Calix[4]arene-derived diphosphines, diphosphinites and diphosphites as chelating ligands for transition metal ions. Encapsulation of silver(I) in a calix-crown diphosphite

Abstract

A series of lower rim-functionalised calix[4]arenes bearing 1,3-positioned phosphorus(III) ligands L¹111–L⁹99 have been synthesized and their coordinative properties examined. L¹111 and L²22 {5,11,17,23-tetra-tert-butyl-25,27-bis[2-(diphenylphosphino)ethoxy]- and -25,27-bis(diphenylphosphinomethoxy)-26,28-bis(3-oxabutyloxy)calix[4]arene} react with [Rh(nbd)(THF)₂]BF₄ (nbd = 1,5-norbornadiene; THF = tetrahydrofuran) to afford in high yield the complexes [Rh(nbd)L¹111]BF₄ and [Rh(nbd)L²22]BF₄, respectively, where the calixarene behaves as a P,P′ chelator. Both complexes catalyse hydroformylation of styrene at comparable rates, the linear∶branched aldehyde ratio being 5∶95. The presence of the ether side groups did not exert a noticeable effect on the selectivity nor the catalytic activity. Reaction of L¹111–L⁸88 with [Pd(η³-C₃H₄Me)(THF)₂]BF₄ gave the corresponding cationic chelate complexes [Pd(η³-C₃H₄Me)Lⁱii]BF₄ that are active in the catalytic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate. Owing to the presence of a non-planar Pd–allyl fragment, the achiral calixarene subunits of some of these complexes are no longer C_2v-symmetrical, as evidenced by the ¹H and ¹³C NMR spectra that show non-equivalent side groups. Selective chelation via the two phosphorus atoms was also observed in the complexes [RuCl(p-MeC₆H₄Prⁱ)Lⁱii]BF₄ (Lⁱii = L³33 or L⁴44) obtained by reaction of the amide phosphines Lⁱii with [RuCl(p-MeC₆H₄Prⁱ)(THF)₂]BF₄ [L³33 = 5,11,17,23-tetra-tert-butyl-25,27-bis(diethylcarbamoylmethoxy)-26,28-bis(diphenylphosphinomethoxy)- and L⁴44 = 5,11,17,23-tetra-tert-butyl-25,27-bis(diphenylphosphinomethoxy)-26,28-bis{(1-(R)-phenylethyl)carbamoylmethoxy}-calix[4]arene]. Reaction of L³33 or L⁴44 with neutral [RuCl₂(p-MeC₆H₄Prⁱ)]₂ afforded the bimetallic complexes [{RuCl(p-MeC₆H₄Prⁱ)}₂Lⁱii] where the calixarene acts as a P,P′ bridging ligand. Reaction of AgBF₄ with calix-crown L⁹99 {25,27-bis(diethoxyphosphinoxy)-26,28-(3,6,9-trioxaundecane-1,11-dioxy)calix[4]arene} resulted in quantitative formation of the complex [AgL⁹99]BF₄ in which the silver(I) ion lies inside the cavity constituted by the crown ether fragment and the two phosphorus arms. As revealed by a single crystal X-ray diffraction study, the Ag⁺ ion has a trigonal P₂O coordination environment with a P–Ag–P angle of 134.74(4)°.