A molecular paddlewheel with a sliding organometallic latch: syntheses, X-ray crystal structures and dynamic behaviour of [Cr(CO)3{eta(6)-2-(9-triptycyl)indene}], and of [M(CO)3{eta(5)-2-(9-triptycyl)indenyl}] (M = Mn, Re).

In [eta(6)-2-(9-triptycyl)-indene]tricarbonylchromium (2a), the indenyl-chromium moiety is linked directly to the axis of the three-bladed triptycene paddlewheel. However, the molecular structure of 2a reveals that there is no steric interaction between these components, and the paddlewheel is free to rotate. Accordingly, its NMR spectrum indicates the full equivalence of the blades of the triptycene. Deprotonation of the indene induces a haptotropic shift of the organometallic fragment from the six-membered to the five-membered ring of the indene and, in the sodium [eta(5)-2-(9-triptycyl)-indenyl]tricarbonylchromium salt (3a), so formed, rotation of the three-bladed molecular paddlewheel is now blocked by the bulky tripod. NMR data for 3a, and also for the isostructural eta(5)-Mn(CO)(3) and eta(5)-Re(CO)(3) complexes, 3b and 3c, respectively, reveal a 2:1 splitting of the blades of the triptycyl moiety, thus breaking its original threefold symmetry. The X-ray crystal structures of the chromium complex, 2a, and of the manganese and rhenium complexes, 3b and 3c, provide pictures of the system in both its "ON" and "OFF" states, whereby the M(CO)(3) tripod has moved about 2 A towards the triptycene, thus blocking its rotation. Comparison of the rotational barriers in 2-(9-triptycyl)indene (1) and its complexes 2 and 3, suggests that rotation of the paddlewheel can be slowed by a factor of approximately 10(8).