Cocrystal Formation Precedes the Mechanochemically Acetate‐Assisted C−H Activation with [CpRhCl2]2

This work reports the experimentally studied mechanochemical formation of rhodacycles by ball milling pyridine‐ and quinoline‐derived substrates and [Cp*RhCl2]2 in the presence of NaOAc. Ex‐situ analysis of the mechanochemical reactions using powder X‐ray diffraction (PXRD), solid‐state UV‐vis spectroscopy and ATR‐FTIR spectroscopy revealed the formation of unexpected cocrystals between the substrates and the rhodium dimer prior to the C−H activation step. This sequence of events differs from the generally accepted steps in solution in which cleavage of [Cp*RhCl2]2 is initiated by acetate ions. Additionally, the mechanochemical approach enabled the synthesis of the six‐membered rhodacycle [Cp*Rh(2‐benzilpyridine)Cl], a metal complex repeatedly reported as inaccessible in solution. Altogether, the results of this investigation clarify some of the fundamental aspects of mechanochemical cyclometallations. Mechanistic insights into the mechanochemically acetate‐assisted C−H activation of nitrogen containing substrates with [Cp*RhCl2]2 are reported in this work. Ex‐situ analysis revealed the formation of unexpected crystalline phases prior to the C−H activation step, making the mechanochemical pathway different from the mechanism in solution. Moreover, the mechanochemical protocol enabled the synthesis of Cp*Rh(2‐benzilpyridine)Cl, a the six‐membered rhodacycle previously reported as impossible to access in solution.