Assembly of Carbohydrates on a Nickel(II) Center by Utilizing N-Glycosidic Bond Formation with Tris(2-aminoethyl)amine (tren). Syntheses and Characterization of [Ni{N-(aldosyl)-tren}(H2O)]2+, [Ni{N,N‘-bis(aldosyl)-tren}]2+ and [Ni{N,N‘,N‘‘-tris(aldosyl)-tren}]2+

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Abstract
Reactions of [Ni(tren)(H2O)2]X2 (tren = tris(2-aminoethyl)amine; X = Cl (1a), Br (1b); X2 = SO4 (1c)) with mannose-type aldoses, having a 2,3-cis configuration (d-mannose and l-rhamnose), afforded {bis(N-aldosyl-2-aminoethyl)(2-aminoethyl)amine}nickel(II) complexes, [Ni(N,N‘-(aldosyl)2-tren)]X2 (aldosyl = d-mannosyl, X = Cl (2a), Br (2b), X2 = SO4 (2c); aldosyl = l-rhamnosyl, X2 = SO4 (3c)). The structure of 1c was confirmed by X-ray crystallography to be a mononuclear [NiIIN4O2] complex with the tren acting as a tetradentate ligand (1c·2H2O: orthorhombic, Pbca, a = 15.988(2) Å, b = 18.826(4) Å, c = 10.359(4) Å, V = 3118 Å3, Z = 8, R = 0.047, and Rw = 0.042). Complexes 2a,c and 3c were characterized by X-ray analyses to have a mononuclear octahedral Ni(II) structure ligated by a hexadentate N-glycoside ligand, bis(N-aldosyl-2-aminoethyl)(2-aminoethyl)amine (2a·CH3OH: orthorhombic, P212121, a = 16.005(3) Å, b = 20.095(4) Å, c = 8.361(1) Å, V = 2689 Å3, Z = 4, R = 0.040, and Rw = 0.027. 2c·3CH3OH: orthorhombic, P212121, a = 14.93(2) Å, b = 21.823(8) Å, c = 9.746(2) Å, V = 3176 Å3, Z = 4, R = 0.075, and Rw = 0.080. 3c·3CH3OH: orthorhombic, P212121, a = 14.560(4) Å, b = 21.694(5) Å, c = 9.786(2) Å, V = 3091 Å3, Z = 4, R = 0.072, and Rw = 0.079). The sugar part of the complex involves novel intramolecular sugar−sugar hydrogen bondings around the metal center. The similar reaction with d-glucose, d-glucosamine, and d-galactosamine, having a 2,3-trans configuration, resulted in the formation of a mono(sugar) complex, [Ni(N-(aldosyl)-tren)(H2O)2]Cl2 (aldosyl = d-glucosyl (4b), 2-amino-2-deoxy-d-glucosyl (5a), and 2-amino-2-deoxy-d-galactosyl (5b)), instead of a bis(sugar) complex. The hydrogen bondings between the sugar moieties as observed in 2 and 3 should be responsible for the assembly of two sugar molecules on the metal center. Reactions of tris(N-aldosyl-2-aminoethyl)amine with nickel(II) salts gave the tris(sugar) complexes, [Ni(N,N‘,N‘‘-(aldosyl)3-tren)]X2 (aldosyl = d-mannosyl, X = Cl (6a), Br (6b); l-rhamnosyl, X = Cl (7a), Br (7b); d-glucosyl, X = Cl (9); maltosyl, X = Br (10); and melibiosyl, X = Br (11)), which were assumed to have a shuttle-type C3 symmetrical structure with Δ helical configuration for d-type aldoses on the basis of circular dichroism and 13C NMR spectra. When tris(N-rhamnosyl)-tren was reacted with NiSO4·6H2O at low temperature, a labile neutral complex, [Ni(N,N‘,N‘‘-(l-rhamnosyl)3-tren)(SO4)] (8), was successfully isolated and characterized by X-ray crystallography, in which three sugar moieties are anchored only at the N atom of the C-1 position (8·3CH3OH·H2O: orthorhombic, P212121, a = 16.035(4) Å, b = 16.670(7) Å, c = 15.38(1) Å, V = 4111 Å3, Z = 4, R = 0.084, and Rw = 0.068). Complex 8 could be regarded as an intermediate species toward the C3 symmetrical tris(sugar) complexes 7, and in fact, it was readily transformed to 7b by an action of BaBr2.

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