For many years, understanding the behavior of reactive intermediates—species too unstable to isolate and put in a bottle—has driven the development of new synthetic methods in organic chemistry. Here I am referring to classic reactive species such as carbenes, nitrenes, cyclobutadiene, benzyne. In this project, we synthesized new molecules designed to be precursors to new classes of reactive intermediates. Building on our successful synthesis of a molecular precursor to the P₂ molecule, we showed that it is possible to transfer the reactive diphosphorus unit to an acceptor such as the inorganic azide ion. This resulted in a “click” cycloaddition reaction resulting in a new diphosphapentazolate ion, an aromatic species composed entirely of nitrogen and phosphorus. This broke new ground in extending the concept of aromaticity from organic chemistry to inorganic chemistry, and portends a future in which certain inorganic materials may conduct electricity when arranged in planar sheets analogous to graphite. This work simulated press coverage and also led to a new collaboration with scientists working at one of the national labs. This enabled us to study the electronic structure of the new aromatic species in exquisite detail by a technique known as negative ion photoelectron spectroscopy (NIPES). Having established this collaboration, we went on to study related species. For example, our molecular precursor to HCP, a known interstellar molecule about which little is known in terms of reaction chemistry, enabled us to produce the new aromatic ion 1-phosphatriazolate, whose electronic structure we were able to probe using NIPES. Phosphaethynolate is unusual in that it possesses a low-coordinate phosphorus center yet is stable enough to isolate even though it lacks bulky protecting groups; to understand the electronic structure responsible for this stability, in another study we carried out a NIPES investigation on this and two related ions. In a further synthetic investigation we showed that these pnictaethynolate ions can transfer the pnictogen to a transition-metal as a new method for forming a terminal metal-pnictogen triple bond that is accompanied by decarbonylation. Teaming up with an astrochemist, we were able to determine for the first time the structure of the simple HSNO molecule, a species postulated to be at the nexus of H₂S and NO chemistries in biological systems. In targeting for synthesis a metal complex having the simple PN diatomic molecule as a ligand, we showed that the new reactive intermediate thereby produced had reactivity patterns typically associated with singlet phosphinidenes. We also set our sights on aminphosphinidenes as reactive intermedates and used our corresponding molecular precursors to demonstrate aminophosphinidene transfer to olefins and alkynes; this constitutes a new synthetic route to phosphiranes, valuable three-membered ring compounds containing one phosphorus atom in the ring. Our mechanistic studies implicate free aminophosphinidenes as reactive intermediates in the new process.

These studies formed the basis for the Ph.D. thesis research of three graduate students. One recently joined the faculty at the University of Washington, one has gone on to postdoctoral studies at Harvard, and one is still in my research group. The studies have been carried out with the participation of MIT undergraduate students likely to pursue graduate studies in chemistry. Also participating were postdoctoral researchers and visiting students from overseas who came with their own fellowship funding. The studies laid the groundwork for our current and future research investigations, as well as for several ongoing collaborations. Together with our published fundamental scientific findings, the education of undergraduate and graduate students, and the advanced training of postdoctoral scientists and visiting scholars, make up the most significant outcomes from the award.

Last Modified: 09/13/2017
Modified by: Christopher C Cummins