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.