PdNP@Cyclodextrin on Cu/Al LDH-containing nanocomposites: Cage effect, crystallite size tuning and composite topology towards cross-couplings

Abstract

A series of Cu/Al layered double hydroxides and Pd(0/II) composites containing native and modified cyclodextrins were prepared under different conditions of solvent (water or N,N-dimethylformamide), Pd precursors (Na2PdCl4 or Pd2dba3) and cyclodextrin/Pd ratios for the catalytic evaluation in Pd/Cu catalyzed cross-couplings aiming for the preparation of aryl thiophenes and N-arylamines. According to XPS measurements, the nanocomposites prepared in water presented a higher Pd(II)/Pd(0) ratio and subtle evidence of [PdCl4]2− intercalation by analyses of X-ray diffractograms. On the other hand, those prepared in N,N-dimethylformamide showed a higher content of Pd(0), probably dispersed on layered double hydroxides surface. SEM-FEG analyses combined with particle size distribution revealed that cyclodextrins were able to tune the size distribution, especially the β-form (15 nm in the presence of cyclodextrins). Powder X-rays diffraction data refinement using Rietveld method revealed that controlled palladium crystallite sizes of 11.53 nm and 10.22 nm were obtained in the presence of hydroxypropylated cyclodextrins (β and γ forms). Notably, the Rietveld refinement of the Pd nanocomposites revealed the presence of malachite, gibbsite, and sperniite phases in 23.05%, 23.05%, 15.23% contents. It suggests that Pd introduces a distortion in the initial LDH lattice. The catalytic activities of these nanocomposites were evaluated in Suzuki-Miyaura reaction in water/ethanol mixture, under low Pd loadings and room-temperature. Noteworthy, the nanocomposite prepared in N,N-dimethylformamide with native β-cyclodextrin showed higher catalytic activity (turnover number up to 7379) in Suzuki-Miyaura and Buchwald-Hartwig reactions for the synthesis of 2-aryl thiophenes, a component of several classes of functional organic electronic materials, and N-(4-nitrophenyl)-1,2-ethanediamine, respectively. This same nanocomposite was recycled up to three runs without loss in catalytic activity. A tentative test envisaging a one-pot borylation-Suzuki-Miyaura strategy indicated that the Miyaura borylation was conceivably affected by the coexisting malachite and spertiniite phases in the nanocomposite. However, control experiments in the presence of Cu(OH)2, Al(OH)3 and malachite indicate a lack of catalytic activity for these phases in Suzuki-Miyaura reactions. Thus, the catalytic activity is mostly attributed to the nanocomposite Cu–Al LDHs containing CDs. The enhancement in catalytic activity is suggested to occur as a result of deprotonation/adsorption of phenylboronic acid, transmetallation facilitated by a relatively high surface hydroxyl sites, improved substrate complexation by β-CDs and potential soluble Cu(II) active species from the layered matrix.