Green Woes of an Inorganic Synthetic Graduate Student

In a chemistry research lab, there is an obvious bulk entity that exists no matter the discipline: chemicals. In the case of a synthetic lab, these chemicals are the tools which chemists use to make new molecules, for the purposes of advancing fundamental ideas, and making functional and applicable materials. All of this sounds exciting and great, but typical syntheses usually have a drawback that is a common problem that violates many aspects of the 12 Principles of Green Chemistry: waste. This is mainly due to most chemistry being performed in a solution, where the solvent acts as a medium for a reaction between two or more substances. This provides a somewhat controlled environment. However, there are other methods of performing (some) reactions that follow green principles. For example, some reactions can be performed solvent-free with comparable, if not better, product yields to using a solution. Aspirin, a well-known painkiller, is a great example of this concept. Table 1 shows results from the solid-state synthesis of aspirin using microwave radiation;(1) note the first column of yields corresponds to solution conditions and recrystallization, while the far right column corresponds to solvent-free conditions. While you take a hit on energy waste with a microwave, you eliminate the need to dispose of hazards solvents.

Synthesis of Aspirin Table

Solvent-free synthesis is common in my research, which focuses on synthetic inorganic chemistry; or in other words, I make molecules that contain metals. I am not writing this to discuss my research, but rather to provide my thoughts on how my personal research starts off green, and then due to the current goals of the project, must be made the exact opposite. This is rather upsetting. The shorthand balanced equation for the solid-state synthesis of the starting material of my research, which I will call I6 for short, is the following:

6 Re(s)  +  8 Se(s)  +  4 CsI(s)  +  I2(s)  à  Cs4[Re6Se8I6](s)  (I6)

There are three green principles that are reinforced in this reaction; 1) waste prevention – this reaction is a solid state reaction, a so-called “shake-and-bake”, therefore, no solvents are used and no waste is formed; 2) atom economy – ALL of the reactant atoms end up in the product molecule (100% atom economy); 3) less hazardous synthesis – performed in a quartz tube under vacuum, thus there is not necessarily a hazard present (though one of the materials inside the tube is hazardous). With the pros of this reaction comes one con: the reaction requires a temperature of 850 °C in a furnace for one full week. That’s a large amount of energy used for maybe 20 grams of starting material (see Power Consumption in the Lab article).

The I6 above is readily soluble in water and a bonus in terms of being environmentally friendly. However, this is where the road ends and my green frustrations begin. The goals of the project require I6 to be soluble in organic solvents in order for synthetic manipulations to be made. After this synthetic manipulation, the major solvent used in all of my chemistry is dichloromethane (DCM), a halogenated solvent that is terrible for the environment, is toxic, and is a very undesirable solvent with regards to being green. In fact, the main organic solvents used in all of my work are DCM), acetonitrile, dimethylformamide (DMF), chlorobenzene, and nitromethane. None of these are desirable solvents and three are undesired (see Solvents in the Environment article). What a complete switch from the benign synthesis at the beginning!

Many of these I6-like compounds are purified via column chromatography – a separations method – and I can personally use close to 4 L of DCM on one column in a single week (not to mention a decent amount of acetonitrile as well). This is the point where I literally get depressed about my work and want to do/change SOMETHING to ease the pain of this wasteful expenditure. However, all methods seem inefficient and time-consuming, something PhD advisors and thus their graduate students usually avoid. I have tried to use desirable and/or acceptable solvents with my compounds, but they either react with the solvent under some required condition or it is insoluble. These complexes are almost exclusively soluble in DCM, even requiring deuterated DCM for Nuclear Magnetic Resonance (NMR) measurements. For reference, 100 g of deuterated chloroform is ~$12 (a very common and somewhat inexpensive solvent for these measurements) and the same amount for deuturated DCM is $415. This is a double whammy that hits both the wallet and the environment negatively.

One purpose for the materials I am attempting to synthesize is the creation of an efficient, reusable catalyst for organic reactions. Due to the solubility of my compound, catalysis can be performed in a polar solvent. Once the reaction is complete, a miscible, nonpolar solvent can be added in which the organic product is soluble. My compound, however, would precipitate separate for reuse, thereby providing a clean (and greener) reaction.

My short-term goal for this large problem is to extrapolate ideas on possible synthetic manipulations that can be performed in the aqueous phase using the initial starting material. Until this goal is realized I will continue to ask myself these two questions: 1) what can I do to make a difference in my research and my lab? and 2) do the ends justify the means with regards to my personal research? Being a member of GREEN and utilizing its resources and knowledge will help me figure out the means to solve research problems such as these.

1 Montes, I. J. Chem. Educ. 2006, 83, 628