Stereoisomerism as an Origin of Different Reactivities of Ir(III) PC(sp3)P Pincer Catalysts

Article abstract of DOI:10.1021/acs.inorgchem.0c00797

Two stereoisomers of pentacoordinate iridium(III) hydridochloride with triptycene-based PC(sp3)P pincer ligand (1,8-bis(diisopropylphosphino)triptycene), 1 and 2, differ by the orientation of hydride ligand relative to the bridgehead ring of triptycene. According to DFT/B3PW91/def2-TZVP calculations performed, an equatorial Cl ligand can relatively easily change its position in 1, whereas that is not the case in 2. Both complexes 1 and 2 readily bind the sixth ligand to protect the empty coordination site. Variable temperature spectroscopic (NMR, IR, and UV-visible) studies show the existence of two isomers of hexacoordinate complexes 1·MeCN, 2·MeCN, and 2·Py with acetonitrile or pyridine coordinated trans to hydride or trans to metalated C(sp3), whereas only the equatorial isomer is found for 1·Py. These complexes are stabilized by various intramolecular noncovalent C-H···Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with faster reactions of 1·L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes' isomerization observed for allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic mechanism.

Full text of DOI:10.1021/acs.inorgchem.0c00797

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Article
Stereoisomerism as an Origin of Different Reactivities of Ir(III)
PC(sp3)P Pincer Catalysts
Vladislava A. Kirkina, Gleb A. Silantyev, Sophie De-Botton, Oleg A. Filippov, Ekaterina M. Titova,
Alexander A. Pavlov, Natalia V. Belkova,* Lina M. Epstein, Dmitri Gelman,* and Elena S. Shubina*
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ABSTRACT: Two stereoisomers of pentacoordinate iridium(III)
hydridochloride with triptycene-based PC(sp3)P pincer ligand (1,8-
bis(diisopropylphosphino)triptycene), 1 and 2, differ by the
orientation of hydride ligand relative to the bridgehead ring of
triptycene. According to DFT/B3PW91/def2-TZVP calculations
performed, an equatorial Cl ligand can relatively easily change its
position in 1, whereas that is not the case in 2. Both complexes 1
and 2 readily bind the sixth ligand to protect the empty
coordination site. Variable temperature spectroscopic (NMR, IR,
and UV−visible) studies show the existence of two isomers of
hexacoordinate complexes 1·MeCN, 2·MeCN, and 2·Py with
acetonitrile or pyridine coordinated trans to hydride or trans to
metalated C(sp3), whereas only the equatorial isomer is found for 1·
Py. These complexes are stabilized by various intramolecular
noncovalent C−H···Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by
pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with
faster reactions of 1·L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes’ isomerization observed for
allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic
mechanism.
prefer cisoid arrangement of phosphine ligands around the
octahedral metal center (Figure 1) atypical for the traditional
PCP complexes, converting into transoid isomers in coordinat-
ing solvents.30,32 Besides these two isomers show different
catalytic activities, e.g., in alcohols dehydrogenation and
alkenes hydroformylation.20,33 In contrast, triptycene-based
PC(sp3)P pincer Ir(III) catalysts appear to be more rigid and
feature transoid ligand geometry. They can be obtained as the
different stereoisomers in which the position of the hydride
ligand with respect to the upper ligand rim leads to markedly
different reaction rates. On the outset of our studies, we
demonstrated unusual activity of such complexes in the
transfer hydrogenation of ketones (Scheme 1).34−37 More
recently, we also showed different reactivity of two stereo-
isomers in oxidative addition and reductive elimination
reactions and, consequently, in the transfer dehydrogenation
INTRODUCTION
■
Carbometalated pincer-type complexes belong to a family of
organometallic compounds in which the intrinsically reactive
carbon−metal bond is stabilized by two neighboring neutral
donor groups.1 This intramolecular stabilization makes for the
overall stability of the complexes, which spots them as
attractive candidates for various applications.2−9 By far, the
most popular scaffold used to design pincer ligands has relied
on a planar aromatic moiety mainly because of the high
sensitivity of the arene rings toward the electronic nature of the
substituents and availability of the synthetic methods for their
simple installation. However, several classes of more
structurally complex aliphatic C(sp3)-metalated pincer com-
plexes have emerged in recent years (Chart 1) and have been
tested in numerous catalytic reactions.10−18
Among others, PC(sp3)P pincer complexes of iridium were
reported by Gelman’s group as efficient catalysts in the transfer
(de)hydrogenation of polar and nonpolar substrates.19−23
Notably, unlike the substantially planar aromatic pincer
catalysts, the stereochemistry of the three-dimensional aliphatic
ones is more complex, leading to the existence of several
stereoisomers that may have different reactivity.24−31 For
example, dibenzobarrelene-based PC(sp3)P iridium complexes
Received: March 16, 2020
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Chart 1. C(sp2)- vs C(sp3)-Based Pincer Preligands
Ir(III) complexes bearing a prototypical 1,8-bis-
(diisopropylphosphino)triptycene ligand and the influence of
their stereochemistry on their catalytic activity in alkenes
isomerization.
EXPERIMENTAL SECTION
■
All manipulations were performed under a dry argon atmosphere
using standard Schlenk technique. Commercially available argon
(99.9%) was additionally purified from traces of oxygen and moisture
by sequential passage through a Ni/Cr catalyst column and 4 Å
molecular sieves. The HPLC grade dichloromethane (DCM) and
toluene (Acros Organics) were dehydrated over CaH2 and Na/
benzophenone, respectively, and freshly distilled under argon before
use. Deuterated solvents (CD2Cl2 and C6D6/toluene-d8) were
distilled and degassed by three freeze−pump−thaw cycles before use.
The high purity CH3CN and Py were distilled under argon. The
complexes [(PCP)IrH(Cl)] (1)38 and [(PCP)IrH(Cl)(MeCN)] (2·
MeCN)29 were prepared following the published procedures.
Commercially available CD3CN (Sigma-Aldrich) was used without
additional purification. The 15N-labeled pyridine was prepared from
15NH4Cl (Sigma-Aldrich).39 Hexacoordinate complexes with L =
MeCN and Py were obtained in situ by the addition of the equimolar
amount of L to dichloromethane solution of 1 or 2·MeCN.
Variable temperature (VT) IR spectra were measured on the
Nicolet iS50 FTIR spectrometer using CaF2 cells. UV−vis spectra
were recorded at different temperatures (190−300 K) on a Varian
Cary 50 spectrometer using CaF2 cells. Home-modified Carl Zeiss
Jena cryostat was used for VT-IR and VT-UV−vis measurements.
NMR spectra were recorded on Bruker Avance 600, Bruker Avance
3
Figure 1. X-ray structure of dibenzobarrelene-based (MeOCH PC(sp )-
P)IrH(Cl) complex featuring the cisoid arrangement of phosphine
ligands. Hydrogen atoms of the ligand are removed for clarity.
Isopropyl substituents are shown as a wireframe. Data from ref 32.
2
1
300, and Bruker Avance 400 FT-NMR spectrometers. H chemical
shifts are reported in parts per million (ppm) downfield to
tetramethylsilane (TMS) and were calibrated against the residual
solvent resonance, while 31P{1H} signals were referenced to 85%
H3PO4.
Scheme 1. Previous Work-Transfer Hydrogenation of
Ketones
The isomerization of terminal alkenes was catalyzed by 1 or 2 at
room temperature or at 80 °C. The reactions were run in 1,2-
dimethoxyethane (DME) using mesitylene as an internal standard. In
a standard experiment, 0.013 mol of the corresponding alkene was
injected to 3 mL of DME solution of the catalyst (0.26 mmol, 2 mol
%) and the reaction mixture was analyzed by GC after the indicated
period of time.
COMPUTATIONAL DETAILS
■
All structures of complexes and transition states were
optimized by Gaussian 1640 utilizing B3PW91 functional41−43
with def2-TZVP basis set44 supplied with ECP for Ir atom45
without any symmetry restrictions. The tight SCF convergence
criteria and ultrafine grid were applied. All minima on the PES
were confirmed to have no negative values in their
diagonalized force constants matrix. The transition state
structures showed only one negative eigenvalue in their
of alkanes (Scheme 2).32 In this work, we inquire about
possible origins of different reactivities of the stereoisomeric
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Scheme 2. Previous Work−Stereochemistry-Controlled Reductive Elimination
Scheme 3. Synthesis of 1 and 2·MeCN and the Atoms’ Numbering Scheme
diagonalized force constant matrices, and their associated
eigenvectors were confirmed to correspond to the motion
along the reaction coordinate. For estimation of ligand
dissociation the same method was applied but with def2-SVP
basis set.
strated for other iridium(III) hydridochloride com-
plexes,30,54−57 there are two possibilities for a Lewis base
binding to 1an axial position trans to the hydride ligand and
an equatorial position trans to the metalated carbon (Scheme
4). The evidence of their formation can be found in variable
The QTAIM analysis was performed using the AIMALL
program package46 based on the wave function obtained by the
B3PW91 calculations. The energies of noncovalent intra-
molecular interactions were calculated using the correlation
between the binding energy (EBCP) and the value of the
potential energy density function V(ρc) in the corresponding
critical point (3, −1): EBCP = 0.5V(ρc).47,48
Scheme 4. Structure of 1·MeCN Isomers
RESULTS AND DISCUSSION
■
Spectroscopic Study of MeCN Derivatives of Com-
plexes 1 and 2. As was reported earlier, the stereochemical
outcome of the oxidative addition of [IrCl(COE)2]2 across the
C(sp3)−H of the 1,8-bis(diisopropylphosphino)triptycene
ligand is solvent-dependent (Scheme 3). Whereas the reaction
in toluene leads to the formation of orange crystals of the
pentacoordinate iridium complex 1 with the hydride ligand
pointing outward of the bridgehead ring of the triptycene
scaffold,38,49 the reaction in refluxing acetonitrile yields
colorless hexacoordinate complex 2·MeCN.32,35 In 2·MeCN
the first coordination sphere of iridium is a slightly distorted
octahedron in which we consider the P,C,P ligand plane as the
equatorial. The hydride ligand occupies an axial position facing
the central phenyl ring. Throughout this work, we use the
axial/equatorial notations to define the position of the sixth
ligand in 1·L and 2·L.
temperature IR50 and NMR spectra. In the 1H NMR spectrum
the addition of 1 equiv of CD3CN at room temperature leads
to the disappearance of the hydride resonance of the
pentacoordinate 1 and appearance of a new broad unresolved
signal at δH −22.18 (Figure 2). Upon cooling, this hydride
signal becomes a well-resolved triplet and the second broad
hydride resonance appears at −24.5 ppm (Figure 2). The latter
2
also resolves into the triplet (δH = −24.50; JP−H = 15 Hz)
below 250 K, and the ratio between two isomers becomes
constant (2.5:1; Supporting Information Figure S8). The
corresponding signals in the 31P{1H} NMR spectrum appear at
37.70 and 38.16 ppm at 210 K (Figure S7).
The resonance of the aromatic proton at C(16) of the
triptycene ligand in 1 (δH 7.46, d, JH−H = 7.34 Hz) appears to
be sensitive to the changes in the ligand environment. Addition
of 1 equiv of acetonitrile at room temperature shifts this signal
from 7.46 to 8.00 ppm. Simultaneously a new signal appears at
9.27 ppm, which becomes a doublet at 250 K (δH = 9.23 ppm,
d, JH−H = 7.32 Hz) (Figure 3). The data obtained for the
interaction of 1 with pyridine (vide infra) allow the assignment
The pentacoordinate complex 1 has a characteristic orange
color in the solid state and in solution in dichloromethane at
room temperature. However, it gives colorless hexacoordinate
species upon cooling in dichloromethane50 or upon addition of
MeCN, which has much higher basicity51−53 and stronger
coordinating ability than dichloromethane. As was demon-
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interacts with the chloride ligand, which is pushed closer to the
upper triptycene ring after the coordination of acetonitrile
molecule (Scheme 4).
When acetonitrile is used as a solvent for the synthesis, the
major reaction product is hexacoordinate iridium hydride 2·
MeCN. That exists as a mixture of two isomers and contains
about 8% 1·MeCN (Figure S11).50 Two hydride resonances
are observed in the 1H NMR spectrum of 2·MeCN in CD2Cl2
at 280 K at δ −21.02 (broad) and −21.99 (t, 2JP−H = 13 Hz) in
1.5:1 ratio that does not change with temperature (Figure 4,
1
Figure 2. Variable temperature H NMR spectra (300 MHz, hydride
region) of 1·MeCN in CD2Cl2.
1
Figure 4. Variable temperature H NMR spectra (600 MHz, hydride
region) of 2·MeCN in CD2Cl2.
Table 1, and Figure S12). In both of these isomers, the hydride
ligand is placed close to the aromatic proton of the triptycene
ligand that is confirmed by the cross-peaks between the
hydride and aromatic hydrogen signals in the NOESY
spectrum at 250 K when both δH are well-resolved (Figure
S13).
Above 250 K the fast exchange equilibrium between hexa-
and pentacoordinate species is observed for the axial isomer 2·
MeCN-ax as evidenced by broadening and drift of its hydride
and CH3CN signals (δIrH −21.02; δCH 1.99 at 280 K; Figure
3
4). The position of the equilibrium and exchange rate depend
on the temperature and amount of the residual CH3CN in the
sample. Indeed, the presence of the higher amount of
acetonitrile in the solution of 2·MeCN in CD2Cl2 causes the
narrowing of hydride resonance of 2·MeCN-ax (full width at
half-height is approximately 96 Hz at 310 K vs >350 Hz at 298
K at equimolar amount CH3CN) and thus allows its
observation at ambient temperatures (Figure S14). Warming
the solution of 2·MeCN from 200 to 310 K results in a
progressive high field shift of 2·MeCN-ax hydride signal from
−20.66 to −21.56 ppm (Figure S15), indicating the
equilibrium shift toward the pentacoordinate 2, which should
Figure 3. Variable temperature 1H NMR spectra (300 MHz, aromatic
protons region) of 1·MeCN in CD2Cl2.
of the former set of resonances to the axial isomer 1·MeCN-ax
and the other set of signals to the equatorial isomer 1·MeCN-
eq (Figure 3, Table 1, and Scheme 4). The lower field shift of
δH C(16) may be explained by its involvement in noncovalent
interactions. As confirmed by DFT calculations (vide infra) in
the 1·MeCN-ax complex, there is the interaction of C(16)
proton with the CN bond, whereas in 1·MeCN-eq it
Table 1. Selected NMR Data (Chemical Shifts, δ (ppm), Spin−Spin Coupling Constants, J (Hz), for 1, 1·MeCN, and 2·MeCN
in CD2Cl2 at 250 K
1
1·MeCN-ax
1·MeCN-eq
2·MeCN-ax
2·MeCN-eq
b
b
2
2
2
2
δH(IrH)
δH(C(16)H)
δP
−35.47 (broad)
−21.95 (t, JP−H = 16)
−24.50 (t, JP−H = 15)
−21.02 (t, JP−H = 14)
−21.99 (t, JP−H = 13)
7.44 (broad)
7.95 (d, JH−H = 7.32)
9.23 (d, JH−H = 7.32)
7.48 (d)
33.4 (s)
7.49 (d)
37.9 (s)
a
b
b
64.30 (broad)
38.34 (s)
38.20 (s)
a
b
At 298 K. At 280 K.
D
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Figure 5. DFT optimized structures of complexes 1 (left) and 2 (right) with key geometric parameters (distances in angstroms, angles in degrees).
Figure 6. DFT optimized structures of complexes 1·MeCN and 2·MeCN showing the hydrogen bonds of the C(16)−H fragment. Selected bond
distances (Å) are given. Hydrogen atoms (except C(16)−H and MeCN) are omitted for clarity; the triptycene backbone is shown as a wireframe.
have the hydride resonance in higher field (ca. −35 ppm,
similar to 1 and benzene-based pincer (PCsp2P)IrH(Cl)56,57).
Theoretical Study of Complexes 1 and 2. The DFT
analysis on the B3PW91/def2-TZVP level gives two stereo-
isomeric complexes 1 and 2 with the meridional configuration
of PCP ligand.58 The hydride ligand takes the axial position in
the distorted square pyramidal environment pointing outward
of the bridgehead ring of the triptycene scaffold in 1 and
occupies the opposite vertex in 2 (Figure 5 and Table S2).
Complex 1 is slightly favored over 2 (by 3.9 kcal·mol−1 in the
ΔE/ΔH scale and 4.8 kcal·mol−1 in the ΔG scale). Both
pentacoordinate hydridochlorides have a disphenoidal (see-
saw) geometry of the P2IrHC(sp3) core (Figure S20), with P
atoms in axial positions and equatorial arrangement of hydride
and ipso-carbon with H−Ir−C angle ca. 70−80°. As with other
triptycene complexes, 1 and 2 are asymmetric, featuring
different orientations of i-Pr groups at the upside and downside
of the PIrP plane. P−Ci-Pr bonds at the upside part (same as
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1H−15N coupling of hydride resonance is seen. Such spectral
observations should be caused by the coordination of pyridine
molecule cis to the hydride ligand.57,60,61 The pyridine
coordination causes a significant low field shift of δH(C(16))
(from 7.46 to 9.47 ppm, d, JH−H = 7.7 Hz, 250 K) (Figure 8).
Such a shift can result from the interaction between chloride
ligand and C(16) proton when pyridine occupies the
equatorial position, as suggested by the DFT calculations
(Figure 9). The position of hydride and C(16) aromatic
proton resonances of 1·Py-eq complex allows assigning the
signals observed in 1H NMR spectra of 1·MeCN to equatorial
or axial isomers (vide supra).
bridgehead arene ring) are almost perpendicular to the PIrP
plane with the C−Ir−P−Ci-Pr dihedral angle of 90−100°,
which facilitates some intramolecular contacts involving C−H
atoms of isopropyl groups. At the downside the second i-Pr
substituent has an intermediate position with the C−Ir−P−
C
i-Pr dihedral angle ca. 130−135°.59 Such a difference in the i-
Pr groups arrangement could impose different steric and
electronic properties at the upper and lower parts of the
molecule.
Both 1 and 2 form complexes with MeCN, and three
different isomers were found for each. The isomers where
nitrogen ligand occupies the position trans to chloride are
highly unfavorable (by ca. 20 kcal·mol−1) and not considered
hereafter. Thus, there are two isomers for each hexacoordinate
complex 1·MeCN and 2·MeCN with acetonitrile in the axial
and equatorial positions. These isomers feature a variety of
hydrogen bonds involving C(16)−H (Figure 6 and Table S4),
which were noticed experimentally. The complexes with the
axial coordination of MeCN are favored for both 1 and 2
(Table 2). However, the Ir···N distance is shorter in equatorial
Analysis of δH resonances of pyridine-15N in 1·Py-eq
evidence the interaction of one of its ortho-protons (o-protons)
with the chloride ligand: below 250 K its signal is significantly
shifted to lower field (from 8.61 ppm in free Py-15N to 10.55
ppm in 1·Py-eq at 190 K). At the same time the position of the
second o-proton matches that in free Py-15N. The temperature
increase leads to the broadening and then coalescence of these
two signals into one broad singlet δH 9.7 at 298 K.
isomers, which leads to a stronger low field shift of δCH signals
DFT calculations confirm the formation of only equatorial
isomer, 1·Py-eq, because of pyridine ligand’s larger steric
volume in contrast with acetonitrile ligand (Figure 9, Table
S4). They also show the presence of close contact of chloride
ligand with pyridine o-proton and C(16) bound proton
(Figure 9). AIM analysis shows these interactions are rather
strong with interaction energies EBCP −4.3 and −4.7 kcal·
mol−1, respectively (Table S6).47,48 In addition, pyridine can
rotate in the metal coordination sphere, which causes the
3
of 2·MeCN-eq.
Table 2. Computed Formation Energies (kcal·mol−1) of
Hexacoordinate Complexes of 1 and 2 with N-Bases
Calculated Relative to Isolated Reactants
L
MeCN
Py
1·L-ax 1·L-eq 2·L-ax 2·L-eq 1·L-eq 2·L-ax 2·L-eq
1
ΔE
ΔH
ΔG298 K
−9.6
−7.9
5.4
−7.4
−5.9
7.2
−9.3
−7.5
3.9
−7.4
−5.9
4.2
−5.4
−3.5
11.4
−8.1
−6.0
7.8
−9.8
−8.1
5.3
broadening of the H NMR signals at ambient temperatures.
Gibbs free energy of Py rotation, ΔG⧧rot, estimated from the
coalescence temperature (Tc = 265 K) is 11.3 kcal·mol−1,50
which is in agreement with the DFT calculations that give
ΔG⧧rot(265 K) = 13.1 kcal·mol−1 (see Figure S25 for the TS
structure).
Interaction of Complexes 1 and 2 with Pyridine. The
addition of pyridine to the solution of 1 in dichloromethane
leads to the quantitative formation of hexacoordinate complex
1·Py already at 295 K, which is indicated by the disappearance
of d−d* transitions in UV−vis spectrum upon the addition of
equimolar pyridine amount (Figure 7).
IR spectra reveal another dynamic process in 1·Py-eq. The
Ir−H stretching vibration band has two components with
maxima at 2228 and 2203 cm−1 (at 190 K) that change in a
different way with temperature (Figure 10). On the basis of
DFT calculations, we assign these two bands to two rotamers
of 1·Py-eq, which differ by the orientation of i-Pr groups at
phosphorus (1·Py-eq and 1·Py-eq-a, respectively; see Figure
S26 and Table S5). Quantitative analysis of their temperature
changes gives thermodynamic parameters of a 1·Py-eq ↔ 1·Py-
eq-a process in dichloromethane, ΔH° = 0.71
0.03 kcal·
mol−1, ΔS° = 3.4 0.2 cal·mol−1·K−1 (ΔG190 K = 0.07 kcal·
mol−1) (Figure S27, Table 3). Almost the same spectral
changes are observed in toluene (νIrH(1·Py-eq) = 2216 cm−1,
νIrH(1·Py-eq-a) = 2194 cm−1 (Figure S28, Table 3); ΔH° =
1.11
0.07 kcal·mol−1, ΔS° = 5.4
0.3 cal·mol−1·K−1,
ΔG190 K = 0.08 kcal·mol−1) (Figure S27, Table 3). Isomer-
ization enthalpies obtained in DCM and toluene nicely
correlate with the DFT computed values. The DFT computed
free energies of 1·Py-eq → 1·Py-eq-a isomerization are slightly
positive within the temperature range studied (Table 3);
however, they follow the same trend as the experimental
onesthe 1·Py-eq isomer is less favorable at room temper-
ature (RT) and more favorable at low temperatures. This
misfit is connected with the underestimation of isomerization
entropy in the gas phase calculations. The predicted isomer-
Figure 7. UV−vis spectra of complex 1 (red line) titrated by pyridine
(from 0.1 (dashed violet) to 1.0 equiv (burgundy)) in CH2Cl2 at 295
K. c(1) = 0.002 M; l = 0.2 cm.
1H NMR spectra demonstrate the formation of a single
isomer of 1·Py with the addition of 1 equiv of pyridine-15N to 1
at 190 K, which is evidenced by the presence of only one high
field triplet δH −24.64 (2JP−H = 15.4 Hz). It moves to the lower
field with the temperature increase and loses the multiplicity at
298 K (Figure S23). In HMBC(1H−15N) experiment the
cross-peak between 15N of the bound pyridine (−147.84 ppm)
and the hydride ligand is observed (Figure S24), but no
ization barrier for the 1·Py-eq → 1·Py-eq-a process, ΔG⧧
=
298 K
4.6 kcal·mol−1 (see Figure S29 for the TS structure), is
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1
Figure 8. Variable temperature H NMR spectra (600 MHz, aromatic protons region) of 1·Py in CD2Cl2. Shaded areas highlight the signals of
C(16) proton of the triptycene ligand and protons of coordinated pyridine. For color coding see the scheme inserted.
Figure 10. FTIR spectra in the νIrH region of complex 1·Py-eq (solid
lines) obtained in situ from 1 (c = 0.015 M) and pyridine-15N (c =
0.02 M) and their deconvolution bands (dashed lines). CH2Cl2, l =
0.2 cm.
Table 3. Spectral Characteristics of Rotamers for 1·Py-eq
(νIrH (cm−1)) and Thermodynamic Parameters of 1·Py-eq
→ 1·Py-eq: A Transformation (Relative to 1·Py-eq, in kcal·
mol−1 and cal·mol−1·K−1) Measured in Different Solvents
and Calculated by DFT
Figure 9. DFT optimized structure of 1·Py-eq complex showing the
CH···Cl hydrogen bonds. Selected bond distances (Å) are given.
Hydrogen atoms (except C(16)−H and Py) are omitted for clarity;
the triptycene backbone is shown as a wireframe.
DCM
toluene
DFT
1·Py‑eq
νIrH
νIrH
2228
2203
0.07
−0.17
−0.29
0.7
2216
2194
0.08
−0.29
−0.50
1.1
2254
2247
0.66
0.56
0.51
0.9
consistent with the observation of two vibrational bands
distinguishable in the IR spectroscopy time scale.62
1·Py‑eq‑a
ΔG190
ΔG260
ΔG298
ΔH°
The addition of pyridine to 2·MeCN causes the substitution
of acetonitrile ligand, but the reactivity of 2·MeCN-ax and 2·
MeCN-eq is different. Thus, isomer 2·MeCN-ax reacts with
pyridine instantaneously already at 200 K, transforming into a
new pyridine-containing complex 2·Py-ax (Scheme 5), which
ΔS°
3.4
5.4
1.4
1
gives a triplet hydride signal in the H NMR spectrum at δ
1
2
−22.21 (200 K, 2JP−H = 14 Hz) (Figure S30). Another isomer,
2·MeCN-eq, transforms slowly into the same product 2·Py-ax
only above 255 K (Figure 11). However, under these
conditions, the 2·Py-ax complex undergoes a slow trans-
formation into new species 2·Py-eq (Scheme 5) giving a triplet
in the H NMR spectrum at δIrH −22.00 (255 K, JP−H = 13
Hz) (Figure S31). The NOESY spectrum of the 2·Py-ax/2·Py-
eq mixture prepared in situ at 260 K shows the cross-peaks
between the hydride and C(16) hydrogen signals (Figure S32),
evidencing the hydride ligands are close to the bridgehead
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aromatic ring both in 2·Py-ax and 2·Py-eq. With 15N-labeled
Scheme 5. Observed Transformations of MeCN and
Pyridine Derivatives of 2
pyridine an additional coupling of the hydride resonance into a
2
1
doublet with JN−H = 20 Hz is observed in the H NMR
spectrum for isomer 2·Py-ax (Figure 12), indicating trans-
Figure 12. 1H NMR spectrum (600 MHz, hydride region) of complex
2·MeCN in the presence of 10 equiv of pyridine-15N in CD2Cl2 at 270
K.
coordination of pyridine to the hydride ligand (Scheme 5).60,61
DFT calculations confirm the preference of 2·Py-eq isomer
(Table 2) and the presence of different hydrogen bonds that
stabilize these structures (Figure 13). According to the AIM
analysis Cl···HPy interactions are twice stronger than those of
C(16)H···HIr, with interaction energies −3.6 and −4.1 kcal·
mol−1 for 2·Py-eq and 2·Py-ax, respectively (Table S6).
The plot of kinetic curves for the components of the 2·
MeCN/pyridine mixture obtained at 255 K (Figure 14) is
typical of two consecutive first-order reactions 2·MeCN-eq →
2·Py-ax → 2·Py-eq with rate constants k2 and k3 (Scheme 5)
and the initial mole fraction of 2·Py-ax of ca. 66%. Both steps
obey the irreversible first-order reaction law; the corresponding
rate constants k2 and k3 are gathered in Table 4. The estimated
activation enthalpy and entropy obtained by the Eyring
analysis (Figure S33) are ca. 26 2 kcal·mol−1 (ΔH⧧ ), 29
7 cal·mol−1·K−1 (ΔS⧧ ) and 38 4 kcal·mol−1 (ΔH⧧2), 68
2
3
13 cal·mol−1·K−1 (ΔS⧧ ).63 Large positive values of the
3
activation entropy for both steps indicate the dissociative
mechanism of these transformations. The pyridine concen-
tration does not affect the k2 and k3 values (Table S3), which is
also in support of the dissociative mechanism.
A similar difference in the reactivity is observed for two
isomers of the 1·MeCN complex. At 190 K pyridine substitutes
acetonitrile molecule in 1·MeCN-ax yielding 1·Py-eq with k1 =
(1.2 0.1) × 10−4 s−1 (ΔG⧧ 190 K = 14.4 0.1 kcal·mol−1),
1
whereas transformation of 1·MeCN-eq is slower with k2 = (4.1
0.4) × 10−5 s−1 (ΔG⧧ 190 K = 14.8 0.1 kcal·mol−1) (Figure
2
S34−S36).
DFT calculations show that the parent pentacoordinate
complex is the key intermediate in the interconversion of
MeCN and pyridine complexes. They predict higher stability
for axial MeCN isomers (Table 2), the amount of which in the
mixture is higher according to the integration of NMR spectra.
Moreover, the signal broadening is observed, namely, for axial
MeCN complexes, although a stronger complex should be less
affected by the dissociative exchange. In the present case, the
dissociation of acetonitrile ligand from the axial position
should occur without significant perturbation of geometry of
the remaining fragment, whereas the removal of the ligand
from the equatorial position entails the restoration of the
Figure 11. Time-dependent 1H NMR spectra of 2·MeCN in the
presence of 10 equiv of pyridine showing the disappearance of 2·
MeCN-eq isomer. CD2Cl2, 260 K.
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Figure 13. DFT optimized geometries of 2·Py-ax and 2·Py-eq complexes. Selected bond distances (Å) are given. Hydrogen atoms (except C(16)−
H and Py) are omitted for clarity; the triptycene backbone is shown as a wireframe.
Therefore, 2·MeCN-eq is the only acetonitrile complex
which has a significant dissociation barrier. The energetics of
nitrile dissociation is connected with the migration of chloride
(monitored by changes of C−Ir−Cl angle). These changes do
not exceed 22° for all isomers except 2·MeCN-eq, where C−
Ir−Cl angle changes from 89° to 169° upon dissociation
(Figure S37). This suggests a lower impact of trans effect of
hydride ligand compared to the chloride flexibility.50 This,
together with the dissociative character of the experimentally
found activation barriers, allows us to present energy profiles
for ligand exchange in these complexes (Figure 15). The whole
scheme assumes that all processes occur via pentacoordinate
complexes. The axial complexes undergo faster transformations
compared to their equatorial pairs due to the absence of the
Figure 14. Plot of kinetic curves for the components of the 2·MeCN/
H−Ir−Cl isomerization substep.
pyridine (10 equiv) mixture in CD2Cl2 at 255 K.
Acetonitrile dissociation from 1·MeCN-ax occurs without a
Table 4. Values of Rate Constants (s−1) and Activation Free
Energies (kcal·mol−1) for 2·MeCN-eq to 2·Py-ax and 2·Py-
ax to 2·Py-eq Transformations at Different Temperatures
barrier (red curve, Figure 15), whereas the formation 1·Py-eq
overcomes a small barrier (green curve, Figure 15). As the
190 K
result, ΔG⧧
= 14.4
0.1 kcal·mol−1 is found
1
experimentally for the overall process of ligand exchange.
2·MeCN-eq → 2·Py-ax
2·Py-ax → 2·Py-eq
The transformation of 1·MeCN-eq into 1·Py-eq is slightly
slower with ΔG⧧ 190 K = 14.8 0.1 kcal·mol−1 related to the
T, K
103k2
ΔG⧧
103k3
ΔG⧧
2
2
3
acetonitrile dissociation from the less favored equatorial
complex (blue curve, Figure 15). In the case of 2 the first
(kinetic) exchange product (2·Py-ax) forms with a barrier
equal to MeCN dissociation energy from 2·MeCN-ax (red and
magenta curves, Figure 15). The barriers connecting equatorial
complexes with pentacoordinate 2 are ca. 6−11 kcal·mol−1
higher than those for 1. At 190 K the transformation of 2·
MeCN-eq into 2·Py-ax (blue and magenta curves, Figure 15)
and 2·Py-ax into 2·Py-eq (magenta and green curves, Figure
15) would be rather slow with estimated barriers ΔG⧧190 K = 21
3 and 25 7 kcal·mol−1 respectively. As a result the final
productthe thermodynamically preferred complex 2·Py-
eqforms at higher temperatures when passing the barriers
of dissociation/reorganization is easier.
255.0
257.5
260.0
262.5
270.0
272.5
275.0
0.45 0.03
0.78 0.03
1.39 0.04
1.97 0.03
18.7
18.6
18.5
18.5
0.54 0.03
0.82 0.02
2.00 0.03
19.8
19.7
19.4
geometry of pentacoordinate 1 or 2 with H−Ir−Cl angle 110−
140° (from ca. 180° in the equatorial complexes). Indeed, the
relaxed PES scan of MeCN dissociation from 2·MeCN-eq and
2·MeCN-ax complexes reveals the absence of barrier for the
axial complex, where the highest energy point (ca. 11 kcal·
mol−1) corresponds to the separated reactants. For the
equatorial isomer the barrier (ca. 16 kcal·mol−1) is located
above the energies of the separated reactants (Figure S37).
Catalysis. Previously reported transfer dehydrogenation of
linear alkanes has demonstrated the reaction was accompanied
by the significant isomerization of the kinetic terminal alkenes
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Figure 15. Schematic energy profiles for transformations of the complexes 1 (left) and 2 (right) to their hexacoordinate derivatives. Computed
electronic energies (E, kcal·mol−1) are given next to the energy levels relative to the corresponding MeCN-ax isomer. Activation Gibbs free energies
at 190 K (ΔG⧧190 K, kcal·mol−1) are derived from the experimental data.
into a thermodynamic mixture of internal regioisomers
(Scheme 6).32,38,49 Although the regioselective formation of
Scheme 7. Alkene Isomerization Reactions Catalyzed by 1
and 2
Scheme 6. Transfer Dehydrogenation of Linear Alkanes
Using 1 as a Catalyst
the terminal olefins is industrially more important,64,65 most of
the known catalysts produce statistical mixtures of the
products.64,66−69 On the other hand, internal olefins are
perspective intermediates of remote functionalization.70 Due to
the fact that metal hydrides are often active catalysts for the
isomerization of alkenes, it would be interesting to get insight
into the nature of the exact catalytic species responsible for the
isomerization process to suppress if it appears as a side reaction
or, on the contrary, to enhance when desired.66,70−72
To see the difference in the activity of the stereoisomeric
iridium hydride complexes, 1 and 2·MeCN were investigated
as catalysts for the isomerization of several representative
alkenes. Thus, the reaction of 50 equiv of allylbenzene with 1
in anhydrous benzene resulted in complete isomerization to β-
methylstyrene over 48 h at RT (eq 1, Scheme 7). Similarly, 1 is
an active catalyst for the isomerization of 1-octene to a mixture
of the octene isomers in 71% yield.73 On the other hand, the
employment of the catalyst 2·MeCN in these reactions
demonstrated only modest reactivity. Thus, conversions of
54% and 3%, respectively, were measured at room temperature,
while full conversions of allylbenzene were reached only under
reflux. The more stable 1-octene showed little isomerization
even under heating conditions. To exclude the influence of the
coordinated acetonitrile molecule on the catalytic activity of 2·
MeCN, we employed pent-4-enenitrile as a substrate.
Immediate color disappearance of the dark orange 1 is found
with the coordinating substrate (58% yield using 1 vs 3% using
2·MeCN). The addition of 10 mol % of a base (e.g., LiHMDS,
tBuONa, or DBU) or halogen-labilizing agents (e.g., NaBPhF4
or AgBF4) had negligible effect on the reaction rate,
conversion, or selectivity. This suggests that cationic
complexes74−78 do not form under the reaction conditions
due to steric hindrance around the metal center.
In view of the above experimental observations, we propose
that the mechanism underlying this reaction is based on an
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insertion/elimination sequence rather than on the allylic
mechanism (Scheme 8). For this mechanism only equatorial
favorable 2, the hydride ligand is facing the bridgehead
aromatic ring but points in the opposite direction in 1. These
two stereoisomers do not interconvert and feature different
reactivity. To understand the origin of that, we analyzed the
structural peculiarities of 1 and 2 and their derivatives.
Scheme 8. Proposed Mechanism of Alkenes Isomerization
and the Structures of Key Intermediates Derived from 1 and
2
Both pentacoordinate hydridochlorides have a disphenoidal
(seesaw) geometry of the P2IrHC(sp3) core, with P atoms in
axial positions and equatorial arrangement of hydride and ipso-
carbon with H−Ir−C angle ca. 70−80°. Cl ligand can relatively
easily change its position in 1, migrating toward the upper
triptycene ring, whereas that is not the case in 2. This entails
the easier formation/dissociation of hexacoordinate equatorial
complexes based on 1. Complexes 1 and 2 readily bind
nitrogen-containing Lewis base ligands such as acetonitrile and
pyridine, which can coordinate trans to hydride (axial position)
or trans to metalated C(sp3) (equatorial position). The
dissociative exchange of the sixth ligand observed on the
NMR time scale assumes the pentacoordinate 1 and 2 as key
intermediates. The substitution of MeCN by pyridine unveils
the difference in the reactivity of axial and equatorial isomers of
1·L and 2·L and faster reactions of 1·L. Whereas axial isomers
undergo “traditional” dissociation where the barrier is close to
the 1·L/2·L bond energy, the equatorial ones show a different
pattern. For 1·L-eq and 2·L-eq the barriers are increased due to
more pronounced structural reorganization entailed by the
ligand dissociation, but they are still quite low for more
reactive 1·L. These studies provided important hints for
understanding the mechanism of the catalytic isomerization of
terminal alkenes, which proceeds as an insertion/elimination
sequence via equatorial substrate coordination. As a result of a
different steric hindrance at free coordination site and
additional C(16)−H···Cl stabilization in hexacoordinate 1,
these three-dimensional pincer catalysts can precisely differ-
entiate the steric hindrance of reaction sites. Besides, the
different stereochemistries of 1 and 2 determine the difference
in their catalytic activity in this isomerization process.
alkene coordination will give reactive intermediates A. The
catalytic cycle starts from the coordination of the alkene to the
iridium center of either 1 or 2 (obtained as the result of
predissociation of the acetonitrile molecule from 2·MeCN),
addition of the Ir−H bond across the double bond, followed
by the β-hydride elimination of Ha or Hb that would give
isomerized olefin products and restore original stereoisomery
of iridium catalyst (Scheme 8). In both 1 and 2, bulky alkene
coordination at the equatorial position (i.e., cis to H) is
required to afford its successful insertion into the Ir−H bond.
The equatorial coordination mode in 1-A entails the migration
of chloride ligand to the axial position that costs minimal
energy. This configuration can be additionally stabilized by
intramolecular hydrogen bonds as in 1·Py (Scheme 8). Thus, it
is reasonable to suggest that formation 1-A will be
thermodynamically less uphill than that of the analogous 2-
A. Moreover, the efficient C(16)−H···Cl stabilization in 1
diminishes the steric hindrance at the free coordination site
that facilitates the substrate binding and assists the subsequent
migratory insertion step to form 1-B. As a result, the applied
three-dimensional pincer catalyst can precisely differentiate the
steric hindrance of reaction sites, and the different stereo-
chemistry of 1 and 2 determines the difference in their catalytic
activity in this isomerization process.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00797.
■
sı
Variable temperature NMR and IR spectra and
supplementary computational details (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Natalia V. Belkova − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia; orcid.org/0000-0001-9346-8208;
Email: nataliabelk@ineos.ac.ru
Elena S. Shubina − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia; orcid.org/0000-0001-8057-3703;
Email: shu@ineos.ac.ru
Dmitri Gelman − Institute of Chemistry, The Hebrew
University, 91904 Jerusalem, Israel; orcid.org/0000-0001-
7185-1345; Email: dmitri.gelman@mail.huji.ac.il
CONCLUSIONS
■
Thus, our study shows that the stereochemical outcome of the
triptycene ligand metalation (Scheme 3) is controlled by the
presence of additional ligand (MeCN) or hemilabile group32
that coordinates to iridium at a previous step and directs
cyclometalation. For this reason, in thermodynamically less
Authors
Vladislava A. Kirkina − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia
K
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Article
(7) Singleton, J. T. The uses of pincer complexes in organic
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Gleb A. Silantyev − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia
Sophie De-Botton − Institute of Chemistry, The Hebrew
University, 91904 Jerusalem, Israel
Oleg A. Filippov − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia; orcid.org/0000-0002-7963-2806
Ekaterina M. Titova − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia
Alexander A. Pavlov − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia; orcid.org/0000-0002-4612-0169
Lina M. Epstein − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russia
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The spectroscopic studies were financially supported by the
Russian Science Foundation (Grant No. 19-13-00459). The
synthetic and catalytic work was supported by the Israel
Science Foundation (Grant N 917/15) and by the Esther K.
and M. Mark Watkins Chair for Synthetic Organic Chemistry.
NMR spectroscopic data for 15N-labeled pyridine were
obtained under support of the German−Russian Interdiscipli-
nary Scientific Center (G-RISC) in the laboratory of Prof.
Klaus Weisz (Institut fur Biochemie, Ernst-Moritz-Arndt-
̈ ̈
̈
Universitat Greifswald) and with the financial support from
the Ministry of Science and Higher Education of the Russian
Federation using the equipment of the Center for Molecular
Composition Studies of INEOS RAS.
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carbon-hydrogen bonds with {cis, cis-1,3,5-tris[(diphenylphosphino)-
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DEDICATION
■
This article is dedicated to Prof. P.H. Dixneuf for his
outstanding contribution to organometallic chemistry and
catalysis.
(20) Musa, S.; Filippov, O. A.; Belkova, N. V.; Shubina, E. S.;
Silantyev, G. A.; Ackermann, L.; Gelman, D. Ligand−Metal
Cooperating PC(sp3)P Pincer Complexes as Catalysts in Olefin
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