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

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

Kengo Hirasawa, Shinya Tanaka*, Takeru Horiuchi, Takahiro Kobayashi, Takumi Sato, Naoya Morohashi, and Tetsutaro Hattori*
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan
Organometallics, Article ASAP
DOI: 10.1021/acs.organomet.5b00999
Publication Date (Web): January 22, 2016

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.