Defective dicubanes of Co~(II)/Co~(III) complexes with triethanolamine and N-donors

摘要

The mixed valence Co~(II)/Co~(III) tetranuclear clusters [Co~(II) _2Co~(III) _2(tea) _2(pyr)_2(NO_3)_4]·2CH _3CN (1), [Co~(II) _2Co~(III) _2(μ_3-OH)_2(Htea)_2(bpy) _4](NO_3)_4 (2), and [Co~(II) _2Co~(III) _2(μ_3-OH) _2(Htea)_2(phen)_4](NO_3) _4·2CH_3CN·2CH_3OH (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~(II) _2Co~(III) _2O_6 core and can be described as defective dicubanes with different imperfectness. In 1, the central rhombic core Co_2O_2 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~(II) _2Co ~(III) _2 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 (Seff = 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.