Spectroscopic Study of δ Electron Transfer between
Two Covalently Bonded Dimolybdenum Units via a Conjugated Bridge:
Adequate Complex Models to Test the Existing Theories for Electronic
Coupling
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Abstract
Three symmetrical and one unsymmetrical
dimolybdenum dimers, namely,
[Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>(E<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>CE<sub>2</sub>) (DAniF = <i>N</i>,<i>N</i>′-di(<i>p</i>-anisyl)formamidinate and
E = O or S), are structurally and electronically closely related.
The mixed-valence cation radicals display well-defined metal to ligand
(ML), ligand to metal (LM), and metal to metal (MM) charge transfer
absorption bands. Successive thiolations of the complexes result in
steady increases of the electronic coupling between the two [Mo<sub>2</sub>] units. The electronic coupling matrix elements (<i>H</i><sub>ab</sub>) calculated from the Hush model fall in the
range of 600–900 cm<sup>–1</sup>, which are remarkably
consistent with the results from the CNS superexchange formalism.
Spectroscopic analyses suggest that the intramolecular electron transfer
occurs by electron-hopping and hole-hopping in concert. The rate constants
(<i>k</i><sub>et</sub>) are estimated in the range of 10<sup>11</sup>–10<sup>12</sup> s<sup>–1</sup> for the symmetrical
analogues and 10<sup>7</sup> s<sup>–1</sup> for the unsymmetrical
species. The ultrafast electron transfer in such a weakly coupled
system (<i>H</i><sub>ab</sub> < 1000 cm<sup>–1</sup>) is attributed to the d(δ)–p(π) conjugation between
the dimetal centers and the bridge