High nuclearity Ni(ii) cages from hydroxamate ligands

Abstract

The synthesis, structural and magnetic characterisation of a family of Ni(II) cages built from hydroxamate ligands is presented. Two pentanuclear 12-MC_Ni(II)-4 metallacrowns [Ni₅(L₁)₄(MeOH)₄](ClO₄)₂·2MeOH (1) and [Ni₅(L₁)₄(py)₅](ClO₄)₂·H₂O (2) (where L₁H₂ = 2-(dimethylamino)phenylhydroxamic acid) share analogous, near-planar {Ni₅(L₁)₄}²⁺ cores, but differ in the number and nature of the ligands located at the axial Ni(II) sites; the addition of pyridine converting square planar Ni(II) ions to square-based pyramidal and octahedral Ni(II) ions, introducing extra paramagnetic metal centres which ‘switch on’ additional magnetic superexchange pathways. Subtle variations in the reaction scheme used to produce complexes 1 and 2 result in both a change of topology and an increase in nuclearity, through isolation of the hepta- and nonametallic complexes [Ni₇(L₁H)₈(L₁)₂(H₂O)₆](SO₄)·15H₂O (3), [Ni₉(μ-H₂O)₂(L₂)₆(L₂H)₄(H₂O)₂](SO₄)·29H₂O (4) and [Ni₉(μ-H₂O)₂(L₂)₆(L₂H)₄(H₂O)₂](ClO₄)₂·2MeOH·18H₂O (5) (where L₂H₂ = 2-(amino)phenylhydroxamic acid). Complementary dc magnetic susceptibility studies and DFT analysis indicate dominant antiferromagnetic exchange interactions in 1, 2, 4 and 5, but competing ferro- and antiferromagnetic exchange in 3.