Selenium-Containing N-Heterocyclic Carbenes and Their First Palladium(II) Complexes: Synthesis, Structure, and Pendent Alkyl Chain Length Dependent Catalytic Activity for Suzuki-Miyaura Coupling

Three new N-alkyl-N'-(2-ethyl-1-selenophenyl)imidazolium salts, L1-L3 (differing in the chain length of alkyl substituent of N.), which are precursors to (Se, C-NHC) ligands and their N-heterocyclic carbene (NHC) complexes [Pd(L-HX)Cl-2] (1-3) [L = L1-L3, X = I/Br], have been synthesized and characterized by multinuclear NMR and HR-MS. For the synthesis of Pd complexes 1-3 from appropriate imidazolium salts (L1-L3), the route of the silver carbene transfer reaction has been adopted. Single-crystal structures of 1 and 2 have been established with X-ray diffraction. Thermally stable, moisture- and air-insensitive complexes 1-3 have been found to be efficient catalysts for Suzuki-Miyaura coupling reactions (yield up to 96% in 2 h at 80 degrees C). Nanoparticles (NPs) (size of 80-85% particles similar to 2-5 nm) formed in the beginning of these reactions (i.e., when the temperature of the reaction mixture reaches 80 degrees C) appear to be important for catalytic coupling, probably as dispensers of Pd(0), and contain Pd and Se. The Pd:Se is nearly 3:2, 4:5, and 1:1 in NPs formed from I, 2, and 3, respectively. The catalytic activity of 3, having the longest alkyl chain containing (Se C-NHC) ligand among the three complexes, has been found to be higher relative to those of 1 and 2. The length of the alkyl chain present in the complex molecule probably controls the catalytic activity by influencing the dispersion of NPs (containing Pd and Se) generated in situ during the catalytic process. The two-phase test has indicated a homogeneous nature of catalytic process, which probably occurs via leaching of Pd(0) from these NPs. This is first example where Pd-Se NPs catalyze Suzuki coupling largely via a homogeneous process, which is probably combined with oxidative addition to Pd(0) at the surface. The effect of alkyl chain length on catalytic efficiency may be useful in designing ligands/Pd complexes that efficiently catalyze Suzuki-Miyaura coupling.