**6. Conclusions**

In our studies of sterically hindered molecules, in particular with their relevance to molecular machines, following the examples of our eminent predecessors, we have taken advantage of the versatile palladium cross-coupling approaches of Stille, Heck, Suzuki and Sonogashira to develop convenient, often high-yield, routes to indenyl and ferrocenyl anthracenes and triptycenes. Their structures, reactivity and dynamic behaviour have been elucidated by X-ray crystallography and variable-temperature NMR spectroscopy and have revealed how rotational barriers between molecular fragments can be manipulated by the controlled migration of organometallic moieties.

Moreover, copper-catalysed coupling of substituted 2-indenyl systems to form *racemic* and *meso* biindenyls led to an apparently heavily congested molecule, in which, surprisingly, correlated gear rotation of adjacent triptycyl moieties continues unhindered. Finally, the computationally predicted large rotation barrier in 9-phenylanthracene has been experimentally verified by judicious symmetry breaking.

**Author Contributions:** Conceptualization, writing, editing and reviewing, M.J.M. and K.N., who contributed equally. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Science Foundation Ireland (SFI) (grant number RFP/CHE0066).

**Acknowledgments:** We thank University College Dublin and the UCD School of Chemistry for additional financial support, the Centre for Synthesis and Chemical Biology (CSCB) for the use of analytical facilities, and the reviewers for their helpful comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
