Thermodynamic stabilities of Cu⁺ and Li⁺ complexes of dimethoxyalkanes (MeO(CH₂)_<i>n</i>OMe, <i>n</i> = 2–9) in the gas phase: conformational requirements for binding interactions between metal ions and ligands

Abstract

Abstract The relative free energy changes for the reaction ML + = M + + L (M = Cu + and Li + ) were determined in the gas phase for a series of dimethoxyalkanes (MeO(CH 2 ) n OMe, n = 2–9) by measuring the equilibrium constants of ligand‐transfer reactions using a FT‐ICR mass spectrometry. Stable 1:1 Cu + ‐complexes (CuL + ) were observed when the chain is longer than n = 4 while the 1:2 complexes (CuL) were formed for smaller compounds as stable ions. The dissociation free energy for CuL + significantly increases with increasing chain length, by 10 kcal mol −1 from n = 4 to 9. This increase is attributed to the release of constrain involved in the cyclic conformation of the Cu + ‐complexes. This is consistent with the geometrical and energetic features of the complexes obtained by the DFT calculations at B3LYP/6‐311G level of theory. On the contrary, the corresponding dissociation free energy for LiL + increases only 3 kcal mol −1 from n = 2 to 9, although the structures of the 1:1 Li + ‐complexes are also considered to be cyclic. From these results it is concluded that the Cu[MeO(CH 2 ) n OMe] + requires linear alignment for OCuO, indicating the importance of sd σ hybridization of Cu + in the first two ligands binding energy, while the stability of the Li + complex is less sensitive to binding geometries except for the system forming a small ring such as n = 1 and 2. Copyright © 2006 John Wiley &amp; Sons, Ltd.