Structural tuning of meso-hexamer, chiral-trimer and chiral-chain by anion directed supramolecular interactions

Abstract

Reactions of the mono-branched tripodal ligand, N-[N′-(carboxymethyl)benzimidazol-2-ylmethyl]-N,N-bis(benzimidazol-2-ylmethyl)amine (HAcNTB), with different Cu²⁺ salts under varied conditions have resulted in six new Cu²⁺ coordination assemblies, namely, [Cu₆(AcNTB)₆](BF₄)₆·30H₂O (1-BF₄4), [Cu₃(AcNTB)₃](SiF₆)_1.5·CH₃CN·CH₃OH·8H₂O (2-SiF₆6), [Cu(AcNTB)](CH₃C₆H₄SO₃)·CH₃OH (3-OTs), [Cu(AcNTB)](ClO₄)·0.5C₆H₅NO₂·2.5H₂O (3-ClO₄4-PhNO₂2), [Cu(AcNTB)](NO₃)·0.5C₆H₅NO₂·4.25H₂O (3-NO₃3-PhNO₂2) and [Cu₂(AcNTB)₂](NO₃)₂·0.5CH₃C₆H₅·2CH₃OH·H₂O (3-NO₃3-MePh). The common feature of these complexes is that, similar chiral [Cu(AcNTB)]⁺ building blocks are formed with three arms of the tripodal ligand wrapping around the Cu²⁺ centers in either clockwise (Δ-conformer) or anticlockwise (Λ-conformer) fashion, and such building blocks are further assembled with the carboxylate branch (Ac group) acting as bridge by forming Cu–O coordination bond. As a consequence, the assembly process is controlled by the bridging fashion of Ac group between the [Cu(AcNTB)]⁺ building blocks. In complex 1-BF₄4 three Δ-conformers and three Λ-conformers are alternately connected to form a cyclic meso-hexamer, in complex 2-SiF₆6 three Δ-conformers (or three Λ-conformers) are connected to form discrete chiral-trimers (ΔΔΔ-trimer or ΛΛΛ-trimer), while in complexes 3-OTs, 3-ClO₄4-PhNO₂2, 3-NO₃3-PhNO₂2 and 3-NO₃3-MePh the same handed conformers are connected into one-dimensional (1D) chiral-chains (ΔΔΔ-chain or ΛΛΛ-chain). Structural analyses disclose that the assembly fashion and chirality transfer in theses complexes are modulated by the supramolecular interactions including N–H⋯F hydrogen bonds, intra- and inter-molecular π–π interactions which are directed by the counter anions as well as additional aromatic molecules.