An improved route to osmium(IV) tetraaryl complexes

Investigations into the reactivity, properties, and applications of osmium(IV) tetraaryl complexes have been hampered by their low yielding syntheses from volatile and toxic OsO4 (typically ≤34%). Here we show that air-stable M(aryl)4 compounds (M = Os, Ru; aryl = 2-tolyl, 2,5-xylyl) can be prepared in improved yields (≤73%) starting from new, less hazardous (Oct4N)2[MX6] precursors (M = Os, Ru; X = Cl, Br). This approach also facilitates the preparation of Os(mesityl)4 (Os3), a complex comprising bulky 2,6-methyl substituted aryl ligands, for the first time (5% yield). We evaluate the X-ray crystal structures of Os(2,5-xylyl)4 (Os2) and Os3, employing a "tetrahedricity" metric that shows Os3 has a particularly distorted tetrahedral geometry compared to other Ru(aryl)4 and Os(aryl)4 materials. The 1H NMR spectrum of Os3 exhibits significantly broadened features at 298 K that split into distinct resonances at low temperatures, indicative of restricted rotation of the mesityl ligands around the Os-aryl 𝜎-bond. Solution cyclic voltammograms of Os(aryl)4 reveal that the potentials of their known reversible redox features can be fine-tuned by varying the number of ligand methyl substituents, and that Os3 exhibits an additional 1+/2+ redox event not previously observed in this class of compounds. This work aims to help advance the potential application of these relatively underexplored organometallic complexes in established and emerging areas of molecular materials science, such as extended molecular frameworks and self-assembled monolayers, where analogous tetraphenylmethane and silane species (M = C, Si) have been frequently targeted.