Displacement kinetics of η²-bound furan and 2,3-dihydrofuran from Mn and Cr centers: Evidence for the partial dearomatization of the furan ligand

The displacement of η²-coordinated ligands from the photolytically generated CpMn(CO)₂L and BzCr(CO)₂L [Cp = η⁵-C₅H₅, Bz = η⁶-C ₆H₆, L = 2,3-dihydrofuran (DHF), furan] complexes by pyridine has been studied. The displacement reactions span a wide range of time scales from microseconds to hours and were studied using a range of time-resolved IR spectroscopic techniques. The substitution reactions follow a dissociative pathway and the measured activation enthalpies provide an estimate for the strength of the metal - (η²-furan) and metal - (η²-DHF) interactions. In these complexes, the Cr center binds both ligands weaker than the Mn center. There is a ∼6-10 kcal/mol difference in the binding enthalpies of η²-furan and η²-DHF to both metals suggesting that this difference is the result of a partial loss of resonance energy in the case of the aromatic furan ligand upon interaction with the metal.