High and Low Rotational Barriers in Metal Tricarbonyl Complexes of 2- and 3‑Indenyl Anthracenes and Triptycenes: Rational Design of Molecular Brakes

Syntheses and X-ray crystal structures are reported for a series of M­(CO)₃ derivatives (M = Cr, Re) of phenyl and also 2- and 3-indenyl anthracenes and triptycenes. In each case, the rotational barrier about the bond linking the two organic fragments was evaluated both experimentally by VT or 2D-EXSY NMR and by calculation at the DFT level. Attachment of the metal tripod to the indenyl moiety in an η⁶ fashion does not markedly change the barrier relative to that for the free ligand but lowers the symmetry so as to facilitate its direct measurement. Interestingly, an η⁶ → η⁵ haptotropic shift of the Cr­(CO)₃ moiety in 9-indenylanthracenes led to a somewhat lowered barrier, probably attributable to an increase in the ground state energy rather than to decreased steric interactions in the transition state. In contrast, in indenyltriptycenes η⁶ → η⁵ migration of the M­(CO)₃ unit along the indenyl skeleton and closer to a paddlewheel leads to a very significant increase in the rotational barrier. These effects can be rationalized in terms of angular steric strain and multiple interactions in the ground state and in the transition state. The results not only provide semiquantitative data on the steric effects of η⁶-phenyl and η⁶- or η⁵-indenyl M­(CO)₃ fragments but are also discussed with relevance to their role in organometallic molecular brakes.