Stabilization of the Compressed Planar Benzene Dianion in Inverse-Sandwich Rare Earth Metal Complexes

For the first time, the capture of the planar antiaromatic parent benzene dianion in between two trivalent rare earth (RE) metal cations (RE^III), each stabilized by two guanidinate ligands, is reported. The synthesized inverse-sandwich complexes [{(Me₃Si)₂NC(NⁱPr)₂}₂RE]₂(μ-η⁶ : η⁶-C₆H₆), (RE=Y (1), Dy (2), and Er (3)) were crystallized from aprotic solvents and feature a remarkably planar parent benzene dianion, previously not encountered for any metal ion prone to low or absent covalency. The -2 charge localization at the benzene ligand was deduced from the results obtained by single-crystal X-ray diffraction analyses, spectroscopy, magnetometry, and Density Functional Theory (DFT) calculations. In the ¹H NMR spectrum of the diamagnetic Y complex 1, the equivalent proton resonance of the bridging benzene dianion ligand is drastically shifted to higher field in comparison to free benzene. This and the calculated highly positive Nucleus-Independent Chemical Shift (NICS) values are attributed to the antiaromatic character of the benzene dianion ligand. The crucial role of the ancillary guanidinate ligand scaffold in stabilizing the planar benzene dianion conformation was also elucidated by DFT calculations. Remarkably, the planarity of the benzene dianion originates from the stabilization of the π-type orbitals of the d-manifold and compression through its strong electrostatic interaction with the two RE^III sites.