Thermal Instability and Associated Potential Safety Hazards of Rhodium(I) Precatalyst Complexes with Weakly Coordinated Ligands (vol 25, pg 1054, 2021)

Rhodium is a critical transition metal in catalyzing chemical transformations in both academia and industry. Over the years Rh(I)-catalyzed chemical transformations such as hydroformylation, conjugate addition, C-H functionalization, and transfer hydrogenation have found broad application in the fine chemical industry. However, Rh(I) precatalyst complexes with weakly coordinated ligands are inherently unstable and thermally decompose to generate a variety of noncondensable, flammable, and/or toxic products. Exposure of [Rh(ethylene)(2)Cl](2) to air promotes its thermal decomposition at temperatures as low as similar to 44 degrees C by differential scanning calorimetry (DSC) analysis, thus significantly increasing the thermal instability hazards and leading to high potential for fire or explosion risk. A thermokinetic model predicts that the adiabatic induction time of [Rh(ethylene)(2)Cl](2) in the presence of air is only 3.1 days at 25 degrees C and 24 h at 31.7 degrees C, indicating that exposure of [Rh(ethylene)(2)Cl](2) to air significantly increases the thermal instability hazards of this catalyst. DSC screening of a variety of other Rh(I) precatalyst complexes with loosely coordinated ligands revealed similar thermal instability hazards, and these hazards were significantly increased when the complexes were evaluated with an air headspace compared with those under an inert atmosphere. We expect this contribution to promote awareness of these potential safety hazards within the wider scientific community. Scientists should understand the thermal instability hazards of their specific Rh(I) precatalyst complexes and employ safer measures, for example, inert atmosphere and safe temperature windows, to prevent related incidents when handling such Rh(I) precatalyst complexes during their research, especially on scale.