61st Annual Report on Research 2016

This fundamental research program initiated the first examination of open geodesic polycyclic aromatic hydrocarbons (PAHs) having variable surface size and curvature as potential ligands for preparation of novel organometallic complexes with f-elements. The project proposed to utilize unique geometry of bowl-shaped PAHs that can support unusual coordination environments and various oxidation states of metal ions in their organometallic complexes. The resulting products are expected to exhibit single-molecule magnet (SMM) behavior but are not available for investigations. To access novel organometallic complexes of lanthanides with carbon-rich curved pi-ligands several different synthetic schemes have been proposed. All of these synthetic routes require bulk quantities of open geodesic PAHs (often referring to as buckybowls or pi bowls) with variable bowl depth and different carbon frameworks, none of which is yet commercially available. Therefore, during the first stage of the project we focused on the development of scalable multi-step organic reactions to prepare a set of pi bowls with the increasing surface area and bowl depth. A family of benzannulated corannulenes with a double size increase from twenty (parent corannulene) to forty carbon atoms (pentabenzocorannulene derivative) has been accessed and structurally characterized (manuscript is under preparation). Next, the controlled reduction reactions of several selected bowls using Group 1 metals as reducing agents have been developed to prepare sets of alkali metal salts with different negatively charged non-planar carbanions. These sets provide new and unique ligand transfer agents that are also not commercially available. Their full characterization has been accomplished using X-ray crystallographic and spectroscopic tools, augmented by DFT calculations. This allowed us to reveal alkali metal binding trends of curved carbanions, along with their stability and solution behavior, and thus to select the suitable mono-, di- and triply-reduced non-planar anions for the ligand exchange reactions with suitable lanthanide-based reagents. DFT-based computational methods have been used for calculating the molecular geometries of multi-charged bowl-shaped anions and the results were compared with X-ray single crystal diffraction data, revealing the reliability of the selected DFT methods (manuscript is under preparation). These investigations of new organometallic products gave valuable insights into metal-piinteractions with an emphasis on the perturbation of structures and reactivity induced by metal coordination to non-planar carbon-rich polyaromatic surfaces.

While several alkali metal salts of mono- and doubly-reduced corannulene, the smallest bowl-shaped PAH used in our work, have been prepared and structurally characterized in our group over the last few years, the first bulk isolation of triply-reduced state of corannulene has been accomplished in the frame of this project. The target trianion was isolated as the cesium salt in the form of single crystals suitable for the first X-ray crystallographic characterization that revealed the geometry perturbation of corannulene core upon addition of three electrons along with the formation of remarkable self-assembled structure having multiple encapsulated cesium ions (Chem. Sci. 2016). The structural investigation was augmented by full characterization and in-depth theoretical investigation to give insights into geometrical features and electronic structure of the product. Notably, our work has provided the first reliable spectroscopic characteristics of corannulene trianion which now can be used for in situ monitoring of the subsequent reactions. The investigation of ligand exchange reactions of this highly-reactive carbanion with suitable lanthanide reagents is now on-going.

This project started to open up the first opportunities for practical use of curved carbon-rich hydrocarbons in synthesis of new magnetic materials, the area which is not yet explored. It should help to uncover the underexploited potential of organometallic chemistry in the development of new and advanced Ln-SMMs. The systematic investigation of p bowls in Ln-based metalation reactions is a new and challenging area of research that requires multiple experimental skills. Therefore, in the course of these research activities a program is created for training and educating young chemists, including several local New York State residents, who combine expertise in organic and inorganic syntheses, crystal growth, various characterization techniques, powder and single crystal diffraction with good understanding of current needs and challenges in design and preparation of new magnetic materials. One postdoctoral associate, one staff-member (X-ray crystallographer), one undergraduate and two graduate students participated in the first year of this program. Their involvement significantly enhanced professional development and assisted in preparing the participants for future research and educational careers. This program also contributed to the efforts of the Department of Chemistry to diversify its research portfolio and strengthen the materials chemistry emphasis at the University at Albany.