Abstract:The results of X-Ray structural determinations, electrochemical measurements, near-IR, 57Fe Mossbauer measurements, and an explanation of the influence of steric factors o n the electron-transfer rates for a series of polyalkyl substituted mixed-valence biferrocenium triiodides are reported.
“…There are numerous studies on other bridged biferrocene systems (e.g., Fc-Fc, , Fc-S-Fc, Fc-PR-Fc, Fc-CR 2 -Fc, , and Fc-C 2 R 4 -Fc 34,39 ) for which two redox couples are generally observed by cyclic voltammetry. The number of electrochemical investigations on isolated bridged oligo(ferrocenes) decreases rapidly with an increasing number of linked ferrocene units present.…”
Section: Resultsmentioning
confidence: 99%
“…Binuclear transition metal complexes in which the metal atoms are in close enough proximity to permit through space or bridging ligand-mediated interactions have been well-studied. − This is particularly the case for biferrocenes which yield mixed-valent monocations (e.g., 1 and 2 ) , that possess delocalization on a variety of different time scales depending on subtle structural factors and even solid state environmental effects. , Recently, oligomeric metallocene-based species have attracted attention with respect to their electrochemical, electronic, and magnetic properties. − …”
In order to gain insight into the electrochemical and
conformational properties of the prototypical high
polymeric poly(ferrocenylsilane),
poly(ferrocenyldimethylsilane)
[Fe(η-C5H4)2SiMe2]
n
6, three series of oligo(ferrocenylsilanes)
R-fc-[SiMe2-fc]
n
-
1-R‘
(fc = Fe(η-C5H4)2)
7
2
−7
9
(R = R‘ = H),
8
2
−8
7
(R = H and R‘ = SiMe3),
and
9
2
−9
7
(R = R‘ = SiMe3) have been prepared and studied (the
subscript n in the oligomer refers to the number
of ferrocene units present). These species were prepared via the
anionic ring-opening oligomerization of the silicon-bridged [1]ferrocenophane
Fe(η-C5H4)2SiMe2
5. Initiation with ferrocenyllithium FcLi (Fc =
Fe(η-C5H5)(η-C5H4))
followed by quenching with H2O or SiMe3Cl
afforded
H-fc-[SiMe2-fc]
n
-
1-H
(7
2
-9
) or
H-fc-[SiMe2-fc]
n
-
1-SiMe3
(8
2
-7
), respectively.
Initiation with the dilithioferrocene complex
fcLi·2/3TMEDA followed by quenching with
H2O
or SiMe3Cl similarly afforded the oligomers
H-fc-[SiMe2-fc]
n
-
1-H
(7
2
-9
) or alternatively
the bis(silyl)-capped species
Me3Si-fc-[SiMe2-fc]
n
-
1-SiMe3
(9
2
-7
), respectively.
The individual molecular compounds of these three series
of
oligomers
7
2
−7
9
,
8
2
−8
7
,
and
9
2
−9
7
were isolated in pure form from the oligomeric mixtures by
column
chromatography and these were structurally characterized by
1H, 13C, and 29Si NMR
spectroscopy, mass spectrometry,
and in selected cases by elemental analysis. The structure of the
linear pentamer 7
5
has also been
determined by
single crystal X-ray diffraction. The central portion of this
species possesses a trans planar zigzag conformation in
the solid state and appears to be a valuable model for the analogous
conformation of the high polymer 6 in
crystalline
domains. The electrochemical behavior of each pure oligomer was
studied by cyclic and differential pulse voltammetry
and was found to depend on whether an odd or an even number of
ferrocene units were present. For oligomer
systems containing an odd number of iron centers two reversible redox
processes of varying intensities at −0.02−0.00 V and 0.21−0.23 V (vs ferrocene) were observed with a redox
splitting of 0.21−0.23 V. For oligomer systems
containing an even number of iron centers larger then two, three
reversible redox processes of varying intensities
were observed at ca. 0.00, 0.13, and 0.24 V vs ferrocene. As the
oligomer chain length increased, the electrochemical
behavior for both the “odd” and “even” series approached that
of the high polymer 6 for which two reversible
redox
processes at 0.00 and 0.24 V (vs ferrocene) of equal intensity exist.
These results are completely consistent with the
previously proposed theory that initial oxidation of 6
affords a product in which alternating iron sites are
oxidized.
Spectroelectrochemical experiments show an intervalence electron
transfer absorption (1100−1350 nm, εmax ⩽
150
M-1 cm-1) for
partially oxidized oligo(ferrocenylsilanes) that is typical for
class II mixed-valent compounds.
Additionally, single crystals were obtained of the m...
“…There are numerous studies on other bridged biferrocene systems (e.g., Fc-Fc, , Fc-S-Fc, Fc-PR-Fc, Fc-CR 2 -Fc, , and Fc-C 2 R 4 -Fc 34,39 ) for which two redox couples are generally observed by cyclic voltammetry. The number of electrochemical investigations on isolated bridged oligo(ferrocenes) decreases rapidly with an increasing number of linked ferrocene units present.…”
Section: Resultsmentioning
confidence: 99%
“…Binuclear transition metal complexes in which the metal atoms are in close enough proximity to permit through space or bridging ligand-mediated interactions have been well-studied. − This is particularly the case for biferrocenes which yield mixed-valent monocations (e.g., 1 and 2 ) , that possess delocalization on a variety of different time scales depending on subtle structural factors and even solid state environmental effects. , Recently, oligomeric metallocene-based species have attracted attention with respect to their electrochemical, electronic, and magnetic properties. − …”
In order to gain insight into the electrochemical and
conformational properties of the prototypical high
polymeric poly(ferrocenylsilane),
poly(ferrocenyldimethylsilane)
[Fe(η-C5H4)2SiMe2]
n
6, three series of oligo(ferrocenylsilanes)
R-fc-[SiMe2-fc]
n
-
1-R‘
(fc = Fe(η-C5H4)2)
7
2
−7
9
(R = R‘ = H),
8
2
−8
7
(R = H and R‘ = SiMe3),
and
9
2
−9
7
(R = R‘ = SiMe3) have been prepared and studied (the
subscript n in the oligomer refers to the number
of ferrocene units present). These species were prepared via the
anionic ring-opening oligomerization of the silicon-bridged [1]ferrocenophane
Fe(η-C5H4)2SiMe2
5. Initiation with ferrocenyllithium FcLi (Fc =
Fe(η-C5H5)(η-C5H4))
followed by quenching with H2O or SiMe3Cl
afforded
H-fc-[SiMe2-fc]
n
-
1-H
(7
2
-9
) or
H-fc-[SiMe2-fc]
n
-
1-SiMe3
(8
2
-7
), respectively.
Initiation with the dilithioferrocene complex
fcLi·2/3TMEDA followed by quenching with
H2O
or SiMe3Cl similarly afforded the oligomers
H-fc-[SiMe2-fc]
n
-
1-H
(7
2
-9
) or alternatively
the bis(silyl)-capped species
Me3Si-fc-[SiMe2-fc]
n
-
1-SiMe3
(9
2
-7
), respectively.
The individual molecular compounds of these three series
of
oligomers
7
2
−7
9
,
8
2
−8
7
,
and
9
2
−9
7
were isolated in pure form from the oligomeric mixtures by
column
chromatography and these were structurally characterized by
1H, 13C, and 29Si NMR
spectroscopy, mass spectrometry,
and in selected cases by elemental analysis. The structure of the
linear pentamer 7
5
has also been
determined by
single crystal X-ray diffraction. The central portion of this
species possesses a trans planar zigzag conformation in
the solid state and appears to be a valuable model for the analogous
conformation of the high polymer 6 in
crystalline
domains. The electrochemical behavior of each pure oligomer was
studied by cyclic and differential pulse voltammetry
and was found to depend on whether an odd or an even number of
ferrocene units were present. For oligomer
systems containing an odd number of iron centers two reversible redox
processes of varying intensities at −0.02−0.00 V and 0.21−0.23 V (vs ferrocene) were observed with a redox
splitting of 0.21−0.23 V. For oligomer systems
containing an even number of iron centers larger then two, three
reversible redox processes of varying intensities
were observed at ca. 0.00, 0.13, and 0.24 V vs ferrocene. As the
oligomer chain length increased, the electrochemical
behavior for both the “odd” and “even” series approached that
of the high polymer 6 for which two reversible
redox
processes at 0.00 and 0.24 V (vs ferrocene) of equal intensity exist.
These results are completely consistent with the
previously proposed theory that initial oxidation of 6
affords a product in which alternating iron sites are
oxidized.
Spectroelectrochemical experiments show an intervalence electron
transfer absorption (1100−1350 nm, εmax ⩽
150
M-1 cm-1) for
partially oxidized oligo(ferrocenylsilanes) that is typical for
class II mixed-valent compounds.
Additionally, single crystals were obtained of the m...
“…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 …”
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.
“…The mixed-valence states of conjugated ferrocene dimers have attracted considerable attention for decades because they are valuable examples affording intrinsic information regarding intramolecular electron-exchange reactions. − It has been shown that the chemical structure of the conjugated spacer group and its length dramatically affect the electronic interaction between the ferrocene units. Solvation effects on the electron exchange rate are also significant, and those for biferrocene and Fc−C⋮C−Fc have been analyzed by Powers and Meyer, by Blackbourn and Hupp, and by Weaver and co-workers 13 based on the Marcus−Hush theory .…”
New azo-bridged ferrocene trimers, Fc-Fc'-N=N-Fc (2) and Fc-N=N-Fc'-N=N-Fc (3), where Fc and Fc' refer to (eta(5)-C(5)H(5))Fe(eta(5)-C(5)H(4)-) and Fe(eta(5)-C(5)H(4)-)(2), respectively, were obtained in the reaction of a mixture of lithioferrocene and 1,1'-dilithioferrocene with N(2)O. X-ray crystallography of azoferrocene (1) has determined that the Fe-Fe distance is 6.80 Å in the trans form. Cyclic voltammograms of 3 in aprotic solvents such as CH(2)Cl(2) or THF exhibit reversible 2e(-) and 1e(-) oxidation waves, indicating that the positive charge in the monocation is localized mostly on the terminal ferrocene unit (correspondingly, Fc(+)-N(2)-Fc'-N(2)-Fc) due to a strong electron-withdrawing effect of the azo group. This charge distribution in the mixed-valence state is supported by the characteristics of intervalence-transfer (IT) bands. An asymmetrical complex, 2, undergoes a three-step 1e(-) oxidation, and the two mixed-valence forms can be roughly expressed as Fc(+)-Fc'-N(2)-Fc and Fc(+)-Fc'-N(2)-Fc(+). The redox potentials and IT band characteristics of 1(+), 2(+), and 2(2+) depend markedly on the solvent. The solvent effect of the IT band on nu(max) cannot be interpreted only by the parameters in the Marcus-Hush theory, indicating that the nature of the solvent as donor or acceptor should be taken into account in the electron-exchange process in the mixed-valence states. More donating solvent affords higher IT and LMCT energy, indicating the hole-transfer mechanism. The reaction of 1,1'-dilithioferrocene and N(2)O gives a polymer composed of [-(Fc'-N=N-Fc')(0.6)-(Fc'-Fc')(0.4)-](n)().
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