Palladium(ii)-1-phenylthio-2-arylchalcogenoethane complexes: palladium phosphide nano-peanut and ribbon formation controlled by chalcogen and Suzuki coupling activation

Abstract

The ligands PhSCH₂CH₂EAr (L1–L3; E = S, Se or Te) and their Pd-complexes [PdLCl₂] (1–3) have been synthesized and authenticated with their ¹H, ¹³C{¹H}, ⁷⁷Se{¹H} and ¹²⁵Te{¹H} NMR spectra. Single crystal structures of 1 and 3 reveal the geometry of donor atoms around palladium as nearly square planar. Thermolysis of all three complexes in trioctylphosphine (TOP) at 350, 320 and 280 °C, respectively, results in a single phase of crystalline PdP₂. The morphology of the phase varies with ‘E’ to some extent. The nanopeanuts (size ∼30 and ∼35 nm) were formed with 1 and 2 as precursor complexes. On using complex 3 as a precursor nanoribbons are formed. The preferential growth in the (202) plane in the case of all the three precursor complexes has been rationalized in terms of texture coefficient and average crystallite size. All three complexes and PdP₂ NPs have been explored for Suzuki–Miyaura coupling of several aryl halides. Complexes 1 and 2 show good catalytic activity but complex 3 does not. The activity appears to result due to in situ generated palladium containing nanoparticles (NPs) in the case of 1 and 2. The formation of inactive large Pd aggregates in the case of 3 appears to be responsible for the difference. The PdP₂ NPs have been found to show good catalytic activity and recyclability up to six reaction cycles. The results of the three phase test suggest the involvement of both homogeneous and heterogeneous pathways in the activation of Suzuki coupling. DFT based free energy calculations are consistent with the results of catalysis via Pd(0) protected with the ligand. This palladium may also be released from in situ generated NPs. In the case of 3, negligible reactivity may be due to non-release of Pd.