Preparations And Properties Of Transition-Metal Pterin Complexes - Models For The Metal Site In Phenylalanine-Hydroxylase

Syntheses and physical properties of pterin and pteridine complexes of first-row transition metals are described. Characterization used single-crystal x-ray diffraction, spectroscopic, and microanalytical methods. These data all indicate that pterin and pteridine ligands chelate via oxygen and nitrogen atoms. Three copper complexes have been structurally determined. Cu(tppb)(pterin) (1)[tppb- = tris(3-phenylpyrazolyl)hydroborate] crystallizes in the triclinic space group P1BAR with cell dimensions a = 11.835 (2) angstrom, b = 12.062 (2) angstrom, c = 12.831 (2) angstrom, alpha = 66.91 (1)-degrees, beta = 83.68 (1)-degrees, and gamma = 77.10(1)-degrees defining a volume of 1641.9 angstrom 3 for Z = 2. The five-coordinate cupric ion has a square-pyramidal geometry, and the pterin ligand occupies two equatorial positions. Pterin coordination in 1 significantly differs from the only extant report of a cupric pterin complex both in orientation and in Cu-O bond strength. Cu(ethp)2(phen) (4) [ethp = 2-(ethylthio)-4-oxopteridine; phen = 1, 10-phenanthroline] also crystallizes in the triclinic space group PlBAR. Unit cell parameters a = 12.414 (2) angstrom, b = 12.882 (2) angstrom, c = 11.371 (2) angstrom, alpha = 112.55 (2)-degrees, beta = 92.58 (1)-degrees, and gamma = 83.96 (1)-degrees define a volume of 1670.1 angstrom 3 for Z = 2. The copper coordination sphere has an elongated octahedral geometry defined by four equatorial nitrogen atoms (two from the pteridines and two from phenanthroline) with axial positions filled by oxygen atoms of the pteridine chelates. The third structure reported is for the compound [Cu(phen)2(acetate)] [acetate-H-ethp] (5). In this structure, the pteridine does not chelate copper but is incorporated into the anionic counterion. The compound crystallizes in the monoclinic cell P2(1)/c with parameters a = 11.755 (2) angstrom, b = 19.079 (2) angstrom, c = 16.329 (2) angstrom, and beta = 110.36 (1)-degrees, resulting in a cell volume of 3343.6 angstrom 3 for Z = 4. The copper atom is coordinated in a distorted square-pyramidal geometry composed of two phenanthroline chelates and one monodentate acetate ligand. The anion can be formulated as an acetate hydrogen-bonded to the hydroxyl group of the pteridine enol tautomer. A comparison of EPR parameters for the copper complexes reported in this manuscript with previous data for copper pterin complexes reveals that only the value of A parallel-to is sensitive to variation in copper environments. Spectroscopic and structural data point to a stronger metal-pterin interaction for equatorially bound pterinate ligands. Preliminary results of copper(II) reduction by tetrahydropterin are also presented.