MAGNETIC-PROPERTIES OF A LAYERED MOLECULAR MATERIAL COMPRISING MANGANESE HEXAFLUOROACETYLACETONATE AND NITRONYL NITROXIDE RADICALS

By reaction of Mn(hfac)2, hfac = hexafluoroacetylacetonate, with the NITBzald nitronyl nitroxide radical, NITBzald = 2-(4-benzaldehyde)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, a compound of formula [Mn(hfac)2]3(NITBzald)2.0.5CHCl3 was synthesized which was found to crystallize in the monoclinic P2(1)/n space group with a = 11.277(8) angstrom, b = 14.07(1) angstrom, c = 26.44(2) angstrom, beta = 95.88(6)-degrees, and Z = 2. The structure consists of chains in which the Mn(hfac)2 units are bridged by the nitronyl nitroxide radicals through the two oxygen atoms of the NO groups. The chains are connected by another Mn(hfac)2 unit which is coordinated by the aldehyde oxygens of two NITBzald radicals belonging to different chains, in such a way that planes parallel to the [201] crystallographic plane are formed. The magnetic susceptibility shows a marked increase of the chiT product on decreasing temperature, which can satisfactorily be reproduced by assuming that the S=512 spins of the manganese atoms within chains and S = 1/2 spins of the radicals are strongly coupled, J = 255 cm-1 with the H = JS1.S2 Hamiltonian, while the spins of the manganese atoms bridging the chains are considered uncorrelated. The low value for the upper limit of the exchange interaction between chains through the benzaldehyde oxygen, 0.01 cm-1, suggested by the EPR spectra is in agreement with the observed critical temperature to reach 3-D ferromagnetic order, T(c) = 6.4 K, which is in the same range of T(c) observed for previously reported 1-D Mn-NITR compounds. A small hysteresis was detected below T(c) with a coercive field H(c) = 22 Oe at 2 K.