Inter- and Intramolecular Spin Transfer in Molecular Magnetic Materials. Solid-State NMR Spectroscopy of Paramagnetic Metallocenium Ions
journal contributionposted on 2002-08-14, 00:00 authored by Henrike Heise, Frank H. Köhler, Martin Herker, Wolfgang Hiller
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)2MxCo1-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.