Copper(II) complexes of mono-anionic glutamate: anionic influence in the variations of molecular and supramolecular structures

Abstract

Three new polynuclear copper(II) complexes of singly deprotonated L-glutamic acid (L-glu), {[Cu(bipy)₂][Cu(bipy)(L-glu)H₂O]₂(BF₄)₄·(H₂O)₃}_n (1), {[Cu(bipy)(L-glu)H₂O][Cu(bipy)(L-glu)(ClO₄)](ClO₄)·(H₂O)₂}_n (2) and [Cu(phen)(L-glu)H₂O]₂(NO₃)₂·(H₂O)₄ (3) (bipy = 2,2′-bipyridine, phen = 1,10-phenanthroline), were synthesized in acidic pH (ca. 2.5) and characterized structurally. In all the complexes, L-glutamic acid acts as a bidentate chelating ligand, leaving the protonated carboxylic acid free. Both in 1 and 2, two different types of species [Cu(bipy)₂](BF₄)₂ and [Cu(bipy)(L-glu)H₂O]BF₄ for 1 and [Cu(bipy)(L-glu)H₂O]ClO₄ and [Cu(bipy)(L-glu)(ClO₄)] for 2 coexist in the solid state. In complex 1, the [Cu(bipy)(L-glu)H₂O]⁺ units are joined together by syn–anticarboxylate bridges to form an enantiopure (M) helical chain and the [Cu(bipy)₂]²⁺ presents a very rare example of the four-coordinate distorted tetrahedral geometry of Cu(II). In complex 2, the [Cu(bipy)(L-glu)(ClO₄)] units are joined together by weakly coordinating perchlorate ions to form a 1D polymeric chain while the [Cu(bipy)(L-glu)H₂O]⁺ units remain as mononuclear species. The different coordinating ability of the two counter anions along with their involvement in the H-bonding network seems likely to be responsible for the difference in the final polymeric structures in the two compounds. Variable-temperature (2–300 K) magnetic susceptibility measurements show negligible coupling for both the complexes. The structure of 3 consists of two independent monomeric [Cu(phen)(L-glu)H₂O]⁺ cations, two nitrate anions and four water molecules. The copper atom occupies a five-coordinate square pyramidal environment with a water molecule in the axial position.