Phosphine-substituted sterically encumbered pyrrolyl ligands – synthesis and reactivity studies

Abstract

The sterically encumbered pyrrolyl KPyr^tBu₂ (1-K, Pyr^tBu₂ = 2,5-(Me₃C)₂C₄H₂N) reacts with PCl₃, (iPr)₂PCl and Ph₂PCl exclusively at the C3-position to yield a series of phosphine-substituted pyrroles HPyr^tBu₂R (R = PCl₂ (2-H), (iPr)₂P (4-H) and Ph₂P (5-H)). Pyrrole 2-H can further be functionalized with MeLi (3 equiv.) to yield LiPyr^tBu₂PMe₂ (3-Li). The coordination chemistry of these phosphine-substituted pyrroles was explored and the resulting complexes were characterized by various spectroscopic techniques and in some cases using single crystal X-ray diffraction. The neutral pyrroles 4-H and 5-H react cleanly with [Mo(CO)₆] to form the κP-coordinate complexes 4-H–Mo(CO)₅ and 5-H–Mo(CO)₅, respectively. IR spectroscopy of these complexes shows that C3-substitution barely changes the π-acceptor properties of the phosphine moiety. The η⁵-coordination was achieved when pyrrolides 4-K and 5-K were reacted with [Mn(CO)₅Br] to give 4-Mn(CO)₃ and 5-Mn(CO)₃, respectively. The energy of the HOMOs in these three legged-piano stool complexes decreases on PPh₂-substitution (5-Mn(CO)₃) stabilizing the Mn–CO bonds, whereas the inverse effect is noticed for P(iPr)₂-substitution (4-Mn(CO)₃).