Complexes with Tunable Intramolecular Ferrocene to TiIV Electronic Transitions: Models for Solid State FeII to TiIV Charge Transfer

Article abstract of DOI:10.1021/acs.inorgchem.5b02587

Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO₂ by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti^IV has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp₂Ti(C₂Fc)₂, Cpa?₂Ti(C₂Fc)₂, and Cp₂Ti(C₄Fc)₂. Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, ^TMSCp, and comprise the complexes ^TMSCp₂Ti(C₂Fc)(C₂R), where R = C₆H₅, p-C₆H₄CF₃, and CF₃. The complexes are compared to Cp₂Ti(C₂Ph)₂, which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti^IV/III reduction wave and an Fe^II/III oxidation that is irreversible for all complexes except ^TMSCp₂Ti(C₂Fc)(C₂CF₃). All of the complexes with both Fc and Ti show an intense absorption (4000 M^-1cm^-1 < ε < 8000 M^-1cm^-1) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti^IV/III and Fe^III/II reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, λ, have been determined using band shape analysis (2600 cm^-1 < λ < 5300 cm^-1) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti^IV and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp₂Ti(C₂Ph)₂ rapidly decomposes upon photolysis.

Full text of DOI:10.1021/acs.inorgchem.5b02587

Article
pubs.acs.org/IC
IV
Complexes with Tunable Intramolecular Ferrocene to Ti Electronic
II
IV
Transitions: Models for Solid State Fe to Ti Charge Transfer
†
,¶
†,¶
†
†
Michael D. Turlington, Jared A. Pienkos, Elizabeth S. Carlton, Karlee N. Wroblewski,
†
‡
§
∥
,†
Alexis R. Myers, Carl O. Trindle, Zikri Altun, Jeffrey J. Rack, and Paul S. Wagenknecht*
†
Department of Chemistry, Furman University, Greenville, South Carolina 29613, United States
‡
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
§
Department of Physics, Marmara University Go
̈
ztepe Kampus, 34772 Istanbul Turkey
Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States
S Supporting Information
∥
*
ABSTRACT: Iron(II)-to-titanium(IV) metal-to-metal-charge transfer
MMCT) is important in the photosensitization of TiO2 by
(
ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts,
and has been invoked to explain the blue color of sapphire, blue kyanite,
and some lunar material. Herein, a series of complexes with alkynyl
IV
linkages between ferrocene (Fc) and Ti has been prepared and
characterized by UV−vis spectroscopy and electrochemistry. Complexes
with two ferrocene substituents include Cp Ti(C Fc) , Cp* Ti(C Fc) ,
2
2
2
2
2
2
and Cp Ti(C Fc) . Complexes with a single ferrocene utilize a
2
4
2
TMS
titanocene with a trimethylsilyl derivatized Cp ring,
Cp, and
TMS
comprise the complexes
C H CF , and CF . The complexes are compared to Cp Ti(C Ph) ,
Cp Ti(C Fc)(C R), where R = C H , p-
2
2
2
6
5
6
4
3
3
2
2
2
which lacks the second metal. Cyclic voltammetry for all complexes
IV/III
II/III
TMS
reveals a reversible Ti
reduction wave and an Fe
oxidation that is irreversible for all complexes except
Cp Ti-
2
−
1
−1
−1
−1
(
C Fc)(C CF ). All of the complexes with both Fc and Ti show an intense absorption (4000 M cm < ε < 8000 M cm )
2
2
3
between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the
IV/III
III/II
difference between the Ti
and Fe
reduction potentials, shifting to lower energy as the difference in potentials decreases.
−1
−1
Reorganization energies, λ, have been determined using band shape analysis (2600 cm < λ < 5300 cm ) and are in the range
observed for other donor−acceptor complexes that have a ferrocene donor. Marcus−Hush-type analysis of the electrochemical
and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also
supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift
IV
upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti and the
identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas
Cp Ti(C Ph) rapidly decomposes upon photolysis.
2
2
2
5
−8
INTRODUCTION
photocatalysis.
In particular, several investigations of
■
MMCT excited states in solid state Ti−O−M architectures
involving Ti acceptors oxo-bridged to donors (M) such as
Fe , Cr , and Mn have recently appeared, leading to interest
in molecular examples of such Ti−O−M structures.
Of additional relevance is that ferrocyanide photosensitizes
TiO , with the photosensitization being ascribed to an Fe to
Ti charge transfer. Furthermore, such an Fe to Ti MMCT
has been implicated as being responsible for the color observed
Charge-transfer (CT) excited states where charge is separated
over a long distance are interesting from a fundamental
standpoint and for applications involving solar energy
conversion and photocatalysis.
complexes of ruthenium(II) have been exploited in dye-
sensitized solar cells (DSSCs). Here, the metal-to-ligand CT
IV
II
III
II
6
7
1
−8
For example, polypyridyl
II
2
IV
8
II
IV
(
MLCT) excited state serves to inject an electron into the
conduction band of the TiO semiconductor. Recently, much
2
2
9
in minerals such as blue sapphire (a corundum-based
effort has been devoted toward studying CT excited states in
completely organic systems where a donor group is bridged to
an acceptor through a π-conjugated bridge (termed donor-π-
bridge-acceptor systems, D−π−A). Such systems are promising3
candidates to replace the expensive ruthenium-based dyes.
Metal-to-metal CT (MMCT) excited states have also been of
9e,10
gemstone) and blue kyanite
(an aluminosilicate-based
II
IV
gemstone). Fe to Ti charge transfer is also reported to
contribute to the relative blueness of the lunar continuum in
Received: November 9, 2015
4
general fundamental interest and for applications involving
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02587
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Inorganic Chemistry
Article
1
1
by 1H NMR after precipitation from THF and hexanes,
elemental analysis revealed that LiCl was present. Sonication o+f
the products in water did not remove LiCl, suggesting Li
coordination to the alkynyl-complex, similar to the tweezer
complexes described by Lang et al. Attempted purification on
untreated silica gel resulted in decomposition of the titanocene.
telescopic reflectance measurements. Consistent with these
assignments, the blue color of anhydrous FeTi(SO ) has also
been attributed to an Fe to Ti MMCT. Evaluation of
spectroscopic data indicates that in solution this species exists
and EPR intensity data suggests that the
Fe /Ti state predominates. However, recent DFT theoreti-
cal evaluations on blue sapphire suggest that in the corundum
lattice, the ground state is Fe /Ti and a MMCT results in
Fe /Ti .
In spite of the possible technological significance of the Fe
to Ti MMCT and the questions surrounding its importance in
mineralogy, very few examples exist of molecular complexes
displaying such an optical transition (Figure 1).
4
3
II
IV
12
2
3
−
17
as FeTiO(SO )
4
II
IV
13
For example, chromatographing Cp Ti(C Fc) on silica yielded
2 2 2
III
III
1,4-bis(ferrocenyl)butadiyne (orange band) as the major
product. These compounds also decomposed on neutral
alumina. Elution through base-treated (triethylamine) silica
gel prevented the aforementioned decomposition and also
resulted in removal of the LiCl impurity.
II
IV 14
II
IV
15−17
Synthesis of the monosubstituted complexes, TMSCp
Our
2
Ti-
(
C Fc)(C R), proceeds through a common intermediate,
recent interest in electron transfer through alkynyl bridged
organometallic frameworks led us to investigate the Cp Ti-
2
Cp
2
18
TMS
Ti(C Fc)Cl. Lang et al. demonstrated that substituted
2
2
2
(
C Fc) architecture.
Cp rings are necessary to prevent disproportionation of
2
n
2
19
Cp Ti(C R)Cl to Cp TiCl and Cp Ti(C R) . Their
synthetic procedure for
2
2
2
2
2
2
2
TMS
Cp Ti(C Ph)Cl involved slow
2 2
addition of a solution containing 1 eq of lithiated
TMS
ethynylbenzene to
Cp TiCl in Et O. We have found that
2 2 2
this reaction can also be controlled by maintaining a
temperature of −15 °C, thus obviating the need for slow
addition.
II
IV
Figure 1. Existing complexes displaying an Fe to Ti MMCT where
TMS
Cp Ti(C Fc)Cl decomposes even on base-treated silica
15
16
2
2
Fc = ferrocenyl; n = 0 and X = H, n = 2 and X = Me Si, n = 1 and
3
gel, so optimal elemental analysis results were not obtained
17
X = Me Si.
3
1
despite clean H NMR spectra. However, the asymmetric
TMS
titanocene products,
Cp Ti(C Fc)(C R), obtained from
2 2 2
16
Such systems were studied by Wakatsuki (n = 2) for
ground-state investigations into long-range electron-transfer
and by Lang (n = 1) as molecular tweezers for cations. For
both complexes, an intense absorption band in the red region
of the visible spectrum is mentioned only briefly, it being
ascribed to an Fe to Ti MMCT band based on its
Because such an excited state might be exploited
for applications involving long-range electron-transfer, we
report here an in-depth chemical and photophysical character-
ization of a series of Fe /Ti complexes (Figure 2). The data
herein clearly support the presence of an Fe to Ti charge
transfer transition and represent the first significant inves-
tigation into such a transition in molecular species.
this starting material exhibited excellent purity following elution
through base treated silica gel and recrystallization from
hexanes.
17
In the solid state, the complexes are air stable at room
temperature for days to weeks, but slowly decompose over a
period of months if not stored at low temperature. In room-
temperature THF, CH Cl , or toluene solution that has neither
II
IV
1
5,17b
intensity.
2
2
been dried nor degassed, most of the complexes are thermally
and photochemically stable and thus can be handled in solution
over periods of minutes to hours without protection from room
light. Only Cp* Ti(C Fc) in CH Cl solution is thermally
II
IV
II
IV
2
2
2
2
2
unstable, undergoing complete decomposition within 4 h.
Moreover, all of the bimetallic complexes studied (with the
exception of Cp* Ti(C Fc) in CH Cl ) are stable to direct
2
2
2
2
2
RESULTS AND DISCUSSION
■
(
photolysis in dried degassed THF, CH
Cl , or toluene. In
2 2
Synthesis. The bisalkynylferrocenyl complexes, Cp Ti-
C Fc) , Cp Ti(C Fc) , and Cp* Ti(C Fc) (Figure 2) were
prepared following a modified procedure for the trimethylsilyl
contrast, the monometallic complex, Cp Ti(C Ph) , is unstable
2
2
2
2
with respect to photolysis (vide infra). Finally, all of the
complexes are subject to acid hydrolysis. Because of this
2
2
2
4
2
2
2
2
TMS
16,17
substituted analogues,
Cp Ti(C Fc) .
In all cases,
sensitivity, all CDCl used for spectroscopy was passed through
2
2n
2
3
addition of the appropriate titanocene dichloride to an Et O
solution containing 2 equiv of the lithiated alkyne resulted in a
deeply colored precipitate. Although the product appeared pure
activated alumina prior to sample preparation.
2
Electrochemistry. The cyclic voltammograms (vs FcH+/0)
of the bimetallic Cp Ti(C Fc) complexes (Figure 3) show
2
2n
2
Figure 2. General synthetic route for D−π−A complexes with a ferrocene donor and titanocene acceptor.
B
DOI: 10.1021/acs.inorgchem.5b02587
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Inorganic Chemistry
Article
−
IV/III
There is also a cathodic shift of the 1e Ti
reduction of
Cp Ti(C Fc) vs Cp Ti(C Fc) . This can be explained by the
2
2
2
2
4
2
IV
relative proximity of the electron-rich ferrocene to Ti .
Not surprisingly, replacement of the two Cp ligands on
titanocene with Cp* (more electron-rich) results in a
IV/III
significant cathodic shift of the Ti
reduction and a milder
shift of the initial Fc0 oxidation (Figure 3, bottom). The
/+
second oxidation wave remains unshifted, consistent with
assignment of this wave to oxidation of the resulting FcC Fc
4
product. The cyclic voltammograms for all complexes
(
including the monoferrocenyl complexes discussed below) in
THF are qualitatively similar (Supporting Information).
In an attempt to suppress the elimination reaction following
−
2
e
oxidation, complexes with a single ethynylferrocene,
TMS
Cp Ti(C Fc)(C R), were investigated. Cyclic voltammetry
2
2
2
Figure 3. Cyclic voltammograms of complexes with two alkynylferro-
cenyl ligands appended to the titanocene. Conditions: [Ti] ∼ 1 mM in
demonstrates that decomposition was suppressed only for
Cp Ti(C Fc)(C CF ) where now a one-electron ferrocenyl
2
2
2
3
+
CH Cl (0.1 M TBAP), glassy carbon working electrode, Ag/Ag
2
2
oxidation wave is observed with the same peak current as the
reference electrode. The x-axis reports voltages vs ferrocene as
determined by running a voltammogram of FcH before and after data
collection. Voltage sweeps are initiated in the cathodic direction.
IV/III
Ti
reduction wave (Figure 4, top). For R = C H and p-
6 5
features representative of both the chemically reversible one-
IV/III
reduction and of the chemically irreversible 2e−
electron Ti
oxidation of the two ethynylferrocenyl ligands.
TMS
17a
and TMSCp Ti-
For the closely related
Cp Ti(C Fc)
2 2 2
2
1
6a
−
(
C Fc)2
4
complexes, the irreversible nature of this 2e
oxidation has been ascribed to subsequent rapid heterolytic
Ti−C bond cleavage resulting in elimination of Fc(C ) Fc (eq
2n 2
1
). This hypothesis was confirmed by isolating the Fc(C ) Fc
2n 2
−
16a,17a
During this cleavage, 1e−
product following 2e oxidation.
+
from each Ti−C bond serves to reduce Fc back to Fc, while
−
the remaining e is involved in formation of the new C−C
Figure 4. Cyclic voltammograms for complexes with a single
bond. In the voltammogram of Cp Ti(C Fc) , the reversible
IV TMS
2
2
2
ethynylferrocene ligand on the Ti ,
Cp Ti(C Fc)(C R). Exper-
2 2 2
oxidation of the FcC Fc product appears as two nearly
4
imental conditions are identical to Figure 3.
superimposed 1e− waves slightly anodic of the first 2e Fc
−
oxidation (confirmed by comparison with an authentic sample
of FcC Fc, Supporting Information). The fact that the first
C H CF , the Fc oxidation remains chemically irreversible.
6 4 3
4
−
irreversible 2e wave is unsplit indicates a lack of electronic
communication between the two ferrocenes via the C Ti C
Note that the second oxidation wave is not split, indicating that
the Fc-containing product does not contain two Fc moieties in
communication with one another. This suggests 1-aryl-4-
IV
2
2
linkage, whereas the 90 mV separation of oxidation peaks for
FcC Fc (the second oxidation wave) indicates electronic
ferrocenylbutadiyne as the main product. Indeed, oxidation of
4
17a,20
TMS
communication through the organic alkynyl bridge.
This
Cp Ti(C Fc)(C Ph) with one equivalent of AgPF resulted
2 2 2 6
separation has been confirmed by differential pulse voltamme-
try (Supporting Information). Finally, increasing scan rates to
in FcC Ph as the main isolated organic product, with trace
4
amounts of FcC Fc also forming (indicating the possibility of a
4
1
000 mV/s did not result in observation of a return wave for
minor bimolecular elimination pathway). FcC Ph was identified
4
−
1
the first 2e oxidation step.
In contrast to the ethynylferrocene complex, the voltammo-
gram of the butadiynylferrocene complex, Cp Ti(C Fc) ,
by mass spectrometry and by comparison of the H NMR
21
spectrum with an independently prepared sample. Such a
−
product trajectory has precedent in that one e oxidation of cis-
2
4
2
−
(Figure 3, middle) reveals that the irreversible 2e oxidation
(FcC )PhPt(dppe) involves Fc oxidation followed by elimi-
2
22
occurs at nearly the same potential as the oxidation of the
product FcC Fc. The fact that the initial 2e oxidation of the
nation of FcC Ph.
2
1
6a
−
The fact that the Fc oxidation wave is chemically reversible
8
ethynylferrocene complex, Cp Ti(C Fc) , is more cathodic
for R = CF suggests that the reversibility is impacted by the
electron density at Ti . The presence of the electron-
2
2
2
3
IV
IV
than that of Cp Ti(C Fc) suggests that the Ti -alkynyl bond
2
4
2
is electron-rich and its proximity facilitates Fc oxidation. This
implies ionic character of the Ti -alkynyl bond, with significant
withdrawing trifluoropropynyl ligand decreases the electron
density at Ti , and might increase the degree of ligand to metal
IV
IV
negative charge density remaining on the terminal alkynyl
σ- and π- donation from the ethynylferrocene ligand,
carbon. This is consistent with a slight upfield shift of the Fc
strengthening that bond. In further support of this hypothesis,
1
IV/III
resonances in the H NMR of Cp Ti(C Fc) vs Cp Ti(C Fc) .
the Ti
reduction wave of the synthetic intermediate,
2
2
2
2
4
2
C
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Inorganic Chemistry
Article
TMS
Cp Ti(C Fc)Cl, (Supporting Information) is anodically
2
2
shifted to nearly the same extent as the complex with R = CF3,
and also shows a chemically reversible Fc oxidation. However, it
−
would appear that simply decreasing the e density at Ti is not
sufficient to suppress the elimination reaction given that such
−
an elimination still occurs after 2e ferrocenyl oxidation for the
bis-butadiynylferrocene complex, Cp Ti(C Fc) .
2
4
2
UV−vis Spectroscopy. Cp Ti(C Ph) is bright orange in
2
2
2
appearance with no absorbance features in the red region of the
spectrum. Replacement of both phenylethynyl ligands with
alkynylferrocenes results in deeply colored compounds, some
appearing almost black in the solid state. The UV−vis spectra
in CH Cl (Figure 5) clearly show a low energy (LE)
2
2
absorbance feature for the bimetallic complexes (centered
near 580 nm) that is absent for Cp Ti(C Ph) .
Figure 6. UV−vis absorption spectra of Cp* Ti(C Fc) and
2
2
2
TMS
2
2
2
Cp Ti(C Fc)(C R) complexes in CH Cl . The LE absorption
2
2
2
2
2
band shifts to higher energy with electron-rich ligands on Ti (Cp*)
and to lower energy with electron-poor ligands on Ti (C R = C CF ).
2
2
3
Line color approximates observed complex color.
between the Fc0/+ oxidation and the TiIV/III reduction. Due to
the chemically irreversible nature of the Fc oxidation, its
thermodynamic potential can only be estimated for most
complexes herein (Table 1). Regardless, with the exception of
Cp Ti(C Fc) (2nd entry in Table 1), there is a clear increase
2
4
2
in the energy of the LE band as the difference between the
IV/III
+/0
Ti
and Fc redox potentials (ΔE1 Table 1) increases.
/2
Furthermore, the increase in the LE band energy upon
switching solvent from CH Cl to THF is mirrored by the
2
2
increase in ΔE1 values between these two solvents (Table 1),
/2
Figure 5. UV−vis absorption spectra of Cp Ti(C R) complexes in
2
2
2
thus providing additional support for the MMCT assignment.
Explaining the anomalous behavior of Cp Ti(C Fc) requires
CH Cl . Probable assignments given. Line color approximates
2
2
2
4
2
observed complex color.
consideration of the potential well diagram (Figure 7) showing
the relationship between the energy of the optical MMCT
transition, E , the energy of the thermal transition, ΔG°, and
op
As mentioned above, this LE band has been ascribed to an
II
IV
the reorganization energy, λ. Here, λ is the excess energy
required (over and above ΔG°) to reach the excited state
configuration with the same molecular and solvation geometry
as the ground state, thus ensuring a vertical transition. This
energy can be estimated from the full-width at half-maximum,
Fe to Ti MMCT. Another possibility is that this LE band is a
ferrocene d−d band that borrows intensity from a higher-
energy CT band. Such a model was originally suggested for the
LE absorption in a ferrocene-π-bridge-acceptor complex, cis-1-
ferrocenyl-2-(4-nitrophenyl)ethylene, studied as a second-order
4a,b,28
23
Δν1/2, of the LE band (eq 2).
nonlinear optical chromophore. Later studies suggested that
metal to nitrophenyl CT was more consistent with the
λ = (Δv1/2)2/2310 cm−1
(2)
24
experimental data. A more recent TDDFT study suggests
The values thus calculated for the reorganization energy
Table 2) fall into or near the range of values in the literature
that the LE band for this complex has both d−d and CT
2
5
(
character. Thus, it seems important to experimentally
II
IV
for D−π−A complexes in CH Cl involving a Fc donor with
interrogate the Fe to Ti MMCT hypothesis for the bimetallic
complexes herein.
2
2
−1
29,30
either organic or inorganic acceptors (3800 to 7000 cm ).
The resulting values can be used to calculate ΔG° (eq 3).
If the LE band is due to a MMCT, then only a single
ferrocene should be necessary. Indeed, the TMSCp Ti(C Fc)-
2
2
ΔG° = Eop − λ
(3)
(
C R) complexes show the same LE absorption feature
2
observed for the bis-alkynylferrocene complexes, albeit with
about half the molar absorptivity (Figure 6). This observation is
consistent with the theoretical treatment by Parker and Crosby
These results are compared with ΔE (Table 2). Whereas
1
/2
ΔE is a reasonable estimate for ΔG°, and is often used as
1/2
such, it fails to take into account the modified electrostatic
26
29,31
for two degenerate acceptors and a single donor (A−D−A).
interactions in the excited state.
The difference between
The model demonstrates that for a centrosymmetric (linear)
system, one expects twice the intensity for the A−D−A charge
transfer than for the nondegenerate D−A system. For nonlinear
A−D−A complexes, the model predicts two transitions whose
intensities sum to twice that of the D−A system. However,
often only a single transition is observed, likely due to the
ΔE and ΔG°, a Coulombic energy correction work term, w,
1/2
should decrease with distance between the donor and acceptor,
and thus, it is not surprising that the smallest calculated value of
w occurs for the butadiynyl complex, Cp Ti(C Fc) .
2
4
2
These values for w appear relatively solvent independent and
are in a very similar range to those calculated for complexes
with an ethylene bridge between ferrocene or permethylferro-
cene donors and a polychlorotriphenylmethyl radical acceptor
27
magnitude of the matrix element coupling the two states.
If the LE band has significant MMCT character, then its
energy should be directly related to the potential difference
29
(0.04−0.2). As expected, the trend of energy of the LE band
D
DOI: 10.1021/acs.inorgchem.5b02587
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Inorganic Chemistry
Article
Table 1. Comparison of Redox Potentials with Wavelength of the CT Bands
E1/2 (TiIV/III
)
a
E1/2 (Fc+/0
)
a
ΔE1/2
MMCT (nm)
Cp to TiIV (nm)
complex
Cp* Ti(C Fc)
CH Cl
THF
CH Cl2
THF
CH Cl2
THF
CH Cl2
THF
CH Cl
THF
2
2
2
2
2
2
2
b
b
−2.28
−1.45
−1.72
−1.75
−1.67
−1.54
−2.13
−1.50
−1.76
−1.82
−1.69
−1.55
−0.22
0.12
−0.08
0.14
2.06
1.57
1.70
1.67
1.67
1.59
2.05
1.64
1.80
1.76
1.73
1.60
544
581
583
584
593
630
542
563
573
571
586
623
372
441
420
408
418
425
373
439
417
408
416
425
2
2
2
b
b
Cp Ti(C Fc)
2
2
4
TMS
b
b
b
Cp Ti(C Fc)(C Ph)
−0.02
−0.08
0.00
0.04
2
2
2
b
Cp Ti(C Fc)
−0.06
0.04
2
2
2
TMS
b
b
Cp Ti(C Fc)(C PhCF )
TMS
2
2
2
3
Cp Ti(C Fc)(C CF )
0.05
0.05
2
2
2
3
a
Volts vs FcH+/0. Irreversible wave. E1 estimated as the half-wave potential.
b
/2
Computational Modeling. To further interrogate the CT
3
3
assignments discussed above, DFT and TD-DFT calculations
have been performed on the parent bimetallic complex,
Cp Ti(C Fc) , the more electron-rich Cp* Ti(C Fc) , and
2
2
2
2
2
2
Cp Ti(C Ph) . Molecular structures were obtained by geom-
2
2
2
34
etry optimization with the ωB97XD/def2-TZV functional
3
5
and basis combination, which in our experience, reproduces
bond lengths within 30 pm. Our choice of density functional
and basis for TD-DFT spectroscopic modeling is guided by
benchmarks that suggest either PBE0 or B3PW91 in a
Pople basis can represent the spectra of ferrocenyl systems
36
37
Figure 7. Potential well diagram showing relationship between
observed electrochemical and optical parameters, assuming a
MMCT model.
2
5,38
well.
The medium is represented by a Tomasi polarizable
39
continuum assigned the macroscopic dielectric constant of
CH Cl . All geometry optimization and spectroscopic calcu-
2
2
lations include medium effects.
with ΔG° rather than ΔE
is more consistent, with
/2
Cp Ti(C Fc) no longer being an outlier. The slight
Some of the frontier orbitals for Cp Ti(C Fc) are depicted
2 2 2
1
in Figure 8 and Supporting Information. The HOMO has a
dominant iron d-orbital contribution with significant contribu-
tion also from the ethynyl linkage. The HOMO−1, HOMO−2,
and HOMO−3 are predominantly iron d-orbital based and
nearly degenerate (within 0.15 eV), consistent with MOs on
isolated ferrocene. The LUMO is dominated by the titanium d-
orbitals, with little amplitude on the titanocene Cp rings,
significant participation by the ethynyl linkage, and minor
amplitude on the iron d-orbitals. The LUMO+1 and LUMO+2
are largely titanocene based. Frontier orbitals for the more
electron-rich Cp* Ti(C Fc) are qualitatively similar (Support-
2
4
2
discrepancy of Cp Ti(C Fc) with respect to this trend can
2
2
2
readily be explained by its relatively low value for λ.
It is worth reiterating that the spectroscopically determined
values for ΔG° (eq 3) are slightly smaller than the
electrochemically determined ΔE values, but the difference
1
/2
corresponds to an expected Coulombic energy correction work
29
term that is consistent with those in related systems. This
correspondence of the electrochemically determined energy
II
IV
III
III
difference between the Fe /Ti ground state and Fe /Ti
excited state with the spectroscopically measured parameters
offers further evidence that the observed LE band has
significant MMCT character. Finally, some caution should be
exercised in interpreting these results given that the E1/2 values
2
2
2
ing Information) with the LUMO lying 0.51 eV higher in
energy than that for Cp Ti(C Fc) , consistent with the
2
2
2
replacement of Cp with the more electron-rich Cp*.
for the Fc oxidation were estimated from the irreversible waves
The vertical transition energies and oscillator strengths
predicted by TD-DFT for Cp Ti(C Fc) , (Figure 9 and
TMS
in all cases except for
Lastly, in agreement with previous reports of titanocenes,
Cp Ti(C Fc)(C CF ).
2 2 2 3
2
2
2
32
Supporting Information) are in reasonable agreement with
the experimental spectrum but slightly underestimate the CT
transition energies owing to poor long-range behavior of the
the absorption feature centered between 370 and 440 nm is
assigned to the Cp to TiIV LMCT. Consistent with this
40
assignment, a general decrease in the energy of this feature
functional. In agreement with experimental results, the
IV/III
accompanies anodic shifts of the Ti
reduction (Table 1).
calculated spectrum of Cp Ti(C Fc) shows a low-energy
2
2
2
Table 2. Spectroscopic Parameters and Calculated Coulombic Energy Work Term, w
a
λ (cm−1
b
ΔG° (cm−1
c
d
Δν1/2
)
)
ΔG° (V)
ΔE1/2 (w)
complex
CH Cl
THF
CH Cl2
THF
CH Cl2
THF
CH Cl2
THF
CH Cl2
THF
2
2
2
2
2
2
Cp* Ti(C Fc)
2
2540
3320
3300
3050
3400
3450
2540
3220
3260
3070
3300
3470
2790
4770
4710
4030
5000
5150
2600
4490
4600
4080
4710
5210
15600
12400
12400
13100
11900
10800
15850
13270
12790
13400
12350
10950
1.93
1.54
1.54
1.62
1.48
1.34
1.97
1.65
1.59
1.66
1.53
1.36
2.06 (0.13)
1.57 (0.03)
1.70 (0.16)
1.67 (0.05)
1.67 (0.19)
1.59 (0.25)
2.05 (0.08)
1.64 (−0.01)
1.80 (0.21)
1.76 (0.10)
1.73 (0.20)
1.60 (0.24)
2
2
e
Cp Ti(C Fc)
2
2
4
TMS
Cp Ti(C Fc)(C Ph)
2
2
2
Cp Ti(C Fc)
2
2
2
TMS
Cp Ti(C Fc)(C PhCF )
2
2
2
3
TMS
Cp Ti(C Fc)(C CF )
2
2
2
3
a
b
c
d
Estimated from a Gaussian fit of the low-energy side of the MMCT. Determined using eq 2 Determined using eq 3. Value of w determined from
e
ΔE1/2 − ΔG° (V). Values for w cannot be less than zero. The negative value here results from experimental uncertainty.
E
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Inorganic Chemistry
Article
the more electron-rich Cp* Ti(C Fc) , again consistent with
2
2
2
experimental results.
The assignment of this LE band is most germane to this
study. TD-DFT calculations suggest that this transition cannot
be represented as a simple excitation from an origin MO to a
destination MO. In this case, natural transition orbitals
41
(
NTOs) (Figure 10) can provide a clearer picture of the
Figure 10. Dominant destination (left) and origin (right) NTOs for
the LE transition of Cp Ti(C Fc) . The occupation value represents
2
2
2
the contribution of the NTOs shown.
charge transfer. The origin NTOs (including two minor
components, Supporting Information) are entirely located on
the ferrocene fragments with major population on the Fe d set
and substantial admixture of the Cp rings’ pi set. The
destination NTO with the largest contribution to the LE
absorption band has a large presence on the Ti fragment
Figure 8. Frontier MOs for Cp Ti(C Fc) . Additional MOs appear in
the Supporting Information.
2
2
2
(
predominantly of Ti d-orbital character) but has considerable
population on the ferrocenes as well. The destination NTO
with the second largest contribution is entirely located on the
ferrocene. This is also the case for the two minor components
(
Supporting Information). We can infer that the LE band is
II
IV
largely Fe to Ti CT (in agreement with the experimental
results above), but with an important contribution of local
excitation on the ferrocenes. This is consistent with the
aforementioned cis-1-ferrocenyl-2-(4-nitrophenyl)ethylene,
where TDDFT calculations suggest that the LE band for this
25
complex has both d−d and CT character.
NTO occupation numbers indicate the relative amounts of
charge involved in each origin and destination NTO and thus
sum to 1.00. They are not an indication of the relative
contribution of each NTO to the overall intensity of the given
transition. In the case of the composite LE band, although it
contains significant d−d character, the MMCT character is
likely the dominant contributor to the intensity owing to the
relative oscillator strengths of CT transitions vs d−d transitions.
NTO analysis of the bands computed to lie at 450 and 485
nm for Cp Ti(C Ph) indicate that these absorptions are each
Figure 9. Predicted vertical transition energies, oscillator strengths,
and UV−vis spectra for Cp Ti(C Ph) (top) and Cp Ti(C Fc)
2
2
2
2
2
2
(
bottom). The UV−vis spectrum is obtained by assuming a Gaussian
−1
width of 1000 cm for each transition.
(
LE) band not present for Cp Ti(C Ph) (Figure 9). This LE
2 2 2
2
2
2
band blue-shifts by approximately 0.2 eV (620 to 560 nm) for
mixtures of two LMCT single excitations, phenyl to Ti and Cp
F
DOI: 10.1021/acs.inorgchem.5b02587
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
to Ti (Supporting Information). The ethynyl bridge is involved
in each transition. Likewise, NTO analysis for the intense band
at 485 nm for Cp Ti(C Fc) demonstrates that the dominant
destination NTO (occupation = 0.65) is predominantly Ti d-
orbital in character and that the dominant origin state is Cp (on
the Ti) ligand in character but with some contribution from
ethynyl bridge and Fe d-orbitals. This again suggests
predominant Cp to Ti LMCT character for this absorption
band. The minor destination and origin states are localized on
the ferrocenes.
with R values ≤0.25 (Supporting Information). Though the fit
2
to π* was very poor (R = 0.05), we attempted to further refine
that model using Kamlet and Taft’s multivariable model (eq
2
2
2
50
4)
−1
E(cm ) = E° + s(π* + dδ) + aα + bβ
(4)
where π* combines both the polarity and polarizability of the
solvent, α describes the hydrogen bond donor acidity of the
solvent, β describes the hydrogen bond acceptor basicity, and δ
is a polarizability correction term (0 for all nonhalogenated
aliphatic solvents, 0.50 for all polyhalogenated aliphatics, and
The model choice B3PW91/6311G(d,p) recommended by
2
5
50
Barlow has produced reasonable transition frequencies and
qualitatively useful relative intensities for the lower energy
transitions in the Cp Ti(C Fc) , Cp* Ti(C Fc) , and Cp Ti-
1.00 for all aromatic solvents). The coefficients s, d, a, and b,
determined from multiple linear regression, describe the
susceptibility of the transition energy to each of the solvent
parameters. Statistically, increasing the number of fit parameters
will always increase the quality of the fit. Thus, we separately
tested the addition of each parameter to π* to interrogate
which variable had the greatest impact on the goodness of fit
Supporting Information). Whereas π* (R = 0.05) and the
addition of α (R = 0.08) do very little to explain the trend in
2
2
2
2
2
2
2
(
C Ph) systems. We explored the ωB97XD and CAM-B3LYP
2 2
functionals with the def2-TZV basis as well. Despite the greater
sophistication of these functionals, which contain range
corrections and dispersion energies, their results were not
even qualitatively informative.
2
(
2
Solvatochromism. Solvatochromism has been reported for
the CT bands in many Fc−π−A complexes. For complexes
where the acceptor is cationic, negative solvatochromism (a
shift to higher energy with increased solvent polarity) is
solvatochromism, improvements in the correlation associated
2
2
with the addition of β (R = 0.51), or δ (R = 0.64) are
statistically significant, with the polarizability correction term δ
having a greater impact. Thus, the aforementioned correlation
of MMCT transition energy with molar mass is likely due to
the known correlation of polarizability with molar mass (due to
the increased number of e in higher molar mass materials).
The above analysis suggests investigating the correlation of
the MMCT energy with solvent polarizability using Catalan’s
SP scale. Clearly the correlation between the MMCT energy
and this scale is also poor (Table 3, Figure 11), but the R value
2
4,30,42−44
typically observed,
neutral acceptor moieties, positive solvatochromism is typically
observed.
whereas for complexes with
2
4,29,42,45−47
It is worth noting that some of these
−
conclusions are based on just two or three solvents. Analyses of
extensive solvent sets with such Fc−π−A complexes sometimes
43,47
́
results in irregular fits to any known solvent parameters.
53
For Cp Ti(C Fc) , the MMCT band is sensitive to solvent,
2
2
2
2
apparently shifting to lower energy with increasing solvent
molar mass. This trend led us to investigate less common but
higher molar mass solvents, 1,2-dibromobenzene and 1,2-
diiodobenzene. The observed trend of a lowered energy for the
MMCT with increased solvent molar mass continued (Table
3). Higher polarity solvents such as alcohols cannot be included
due to poor solubility of the complexes.
In contrast, correlations with solvent scales such as the
48
solvent dielectric constant, Kosower’s Z, Dimroth and
4
9
50
Reichardt’s E (30), Kamlet’s π*, Gutmann’s AN and
T
5
1
52
DN, and Swain’s acity and basity scales were very poor,
Table 3. Solvatochromism for Cp Ti(C Fc) and
2
2
2
Figure 11. Energy of the MMCT transition as a function of Catalan
́
’s
Cp Ti(C Fc)
2
4
2
SP scale. The halocarbon solvents, CCl , CHCl , and CH Cl , are
4
3
2
2
Cp Ti(C Fc)
Cp Ti(C Fc)
2
indicated by red triangles.
2
2
2
2
4
MW
(g/mol)
MMCT LMCT MMCT LMCT
solvent
SP
(nm)
(nm)
(nm)
(nm)
o-C H I
329.9
235.9
153.8
120.5
120.1
92
-
598
592
588
586
584
583
584
581
573
572
567
910
411
405
410
410
602
588
584
585
584
578
585
577
566
563
557
1340
451
443
444
444
441
442
441
442
437
439
436
760
6
4 2
(
(
0.46) was the highest of all the single parameter models tested
other than molar mass, where R = 0.71). Closer analysis of
o-C H Br
-
6
4
2
2
CCl4
CHCl3
0.768
0.783
0.891
0.782
0.761
0.842
0.759
0.714
0.651
the plotted data indicates that the halocarbon solvents (CCl4,
CHCl , and CH Cl : red triangles) are largely responsible for
the poor correlation. Though we have no current explanation
for why this solvent class might not fit this model, their
C H NO
3
2
2
6
5
2
toluene
410
408
409
407
407
406
300
CH Cl2
85
2
2
exclusion increases R to 0.86.
pyridine
DMF
79.1
73.1
72
This again points to a model where solvent polarizability
dominates in the solvatochromism, suggesting that the excited
state is more polarizable than the ground state.
bathochromic shift with increasing solvent polarizability has
also been observed for another D−π−A system with a Fc
donor, namely, 4-(ferrocen-1-yl)benzylidene-malonitrile, albeit
THF
53a
Such a
acetone
58
a
(cm−1
Δ
)
a
−1
Energy difference (cm ) between absorption maxima in acetone and
C H I
54
with only three solvents.
6
4 2
G
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Inorganic Chemistry
Article
Table 4. Solvatochromism for TMSCp Ti(C Fc)(C R) and Cp* Ti(C Fc) Complexes
2
2
2
2
2
2
TMS
TMS
TMS
Cp Ti(C Fc)(C PhCF )
Cp Ti(C Fc)(C CF )
Cp Ti(C Fc)(C Ph)
Cp* Ti(C Fc)
2 2 2
2
2
2
3
2
2
2
3
2
2
2
solvent
C H I
MW (g/mol) MMCT (nm) LMCT (nm) MMCT (nm) LMCT (nm) MMCT (nm) LMCT (nm) MMCT (nm) LMCT (nm)
a
329.9
153.8
85
612
602
593
586
580
900
423
418
418
416
416
400
650
638
629
619
611
431
424
424
425
423
440
602
586
581
575
570
930
423
419
420
418
417
340
552
548
543
542
538
470
-
6
4 2
CCl4
374
372
373
370
-
CH Cl2
2
THF
72
acetone
58
b
(cm−1
Δ
)
980
a
b
−1
Due to a limited solvent window, this peak was not observed. Energy difference (cm ) between absorption maxima in acetone and C H I .
6
4 2
The magnitude of the solvatochromism increases with
increased distance between the donor and acceptor. Chiefly,
between the two solvents that generate the largest solvato-
chromic shift, acetone and o-diiodobenzene, there is a shift of
10 cm− for Cp Ti(C Fc) . This increases to 1340 cm for
1
−1
9
2 2 2
Cp Ti(C Fc) where an additional ethynyl linkage has been
2
4
2
inserted between the donor and acceptor.
For the three monosubstituted complexes,
TMS
Cp Ti(C Fc)-
2
2
(
C R), the magnitude of the solvatochromic shift for the
2
MMCT is very similar to that for Cp Ti(C Fc) , ranging from
2
2
2
−1
9
00 to 980 cm (Table 4). Note that these four complexes all
IV/III
have Ti
reduction potentials within 0.21 V of one another.
IV/III
In contrast, Cp* Ti(C Fc) has a Ti
reduction potential
2
2
2
more than 0.5 V further negative and has a significantly smaller
−
1
magnitude for the solvatochromic shift (470 cm ). Clearly the
charge distribution in the ground state has a significant impact
on the magnitude of the solvatochromism and may also play a
role in the larger magnitude of solvatochromism observed for
Cp Ti(C Fc) . The magnitude of the solvatochromic shift for
Figure 12. Scheme for the photochemical decomposition of
Cp Ti(C Ph) .
2
2
2
2
4
2
the LMCT is smaller than that of the MMCT in every case,
likely reflecting the shorter distance of charge separation in this
excited state. Additional TDDFT calculations are being pursued
to characterize these charge distributions and their relationship
to polarizability and solvent influences.
−
IV
analysis suggest an energy ordering where Cp to Ti and
−
IV
PhCC to Ti LMCT states are higher in energy than the
MMCT states. In the bimetallic complexes, the LMCT state(s)
may undergo rapid internal conversion to the lower energy
MMCT state before dissociation. Even the complex with one
ethynylferrocene and one ethynylbenzene ligand is quite
photostable, suggesting that the PhCC to Ti LMCT
state rapidly funnels into the MMCT prior to reaction out of
the LMCT.
Photochemistry. The bimetallic Fe−Ti systems are stable
to photolysis with broad band irradiation from a tungsten lamp,
contrasting the related Cp Ti(C Ph) complex, which rapidly
−
IV
2
2
2
decomposes in THF, benzene, or toluene solution upon
photolysis. The chief organic product of photolysis is III
1
(
Figure 12), as demonstrated by comparison of the H NMR of
As a further test of this, irradiation of dilute absorbance
55
the product with an independently prepared sample.
matched (A419 = 1.0) solutions of Cp
2
Ti(C
Ph) in THF was conducted
at 419 nm, a wavelength that excites into the LMCT state(s). In
the time required for Cp Ti(C Ph) to undergo complete
2 2 2
Ph) , Cp Ti-
TMS
This product likely forms from reductive elimination of two
phenylethynyl ligands to give I, followed by reduction of I with
the resulting “titanocene”, II. Reductive elimination is
precedented in that photolysis of Cp TiPh results in biphenyl
(C
2
Fc)
2
, and
Cp Ti(C Fc)(C
2 2 2
2
2
2
photobleaching, the bimetallic complexes showed <1% change
in absorption values across the UV−vis spectrum. Photolysis of
the bimetallic complexes at 575 nm (MMCT) also resulted in
<1% change in absorption values across the UV−vis spectrum.
2
2
II
56
and a Ti product. The so-called “titanocene” resulting from
such a reaction has been demonstrated to have a dihydride
5
7
structure, II, and has been shown to act as a reducing agent
58
toward ethylene and CO2. The hydrides in II result from the
coupling of the two Cp rings to the fulvalene structure. To gain
further evidence that the reducing hydrogen equivalents are
coming from the starting material, the photolysis was
performed in d -benzene and d -THF. This did not result in
CONCLUSIONS
■
IV
A series of Fc-alkynyl bridge-Ti complexes with an intense
low-energy visible absorption band has been prepared. The data
suggest that the low-energy absorption involves an Fe to Ti
II
IV
6
8
deuterium incorporation into the product. Also, the substituted
butadiyne, I, was not observed, suggesting either a very efficient
reduction of I by II, or perhaps even a concerted process to
product III.
MMCT. This assignment is supported by UV−vis spectrosco-
py, electrochemistry, a Marcus−Hush-type analysis of the data,
and TD-DFT modeling. Thus, the results reported herein
II
correspond with the experimental data implicating an Fe to
IV
One explanation of the relative photolytic stability of the
Ti charge transfer in the long wavelength absorption in blue
bimetallic complexes vs Cp Ti(C Ph) is that the MMCT state
minerals such as sapphire and blue kyanite. Accordingly, these
complexes may represent molecular examples of the
chromophore responsible for the blue color in such minerals.
2
2
2
is a bound state, whereas one or more of the LMCT states are
5
9
dissociative states. UV−vis spectroscopy and TD-DFT
H
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Inorganic Chemistry
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Figure 13. Additional complexes of Fe and TiIV whose reported colors are consistent with MMCT.
II
1
6b
Recent calculations suggesting that the ground state for
followed the procedure of Hayashi et al. UV−vis absorption spectra
were recorded using a Cary-50 spectrophotometer. NMR spectra were
obtained using a Varian 400-MR or 500-MR spectrometer. Infrared
spectra were measured on solid samples using a PerkinElmer Spectrum
100 series FT-IR spectrometer equipped with an ATR accessory.
Cyclic voltammograms were recorded using a BASi εpsilon electro-
chemical workstation and a BASi cell stand. Wavelength specific
photolysis experiments were conducted using a Rayonet RPR-100
photochemical reactor operating with eight RPR4190 or RPR5750
bulbs. Elemental analyses were performed by Midwest Microlabs in
Indianapolis, IN.
III
III
sapphire is Ti /Fe would dictate that a combination of
ligand electronic differences and constraints of the sapphire
crystal lattice reverse the thermodynamic ordering of the Ti/Fe
II
IV
pair: namely, from the Fe /Ti pair being more stable by
approximately 1.6 eV (as is the case for the molecular case
III
III
herein) to the Fe /Ti pair being more stable by
14
approximately 1.3 eV (as calculated by DFT for sapphire).
It is worth noting that a key difference between sapphire and
II
the systems herein is the spin state of Fe , i.e., high spin vs low
spin, respectively. Still, this question warrants further
investigation.
Syntheses. General Method for Cp Ti(C Fc) and Cp* Ti(C Fc) .
2 2 2 2 2 2
To an oven-dried two-neck round-bottom flask (50 mL) under a
positive pressure of argon was added ethynylferrocene (2.2−3.0 eq
relative to the titanocene) and Et O (8 mL). After the red solution was
The characterization of such a charge transfer suggests that
such Fc to TiIV charge transfer transitions might be more
2
cooled in a dry ice/acetone bath for 10 min, n-butyllithium (1 equiv
relative to the ethynylferrocene) was added. The solution was stirred
in the bath for 5 min and then was removed for 10 min. The
appropriate titanocene was added, and the mixture was returned to the
cold bath for 10 min and then removed and allowed to stir at room
temperature for 4 h. The colored precipitate was collected using
vacuum filtration. The solid was then loaded onto a triethylamine
treated silica gel column (1 × 18 cm) and eluted using a 5% mixture of
general. A survey of the literature reveals several examples of
IV
deeply colored complexes comprising Fc and Ti , though UV−
vis characterization is absent. For example, ML (Figure 13)
1
where M = Mg2 was isolated as orange crystals but the
+
IV
−
corresponding Ti complex (also including two Cl ligands) is
isolated as a “dark red solid”. Furthermore, whereas the Li+
60
and Mg2 salts of L (Figure 13) are golden and yellow crystals,
+
2
respectively, the corresponding Ti complex, TiL , is isolated as
triethylamine in CH Cl . The solvent was then removed using rotary
evaporation. The resulting solid was dissolved in minimal THF
(approximately 10 mL) and precipitated with hexanes (approximately
2 2
2
6
1
a dark purple crystalline solid. Likewise, FcTi(NR )
complexes are reported to be reddish brown to dark red,
2
3
6
2
2
0 mL). The solid was collected using vacuum filtration and dried
and the Ti(IV) ferrocenylaminate complexes are green to
6
3
under vacuum.
blue. It is likely that all of these complexes are demonstrating
Note: Several different timings for addition of the titanocene were
tested for the synthesis of Cp Ti(C Fc) . Addition of the titanocene
IV
a similar Fc to Ti CT absorption. Complexes with carboxylate
2
2
2
IV
linkages between Fc and Ti , however (e.g., Cp TiCl-
(
2
prior to warming the mixture of BuLi and the alkyne gave extremely
poor yields. Thus, allowing the solution to warm for 10 min is likely
important for the formation of the alkynyllithium. Addition of the
titanocene without subsequent recooling of the solution yielded
satisfactory yields. Also, recooling the alkynyllithiuum solution prior to
titanocene addition yielded satisfactory yields.
Cp Ti(C Fc) . Using ethynylferrocene (277 mg, 1.318 mmol) and
Cp TiCl (149 mg, 0.598 mmol) resulted in 200 mg (56%) of a deep
Cl ) λmax (ε); 584 (6640), 408 (9710). H
NMR (500 MHz, CDCl ) δ 4.15 (t, J = 1.9, 4H), 4.19 (s, 10H), 4.30
t, J = 1.9, 4H), 6.39 (s, 10H). Anal. Calcd (found) for C H Fe Ti·
H O: C, 66.49 (66.81); H, 4.92 (4.96). IR (neat) ν
Cp* Ti(C Fc) . Using ethynylferrocene (243 mg, 1.16 mmol) and
Cp* TiCl (150 mg, 0.385 mmol) resulted in 181 mg (64%) of a deep
6
4
i
65
O CFc) and Ti O (O Pr) (O CFc) ), are reported to be
2 4 2 6 2 6
orange solids, reminiscent of the color of ferrocene itself, either
indicating that CT through these carboxylate linkages is either
not significant or of higher energy.
The use of Fc in molecular D−π−A systems is quite
IV
extensive. The unique feature herein is the use of Ti as an
2
2
2
2
2
acceptor. The electrochemical data presented suggest that the
1
IV/III
blue solid. UV−vis (CH
2 2
Ti
reduction potential of the titanocene fragment is well-
3
positioned to inject electrons into TiO . This suggests that
2
(
IV
7c
34 28 2
D−π−A complexes with titanocene or other Ti fragment
−1
= 2053 cm .
2
CC
acceptors may have utility in DSSCs. Thus, the synthesis of
related complexes with organic donors, and investigations into
the excited state evolution using time-resolved transient
absorption spectroscopic measurements are in progress.
2
2
2
2
2
1
red solid. UV−vis (CH Cl ) λ (ε); 544 (7926), 372 (8206). H
2
2
max
NMR (400 MHz, CDCl
) δ 2.07 (s, 30H), 4.10 (t, J = 1.9, 4H), 4.14
3
(
s, 10H), 4.26 (t, J = 1.9, 4H). Anal. Calcd (found) for C H Fe Ti:
C, 71.76 (71.49); H, 6.57 (6.59). IR (neat) ν
Cp Ti(C Fc) . To an oven-dried two-neck round-bottom flask (50
mL) under a positive pressure of argon was added Et O (10 mL) and
diisopropylamine (0.34 mL, 2.43 mmol). The flask was cooled in an
acetonitrile/liquid nitrogen bath. After 5 min, n-butyllithium (0.900
mL, 2.25 mmol) was added. After 30 min, 1-chloro-4-ferrocenylbut-1-
4
4
48
2
−
1
= 2056 cm .
CC
EXPERIMENTAL SECTION
■
2 4 2
Materials and Methods. THF, Et O, and CH Cl were dried and
degassed using an Innovative Technology Inc. solvent purification
2
2
2
2
6
6
system before use. 1-Chloro-4-ferrocenylbut-1-ene-3-yne,
TMS
67
68
69
Cp TiCl , Cp Ti(C Ph) , and FcC Fc were prepared
2
2
2
2
2
4
following literature methods. All other materials were reagent grade
and used as received. Reactions were performed under a dry Ar
atmosphere implementing standard Schlenk procedures unless
otherwise noted. Oxidative decomposition studies using AgPF6
ene-3-yne (0.300 g, 1.1 mmol) and Cp TiCl (0.124 g, 0.50 mmol)
2 2
were added. The flask was transferred to a dry ice/acetone bath for 4 h
and then stirred for an additional 1 h at room temperature. The green
precipitate was collected using vacuum filtration and was washed with
I
DOI: 10.1021/acs.inorgchem.5b02587
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Et O (2 × 10 mL). The solid was then loaded onto a triethylamine
then n-butyllithium (283 μL, 0.706 mmol) was added. The flask
remained in the bath for 15 min, and then trifluoropropyne gas (17
mL at atmospheric T and P, 0.706 mmol) was added over 2 min while
the stopcock on the argon inlet was closed. The inlet was opened 1
2
treated silica gel column (1 × 18 cm) and eluted using a 5% mixture of
triethylamine in CH Cl . The dark green band was collected, and the
2
2
solvent was removed in vacuo. The resulting green solid was dissolved
in minimal THF (10 mL) and precipitated with hexanes (40 mL).
This solid was collected using vacuum filtration and washed with
hexanes (2 × 10 mL), yielding 55 mg (17.4%) of a green solid. UV−
min after the addition had been completed. After 10 min, the flask was
TMS
removed from the bath, and 10 min later,
Cp Ti(C Fc)Cl (0.100 g,
2 2
0.177 mmol) was added. The flask was returned to the bath for 10 min
and then allowed to stir at room temperature for 3 h. The mixture was
filtered, and the solvent was removed from the filtrate using rotary
evaporation. The resulting green solid was purified on base treated
1
vis (CH Cl ) λ (ε); 581 (6910), 441 (12100). H NMR (500 MHz,
2
2
max
CDCl ) δ 4.23 (t, J = 1.8, 4H), 4.24 (s, 10H), 4.44 (t, J = 1.8, 4H),
3
6
.43 (s, 10H). Anal. Calcd (found) for C H Fe Ti·H O: C, 68.92
38 28 2 2
1
−
(
69.02); H, 4.57 (4.74). IR (neat) ν
= 2179 and 2022 cm .
silica gel (1 × 15 cm) using 5% triethylamine in CH Cl . The green
2 2
CC
TMS
Cp Ti(C Fc)Cl. To an oven-dried two-neck round-bottom flask
band was collected, and the solvent removed under vacuum. The
resulting oil was dissolved in minimal hexanes (approximately 2 mL),
chilled in a dry ice/acetone bath for 10 min, and the solid was
2
2
TMS
(
50 mL) under a positive pressure of argon was added
Cp TiCl2
2
(
0.393 g, 1.0 mmol) and Et O (10 mL). To a second oven-dried two-
2
neck round-bottom flask (50 mL) under argon was added
collected using vacuum filtration, yielding 42 mg (38%). UV−vis
1
ethynylferrocene (0.252 g, 1.2 mmol) and Et O (10 mL). This
(CH
CDCl
1.9, 2H), 6.23 (m, 2H), 6.31 (m, 2H), 6.64 (m, 2H), 6.71 (m, 2H).
Anal. Calcd (found) for C31 FeSi Ti: C, 59.62 (59.41); H, 5.65
(5.40). IR (neat) νCC = 2117 and 2047 cm .
Photochemical Decomposition of Cp Ti(C Ph)
pressure of Ar, a solution of Cp Ti(C Ph) (0.031 g, 0.081 mmol) in
2
Cl
2
) λmax(ε); 630 (4080), 425 (8300). H NMR (400 MHz,
2
second flask was cooled in a dry ice/acetone bath for 5 min, then n-
butyllithium (480 μL, 1.2 mmol) was added. After 10 min, the flask
3
) δ 0.34 (s, 18H), 4.20 (s, 5H), 4.27 (t, J = 1.9, 2H), 4.31 (t, J =
was removed from the bath. While the lithiated alkyne was warming,
H
35
F
3
2
TMS
−1
the flask containing
Cp TiCl was cooled in the dry ice/acetone
2 2
bath for 10 min. The solution containing the lithiated alkyne was then
added to the titanocene via cannula. The resulting solution was stirred
at −15 °C (ice/brine bath) for 1 h and then at room temperature for
an additional 1 h. The mixture was then filtered, and the solvent was
removed from the filtrate using rotary evaporation. The resulting blue
solid (0.568 g) was dissolved in hexanes (30 mL), sonicated, and
2
2
. Under a positive
2
2
2
2
THF (5 mL) was placed directly under a 60 W, soft white light bulb.
The bright orange homogeneous solution turned yellow over a period
of 45 min. The solvent was then removed in vacuo. The resulting
yellow/brown oil was chromatographed on silica gel (1 × 10 cm)
using 20% EtOAc in hexanes. The decomposition product was isolated
chilled in a dry ice/acetone bath for 10 min. (Note: Et O is also an
2
1
as a yellow oil (0.010 g, 60%, Rf = 0.68). H NMR and mass spectral
appropriate solvent for recrystallization.) The mixture was filtered, and
data were consistent with identification of the product as cis-1,4-
the solid was dried under vacuum yielding 0.335 g (55%). UV−vis
5
5
1
diphenylbuteneyne. The same product was obtained in dry benzene
(
CH Cl ) λ ; 592, 410. H NMR (400 MHz, CDCl ) δ 0.35 (s,
2
2
max
3
and dry toluene, though decomposition in toluene took approximately
1
2
2
8H), 4.20 (s, 5H), 4.21 (t, J = 1.8, 2H), 4.32 (t, J = 1.8, 2H), 6.27 (m,
2
h.
Computational Modeling. Gaussian 09 was used for all
H), 6.37 (m, 2H), 6.60 (m, 2H), 6.78 (m, 2H). IR (neat) ν
=
CC
7
1
−1
058 cm .
7
2
General Method for TMSCp Ti(C Fc)(C C H ) and Cp Ti(C Fc)(p-
TMS
calculations and Gaussview was used for task preparation and
orbital imaging.
2
2
2
6
5
2
2
C C H CF ). To an oven-dried two-neck round-bottom flask (50 mL)
2
6
4
3
under a positive pressure of argon was added Et O (8 mL) and excess
2
arylacetylene (5 equiv). The flask was cooled in a dry ice/acetone bath
for 5 min and then n-butyllithium (1 equiv relative to the
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02587.
■
*
S
arylacetylene) was added. After 5 min, the flask was removed from
TMS
the bath, and 10 min later,
Cp Ti(C Fc)Cl was added. The flask
2 2
was returned to the bath for 10 min and then allowed to stir at room
temperature for 3 h. The mixture was filtered and the solvent was
removed from the filtrate using rotary evaporation. The resulting dark
solid was purified on base-treated silica gel (1 × 15 cm) using 5%
triethylamine in CH Cl . The colored band was collected and the
Cyclic voltammogram and differential pulse voltammo-
gram for FcC Fc in CH Cl , cyclic voltammograms and
4
2
2
2
2
UV−vis spectra of all complexes in THF, cyclic
solvent removed under vacuum. The resulting oil was dissolved in
minimal hexanes (approximately 10 mL), chilled in a dry ice/acetone
bath for 10 min, and the solid was collected using vacuum filtration.
TMS
voltammograms of
Cp Ti(C Fc)Cl in THF and
2 2
CH Cl , additional solvent parameters and multivariate
2
2
1
TMS
solvatochromism analysis, H NMR spectra; MOs,
NTOs, excitation energies and oscillator strengths,
geometric parameters, and Gaussian log files for
Cp Ti(C Fc) , Cp Ti(C Ph) , and Cp* Ti(C Fc)
Cp Ti(C Fc)(C C H ). Using phenylacetylene (0.145 mL, 1.3
2
2
2 6 5
TMS
mmol) and
Cp Ti(C Fc)Cl (0.150 g, 0.265 mmol) resulted in 71
2 2
mg (42%) of a dark blue solid. UV−vis (CH Cl ) λmax(ε); 583 (4220),
2
2
1
4
5
2
20 (11300). H NMR (400 MHz, CDCl ) δ 0.36 (s, 18H), 4.18 (s,
3
2
2
2
2
2
2
2
2
2
H), 4.20 (t, J = 1.9, 2H), 4.29 (t, J = 1.9, 2H), 6.21 (m, 2H), 6.28 (m,
H), 6.69 (m, 2H), 6.75 (m, 2H), 7.21−7.24 (m, 5H). Anal. Calcd
(PDF)
(
(
found) for C H FeSi Ti: C, 68.35 (68.39); H, 6.37 (6.29). IR
neat) ν
3
6
40
2
−
1
AUTHOR INFORMATION
Corresponding Author
*E-mail: paul.wagenknecht@furman.edu.
Author Contributions
= 2053 cm .
■
CC
TMS
Cp Ti(C Fc)(p-C C H CF ). Using 4-ethynyl-α,α,α-trifluoroto-
2
2
2
6
4
3
TMS
luene (0.145 mL, 0.883 mmol) and
Cp Ti(C Fc)Cl (0.100 g,
2
2
0
.177 mmol) resulted in 56 mg (45%) of a dark green solid. UV−vis
1
(
CH Cl ) λ (ε); 593 (3830), 418 (10200). H NMR (400 MHz,
2 2 max
¶
M.D.T. and J.A.P. contributed equally.
CDCl ) δ 0.35 (s, 18H), 4.19 (s, 5H), 4.22 (t, J = 1.9, 2H), 4.30 (t, J =
3
1
.9, 2H), 6.23 (m, 2H), 6.29 (m, 2H), 6.70 (m, 2H), 6.72 (m, 2H),
Notes
7
.29 (d, J = 8.2, 2H), 7.46 (d, J = 8.2, 2H). Anal. Calcd (found) for
The authors declare no competing financial interest.
C H F FeSi Ti: C, 63.43 (63.42); H, 5.61 (5.53). IR (neat) ν
=
3
7
39
3
2
CC
−
1
2
052 cm .
ACKNOWLEDGMENTS
TMS
■
Cp Ti(C Fc)(C CF ). Addition of the trifluoropropynyl ligand
2
2
2
3
This material is based upon work supported by the National
Science Foundation under Grants CHE-1362516 (P.S.W.) and
CHE-1112250 (to J.J.R.). P.S.W. also acknowledges financial
support of this effort through institutional commitments to the
followed a modification of the procedure used to add this ligand to
metallocyclam complexes. To an oven-dried two-neck round-bottom
flask (50 mL) under a positive pressure of argon was added Et O (8
mL). The flask was cooled in a dry ice/acetone bath for 5 min, and
70
2
J
DOI: 10.1021/acs.inorgchem.5b02587
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
South Carolina NIH-INBRE Award, 8 P20 GM103499. M.D.T.
was supported by the Arnold and Mabel Beckman Foundation
Beckman Scholars Award. Z.A. and C.O.T. are grateful for
support from the Body Foundation. We thank Yavuz Tikman
and Erdi Ata Bleda of Marmara University for computational
assistance and Nathan Cook of Furman University for
assistance with multivariate fits.
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DOI: 10.1021/acs.inorgchem.5b02587
Inorg. Chem. XXXX, XXX, XXX−XXX