Tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl metal complexes, [TismPriBenz]M: a new class of metallacarbatranes, isomerization to a tris(N-heterocyclic carbene) derivative, and evidence for an inverted ligand field

Abstract

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [Tism^PriBenz], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [Tism^PriBenz]Li, [Tism^PriBenz]MgMe, [Tism^PriBenz]Cu and [Tism^PriBenz]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ³-coordination, namely [κ³-Tism^PriBenz]ZnMe, has also been obtained. Furthermore, the [Tism^PriBenz] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [Tism^PriBenz]Cu to [κ⁴-C₄-Tism^PriBenz*]Cu. The transannular M–C bond lengths in the atrane compounds are 0.19–0.32 Å longer than the sum of the respective covalent radii, which is consistent with a bonding description that features a formally zwitterionic component. Interestingly, computational studies demonstrate that the Cu–C_atrane interactions in [Tism^PriBenz]Cu and [κ⁴-C₄-Tism^PriBenz*]Cu are characterized by an “inverted ligand field”, in which the occupied antibonding orbitals are localized more on carbon than on copper.