Bis- and Tris(2-oxobenzimidazolyl)hydroborato Complexes of Sodium and Thallium: New Classes of Bidentate and Tridentate Oxygen Donor Ligands.

A series of bis- and tris(oxobenzimidazolyl)hydroborato compounds, namely, [BoRBenz ]Na and [ToRBenz ]-Na (R = Me, But , Ad), which feature uncommon sterically demanding LX [O2 ] and L2 X [O3 ] donor ligands, have been obtained via the reactions of NaBH4 with 1-R-1,3-dihydro-2H-benzimidazol-2-ones. Evidence that the alkyl substituents are suitably located to have a significant impact on the coordination environment is provided by the observation that the methyl derivative [ToMeBenz ]Na(κ3 -diglyme) exhibits κ3 -coordination of the diglyme, whereas the t-butyl and adamantyl derivatives, [ToBut Benz ]Na(κ2 -diglyme) and [ToAdBenz ]Na(κ2 -diglyme), exhibit κ2 -coordination. The [BoRBenz ] and [ToRBenz ] ligands also allow for isolation of discrete mononuclear thallium compounds, [BoRBenz ]Tl and [ToRBenz ]Tl, for which the steric demands of the ligands have been quantified in terms of both cone angle and buried volume concepts.