Syntheses and structures of bowl-shaped triarylphosphines and their palladium(II) complexes

Novel bowl-shaped triarylphosphines, tris(2,2″,6,6″-tetraalkyl[1,1′:3′,1″-terphenyl]-5′-yl)phosphines (TRMP: alkyl = methyl; TRIP: alkyl = isopropyl), were prepared by lithiation of the corresponding m-terphenyl bromides followed by reaction with PCl3. X-ray crystallographic analysis revealed that the phosphorus center of TRMP is embedded in the shallow bowl-shaped cavity of 16 Å diameter and 2.1 Å depth formed by three radially extended m-terphenyl units. Its cone angle was estimated to be as large as 174°. In the crystal structure of TRIP, the depth of the cavity and cone angle increased to 3.3 Å and 206°, respectively, because of the different arrangement of the m-terphenyl units. In contrast with TRMP, which can form the mononuclear complex, PdCl2(TRMP)2 (6), in the reaction with PdCl2, treatment of TRIP (1 or 3 eq.) with PdCl2 produced the trinuclear palladium(II) chloride complex, [(PdCl2)3(TRIP)2] (8), as a single product. X-ray crystallography established the structure of 8, where the trimer of PdCl2 is terminated by two TRIP ligands. The formation of the different types of PdCl2 complexes was explained in terms of the difference in the cavity shape of TRMP and TRIP.