Long-Range Electronic Coupling of MM Quadruple Bonds (M = Mo or W) via a 2,6-Azulenedicarboxylate Bridge

The preparation of 2,6-azulenedicarboxylic acid (I) from its diester, 2-CO2tBu-6-CO2-C10H6 (II),
is reported together with the crystal and molecular structure of the ester, II. From the reactions between
the dicarboxylic acid I and the MM quadruply bonded complexes M2(O2CtBu)4, where M = Mo or W, the
azulenedicarboxylate bridged complexes [M2(O2CtBu)3]2(μ-2,6-(CO2)2−C10H6) have been isolated, III (M =
Mo) and IV (M = W). The latter compounds provide examples of electronically coupled M2 centers via a
polar bridge. The compounds show intense electronic absorptions due to metal-to-bridge charge transfer.
This occurs in the visible region of the spectrum for III (M = Mo) but in the near-IR for IV (M = W). One
electron oxidation with Ag+PF6- in THF generates the radical cations III+ and IV+. By both UV−vis−NIR
and EPR spectroscopy the molybdenum ion III+ is shown to be valence trapped or Class II on the Robin
and Day classification scheme. Electrochemical, UV−vis−NIR, and EPR spectroscopic data indicate that,
in the tungsten complex ion IV+, the single electron is delocalized over the two W2 centers that are separated
by a distance of ca. 13.6 Å. Furthermore, from the hyperfine coupling to 183W (I = 1/2), the singly occupied
highest molecular orbital is seen to be polarized toward one W2 center in relationship to the other. Electronic
structure calculations employing density functional theory indicate that the HOMO in compounds III and IV
is an admixture of the two M2 δ orbitals that is largely centered on the M2 unit having proximity to the C5
ring of the azulenedicarboxylate bridge. The energy of the highest occupied orbital of the bridge lies very
close in energy to the M2 δ orbitals. However, this orbital does not participate in electronic coupling by a
hole transfer superexchange mechanism, and the electronic coupling in the radical cations of III and IV
occurs by electron transfer through the bridge π* system.