The effect of bending modes on the intramolecular electron-transfer rates of mixed-valence 1′,3′,1‴,3‴-tetraethyl- and 1′,2′,1‴,2‴-tetraethyl-biferrocenium triiodides
Abstract:The results of an X-ray structure determination of 1 ',3',1 "',3"'-tetraethylbiferrocenium triiodide, the effects of changing the relative positions of ethyl substituents from 1 ',3',1 "',3"' t o 1 ',2'"''',2'" o n the intramolecular electron-transfer rates of biferrocenium salts in the solid state, and a theoretical explanation of the influence of steric factors o n the electron-transfer rates are reported.
“…Furthermore, compound 11 has a valence-trapped electronic structure at temperatures below 150 K and becomes a valence-detrapped electronic structure at a temperature of ∼200 K. In comparison with para-substituted compounds ( 10 − 12 ), a valence-detrapped electronic structure is seen even at 77 K for ortho-substituted mixed-valence compounds of 13 and 14 . We suggested that relatively minor perturbations caused by interactions between neighboring cations and anions have pronounced effects on electron transfer. ,− 1 …”
Section: Introductionmentioning
confidence: 93%
“…We suggested that relatively minor perturbations caused by interactions between neighboring cations and anions have pronounced effects on electron transfer. 22,[28][29][30] A recent interesting finding is that there is a significant influence on the electron-transfer rate in the mixedvalence biferrocenium salts (15-18) when the cyclopentadienyl (Cp) rings in each ferrocenyl moiety are tilted from a parallel relation between the two Cp rings around the Fe ion. Such a structural modification would lead to greater metal-ligand interactions as the rings tilt.…”
Relatively minor perturbations caused by Cp ring
substituents in a series of mixed-valence
biferrocenium cations have pronounced effects on the electronic
structure and rate of
intramolecular electron transfer. The X-ray structures of
1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocene,
1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocene,
1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocene,
1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocene, and their corresponding mixed-valence biferrocenium
triiodide salts have been
determined at 298 K. The rates of intramolecular electron transfer
in the mixed-valence
cations were estimated by variable-temperature 57Fe
Mössbauer spectroscopy (time scale
∼107 s-1). The features in
all of the 80 K spectra include two doublets, one with a
quadrupole
splitting (ΔE
Q) of ∼2 mm
s-1 (Fe(II) site) and the other with
ΔE
Q = ∼0.6 mm
s-1 (Fe(III)
site). This pattern of two doublets is expected for a
mixed-valence biferrocenium cation
which is valence-trapped on the time scale of the Mössbauer
experiment (electron-transfer
rate < ∼107 s-1 in the solid
state). Increasing the sample temperature in
1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocenium triiodide,
1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocenium triiodide, and
1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocenium triiodide causes the two
doublets to move together with
no discernible line broadening, eventually becoming a single
“average-valence” doublet at
temperatures of 295, 265, and 190 K, respectively. In the case of
1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocenium triiodide, the two doublets at low temperatures do not
coalesce into an average-valence doublet at 300 K (electron-transfer rate < ∼10
s-1 at 300 K). Thus, the
micromodification of the relative positions of the alkyl substituents
and the nature of the
alkyl substituents has a dramatic influence on the rate of
intramolecular electron transfer.
The deviations of the Cp rings from the parallel position
correlate quite well with the
Mössbauer critical temperature for electronic
delocalization−localization in mixed-valence
cations. Qualitatively, we suggest that the metal nonbonding
orbitals
(d
x
2
-
y
2
,
d
xy
) start to
interact with the ligand π orbitals as the Cp rings tilt from the
parallel position. On the
basis of the density functional calculations, a theoretical explanation
of the influence of
bending back the Cp rings on the electron-transfer rates is presented.
The electrochemical
measurements, IR data, and near-IR spectra for these mixed-valence
biferrocenium cations
are also presented.
“…Furthermore, compound 11 has a valence-trapped electronic structure at temperatures below 150 K and becomes a valence-detrapped electronic structure at a temperature of ∼200 K. In comparison with para-substituted compounds ( 10 − 12 ), a valence-detrapped electronic structure is seen even at 77 K for ortho-substituted mixed-valence compounds of 13 and 14 . We suggested that relatively minor perturbations caused by interactions between neighboring cations and anions have pronounced effects on electron transfer. ,− 1 …”
Section: Introductionmentioning
confidence: 93%
“…We suggested that relatively minor perturbations caused by interactions between neighboring cations and anions have pronounced effects on electron transfer. 22,[28][29][30] A recent interesting finding is that there is a significant influence on the electron-transfer rate in the mixedvalence biferrocenium salts (15-18) when the cyclopentadienyl (Cp) rings in each ferrocenyl moiety are tilted from a parallel relation between the two Cp rings around the Fe ion. Such a structural modification would lead to greater metal-ligand interactions as the rings tilt.…”
Relatively minor perturbations caused by Cp ring
substituents in a series of mixed-valence
biferrocenium cations have pronounced effects on the electronic
structure and rate of
intramolecular electron transfer. The X-ray structures of
1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocene,
1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocene,
1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocene,
1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocene, and their corresponding mixed-valence biferrocenium
triiodide salts have been
determined at 298 K. The rates of intramolecular electron transfer
in the mixed-valence
cations were estimated by variable-temperature 57Fe
Mössbauer spectroscopy (time scale
∼107 s-1). The features in
all of the 80 K spectra include two doublets, one with a
quadrupole
splitting (ΔE
Q) of ∼2 mm
s-1 (Fe(II) site) and the other with
ΔE
Q = ∼0.6 mm
s-1 (Fe(III)
site). This pattern of two doublets is expected for a
mixed-valence biferrocenium cation
which is valence-trapped on the time scale of the Mössbauer
experiment (electron-transfer
rate < ∼107 s-1 in the solid
state). Increasing the sample temperature in
1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocenium triiodide,
1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocenium triiodide, and
1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocenium triiodide causes the two
doublets to move together with
no discernible line broadening, eventually becoming a single
“average-valence” doublet at
temperatures of 295, 265, and 190 K, respectively. In the case of
1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocenium triiodide, the two doublets at low temperatures do not
coalesce into an average-valence doublet at 300 K (electron-transfer rate < ∼10
s-1 at 300 K). Thus, the
micromodification of the relative positions of the alkyl substituents
and the nature of the
alkyl substituents has a dramatic influence on the rate of
intramolecular electron transfer.
The deviations of the Cp rings from the parallel position
correlate quite well with the
Mössbauer critical temperature for electronic
delocalization−localization in mixed-valence
cations. Qualitatively, we suggest that the metal nonbonding
orbitals
(d
x
2
-
y
2
,
d
xy
) start to
interact with the ligand π orbitals as the Cp rings tilt from the
parallel position. On the
basis of the density functional calculations, a theoretical explanation
of the influence of
bending back the Cp rings on the electron-transfer rates is presented.
The electrochemical
measurements, IR data, and near-IR spectra for these mixed-valence
biferrocenium cations
are also presented.
“…Recently, an interesting finding was reported that there is a significant influence on the electron-transfer rate in mixed-valence biferrocenium triiodide salts when the two cyclopentadienyl (Cp) rings associated with each ferrocenyl moiety are tilted from a parallel geometry. − Such a structural modification of the coplanar relation between the two Cp rings around the Fe ion leads to greater metal−ligand interactions as rings tilt. We previously suggested 4 that the difference in electron-transfer rates for the series of alkylbiferrocenium cations ( 1 − 3 ; Chart ) is mainly due to the degree of tilting of the Cp rings.…”
The X-ray structure determinations of the mixed-valence
2,1‘:3,2‘:2‘‘,1‘‘‘:3‘‘,2‘‘‘-tetrakis(propane-1,3-diyl)-1,1‘‘-biferrocenium triiodide and the corresponding
neutral biferrocene and
the effects of the interannular trimethylene bridges on the
intramolecular electron-transfer
rates in both solid and solution states are reported. The X-ray
structure of the mixed-valence
triiodide salt has been determined at 298 K. The corresponding
neutral biferrocene
crystallizes in a hexagonal space group. In common with most
mixed-valence compounds,
the triiodide salt in CH2Cl2 have an
intervalence transition band at 4904 cm-1.
The
calculated rate constant of intramolecular electron transfer is 1.21
× 1012 s-1. The
Mössbauer
results indicate that the mixed-valence triiodide salt has a
valence-trapped electronic
structure (electron-transfer rate less than ∼107
s-1 in the solid state). We believe that
the
most important factor controlling the rate of electron transfer in the
solid state in the triiodide
salt is the degree of coplanarity between the two Cp rings in the
fulvalenide bridge.
“…Recently, we reported systematic studies for ET in the solid state for a series of mixed-valence biferrocenium cations ( 1 − 8 ), which have led to a wealth of new experimental results. − Major progress has been made in studying the dependence of ET rates on the lattice dynamics 3-6 and on the cation−anion interactions, − as well as on the structural micromodification . More recently, we prepared a series of polyalkylbiferrocenium triiodide salts ( 9 − 14 ) to study the influence of ring tilt between two least-squares-fitted cyclopentadienyl (Cp) rings on ET and to investigate energetic control of ET rates. − Deviations of the Cp rings from the parallel position were found to correlate quite well with the critical temperature for electronic delocalization−localization in mixed-valence biferrocenium salts.…”
The syntheses, characterizations, and physical
properties of asymmetric polyethyl-substituted
mixed-valence
1‘,2‘,1‘‘‘-triethylbiferrocenium triiodide (15),
1‘,3‘,1‘‘‘-triethylbiferrocenium triiodide (16), and
1‘,2‘,1‘‘‘,3‘‘‘-tetraethylbiferrocenium triiodide (17) are described.
Complexes 15−17 were characterized by
electrochemical
measurements and by near-IR, 57Fe Mössbauer, and
paramagnetic 1H NMR spectroscopy. These complexes
are
found to be localized on the 57Fe Mössbauer time
scale (electron-transfer rates less than 107
s-1). The cations in
complexes 15−17 are not in equivalent
environments. This asymmetry results in a nonzero zero-point
energy
barrier for intramolecular electron transfer. Analysis of the sign
of contact shifts suggests that the electron
delocalization in 15−17 is based on competing
σ and π delocalization mechanisms. The X-ray structure of
15
has been determined at 298 K: monoclinic,
P21/n, a = 9.962(3)
Å, b = 20.610(3) Å, c = 13.326(3)
Å, β =
92.74(2)°, Z = 4, D
calcd =
2.029 g cm-3, RF
= 0.033,
R
wF
= 0.036. The neutral
compound 1‘,2‘,1‘‘‘-triethylbiferrocene crystallizes in the triclinic space group
P1̄ with two molecules in a unit cell with dimensions
a =
7.465(3), b = 8.419(2), c =
18.581(4) Å and α = 87.83(2), β = 87.98(3), γ
= 64.27(2)°; RF
= 0.021
and
R
wF
= 0.029.
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