Defective dicubanes of CoII/CoIII complexes with triethanolamine and N-donors

Abstract

The mixed valence Co^II/Co^III tetranuclear clusters [Co^II₂Co^III₂(tea)₂(pyr)₂(NO₃)₄]·2CH₃CN (1), [Co^II₂Co^III₂(μ₃-OH)₂(Htea)₂(bpy)₄](NO₃)₄ (2), and [Co^II₂Co^III₂(μ₃-OH)₂(Htea)₂(phen)₄](NO₃)₄·2CH₃CN·2CH₃OH (3) are described where tea and Htea are the fully and the doubly deprotonated form of triethanolamine, while as N-donors are pyridine, 2,2′-bipyridine and 1,10-phenanthroline. Complexes 1–3 contain the Co^II₂Co^III₂O₆ core and can be described as defective dicubanes with different imperfectness. In 1, the central rhombic core Co₂O₂ is occupied by two Co^III ions while the external cobalt atoms display Co^II oxidation states; meanwhile 2 and 3 exhibit a reversal in their Co^II₂Co^III₂ oxidation state distribution. Two different theoretical models were used to explain the magnetic behavior: (i) spin–spin interaction model with local anisotropy terms where S = 3/2 for both metal centers and (ii) an anisotropic spin–spin interaction model applicable in the low temperature range (T < 40 K) using effective spins (S_eff = 1/2) for both metal centers. For 1 a relatively strong next-nearest-neighbour antiferromagnetic exchange interaction between the Co(II) centers which are connected via diamagnetic Co(III) ion was found while for 2 and 3 the presence of ferromagnetic interaction is confirmed. The fitting results, concerning the first model, gave: J = 2.0(2)/3.2(2)/3.8(2) cm⁻¹, g = 2.35(1)/2.52(1)/2.57(1) and D = 11.0(1)/8.5(1)/7.8(1) cm⁻¹ while concerning the second model are: J_z = −7.1(2)/19.2(2)/22.1(2) cm⁻¹, g_z = 6.8(1)/8.1(1)/8.3(1), J_xy/J_z = 0.34(2)/0.11(2)/0.14(2), and g_xy/g_z = 0.52(2)/0.28(2)/0.36(2) for 1–3. X-Band EPR spectrum of 1 has a very broad derivative centered at g = 5.3 while for 2 and 3 large g-variations were found in the range 20.0–1.0, indicative of an exchange interaction between Co(II) ions.