Synthesis and conformational studies of a chiral octadentate ligand derived from (R)-1,1 '-binaphthyl-2,2 '-diamine and its dinuclear zinc(II) and nickel(II) complexes

The synthesis of the octadentate ligand (R)-(-)-N,N'-dimethyl-N,N'-bis{3-[bis(1-methyl-2-benzimidazolylmethyl)]- aminopropyl}-1,1'-binaphthyl-2,2'-diamine (L), containing a central 1,1'-binaphthalenediamine core connected to two aminobis(benzimidazole) arms by flexible trimethylene carbon chains, and its dinuclear complexes [Zn2L](ClO4)(4) and [Ni2L](ClO4)(4) is reported. The structural features of the free ligand and the metal complexes have been investigated by NMR, UV/Visible, CD, and computational methods, including simulated annealing-molecular mechanics and semiempirical PM3 calculations. The preferred conformation of the free ligand is characterized by a relatively small dihedral angle of about 75degrees between the two naphthyl rings, which is imposed by favorable stacking interactions between benzimidazole rings from different arms. In the dinuclear zinc(II) complex each metal ion is pseudotetrahedral and bound to three N donors of one aminobis(benzimidazole) arm and one N donor of the binaphthalenediamine residue. Coordination of the benzimidazole groups to the metal ions prevents the stacking interactions seen in the free ligand and, consequently, the dihedral angle between the naphthyl groups increases to the more usual ca. 90degrees. In the dinuclear nickel(II) complex, the ligand still provides four nitrogen donors to each metal centers, but the stereochemical preference of the nickel(II) ions imposes a ligand field of tetragonal symmetry, with pseudo-octahedral metal centers, and this occurs with pronounced flattening of the dihedral angle between the naphthyl groups. The change in conformation, from L to [Zn2L](4+) to [Ni2L](4+), of the critical 1, 1'-binaphthalenediamine residue is clearly indicated by CD spectra. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).