The story of a mechanism-based solution to an irreproducible synthesis resulting in an unexpected closed-system requirement for the LiBEt3H-based reduction: The case of the novel subnanometer cluster, [Ir(1,5-COD)(μ-H)]4, and the resulting improved, independently repeatable, reliable synthesis

Abstract

Tetra-iridium subnanometer cluster Reproducible syntheses Mechanism and radical-cage chemistry Super hydride reductions Hydrogen formation from super hydride a b s t r a c t Reproducibility is the hallmark of reliable science. Reproducible synthetic procedures are of central importance in the chemical sciences, yet P12% of syntheses submitted to publications that explicitly check procedures before their publication, such as Inorganic Syntheses and Organic Syntheses, are reported as having to be rejected since the submitted synthesis could not be repeated. In the present contribution paper we re-examine our own 2012 synthesis of [Ir(1,5-COD)(l-H)] 4 which, frustratingly, we were unable to reproducible once the first author of the original publication completed his stent in our labs. We detail the approach we took to uncover the problems in the synthesis, a key step in which was constructing a ''paper mechanism'' that led to the key hypothesis of what was going wrong in our attempts to repeat the published synthesis. The results have led to an improved synthesis, one shown to be reproducible by a second researcher working only from the detailed written procedure. Also detailed are the 4 conceptual Steps that were used to find the main problems in the synthesis, as well as the 7 specific alternative hypotheses that were involved and 20 experimental trials which discovered the missing detail in the original synthesis-the previously unknown need to employ a closed reaction system-and which led to the present, further improved synthesis which we demonstrate can be reproduced by an independent researcher with no prior Schlenk-technique experience working from only the written procedure. The hard-won insight of the need for a closed system is likely of broader significance and applicability to analogous syntheses involving metal-reductions by LiBEt 3 H and probably other anionic hydrides. A summary and conclusion section is included, one striving toward assisting the design and reporting of more reproducible syntheses in the chemical sciences.

Parts–per–million of ruthenium catalyze the selective chain–walking reaction of terminal alkenes

“…It must be notice here that, at this point, it is difficult to exactly know the structure of the active complex, however, according to previous results, a fully coordinated, presumably square planar 16–electron per –alkene Ru II complex is plausible a resting state while a more reactive 14–electron tris –alkene Ru II would be more favorable for catalysis 51 . A Ru II (olefin) x H 2 and its possible tetramer, {Ru II (olefin) x H 2 } 2 , must not be discarded as a potential catalyst resting state or actual catalyst on the basis of literature precedents with related {HIr(olefin)2} 4 complexes 52 , 53 . The fact that not only the catalytic activity but also the selectivity for a plethora of different Ru complexes is extraordinarily similar (see Fig.…”

LaMer's 1950 model of particle formation: a review and critical analysis of its classical nucleation and fluctuation theory basis, of competing models and mechanisms for phase-changes and particle formation, and then of its application to silver halide, semiconductor, metal, and metal-oxide nanoparticles

“…In our own, collaborative work on SAXS of Ir nanoparticle formation, an important control experiment reveals that even a small Ir 4 (isolable, fully characterized, and hence authentic) cluster 184,185 179,180 holds considerable promise for adding to our insights about especially nucleation, but also growth and agglomeration, of nanoparticles.…”

Nanoparticle Formation Kinetics, Mechanisms, and Accurate Rate Constants: Examination of a Second-Generation Ir(0)n Particle Formation System by Five Monitoring Methods Plus Initial Mechanism-Enabled Population Balance Modeling

“…1.5 nm length of the cigar-shaped POM, P 2 W 15 Nb 3 O 62 9– , interferes with small-angle X-ray scattering (SAXS) observation of early nucleation events. Those early nucleation clusters can easily be below 1.0 nm, as for example, as modeled by a crystallographically characterized tetra-iridium-hydride, Ir 4 H 4 (1,5-COD) 4 cluster , that is 0.8 ± 0.1 nm by SAXS observation control experiments (performed in collaboration with Karim and his group ,, ). An additional limitation of the {(1,5-COD)­Ir I ·POM} 8– system is (ii) that W in the POM interferes with the Ir X-ray absorption fine structure (XAFS) (specifically, the overlap of the W L 2 -edge with the preferred Ir L 3 -edge).…”

“…The simplest, discovered first, two-step mechanism contained within Scheme and explicitly given back in Scheme , namely that of A → B (rate constant k 1 ) and A + B → 2B (rate constant k 2 ), is especially well-tested in a number of other particle formation and growth systems across nature, including homogeneous catalyst formation, − heterogeneous catalyst formation, − protein aggregation, − solid-state kinetics, , dye aggregation, and other areas of nature showing “cooperative”, autocatalytic phenomena . The use to date of pretty much any and all applicable physical methods in those >560 citations of the 1997 paper documents that the two-step mechanism is the best-tested, best-supported, and currently most accepted kinetic model for the initial treatment of particle formation kinetic data at the PEStep level for a broad variety of nucleation and growth systems across nature. − ,− However, it is not yet clear which physical methods are both necessary and sufficient to yield a reliable particle formation mechanism? Additionally, not yet addressed are which physical methods in what combinations are needed to yield what level of precision and, notably, what accuracy in the resultant rate constants?…”

“…With the required background now in hand, we can summarize the focus of the present study by listing the seven specific questions addressed herein: Will XANES and SAXS obtained kinetics data for the {[(1,5-COD)­Ir I ·HPO 4 ] 2 } 2– precursor/Ir(0) n ·(HPO 4 2– ) x nanoparticle system both be (i) sigmoidal as expected, and (ii) well-fit by the two-step mechanism as is found for the CHCRR and GLC-derived kinetics data? What level of at least semiquantitative agreement is there between the rate constants obtained by CHCRR, GLC, XANES, and SAXS monitoring methods? What is the implied level of precision and, ideally, accuracy of the resultant rate constants? When looking in more detail at the now well-established, prototype Ir I (1,5-COD) + -containing precursor systems, − ,,,− ,− what are the known, primary sources of experimental error in measuring accurate nucleation and growth rate constants? What more generally are the error bars on the nucleation and growth rate constants and how do those error bars compare to the most precise nucleation and growth kinetics in the literature? , Are synchrotron-based methods alone, specifically XANES and SAXS, sufficient for the present example to obtain the ostensibly “correct” mechanism according to all the available data?…”