Thermal Jahn–Teller Distortion Changes and Slow Relaxation of Magnetization in Mn(III) Schiff Base Complexes

The impact that the anion and alkyl group has on the electronic structures and magnetic properties of four mononuclear Mn­(III) complexes is explored in [Mn­(salEen-Br)2]Y (salEen-Br = 2-{[2-(ethylamino)­ethylimino]­methyl}-4-Br-phenol; Y = ClO4 – 1 and BF4 –·1/3CH2Cl2 2) and [Mn­(salBzen-Br)2]Y (salBzen-Br = 2-{[2-(benzylamino)­ethylimino]­methyl}-4-Br-phenol; Y = ClO4 – 3 and BF4 – 4). X-ray structures of [Mn­(salEen-Br)2]­ClO4·0.45C6H14 1-hexane, [Mn­(salEen-Br)2]­BF4·0.33CH2Cl2·0.15C6H14 2-dcm-hexane, and 3–4 reveal that they crystallize in ambient conditions in the monoclinic P21/c space group. Lowering the temperature, 2-dcm-hexane uniquely exhibits a structural phase transition toward a monoclinic P21/n crystal structure determined at 100 K with the unit cell trebling in size. Remarkably, at room temperature, the axially elongated Jahn–Teller axis in 2-dcm-hexane is poorly defined but becomes clearer at low temperature after the phase transition. Magnetic susceptibility measurements of 1–4 reveal that only 3 and 4 show slow relaxation of magnetization with Δeff/k B = 27.9 and 20.7 K, implying that the benzyl group is important for observing single-molecule magnet (SMM) properties. Theoretical calculations demonstrate that the alkyl group subtly influences the orbital levels and therefore very likely the observed SMM properties.