Phloroglucinol-bridged trinuclear complexes with three paramagnetic octahedral nickel(II) ions: Syntheses, crystal structures, and magnetic properties

Three new C-3-symmetric tritopic ligands with a central phloroglucinol bridging unit have been synthesized and characterized. The ligands are accessible through Schiff-base condensation of 2,4,6-triformylphloroglucinol with 2-aminomethylpyridine (H(3)tfpg-ampy), N,N-bis(pyridin-2-ylmethyl)-ethylenediamine (H(3)tfpg-unspenp), and benzhydrazide (H(6)tfpg-bhy). These ligands differ in nature and number of the donor atoms within the resulting binding pockets. Based on these ligands the synthesis of the first trinuclear phloroglucinol-bridged nickel(II) complexes with three octahedrally coordinated nickel centers is reported. The ligands H(3)tfpg-ampy and H(6)tfpg-bhy, which provide tridentate binding pockets, react with nickel(II) perchlorate in the presence of bis(pyridin-2-ylethyl)-amine (bpea) as an additional tridentate capping ligand leading to the formation of the trinuclear complexes [Ni-3(tfpg-ampy)(bpea)(3)](ClO4)(3) and [Ni-3(tfpg-bhy)(b-pea)(3)](ClO4)(3), respectively. Due to the pentadentate binding pocket in ligand H(3)tfpg-unspenp, no additional coligand is needed and a water molecule occupies the sixth coordination site at the nickel(II) ion resulting in the complex [Ni-3(tfpg-unspenp)(H2O)(3)](ClO4)(3). Temperature-dependent magnetic measurements reveal overall weak antiferromagnetic exchange interactions within the trinuclear complex together with a rather strong zero-field splitting (ZFS) for the nickel(II) ions. The observed isotropic coupling constants for the three complexes are in the range of 0.14 < - J < 0.37 cm (1), whereas for the zero-field splitting parameter vertical bar D vertical bar values between 1.8 and 5.5 cm (1) are found. This is indicative for competitive spin-polarization and superexchange mechanisms, with the latter prevailing the interaction between the nickel(II) ions through the meta-phenylene-linkage for the complexes reported. (C) 2011 Elsevier B.V. All rights reserved.