Lanthanide cation binding to a phosphoryl-calix[4]arene: the importance ofsolvent and counterions investigated by molecular dynamics and quantum mechanical simulations

Molecular dynamics simulations on the 1:1 M3+ lanthanide (La3+, Eu3+ and Yb 3+) "inclusion'' complex of a t-butyl-calix[4]arene L substituted at the na rrow rim by four CH2-P(O)Ph-2 arms demonstrate the role of hydration and co unterions on the cation binding mode and shielding. In dry chloroform and i n the absence of counterions, the cation is "endo'', fully encapsulated wit hin the pseudo-cavity delineated by the four phosphoryl arms and the four p henolic oxygens. This "endo'' bidentate binding mode is supported by full a b initio quantum mechanical optimization of the calixarene M3+ complexes. I n biphasic solution, the complexes are shown to be surface active and to ad sorb at an "oil''/water interface with the cationic site pointing towards w ater and the hydrophobic t-butyl groups in "oil''. The cation is not encaps ulated, but adopts an "exo'' position, coordinated to the four P=O oxygens of L, to water molecules, and to counterions. This complex is too hydrophil ic to be extracted from the interface to an organic phase. The unexpected b inding mode has important implications concerning the mechanism of liquid-l iquid ion extraction and the microscopic state of the extracted complex in the organic phase.