**Michael J. McGlinchey \* and Kirill Nikitin \***

School of Chemistry, University College Dublin, Belfield, D04 V1W8 Dublin 4, Ireland **\*** Correspondence: michael.mcglinchey@ucd.ie (M.J.M.); kirill.nikitin@ucd.ie (K.N.)

Received: 10 April 2020; Accepted: 19 April 2020; Published: 22 April 2020

**Abstract:** Pd-catalysed Stille and Suzuki cross-couplings were used to prepare 9-(3-indenyl)-, 6, and 9-(2-indenyl)-anthracene, 7; addition of benzyne led to the 9-Indenyl-triptycenes, 8 and 9. In 6, [4 + 2] addition also occurred to the indenyl substituent. Reaction of 6 through 9 with Cr(CO)6 or Re2(CO)10 gave their M(CO)3 derivatives, where the Cr or Re was complexed to a six- or five-membered ring, respectively. In the 9-(2-indenyl)triptycene complexes, slowed rotation of the paddlewheel on the NMR time-scale was apparent in the η5-Re(CO)3 case and, when the η6-Cr(CO)3 was deprotonated, the resulting haptotropic shift of the metal tripod onto the five-membered ring also blocked paddlewheel rotation, thus functioning as an organometallic molecular brake. Suzuki coupling of ferrocenylboronic acid to mono- or dibromoanthracene yielded the ferrocenyl anthracenes en route to the corresponding triptycenes in which stepwise hindered rotations of the ferrocenyl groups behaved like molecular dials. CuCl2-mediated coupling of methyl- and phenyl-indenes yielded their *rac* and *meso* 2,2 -biindenyls; surprisingly, however, the apparently sterically crowded *rac* 2,2 -Bis(9-triptycyl)biindenyl functioned as a freely rotating set of molecular gears. The predicted high rotation barrier in 9-phenylanthracene was experimentally validated via the Pd-catalysed syntheses of di(3-fluorophenyl)anthracene and 9-(1-naphthyl)-10-phenylanthracene.

**Keywords:** restricted rotations; M(CO)3 tripods; molecular brakes and gears; X-ray; V-T NMR
