A Bis-Thiosemicarbazone Pd(II) SiO2 Nanoparticle Conjugate for CC Coupling Catalysis

Pd(II) complexes of thiosemicarbazones (TSCs) have previously been used as homogenous precatalysts for constructing CC bonds. Herein, the first SiO₂-supported homogeneous Pd(II)-TSC catalyst [PdL]–APTES–SiO₂ (APTES = (3-aminopropyl)triethoxysilane) is reported for this purpose. This Pd complex–SiO₂ nanoparticle (NP) conjugate is synthesized through covalent binding of the pentadentate bis(N4-(2-ethylphenyl)) 2,6-diacetyl-pyridine TSC onto monodispersed SiO₂, followed by coordination of Pd(II). Characteristic shifts of resonances in the Fourier-transform infrared spectra allowed confirming the SiO₂ functionalization and the Pd-binding. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of N 1s, C 1s, and S 2p shows that the binding energies of the atoms in the CN, CS, CN, NH, pyridine-N, and imine-N groups shift to lower values upon Pd coordination. The absence of chloride in the XPS of the H₂L–APTES–SiO₂ conjugate strongly supports the successful covalent binding of the TSC onto the SiO₂ NPs and is in line with the S_thiolate^N_imine^N_pyridine^N_amide coordination of Pd(II) in the conjugate that is also found in the model complex [Pd(L)]. The [PdL]–APTES–SiO₂ NPs are used as precatalyst in Suzuki–Miyaura-type cross-coupling reactions with yields ranging from 70% to 83%.