Molecular magnets based on two-dimensional Mn(II)–nitronyl nitroxide frameworks in layered structures

A series of layered compounds of the formula { [ Mn 2 II ( NITBzIm ) 3 ] ( X ) } n [X = ClO4 (1) or CnH2n+1SO4 with n = 10 (2), 11 (3), 12 (4), 13 (5), 14 (6), 18 (7)] were obtained by the reaction of the 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazolidinyl-1-oxy-3-oxide nitronyl nitroxide radical (NITBzImH) with manganese(II) acetate in methanol and by the successive addition of sodium perchlorate or n-alkylsulfate. The crystal structures of all compounds have been established by Rietveld refinement of their X-ray powder diffraction patterns using the previously reported analogue compound { [ Mn 2 II ( NITIm ) 3 ] ( ClO 4 ) } n obtained with 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazolidinyl-1-oxy-3-oxide (NITImH) as model. Accordingly, all compounds contain the 2D cationic framework of the formula { [ Mn 2 II ( NITBzIm ) 3 ] + } n exhibiting a honeycomb-like structure with alternating manganese(II) ions and radicals and crystallizing in the form of layered compound with perchlorate or n-alkylsulfate anions in between. The series of compounds with n-alkylsulfate anions evidence a linear dependence of the spacing between the layers in respect to the length of the alkyl chain. The variation is small because the chains are almost parallel to the mean plane of the layers. The study of the magnetic behaviors shows that the spacing between the layers deeply influences the magnetic properties. Compounds 1–6 behave as magnets with Curie temperature decreasing when the interlayer spacing increases [52K (1)–30 K (6)]. No 3D ordering was evidenced for compound 7 with larger spacing. A close examination of the magnetization versus magnetic field indicate that compounds 1–6 are a case of weak ferromagnetism due to spin canting within the 2D framework layers. Interestingly, it is found that the canting as well as the interlayers magnetic coupling of dipolar origin increases when the layers come closer. One explanation is that an increase in the dipolar magnetic interaction causes a more accented canting. Alternatively, it may be that when the layers come closer this induces stronger anisotropy through crystal packing effects.