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

The mixed valence Co(II)/Co(III) tetranuclear clusters [Co(II)2Co(III)2(tea)2(pyr)2(NO3)4]·2CH3CN (1), [Co(II)2Co(III)2(μ3-OH)2(Htea)2(bpy)4](NO3)4 (2), and [Co(II)2Co(III)2(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(II)2Co(III)2O6 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(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: Jz = -7.1(2)/19.2(2)/22.1(2) cm(-1), gz = 6.8(1)/8.1(1)/8.3(1), Jxy/Jz = 0.34(2)/0.11(2)/0.14(2), and gxy/gz = 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.