Novel nickel(II) complexes with diazamesocyclic ligands functionalized by additional phenol donor pendant(s): synthesis, characterization, crystal structures and magnetic properties

Abstract

The formation of two unique μ-phenoxo-bridged nickel(II) complexes with l,5-diazacyclooctane (DACO) ligands functionalized by additional phenol donor pendants has been achieved and the complexes have been characterized by IR, elemental analyses, conductivity, thermal analysis, and UV-Vis techniques. Single-crystal X-ray diffraction analyses reveal that HL¹11 forms a phenoxo-bridged binuclear complex (in the presence of N₃⁻ anions) [NiL¹11(N₃)]₂·0.5H₂O 1, H₂2L²2 a phenoxo-bridged edge-sharing linear trinuclear complex [Ni₃(L²2)₂(C₂H₅OH)₂Cl₂] 2, where HL¹11 = 1-(2-hydroxybenzyl)-1,5-diazacyclooctane and H₂2L²2 = 1,5-bis(2-hydroxybenzyl)-1,5-diazacyclooctane. In 1 the two Ni^II are in five-coordinated distorted square-pyramidal environments with a intramolecular Ni ⋯ Ni distance of 3.1232(13) Å. However in 2 the two terminal Ni^II are in nearly ideal square-pyramidal coordination environments and the central Ni^II takes an octahedral configuration by axial coordination of two ethanol molecules, the adjacent intramolecular Ni ⋯ Ni distance being 3.132(2) Å. Such a trinuclear nickel(II) complex with square-pyramidal/octahedral/square-pyramidal coordination is unprecedented. These results also indicate that the coordination chemistry of the nickel(II) complexes with DACO ligands can be controlled by altering the pendant donors. Variable-temperature magnetic data show that both complexes display modest intramolecular antiferromagnetic coupling between adjacent nickel(II) centers, which is unusual for such linear trinuclear nickel(II) complexes. The magneto-structural correlation has been investigated for both complexes, and a rough correlation between the exchange coupling and the Ni–O–Ni angle of phenoxo-bridged binuclear nickel(II) complexes obtained and discussed.

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