The trimetallic structural motif in group 13 chemistry

In the context of this Comment, a trimetallic complex will be defined as one in which three group 13 metals are contained in one complex by either one or two multidentate ligands. This motif has been observed sporadically in the literature throughout the years. Up until the present work, however, this unique structural arrangement has not been the focus of a systematic study. This Comment will comprise a discussion of such complexes, focusing on those employing the Salan class of ligands. Members of the Salan class of tetradentate (-N₂O₂) ligand, N,N′-bis(o-hydroxybenzyl)-1,2-diaminoethane (SaleanH₄), N,N′-bis(o-hydroxybenzyl)-1,3-diaminopropane (SalpanH₄), N,N′-bis(o-hydroxybenzyl)-1,2-diaminobenzene (SalophanH₄), N,N′-bis(o-hydroxybenzyl)-1,2-diamino(4,5-dimethyl)benzene (SalomphanH₄), form unique, monomeric trimetallic complexes when combined with three equivalents of MR₃ (where M = Al, Ga; R = Me, Et, ⁱBu). The aluminum complexes form cis ligand complexes with Salpan, Salophan and Salomphan and trans complexes when Salean is employed. However, when the Salean ligand is used with ⁱBu₃Al, a sterically congested cis complex results. By way of contrast, the gallium derivatives adopt trans geometries in every case. A general feature of these compounds is the presence of a rigid solution state geometry as evidenced by the ¹H NMR. The presence of a shielding interaction between the protons on the alkyl group 13 reagent and the aryl amine backbones of the Salophan and Salomphan ligands will be discussed.