Formation of Racemate and Mesocate Complexes from an Achiral Tripodal Ligand Containing Three Benzimidazole Groups

Two complexes of the achiral tripodal ligand tris(1-benzimidazolylethyl)amine (nteb), viz., [Mn(nteb)₂(H₂O)₂](ClO₄)₂·CH₃OH, 1, and [Ag₂(nteb)₂](CF₃CO₂)₂·2H₂O, 2, have been synthesized and characterized by IR, ¹H NMR, and EPR spectroscopy (1), electrospray mass spectrometry, thermogravimetric analysis, and X-ray crystallography. Compound 1 contains chiral complex cations due to the same handedness of two nteb ligands (δδ or λλ) but crystallizes as a racemate, while 2 contains a dinuclear mesocate because of the opposite handedness (δλ) of two nteb ligands. Crystal data for 1:  space group P1̄ with a = 9.471(4) Å, b = 16.884(7) Å, c = 19.906(8) Å, α = 77.930(7)°, β = 88.063(7)°, γ = 89.706(7)°, V = 3111(2) ų, and Z = 2. Crystal data for 2:  space group P1̄ with a = 9.3390(12) Å, b = 10.2168(13) Å, c = 16.382(2) Å, α = 75.721(3)°, β = 87.857(3)°, γ = 71.738(3)°, V = 1437.2(3) ų, and Z = 1. In 1, water serves as an auxiliary ligand, connected to one imidazole N atom of each nteb ligand through a H-bridge; the cationic complexes are associated intermolecularly via π···π interactions. In contrast to 1, the coordination to the Ag⁺ ions in 2 occurs in a heterochiral manner and produces a dinuclear mesocate.