Reports: ND354967-ND3: New Class of Organometallic Single-Ion Lanthanide Molecular Magnets

Marina A. Petrukhina, State University of New York at Albany

This fundamental research program targets the first exploration of open geodesic polycyclic aromatic hydrocarbons (PAHs) having variable surface size and curvature as potential new ligands for preparation of novel organometallic complexes with f-elements. This project utilizes unique geometry of bowl-shaped PAHs that can support unusual coordination environments and various oxidation states of metal ions and thus may add new examples of single-molecule magnet (SMM) behavior for lanthanides. To access novel organometallic complexes of f-metals with carbon-rich curved p-ligands several different synthetic schemes have been developed. All of these preparative routes require bulk quantities of open geodesic PAHs (often referring to as buckybowls or p bowls) with variable bowl depth and different carbon frameworks, which are not commercially available. Therefore, the group has focused on the development of scalable multi-step organic reactions to prepare sets of p 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 prepared, structurally characterized and now used in metalation reactions (manuscript in preparation). In addition, suitable starting mixed ligand COT-based complexes of several lanthanides have been synthesized. The Sc-analogue prepared for NMR studies by an undergraduate student in the group has been structurally characterized and compared with known Ln-complexes (Acta Cryst. 2017). In addition, 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 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 (manuscript in preparation). 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. Furthermore, we have recently started to explore the use of dual metal combinations in chemical reduction reactions of non-planar PAHs, revealing unique examples of their synergism. As a result, we prepared and structurally characterized the first mixed metal product with triply-reduced corannulene radicals. Its successful synthesis stems from the fascinating coordination selectivity of bowl-shaped carbanions towards different metal ions. Analysis of structural and magnetic data of that unique product has opened the discussion of the role of concave and convex bound metals on perturbation of geometry and interaction of bowl-shaped radicals within the sandwich-type architectures. Using comprehensive theoretical calculations, the consequences of internal metal binding have been investigated for the Group 1 metals, revealing how downsizing of the sandwiched metal belts can be used for fine-tuning of radical magnetic coupling (Chem. Sci. 2017). In addition, the dual combination of two alkali metals having different ion size and binding abilities, has been used in reactions with sumanene for the designed synthesis of a novel sandwich-type architecture with an unprecedented double concave encapsulation of a metal ion by sumanenyl bowls (Angew. Chem. 2017). Investigation of ligand exchange reactions of this highly-reactive carbanion with suitable lanthanide reagents is now on-going.

This project opens up the first opportunities for practical use of curved carbon-rich PAHs 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), two undergraduate and two graduate students participated in 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.