The new air-stable chelating diphosphine ligands 1,8-bis(diisopropylphosphino)triptycene ( 2 ) and 1-(diisopropylphosphino)-8-(diphenylphosphino)triptycene ( 3 ) were synthesized in 51−73% yield from readily available starting materials. The examination of their coordination modes in the palladium(II) complexes [PdCl 2 ( 2 )] ( 4 ) and [PdCl 2 ( 3 )] ( 5 ) by means of 1 H, 13 C, and 31 P NMR is consistent with the formation of trans-spanned complexes. X-ray analysis of 4 and 5 confirmed the structure but disclosed a strong distortion of the palladium centers from the expected square-planar geometry toward a butterfly-like environment. Further investigation revealed that 2 can also behave as a binucleating ligand, forming the quasi-closed halogen-bridged dipalladium complex 6, featuring an unusual nonplanar Pd 2 (μ-Cl) 2 Cl 2 core. The catalytic activity of the new ligands and complexes was tested in palladium-catalyzed cross-coupling reactions of aryl chlorides with phenylboronic acid.
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Grossman et al. (2005) studied this question.