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), [graphic omitted]HCH₂(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.