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 eta(2)-coordinated ligands from the photolytically generated CpMn(CO)(2)L and BzCr(CO)(2)L [Cp = eta(5)-C5H5, Bz = eta(6)-C6H6, 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-(eta(2)-furan) and metal-(eta(2)-DHF) interactions. In these complexes, the Cr center binds both ligands weaker than the Mn center, There is a similar to 6-10 kcal/mol difference in the binding enthalpies of eta(2)-furan and eta(2)-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.