Magneto-Structural Characterization of Metallocene-Bridged Nitronyl Nitroxide Diradicals by X-Ray, Magnetic Measurements, Solid-state NMR Spectroscopy, and Ab Initio Calculations

Crystallization of ferrocene and ruthenocene substituted in the 1‐ and 1′‐positions by two nitronyl nitroxide radicals gave the new crystal phases β‐1 (besides the known phase α‐1), α‐2, and β‐2 whose structures were determined by X‐ray analysis. In β‐1 the radical moieties adopt transoid positions, whereas two different cisoid conformations are adopted by α‐2 and β‐2. These conformations result from inter‐ and intramolecular hydrogen bonds, respectively. All compounds experience antiferromagnetic interactions, and J/kB values up to −7 K have been found by fitting the experimental magnetic susceptibilities to a modified Bleaney–Bowers equation. The solid diradicals α‐1, β‐1, α‐2, and β‐2 as well as the ferrocene 3, which was substituted by a unique nitronyl nitroxide, were investigated by 13C and 1H NMR spectroscopy with magic angle spinning. The carbon signals cover a range of 2000 ppm, and are well resolved such that the structure could be confirmed. Conversion of the signal shifts into spin densities disclosed the mechanisms by which spin delocalization from the nitronyl nitroxide substituents to the metallocene core occurs. The spin density distribution in α‐1, β‐1, and 3 was also predicted by DFT calculations. There is good agreement between the experimental and theoretical trends of the spin delocalization. The magnetic interactions were discussed in the light of intramolecular spin transfer and its dependence on geometric constraints, demonstrating that the 1,1′‐metallocenylene bridge is not a robust magnetic coupler. Subtle changes in the framework of hydrogen bonds have a strong impact on the conformations and structure of metallocenes (e.g., 1 and 2) that are substituted by two nitronyl nitroxide radicals. Experimentally these changes can be triggered by different conditions of crystallization. The molecular geometries differ in the twist angle between the nitronyl nitroxide and the Cp rings of a given ligand, in the cisoid or transoid arrangement of the two ligands, and in the puckering of the nitronyl nitroxide rings.