Hybrid NCS palladium pincer complexes of thiophosphorylated benzaldimines and their ketimine analogs

A series of novel hybrid pincer ligands bearing thiophosphoryl and imino groups as donating arms was obtained by the condensation of 3-diphenylthiophosphorylbenzaldehyde with RNH 2 (R = Ph, t Bu) to afford aldimine derivatives 2a , b or by the reaction of lithiated 3-(thiophosphoryl)bromobenzene with an appropriate substituted lactam yielding their ketimine analogs 2c – e with 5–7-membered azacycloalkene moieties. The direct cyclopalladation of the ligands with (PhCN) 2 PdCl 2 in MeCN under reflux led to κ 3 - NCS pincer complexes 3b – e with two five-membered fused metallacycles, isolated in low to moderate yields (12–53%); their structures were confirmed by multinuclear NMR and X-ray diffraction study. The palladacycles demonstrated high activity as (pre)catalysts for the Suzuki cross-coupling of phenylboronic acid with aryl bromides, which was found to increase in the series 3e  ∼  3d 3c 3b , i.e. passing from ketimine derivatives 3d , e with larger azacycloalkene to their analog with smaller five-membered cyclic imine moiety and further to benzaldimine complex 3b . The tendency observed was explained by the controlled release of Pd(0) catalytically active species in the case of less sterically hindered complexes.