Mechanistic Study of a Thermal- and Light-Driven Single-Crystal-to-Single-Crystal Transformation of an Organocerium...

In crystallo synthetic and analytical methods are remarkably effective for stabilizing and structurally characterizing otherwise reactive compounds, and elucidating transient intermediates in catalytic cycles. In this work, we investigate the mechanism of the ring-opening isomerization of a CeIV-cyclopropenyl complex, CeIV(3,3-diphenylcyclopropenyl)(TriNOx) (Ce-1), to the corresponding CeIV-allenyl isomer, CeIV(CH═C═CPh2)(TriNOx) (Ce-2), where TriNOx3-═[((2-tBuNOH)C6H4CH2)3N]3-. The isomerization occurs by a single-crystal-to-single-crystal (SCSC) transformation under thermal and photochemical conditions, allowing for the direct extraction of kinetic parameters from sequential single-crystal X-ray diffraction (SC-XRD) measurements. In crystallo kinetic isotope effect (KIE) experiments using a deuterium-labeled isotopologue of Ce-1 revealed a modest KIE (1.01) for the thermal reaction and an inverse KIE (0.74) for the photochemical reaction. Additionally, variable-temperature studies demonstrated that the photochemical SCSC reaction occurs without a detectable thermal activation energy barrier over a temperature range of 163 K. An investigation of the solution-phase thermal and photochemical decomposition products yielded results that were consistent with a multistep reaction initiated by the ring-opening isomerization reaction of Ce-1 to Ce-2. Complementary solution-phase KIE studies tracking the consumption of the two isotopologues of the CeIV-cyclopropenyl complex revealed, qualitatively, an inverse KIE for the photochemical process, consistent with the results obtained from the photocrystallographic studies. The combined results from the in crystallo and solution-phase studies provide experimental support for the previously computed thermal mechanism, clarify the distinct photochemical mechanism, and illustrate how single-crystal X-ray diffractometry can be used to obtain detailed kinetic and thermodynamic insights into SCSC transformations for reactions on appropriate time scales.