Accelerated Arene Ligand Exchange in the (Arene)Cr(CO)₂L Series

Arene ligand exchange in the (η6-arene)Cr(CO)2L series can be accelerated if the ligand L is an
electronically unsymmetrical bidentate ligand. The system evaluated here employs derivatives of tris(pyrrolyl)phosphine as L. A series of 2-L‘-substituted pyrroles was prepared, where the substituents include: 
L‘ = −SMe, −CH2SMe, −SPh, −CH2SPh, −SCF3, −S-tBu, −CO2Me, −CONMe2, −2-pyridinyl, and −PPh2.
Reaction with ClP(pyrrolyl)2 gave a new series of phosphines, (2-L‘-pyrrolyl)(pyrrolyl)2P. Each of these
phosphines was converted to (arene)Cr(CO)2[P(2-L‘-pyrrolyl)(pyrrolyl)2P) complexes. The substituents L‘
are proposed to provide temporary coordination to the Cr and to lower the barrier to arene exchange. The
series was evaluated where the arene in the complex (departing) is benzene, fluorobenzene, toluene,
o-xylene, m-xylene, or p-xylene and the incoming arene is C6D6, chlorobenzene-d5, anisole-d8, fluorobenzene-d5, toluene-d8, o-xylene-d10, m-xylene-d10, p-xylene-d10, or mesitylene-d12. Most of the new complexes
showed a significant increase in the rate of arene exchange due to the side chain unit L‘. The strongest
effects were seen with the examples where X = −CO2Me, −CONMe2, and −(2-pyridinyl), allowing exchange
with a half lifetime as low as 8 h/22 °C.