Fusing Rigid Spin Probes to Paramagnetic Sandwiches. Synthesis, Crystal Structures, and NMR Spectroscopy of Bis(isodicyclopentadienyl)metal Compounds†
journal contributionposted on 21.04.1999 by Irene Gattinger, Martin A. Herker, Wolfgang Hiller, Frank H. Köhler
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To probe the sign of the unpaired spin density on the ligands of metallocenes neutral and cationic derivatives which contained the isodicyclopentadienyl (Isodicp) ligand have been synthesized. The resulting compounds have the composition (Isodicp)2M [M = Fe (3), V (4), Cr (5), Ni (7)], and the corresponding salts are (Isodicp)2M+PF6- (M+ = Fe+) (3+PF6-), Cr+ (5+PF6-), Co+ (6+PF6-), Ni+ (7+PF6-). While the exo,exo-, exo,endo-, and endo,endo-isomers (a, b, and c, respectively) are conceivable, 4a, 5a, 5a+PF6-, 7, and 7a+PF6- have been isolated as the only isomers. By contrast, mixtures of 3a and 3b, 3a+PF6- and 3b+PF6-, as well as 6a+PF6-, 6b+PF6-, and 6c+PF6- have been obtained; 3a and 3a+PF6- could be separated by crystallization. Metal-dependent ligand exchange is proposed to be responsible for the isomer distribution. All compounds have been investigated by 1H and 13C NMR spectroscopy. The diamagnetic species 3a, 3b, 6a+PF6-, and 6b+PF6- served as NMR standards for the neutral and cationic paramagnetic species, respectively. X-ray crystal structure analyses of 3a+PF6- and 5a+PF6- provided similar geometrical data which were used to calculate the dipolar signal shifts and subsequently the contact shifts (δcon) of all paramagnetic compounds. The two different carbons which are separated from the Cp ring by two bonds (β carbons) have been used as probes for the spin sign. From the difference of their δcon values negative spin could be established in the ligand π orbitals of the vanadocene, chromocene, chromocenium ion, and ferrocenium ion; for the nickelocene and nickelocenium ion the spin was positive. This method proved to be most reliable, because the perturbation by σ spin delocalization could be strongly reduced. The spin-carrying orbitals were analyzed on the basis of MO calculations.