A Convergent Synthesis Approach to Uranium-Carbon Triple Bonds for Alkyne Metathesis Reactions.
Final Report Abstract
The proposal for grant application was put forward in response to a joint NSF-DFG call for collaborative research proposals. Based upon the NSF application titled “A Convergent Synthesis Approach to Uranium-Carbon Triple Bonds for Alkyne Metathesis Reactions”, by Prof. Christopher C. Cummins, the research plan proposed collaboration between the Cummins’ research group at the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA, and my own group at the Friedrich-Alexander-University Erlangen-Nuremberg (FAU). Alkylidyne complexes are of interest for the role that they play in transition metal-mediated alkyne metathesis catalysis. The most prominent and commercially available catalysts for alkyne metathesis reactions were originally discovered by R. Schrock and have the formula [(OR)3M≡CR] with M = Mo, W. The Cummins group is well-experienced with and has made a number of contributions to alkyne metathesis by employing an active catalysts of the general formula [(OR2)3Mo≡CR1]. For the proposed research, the principle of “convergent synthesis” was thought to be implemented by combination of our well-established tacn-triphenolate uranium platform with Cummins’ early transition-metal reagents for group transfer chemistry to uranium. The overall goal of the synthetic efforts was to uncover uranium complexes with multiple-bonded C-ligands, including the first uranium alkylidyne (U≡CR) complexes and to probe their activity in alkyne metathesis. Therefore, synthesis of uranium complexes with multiple-bonded carbon ligands was attempted through a variety of methods. Among more conventional attempts like activation of diazomethane derivatives, the proposed M=CR2 / M≡CR group transfer chemistry employing Cummins’ [Mo] and our tacn-based [((RArO)3tacn)U] complexes, was thoroughly explored but unfortunately has not met with success. This is most likely due to an insufficient thermodynamic driving force and the inability of the soft multiply-bonded C-ligand to stabilize mid- and high-valent oxidation states at the U ion. Triggered by this initial lack of success in isolating high-valent uranium complexes with U–C multiple bonds, stabilized by soft pi-donating alkylidynes (RC3–) and / or alkylidenes and (R2C2–) ligands, we turned our focus towards gaining a deeper understanding of the factors and electronic structures that would enforce the proclivity of stabilizing multiply-bonded (soft) ligands in high-valent uranium complexes. For this purpose a wide variety of potential reagents (CO2 and related heterocumulenes) and multiply-bonded terminal ligands, such as the heavy chalcogenides (S, Se, Te) and pnictogenides (P, As), was tested for their coordination potential with respect to our established tacn-based uranium complexes. In summary, the diverse results gathered in the course of our studies, clearly unveiled the fundamental limitations of the experimental attempts so far but also opened many unforeseen avenues towards exciting new uranium chemistry. Most importantly, the discovery of the so called inverse trans-influence (ITI) as a guiding principle for successfully stabilizing high-valent uranium complexes with a variety of strong σ- and π-donating ligands is important and will help us to plan future synthetic efforts. The ITI is a thermodynamic, ground state phenomenon, in which the M–Xtrans bond is shortened and strengthened in comparison to the M–Xcis bond (X identifies identical anionic ligands under the closed shell formalism). Consequently, the loosely bound – but sterically rather rigid – tacn-supported ligand framework has not been the ideal choice for synthesizing novel high-valent uranium ligand multiple bonds such as nitrides, alkylidenes, or alkylidynes. In our synthetic work on the ITI and terminal high-valent uranium-ligand multiple bonds so far, we have prepared three new crystallographically characterized examples of the ITI. In continuation of this work, we now devised a plan to depart from the rigid triazacyclononane-based tris(aryloxide) ligand system to the more flexible and coordinatively less saturated single N-anchored tris- and bis(aryloxide) ligand derivatives and their corresponding U(III) complexes, namely [((RArO)3N)UIII] and [(R’(RArO)2N)UIII(X)]. The respective [(RArO)3N)UIII] system has recently been established and the synthesis of the next generation ligand (RArO)2NR’)2– (with R’ = pyridine) and its uranium coordination complexes is currently ongoing. We are optimistic that based on the lessons learned in the past funding period, novel high-valent uranium alkylidenes, or alkylidynes will be synthesized in the near future. For this we must apply both steric, kinetic protection, as has been successful in transition metal chemistry and, recently, in the synthesis of uranium (V) nitride complex and electronic, thermodynamic stabilization. This approach requires the further development of ligand systems capable of placing electronegative σand π-donating ligands trans to the desired uranium ligand multiple bond.
Publications
- "Carbonate Formation from CO2 via Oxo versus Oxalate Pathway: Theoretical Investigations into the Mechanism of Uranium-Mediated Carbonate Formation". Organometallics, 2010, 29, 5504
L. Castro, O.P. Lam, S.C. Bart, K. Meyer, L. Maron
- "Activation of elemental S, Se and Te with uranium(III): bridging U-E-U (E = S, Se) and diamond-core complexes U-(E)2-U (E = O, S, Se, Te)". Chem. Sci., 2011, 2, 1538
O.P. Lam, F.W. Heinemann, K. Meyer
- "C-C Bond Formation through Reductive Coupling of CS2 to Yield Uranium Tetrathiooxalate and Ethylenetetrathiolate Complexes". Angew. Chem. Int. Ed., 2011, 50, 5965
O. P. Lam, F.W. Heinemann, K. Meyer
- "Formation of a Uranium Trithiocarbonate Complex via the Nucleophilic Addition of a Sulfide-Bridged Uranium Complex to CS2". 2012, 51, 781
O.P. Lam, L. Castro, B. Kosog, F.W. Heinemann, L. Maron, K. Meyer
(See online at https://doi.org/10.1021/ic202535e) - "Uranium(III)-Mediated C-C-Coupling of Terminal Alkynes: Formation of Dinuclear Uranium(IV) Vinyl Complexes". J. Am. Chem. Soc., 2012, 134, 12792
B. Kosog, C.E. Kefalidis, F.W. Heinemann, L. Maron, K. Meyer
(See online at https://doi.org/10.1021/ja3047393)
