Organometallics perspective

By Brandon Ackley

Organometallics is a very broad field, with important advances being made in fundamental studies, catalyst development, and small molecule activation by non-metals in the form  of Lewis-pair activation. The success of the field relies on all of these smaller topics, as well as new discoveries which have yet to be made. Attempts to discuss all of these topics would make for a bland, nonspecific opinion. Instead, this piece will focus on fundamental studies of heavier main group elements.

Heavy main group elements, such as arsenic and antimony, find the most use in organic / metalloid based electronic devices. A good example of main group electronics are found in phospholes, the phosphorus equivalent of pyrroles. This 5-membered “aromatic” core can be found in optics and organic LED’s, and finds utility as a building block to more complex structures. Despite the utility of these compounds, the analogous heavier analogs are far understudied. For a quick back of the envelope comparison, the simplest phosphole (1-H- phosphole) has 218 hits on SciFinder, while the simplest arsole has only 29. This is despite the precedence for these heterole compounds to possess bizarre activity- the Tang group has demonstrated that 1,2,3,4,5-pentaphenylsilole demonstrates remarkable fluorescence in the solid state, and that the phenyl groups rotation quenches the fluorescence when in solution. When stepping further down to germanium, we know enough from computation to know that the germoles will likely possess useful properties, but the chemistry of these heavier analogs has not been sufficiently developed to make these materials on a commercial scale.

Stepping away from electronic devices, another important field of study is found in low- valent main group compounds (for the purposes of this opinion, low-valent refers to a compound which is substituted but does not possess the most stable oxidation state for its group number). The ultimate example of a low valent main group element is found in carbon- despite how much it pains me to refer to carbon as a main group element. The landmark discovery of carbenes opened a door for chemists, and now carbenes are found in some of the most important homogenous catalysts in industry- all of Grubb’s catalysts possess a carbene moiety, with later generations featuring an N-heterocyclic carbene on a ruthenium center. The reader can infer the two directions this is going; the first being that the understanding of low valent compounds leads to new reactivity which may prove to result in a range of new useful products. The second area for development lays in the N-heterocyclic carbene; substituting the nitrogen in the ring for heavier (or lighter, in the case of boron) elements will undoubtedly change the properties of these classic stable carbenes.

There are valid reasons for the study of these heavier elements to be neglected- the compounds tend to be easily reduced to the inert bulk element, tend to be photosensitive, and frankly, the heavier elements tend to make weaker bonds. In those drawbacks, though, are opportunities. The ease of reduction observed with these heavier elements means that their HOMO-LUMO gap simply is not in the same range as the lighter elements, meaning that in the bulk phase, their band gap will be different. That is a huge positive for electronics applications. The photosensitivity, while frustrating, does mean that these compounds are active in the visible spectrum- something that is uncommon in classical organic chemistry. The weak bonds of the heavier elements show use as catalysts, potentially enabling new activity in organic chemistry without the use of transition metals.

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