Hexatriynediyl Chain Spanning Two Cp*(dppe)M Termini (M = Fe, Ru): Evidence for the Dependence of Electronic and Magnetic Couplings on the Relative Orientation of the Termini

The binuclear complexes Cp*­(dppe)­FeCCCCCCM­(dppe)­Cp* (<b>6</b>, M = Fe; <b>8</b>, M = Ru) were obtained in good yield by treatment of the iron chloro complex Cp*­(dppe)­Fe–Cl (<b>5</b>) in the presence of KF with the bis­(silylated) hexatriyne Me₃SiCCCCCCSiMe₃ and the ruthenium complex Cp*­(dppe)­RuCCCCCCSiMe₃ (<b>7</b>), respectively. The oxidized species <b>6­(PF</b>_<b>6</b><b>)</b>_{<b><i>n</i></b>} (<i>n</i> = 1, 2) and <b>8­(PF</b>_<b>6</b><b>)</b> were obtained in ca. 80% yield by treatment of the parent neutral compounds with 1 or 2 equiv of [Cp₂Fe]­(PF₆) in THF or dichloromethane at −78 °C. The CV of these compounds show three reversible waves with a separation larger than 0.5 V. The salts <b>6­(PF</b>_<b>6</b><b>)</b>_{<b><i>n</i></b>} (<i>n</i> = 1, 2), and <b>8­(PF</b>_<b>6</b><b>)</b> were characterized by XRD. Quantum chemistry calculations performed at the DFT level on the oxidized species show a strong contribution of the −C₆– spacer to the delocalization of the spin density. IR spectra analyzed with the support of TD-DFT calculations are consistent with the delocalization of the odd electron on the fast IR time scale for the two mixed-valence complexes <b>6­(PF</b>_<b>6</b><b>)</b> and <b>8­(PF</b>_<b>6</b><b>)</b>. Combined ESR measurements on rigid glass and on single crystal samples clearly establish that the electronic properties of MV species and particularly their magnetic anisotropies depend on the conformation of the molecules. In the case of the doubly oxidized species <b>6­(PF</b>_<b>6</b><b>)</b>_<b>2</b>, which carries two unpaired electrons, it is shown that the singlet vs triplet ground states can be inverted by the rotation of one metal end with respect to the other around the all-carbon chain axis. Very strong NIR bands are found for the symmetric <b>6­(PF</b>_<b>6</b><b>)</b> and nonsymmetric <b>8­(PF</b>_<b>6</b><b>)</b> MV (mixed-valence) derivatives allowing the determination of very large electronic couplings (<i>H</i>_ab = 3070 and 4025 cm^–1, respectively)