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 Mn(3)O(O(2)CR)(6)(py)(3)](ClO(4)) (R = Me (1); R = Et (2); R = Ph (3)) has afforded the new tetranuclear mixed-valent complexes Mn(4)O(2)(O(2)CR)(4)L(2)](ClO(4))(2) (R = Me (4); R = Et (5); R = Ph (6)) and Mn(4)O(2)(OMe)(3)(O(2)CR)(2)L(2)(MeOH)](ClO(4))(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 x 2THF and 9 x 3MeOH x H(2)O crystallize in the triclinic space group P1 macro and contain Mn(4)(mu(3)-O)(2)](6+) and Mn(4)(mu(3)-O)(2)(mu-OMe)(2)](5+) cores, respectively, the latter being a new structural type in the family of Mn(4) 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.