Determination of the Dominant Catalyst Derived from
the Classic [RhCp*Cl<sub>2</sub>]<sub>2</sub> Precatalyst System:
Is it Single-Metal Rh<sub>1</sub>Cp*-Based, Subnanometer Rh<sub>4</sub> Cluster-Based, or Rh(0)<i><sub>n</sub></i> Nanoparticle-Based
Cyclohexene Hydrogenation Catalysis at Room Temperature and Mild Pressures?
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Abstract
Determining the kinetically dominant
catalyst in a given catalytic
system is a forefront topic in catalysis. The [RhCp*Cl<sub>2</sub>]<sub>2</sub> (Cp* = [η<sup>5</sup>-C<sub>5</sub>(CH<sub>3</sub>)<sub>5</sub>]) system pioneered by Maitlis and co-workers is a classic
precatalyst system from which homogeneous mononuclear Rh<sub>1</sub>, subnanometer Rh<sub>4</sub> cluster, and heterogeneous polymetallic
Rh(0)<i><sub>n</sub></i> nanoparticle have all arisen as
viable candidates for the true hydrogenation catalyst, depending on
the precise substrate, H<sub>2</sub> pressure, temperature, and catalyst
concentration conditions. Addressed herein is the question of whether
the prior assignment of homogeneous, mononuclear Rh<sub>1</sub>Cp*-based
catalysis is correct, or are trace Rh<sub>4</sub> subnanometer clusters
or possibly Rh(0)<i><sub>n</sub></i> nanoparticles the dominant,
actual cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm
initial H<sub>2</sub> pressure? The observation herein of Rh<sub>4</sub> species by in operando-X-ray absorption fine structure (XAFS) spectroscopy,
at the only slightly more vigorous conditions of 26 °C and 8.3
atm H<sub>2</sub> pressure, and the confirmation of Rh<sub>4</sub> clusters by ex situ mass spectroscopy raises the question of the
dominant, room temperature, and mild pressure cyclohexene hydrogenation
catalyst derived from the classic [RhCp*Cl<sub>2</sub>]<sub>2</sub> precatalyst pioneered by Maitlis and co-workers. Ten lines of evidence
are provided herein to address the nature of the true room temperature
and mild pressure cyclohexene hydrogenation catalyst derived from
[RhCp*Cl<sub>2</sub>]<sub>2</sub>. Especially significant among those
experiments are quantitative catalyst poisoning experiments, in the
present case using 1,10-phenanthroline. Those poisoning studies allow
one to distinguish mononuclear Rh<sub>1</sub>, subnanometer Rh<sub>4</sub> cluster, and Rh(0)<i><sub>n</sub></i> nanoparticle
catalysis hypotheses. The evidence obtained provides a compelling
case for a mononuclear, Rh<sub>1</sub>Cp*-based cyclohexene hydrogenation
catalyst at 22 °C and 2.7 atm H<sub>2</sub> pressure. The resultant
methodology, especially the quantitative catalyst poisoning experiments
in combination with in operando spectroscopy, is expected to be more
broadly applicable to the study of other systems and the “what
is the true catalyst?” question