Copper Ion Complexes with Derivatized Chelating Pterin Ligands

Pterin−copper 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)(CH 3 CN)](PF 6 ) ( 3 -PF 6 ) is described [space group P 2 1 / c; a = 10.606(3) Å, b = 21.683(3) Å, c = 12.188(3) Å; β = 96.67(2)°; Z = 4; V = 2784(1) Å 3 ] and compared with the previously reported structure of [ L Cu(Cl)](PF 6 )(NaPF 6 ) ( 1 -PF 6 ·NaPF 6 ). In 3 -PF 6, the Cu(II) is pentacoordinate, including equatorial ligation to the pterin N5 atom, achieving one of the goals of the ligand design. Unlike other pterin−Cu(II) structures, neither 1 -PF 6 ·NaPF 6 nor 3 -PF 6 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 [ L Cu I ](PF 6 ) ( 5 -PF 6 ) and [ L ‘ Cu I ](PF 6 )·0.25CH 2 Cl 2 ( 6 -PF 6 ·0.25CH 2 Cl 2 ) have been generated and characterized. Observed coordination-induced 1 H NMR chemical shifts in 6 -PF 6 ·0.25CH 2 Cl 2 ) 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 -PF 6 ·0.25CH 2 Cl 2 (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 (CH 3 CN)](CF 3 SO 3 ) 2 ( 4 -(CF 3 SO 3 ) 2 ) exhibits the identical cyclic voltammogram, indicating common solution structures. The analogue complex [( Me L )Cu I (CH 3 CN)](ClO 4 ) ( 7 -ClO 4 ) (Me L = (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 O 2 reactivity of copper(I) complexes of L and L ‘ .