Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes

Article abstract of DOI:10.1021/acs.inorgchem.0c02914

Six rhenium(I) κ3N-dicarbonyl complexes with 4′-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths - from CN to NMe2 - were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground- and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett σp substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous κ2N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

Full text of DOI:10.1021/acs.inorgchem.0c02914

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Coordination Environment Prevents Access to Intraligand Charge-
Transfer States through Remote Substitution in Rhenium(I)
Terpyridinedicarbonyl Complexes
́
́
́
Ricardo J. Fernandez-Teran* and Laurent Severy
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ABSTRACT: Six rhenium(I) κ3N-dicarbonyl complexes with 4′-
(4-substituted phenyl)terpyridine ligands were evaluated in their
ground and excited states. These complexes, bearing substituents
of different electron-donating strengthsfrom CN to NMe2
were studied by a combination of transient IR (TRIR),
electrochemistry, and IR spectroelectrochemistry, as well as time-
dependent density functional theory (TD-DFT). They exhibit
panchromatic absorption and can act as stronger photoreductants
than their tricarbonyl counterparts. The ground- and excited-state
potentials, absorption maxima, and lifetimes (250−750 ps) of these
complexes correlate well with the Hammett σp substituent
constants, showing the systematic effect of remote substitution in
the ligand framework. TRIR spectroscopy allowed us to assign the
lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous
report on analogous κ2N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge
transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine,
local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived
triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state
switching strategy via remote substitution.
INTRODUCTION
■
Rhenium(I) carbonyl complexes have found widespread
applications in chemistry, ranging from their ubiquitous role
as photosensitizers in diverse photocatalytic transforma-
tions1−12 to their use as CO2 reduction catalysts.13−18
Complexes of the type fac-[Re(N^N)(CO)3L]+ (where N^N
Figure 1. Different strategies for tuning the properties of rhenium(I)
is a bidentate diimine ligand) have been studied to a great
carbonyl complexes: (A) by substitution on the diimine ligands, (B)
extent since the pioneering work by Wrighton and co-
by tuning of the axial ligands, (C) by remote substitution in
workers19−21 as well as Lehn and co-workers.22
tricarbonyl complexes and (D) in their dicarbonyl analogues with a
While the photochemical and photophysical properties of
fac-tricarbonylrhenium(I) complexes have been studied
terpy framework.
extensively,1 9 , 2 3 − 3 1 those of mononuclear cis-
acting as stronger photoreductants than conventional rhenium-
dicarbonylrhenium(I) complexes, in contrast, remain to date
(I) tricarbonyl complexes. Hightower and co-workers37,38 as
relatively unexplored.32 The groups of Castellano33 and
well as Richeson and co-workers39 have shown that cis-
Sullivan34 have recently reported on the substituent effects in
dicarbonylrhenium(I) complexes of nitrogen-donor ligands
rhenium(I) bis(diimine)dicarbonyl complexes with the general
formula [Re(N^N)2(CO)2]+ (Figure 1A). In addition,
Morimoto and Ishitani35 have reported on supramolecular
Received: September 30, 2020
assemblies of rhenium(I) dicarbonyl complexes with phos-
phine ligands. Furthermore, Dempsey and co-workers36 have
reported on a series of rhenium(I) bipyridinedicarbonyl
complexes with axial ligands of different donating strengths
(Figure 1B), exhibiting broadband visible absorption and
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Scheme 1. Synthetic Route and Structures of the Compounds Studied in the Present Work
bearing a κ3N-coordination motif exhibit enhanced absorption
properties throughout the entire visible spectruma common
feature of these complexes that makes them attractive
candidates for applications in photochemical energy con-
version.
We have recently reported on a complementary remote
substitution strategy to tune the photophysical and photo-
chemical properties of rhenium(I) tricarbonyl complexes, using
a ligand framework based on bidentate 4′-(4-R1-phenyl)-
2,2′:6′,2″-terpyridine (terpy) ligands (Figure 1C), decorated
with electron-withdrawing and -donating substituents (ranging
from CN to NMe2).40 In the present work, we extend our
approach to their rhenium(I) cis-dicarbonyl analogues (Figure
1D).
We report herein the synthesis, spectroscopic, crystallo-
graphic, and electrochemical characterization of a series of
rhenium(I) κ3N-dicarbonyl complexes (3a−3f; Scheme 1)
with substituted terpy ligands (La−Lf). The present study
brings new insights into the photophysical and photochemical
properties of this family of complexes. By combining
experimental and computational methods, including UV
pump−IR probe, Fourier transform infrared (FT-IR) spec-
troelectrochemistry, and time-dependent density functional
theory calculations (TD-DFT), we establish herein the key
differences between the photophysics of the κ2N-tricarbonyl
and κ3N-dicarbonyl terpy frameworks.
complexes (2a−2f, obtained from ReI(CO)5Cl and the
corresponding ligand).
The κ3N-dicarbonyl complexes studied in this work (3a−3f)
were obtained as air-stable black solids in quantitative yields by
thermal decarbonylation of 2a−2f under vacuum (Scheme 1),
an improved protocol with respect to previous literature
reports.37−39 The key aspect to quantitative conversion is
heating the complexes under vacuum (instead of under an
inert atmosphere) because this is expected to promote CO
loss. Complete experimental details are provided in the
Supporting Information (SI).
It is worth mentioning that both the κ2N-tricarbonyl and
κ3N-dicarbonyl complexes show remarkable photochemical
and thermal stability. In comparison, manganese(I) tricarbonyl
complexes with terpy ligands readily release CO upon
photoexcitation, forming the corresponding κ3N-dicarbonyl
complex.43
The crystal structures of these complexes (illustrated by that
of 3f in Figure 2) confirm the mer,cis-coordination geometry
We assess the impact of the different coordination
geometries and local symmetries around the Re(I) center
and correlate the observed properties with the electronic
density on the terpy ligand. These two complementary
approaches (i.e., remote substitution and coordination
geometry) provide full control over the absorption properties
of these complexes, while allowing us to look at their excited-
state structure and dynamics.
Figure 2. Displacement ellipsoid representation (at the 50%
probability level) of the crystal structure of complex 3f. H atoms
and disordered solvent molecules are omitted for clarity.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic route to
the κ3N-dicarbonyl complexes is shown in Scheme 1. In our
previous report,40 we have described the synthesis of the
around the Re(I) center. In the complexes studied in the
present work, the plane of the {Re(CO)2}+ moiety is almost
perpendicular to the plane of the terpy ligand, in contrast to
other reports of rhenium(I) dicarbonyl complexes, where the
two CO groups are coplanar with the bis(diimine) ligand.36
The N5−Re1−N15 angles close to 150° indicate a slight
̈
ligands (La−Lf, based on a modified Krohnke strategy with 4-
acetylpyridine and a 4-substituted benzaldehyde deriva-
tive)41,42 as well as the synthesis of the κ2N-tricarbonyl
B
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distortion from the ideal octahedral geometry. In general, bond
lengths and angles are in line with those previously reported
for similar complexes.34,44−47 Additional crystallographic
details are given in the SI.
delocalization of the CO stretching modes observed in their
tricarbonyl counterparts.48 The red shift in the absorption
frequencies can be explained by a significant increase in the
electron density around the metal center due to replacement of
an electron-accepting CO group by a donating imine group
(from the now meridionally coordinated terpy ligand).
Complexes 3a−3e show consistent hypsochromic (blue)
shifts in all UV−vis absorption maxima as the electron-
donating character of the substituent is increasedshifting by
614 (ν1), 390 (ν2), and 700 cm−1 (ν3), respectively, upon
going from CN to OMeaccompanied by a slight increase in
the extinction coefficients. These shifts are similar to those
observed for 2a−2e. Complete UV−vis and IR spectra are
provided in Figures S1 and S2.
In contrast to its tricarbonyl counterpart, the NMe2-
substituted complex (3f) does not show any significant new
bands in its UV−vis absorption spectrum. The positions
(cm−1) of the lowest-energy band (ν1) of the UV−vis spectra
show an excellent linear correlation with the Hammett σp
substituent constants (R2 = 0.9983; Figure S3), while those of
ν2 and ν3 deviate from linearity for the extremely donating or
withdrawing substituents.
The 1H NMR spectra of all κ3N complexes (3a−3f) show a
single set of seven signals for the terpy ligand framework, also
in agreement with the crystallographically observed mer,cis-
(κ3N-terpy) coordination geometry and an apparent Cs
symmetry around the Re(I) center. The chemical shifts of
the 3′ and 5′ protons of the terpy ligand show a linear
correlation with the Hammett σp substituent constants,
suggesting good electronic communication between the ligand
subunits. A similar correlation was also observed for the 13C
NMR shifts of the equatorial carbonyl ligands of the κ3N
complexes. The NMR spectra of all complexes are given in the
SI.
Steady-State Spectroscopic Properties. Compared to
their κ2N-tricarbonyl counterparts, the κ3N-dicarbonyl com-
plexes show panchromatic absorption (throughout the entire
visible spectrum), manifested in three strong metal-to-ligand
charge transfer (MLCT) bands around ca. 720, 480, and 410
nm (denoted herein as ν1, ν2, and ν3, respectively),
accompanied by a shoulder around 575 nm (Figure 3A).
Complexes 3a−3f are nonemissiveneither at room
temperature nor at 77 K, in contrast to their κ2N-tricarbonyl
analogues (2a−2f). We attribute this to a faster nonradiative
deactivation due to the significantly red-shifted absorption (as
expected from the energy gap law).
Ground- and Excited-State Redox Properties. Electro-
chemical studies of these complexes were carried out in 0.1 M
[Bu4N][PF6] in N,N-dimethylformamide (DMF; Figure 4).
Figure 4. Cyclic voltammograms of selected dicarbonyl complexes in
0.1 M [Bu4N][PF6] in DMF. Scan rate: 100 mV s−1.
The κ3N-dicarbonyl complexes show a one-electron chemically
reversible oxidation around ∼0 V versus ferrocenium/
ferrocene (Fc+/0) and a stepwise two-electron reduction
below −1.5 V versus Fc+/0, analogous to that observed in
their κ2N-tricarbonyl analogues.40 The dicarbonyl complexes
show, however, a more evident separation of the two
overlapping consecutive reduction waves, especially on the
more electron-rich complex 3f.
The partial irreversibility of the second reduction is
attributed to loss of the chloride ligand. This was well
reproduced in the DFT calculations, performed in Gaussian 16,
revision B.01,49 and in agreement with previous reports of
related complexes.50−52 The redox potentials of the oxidative
and reductive events shift to more negative values with more
electron-donating substituents. Peak shifts of −160 and −60
mV were recorded for the reduction and oxidation waves,
respectively, between CN and NMe2 as substituents (Figure
Figure 3. (A) UV−vis and (B) FT-IR absorption spectra in DMF of
the unsubstituted κ2N-tricarbonyl (2d, blue) and κ3N-dicarbonyl (3d,
red) complexes.
In the IR, the change from the fac-{Re(CO)3}+ moiety to
the cis-{Re(CO)2}+ moiety leads to an average red shift of ca.
85 cm−1 in the CO frequencies (i.e., average of all CO
stretching frequencies). The two νCO absorption bands
expected for the cis-dicarbonyl complexes are observed around
ca. 1820 and 1895 cm−1 (Figure 3B).
These two vibrations have antisymmetric and symmetric
character (formally both of A′ symmetry in the Cs point
group)consisting of out-of-phase and in-phase stretching of
the two CO groups, respectively. They preserve the large
C
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S4). The ground- and excited-state redox properties of these
complexes are summarized in Table 1 and discussed below.
Table 1. Ground- and Excited-State Redox Properties of the
Studied Complexes in DMF
theo
E
°′
E °′
ox a
ΔG
(Ered°′)*
(Eox°′)*
ST
red a
b
c
c
R1
(V)
(V)
(eV)
(V)
(V)
3a
3b
3c
3d
3e
3f
CN
CF3
Br
H
OMe
−1.58
−1.60
−1.62
−1.65
−1.67
−1.71
0.09
0.07
0.06
0.05
0.05
0.03
1.35
1.39
1.42
1.44
1.47
1.51
−0.19
−0.20
−0.20
−0.21
−0.19
−0.20
−1.26
−1.32
−1.36
−1.39
−1.42
NMe2
−1.48
b
Figure 5. Experimental (solid lines) and calculated (dashed lines)
difference IR spectra of the redox states of 3d. Experimental spectra
a
Electrochemical potential in volts versus Fc+/0 in DMF. Energy
difference between the optimized S0 and T1 structures. Excited-state
potentials estimated from eqs 1 and 2.
c
measured in 0.1 M [Bu4N][PF6] in DMF. Potentials versus Fc+/0
.
Theoretical frequencies were scaled by 0.969 to better match the
experimentally obtained values.
Any potential application of these complexes for photo-
induced electron-transfer reactions requires knowledge of their
excited-state redox potentials, which can be estimated by eqs 1
and 2.
Overall, the shifts observed upon electrochemical oxidation
and reduction of 3a−3f correlate well with those of similar
complexes, being significantly higher for the one-electron
oxidation. These shifts were almost quantitatively reproduced
in DFT calculations of the different redox states, allowing us to
safely assign them as described above.
Excited-State Dynamics. We now turn to the excited-
state dynamics of complexes 3a−3f. Transient absorption
experiments were performed with a UV pump and ultrashort
broadband mid-IR pulses derived from a home-built optical
parametric amplifier as the probe (complete experimental
details are given in section 1.3 of the SI).54−56 Transient IR
(TRIR) spectra of complexes 3a−3f in air-saturated DMF
were recorded upon excitation with 420 nm ultrashort pulses
and are illustrated in Figure 6 by those of 3d as a representative
Ered°′(3[ReI]*) = Ered°′ + ΔGST
(1)
Eox°′(3[ReI]*) = Eox°′ − ΔGST
(2)
Because the κ3N complexes are nonemissive, we estimated
the ΔGST values of these complexes directly from the energy
differences between the optimized ground-state and lowest
triplet excited-state structures (without further corrections).
We base this approach on the excellent agreement between the
experimental and calculated values obtained in our previous
report for the κ2N complexes (with a mean deviation of 0.02
3
eV for complexes 2a−2e, operating from the lowest MLCT
state).40 Considering the reported (Eox°′)* values for
rhenium(I) tricarbonyl complexes (≈1 V vs Fc+/0 in
acetonitrile),36 we conclude that the κ3N complexes are
stronger photoreductants but weaker photooxidants than their
κ2N analogues.
Spectroelectrochemical studies in the IR (IR-SEC) revealed
the absorption features of the singly oxidized, as well as the
singly and doubly reduced species. The experimental and
calculated IR-SEC difference spectra of 3d (as a representative
example) are shown in Figure 5.
Upon oxidation, the antisymmetric and symmetric CO
stretches blue shift by ca. 95 and 120 cm−1, respectively.
Similarly, upon reduction, both carbonyl stretching bands
exhibit red shifts of ∼18 cm−1 (first reduction) and ∼55 cm−1
(second reduction). Ishitani and co-workers reported red shifts
of 27 and 33 cm−1 for the first reduction and red shifts of 55
and 66 cm−1 for the second (irreversible) reduction of cis,trans-
Figure 6. FT-IR spectrum (top) and magic-angle TRIR spectra
(bottom) of 3d (5 mM in DMF) at different pump−probe delays.
[Re(CO)2(bpy){P(OEt)3}2]+.51 In a similar fashion, Hartl and
−1
̌
Vlcek observed blue shifts of 78 and 92 cm (for the first
oxidation) and red shifts of 42 and 53 cm−1 (for the first
reduction) for a rhenium(I) diphosphinedicarbonyl complex
with a quinone ligand.50
example. Overall, the TRIR spectra of all complexes show two
excited-state absorption (ESA) bands, centered around 1871
and 1970 cm−1, and the corresponding ground-state bleach
signatures at 1820 and 1890 cm−1.
We tentatively conclude that complexes 3a−3f undergo
similar structural changes upon reduction as their tricarbonyl
counterparts (2a−2f),40,53 leading to Cl− dissociation after the
second reduction. DFT optimization of the structure of the
doubly reduced species also led to dissociation of the Cl ligand.
In this case, the doubly reduced complexes were reoptimized
after manually removing the Cl ligand.
(TD-)DFT calculations of the TRIR spectroscopic signa-
tures for these complexes almost quantitatively matched the
experimental TRIR spectra (Figure 7). On this basis, we
assigned the observed spectroscopic signatures to population
3
of the lowest MLCT state of the κ3N-dicarbonyl complexes
after ultrafast intersystem crossing (ISC) and/or internal
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(OEt)3}2]+.51 In their work, they showed that, upon photo-
excitation, the νCO antisymmetric (lower energy) and
symmetric (higher energy) bands of this complex blue shift
by ca. 45 and 56 cm−1. Similarly, Castellano and co-workers
observed blue shifts of ca. 40 and 75 cm−1 upon photo-
excitation of rhenium(I) dicarbonyl complexes with sub-
stituted bis(diimine) ligands.33 In the excited MLCT state,
3
complexes 3a−3f display a blue shift of the CO stretching
bands of ca. 49 and 77 cm−1, in line with previous reports for
similar complexes.
We observe, across the series of complexes 3a−3f, a
systematic modification of the lifetimes of the lowest triplet
excited state (Figure 8), analogous to that observed for 2a−2e.
Figure 7. Calculated (A) absolute and (B) difference spectra of the
ground and lowest singlet and triplet excited states. (C) Experimental
TRIR spectra at early and late population delays. All spectra
correspond to 3d in DMF. Calculated frequencies were scaled by
0.971 to better match the experiment. Vertical colored bars indicate
the positions of the calculated (scaled) frequencies for each state.
3
Figure 8. Linear correlations between the MLCT lifetimes (from
TRIR) and the Hammett σp substituent constants for the κ2N-
tricarbonyl complexes (2a−2e, in blue; 2f excluded for clarity) and
κ3N-dicarbonyl complexes (3a−3f, in red).
The lifetimes increase from 251 to 762 ps between the CN-
and OMe-substituted complexes, up to a maximum of 947 ps
for the NMe2-substituted complex (3f). The fitted lifetimes of
all complexes can be found in Table S2 and the corresponding
TRIR kinetic traces in Figure S6.
When comparing the slopes of the lifetimes and Hammett σp
substituent constants, we observe that the change for κ2N-
tricarbonyl complexes (2a−2e) is more prominent than that
for the κ3N-dicarbonyl complexes (3a−3f). These results are
intriguing, considering that both series show similar spectral
shifts upon substitution (717 and 614 cm−1, respectively, upon
going from CN to OMe), excluding the NMe2 complexes.
Computational Aspects. We now turn to TD-DFT
calculations to rationalize the observed properties and the
differences between the κ2N-tricarbonyl complexes and their
κ3N-dicarbonyl counterparts.
Neither the energy levels of the lowest unoccupied
molecular orbital (LUMO) nor those of the highest occupied
molecular orbital (HOMO) of complexes 3a−3f correlate with
the electron-donating/withdrawing properties of the substitu-
ents. This contrasts the trends observed for both HOMO and
LUMO of their tricarbonyl analogues (2a−2f). On average,
while the LUMO orbitals of 3a−3f remain at similar energies
compared to those of 2a−2f, the HOMO orbitals are ∼1 eV
higher in energy (making the HOMO−LUMO gap corre-
spondingly smaller). These results are in line with the similar
reduction potentials observed on both series and the
significantly lower oxidation potential of the dicarbonyl
complexes. Orbital energy diagrams are shown in Figure S7.
Apart from the significant change in the electronic density
around the Re(I) center introduced by replacing a CO
ligand for a pyridine ligand (evidenced by the large red shift of
conversion from an initially populated higher singlet excited
state. Evidence for this assignment comes from the
pronounced time-dependent blue shift of the two ESA bands
(also reproduced in the DFT calculations; Figure 7A,B). This
process, taking place on a time scale of ca. 10 ps, is attributed
to a convolution of relaxation on the excited state(s) (internal
conversion) and ISC, analogous to that observed for
rhenium(I) tricarbonyl complexes.28,57,58
It is important here to emphasize that excitation with 420
nm initially populates higher singlet excited states in the κ3N-
dicarbonyl complexes. According to TD-DFT calculations, all
of these excited states (up to at least S5) have MLCT character,
irrespective of the substituent on the ligand scaffold. This
sharply contrasts the excited-state character switching observed
with the NMe2 substituent in the κ2N-tricarbonyl complexes
from our previous report.40
The excellent agreement between the experimental and
calculated UV−vis absorption spectra (Figure S5) shows that
our computational approach accurately reproduces the ground-
and excited-state properties.
Considering current and previous observations on rhenium-
(I) κ2N-tricarbonyl complexes, we conclude that the photo-
physics of κ3N-dicarbonyl complexes follow a very similar
1
pathway: excitation to one of the MLCT states leads to
ultrafast internal conversion and ISC to the lowest-lying
3MLCT state, independent of the substituent on the terpy
ligand framework.
Time-resolved investigations of mononuclear rhenium(I)
dicarbonyl complexes are relatively scarce in the literature.
Ishitani and co-workers have reported on the photophysical
and photochemical properties of cis,trans-[Re(CO)2(bpy){P-
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all IR absorption bands), the local symmetry of the orbitals
around the metal center is increased upon going from the κ2N
to κ3N geometry. Planarization of the terpy ligand generates
also a significant increase in conjugation, which now spans
across the three aromatic rings of the terpy ligand (also
evidenced in the crystal structures of these complexes). This
change in the geometry suffices to introduce additional
electronic transitions, as evidenced by inspection of the UV−
vis absorption spectra (Figure 3A) and reproduced by TD-
DFT calculations (Figure S5).
The charge density difference (CDD) isosurfaces of
complexes 3d−3f further support the observed TRIR results:
no significant character change of the excited states was
observed upon going from the unsubstituted terpy ligand (R1 =
H, 3d) to the ligand substituted with the most electron-
donating group (R1 = NMe2, 3f). This contrasts with the case
of the tricarbonyl complex 2f. The CDD isosurfaces for the S1
← S0 transitions of 2a−2f and 3d−3f, calculated with
MultiWfn, version 3.7,59 are shown in Figure 9 as
representative examples.
Figure 10. CDD isosurfaces of the Sn ← S0 transitions for the first five
singlet states of complex 3d, shown at |Δρ| = 0.001 au.
The computational results presented herein serve to explain
the observed changes in the electronic structure of these
complexes upon going from the κ2N-tricarbonyl to κ3N-
dicarbonyl coordination geometry. In addition, the limitations
of the excited-state switching strategy (to avoid the energy gap
law), toward increasing the lifetime and red shifting the
absorption, are dissected in terms of symmetry and
conjugation.
CONCLUDING REMARKS
■
Herein we studied the ground- and excited-state properties of a
series of rhenium(I) dicarbonyl complexes with substituted 4′-
(4-R1-phenyl)-2,2′:6′,2″-terpyridine ligands in a κ3N coordi-
nation motif, by employing a combination of ultrafast and
steady-state spectroscopic methods ranging from TRIR to IR
spectroelectrochemistry and DFT calculations.
Figure 9. CDD isosurfaces of the S1 ← S0 transition of complexes
2d−2f (top row) and 3d−3f (bottom row), shown at |Δρ| = 0.002 au.
In contrast to our previous report on analogous κ2N-
tricarbonyl complexes,40 we did not observe excited-state
character switching when increasing the electron-donating
strength of the substituent from CN to NMe2. In the present
case, all excited states preserved their MLCT character, as
evidenced by TRIR spectroscopy and supported by TD-DFT
calculations. The photophysical and electrochemical properties
of these complexes showed excellent linear correlations with
the Hammett σp substituent constants, in line with the results
observed for their κ2N-tricarbonyl analogues operating from an
MLCT excited state (2a−2e).
TD-DFT calculations revealed the fundamental differences
in the electronic structure between complexes of the two series.
We show that a combination of destabilization of the HOMO
orbitals (by replacing one CO with a pyridine) and an increase
in the conjugation length is responsible for the observed ca.
1.55 eV red shift in the absorption. This shift consequently
lowers the excited-state lifetimes according to the energy gap
law. We also show that, for the remote substitution strategy to
switch the lowest excited state to intraligand charge-transfer
(ILCT) character in the excited state, a third carbonyl is
Two parallel effectsincreased electron density and local
symmetryexplain the more complex UV−vis absorption
spectrum of the dicarbonyls and the red shift of ca. 1.55 eV in
the energy of the S1 ← S0 transition. We attribute ca. 1 eV of
this shift to the increase in the HOMO relative energies and
the remainder to the increased conjugation. Furthermore, we
believe the increased electron density on the {Re(CO)2}+
moiety compared to the {Re(CO)3}+ core hinders charge
transfer from the NMe2 group because the former is a more
potent donor in the excited state.
The impact of the increased local symmetry around the
metal center in the dicarbonyl complexes 3a−3f can be further
evidenced in the CDD isosurfaces of the higher excited states
(Sn ← S0), shown in Figure 10.
The fact that the CDD isosurfaces bear strong similarities
results from an increase in the local symmetry of the complexes
and also serves to illustrate that these higher excited states all
have MLCT character.
F
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needed, since the electron-rich {Re(CO)2}+ moiety out-
competes the electron-donating power of the NMe2 sub-
stituent, thus hindering access to the ILCT states in these
complexes.
for insightful discussions. Prof. Dr. Bernhard Spingler and Prof.
Dr. Anthony Linden are acknowledged for helpful crystallo-
graphic discussions.
We believe that this work illustrates the boundaries of the
MLCT-to-ILCT switching strategy to enhance the lifetimes
and absorption properties of rhenium(I) carbonyl complexes.
This brings fundamental insights into the rich photochemistry,
photophysics, and electronic structure of rhenium(I) carbonyl
complexes with substituted terpyridine ligands.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02914.
■
sı
Full experimental details, including synthetic procedures,
characterization details, crystallographic discussions,
details about the transient absorption and electro-
chemistry experiments; UV−vis absorption, NMR and
FT-IR spectra of all complexes; details of the computa-
tional calculations; and additional tables and figures
(PDF)
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Accession Codes
CCDC 2032841 and 2032842 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Author
(8) Probst, B.; Guttentag, M.; Rodenberg, A.; Hamm, P.; Alberto, R.
Photocatalytic H2 production from water with rhenium and cobalt
complexes. Inorg. Chem. 2011, 50, 3404−3412.
́
́
Ricardo J. Fernandez-Teran − Department of Chemistry,
University of Zurich, CH-8057 Zurich, Switzerland;
orcid.org/0000-0002-4665-3520;
(9) Guttentag, M.; Rodenberg, A.; Kopelent, R.; Probst, B.;
Email: Ricardo.FernandezTeran@gmail.com,
Ricardo.Fernandez@chem.uzh.ch
̈
Buchwalder, C.; Brandstatter, M.; Hamm, P.; Alberto, R. Photo-
catalytic H2 production with a rhenium/cobalt system in water under
acidic conditions. Eur. J. Inorg. Chem. 2012, 2012, 59−64.
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P.; Alberto, R. A highly stable polypyridyl-based cobalt catalyst for
homo- and heterogeneous photocatalytic water reduction. Dalt. Trans.
2013, 42, 334−337.
Author
́
Laurent Severy − Department of Chemistry, University of
Zurich, CH-8057 Zurich, Switzerland; orcid.org/0000-
0002-6546-379X
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Alberto, R.; Baldridge, K. K.; Hamm, P. Mechanism of photocatalytic
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solution. Inorg. Chem. 2015, 54, 646−657.
(12) Rodenberg, A.; Orazietti, M.; Mosberger, M.; Bachmann, C.;
Probst, B.; Alberto, R.; Hamm, P. Quinones as Reversible Electron
Relays in Artificial Photosynthesis. ChemPhysChem 2016, 17, 1321−
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02914
Funding
This research was funded through the Swiss National Science
Foundation (Sinergia Project CRSII2 160801/1 and AP
Energy Grant PYAPP2 160586) and the University Research
Priority Program for Solar Light into Chemical Energy
Conversion (LightChEC) of the University of Zurich.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Dr. Peter Hamm and Prof. Dr. S. David Tilley
for their valuable input and encouragement. Estefanía Sucre-
Rosales is greatly acknowledged for her support, in part, of the
̌
́
computational calculations. We also thank Olga Bozovic and
Jeannette Ruf for proofreading the manuscript and providing
helpful comments. Dr. Benjamin Probst-Ru
̈
d, Prof. Dr. Roger
̈
Alberto, Dr. Jan Helbing, and Gokce
̧
n Tek are acknowledged
G
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