Pd(II) Complexes Ligated by 1,3-Bis(diphenylphosphino)calix[4]arene: Preparation, X‑ray Structures, and Catalyses

syn-1,3-Bis­(diphenylphosphino)-2,4-di­meth­oxy­calix­[4]­arene
(7) was synthesized via an Ullmann-type phosphinoylation
of the syn-1,3-bis­(triflate ester) (2) of p-tert-butylcalix­[4]­arene, followed by reduction
of the phosphinoyl moieties and subsequent methylation of the remaining
hydroxy groups. The treatment of diphosphine 7 with 2
molar equiv of PdCl2(MeCN)2 in benzonitrile
afforded di-μ-chloro-bridged dinuclear palladium complex 8, formulated as Pd2(7)­Cl4, whereas the same reaction conducted in acetonitrile with 1 molar
equiv of PdCl2(MeCN)2 yielded mononuclear palladium
complex 9, formulated as [Pd­(7)­Cl­(MeCN)]­Cl.
Complex 9 was transformed into dicationic complex 11, formulated as [Pd­(7) MeCN)2]­(BF4)2, by treatment with AgBF4 in dichloromethane,
followed by the addition of acetonitrile. On the other hand, refluxing
complex 9 in 1,2-dichloroethane yielded neutral complex 12, formulated as Pd­(7)­Cl2. In each
of the mononuclear complexes, the palladium ion adopts a tetracoordinated
square-planar geometry perpendicular to the mean plane defined by
the macrocycle, and two phosphorus atoms occupy trans coordination
sites. Consequently, an acetonitrile or chlorine ligand coordinates
through the cavity of the calixarene, while another acetonitrile or
chlorine ligand occupies the exo position. Dicationic complex 11 promoted Suzuki–Miyaura coupling between aryl chlorides
and phenylboronic acid and the ring-opening reaction of an epoxide
with thiophenol.