60th Annual Report on Research 2015

            The reversible binding processes supported by hemilabile ligands often engender catalysts with an exceptional blend of high activity and good stability. Despite the impact that catalysts with hemilabile ligands have had on petrochemical transformations, rational design of hemilability is difficult. The goal of this ACS PRF New Doctoral Investigator project is to synthesize pincer-crown-ether catalysts that feature a hemilabile aza-crown ether group that enables modulation of hemilability through cation-macrocycle interactions.

            In the third year, made possible by a no-cost extension, we carried out mechanistic studies to better understand the cation-responsive reactivity of iridium and nickel pincer-crown-ether complexes. These mechanistic studies are currently guiding efforts towards cation-tunable catalysis, building on a proof-of-concept result that the rate of H₂ activation can be controlled based on the choice of cation added to solution.

            In order to successfully control hemilability, the system must support (a) reversible binding events to change the ligand hapticity, and (b) substantial cation-macrocycle binding processes (Scheme 1). Prior work has shown that the pincer-crown-ether ligand can move between tridentate, tetradentate, and pentadentate binding modes on iridium. Similarly, nickel complexes that feature tridentate and tetradentate binding modes have been synthesized. Donating solvents such as acetonitrile can displace the ether ligands in the high-coordinate chelates.

Scheme 1.

            Further studies to quantify the cation-crown ether interactions are currently underway (Scheme 1). The complex (k³-^15c5NCOP^iPr)Ni(Br), with an aza-15-crown-5 macrocycle that contains four potential oxygen donors, was treated with increasing amounts of LiPF₆. Based on the change in chemical shift of the crown ether protons, a binding affinity could be obtained. Comparative studies with Na⁺ and K⁺ salts revealed that Li⁺ has the highest binding affinity, as expected for a relatively small macrocycle. When analogous titrations were carried out with the cationic complex (k³-^15c5NCOP^iPr)Ni(NCCH₃)]⁺, the binding affinity with Li⁺ was substantially lower. The results suggest that Li⁺ gives the largest rate enhancements for H₂ cleavage because it boasts the highest binding affinity with the pincer-crown-ether ligand.

Scheme 2.

            After discovering that (k³-^15c5NCOP^iPr)Ni(O^tBu) is an active catalyst for the insertion of benzaldehyde into a C−H bond of acetonitrile, a series of mechanistic studies was undertaken to better understand this reaction. The behavior of neutral precatalyst (k³-^15c5NCOP^iPr)Ni(O^tBu) was compared with cationic precatalyst (k³-^15c5NCOP^iPr)Ni(NCCH₃)]⁺. The neutral precatalyst does not require any added base, and gives over 100 turnovers in 24 hours. On the other hand, the cationic precatalyst requires base (equimolar with respect to the catalyst), and even then it takes roughly 120 hours to achieve 100 turnovers.

            In situ NMR spectroscopy studies confirmed that the tert-butoxide complex (k³-^15c5NCOP^iPr)Ni(O^tBu) rapidly activates acetonitrile to form the cyanomethyl complex (k³-^15c5NCOP^iPr)Ni(CH₂CN) before undergoing benzaldehyde insertion to afford the cyanoalkoxide complex (k³-^15c5NCOP^iPr)Ni(OCH(Ph)CH₂CN). The latter species had not been observed in prior studies of Ni catalysts. Interestingly, the same intermediates are observed during catalysis by neutral and cationic species, suggesting some shared pathways. A unified mechanistic picture is shown in Scheme 2.

            The ACS PRF DNI award has had a positive impact on the supported personnel. With DNI support, the PI has presented research at ACS National Meetings and at the Organometallic Chemistry Gordon Research Conference (GRC) in 2015. A graduate student was also able to attend the Organometallic Chemistry GRC this year. The ACS-supported research has also attracted the interest of an industrial sponsor. A strong research foundation for catalysis with the pincer-crown-ether ligand framework has been established with ACS support.