Tris(carbene)borates; alternatives to cyclopentadienyls in organolanthanide chemistry

Abstract

The chemistry of the tris-carbene anion phenyltris(3-alkyl-imidazoline-2-yliden-1-yl)borate, [C3^Me]⁻ ligand, is initiated for f-block metal cations. Neutral, molecular complexes of the form Ln(C3)₂I are formed for cerium(III), while a separated ion pair [Ln(C3)₂]I forms for ytterbium(III). DFT/QTAIM computational analyses of the complexes and related tridentate tris(pyrazolyl)borate (Tp) – supported analogs demonstrates the anticipated strength of the σ donation and confirms greater covalency in the metal–carbon bonds of the [C3^Me]⁻ complexes in comparison with those in the Tp^Me,Me complexes. The DFT calculations demonstrate the crucial role of THF solvent in accurately reproducing the contrasting molecular and ion-pair geometries observed experimentally for the Ce and Yb complexes.