Synthesis of Heterogeneous Ir⁰_∼600–900/γ-Al₂O₃ in One Pot From the Precatalyst Ir(1,5-COD)Cl/γ-Al₂O₃: Discovery of Two Competing Trace “Ethyl Acetate Effects” on the Nucleation Step and Resultant Product

In 2010 we reported a two-step synthesis of a Ir⁰_∼900/γ-Al₂O₃ supported-nanoparticle catalyst. In that study, a well-defined Ir­(1,5-COD)­Cl/γ-Al₂O₃ precatalyst was isolated and characterized before being reduced in contact with acetone solvent and cyclohexene and under H₂ in a second step. Synthetically, one would like to remove the Ir­(1,5-COD)­Cl/γ-Al₂O₃ precatalyst isolation step, shortening the precatalyst synthesis and allowing the overall synthesis to be accomplished more efficiently in one pot. However, herein we report that the one-pot synthesis starting from commercially available [Ir­(1,5-COD)­Cl]₂ and γ-Al₂O₃ yields an order of magnitude increase in the observed nucleation rate constant, k_1,obs, as well as a decrease in the average particle size from Ir⁰_∼900 to Ir⁰_∼600. Mechanistic experiments reveal that the origin of this effect, amazingly, is the presence of residual ethyl acetate employed in the isolated precatalyst synthesis, which is not present in the one-pot synthesis. Additional mechanistic probing, along with multiple control experiments, reveals that the presence of even small levels of EtOAc has two, competing effects: a nucleation enhancing effect of increasing the amount of solvated Ir­(1,5-COD)­Cl­(solvent) dissociated off of the γ-Al₂O₃ support (a step known to be involved in nucleation in solution on the basis of a second paper published in 2011), but then also a more dramatic effect of EtOAc reacting with Ir⁰_n (or possibly Ir_xH_y) nuclei to inhibit nucleation. Armed with these mechanistic insights, we achieved the goal of one-pot syntheses by controlling the presence or absence of the EtOAc. Overall, seemingly innocent solvents such as EtOAc are hereby added to an increasing list of variables crucial to achieving reproducible nanoparticle nucleation- and growth-based syntheses. A conclusions section summarizes those variables along with five additional noteworthy findings and recommendations from the present study.