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Objective

The objective of this research is to use computational and synthetic chemistry to understand the factors that affect the binding of organic molecules, particularly thiophene and benzothiophenes, to copper(I) centers. Such organometallic binding interactions are proposed to play an important role in adsorption processes that utilize copper salts to bind to and remove sulfur-containing impurities from petroleum fuels. Moreover, the binding of copper to other related organic molecules like alkenes is a critical step in a number of important catalytic reactions. Thus, gaining a detailed understanding of the fundamental nature of bonding in organometallic copper complexes will help to further research in a variety of fields.

Research Progress

Our initial goal during this first year of the grant period was to use DFT calculations to gain a fundamental insight into how ligand electron-donating ability affects the binding strength of thiophene to a copper(I) ion. Because there are no known examples of synthetic copper-thiophene complexes, we first sought to understand the nature of bonding between copper(I) ions and the simple alkene ethylene, with particular interest in understanding the impact of ligand basicity.

We performed DFT calculations on a series of copper(I)-ethylene complexes having a variety of substituted 1,10-phenanthroline ligands of different electron-donating abilities. Our calculations showed that the covalent stabilization between the [LCu^I]⁺ fragment and ethylene involves predominantly π-backbonding from the metal to the alkene (~60%), while σ-donation from the alkene to copper(I) accounts for a lesser amount (~40%). This is in sharp contrast to the relative contributions calculated for ethylene bound to the free Cu^I ion (29% Cu-to-ethylene π-backbonding; 71% ethylene-to-Cu σ-donation). Moreover, while increasing ligand basicity increases the relative amount of π-backbonding from the copper to the ethylene, the amount of ethylene-to-copper σ-donation and the electrostatic attraction between the copper center and the ethylene molecule decrease with increasing ligand basicity.  Thus, “tuning” a copper(I) ion to bind strongly with ethylene (and, presumably, other related organic molecules like thiophene) requires a delicate balance of covalent and electrostatic interactions.

To apply this information to copper(I)-thiophene interactions, undergraduate student Mary Jane Simpson worked during 2008-2009 conducting DFT calculations and synthetic studies comparing the binding of ethylene and thiophene to [LCu^I(MeCN)]⁺ complexes where L = 1,10-phenanthroline and the related 2,2'-bipyridine (bipy) ligand. These computational results suggested that the binding of thiophene to the copper centers would be slightly less favorable (by ~3 kcal/mol) than the binding of ethylene. Moreover, these calculations suggested that the binding of thiophene to the copper(I) center would adopt an η²-coordination mode through a double-bond of the thiophene molecule, much like the binding of copper(I) to ethylene. Because ethylene readily reacts with [(bipy)Cu^I(MeCN)]⁺ to form [(bipy)Cu^I(ethylene)]⁺, we attempted to synthesize [(bipy)Cu^I(thiophene)]BF₄ by addition of thiophene to anaerobic solutions of [(bipy)Cu^I(MeCN)]BF₄. Attempts were also made using other copper(I) salts, including copper(I) triflate and copper(I) hexafluorophosphate. Preliminary characterization of the isolated solid reaction products using IR-spectroscopy have thus far proved inconclusive due to the air sensitivity of these compounds, and further attempts to synthesize and characterize these [(bipy)Cu^I(thiophene)]⁺ are ongoing in our lab.

A second undergraduate student, Naomi Pernicone, worked during 2009 exploring the binding of thiophene and ethylene to [(TMEDA)Cu^I(MeCN)]BF₄ where TMEDA = the more strongly electron-donating N,N,N’,N’-tetramethylethylendiamine ligand. DFT calculations were used to examine the binding of both ethylene and thiophene to [(TMEDA)Cu^I(MeCN)]⁺ and the results were similar to those obtained for  [(bipy)Cu^I(thiophene)]⁺.  This study, including attempts to synthesize [(TMEDA)Cu^I(thiophene)]⁺_, is ongoing and will be continued by Ms. Pernicone during 2009-2010.

A third undergraduate student Ashli Simone, began conducting DFT calculations during the summer of 2009 to understand how different ligands, such as thioethers and imidazoles, could impact the bonding of ethylene, thiophene, and other alkenes, to copper(I) centers. This facet of the project is being continued by Ms. Simone during 2009-2010.

Impact, Presentations, and Publications

The preliminary results of the copper-ethylene binding study were presented by the P.I. at two meetings of the American Chemical Society (the 237^th National Meeting on March 10^th 2009 in Salt Lake City, and the Florida Regional Meeting on May 16, 2009 in Orlando Florida). In addition, a manuscript detailing the full results of this study is currently being finished for submission for publication, with Naomi Pernicone and Mary Jane Simpson as co-authors.

Mary Jane Simpson presented the results of her research at the Stetson Undergraduate Research and Creative Arts Symposium (SURCAS) on April 8, 2009. Mary Jane also presented these results at the Florida Regional Meeting of the American Chemical Society on May 16, 2009 in Orlando Florida. Mary Jane is now continuing her graduate studies in chemistry at Duke University .

Naomi Pernicone plans to present the results of her studies on copper-thiophene and copper-ethylene binding at the upcoming spring National Meeting of the American Chemical Society in San Francisco in 2010. She plans to attend graduate school in chemistry upon gradation in spring 2010.

Based on the preliminary ethylene binding study, undergraduate Ashli Simone was able write a related research proposal and received funding through a Stetson Undergraduate Research Experience (SURE) grant to support her full-time work on this project over the 2009 summer session. This is a prestigious award at Stetson University, which would not have been possible without the preliminary work supported by this ACS-PRF grant. She plans to attend graduate school in chemistry upon gradation in spring 2010.

This ACS-PRF grant has made a significant impact on the career and research program of the P.I. The funds have been used to purchase vital equipment and supplies necessary to complete this research and to support the P.I. during the summer of 2009. These funds will also allow the P.I. and students to attend regional and national ACS meetings in the upcoming year to present these results, which will give the students national exposure that will be vital to their continued careers in chemistry.