Synthesis, photophysical and electrochemical investigation of dinuclear tetrazolato-bridged rhenium complexes

Article abstract of DOI:10.1021/om300870a

Starting from anionic tetrazole-based ligands, namely, 5-(4′- cyanophenyl)tetrazolate and 5-(4′-pyridyl)tetrazolate, mononuclear and dinuclear complexes of fac-[Re(CO)₃(phen)]⁺ (phen = 1,10-phenanthroline) were prepared and characterized. For the mononuclear complexes, regioselective coordination of the metal fragments on the negatively charged tetrazolato ring is exclusively obtained. Coordination to the benzonitrile and pyridine groups was achieved by previous alkylation of the tetrazole ring. Dinuclear complexes were obtained by treatment of the corresponding mononuclear tetrazole-bound complexes with fac-[Re(CO) ₃(phen)(THF)]⁺. The second rhenium fragment coordinated either to the pyridine ring or, in the case of the benzonitrile ligand, to the tetrazole ring. The electrochemical properties were probed in an imidazolium ionic liquid, highlighting reduction processes centered on the phen ligand and oxidation processes localized on the metal. The photophysical properties of the complexes are characterized by phosphorescent emission from triplet metal-to-ligand charge transfer excited states, with trends in the lifetime and quantum yield in qualitative agreement with the energy gap law. The two dinuclear complexes show almost superimposable emission profiles: in the 5-(4′-cyanophenyl)tetrazolate-bridged complex, the two metal fragments coordinated to the tetrazole are equivalent and share a positive charge of +1. On the other hand, the photophysical properties of the 5-(4′-pyridyl) tetrazolate-bridged dinuclear complex suggest energy transfer between the two metal centers.

Full text of DOI:10.1021/om300870a

Article
pubs.acs.org/Organometallics
Synthesis, Photophysical and Electrochemical Investigation of
Dinuclear Tetrazolato-Bridged Rhenium Complexes
Phillip J. Wright,† Sara Muzzioli,‡ Melissa V. Werrett,† Paolo Raiteri,† Brian W. Skelton,§
Debbie S. Silvester,† Stefano Stagni,*,‡ and Massimiliano Massi*,†
†Department of Chemistry, Curtin University, Kent Street, Bentley WA 6102, Australia
‡Department of Physical and Inorganic Chemistry, University of Bologna, Viale del Risorgimento, I-40126 Bologna, Italy
§Center for Microscopy, Characterization and Analysis, University of Western Australia, Crawley 6009 WA, Australia
S
* Supporting Information
ABSTRACT: Starting from anionic tetrazole-based ligands, namely, 5-(4′-
cyanophenyl)tetrazolate and 5-(4′-pyridyl)tetrazolate, mononuclear and
dinuclear complexes of fac-[Re(CO)3(phen)]+ (phen = 1,10-phenanthroline)
were prepared and characterized. For the mononuclear complexes,
regioselective coordination of the metal fragments on the negatively charged
tetrazolato ring is exclusively obtained. Coordination to the benzonitrile and
pyridine groups was achieved by previous alkylation of the tetrazole ring.
Dinuclear complexes were obtained by treatment of the corresponding
mononuclear tetrazole-bound complexes with fac-[Re(CO)3(phen)(THF)]+.
The second rhenium fragment coordinated either to the pyridine ring or, in
the case of the benzonitrile ligand, to the tetrazole ring. The electrochemical
properties were probed in an imidazolium ionic liquid, highlighting reduction
processes centered on the phen ligand and oxidation processes localized on
the metal. The photophysical properties of the complexes are characterized by phosphorescent emission from triplet metal-to-
ligand charge transfer excited states, with trends in the lifetime and quantum yield in qualitative agreement with the energy gap
law. The two dinuclear complexes show almost superimposable emission profiles: in the 5-(4′-cyanophenyl)tetrazolate-bridged
complex, the two metal fragments coordinated to the tetrazole are equivalent and share a positive charge of +1. On the other
hand, the photophysical properties of the 5-(4′-pyridyl)tetrazolate-bridged dinuclear complex suggest energy transfer between
the two metal centers.
While a vast number of luminescent Re(I) complexes have
been reported to date, the investigation of monometallic
dinuclear or multinuclear Re(I) complexes seems in compar-
ison more limited.1,7,16−23 Aside from some dinuclear Re
complexes bound to, for example, substituted pyridazine or
4,4′-bipyridine ligands,24−27 the photophysical properties of
multinuclear assemblies of bridged fac-[Re(CO)3(diim)]+ are
sometimes inferior to the corresponding mononuclear
complexes. This trend has been assigned to enhanced
nonradiative pathways favored by vibrational deactivation due
to an increased molecular complexity.1 Studies of multinuclear
complexes where the Re centers are nonequivalent are even
rarer. In some examples, Meyer and co-workers synthesized a
series of dinuclear Re complexes bridged by a variety of 4,4′-
bipyridine ligands.25,28 In those cases, the nonequivalent nature
of the two Re complexes was achieved by the use of differently
substituted 2,2′-bipyridyne ligands bound to the Re centers.
The addition of electron-donating or electron-withdrawing
substituents resulted in the destabilization or stabilization of the
INTRODUCTION
■
The photophysical properties of Re complexes have attracted
considerable interest stemming from their potential application
in a variety of fields, including optical displays, sensors, catalysis,
molecular electronics, and biological labeling.1−9 The most
investigated Re complexes are the Re(I) tricarbonyl type with
the general formulation fac-[Re(CO)3(diim)(L)]0/+, where
diim is a diimine ligand [e.g., 1,10-phenathroline (phen)] and
L a neutral or anionic monodentate ancillary ligand [e.g.,
pyridine (py)].1 The emission of these complexes is ascribed to
the phosphorescent decay from the lowest energy triplet
excited state, which has generally a metal-to-ligand charge
transfer character (3MLCT).10−12 Modulation of the HOMO−
LUMO gap is achieved through chemical changes in either the
diim ligand, thus directly affecting the LUMO orbital, and/or
the ancillary ligand L, with a consequent indirect shift of the
HOMO orbital.1,13,14 Studies on a large library of Re(I)
complexes have shown that the photophysical properties follow
the trend dictated by the energy gap law;15 hence cationic fac-
[Re(CO)3(diim)(L)]+ complexes exhibit blue-shifted emis-
sions, longer excited-state lifetimes (τ), and increased quantum
yields (Φ) with respect to their neutral analogues.1
Received: September 10, 2012
Published: October 29, 2012
© 2012 American Chemical Society
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Figure 1. Synthetic pathways for the preparation of the neutral complexes [Re1] and [Re2].
Figure 2. Synthetic pathways for the preparation of the ligands 3 and 4 and the cationic complexes [Re3]PF6 and [Re4]PF6.
LUMO level, hence increasing or decreasing the relative energy
of the MLCT transition, respectively. By combining cyclic
voltammetry, electronic spectroscopy, and transient detection,
it was demonstrated that in such dinuclear complexes the two
metal centers undergo electronic coupling via energy transfer.
The energy transfer was found to be mediated by electron
transfer, the latter being favored by the presence of a bridging
ligand with an accessible π* delocalized system.28
To the best of our knowledge, no example has been reported
of dinuclear rhenium complexes, bridged by π-conjugated
ligands, where the nonequivalence of the two emissive rhenium
centers originates from the direct modulation of the HOMO
levels. Following our recent work on mononuclear Re(I)
tetrazolato complexes,29,30 we have extended the investigation
to dinuclear Re(I) complexes bridged by the ligands 5-(4′-
cyanophenyl)tetrazolate (1−) and 5-(4′-pyridyl)tetrazolate
(2−). Our focus is to achieve bridged dinuclear Re(I)
complexes where the two metal centers are electronically
nonequivalent, e.g., one neutral complex bound to the anionic
tetrazolato ring and the other cationic complex bound to either
the nitrile or pyridine group. While the dinuclear complex
bridged by 5-(4′-pyridyl)tetrazolate could be obtained, in the
case of 5-(4′-cyanophenyl)tetrazolate the desired complex was
not formed and instead an unprecedented doubly coordinated
tetrazole was obtained. To gain insight into the photophysical
and electrochemical properties of the dinuclear complexes, we
have also attempted to synthesize the corresponding four
mononuclear complexes, where the rhenium fragments are
coordinated to the tetrazole ring in both 5-(4′-cyanophenyl)-
tetrazolate and 5-(4′-pyridyl)tetrazolate, thus forming neutral
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Figure 3. Synthetic pathways for the preparation of the dinuclear complexes [Re21]PF6 and [Re2Re]PF6.
mononuclear complexes, or to the nitrile and pyridine
substituents, thus forming cationic mononuclear complexes.
Supporting Information, Figure S1). The preparation of the
pyridine-coordinated complex [Re4]PF6 was obtained follow-
ing the same procedure, and the complex was isolated as the
−
corresponding PF6 salt. The successful salt metathesis could
RESULTS AND DISCUSSION
■
be monitored by the appearance of a broad band at 828 cm−1 in
Synthesis of the Tetrazole Ligands and the Re
Complexes. The synthesis of the tetrazole-based ligands 1H
and 2H was achieved by 1,3-dipolar cycloaddition of NaN3 to
the corresponding nitrile-containing substrates, according to a
previously reported methodology.31 To prepare the neutral
rhenium complexes [Re1] and [Re2] (Figure 1), the
halogenated precursor fac-[Re(CO)3(phen)Cl]11 was directly
treated with the corresponding tetrazole ligand and triethyl-
amine in refluxing acetonitrile. Compared to previously
published procedures using solvato or triflato rhenium
precursors,30,32−34 this one-step protocol avoids the use of
light-sensitive silver salts to remove the chloro ligand from the
rhenium center and, more importantly, the starting fac-
[Re(CO)3(phen)Cl] complex can be readily recovered in the
purification step and recycled for further syntheses.
the IR spectrum of [Re4]PF6.37
The preparation of the dinuclear cationic complexes was
attempted by treating the solvato complex fac-[Re-
(CO)3(phen)(THF)]+ with either [Re1] or [Re2] in refluxing
THF (Figure 3). In the case of fac-[Re(CO)3(phen)(THF)]+
and [Re1], the targeted dinuclear species bearing the rhenium
centers coordinated to the tetrazole and to the nitrile group was
not obtained, since the IR spectrum displayed the presence of a
nitrile band at 2233 cm−1.36 Nevertheless, the spectroscopic
1
data suggested the presence of two rhenium centers, as the H
NMR spectrum shows two phen systems versus one tetrazole
ligand. The spectroscopic evidence of the dimeric species could
be rationalized by the analysis of the X-ray crystal structure (see
below), where the unexpected formation of a doubly
coordinated tetrazole in [Re21]PF6 was observed, as schema-
tized in Figure 3. The synthesis of [Re2Re]PF6 was achieved
using the same methodology. The structural feature was indeed
demonstrated by the X-ray crystal structure in the solid state
(see below). However, 1H NMR data also proved that the same
structure is retained in solution. The difference between the
coordination modes in [Re21]PF6 and [Re2Re]PF6 might be
ascribed to (i) a significantly lower nucleophilicity of the nitrile
group with respect to the pyridine and (ii) a much weaker π-
accepting nature of the nitrile. These characteristics make the
nitrile a labile ligand for a Re(I) center, and therefore, despite
the high steric hindrance, the second metal center coordinates
to the tetrazole ring in [Re21]PF6. This behavior seems to be
unique for rhenium, as analogous monometallic dinuclear
complexes of iron and ruthenium bridged by the same ligand 1
did not show double coordination at the tetrazole ring.38,39
Interestingly though, when comparing the structure of
[Re21]PF6 with the product obtained by treating fac-[Re-
(CO)3(phen)(THF)]+ with 3, in the former case a double
coordination of the tetrazole ring occurred, whereas in the
latter the rhenium coordinates to the nitrile group. The higher
The synthesis of the cationic mononuclear rhenium
complexes required the preparation of the N2-alkylated version
of the 1H and 2H ligands (Figure 2). The alkylation was
successfully achieved by treating 2H with H2SO4 and
CF3COOH in tert-butanol,35 thus introducing a bulky tert-
butyl substituent on the N2 atom of the tetrazole ring. On the
other hand, when 1H was reacted under the same conditions,
an acid-catalyzed hydrolysis of the nitrile group took place to
yield the corresponding secondary N-tert-butyl amide. We
therefore decided to alkylate the tetrazole ring in 1H with a
benzyl group. The preparation of the corresponding cationic
complexes was attempted in a two-step procedure, involving
the generation of the solvato complex fac-[Re(CO)3(phen)-
(THF)][OSO2CF3] followed by exchange of the labile THF
molecule with the ligand 3 or 4 (Figure 2). In the case of the
ligand 3, the spectroscopic data suggest that the nitrile group
coordinates to the rhenium center, as witnessed by the shift to
higher wavenumbers that is displayed by the CN group upon
coordination, from 2230 to 2263 cm−1.36 However, the
coordination of the nitrile group was found to be rather labile,
and its dissociation was monitored by 1H NMR (see
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nucleophilicity of the coordinated tetrazole in [Re1] might be
ascribed to the presence of π-backbonding from the Re to the
tetrazole, which is absent in the case of a tetrazole ring alkylated
by a tert-butyl group. This argument is formulated on the
assumption that the steric bulkiness of the fac-[Re-
(CO)3(phen)]+ fragment is not significantly lower than the
bulkiness of a tert-butyl substituent; henceforth the difference in
the coordination modes does not have a kinetic origin.
Spectroscopic Characterization. All the complexes show
characteristic carbonyl stretching bands as expected from the
local Cs point group, e.g., the totally symmetric in-phase
stretching A′(1), the totally symmetric out-of-phase stretching
A′(2), and the asymmetric stretching A″.40−43 The values of the
recorded frequencies are summarized in Table 1. The relative
that the species exists in solution as an equilibrium of
“tautomers”, where the rhenium fragment coordinates to the
N1 and N2 atoms of the tetrazole ring as schematized in Figure
4. On the other hand, no equilibrium was present in the NMR
data of [Re2Re]PF6, in support of the coordination of one
rhenium center to the N2 atom of the tetrazole ring with the
second rhenium fragment bound to the pyridine N atom. See
Supporting Information and Figure S2 for the detailed NMR
studies of the two dinuclear complexes.
Structural Characterization. The structures of the
prepared complexes are reported in Figure 5 (see Supporting
Information for selected bond lengths and angles). [Re1]
crystallizes in the orthorhombic space group Pbca, while [Re2]
crystallizes in the monoclinic space group C2/c. In both
complexes the rhenium centers are hexacoordinated with
slightly distorted octahedral geometries. The carbonyl ligands
are orientated in facial configuration, in agreement with the
spectroscopic data. The structures also confirm the coordina-
tion of the Re center to the N2 atom of the tetrazolato ligands
1− and 2−. The crystal packing of [Re2] is such that the phen
ligands are involved in π-stacking between neighboring
complexes related by an inversion center, with a plane-to-
plane distance of 3.473(4) Å.
Table 1. Carbonyl Ligand Stretching Frequencies (cm−1) for
the Synthesized Rhenium Complexes in the Solid State
complex
[Re1]
A′(1)
A′(2)
A″
2017
2020
2030
2027
2025
1912
1896
a
a
a
a
[Re2]
1898
1906
1906
1895
[Re4]PF6
[Re21]PF6
[Re2Re]PF6
The cationic complex [Re4]PF6 crystallizes in the monoclinic
space group P21/n. The coordination environment of the
rhenium center is analogous with the previous Re tetrazolato
complexes. The bond length between the Re and the pyridine
N atom is 2.207(3) Å and slightly elongated with respect to the
Re−N(tetrazole) bonds in complexes [Re1] and [Re2],
2.174(2) and 2.176(2) Å, respectively. The tetrazole and
phenyl rings lie planar with respect to each other, thus favoring
interannular conjugation.
a
The A′(2) and A″ modes are superimposed into a single broad band.
frequencies of the bands, especially in the case of the A′(1), also
reflect the nature of the compounds in terms of electron
density on the rhenium center. In the case of [Re21]PF6 and
[Re2Re]PF6, only one set of bands is visible in the IR spectra,
whose frequency values lie between the ones found for the
mononuclear complexes and the cationic [Re4]PF6 complex.
While it is expected that the dinuclear complex [Re21]PF6
exhibits only one set of carbonyl bands, it is surprising that the
same happens for [Re2Re]PF6 due to the formally non-
equivalent nature of the two metal centers. The result suggests
that in [Re2Re]PF6 the ligand favors the electronic conjugation
between the two metal centers with consequent coalescence of
the carbonyl bands.
The complex [Re21]PF6 crystallizes in the triclinic space
group P1. The two rhenium centers are bridged via the
̅
tetrazole ring, binding to the N1 and N4 atoms, respectively, in
a μ2-η1(N1):η1(N4) fashion. This linkage isomer seems to be
the only one present in the lattice, and no evidence of the μ2-
η1(N1):η1(N3) binding mode is found. The complex possesses
pseudomirror symmetry about the bridging ligand. The
dihedral angles between the plane of the benzonitrile group
and the planes of the two phen groups are 9.4(2)° and 3.1(2)°,
with the dihedral angle between the planes of the two phen
ligands being 8.2(1)°. The dihedral angle between the
benzonitrile group and the tetrazole ring is 81.4(2)°. The
two rings are forced to twist from a coplanar arrangement as an
effect of the steric hindrance caused by the coordination of the
two Re centers to the N1 and N4 atoms of the tetrazole ring.
The Re−N bonds to the tetrazole ring are not colinear, the
structure being bent at the tetrazole group. The angles between
these bonds and those normal to the plane of the tetrazole ring
are 79.5(1)° and 95.7(1)° respectively.
A summary of the NMR data including Hortho, Hmeta, and C5
(see Experimental Section for the assignment of the peaks) is
reported in Table 2. The NMR spectra readily confirm the
Table 2. Selected NMR Data Including Hortho, Hmeta, and C5
for the Synthesized Complexes
complex
[Re1]
Hortho
Hmeta
C5
solvent
7.67
7.42
7.79
7.19
7.27
7.75
8.48
8.45
7.62
8.11
161.1
160.6
d6-DMSO
d6-DMSO
CD3CN
CDCl3
[Re2]
[Re4]PF6
[Re21]PF6
[Re2Re]PF6
161.5
a
160.5
CD3CN
The complex [Re2Re]PF6 crystallizes in the triclinic space
a
The signal could not be observed due to very weak intensity.
group P1. The two rhenium centers are analogous to all the
̅
previously described complexes. The structure confirms the μ2-
η1(N2):η1(Npy) binding mode of the ligand 2−. The py and
tetrazole rings are almost coplanar, with the dihedral angle
between the two planes being 7.3(5)°. Although the Re1 atom
is virtually coplanar with the tetrazole ring (δRe1 0.03(2) Å),
Re2 is considerably out of plane of the py ring (δRe2 0.43(3)
Å). The two phen ligands are not coplanar, the dihedral angle
between the two being 25.2(3)°.
proposed structures of the mononuclear complexes [Re1] and
[Re2] in agreement with the X-ray diffraction data (see below).
In the 13C NMR, the δ value of the tetrazolic C5 atom between
161 and 160 ppm indicates coplanarity between the two rings
in the ligand and hence confirms the regioselective complex-
ation of the tetrazole ring at the N2 position.44−48 The same
argument can be used in the case of [Re4]PF6 to demonstrate
the coordination of the rhenium fragment to the pyridine N
atom. The NMR data for the dinuclear [Re21]PF6 evidenced
Electrochemical Properties. The electrochemical behav-
ior of the complexes was investigated using cyclic voltammetry
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Figure 4. Equilibrium between the N1,N4 and N1,N3 linkage isomers of the dinuclear complex [Re21]PF6.
Figure 5. X-ray crystal structures of the synthesized complexes with thermal ellipsoids drawn at 50% probability: (a) [Re1], (b) [Re2], (c) [Re4]PF6,
(d) [Re21]PF6, (e) [Re2Re]PF6. Solvent molecules and counteranions, when present, have been omitted for clarity.
(CV), and a summary of the data is reported in Table 3. Two
more complexes, fac-[Re(CO)3(phen)Cl] and the previously
reported fac-[Re(CO)3(phen)(TzPh)]30 (where TzPh = 5-
phenyltetrazolate), were investigated to analyze the electro-
chemical trend on changing the nature of the ancillary ligand in
the rhenium complexes. We have already established the
advantages of performing these types of measurements in room
temperature ionic liquids (RTILs) instead of common organic
solvents.29 In this work, the solvent chosen was the
hydrophobic RTIL [C6mim][FAP], namely, 1-hexyl-3-methyl-
imidazolium tris(pentafluoroethyl)trifluorophosphate.49
The voltammetric curves of all the mononuclear complexes
are reported in the Supporting Information (see Figures S3−5).
In all the neutral mononuclear complexes the reduction peak
(R1/R1′) around −1.62 to −1.60 V is assigned to phen-
centered one-electron transfer.11 Two oxidation processes
(O1/O1′ and O2/O2′) are observed for fac-[Re(CO)3(phen)-
Cl], fac-[Re(CO)3(phen)(TzPh)], and [Re1], and they are
assigned to oxidations occurring mainly on the rhenium center
(e.g., Re(I) → Re(II) + e− and Re(II) → Re(III) + e−) with the
involvement of the tetrazolato ligand (aside from the chloro
complex).49 The trend in the oxidation potential values of O1/
O1′ is in agreement with the relative electron density on the
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irreversibility due to the smaller sized reverse peak, suggesting
some degree of follow-up chemistry, but this appears to be
relatively slow on the voltammetric time scale.
Table 3. Half-Wave Potentials (E1/2) for the Prepared Mono-
and Dinuclear Rhenium Complexes
E1/2(O1/O1′)
E1/2(O2/O2′) E1/2(R1/R1′) [V]
vs (Fc/Fc+)
[V] vs (Fc/Fc+) [V] vs (Fc/Fc+)
Figure 6 shows the cyclic voltammograms for the dinuclear
complexes [Re21]PF6 and [Re2Re]PF6. A two-electron R1/R1′
reduction process appears for [Re21]PF6 at −1.60 V and is
again assigned to the reduction of both phen ligands. The R1/
R1′ process of [Re1] and [Re21]PF6 appear to be unchanged in
terms of electrochemical reversibility. The oxidative region of
[Re21]PF6 appears more complicated, with at least four distinct
oxidative processes in the region 0.95−1.47 V. We have
tentatively assigned the four peaks at higher potentials to the
first and second metal oxidations for both of the two separate
metal centers. The other peaks present in the voltammogram of
[Re21]PF6 (from −0.2 to 0.8 V) may be a result of the fact that
this compound exists as an equilibrium between two isomers,
and there are oxidation steps associated with both isomers,
potentially a result of their interaction and oxidation at the Au
electrode surface. Remarkably, the first oxidation potential of
[Re21]PF6 occurs at 0.95 V despite the positively charged
nature of the complex. We have ascribed the low potential to
the fact that the tetrazole ring is unable to donate electronic
charge to the benzonitrile ring as a result of the almost
perpendicular arrangement of the two rings. The tetrazole ring
has therefore an increased electron density that facilitates the
oxidation of the complex.
In the case of [Re2Re]PF6, two closely lying one-electron
reduction processes centered on the phen ligands appear at
−1.50 and −1.66 V. Despite their overlapping nature, both of
these peaks show some degree of chemical reversibility. The
oxidation region shows two processes, both of which are
ascribed to a one-electron O1/O1′ process occurring on each
metal center. This conclusion was obtained by comparing the
data with the voltammograms of [Re2] and [Re4]PF6, both of
which show only one oxidation occurring within the available
potential window (see SI Figures 3 and 4). The peak-to-peak
separations of the O1/O1′ redox couple are 68 and 199 mV,
respectively, suggesting fast kinetics for the first metal oxidation
and intermediate kinetics for the second metal oxidation. Also
noteworthy is that the oxidation of the Re(−py) in [Re2Re]PF6
occurs at a lower potential with respect to the oxidation of
complex
fac-
0.96
1.12
1.56
1.45
1.52
−1.62
[Re(CO)3(phen)
Cl]
fac-
−1.61
[Re(CO) (phen)
(TzPh)]
a3
[Re1]
1.14
−1.62
−1.60
−1.49
−1.60
[Re2]
1.22
[Re4]PF6
[Re21]PF6
[Re2Re]PF6
1.66
0.95, 1.13
1.18, 1.51
1.29, 1.47
b
b
−1.50 , −1.66
a
The synthesis and photophysical properties of this complex are
described elsewhere.30 The value corresponds to the position of the
peaks instead of the half-wave potential, as the back peaks cannot be
precisely located.
b
rhenium center, which is modulated by variation of the π-
donating or π-accepting nature of the ancillary ligand. Only one
oxidation process is observed for [Re2] within the available
potential window. The same trend is also observed for the O1/
O1′ process on [Re4]PF6 that occurs at E1/2 = 1.66 V, in
agreement with the positive charge of the complex and hence
the reduced electron density on the metal center. Both the
reduction and oxidation processes appear to be diffusion
controlled, since a plot of peak current against the square root
of scan rate was linear over a range of 5 to 1000 mV s−1 (plots
not shown). The peak-to-peak separations of R1/R1′ and O1/
O1′ are typically 80−100 mV, consistent with a kinetically fast
electrochemical step. This separation is comparable to the
separation of the Fc/Fc+ redox couple (90 mV), which is
known to have relatively fast kinetics in RTILs, but larger than
the 60 mV expected for fast, one-electron processes in
traditional solvents,50 due to different contributions from
double-layer effects in ionic liquids.51,52 The second metal-
centered oxidation O2/O2′ has quasi-reversible kinetics with a
peak-to-peak separation of ∼160−250 mV. The redox couple
R1/R1′ appears to be fully chemically reversible in [C6mim]-
[FAP], but O1/O1′ shows some degree of chemical
Figure 6. Voltammetric curves of [Re21]PF6 (left) and [Re2Re]PF6 (right) on a 11.8 μm radius Au microdisk electrode in [C6mim][FAP] at a scan
rate of 100 mV s−1.
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Table 4. Photophysical Data for the Prepared Rhenium Complexes from Diluted (ca. 10−5 M) Dichloromethane Solutions
absorption
λ [nm] (104ε) [M−1 cm−1
emission 298 K
τ [μs] dear
0.470
emission 77 K
complex
[Re1]
]
λ [nm]
τ [μs] air
Φ air
Φ dear
λ [nm]
τ [μs]
275(4.17)
359(0.44)
400(0.28)
266(5.38)
355(0.63)
395(0.44)
257(35.4)
337(9.54)
385(3.74)
260(35.00)
353(5.76)
260(16.00)
328(7.05)
378(2.41)
584
0.360
0.039
0.041
518
520
504
3.90(21%)
8.70(79%)
[Re2]
580
540
0.390
1.496
0.730
2.866
0.032
0.203
0.063
0.490
10.85
7.34
[Re4]PF6
[Re21]PF6
558
557
0.898
0.712
2.325
1.217
0.068
0.084
0.203
0.246
518
520
8.38(86%)
2.47(14%)
7.83
[Re2Re]PF6
Re(−py) in [Re4]PF6 (E1/2 = 1.51 and 1.66 V, respectively).
This shift could be ascribed to the presence of a π-donating Re
center bound to the tetrazole ring in [Re2Re]PF6 instead of an
alkyl substituent in [Re4]PF6.
the charge on each rhenium can be considered as approximately
equal to +1/2. The absorption spectrum of the dinuclear
complex [Re21]PF6 shows a higher energy IL transition and a
lower energy MLCT transition.
The absorption profile for the [Re2Re]PF6 complex shows
higher energy IL transitions, and similarly to the spectrum of
[Re4]PF6, two bands tail off in the 300−400 nm region. The
lower energy band is ascribed to mixed MLCT transitions
occurring in the two nonequivalent metal centers (of the type
Re→phen), whereas the higher energy band is attributed to the
Re→py charge transfer.
Photophysical Properties. A summary of the photo-
physical data is provided in Table 4, and all the individual
absorption, excitation, and emission profiles recorded from
diluted dichloromethane solutions (ca. 10−5 M) are available in
the Supporting Information (see Figures SI6−10). The steady-
state absorption profiles are similar for all the prepared
complexes, with differences arising from the specific nature of
each complex (Figure 7). In general, the absorption profiles
show a higher energy intense band in the 250−300 nm region
tailing off into a lower energy band in the 300−450 nm region.
The higher energy band is attributed to intraligand (IL) π→π*
transitions localized on the phen and tetrazole ligands.11 On the
other hand, the lower energy band is attributed to the spin-
allowed metal-to-ligand charge transfer involving the rhenium
center and the coordinated aromatic ligands with accessible π*
orbitals.11
The absorption profiles for [Re1] and [Re2] are virtually
identical, showing typical structureless charge transfer (CT)
bands. In this case, the transition is rationalized as an admixture
of metal-to-ligand and ligand-to-ligand CT, the latter involving
the promotion of an electron from the tetrazole ring to the π*
orbitals of the phen ligand. This effect is analogous to
previously studied rhenium tetrazolato complexes, and the
transition is better described as a metal-ligand-to-ligand charge
transfer (MLLCT).30,53
The absorption spectrum of the cationic complex [Re4]PF6
shows an intense IL transition at 257 nm, followed by two
shoulders in the 300−400 nm region. The presence of these
two bands was ascribed to two distinct MLCT processes. The
lower energy band is associated with the same MLCT Re→
phen occurring in the neutral complexes [Re1] and [Re2]. The
shoulder at higher energy is instead associated with an MLCT
process where one electron is promoted from the Re-centered
orbitals to the π* system of the py, which is lowered in energy
due to the coordination of the positively charged metal center
to the N atom.
Following excitation to the singlet manifolds and internal
conversion to the lowest excited state, the corresponding triplet
manifold is rapidly populated via intersystem crossing. The
3
radiative decay from these M(L)LCT states produces broad
and structureless emissions in all cases (Figure 8).10 All the
complexes exhibit a monoexponential decay in solution at room
temperature. The τ and Φ values in air-equilibrated and
deareated solutions demonstrate that dissolved O2 causes
quenching of the emissive excited states, in agreement with the
phosphorescent nature of the emission from the triplet
M(L)LCT excited state to the singlet ground state. The
trend in the values of λem is generally in agreement with the
electronic nature of the specific rhenium complex. [Re1] and
[Re2] have almost identical λem that are red-shifted by about 40
nm with respect to the emission profile of [Re4]PF6. The blue-
shift of [Re4]PF6 can be readily explained by the wider
HOMO−LUMO gap in the positively charged complex.30 For
the mononuclear [Re1] and [Re2], the τ and Φ values are
similar to the previously reported rhenium tetrazolato
complexes.30 On the other hand, the cationic complex
[Re4]PF6 exhibits increased τ and Φ with respect to the
neutral complexes: this trend can be explained by recalling the
energy gap law.15
Remarkably, [Re21]PF6 and [Re2Re]PF6 have almost
superimposable emission profiles, with values of λem in between
the blue-shifted cationic [Re4]PF6 and the red-shifted neutral
[Re1] and [Re2] complexes. The averaged λem of [Re21]PF6
can be readily interpreted following the assumption that the
two metals possess an approximate charge of +1/2; therefore
the HOMO−LUMO gap progressively widens in the order
[Re1] ≈ [Re2] < [Re21]PF6 < [Re4]PF6. The same trend is
observed in the values of τ and Φ for [Re21]PF6, again in
qualitative agreement with the energy gap law.15 The emission
In [Re21]PF6 the two metal centers are positioned in
equivalent (N1,N4 isomer) or pseudoequivalent (N1,N3
isomer) positions, therefore sharing the overall positive charge
of the complex. The direct consequence is that in [Re21]PF6
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Figure 8. Normalized emission profiles for the prepared complexes:
(1) [Re1]; (2) [Re2]; (3) [Re4]PF6; (4) [Re21]PF6; (5) [Re2Re]PF6.
localized on one of the two rhenium centers. The nature of the
emissive state is again described as a 3M(L)LCT, which can be
populated either by direct excitation or efficient energy transfer
3
from the M(L)LCT localized on the other rhenium center.
The occurrence of efficient energy transfer is supported by the
wavelength-independent nature of the emission.25,28 By direct
comparison with [Re2] and [Re4]PF6, the lowest excited state
would seem to be located on the Re center coordinated to the
tetrazole ring. However, it cannot be excluded that the
coordination of a rhenium center on the tetrazole ring, as
compared to an alkyl group in [Re4]PF6, favors a partial
redistribution of charge, leading to a shortening of the
HOMO−LUMO gap for the Re coordinated to the py ring.
For all the complexes, the emission maxima at 77 K in a
frozen matrix appear blue-shifted with respect to the values
obtained at room temperature. The behavior can be explained
with the rigidochromic effect, which is responsible for raising
3
the energy of the emissive M(L)LCT due to the lack of
solvent reorganization following excitation.54 The emissive
bands also show the appearance of partial vibronic coupling
3
features, indicating some degree of mixing between IL and
3M(L)LCT states.1 The excited-state lifetime values are
significantly elongated in the frozen matrix at 77 K due to
suppression of vibrational deactivation to the ground state.
Computational Calculations. To validate the interpreta-
tion of the photophysical data, the energetics and absorption
spectra of the complexes were simulated with time-dependent
density functional theory (TDDFT), using Gaussian 09.55
While the transitions below 300 nm are well understood and
assigned to IL transitions, TDDFT calculations can be
effectively used to help the interpretation of the M(L)LCT
transitions. The overall appearance of the simulated spectra and
orbital localization (see Supporting Information Figures S11−
19) is generally in good agreement with the experimental
results, although a quantitative analysis reveals differences in the
precise position of the peaks relative to the experimental data.
This is especially noted in the [Re2] complex, for which some
of the CT transitions seem to be red-shifted compared to the
experimental results. These inconsistencies were explained by
taking into account the limitations in the exchange−correlation
description as well as the approximate nature of the implicit
solvent model,56 which is used to describe the molecules of
dichloromethane surrounding the complexes.
Figure 7. Absorption plots of the synthesized complexes: (a) [Re1]
(red) and [Re2] (black); (b) [Re4]PF6; (c) [Re21]PF6; (d)
[Re2Re]PF6.
profile of [Re2Re]PF6 is independent from the excitation
wavelength within the range 250−400 nm and is characterized
by a monoexponential decay, suggesting that the radiative decay
in the complex likely originates from a unique excited state
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For the neutral complexes [Re1] and [Re2], the transitions
occurring in the region above 300 nm mainly originate from
charge transfer processes involving the various frontier orbitals
from HOMO−1 to LUMO+2. An analysis of the major
contributors to these bands reveals that the transitions originate
from the promotion of an electron from metal-based 5d
orbitals, partially mixed with the π system of the tetrazole ring,
to the π* system of either the phen or the aryl portion of the
ligands 1− (benzonitrile) and 2− (py), respectively. These
results are in agreement with the MLLCT nature attributed to
these transitions. In any case, the lowest excited state is
obtained via the MLLCT(Re→phen) transitions.
The predicted spectrum of [Re4]PF6 highlights two distinct
bands above 300 nm, which is in agreement with the
experimental data. The lower energy band originates from the
HOMO→LUMO transition; hence it confirms its assignment
as a MLCT(Re→phen) process contributing to the population
of the emissive excited state. On the other hand, the higher
energy band is associated with the HOMO→LUMO+2
transition, thus corresponding to the other MLCT(Re→py)
that is now lowered in energy with respect to [Re2], resulting
from the coordination of the py ring to the positively charged
metal complex.
The predicted spectra of the N1,N4 and N1,N3 linkage
isomers of [Re21]PF6 show superimposable profiles, indicating
that the nature and relative energies of the electronic transitions
are unaffected by the positions of the two metal centers on the
tetrazole ring. In agreement with the photophysical inter-
pretation, the orbitals involved in the electronic transitions in
the region 340−400 nm range from the HOMO−3 to the
LUMO+3. The HOMO−n (n = 3−0) orbitals are formed by a
combination of the 5d orbitals on the metal centers with a
minor contribution of the π system of the tetrazole, whereas the
LUMO+m (m = 0−3) are mainly localized on the π* system of
the phen ligands. The results support the interpretation of the
absorption band as an admixture of MLLCT transitions
involving the two complexes.
The spectrum of the dinuclear [Re2Re]PF6 has similar
features to the absorption profile of [Re4]PF6, with two close
lying bands in the charge transfer region between 300 and 400
nm. An analysis of the calculated electronic transitions reveals
that the low-energy band is an admixture of M(L)LCT
transitions involving the 5d orbitals of the rhenium centers, the
tetrazole π system, and the phen π* system. Mixed with these
transitions, the theory predicts the contribution from remote
M(L)LCT, where one electron is promoted from the 5d
orbitals on the Re center to the π* orbitals of the phen ligand
of the second bridged complex. These kinds of transitions were
previously reported for dinuclear and trinuclear Ru complexes
bridged by cyano ligands.57 Noteworthy, the lowest transition
with appreciable value of oscillator strength seems to be a
mixture of MLCT occurring on the Re complex coordinated to
the py ring (35%) and a remote MLLCT originating from the
Re complex coordinated to the tetrazole ring (62%). The
transitions HOMO→LUMO+4 and HOMO−2→LUMO+4
are the main contributors to the higher energy band, and these
are attributed to a remote MLLCT [Re(tetrazole)→py] and a
MLCT (Re→py) transfer.
dinuclear complexes. In the case of the mononuclear
complexes, the coordination of the rhenium fragments occurs
preferentially on the anionic tetrazolato ligands or, after
alkylation of the tetrazole ring, on the neutral pyridine or
benzonitrile substituents. The dinuclear complexes were
obtained by direct coordination of the rhenium fragments to
the preformed tetrazolato-bound mononuclear complexes. The
structural determinations revealed, for the mononuclear
complexes, the expected regioselectivity for the coordination
of the metal fragments to the N2 atom of the tetrazole. The
coordination of the metal fragment to the nitrile group of
ligand 4 yielded a labile compound characterized by solvolysis
of the Re−NCR bond in polar solvents. On the other hand, an
unprecedented double coordination of the tetrazole ring in
ligand 1 was obtained, and the resulting dinuclear complex was
found to exist as an equilibrium of two linkage isomers in
solution. The electrochemical behavior evidenced reduction
processes centered on the phen ligand, and oxidation processes
centered on the rhenium cations with the participation of the
tetrazole ring for the complexes [Re1], [Re2], and [Re21]PF6.
The oxidation and reduction potentials are in general good
agreement with the variations in the electron density on the
metal centers; however the voltammogram of [Re21]PF6
appears rather complicated, which is likely to be due to the
equilibrating behavior of the complex. The complexes are
characterized by phosphorescent emission from their respective
3M(L)LCT excited states. The photophysical properties of
[Re1], [Re2], [Re4]PF6, and [Re21]PF6 in terms of emission
maxima, excited-state lifetime, and quantum yield showed
trends in qualitative agreement with the energy gap law. On the
other hand, the emission of the dinuclear complex [Re2Re]PF6
seems to originate from a unique excited state that is populated
by efficient energy transfer between the two metal centers.
Computational calculations corroborate the interpretation of
the photophysical data.
EXPERIMENTAL SECTION
■
General Remarks. All reagents and solvents were purchased from
Sigma Aldrich and used as received without further purification. The
room temperature ionic liquid 1-hexyl-3-methylimidazolium tris-
(pentafluoroethyl)trifluorophosphate ([C6mim][FAP]) was purchased
from Merck and used as received. THF was distilled over sodium/
benzophenone according to general laboratory procedures. All
procedures involving rhenium complexes were carried out under a
nitrogen atmosphere using standard Schlenk techniques. Both the
acidic and basic alumina for column chromatography were of
Brockmann I activity unless otherwise specified. Alumina of
Brockmann II activity was prepared by adding water to Brockmann
I alumina at a ratio of 3% w/w, shaking until clumping stopped, and
leaving in a sealed container for two days. 1H,31 2H,31 4,35 and fac-
[Re(CO)3(phen)Cl]11 were prepared according to previously
published procedures. Nuclear magnetic resonance spectra, consisting
of 1H and 13C, were 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. The assignments of
the peaks for the phen and the various tetrazole groups are based on
the referencing scheme shown below.
CONCLUSIONS
■
The coordination of rhenium fragments of the type fac-
[Re(CO)3(phen)]+ to 5-(4′-pyridyl)tetrazolate and 5-(4′-
cyanophenyl)tetrazolate yielded mononuclear and bridged
Infrared spectra were recorded in the solid state, using an attenuated
total reflectance Perkin-Elmer Spectrum 100 FT-IR with a diamond
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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), with broad (br) bands also specified. Melting points were
determined using a BI Barnsted Electrothermal 9100 apparatus and are
reported uncorrected. 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 (185−850 nm). 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,58 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:
apparatus along with the ionic liquid containing the rhenium complex.
Cyclic voltammetry experiments were performed using a
PGSTAT302N potentiostat (Eco-Chemie, The Netherlands) inter-
faced 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 recommendations61 and established in
ionic liquids.51,62 Scans were performed using a potential window
within the range of −1.7 to 2.5 V reliant on the compound
investigated. Reported oxidation and reduction potentials were
obtained from a scan rate of 100 mV s−1.
Computational Calculations. Time-dependent density functional
theory calculations were performed with GAUSSIAN 0955 in order to
calculate the absorption spectra of all compounds. Prior to these
calculations the structures were relaxed at the 6-311G** level of
theory. Re atoms were treated with the Stuttgart−Dresden (SDD)
effective core potential,63 and the effect of the solvent was mimicked
with the PCM solvation model.56 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 the calculated intensities and a variance of
11 nm.
2
⎡
⎤
⎥
⎦
⎡
⎤⎡
⎤
⎡
⎤
Ar(λr) Ir(λr) nx
D
D
r
Synthesis of 3. Potassium carbonate (0.13 g, 1.0 mmol), benzyl
bromide (0.20 g, 1.2 mmol), and 1H (0.17 g, 1.0 mmol) were
combined in acetone (20 mL) and stirred at reflux for 24 h. The
reaction was cooled, and the solvent was reduced; the resulting solid
was dissolved in dichloromethane and extracted with a 5% sodium
carbonate solution, water, and brine (each for 20 mL). The organic
phase was collected and dried over magnesium sulfate, and the solvent
was removed under vacuum. The resulting solid was recrystallized
from hexanes, yielding a white solid. Yield: 0.145 g (56%); mp 112 °C
x
⎢
Φ = Φ⎢
⎥⎢
A (λ ) I (λ )
⎥
⎢
⎣
⎥
⎦
x
r
2
⎣
⎦⎣
⎦ n
⎣
x
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)3Cl2 used as reference (Φr =
0.028).59 Emission lifetimes (τ) were determined with the single
photon counting technique (TCSPC) with the same Edinburgh
FLSP920 spectrometer using pulsed picosecond LEDs (EPLED 295 or
EPLED 360, fhwm <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
quartz tubes (2 mm diameter) and inserted in a special quartz dewar
filled with liquid nitrogen. The solvent (dichloromethane) used in the
preparation of the solutions for the photophysical investigations was of
spectrometric grade. All the prepared solutions were filtered through a
0.2 μm syringe filter before measurement. Degassed solutions were
prepared by the freeze−pump−thaw technique. Experimental
uncertainties are estimated to be 8% for lifetime determinations,
20% for quantum yields, and 2 and 5 nm for absorption and
emission peaks, respectively.
Electrochemical Measurements. A gold microelectrode (made
in-house and kindly donated by the group of Prof. Richard Compton
at Oxford University, UK) was polished and modified with a 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.60 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) 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 micropipet. 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
1
3
(dec). H NMR (CDCl3): δ 8.25 (d, J(H,H) = 8.4 Hz, 2H; Hmeta),
3
7.75 (d, J(H,H) = 8.4 Hz, 2H; Hortho), 7.41−7.26 (m, 5H; Ph-H),
5.82 (s, 2H; CH2) ppm. 13C NMR (CDCl3): δ 164.0 (C5), 133.0,
132.8, 131.7, 129.3, 129.2, 128.6, 126.5, 118.5, 114.0 ppm. IR: νmax
2996 w, 2230 s (CN), 1496 m, 1464 s, 1456 s, 1438 m, 1423 m, 1340
m, 1202 m, 1043 m, 856 m cm−1. Anal. Calcd (%) for 3: C 68.95, H
4.24, N 26.80. Found: C 69.05, H 4.01, N 26.71.
Synthesis of [Re1]. 1H (0.07 g, 0.4 mmol) was combined with
triethylamine (55 μL, 0.4 mmol) in 10 mL of acetonitrile, and fac-
[Re(CO)3(phen)Cl] (0.10 g, 0.2 mmol) was added to the solution.
The solution was stirred at reflux for 48 h, then allowed to cool to
room temperature. The solvent was removed and the solid was
purified via column chromatography with acidic alumina as the
stationary phase using a dichloromethane/ethyl acetate (4:1) solvent
system as the eluent to elute the first fraction and a toluene/ethanol
(5:1) mixture to elute the second fraction. The first fraction was
collected and identified as unreacted starting material, the second
fraction was collected, and the solvent was removed to yield a bright
yellow solid. Single crystals suitable for X-ray diffraction were grown
from a slowly evaporating solution of [Re1] in chloroform. Yield: 64
mg (51%); mp 258 °C (dec). 1H NMR (d6-DMSO): δ 9.57 (d,
3
3J(H,H) = 5.2 Hz, 2H; H2 H9), 8.99 (d, J(H,H) = 8.2 Hz, 2H; H4
H7), 8.31 (s, 2H; H5 H6), 8.15−8.11 (m, 2H; H3 H8), 7.75 (d,
3J(H,H) = 8.8 Hz, 2H; Hmeta), 7.67 (d, 3J(H,H) = 8.8 Hz, 2H; Hortho
)
ppm;. 13C NMR (d6-DMSO): δ 195.9 (CO), 193.1 (CO), 161.1 (C5),
154.4, 146.4, 139.8, 133.3, 132.8, 130.3, 127.8, 126.8, 126.0, 118.6
ppm. IR: νmax 2923 w, 2228 m (CN), 2017 s (CO A′(1)), 1912 s (CO
A′(2)), 1896 s (CO A″), 1615 w, 1521 w, 1430 w, 1417 w, 1248 w,
1042 w, 846
m . Anal. Calcd (%) for [Re1]·(-
cm−1
C6H5Me)1/4(EtOH)1/4: C 46.29, H 2.38, N 14.97. Found: C 46.29,
H 2.27, N 14.76.
Synthesis of [Re2]. 2H (0.09 g, 0.6 mmol) was combined with
triethylamine (83 μL, 0.6 mmol) in 10 mL of acetonitrile, and fac-
[Re(CO)3(phen)Cl] (0.15 g, 0.3 mmol) was added to the solution
and stirred at reflux for 48 h. The solution was then allowed to cool to
room temperature, and the solvent was evaporated under reduced
pressure. The solid was dissolved in a minimal amount of
dichloromethane and purified by column chromatography using
basic alumina as the stationary phase and a mixture of dichloro-
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methane/ethyl acetate (4:1) as the eluent. The first fraction was
collected and identified as unreacted fac-[Re(CO)3(phen)Cl]. The
second fraction was collected, and the solvent was removed to give a
yellow solid. Single crystals suitable for X-ray diffraction were grown
from a slowly evaporating solution of [Re2] in chloroform. Yield: 105
mg (59%); mp 270 °C (dec). 1H NMR (d6-DMSO): δ 9.57 (d,
Synthesis of [Re2Re]PF6. fac-[Re(CO)3(phen)Cl] (0.04 g, 0.1
mmol) and AgSO3CF3 (0.02 g, 0.1 mmol) were combined in dry THF
(5 mL), while shielded from light and under a nitrogen atmosphere,
and heated at reflux for 4 h. To this mixture was added [Re2] (0.05 g,
0.1 mmol), and the mixture was stirred at reflux for 24 h. The reaction
mixture was cooled to room temperature and filtered through Celite.
The solution was reduced in volume, and 1 mL of methanol was
added. This mixture was added to 2 mL of methanol containing 0.3 g
of NH4PF6. The solid formed was purified by column chromatography
using Brockmann II basic alumina as the stationary phase. A mixture of
dichloromethane/ethyl acetate (4:1) was used to elute any unreacted
fac-Re(CO)3(phen)Cl as the first fraction. A mixture of ethyl acetate/
acetonitrile (1:1) was used to elute the second fraction, which was
collected, the solvent volume was reduced, and diethyl ether was
added to precipitate the yellow solid. Single crystals suitable for X-ray
diffraction were grown from layering hexanes on a dichloromethane
3
3J(H,H) = 5.3 Hz, 2H; H2 H9), 9.00 (d, J(H,H) = 8.4 Hz, 2H; H4
3
H7), 8.48 (d, J(H,H) = 6.4 Hz, 2H; Hmeta), 8.31 (s, 2H; H5 H6),
3
8.11−8.14 (m, 2H; H3 H8), 7.42 (d, J(H,H) = 6.4 Hz, 2H; Hortho
)
ppm. 13C NMR (d6-DMSO): δ 196.3 (CO), 193.4 (CO), 160.6 (C5),
154.3, 150.2, 146.5, 139.9, 136.1, 130.3, 127.8, 126.8, 119.7 ppm. IR:
νmax 2853 w, 2020 s (CO A′(1)), 1898 s (CO A′(2) and A″), 1609 m,
1520 m, 1429 m, 1415 m, 1039 w, 849 m cm−1. Anal. Calcd (%) for
[Re2]·(CH3CO2Et)1/3: C 42.85, H 2.36, N 15.89. Found: C 42.98, H
2.00, N 15.91.
Synthesis of [Re4]PF6. fac-[Re(CO)3(phen)Cl] (0.10 g, 0.2
mmol) and AgSO3CF3 (0.05 g, 0.2 mmol) were combined in dry THF
(5 mL), while shielded from light and under a nitrogen atmosphere,
and heated at reflux for 4 h. To this mixture was added 3 (0.04 g, 0.2
mmol), and the mixture was stirred at reflux for 24 h. The reaction
mixture was cooled and filtered through Celite, the solution was
reduced in volume, and 1 mL of methanol was added. This mixture
was added to 2 mL of methanol containing 0.3 g of NH4PF6. The solid
formed was filtered and redissolved in dichloromethane and purified
by column chromatography using acidic alumina as the stationary
phase and a mixture of dichloromethane/ethyl acetate (4:1) as the
eluent. The first fraction was collected and identified as unreacted fac-
[Re(CO)3(phen)Cl]. The second fraction was collected, the solvent
volume was reduced, and diethyl ether was added to precipitate the
yellow solid. Single crystals suitable for X-ray diffraction were grown
from layering hexanes on a dichloromethane solution of [Re4]PF6.
Yield: 32 mg (20%); mp 165 °C (dec). 1H NMR (CD3CN): δ 9.66 (d,
1
solution of [Re2Re]PF6. Yield: 20 mg (20%); mp 212 °C (dec). H
3
NMR (CD3CN): δ 9.54 (d, J(H,H) = 5.2 Hz, 2H; H2 H9), 9.41 (d,
3
3J(H,H) = 5.2 Hz, 2H; H2 H9), 8.79 (d, J(H,H) = 8.4 Hz, 2H; H4
3
H7), 8.70 (d, J(H,H) = 8.2 Hz, 2H; H4 H7), 8.10 (s, 2H; H5 H6),
8.14−8.06 (m, 8H; Hmeta, (H3 H8) (H5 H6 × 2)), 8.08 (s, 2H; H5
3
H6), 7.92−7.88 (m, 2H; H3 H8), 7.27 (d, J(H,H) = 5.4 Hz, 2H;
Hortho) ppm. 13C NMR (CD3CN): δ 197.8 (CO), 196.6 (CO), 160.5
(C5), 155.3, 155.0, 153.2, 184.1, 147.5, 141.2, 140.8, 140.4, 132.2,
131.6, 129.0, 128.6, 128.0, 127.2, 123.1 ppm. IR: νmax 3096 w, 2025 s
(CO A′(1)), 1895 s (CO A′(2) and A″), 1622 w, 1520 w, 1429 m,
1224 w, 1148 w, 1110 w, 1042 w, 1012 w, 835 w cm−1. Anal. Calcd
(%) for [Re2Re]PF6: C 36.27, H 1.69, N 10.58. Found: C 36.48, H
1.45, N 10.42.
X-ray Crystallography. Crystallographic data for the structures
were collected at 100(2) K on an Oxford Diffraction Gemini
([Re4]PF6, [Re21]PF6, [Re2Re]PF6) or Oxford Diffraction Xcalibur
diffractometer ([Re1] and [Re2]) fitted with Mo Kα radiation (λ =
0.71073 Å). Following analytical absorption corrections and solution
by direct methods, the structures were refined against F2 with full-
matrix least-squares using the program SHELXL-97.64 All hydrogen
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. The crystal structures are depicted in Figure 5, where
displacement ellipsoids are drawn at the 50% confident level and with
hydrogen atoms omitted.
3
3J(H,H) = 5.2 Hz, 2H; H2 H9), 8.89 (d, J(H,H) = 8.4 Hz, 2H; H4
H7), 8.45 (d, 3J(H,H) = 5.4 Hz, 2H; Hmeta), 8.20 (s, 2H; H5 H6), 7.79
(d, 3J(H,H) = 5.4 Hz, 2H; Hortho), 1.68 (s, 9H; CH3) ppm. 13C NMR
(CD3CN): δ 196.8 (CO), 192.4 (CO), 161.5 (C5), 155.6, 154.0,
147.5, 141.4, 139.1, 132.3, 129.1, 128.1, 128.9, 66.0 (C(CH3)3), 29.3
(CH3) ppm. IR: νmax 2924 w, 2853 w, 2030 s (CO A′(1)), 1906 s (CO
A′(2) and A″), 1708 w, 1627 w, 1522 w, 1460 w, 1431 m, 1222 w,
1046 w, 1028 w, 1212 w, 828 m cm−1. Anal. Calcd (%) for [Re4]PF6:
C 37.60, H 2.65, N 12.28. Found: C 37.77, H 2.48, N 12.11.
Crystal Data and Structure Refinement for [Re1]. Empirical
formula C24.50H13.50Cl4.50N7O3Re. MW = 799.65. Orthorhombic, space
group Pbca, a = 12.5208(2) Å, b = 18.2828(3) Å, c = 24.9107(5) Å, V
= 5702.44(17) Å3, Z = 8, ρC = 1.863 g cm−3, μ = 4.724 mm−1. Crystal
size = 0.54 × 0.46 × 0.06 mm3. Reflections collected = 20 0961,
unique reflections = 18 207 [R(int) = 0.0610]. Max. and min.
transmission = 0.769 and 0.172. No. parameters = 407. GoF = 1.052.
Final R indices [I > 2σ(I)] R1 = 0.0437, wR2 = 0.0966. R indices (all
data) R1 = 0.0628, wR2 = 0.1026. Largest diff peak and hole = 4.250
and −2.189 e Å−3. Both CHCl3 solvent molecules were modeled as
being disordered. The site occupancies for the two sets of chlorine
atoms for molecule 1 refined to 0.686(3) and its complement. Solvent
molecule 2 is disordered about a crystallographic inversion center.
Crystal Data and Structure Refinement for [Re2]. Empirical
formula C21H12N7O3Re. MW = 596.58. Monoclinic, space group C2/c,
a = 20.3187(6) Å, b = 8.6848(5) Å, c = 24.4550(7) Å, β =
108.812(3)°, V = 4084.9(3) Å3, Z = 8, ρC = 1.940 g cm−3, μ = 5.990
mm−1. Crystal size = 0.25 × 0.17 × 0.14 mm3. Reflections measured =
31 770, unique reflections = 9254 [R(int) = 0.0421]. Max. and min.
transmission = 0.543 and 0.392. No. parameters = 289. GoF = 0.858.
Final R indices [I > 2σ(I)] R1 = 0.0267, wR2 = 0.0510. R indices (all
data) R1 = 0.0411, wR2 = 0.0525. Largest diff peak and hole = 2.218
and −0.995 e Å−3.
Synthesis of [Re21]PF6. fac-[Re(CO)3(phen)Cl] (0.04 g, 0.1
mmol) and AgSO3CF3 (0.02 g, 0.1 mmol) were combined in dry THF
(5 mL), while shielded from light and under a nitrogen atmosphere,
and heated at reflux for 4 h. To the mixture was added [Re1] (0.05 g,
0.1 mmol), and the mixture was stirred at reflux for 24 h. The reaction
mixture was cooled and filtered through Celite, the solvent was
removed, and 1 mL of methanol was added. This solution was added
to 2 mL of methanol containing 0.3 g of NH4PF6. The solid formed
was purified by column chromatography using Brockmann II acidic
alumina as the stationary phase, and a mixture of dichloromethane/
ethyl acetate (4:1) was used to elute any unreacted fac-[Re-
(CO)3(phen)Cl] as the first fraction. A mixture of ethyl acetate/
acetonitrile (4:1) was used to elute the second fraction, which was
collected, the solvent volume was reduced, and diethyl ether was
added to precipitate the yellow solid. Single crystals suitable for X-ray
diffraction were grown from layering hexanes on a dichloromethane
1
solution of [Re21]PF6. Yield: 25 mg (26%); mp 130 °C (dec). H
3
NMR (CDCl3): δ 8.96 (d, J(H,H) = 4.7 Hz, 2H; H2 H9), 8.87 (d,
3
3J(H,H) = 5.5 Hz, 2H; H2 H9), 8.80 (d, J(H,H) = 8.2 Hz, 2H; H4
3
H7), 8.65 (d, J(H,H) = 8.2 Hz, 2H; H4 H7), 8.27 (s, 2H; H5 H6),
8.03 (s, 2H; H5 H6), 7.89−7.84 (m, 4H; H3 H8 × 2), 7.62 (d,
3J(H,H) = 8.4 Hz, 2H; Hmeta), 7.19 (d, 3J(H,H) = 8.4 Hz, 2H; Hortho
)
ppm. 13C NMR [(CD3)2CO; CO and C5 peaks cannot be clearly
distinguished]: δ 155.2, 147.7, 140.9, 140.6, 133.2, 131.8, 130.9, 128.9,
127.5 ppm. IR: νmax 2970 w, 2233.1 w (CN), 2027 s (CO A′(1)), 1906
s (CO A′(2) and A″), 1633 w, 1521 w, 1430 w, 838 m cm−1. Anal.
Calcd (%) for [Re21]PF6: C 37.53, H 1.66, N 10.19. Found: C 37.49,
H 1.49, N 10.19.
Crystal Data and Structure Refinement for [Re4]PF6. Empirical
formula C25H21F6N7O3PRe. MW = 798.66. Monoclinic, space group
P21/n, a = 11.5325(3) Å, b = 14.9990(5) Å, c = 16.5483(4) Å, β =
95.276(2)°, V = 2850.33(14) Å3, Z = 4, ρC = 1.861 g cm−3, μ = 4.400
mm−1. Crystal size = 0.55 × 0.10 × 0.10 mm3. Reflections measured =
36 589, unique reflections = 9893 [R(int) = 0.0645]. Max. and min.
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Organometallics
Article
transmission = 0.682 and 0.254. No. parameters = 391. GoF = 0.861.
Final R indices [I > 2σ(I)] R1 = 0.0337, wR2 = 0.0555. R indices (all
data) R1 = 0.0677, wR2 = 0.0591. Largest diff peak and hole = 1.606
and −1.298 e Å−3.
used for the electrochemical experiments. P.W. and M.W. wish
to thank Curtin University for their APA. The authors
acknowledge the facilities and scientific and technical assistance
of the Australian Microscopy & Microanalysis Research Facility
at the Center for Microscopy, Characterization & Analysis, The
University of Western Australia, a facility funded by the
University and State and Commonwealth Governments.
Crystal Data and Structure Refinement for [Re21]PF6. Empirical
formula C41H26F6N9O7PRe2. MW = 1274.08. Triclinic, space group
P1, a = 11.276(5) Å, b = 13.079(5) Å, c = 14.516(5) Å, α = 92.170(5),
̅
β = 92.134(5), γ = 99.976(5)°, V = 2104.8(14) Å3, Z = 2, ρC = 2.010 g
cm−3, μ = 5.873 mm−1. Crystal size = 0.75 × 0.44 × 0.02 mm3.
Reflections measured = 51 348, unique reflections = 15 841 [R(int) =
0.0528]. Max. and min. transmission = 0.886 and 0.088. No.
parameters = 629. GoF = 1.064. Final R indices [I > 2σ(I)] R1 =
0.0497, wR2 = 0.1099. R indices (all data) R1 = 0.0794, wR2 = 0.1192.
Largest diff peak and hole = 4.020 and −3.252 e Å−3. The solvent was
modeled as a molecule of acetone disordered over two sets of sites
with equal occupancies.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed NMR studies. Bond length and angle values for the
crystallographically characterized complexes. Cyclic voltammo-
grams for the mononuclear complexes. Detailed absorption,
excitation, and emission profiles for all the complexes. TDDFT-
simulated absorption profiles, list of electronic transitions, and
molecular orbital representations for all the complexes. Full
details of the structure determinations (except structure
factors) have been deposited with the Cambridge Crystallo-
graphic Data Center as CCDC 882648 ([Re1]), 882649
([Re2]), 882650 ([Re4]PF6), 882651 ([Re21]PF6), 882652
([Re2Re]PF6). Copies of this information may be obtained free
of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: + 44 1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk; or www: http://www.ccdc.cam.ac.
uk). This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: m.massi@curtin.edu.au, stefano.stagni@unibo.it.
■
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The work was supported by the Australian Research Council
(DP0985481, which includes an APD Fellowship to M.M.).
P.R. thanks the ARC for his ARF fellowship (DP0986999) and
iVEC and the National Computational Infrastructure for the
provision of computer time. D.S. thanks Curtin University for a
Research Fellowship and the ARC for a DECRA
(DE120101456). The authors thank Prof. R. G. Compton at
Oxford University for the kind donation of the microelectrode
7577
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