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 13 C‐ and 15 N‐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 13 C‐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 T 1 ρ , the 13 C 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 (D 12 )‐ 3 , the rotation of the CD 3 groups has also been studied, and a barrier E a of 14.1 kJ mol −1 was found.