Spin lattice relaxation time studies of ring rotation in cyclopentadienylmetal complexes

Artificial Molecular Rotors

“…Increasing the steric bulk around the titanium atom by replacing the chlorine ligands with cyclopentasulfide (Cp 2 TiS 5 ; 169) increases the barriers (∆G q ) to rotation about the Cp rings. 675 (171), the barriers (∆G q ) in solution were found by the coalescence NMR technique to be ∼8.9 and ∼9.0 kcal mol -1 , respectively. When three trimethylsilyl groups were investigated (172), the barrier (∆G q ) increased predictably to 11.2 kcal mol -1 for the zirconium complex and 11.0 kcal mol -1 for the hafnium complex (Hf has a larger ionic radius).…”

“…Increasing the steric bulk around the titanium atom by replacing the chlorine ligands with cyclopentasulfide (Cp 2 TiS 5 ; 169) increases the barriers (∆G q ) to rotation about the Cp rings. 675 Two barriers were found, 1.8 and 2.1 kcal mol -1 , for equatorially and axially positioned Cp ligands. The two rings have different steric environments and rotate at different rates.…”

“…Similarly, Gilson et al measured the barriers to rotation in 159 and 160 by spin−lattice relaxation NMR and found similar values (Δ G ⧧ ) of 1.71 and 1.70 kcal mol -1 , respectively. Earlier, they had measured the barrier in 161 and found it to be a seemingly high 2.3 kcal mol -1 , which is the upper limit measured for ferrocene . Several half-sandwich complexes with cyclooctadiene (COD) “stools” have been measured by Mann and co-workers using 13 C spin−lattice relaxation experiments.…”

“…Earlier, they had measured the barrier in 161 and found it to be a seemingly high 2.3 kcal mol -1 , which is the upper limit measured for ferrocene. 675 Several half-sandwich complexes with cyclooctadiene (COD) "stools" have been measured by Mann and co-workers 676 using 13 C spin-lattice relaxation experiments. For the rhodium complex (162), these authors obtained a barrier (∆G q ) of ∼1.9 kcal mol -1 , and for the iridium complex (163) it was ∼1.7 kcal mol -1 .…”

“…In 1959, the barrier to rotation of cyclopentadiene groups in Cp 2 TiCl 2 ( 168 ) was measured 646 in the solid state using the T 1 relaxation NMR method and the barrier (Δ G ⧧ ) was found to be a scant 0.5 kcal mol -1 . Increasing the steric bulk around the titanium atom by replacing the chlorine ligands with cyclopentasulfide (Cp 2 TiS 5 ; 169 ) increases the barriers (Δ G ⧧ ) to rotation about the Cp rings . Two barriers were found, 1.8 and 2.1 kcal mol -1 , for equatorially and axially positioned Cp ligands.…”