Fusing rigid spin probes to paramagnetic sandwiches. Synthesis, crystal structures, and NMR spectroscopy of bis(isodicyclopentadienyl)metal compounds

To probe the sign of the unpaired spin density on the ligands of metallocen es neutral and cationic derivatives which contained the isodicyclopentadien yl (Isodicp) ligand have been synthesized. The resulting compounds have the composition (Isodicp)(2)M [M = Fe (3), V (4), Cr (5), Ni (7)], and the cor responding salts are (Isodicp)(2)M+PF6- (M+ = Fe+) (3(+)PF(6)(-)), Cr+ (5()PF(6)(-)), Co+ (6(+)PF(6)(-)), Ni+ (7(+)PF(6)(-)). While the exo,exo-, exo ,endo-, and endo,endo-isomers (a, b, and c, respectively) are conceivable, 4a, 5a, 5a(+)PF(6)(-), 7, and 7a(+)PF(6)(-) have been isolated as the only isomers. By contrast, mixtures of 3a and 3b, 3a(+)PF(6)(-) and 3b(+)PF(6)(- ), as well as 6a(+)PF(6)(-), 6b(+)PF(6)(-), and 6c(+)PF(6)(-) have been obt ained; 3a and 3a(+)PF(6)(-) could be separated by crystallization. Metal-de pendent ligand exchange is proposed to be responsible for the isomer distri bution. All compounds have been investigated by H-1 and C-13 NMR spectrosco py. The diamagnetic species 3a, 3b, 6a(+)PF(6)(-), and 6b(+)PF(6)(-) served as NMR standards for the neutral and cationic paramagnetic species, respec tively. X-ray crystal structure analyses of 3a(+)PF(6)(-) and 5a(+)PF(6)(-) provided similar geometrical data which were used to calculate the dipolar signal shifts and subsequently the contact shifts (delta(con)) Of all para magnetic compounds. The two different carbons which are separated from the Cp ring by two bonds (beta carbons) have been used as probes for the spin s ign. From the difference of their delta(con) values negative spin could be established in the ligand pi orbitals of the vanadocene, chromocene, chromo cenium 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 sigma spin delocalization could be strongly reduced. The sp in-carrying orbitals were analyzed on the basis of MO calculations.