60th Annual Report on Research 2015

Overview of Research Project. The replacement of precious metal catalysts with earth-abundant metal alternatives is a currently focusing research topic in chemistry, in particular with regard to renewable and sustainable energy. Although precious metal catalysts have been known for most of organic transformations, there are great opportunities for developing well-defined non-precious metal hydrogenation catalysts for a class of energy-related transformations from a viewpoint of sustainable chemistry, such as hydrogenation (or transfer hydrogenation) reactions for biomass conversion. This project supported by the PRF grant has allowed our undergraduates to participate in the experimental development of synthesizing novel earth abundant metal complexes as catalysts that enable efficient catalytic reduction of unsaturated bonds through a supramolecular principle of catalyst design, and preliminary publishable results have been obtained within the first year of the grant period.

Current Progress. The proposed work was conceived based upon the PI’s previous observation on a highly active Co^II(PNP)(alkyl) complex that enables the hydrogenation and transfer hydrogenation of C=C, C=O and C=N bonds of a number of substrates under mild conditions. Supported by the PRF grant, we have been able to work on the proposed project and have extended the research significantly. We explored a class of readily available Schiff-base type PNN pincer ligand for replacing the relatively expensive PNP ligand reported. Thus, the one-step Schiff base condensation of a simple diamine and a phosphine-containing aldehyde afforded a tridentate PNN ligand in high yield, and the corresponding complex Co^II(PNN)(alkyl) was obtained via a procedure similar to the known cobalt(II) catalyst. The resulting reactivity tests confirmed that this complex exhibited high efficiency for hydrogenation of both C=C and C=O bonds. Several aliphatic and aromatic alkenes have been examined by using 2 mol% pre-catalyst under 1 atm. hydrogen, and almost quantitative yields were achieved. Hydrogenation of aldehydes and ketones with various substituents was also furnished under the same reaction condition, suggesting this complex is a highly active catalyst. In addition, a trial to transfer hydrogenate acetophenone by using 2 mol% of cobalt catalyst and isopropanol as a hydrogen source proved to be successful as well. This expansion of research is rather significant as it proves that two phosphine units in the ligand framework are in fact not necessary for high catalytic activity, instead it is feasible to gain good catalytic efficacy by designing relevant PNN-, PPN-, or PSN-containing ligands.

To develop novel cobalt(II) catalysts containing a thiourea moiety we performed preliminary work on a related ligand backbone. Addition of a solution of phenyl thioisocyanate to a slightly excess of o-phenylenediamine in dichloromethane resulted in the formation of yellow precipitate of an intermediate after being stirred for 2 hours. Accordingly, the reaction with salicylaldehyde afforded pure ligand in 86% yield after recrystallization. The ligand was subsequently reacted with cobalt(II) acetate and single crystals of a complex were obtained in high yield. Its crystal structure has been determined by X-ray crystallography. The crystal structure reveals a tetra-coordinate cobalt(II) complex, where cobalt is bound to O, N_imine, and N,S atoms from thiourea. It was interesting to note that the coordinated thiourea-N was deprotonated, giving a neutral, distorted square-planar cobalt(II) complex. The involvement of thiourea sulfur in coordination with metals is unexpected from our previous proposal, which certainly deserves further investigation. However, it should be mentioned that the remaining NH group in the complex could still provide extra intermolecular hydrogen bonds for substrate activation. We are currently working on the structural modification by incorporating a phosphine-containing ligand as well as introducing a chiral diamine backbone, and hopefully we will produce new chiral Co-NPS complex for asymmetric catalysis.

In addition, the design of multidentate nitrogen ligands for multimetallic complexes as active catalysts is another objective of the proposed research. First, by utilizing a simple ligand 4'-Cl-2,2':6',2''-terpyridine we have serendipitously isolated and characterized a novel mixed-valent Cu^ICu^II polymeric assembly when the crystallization experiment was conducted under radical-mediated conditions. We then succeeded in synthesizing novel copper complexes based on new terpyridine-derived ligands, and several X-ray structures were obtained. The synthesis of ligands was straightforward through the one-pot Kröhnke condensation of a substituted aldehyde with two equivalents of 2-acetylpyridine or 2-acetylpyrazine, respectively. It was observed that the ligand containing two pyrazinyl groups on the central pyridine is versatile for the formation of diverse mononuclear and trinuclear copper complexes. A tetranuclear cyclic supramolecule and polymeric network were also obtained from related ligands. All of these new metal complexes have been characterized by standard spectroscopic techniques and X-ray crystallography. Preliminary catalytic tests show significant reactivity of the copper complexes in aerobic oxidation of some alcohol substrates. In the next step, we will synthesize new cobalt(II) complexes based on these ligands and investigate their catalytic activity for reduction reactions. The synthesis of chiral nitrogen ligands will be another goal of the proposed research in the second year.

Impact. The current project on non-precious metal catalysis provides arrays of opportunities for undergraduate researchers to conduct cutting-edge research at John Jay College, a certified Minority- and Hispanic-Serving Institution and a Primarily Undergraduate Institution. The existing science Program for Research Initiatives for Science Majors (PRISM) within the college partially benefits this project by providing additional financial support to undergraduates. One undergraduate has participated in the preliminary work described above and is co-authored in three recent publications, which in turn inspires the student to pursue a chemistry PhD degree after graduation. Posters on this work have been presented at the on-campus annual research symposium and another one will be presented at a national undergraduate conference soon. Furthermore, three minority undergraduate researchers are recently engaged in the project and will be awarded with stipends for the 2015-2016 academic year.