Comparison of Magnesium and Zinc in Cationic π‑Arene and Halobenzene Complexes

The reaction of (tBuBDI)­ZnEt with [Ph3C+]­[B­(C6F5)4−] yielded the cation (tBuBDI)­Zn+ (tBuBDI = CH­[C­(tBu)­N-DIPP]2, DIPP = 2,6-diisopropylphenyl).
The cation is sterically too shielded to interact with the ion B­(C6F5)4– but forms complexes
with arenes (benzene, toluene, m-xylene) or halobenzenes
(PhX: X = F, Cl, Br, I). Crystal structures of these complexes are
compared with those of the corresponding Mg complexes. Although Mg2+ and Zn2+ are of equal size, the Zn···arene
and Zn···XPh contacts are generally 0.1–0.2
Å shorter than comparable contacts to (tBuBDI)­Mg+. This originates from differences in bond character: bonding
to Mg has a more electrostatic nature. A major difference between
Mg and Zn is observed for PhF complexation. While the hard Mg2+ cation prefers Mg···FPh bonding, the softer
Zn2+ shows a Zn···(π)­PhF interaction.
Heavier halobenzenes with softer halogens (Cl, Br, I) show Zn···XPh
bonding. DFT calculations on (tBuBDI)­Zn+···XPh
(X = F, Cl, Br, I) show decreasing Zn···X–Ph
angles from PhF to PhI on account of the increase in the halogen’s
σ-hole. Zn···XPh interactions result in C–X
bond lengthening, but C–X bond activation is less pronounced
than in corresponding Mg···XPh complexes. Weak (tBuBDI)­Zn+···XPh bonding could not
be detected in solution but is believed to play a role in the functionalization
of organohalides by Zn reagents.