The first pentagonal-bipyramidal vanadium(iii) complexes with a Schiff-base N3O2 pentadentate ligand: synthesis, structure and magnetic properties

Abstract

A series of three mononuclear pentagonal-bipyramidal V(III) complexes with the equatorial pentadentate N₃O₂ ligand (2,6-diacethylpyridinebis(benzoylhydrazone), H₂DAPBH) in the different charge states (H₂DAPBH⁰, HDAPBH¹⁻, DAPBH²⁻) and various apical ligands (Cl⁻, CH₃OH, SCN⁻) were synthesized and characterized structurally and magnetically: [V(H₂DAPBH)Cl₂]Cl·C₂H₅OH (1), [V(HDAPBH)(NCS)₂]·0.5CH₃CN·0.5CH₃OH (2) and [V(DAPBH)(CH₃OH)₂]Cl·CH₃OH (3). All three complexes reveal paramagnetic behavior, resulting from isolated S = 1 spins with positive zero-field splitting energy expected for the high-spin ground state of the V³⁺ (3d²) ion in a PBP coordination. Detailed high-field EPR measurements for compound 3 show that its magnetic properties are best described by using the spin Hamiltonian with the positive ZFS energy (D = +4.1 cm⁻¹) and pronounced dimer-like antiferromagnetic spin coupling (J = −1.1 cm⁻¹). Theoretical analysis based on superexchange calculations reveals that the long-range spin coupling between distant V³⁺ ions (8.65 Å) is mediated through π-stacking contacts between the planar DAPBH²⁻ ligands of two neighboring [V(DAPBH)(CH₃OH)₂]⁺ complexes.