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

A series of 26,28-di(alkoxy)-5,11,17,23-tetra-tert-butyl-25,27-bis(diphenylphosphinooxy)calix[4]arenes [alkoxy = OH L(10), OMe L(11), OEt L(12),OPr(n)L(13), OCH(2)CO(2)MeL(14), OCH(2)CO(2)EtL(15) or (-)-OCH(2)CO(2)C(10)H(19)L(16)(C10H19 = menthyl = 2-isopropyl-5-methylcyclohexyl)] and 28-(alkoxy)-5,11,17,23-tetra-tert-butyl-25,27-bis(diphenylphosphinooxy)-26-ethoxycalix[4]arenes [alkoxy = OCH(2)CO(2)Et L(17) or (-)-OCH2CO2C10H19 L(18)] have been prepared selectively as cone conformers b; treating the corresponding 25,27-dihydroxycalix[4] arene precursor with LiNPr2i or LiBu at temperatures below -50 degrees C, followed by reaction with PPh(2)Cl. All compounds exist in solution in a stable cone conformation, except L(11) 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 L(6) using NEt(3) instead of LiNP2i, gave 5,11,17,23-tetra-tert-butyl-25-diphenylphos 27-hydroxycalix[4]arene L(19). When the reaction leading to L(10) was performed in refluxing tetrahydrofuran (thf), the 1,2-alternate conformer L(21) was formed in addition to L(10). As shown by a variable-temperature NMR study, L(21) undergoes fast homomerization in solution. Reaction of [MCl(2)(PhCN)(2)] (M = Pt or Pd) with 2 equivalents of monophosphinite L(19) gave selectively the corresponding trans-[MCl(2)L(2)(19)] 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 C-2 symmetrical diphosphinites L(12), L(15) and L(16) 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 L(15) with [PdCl2(PhCN)(2)] gave [(trans-PdCl(2)L(17))(2)]. When the unsymmetrically substituted diphosphinite L(17) was treated with [PtCl2(PhCN)(2)], the dimer [(trans-PtCl(2)L(17))(2)] was formed. Diphosphinite L(18) and [PdCl2(PhCN)(2)] gave [(trans-PdCl(2)L(18))(2)]. Chelating behaviour was found for the chiral diphosphinite L(16) in [Rh(cod)L(16)]BF4 (cod = cycloocta-1,5-diene) obtained by reaction of [{RhCl(cod)}(2)]with 2 equivalents of AgBF4 and 2 equivalents of L(16) in thf.