Solvent-Induced Electron Transfer and Electronic Delocalization in Mixed-Valence Complexes. Spectral Properties

Spectra of the mixed-valence complexes
[(bpy)2ClOs(L)Ru(NH3)5]4+
(L = 4,4‘-bipyridine, pyrazine; bpy
= 2,2‘-bipyridine) are highly solvent dependent. They provide
oxidation state specific spectral markers which show
that oxidation states for L = 4,4‘-bpy are
OsIII−RuII in solvents of donor number (DN)
< 14. In solvents of high
donor number (DN > 15) they are OsII−RuIII.
The isomers coexist at donor numbers 14−15 and the
distribution
between them tuned in mixtures of acetonitrile and propylene carbonate
demonstrating that intramolecular electron
transfer can be induced by varying the solvent. Both
OsIII−RuII and
OsII−RuIII display broad,
solvent-dependent
intervalence transfer (IT) bands in the near infrared (NIR). There
are two isomers of
[(bpy)2ClOs(pz)Ru(NH3)5]4+
as well. In one, which is dominant in solvents of DN < 22, the
oxidation states are
[(bpy)2ClOsIII(pz)RuII(NH3)5]4+.
This is shown by the appearance of dπ → dπ marker bands for
OsIII (at 4200 and 5800 cm-1 in
CD3CN), ν(bpy)
OsIII bands in the mid-IR, and the pattern of UV−visible
bands. For this isomer a narrow, structured IT band
is
observed at ∼8100 cm-1 which is nearly
solvent independent in solvents of DN ≥ 11.9. The second
isomer,
[(bpy)2ClOsII(pz)RuIII(NH3)5]4+,
is dominant at DN > 24. It has IT bands in the NIR which are
broad and solvent
dependent. It is concluded that there is significant
through-pyrazine electronic coupling between OsIII and
RuII in
[(bpy)2ClOsIII(pz)RuII(NH3)5]4+,
but localized oxidation states, and a residual solvent−vibrational
barrier to
intramolecular electron transfer continue to exist. The exchanging
electron is in a dπ(RuII) orbital largely
orthogonal
to the bridge. In
[(bpy)2ClOsII(pz)RuIII(NH3)5]4+
electronic coupling is much less. Its properties are consistent
with
those of Class II in the Robin and Day mixed-valence classification
scheme. The two isomers coexist in trimethyl
phosphate (DN = 23.0) and formamide (DN = 24.0). From the
temperature dependence of the mixed-valence
equilibrium in trimethyl phosphate,
[(bpy)2ClOsIII(pz)RuII(NH3)5]4+
⇌
[(bpy)2ClOsII(pz)RuIII(NH3)5]4+,
K(297K) =
2.8, ΔH° = −8.2 ± 2 kcal/mol, and
ΔS° = −27 ± 6 cal mol-1
deg-1. Because of the difference in the
extent of
electronic delocalization between the two isomers, intramolecular
electronic transfer is a complex event involving
coupled electronic and nuclear motions. There is no simple
relationship between optical and thermal electron
transfer.