Exchange-coupled high-spin, low-spin and spin-crossover dicobalt(ii) complexes of a pyridazine-containing Schiff-base macrocycle: control of cobalt(ii) spin state by choice of axial ligands

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Abstract
A series of doubly pyridazine-bridged dicobalt(II) macrocyclic complexes, 1–7, is reported. The pyridazine-containing Schiff-base macrocycle L1 [derived from the (2 + 2) condensation of 3,6-diformylpyridazine and 1,3-diaminopropane] provides four nitrogen donor atoms, disposed in an approximately square-planar manner, to each cobalt ion. Variation of the axial donors alters the spin state of the cobalt(II) ions from high spin, [Co2L1(H2O)4](ClO4)4 (2), [Co2L1(H2O)4](S2O6)2·4(H2O) (3·8H2O), [Co2L1(N3)2](ClO4)2 (4) and [Co2L1(NCO)3]ClO4 (5), to spin crossover, [Co2L1(MeCN)4](ClO4)4 (1·4MeCN) and [Co2L1(NCS)2(SCN)2] (6), to low spin, [Co2L1Cl2](ClO4)2 (7). Detailed magnetic susceptibility studies have yielded small, negative 2J values (−7 to −20 cm−1) for complexes 2 to 5. Compounds 1, 1·4MeCN and 6 are rare examples of binuclear complexes displaying simultaneous spin crossover (3/2 ⇔ 1/2) at high temperatures and antiferromagnetic exchange of the low-spin d7–d7 ions at low temperatures (2J = −10 to −14 cm−1).