Ligand Dynamics in the Solid: Lithium-fluorenide and Lithium-benzo[ b]fluorenide N, N, N′, N′-Tetramethylethylenediamine (tmeda) Complexes.

The dynamic behavior of the N, N, N′, N′-tetramethylethylenediamine (tmeda) ligand has been studied in solid lithium-fluorenide(tmeda) ( 3) and lithium-benzo[ b]fluorenide(tmeda) ( 4) using CP/MAS solid-state 13C- and 15N-NMR spectroscopy. It is shown that, in the ground state, the tmeda ligand is oriented parallel to the long molecular axis of the fluorenide and benzo[ b]fluorenide systems. At low temperature (<250 K), the 13C-NMR spectrum exhibits two MeN signals. A dynamic process, assigned to a 180° rotation of the five-membered metallacycle ( π-flip), leads at elevated temperatures to coalescence of these signals. Line-shape calculations yield Δ H‡=42.7 kJ mol−1, Δ S‡=−5.3 J mol−1 K−1, and $\rm{\Delta G_{298}^{\ddag}}$=44.3 kJ mol−1 for 3, and Δ H‡=36.8 kJ mol−1, Δ S‡=−17.7 J mol−1 K−1, and $\rm{\Delta G_{298}^{\ddag}}$=42.1 kJ mol−1 for 4, respectively. A second dynamic process, assigned to ring inversion of the tmeda ligand, was detected from the temperature dependence of T1 ρ, the 13C spin-lattice relaxation time in the rotating frame, and led to Δ H‡=24.8 kJ mol−1, Δ S‡=−49.2 J mol−1 K−1, and $\rm{\Delta G_{298}^{\ddag}}$=39.5 kJ mol−1 for 3, and Δ H‡=18.2 kJ mol−1, Δ S‡=−65.3 J mol−1 K−1, and $\rm{\Delta G_{298}^{\ddag}}$=37.7 kJ mol−1 for 4, respectively. For (D12)- 3, the rotation of the CD3 groups has also been studied, and a barrier Ea of 14.1 kJ mol−1 was found. [ABSTRACT FROM AUTHOR]