STRUCTURAL AND THERMODYNAMIC STUDY OF THE EFFECT OF STERICALLY HINDERING ALKYL-GROUPS ON COMPLEX STABILITY

The balance between steric and inductive effects on the thermodynamic stability of complexes in aqueous solution has been examined as the bulk of N-alkyl substituents is increased along the series methyl, ethyl, isopropyl, tert-butyl. The ligands 7,16-diisopropyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (L3), and RN[CH2CH(CH3)OH]2 (R = Me, L4; Pr(i), L5, or Bu(t), L6) have been prepared. The formation constants of these ligands, and of other ligands with bulky N-alkyl groups, have been determined with the metal ions Cu(II), Zn(II), Cd(II), Pb(II) and with Ca(II), Sr(II) and Ba(II) (L3 only). the N-isopropyl groups of ligand L3 cause a sharp drop in thermodynamic complex stability for all metal ions, whereas in the series L2-L4 lead(II) is unusual in showing a strong increase in complex stability in response to the inductive effects of the larger alkyl groups. In an attempt to understand the lowering of complex stability by the N-isopropyl groups of L3 the crystal structure of the complex [KL3]l was determined: monoclinic, space group P2(1)/c, a = 12.603(2), b = 14.875(3), c = 13.877(3) angstrom, beta = 112.81(1)-degrees, Z = 4, final R = 0.042. The structure indicates some stretching of the K-N bond by the presence of the N-isopropyl groups. The parallel between steric hindrance effects by bulky N-alkyl groups and hard and soft acid and base behaviour is discussed.