Bis-phosphites and bis-phosphinites based on distally-functionalised calix[4]arenes: coordination chemistry and use in rhodium-catalysed, low-pressure olefinhydroformylation

Abstract

Six calix[4]arenes each bearing two non-cyclic PR₂ units attached at distal phenolic oxygen atoms, p-Bu^t-calix[4]arene-25,27-(OPR₂)₂-26,28-(OR′)₂ (R = OPh; R′ = Prⁿ, L¹111; R = OPh; R′ = CH₂CO₂Et, L²22; R = OPh; R′ = CO₂cholesteryl, L³33; R = Ph; R′ = Prⁿ, L⁴44; R = Ph; R′ = CH₂CO₂Et, L⁵55; R = Ph; R′ = CO₂cholesteryl, L⁶66) have been synthesized and their coordinative properties investigated. The diphosphites L¹111–L³33, where the P centres are separated by 12 bonds, readily form chelate complexes provided the complexation reaction is achieved either by using a starting complex that possesses good leaving groups or by operating under high dilution in order to avoid oligomer formation. Thus, the cationic complexes [Rh(COD)L¹111]BF₄ and [Rh(COD)L³33]BF₄ were both formed in high yield by reacting the appropriate diphosphite with either [Rh(COD)(THF)₂]BF₄ or [Rh(COD)₂]BF₄. At high dilution, reaction of L³33 with the neutral complex [PdCl₂(COD)] afforded the chelate complex [PdCl₂L³33] in 90% yield. The reaction of one equiv. of L¹111 with [Rh(acac)(CO)₂] resulted in the formation of [Rh(acac)L¹111] without requiring high dilution conditions. When the latter reaction was carried out with 0.5 equiv. of L¹111, the bimetallic complex [{Rh(acac)(CO)}₂(η¹-P,η¹-P′-L¹111)] was formed instead. Reaction at high dilution of L²22 with the cyclometallated complex [Pd(o-C₆H₄CH₂NMe₂)(THF)₂]BF₄ gave the expected chelate complex [Pd(o-C₆H₄CH₂NMe₂)L²22]BF₄. The latter slowly converts in solution to an oligomer in which the ligand behaves as a (η¹-P,η¹-P′) bridging ligand, thus leading to a less strained structure. All six ligands, when mixed with [Rh(acac)CO)₂], effectively catalyse the hydroformylation of octene and styrene. In the hydroformylation of octene, the linear aldehyde selectivities observed with L²22 and L³33 are significantly higher (linear : branched = ca. 10) than those obtained with the other 4 ligands of this study and also with respect to PPh₃. Molecular modelling shows that the lower rim substituents of L²22 and L³33 form tighter pockets about the metal centre than do the other ligands and so sterically favour the formation of Rh(n-alkyl) intermediates over that of Rh(i-alkyl) ones. In styrene hydroformylation, all ligands result in the formation of unusually high amounts of the linear aldehyde, the b : l ratios being all close to 65 : 35. The highest activities were found when using an L/Rh ratio of 1/1.