Vibrational Couplings in Hydridocarbonyl Complexes: A 2D-IR Perspective

Article abstract of DOI:10.1021/acs.inorgchem.0c00750

Hydridocarbonyl complexes, a class of industrially relevant catalysts, contain both the M-H and M-CO moieties. Here, using two-dimensional infrared spectroscopy, we examine the coupling of the typically weak M-H stretching mode and the intense M(CO) mode. By studying a series of Ir(I)- and Ir(III)-based hydridocarbonyl complexes, we show that the arrangement of the H and CO ligands in a trans configuration leads to strong vibrational coupling and mode delocalization. In contrast, a cis arrangement leads to no coupling, with the localized M-H mode having a much larger anharmonicity. These results highlight a promising strategy for enhancing the M-H vibration by intensity borrowing from the strong CO modes in a trans configuration, allowing for direct determination by infrared spectroscopy of both the oxidation (by frequency shifts) and the protonation state (via vibrational coupling) of the complex, in mechanistic studies of proton-coupled electron transfer reactions.

Full text of DOI:10.1021/acs.inorgchem.0c00750

pubs.acs.org/IC
Article
Vibrational Couplings in Hydridocarbonyl Complexes: A 2D-IR
Perspective
́
́
Ricardo Fernandez-Teran,* Jeannette Ruf, and Peter Hamm
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c00750
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Hydridocarbonyl complexes, a class of industrially
relevant catalysts, contain both the M−H and M−CO moieties.
Here, using two-dimensional infrared spectroscopy, we examine the
coupling of the typically weak M−H stretching mode and the intense
M(CO) mode. By studying a series of Ir(I)- and Ir(III)-based
hydridocarbonyl complexes, we show that the arrangement of the H
and CO ligands in a trans configuration leads to strong vibrational
coupling and mode delocalization. In contrast, a cis arrangement
leads to no coupling, with the localized M−H mode having a much
larger anharmonicity. These results highlight a promising strategy for
enhancing the M−H vibration by intensity borrowing from the
strong CO modes in a trans configuration, allowing for direct
determination by infrared spectroscopy of both the oxidation (by frequency shifts) and the protonation state (via vibrational
coupling) of the complex, in mechanistic studies of proton-coupled electron transfer reactions.
variable (and typically low) extinction coefficients, with the
latter often precluding their study.10
INTRODUCTION
■
Transition metal hydride complexes play a central role as
fundamental intermediates in many chemical transformations.
Among these, proton and CO2 reduction are the most
prominent examples for their relevance in addressing the
global energy challenge through solar fuel production.1,2
A subset of transition metal hydrides, containing both M−H
and M−CO moietiesalso known as hydridocarbonyl
complexes, not only are very active and industrially relevant
hydroformylation catalysts (especially those of Ir and Rh),3−5
but also have been used as mechanistic probes for proton-
coupled electron transfer (PCET) reactions (mostly tungsten-
based complexes). These reactions constitute fundamental
steps in proton reduction and solar fuel production, among
many other chemically relevant transformations.6 In this
context, it has been shown that electron transfer driving
force, applied pressure, and the ligand environment can all
vastly influence the PCET pathways,7−9 illustrating the delicate
interplay between such variables, and their role in steering
reactivity.
Infrared (IR) spectroscopy has been the tool of choice for
the study of transition metal carbonyl complexes, due to the
very strong transition dipole of the CO stretching modes,
which serve as ubiquitous probes for electronic density (i.e.,
oxidation state) at a transition metal center.5 Their character-
istic absorption bands around 1900−2100 cm−1 (for terminal
carbonyls) situate them in a nearly overlap-free and vastly
solvent-transparent region. Terminal M−H stretches are one of
the few modes with similar absorption frequencies (ranging
from 1700 to 2250 cm−1) and are known to have extremely
Aspects such as couplings, exchange, and orientations, not
evident in a Fourier transform infrared (FT-IR) absorption
spectrum, become observable through multidimensional
spectroscopic methods. For instance, two-dimensional infrared
(2D-IR) spectroscopy provides insight into the coupling of
different vibrational modes. By frequency resolving the pump
(or excitation) axis, a series of diagonal and off-diagonal
responses are obtained. A cross-peak is observed whenever
excitation of one mode influences the response of other modes,
through either coupling or exchange. In addition, diagonal
peaks yield information about solvation and allow the
distinction of inhomogeneous versus homogeneous broad-
ening mechanisms.11
In this regard, 2D-IR spectroscopy has been used to study
many aspects of metal carbonyl complexes,12−17 as well as
solvation dynamics of Vaska’s complex and its O2 and I2
adducts in binary solvent mixtures.18−22 Recently, 2D-IR
spectroscopy has also been used to distinguish exchanging
conformations of a Rh(I) hydridocarbonyl complex.23 A recent
report from our group on the Pt−H stretching mode on a
Received: March 12, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c00750
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
detector, yielding an ω3 resolution of ∼10.4 or 5.5 cm−1 and an ω1
resolution of ∼2 cm−1, respectively. In all experiments, the pump and
probe beams were p-polarized.
metal surface showed, by means of 2D-IR spectroscopy, that
this mode can be characterized by its frequency, lifetime, and
large anharmonicity: ∼90 cm−1 for Pt−H versus 20−25 cm−1
for Pt(CO). At the same time, it was shown that the
stretching mode of the surface-bound hydride has an
absorption cross section comparable to that of the strong
Pt(CO) mode.24
Computational Details. Density functional theory (DFT)
calculations were performed using Gaussian 09 rev. D.0129 with the
B3LYP functional, the 6-311G(d,p) basis set for all light atoms, and
the LANL2DZ effective core potential for the Ir atoms.30−32 The IEF-
PCM solvation model (as implemented in Gaussian) was used for
calculations in solution.33 Harmonic vibrational analysis revealed no
negative frequencies, confirming the structures to be true minima.
Anharmonic vibrational analysis was performed on the reduced
subspace of the Ir−H and Ir(CO) normal modes. The anharmonic
treatment of the vibrations was performed using the Gaussian 09 rev.
D.01 implementation of the generalized second-order vibrational
perturbation theory (GVPT2), including terms up to the third and
fourth derivatives of the potential energy with respect to the normal
mode coordinates.34
Considering the structural specificity and time resolution
provided by IR spectroscopy, we chose to take a closer look
into the ultrafast dynamics of the M−H and M(CO) bands
of transition metal hydridocarbonyl complexes in solution.
Herein, we employ 2D-IR spectroscopy to study a series of Ir-
based hydridocarbonyl complexes in order to understand the
vibrational couplings between the M−H and M(CO)
modes, revealing their dependence on the relative orientation
of the CO and H ligands.
RESULTS AND DISCUSSION
■
EXPERIMENTAL SECTION
■
Intermode Coupling in IrHCOP3. As a starting point, we
turn to the absorption and 2D-IR spectra of IrHCOP3 (Figure
1). This Ir(I) complex, containing only one CO and one
hydride ligand in a trans configuration, constitutes the simplest
model system.
Chemicals and Solvents. All reactions were performed under a
nitrogen atmosphere using standard Schlenk techniques. Solvents
used for the synthesis were of reagent grade or higher and were used
as received. [Ir(CO)Cl(PPh3)2] (Vaska’s complex, VC; Strem
Chemicals) and [HIr(CO)(PPh3)3] (IrHCOP3; Acros Organics)
were used without further purification. Solvents for 2D-IR and FT-IR
measurements were degassed (when needed) by freeze−pump−thaw
cycles (3×) and handled under an inert atmosphere. H2 (PanGas AG,
grade 6.0) and D2 (PanGas AG, grade 3.0) were passed through
ambient-temperature point-of-use gas purifiers (MC1-904F, SAES
Pure Gas), reducing the impurity levels in the gas stream to less than
1 ppb.
Synthesis of the Complexes. The dihydrogen adduct of Vaska’s
complex, [IrH2(CO)Cl(PPh3)2] (VC-H2), was prepared in situ by
bubbling a solution of VC (∼10 mM in CHCl3) with H2 (or D2 for
VC-D2) until the νCO band of the starting material decayed to a
constant value (typically within 30 min, resulting in >90%
conversion). During the measurements, an atmosphere of H2 was
maintained over the solution to avoid decomposition of the product.
In a similar manner, the HCl adduct of Vaska’s complex,
[IrH(CO)Cl2(PPh3)2] (VC-HCl), was prepared in situ by adding
one drop of concentrated HCl (35% aq) to a solution of VC (∼20
mM in CHCl3), resulting in an discoloration after mixing and in
quantitative conversion into VC-HCl. Upon standing, a white
precipitate was formed on a 10 min time scale. In both cases,
degassing the solvent was crucial to minimize the formation of the
oxygen adduct of Vaska’s complex, [Ir(η2-O2)(CO)Cl(PPh3)2] (VC-
O2).
Steady-State Spectroscopic Characterization. FT-IR spectra
were collected on a Bruker Vertex 80v spectrometer. The measure-
ments were performed in a home-built flow cell consisting of two 2
mm thick CaF2 windows separated by a 200 μm thick polytetrafluoro-
ethylene (PTFE) spacer. For the very insoluble VC-HCl complex, a 1
mm PTFE spacer was used instead (also for 2D-IR; see below).
Ultrafast 2D-IR Spectroscopy. The ultrafast 2D-IR spectrometer
used in this work is based on the output of a 5 kHz Ti:sapphire
amplifier producing ∼100 fs pulses centered at 800 nm, which was
used to pump a home-built mid-IR OPA,25,26 delivering ∼2 μJ, ∼120
fs pulses centered around 2000−2150 cm−1, with a bandwidth of
∼200 cm−1. A small fraction of the mid-IR light was split from a BaF2
wedge to be used as the probe and reference beams. Absorptive 2D-IR
spectra were obtained by fast scanning the coherence delay (t1)
between the two collinear mid-IR pump pulses (up to 3.5 ps)
generated in a Mach−Zehnder interferometer for a fixed population
delay (t2).27 Scattering suppression was achieved by quasi-phase
cycling using a librating ZnSe window introduced at a Brewster angle
in the pump beam.28 The pump and probe beams were overlapped at
the sample position, and afterward, the probe and reference beams
were dispersed in a spectrograph with a 100 or 150 l mm−1 grating.
Both were simultaneously recorded with a 2 × 32 pixel MCT array
Figure 1. (A) Normal modes and scaled displacement vectors.
Absorption (B) and 2D-IR spectra (C) of IrHCOP3 in DMF, with 1.5
equiv of PPh3 (at t2 = 3 ps).
Its absorption spectrum in DMF shows two strong
absorption bands centered around ν1 = 1925 and ν2 = 2073
cm−1, corresponding to the symmetric and antisymmetric H−
Ir(CO) stretching modes, respectively, according to the
DFT calculations (Figure 1A,B). In the 2D-IR spectra, strong
instantaneous cross-peaks can be observed between the ν1 and
B
https://dx.doi.org/10.1021/acs.inorgchem.0c00750
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
ν2 bands, indicative of direct coupling between these modes
(Figure 1C).
To begin, the appearance of a 2D-IR spectrum relies on
anharmonicity, as the 2D-IR response of a strictly harmonic
system would vanish. The anharmonic coupling between two
modes can be quantified by the cross-anharmonicity (Δij)
defined as11,35
Δij = (ν + ν) − ν
i
j
ij
(1)
where νi is the fundamental anharmonic frequency and νij the
anharmonic frequency of the combination band of modes i and
j. The diagonal anharmonicity (Δi) refers to the difference
between the ν1→2 and the ν0→1 transition energies. All of these
quantities can be (and have been) calculated from anharmonic
vibrational calculations within Gaussian, which start from a
(harmonic) normal mode calculation and treat anharmonicity
perturbatively.29
It can be shown that the size of the cross-anharmonicity (eq
1) tentatively reflects the degree to which the harmonic normal
modes are delocalized.11,35−37 This is because the source of
anharmonicity is localdue to the very fact that the potential
energy surface of a chemical bond is better described by a
Morse potential rather than a parabolic (harmonic) potential.
Upon normal mode transformation, which delocalizes local
modes in a harmonic sense, the local anharmonicity delocalizes
accordingly, revealing the cross-terms Δij (eq 1). The
relationship between cross-anharmonicity and degree of
delocalization is not strict, as many parameters may start to
play a role once anharmonicity is considered. However, we will
report both cross-anharmonicity and degree of delocalization
for all molecular examples we consider, and we will see that
they are indeed correlated to a significant extent.
Figure 2. (A) Normal modes and scaled displacement vectors.
Absorption (B) and 2D-IR spectra (C) of VC-HCl in CHCl3 (at t2 =
3 ps). The scale of the 2D-IR spectrum has been expanded to
approximately 8% of the maximum, in order to show the very weak
Ir−Hcis band (ν2). Phosphine ligands are simplified for clarity.
Anharmonic vibrational calculations for IrHCOP3 in DMF
calc
revealed Δ12 = 14 cm−1, in accordance with the result that
harmonic normal modes are delocalized among the Ir−H and
the −CO chemical bonds (see red displacement vectors in
calc
Figure 1A). The calculated diagonal anharmonicities (Δ1
=
discussion above, no cross-peaks are observed between the
Ir(CO) and Ir−Hcis modes in the 2D-IR spectrum of this
complex (Figure 2C).
22 cm−1 and Δ2calc = 61 cm−1) are also in good agreement with
exp
the experimentally observed results (Figure 1C; with Δ1
=
exp
exp
22, Δ2 = 45, and Δ12 = 19 cm−1).
Anharmonic DFT calculations indeed yield Δ12calc ≈ 0 cm−1
for the cross-anharmonicity, whereas the diagonal anharmo-
nicities are once more in excellent agreement with the
To assess the extent of localization of normal modes, we
have chosen the mass-weighted length of the displacement
vectors (a more detailed explanation is given in the Supporting
Information). We found, using this approach, that for
IrHCOP3 the fractional contributions of the Ir−Htrans and
Ir(CO) moieties are, respectively, 19:81 for ν1 and 70:30 for
ν2, meaning that the higher frequency band is more localized in
the hydride, whereas the lower frequency band is more
localized in the carbonyl moiety, also in line with the observed
diagonal anharmonicities.
Intermode Coupling in VC-HCl. The next complex we
consider, VC-HCl, constitutes a counterexample to the
discussion above. Addition of HCl to Vaska’s complex has
been shown to proceed in a manner analogous to that for H2,
yielding [IrH(CO)Cl2(PPh3)2] (VC-HCl).38,39 In this case,
only the cis-Cl2 isomer of VC-HCl is obtained with the hydride
being cis to the carbonyl ligand. This stereochemical
assignment has been confirmed crystallographically.40
experiment (Δ1 = 24 and Δ2 = 82 cm−1, vs Δ1 = 24
calc
calc
exp
and Δ2
= 71 cm−1). For this complex, the fractional
exp
contributions of the Ir−Hcis and Ir(CO) moieties are,
respectively, 1:99 for ν1 and 100:0 for ν2, reflecting the totally
localized characters of these vibrations.
We also see that a localized M−H vibration has a
significantly larger anharmonicity (typically >80 cm−1), in
comparison to that of the M(CO) modes (typically ∼20
cm−1). Similarly large anharmonicities are found in the O−H
stretches of water,41 phenol,42 and other oscillators involving a
H atom. In particular, the Ir−H anharmonicity in solution
(ΔIr−H ≈ 70 cm−1) compares favorably with that of Pt−H on a
metal surface (ΔPt−H ≈ 90 cm−1).24 This can be explained in
simple terms: the M−H fragment, having a much lower
reduced mass, explores a larger extent of its potential energy
surface.
The Ir−H and Ir(CO) modes are now decoupled (see the
red displacement vectors in Figure 2A), yielding an IR
absorption spectrum (Figure 2B) with a strong Ir(CO)
stretching band (ν2 = 2048 cm−1) and a very weak and broad
Ir−H stretching band (ν2 = 2226 cm−1). In line with the
Intermode Coupling in VC-H2. The final example, VC-
H2, combines both situations with two hydride ligands, one of
which is trans to the CO ligand and the other cis. Concerted
oxidative addition of H2 to the Ir(I) center in Vaska’s complex
leads to a cis-dihydride complex (Figure 3).39 The two
C
https://dx.doi.org/10.1021/acs.inorgchem.0c00750
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 3. (A) Normal modes of VC-H2 with their corresponding scaled displacement vectors (phosphine ligands simplified for clarity). (B)
◇
Normalized absorption spectra of VC (black), VC-H2 (blue), and VC-D2 (red) in CHCl3. The symbols * and
in the spectra refer to traces of
VC and VC-O2, respectively. In the spectrum of VC-D2, the additional, very weak bands due to partially deuterated products or traces of VC-H2 are
indicated with †. (C) Absorption and (D) 2D-IR spectra of VC-H2 in CHCl3 in the 1960−2280 cm−1 spectral region (for D, t2 = 3 ps). The dashed
circles emphasize the missing cross-peaks between ν3 and the other modes; see text for discussion.
hydrides add to the metal in a mutually cis fashion due to
homolytic cleavage of the H−H bond, effectively transforming
the complex from square-planar to octahedral geometry and
increasing the oxidation state of the metal to Ir(III). These
changes are evident in the FT-IR spectra of the VC-H2
complex, where the νCO band of VC at 1966 cm−1 disappears
and three new bands appear at 1998, 2098, and 2221 cm−1
(Figure 3B, black vs blue). Upon deuteration, the νCO band
shifts to 2033 cm−1 without the appearance of other bands in
this region (Figure 3B, red). Complete FT-IR spectra for these
complexes are provided in the Supporting Information (Figure
S1).
Ir−Dtrans mode becomes more difficult to locate as it would
appear in a region around 1470−1500 cm−1. This region is
particularly crowded with CC stretching and bending modes
from the phenyl rings of the PPh3 ligands, probably leading to
mixing of the Ir−Dtrans band with the former. Our observations
are in agreement with previous reports of this complex.44 The
Ir(CO) mode, in contrast, localizes in VC-D2. The 2D-IR
spectrum in the νCO region of VC-D2 in CHCl3 (Figure S2 in
the Supporting Information) reveals a diagonal anharmonicity
of ∼25 cm−1 for this mode.
Previously, the ν1 and ν2 bands were assigned to the Ir(C
O) and Ir−H stretching modes.22,43−45 Here, it becomes clear
that these bands correspond instead to the νHCO,sym. and
νHCO,asym. modes, which are strongly coupled and largely
delocalized. For this complex, the fractional contributions of
the Ir−Hcis, Ir−Htrans, and Ir(CO) moieties are, respectively,
1:25:74 for ν1, 3:57:39 for ν2, and 95:5:0 for ν3. The absence of
coupling with ν3 explains also its larger anharmonicity and
lower intensity, reflecting the local character of this mode.
Two mechanisms may, in principle, lead to mode
delocalizationnamely, kinematic and electronic coupling.
Kinematic coupling is dominant in small molecules such as
CO2 and refers to the fact that the vibration of one CO local
mode affects that of the other, as they share the same C atom
(in the case of CO2). One can easily verify that this effect is
negligible here by setting the mass of the Ir atom to
(effectively) infinity in the normal mode calculation, which
does not result in any significant change of the calculated
spectra and displacement vectors. This leaves electronic
coupling of the Ir−H and Ir(CO) modes as the dominating
effect in the present case.
The corresponding 2D-IR spectrum is shown in Figure 3D,
with a cross-peak between ν1 and ν2 but none between ν3 to
any of the other two modes. Anharmonic DFT calculations for
VC-H2 revealed Δ12 = 27 cm−1 (vs Δ12 = 30 cm−1) and
calc
exp
Δ23 = Δ13 ≈ 0 cm−1, in agreement with the notion that ν1
calc
and ν2 are the delocalized symmetric and antisymmetric H−
Ir(CO) modes, whereas ν3 remains a localized Ir−H mode
for the cis hydride (see red displacement vectors in Figure 3A).
The diagonal anharmonicities are also reasonably well
calc
calc
reproduced in the DFT calculations (Δ1 = 19, Δ2 = 54,
Δ3 = 86 cm−1; vs Δ1 = 21, Δ2 = 26, and Δ3 = 73
calc
exp
exp
exp
cm−1).
Comparing the FT-IR spectra of VC-H2 and VC-D2, we
note that the ν1 band of the former is closer to νCO of the
latter. The magnitude of the splitting of the two modes (∼100
cm−1) is an indicator of the coupling between them43,44 and
can be related to the cross-anharmonicity (Δ12) obtained from
2D-IR spectroscopy. This shows the potential for 2D-IR to
reveal these couplings without the need for isotope labeling,
showing the complementarity between both approaches.
Upon deuteration, one would expect the ν3 band to shift to
∼1571 cm−1 (based on the change of reduced mass). Indeed,
we observe a weak and broad band in this region (Figure S1 in
the Supporting Information), which overlaps with several other
modes. We attribute this band to the Ir−Dcis stretching mode,
whereas the weaker bands are CC stretching modes. The
Hydride ligands are among the strongest σ-donors. At the
same time, bonding to carbonyl ligands has a σ and a π
component (the CO ligand being a very good π-acceptor). The
observed strong vibrational coupling between the M−H and
M(CO) modes in a trans geometry (as well as their absence
in the cis configuration) can be rationalized by considering the
trans influence of the hydride ligandthat is, electronic
D
https://dx.doi.org/10.1021/acs.inorgchem.0c00750
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
communication through the σ-bonding orbitals of the complex
allows the displacement along the M−H bond coordinate to
influence the extent of back-bonding on the CO ligand (and
hence the C−O distance), effectively coupling the two
potential energy surfaces and delocalizing the vibration.11,46,47
It is important here to distinguish the trans influence (a
thermodynamic aspect) from the trans effect (a kinetic aspect),
especially when discussing equilibrium properties as above (in
our case, electronic communication, orbital interactions, and
bonding).48,49 The trans influence is maximized in octahedral
d6 complexes (such as VC-HCl and VC-H2) and square-planar
d8 complexes. Also, for σ-donor ligands, a trigonal bipyramidal
geometry with a d8 electronic configuration (like that of
IrHCOP3) gives rise to a strong trans influence. These aspects
have been previously described in ref 50 from an orbital
interaction point of view and in ref 43 in a more general
perspective.
Full FT-IR spectra of VC, VC-H2, and VC-D2; 2D-IR
spectrum of VC-D2; and a discussion concerning the use
of mass-weighted displacement vectors to assess local-
ization of vibrational modes (PDF)
DFT-optimized Cartesian coordinates of IrHCOP3 in
DMF, VC-H2 in CHCl3, and VC-HCl in CHCl3; these
can be visualized with any structure viewer (e.g.,
Mercury) (XYZ)
AUTHOR INFORMATION
■
Corresponding Author
́
́
Ricardo Fernandez-Teran − Department of Chemistry,
University of Zurich, Zurich 8057, Switzerland; orcid.org/
0000-0002-4665-3520; Phone: +41 44 635 44 73;
Email: ricardo.fernandez@chem.uzh.ch; Fax: +41 44 635 68
38
The coupling and mode delocalization is different in
complexes containing, for example, the fac-{Re(CO)3}+ core,
where the M(CO) modes are strongly coupled and totally
delocalized, despite the mutual cis arrangement of the CO
ligands. In a complementary manner, this can be explained by
the contribution of π-back-bonding as electronic communica-
tion takes place through the π orbitals instead.
Authors
Jeannette Ruf − Department of Chemistry, University of Zurich,
Zurich 8057, Switzerland; orcid.org/0000-0003-0495-
6459
Peter Hamm − Department of Chemistry, University of Zurich,
Zurich 8057, Switzerland; orcid.org/0000-0003-1106-
6032
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00750
CONCLUSIONS
■
In this work, we use two-dimensional infrared spectroscopy to
show that the trans configuration of the carbonyl and hydride
ligands is favorable for vibrational coupling between the
M(CO) and M−H vibrational modes. When the two ligands
are in a cis configuration, no vibrational coupling is observed.
2D-IR spectroscopy reveals, in this manner, the extent of
coupling and delocalization of the M−H and M(CO)
modes, yielding complementary information as had been
obtained by isotope labeling in the past. In turn, the coupling
and mode delocalization are responsible for the enhancement
of the weaker M−H stretching mode. These trends were
illustrated by IrHCOP3 and VC-HCl, having purely trans or cis
H−Ir−CO configurations, respectively. In VC-H2, having
hydride ligands both cis and trans to the carbonyl ligand,
strong vibrational coupling was observed exclusively with the
hydride in the trans position. These couplings take place both
in Ir(I) as well as in Ir(III) complexes, highlighting the
apparent independence of this effect on the oxidation state of
the metal center. We attribute the directionality of this effect to
pure electronic coupling of the modes, due to communication
through the σ-bonding orbital joining the CO−M−H
fragment. In addition, we find that the very high anharmonicity
of the M−H stretches is a good indicator of the local character
of these modes. Overall speaking, placing these ligands in a
trans orientation is shown here to be beneficial to enhance the
M−H stretching mode, leading to a promising strategy for the
design of novel hydridocarbonyl complexes acting as PCET
model systems, which allows one to study the proton/hydride
transfer steps directly from the metal hydride perspective using
IR spectroscopy.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Dr. Robin Perutz, Prof. Dr. Roger Alberto,
̈
Gokce
̧
n Tek, and Dr. Jan Helbing for insightful discussions.
The work was funded by the Swiss National Science
Foundation (Grant CRSII2_160801/1) and the University
Research Priority Program (URPP) for Solar Light to
Chemical Energy Conversion (LightChEC) of the University
of Zurich.
REFERENCES
■
(1) Gong, J.; Li, C.; Wasielewski, M. R. Advances in solar energy
conversion. Chem. Soc. Rev. 2019, 48, 1862−1864.
(2) Roy, N.; Suzuki, N.; Terashima, C.; Fujishima, A. Recent
improvements in the production of solar fuels: From CO 2 reduction
to water splitting and artificial photosynthesis. Bull. Chem. Soc. Jpn.
2019, 92, 178−192.
(3) Evans, D.; Osborn, J. A.; Wilkinson, G. Hydroformylation of
alkenes by use of rhodium complex catalysts. J. Chem. Soc. A 1968,
3133−3142.
(4) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to
Catalysis; University Science Books: Sausalito, CA, 2010.
(5) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 6th ed.; Wiley Blackwell, 2014.
(6) Adams, R. E.; Grusenmeyer, T. A.; Griffith, A. L.; Schmehl, R. H.
Transition metal hydride complexes as mechanistic models for proton
reduction catalysis. Coord. Chem. Rev. 2018, 362, 44−53.
̈
(7) Bourrez, M.; Steinmetz, R.; Ott, S.; Gloaguen, F.; Hammarstrom,
L. Concerted proton-coupled electron transfer from a metal-hydride
ASSOCIATED CONTENT
complex. Nat. Chem. 2015, 7, 140−145.
■
(8) Huang, T.; Rountree, E. S.; Traywick, A. P.; Bayoumi, M.;
Dempsey, J. L. Switching between Stepwise and Concerted Proton-
Coupled Electron Transfer Pathways in Tungsten Hydride Activation.
J. Am. Chem. Soc. 2018, 140, 14655−14669.
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00750.
E
https://dx.doi.org/10.1021/acs.inorgchem.0c00750
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
́
́
(9) Liu, T.; Tyburski, R.; Wang, S.; Fernandez-Teran, R.; Ott, S.;
(31) Wadt, W. R.; Hay, P. J. Ab initio effective core potentials for
molecular calculations. Potentials for main group elements Na to Bi. J.
Chem. Phys. 1985, 82, 284−298.
̈
Hammarstrom, L. Elucidating Proton-Coupled Electron Transfer
Mechanisms of Metal Hydrides with Free Energy- And Pressure-
Dependent Kinetics. J. Am. Chem. Soc. 2019, 141, 17245−17259.
(10) Kaesz, H. D.; Saillant, R. B. Hydride complexes of the transition
metals. Chem. Rev. 1972, 72, 231−281.
(11) Hamm, P.; Zanni, M. Concepts and Methods of 2D Infrared
Spectroscopy; Cambridge University Press: Cambridge, U.K., 2011.
(32) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for
molecular calculations. Potentials for the transition metal atoms Sc to
Hg. J. Chem. Phys. 1985, 82, 270−283.
(33) Scalmani, G.; Frisch, M. J. Continuous surface charge
polarizable continuum models of solvation. I. General formalism. J.
Chem. Phys. 2010, 132, 114110.
(34) Bloino, J. A. VPT2 route to near-infrared spectroscopy: The
role of mechanical and electrical anharmonicity. J. Phys. Chem. A
2015, 119, 5269−5287.
(35) Woutersen, S.; Hamm, P. Nonlinear two-dimensional vibra-
tional spectroscopy of peptides. J. Phys.: Condens. Matter 2002, 14,
R1035.
(36) Lehmann, K. K. On the relation of Child and Lawton’s
harmonically coupled anharmonic-oscillator model and Darling-
Dennison coupling. J. Chem. Phys. 1983, 79, 1098.
(37) Mills, I. M.; Robiette, A. G. On the relationship of normal
modes to local modes in molecular vibrations. Mol. Phys. 1985, 56,
743−765.
(38) Vaska, L.; DiLuzio, J. W. Carbonyl and hydrido-carbonyl
complexes of iridium by reaction with alcohols. Hydrido complexes by
reaction with acid. J. Am. Chem. Soc. 1961, 83, 2784−2785.
(39) Vaska, L.; DiLuzio, J. W. Activation of Hydrogen by a
Transition Metal Complex at Normal Conditions Leading to a Stable
Molecular Dihydride. J. Am. Chem. Soc. 1962, 84, 679−680.
(40) Charmant, J. P.; Norman, N. C.; Orpen, A. G.; Whittell, G. R.
Carbonyldichlorohydridobis(triphenyl-phosphine) iridium(III). Acta
Crystallogr., Sect. E: Struct. Rep. Online 2004, 60, m162−m163.
(41) Perakis, F.; De Marco, L.; Shalit, A.; Tang, F.; Kann, Z. R.;
̈
Kuhne, T. D.; Torre, R.; Bonn, M.; Nagata, Y. Vibrational
Spectroscopy and Dynamics of Water. Chem. Rev. 2016, 116,
7590−7607.
(42) Kwac, K.; Lee, C.; Jung, Y.; Han, J.; Kwak, K.; Zheng, J.; Fayer,
M. D.; Cho, M. Phenol-benzene complexation dynamics: Quantum
chemistry calculation, molecular dynamics simulations, and two
dimensional IR spectroscopy. J. Chem. Phys. 2006, 125, 244508.
(43) Appleton, T. G.; Clark, H. C.; Manzer, L. E. The trans-
influence: its measurement and significance. Coord. Chem. Rev. 1973,
10, 335−422.
(44) Vaska, L. Infrared Spectra and Structures of Hydridocarbonyl
Complexes of Transition Metals. J. Am. Chem. Soc. 1966, 88, 4100−
4101.
(45) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds: Part B: Applications in Coordination,
Organometallic, and Bioinorganic Chemistry, 6th ed.; John Wiley and
Sons: Hoboken, NJ, 2008.
(46) Miessler, G. L.; Fischer, P. J.; Tarr, D. A. Inorganic Chemistry,
5th ed.; Pearson: Boston, MA, 2014.
(47) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and Reactivity, 4th ed.; HarperCollins College
Publishers: New York, 1993.
̈
(12) Khalil, M.; Demirdoven, N.; Tokmakoff, A. Coherent 2D IR
spectroscopy: Molecular structure and dynamics in solution. J. Phys.
Chem. A 2003, 107, 5258−5279.
(13) Stewart, A. I.; Clark, I. P.; Towrie, M.; Ibrahim, S. K.; Parker, A.
W.; Pickett, C. J.; Hunt, N. T. Structure and vibrational dynamics of
model compounds of the [FeFe]-hydrogenase enzyme system via
ultrafast two-dimensional infrared spectroscopy. J. Phys. Chem. B
2008, 112, 10023−10032.
(14) Baiz, C. R.; McRobbie, P. L.; Anna, J. M.; Geva, E.; Kubarych,
K. J. Two-Dimensional Infrared Spectroscopy of Metal Carbonyls.
Acc. Chem. Res. 2009, 42, 1395−1404.
(15) Hunt, N. T. 2D-IR spectroscopy: Ultrafast insights into
biomolecule structure and function. Chem. Soc. Rev. 2009, 38, 1837−
1848.
(16) Kiefer, L. M.; King, J. T.; Kubarych, K. J. Dynamics of rhenium
photocatalysts revealed through ultrafast multidimensional spectros-
copy. Acc. Chem. Res. 2015, 48, 1123−1130.
(17) Kiefer, L. M.; Kubarych, K. J. Two-dimensional infrared
spectroscopy of coordination complexes: From solvent dynamics to
photocatalysis. Coord. Chem. Rev. 2018, 372, 153−178.
(18) Jones, B. H.; Huber, C. J.; Massari, A. M. Solvation dynamics of
Vaska’s complex by 2D-IR spectroscopy. J. Phys. Chem. C 2011, 115,
24813−24822.
(19) Jones, B. H.; Huber, C. J.; Spector, I. C.; Tabet, A. M.; Butler,
R. A. L.; Hang, Y.; Massari, A. M. Correlating solvent dynamics and
chemical reaction rates using binary solvent mixtures and two-
dimensional infrared spectroscopy. J. Chem. Phys. 2015, 142, 212441.
(20) Jones, B. H.; Massari, A. M. Origins of spectral broadening in
iodated Vaska’s complex in binary solvent mixtures. J. Phys. Chem. B
2013, 117, 15741−15749.
(21) Jones, B. H.; Huber, C. J.; Massari, A. M. Solvent-mediated
vibrational energy relaxation from Vaska’s complex adducts in binary
solvent mixtures. J. Phys. Chem. A 2013, 117, 6150−6157.
(22) Huber, C. J.; Anglin, T. C.; Jones, B. H.; Muthu, N.; Cramer, C.
J.; Massari, A. M. Vibrational solvatochromism in Vaska’s complex
adducts. J. Phys. Chem. A 2012, 116, 9279−9286.
́
(23) Panman, M. R.; Vos, J.; Bocokic, V.; Bellini, R.; De Bruin, B.;
Reek, J. H.; Woutersen, S. Exchanging conformations of a
hydroformylation catalyst structurally characterized using two-dimen-
sional vibrational spectroscopy. Inorg. Chem. 2013, 52, 14294−14298.
̌
(24) Palecek, D.; Tek, G.; Lan, J.; Iannuzzi, M.; Hamm, P.
Characterization of the Platinum-Hydrogen Bond by Surface-
Sensitive Time-Resolved Infrared Spectroscopy. J. Phys. Chem. Lett.
2018, 9, 1254−1259.
(25) Hamm, P.; Lim, M.; Hochstrasser, R. M. Vibrational energy
relaxation of the cyanide ion in water. J. Chem. Phys. 1997, 107,
10523−10531.
(26) Hamm, P.; Kaindl, R. A.; Stenger, J. Noise suppression in
femtosecond mid-infrared light sources. Opt. Lett. 2000, 25, 1798.
(27) Helbing, J.; Hamm, P. Compact implementation of Fourier
transform two-dimensional IR spectroscopy without phase ambiguity.
J. Opt. Soc. Am. B 2011, 28, 171.
(28) Bloem, R.; Garrett-Roe, S.; Strzalka, H.; Hamm, P.; Donaldson,
P. Enhancing signal detection and completely eliminating scattering
using quasi-phase-cycling in 2D IR experiments. Opt. Express 2010,
18, 27067.
(48) Basolo, F.; Pearson, R. G. Progress in Inorganic Chemistry; John
Wiley & Sons, Ltd., 1962; Vol. 4, _pp 381−453.
(49) Pidcock, A.; Richards, R. E.; Venanzi, L. M. 195Pt-31P nuclear
spin coupling constants and the nature of the trans-effect in platinum
complexes. J. Chem. Soc. A 1966, 0, 1707−1710.
(50) Burdett, J. K.; Albright, T. A. Trans Influence and Mutual
Influence of Ligands Coordinated to a Central Atom. Inorg. Chem.
1979, 18, 2112−2120.
(29) Frisch, M. J.; et al. Gaussian 09, revision D.01; Gaussian, Inc.:
Wallingford, CT, 2009; http://gaussian.com/.
(30) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for
molecular calculations. Potentials for K to Au including the outermost
core orbitale. J. Chem. Phys. 1985, 82, 299−310.
F
https://dx.doi.org/10.1021/acs.inorgchem.0c00750
Inorg. Chem. XXXX, XXX, XXX−XXX