New members of the polynuclear manganese family: Mn II2Mn III2 single-molecule magnets and Mn II3Mn III8 antiferromagnetic complexes. Synthesis and magnetostructural correlations

Abstract

The synthesis, crystal structures and magnetic properties are reported for three novel mixed-valence tetranuclear [Mn^II₂Mn^III₂(HBuDea)₂(BuDea)₂(EBA)₄] (1), [Mn^II₂Mn^III₂(HBuDea)₂(BuDea)₂(DMBA)₄] (2) and undecanuclear [Mn^II₃Mn^III₈O₄(OH)₂(BuDea)₆(DMBA)₈] (3) clusters, where H₂BuDea 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 {M₄(μ₃-O)₂(μ-O)₄} core, while 3 discloses the unprecedented {M₁₁(μ-O)₄(μ₃-O)₁₂} one. The Mn₄ 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_{Mn^III–Mn^III}/hc = 25.7 cm⁻¹ and two J_{Mn^III–Mn^II}/hc constants of 3.1 and −0.93 cm⁻¹ (data correspond to the Ĥ = −JŜ₁·Ŝ₂ formalism). The Mn₁₁ 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 {Mn^II₂Mn^III₂(μ₃-O)₂(μ-O)₄} cores, supported by broken symmetry DFT calculations, disclose the X–Mn^III⋯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_{Mn^III–Mn^III} exchange coupling. It is shown that two J_{Mn^III–Mn^II} constants are necessary for the correct description of magnetic exchange couplings in the {Mn^II₂Mn^III₂(μ₃-O)₂(μ-O)₄} tetranuclear unit.