1H nuclear magnetic resonance studies on structural phase transitions and molecular dynamics of five-membered rings in solid ferrocene, azaferrocene and ruthenocene
The temperature variations of the 1H spin–lattice relaxation time T1 and the second moment M2 of 1H n.m.r. absorptions have been studied for solid ferrocene, azaferrocene and ruthenocene. Differential thermal analysis and 1H T1 measurements have revealed that azaferrocene has a new low-temperature stable phase below 281 K, while its monoclinic phase, stable at room temperature, is easily supercooled to extremely low temperatures to form a metastable phase, and ruthenocene shows a second-order phase transition at 392 K. The 1H n.m.r. results obtained indicate that the five-membered rings of the respective phases of these metallocenes perform in-plane C5 or pseudo-C5 reorientations with different activation energies, which were estimated from the 1H T1 data. For the low-temperature stable phases of ferrocene and azaferrocene, large activation energies for the C5(including pseudo-C5) reorientation of the rings were obtained, indicating that they form closely packed crystals. The metastable phase of azaferrocene indicates the presence of a dipole-glass-like state at lower temperatures for the electric dipoles of C4H4N rings randomly frozen. The solid-state properties of each phase and the mechanism by which phase transitions occur in the three metallocenes are discussed with reference to the dynamics of five-membered rings. A new explanation of the transition entropies reported for solid ferrocene is proposed based on the present n.m.r. results.