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.

(η⁶-芳烃（η⁶-arene）)Cr(CO)₂L系列中的芳烃配体交换反应，当配体L为电子不对称双齿配体（bidentate ligand）时，反应速率可得到提升。本研究所评估的体系以三(吡咯基)膦（tris(pyrrolyl)phosphine）的衍生物作为配体L。我们制备了一系列2-L’取代吡咯，其取代基L’包括：−SMe、−CH₂SMe、−SPh、−CH₂SPh、−SCF₃、−S-tBu、−CO₂Me、−CONMe₂、−2-吡啶基以及−PPh₂。将其与ClP(吡咯基)₂反应，得到一系列新型膦配体(2-L’-吡咯基)(吡咯基)₂P。将每一种上述膦配体转化为(芳烃)Cr(CO)₂[P(2-L’-吡咯基)(吡咯基)₂]配合物。研究认为，取代基L’可通过与Cr中心的临时配位，降低芳烃配体交换的能垒。本系列配合物的评估体系涵盖：离去芳烃为苯、氟苯、甲苯、邻二甲苯、间二甲苯或对二甲苯，进入芳烃则为C₆D₆、五氘代氯苯、八氘代苯甲醚、五氘代氟苯、八氘代甲苯、十氘代邻二甲苯、十氘代间二甲苯、十氘代对二甲苯或十二氘代均三甲苯。绝大多数新型配合物的芳烃交换速率因侧链单元L’的存在得到显著提升，其中效果最为显著的为L’=−CO₂Me、−CONMe₂以及−2-吡啶基的案例，其交换反应的半衰期最低可达8 h/22 ℃。