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Ligand-Parameterization

Catalytic reactions provide routes to access important chemical compounds more efficiently. Developing methods to improve catalysts that run those reactions is the primary focus of our current research. When evaluating efficiency and making alterations to a catalyst, there are typically two primary handles: sterics and electronics. Since there are a few options that exist for comparing sterics of ligands, it seems the more challenging hurdle is comparison of ligand-metal electronic interaction. This is where some of our current research is focused.


Determining how much electron density a ligand can donate or take from a metal center can be very challenging. To address this problem, we began work on classifying ligand electronic donation to an early transition metal center. Experimentally, this is accomplished using the chromium molecule below.


Because this complex is nearly electronically saturated, there exists a constant competition between donation from the ligand under study, X, and the ancillary diisopropylamido. As a result of the electronic saturation, the amount of electron density X can donate to the chromium center can significantly affect bonding between the amido and chromium, shown as the equilibrium above. The ligand in study, X, competes with donation from the diisopropylamido, and the N–Cr bond is weakened causing faster rotation of the amide ligand. When X is a poor donor, the nitrogen can donate more density to make an effective double bond, the N-Cr bond rotation is more difficult.


Conveniently, this rotation is easily monitored on the nuclear magnetic resonance, NMR, timescale. Using spin saturation transfer, the rate of rotation about the iPr2N-Cr bond can be determined. From this rate, the enthalpy barrier for rotation (ΔH) can be obtained. The value of ΔH for that rotation has been dubbed the ligand donation parameter (LDP), which allows for the direct, quantitative comparison of electronic donation of various ligands to an early transition metal. When the LDP is high, the ligand donates electon density to the metal center weakly, when the LDP is low the ligand is a good donor. The chart below shows some of the ligands examined on the same scale.


Current work is focused on applications of LDP to catalyst optimization in a variety of different areas. 

 

 

Selected references on this topic from our group:

"Weakly Coordinating yet Ion Paired: Anion Effects on an Internal Rearrangement", Billow, B. S.; McDaniel, T. J.; Odom, A. L.; Nat. Chem., 2017, 9, 837. Link

"Weakly Coordinating yet Ion Paired: Anion Effects on an Internal Rearrangement", Aldrich, K. E.; Billow, B. S.; Holmes, D.; Bemowski, R. D.; Odom, A. L.; Organometallics, 2017, 36 1227. Link

"A Complex with Nitrogen Single, Double, and Triple Bonds to the Same Chromium Atom: Synthesis, Structure, and Reactivity", Beaumier, E. P.; Billow, B. S.; Singh, A. K.; Biros, S. M.; Odom, A. L.; Chem. Sci., 2016, 7, 2532. Link

"Synthesis and Structure of Chromium(VI) Nitrido Cyclopentadienyl Complexes", Billow, B. S.; Bemowski, R. D.;DiFranco, S. A.; Staples, R. J.; Odom, A. L.; Organometallics, 2015, 34, 4567. Link

"Effective donor abilities of E-t-Bu and EPh (E = O, S, Se, Te) to a high valent transition metal", Bemowski, R. D.; Singh, A. K.; Bajorek, B. J.; DePorre, Y.; Odom, A. L. Dalton Trans. 2014, 43, 12299. Link

"Evaluation of Donor and Steric Properties of Anionic Ligands on High Valent Transition Metals", DiFranco, S. A.; Maciulis, N. A.; Staples, R. J.; Batrice, R. J.; Odom, A. L. Inorg. Chem. 201251, 1187-1200. Link