CHEMISTRY AND PHYSICS OF A MOLECULAR-BASED MAGNET CONTAINING 3 SPIN CARRIERS, WITH A FULLY INTERLOCKED STRUCTURE

The compound of formula (rad)(2)Mn-2[Cu(opba)](3)(DMSO)(2).2H(2)O (hereafter abbreviated as (rad)(2)Mn2Cu3) has been synthesized; rad(+) stands for the radical cation 2-(1-methylpyridinium-4-yl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, and opba stands for o-phenylenebis(oxamato). The crystal structure has been solved (space group, Cc; lattice parameters, a = 25.379(3) Angstrom, b = 25.146(3) Angstrom, c = 18.845(6) Angstrom, beta = 131.52(4)degrees, Z = 4) and has revealed quite an unusual architecture. The structure consists of two nearly perpendicular graphite-like networks with edge-sharing hexagons. The corners of each hexagon are occupied by Mn(II) ions, and the middles of the edges by Cu(II) ions. The two networks are interlocked, the center of each hexagon being occupied by a Cu(II) ion belonging to a perpendicular hexagon. (rad)(2)Mn2Cu3 contains three kinds of spin carriers: Mn(II) and Cu(II) ions, antiferromagnetically coupled through oxamato bridges, and rad(+) radical cations, bridging the Cu(II) ions through the nitronyl nitroxide groups and forming rad(+)-Cu(II) chains. The magnetic and EPR properties of (rad)(2)Mn2CU3 have been studied in some detail. The temperature dependence of chi(M) T (chi(M) being the molar magnetic susceptibility and T the temperature) shows a minimum around 115 K characteristic of a ferrimagnetic behavior and then a divergence as T approaches the very low temperatures. The temperature dependence of the magnetization has confirmed that a long-range magnetic ordering occurs at T-c = 22.5 K. The field dependence of the magnetization, M = f(H), has been measured at 4.2 K up to 200 kOe. The compound behaves as a soft magnet, with a very weak coercive field. The M = f(H) curve shows an extremely high zero-field susceptibility, as expected for a magnet and then a smooth increase of M as H increases. This behavior has been attributed to the progressive alignment of the rad(+) spins along the field direction. In zero field, the radical cations are antiferromagnetically coupled with the Mn2Cu3 metal core, a situation which probably results from ferromagnetic interactions between nearest neighbor spin carriers along the rad(+)-Cu(II) chains. The average value of the rad(+)-Cu(II) interaction parameter has been estimated from the high-field range of the M = f(H) curve and found as J(radCu) = 11.5 cm(-1) (H=-J(radCu)S(rad).S-Cu) The X-band powder EPR spectra have also been investigated. Above T-c the spectra show a single resonance centered at g = 1.995 at 290 K and at g = 1.955 at 23 K. At T-c new features appear, which are strongly temperature dependent. A qualitative picture of the ferromagnetic resonance in (rad)(2)Mn2Cu3 has been proposed. All the structural and physical data have been discussed, and some perspectives of our findings have been presented.