Fusing Rigid Spin Probes to Paramagnetic Sandwiches. Synthesis, Crystal Structures, and NMR Spectroscopy of Bis(isodicyclopentadienyl)metal Compounds

Abstract

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 (δ con) 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 δ con 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.

Why Are Dithienylethene‐Linked Biscobaltocenes so Hard to Photoswitch?

“…In addition, the corresponding hydrogen atoms, which are expected to support a negative spin density, appear at high field (Table ). These observations agree with the work of Köhler et al …”

Solid-State NMR Spectroscopy of Paramagnetic Metallocenes

“…Instead, polarization of the fully occupied bonding e 1g orbitals induces negative spin density in the π system. These features have been established experimentally for various substituted neutral and cationic metallocenes by solution NMR spectroscopy (19,20). After some initial solid-state MAS NMR studies of chromocenes (24) and vanadocene (25) we now report on detailed studies of neutral paramagnetic first-row metallocenes and their permethylated analogues.…”

“…Quite strikingly, the positive signal shift of the methyl carbon signal is roughly twice as large as the negative signal shift of the neighboring methyl ring carbon atom. This phenomenon is ascribed to delocalization into suitable σ orbitals of the Cp * ligand, which transfers positive spin to all nuclei (20). It adds to π polarization, which entails negative and positive spin at the ring and methyl carbon atoms, respectively.…”

“…In contrast, solidstate NMR spectroscopy also detects spin density in p and d orbitals that contribute to the total anisotropy of the signal shift. If the geometry of the molecule is known, a tensor analysis of the signals can give information about unpaired electrons outside the s core of the molecules (15,16).In this context we were interested in applying solid-state NMR spectroscopy to open-shell bis(cyclopentadienyl)metal compounds (metallocenes), because they are promising building blocks for magnetic materials (17, 18) and because intermolecular magnetic interactions depend on the sign and the magnitude of the electron spin density in the π system of the cyclopentadienyl ligands (8,(19)(20)(21)(22). It follows that determination of the π spin density is of considerable interest.…”

Polymer Rings and Chains Consisting of Doubly Silyl-Bridged Metallocenes

“…For ferrocenium ions the signal half widths of the cyclopentadienyl protons is generally much broader than those of the substituents. [31] Therefore, as the oxidation proceeds, the corresponding signals of [* 7 ] n seem to disappear in the baseline at the amplification level shown in Figure 10. The signal assignment is derived from that of pure * 7 except for the silyl protons.…”