Solvent-Induced Facile Synthesis of Cubic-, Spherical-, and Honeycomb-Shape Palladium N-Heterocyclic Carbene Particles and Catalytic Applications in Cyanosilylation.

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Carbene

Solvent-Induced Facile Synthesis of Cubic-, Spherical-, and Honeycomb-Shape Palladium N-Heterocyclic Carbene Particles and Catalytic Applications in Cyanosilylation Huaixia Zhao, Xinxiong Li, Liuyi Li, and Ruihu Wang* Nanometer- and micrometer-sized particles of organic and inorganic materials have attracted considerable attention owing to their tailorable applications in gas storage, heterogeneous catalysis, bioengineering, drug delivery, and data storage.[1–10] As their promising alternatives, metal-organic coordination polymer particles are of considerable significance because they combine the advantages of polydentate organic ligands with metal ions or metal clusters. Just as metal-organic frameworks (MOFs), various morphological particles are readily formed through coordination-directed assembling,[11–14] and unique functional groups are easily introduced into the target particles through judicial selection of predefined organic building units and metal connectors. Besides organic ligands and metal ions, other factors, such as pH, temperature, concentration, and solvent, have important effects on size, shape, chemical, and physical properties of resultant particles.[11,12] Recent advances have realized the control over shape and size of coordination polymer particles, but the reversible nature of metal-coordination chemistry usually makes them unstable enough in most of liquid-phase heterogeneous catalysis. N-heterocyclic carbenes (NHCs) are known to form exceptionally stable complexes with many metals; their superb σ donating ability in conjunction with the imposed steric protection of the bonded elements has conferred them promising application in transition-metal catalysis.[15–18] Recently, palladium NHCs have emerged as a family of very popular connectors in the construction of main-chain organometallic polymers. Several 1D and 3D main-chain palladium

Dr. H. Zhao, Dr. X. Li, Prof. L. Li, Prof. R. Wang State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences Fuzhou, Fujian 350002, China E-mail: [email protected] Dr. H. Zhao University of Chinese Academy of Sciences Beijing 100049, China DOI: 10.1002/smll.201500658 small 2015, DOI: 10.1002/smll.201500658

NHC polymers have been generated through a concerted process of deprotonation of di- or poly-imidazolium salts and subsequent metalation.[19–27] These main-chain palladium NHC polymers have solved problems in traditional solid-supported palladium NHCs, such as tedious synthesis for post-immobilization process, uneven distribution of active sites, low catalyst loading, and irreversible leaching of metal atoms from the supports.[28,29] There is rapid development in palladium NHC polymers, but either the tailorable synthesis of palladium NHC particles with specific morphologies or the study on the control over their outer-shape evolution remains largely unexplored. The outer shape of particles is well known to be a key factor for determining their physical properties and ultimate applications. For example, the dispersion ability of particles in solvents and the accessibility of catalytic active sites to substrates are dependent on the outer shape of particles. In addition, a detailed understanding of structural connectivity and transition metal coordination environments in these particles is still unclear. In comparison with MOFs, the simple coordination mode of palladium-NHC units will be helpful to set up relationship between X-ray crystal structures and particle morphologies. Imidazolium-based ionic salts are well known to be excellent precursors of metal NHCs; the structures and performances of palladium NHC polymers are primarily dependent on poly-imidazolium salts. The flexible star-like tri(4-imidazolylphenyl)amine is easily quaternized by methyl iodide to form an N-centered tripodal imidazolium salt (TIS-3HI), in which phenyl-imidazolium arms can freely rotate and sterically adjust itself around the central nitrogen atom to form different types of palladium NHC complexes.[30] In our continuous effort to develop highly efficient palladium NHC catalytic systems,[31–33] herein, we present a new strategy for the controllable synthesis of palladium NHC particles with spherical (PNC-1), cubic (PNC-2), and honeycomb (PNC-3) shapes. Particle morphologies show an obvious effect on catalytic activity and recyclability of cyanosilylation. The structures of cubic and honeycomb particles are confirmed as an unprecedented trinuclear Pd(II)–NHC complex. The synthetic routes of palladium NHC particles are depicted in Scheme 1; direct treatment of TIS-3HI with 1.5 equivalent of Pd(OAc)2 in DMF and MeCN (1:2 in

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Scheme 1. General synthetic illustration of palladium NHC particles.

volume) for 24 h produced spherical particles of PNC-1. The size of the particles is ≈400 nm (Figure 1a). As solvent volume is decreased to half of original volume, the size of spherical particles increases to ≈1 µm (PNC-1-1) owing to the increment of concentration of TIS-3HI and Pd(OAc)2 (Figure 1b). The formation process of these spherical particles was further explored by taking SEM images at different reaction times (Figure S1, Supporting Information). In the initial stage of the reaction between TIS-3HI and Pd(OAc)2, innumerable small particles were generated, followed by the subsequent coalescence and fusion, resulting in the formation

of uniform and smooth particles to reduce surface energy (Scheme S1, Supporting Information).[34–36] When concentration of starting materials was increased, more particles were involved in the fusion process, giving rise to larger spherical particles. It should be mentioned that the extension of reaction time from 10 h to 48 h has no obvious effect on the size and morphology of the resultant particles (Figure S1d–f, Supporting Information). It is noteworthy that no precipitate was generated when the mixture of DMSO and MeCN (1:1 in volume) was used as a reaction medium under the same conditions,

Figure 1. SEM images for PNC-1 (a), PNC-1-1 (b), PNC-2 (c), and PNC-3 (d).

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but the addition of diethyl ether and dichloromethane to the resultant solution gave rise to yellow cubic particles of PNC-2 (Figure 1c). Interestingly, when the solution was kept untouched in air for 2 weeks, yellow block crystals suitable for X-ray diffraction were obtained. Single-crystal X-ray analysis shows that it is a trinuclear complex (Figure 2), in which Pd(II) ion is in a slightly distorted square geometry and is coordinated by two carbene carbon atoms from different ligands and two iodide anions. Two TIS bridge three Pd(II) ions, resulting in the formation of the unprecedented trinuclear palladium NHC complex. However, the reaction of TIS-3HI with Pd(OAc)2 in DMSO, followed by the addition of diethyl ether and dichloromethane, generated honeycomb particles of PNC-3 (Figure 1d), which were further confirmed by TEM images (Figure S2, Supporting Information). 1H NMR, 13C NMR, mass spectra, and elementary analysis (vide infra) revealed PNC-2 and PNC-3 possessed the same structures as that of crystals. Notably, the reaction medium is the only difference of reaction conditions in the preparation of such three kinds of morphological particles. It is well known that solvent polarity has an important effect on the solubility of the resultant particles. The use of the mixed DMF and MeCN (1:2 in volume) led to direct formation of PNC-1 as yellow precipitates owing to relatively low polarity of the solvent, while the reaction in higher polarity of DMSO and MeCN (1:1 in volume) has not generated precipitates of the resultant particles. The slow addition of weakly polar initiation solvents to the polar precursor solutions lowered their polarity, resulting in gradual formation of yellow solids of PCN-2. However, when DMSO was used as a reaction medium, the incipient addition of diethyl ether and dichloromethane did not generate precipitates of PCN-3. After a critical point of solution saturation was reached, further addition of diethyl ether and dichloromethane quickly produced large amount of yellow precipitates. In order to further investigate the effects of solvents on the size and morphology of palladium NHC particles, a series of experiments in different solvents were explored. When

Figure 2. The molecular structure of PNC-2. small 2015, DOI: 10.1002/smll.201500658

the reaction of TIS-3HI and 1.5 equivalent of Pd(OAc)2 was performed in MeCN instead of DMF/MeCN, the larger spherical particles (1.0–1.7 µm) than PCN-1 were obtained in 24 h (Figure S3a, Supporting Information), which was attributed to low solubility of palladium NHC particles in MeCN, leading to fast precipitation of resultant product from MeCN. However, the use of DMF gave spherical particles with smaller size (≈300 nm) in a low yield (Figure S3b, Supporting Information), which was ascribed to relatively high polarity of DMF. Interestingly, when DMSO and MeCN with volume ratios of 1:3 and 1:2 were used as reaction media, inhomogeneous spherical particles were generated as yellow precipitates in 24 h (Figure S3c,d, Supporting Information). Further increment of DMSO to volume ratio of 1:1.5 gave a mixture of spherical and cubic particles (Figure S3e, Supporting Information). It should be mentioned that these particles were directly formed without the addition of weakly polar initiation solvents in their mother solutions, which were much different from that of PNC-2 and PNC-3 owing to different polarity of reaction media. As expected, the reaction of TIS3HI and Pd(OAc)2 in DMSO and MeCN with a volume ratio of 6:1 did not give the precipitates under the same conditions; the addition of diethyl ether and dichloromethane produced porous block particles (Figure S3f, Supporting Information). These results have demonstrated that the reaction solvents have an important effect on nucleation and precipitation processes of palladium NHC complexes, which ultimately determines the size and morphology of the resultant particles.[37,38] All of the particles are stable in air, but single crystals are ready to turn into yellow powders when they are out of mother solution. PNC-1 is insoluble in most of common solvents including DMF and DMSO, while PNC-2 and PNC-3 are soluble in DMSO. The formation of PNC-1, PNC-2, and PNC-3 is confirmed by IR, NMR, elementary analysis, and thermogravimetric analysis (TGA). IR spectra of such three kinds of particles are close to each other (Figure S4, Supporting Information). In comparison with the IR spectrum of TIS-3HI, the appearance of new bands at 670, 1106, and 1605 cm−1, together with the decrement of quaternary imidazolium bands at 614, 1070, and 1552 cm−1, suggests successful conversion from imidazolium salt to palladium NHCs.[38] The solid-state 13C NMR spectrum of PNC-1 (Figure S5, Supporting Information) is similar to the liquid-state 13C NMR of PNC-2 and PNC-3 (Figure S6, Supporting Information). The characteristic peak at ≈167 ppm is assigned as a carbene carbon atom, which matches well with the reported value,[38] clearly supporting the formation of palladium NHC species. The peak at ≈39 ppm corresponds to methyl group, and the remaining peaks at 105–155 ppm are attributed to carbon atoms of benzyl and imidazolyl rings. 1H NMR spectra of PNC-2 and PNC-3 are identical to that from the trinuclear palladium NHC complex (Figure S7, Supporting Information); the disappearance of imidazolium C2–H at 9.74 ppm further shows the formation of palladium NHCs. Mass spectra of trinuclear palladium NHC complex, PNC-2, and PNC-3 show the mass of cationic [Pd3(TIS)2I4]2+ H+ and [Pd3(TIS)2I5]+ occurs as the peaks of the highest intensity at 1797 and 1925, resepctively (Figure S8, Supporting Information). Elemental analyses display the contents of carbon,



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hydrogen, and nitrogen match well with the theoretic values with guest molecules. ICP analyses show that palladium contents in PNC-1, PNC-2, and PNC-3 are 1.13, 1.35, and 1.32 mmol g−1, respectively, which are very close to the calculated values of 1.35, 1.43, and 1.41 mmol g−1, respectively. The presence of guest molecules is further confirmed by TGA (Figure S9, Supporting Information). The weight losses of 7.3, 1.0, and 3.6% before 200 °C were observed in the TGA curves of PNC-1, PNC-2, and PNC-3, respectively, which are close to the respective theoretic values of 8.2%, 0.9%, and 3.7%. PNC-1 starts to decompose after 350 °C, while PNC-2 and PNC-3 are stable up to 250 °C. On the basis of the above results, it could be deduced that the structures of PNC-2 and PNC-3 are composed of trinuclear palladium NHCs, while PNC-1 can be regarded as an infinite palladium NHC polymer.[19,21] The valence state and distribution of palladium in PNC-1, PNC-2, and PNC-3 were investigated by XPS and SEM/EDS mapping. In their XPS spectra, two similar peaks at 343.0 and 337.1 eV can be ascribed to Pd 3d3/2 and Pd 3d5/2, respectively (Figure S10, Supporting Information). A negative shift about 1.3 eV was observed in comparison with Pd 3d5/2 peak of free Pd(OAc)2 at 338.4 eV,[39] clearly corroborating only existence of Pd(II) and the formation of palladium NHCs. High-annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) mapping images revealed homogeneous distribution of carbon, nitrogen, palladium, and iodide in a wide region in these palladium NHC particles (Figure 3). The porous properties of these palladium NHC particles were investigated by N2 adsorption analyses at 77 K. As

shown in Figure S11 (Supporting Information), N2 adsorption isotherms of PNC-1 and PNC-2 show a steep rise at high relative pressure (P/P0 > 0.9), indicating the presence of some mesopores and/or macropores in both particles, which probably results from the interparticle porosity or void.[40–44] The observation was also confirmed by their pore size distributions with pore width >2 nm. The specific surface areas of PNC-1 and PNC-2 are 28.4 and 28.0 m2 g−1, respectively. As expected, PNC-3 exhibits a different isotherm with a higher specific surface area of 87.9 m2 g−1. It should be mentioned that PNC-1 and PNC-2 show little hysteresis upon desorption, while significant hysteresis without convergence was observed for PNC-3, indicative of the presence of mesoporosity or swelling as a result of gas absorption.[45,46] Cyanohydrins are one of the most valuable synthons in organic synthesis and life science, and cyanosilylation is the most common method for their synthesis.[47–49] Various catalysts including NHC complexes have been applied in cyanosilylation of carbonyl compounds.[24,25] However, heterogeneous catalytic systems based on palladium NHC complexes have not been reported hitherto. The catalytic performances of PNC-1, PNC-2, and PNC-3 were initially evaluated using cyanosilylation of aldehydes under neat conditions at 25 °C. As shown in Figure 4, when the reaction of benzaldehyde and Me3SiCN was performed in the presence of spherical PNC-1 and cubic PNC-2 for 1 h, 2-phenyl2-(trimethylsilyloxy)ethanenitrile was obtained in 92% and 96% GC yields, respectively, while honeycomb PNC-3 gave a quantitative GC yield under the same conditions, suggesting that particle morphology has an important effect on

Figure 3. High-annular dark-field scanning TEM and energy-dispersive X-ray mapping images of PNC-1 with scale bar at 200 nm (a), PNC-2 (b), and PNC-3 (c) with scale bar at 1 µm.



small 2015, DOI: 10.1002/smll.201500658

www.MaterialsViews.com Table 1. Cyanosilylation of carbonyl substrates in the presence of PNC-3. Entry

Figure 4. Recyclability of PNC-1, PNC-2, and PNC-3 in cyanosilylation of benzaldehyde and Me3SiCN. Reaction conditions: benzaldehyde (5 mmol), Me3SiCN (10 mmol), Pd in catalyst (0.01 mmol), 25 °C, 1 h.

cyanosilylation. High catalytic activity in PNC-3 is probably ascribed to more active sites available for the substrates in the mesoporous honeycomb structure, which allows substrates and products to diffuse in and out of the pores with little restriction. However, substrates may be difficult to contact with the active sites in the inner of spherical PNC-1 and cubic PNC-2. The stability and reusability are very important parameters for heterogeneous catalytic systems. The good catalytic activity of these palladium NHC particles has encouraged us to investigate their recyclability in cyanosilylation. After the reaction between benzaldehyde and Me3SiCN was finished, the product was isolated through simple extraction. ICP analyses showed that palladium leaching from PNC-1, PNC-2, and PNC-3 was 0.2, 0.6, and 0.4 wt%, respectively, which was negligible with respect to their respective total palladium content. Interestingly, PNC-1 and PNC-3 still remained good catalytic activity after the catalytic system was used at least six times, while an obvious decline of conversion appeared after the second run for PNC-2. As comparisons, a series of control experiments were also performed. When palladium NHC particles were replaced by a mixture of TIS-3HI and Pd(OAc)2, the reaction between benzaldehyde and Me3SiCN under the same conditions gave 2-phenyl-2- (trimethylsilyloxy)ethanenitrile in a 94% GC yield, which was comparable with those of palladium NHC particles. However, only trace amount of the product was detected when the reaction was recycled, suggesting that palladium NHC polymers were not formed during the catalytic reaction. As a result, the anchoring of palladium in the palladium NHC particles is crucial for superior recyclability in the catalytic system. Notably, the use of Pd(OAc)2 under the same conditions provides the target product in a 90% GC yield, while negligible conversion of benzaldehyde was observed in the presence of TIS-3HI or the absence of palladium NHC particles. The scope and generality of the catalytic system were further explored using PNC-3 at 25 °C. As shown in Table 1, the reactions of electron-deficient aldehydes, such as 4-chlorobenzaldehyde and 4-bromobenzaldehyde, afforded the target products in 100% and 97% GC yields, respectively (entries 1 and 2). Similarly, cyanosilylation of the aldehydes containing electron-rich substituents, such as hydroxyl, small 2015, DOI: 10.1002/smll.201500658

Substratea)

Product

Yieldb)[%]

1

100

2

97

3

95

4

97

5

94

6

100

7

100

8

100

9

95

10

100

11

100

12

100

13

100

a)

Reaction conditions: aldehydes (1.0 mmol), Me3SiCN (2.0 mmol), PNC-3 (0.002 mmol Pd),

25 °C, 1 h; b)GC yield based on aldehyde.

tert-butyl, methyl, and methoxyl, also gave corresponding cyanohydrin trimethylsilyl ethers in excellent to quantitative yields (entries 3–8), and no desilylated products were detected in all cases. Interestingly, quantitative GC yields were achieved when the sterically hindered aldehydes, such as 2-methoxybenzaldehyde and 2-methylbenzaldehyde, were used as substrates (entries 7 and 8). However, the use of 1-naphthaldehyde gave a 95% GC yield (entry 9). The catalytic system was also applicable for heteroatom-containing pyridinecarboxyaldehyde; the target products were obtained in quantitative GC yields regardless of the difference of their electronic and steric characters (entries 10–12). The cyanosilylation of disubstituted aldehydes was also examined under the same conditions. The reaction of isophthalaldehyde with Me3SiCN generated the disubstituted product in a quantitative yield (entry 13).



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The controllable synthesis of palladium NHC particles with different sizes and morphologies was presented for the first time. The nano- and micrometer-sized organometallic particles with spherical, cubic, and honeycomb morphologies can readily be prepared through a one-step reaction of a tripodal imidazolium salt and palladium acetate. The solvent has an important effect on the formation, size, and morphology of these particles. The structures of cubic and honeycomb particles are confirmed by a single-crystal X-ray diffraction analysis and are an unprecedented trinuclear palladium NHC complex. To the best of our knowledge, this is the first report of relationship between X-ray crystal structures and organometallic particles with specific morphologies. An obvious effect of particle morphologies on catalytic activity and recyclability was observed in heterogeneous cyanosilylation. In summary, this study not only is of significant assistance in understanding the factors controlling structures and morphologies of organometallic particles, but also greatly widens the scope of nano- and micrometer-sized particles.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors acknowledge 973 Program (2011CBA00502), National Natural Science Foundation of China (21273239, 21471151) for ﬁnancial support.

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Received: March 7, 2015 Revised: March 24, 2015 Published online:

small 2015, DOI: 10.1002/smll.201500658

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