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Directed Synthesis of a Heterobimetallic Complex Based on a Novel Unsymmetric Double-Schiff-Base Ligand: Preparation, Characterization, Reactivity and Structures of Hetero- and Homobimetallic Nickel(II) and Zinc(II) Complexes

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Abstract

A series of bimetallic zinc(II) and nickel(II) complexes based on the novel dinucleating unsymmetric double-Schiff-base ligand benzoic acid [1-(3-{[2-(bispyridin-2-ylmethylamino)ethylimino]methyl}-2-hydroxy-5-methylphenyl)methylidene]hydrazide (H2bpampbh) has been synthesized and structurally characterized. The metal centers reside in two entirely different binding pockets provided by the ligand H2bpampbh, a planar tridentate [ONO] and a pentadentate [ON4] compartment. The utilized ligand H2bpampbh has been synthesized by condensation of the single-Schiff-base proligand Hbpahmb with benzoic acid hydrazide. The reaction of H2bpampbh with two equivalents of either zinc(II) or nickel(II) acetate yields the homobimetallic complexes [Zn2(bpampbh)(μ,η1-OAc)(η1-OAc)] (ZnZn) and [Ni2(bpampbh)(μ-H2O)(η1-OAc)(H2O)](OAc) (NiNi), respectively. Simultaneous presence of one equivalent zinc(II) and one equivalent nickel(II) acetate results in the directed formation of the heterobimetallic complex [NiZn(bpampbh)(μ,η1-OAc)(η1-OAc)] (NiZn) with a selective binding of the nickel ions in the pentadentate ligand compartment. In addition, two homobimetallic azide-bridged complexes [Ni2(bpampbh)(μ,η1-N3)]ClO4 (NiNi(N3)) and [Ni2(bpampbh)(μ,η1-N3)(MeOH)2](ClO4)0.5(N3)0.5 (NiNi(N3)(MeOH)2) were synthesized. In all complexes, the metal ions residing in the pentadentate compartment adopt a distorted octahedral coordination geometry, whereas the metal centers placed in the tridentate compartment vary in coordination number and geometry from square-planar (NiNi(N3)) and square-pyramidal (ZnZn and NiZn), to octahedral (NiNi and NiNi(N3)(MeOH)2). In the case of complex NiNi(N3) this leads to a mixed-spin homodinuclear nickel(II) complex. All compounds have been characterized by means of mass spectrometry as well as IR and UV/Vis spectroscopies. Magnetic susceptibility measurements show significant zero-field splitting for the nickel-containing complexes (D=2.9 for NiZn, 2.2 for NiNi(N3), and 0.8 cm−1 for NiNi) and additionally a weak antiferromagnetic coupling (J=−1.4 cm−1) in case of NiNi. Electrochemical measurements and photometric titrations reveal a strong Lewis acidity of the metal center placed in the tridentate binding compartment towards external donor molecules. A significant superoxide dismutase reactivity against superoxide radicals was found for complex NiNi.

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