Anion coordination directed synthesis patterns for [Ni4] aggregates: structural changes for thiocyanate coordination and ligand arm hydrolysis

Abstract

The coordination reactivity of ligand H₅L1 (2,6-bis-{(1,3-dihydroxy-2-methylpropan-2-ylimino)methyl}-4-methylphenol) toward nickel(II) ions in the presence of two different anions resulted in cubane and fused dicubane types of tetranuclear coordination aggregates. In the first case, utilization of two carboxylate (R = Me and Et in RCO₂⁻) anions led to cubane type Ni₄O₄ aggregates [Ni₄(H₄L1)₂(μ₃-OH)₂(μ_1,3-O₂CCH₃)₂](NO₃)(CH₃COO)·3H₂O (1) and [Ni₄(H₄L1)₂(μ₃-OH)₂(μ_1,3-O₂CC₂H₅)₂](NO₃)(C₂H₅COO)·CH₃CN·1.5H₂O (2). Whereas the use of thiocyanate anions triggered single imine arm hydrolysis of H₅L1 and gave partial dicubane type Ni₄N₂O₄ aggregates [Ni₄(H₂L2)₂(HAMP)₂(μ_1,1-NCS)₂](NO₃)₂ (3a) and [Ni₄(H₂L2)₂(HAMP)₂(μ_1,1-NCS)₂]Cl₂·CH₃OH·H₂O (3b) (H₂L2 = 3-{(1,3-dihydroxy-2-methylpropan-2-ylimino)methyl}-2-hydroxy-5-methylbenzaldehyde). Liberated 2-amino-2-methyl-1,3-propanediol from imine arm hydrolysis was successfully trapped in 3a and 3b for the growth and stabilization of different molecular architectures. Mass spectral evidence established that two types of dinuclear fragments Ni₂(H₄L1) and Ni₂(H₂L2), from the originally used and partially hydrolyzed ligand anions have been self-assembled for two types of Ni^II based coordination aggregates. Two sets of complexes were characterized by elemental analysis, FT-IR and optical spectroscopy, variable temperature magnetic measurements, and single crystal X-ray analysis. The magnetic behavior examined through the temperature interval of 1.9–300 K with experimental and theoretical corroboration support the exchange pathways within the two types of [Ni₄] coordination aggregates.