Dinuclear Co(II)Y(III)vs. tetranuclear CoII2YIII2 complexes: the effect of increasing molecular size on magnetic anisotropy and relaxation dynamics

A new CoII2YIII2 complex with the formula [{Co(-L)Y(NO3)}2(-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 tetradentate coordination mode. The CoII ion exhibits a slightly trigonally distorted CoN3O3 coordination environment. From direct-current magnetic data a large and positive axial anisotropy parameter was extracted (D = +82.62 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 CoIIYIII dinuclear complexes, which agrees with the fact that the CoII ion in the CoII2YIII2 complex exhibits the lower distortion from the octahedral geometry in this family of CoIInYIIIn 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 below and above 4 K, respectively. 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 Co2Y2 complex is faster that those observed for the CoY dinuclear counterparts. This fact and the existence of the persistent direct process at low temperature could be because the former molecule is larger and flexible than the latter ones