Synthesis, structure and magnetic properties of [Fe(4)(III)Ln(2)(III)] (Ln = Gd, Tb, Dy, Ho) and [(Fe4Y2III)-Y-III] clusters
Résumé
The employment of alcohol-containing chelates and carboxylates in mixed Fe/Ln (Ln = lanthanide) and Fe/Y reactions has afforded a new family of [Fe(4)(III)Ln(2)(III)] (Ln = Gd, Tb, Dy, Ho) and [(Fe4Y2III)-Y-III] clusters. The reaction of [Fe3O(O2CPh)(6)(H2O)(3)](NO3), Ln(NO3)(3) (Ln = Gd, Tb, Dy, Ho), 2-(hydroxymethyl)pyridine (hmpH) and NEt3 in a 1:1:4:4 molar ratio in MeCN/MeOH gave [Fe(4)Ln(2)O(2)(hmp)(8)(O2CPh)6(]) (Ln = Gd (1), Tb (2), Dy (3), Ho (4)). The same reaction with Y(NO3)(3) gave [Fe4Y2O2(hmp)(8)(O2CPh)(6)] (5). The crystal structure of representative complex 1 was solved and reveals a centrosymmetric structure with an Fe-4-Gd-2 unit in a chair conformation with the two Gd atoms at opposite ends, above and below an Fe-4 rectangular plane. Each Fe2Gd triangle is bridged by a mu(3)-O2- ion, and eight hmp(-) groups bind in an eta(1):eta(2):mu fashion. Ligation is completed by two PhCO2- group at each Gd, one bound eta(1) and the other eta(2). Analysis of the variable-temperature dc and ac magnetic susceptibility data revealed that the central Fe-4 rectangular sub-unit has an S=0 ground state due to strong Fe center dot center dot center dot Fe exchange interactions, so that at low temperatures the two well separated Ln(III) behave as magnetically isolated ions. The Fe center dot center dot center dot Fe interactions were estimated using the magnetostructural correlation reported by Weihe and Gudel, and then determined more accurately from the experimental data for 5 center dot 2H(2)O using the program MAGPACK. The obtained parameters were J(1) = -20.5 cm(-1) and J(2) = -41.3 cm(-1), with g held constant at 2.0, where J(1) and J(2) are for the Fe center dot center dot center dot Fe interactions between and within the two Fe2Y triangles, respectively. The Fe4Dy2 complex (3) exhibits weak out-of-phase signals and very small hysteresis in magnetization versus field scans at low temperature.