Self-Assembled Tetra- and Pentanuclear Nickel(II) Aggregates From Phenoxido-Based Ligand -Bound {Ni2} Fragments: Carboxylate Bridge Controlled Structures

Abstract

Three different carboxylato bridges (R = C2H5, CF3, and PhCH2 in RCO2(-)) have been used to obtain the supramolecular aggregates [Ni5(μ-H2bpmp)2(μ3-OH)2(μ1,3-O2CC2H5)6]·2H2O·4DMF (1·2H2O·4DMF), [Ni4(μ3-H2bpmp)2(μ3-OH)2(μ1,3-O2CCF3)2](CF3CO2)2·H2O (2·H2O), and [Ni4(μ3-H2bpmp)2(μ3-OH)2(μ1,3-O2CCH2Ph)2](PhCH2CO2)2·4H2O (3·4H2O) (H3bpmp =2,6-bis-[(3-hydroxy-propylimino)-methyl]-4-methyl-phenol) from the hydroxido-bridged dinuclear motif [Ni2(μ-H2bpmp)(OH)](2+). These complexes have been characterized by X-ray crystallography and magnetic measurements. A change from propanoate group to trifluoroacetate and phenylaceate groups provided different course of cluster assembly based on Ni2(μ-H2bpmp)2 fragments. The {Ni5(μ3-OH)2(μ1,3-O2CC2H5)6}(2+) core in 1 contains five Ni(II) ions in an hourglass (pentanuclear vertex-shared double cubane) arrangement. These compounds are new examples of [Ni5] and [Ni4] complexes where aggregation of the building motifs are guided by the nature of the carboxylate anions, which allows an effective tuning of the self-aggregate process within same ligand environment. The study of the magnetic properties reveals that 1 exhibits an S = 3 ground state. Nevertheless, the magnetization increases above the expected saturation value of 6 μB at higher fields, because of the suppression of antiferromagnetic exchange between the central and peripheral Ni(II) ions. Complexes 2 and 3 exhibit ferromagnetic exchange interactions that result in the S = 4 ground state. Examination of AC magnetic susceptibility showed that complex 2 in finely ground form behaves as spin glass with the spin-freezing temperature of ∼5.5 K. This behavior was attributed to the collapse of the structure upon the loss of interstitial solvent. Such property was not observed for complex 3, in which the bulkier carboxylate ligands provide for a more robust crystal packing and larger separation between the [Ni4O4] clusters.