Angular Distortions at Benzylic Carbons Due to Intramolecular Polarization-Induced Metal–Arene Interactions: A Case Study with Open-Shell Chromium(II) NHC Complexes

The synthesis and full characterization of the unprecedented
open-shell
Cr­(II) benzyl organometallic complexes [Cr­(NHC)2(benzyl)2] (2) and [Cr­(NHC*)­(benzyl)2] (3) (NHC = N,N′-diisopropylimidazol-2-ylidene;
NHC* = N,N′-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene)
from [Cr­(benzyl)3(THF)3] and [CrCl2(THF)2]/[Mg­(benzyl)2], respectively, uncovered
unusually acute angles (93° in 2 and 76° in 3) at the sp3 benzylic C of the coordinated benzyl
ligands. Detailed theoretical analyses (DFT and CASPT2) of the four-
and three-coordinate Cr­(II) species were performed to elucidate the
physical origin of the benzyl bending and led to the recognition of
a noncovalent, intramolecular polarization-induced metal–arene
(PIMA) interaction as being responsible for it. The energetic contribution
from a single PIMA interaction is estimated to be ca. 50 kJ/mol. A
comparison with the origin of the angular distortions in the d0 [Zr­(benzyl)4] complex will also be presented.
Sharing the common origin of an induced-dipole charge density scheme
with intermolecular anion−π interactions, the intramolecular
PIMA interaction concept involving a transition metal unpaired d electron
and arene−π orbitals can be viewed as an extension of
intermolecular anion−π interactions and leads to remarkable
quantitative prediction of the observed structural distortions. PIMA
interactions may manifest themselves in diverse structural and dynamic
phenomena and have broad implications in chemical sciences.