Bifunctional pyrimidine-amino-acid ligands: solution study and crystal structure of a Mn(II) chain alternating six- and sevenfold coordination environments

Abstract

The acid–base characterization in aqueous solution of the N-2-[4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)methionine, a member of a family of bifunctional N-pyrimidine α-amino acids ligands, has been carried out by potentiometric and UV–Vis techniques in the 2.5–9.0 pH range, indicating a quasi-zwitterionic structure. The solution study of the HL/Mn(II) system at 1:2, 2:1 and 4:1 molar ratios (25 °C and pH<6), reveals that both moieties are active in coordinating to the metal ion, through the carboxylate group when the ligand is deprotonated or through the pyrimidinic fragment when acting in its molecular form. A solid complex with MnL 2·6 1 2 H 2O stoichiometry was isolated from an aqueous 1:3 [HL]/[Mn(II)] mixture at pH 6. The X-ray single-crystal characterization has revealed that this complex can be formulated as {[Mn(H 2O) 4(μ-L) 2Mn(L) 2(H 2O)]·8H 2O} n , an infinite chain in which two different Mn(II) ions having six- and sevenfold coordination environments alternate along the chain. The asymmetric unit contains two ligands coordinating in different fashion. One of these coordinates monodentately through the oxygen atom belonging to the exocyclic nitroso group while the other exhibits a 3η-bridging pattern between the six- and sevenfold Mn(II) arrangements. The versatile coordination modes of this ligand is discussed and compared to other complexes of this bifunctional family of ligands.