Stacked Nickelocenes: Synthesis, Structural Characterization, and Magnetic Properties

Abstract

The disubstitution of 1,8-diiodonaphthalene (1) with cyclopentadienyl nucleophiles reveals 1,8-(dicyclopentadienyl)-naphthalene, which rapidly undergoes Diels-Alder reaction forming 1,8-(3a',4',7',7a'-tetrahydro-4',7'-methanoindene-7a',8'-diyl)-naphthalene (2). A subsequent retro-Diels-Alder reaction in the presence of sodium hydride yields the disodium salt of 1,8-(dicyclopentadiendiyl)-naphthalene 3. The disodium salt 3 was the starting material to obtain the paramagnetic bisnickelocene derivative 4, which structure was obtained by X-ray structure analysis, revealing two nickelocenes kept together in a stacked fashion by a 1,8-naphthalene clamp. An electronic interaction between the two nickel atoms is found as a result of cyclic voltammetry, indicating five different oxidation states +4, +3, +2, +1, and 0. The magnetic properties of 4 in solution were studied by variable temperature paramagnetic (1)H NMR spectroscopy and Evans method and revealed Curie behavior between 213 and 293 K. The magnetic susceptibility of a powdered sample of 4 was measured, and an antiferromagnetic interaction with an exchange coupling of J(12) = -31.49 cm(-1) is found. In accord with experimental data, broken symmetry density functional theory (DFT) calculations revealed four antiferromagnetically coupled electrons resulting in an open shell singlet ground state.

Electronic Coupling through Intramolecular π–π Interactions in Biscobaltocenes: A Structural, Spectroscopic, and Magnetic Study

“…[14] To synthesize the decamethylated congener of compound 2, a pathway previously utilized in our group was chosen. [13] Disodium-1,8-bis(cyclopentadiendi-1'-yl)naphthalene was allowed to react with [ -cyclopentadiendiyl)cobalt(II)]naphthalene (4) were synthesized. The molecular structures were characterized by X-ray structure analysis and consisted of two cobaltocenes linked through a distorted naphthalene clamp.…”

“…The twist of the naphthalene bridges, common to 1,8-bismetallocenylsubstituted naphthalenes, is caused by the repulsion of the two naphthalene-bonded Cp ligands. [11,13,14,15,17,19] The magnitude of the twist correlates roughly with the relative electron density on the naphthalene bonded Cp rings, which is demonstrated by the angle between the best planes of the Cp ligands. It is smallest for the dicationic species 1 and 3, 20.3 and 22.18, respectively, and increases for the neutral compounds 2 and 5.…”

“…(h 2 -Acetylaceto-A C H T U N G T R E N N U N G nato)(pentamethyl-h 5 -cyclopentadienyl)cobalt(II) [16] and 1,8-diiodonaphthalene [35] were prepared according to literature procedures; disodium-1,8-bis(cyclopentadiendi-1'-yl)naphthalene was prepared as described previously, but contained one equivalent of THF. [13] Compound 2: Sodium amalgam (1.1 % Na, 0.7 mL) was added to a suspension of 1 (1.13 g, 1.66 mmol) in THF (120 mL). The yellow mixture was stirred at room temperature for 21 h. The mixture was decanted from mercury and filtered through Celite 521.…”

“…However, the dicationic complex 3 and the corresponding decamethylated Ni complex 1,8-bis[(pentamethyl-h 5 -cyclopentadienyl)(h 5 -cyclopentadiendiyl)nickel(II)]naphthalene do not show this extraordinary distortion. [13] In the case of complex 3, the electron density is decreased and the repulsion between the Cp ligands is weaker, which causes a less bending of the metallocenes. The analogous Ni complex crystallizes in the triclinic space group P1 with an intermolecular nearly coplanar head-to-head arrangement of the molecules generating one-dimensional stacks with similar inter-and intramolecular Ni À Ni distances of 741 and 697 pm, respectively.…”

“…[12] Recently, we reported the properties of a 1,8-bis(nickelocenyl)naphthalene, which showed antiferromagnetic coupling in the undiluted crystalline material. [13] Theoretical calculations indicate that through-space interactions are the main pathway of the antiferromagnetic coupling.…”