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Objective

The goal of this project is to use a combination of computational and synthetic chemistry to understand how different factors affect the binding of alkenes, thiophenes, and benzothiophenes to copper(I) centers. These types of organometallic interactions are believed to play an important role in adsorption processes that utilize copper salts to bind to and remove sulfur-containing impurities from petroleum fuels, as well as mimic important steps in important catalytic reactions. Thus, gaining a detailed understanding of the fundamental nature of bonding in organometallic copper complexes is a worthy objective.

Research Progress

During this second year of the grant period, we have completed our investigation of series of [LCu(I)-ethylene]⁺ complexes having a variety of substituted 1,10-phenanthroline ligands with systematically varying ligand basicities. Using DFT calculations including natural bond orbital (NBO) and energy decomposition analysis (EDA), we have quantified the various components of the bonding interaction including electrostatic and covalent contributions. Both the NBO and EDA calculations suggest that, while the amount of LCu→ethylene π-backbonding indeed increases with ligand basicity, the electrostatic attraction between the copper(I) center and the ethylene molecule significantly decreases with increasing ligand basicity. Thus, increasing ligand basicity in LCu(I)-ethylene complexes might actually result in a decrease in copper(I)-alkene bond strength rather than an increase as is often assumed and reported for synthetic complexes.

Undergraduate students Naomi Pernicone and Ashli Simone further explored this phenomenon during this second year of the grant period by performing comprehensive calculations on LCu(I)-ethylene complexes having a wide range of common N-donor ligands, including diamines, triamines, b-diketiminate, and tris(pyrazolyl)borate ligands.  Results of this study using NBO analysis and EDA calculations were consistent with the findings for the phenanthroline ligands, with increasing ligand basicity increasing Cu→ethylene π-bacbonding, but decreasing favorable electrostatic attraction by a significant amount. For example, the binding energy of ethylene to the [LCu(I)]⁺ fragment where L = the weakly basic 2,2’-bipyridine ligand was predicted to be roughly -150 kJ/mol, significantly more favorable thanthat when L = a strongly donating anionic b-diketiminate ligand (-100 kJ/mol). In this case, the increase in Cu→ethylene donation is significantly outweighed by a decrease in electrostatic attraction.

Undergraduate student Jacob Geri applied this new insight into studying copper(I)-thiophene interactions. Jacob used DFT to study the impact of increasing ligand basicity on the binding of [LCu(I)-thiophene]⁺ complexes having the series of substituted 1,10-phenanthroline ligands. This computational exploration demonstrated that copper preferentially binds to thiophene through an η­­² geometry in which copper is positioned between the carbons α and β to the sulfur heteroatom. Moreover, these calculations predict that overall copper-thiophene bond strength, like copper(I)-ethylene bond strength, decreases with increasing ancillary ligand basicity due to a significant decrease in electrostatic attraction. Additionally, copper(I)-thiophene binding was predicted to be weaker than related [LCu(I)-ethylene]⁺ or [LCu(I)-thioether]⁺ binding due to effects stemming from the aromatic nature of the thiophene molecule.

We are also actively pursuing the synthesis of a LCu(I)-thiophene adduct in addition to our computational investigations. We have attempted many of the standard routes that are generally used for such syntheses, including the addition of thiophene to [LCu(MeCN)]⁺ complexes having a variety of different N-donor ligands. One avenue that is currently being explored and is showing promise is the oxidation of metallic copper by silver salts in the presence of N-donor ligand and excess thiophene. This procedure has been used successfully to synthesize other copper(I) adducts of weakly-binding ligands and is being thoroughly explored for our system as well.

Impact, Presentations, and Publications

The detailed results of the [LCu(I)-ethylene]⁺ binding study were presented by the P.I. at the national meeting of the American Chemical Society in March of 2010 in San Francisco. In addition, a manuscript detailing the full results of this study is currently being finalized for submission as a full article to Organometallics, with undergraduate students Naomi Pernicone and Mary Jane Simpson as co-authors.

Naomi Pernicone presented the results of her combined studies on copper(I)-thiophene and copper(I)-ethylene binding interactions as a research poster at the spring National Meeting of the American Chemical Society in San Francisco in March of 2010.  Moreover, Ms. Pernicone presented her results at the Stetson Undergraduate Research and Creative Arts Symposium (SURCAS) in April of 2010 at Stetson University. She plans to attend graduate school in chemistry beginning in the fall of 2011. The results of her studies on LCu(I)-ethylene complexes are currently being prepared for submission to the Journal of Molecular Structure: Theochem.

Undergraduate student Ashli Simone presented the results of her studies on copper(I)-alkene binding at the Stetson Undergraduate Research and Creative Arts Symposium (SURCAS) in April of 2010 at Stetson University. Ashli was awarded a prestigious Maris prize for the best research project/presentation in her section. Ashli graduated in May 2010 and is currently pursuing a PhD in chemistry at Colorado State University.

Undergraduate student Jacob Geri wrote a research proposal involving copper-thiophene binding and received funding through a Stetson Undergraduate Research Experience (SURE) grant to support his full-time work on this project during the summer of 2010. This grant would not have been possible without his preliminary work supported by this ACS-PRF grant. Jacob will present the results of his study at the spring national meeting of the American Chemical Society in March 2011 and a manuscript detailing the computational study of [LCu(I)-thiophene]⁺ binding is being prepared for submission as a full article to the Journal of Physical Chemistry B.

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, supplies, and computational tools necessary to complete this research and to support the P.I. during the summer of 2010. Continued use of these funds will 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.