1H nuclear magnetic resonance studies on structural phase transitions and molecular dynamics of five-membered rings in solid ferrocene, azaferrocene and ruthenocene

Abstract

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.