Cationic Mn12 single-molecule magnets and their polyoxometalate hybrid salts

A carboxy-substituted alkylammonium salt, namely, (4-carboxybenzyl)tributylammonium hexafluorophosphate, ZHPF(6), was prepared and used as incoming carboxylate ligand in a ligand-exchange reaction with [Mn12O12(O2CCH3)(16)-(H2O)(4)] (1) to afford a new Mn-12 single-molecule magnet (SMM), [Mn12O12(Z)(16)(H2O)(4)][PF6](16) (2), bearing 16 cationic units appended in the periphery. This compound behaves as a single-molecule magnet, exhibiting an out-of-phase ac magnetic susceptibility chi"(M) signal that shows a single maximum in the 3.1-5.4 K temperature range. The frequency dependence of the maximum follows an Arrhenius law, with an effective energy barrier for reorientation of the spins U-eff = 53 K. The reduced magnetization versus H/Tdata at different temperatures were fitted by using a Hamiltonian containing Zeeman, axial, and quartic zero-field splitting terms. The expected spin ground state S = 10 was found, and the least-squares fit afforded the following zero-field-splitting parameters: D = -0.44 cm(-1); B-4(0) = 0.12 x 10(-4) cm(-1). Magnetization hysteresis loops were observed for 2, with a coercive field H-c = 0.34 T. Complex 2 has been used as countercation in the preparation of a family of hybrid salts containing different polyoxometalate anions, [Mn12O12(Z)(16)(H2O)(4)][W6O19](8) (3), [Mn12O12(Z)(16)(H2O)(4)][PW12O40](16/3) (4), [Mn12O12(Z)(16)(H2O)(4)][(H3O)PW11O39Ni](4) (5), and [Mn12O12(Z)(16)(H2O)(4))][(H3O)PW11O39CO](4) (6). 3-6 exhibit typical magnetic hysteresis loops with higher coercive fields for the complexes containing diamagnetic polyanions: H-c = 0.075 T (3), 0.046 T (4), 0.016 T (5), and 0.0075 T (6). However, the dynamics of the magnetic behavior below the blocking temperature is similar in all compounds. Broad frequency-dependent out-of-phase ac susceptibility signals are observed, presumably due to mixtures of different Jahn-Teller isomers. Their temperature dependence is also typical of an activated-energy process, with effective energy barriers in the 50 K range, irrespective of the nature of the polyoxoanion (diamagnetic, as in 3 and 4, or paramagnetic, as in 5 and 6). These findings seem to discard any influence of the polyoxometalate in the magnetic properties of the SMM.