Engineering Electronic Structure of Single-Atom Pd Site on Ti _0.87 O ₂ Nanosheet via Charge Transfer Enables C–Br Cleavage for Room-Temperature Suzuki Coupling

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jun 2021Engineering Electronic Structure of Single-Atom Pd Site on Ti0.87O2 Nanosheet via Charge Transfer Enables C–Br Cleavage for Room-Temperature Suzuki Coupling Yangxin Jin†, Fei Lu†, Ding Yi†, Junmeng Li, Fengchu Zhang, Tian Sheng, Zhan, Ya’nan Duan, Gaochao Huang, Jinyang Dong, Bo Zhou, Xi Wang and Jiannian Yao Jin† Department Physics, School Science, Beijing Jiaotong University, 100044 †Y. Jin, F. Lu, D. Yi contributed equally to this work.Google Scholar More articles by author , Lu† Yi† Li Chemical Engineering Technology, Molecular Plus Collaborative Innovation Center Science Engineering, Tianjin 300072 Google Zhang Sheng College Chemistry Materials Anhui Normal Wuhu 241000 Zhan Guangdong Laboratory, Shantou 515031 Duan Huang Dong Zhou *Corresponding authors: E-mail Address: [email protected] Key Laboratory Photochemistry, National Sciences (BNLMS), Institute Chemistry, Chinese Academy Sciences, 100190 https://doi.org/10.31635/ccschem.020.202000388 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd favoritesTrack Citations ShareFacebookTwitterLinked InEmail The palladium (Pd)-catalyzed reaction is widely applied in the pharmaceutical industry, where constructing highly active low-cost sites are impendent. Here, we report fabrication a heterogeneous Pd/Tio2 catalyst engineering an electronic structure single Pd1 atom monolayered nanosheet (Pd1-Ti0.87O2). This motivated kinetically sluggish cleavage, thus boosting at room temperature. Pd1-Ti0.87O2 exhibited outstanding activity with turnover frequency (TOF) 11,110 h−1, exceeding that PdCl2 Pd(OAc)2 catalysts factor >200. Various situ techniques were employed investigate activation process, which showed kinetic-feasibly dissociated chemisorbed bromobenzene, especially bond cleavage. Theoretical calculations further revealed improved ascribed optimized charge state within Pd1O4 realm transfer. Download figure PowerPoint Introduction discovery Suzuki–Miyaura cross-couplings breakthrough due its vital value industries,1,2 since first 1979s.3 Homogenous have been well studied coupling. However, they generally suffer from high cost, residue formation, nonrecyclability, reactivity aryl halides.4–8 With regards supporting complexes solid carriers, could be recycled, but give worse performances after each cycle, mainly aggregation Pd.9,10 third choice supported nanoparticle catalysts; however, do not enough under mild conditions their determined, dynamic structures.11–20 Inspiringly, single-atom emerged as most potential candidate settle these challenges, precisely, providing bridge between homogeneous catalysis.21,22 Informatively, Chen et al.23 reported promising anchor atoms exfoliated graphitic carbon nitride (Pd-ECN) 549 h−1 robust stability, surpassing continuing system. Last least, defects along metal–support interactions stabilize manipulate structure, regulating catalytic activity.24,25 In study, cation-deficient was engineered support. specific Ti vacancies used toward cation anchorage, strong electrostatic interaction cations ensured atomic dispersion surface Then room-temperature reaction, showing extraordinarily (TOF up h−1). TOF over 200 times higher than traditional homogenous [e.g., Pd(OAc)2]. refined high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) X-ray absorption fine (XAFS). characterizations, such XAFS Fourier transform infrared spectroscopy (FTIR), observe cleavage process. These results suggested indeed promoted hybridization halides site. there preferred transfer realm, compared Pd-related [PdCl2 As result, increased density site boosted dramatically. study indicates plays crucial role activity. Experimental Methods Preparation titania (Ti0.87O2) synthesized according delamination process stacked titanate materials, nonstoichiometry formula indicated existence deficiencies.26–28 Typically, stoichiometric chemical mixture Li2CO3, K2CO3, TiO2 (anatase) molar ratio 0.14∶0.4∶1.73 fully ground then calcined 1173 K 1 h. After cooling temperature, calcinated 1273 20 h, followed another grinding step. precursor K0.8Ti1.73Li0.27O4 produced. composite metal generated stirred homogeneously mol dm−3 HCl solution temperature 2 days through protonation acid liquor renewed daily fresh one. protonated solids gleaned successive centrifugal separation washed several ddH2O remove residual acid, dried vacuum drying oven 353 overnight. Thereafter, gathered scattered sufficiently even tetrabutylammonium hydroxide [(C4H9)4NOH; TBAOH, 1.5M] aqueous get nanosheet. proton TBAOH 1∶1. A typical impregnation approach farther executed attain final Pd1-Ti0.87O2: quantificational [tetraamminepalladium(II) hydrogen carbonate] added into colloidal suspensions nanosheets. Other precursors also poor solubility water made it difficult provide free Pd2+. Finally, prepared successfully freeze-drying compounded suspensions. actual loading quantity 0.10 wt%, determined inductively coupled plasma optical emission spectrometer (ICP-OES). FTIR experiment recorded model Nicolet Avatar 360 (Thermo, NYC, USA) spectrophotometer equipped sensitive mercury cadmium telluride (MCT) detector. observed applying attenuated total reflectance (ATR) method. Bromobenzene dropwise sample subjected immediately (IR) gather spectral information changes. test near-edge (XANES) extended (EXAFS) measurements K-edges performed fluorescence mode beamline BL14W1. PdO foils reference samples. Quantitative curve fittings carried out k2-weighted EXAFS oscillation range 0–6 Å using ARTEMIS module IFEFFIT software package. fit, lattice parameters optimized. respect investigation, tested, serving contrast, two drops bromobenzene surface. Compared one, noticeable change pre-edge peak short time. Density functional theory calculation (DFT) Vienna ab-initio simulation package (VASP, University Vienna, Austria). Perdew–Burke–Ernzerhof generalized gradient approximation exchange-correlation (PBE-GGA) used, electron–core treated projector augmented wave (PAW) method.29–32 on-site Hubbard U term 3d orbitals 3.5 eV. Structures until forces smaller 0.01 eV Å−1 kinetic cutoff energy 400 Reaction barriers nudged elastic band constants x–y plane unit cell, including four O atoms, 3.867 × 3.057 Å, good agreement experimental results. modeled 40 80 atoms. One neighboring removed create one hole occupied atom. All adsorbed H2O, relaxed during geometry optimizations. layer z-direction 25 Å. Monkhorst–Pack k-point sampling dipole correction z-direction. adsorption defined follows: E(ad) = E(ad/surf) − E(surf) E(ad), E(ad/surf), energies adsorbate binding surface, gas phase, clean respectively. species obtained G E + ZPE TS, species, TS entropy zero-point energy. vibrational frequencies, vi (Hz), calculated based harmonic oscillators approximation.33 Results Discussion Generally, deficiency rich unilamellar soft-chemical exfoliation method (Figure 1a Supporting Information Figure S1). Holistically, featured negative charge, enabled divalent ions (Pd2+) attach deficiencies interaction.34 powder diffraction pattern (PXRD) ( S2) retained consistency pure Ti0.87O2, no signs nanoparticle, indicating species. estimated wt %, quantified ICP-OES. depicted force (AFM) image 1b), feature uniform thickness ∼1.1 nm 1c). (SEM) images S3) confirmed flake-like morphology (2–10 μm diameter). An energy-dispersive (EDS) element mapping 1d) dispersed Each bright spot (in red circles) HAADF-STEM represented 1e S4). Thus, evenly | (a) Schematic diagram preparation catalyst; red, gray, green, blue, yellow balls indicate O, Ti, K, Pd, N [(C4H9)4N+] (b) AFM (c) corresponding height profiles (d) Representative STEM EDS elemental sample. (e) AFM, microscopy; STEM, EDS, spectroscopy; HAADF-STEM, microscopy. We accurate location investigation (STEM) 2a). blue cycles) nanosheet, exclusively located sites, demonstrated relevant intensity variation line scan profile 2b). local coordination environment spectrometry. shown XANES spectra 2c), white slightly lower foil (the inset right verifying moderate rather metallic Pd(II) PdO, induced substrate.25,35 Meanwhile, structural characteristics similar localized square-planar PdO4 structure. should PdO-like tetra-coordinate configuration, 2c (top left). Evidently, only scattering path 1.5 attributable Pd–O Pd–Pd S5a), excluding possible clusters. Pd1-Ti0.87O2. profile, measured (a), cycles vacancies. K-edge, references. 2p XPS Ti0.87O2. difference Pd-substituted slab, colors represent increment decrement, respectively, isosurface 0.002 e Bohr−3. XANES, structure; XPS, photoelectron spectroscopy. average number (CN) fitting shell (red line, S5b), result relative displayed Table S1. CN (Pd–O) 3.64 Pd1-Ti0.87O2, medially bonded oxygen contribution missing ∼3.0 signified atomical According analyses above, act anchoring centers capture atomically atom.36 configurationally, unsaturated exposed fix individual leading formation thermodynamically stable configuration.37,38 (XPS) unveil 2d S6, peaks shift energy, Accordingly, shifted low normal Pd(II). augmentation anchored beneficial promote cleavage.39–41 DFT uncover property Pd1. chosen theoretical coordinated TiO2, observations 2c). inspiration basal assays, conducted distribution before doping 2e). Considering incompletely filled 4d states might strongly relatively significant overlap orbitals. viewpoint has verified projected (PDOS) slab S7). shaded area region −1.5 1.0 bonding O. Moreover, judging analysis, accumulation (electron cloud yellow). consistent analyses, suggesting enriched appropriate extent, propitious chemisorption reactant.41 commonly evaluated (Table 1). p-Bromotoluene excessive phenylboronic substrates evaluate selectivity conversion title reaction. It came trace 4,4′-dimethylbiphenyl detected, possibility Ullman-coupling p-bromotoluene.42 Noticeably, proved efficient remarkable (Entry 1, Pd(PPh3)4 yielded amounts target product 2, PdCl2, frequently catalysts, afforded cross-coupling impressive yield (99.9% 99.9%) our (Entries 3 4, 1), published results.43,44 far S8), specifically following sequence: > PdCl2. Deductively, favorable degree, played promoting performance. Catalytic Performance Pd-Based Catalysts Phenylboronic Acid Room Temperature Entry Sample (wt %)a Conversion (%)b (h−1) 0.1c 95.5 11,110.2d 9.2 Trace I 47.4 99.9 53.9 4 60.0 42.6 condition: 5 mg 0.1 mmol p-bromotoluene, 0.2 0.5 mL DMF, H2O solvent, open air. aValues atom/weight ratios referring nominal value. bDetermined chromatography (based content p-bromotoluene). cTested ICP-OES measurement. dCalculated 17.4% p-bromotoluene minutes. dimethylformamide. Heterogeneous (Pd/C PdO) tested 3a), consumed entirely coupling produce 4-methylbiphenyl presence 30 min while gained Pd/C PdO. 3b shows efficiency maintained five cycles, anchorage excellent cycle performance Pd2+ consumable, instead, recoverable To direct evidence both measured, found them S9a). S9b, formed. From derivative K-edge S9c), seen valance changed use. Furthermore, extra stress S10) confirm stability continuously pumping air (20 mL/min), impact Pd(0)/Pd(II) improving oxidation Pd(0) (if existed system). affected appreciably. facile application S11). Pd/C, product, Cyclic (five cycles). substrate scope investigated optimal conditions. Diverse bromides iodides bearing electron-withdrawing groups or electron-donating all reacted yields (up 99.9% yield), (>99%) para substituent iodobenzene effect catalyzed Substrates substituents like methyl, methoxyl, hydroxyl, amino inferior when b, g, f, e). reason lies possibly makes stronger. Conversely, chloride nitro smoothly d h) weakening substrates. Scope halide, boronic acids, DMF catalyst, condition ability dissociate halides. focused step, considered rate-determining step.38,45,46 this, detailed (ATR mode). 4a, normalized 1068 cm−1, signal stretching vibration bond,47 decreased gradually happen 1019 in-plane bending C–H benzene ring,48 deliberately absence 4b, 1019–1068 cm−1 gradual oxidative addition step illustration ring bond. (fresh reaction). changes 4c. FTIR, Fourier-transform Another convincing proof technique visualize alteration geometric structures Pd. During test, off-line spectrum collected initially serve contrastive data (orange line). Then, slight amount ambient illustrated 4c (green-dotted inset), apparent near 24,330 observed. configuration triggered reduction central symmetry. enhanced transition 1s favored (sp orbitals) (4d orbitals). R-space S12), S2. worth noting initial (3.64) 6 (5.74). explicated 4d, Ar–Br (Ar phenyl) transformed Ar–Pd1O4–Br namely, had occurred catalyst. mechanism clarified calculations. Step 5a, denotes state. Namely, onto C–Br–Pd represents state, intermediate breaking trend. II complete form new C–Pd–Br. barrier (∼0.45 eV), meant readily implied exothermal happened spontaneously. 4). clarify detail, (DOS) analysis performed, 5b. There notable ∼−0.5 blue-dashed frame), finding agreed 5a. By inferred Pd–Br weakening, C–Pd heightening. manifested well-fabricated strategy