Bis(1-methylimidazol-2-yl)propionates and bis(1-methylbenzimidazol-2-yl)-propionates:: A new family of biomimetic N,N,O Ligands -: Synthesis, structures and CuII coordination complexes

A general synthetic route towards new tridentate, tripodal N,N,O ligands, bearing one carboxylate group and two of the biologically relevant imidazole moieties, is presented. The parent compounds of this new family of ligands, A[MIm(2)Pr] (1) and A[BenzMIm(2)Pr] (2) (A = H, K, Bu4N) were synthesized in two steps and high overall yield. The structures of HMIm(2)Pr (1a) and HBenzMIm(2)Pr (2a) were confirmed by X-ray crystal structure determination, both molecules forming infinite one-dimensional hydrogen bonded chains. The synthetic route allows for the fine-tuning of the physical properties of both ligand systems, e.g., solubility and ease of crystallization. The capability of the two ligands to coordinate facially through all donor atoms was investigated and two isostructural mononuclear copper(II) complexes of the type [CuL2] were obtained. Both [Cu(MIm(2)Pr)2](.)2H(2)O (3) and [Cu(BenzMIm(2)Pr)(2)](.)3.12(H2O)(.)1.74(C2H5OH) (4) have been characterized by X-ray crystallography, ESI-MS, elemental analysis, UV/Vis, IR, and EPR spectroscopy. In both structures the copper atoms are on an inversion center, which results in a tetragonally distorted octahedral coordination geometry. The CuN4O2 chromophores consist of four equatorially coordinated imidazole nitrogen atoms and two axially coordinated carboxylate groups. The non-coordinated water molecules of both crystal structures are involved in a one-dimensional hydrogen-bonded network. Two neighboring octahedrons are connected by two water molecules, each water molecule forming two hydrogen bonds with the non-coordinated oxygen atoms of the carboxylate group. This results in the formation of infinite one-dimensional chains. EPR spectroscopy and ESI-MS measurements provide evidence for the integrity of the complexes in solution. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.