Syntheses, structures, and magnetic properties of the lanthanide complexes of imidazole-substituted nitronyl nitroxide biradicals

Abstract

Two new bis-bidentate imidazole-substituted nitronyl nitroxide biradicals, BNITIm-C2 (BNITIm-C2 = 1,1′-(1,2-ethanediyl)bis(1H-imidazole-2–4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)) and BNITIm-C4 (BNITIm-C4 = 1,1′-(1,4-butanediyl)bis(1H-imidazole-2–4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)), and two series of lanthanide complexes, namely [(BNITIm-C2)Ln(NO₃)₃](MeOH) (Ln = Gd (1_Gd) and Tb (2_Tb)), (BNITIm-C2)Dy(NO₃)₃ (3_Dy) and (BNITIm-C4)[Ln(hfac)₃]₂(C₇H₈)₂ (Ln = Gd (4_Gd), Tb (5_Tb) and Dy (6_Dy), hfac = hexafluoroacetylacetonate), have been prepared and characterized structurally and magnetically. Single crystal X-ray crystallographic analyses revealed that complexes 1_Gd–3_Dy exhibit 1D chain structures where the Ln(NO₃)₃ units are bridged by the BNITIm-C2 bis-bidentate biradical, while complexes 4_Gd–6_Dy exhibit binuclear structures with two Ln(hfac)₃ units bridged by the BNITIm-C4 biradical. The bulky hfac anions prohibit the further coordination of Ln^III to another NIT ligand and the formation of a similar 1D chain structure. Due to the very long intra- and intermolecular distances of the spin centers, complexes 1_Gd–3_Dy can be magnetically regarded as an isolated 2p–4f–2p tri-spin system while complex 4_Gd–6_Dy can be regarded as an isolated 2p–4f bi-spin system. Magnetic analyses on the two Gd^III compounds revealed the ferromagnetic Gd^III–NIT interactions and antiferromagnetic NIT–NIT interactions through the Gd^III ion in 1_Gd. Alternating-current (ac) magnetic susceptibility investigations revealed that complex 5_Tb exhibits the typical SMM behavior under a zero dc field while complex 6_Dy was proved to be a field-induced SMM. Ab initio calculations were performed on complexes 2_Tb and 5_Tb to understand their magnetic anisotropy together with their different magnetic dynamics.