ic5022559_si_001.pdf (5 MB)
Thermodynamic N‑Donor trans Influence in Labile Pseudo-Octahedral Zinc Complexes: A Delusion?
journal contribution
posted on 2014-12-15, 00:00 authored by Lilit Aboshyan-Sorgho, Timothée Lathion, Laure Guénée, Céline Besnard, Claude PiguetWhile the forces responsible for
the chelate effect are well-established
in coordination chemistry, the origin and implementation of the related
thermodynamic trans influence remains debatable. This work illustrates
a simple approach for quantifying this effect in labile pseudo-octahedral
[Zn(Lk)3]2+ complexes
lacking stereochemical preferences (Lk = L1–L4 are unsymmetrical didentate α,α′-diimine
ligands). In line with statistics, the triply degenerated meridional
isomers mer-[Zn(Lk)3]2+ are stabilized by 0.8 ≤ ΔGexchmer→fac ≤ 4.2 kJ/mol over their nondegenerated facial analogues fac-[Zn(Lk)3]2+ and therefore display no apparent trans influence at room temperature.
However, the dissection of the free energy terms into opposite enthalpic
(favoring the facial isomers) and entropic (favoring the meridional
isomers) contributions reveals a trans influence assigned to solvation
processes occurring in polar solvents. Altogether, the thermodynamic
trans influence operating in [Zn(α,α′-diimine)3]2+ complexes is 1–2 orders of magnitude
smaller than the chelate effect. A weak templating effect provided
by a noncovalent lanthanide tripod is thus large enough to produce
the wanted facial isomer at room temperature.