Transition–metal complexation by calix[4]arene-derived phosphinites

Abstract

A series of 26,28-di(alkoxy)-5,11,17,23-tetra-tert-butyl-25,27-bis(diphenylphosphinooxy)calix[4]arenes [alkoxy = OH L¹⁰, OMe L¹¹, OEt L¹², OPrⁿ L¹³, OCH₂CO₂Me L¹⁴, OCH₂CO₂Et L¹⁵ or (–)-OCH₂CO₂C₁₀H₁₉ L¹⁶(C₁₀H₁₉= methyl = 2-isopropyl-5-methylcyclohexyl)] and 28-(alkoxy)-5,11,17,23-tetra-tert-butyl-25,27-bis(diphenylphosphinooxy)-26-ethoxycalix[4]arenes [alkoxy = OCH₂CO₂Et L¹⁷ or (–)-OCH₂CO₂C₁₀H₁₉ L¹⁸] have been prepared selectively as cone conformers by treating the corresponding 25,27-dihydroxycalix[4]arene precursor with LiNPrⁱ₂ or LiBu at temperatures below –50 °C, followed by reaction with PPh₂Cl. All compounds exist in solution in a stable cone conformation, except L¹¹ for which a fast exchange between the cone conformer and a partial-cone isomer occurs. Phosphination of 5,11,17,23-tetra-tert-butyl-25,27-di(ethoxycarbonylmethoxy)-26,28-dihydroxycalix[4]arene L⁶ using NEt₃ instead of LiNPrⁱ₂ gave 5,11,17,23-tetra-tert-butyl-25-diphenylphosphinooxy-26,28-di(ethoxycarbonylmethoxy)-27-hydroxycalix[4]arene L¹⁹. When the reaction leading to L¹⁰ was performed in refluxing tetrahydrofuran (thf), the 1,2-alternate conformer L²¹ was formed in addition to L¹⁰. As shown by a variable-temperature NMR study, L²¹ undergoes fast homomerization in solution. Reaction of [MCl₂(PhCN)₂](M = Pt or Pd) with 2 equivalents of monophosphinite L¹⁹ gave selectively the corresponding trans-[MCl₂L¹⁹₂] complexes (M = Pt or Pd). For the diphosphinites, it was found that their complexation properties depend on both the calixarene substituents and the nature of the starting complex. Thus, reaction of the C₂ symmetrical diphosphinites L¹², L¹⁵ and L¹⁶ with [PtCl₂(PhCN)₂] gave cyclooligomeric complexes of formula [{trans-PtCl₂(diphosphinite)}₄] in which the diphosphinites behave as bridging ligands between two metal centres. Reaction of L¹⁵ with [PdCl₂(PhCN)₂] gave [(trans-PdCl₂L¹⁵)₄]. When the unsymmetrically substituted diphosphinite L¹⁷ was treated with [PtCl₂(PhCN)₂], the dimer [(trans-PtCl₂L¹⁷)₂] was formed. Diphosphinite L¹⁸ and [PdCl₂(PhCN)₂] gave [(trans-PdCl₂L¹⁸)₂]. Chelating behaviour was found for the chiral diphosphinite L¹⁶ in [Rh(cod)L¹⁶]BF₄(cod = cycloocta-1,5-diene) obtained by reaction of [{RhCl(cod)}₂] with 2 equivalents of AgBF₄ and 2 equivalents of L¹⁶ in thf.