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 TpNpCo-C 2O4-CoTpNp (1), TpNpCo-C 2O4-NiTpCy (2), TpCyNi-C 2O4-NiTpCy (3), TpNpCo-C 2O2(NH)2-CoTpNp (4), Tp CyNi-C2O2(NH)2-NiTpCy (5), TpNpCo-C2S2(NH)2-CoTp Np (6), TpNpCo-C2S2(NH) 2-NiTpCy (7), TpCyNi-C2S 2(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 Ĥex. = -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. © 2013 The Royal Society of Chemistry.