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

Abstract

Three symmetrical and one unsymmetrical dimolybdenum dimers, namely, [Mo2(DAniF)3]2(E2CC6H4CE2) (DAniF = N,N′-di(p-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 [Mo2] units. The electronic coupling matrix elements (H ab) calculated from the Hush model fall in the range of 600–900 cm–1, 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 (k et) are estimated in the range of 1011–1012 s–1 for the symmetrical analogues and 107 s–1 for the unsymmetrical species. The ultrafast electron transfer in such a weakly coupled system (H ab < 1000 cm–1) is attributed to the d(δ)–p(π) conjugation between the dimetal centers and the bridge.

Tuning the Electronic Coupling and Electron Transfer in Mo2 Donor–Acceptor Systems by Variation of the Bridge Conformation

“…In the case in which this technique is not available, we estimated for Mo 2 dimeric systems from the size of the spacer, because the δ electrons in the complex unit [Mo 2 ] are fully delocalized through d(δ)–p(π) conjugation and the spacer connecting the two [Mo 2 ] units is the real bridge. For instance, from the geometric length of the −CC 6 H 4 C− group, =5.8 Å is determined for the phenylene‐ spaced complexes . This treatment gave H ab parameters in excellent agreement with the data derived from the CNS formalism.…”

“…The two thiolated species show a weak ligand‐to‐metal charge‐transfer (LMCT) band on the low‐energy side. The LMCT band for the dicarboxylate‐bridged [ O 2 –(1,4‐naph)–O 2 ] + is blueshifted and overlaps with the MLCT band (Figure ), which is similar to that for its phenyl counterpart [ O 2 –ph–O 2 ] + . According to the superexchange formalism, the high MLCT and LMCT energies would suppress the D–A EC through the bridge …”

“…Each of the spectra shows an intense negative absorption band around 3400 cm −1 , which can tentatively be assigned to the vibrational fine structure . Interestingly, the three MV complexes show similar optical characters, such as the IVCT band energy E IT , intensity ɛ IT , and shape Δ ν 1/2 (Figure and Table ), corresponding to their similar redox properties, whereas the optical behaviors of the three phenyl analogues are quite different . For [ O 2 –(1,4‐naph)–O 2 ] + and [ OS–(1,4‐naph)–OS ] + , the band maxima are blueshifted by about 200 cm −1 relative to those of the phenylene‐bridged counterparts, but the IVCT band for [ S 2 –(1,4‐naph)–S 2 ] + is much higher in energy ( E IT =3680 cm −1 ) than that of [ S 2 –ph–S 2 ] + ( E IT =2640 cm −1 ; Table ).…”

“…Diverse D–B–A models consisting of quadruply bonded dimetal complex units (M 2 , M=Mo and W) have been developed and studied . In these systems, owing to the unique electronic configuration (σ 2 π 4 δ 2 ) of the M 2 4+ units, the δ electron(s) are solely responsible for the electrochemical and optical behaviors, and this allows electrochemical and spectroscopic analyses with more certainty.…”

“…By modifying the geometric topology of the bridge, the π conjugation along the CT platform is mediated, and consequently the TB coupling is controlled while the TS interaction remains. On the other hand, when the [Mo 2 ] complex units are viewed as the electron D and A, their electron‐donating and electron‐accepting abilities are tuned by alternation of O/S ligating atoms on the bridging ligand . Therefore, by developing systems with these two variables, the effect of the interplay between B and D (A) on the EC and ET can be further clarified.…”