Giant, Submicron Size, Yet Nearly Uniform Ir⁰ _30,000,000 Particles: Synthesis, "Raspberry" Structure, and Resultant Insights into Their Mechanism of Formation

Synthetic conditions have been discovered that yield previously unknown massive, 0.230 +/- 0.029 mu m (i.e., submicron-sized), yet still near-uniform size, still near-monodisperse, ca. Ir-similar to 30,000,000(0) particles. These are the first giant yet still near-uniform Ir-0 particles to be described. The reaction conditions that provide the giant particles are summarized in the following generalized equation, where PS stands for proton sponge: 0.5 [(1,5-COD)IrCl](2) + 1.0 PS + 0.2 Bu3N + 2.5 H-2 -> 1/n {[(Ir)(1)(NBu3)(b)(Cl)(c)](c-)}(n) + 1.0 PS-H+ + 1.0 cyclooctane + (0.2-b) Bu3N + (1.0-c) Cl-. A key control leaving out the PS, but with 1 equiv of Bu3N per mole of iridium, yields only primary, unagglomerated, nearly uniform nanoparticles of only similar to 1.6 nm, hence on-average Ir-similar to 150(0). Because the 1.0 Bu3N is consumed by the 1.0 H+ produced in this experiment (i.e., without a stronger base PS to scavenge the H+ as PS-H+), this control demonstrates the key role of the deliberately added Bu3N in the formation of the massive submicron-sized particles. Characterization of the particles was accomplished by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The microscopy results reveal that the submicrometer, on-average roughly similar to Ir-similar to 30,000,000(0) particles are agglomerates of, ultimately, a similar to 1.5 nm metal core, ca. Ir-similar to 125(0) nanoparticles stabilized by capping Bu3N in, ultimately, a "raspberry" structure for the Ir-similar to 30,000,000(0) particles. That raspberry structure provides additional evidence consistent with agglomeration of the initially formed, Ir-similar to 125(0) nanoparticles when Bu3N is available (i.e., when the PS is added so that PS, and not Bu3N, scavenges the 1.0 H+ generated in the reaction). The raspberry structural finding also provides strong evidence for a mechanism that must include (i) the formation of meta-stabilized, near-monodisperse primary particles, and (ii) their agglomeration into a raspberry structural arrangement. Additional details of the proposed mechanism are presented and discussed, an important detail of which is that size-dependent focusing of the initial particles and then size-dependent agglomeration can (a) yield giant but still near-monodisperse particles, while also (b) saving somewhere between 99.99 and 99.999% of the otherwise similar to 30,000,000 steps of a strictly monomer addition mechanism. References and discussion are provided strongly suggesting that the proposed mechanism of formation is likely much more general, accounting for the formation of other massive, yet still near-uniform, particles described in the literature.