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

Abstract

The reaction of benzyl 2-amino-4,6-O-benzylidene-2-deoxy-alpha-D-glucopyranoside (HL) with the metal salts Cu(ClO(4))(2)6 H(2)O and Ni(NO(3))(2)6 H(2)O affords via self-assembly a tetranuclear mu(4)-hydroxido bridged copper(II) complex [(mu(4)-OH)Cu(4)(L)(4)(MeOH)(3)(H(2)O)](ClO(4))(3) (1) and a trinuclear alcoholate bridged nickel(II) complex [Ni(3)(L)(5)(HL)]NO(3) (2), respectively. Both complexes crystallize in the acentric space group P2(1). The X-ray crystal structure reveals the rare (mu(4)-OH)Cu(4)O(4) core for complex 1 which is mu(2)-alcoholate bridged. The copper(II) ions possess a distorted square-pyramidal geometry with an [NO(4)] 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 [N(4)O(2)] donor set is observed at the nickel(II) ions with a distorted octahedral geometry. The trinuclear isosceles Ni(3) core is bridged by mu(3)-alcoholate 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 (4)C(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(1,2)=2.183, g(3)=2.247). For the high-spin nickel(II) centers a zero-field splitting of D(1,2)=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.

A New Family of Trinuclear Nickel(II) Complexes as Single‐Molecule Magnets

“…In addition to the bridging atoms, as mentioned above, two symmetry related nickel atoms, Ni(1) and Ni(1)', are bonded to the facially coordinated monoanionic tridentate L through the amine nitrogen atom N(1), the imine nitrogen atom N(2), and the phenoxido oxygen atom O(1), whereas Ni(2) is coordinated to three donor atoms of L in a meridional configuration. [21,32] A similar oxido-bridged core has also been found in the two heterometallic clusters Ni 3 M'O 4 (M' = K and Na). The geometry around Ni (2) Trinuclear nickel(II) complexes with a Ni 3 core motif similar to that of the present complexes have been reported with bridging oxygen atoms of phenoxido, [30] amino alcohol, [31] and carbohydrate systems.…”

“…Trinuclear nickel(II) complexes with a Ni 3 core motif similar to that of the present complexes have been reported with bridging oxygen atoms of phenoxido, [30] amino alcohol, [31] and carbohydrate systems. [21,32] A similar oxido-bridged core has also been found in the two heterometallic clusters Ni 3 M'O 4 (M' = K and Na). [20] However, to our knowledge, complexes 1-3 are the first examples of such motifs in which a chlorine or nitrogen atom acts as a bridging atom along with the oxido bridges, making the bridging system heteroatomic.…”

“…Single-crystal structural analyses revealed that all three complexes have a similar Ni 3 core motif with three different types of bridging, namely phenoxido (m 2 and m 3 ), hyA C H T U N G T R E N N U N G drox-A C H T U N G T R E N N U N G ido (m 3 ), and m 2 -Cl (1), m 1,1 -NCO (2), or m 1,1 -N 3 (3). [20,21] Herein we report the synthesis, crystal structures, and magnetic behavior of a new family of heterobridged (phen-A C H T U N G T R E N N U N G oxido, hydroxido, and halo or pseudohalo) Ni 3 trinuclear complexes with the formulae [Ni 3 L 3 (OH) (Cl) (HL). Single-crystal magnetization measurements on complex 1 revealed that the pseudo-three-fold axis of Ni 3 corresponds to a magnetic easy axis, being consistent with the magnetic anisotropy expected from the coordination structure of each nickel ion.…”

“…A few ferromagnetically coupled oxido-bridged complexes containing the trinuclear Ni 3 core have been reported, but they possessed positive D values and consequently did not behave as SMMs. [20,21] Herein we report the synthesis, crystal structures, and magnetic behavior of a new family of heterobridged (phen-A C H T U N G T R E N N U N G oxido, hydroxido, and halo or pseudohalo) Ni 3 trinuclear complexes with the formulae [Ni 3 L 3 (OH) (Cl)…”

Antiferro- to ferromagnetic crossover in diphenoxido bridged NiII2MnII complexes derived from N2O2 donor Schiff base ligands

“…From the experimental observations and theoretical calculations, it has been found that as Ni-O-Ni angle increases from 90º, the ferromagnetic coupling decreases and becomes antiferromagnetic at ca. 93.5º for µ 2 -O bridged dinuclear Ni II complexes [5].…”