Structural and Thermodynamic Study of the Effect of Sterically Hindering Alkyl Groups on Complex Stability

Article abstract of DOI:10.1039/DT9910002717

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 RN2 (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 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 3>I was determined: monoclinic, space group P2₁/c, a = 12.603(2), b = 14.875(3), c = 13.877(3) Angstroem, β = 112.81(1) deg, 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.

Full text of DOI:10.1039/DT9910002717

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