Chemistry
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Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy
Chapters
Summary February 20th, 2020
Here, we present a method for the observation of solution interactions between Lewis acids and bases by employing in situ infrared spectroscopy as a detector for titration under synthetically relevant conditions. By examining solution interactions, this method represents a complement to X-ray crystallography, and provides an alternative to NMR spectroscopy.
Transcript
This method probes the solution interactions between Lewis acids and carbonyls under synthetic conditions. This technique can be applied for mechanistic insight into carbonyl-centered reactions. We can observe Lewis acid carbonyl complexes in real time under reaction conditions, allowing for the observation of the behavior of substrates and products with the catalyst.
This method provides insight into the fundamental interactions of molecules, giving the synthetic chemist the necessary information to design high-yielding procedures. This system can be applied to any solution interaction that results in a change in equilibrium that can be detected in the infrared spectrum. To begin, open the data acquisition software, click instrument, under the configure tab, click collect background, click continue, set scans to 256 and click OK to collect a background.
Then, in the data acquisition software, click file, click new, click quick start. Set duration to 15 minutes and sample interval to 15 seconds. Click create to create the experiment.
Put a flame-dried 25 millimeter two-neck round-bottom flask charged with a stir bar into a glove box. Under the inert atmosphere, add 324 milligrams of iron trichloride. Cap the necks of the flask with rubber septa and take the flask out of the glove box.
Attach an argon-filled balloon to the flask through a needle in the rubber septa. Add 12 milliliters of anhydrous solvent DCE via syringe. Next, remove one septum and attach the flask to the in situ IR probe.
Place the flask in a temperature-controlled bath, set to the desired temperature of 30 degrees celsius. Start the experiment in the data acquisition software by clicking the start button to begin collecting data for the solvent reference spectrum. After two minutes, stop collecting data.
First, to create a new titration experiment in the data acquisition software, click file, new, quick start. Set duration to eight hours and sample interval to 15 seconds. Click create to create the experiment.
In the data acquisition software, go to the spectra tab and click add spectra. Click from file and open the appropriate solvent reference spectrum obtained previously. Check the box with the time signature and click OK.Start the experiment in the data acquisition software by clicking the start button to begin collecting data.
Click solvent subtraction and select the edit appropriate reference spectrum. Stir for 15 minutes to reach the temperature of 30 degrees celsius. Use the in situ IR probe to determine the temperature.
Add 10 microliters of carbonyl analyte into the flask via a syringe. Observe the signal response on the data acquisition. System shifts from equilibrium and changes with time.
When the IR signal stabilizes and remains constant, additional carbonyl analyte is added. Add additional carbonyl analyte into the flask with the amount of 10 microliters each time and wait for the system to equilibrate. To export data for the data acquisition software, click file, export, multi-spectrum file.
Under format, check CSV and under data, check raw. Click export to export the IR data to a spreadsheet or mathematical processing software. Select data where the system had reached equilibrium after each addition of analyte.
Plot the desired region of the IR spectrum. Examine the spectrum for transitions and isosbestic points. Plot data that pertains to a particular transition period by progression separately.
The best way to examine the spectra for transition periods is to plot each segment of the titration incrementally until you can visualize only one transition occurring. To analyze the component, identify the lambda max of each species of interest. To account for dilution, multiply the absorbents by the total volume of the solution for each spectrum.
Plot the products of absorbents times volume as a function of equivalence of analyte. For in situ generated species that can be identified, plot a Beer-Lambert relationship with absorbents on the Y axis and concentration on the X axis. For known species, measure the impact of concentration on absorbents at the desired lambda max and plot a Beer-Lambert relationship.
Using the two Beer-Lambert relationships, determine the observed in situ amounts of the species of interest. C-max equals two millimoles as defined by the amount of iron chloride present. C-add is the moles of acetone added.
C-coord is the moles of iron chloride acetone complex. C-observed is the moles of unbound species one. C-ND is the moles of species one not detected.
C-max minus C-coord is the moles of species three that have been consumed. Plot C-ND versus C-max minus C-coord to determine if there is a correlation. In this study, in situ IR-monitored titration was used to observe the interactions of species one and gallium trichloride, as well as species one and iron chloride.
Gallium trichloride and species one formed a one-to-one complex two in solution. Alternatively, when iron chloride and species one were combined, more complex behavior was observed. This figure displays the raw feed of data obtained by the in situ IR using the data acquisition software for the titration of iron chloride with species one.
Here, the process of extracting the transitions that result from this titration method is shown. The extraction of lambda max data of the titration of gallium trichloride with species one and the titration of iron chloride with species one demonstrates only one-to-one complex two was formed when gallium trichloride is combined with species one, whereas one-to-one complex three was initially formed when combined with iron trichloride but was then consumed. With these protocols, the examination of competitive access to a Lewis acid was achieved.
When performing spectra analysis, it is best to look for transitions by incrementally plotting time points until a transition period is found. This procedure could be applied to multiple Lewis acid carbonyl systems, furthering insight into fundamental interactions between catalyst and carbonyl-centered substrates and byproducts. We have employed this technique to gain mechanistic insight into the competitive interactions of substrate and byproduct in metal-catalyzed carbonyl-olefin metathesis, which led us to revise our initial mechanistic proposal.
Many Lewis acid catalysts are moisture-sensitive and can produce hydrochloric acid. So, ensure that these systems are kept under inert atmosphere and proper protective equipment is worn.
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