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 L2X [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, [ToButBenz]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.