Hydrogen Bonds as Structural Directive towards Unusual Polynuclear Complexes: Synthesis, Structure, and Magnetic Properties of Copper(II) and Nickel(II) Complexes with a 2-Aminoglucose Ligand

The reaction of benzyl 2-amino-4,6-O-benzylidene-2-deoxy-alpha-D-glucopyranoside (HL) with the metal salts Cu(ClO4)(2)center dot 6H(2)O and Ni-(NO3)(2)center dot 6H(2)O affords via self-assembly a tetranuclear mu(4)-hydroxido bridged copper(II) complex [(mu(4)-OH)Cu-4(L)(4)center dot (MeOH)(3)(H2O)](ClO4)(3) (1) and a trinuclear alcoholate bridged nickel(II) complex [Ni-3(L)(5)(HL)]NO3 (2), respectively. Both complexes crystallize in the acentric space group P2(1). The X-ray crystal structure reveals the rare (mu(4)-OH)(Cu4O4 core for complex I which is mu(2)-alcoholate bridged. The copper(II) ions possess a distorted square-pyramidal geometry with an [NO4] donor set. The core is stabilized by hydrogen bonding between the coordinating amino group of the glucose backbone and the benzylidene protected oxygen atom O4 of a neighboring (Cu(L)) fragment as hydrogen-bond acceptor. For complex 2 an [N4O2] donor set is ob served at the nickel(II) ions with a distorted octahedral geometry. ne trinuclear isosceles Ni-3 core is bridged by mu(3)-alcollolate O3 oxygen atoms of two glucose ligands. The two short edges are capped by mu(2)-alcoholate O3 oxygen atoms of the two ligands coordinated at the nickel(II) ion at the vertex of these two edges. Along the elongated edge of the triangle a strong hydrogen bond (244 pm) between the O3 oxygen atoms of ligands coordinating at the two relevant nickel(II) ions is observed. The coordinating amino groups of the these two glucose ligands are involved in additional hydrogen bonds with O4 oxygen atoms of adjacent ligands further stabilizing the trinuclear core. The carbohydrate backbones in all cases adopt the stable C-4(1) chair conformation and exhibit the rare chitosan-like trans-2,3-chelation. Temperature dependent magnetic measurements indicate an overall antiferromagnetic behavior for complex 1 with J(1) = -260 and J(2) = 205 cm(-1) (g-2.122). Compound 2 is the first ferromagnetically coupled trinuclear nickel(II) complex with J(A) = 16.4 and J(B) = 11.0 cm (1) (g(12) = 2.183, g(3) = 2.247). For the high-spin nickel(II) centers a zero-field splitting of D-1,D-2 = 3.7 cm (1) and D-3 = 1.8 cm (1) is observed. The S = 3 ground state of complex 2 is consistent with magnetization measurements at low temperatures.