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, CNHC) ligands and their N-heterocyclic carbene
(NHC) complexes [Pd­(L-HX)­Cl2] (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 °C). Nanoparticles
(NPs) (size of 80–85% particles ∼2–5 nm) formed
in the beginning of these reactions (i.e., when the temperature of
the reaction mixture reaches 80 °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 1, 2, and 3, respectively. The catalytic
activity of 3, having the longest alkyl chain containing
(Se CNHC) 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.