Molecular design of C-pivot tripodal ligands: Importance of the glycerol structure for effective complexation toward alkali metal cations

A new series of C-pivot tripodal ligands 1-4 containing a 2-methylglycerol unit were prepared and their complexation properties toward alkali metal cations were examined by measuring the extractability, stability constant in THF, characteristic absorption in the UV spectrum, change in the chemical shia in the H-1 NMR, and competitive passive transport. This type of compound (1-4) was found to possess excellent complexing ability and higher selectivity than another type of tripodal Ligand 5 derived from tris(hydroxymethyl)ethane. This remarkable difference in the stability constants was reasonably explained by considering that the former takes a three-dimensional coordination toward alkali metal cations by cooperatively using the three electron-donating arms but the latter does not. This finding clearly shows that a proper selection of the structure of the basic skeleton is important for the molecular design of C-pivot tripodal ligands.