Unintuitive Inverse Dependence of the Apparent Turnover Frequency on Precatalyst Concentration: A Quantitative Explanation in the Case of Ziegler-Type Nanoparticle Catalysts Made from [(1,5-COD)Ir(μ-O₂C₈H₁₅)]₂ and AlEt₃

The Ziegler-type hydrogenation precatalyst dimer, [(1,5-COD)Ir(μ-O2C8H15)]2 (1,5-COD = 1,5-cyclooctadiene; O2C8H15 = 2-ethylhexanoate) plus added AlEt3 stabilizer has recently been shown to form AlEt3-stabilized, Ziegler-type Ir(0)∼4–15 nanoparticles initially, which then grow to larger Ziegler-type Ir(0)∼40–50 nanoparticles during the catalytic hydrogenation of cyclohexene (Alley, W. M.; Hamdemir, I. K.; Wang, Q.; Frenkel, A. I.; Li, L.; Yang, J. C.; Menard, L. D.; Nuzzo, R. G.; Ozkar, S.; Johnson, K. A.; Finke, R. G. Inorg. Chem. 2010, 49, 8131–8147). An interesting observation for this Ziegler-type nanoparticle catalyst system is that the apparent TOF (TOFapp = kobs/[Ir]) for cyclohexene hydrogenation increases with decreasing concentration of the precatalyst, [Ir] (defined as 2[{(1,5-COD)Ir(μ-O2C8H15)}2], that is, twice the starting precatalyst concentration since that dimer contains 2 Ir). A perusal of the literature reveals that such an intuitively backward, inverse relationship between the apparent...