Ligand-induced structural, photophysical, and electrochemical variations in tricarbonyl rhenium(I) tetrazolato complexes

Article abstract of DOI:10.1021/om400356n

Treatment of [Re(CO)₅X] (X = Cl, Br) with 2-(2-tert- butyltetrazol-5-yl)pyridine yielded neutral mononuclear complexes by exchange of two CO ligands for the chelating tetrazolato ligand. Treatment of [Re(CO) ₅Br] with phenyltetrazolate resulted in the assembly of an anionic dinuclear rhenium tricarbonyl species bridged by three tetrazole rings. The reaction of [Re(CO)₅Br] with (2-tert-butyltetrazol-5-yl)benzene formed an analogous neutral dinuclear complex bridged by a tetrazole ring as well as two bromide ligands; however this complex was found to be rather unstable in solution and was only structurally characterized via X-ray diffraction. The first three complexes were investigated for their photophysical properties, highlighting phosphorescent emission from their triplet metal-to-ligand charge transfer excited states, although in the case of the dinuclear species the quantum yield was found to be extremely low. The complexes are also characterized for their electrochemical behavior, and while the neutral mononuclear species show irreversible oxidations, the dinuclear complex displays one reversible and simultaneous two-electron oxidation.

Full text of DOI:10.1021/om400356n

Article
pubs.acs.org/Organometallics
Ligand-Induced Structural, Photophysical, and Electrochemical
Variations in Tricarbonyl Rhenium(I) Tetrazolato Complexes
Phillip J. Wright,† Mark G. Affleck,† Sara Muzzioli,‡ Brian W. Skelton,§ Paolo Raiteri,† Debbie S. Silvester,†
Stefano Stagni,*,‡ and Massimiliano Massi*,†
†Department of Chemistry, Curtin University, Kent Street, Bentley 6102 WA, Australia
‡Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Viale del Risorgimento 4, Bologna 40126, Italy
§Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley 6009 WA, Australia
S
* Supporting Information
ABSTRACT: Treatment of [Re(CO)5X] (X = Cl, Br) with 2-
(2-tert-butyltetrazol-5-yl)pyridine yielded neutral mononuclear
complexes by exchange of two CO ligands for the chelating
tetrazolato ligand. Treatment of [Re(CO)5Br] with phenyl-
tetrazolate resulted in the assembly of an anionic dinuclear
rhenium tricarbonyl species bridged by three tetrazole rings.
The reaction of [Re(CO)5Br] with (2-tert-butyltetrazol-5-
yl)benzene formed an analogous neutral dinuclear complex
bridged by a tetrazole ring as well as two bromide ligands;
however this complex was found to be rather unstable in
solution and was only structurally characterized via X-ray diffraction. The first three complexes were investigated for their
photophysical properties, highlighting phosphorescent emission from their triplet metal-to-ligand charge transfer excited states,
although in the case of the dinuclear species the quantum yield was found to be extremely low. The complexes are also
characterized for their electrochemical behavior, and while the neutral mononuclear species show irreversible oxidations, the
dinuclear complex displays one reversible and simultaneous two-electron oxidation.
energy of the MLCT manifold, whereas neutral electron-
withdrawing ligands raise the energy of the MLCT manifold.
The variation of the relative energy of the emissive 3MLCT has
also profound consequences on the photophysical performance
in terms of quantum yield (Φ) and excited-state lifetime (τ) of
these rhenium complexes, a trend that is found in general
agreement with the energy gap law.13 Therefore, positively
charged complexes with electron-withdrawing ancillary ligands
(e.g., pyridine) have generally a higher quantum yield and
longer excited-state lifetime compared to neutral complexes,
which is the consequence of a lower nonradiative decay
constant (knr) for the former due to a larger energy gap
INTRODUCTION
■
Emissive tricarbonyl rhenium(I) complexes of formulation fac-
[Re(CO)3(diim)L]0/+, where diim represents a neutral
chelating diimine-type ligand such as 2,2′-bipyridine and L a
neutral or anionic monodentate ancillary ligand, have been the
focus of intense investigation.1,2 The interest in these
complexes largely stems from their favorable photophysical
properties that make them amenable to a variety of areas, from
the fabrication of organic light-emitting devices to cellular
labeling.3−7 The photophysics of these species has now been
well established, and their emissive properties are in general
attributed to a spin-forbidden radiative decay originating from
excited states possessing a triplet metal-to-ligand charge transfer
character [3MLCT; 5d(Re) → π*(diim)].8−12 This excited
state can sometimes be mixed with excited states of ligand-
centered (LC) origin, such as π−π* transitions localized on the
diim ligand, or of ligand-to-ligand charge transfer character
[LLCT; L → π*(diim)] when the ancillary ligand is a halogen
or electron-rich species.1,12 The photophysical features of these
complexes, in terms of absorption and emission maxima, can be
readily tuned by opportune chemical modifications on the diim
and/or L ligands.1,2 Rendering the diim ligand more electron-
rich or electron-poor directly manipulates the energy of the
LUMO, thus inducing a hypsochromic or bathochromic shift,
respectively. On the other hand, electron-rich anionic ancillary
ligands destabilize the 5d orbitals of the Re center, lowering the
1,13
3
between the MLCT excited state and the ground state.
Despite this trend, highly emissive blue rhenium complexes
(e.g., λem < 450 nm) are extremely rare since as the energy of
3
the MLCT increases, the thermal population of quenching
excited states becomes progressively more efficient.14
We have been interested in the photophysical properties of
mononuclear and dinuclear tricarbonyl rhenium(I) tetrazolato
complexes, and we have recently reported the indirect influence
of the tetrazolato ancillary ligand in governing the relative
3
energy of the MLCT [5d(Re) → π*(diim)] excited states via
stabilization or destabilization of the 5d orbitals of the rhenium
Received: April 24, 2013
© XXXX American Chemical Society
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Figure 1. Synthesis of the complexes reported in this work.
centers.15,16 Furthering these studies, we have sought rhenium
tetrazolato complexes of alternative nature, so that the
tetrazolato ligand is directly involved in the MLCT excited
state, in a similar fashion to what we have previously reported
in the case of sky-blue-emitting platinum(II) complexes17 and
what others have reported for rhenium(I) complexes bound to
triazoles.18−23 To achieve this goal, we have tried to synthesize
complexes of analogous formulation to fac-[Re(CO)3(diim)-
L]0/+, where the diim ligand is substituted by a chelating
aryltetrazolato ligand. Our first attempt resulted in the
serendipitous formation of a metallacalix[3]arene by treatment
of [Re(CO)5X] (X = Cl, Br) with 2-pyridyltetrazolate, which is
characterized by a relatively efficient aqua-colored emission,
demonstrating the direct involvement of the tetrazolato ligand
in the MLCT excited state. As a continuation of this work,
we now report the complexes obtained by treating [Re(CO)5X]
(X = Cl, Br) with the neutral tert-butylated phenyl- and
pyridyltetrazole as well as the anionic phenyltetrazolate (Figure
1). While in the case of the neutral tert-butylated pyridylte-
trazole the expected mononuclear tricarbonyl of facial arrange-
ment is isolated, for the remaining ligands we isolated dinuclear
structures bridged by three phenyltetrazolato ligands or by one
tert-butylated phenyltetrazole and two bromide ligands. The
diverse photophysical and electrochemical properties of these
species were also investigated and corroborated by computa-
tional calculations.
sulfuric acid or in the case of 2-(1H-tetrazol-5-yl)pyridine
with additional trifluoroacetic acid, in tert-butyl alcohol.16
The preparation of the complexes 1 and 2 was achieved by
reaction of [Re(CO)5X] (X = Cl, Br) with 2-(2-tert-
butyltetrazol-5-yl)pyridine in refluxing toluene, as shown in
Figure 1. Both of these complexes were structurally
characterized by X-ray diffraction, and their structural
formulation appeared consistent by means of IR and NMR
spectroscopy as well as elemental analysis. In particular, the IR
spectra reveal the presence of three carbonyl bands (with the
two lower frequency bands overlapping), as expected from a
tricarbonyl rhenium complex of facial arrangement and
belonging to the C1 point group, as further confirmed in the
presence of three distinct peaks in the 13C NMR spectra
corresponding to the CO ligands.
Complex 3 was obtained by reaction of [Re(CO)5Br] with 5-
phenyltetrazolate in refluxing toluene, as shown in Figure 1.
The X-ray structural determination revealed an anionic
dinuclear species counterbalanced by a triethylammonium
cation. This motif was further supported by means of IR and
NMR spectroscopy as well as elemental analysis. The 1H NMR
spectrum evidences a 1:3 ratio between the triethylammonium
cation and the tetrazole ligands of the dinuclear anionic
complex. The 13C NMR shows the presence of one single peak
at 196.6 ppm corresponding to the six CO ligands, suggesting
the equivalence of these six ligands. Also, the signal at 167.0
ppm assigned to the tetrazolic C atom is typical of extended
conjugation between the tetrazole and the phenyl ring, which
can only be achieved through a coplanar arrangement of the
two rings.26−29 The IR spectrum highlights two bands in the
carbonyl fingerprint region at 2024 and 1901 cm−1, suggesting
either a D3h point group for the entire assembly or two
noninteracting rhenium tricarbonyl fragments of C3v symmetry.
Noteworthy, there was no evidence of formation of the same
dinuclear species when the reaction was attempted starting
24
3
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The tetrazole ligands, 2-
(1H-tetrazol-5-yl)pyridine and 5-phenyl-1H-tetrazole, were
synthesized via a 1,3-dipolar cycloaddition of NaN3 to the
corresponding aryl nitriles, according to previously published
procedures.25 The tert-butylation of the tetrazole rings was
performed by treating the corresponding substrates with
B
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Figure 2. X-ray crystal structures of the complexes, with ellipsoids drawn at the 50% probability level. Solvent molecules and the triethylammonium
cation in the case of 3 are omitted for clarity.
from [Re(CO)5Cl] instead of [Re(CO)5Br], hinting to the fact
that the higher lability of the bromide ligand might be favoring
the dinuclear assembly by displacement with the negatively
charged tetrazolato ligand. Assemblies with loss of two CO and
a bromide ligand have in fact been reported by treatment of
[Re(CO)5Br] with anionic ligands such as 2-pyridyltetrazolate
or monothiobenzoate.24,30
the analysis of the structure of complex 4 shows that a
migration of the tert-butyl group occurred, which moved from
its original position on the N2 atom to the N1 atom of the
tetrazole ring. While we have previously reported migration of
the rhenium fragment between the N1 and N2 atoms of a
tetrazolato ligand,16 the migration of an alkyl group has never
been previously observed in our studies. However, it should be
noted that while the alkylation of the tetrazole ligand was
generally achieved using a methyl substituent,29,34 in this case
the higher stability of the tert-butyl cation could be taken into
account to rationalize the migration. A space-fill analysis of the
structure reveals that without the N2 to N1 migration of the
tert-butyl substituent the structure would not form due to steric
hindrance between the alkyl group and the close CO ligands.
Furthermore, the N1 tert-butylation causes the rotation of the
phenyl ring of ca. 90° with respect to the tetrazole ring. Again,
this rotation is necessary to accommodate the phenyl ring close
to the CO ligands.
In an attempt to obtain a cyclometalated rhenium(I)
complex, (2-tert-butyltetrazol-5-yl)benzene was combined
with [Re(CO)5Br] in refluxing toluene (Figure 1). While
single crystals could be grown, the bulk was always found
contaminated with other unidentified products that could not
be easily removed. Furthermore, the compound does not seem
to be stable over time in solution, as witnessed by changes
occurring in the NMR spectra, a behavior that was observed
before for dinuclear rhenium(I) complexes of analogous
structures bound to 1,2-diazine bridging ligands.7 Despite the
1
lack of purity, the H NMR spectroscopic data seem to be in
agreement with the structural details obtained via X-ray
diffraction. Only a structural investigation is provided within
this work. Interestingly, the structure of 4 is again a dinuclear
complex with an arrangement analogous to complex 3, but this
time the two bromide ligands are retained in the structure. The
presence of the bridging halogens reinforces the argument that
their displacement is favored by anionic ligands, such as 5-
phenyltetrazolate in the case of 3, but not when the
pentacarbonyl precursor is treated with neutral ligands such
as (2-tert-butyltetrazol-5-yl)benzene. The same trend is
observed by previously published dinuclear complexes of
analogous structures reported by others.7,31−33 Remarkably,
X-ray Structural Determination. The structures of the
prepared complexes are shown in Figure 2 (see Supporting
Information for bond lengths and angles tables). For the
mononuclear complexes, 1 crystallizes in the monoclinic space
group P21/n, and 2 crystallizes in the monoclinic space group
P21/c. Both complexes have an analogous octahedral
coordination around the metal centers with the three CO
ligands in a facial configuration. In both crystal structures, the
tert-butyl substituent is connected to the N2 atom of the
tetrazole ring, while the rhenium coordinates to the N4 atom.
No evidence of the other possible linkage isomer where the tert-
butyl is attached to the N2 atom and the rhenium is
C
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Table 1. Summary of the Photophysical Data
absorption
emission, 298 K
emission, 77 K
a
b
a
b
λab (104ε) [nm (M−1 cm−1)]
λem[nm]
τ
[ns]
167
τ
[ns]
Φ
Φ
λem [nm]
τ [μs]
1
2
3
271 (1.4)
353 (0.4)
268 (1.1)
358 (0.3)
253 (6.85)
574
291
0.033
0.077
510
5.81
568
177
10
323
12
0.024
0.058
508
474
6.28
4.30
c
c
354
515
−
−
312 (0.6)
a
b
c
From air-equilibrated solution. From degassed solution. Value too low to be accurately determined.
coordinated to the N1 atom was found. This isomer is probably
not formed due to the steric hindrance caused by the vicinity of
the tert-butyl substituent and the CO ligand.
The dinuclear anionic complex 3 crystallizes in the
monoclinic space group C2/c. The two metal centers are
bridged by the tetrazole rings of three ligands, binding to the
N2 and N3 atoms of each ligand respectively with an μ2-
η1(N2):η1(N3) arrangement, resembling the bonding motif
that is commonly encountered in the structures of several
tetrazolate and tetrazole-based MOFs.35−37 There are water
molecules present in the lattice that are hydrogen bonded to
the triethylammonium cations. In the solid state, it seems that
the coplanarity between the tetrazole and phenyl rings is lost,
with twisting angles of ca. 16°, 34°, and 32°. While the
direction of the twisting is the same and would cause helical
chirality to the species, this rotation is likely to be lost in
solution. The distance between the two rhenium centers is ca.
4.0 Å; thus the presence of a Re−Re bond can be excluded.31
The dinuclear complex 4 crystallizes in the monoclinic space
group P21/n. The structure is analogous to complex 3, with the
tetrazole ligands coordinated in a μ2-η1(N3):η1(N4) fashion
and the two bridging bromide ligands. The Re···Re separation
of ca. 3.7 Å again excludes the presence of any Re−Re bond.31
Photophysical Properties. A summary of the photo-
physical data can be found in Table 1, and the individual
absorption and emission profiles from dilute air-equilibrated
dichloromethane solutions (ca. 10−5 M) can be found in Figure
3 (see Supporting Information for the excitation profiles and
emission profiles measured at 77 K).
The complexes 1 and 2 appear to have almost identical
absorption profiles, generally displaying an intense band in the
250−280 nm region and a lower energy band of reduced
intensity peaking around 355 nm. The higher energy band is
attributed to the spin-allowed LC transition localized on the
tetrazole chelating ligand. The lower energy band is, on the
other hand, attributed to the spin-allowed MLCT transition
[5d(Re) → π*(pyridyltetrazole)].
For compound 3 the absorption profile has an intense high-
energy band at 253 nm, which tails off to a lower energy
shoulder at 312 nm. The peak at 250 nm is attributed to the LC
transition localized on the 5-phenyltetrazolate ligand. The
lower energy shoulder is attributed to an MLCT manifold, and
this conclusion is corroborated by the fact that an absorption
spectrum of phenyltetrazolate is characterized by the intense
band around 250 nm without any shoulder. Due to the
proximity of these excited states, the band is probably a
convoluted admixture of LC/MLCT transitions.12 The blue-
shifted MLCT of 3 with respect to those of 1 and 2 might be
ascribed to a higher energy of the empty π* orbitals of
phenyltetrazolate with respect to the pyridyltetrazole system,
Figure 3. Absorption and normalized emission profiles for 1 (top), 2
(middle), and 3 (bottom) from diluted (ca. 10−5 M) dichloromethane
solutions at room temperature.
D
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the latter further stabilized by the coordination of the metal
center to both the tetrazole and pyridine ring.
For complexes 1 and 2, following excitation to the singlet
manifolds and internal conversion to the lowest singlet excited
state, the corresponding triplet manifold is populated via
intersystem crossing.1 The radiative decay from this excited
state produces similar broad and structureless emission profiles
centered at λem = 574 and 568 nm for 1 and 2, respectively. The
emission profiles are typical of excited states characterized by a
charge transfer nature, and they are independent from the
excitation wavelength. The excited-state lifetime decays (τ) are
monoexponential for both cases, with values suggesting
phosphorescent emission from excited states of triplet multi-
plicity (3MLCT). The lifetime and quantum yield (Φ) values in
air-equilibrated and degassed solutions show that the excited
states are partially quenched by the presence of oxygen,
consistently with their triplet multiplicity. In a rigid matrix at 77
K there is a significant hypsochromic shift in the emission
maxima of ca. 60 nm caused by rigidochromism. The band at
77 K remains structureless, suggesting a lack of mixing with
excited states of LC nature.1
Complex 3 exhibits a very weak, short-lived (τ ≈ 10 ns), and
almost O2-insensitive emission in dichloromethane solution at
room temperature. This could be due to the efficient
population of quenching higher excited states as a consequence
of the blue-shifted lower-lying MLCT states,12,14 as evidenced
on comparing the absorption spectra of 1 and 2 with that of 3,
or by low-frequency vibrations of the assembly. The complex is
also weekly emissive at 77 K, from where a low-intensity
structured band can be detected, suggesting mixing of MLCT
and LC excited states.1
Electrochemical Properties. The electrochemical behav-
ior of the complexes was investigated using cyclic voltammetry,
and a summary of the data can be found in Table 2. The
Table 2. Summary of the Electrochemical Data (Ep = peak
potential, ΔEpp = peak-to-peak separation)
oxidation peak
reduction peak
Ep vs Fc/Fc+ (V) ΔEpp (mV) Ep vs Fc/Fc+ (V) ΔEpp (mV)
a
1
2
3
1.04
1.03
1.10
−
−
−1.89
−1.88
−1.62
110
110
a
a
80
−
a
Peak-to-peak separations could not be elucidated due to the
irreversible nature of the voltammetry.
voltammograms were recorded from solutions of the complexes
in the ionic liquid 1-hexyl-3-methylimidazolium tris-
(pentafluoroethyl)trifluorophosphate, [C6mim][FAP], whose
advantages have been previously described in our work.16,38
The voltammetric curves can be seen in Figure 4 for the
oxidation region and Figure 5 for the reduction region. For
complexes 1 and 2, the oxidation potentials are at 1.04 and 1.03
V, respectively, and are attributed to the oxidation occurring on
the rhenium center, Re(I) → Re(II) + e−, following
electrochemical studies reported on analogous neutral mono-
nuclear complexes fac-[Re(CO)3(diim)L].39 The oxidation
peaks appear to have no reversible character at 100 mV s−1,
and potentially occurring follow-up chemistry appears to be
very fast on the voltammetric time scale. Similar irreversible
oxidation peaks were observed for tricarbonyl Re(I) complexes
bound to the analogous pyridyltriazolato ligands.40 On the
other hand, the oxidation process of 3 is characterized by a
Figure 4. Oxidation range of the cyclic voltammograms for solutions
of 1 (top), 2 (middle), and 3 (bottom) in [C6mim][FAP].
reversible nature with a peak-to-peak separation of approx-
imately 80 mV, consistent with fast kinetics and indicating that
any follow-up chemistry is relatively slow on the electro-
chemical time-scale. The oxidation in this case is tentatively
attributed to a simultaneous two-electron process involving the
rhenium centers, a behavior that was previously observed by
dinuclear rhenium(I) species bridged by two halogen ligands
and 1,2-diazine.7 Compared to our previously published results
on trinuclear metallacalix[3]arene rhenium species bound by
pyridyltetrazolato ligands,24 where the sequential oxidation of
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less polar solvent, dichloromethane (results not shown).
Therefore the lack of splitting of the two electron oxidation
wave may more likely indicate a lack of interaction between the
two Re centers.
The reduction peaks of complexes 1 and 2 have peak
potentials at −1.89 and −1.88 V, respectively, and are
attributed to reduction processes occurring on the pyridylte-
trazole ligands. The reduction appears to be have a slightly
reversible character with a peak-to-peak separation of
approximately 110 mV, consistent with a moderately fast
electrochemical step when compared to the separation of the
Fc/Fc+ couple in ionic liquids (90 mV), which is known to have
fast kinetics.41
For complex 3 the reduction peak is broader and appears to
be characterized by a chemically irreversible process, and is
ascribed to a reduction of the phenyltetrazolato ligand.
The differences in the peak currents for both the oxidation
and reductions are attributed to the different concentration of
the complexes in the ionic liquid solutions (see Experimental
Section).
Computational Calculations. In order to further under-
stand the electronic transitions of the complexes and validate
the interpretation of the photophysical and electrochemical
results, the absorption spectra of the complexes and the orbital
contours were simulated with time-dependent density func-
tional theory (TDDFT). The energy-minimized structures,
obtained using the implicit solvent model (PCM),42 resulted in
good agreement with the structural data obtained by X-ray
diffraction: differences in the bond lengths were calculated
within 0.01 and 0.1 Å, and differences in bond angle values
were calculated between 0.9° and 2°. These differences can be
related to the presence of crystal packing effects in the X-ray
determined structures, which are not present in the case of
isolated molecules used in the calculations.
For complexes 1 and 2, the overall appearance of the
simulated absorption spectra (reported in the Supporting
Information along with the relative tables of calculated
transitions) is in agreement with the experimental results.
The lower energy band was determined to be composed by an
envelope of transitions between the HOMO−n and LUMO+m
orbitals (n = 0, 1; m = 0, 1), with the main contributor being
the HOMO−1 → LUMO transition (Figure 6). An analysis of
the orbital contours for this transition reveals that the HOMO
and HOMO−1 orbitals are mainly localized on the rhenium
center and halogen ligand, whereas the LUMO and LUMO+1
are localized exclusively on the pyridyltetrazole ligand, with a
higher contribution from the pyridine ring (especially in the
case of the LUMO). This can be explained by the electron-rich
nature of the tetrazole ring, and it is in agreement with orbital
contours obtained from tricarbonyl rhenium(I) frangments
bound to chelating pyridyltriazolato ligands.20−23 The calcu-
lations therefore agree with the MLCT character of the lowest
excited state and suggest partial mixing with ligand-to-ligand
charge transfer [LLCT, X → π*(pyridyltetrazole)].
Figure 5. Reduction range of the cyclic voltammograms for solutions
of 1 (top), 2 (middle), and 3 (bottom) in [C6mim][FAP].
the rhenium centers was characterized by three reversible
processes at progressively higher potentials, in this case only
one wave is visible. The fact that the second oxidation is
occurring almost simultaneously and not at higher potential for
the negatively charged complex 3 could be attributed to the
highly polar and ionic nature of the medium, where stabilization
by solvation is more favorable to a cationic species (obtained
via a two-electron oxidation from 3) rather than a neutral
species (obtained via a one-electron oxidation from 3).7
However we also observed only a single oxidation peak in a
In the case of the dinuclear complex 3, the shoulder around
300 nm appearing in the experimental absorption spectrum
seems to originate from transitions involving all the orbitals
from the HOMO−5 to the LUMO+5, with the main
contributor being the HOMO → LUMO +2. All the
HOMO−n (n = 0, 5) orbitals are localized on the rhenium
centers, whereas all the LUMO+m (m = 0, 5) are distributed on
the phenyltetrazolato ligand. Therefore, the calculations
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However, the reversible oxidation of 3 makes this dinuclear
compound appealing in the construction of assemblies that
exhibit photoinduced electron transfer.
EXPERIMENTAL SECTION
■
General Remarks. All reagents and solvents were purchased from
Sigma Aldrich and used as received without further purification. 2-
(1H-Tetrazol-5-yl)pyridine and 5-phenyl-1H-tetrazole were synthe-
sized according to previously published procedures.25 2-(2-tert-
Butyltetrazol-5-yl)pyridine and 5-phenyl-2-tert-butyltetrazole were
also synthesized according to published procedures.16,43 Acidic
alumina for column chromatography was of Brockmann I activity.
Nuclear magnetic resonance spectra, consisting of H and 13C, were
1
recorded using a Bruker Avance 400 spectrometer (400.1 MHz for 1H,
100 MHz for 13C) at 300 K. 1H and 13C chemical shifts were
referenced to residual solvent signals. Infrared spectra were recorded in
the solid state, using an attenuated total reflectance Perkin-Elmer
Spectrum 100 FT-IR, equipped with a diamond stage. Compounds
were scanned from 4000 to 650 cm−1. The intensities of the IR bands
are reported as strong (s), medium (m), or weak (w). Elemental
analyses were obtained at the Central Science Laboratory, University
of Tasmania, using a Thermo Finnigan EA 1112 Series Flash.
Photophysical Measurements. Absorption spectra were re-
corded at room temperature using a Perkin-Elmer Lambda 35 UV/vis
spectrometer. Uncorrected steady-state emission and excitation spectra
were recorded on an Edinburgh FLSP920 spectrometer equipped with
a 450 W xenon arc lamp, double excitation and single emission
monochromators, and a Peltier-cooled Hamamatsu R928P photo-
multiplier tube. Emission and excitation spectra were corrected for
source intensity (lamp and grating) and emission spectral response
(detector and grating) by a calibration curve supplied with the
instrument. According to the approach described by Demas and
Crosby,44 luminescence quantum yields (Φem) were measured in
optically dilute solutions (OD < 0.1 at excitation wavelength) obtained
from absorption spectra on a wavelength scale [nm] and compared to
the reference emitter by the following equation:
Figure 6. Selected frontier orbital contours for 1 (left), 2 (center), and
3 (right).
2
⎡
⎤
⎥
⎦
⎢
⎥⎢
⎥
⎡
⎢
⎣
⎤
⎥
⎦
Ar(λr) Ir(λr) nx
D
D
r
x
⎢
⎥⎢ ⎥
A (λ ) I (λ )
x
Φ = Φ⎢
x
r
2
⎣
⎦⎣
⎦ n
⎣
x
x
x
r
where A is the absorbance at the excitation wavelength (λ), I is the
intensity of the excitation light at the excitation wavelength (λ), n is
the refractive index of the solvent, D is the integrated intensity of the
luminescence, and Φ is the quantum yield. The subscripts r and x refer
to the reference and the sample, respectively. The quantum yield
determinations were performed at identical excitation wavelength for
the sample and the reference, therefore canceling the I(λr)/I(λx) term
in the equation. All the Re complexes were measured against an air-
equilibrated water solution of [Ru(bpy)3]Cl2 used as reference (Φr =
0.028).45 Emission lifetimes (τ) were determined with the single
photon counting technique with the same Edinburgh FLSP920
spectrometer using pulsed picosecond LEDs (EPLED 295 or EPLED
360, FWHM < 800 ps) as the excitation source, with repetition rates
between 10 kHz and 1 MHz, and the above-mentioned R928P PMT
as detector. The goodness of fit was assessed by minimizing the
reduced χ2 function and by visual inspection of the weighted residuals.
To record the 77 K luminescence spectra, the samples were put in
glass tubes (2 mm diameter) and inserted in a special quartz Dewar
filled up with liquid nitrogen. The solvent (dichloromethane) used for
the preparation of the solutions for the photophysical investigations
was of spectrometric grade. The prepared solution was filtered through
a 0.2 μm syringe filter before measurement. Degassed samples were
prepared by the freeze−pump−thaw technique. Experimental
uncertainties are estimated to be 8% for lifetime determinations,
20% for quantum yields, and 2 nm and 5 nm for absorption and
emission peaks, respectively.
corroborate that the lowest excited state of 3 has an MLCT
character.
CONCLUSIONS
■
In conclusion, this work reports the synthesis of four
tricarbonyl rhenium complexes bound to aryltetrazole ligands.
The intimate chemical nature and charge of the tetrazole
ligands directly influence the structures of the obtained species,
which are either mononuclear complexes with bidentate tert-
butylpyridyltetrazole, anionic dinuclear species bridged by three
phenyltetrazolato ligands, or a dinuclear complex bridged by a
tert-butylated phenyltetrazolato and two bromide ligands. The
latter complex was found to be rather unstable in solution, and
its complete purification and characterization was not possible,
despite the successful growth of single crystals. The complexes
obtained in this work emphasize how the neutral or anionic
charge of the ligand reacting with the [Re(CO)5X] precursor
might determine the exclusive dissociation of two CO ligands
from the rhenium center or the concomitant dissociation of the
anionic halogen ligand. The trend reported here is in agreement
with previously reported structures of rhenium complexes. The
photophysical properties of these complexes were investigated,
clearly demonstrating that the tetrazolato ligand directly
participates in the MLCT manifold. While complexes 1 and 2
possess photophysical properties that are typical of these
species, complex 3 shows an extremely weak emission.
Cyclic Voltammetry in Ionic Liquid. A gold microelectrode
(made in house and kindly donated by the group of Professor Richard
Compton at Oxford University, UK) was polished and modified with a
G
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Organometallics
Article
1
section of disposable micropipet tip into which microliter quantities of
the ionic liquid solvent can be placed. The electrode was then inserted
into a “T-cell” apparatus as described elsewhere.46 A silver wire (0.5
mm diameter) was inserted from the top and acted as a combined
counter and reference electrode. The T-cell was placed inside an
aluminum Faraday cage and connected to a vacuum pump (Edwards
ES50). The samples for electrochemistry were prepared by dissolving
the required amount of rhenium complex in a minimal amount of
dichloromethane and then adding this solution to the ionic liquid (1-
hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate,
purchased from Merck Pty. Ltd., Australia, at ultra high purity
electrochemical grade) in order to obtain a final concentration on the
order of 10 mM. The solution was left in an open container to allow
the dichloromethane to evaporate. A 40 μL amount of this sample was
transferred into the T-cell with the use of a micropipette. Experiments
requiring the use of ferrocene followed the above procedure; however
10 μL of a 10 mM solution of ferrocene in acetonitrile was added into
the T-cell apparatus along with the ionic liquid containing the rhenium
complex. It is noted here that due to the small quantities of
complexes/ferrocene and ionic liquid solvent used in this work, the
exact concentrations used for electrochemistry are not known;
although useful qualitative results can be obtained, the peak currents
cannot be directly compared for a quantitative analysis. Cyclic
voltammetry experiments were performed using a PGSTAT302N
potentiostat (Eco-Chemie, Netherlands) interfaced to a PC with
GPES (General Purpose Electrochemical System) software. The step
potential was fixed at 0.01 V. The potentials are referenced to an
internal reference ferrocene/ferrocenium redox couple according to
IUPAC recommendations47 and established in ionic liquids.46,48 Scans
were performed using a potential window within the range −2.0 to 1.2
V reliant on the compound investigated. Reported potentials were
obtained from a scan rate of 100 mV s−1.
Computational Calculations. Time-dependent density functional
theory calculations were performed with GAUSSIAN 0949 in order to
calculate the absorption spectra for the title compound. Prior to these
calculations, the structures were relaxed at the B3LYP level of theory.
The Re atoms were treated with the Stuttgart-Dresden effective core
potential,50 the Pople 6-311G** basis set was used for C, H, and N
atoms, and the effect of the solvent was mimicked with the PCM
solvation model,42 with parameters adequate for dichloromethane.
The low-lying singlet−singlet excitation energies were calculated at the
same level of theory, and the spectra were reproduced as the
superposition of Gaussian functions with heights proportional to
calculated intensities and a variance of 11 nm.
Synthesis of 1. [Re(CO)5Cl] (100 mg, 0.27 mmol) and 2-(2-tert-
butyltetrazol-5-yl)pyridine (100 mg, 0.49 mmol) were combined in
toluene (15 mL) and stirred at reflux for 6 h. The mixture was cooled,
and the solvent removed under reduced pressure, yielding a yellow oil
residue. The oil was dissolved in dichloromethane, and the addition of
hexanes produced a yellow solid, which was filtered and washed with
hexanes. Single crystals suitable for X-ray diffraction were grown by
layering hexanes on a dichloromethane solution of 1. Yield: 145 mg
(99%). 1H NMR (CDCl3): δ 9.08 (d, nJ(H,H) = 5.4 Hz, 1H; H6(py)),
8.28 (d, nJ(H,H) = 8.1 Hz, 1H; H3(py)), 8.16−8.11 (m, 1H; H(py)),
7.65−7.60 (m, 1H; H(py)), 1.88 (s, 9H; C(CH3)3) ppm; solvent peak
at δ 5.29 (s, CH2Cl2) ppm. 13C NMR (CDCl3): δ 196.5 (CO), 194.7
(CO), 188.5 (CO), 166.1 (CN4), 153.9, 145.2, 139.8, 128.3, 124.0,
68.4, 29.2 ppm; solvent peak at δ 55.6 (CH2Cl2) ppm. IR: νmax 2021 s
(CO), 1884 s (2 overlapping CO) cm−1. Anal. Calcd (%) for
1·(CH2Cl2): C 28.31, H 2.55, N 11.79. Found: C 28.53, H 2.53, N
12.05.
solution of 2. Yield: 35 mg (25%). H NMR (CDCl3): δ 9.09 (d,
nJ(H,H) = 4.8 Hz, 1H; H6(py)), 8.29 (d, J(H,H) = 7.8 Hz, 1H;
n
H3(py)), 8.15−8.10 (m, 1H; H(py)), 7.64−7.59 (m, 1H; H(py)),
1.88 (s, 9H; C(CH3)3) ppm; solvent peak at δ 5.32 (s, CH2Cl2) ppm.
13C NMR (CDCl3): δ 196.1 (CO), 194.3 (CO), 188.0 (CO), 166.2
(CN4), 154.8, 145.4, 139.7, 128.2, 124.0, 68.5, 29.5 ppm; solvent peak
at δ 55.6 (CH2Cl2) ppm. IR: νmax 2020 s (CO), 1881 s (2 overlapping
CO) cm−1. Anal. Calcd (%) for 2·1.2(CH2Cl2): C 26.08, H 2.37, N
10.73. Found: C 25.82, H 2.22, N 11.12.
Synthesis of 3. [Re(CO)5Br] (100 mg, 0.25 mmol), 5-phenyl-1H-
tetrazole (53 mg, 0.36 mmol) and triethylamine (50 μL, 0.36 mmol)
were combined in toluene and stirred at reflux for 12 h. The mixture
was cooled and the solvent removed under reduced pressure. The
residue was added to dichloromethane, and the white solid formed was
filtered, washed with dichloromethane, and recrystallized from water,
yielding the title compound. Yield: 60 mg (45%). 1H NMR (d6-
acetone): δ 8.15−8.10 (m, 6H; H(ph)), 7.53−7.43 (m, 9H; H(ph)),
3.42 (q, nJ(H,H) = 7.2 Hz, 6H; NCH2CH3), 1.39 (t, nJ(H,H) = 7.2 Hz,
9H; NCH2CH3) ppm; solvent peak at δ 2.80 (s, H2O) ppm. 13C NMR
(CDCl3): δ 196.6 (CO), 167.0 (CN4), 130.6, 129.7, 129.0, 127.5,
46.96, 9.03. IR: νmax 2024 s (CO), 1901 s (CO) cm−1. Anal. Calcd (%)
for 3·2(H2O): C 35.58, H 3.17, N 16.34. Found: C 35.82, H 2.89, N
16.32.
Synthesis of 4. [Re(CO)5Br] (100 mg, 0.25 mmol) and 5-phenyl-
2-tert-butyltetrazole (54 mg, 0.27 mmol) were combined in toluene
and stirred at reflux for 12 h. The mixture was cooled and the solvent
removed under reduced pressure. Dichloromethane was added to the
residual solid, and the mixture was filtered and washed with
dichloromethane to give a white solid. Single crystals suitable for X-
ray diffraction were grown by layering hexanes on a solution of 4 in
dichloromethane; the crop of crystals cocrystallized with an
amorphous pale yellow solid. Yield (not optimized): 20 mg. 1H
n
NMR (d6-acetone): δ 8.15 (d, J(H,H) = 6.8 Hz, 2H; Hortho), 7.90−
7.50 (m, 3H; Hmeta and Hpara), 1.76 (s, 9H; C(CH3)3). Impurities
appear to be present within the multiplet at 7.90−7.50 ppm, with
integration corresponding to 3H, and as a singlet at 1.82 ppm, with
integration corresponding to 2H. An accurate determination of the
bulk composition by elemental analysis could not be obtained due to
the presence of impurities.
X-ray Structural Determinations. Diffraction data were collected
at 100(2) K on an Oxford Diffraction Gemini diffractometer fitted
with Mo Kα (λ = 0.71073 Å) radiation. Following analytical
absorption corrections and solution by direct methods, the structures
were refined against F2 with full-matrix least-squares using the program
SHELXL-97.51 All H-atoms were added at calculated positions and
refined by use of a riding model with isotropic displacement
parameters based on those of the parent atoms. Anisotropic
displacement parameters were employed throughout for the non-
hydrogen atoms.
Crystal data for 1: C14H15Cl3N5O3Re; M = 593.86, monoclinic,
space group P21/n, a = 10.8488(5) Å, b = 11.4278(4) Å, c =
16.6365(4) Å, β = 106.769(4)°, V = 1974.85(12) Å3, Z = 4, ρc = 1.997
Mg/m3, μ = 6.583 mm−1. Reflections collected = 64 078, independent
reflections = 10 156 (R(int) = 0.0324), S = 1.176. R1 = 0.0293 (I >
2σ(I)), wR2 = 0.0596 (all data). The tertiary butyl group was found to
be disordered over two sets of sites with occupancies constrained to
0.5 after trial refinement.
Crystal data for 2: C14H14BrCl3N5O3Re; M = 672.76, monoclinic,
space group P21/c, a = 8.6560(2) Å, b = 18.1418(2) Å, c = 13.2050(4)
Å, β = 92.829(3)°, V = 2071.13(8) Å3, Z = 4, ρc = 2.158 Mg/m3, μ =
8.208 mm−1. Reflections collected = 53 110, independent reflections =
13 425 (R(int) = 0.0382), S = 0.921. R1 = 0.0229 (I > 2σ(I)), wR2 =
0.0403 (all data).
Crystal data for 3: C33H35N13O8Re2; M = 1114.14, monoclinic,
space group C2/c, a = 20.5571(5) Å, b = 12.1245(2) Å, c = 16.5533(4)
Å, β = 105.931(3)°, V = 3967.36(15) Å3, Z = 4, ρc = 1.865 Mg/m3, μ =
6.162 mm−1. Reflections collected = 41 968, independent reflections =
5755 (R(int) = 0.0366), S = 0.971. R1 = 0.0180 (I > 2σ(I)), wR2 =
0.0428 (all data). The counterion was modeled as a Et3NH+ cation
with site occupancy constrained to 0.5 after trial refinement and charge
Synthesis of 2. [Re(CO)5Br] (100 mg, 0.25 mmol) and 2-(2-tert-
butyltetrazol-5-yl)pyridine (100 mg, 0.49 mmol) were combined in
toluene (15 mL) and stirred at reflux for 6 h. The mixture was cooled,
and the solvent removed under reduced pressure, yielding a yellow oil.
The oil was dissolved in minimal dichloromethane and purified by
column chromatography using alumina as the stationary phase and
dichloromethane as eluent. The first fraction was collected and
identified as the title compound. Single crystals suitable for X-ray
diffraction were grown by layering hexanes on a dichloromethane
H
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Organometallics
Article
balance considerations. Two peaks in the difference maps were
modeled as water molecules with site occupancies also constrained to
0.5.
Crystal data for 4: C18H16Br2Cl2N4O6Re; M = 987.47, monoclinic,
space group P21/n, a = 10.7387(2) Å, b = 16.7806(2) Å, c =
14.5656(2) Å, β = 104.820(2)°, V = 2537.43(7) Å3, Z = 4, ρc = 2.585
Mg/m3, μ = 12.934 mm−1. Reflections collected = 63 567,
independent reflections = 10 334, (R(int) = 0.0393), S = 1.079. R1
= 0.0209 (I > 2σ(I)), wR2 = 0.0417 (all data).
(16) Wright, P. J.; Muzzioli, S.; Werrett, M. V.; Raiteri, P.; Skelton, B.
W.; Silvester, D. S.; Stagni, S.; Massi, M. Organometallics 2012, 31,
7566−7578.
(17) MaGee, K. D. M.; Wright, P. J.; Muzzioli, S.; Siedlovskas, C. M.;
Raiteri, P.; Baker, M. V.; Brown, D. H.; Stagni, S.; Massi, M. Dalton
Trans. 2013, 42, 4233−4236.
(18) Anderson, C. B.; Elliott, A. B. S.; Mcadam, C. J.; Gordon, K. C.;
Crowley, J. D. Organometallics 2013, 32, 788−797.
(19) Anderson, C. B.; Elliott, A. B. S.; Lewis, J. E. M.; Mcadam, C. J.;
Gordon, K. C.; Crowley, J. D. Dalton Trans. 2012, 41, 14625−14632.
(20) Obata, M.; Kitamura, A.; Mori, A.; Kameyama, C.; Czaplewska,
J. A.; Tanaka, R.; Kinoshita, I.; Kusumoto, T.; Hashimoto, H.; Harada,
M.; Mikata, Y.; Funabiki, T.; Yano, S. Dalton Trans. 2008, 3292−3300.
(21) Boulay, A.; Seridi, A.; Zedde, C.; Ladeira, S.; Picard, C.; Maron,
L.; Benoist, E. Eur. J. Inorg. Chem. 2010, 5058−5062.
(22) Seridi, A.; Wolff, M.; Boulay, A.; Saffon, N.; Coulais, Y.; Picard,
C.; Machura, B.; Benoist, E. Inorg. Chem. Commun. 2011, 14, 238−
242.
ASSOCIATED CONTENT
* Supporting Information
■
S
Bond lengths and angles tables, excitation and emission spectra
at room temperature and 77 K, simulated absorption spectra
and list of calculated transitions, CIF files for the X-ray
diffraction data. Crystallographic data for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre. CCDC 934711 (1), 934712 (2),
934713 (3), and 934714 (4) contain supplementary crystallo-
graphic data for this paper. This material is available free of
charge via the Internet at http://pubs.acs.org.
(23) Wolff, M.; Munoz, L.; Franco̧ is, A.; Carrayon, C.; Seridi, A.;
Saffon, N.; Picard, C.; Machura, B.; Benoist, E. Dalton Trans. 2013, 42,
7019−7031.
(24) Wright, P. J.; Muzzioli, S.; Skelton, B. W.; Raiteri, P.; Lee, J.;
Koutsantonis, G.; Silvester, D. S.; Stagni, S.; Massi, M. Dalton Trans.
2013, 42, 8188−8191.
AUTHOR INFORMATION
Corresponding Author
*E-mail: s.stagni@unibo.it; m.massi@curtin.edu.au.
Notes
■
(25) Koguro, K.; Oga, T.; Mitsui, S.; Orita, R. Synthesis 1998, 910−
914.
(26) Butler, R.; Garvin, V. J. Chem. Soc., Perkin Trans. 1 1981, 390−
393.
́
(27) Butler, R.; Garvin, V.; Lumbroso, H.; Liegeois, C. J. Chem. Soc.,
The authors declare no competing financial interest.
Perkin Trans. 2 1984, 721−725.
(28) Palazzi, A.; Stagni, S.; Bordoni, S.; Monari, M.; Selva, S.
Organometallics 2002, 21, 3774−3781.
ACKNOWLEDGMENTS
■
(29) Palazzi, A.; Stagni, S. J. Organomet. Chem. 2005, 690, 2052−
2061.
The work was supported by the Australian Research Council
(DP0985481 and DP0986999). P.J.W. wishes to thank Curtin
University for the Australian Postgraduate Award. Access to the
facilities at the Centre for Microscopy, Characterisation and
Analysis, University of Western Australia, is kindly acknowl-
edged. P.R. and P.J.W. also acknowledge NCI for the provision
of computer time.
(30) Mattes, R.; Weber, H. J. Organomet. Chem. 1979, 178, 191−196.
(31) Mauro, M.; Panigati, M.; Donghi, D.; Mercandelli, P.; Mussini,
P.; Sironi, A.; D’Alfonso, G. Inorg. Chem. 2008, 47, 11154−11165.
(32) Procopio, E. Q.; Mauro, M.; Panigati, M.; Donghi, D.;
Mercandelli, P.; Sironi, A.; D’Alfonso, G.; De Cola, L. J. Am. Chem.
Soc. 2010, 132, 14397−14399.
(33) Panigati, M.; Mauro, M.; Donghi, D.; Mercandelli, P.; Mussini,
P.; De Cola, L.; D’alfonso, G. Coord. Chem. Rev. 2012, 256, 1621−
1643.
(34) Stagni, S.; Orselli, E.; Palazzi, A.; De Cola, L.; Zacchini, S.;
Femoni, C.; Marcaccio, M.; Paolucci, F.; Zanarini, S. Inorg. Chem.
2007, 46, 9126−9138.
(35) Maspero, A.; Galli, S.; Colombo, V.; Peli, G.; Masciocchi, N.;
Stagni, S.; Barea, E.; Navarro, J. A. R. Inorg. Chim. Acta 2009, 362,
4340−4346.
(36) Ouellette, W.; Liu, H.; O’Connor, C. J.; Zubieta, J. Inorg. Chem.
2009, 48, 4655−4657.
(37) Luo, T.-T.; Tsai, H.-L.; Yang, S.-L.; Liu, Y.-H.; Yadav, R. D.; Su,
C.-C.; Ueng, C.-H.; Lin, L.-G.; Lu, K.-L. Angew. Chem., Int. Ed. 2005,
44, 6063−6067.
(38) Silvester, D. S.; Uprety, S.; Wright, P. J.; Massi, M.; Stagni, S.;
Muzzioli, S. J. Phys. Chem. C 2012, 116, 7327−7333.
(39) Bullock, J. P.; Carter, E.; Johnson, R.; Kennedy, A. T.; Key, S. E.;
Kraft, B. J.; Saxon, D.; Underwood, P. Inorg. Chem. 2008, 47, 7880−
7887.
(40) Kim, T. Y.; Elliott, A. B.; Shaffer, K. J.; McAdam, C. J.; Gordon,
K. C.; Crowley, J. D. Polyhedron 2013, 52, 1391−1398.
(41) Barrosse-Antle, L. E.; Bond, A. M.; Compton, R. G.; O’Mahony,
A. M.; Rogers, E. I.; Silvester, D. S. Chem. Asian J. 2010, 5, 202−230.
(42) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
2999−3094.
REFERENCES
■
(1) Kirgan, R. A.; Sullivan, B. P.; Rillema, D. P. Top. Curr. Chem.
2007, 281, 45−100.
(2) Kumar, A.; Sun, S.; Lees, A. Top. Organomet. Chem. 2010, 29, 1−
35.
(3) Lo, K.; Hui, W.; Chung, C.; Tsang, K.; Ng, D.; Zhu, N.; Cheung,
K. Coord. Chem. Rev. 2005, 249, 1434−1450.
́
(4) Fernandez-Moreira, V.; Thorp-Greenwood, F. L.; Coogan, M. P.
Chem. Commun. 2010, 46, 186−202.
(5) Chou, P.-T.; Chi, Y. Chem. Eur. J. 2007, 13, 380−395.
(6) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer,
T. Coord. Chem. Rev. 2011, 255, 2622−2652.
(7) Donghi, D.; D’Alfonso, G.; Mauro, M.; Panigati, M.; Mercandelli,
P.; Sironi, A.; Mussini, P.; D’Alfonso, L. Inorg. Chem. 2008, 47, 4243−
4255.
(8) Vlce
(9) Stufkens, D.; Vlce
̌ ́
k, A.; Zalis, S. Coord. Chem. Rev. 2007, 251, 258−287.
̌
k, A. Coord. Chem. Rev. 1998, 177, 127−179.
(10) Stufkens, D. J. Comments Inorg. Chem. 1992, 13, 359−385.
(11) Wrighton, M.; Morse, D. J. Am. Chem. Soc. 1974, 96, 998−1003.
̌
(12) Vlcek, A. Top. Organomet. Chem. 2010, 29, 73−114.
(13) Kober, E.; Caspar, J.; Lumpkin, R.; Meyer, T. J. Phys. Chem.
1986, 90, 3722−3734.
(14) Koike, K.; Okoshi, N.; Hori, H.; Takeuchi, K.; Ishitani, O.;
Tsubaki, H.; Clark, I.; George, M.; Johnson, F.; Turner, J. J. Am. Chem.
Soc. 2002, 124, 11448−11455.
(15) Werrett, M. V.; Chartrand, D.; Gale, J. D.; Hanan, G. S.;
MacLellan, J. G.; Massi, M.; Muzzioli, S.; Raiteri, P.; Skelton, B. W.;
Silberstein, M.; Stagni, S. Inorg. Chem. 2011, 50, 1229−1241.
(43) Downard, A.; Steel, P.; Steenwijk, J. Aust. J. Chem. 1995, 48,
1625−1642.
(44) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991−1024.
(45) Eaton, D. Pure Appl. Chem. 1988, 60, 1107−1114.
I
dx.doi.org/10.1021/om400356n | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(46) Silvester, D. S.; Wain, A. J.; Aldous, L.; Hardacre, C.; Compton,
R. G. J. Electroanal. Chem. 2006, 596, 131−140.
(47) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461−466.
(48) Rogers, E. I.; Silvester, D. S.; Poole, D. L.; Aldous, L.; Hardacre,
C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 2729−2735.
(49) Frisch, M. J.; Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone,
V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J.
L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J.
J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.;
Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.;
Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.;
Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A.
̈
D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian09, Revision B.01; Wallingford, CT, 2009.
(50) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123−141.
(51) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112−122.
J
dx.doi.org/10.1021/om400356n | Organometallics XXXX, XXX, XXX−XXX