Iridium Hydride Complexes with Cyclohexyl-Based Pincer Ligands: Fluxionality and Deuterium Exchange

Article abstract of DOI:10.1021/acs.organomet.6b00324

Two hydride compounds with aliphatic pincer ligands, (PCyP)IrH₂ (PCyP = {cis-1,3-bis[(di-tert-butylphosphino)methyl]cyclohexane}^- (1) and (PCyP)IrH₄ (2), have been studied, with emphasis on features where such systems differ from arene-based analogues. Both compounds reveal relatively rapid exchange between α-C-H and Ir-H, which can occur via formation of carbene or through demetalation, with nearly equal barriers. This observation is confirmed by deuterium incorporation into the α-C-H position. Complex 1 can reversibly add an N₂ molecule, which competes with the α-agostic bond for a coordination site at iridium. The hydrogen binding mode in tetrahydride 2 is discussed on the basis of NMR and IR spectra, as well as DFT calculations. While the interpretation of the data is somewhat ambiguous, the best model seems to be a tetrahydride with minor contribution from a dihydrido-dihydrogen complex. In addition, the catalytic activity of 1 in deuterium exchange using benzene-d₆ as a deuterium source is presented.

Full text of DOI:10.1021/acs.organomet.6b00324

Article
pubs.acs.org/Organometallics
Iridium Hydride Complexes with Cyclohexyl-Based Pincer Ligands:
Fluxionality and Deuterium Exchange
Alexey V. Polukeev,† Rocío Marcos,‡ Marten S. G. Ahlquist,‡ and Ola F. Wendt*,†
̊
†Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, 22100 Lund, Sweden
‡School of Biotechnology, KTH Royal Institute of Technology, Stockholm SE-106 91, Sweden
S
* Supporting Information
ABSTRACT: Two hydride compounds with aliphatic pincer
ligands, (PCyP)IrH2 (PCyP = {cis-1,3-bis[(di-tert-
butylphosphino)methyl]cyclohexane}− (1) and (PCyP)IrH4
(2), have been studied, with emphasis on features where such
systems differ from arene-based analogues. Both compounds
reveal relatively rapid exchange between α-C−H and Ir−H,
which can occur via formation of carbene or through
demetalation, with nearly equal barriers. This observation is
confirmed by deuterium incorporation into the α-C−H
position. Complex 1 can reversibly add an N2 molecule, which competes with the α-agostic bond for a coordination site at
iridium. The hydrogen binding mode in tetrahydride 2 is discussed on the basis of NMR and IR spectra, as well as DFT
calculations. While the interpretation of the data is somewhat ambiguous, the best model seems to be a tetrahydride with minor
contribution from a dihydrido−dihydrogen complex. In addition, the catalytic activity of 1 in deuterium exchange using benzene-
d6 as a deuterium source is presented.
IrH4,10 and (POCOP)IrH4.10 While these arene-based
compounds have been properly studied, polyhydride pincer
INTRODUCTION
■
Iridium pincer complexes with terdentate PCP type ligands
complexes with aliphatic ligands have received little attention.
Previously, the synthesis and some studies of
have been shown to be effective catalysts for various
dehydrogenation reactions.1 Depending on the substrate,
both di-1 and tetrahydride1,2 species may be relevant to a
catalytic cycle and thus the study of such compounds has not
only fundamental but also practical importance. Previously, we
have studied many aspects of complexes with aliphatic pincer
ligands based on a cyclohexyl framework3 and in dehydrogen-
ation reactions we found that the catalytic activity of iridium
pincer complexes with the cyclohexyl backbone is smaller than
that of their arene-based counterparts.4 Furthermore, we found
quite spectacular reactivity of these complexes, which included
multiple C−H activations leading to formation of a CC
double bond5 and also completely reversible α-methyl
migration via a carbene intermediate.6 This prompted us to
investigate the dihydride complex (PCyP)IrH2 (1)7 and
tetrahydride complex (PCyP)IrH4 (2; Chart 1) in order to
understand how these systems differ from the arene-based
11
[(tBu2PCH2)2CH]IrH4 and [(tBu2PCH2)2CH]IrH2 as well
12
as [(iPr2P-o-C6H4)2CH]IrH4 were reported, but the rather
limited amount of data provided did not allow an unambiguous
clarification of the nature of these rather complex species. We
have also previously shown, by spectroscopic methods and
DFT calculations, that the geometry of 1 is dependent on the
dielectric permittivity of the medium; ligands around the Ir
atom adopt a trigonal-bipyramidal geometry in nonpolar
solvents and square-pyramidal geometry in polar solvents. We
also found that a weak agostic bond between α-C−H and Ir is
present in 1.7 However, there are still many aspects of the
chemistry of compounds 1 and 2 that are unknown and that
could shed additional light on the general properties and
reactivity of hydride complexes with an aliphatic pincer
framework.
8
analogues (PCP)IrH2 (PCP = [2,6-(tBu2PCH2)2C6H3]−),
9
(POCOP)IrH2 (POCOP = [2,6-(tBu2PO)2C6H3]−), (PCP)-
Here, we report on the exchange process between the α-C−
H and Ir−H in compound 1, as well as on a full characterization
of the tetrahydride 2, particularly with respect to the nature of
the hydrides. Furthermore, we report on their use as deuterium
exchange catalysts.
Chart 1. Structures of Hydride Complexes 1 and 2
Received: April 22, 2016
© XXXX American Chemical Society
A
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Table 1. Chemical Shifts, Isotope Shifts, T1(min), JHD, and Gibbs Energies of Activation for Complex (PCyP)IrH4 (2) in
a
Various Solvents
solvent
methylcyclohexane-d14
toluene-d8
−10.23
dichloromethane-d2
δ IrH4 (+25 °C), ppm
−10.40
−10.58
b
δav IrH4 (ca. −100 °C), ppm
Δδ (−100 °C to +25 °C)
−10.6
−10.3
−10.9
−0.2
−0.1
−0.3
c
δ IrHset 1Hset 2
−9.6 and −11.5
−1.9
not determined
−9.5 and −11.0
−1.5
19.7
not resolved
not resolved
not determined
not determined
ca. 150 (−70 °C)
Δδ (Hset 1−Hset 2
)
d
ΔG⧧ (100 °C), kcal/mol
1
e
ΔG⧧ (−70 °C), kcal/mol
∼8.7
154 (−50 °C)
3.0
∼8.7
159 (−50 °C)
2.9
2
T1(min), ms
f
J
HD, Hz
2.8
f
Δδ (IrH4−IrHD3)
+0.07
+0.07
+0.09
a
For the analogous data for 1, see ref 10. For a brief comparison in aromatic solvent: 1, δ IrH2 −22.85 ppm, δ α-C−H 0.54 ppm, δ P 86.9 ppm; 2, δ
b
c
IrH4 −10.23 ppm, δ α-C−H 2.42 ppm, δ P 71.7 ppm. (δ IrHset 1 + δ IrHset 2)/2. Positions of centers of multiplets for “upper” and “lower” sets of
protons. For α-C−H and IrH4 exchange. For averaged exchange between hydrides. The signal of IrH2D2 was also resolved, with JHD = 2.8 Hz and
Δδ (IrH4−IrH2D2) = +0.06.
d
e
f
Crystallography. Crystals of complex 2 were grown by slow
evaporation of a hexane solution of 2 at −35 °C under an N2
atmosphere. Intensity data were collected at 120 K with an Oxford
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were conducted
under an inert gas atmosphere using standard Schlenk and glovebox
Diffraction Xcalibur 3 system using ω scans and Mo Kα radiation (λ =
0.71073 Å).14 The CCD data were extracted and integrated using
techniques unless otherwise stated. All solvents were distilled under
Crysalis RED.15 The structures were solved using direct methods and
vacuum from Na/benzophenone. Hydrocarbon deuterated solvents
refined by full-matrix least-squares calculations on F2 using SHELXL16
were distilled under vacuum from Na/benzophenone; CD2Cl2 was
distilled under vacuum from calcium hydride. Substrates used for
and SIR-92.17 Non-H atoms were refined with anisotropic displace-
catalytic deuteration were used as received, after degassing and refilling
with nitrogen. Complexes 1 and 2 were synthesized according to
previously published procedures.5,7 NMR spectra were recorded on
Varian Unity INOVA 500 MHz and Bruker Avance 400 MHz
instruments. 1H and 13C NMR chemical shifts are reported in parts per
million and referenced to the signals of deuterated solvents. 31P NMR
chemical shifts are reported relative to external 85% solution of
phosphoric acid. A calibration curve obtained using a neat methanol
sample was used for variable-temperature measurements. Spin−lattice
relaxation times (T1) were measured using a standard inversion
recovery pulse sequence. The line shape analysis was done using
WinDNMR 7.1.13 IR spectra were recorded on a Bruker Alpha FT-IR
spectrometer.
ment parameters. Hydrogen atoms were constrained to parent sites,
using a riding model. Although there is substantial electron density
where the hydrides are expected, we were unable to refine them
properly. The CCDC deposition number is 1473015.
RESULTS AND DISCUSSION
■
Variable-Temperature NMR Studies of 1 and 2. The
dynamic behavior of 1 and 2 in three different solvents,
toluene-d8, methylcyclohexane-d14, and dichloromethane-d2,
was investigated and revealed two types of exchange processes.
The first process is responsible for broadened signals of the
hydride ligands and the α-C−H of complex 1 at room
temperature and is interpreted as an exchange of hydrogen
atoms between these positions. At elevated temperatures this
process results in very broad signals of α-C−H and IrH2, while
upon lowering the temperature to 0 °C and below it is frozen
out. The Gibbs free energies of activation are 16.7, 16.3, and
15.7 kcal/mol in methylcyclohexane-d14, toluene-d8, and
dichloromethane-d2, respectively, at 25 °C. A line shape
analysis through temperatures from 25 to 90 °C (in toluene-
Synthesis of 2-D,HD3. Deuterium gas was slowly bubbled through
a solution of complex 2 (0.017 g, 0.029 mmol) in C6D6 (0.7 mL) in a
rubber-septum-capped NMR tube equipped with an outlet needle for
ca. 50 min. Complex 2-D,HD3 was formed with >95% deuteration of
hydridic positions. The compound suffers from slow incorporation of
t
deuterium into Bu groups which makes JHD irresolvable, and it was
used immediately after preparation.
Synthesis of 1-d38. A solution of complex 1 (0.019 g, 0.032
mmol) in C6D6 (3 mL) was heated on an oil bath in a sealed Straus
flask for ca. 3 h at 120 °C. Complex 1-d38 was formed with >95%
deuteration of hydridic positions.
d8) resulted in ΔH⧧ = 14.3( 0.4) kcal/mol and ΔS⧧
=
−6.6( 1.2) cal/(mol K) (see the Supporting Information for
details). The second dynamic process is an exchange of
hydrides between each other and was discussed previously.7
For tetrahydride complex 2 resonances corresponding to the
α-C−H and Ir−H are sharp at room temperature, indicating
that any exchange process is slower than for 1 (see Table 1).
Indeed, ΔG⧧ measured using line-shape analysis at 100 °C in
toluene-d8 is 19.7 kcal/mol, which is ca. 3 kcal/mol higher than
for complex 1. At low temperatures decoalescence of hydride
resonances of 2 occurs, resulting in the appearance of a rather
General Procedure for Catalytic Deuterium Exchange. A
Straus flask was charged with (PCyP)IrH2 (1; 0.3 mL of a stock
solution in C6D6, 0.00478 mmol), 0.7 mL of C6D6, substrate (0.478
mmol, 100 equiv), and cyclooctane (0.010 mL, 0.074 mmol, internal
standard) inside a nitrogen atmosphere glovebox. The flask was sealed
and heated on an oil bath for 24 h at 150 °C, cooled to room
temperature, and analyzed by NMR. The degree of deuteration was
determined by integration of substrate signals versus cyclooctane prior
to and after heating. In several cases (olefins, cyclooctane) external
standards (both 1H and 2H) were added after the reaction in order to
quantify the degree of deuteration.
1
complex pattern (Figure 1) in the H NMR spectrum; at the
B
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occupying the site opposite to the α-C−H group. Further
cooling to −30 °C transforms this broad triplet into a complex
multiplet with sufficiently sharp lines, which seems to be no
longer affected by exchange and consists of overlapping signals
of nonequivalent hydrides. Interestingly, these signals move
away from each other when the temperature decreases and
finally are separated by 0.11 ppm (ca. 57 Hz) at ca. −100 °C.
1
The H{31P} spectrum recorded at −60 °C revealed an ABX
system, with 8.5 Hz coupling between hydrides (which is
believed to have a negative sign19,20), as well as 4.0 and 2.0 Hz
couplings between α-C−H and anti and syn hydrides,
respectively. The ratio 1:3 changes from 7:1 at 0 °C to ca.
1:1 at −60 °C. Since the coordination of nitrogen replaces the
agostic C−H bond in the coordination sphere of 1, the affinities
of these two ligands for Ir seem to be roughly comparable. In
contrast, complex 2 does not react with 1 atm of N2.
Stoichiometric Deuterium Exchange in 1 and 2. In
order to measure JHD values, which are useful tools for
determining H−H distances in hydride complexes, deuterium-
labeled samples were prepared (Scheme 2). Deuteration of
Figure 1. VT NMR spectra of 2 in methylcyclohexane-d14. Asterisks
indicate possible positions of maxima. The small triplet in the top
spectrum is <0.1% impurity.
Scheme 2. Deuterium Exchange with D2 and C6D6 for
Complexes 1 and 2
same time, only one signal is observed in the 31P{1H} NMR
spectrum. One could expect the existence of four lines in the
proton spectrum of 2 in the case of a tetrahydride structure and
three or six lines in the case of dihydrido−dihydrogen structure,
depending on how fast H2 rotation is on the NMR time scale.
However, at least eight lines are observed in the spectrum of 2;
moreover, the lines in each set of signals (likely corresponding
to different sides of the plane formed by the two chelating five-
membered metallacycles) are moving closer to each other upon
decreasing the temperature. Tentatively, we ascribe such
behavior to the existence of several exchange couplings between
hydrides, which leads to splitting and appearance of additional
lines.18 Such couplings can be fairly large and temperature-
dependent, decreasing with a decrease in the temperature,
which is in line with the behavior of 2. Different line widths
may be a result of several exchange processes. Taking the
centers of the two sets of signals as Δν0, it is possible to roughly
estimate ΔG⧧ as 8.7 kcal/mol at coalescence (ca. −70 °C),
which corresponds to the exchange between iridium hydrides.
Differences between different solvents are fairly small for 2 and
will not be discussed in detail.
tetrahydride 2 was achieved by slow bubbling of deuterium gas
through a solution of the complex for ca. 1 h. Isotope effects on
hydride chemical shifts are small (see Table 1), and signals of
individual isotopomers are severely overlapping. Therefore, the
observation of a single isotopomer, 2-D,HD3, required a high
degree of deuteration (>95%); incorporation of deuterium into
the α-C−H position confirmed the existence of the exchange
process with the hydrides observed in the VT NMR spectra.
The resulting samples of 2-D,HD3 suffer from deuterium
scrambling into tert-butyl groups and were measured
immediately after preparation. Even a small amount of
deuterium which migrates to tert-butyl groups upon evapo-
ration of the solvent makes H−D couplings unresolvable;
interestingly, exchange into one set of tert-butyl groups is
significantly easier than into the other. For this reason, samples
of 1-D,HD cannot be prepared by thermolysis of 2-D,HD3;
exchange with benzene-d6 was used instead. Thus, heating of a
solution of 1 in benzene-d6 at moderate temperatures mainly
results in deuteration of hydride and α-C−H positions, while
above 100 °C the rate of exchange with tert-butyl groups
becomes significant. Within several hours at 120 °C, complete
deuteration of tert-butyl groups in 1 was achieved to give a
sample suitable for JHD measurement.7,21
Complexation of 1 with Nitrogen. Initial VT studies of 1,
which were performed under an N2 instead of an Ar
atmosphere, revealed the formation of the dinitrogen complex
3 at low temperatures (Scheme 1). NMR signals of 3 are almost
Scheme 1. Reaction of Complex 1 with Nitrogen
indistinguishable from the baseline at room temperature, but
upon cooling to 0 °C a new peak at 65.5 ppm in the 31P{1H}
NMR spectrum as well as a broad triplet at −9.85 ppm in the
1H NMR spectrum appear (Figure S1 in the Supporting
Information). Chemical shifts around −10 ppm are character-
istic of a hydride trans to a high-field ligand and indicates a
mutual trans arrangement of hydrides, with the N2 molecule
Mechanism of Exchange between α-C−H and Ir−H in
1 and 2. The sp3 hybridization of the metalated carbon
together with the presence of an α-hydrogen atom results in
some unique properties of iridium pincers with aliphatic
ligands. First of all, the dihydride complex 1 is an 18e
compound in contrast to the 16e arene-based counterparts due
to the agostic interaction between iridium and the α-C−H
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Figure 2. Energy profile for exchange between α-C−H and Ir−H for dihydride complex 1. The energy profile was calculated using LACVP**/
B3LYP and single corrections for solvent (benzene) and dispersion: LACV3P**++/B3LYP-D3. The relative Gibbs free energies are given in kcal/
mol.
Figure 3. Energy profile for exchange between α-C−H and Ir−H for tetrahydride complex 2. The energy profile was calculated using LACVP**/
B3LYP and single corrections for solvent (benzene) and dispersion: LACV3P**++/B3LYP-D3. The relative Gibbs free energies are given in kcal/
mol.
bond. The second important thing is the existence of an
exchange process between the α-C−H and hydride positions
for both 1 and 2. For complex 1 there are two reasonable
mechanisms for this process: demetalation and migration of the
α-C−H hydrogen to give the carbene complex. DFT
calculations provide some insight into the details of the
exchange (Figure 2). The computed energies are in very good
agreement with the experimental results. Thus, by calculations
it is possible to identify two isomers of 1, namely triequatorial
1-eee and diequatorial 1-eae, depending on whether an axial or
equatorial C−H bond is metalated by Ir. The observed spectra
correspond exclusively to 1-eee due to its greater stability, which
agrees with the calculations (1-eae is 7.1 kcal/mol higher in free
energy). DFT calculations show that the complex 1-eee can go
through a demetalation process leading to complex 5, followed
by an oxidative addition to obtain the 1-eae complex. The other
possible mechanism for the formation of 1-eae is 1,2-migration.
The process begins with an α-elimination of a hydrogen atom
D
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to give Ir carbene 4, which via an α-hydride migration
(insertion) leads to the 1-eae complex. Both pathways have
reasonably low barriers for a room-temperature reaction, where
the carbene path has a lower free energy of activation that is
closer to the experimentally observed value (20.3 (a) and 18.1
(b) kcal/mol vs 16.3 kcal/mol (experimental) in aromatic
solvent). In contrast, for tetrahydride 2 the demetalation
pathway (Figure 3) lies slightly lower in free energy than the
dissociation of dihydrogen and exchange through a carbene
(20.7 (a) and 19.7 (b) kcal/mol vs 19.7 kcal/mol
(experimental) in benzene). Other possible mechanisms, for
instance, direct formation of 4 from 2 via H−H coupling, were
considered but were found to be energetically unavailable. It
should be noted that the barriers for reductive elimination
(around 15 kcal/mol), and especially the barriers for oxidative
addition (metalation) (up to 1.2 kcal/mol for equatorial
position and up to 6.9 kcal/mol for axial position) are
remarkably low for both 1 and 2, indicating their fluxional
nature. The related compounds with aromatic backbones are
significantly more rigid; for comparison, barriers for reductive
elimination/oxidative addition of Ph−H bond in the benzene-
based complex trans-[2,6-(H2PCH2)2C6H3]IrH(CO)H (H
as indicated by virtually the same values of JHD and T1(min) in
different solvents. Isotope effects on chemical shift are positive
(to high field) and small and do not exceed 0.09 ppm (for three
deuterium atoms in 2-HD3), which indicates that the
distribution of deuterium in the molecule is close to statistical.
This is also confirmed by the observation of JHD for the 2-H2D2
isomer, which is resolved in dichloromethane-d2 and equal to
JHD in 2-HD3. At the same time, the interpretation of
spectroscopic data is more complex, since the observed
parameters are averaged between several hydride positions.
Three possible structures were considered:10 classical tetrahy-
dride 2A and trans and cis dihydrido−dihydrogen complexes
2B,C (attempts to locate the analogue of 2C with a syn
orientation of the α-C−H and H2 ligand were unsuccessful).
For simplicity, during the analysis of T1(min) and JHD the two
sides of the plane formed by the chelating metallacycles were
taken as equivalent (Chart 2); in the real system fairly small
differences between “upper” and “lower” rHH could be observed
by calculations.
Chart 2. Possible Structures of Tetrahydride Complex 2
i
atoms replaced real Pr groups on P atoms) were calculated
to be 29.9 and 16.4 kcal/mol, respectively.22 These results also
imply that during the synthesis of the parent compound
(PCyP)IrHCl3c the highest barriers should be associated with
bringing the pincer ligand to a desired conformation, since
when this is done the C−H activation is nearly barrierless.
The deuterium scrambling into tert-butyl groups is also easier
for 1 than for arene-based counterparts (for example, 4 h at 120
°C in benzene-d6 for 1 vs 12 h at 150 °C for (p-ArF-
POCOP)IrH2 for complete deuteration23), which is in line with
the increased electron-donating ability of the aliphatic pincer
ligand.24
Given the complexity of the system, it seems reasonable to
start with the computational studies of 2. Tetrahydride 2A is
found to be lowest in energy. Structure 2C lies 3.0 kcal/mol
above 2A and thus most likely should be rejected; 2B, however,
is only 0.14 kcal/mol higher than 2A (Figure 5). This difference
Hydride Bonding Mode in Complex 2. The XRD-based
molecular structure of tetrahydride complex 2 is shown in
Figure 4, revealing the triequatorial conformation of the
Figure 4. Molecular structure of tetrahydride complex 2 with thermal
ellipsoids at the 50% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Ir−C1 2.187(5),
Ir−P1 2.3218(11), Ir−P2 2.3214(11); P−Ir−P 165.64(4), C1−Ir−P1
82.83(12), C1−Ir1−P2 82.95(12).
Figure 5. Free energy profile for 2A−C and hydride exchange. The
energy profile was calculated using LACVP**/B3LYP and single
corrections for solvent (benzene) and dispersion: LACV3P**+
+/B3LYP-D3. The relative Gibbs free energies are given in kcal/mol.
is too small to be taken at face value, but taking into
consideration the good agreement between measured and
calculated energies for the studies of these systems it seems
reasonable that such a difference at least semiquantitatively
describes the reality and 2B is to some extent populated. An
extremely low barrier for interconversion between 2A and 2B is
in agreement with the observation of only one signal in 31P{1H}
NMR spectra also at low temperatures. An exchange between
cyclohexyl ligand. The P−Ir−P angle of 165.64(4)° is close
to typical values for Ir complexes with the PCyP ligand.3 A
fairly long Ir−C bond length of 2.187(5) Å is consistent with its
easy cleavage (see above). Unfortunately, we were not able to
locate the hydrides properly in the Fourier map; thus, we have
to rely on solution-state data. The tetrahydride complex 2
seems to be less affected by solvation effects than dihydride 1,7
E
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Ha and Hb, as well as between Hd and Hc, is thought to be
frozen near −100 °C, since the calculated barrier is 8.1 kcal/
mol; this is in line with the experimental barrier for
decoalescence around 8.6 kcal/mol. At the same time,
according to calculations, exchange between Hb and Hc (with
a barrier of 5.6 kcal/mol) remains fast in comparison to the
NMR time scale even at the lowest accessible temperature
(−110 °C); it should be noted, however, that an error of 1−2
kcal/mol is enough to change the conclusion. We will go
through the different indications concerning the bonding mode
in the following paragraphs.
be around 5−7 Hz, which falls in the range previously observed
for compressed dihydrides. Since the correlation between JHD
and r(H−H) is far from accurate at long distances,27,28 some
additional assumptions are required for a reciprocal solution. It
seems reasonable to take the value of −1 Hz as an estimation
for two-bond trans H-D couplings in 2A,B, because the
corresponding value in 3 is −1.3 Hz. If the principal shape of
curve (4) is correct, this value is the most negative coupling
possible for related systems, and at smaller H−H distances,
corresponding to transoid interactions, couplings should
decrease in module and pass through 0. Thus, we estimate
these couplings as 1 Hz. For smaller distances, eq 4 is
expected to give reasonably accurate results; therefore, the
calculated H−H distances were used to obtain H−D coupling
constants. Such a treatment gives JHDav ≈ 1 Hz for 2A and JHDav
≈ 4 Hz for 2B. From Jobs = 2.9 Hz, a ca. 40:60 ratio between 2A
and 2B under ambient conditions can be roughly estimated, in
good agreement with the 55:45 ratio derived from calculations.
ii. T1(min). This value was calculated using the atomic
positions of DFT optimized tetrahydride structure 2A using
Halpern’s method,; it is only slightly longer than that
experimentally observed (174 ms vs 154−159 ms), and bearing
in mind some underestimation of relaxation rate obtained by
this method for (PCyP)IrHCl,7 these values should be
considered as comparable. At the same time, both isomers of
the dihydrido−dihydrogen structure are inconsistent with the
experimental observations, giving rise to calculated T1(min)
values much smaller than 100 ms (for example, 20 ms for 2B).
Fixing of the hydrides at the calculated positions for structures
B and C and varying the angle between atoms in the
dihydrogen ligand lead to an rHH value above 1.5 Å in order
to make the calculated T1(min) value consistent with
experiment, which argues against the existence of dihydrogen
or elongated dihydrogen ligands. Relaxation rates for 2A,B are
i. JHD. Relationships between observed JHD and individual
couplings are given in eqs 1−3 for 2A−C, respectively (for
details, see refs 9 and 25)
2t
2t‐oid
2c‐oid
J
+ 22cJHD + 2
J
+ 2
J
HD
HD
HD
Jobs
Jobs
Jobs
=
=
=
(1)
(2)
(3)
6
1
2c
2t
JHD + 2 JHD + 2 JHD + 2JHD
*
*
6
JHD + 4 JHD + 2JHD
1
2
*
6
where 2tJHD and 2cJHD are two-bond couplings between mutually
cis and trans H and D atoms (for example, a and b, a and d)
2t‑oid
2c‑oid
and
J
HD
and
J
HD
are two-bond couplings between
cisoid (for example, positions b and c) and transoid (for
example, positions a and c) H and D atoms, while the symbol *
indicates coupling (cis or trans) between a hydride and a
dihydrogen ligand. Typically all two-bond couplings are taken
as 1 Hz;19 for 2B,C this assumption will lead to Jobs = (1JHD
1
5)/6 and thus JHD = 17
5 Hz. This, in turn, implies the
presence of an elongated dihydrogen ligand with rHH = 1.06−
1.31 Å (Table 2 and eq 419,26).
R
2A = 5.7 s−1 (174 ms) and R2B = 50.0 s−1 (20 ms), respectively,
which given Robs = 6.3−6.5 s−1 (154−159 ms) is not consistent
with a significant population of 2B, in contradiction with JHD
data. These calculations are done assuming a slow rotational
regime for the H2 ligand; indeed, the calculated barrier for
rotation of 5.6 kcal/mol is much higher than that of molecular
tumbling. It was noted, however, that if significant torsional
librations of coordinated H2 are present, a somewhat similar
correction factor between 0.794 and 1 should be applied to the
r(H−H)−T1(min) relationship,29 depending on the angle of
libration. Such a correction may increase the calculated
T1(min) value for 2B up to 70 ms and thus may be consistent
with up to 5−10% of population of 2B.
r(H−H) = 0.74 − 0.494
⎛
⎞
16.1447 − 260.65 − 4(1 + 0.32895(JHD))
⎜
⎜
⎝
⎟
⎟
⎠
× ln
2
(4)
Structure 2A also cannot be excluded; if one considers all
pairs of adjacent hydrogens as compressed dihydrides, a similar
procedure would lead to an estimation for 22cJHD
+
J
=
2c‑oid
HD
14−20 Hz, and thus individual couplings between mutually cis
(a and b, c and d) and cisoid (b and c) H and D atoms would
iii. Experimental IR Spectrum. The experimental IR
spectrum of 2 in hexane matches that calculated for 2A quite
well (although the calculated bands are somewhat blue shifted),
but not that for 2B (Figure 6). The experimental band at 1940
cm−1 is slightly asymmetric, which in principle may be
associated with the presence of a very low intensity additional
absorption at ca. 1900 cm−1, for example, from 2B; however,
such a speculation should be treated with caution.
Table 2. H−D Couplings and Possible H−H Distances
within Dihydrogen Ligands in 2 and Related Compounds
iv. Low-Temperature NMR Spectra. The low-temperature
IR spectra can in principle represent 2A,B or be a weighted
average of those two. According to the calculated barriers, one
set of signals should be assigned to Ha and Hd, while another
set can be assigned to Hb and Hc, which are likely to be in
exchange. Since only one large coupling between Hb and Hc
may be present, 2A is expected to give rise to four lines, while
other models give three or six lines. None of these models can
compound
2
J
obs,HD, Hz
2.9 0.1
17
1.06−1.31
3.0 0.211
18
3.5 0.19
22 59
4.4 0.19
26 59
0.90−1.089
a
1JHD, Hz
5
5
b
r(H−H), Å
1.04−1.28
0.96−1.179
a
b
1
Extracted using eqs 2 and 3. Using eq 4 and JHD
.
F
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Organometallics
Article
0.2−0.3 Å.30 If such an underestimation is present, it would
significantly increase T1(min) for 2B and in turn give better
agreement between the various methods.
Catalytic Deuterium Exchange. Catalytic deuterium
exchange is an important method for the preparation of
deuterium-labeled compounds, which find use in many areas.31
As complex 1 is capable of isotope exchange with benzene-d6, it
seemed reasonable that it could also mediate isotope exchange
between benzene-d6 and arenes. In line with this, it was found
that other substrates, including those with activated and to
3
some extent with nonactivated Csp −H bonds, can be
successfully deuterated (Scheme 3).
Scheme 3. Complex 1 as Catalyst for Deuterium Exchange
a
between Benzene-d6 and Various Substrates
Figure 6. Stick diagram representing experimental and calculated IR
spectra. Intensities are normalized to the most intense band.
Experimental spectrum is given in Figure S2.
explain the observed NMR pattern; therefore, we suppose that
additional splitting is a result of quantum-mechanical exchange
couplings, likely between Ha and Hd, as well as Hb and Hc, as
described above.18 While we cannot unambiguously assign the
low-temperature NMR spectra to one of the possible structures,
it is worth noting that T1 values measured for the decoalesced
signals at −80 °C of ca. 360 ms (methylcyclohexane-d14) not
only argue against the dihydrogen nature of any of these signals
but also significantly limit the possible contribution of 2B to the
observed spectra.
v. Reactivity of 2. The reactivity of 2 with deuterium clearly
shows that dihydrogen-containing structures are energetically
available at room temperature but does not prove that they are
significantly populated, because the barriers for interconversion
among 2A−C are negligible in comparison to thermal energy.
All in all, DFT calculations suggest the tetrahydride 2A to be
a global minimum, with the energy gap between 2A and 2B
being comparable to the expected error of calculations. T1(min)
and IR data argue for tetrahydride structure 2A, with small or
negligible contribution from 2B. H−D coupling, at the same
time, is only consistent with significant (around 50%)
population of 2B. In this respect, it is interesting that the
related benzene-based complex (PCP)IrH4 was found to have a
tetrahydride structure at 100 K by neutron diffraction.10
Solution parameters for (PCP)IrH4 were ambiguous; thus, IR
spectra were better consistent with a tetrahydride: JHD for a
dihydrido−dihydrogen complex and T1(min) for both of these
structures. Complex 2 is apparently closer to a “classical”
tetrahydride than (PCP)IrH4, as seen from the smaller JHD and
longer T1(min) values and decoalescence of the hydride
resonances, which were not reported for the latter compound.
Therefore, it seems likely that 2A is the predominating
structure in the solution of 2. A precise estimation of the
contribution of 2B is difficult. It is important to note that an
analysis of JHD is only approximate because several contributing
couplings are unknown and correlation (4) at certain distances
suffers from data scattering. At the same time, the PES is
exceptionally flat toward movement of Hb and Hc; a change of
the distance from 0.97 to 1.75 Å costs only 0.1 kcal/mol. For
such shapes of the PES, occupation of vibrational states may
affect the geometry;26,30 for example, r(H−H) values in
elongated dihydrogen compounds can be underestimated by
a
The values given with the structures show the percentages of
deuteration.
Nonhindered positions in arenes are deuterated almost
quantitatively, and the same is true for olefins which are not
subjected to isomerization such as tert-butylethylene. 1-Hexene
undergoes isomerization into internal hexenes, and deuterium
scrambling takes place in all positions with the percentage of
deuteration decreasing upon moving away from the double
2
bond along the carbon chain. Importantly, not only could Csp −
H bonds be involved in exchange but also activated and
nonactivated Csp −H bonds, a rarely observed event.32 Thus, ca.
3
15% deuteration of methyl groups in toluene and tert-
butylethylene was observed. Up to 60% of C−H bonds of
triethylamine were deuterated using increased catalyst loading
(10 mol %). At the same time, the level of deuteration of purely
aliphatic substrate cyclooctane is fairly low. Deuteration of
arenes presumably proceeds via oxidiative addition of the
substrate C−H bond to 1 with formation of an Ir(V) complex,
followed by reductive elimination. In the case of triethylamine,
traces of dehydrogenated products were observed during the
G
DOI: 10.1021/acs.organomet.6b00324
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Organometallics
Article
3
course of the reaction, and thus an insertion/β-elimination
mechanism can make a contribution for this substrate. Olefins
are likely deuterated through removal of H2 from Ir, resulting in
an Ir(III)/Ir(I) catalytic cycle, similar to that previously
observed by Hartwig for [(tBu2PCH2)2CH]IrH(NH2).33 In
general, the latter complex operates at lower temperatures and
does not isomerize olefins, providing better selectivity. At the
same time, the use of 1 allows for a decrease in catalyst loading
respect, it should be noted that (PCsp P)Ir complexes that do
not have a α-C−H bond were recently found to be significantly
more stable under alkane dehydrogenation conditions.35
Regarding the tetrahydride 2, it has a higher stability with
respect to loss of hydrogen in comparison to (PCP)IrH4 and
especially (POCOP)IrH4. This means that at low temperatures
complex 2 is unlikely to catalyze acceptorless dehydrogenation.
3
SUMMARY
and deuteration of Csp −H bonds to some extent. For
■
cyclooctane, it is not easy to unambigously determine which
mechanism is operative. Deuteration of methyl groups in
toluene and tert-butylethylene clearly shows that the Ir(III)/
Ir(V) cycle is possible, but we cannot completely exclude that
some small unobserved amount of cyclooctene may form under
experimental conditions to promote an Ir(III)/Ir(I) catalytic
cycle.
In summary, we have presented some studies of the dihydride
and tetrahydride iridium complexes (PCyP)IrH2 (1) and
(PCyP)IrH4 (2) with a cyclohexane-based pincer ligand.
These compounds are highly fluxional and complex systems
with several dynamic processes present at room temperature.
Thus, both compounds undergo exchange between α-C−H and
Ir−H positions. For complex 1, two mechanismsthrough
carbene and through demetalationhave reasonably low
barriers to be responsible for such exchange, with the former
lying slightly lower in energy and closer to the experimentally
observed ΔG⧧ values. In contrast, for complex 2 the
demetalation mechanism is slightly more favorable. These
data suggest that for complex 1 at least three additional
structuresaxially metalated isomer 1-eae, carbene 4, and
demetalated complex 5are kinetically accessible at room
temperature (but not significantly populated) and may affect its
reactivity.
NMR spectra of complex 2 are much less solvent dependent
in comparison to those of 1. As for 1, a low-temperature
decoalescence of hydride resonances was achieved, with the
observed spectral pattern being very complex likely due to the
presence of quantum-mechanical exchange couplings. IR
spectra and T1(min) analysis suggest a tetrahydride structure
for 2, while JHD is more consistent with a significant
contribution of a dihydrido−dihydrogen structure. Bearing in
mind the complexity of the system, and the fact that both
T1(min) and JHD interpretation requires some assumptions, we
rely on the more “straightforward” comparison of IR spectra
and conclude that the tetrahydride is the main compound in
solution, with possible minor contribution from the dihydrido−
dihydrogen complex.
Relevance to Catalytic Dehydrogenation. Previously we
reported that the activity in catalytic dehydrogenations for
complex 3 is lower than that of its arene-based counterparts.4
Observations of hydride complexes 1 and 2 shed some light on
t
this problem. The reaction of (PCyP)IrHCl with BuONa in
neat cyclooctane produces exclusively dihydride 1 (together
with 1 equiv of cyclooctene, COE), while mixtures of
(PCP)IrH2 and (PCP)Ir(COE) as well as (POCOP)IrH2 and
(POCOP)Ir(COE) in ratios ca. 9:1 and 1:1, correspondingly,
were formed for complexes with benzene-based phosphine and
phosphinite backbones,23 in line with the increased electron-
donating ability of the aliphatic pincer ligand. This stabilization
of the Ir(III) oxidation state is, presumably together with
increased steric barriers, responsible for the low dehydrogen-
ation rates at moderate temperatures; thus, hydrogenation of
tert-butylethylene, which is an important part of the catalytic
cycle, is slower by approximately 1 order of magnitude for 1 in
34
comparison to (PCP)IrH2 at similar temperatures (65−70
°C). At high temperatures, complex 1 decomposes under
catalytic conditions. While complex 1 is moderately stable in
solution, as indicated by the performance in catalytic deuterium
exchange, the addition of a hydrogen acceptor such as tert-
butylethylene or cyclooctene causes relatively fast decom-
position at temperatures around 150 °C. In addition to
metalation of tert-butyl groups, which is possible for arene
counterparts also, 1 and 2 undergo demetalation and activation
of the α-C−H bond; these processes seem to be responsible for
the low thermal stability of dihydride 1. Decomposition
pathways may include formation of species with a demetalated
cyclohexyl ring or formation of the olefin complex (Scheme 4),
as we previously showed for these systems.5,6 The latter
pathway is somewhat more likely, since addition of a hydrogen
acceptor significantly facilitates decomposition of 1. In this
Finally, we have shown that complex 1 is a good catalyst for
deuterium exchange using deuterobenzene as the deuterium
source; Csp −H bonds of alkenes and arenes are deuterated
almost quantitatively, while a moderate or low degree of
2
3
deuetration was observed for substrates with Csp −H bonds.
The reduced thermal stability of 1 in comparison to its arene-
based counterparts is related to the processes involving the α-
C−H bond; this should be kept in mind while designing new
pincer ligands.
Scheme 4. Proposed Pathway for Decomposition of
Complex 1 at High Temperatures
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications Web site. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.6b00324.
Eyring plot for exchange between α-C−H and Ir−H in
toluene-d8, IR spectrum of complex 2, 1H NMR
spectrum of 3 in toluene-d8, additional computational
results, and atomic coordinates for all of the species
(PDF)
Crystallographic data for 2 (CIF)
H
DOI: 10.1021/acs.organomet.6b00324
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(26) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledos
́
, A.; Pons, V.;
AUTHOR INFORMATION
Corresponding Author
*E-mail for O.F.W.: ola.wendt@chem.lu.se.
■
Heinekey, D. M. J. Am. Chem. Soc. 2004, 126, 8813−8822.
(27) Thus, for dinitrogen complex 3 one could expect a distance of
around 3.2 Å for mutually trans hydrides (calculations give 3.32 Å),
while the observed H−H coupling of −8.5 Hz corresponds to JHD
=
Notes
−1.3 Hz and rHH = 2.39 Å.
The authors declare no competing financial interest.
(28) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173−4184.
ACKNOWLEDGMENTS
■
(29) Morris, R. H.; Wittebort, R. J. Magn. Reson. Chem. 1997, 35,
243−250.
Financial support from the Swedish Research Council, the Knut
and Alice Wallenberg Foundation, and the Crafoord
Foundation is gratefully acknowledged. Computational resour-
ces were provided by the National Supercomputer Centre in
́
(30) (a) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledos, A.;
Heinekey, D. M. J. Am. Chem. Soc. 2005, 127, 5632−5640. (b) Maltby,
P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R. H.; Klooster,
W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc. 1996, 118,
5396−5407.
Linkoping, Sweden.
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(31) (a) Atzrodt, J.; Derdau, V.; Fey, T.; Zimmermann, J. Angew.
Chem., Int. Ed. 2007, 46, 7744−776. (b) Junk, T.; Catallo, W. J. Chem.
Soc. Rev. 1997, 26, 401−406.
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