 and Pd 3d3/2, and a molar ratio of Pd(0)/Pd(II) is still close to one-half based on peak areas of Pd 3d. These results confirm the [CpPd(II)Cl]2 was not reduced to Pd(0) NP and confirm the excellent recycling stability of the Pd(0)−[CpPd(II)Cl]2 nanoreactor in the hydrogenation/acetalization cascade.

2Cp2Ti(IV)Cl 2 + Zn → 2Cp2Ti(III)Cl + ZnCl 2

2Cp2Ti(III)Cl + 3Pd(C2H3O2 )2 → Pd(0 )−[CpPd(II)Cl]2 + 2Ti4 + + 2Cp− + 6C2H3O2− Synthesis of Other Supported Palladium Catalysts. Compared to the activity of the catalyst Pd(0)−[CpPd(II)Cl]2, several active-carbon (carbon nanotube, graphene, acetylene black) and some heterogeneous supports (MoS2, Al2O3)-loaded Pd nanoparticles have been prepared by simple impregnation. General Procedure for Selective Hydrogenation and AcidFree Acetalization of α,β-Unsaturated Aldehydes. The hydrogenation/acetalization of cinnamaldehyde (CAL) was performed in a stainless autoclave. Cinnamaldehyde (0.4 mmol), 3 mL of methanol, and 0.5 mg of Pd(0)−[CpPd(II)Cl]2 nanocatalyst were added to a reaction tube. The autoclave was purified with hydrogen and then under stirring with 5 bar of H2 at room temperature for 25 min. The products were measured by gas chromatograph (GC) and GC-MS. To study the catalytic recycling performance, the nanocatalyst was filtered, washed, and then dried for the next run. The hydrogenation/ acetalization of 3-methyl-2-butenal and furfural experiment were carried out with the same procedure. For the ethylene glycol system, the reaction time was increased to 40 min. Characterization. The as-synthesized samples were characterized by scanning electron microscopy (SEM) images, using a fieldemission SEM (JSM-6701F, JEOL, 5 kV) and TEM (JEOL-2010, 200 kV). HRTEM images and elemental mapping were carried out using a JEM-2100F field-emission high-resolution transmission electron microscope operated at 200 kV. The XRD patterns of the nanomaterials were performed on a Rigaku/Max-3A X-ray diffractometer. Raman spectra were measured using a PerkinElmer 400F Raman Spectrometer. The XPS was measured at a PerkinElmer RBD. FTIR spectra were measured using a Nicolet Nexus spectrometer. The content of Ti and Pd was carried out by ICP-MS. TGA was measured using a Shimadzu-50 thermoanalyser under air gas flow. The surface features were analyzed with nitrogen adsorption− desorption isotherms, which were obtained with a Micromeritics Gemini apparatus (ASAP 2020M+C, Micromeritics Co. U.S.A.). The surface areas of the nanomaterials were calculated through the Brunauer−Emmett−Teller method, and their pore volumes were measured by the amount of nitrogen adsorbed. The static H2 chemisorption was carried out on an Autosorb IQ S/N (Quantachrome Instruments). The conversion of α,β-unsaturated aldehyde and selectivity for specific products were measured by gas chromatography−mass spectrometry and gas chromatography (Agilent 7890B).

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CONCLUSIONS In conclusion, a bifunctional nanoreactor has been designed and synthesized by integrating metallic Pd(0) and [CpPd(II)Cl]2 capping ligands and self-assemblies, which are obtained by a simple redox and in situ transmetalation reaction, thus endowing intimate contact and interaction between ultrafine Pd nanoparticles and an organic carrier. Owing to the combination of two functions of metallic Pd(0) NPs and Lewis acid Pd(II) ions in the [CpPd(II)Cl]2 ligands, the interesting palladium-catalyzed procedure for atom-economic cascade reactions of selective hydrogenation and acid-free acetalization of broad unsaturated aldehydes has been realized. Both unsaturated aromatic aldehydes and unsaturated aliphatic as well as heterocyclic aldehydes can participate in this cascade reaction. The results of hydrogenation/acetalization reactions reveal that Pd(0)−[CpPd(II)Cl]2 shows excellent catalytic activity and selectivity. The recycling stability experiments prove that the self-assemblies can serve as supports to provide physical barriers against aggregation of ultrafine Pd NPs while keeping Pd NP core highly accessible. This nanocatalyst could be determined to be a sustainable and alternative catalyst for the synthesis of acetals as compared to traditional catalysts thanks to its great potential. This study endows deep understanding for the design and construction of an efficient heterogeneous catalytic system in sophisticated organic cascade reactions.

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EXPERIMENTAL SECTION

Chemicals and Reagents. Palladium(II) acetate (Pd(C2H3O2)2, 99%), titanocene dichloride (Cp2TiCl2, 98%), cinnamaldehyde (97%), and 3-methyl-2-butenal (96%) were obtained from J&K Scientific Ltd. Zinc powder (99.5%), methanol (99.5%), ethanol (99.5%), tetrahydrofuran (99.5%), ethylene glycol (99%), and furfural (99%) were obtained from Sinopharm Chemical Reagent Beijing Co. Ltd. All chemicals were used as received without further purification. Synthesis of the Metal−Organic Ligand Networks of Pd(0)[CpPd(II)Cl]2 NPs. In a typical synthesis, 11.2 mg of palladium(II) acetate was first added into 3 mL of tetrahydrofuran (THF). Titanocene dichloride (8.5 mg) was added to 10 mL of the THF solution. The formed red solution was degassed by purging with nitrogen and then stirred. Whereafter, 10 mg of Zn powder was added until this mixture solution color changed from red to light green, which indicated that tetravalent titanium was reduced to trivalent

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b01169. XRD pattern, SEM and TEM images, IR and Raman spectra, TGA curves of the nanomaterial Pd(0)− [CpPd(II)Cl]2 and their catalytic performance, N2 adsorption−desorption isotherm, mass spectrometry details, more comparisons of different catalysts, and theoretical section (PDF) G


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.B.L). *E-mail: [email protected] (A.W.X.). ORCID

Hong-Bao Li: 0000-0002-0911-7251 An-Wu Xu: 0000-0002-4950-0490 Author Contributions §

S.Z. and X.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the special funding support from the National Natural Science Foundation of China (51572253, 21771171, 21603206), a Scientific Research Grant of Hefei National Synchrotron Radiation Laboratory (UN2017LHJJ), the Fundamental Research Funds for the Central Universities (YD2340002001), and cooperation between NSFC and Netherlands Organization for Scientific Research (51561135011). The density functional theoretical calculations were performed at the Supercomputing Center of the University of Science and Technology of China.

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Pd Nanoparticles Capped with [CpPd(II)Cl]2 Complexes for

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