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?

Dalton Transactions Pub Date: 2012-08-10 DOI: 10.1039/C2DT31121F

Abstract

Six homodinuclear and two heteronuclear complexes TpNpCo–C2O4–CoTpNp (1), TpNpCo–C2O4–NiTpCy (2), TpCyNi–C2O4–NiTpCy (3), TpNpCo–C2O2(NH)2–CoTpNp (4), TpCyNi–C2O2(NH)2–NiTpCy (5), TpNpCo–C2S2(NH)2–CoTpNp (6), TpNpCo–C2S2(NH)2–NiTpCy (7), TpCyNi–C2S2(NH)2–NiTpCy (8) (TpNp = tris(3-neopentylpyrazolyl)borate, TpCy = 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 ?ex. = ?2J?1?2) 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.

Graphical abstract: 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
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