59th Annual Report on Research 2014

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. The next step will be to react these 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: Synthesis of iridium catalysts for transfer hydrogenation catalysis

Additionally, funds have supported a new project in my group that is focused on transfer hydrogenation catalysis. The reduction of achiral polar substrates is an important reaction in organic synthesis. Reduction of carbonyl and imine compounds to generate alcohols and amines generates value-added chiral-products that are important building blocks for the pharmaceutical and fine chemical industries. 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. It is known that Cp* (Cp* = pentamethylcyclopentadienyl) iridium complexes with chelating ligands are quite active in transfer hydrogenation. Thus, we became interested in using new water and oxygen tolerant Cp* iridium complexes to investigate hydrogen transfer catalysis. This work will complement previous studies in which iridium catalysts exhibit a cooperative effect for hydrogen delivery between an NH amido group on the ligand and the iridium center, by providing key data to support hydrogen transfer between the iridium center and the NR group of a sulfonamide moiety, which has not been reported.

A library of 18-electron Cp*Ir^IIICl complexes containing pyridinesulfonamide ligands were synthesized (Scheme 2). Although pyridine ligands are ubiquitous in the literature, few examples of complexes containing second and third row transition metals with pyridinesulfonamide ligands exist and no applications of these complexes to catalysis have been reported. The structures of complexes 4, 7, and 11 were confirmed by X-ray crystallography (Figure 1) and exhibit the expected three-legged piano stool geometry. Additional structural support obtained using ¹H NMR spectroscopy verified deprotonation of the sulfonamide.

Mechanistic studies were conducted to evaluate hydrogen transfer from isopropanol to the iridium complex. Exposure of Cp*Ir(pyridine-sulfonamide)Cl complex 8 to excess water or EtOH at room temperature resulted in protonation of the sulfonamide nitrogen. Heating complexes 4-8 in isopropanol at reflux overnight resulted an iridium hydride complex, which was detected by a singlet at –13.7 ppm in the ¹H NMR.

The Cp*Ir^III chloride complexes (4-11) were screened as precatalysts in the transfer hydrogenation of acetophenone in isopropanol without base at 83¼C (refluxing) for 24 h using 1,4-dimethoxybenzene as an internal standard (Scheme 3). The solvent and substrates were not degassed prior to catalysis screening and all experiments were set up in gas tight vials in air. The product yield for the ethylene-based precatalysts was 84-87% and variation of the sulfonamide substituent did not impact the yield of the alcohol product (Table 1). Base is not required to observe catalytic turnover, as there was no significant difference in product yield with (87%) or without (88%) base. Yields were increased to 95% under dilute conditions (0.5 M) with catalyst 8.  The ethylene-linked catalysts reduce acetophenone in higher yields than the methylene linked catalysts. Future work will focus on increasing the substrate scope and applications in asymmetric transfer hydrogenation.

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

Figure 1. X-ray crystal structures of select Cp*Ir complexes.

Scheme 3. Catalytic reduction of acetophenone using Cp*Ir(III) pyridinesulfonamide chloride precatalysts.

Table 1. Catalytic reduction of acetophenone using Cp*Ir^III pyridinesulfonamide chloride precatalysts.  Yields are an average of three trials. All reactions were run for 24 h.

^a Run with 1 mol% KOH. b Run with 1 eq NaPF₆ relative to catalyst (1 mol%). Reactions without error bars were run once. Substrate concentration = 1.3 M for all reactions.

To date this work has supported research done by 5 undergraduates in my research group. In addition, this work has also been brought into the classroom at The College of New Jersey. The PI also engaged 31 students in independent research project related to the preparation of coordination complexes containing pyridine-based ligands related to this project. The work has been presented by undergraduate students and the PI at the American Chemical Society Meeting and at local college-wide functions. Preliminary results obtained for the transfer hydrogenation project are included in a proposal submitted to the National Science Foundation.