Dinuclear CoIIYIIIvs. tetranuclear Co II2Y III2 complexes: the effect of increasing molecular size on magnetic anisotropy and relaxation dynamics

Abstract

A new Co^II₂Y^III₂ complex with the formula [{Co(μ-L)Y(NO₃)}₂(μ-CO₃)₂]·2CH₃OH·2H₂O (where H₂L = N,N′,N′′-trimethyl-N,N′′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine) has been prepared and its structure solved by single-crystal X-ray diffraction. The tetranuclear structure is formed by the connection of two [Co(μ-L)Y(NO₃)] dinuclear units through two carbonate bridging ligands, which exhibit a μ₃-κ²-O,O′:κ-O:κ-O′′ tetradentate coordination mode. The Co^II ion exhibits a slightly distorted octahedral CoN₃O₃ coordination environment. From direct-current magnetic data a large and positive axial anisotropy parameter was extracted (D = +80.6 cm⁻¹) and its sign unambiguously confirmed by HFEPR spectra and ab initio calculations. The extracted D value is rather larger than those previously reported for the analogous Co^IIY^III dinuclear complexes, which agrees with the fact that the Co^II ion in the Co^II₂Y^III₂ complex exhibits a lower distortion from the octahedral geometry in this family of Co^II_nY^III_n complexes. Dynamic ac magnetic measurements show that the reported compound presents field-induced slow relaxation for magnetization reversal, through a combination of direct and Raman processes. Magnetic measurements on the diluted magnetic counterpart (Zn/Co = 10/1) show the persistence of these processes, pointing out their single-ion origin. The Raman relaxation process for the Co^II₂Y^III₂ complex is faster that those observed for its Co^IIY^III dinuclear counterparts. This fact and the existence of the persistent direct process at low temperature could be attributed to the former molecule being larger and more flexible than the latter ones.