60th Annual Report on Research 2015

This work explores the development of new (allyl)nickel cations to potentially serve as catalysts in hydrogenation reactions as little progress has been made to identify alternate ways to hydrogenate olefins using more sustainable homogeneous catalysts. To date our group has synthesized eight new cationic (2-alkyl-phosponate-allyl)nickel cations with either tert-butylnitrile, benzonitrile, or triphenylphosphine ligands and different non-coordinating counteranions  via a convenient one-pot strategy. The complexes were characterized by ¹H, ³¹P, and ¹³C NMR spectroscopy and X-ray crystallography. Our preliminary results evaluating the isomerization of 1-pentene with each complex show rapid catalytic isomerization at room temperature in the presence of the nickel complexes. This reaction verifies the ability of our complexes to react with alkenes, an important piece of the hydrogenation mechanism. In the future, we will expose nickel complexes with H₂ and then in conjunction with alkenes to study if catalytic hydrogenation is possible.

Scheme 1. One-pot synthetic strategy employed to prepare of (2-alkyl-phosphonate-allyl)nickel(II) cations. The solid-state structure was verified by X-ray crystallography.

Project 2: Base-free transfer hydrogenation using Cp*Ir(III) precatalysts containing pyridinesulfonamide precatalysts

Additionally, funds continue to support another project in my group focused on transfer hydrogenation catalysis looking to selectively convert aldehydes and ketones, molecules isolated from petroleum feedstocks, to alcohols without the use of hydrogen gas. It is well documented that metal-ligand cooperative effects in the catalyst allow for facile hydrogen transfer from the donor molecule to substrate. This is often facilitated by the presence of both the basic nitrogen atom on the ligand and transition metal center. Thus, we became interested in using new water and oxygen tolerant Cp* iridium complexes to investigate hydrogen transfer catalysis with isopropanol as a benign hydrogen source. In addition, selective transfer hydrogenation of aldehydes is challenging because in general this catalysis occurs under basic conditions, which can lead to multiple side products from the aldol condensation.

A library of 18-electron Cp*Ir^IIICl complexes containing pyridinesulfonamide ligands with different electronic parameters were synthesized (Scheme 1). The Cp*Ir^III chloride complexes (1-7) serve as precatalysts in the transfer hydrogenation of aryl, diaryl, dialkyl, linear, and cycloaliphatic ketones, linear and aromatic aldehydes, a,b-unsaturated ketones, diones, β-ketoesters, and biomass-derived substrates, with 2-propanol as the hydrogen source in presence of 1 mol% catalyst (Tables 1 and 2), although it was discovered that 0.1 mol% can also be employed. Remarkably, all catalysis experiments could be conducted in air without dried and degassed solvents. Basic additives and halide abstractors are not required for high transfer hydrogenation activity and additives such as silver salts and KOH impeded catalysis. Control and mercury poisoning experiments support a homogeneous catalyzed pathway. Overall, the fastest reactions and greatest conversions were observed using electron-poor substrates, and precatalysts bearing electron-rich substituents on the sulfonamide moiety (1, 4, 6). Thus, an iridium complex with bound ligand must be intimately involved in transfer hydrogenation catalysis, but if ligand remains bound at both nitrogen sites or completely during catalysis remains unknown. The synthesis of the precatalysts and transfer hydrogenation of ketones was recently published in Organometallics.

Scheme 2. Synthesis of Cp*Ir complexes containing pyridinesulfonamide ligands.

Table 1. Substrate Scope for transfer hydrogenation of ketones.

¹H NMR spectroscopy used to determine yield. Yields reported as an average of 3 trials. 1,4-Dimethoxybenzene used as standard. 1.0 M solution of substrate in isopropanol. ^a GC-MS used to determine yield.

Table 2. Reduction of Aromatic Aldehydes.

¹H NMR spectroscopy used to determine yield. Yields reported as an average of 3 trials. 1,4-Dimethoxybenzene used as standard. 1.0 M solution of substrate in isopropanol. ^aReaction run for 20h.

Initial mechanistic studies on the transfer hydrogenation of acetophenone using Cp*Ir(pyridinesulfonamide)Cl precatalysts (5) revealed the length and flexibility of the linker between the sulfonamide and pyridine moieties of the ligand impacts the rate of catalysis. Precatalysts possessing the more flexible ethylene linker (5) yield the highest amount of 1-phenylethanol in 3h, when compared to their methylene- (5a) and phenyl- (5b) linked analogs (Scheme 3), strongly suggesting that the ability for the pyridinesulfonamide ligand to dissociate from the metal plays a role in the high activity observed. In addition, metal hydride species have been identified and experiments are underway to isolate these complexes.

Scheme 3. Assessing Ligand Flexibility in Transfer Hydrogenation Catlaysis.

To date this work has supported research done by 7 undergraduates in my research group. In addition, this work has also been brought into the classroom at The College of New Jersey. Three TCNJ graduates supported by this grant are currently chemistry graduate students at Minnesota, UC Santa Barbara, and Brandeis. Work relevant to this grant was presented by undergraduate students and the PI at the American Chemical Society (ACS) Meetings in Denver and Dallas and at local college-wide functions. Additionally, the PI was invited to speak at the Mid-Atlantic Regional Meeting (MARM) of the ACS and at the Philadelphia Inorganic Colloquium (PIC) on topics funded by this grant. Preliminary results obtained for the transfer hydrogenation project were included in two proposals submitted to the National Science Foundation. During this grant period the PI was granted tenure and promoted to Associate Professor.