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 4.2THF and 9.3MeOH.H 2 O crystallize in the triclinic space group P1 and contain [Mn 4 (μ 3 -O) 2 ] 6 + and [Mn 4 (μ 3 -O) 2 (μ-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.