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 CIP(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-oxylene, m-xylene, or p-xylene and the incoming arene is C₆D₆, chlorobenzene-d₅, anisole-d ₈, fluorobenzene-d₅, toluene-d₈, o-oxylene-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.