Palladium Complexes Bearing Novel Strongly Bent Trans-Spanning Diphosphine Ligands: Synthesis, Characterization, and Catalytic Activity

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 [PdCl2(2)] (4) and [PdCl2(3)] (5) by means of 1H, 13C, and 31P 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 Pd2(μ-Cl)2Cl2
core. The catalytic activity of the new ligands and complexes was tested in palladium-catalyzed cross-coupling reactions of aryl chlorides with phenylboronic acid.