New members of the polynuclear manganese family: Mn2II Mn2III single-molecule magnets and Mn3II Mn8III antiferromagnetic complexes. Synthesis and magnetostructural correlations

The synthesis, crystal structures and magnetic properties are reported for three novel mixed-valence tetranuclear [(Mn2Mn2III)-Mn-II(HBuDea)2(BuDea)(2)(EBA)(4)] (1), [(Mn2Mn2III)-Mn-II(HBuDea)(2)(BuDea)(2)(DMBA)(4)] (2) and undecanuclear [(Mn3Mn8O4)-Mn-II-O-III(OH)(2)(BuDea)(6)(DMBA)(8)] (3) clusters, where H(2)BuDea is N-butyldiethanolamine, HEBA is 2-ethylbutyric acid and HDMBA is 2,2-dimethylbutyric acid. The compounds have been prepared through self-assembly reactions of manganese(II) chloride with H2BuDea and respective carboxylic acid in methanol solution in air, affording 1 with HEBA, and 2 or 3 with HDMBA, depending on the experimental conditions. The single crystal X-ray analysis reveals that 1 and 2 have similar centrosymmetric structures based on the {M-4(mu(3)-O)(2)(mu-O)(4)} core, while 3 discloses the unprecedented {M-11(mu-O)(4)(mu(3)-O)(12)} one. The Mn-4 complexes display single-molecule magnet (SMM) behavior with a S = 9 spin ground state and a high energy barrier U-eff/k(B) of up to 51 K. The magnetic properties of 2 are successfully modeled with J(MnIII-MnIII)/hc = 25.7 cm-1 and two J(MnIII-MnII)/hc constants of 3.1 and-0.93 cm(-1) (data correspond to the H =-J (S) over cap (1).(S) over cap (2) formalism). The Mn-11 cluster exhibits a paramagnetic behavior with dominant antiferromagnetic coupling. A possible influence of intermolecular effects and of different peripheries of the magnetic cores designed by using 2-ethylbutyrate (in 1) or 2,2-dimethylbutyrate (in 2) on the magnetic properties of 1 and 2 is discussed. The experimental magnetostructural correlations for the {(Mn2Mn2III)-Mn-II(mu(3)-O)(2)(mu-O)(4)} cores, supported by broken symmetry DFT calculations, disclose the X-Mn-III center dot center dot center dot Mn-III angle and Mn-III-O distance (where Mn-III-X and Mn-III-O are axial Jahn-Teller bonds) as the structural factors having the strongest influence on J(MnIII-MnIII) exchange coupling. It is shown that two JMnIII-MnII constants are necessary for the correct description of magnetic exchange couplings in the {(Mn2Mn2III)-Mn-II(mu(3)-O)(2)(mu-O)(4)} tetranuclear unit.