CRYSTAL-STRUCTURE AND EPR-SPECTRA OF GLYCILGLYCILGLYCINOCOPPER(II) BROMIDE SESQUIHYDRATE

The title compound, Cu(glyglygly)Br . 1 . 5H(2)O, crystallizes in the space group C2/c, with a = 21.468(7), b = 6.716(5), c = 16.166(6) Angs trom, beta = 98.39 degrees, and Z = 8. The tripeptide is bonded to one Cu(II) ion through the nitrogen [Cu-N = 1.97(1) Angstrom] and oxygen [Cu-O = 2.019(8) Angstrom] atoms of the amino end glycine residue and to another Cu(II) through one oxygen atom [Cu-O = 1.931(9) Angstrom] o f the terminal carboxyl group. This give rise to covalently bonded and infinite ...-Cu-tripeptide-Cu-... chains. These chains are linked to one another by a network of H-bonds involving the water molecules and bromide ions. The Cu(II) ion is in a distorted tetragonal pyramidal co ordination polyhedron. At the corner of the base of the pyramid are th e terminal glycine nitrogen and oxygen atoms of one tripeptide, a carb oxylic oxygen of another tripeptide and a bromide ion. ?he fivefold co ordination is completed with a water molecule at the top of the pyrami d [Cu-Ow = 2.286(9) Angstrom]. For all orientations of the applied mag netic field the single crystal EPR spectra display a single anisotropi c exchange collapsed resonance without hyperfine structure. Its positi on was measured in three perpendicular planes and the crystal g-tensor evaluated from the data. This tensor is interpreted in terms of the c ontributing Cu(II) complexes in the unit cell to deduce the principal values g(1) = 2.273, g(2) = 2.050 and g(3) = 2.131 for the molecular g yromagnetic tenser. We also discuss the magnitude of the exchange inte raction between neighboring copper ions in the lattice on the basis of the features in the EPR spectra and the structural information.