Diferrocenylmercury diphosphine diastereomers with unique geometries: trans-chelation at Pd(ii) with short Hg(ii)⋯Pd(ii) contacts†
Diphosphine chelate ligands are essential in many catalytic processes with both the electronic structure and bite angle having a dramatic influence on the coordination behavior and catalytic performance. The synthesis of a new class of diferrocenylmercury-supported diphosphine chelate ligands was accomplished by the reaction of (ortho-diphenylphosphino)ferrocenyl sulfinate (2) with t-BuLi, followed by treatment with mercury(II) chloride. Two diastereomers, 4a (pSpR-, meso-isomer) and 4b (pSpS-isomer), differ in the orientation of the ferrocene moiety relative to the central Ph2PC5H3-Hg-C5H3PPh2 bridging entity. They were isolated independently and fully characterized in solution and in the solid state by single crystal X-ray diffraction analysis. Key characteristics of these ligands are their exceptionally wide and flexible bite angles and the unique stereochemical environment that is achieved upon coordination to transition metals. Complexation to Pd(II)Cl2 gives rise to unusual square-planar trans-chelate complexes 5a (meso) and 5b (pSpS). In competition reactions, 4a and 4b show similar reactivity toward Pd(II)Cl2. The molecular structures of 5a and 5b exhibit short Pd⋯Hg contacts, possibly indicating secondary metallophilic interactions as further evidenced by bond-critical points between Pd and Hg that were identified by AIM analyses.