Alignment of axial anisotropy of a mononuclear hexa-coordinated Co(II) complex in a lattice shows improved single molecule magnetic behavior over a 2D coordination polymer having a similar ligand field.

Two CoII based complexes namely [Co(IAP)(SCN)2] (1) and {[Co(IPEH)2(SCN)2]·H2O}n (2) (where IPEH = (((1E,2E)-1,2-bis(1-(4-(1H-imidazol-1-yl)phenyl)ethylidene)hydrazine) and IAP = (4'-(imidazol-1-yl)acetophenone)) have been synthesized and characterized by single-crystal X-ray diffraction, magnetic measurements and ab initio calculations. Structural analysis revealed that complex 1 has a zero-dimensional mononuclear structure and complex 2 has a two-dimensional framework where the CoII centers are bridged by bis(monodentate) ligand IPEH. In both the complexes, the CoII center has a distorted octahedral geometry with a CoN6 coordination environment, formed by four equatorial N atoms from the neutral ligand and two NCS- at the axial positions. Detailed magnetic measurements reveal the presence of easy-plane magnetic anisotropy for both the complexes, whereas the field-induced slow relaxation of magnetization is observed at a relatively higher temperature in 1 (above 2 K, Ueff = 30 K) as compared to 2 (below 2 K, Ueff = 6.5 K). The ab initio calculations show the negligible effect of the first coordination sphere as similar anisotropic parameters are obtained for both the complexes. Additionally, it was found that the anisotropic axes are aligned well in a particular direction in complex 1 whereas it is randomly oriented in the framework of 2. The parallel orientation of the anisotropic axes enhances the axial anisotropy and reduces the transverse component, resulting in a higher energy barrier (Ueff = 30 K) in complex 1. The detailed analysis of field and temperature dependence of relaxation time indicates that Raman, direct and QTM processes mainly play an important role in the relaxation dynamics of complex 1.