A Comprehensive Solution and Solid-State NMR Study of Proton Spin Lattice Relaxation in Paramagnetic Metallocenes

Solid solutions of the metallocenes ferrocene (Cp2Fe), nickelocene (Cp2Ni), and cobaltocene (Cp2Co) have been prepared by manually grinding the components together, or by co-crystallizing them from solution. In the solid solutions Cp2Fe/Cp2Ni and Cp2Co/Cp2Ni, the cyclopentadienyl (Cp) protons relax via dipolar electron–proton interactions, which represent the dominant relaxation mechanism. The 1H T1 relaxation times of the molecules Cp2Ni and Cp2Co, dissolved in CDCl3, and in the solid solutions, show that the relaxation takes place intramolecularly. The relaxation of the protons is propagated exclusively via the unpaired electrons of the metal centers to which their Cp rings are coordinated, due to the large intermolecular distances that are greater than 3.91 Å. In contrast, the intramolecular distances between the electrons of the metal atoms and the protons of their coordinated Cp rings are merely 2.70 Å. Using these intramolecular distances and the 1H T1 relaxation times, the electron relaxation times T1e have been determined as 17 × 10−13 s in CDCl3 solutions and 45 × 10−13 s in the solid state for Cp2Ni. The corresponding T1e times for Cp2Co are calculated as ca. 5 × 10−13 s and 20 × 10−13 s. Grinding Cp2Fe and Cp2Ni together leads to two different 1H T1 relaxation times for the protons of Cp2Fe. The longer T1 relaxation time indicates domains that consist mostly of Cp2Fe molecules. The short T1 times show a close contact of Cp2Fe and Cp2Ni molecules. An analysis of the short 1H T1 times reveals the presence of at least two to three short distances of 3.91 Å between Cp2Fe and Cp2Ni molecules. These results support the hypothesis that dry grinding of the metallocenes Cp2Fe and Cp2Ni in ratios that were changed in 10% increments from 90%/10% to 30%/70% leads to domains that mostly consist of Cp2Fe molecules, and additionally to domains that contain a mixture of the components on the molecular level.