Cation coordination by calix[4]arenes bearing amide and/or phosphine oxidependant groups: how many arms are needed to bind Li+ vs. Na+? A combined NMR and molecular dynamics study

Combined spectroscopic and theoretical studies have been performed on two r ecently developed calix[4]arenes in the cone conformation, L1 (bearing two -CH2C(O)NEt2 and two -CH2P(O)Ph-2 substituents occupying respectively dista l phenolic positions) and L2 (with four -CH2P(O)Ph-2 substituents), in orde r to compare the Li+ vs. Na+ cation binding mode. Molecular dynamics simula tions indicate that coordination of the Li+ cation involves three of the fo ur substituents (the two phosphoryl groups and one of the two amide functio ns of L1; three phosphoryl arms of L2). A variable temperature NMR study ca rried out with L1 . Li+ confirms this fourfold coordination and reveals tha t in solution the lithium cation moves between the two adjacent OPOPOamide units. The weaker binding of the Na+ cation results in a more symmetrical c oordination of the four phenolic oxygen atoms and two carbonyls of L1 or fo ur phosphoryls of L2.