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Trinuclear Zn(II) and Cu(II) Homo and Heterotrimetallic Complexes Involving d -Glucopyranosyl and Biscarboxylate Bridging Ligands. A Substrate Binding Model of Xylose Isomerases

2001; American Chemical Society; Volume: 40; Issue: 16 Linguagem: Inglês

10.1021/ic001419t

1520-510X

Tomoaki Tanase, Hiromi Inukai, Tomoko Onaka, Masako Kato, Shigenobu Yano, Stephen J. Lippard,

Carbohydrate Chemistry and Synthesis

Reactions of MCl2·nH2O with N,N'-bis(d-glucopyranosyl)-1,4,7-triazacyclononane ((d-Glc)2-tacn), which was formed from d-glucose and 1,4,7-triazacyclononane (tacn) in situ, afforded a series of mononuclear divalent metal complexes with two β-d-glucopyranosyl moieties, [M{(d-Glc)2-tacn}Cl]Cl (M = Zn (11), Cu (12), Ni (13), Co (14)). Complexes 11−14 were characterized 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, (β-d-glucopyranosyl)2-tacn, and a chloride anion. Each d-glucose moiety is tethered to the metal center through the β-N-glycosidic bond with tacn and additionally coordinated via the C-2 hydroxyl group, resulting in a λ-gauche five-membered chelate ring. When l-rhamnose (6-deoxy-l-mannose) was used instead of d-glucose, the nickel(II) complex with two β-l-rhamnopyranosyl moieties, [Ni{(d-Man)2-tacn}(MeOH)]Cl2 (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 heterotrimetallic complexes formulated as [Zn2M'{(d-Glc)2-tacn}2(XDK)]Cl2 (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)]Cl2 (M' = Cu (33), Zn (34)). An X-ray crystallographic study revealed that complexes 31 and 34 have either ZnII3 or CuIIZnIICuII trimetallic centers bridged by two carboxylate groups of XDK and two d-glucopyranosyl residues. The M···M' separations are 3.418(3)−3.462(3) Å (31) and 3.414(1)−3.460(1) Å (34), and the M···M'···M angles are 155.18(8)° (31) and 161.56(6)° (34). The terminal metal ions are octahedrally coordinated 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 carboxylate oxygen atom of XDK ligand. The central metal ions sit in a distorted 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 β-d-glucopyranosyl unit at the C-2 hydroxyl group bridges the terminal and central ions with the C-2 μ-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 very useful substrate binding models of xylose or glucose isomerases, which promote d-glucose d-fructose isomerization by using divalent dimetallic centers bridged by a glutamate residue.