Organometallics Research on ChemRxiv

ChemRxiv is proud to sponsor the 2019 Organometallics Gordon Research Seminar & Conference — in honor of the occasion, we would like to highlight some of the fantastic organometallic chemistry that has been submitted and posted to ChemRxiv. Here are three highlights, some of which feature authors who will be presenting at the upcoming conference.

Talk to Dave at the GRC meeting — his presentation is on Sunday at 8:40 pm.

In this preprint from January, David Powers’ group at Texas A&M present a fascinating strategy for gaining an atomistic understanding of catalysis in porous polymers and metal-organic frameworks (MOFs). MOF chemists have known for years that MOFs offer specific advantages in catalysis; their ability to noncovalently co-localize substrates and catalyst sites and their inherent recyclability are but two of those. However, MOF catalysts are typically not able to be systematically optimized for specific catalysis reactions. This is due to a weakness inherent to MOF synthesis: the synthesis of these highly crystalline materials requires reversible binding of weak ligands to metal nodes. This limits the metal sites in these materials to coordination spheres featuring weak donors such as carboxylates, pyridyl groups, and azolates. Complicating matters is that many of the most reactive homogeneous catalysts (those which would ideally be incorporated into MOFs for catalysis) feature strong donors such as amines, phosphines, and alkoxides that are irreversibly bound. Thus, any site accessed through conventional MOF synthesis would lack the advantages provided by an optimized homogeneous catalyst.

In the preprint, the Powers group developed a fascinating solution to the synthesis of such a catalyst system that features both site isolation and co-localization in nanoporous materials — their approach circumvents the difficulties inherent to the synthesis of these porous polymers by utilizing preformed metallomonomers featuring ruthenium dimers chelated by four different supporting ligands. They then polymerize these metallomonomers through their ligand backbones to alkyne-bearing supports via a Sonogashira reaction. Rigorous structural and spectroscopic characterization demonstrated that these resulting amorphous polymers were indeed porous and that the electronic structures of the Ru2 sites in the polymer were maintained between the monomer and the polymer. Furthermore, the resulting porous polymers maintained the same selectivity pattern for intramolecular amination catalysis as the metallomonomers after polymerization.

Talk to Alex at the GRC meeting — his presentation is on Tuesday at 7:30 pm.

One of the first preprints to hit the server this month was a beautiful preprint from Alexander Spokoyny’s group at UCLA. In their manuscript, they premiered a nucleophilic boron reagent which is not only unencumbered by organic foliage, but is also bench stable and useful in a wide variety of reaction schemes. Bemoaning the lack of unencumbered nucleophilic boron reagents in the literature, Spokoyny’s group approached the problem of stabilizing electron density on boron through delocalization across three dimensions in a cluster. While such clusters are typically thought of as being quite stable, particularly in higher nuclearity systems such as [B12H12]2−, as the nuclearity decreases, the energy of the HOMO increases. Leveraging this insight, the team investigated a series of nucleophilic substitution reactions and found that not only could the boron clusters act as nucleophiles, but that they could be disassembled in the presence of oxidants. This releases individual unencumbered boryls (coordinated only by pinacol) which can readily react with organic electrophiles. Further analysis and optimization demonstrated that this protocol is widely applicable and can be useful for providing non-reducing nucleophilic boryls to organic electrophiles. However, the scope of the reaction doesn’t stop at organic systems — the protocol is so robust that the chemistry can even be applied to main group elements such as phosphorus and selenium, allowing the creation of B-P and B-Se bonds without reduction.

While mass spectrometry is a frequent tool in the kit of any organometallic chemist, it isn’t quite as common in detailed mechanistic work. The McIndoe group at the University of Victoria seek to change that with a fascinating mechanistic investigation using multiple reaction monitoring modes in a triple quadrupole mass spectrometer. This technique allows for the stepwise observation of transient catalytic intermediates on a relatively fast timescale. While such techniques require charged species in order to be detected, the McIndoe group circumvents that problem with the judicious choice of a sulfonated ligand on their palladium catalyst.

The reaction they use to illustrate this technique is one which has seen extensive mechanistic investigation over the years: the Buchwald-Hartwig amination. The reaction is generally understood to proceed in the same manner as most cross coupling reactions: with catalyst activation, oxidative addition of an aryl halide, coordination of the aniline of choice, base-assisted deprotonation of the amine, and finally reductive elimination to form the amination product and reform the active catalyst. Using this new technique, we can follow these reactions in unprecedented detail and with impressive time resolution. This promises that through use of this new technique, new insights into and subsequent optimization of catalytic reactions are on the horizon. What’s more is that the technique can be performed in the absence of air — a requirement for sensitive catalysts that is often difficult to meet with mass spectrometry.

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