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, [Bo^RBenz]Na and [To^RBenz]-Na (R = Me, Bu^t, 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 [To^MeBenz]Na(κ³-diglyme) exhibits κ³-coordination of the diglyme, whereas the t-butyl and adamantyl derivatives, [To^ButBenz]Na(κ²-diglyme) and [To^AdBenz]Na(κ²-diglyme), exhibit κ²-coordination. The [Bo^RBenz] and [To^RBenz] ligands also allow for isolation of discrete mononuclear thallium compounds, [Bo^RBenz]Tl and [To^RBenz]Tl, for which the steric demands of the ligands have been quantified in terms of both cone angle and buried volume concepts.