Heterobimetallic cyanide-bridged FeIII(μ-CN)MII complexes (M = Mn and Cu): synthesis, structure and magnetism

Abstract

A controlled reaction in DMF/CH₃CN between a facially disposed tricyanide-containing building block (ⁿBu₄N)[Fe^III(Tp)(CN)₃] (Tp(–) = tris(pyrazol-1-yl)borate anion) and tailor-made Mn(II) and Cu(II) complexes of two tripodal tetradentate ligands providing two 1-methylbenzimidazole, a pyridyl, and a tertiary amine as donor sites, differing only in the (2-pyridyl)alkylamine chain length, has been employed to assemble cyanide-bridged complexes. The heterobimetallic complexes [Fe^III(Tp)(CN)₂(μ-CN)Mn^IICl(L¹)]·3DMF·3H₂O (1), [Fe^III(Tp)(CN)₂(μ-CN)Mn^IICl(L²)]·DMF (2), [Fe^III(Tp)(CN)₂(μ-CN)Cu^II(L¹)](ClO₄)·2CH₃OH (3), and [Fe^III(Tp)(CN)₂(μ-CN)Cu^II(L²)](ClO₄)·2CH₃CN (4) [L¹ = bis{(N-methyl-benzimidazol-2-yl)methyl}(2-pyridylmethyl)amine; L² = bis{(N-methyl-benzimidazol-2-yl)methyl}(2-pyridylethyl)amine (L²)] have been structurally authenticated, revealing single cyanide-bridged discrete dimers with Fe(III)(μ-CN)Mn(II) and Fe(III)(μ-CN)Cu(II) structural motifs. In 1 and 2, both the metal ions have a distorted octahedral geometry. In 3 and 4, the Fe(III) centres are six-coordinate, but the two five-coordinate Cu(II) centres assume two different geometries: distorted trigonal bipyramidal in 3 and almost ideal square pyramidal in 4. Investigation of the magnetic properties reveals an antiferromagnetic interaction through the cyanide bridge between the six-coordinate low-spin Fe(III) (S = 1/2) ions and high-spin six-coordinate Mn(II) (S = 5/2) ions in 1 and 2 with the total spin S_t = 2 ground state; while 3 exhibits a ferromagnetic interaction between the cyanide-bridged Fe(III) (S = 1/2) and Cu(II) (S = 1/2) ions with the S_t = 1 state, 4 displays an antiferromagnetic interaction leading to the S_t = 0 state.