Structural and thermodynamic study of the effect of sterically hindering alkyl groups on complex stability

Abstract

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-diazacyclo-octadecane (L³), and RN[CH₂CH(CH₃)OH]₂(R = Me, L⁴; Prⁱ, 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]I was determined: monoclinic, space group P2₁/c, a= 12.603(2), b= 14.875(3), c= 13.877(3)Å, β= 112.81(1)°, 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.