Ferromagnetic phase transitions of two one-dimensional ferrimagnets formed by manganese(II) and nitronyl nitroxides cis octahedrally coordinated

We report the structure of two chain compounds formed by manganese(I1) hexafluoroacetylacetonate and nitronyl nitroxide radicals 2-R-4,4,5,5-tetramethyl-4,5-dihydro-lH-imidazolyl-l3--ooxxiyd e, Mn(hfac),NITR, with R ethyl, Et (I), and n-propyl, n-Pr (11), which crystallize in the monoclinic P21/n space group with a = 8.890 (3) A, b = 25.573 (5) A, c = 12.208 (9) A, @ = 103.52 ( 5 ) O , and Z = 4 for I and a = 9.297 (6) A, b = 25.367 (7) A, c = 12.215 (4) A, p = 104.38 (5)O, and Z = 4 for 11. In both cases Mn ( h f a ~u)n~it s are bridged by NITR radicals which are cis-coordinated toward the manganese originating a zigzag chain. The magnetic susceptibility in the range of temperature 18-300 K is typical of a ferrimagnetic chain formed by alternating spins 5 / 2 and spins The antiferromagnetic coupling constant has been estimated to be on the order of 260 cm-I for both compounds. Low-field magnetic measurements reveal that both compounds undergo a magnetic phase transition at 8.1 and 8.6 K for I and 11, respectively. The magnetization curves indicate that the transition is ferromagnetic. The magnetic anisotropy, which is of the Ising type, is originated mainly by the dipolar interaction between manganese and radical. The EPR spectra at room temperature do not show the typical behavior of one-dimensional systems due to the fact that the chains are not linear but zigzag; however, effects due to short-range order are visible in the EPR resonance fields on lowering the temperature