Mononuclear complexes of dysprosium(iii) with 2,6-diacetylpyridine bis(isonicotinoylhydrazone): synthesis, crystal structure, and magnetic properties

Abstract

Four discrete mononuclear Dy(III) coordination compounds with the pentadentate N₃O₂ equatorial ligand 2,6-diacetylpyridine-bis(isonicotinoylhydrazone) (H₂dapin) in different charge states, containing the strong-field ligands Ph₃PO and Cy₃PO at the apical positions, have been synthesized: [Dy(H₂dapin)(Ph₃PO)₃](CF₃SO₄)₃ (1), [Dy(Hdapin)(Ph₃PO)₃](ClO₄)₂·C₂H₅OH·1.9H₂O (2), [Dy(Hdapin)(Cy₃PO)₂](CF₃SO₃)₂ (3) and [Dy(Hdapin)(Cy₃PO)₂] [Dy(dapin)(Cy₃PO)₂](CF₃SO₃)₃ (4). The crystal structures of the complexes and DC and AC magnetic properties of 1 and 3 have been investigated. Complexes 1 and 2 with triphenylphosphinoxide apical ligands are eight-coordinated, whereas 3 and 4 with bulkier tricyclohexylphosphinoxide ligands are rare seven-coordinated Dy(III) complexes. The pyridine-substituted pentadentate N₃O₂ ligand exhibits variability in composition and charge: neutral doubly-protonated H₂dapin in complex 1, monodeprotonated negatively charged Hdapin⁻ in 2 and 3, and monodeprotonated Hdapin⁻ and fully deprotonated dapin²⁻ in 4. The proton at one of the hydrazide groups of the H₂dapin ligand in 1 is transferred to the terminal pyridine N atom. The resulting neutral H₂dapin ligand adopts a zwitterionic form with 1− charge localized on the enolate O atom of the deprotonated part and 1+ charge on the N atom of the protonated pyridine. The proton transfer to the pyridyl N-atom is observed in all Hdapin⁻ ligands in 2–4. According to SHAPE analysis, complexes 1 and 3 have significantly different local symmetries – low D_2d (highly distorted 8-vertex triangular dodecahedron) and high D_5h (7-vertex pentagonal bipyramid), respectively. In contrast to 1, complex 3 is a single-ion magnet and shows slow magnetic relaxation. The experimental study of the magnetic properties was complemented by theoretical analysis based on ab initio calculations.