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A family of Mn-III dimers of general formula [Mn-2(III)(R-sao)(2)(dpa)(2)](ClO4)(2) (1-5) has been synthesised using derivatised phenolic oximes (R-saoH(2), where R = H, Me, Et, Ph) in combination with di-(2-picolyl)-amine (dpa). Their structures reveal a double-oxime bridged [Mn-III(NO)](2) magnetic core in which the Jahn-Teller axes lie perpendicular to the bridging plane, in contrast to two previously reported family members (6, 7). The switch in the orientation of the Jahn-Teller axes is enforced through the use of the chelating ligand which is present in 1-5 and absent in 6-7. Dc magnetic susceptibility measurements reveal that the exchange interactions between the MnIII metal centres in 1-5 are antiferromagnetic in contrast to that observed for 6 and 7 which are ferromagnetic. DFT calculations performed on complexes 1-6 reproduce both the sign and strength of the J values found experimentally. Molecular orbital analysis unlocks a common mechanism of magnetic coupling based upon the orientation of the Jahn-Teller axis, with the magneto-structural correlation also dependent upon the Mn-N-O-Mn angles - with ferromagnetic interactions at smaller dihedral angles.

The reaction of Mn(ClO4)(2)center dot 6H(2)O, a derivatised phenolic oxime (R-saoH(2)) and the ligand tris(2-pyridylmethyl)amine (tpa) in a basic alcoholic solution leads to the formation of a family of cluster compounds of general formula [(Mn2O)-O-III(R-sao)(tpa)(2)](ClO4)(2) (1, R = H; 2, R = Me; 3, R = Et; 4, R = Ph). The structure is that of a simple, albeit asymmetric, dimer of two Mn-III ions bridged through one mu-O2- ion and the -N-O- moiety of the phenolic oxime. Magnetometry reveals that the exchange interaction between the two MnIII ions in complexes 1, 3 and 4 is antiferromagnetic, but that for complex 2 is ferromagnetic. A theoretically developed magneto-structural correlation reveals that the dominant structural parameter influencing the sign and magnitude of the pairwise interaction is the dihedral Mn-O-N-Mn (torsion) angle. A linear correlation is found, with the magnitude of J varying significantly as the dihedral angle is altered. As the torsion angle increases the AF exchange decreases, matching the experimentally determined data. DFT calculations reveal that the (dyz)vertical bar pi vertical bar d(yz) interaction decreases as the dihedral angle increases leading to ferromagnetic coupling at larger angles.

International audience; A series of isostructural octanuclear lanthanide complexes of general formula [Ln(8)(sao)(4)(mu(3)-OH)(4)( NO3)(12)(DMF)(12)] (Ln = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), Ho (7), Er (8); DMF = dimethylformamide) have been prepared via reactions of salicylaldoxime (saoH(2)), tetramethylammonium hydroxide (Me4NOH) with the appropriate lanthanide nitrate salt (Ln(NO3)(3)center dot 6H(2)O). The metallic skeletons of the complexes describe [Ln(4)] tetrahedra encapsulated inside a [Ln(4)] square with the inner core stabilised through mu(3)-OH-ions and the periphery by mu(4)-sao(2)-ligands. The magnetic properties of compounds 2-8 were investigated by dc and ac magnetometry. Temperature dependent ac magnetic susceptibility data reveal that the dysprosium analogue (6) displays an out-of-phase signal in the absence of an applied magnetic field indicative of slow relaxation of the magnetization typical of a Single-Molecule Magnet (SMM). Micro-SQUID measurements reveal temperature and sweep rate dependent hysteresis below 1.0 K.