The role of the bridging group in exchange coupling in dinuclear homo- and heterometallic Ni(ii) and Co(ii) complexes with oxalate, oxamidate and dithiooxamidate bridges

Abstract

Six homodinuclear and two heteronuclear complexes Tp^NpCo–C₂O₄–CoTp^Np (1), Tp^NpCo–C₂O₄–NiTp^Cy (2), Tp^CyNi–C₂O₄–NiTp^Cy (3), Tp^NpCo–C₂O₂(NH)₂–CoTp^Np (4), Tp^CyNi–C₂O₂(NH)₂–NiTp^Cy (5), Tp^NpCo–C₂S₂(NH)₂–CoTp^Np (6), Tp^NpCo–C₂S₂(NH)₂–NiTp^Cy (7), Tp^CyNi–C₂S₂(NH)₂–NiTp^Cy (8) (Tp^Np = tris(3-neopentylpyrazolyl)borate, Tp^Cy = tris(3-cyclohexylpyrazolyl)borate), were synthesized and characterized by mass spectrometry, electronic spectroscopy and X-ray crystallography. These compounds possess similar molecular structures, with the metal ions linked by bridging oxalate (1–3), oxamidate (4 and 5) or dithiooxamidate (6–8) ions. The heteronuclear nature of compounds 2 and 7 was additionally confirmed by high-resolution mass spectrometry. The magnetic properties of the Co²⁺ complexes were modelled taking into account zero-field splitting of this ion, yielding D-values for Co²⁺ in the range −17(1) to −50(1) cm⁻¹. All the metal ion pairs in compounds 1–8 are antiferromagnetically-coupled, with J values between −10.0(1) and −45.0(2) cm⁻¹ (via the exchange Hamiltonian Ĥ_ex. = −2JŜ₁Ŝ₂) and |J| increasing in the order oxalate < oxamidate < dithiooxamidate. This tendency can be attributed to greater M–S bond covalency compared to M–N or M–O bonds (M = Co²⁺ and Ni²⁺). It was found that this antiferromagnetic coupling of Co²⁺ and Ni²⁺ ions through oxalate is more efficient for these tris(pyrazolyl)borate complexes than for similar oxalate-bridged systems with neutral aliphatic amine ligands.