The Effect of a Single Trifluoromethyl Substituent on the Reactivity of Chelating C2 and Cs‑Symmetric Bis(alkoxide) Ligands on a Terphenyl Platform

Herein we describe the synthesis and preliminary reactivity studies of new racemic C2-symmetric and meso Cs-symmetric bis(alkoxide) ligands on a para-terphenyl platform. The ligands were synthesized by reaction of 2,2′-dilithium-p-terphenyl with trifluoroacetophenone, separated by column chromatography, and obtained in 36% (racemic, rac-Lig2H2) and 26% (meso, meso-Lig2H2) isolated yields. The reaction with n-butyl-sec-butylmagnesium led to formation of the expected C2-symmetric and Cs-symmetric mononuclear magnesium complexes. In contrast, the reaction with Cr(N(SiMe3)2)2(THF)2 exhibited a profoundly different coordination chemistry from that of all-phenyl chelating bis(alkoxide) or monodentate alkoxides. While the latter generally form Cr2(OR)4 dimers in which the geometry at Cr(II) is Y-shaped, these new ligands lead to square-planar Cr(II) complexes. DFT calculations help rationalize the distinct dimeric species Cr2(rac-Lig2)2(THF)4 and Cr2(meso-Lig2)2 observed for these new ligands.