Syntheses, crystal structures and magnetic properties of Mn₁₂ single-molecule magnets with diphenylphosphate and their tetraphenylphosphonium salts

The preparation and physical characterization are reported for a family of novel Mn₁₂ single-molecule magnets having diphenylphosphate (labeled dppH) bridges, [Mn₁₂O₁₂(O₂CR) _{16 - x}(dpp)_x(H₂O)_y] (R = CH ₃: x = 4, y = 0 (1); x = 8, y = 2 (2), R = Ph: x = 4, y = 4 (5); x = 8, y = 4 (6), and R = Et: x = 4, y = 4 (9)) and their tetraphenylphosphate salts [Ph₄P][Mn₁₂O₁₂(O₂CR) _{16 - x}(dpp)_x(H₂O)₄] (R = CH ₃: x = 4 (3); x = 8 (4) and R = Ph: x = 4 (7); x = 8 (8)). The crystal structures of 1 and 5 were characterized by X-ray. The X-ray structure analysis of 1 reveals that there are four five-coordinate Mn^III ions with Mn⋯H approaches and without coordinating water molecules. The ¹H NMR measurements show H₂O can coordinate to a vacant site of 1 in CD₂Cl₂ solution. Ac magnetic susceptibility measurements indicate that nine complexes are single-molecule magnets. The increase of the number of dpp^- ligands resulted in a decrease of the U_eff values of neutral Mn₁₂ SMMs estimated from ac measurements. Complexes 1 and 5 show a quantum tunneling behavior, whose ground states were tentatively assigned to S = 10 with g = 1.78, and D = -0.60 K; S = 10 with g = 1.92, and D = -0.63 K, respectively.