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

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [Tism(Pr(i)Benz)], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [Tism(Pr(i)Benz)]Li, [Tism(Pr(i)Benz)]MgMe, [Tism(Pr(i)Benz)]Cu and [Tism(Pr(i)Benz)]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ(3)-coordination, namely [κ(3)-Tism(Pr(i)Benz)]ZnMe, has also been obtained. Furthermore, the [Tism(Pr(i)Benz)] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [Tism(Pr(i)Benz)]Cu to [κ(4)-C(4)-Tism(Pr(i)Benz*)]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(Pr(i)Benz)]Cu and [κ(4)-C(4)-Tism(Pr(i)Benz*)]Cu are characterized by an “inverted ligand field”, in which the occupied antibonding orbitals are localized more on carbon than on copper.