Trinuclear Zn(II) and Cu(II) homo and heterotrimetallic complexes involving D-glucopyranosyl and biscarboxylate bridging ligands. A substrate bindingmodel of xylose isomerases

Reactions of MCl2. nH(2)O with N,N ' -bis(D-glucopyranosyl)-1,4,7-triazacyc lononane ((D-Glc)(2)-tacn), which was formed from D-glucose and 1,4,7-triaz acyclononane (tacn) in situ, afforded a series of mononuclear divalent meta l complexes with two beta -D-glucopyranosyl moieties, [M{(D-Glc)(2)-tacn}Cl ]Cl (M = Zn (11), Cu (12), Ni (13), Co(14)). Complexes 11-14 were character ized by analytical and spectroscopic measurements and X-ray crystallography and were found to have a distorted octahedral M(II) center ligated by the pentacoordinate N-glycoside ligand, (beta -D-glucopyranosyl)(2)-tacn, and a chloride anion. Each D-glucose moiety is tethered to the metal center thro ugh the beta -N-glycosidic bond with tacn and additionally coordinated via the C-2 hydroxyl group, resulting in a A-gauche five-membered chelate ring. When L-rhamnose (6-deoxy-L-mannose) was used instead Of D-glucose, the nic kel(II) complex with two beta -L-rhamnopyranosyl moieties, [Ni{(D-Man)(2)-t acn}(MeOH)]Cl-2 (15), was obtained and characterized by an X-ray analysis. Reactions of 11 (M = Zn) with [Zn(XDK)(H2O)] (21) or [Cu(XDK)(PY)(2)] (22) (H2XDK = m-xylylenediamine bis(Kemp's triacid imide)) yielded homo, and het erotrimetallic complexes formulated as [Zn2M '{(D-Glc)2-tacn}(2)(XDK)]Cl-2 (M ' = Zn (31), Cu (32)). The similar reactions of 12 (M = Cu) with complex 21 or 22 afforded [Cu2M '{(D-Glc)(2)-tacn}(2)(XDK)]Cl-2 (M ' = Cu (33), Zn (34)). An X-ray crystallographic study revealed that complexes 31 and 34 h ave either Zn-3(II) or (CuZnCuII)-Zn-II-Cu-II trimetallic centers bridged b y two carboxylate groups of XDK and two D-glucopyranosyl residues. The (MM) -M-... ' separations are 3.418(3)-3.462(3) Angstrom (31) and 3.414(1)-3.460 (1) Angstrom (34), and the (MM)-M-... ' M-... angles are 155.18(8)degrees ( 31) and 161.56(6)degrees (34). The terminal metal ions are octahedrally coo rdinated by the (D-Glc)2-tacn ligand through three nitrogen atoms of tacn, two oxygen atoms of the C-2 hydroxyl groups of the carbohydrates, and a car boxylate oxygen atom of XDK ligand. The central metal ions sit in. a distor ted octahedral environment ligated by four oxygen atoms of the carbohydrate residues in the (D-Glc)2-tacn ligands and two carboxylate oxygen atoms of XDK. The deprotonated beta -D-glucopyranosyl unit at the C-2 hydroxyl group bridges the terminal and central ions with the C-2 mu -alkoxo group, with the C-1 N-glycosidic amino and the C-3 hydroxyl groups coordinating to each metal center. Complexes 31-34 are the first examples of metal complexes in which D-glucose units act as bridging ligands. These structures could be v ery useful substrate binding models of xylose or glucose isomerases, which promote D-glucose D-fructose isomerization by using divalent dimetallic cen ters bridged by a glutamate residue.