Influence of pendent alkyl chains on Heck and Sonogashira C–C coupling catalyzed with palladium(II) complexes of selenated Schiff bases having liquid crystalline properties

Abstract

The effect of pendent alkyl chain lengths on Heck and Sonogashira coupling has been investigated for the first time using air and moisture insensitive complexes, [PdLCl] (1–4), of selenated Schiff bases (L = L1–L4), differing in length and number of pendent alkyl chain(s) and behaving as a (Se, N, O−) type of ligand. Differential scanning calorimetric (DSC) and polarized optical microscopic (POM) investigations show liquid crystalline nature of L1 and L2, which have one pendent alkyl chain of size C18 and C10 respectively. The yields of coupled products in catalytic Heck and Sonogashira coupling reactions were found good when amount of 1–4 used for them was 1.0 and 0.5 mol% respectively. The catalytic efficiency decreases with alkyl chain length of pendent arm of ligand for Heck coupling but remains unaffected in case of Sonogashira coupling. The in situ generation of palladium nanoparticles (NPs) protected with organoselenium species occurs in the case of Heck coupling. The length of pendent alkyl chain appears to control the dispersion and composition of these NPs and consequently the catalytic efficiency for Heck coupling. The black residues formed in the case of Sonogashira coupling catalyzed with 1 and 4 were found to have small fraction of NPs and a large proportion of big size aggregates. Their compositions for both the catalysts have been found nearly the same and due to predominance of aggregates, the efficiency remains almost unchanged on varying length of pendent alkyl chain. The two phase test for Heck coupling indicates that catalysis is largely heterogeneous.