Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C-Cl bond activation

Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd⁰complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines1a(pSpR,meso-isomer) and1b(pSpS-isomer) differ in the orientation of the ferrocene moieties relative to the central Ph₂PC₅H₃-Hg-C₅H₃PPh₂bridging entity. The structurally distinct trigonal LPd⁰(dba) complexes2a(meso) and2b(pSpS) are obtained upon treatment with Pd(dba)₂. A competition reaction reveals that1breacts faster than1awith Pd(dba)₂. Unexpectedly, catalytic interconversion of1a(meso) into1b(rac) is observed at room temperature in the presence of only catalytic amounts of Pd(dba)₂. Both Pd⁰complexes,2aand2b, readily undergo oxidative addition into the C-Cl bond of CH₂Cl₂at moderate temperatures with formation of the square-planartrans-chelate complexes LPd^IICl(CH₂Cl) (3a,3b). Kinetic studies reveal a significantly higher reaction rate for themeso-isomer2ain comparison to (pSpS)-2b.