Metal(IV) tetraaryl (M(aryl)4) complexes are a relatively unexplored class of organometallic compounds with distinct electrochemical, magnetic, and optical properties attributed to their tetrahedral coordination geometry. A key feature of such complexes is their ortho-methyl group, which sterically protects against decomposition pathways such as reductive elimination. Recognizing that organic tetraaryl complexes (M = C and Si) are common building blocks for covalent organic frameworks (COFs), we propose that metal tetraaryls can be utilized as alternative, redox-active and panchromatic, tetrahedral nodes for a new class of “metal-covalent organic frameworks” (MCOFs) with distinct properties. To evaluate the role of the central atom (M = C, Si, Os, Ru) in M(aryl)4 for mediating conducting tunnelling electrons, we are installing electrode linker groups on the aryl ligands to measure and contrast their transport properties at the single-molecule limit. While complexes comprising different transition metals can be prepared, our initial work is focused on developing the chemistry of Os(aryl)4, as these complexes have been shown to be robust and exhibit reversible redox properties. Using the novel osmium salt, (Oct4N)2[OsBr6], we show that yields of previously reported Os(aryl)4 can be increased from ~30% to ≤76%. We have also developed new routes to elaborate the family of halogen-functionalized osmium tetraaryl complexes, identifying these as valuable precursors for novel materials. Critical to the preparation of halogenated Os(aryl)4 is the use of pre-functionalized (halogenated) aryl Grignard reagents (utilized here for the preparation of M(aryl)4 for the first time), which we synthesize via magnesium insertion or magnesium-halogen exchange. We show our synthetic approach can be used to prepare different Os(aryl)4 bearing aryl ligands: (1) functionalized at different positions; (2) with varying numbers of methyl substituents; and (3) with different stabilizing groups other than an ortho-methyl. We will discuss ongoing efforts to convert these halogenated Os(aryl)4 into an increasingly more diverse family of complexes that may be utilized for numerous applications in materials science.

Functional Os(aryl)4 complexes via Grignard chemistry

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