Tetranuclear Manganese Complexes with Dimer-of-Dimer and Ladder Structures from the Use of a Bis-Bipyridyl Ligand

The reaction of the bis-chelating ligand 1,2-bis(2,2‘-bipyridine-6-yl)ethane (L) with the trinuclear species of formula
[Mn3O(O2CR)6(py)3](ClO4) (R = Me (1); R = Et (2); R = Ph (3)) has afforded the new tetranuclear mixed-valent
complexes [Mn4O2(O2CR)4L2](ClO4)2 (R = Me (4); R = Et (5); R = Ph (6)) and [Mn4O2(OMe)3(O2CR)2L2(MeOH)](ClO4)2 (R = Me (7); R = Et (8); R = Ph (9)). Complexes 4−6 were obtained in yields of 20%, 44%, and 37%,
respectively. They are mixed-valent, with an average Mn oxidation state of +2.5. Complexes 7−9 were obtained
in yields of 57%, 65%, and 70%, respectively. They are also mixed-valent, but with an average Mn oxidation state
of +2.75. Complexes 4·2THF and 9·3MeOH·H2O crystallize in the triclinic space group P1̄ and contain [Mn4(μ3-O)2]6+ and [Mn4(μ3-O)2(μ-OMe)2]5+ cores, respectively, the latter being a new structural type in the family of Mn4
complexes. Reactivity studies of 4−9 have shown that 4−6 can be converted into 7−9, respectively, and vice
versa. The magnetic properties of 5 and 9 have been studied by dc and ac magnetic susceptibility techniques.
Complex 5 displays antiferromagnetic coupling between its Mn ions resulting in a spin ground state of S = 0.
Complex 9 also displays antiferromagnetic coupling, but the resulting ground state is S = 7/2, as confirmed by
fitting magnetization versus field data collected for 9 at low temperatures, which gave S = 7/2, D = −0.77 cm-1,
and g = 1.79. Complex 9 exhibits a frequency-dependent out-of-phase ac susceptibility peak, indicative of the
slow magnetization relaxation that is diagnostic of single-molecule magnetism behavior.