Cation coordination by calix[4]arenes bearing amide and/or phosphine oxide pendant 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 recently developed calix[4]arenes in the cone conformation, L1 (bearing two –CH2C(O)NEt2 and two –CH2P(O)Ph2 substituents occupying respectively distal phenolic positions) and L2 (with four –CH2P(O)Ph2 substituents), in order to compare the Li+vs. Na+ cation binding mode. Molecular dynamics simulations indicate that coordination of the Li+ cation involves three of the four substituents (the two phosphoryl groups and one of the two amide functions of L1; three phosphoryl arms of L2). A variable temperature NMR study carried out with L1·Li+ confirms this fourfold coordination and reveals that in solution the lithium cation moves between the two adjacent OPOPOamide units. The weaker binding of the Na+ cation results in a more symmetrical coordination of the four phenolic oxygen atoms and two carbonyls of L1 or four phosphoryls of L2.