Control of metal-ion size-based selectivity through the structure of the oxygen-donor pendant groups on lariat ethers. A crystallographic and thermodynamic study

Abstract

Ligand protonation constants and formation constants of complexes of Cu2+, Cd2+, Ca2+, Sr2+, Pb2+ and Ba2+ with 7,16-disubstituted 1,4,10,13-tetraoxa-7,16-diazacyclooctadecanes where the substituents are MeOCH2CH2 (L4), CH2CH2CH2OCHCH2 (tetrahydrofurfuryl, L5), and HOCH2CMe2 (L7) have been determined. Steric and inductive effects alter the selectivities of the ligands such that the stability order for L4 is Ba2+ > Sr2+ > Ca2+, but the reverse for L7. The structure of the complex [KL7]I has been determined: colourless crystals, orthorhombic space group P12121, with a = 11.507(4), b = 13.222(6) and c = 17.911(4) Å, Z = 4 and R = 0.049. The absolute structure was determined by statistical analysis. The K-L bond lengths of the potassium complexes of L2 (substituent HOCH2CH2), L4 and L7 vary considerably. The origins of this variation have been analysed using molecular mechanics calculations, and different approaches to modelling the K-O and K-N bonds are discussed.