Inter- and Intramolecular Spin Transfer in Molecular Magnetic Materials. Solid-State NMR Spectroscopy of Paramagnetic Metallocenium Ions

To shed light on the interaction in molecule-based magnetic materials, the decamethylmetallocenium hexafluorophosphates, [(C₅Me₅)₂M]⁺ [PF₆]^- with M = Cr, Mn, Fe, Co, and Ni, as well as the tetracyanoethenides, [(C₅Me₅)₂M]⁺ [TCNE]^- with M = Cr, Mn, Fe, and Co, have been investigated in the solid state by using ¹H, ¹³C, ¹⁹F, and ³¹P NMR spectroscopy under magic angle spinning (MAS). The isotropic ¹³C and ¹H NMR signals cover ranges of about 1300 and 500 ppm, respectively. From the shift anisotropies of the ring carbon signal of the [(C₅Me₅)₂M]⁺ cations, the total unpaired electron spin density in the ligand π orbitals has been calculated; it amounts up to 36% (M = Ni) and is negative for M = Cr, Mn, and Fe. The radical anion of [(C₅Me₅)₂M]⁺ [TCNE]^- shifts the ¹³C NMR signals of all [(C₅Me₅)₂M]⁺ cations to high frequency, which establishes transfer of positive spin density from the anions to the cations. The ¹⁹F and ³¹P NMR signals of the paramagnetic salts [(C₅Me₅)₂M]⁺ [PF₆]^- are shifted up to 13.5 ppm relative to diamagnetic [(C₅Me₅)₂Co]⁺ [PF₆]^-. The signs of these shifts are the same as those of the π spin density in [(C₅Me₅)₂M]⁺. After consideration of interionic ligand- and metal-centered dipolar shifts, this establishes cation−anion spin delocalization. The mixed crystals [(C₅Me₅)₂M_xCo_1-x]⁺ [PF₆]^- have been prepared for M = Cr and Ni. They are isostructural with [(C₅Me₅)₂Co]⁺ [PF₆]^- whose single-crystal structure has been determined by X-ray diffraction. The ¹³C, ¹⁹F, and ³¹P MAS NMR spectra of the mixed crystals show that the respective two closest paramagnetic ions in the lattice delocalize spin density to [(C₅Me₅)₂Co]⁺, [(C₅Me₅)₂Ni]⁺, and [PF₆]^-. In [(C₅Me₅)₂M]⁺, about 10^-4 au per carbon atom are transferred.