A New ternary organometallic Pd(ii)/Fe(iii)/Ru(iii) self-assembly monolayer: the essential ensemble synergistic for improving catalytic activity

The synergistic catalytic effect in a hetero-trimetallic catalytic monolayer is one of the intriguing topics because the additive effects of the second or third component play an important role in improving the activity. In this paper, a new Schiff-base organometallic nanosheet containing Pd/Fe/Ru immobilized on graphene oxide (GO@H-Pd/Fe/Ru) was prepared and characterized. The catalytic performance of GO@H-Pd/Fe/Ru and synergistic effect were systematically investigated. GO@H-Pd/Fe/Ru was found to be an efficient catalyst with higher turnover frequency (TOF) (26 892 h−1) and stability with recyclability of at least 10 times in the Suzuki–Miyaura coupling reaction. The deactivation mechanism was caused by the aggregation of the active species, loss of the active species, the changes of the organometallic complex, and active sites covered by adsorbed elements during the catalytic process. GO@H-Pd/Fe/Ru was a heterogeneous catalyst, as confirmed by kinetic studies with in situ FT-IR, thermal filtration tests and poisoning tests. The real active center containing Pd, Ru and Fe arranged as Fe(iii)–Ru(iii)–Pd(ii)–Fe(iii) was proposed. Although Ru(iii) and Fe(iii) were shown to be less active or inactive, the addition of Fe and Ru could effectively improve the entire activity by their ‘‘indirect’’ function, in which Fe or Ru made Pd more negative and more stable. The ensemble synergistic effect between metals, the ligand and support was described as a process in which the electron was transferred from GOvia ligand to Ru, and then to Pd or from Fe to Pd to make Pd more negative, promoting the oxidation addition with aryl halide. Also, the vicinity of Ru around Pd as the promoter adsorbed aryl boronic acid, which facilitates its synergism to react with the oxidation intermediate to the trans-metallic intermediate.


General methods
Chemical regents were obtained from commercial sources. Solvents were distilled using appropriate drying agents under nitrogen. X-Ray diffraction (XRD) was performed on a PAN analytical X-Pert PRO instrument. Fourier transform infrared (FTIR) spectroscopy was carried out on a BRUKER TENSOR FTIR spectrometer using KBr pellets. Raman spectra were measured with a Thermo Scientific DXR Raman microscope with an excitation laser wavelength of 532 nm. XPS data were obtained using an ESCALab220i-XL electron spectrometer from VG Scientific with 300 W Al Kα radiation. Scanning electron microscopy (SEM) images were recorded using a Hitachi S-4800 system. Transmission electron microscopy (TEM) images were recorded using a JEM-2100F transmission electron microscope operating at 200 kV. The Pd and Co content in the catalysts before and after the crosscoupling reactions was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with an ICAP 6000 Series (Thermo Scientific). The sample treatments were as follows: the sample was broken down with nitrolysis, and then residual solid was dissolved with 2 M hydrochloric acid and transferred into a 10 mL volumetric flask to fix its quantity in water. 1 H NMR and 13 C NMR spectra were recorded on a Bruker Advance III 400 MHz spectrometer in CDCl 3 with tetramethylsilane as an internal standard. A Vertex 70 V spectrometer (Bruker Optik, Ettlingen, Germany) at 293 K, with a spectral resolution of 4 cm −1 and a scanner Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2020 velocity of 10 kHz was used for monitoring the coupling reaction of 4-bromotoluene with phenylboronic acid. AFM images were measured on AIST-NT，AC(tapping) mode.

Fabrication and characterization of GO@H-Pd/Fe/Ru monolayer
Preparation of GO@H-Pd/Fe/Ru monolayer was carried out according the literature reported [16] .
Ref. Mungse H P, Verma S, Kumar N, et al. Grafting of oxo-vanadium Schiff base on graphene nanosheets and its catalytic activity for the oxidation of alcohols [J]. Journal of materials chemistry, 2012, 22(12): 5427-5433. Then, GO (1g) was dispersed in toluene (100 mL) by ultrasonic processing about 2h. Then, nitrogen group (1.6 mmol) was added to round-bottom flask and refluxed at 80°C for 24h. The product was separated by filtration and washed with appropriate solvent three times. Nitrogen group modificatory graphene oxide (denoted as H-GO) was dried in a vacuum oven at 40°C for 12h.

Preparation of GO@H-Pd/Fe/Ru
H-GO (1g) was dissolved in anhydrous methyl alcohol (30mL) sonicated for 0.5h. Then, Different ratio of Li 2 PdCl 4 /FeCl 3 . 6H 2 O/RuCl 3 mixture (total 1.6 mmol) was added to mixture with continuous stirring at 40°C for 24h. The product was separated by filtration and washed with appropriate solvent three times. GO@H-Pd/Fe/Ru was obtained by drying in a vacuum oven at 40°C for 12h.

Procedure for Suzuki reaction and recycling
Si@H-Pd/Fe/Ru(1mg), base, and reactant were added to a 10 mL round-bottom flask with 4 mL solvent. The reaction was carried out in an oil bath at 80 °C for a certain time. The catalysts were separated from the reaction mixture. For the recycling experiments, the reactions were carried out under the above conditions. After each run, the used catalyst was recovered from the reaction mixture and reused in sequential runs after washing with ethyl acetate, methyl alcohol, and water three times.

ReactIR dynamic analysis
ReactIR analysis are carried out as follow: A 10 mL two-neck flask with a magnetic stirrer bar was equipped with the ReactIR probe to monitor the reaction in a 3 70 °C oil bath. 4 mL solvent, 1.0 mmol reactant, and 2.0 mmol K 2 CO 3 were added to the two-neck flask and dispersed by rapid ultrasonic processing. After that, a background spectrum was recorded. Then scans were performed in the time resolved spectroscopy (TRS) mode with 60 s intervals for 30 min.             N 7 a Reaction condition: PhB(OH) 2 derivatives (0.5mmol), 4-bromotoluene derivatives (0.5 mmol), Na 2 CO 3 1 mmol), catalyst 1 mg, solvent (25% aqueous alcohol 4 mL) at 80℃ for 1 h.

5.Investigation on catalytic mechanism
Figure S11 Hot filtration experiment.