Copper ion complexes with derivatized chelating pterin ligands

Pterin̄copper ion interactions are of interest for possible modeling of enzyme active site chemistry or in coupling reactions of a redox-active metal ion with a redox-active ligand cofactor, Here, we describe the synthesis of derivatized chelating pterins, provided with enhanced organic solubility by introduction of a pivalolyl group at the pterin C2 position and supplied with a good copper ion chelator, through reaction of the 6-(bromomethyl)pterin with a 2-(2-(methylamino)ethyl)pyridine (to give L') or bis(2-pyrid-2-ylethyl)amine (to give L′). Copper(I) and copper(II) complexes have been synthesized and characterized. The X-ray structure of [L′Cu(Cl)(CH3CN)](PF₆) (3-PF₆) is described [space group P2₁lc; a = 10.606(3) ̊, b = 21.683(3) ̊, c = 12.188(3) ̊; β= 96.67(2)°; Z = 4; V = 2784(1) ̊³] and compared with the previously reported structure of [LCu(Cl)](PF₆)-(NaPF₆) (l-PF₆·NaPF₆). In 3-PF₆, the Cu(II) is pentacoordinate, including equatorial ligation to the pterin N5 atom, achieving one of the goals of the ligand design. Unlike other pterin̄Cu(II) structures, neither 1-PF₆·-NaPF₆ nor 3-PF₆ binds to the pterin carbonyl oxygen atom (off pterin C4). Other solution spectroscopic data (e.g., IR, UV-vis, EPR) and a magnetic moment of 1.91 μ_B are provided and are consistent with the solid-state structure observed. Copper(I) complexes [LCu¹](PF₆) (5-PF₆) and [L′Cu¹](PF₆)·0.25CH₂Cl₂ (6-PF₆·0.25CH₂Cl₂) have been generated and characterized. Observed coordination-induced ¹H NMR chemical shifts in 6-PF₆·0.25CH₂-C1₂) are consistent with binding through the pterin pyrazine N5 donor; a UV-vis feature at 466 nm (sh, ε = 1200) is considered a MLCT transition. A cyclic voltammogram of 6-PF₆·0.25CH₂Cl₂ (DMF solvent) reveals a reversible redox process with E_1/2 = 286 mV (E_1/2 = 546 mV for ferrocene/ferrocenium; vs Ag/AgCl). [L'Cu_II(CH₃-CN)](CF₃SO₃)₂ (4-(CF₃SO₃)₂) exhibits the identical cyclic voltammogram, indicating common solution structures. The analogue complex [(MeL)Cu¹(CH₃CN)](C1O₄) (7-C1O₄) (MeL = (2-(2-pyridyl)ethyl)(2-pyridylmethyl)-methylamine) displays a more negative redox potential, E_1/2 = 24 mV, indicating that the pterin pyrazine ring has an electron-withdrawing effect, which may explain the lack of O₂ reactivity of copper(I) complexes of L and L'.