Solid-state molecular Organometallic Chemistry of Group 7 Carbonyls: Taming Reactive Complexes in an Anionic Cage

Background: Organometallic synthesis, reactivity and catalysis is nearly always performed in the solution phase, where the complex, or catalyst, of interest is dissolved in a solvent of choice. While this is convenient it often means that very reactive intermediates in catalysis, or complexes in synthesis, are challenging to observe, let alone isolate, as the solvent often binds, or irreversibly reacts, with the metal centre. The Weller group has recently developed a technique to avoid solvent binding, and thus isolate, characterise and study highly reactive complexes that cannot be made by traditional solution routes. The technique is deceptively simple: all the chemistry, characterisation, reactivity and even catalysis is carried out in single-crystal to single-crystal transformations in which gases penetrate the crystal and react with the metal centre to generate the target complex, or undergo the catalytic transformation of interest. We term this solid-state molecular organometallic chemistry (SMOM-Chem). For example this approach allows for cationic sigma-alkane complexes to be synthesised, key - but normally transient - intermediates in C-H activation process. A stabilising "nanoreactor" of [BArF4]- anions around a reactive cation in the crystal lattice (similar to enzyme reactive sites) is key to this. The power of this approach is demonstrated by the synthesis on gram scale of such alkane complexes that are indefinitely stable at room temperature, whereas in solution lifetimes are (at best) minutes at temperatures of -100 degrees C and mg quantities. SMOM techniques thus allow for the isolation of "impossible" complexes.
Objectives: Up until now this chemistry has focussed on group 9 complexes. The question is can SMOM chemistry be extended to other transition metals, and can very reactive, synthetically challenging, and catalytically interesting, systems be developed? We will answer this in an exciting new project that combines SMOM techniques with group 7 cations (Mn, Re) using photophysical methods to generate and interrogate the reactive metal centres.
Experimental Approach: Very recently the design motifs for stable SMOM systems has been determined. Informed by these criteria new cationic Mn- or Re-carbonyl complexes will be synthesised and their reactivity studied in the solid-state, e.g. [M(chelating-ligand)(CO)n][BArF4]. A wide variety of ligand motifs (chelate, pincer, non-innocent ligands) will be used to generate a library of new group-7 SMOM systems. These may already be coordinatively unsaturated (agostic) or, uniquely, can be activated in situ by CO-loss using photolysis. These will be fascinating complexes in their own right, being highly reactive 16 or 14-electron species that are primed for reactivity in solid/gas processes, as well as "drop-in" catalysts for traditional solution processes. In addition to the fundamental structure bonding in these systems there is much reactivity to explore, e.g. alkane, H2 and noble-gas complexes, C-H activation and solid/gas catalysis (e.g. hydrogenation, dehydrogenation of alcohols, isomerisation). It also opens the opportunity to study events within the solid-state using time-resolved infra-red spectroscopy (Lynam is an expert in the area).
Novelty: The study of solid-state molecular chemistry of group-7 complexes, and their photophysical properties, is novel; and will result in high-impact publications. It will push the boundaries of what is achievable in SMOM chemistry, develop new chemistry (SMOM-photochem) and potentially unlock new catalytic systems. It takes 3d-metal chemistry in a new direction.
Training: The PhD student will become expert in organometallic synthesis, NMR (solution and solid-state), x-ray crystallography and new photophysical techniques. There will be plenty of opportunities to explore solid/gas catalysis.