Magneto-structural studies of two new cobalt(ii)-N,N-diisobutylisonicotinamide compounds: [CoLCl2]n and [Co(L)2(H2O)4][CoLBr3]2·2H2O

Abstract

Two similar synthetic pathways using the ligand N,N-diisobutylisonicotinamide (L) with anhydrous CoX₂ salts (being X = Cl⁻, Br⁻) led to different species: a one-dimensional system, [CoLCl₂]_n, 1, and an ionic product [Co(L)₂(H₂O)₄][CoLBr₃]₂·2H₂O, 2, respectively. Compound 1 is a polymer in which ligand L coordinates to tetrahedral Co^II ions in a bidentate bridging fashion using the pyridine nitrogen and carbonyl oxygen atoms. Compound 2 consists of one octahedral cationic [Co(L)₂(H₂O)₄]²⁺ entity and two tetrahedral anionic [CoLBr₃]⁻ units. In this system, the ligand molecules coordinate only through the pyridine nitrogen atoms. The magnetic properties of 1 and 2 were investigated in the temperature range of 2.0 to 300.0 K and correlations between both (due to the existence of similar features) examined. The study of the magnetic properties of 1 was carried out by considering each Co^II ion as a perfectly isolated system, hence, J = 0, but taking into account a significant zero-field splitting contribution due to distortions on the tetrahedral environment of the cobalt atoms. The fit of the magnetic susceptibility data together with reduced magnetization vs H/T measurements provided similar parameters (|D| = 10.8 cm⁻¹, g_⊥ = 1.92, g_‖ = 2.92 for the former and |D| = 11.04 cm⁻¹and g = 2.05 for the latter, respectively). On the other hand, the magnetic response of compound 2 has been analyzed using a model which considers the presence of two tetrahedral and one octahedral Co(II) ions (Co_Td and Co_Oh). The study was carried out in two separated blocks, above and below 80 K, where only the most significant effects at each interval of temperature were considered. As a result, the analysis of the magnetic data shows weak antiferromagnetic interactions between the Co_Ohand the two Co_Td ions (J = −0.41 cm⁻¹) in 2. The best fit parameters were g_CoTd = 2.89, g_CoOh = 3.50, |D_CoTd| = 10.62 cm⁻¹, |E_CoTd| = 2.95 cm⁻¹, Δ = 240.9 cm⁻¹ and J_L-S = −107.1 cm⁻¹, from where λ was calculated with a final value of −144.8 cm⁻¹ (J_L-S = Aκλ). The approximations performed to obtain these values provide reasonable results in agreement with compound 1 and also to other systems in the literature.