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 D2d (highly distorted 8-vertex triangular dodecahedron) and high D5h (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.
