Synthesis, redox, and magnetic properties of a neutral, mixed-valent heptanuclear manganese wheel with S=27/2 high-spin ground state

Reaction of lithium tetrachloromanganate(II) with N-n-butyldiethanolamine H2L3 (3) in the presence of LiH leads to the formation of wheel-shaped, mixed-valent heptanuclear, neutral complex {Mn-II subset of[(Mn2Mn4Cl6)-Mn-II-Cl-III(L-3)(6)]} (4). The manganese wheel crystallizes in the triclinic space group P (1) over bar as 4 center dot 2CHCl(3) or 4 center dot 3THF when either diethyl ether or n-pentane was allowed to diffuse into solutions of 4 in chloroform or tetrahydrofuran. The oxidation states of each manganese ion in 4 center dot 2CHCl(3) or 4 center dot 3THF were assigned on the basis of detailed symmetry, bond length, and charge considerations, as well as by the Jahn-Teller axial elongation observed for the manganese(III) ions, and were further supported by cyclic voltammetry. The analysis of the SQUID magnetic susceptibility data for complex 4 center dot 2CHCl(3) showed that the intramolecular magnetic coupling of the manganese(II,III) ions is dominated by ferromagnetic exchange interactions. This results in an S = 27/2 ground-state multiplet at low magnetic field. At fields higher than 0.68 T, the energetically lowest state is given by the m(S) = 31/2 component of the S = 31/2 multiplet due to the Zeeman effect. The ligand-field-splitting parameters were determined by anisotropy SQUID measurements on single crystalline samples along the crystallographic x, y, and z axes (D = -0.055 K, E = 6.6 mK) and by high-frequency electron spin resonance measurements on a polycrystalline powder of 4 center dot 2CHCl(3) (D = -0.068 K, E = 9.7 mK). The resulting barrier height for magnetization reversal amounts to U approximate to 10 K. Finally, 2DEG Hall magnetization measurements revealed that 4 center dot 2CHCl(3) shows single-molecule magnet behavior up to the blocking temperature of about 0.6 K with closely spaced steps in the hysteresis because of the quantum tunneling of the magnetization.