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.
The reaction of naphthalene‐1,8‐diylbis(cyclopentadienylsodium) (1) with the nickel triple‐decker compound [Ni2Cp3]BF4 affords the paramagnetic ansa‐nickelocene 1,1′‐(naphthalene‐1″,8″‐diyl)nickelocene (2). Alternatively, 2 can directly be obtained by a salt metathesis reaction of the disodium salt 1 with hexaammine nickel(II) dichloride [Ni(NH3)6]Cl2. For magnetic dilution experiments, the corresponding but diamagnetic ansa‐ruthenocene 3 was synthesized by reaction of the disodium salt 1 with dichloridotetrakis(dimethyl sulfoxide)ruthenium(II). The solid‐state molecular structures of 2 and 3 were determined by single‐crystal X‐ray diffraction analysis and revealed bent sandwich complexes. Cyclic voltammetry studies of 2 indicate three oxidation states. The electronic structure of complex 2 has been investigated by variable‐temperature 1H NMR spectroscopy, magnetic measurements, and DFT calculations. Superconducting quantum interference device (SQUID) magnetometry has been performed for the neat and diluted samples of 2. The neat crystalline material of 2 displays antiferromagnetic behavior, whereas the susceptibility of the magnetically diluted sample runs into saturation at T < 4 K. This property is in agreement with an intermolecular antiferromagnetic interaction in the bulk material of compound 2 and the triplet ground state calculated by broken‐symmetry (BS)‐DFT calculations.
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