The Missing Link: Manganese Complexes with Sterically Demanding Pyrrolyl Ligands

Reaction of NaPyrtBu2 [PyrtBu2 = 2, 5-(Me3C)2C4H2N] and KPyrtBu2Me2 [PyrtBu2Me2 = 2, 5-(Me3C)2-3, 4-Me2C4N] with MnI2(thf)3 resulted in the isolation of the first bis(pyrrolyl)manganese(II) complexes. However, different coordination modes are realized depending on the identity of the Pyr-ligand. For example, the MnII atom is κN coordinate in [(κN-PyrtBu2)2Mn] (1-Mn), whereas several different species were isolated with the more bulky PyrtBu2Me2 ligand. Depending on the reaction conditions the κN-coordinate complex [(κN-PyrtBu2Me2)2Mn] (2-Mn′) is initially formed, but converts on sublimation to the η3 coordinate isomer [(η3-PyrtBu2Me2)2Mn] (2-Mn′). In some cases we could also identify a THF adduct of 2-Mn to give [(κN-PyrtBu2Me2)(η2-PyrtBu2Me2)Mn(thf)] [2-Mn(thf)], in which one PyrtBu2Me2 ligand binds in a κN coordination mode, while the other is best described as η2-PyrtBu2Me2. These complexes were characterized by elemental analyses and X-ray diffraction studies. Density functional theory (DFT) computations confirm that the potential energy surface between the different hapticities is rather flat for these MnII high-spin species (S = 5/2).