Elucidating the Reactivity of Open-Shell Polynuclear Iron Hydride Clusters

Abstract

Polynuclear reaction sites are used by metalloenzymes as well as heterogeneous catalysts to facilitate multi-electron small molecule activation via cooperative redox chemistry. Specifically, polynuclear transition metal hydrides are invoked as key intermediates in many biological and synthetic systems. For example, enzymatic catalysts that invoke transition metal hydrides are the bridging NiFe-hydride moieties found in the dehydrogenase ([NiFe]-CODHase) and hydrogenase ([NiFe]-hydrogenase), the terminal hydride invoked in [FeFe]-hydrogenase, as well as proposed bridging hydride reactive intermediates in nitrogenase. Heterogeneous transition metal hydrides are invoked in various industrial transformations like the Haber-Bosch process. Because transition metal hydrides are continuously invoked as intermediates in reaction chemistry, understanding their reactivity profile is of utmost importance. Our lab has demonstrated that the hexadentate ligand tbsLH6 (1,3,5-C6H9(HNC6H4-o-HNSiMe2tBu)3) allows for the synthesis of open-shell triiron clusters capable of multinuclear, multi-electron reaction chemistry. Additionally, we have shown that bridgehead ligands can be installed onto the triiron core. To this end, we have developed a synthetic strategy to assemble anionic triiron hydride complexes and explore their reactivity.

With an anionic triiron hydride complex in hand, we demonstrated the anionic triiron hydride cluster is chemically distinct from the related isostructural anionic cluster through 1H NMR, 57Fe Mössbauer, magnetometry, cyclic voltammetry, and X-ray crystallography. The triiron hydride cluster is capable of mediating small molecule reduction chemistry including the cleavage of azobenzene with concurrent dihydrogen formation, stoichiometric hydrogenation of isonitriles and catalytic hydrosilylation of aldehydes.

Changes in metal identity, oxidation state, and ligand design can have significant impacts on metal-metal interactions, delocalization of redox within the transition metal cluster core, and reactivity with small molecule substrates. The modular polydentate ligand synthesis permits facile ligand modification, which allows for the synthesis of a ligand series bearing both electron-withdrawing and electron-donating substituents. With the ligand series in hand, we synthesized four anionic triiron clusters to examine the impact ligand modifications have on the reactivity of open-shell polynuclear iron hydride clusters. The baseline reactivity was determined to be similar across the series, which may reflect similarities in the underlying thermodynamic hydricity; although, the rate of hydride transfer was impacted by the relative ligand electron-richness. We estimate the acidity and bond dissociation free energy using thermochemical cycles. The cluster hydride acidity decreases from ligands featuring electron-withdrawing substituents to electrondonating, as well as an increase in homolytic bond strength.

Finally, we explored changes in the oxidation state via the synthesis of monovalent polynuclear iron hydride clusters. We establish a new platform F, RLH3 (1,3,5-C6H9(HNC6FH3-o-(R)2P)3) (R = phenyl, isopropyl, and tert-butyl) capable of supporting monovalent transition metal clusters. We found that nuclearity was dependent on the phosphine cone angle where smaller ligand variants (R = phenyl) favor mono-iron species; increased steric bulk (R = isopropyl) favors diiron complex formation; culminating in triiron cluster formation supported by R = tert-butyl substituents. The diiron and triiron complexes can stabilize monohydride and polyhydride adduct formation, respectively.