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.