Cyclohexane-Based Phosphinite Iridium Pincer Complexes: Synthesis, Characterization, Carbene Formation, and Catalytic Activity in Dehydrogenation Reactions

Article abstract of DOI:10.1021/acs.organomet.6b00846

Metalation of two cyclohexane-based phosphinite pincer ligands, cis-POCyPO (4) and trans-POCyPO (5) (POCyPO = {1,3-bis-[(di-tert-butylphosphinito]cyclohexane}^?), is reported. In line with previously published results (Dalton Trans. 2009, 8626, DOI: 10.1039/B910798C), ligand 4 undergoes aromatization to give benzene-based complex (POCOP)IrHCl (3) at high temperatures in the presence of [Ir(COD)Cl]₂. However, here we present the isolation of carbene complex (POCyOP)IrCl (6) which is an intermediate in the aromatization process; upon reaction with H₂, 6 can be readily transformed to the corresponding hydrido-chloride 8. Metalation of trans-POCyOP ligand 5 gives hydrido-chloride 13 which only upon further heating can be converted to the corresponding carbene 14. A mechanistic study of hydrogenation of carbene 6 is reported, as well as interesting ambient temperature CO-induced C-H activation in β-position of 6, a process that under other circumstances takes place around 200 °C. The cis complex (POCyOP)IrHCl (8), upon activation with base, revealed moderate activity in transfer dehydrogenation of cyclooctane (144 turnover numbers (TON)), while the performance of trans analog 13 was much better (up to 1684 TON). Carbene complex 6 and in situ generated 14 demonstrated promising activity in acceptorless dehydrogenation of alcohols, presumably operating via a novel metal-ligand cooperation type mechanism. Some of the alcohol dehydrogenations generated large amounts of polystyrene.

Full text of DOI:10.1021/acs.organomet.6b00846

Article
pubs.acs.org/Organometallics
Cyclohexane-Based Phosphinite Iridium Pincer Complexes:
Synthesis, Characterization, Carbene Formation, and Catalytic
Activity in Dehydrogenation Reactions
Alexey V. Polukeev and Ola F. Wendt*
Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, 22100 Lund, Sweden
S
* Supporting Information
ABSTRACT: Metalation of two cyclohexane-based phosphin-
ite pincer ligands, cis-POCyPO (4) and trans-POCyPO (5)
(POCyPO = {1,3-bis-[(di-tert-butylphosphinito]-
cyclohexane}−), is reported. In line with previously published
results (Dalton Trans. 2009, 8626, DOI: 10.1039/B910798C),
ligand 4 undergoes aromatization to give benzene-based
complex (POCOP)IrHCl (3) at high temperatures in the
presence of [Ir(COD)Cl]2. However, here we present the
isolation of carbene complex (POCyOP)IrCl (6) which is an
intermediate in the aromatization process; upon reaction with
H2, 6 can be readily transformed to the corresponding hydrido-
chloride 8. Metalation of trans-POCyOP ligand 5 gives hydrido-
chloride 13 which only upon further heating can be converted
to the corresponding carbene 14. A mechanistic study of hydrogenation of carbene 6 is reported, as well as interesting ambient
temperature CO-induced C−H activation in β-position of 6, a process that under other circumstances takes place around 200 °C.
The cis complex (POCyOP)IrHCl (8), upon activation with base, revealed moderate activity in transfer dehydrogenation of
cyclooctane (144 turnover numbers (TON)), while the performance of trans analog 13 was much better (up to 1684 TON).
Carbene complex 6 and in situ generated 14 demonstrated promising activity in acceptorless dehydrogenation of alcohols,
presumably operating via a novel metal−ligand cooperation type mechanism. Some of the alcohol dehydrogenations generated
large amounts of polystyrene.
that under typical conditions used for benchmark transfer
dehydrogenation of cyclooctane (COA) precatalyst (PCyP)-
IrHCl (1) suffered severely from decomposition and gave only
50 turnovers,17 much fewer than the values obtained for
corresponding arene-based analogues 2 and 3 (Chart 1).18
INTRODUCTION
■
Transition metal pincer complexes constitute a research area of
great interest due to the combination of high stability and
tunabilty of electronic and steric properties of the ligands,
giving rise to remarkable catalytic activity in many reactions.1,2
In particular, iridium pincer complexes are efficient catalysts for
dehydrogenation of alkanes, heterocycles, alcohols, and
ammonia-boranes.3 The majority of systems studied are
arene-based PCP4 and POCOP5 type complexes; some recent
developments include hybrid PCOP,6 POCN,7 and POCSP,8 as
well as CCC9 pincer compounds. At the same time, complexes
with an sp3 carbon in the central position only recently have
begun to attract considerable attention, despite being
synthesized three decades ago during the pioneering studies
by Shaw.10 These compounds are believed to be somewhat
Chart 1. Ir Pincer Complexes Used for Dehydrogenation
Reactions and Phosphinite Ligands in This Work
However, Brookhart reported19 that a triptycene-based
2
more electron-rich compared to their arene Csp counterparts,
3
which may, for example, facilitate oxidative addition.11 Another
interesting feature of aliphatic pincers is their ability to form
reactive CM carbene complexes via α-hydrogen10 or even α-
alkyl elimination.12 Such reactivity patterns are responsible for
(PCsp P)Ir complex which lacks labile α- and β-hydrogens
demonstrated a very high activity (2820 turnover numbers
(TON)) under the same conditions, indicating a good potential
3
for (PCsp P)Ir systems. Later, the same framework was shown
to be active for dehydrogenation of ethers.20 Gelman designed
3
3
many interesting transformations including Csp −H−Csp −H
bond coupling,13 Csp −Csp bond cleavage,12,13 H2,10,14 H2O,15
3
3
and N2O16 activation, but in contrast, the α-Csp −H unit may
Received: November 7, 2016
3
also decrease the thermal stability. Thus, we previously found
© XXXX American Chemical Society
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3
an efficient (PCsp P)Ir dibenzobarrelene based catalyst for
acceptorless dehydrogenation of alcohols working through a
metal−ligand cooperation;21 a somewhat related design was
used by Iluc as a catalyst for alkane dehydrogenation but with a
more limited success.22
ligand, and the structure of 6 was confirmed using X-ray
crystallography (Figure 1), revealing a shorter C−Ir distance
3
Most reported (PCsp P)Ir compounds belong to the
phosphine family of catalysts, with methylene linkers in the
pincer arm. Inspired by the enhancement of the catalytic
activity upon CH2-for-O substitution within the arene-based
PCP pincer ligand to give the POCOP one,18 we have been
3
interested in synthesizing aliphatic phosphinite (POCsp OP)Ir
complexes for some time. Initial attempts to metalate
phosphinite cyclohexyl based ligand 4 under conditions similar
to those used for its phosphine counterpart led to aromatization
of the ligand and formation of benzene-based complex 3, and
only with nickel has cyclometalation been successful.23
Complexation of an acyclic aliphatic ligand with an Ir precursor
afforded the corresponding pincer complex in low yield,24 but it
was very prone to thermal metal mediated ligand decom-
position and, unsurprisingly, revealed no activity in catalytic
dehydrogenation of COA.17 This latter result is well in line with
a report by Zargarian,25 who noted facile decomposition of
Figure 1. Molecular structure of carbene complex 6 (left) and
hydrido-chloride 8 (right). Selected bond distances (Å) and angles
(deg) of 6, Ir1−C1 1.86(1), Ir−Cl 2.403(4), P−Ir−P 163.8(1), and
those of of 8, Ir1−C1B 2.024(7), Ir−Cl 2.4017(18), P−Ir−P
160.23(6).
and a more acute P−Ir−P angle than those of the related
phosphine complexes synthesized by Shaw and Piers (C1−Ir
1.86(1) vs 2.006(4)10 and 1.899(7)14 Å; P−Ir−P 163.8(1) vs
168.1(1)10 and 167.81(7),14 correspondingly). Complex 7 is
characterized by a singlet at 159.5 ppm in the 31P{1H} NMR
3
some acyclic POCsp OP type nickel complexes by C−O bond
rupture and is likely related to an inherent fragility of this type
of ligands.
1
spectrum and reveals H and 13C signals corresponding to a
We thus returned to the cyclohexane-based ligands and here
report a more thorough study of complexation of cis ligand 4
and its trans counterpart 5 with iridium. We show that under
certain conditions the complete aromatization of the ligand can
be prevented to give the desired aliphatic phosphinite iridium
nonmetalated ligand, in addition to two hydrides at −32.89 and
−33.27 appearing as triplets of doublets due to mutual H−H
coupling as well as coupling to two equivalent P nuclei. We
thus formulate 7 as [(POCyHOP)*Ir(H)2Cl]n; HRMS and
DOSY NMR measurements argue for a dimeric composition.
Based on previously reported structures for complexes of 4 and
related ligands with other metals,26 we propose the structure of
7 as depicted in Scheme 1. Along with these two complexes,
small amounts of other compounds are present in solution
prior to chromatography, one of which is hydrido-chloride
(POCyOP)IrHCl 8 (identified by comparison with an
independently prepared sample, see below). It seems
reasonable that cyclometalation of 4 produces complex 8
which extrudes a molecule of dihydrogen to give carbene 6.
The mixture of 6 and 7 is fairly stable at the temperature of
refluxing toluene. Upon raising the temperature to 160−180
°C, some 7 is gradually converted to 6; at the same time 6
slowly transforms into aromatic complex 3. Thus, after several
hours at that temperature, the NMR yield of 6 passes through a
maximum of 70−75%. At 200 °C, complex 3 is formed almost
exclusively within a few hours. In contrast to the phosphine
analogue (PCyP)IrHCl (1), no olefin or diene13 intermediates
could be detected during the formation of 3 from 4. It is likely
that COD from [Ir(COD)Cl]2 serves as a hydrogen acceptor
during this process; as we reported previously for 1,13
aromatization does not proceed to completion when
presynthesized aliphatic hydrido-chloride complexes are heated
in a sealed vessel.
2
pincers in the form of (POCsp OP)IrCl carbenes or
3
(POCsp OP)Ir(H)Cl hydrido-chlorides. The presence of an α-
3
Csp −H unit is probably responsible for the thermal instability
of hydrido-chloride complexes, still their activity in dehydro-
genation of COA can be fairly high under certain conditions.
3
However, the α-Csp −H group can be successfully used to
organize a novel catalytic cycle for dehydrogenation of alcohols.
RESULTS AND DISCUSSION
■
Metalation and Aromatization of Cis Aliphatic Pincer
Ligand 4. Mixing ligand 4 with [Ir(COD)Cl]2 at room
temperature results in the formation of several soluble species,
structures of which could not be reliably determined from
NMR spectra. After heating this mixture in a closed flask at
110−120 °C for several hours, compounds 6 and 7 are
observed as predominate species in ca. 1:1.5 ratio; both were
isolated as pure compounds using column chromatography
(Scheme 1). Complex 6 reveals a singlet at 179.2 ppm in the
31P{1H} NMR spectrum, as well as resonances from −O−CH−
1
at −1.21 ppm and from CIr at 235.9 ppm in the H and
13C{1H} NMR spectra, respectively, which are very character-
istic for a carbene moiety.10,13−15 This is the first example of a
carbene complex derived from an aliphatic pincer phosphinite
Hydrogenation of Carbene 6. At ambient temperature,
Scheme 1. Metallation of the Aliphatic Phosphinite 4
carbene complex 6 reacts with hydrogen to give several species
1
characterized by broad resonances in H and 31P{1H} NMR
spectra. After ca. 3 days, this mixture is converted into a single
product, namely, hydrido-chloride complex 8, which demon-
1
strates a 31P{1H} NMR signal at 170.0 ppm as well as a H
2
3
NMR hydride resonance at −40.37 (td, JPH = 12.4 Hz, JHH
=
3.1 Hz), cf. Scheme 2. The molecular structure of 8 is given in
Figure 1. The cyclohexane unit adopts a chair conformation
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Scheme 3. Upon warming, the signals of 8-syn are broadened,
but above 0 °C begin to become more sharp again; at room
temperature, they are observed as moderately broad singlets at
Scheme 2. Hydrogenation of Carbene 6
1
170.7 and −45.54 ppm in the 31P{1H} and H NMR spectra,
correspondingly. After several hours at room temperature, only
signals from 8-syn and 8 (henceforth called 8-anti) were
observed in the reaction mixture, indicating that the two other
intermediates are formed at some early stage of hydrogenation
and are further converted to 8-syn as the kinetic and 8-anti as
the thermodynamic products. The second intermediate
demonstrated a broadened signal at 178.9 ppm in the
31P{1H} NMR spectrum and a broadened triplet at −48.25
and is somewhat disordered due to an inverted configuration of
ca. 40% of the molecules; therefore, bond lengths and angles
within this part of the structure should be treated with caution.
Compared to 1, 8 reveals the expected lowering of the P−Ir−P
angle (160.28(5) vs 166.74(5)°) and shortening of the C−Ir
distance (2.048 vs 2.094(5) Å). Although the hydride
position(s) could not be located with certainty, there is
electron density in the Fourier map in positions where the
hydride should be found. Most probably the conformation is
anti with respect to the α-hydrogens in the two conformations,
respectively; the H−H coupling constant of 3.1 Hz is similar to
that of the phosphine analog 1 (3.0 Hz) and supports this
conclusion. Interestingly, the same stereochemical result (anti
addition) was observed by Shaw for hydrogenation of the
related phosphine carbene complex;10b it is thus evident that
addition of H2 to 6 is not just a simple syn addition of H2 to Ir
or along the CIr bond. In order to get some mechanistic
insight into the process, we performed a VT NMR study of the
interaction of 6 with H2. Complex 6 does not react with H2 at
temperatures below the −40 °C required to make sharp NMR
signals, so we quickly warmed the J. Young NMR tube charged
with a solution of 6 in toluene-d8 and H2 to room temperature
and then inserted it into the spectrometer precooled to −80
°C. This sequence was repeated several times.
2
1
ppm (t, JPH ≈ 13 Hz) in the H NMR spectrum; given the
almost equal probability of H2 attack from both sides of P1−
P2−α−C−Ir plane, it is reasonable to propose that this
compound is an axially metalated analog of 8-syn, 8-syn-axial
(in 8-anti and 8-syn, equatorial positions are metalated),
resulting from an attack of H2 on the same side as the >CH−O
or β-hydrogens (cis-α-H). The third detected compound was
characterized by a 31P{1H} NMR signal at 160.6 ppm and a
1
resonance in the H NMR spectrum appearing as an apparent
quintet at −13.90 ppm. In addition, a broad signal at −2.55
1
ppm in the H NMR spectra was observed, always integrating
as 2:1 versus the hydride.
We thus propose that this broad signal corresponds to a
coordinated molecule of dihydrogen, and the quintet-like
pattern from the hydride is a coinciding triplet of triplets
resulting from coupling with two equivalent P nuclei and the
H2 unit with similar J of around 14 Hz; the structure of this
complex is formulated as 9 (see Supporting Information for a
longer discussion of its NMR characterization). Upon heating,
the signals from intermediates 8-syn-axial and 9 are broadened
and at ca. −20 °C cannot be distinguished from the baseline. At
room temperature, a very broad phosphorus signal is observed
at ca. 174.1 ppm (without detectable hydride), which we
interpret as a weighted-average resonance from 8-syn-axial and
9. The position of the signal is much closer to that of 8-syn-
axial which likely reflects the disfavored coordination of H2 at
Along with nonreacted 6, three compounds were detected by
NMR spectroscopy. One intermediate demonstrates NMR
signals that are very similar to those of 8, namely, a doublet at
171.3 ppm in the 31P{1H} NMR spectrum (high-field hydride is
not decoupled) and a hydride peak at −46.5 ppm (t, 2JPH ≈ 13
Hz). We thus propose that this compound is a close analog of 8
but with syn orientation of α-C−H and Ir−H bonds, cf.
a
Scheme 3. Possible Mechanism for Hydrogenation of Carbene 6
a
Compounds observed by NMR spectroscopy are marked with frames. Calculated energies are given in Figure 2.
C
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higher temperatures. The overall mechanism of hydrogenation
is thus depicted on Scheme 3. It is likely that the reaction
begins with the formation of complexes with dihydrogens A-1
and A-2, which undergo oxidative addition (B-1 and B-2) and
subsequent migration of hydrogen (α-insertion) to give 8-syn
and 8-syn-axial; the latter is in equilibrium with 9. How are 8-
syn-axial and 8-syn now transformed into 8-anti? During our
previous studies of the dihydride complex (PCyP)IrH2, we
found that it undergoes H/D exchange between Ir−H and α-
C−H positions.27 Experimental and computational studies also
revealed that (PCyP)IrH2 is prone to rapid reversible
demetalation of both equatorial and axial positions, which is
responsible for the observed H/D scrambling. It seems
reasonable that a similar mechanism may be involved in the
isomerization of 8-syn-axial and 8-syn into 8-anti. Overall this
mechanism of reaction between 6 and H2 was corroborated by
DFT calculations giving reasonable barriers as opposed to the
direct H2 addition to the carbene (TS3) which gives
prohibitively high barriers, cf. Figure 2. Note that 8-syn-axial
Scheme 4. Reaction of Carbene 11 with CO
187.35 and 187.09 were assigned as Ir−CO moieties. In the IR
spectra, C−O stretching vibrations were observed at 1961 and
1968 cm−1. After ca. 0.5 h, a new AB system (156.82 and
2
155.86 ppm JPP = 325.4 Hz, ca. 8%) appeared in the 31P{1H}
NMR spectrum. The new compound, 11a, was also unstable
and was never a major component in the mixture, but we
1
managed to identify most of its H and 13C signals.
Specifically, the chemical shift of a hydride (−8.49 ppm,
apparent t, 2JP1H = 2JP2H = 16.3 Hz) points to the trans position
being occupied by a high-field ligand like CO. Complex 11a
was further transformed into 11b (this does not exclude that
some of the 11b may be formed directly from 10b). After
reaction overnight, the 10ab/11a/11b ratio was around 1:1:2,
and after a few days, 11b became the sole product in the
solution. An AMX system in the 31P{1H} NMR spectrum of
11b points to a desymmetrized structure (high-field hydride is
not decoupled); the presence of 13C NMR signals in the olefin
region (154.5 and 127.5 ppm) which reveal a resolvable C−H
coupling with the hydride unambiguously point to migration of
one of the β-hydrogens to Ir with formation of a C−C double
bond. A hydride resonance at −17.96 appears as a doublet of
doublets; such chemical shifts are typical for a hydride trans to a
weak-field ligands like Cl (for instance, −18.7 ppm for
phosphine complex (PCyP)IrH(CO)Cl).28 In agreement, a
2JCH of 4 Hz between Ir−H and CO indicates a mutual cis
arrangement of these ligands.29 Because of the diequatorial
substitution of the cyclohexane ring with −OPtBu2, the β-
hydrogens occupy axial positions and can only approach Ir from
the cis-β-H side, with a resulting syn arrangement of the
remaining >C−H(−O−) bond and Ir−H, as drawn for 11a and
11b.
Figure 2. Calculated free energy barriers for H2 attack cis and trans to
β-hydrogens of 6 (at room temperature). Structures are given in
Scheme 3.
slightly more favorably adds an H2 molecule with formation of
9, compared to the action of 8-syn, which explains the lack of
observation of 8-syn(H2) at low temperature. Finally, a
computational search of the potential energy surface for the
transformation of the 8-syn species to 8-anti shows that it is
extremely complicated with a large number of isomers and
conformers, and we were unable to establish a certain
mechanism. It seems clear, though, that the fully demetalated
species (as seen for the hydrides)27 is high in energy and that
involved intermediates and transition states involve partially
broken Ir−C and Ir−H bonds.
With these result in hand, we attempted the synthesis of
complex 8 directly from ligand 4 under H2 pressure.
Unfortunately, instead of 8, a mixture of unidentified
compounds was formed, none of which could be properly
characterized.
CO-Induced C−H Activation. Carbene complex 6 readily
reacts with 1 atm of CO with a simultaneous formation of
isomeric adducts 10a and 10b (ca. 1:0.8 ratio) which are likely
formed via CO attack on Ir from the trans- or cis-β-H side, cf.
Scheme 4. These complexes are unstable. At ambient
temperature, they undergo further transformations character-
ized in situ by NMR and IR spectroscopy. Two very similar sets
of signals were observed; thus, 31P{1H} NMR spectrum
revealed two peaks at 182.45 and 182.40 ppm. Carbon signals
at 214.3 and 212.0 ppm were assigned as CIr, while those at
In solution, 11b slowly loses HCl, and after 2 weeks, it is fully
converted to 16-electron Ir(I) carbonyl complex 12. Isomer-
izations similar to the transformation of 11a to 11b were
previously observed by us28 and others30 for carbonyl-hydrido-
chloride pincer complexes.
A more interesting process from a mechanistic point of view
is the formation of 11a from 10a, which is a CO-induced C−H
activation.31 In principle, one can think of this process as a β-
elimination, which can be facile for the more flexible acyclic
aliphatic backbone.32 In the bulky and rigid pincer framework, a
simple β-elimination is probably much more difficult which is
manifested in the fact that for 6 the isomerization into olefin
forms has a high barrier and takes hours at temperatures above
180 °C. A possible explanation for the effect of CO
coordination is found in the calculated Ir−Cl bond, which is
fairly long for 10a (2.57 Å) and 10b (2.55 Å). Possibly, this
leads to partial or complete dissociation of the chloride giving a
buildup of some positive charge on Ir, thereby facilitating the
elimination (see the Supporting Information for more details).
D
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Metalation and Aromatization of Trans Aliphatic
Pincer Ligand 5. Similar to the case of 4, addition of
[Ir(COD)Cl]2 to a solution of 5 resulted in formation of
several unidentified compounds. However, refluxing the
reaction mixture in toluene provided mainly hydrido-chloride
complex 13, which was isolated by crystallization. Compound
13 revealed an AMX pattern with some roof effect (173.28 and
169.23 ppm, 2JPP = 366.2 Hz) in the 31P{1H} NMR spectrum in
agreement with a nonsymmetrical structure; a hydride
Scheme 5. Metallation and Aromatization of trans Aliphatic
Pincer Ligand 18
3
resonance was observed at −39.37 with JH−IrH = 2.8 Hz,
consistent with a mutual anti arrangement of α-C−H and Ir−H
bonds. The crystal structure of 13 suffers from disorder in the
cyclohexane unit, but the trans orientation of the −OP(tBu)2
groups can clearly be seen in both components (Figure 3).
Aromatization of phosphinite ligands 4 and 5 is quite similar
to the one for phosphine analog, but has some important
differences. Thus, carbene complexes 6 and 14 are formed at
temperatures significantly lower than those required for
isomerization into olefin forms (roughly around 200 °C for
all compounds) as major products; at the same time, the yield
of the phosphine based carbene never exceeded 10% because of
further transformations. We attribute this difference to a
tightening effect resulting from the CH2-for-O substitution,
which is seen as a decrease of P−Ir−P angles and shorter α-C−
Ir distances, compared to the phosphine counterpart. The
shorter distance apparently lowers the barrier for α-hydrogen
elimination, a key step for the formation of carbene
compounds. It therefore can be concluded that for phoshinite
ligands the carbene chemistry should be more important or
even predominating compared to that for similar phosphine
frameworks (see also the catalytic part). Neither olefin nor
diene intermediates were observed during aromatization of 4
and 5, likely indicating their rapid conversion to complex 3 at
the conditions required for their formation.
Figure 3. Molecular structures of trans pincer complexes 13 (left) and
15 (right, half of the asymmetric unit). Selected bond distances (Å)
and angles (deg) of 13, Ir−C1B 1.998(14), Ir−C1A 2.101(13), Ir−Cl
2.417(2), P−Ir−P 160.61(7), and those of 15, C1−Ir1 2.076(5),
C21−Ir2 2.077(5), P1−Ir1−P2 152.23(5), P3−Ir2−P4 153.54(5).
Catalytic Dehydrogenation of Alkanes. A benchmark
reaction which is used for testing catalytic activity of iridium
pincer complexes is transfer dehydrogenation of cyclooctane in
the presence of tert-butylethylene (TBE) (Scheme 6).
Some ligand decomposition was always observed during the
metalation; in one experiment, possibly because of traces of
methanol left from crystallization, it was sufficient enough to
change the Ir/5 ratio and produce dinuclear complex 15 as the
major product. Fortunately, the crystal structure of 15 was not
disordered and allowed some conclusions about the geometry
of pincer complexes with trans phosphinite ligands, cf. Figure 3.
Two crystallographically independent molecules revealed α-C−
Ir distances of 2.07 and 2.08 Å and P−Ir−P angles of 152.2 and
153.5°. The cyclohexyl ring adopts a twisted-boat conformation
in contrast to related cis systems where the chair conformation
is usually preferred. Although the hydride was not located in
the Fourier map, the position of other atoms unambiguously
points to an anti arrangement with respect to the α-C−H.
Heating 13 at 180 °C leads to formation of carbene complex
14, which has a characteristic carbene peak in the 13C NMR
spectrum at 236.0 ppm. A number of very small unidentified
hydride peaks were detected by NMR during this trans-
formation, indicating that a complicated process occur as
described above for cis-ligand 4. Further heating at even higher
temperatures leads to aromatization and formation of complex
3. Reactions are described in Scheme 5.
Some Remarks about Aromatization of Cyclohexane-
Based Phosphine and Phosphinite Pincer Ligands. We
previously reported the aromatization of phosphine complex
1.13 At temperatures around 200 °C, a carbene complex as well
as olefin and diene complexes were formed, which upon further
heating were converted to benzene-based complex 2. The
presence of a hydrogen acceptor was essential for the
completion of the reaction; otherwise, the mixture of
compounds mentioned above was formed.
Scheme 6. Catalytic Transfer Dehydrogenarion of
Cyclooctane
Common conditions which most of the literature refer to
include 3030/3030/1 COA/TBE/cat. ratio and temperatures
around 200 °C. If the corresponding hydrido-chloride complex
t
is used as catalyst, then 1 equiv of base, typically BuONa, is
required to generate active species. Cis complex 8 demon-
strated only moderate catalytic activity with TON 144, which
can be compared to arene-based (PCP)Ir (2, TON 230)18 and
(POCOP)Ir (3, TON 1512 according to the original report;5a
1730−1918 in our hands)17,33 and therefore was not studied
any further. Trans complex 13 however showed a fairly good
TON of 1223 under the same conditions. Bearing in mind that
thermal decomposition of catalytically active species was a
problem for phosphine complex 1, we gradually decreased the
temperature to 150 °C and then to 120 °C. This was
accompanied by an increase in the turnover numbers at the cost
of reaction rate. Subsequently raising the temperature indicated
that after 32 h at 150 °C there is almost no activity (Table 1,
entries 3 and 4), but after the same time at 120 °C, some
E
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composition (−CH2(Ph)CH−)n, i.e., polystyrene. We assume
Table 1. Transfer Dehydrogenation of Cyclooctane by
Iridium Pincer Complexes
that at lower temperatures similar byproducts are formed. The
1
olefin region of the reaction mixture was almost free from H
entry catalyst conditions [T (°C); t (h)]
COA/TBE/cat
TON
NMR signals, which probably means that polystyrene was
formed via dehydration rather than polycondensation. Pure 1-
phenylethanol is not readily dehydrated at the specified
temperatures, so metal catalysis seems reasonable. While
these side reactions are undesired for the purpose of H2 and/
or carbonyl compounds production, they may have a stand-
alone interest toward bioalcohols functionalization, particularly
the use of alcohols to give polymers.35
1
2
3
4
5
6
7
8
200; 16
3030/3030/1
3030/3030/1
3030/3030/1
3030/3030/1
3030/3030/1
3030/3030/1
20000/5000/1
144
1223
1253
1365
1388
1684
3238
13
13
13
13
13
13
200; 16
150; 32
150; 32 + 180; 3
120; 32
120; 32 + 180; 3
120; 32 + 180; 3
Unfortunately, other tested alcohols demonstrated very low
conversions around 10−15%. We speculate that a benzylic β-
hydrogen (upon coordination of the oxygen) may be important
for the present systems and the low conversion of aliphatic
alcohols may be related to slower transfer of this hydrogen to Ir
which opens up the way for competing deactivating processes.
To test the possibility of HCl loss from 8/13 and formation
of 14-electron (PCP)Ir active species, common for dehydro-
genation of alkanes, we performed a control experiment using
aromatic (POCOP)IrHCl complex 3 and 1-phenylethanol at
190 °C, but less than 1% of acetophenone was detected
indicating that this hypothesis can be rejected. In this respect, it
should be noted that in the absence of base, hydrido-chloride 8
(and therefore carbene 6) is not active in the dehydrogenation
of COA. We thus conclude that the mechanism of dehydrogen-
ation of alcohols by 6 is principally different from the
traditional one operating for alkanes. A possible catalytic
cycle is depicted on Scheme 7. An alcohol reacts with the
catalyst remained in active form (Table 1, entries 5 and 6). The
highest TON thus obtained was 1684. Interestingly, the
formation of COD was detected in this experiment giving a
ca. 84:16 COE/COD ratio. In a diluted solution containing
20 000/5000/1 COA/TBE/cat., the turnover number went up
to 3238, consistent with a less pronounced inhibition by olefins.
For comparison, the best catalyst reported to date demon-
strated a TON of 5901 with 6000/6000/1 COA/TBE/cat.8
Catalytic Dehydrogenation of Alcohols. Acceptorless
alcohol dehydrogenation is a promising way to produce
hydrogen from liquid organic substrates and also a valuable
tool for green oxidation and green synthesis of amides, acetals,
lactams, and many other classes of compounds when combined
with coupling reactions.3b,34 Inspired by the facile oxidative
addition of hydrogen to carbene 6 and regeneration of the latter
3
upon heating, we reasoned that (PCsp P)Ir systems may offer
2
something that their aromatic (PCsp P)Ir analogs cannot: a
possibility of metal−ligand cooperation, where a related
reaction sequence would lead to the dehydrogenation of
alcohols. An obvious problem could be the competing
aromatization of the carbene compounds; in order to
circumvent such reactivity, we decided to take neat alcohols
as the reaction media to maximize probability of the reaction
between carbene and alcohol. When cis carbene complex 6 was
heated with benzyl alcohol at 190 °C in a 4000/1 ratio, a
complete conversion of the substrate was observed with
formation of benzylbenzoate, benzaldehyde, and some amount
of unidentified products (Table 2). Corresponding hydrido-
chloride 8 can be used with virtually the same result; we thus
used more accessible trans complex 13 instead of carbene 14.
The activity of the latter compounds was found to be very
similar to that of 6/8. With secondary substrate 1-phenyl-
ethanol, the reaction was much less clean and produced more
byproducts than desired acetophenone; decreasing the temper-
ature to 175 °C improved the yield of the ketone. The
byproducts from the 190 °C experiment were precipitated and
subjected to elemental analysis, which, together with the
presence of very broad NMR signals in the aliphatic and
aromatic regions suggested that these byproducts have the
Scheme 7. Possible Catalytic Cycle for Dehydrogenation of
Alcohols by Carbene 11
carbene by oxidative addition of the O−H bond to Ir, followed
by hydrogen migration to the α-carbon. Subsequent β-
Table 2. Acceptorless Alcohol Dehydrogenation by Aliphatic Phosphinite Iridium Pincer Complexes
a
entry
catalyst
substrate
conditions [T (°C); t (h)]
conversion (%)
yield
1
2
3
4
5
6
6
PhCH2OH
190; 16
190; 16
190; 16
175; 48
190; 16
190; 16
>99
>99
>99
>99
12
PhCHO 20%, PhCOOBn 68%
13
6
PhCH2OH
PhCHO 13%, PhCOOBn 60%
PhCH(OH)CH3
PhCH(OH)CH3
Cy−OH
PhCOCH3 29%, PS 71%
6
PhCOCH3 52%
6
b
b
6
1-octadecanol
16
a
b
When the sum of the yields is less than 100%, the rest corresponds to condensation/polymerization products. PS = polystyrene. A mixture of
several unidentified compounds was formed.
F
DOI: 10.1021/acs.organomet.6b00846
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
hydride. Alcohols for catalytic experiments were used as received
except for degassing. NMR spectra were recorded on a Varian Unity
INOVA 500 MHz or Bruker Avance 400 MHz instruments. H and
elimination leads to hydrido-chloride 8, which eliminates
hydrogen and regenerates carbene 6. No active species could
be characterized during reaction, and further studies are
required to elucidate the details of the mechanism.
1
13C NMR chemical shifts are reported in parts per million and
referenced to the signals of deuterated solvents. 31P{1H} NMR
chemical shifts are reported relative to external 85% solution of
phosphoric acid. In some 31P{1H} and 13C{1H} NMR spectra
decoupling of high-field hydride ligands was incomplete or totally
absent; in the latter case, coupling constants are indicated in the text.
Assignments of signals were typically confirmed using 2D spectra;
where such spectra are lacking or do not provide reliable assignment,
several possible options are listed. IR spectra were recorded on a
Bruker Alpha spectrometer. Elemental analyses were performed by H.
A very important feature related to the mechanism is the
compatibility of carbenes 6/14 with primary alcohols. In this
respect, it should be noted that seemingly all (PCP)Ir pincers
that can somehow form unsaturated 14-electron species are
more or less prone to decarbonylation of primary alcohols29b,36
due to the high binding energy of CO to electron-rich Ir,
making catalytic dehydrogenation of such alcohols trouble-
some. This limitation is probably relevant to all complexes for
which the catalytic cycle at least partially resembles the one for
dehydrogenation of alkanes (specifically, involving removal of
HCl from the corresponding (PCP)IrHCl complexes),
although for secondary alcohols the catalytic activity can be
very high.29b,36a,37 At the same time, catalysts which work via a
cooperative type of mechanism are much less prone to
decarbonylation.21
Kolbe Microanalytisches Laboratorium, Mulheim an der Ruhr,
̈
Germany. Ligand 4 has been synthesized before,26b and a slightly
modified procedure was used as described in the Supporting
Information. In the characterization, the following numbering was
applied:
CONCLUSIONS
■
In summary, we have presented the synthesis, characterization,
and catalytic activity of aliphatic phosphinite iridium pincer
complexes and their derivatives. While under certain conditions
cyclohexane-based phosphinite pincer ligands apparently
decompose, they can be successfully metalated to give
moderately robust iridium complexes. The thermal stability of
these compounds and their derivatives is lower compared to
that of the arene-based counterparts, but some of them (such as
trans complex 13) still demonstrated good catalytic activity for
dehydrogenation of alkanes, higher than that for aromatic
phosphine complex 2 and only slightly lower than that for
aromatic phosphinite complex 3. Because of facile α-elimination
reactions, the chemistry of the ligands reported here is
dominated by carbene formation rather than the formation of
reactive 14-electrons species as observed for many similar
iridium pincer complexes. The α-elimination is probably
facilitated by the compartively short α-C−Ir distances. On
the one hand, this may open up for specific decomposition
pathways which are likely responsible for the low activity of cis
complex 8 in the dehydrogenation of alkanes, but on the other
hand, the possibility of α-elimination may also provide
reactivity patterns that are unavailable for arene-based
compounds. We argue that such metal−ligand cooperative
pattern is responsible for the catalytic dehydrogenation of
alcohols by carbene 6, hydrido-chloride 13, and related
compounds. An excellent activity was observed in acceptorless
dehydrogenation of benzylic alcohol and good for 1-phenyl-
ethanol, but aliphatic alcohols revealed low conversions. Of
particular interest is the ability of these complexes to transform
alcohols into polymers formally derived from the correspond-
ing olefin.
Synthesis of Trans POCyOP Ligand 5. trans-1,3-Cyclohexane-
diols were separated from the cis/trans mixture using the method of
Aoyama and co-workers.38 Starting from trans diol (0.220 g, 1.894
mmol), 60% NaH in paraffin (0.159 g, 3.977 mmol, 2.1 equiv) and di-
tert-butylchlorophosphine (0.79 mL, 4.167 mmol, 2.2 equiv) ligand 5
was obtained in almost quantitative yield using the same procedure as
for 4. The product exhibited ca. 95% purity by NMR spectroscopy and
was used without further purification. A sample for elemental analysis
was additionally recrystallized from CH2Cl2/MeOH.
1H NMR (500 MHz, toluene-d8) δ 4.05−3.99 (m, 2H, >CH−O−),
1.89 (br s, 1H, Cy−H), 1.67−1.46 (overlapping, m, 4H), 1.39−1.24
3
(m, 2H, Cy−H), 1.11 (d, JPH = 11.0 Hz, 18H, 2 C(CH3)3), 1.08 (d,
3JPH = 11.0 Hz, 18H, 2 C(CH3)3), 0.95−0.87 (m, 1H, Cy−H).
31P{1H} NMR (202 MHz, toluene-d8) δ 155.07 (br s). Anal. Calcd for
C22H46O2P2: C, 65.32; H, 11.46. Found C, 65.63, H, 11.78.
Metalation of Ligand 4 and Synthesis of Carbene 6 and
Complex 7. A Straus flask was charged with POCyOP ligand 4 (0.550
g, 1.360 mmol), [Ir(COD)Cl]2 (0.457 g, 0.680 mmol), and 15 mL of
toluene inside a nitrogen atmosphere glovebox. The flask was sealed,
fully immersed into an oil bath, and heated at 120 °C overnight. The
volatiles were removed in vacuum, and the residue was extracted with
hexane until the extract became pale yellow. The solutions were
combined, evaporated, and chromatographed on alumina using
benzene−hexane 1:1 mixture as eluent. The green fraction was
collected, evaporated, and dried under vacuum to give 6 (0.294 g,
34%) as a dark greenish powder. The residue from the extraction was
suspended in a small amount of hexane, the solid particles were
allowed to sediment, and the mother liquor was decanted. This
procedure was repeated several times. The resulting solid material was
dried under vacuum to give 7 (0.277 g, 32%) as a yellow powder.
When only 6 is desired, the temperature may be raised to 170 °C,
which improves the isolated yield of 6 up to 55%.
1
Characterization of 6. H NMR (500 MHz, C6D6) δ 1.57−1.53
(m, overlapping, 2H, 3HA and 5HA), 1.51 (vt, 18H, |3JPH + 5JPH| = 14.0
5
Hz, 2 C(CH3)3), 1.44 (vt, 18H, |3JPH + JPH| = 13.7 Hz, 2 C(CH3)3),
EXPERIMENTAL SECTION
■
1.25 (apparent qd, 2H, JHH = 12.4 Hz, JHH = 4.4 Hz, 3HB and 5HB),
General Considerations. All manipulations were conducted
under an inert gas atmosphere using standard Schlenk, high-vacuum
line, and glovebox techniques unless otherwise stated. All solvents
were distilled under vacuum from Na/benzophenone except for
CH2Cl2 which was distilled under vacuum from calcium hydride.
Hydrocarbon deuterated solvents were distilled under vacuum from
Na/benzophenone. Cyclooctane was washed with H2SO4 until it was
nearly colorless, dried, and distilled under vacuum from calcium
hydride. tert-Butylethylene was distilled under vacuum from calcium
0.98 (apparent qt, 1H, JHH = 13.2 Hz, 3JHH = 3.4 Hz, 4HA), 0.92−0.86
3
3
(m, 1H, 4HB), −1.21 (dd of vt, 2H, JHH = 11.8 Hz, JHH = 5.8 Hz,
|3JPH + 4JPH| = 5.8 Hz, 2H and 6H). 13C{1H} NMR (126 MHz, C6D6)
δ 235.85 (t, 2JPC = 4.3 Hz, 1C), 106.39 (vt, |2JPC + 3JPC| = 13.8 Hz, 2C
and 6C), 41.47 (vt, |1JPC + 3JPC| = 23.0 Hz, C(CH3)3), 40.14 (vt, |1JPC
+
3JPC| = 24.8 Hz, C(CH3)3), 29.16 (vt, |2JPC + 4JPC| = 7.0 Hz, C(CH3)3),
28.93 (vt, |2JPC + 4JPC| = 6.8 Hz, C(CH3)3), 27.43 (vt, |3JPC + 4JPC| = 4.0
Hz, 3C and 5C), 20.88 (s, 4C). 31P{1H} NMR (202 MHz, C6D6) δ
G
DOI: 10.1021/acs.organomet.6b00846
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2
179.2 (s). Anal. Calcd for C22H44ClIrO2P2: C, 41.93; H, 7.04. Found
C, 42.21, H, 7.44.
Characterization of 7. H NMR (500 MHz, C6D6) δ 4.08−4.00
NMR (126 MHz, C6D6) δ 211.99 (t, JPC = 7.7 Hz, 1C), 187.09 (t,
2JPC = 3.8 Hz, Ir−CO), 99.77 (vt, |2JPC + 3JPC| = 12.6 Hz, 2C and 6C),
1
42.56 (vt, |1JPC + JPC| = 26.6 Hz, C(CH3)3), 40.27 (vt, |1JPC + JPC| =
3
3
26.8 Hz, C(CH3)3), 31.63 (vt, |3JPC + JPC| = 3.2 Hz, 3C and 5C),
4
(m, 2H, 2H and 6H), 2.76−2.72 (m, 1H, 1HA), 2.05−2.00 (m, 2H,
3HA and 5HA), 1.54−1.48 (m, overlapping, 3H, 1HB + 4HA + 4HB),
1.47−1.43 (m, 36 H, 4 C(CH3)3), 1.14−1.04 (m, 2H, 3HB and 5HB),
−32.89 (td, 1H, 2JPH = 10.4 Hz, 2JHH = 8.1 Hz, Ir−H), −33.27 (td, 1H,
29.18 (vt, |2JPC + 4JPC| = 7.6 Hz, C(CH3)3), 27.83 (vt, |2JPC + 4JPC| = 6.0
Hz, C(CH3)3), 20.96 (s, 4C). 31P{1H} NMR (202 MHz, C6D6) δ
182.40 (s).
2
2JPH = 10.7 Hz, JHH = 8.1 Hz, Ir−H). 13C{1H} NMR (126 MHz,
Characterization of 11a. 1H NMR (500 MHz, C6D6) δ 4.15−4.11
(m, 1H, 2H), 2.07 (overlapping, 5HA), 1.94 (overlapping, 5HB), 1.89
(overlapping, 3HA), 1.53 (overlapping, 4HB), 1.5 (overlapping, 2
C(CH3)3), 1.42 (overlapping, 2 C(CH3)3), 1.27 (overlapping, 3HB),
C6D6) δ 77.91 (s, 1C), 41.90 (br s, 1C), 41.71 (apparent td, J = 14.3
Hz, J = 8.6 Hz, C(CH3)3), 33.10 (s, 3C and 5C), 29.41 (vt, |2JPC + 4JPC
|
= 6.0 Hz, C(CH3)3), 29.35 (vt, |2JPC + 4JPC| = 5.8 Hz, C(CH3)3), 21.16
(s, 4C). 31P{1H} NMR (202 MHz, C6D6) δ 159.49 (s). NMR spectra
in THF-d8 are given in the Supporting Information. Anal. Calcd for
C44H96Cl2Ir2O4P4: C, 41.66 H, 7.63. Found C, 42.16, H, 6.99. HRMS:
Calcd for C44H96ClIr2O4P4 ([M − Cl]+): 1233.5206. Found:
1233.5211. NMR signals for a single (POCyHOP)*Ir(H)2Cl unit
are reported, since the number of units cannot be derived from
integration.
2
2
1.20 (overlapping, 4HB), −8.49 (apparent t, 1H, JP1H = JP2H = 16.3
Hz). 13C{1H} NMR (126 MHz, C6D6) δ 180.12 (m, Ir−CO), 148.08
(m, 6C), 111.11 (m, 1C), 86.73 (d, 2JPC = 3.7 Hz, 2C), 29.45 (dd, 2JPC
= 3.5 Hz, 4JPC = 1.8 Hz, C(CH3)3), 29.27 (dd, 2JPC = 3.5 Hz, 4JPC = 1.9
Hz, C(CH3)3), 23.34 (dd, JPC = 6.0 Hz, JPC = 1.8 Hz, 5C). 31P{1H}
3
4
NMR (202 MHz, C6D6) δ 156.82 and 155.86 ppm (AB system, 2JPP
=
325.4 Hz).
Characterization of 11b. 1H NMR (500 MHz, C6D6) δ 4.99
Synthesis of Hydrido-Chloride Complex 8. A J. Young NMR
tube was charged with carbene 6 (0.020 g, 0.032 mmol) inside a
nitrogen atmosphere glovebox, followed by vacuum-transfer of 0.7 mL
of C6D6. The tube was refilled with 1 atm of H2 and left for 3 days at
room temperature, after which NMR spectroscopy indicated that
complex 8 was the only product in the solution. The volatiles were
evaporated and the residue was dried in vacuum to give 8 as a yellow
powder in quantitative yield. 1H NMR (500 MHz, C6D6) δ 3.15
(apparent ddt, 1H, J = 10.0 Hz, J = 5.1 Hz, J = 2.6 Hz, 2H), 2.16−2.12
3
(m, 2H, 5HA + 5HB), 2.12−2.07 (m, 1H, 3HA), 1.64 (d, 9H, JPH
=
14.7 Hz, C(CH3)3), 1.58 (d, 9H, 3JPH = 14.6 Hz, C(CH3)3), 1.55−1.52
(m, 1H, 4HA), 1.38−1.34 (m, 1H, 4HB), 1.26−1.20 (m, 1H, 3HB),
3
3
1.17 (d, 9H, JPH = 13.9 Hz, C(CH3)3), 1.12 (d, 9H, JPH = 13.8 Hz,
2
2
C(CH3)3), −17.96 (dd, JP1H = 14.9 Hz, JP2H = 12.2 Hz, 1H).
13C{1H} NMR (126 MHz, C6D6) δ 179.88 (apparent q, 2JP1C = 2JP2C
=
2JCH = 4.0 Hz, Ir−CO), 154.49 (apparent dt, 2JPC = 7.2 Hz, 3JPC = 3JCH
= 1.6 Hz, 6C), 127.5 (m, 1C), 88.09 (d, 2JPC = 6.5 Hz, 2C), 44.20 (dd,
3
3
(apparent td, 2H, JHH = 10.4 Hz, JHH = 3.6 Hz, 2H and 6H), 2.62
3
3
(td, 1H, JHH = 10.4 Hz, JH−IrH = 3.1 Hz, 1H), 1.98−1.92 (m, 2H,
3HA and 5HA), 1.59−1.54 (m, 1H, 4HA) 1.38 (vt, 18H, |3JPH + 5JPH| =
1JPC = 12.9 Hz, JPC = 8.2 Hz, C(CH3)3), 43.74 (dd, JPC = 15.1 Hz,
3
1
13.8 Hz, 2 C(CH3)3), 1.33 (d, 18H, |3JPH
+
5JPH| = 14.0 Hz, 2
1
3
3JPC = 6.6 Hz, C(CH3)3), 39.42 (ddd, JPC = 30.4 Hz, JPC = 3.3 Hz,
3JCH = 2.2 Hz, C(CH3)3), 38.99 (apparent dt, JPC = 26.6 Hz, JPC
=
1
3
C(CH3)3), 1.20−1.12 (m, 2H, 3HB and 5HB), 1.06−0.96 (m, 1H,
2
3
2
4
3JCH = 2.3 Hz, C(CH3)3), 30.00 (dd, JPC = 4.4 Hz, JPC = 0.9 Hz,
4HB), −40.37 (td, JPH = 12.4 Hz, JH−IrH = 3.1 Hz, 1H, Ir−H).
13C{1H} NMR (126 MHz, C6D6) δ 88.73 (vt, |2JPC + JPC| = 9.6 Hz,
3
2
3
C(CH3)3), 29.57 (d, JPC = 11.0 Hz, 3C), 29.15 (d, JPC = 4.3 Hz,
C(CH3)3), 28.38 (d, 2JPC = 4.3 Hz, C(CH3)3), 28.21 (d, 2JPC = 4.0 Hz,
C(CH3)3), 24.09 (d, 3JPC = 10.1 Hz, 5C), 20.66 (s, 4C). 31P{1H} NMR
(202 MHz, C6D6) δ 168.16 (dd, JPP = 310.5 Hz, JPH = 14.9 Hz),
161.68 (dd, JPP = 310.5 Hz, JPH = 12.2 Hz).
2
2
2C and 6C), 43.54 (dt, JCH = 6.2 Hz, JPC = 2.0 Hz, 1C), 40.46 (vt,
|1JPC + JPC| = 26.8 Hz, C(CH3)3), 40.04 (vt, |1JPC + JPC| = 23.5 Hz,
3
3
C(CH3)3), 32.45 (t, |3JPC + JPC| = 10.6 Hz, 3C and 5C), 28.25 (vt,
4
2
2
|2JPC + JPC| = 6.6 Hz, C(CH3)3), 27.96 (vt, |2JPC + JPC| = 6.0 Hz,
C(CH3)3), 22.36 (s, 4C). 31P{1H} NMR (202 MHz, C6D6) δ 170.0 (d,
2JPH = 12.4 Hz). Anal. Calcd for C22H46ClIrO2P2: C, 41.80; H, 7.33.
Found C, 41.96, H, 7.47.
4
4
2
2
1
Characterization of 12. H NMR (400 MHz, C6D6) δ 4.58−4.52
(m, 1H, 2H), 2.36−2.28 (m, 2H, 5HA + 5HB), 2.17−2.11 (m, 1H,
3HA), 1.70−1.59 (m, 1H, 4HA), 1.39−1.32 (overlapping, m, 36H, 4
C(CH3)3), 1.28−1.18 (overlapping, m, 2H, 3HB + 4HB). 13C{1H}
Reaction of Carbene 6 with CO. A J. Young NMR tube was
charged with carbene 6 (0.019 g, 0.030 mmol) inside a nitrogen
atmosphere glovebox, followed by vacuum-transfer of 0.7 mL of C6D6.
The tube was refilled with 1 atm of CO accompanied by a
simultaneous color change from dark greenish to yellow. Compounds
10a and 10b were initially the only products of reaction and were
characterized by NMR spectroscopy. The low stability of 10a and 10b
requires high concentration in order to get reasonably good 13C and
2D spectra. Subsequently, 11a and 11b were formed, and after several
days, 11b was the only product in the solution. Complex 11a was
never formed as a major product, but the majority of its NMR signals
were reliably identified. After 2 weeks there was complete conversion
of 11b into 12, which was obtained by evaporation of the volatiles in
almost quantitative yield.
2
2
NMR (101 MHz, C6D6) δ 199.23 (apparent t, JP1C = JP2C = 5.3 Hz,
2
2
Ir−CO), 161.54 (apparent t, JP1C = JP2C = 7.3 Hz, 6C) 153.49 (dd,
2JPC = 8.4 Hz, 3JPC = 8.2 Hz, 1C), 88.45 (dd, 2JPC = 5.9 Hz, 3JPC = 5.0
Hz, 2C), 41.62−41.27 (overlapping, 2 C(CH3)3), 40.20 (X part of
ABX, |1JPC + 3JPC| = 25.9 Hz, C(CH3)3), 39.49 (X part of ABX, |1JPC
+
3JPC| = 31.6 Hz, C(CH3)3), 29.69 (X part of ABX, |3JPC + JPC| = 8.8
4
Hz, 3C), 28.85 (X part of ABX, |2JPC + JPC| = 7.4 Hz, C(CH3)3),
4
28.51−28.36 (m, overlapping, 2 C(CH3)3), 28.28 (X part of ABX, |2JPC
+ 4JPC| = 6.3 Hz, C(CH3)3), 23.61 (X part of ABX, |3JPC + 4JPC| = 10.6
Hz, 5C), 20.85 (s, 4C). 31P{1H} NMR (162 MHz, C6D6) δ 155.40 and
2
154.32 ppm (AB system, JPP = 305.5 Hz).
Metalation of Trans Ligand 5 and Synthesis of Complex 13.
A Straus flask was charged with 5 (0.134 g, 0.331 mmol),
[Ir(COD)Cl]2 (0.111 g, 0.165 mmol), and 7 mL of toluene inside a
nitrogen atmosphere glovebox. The flask was sealed, fully immersed
into an oil bath and heated at 120 °C for 8 h. The volatiles were
removed in vacuum, the residue was dissolved in a minimum amount
of hexane at 120 °C, and the flask was allowed to stay overnight at
room temperature. The mother liquor was decanted, and the residue
was washed with a small amount of cold hexane to give 13 (0.089 g,
43%) as dark red crystals of XRD quality. 1H NMR (500 MHz, C6D6)
Characterization of 10a. 1H NMR (500 MHz, C6D6) δ 2.61−2.57
(m, 2H, 2H and 6H), 1.71 (overlapping, 3HA and 5HA), 1.50
(overlapping, 18H, 2 C(CH3)3), 1.38 (overlapping, 18H, 2 C(CH3)3),
1.17 (overlapping, 4HA), 0.91 (overlapping, 3HB and 5HB + 4HB).
2
13C{1H} NMR (126 MHz, C6D6) δ 214.31 (t, JPC = 7.8 Hz, 1C),
187.35 (t, 2JPC = 3.5 Hz, Ir−CO), 99.78 (vt, |2JPC + 3JPC| = 12.6 Hz, 2C
and 6C), 43.58 (vt, |1JPC + 3JPC| = 25.4 Hz, C(CH3)3), 38.81 (t, |1JPC
+
4
3JPC| = 27.2 Hz, C(CH3)3), 33.02 (vt, |3JPC + JPC| = 4.4 Hz, 3C and
5C), 29.61 (vt, |2JPC + 4JPC| = 4.6 Hz, C(CH3)3), 28.36 (vt, |2JPC + 4JPC
|
3
δ 3.91−3.85 (m, 1H, 2H or 6H), 3.43 (apparent td, 1H, JHH = 11.6
= 6.2 Hz, C(CH3)3), 20.37 (s, 4C). 31P{1H} NMR (202 MHz, C6D6)
δ 182.45 (s).
Hz, JHH = 4.8 Hz, 2H or 6H), 3.15 (apparent td, JHH ≈ 3JHH ≈ 10.8
Hz, 3JH−IrH = 2.8 Hz, 1H, 1H), 1.97−1.91 (m, 1H, 3HA or 5HA), 1.46−
1.43 (overlapping, 1H, 3HA or 5HA), 1.45−1.41 (overlapping, 1H,
4HA), 1.40−1.33 (overlapping, m, 36H, 4 C(CH3)3), 1.27−1.22
(overlapping, 1H, 4HB), 1.23−1.18 (overlapping, 1H, 3HB or 5HB),
3
3
Characterization of 10b. 1H NMR (500 MHz, C6D6) δ 2.90−2.85
(m, 2H, 2H and 6H), 1.76 (overlapping, 3HA and 5HA), 1.52
(overlapping, 18H, 2 C(CH3)3), 1.36 (overlapping, 18H, 2 C(CH3)3),
1.26 (overlapping, 3HB and 5HB), 0.91 (overlapping, 4HB). 13C{1H}
2
2
1.12−1.04 (m, 1H, 3HB or 5HB), −39.37 (dd, JPH = 13.9 Hz, JPH
=
H
DOI: 10.1021/acs.organomet.6b00846
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
11.1 Hz, JH−IrH = 2.8 Hz, 1H, Ir−H). 13C{1H} NMR (126 MHz,
SIR-92.41 Non-H atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were constrained to parent sites, using a
riding model. Crystals of 6 and 13 have residual electron density close
to Ir. This is explained by poor crystal quality and a nonmodeled
hydride, respectively. Crystals of 8 and 13 have disordered cyclohexyl
groups, and these were modeled isotropically. The CCDC deposition
numbers are 1474224−1474227.
3
C6D6) δ 88.82 (dd, 2JPC = 5.1 Hz, 3JPC = 2.0 Hz, 2C or 6C), 83.02 (dd,
2JPC = 7.8 Hz, 3JPC = 1.6 Hz, 2C or 6C), 42.37 (dd, 1JPC = 14.5 Hz, 3JPC
1
3
= 6.9 Hz, C(CH3)3), 40.63 (dd, JPC = 18.2 Hz, JPC = 6.3 Hz,
C(CH3)3), 40.46 (dd, JPC = 17.3 Hz, 3JPC = 6.1 Hz, C(CH3)3), 37.83
1
1
3
(dd, JPC = 25.8 Hz, JPC = 6.3 Hz, C(CH3)3), 37.0 (d of apparent t,
2JCH = 5.5 Hz, JP1C
=
2JP2C = 1.4 Hz, 1C), 28.69−28.60 (m,
2 4
2
General Procedure for Transfer Dehydrogenation of Cyclo-
octane. In a typical experiment, the catalyst (0.0095 mmol) and
tBuONa (0.0014 g, 0.0142 mmol, 1.5 equiv) were placed into a Straus
overlapping, 2 C(CH3)3), 28.51 (dd, JPC = 4.9 Hz, JPC = 1.2 Hz,
C(CH3)3), 28.37 (dd, JPC = 5.0 Hz, JPC = 1.2 Hz, C(CH3)3), 26.34
2
4
3
2
(d, JPC = 11.6 Hz, 3C or 5C), 25.65 (d, JPC = 6.6 Hz, 3C or 5C),
17.64 (s, 4C). 31P{1H} NMR (202 MHz, C6D6) δ 173.28 (dd, JPP
=
2
flask and specified amounts of cyclooctane (COA) and tert-
butylethylene (TBE) were added. The flask was sealed and fully
immersed into a preheated oil bath. After completion of the heating,
the flask was cooled by a stream of air and the sample was analyzed by
NMR spectroscopy.
2
2
2
366.2 Hz, JPH = 13.9 Hz), 169.23 (dd, JPP = 366.2 Hz, JPH = 11.1
Hz). Anal. Calcd for C22H46ClIrO2P2: C, 41.80; H, 7.33. Found C,
42.02, H, 7.43.
Synthesis of Complex 14. A J. Young NMR tube was charged
with complex 13 (0.016 g, 0.025 mmol) and 0.7 mL of toluene-d8
inside a nitrogen atmosphere glovebox. The tube was heated on an oil
bath at 180 °C for 24 h, the volatiles were evaporated, and the residue
was dried in vacuum to give 14 as brown powder in almost
quantitative yield. 1H NMR (500 MHz, C6D6) δ 1.57−1.50
General Procedure for Acceptorless Dehydrogenation of
Alcohols. In a typical experiment, the catalyst (0.0095 mmol) and
degassed alcohol (4000 equiv) were placed into a Schlenk flask
equipped with magnetic stirring bar and reflux condenser. The
reaction mixture was stirred for 0.5 h, and then the flask was placed
into an oil bath preheated to the specified temperature. A slow stream
of Ar was passed above the reflux condenser to facilitate the escape of
H2. After the desired reaction time, the Schlenk flask was allowed to
reach room temperature, and the sample was analyzed by NMR
spectroscopy. To obtain a sample of polystyrene, the reaction mixture
was diluted with small amount of acetone, followed by addition of
pentane. The precipitate was washed with pentane and dried in
vacuum. NMR spectra were in agreement with those reported for
polystyrene.42 Anal. Calcd for (C8H8)n: C, 92.26; H, 7.74. Found C,
91.81, H, 7.35.
Computational Details. The electronic structure calculations
were carried out with Gaussian 0943 program using the DFT method
with the PBE44 exchange and correlation functionals. The relativistic,
small-core ECP and corresponding basis set of Dolg et al.45 (SDD)
was used for the Ir atom; D95(d)46 basis set was applied to other
atoms. This model was previously shown to give accurate results for
iridium pincer complexes.47 All stationary points located on the
potential energy surfaces were characterized by normal-mode analysis
as minima or transition states. Natural bond occupation (NBO)
calculation was done as implemented in the Gaussian program
package.48
(overlapping, 2H, 3HA and 5HA), 1.52 (vt, 18H, |3JPH + JPH| = 14.0
5
Hz, C(CH3)3), 1.44 (vt, 18H, |3JPH + JPH| = 13.6 Hz, C(CH3)3),
5
1.28−1.21 (m, 2H, 3HB and 5HB), 0.98−0.87 (m, 2H, 4HA + 4HB),
−1.20 (dd of vt, 3JHH = 11.5 Hz, 3JHH = 5.6 Hz, |3JPH + 4JPH| = 5.6 Hz,
2H, 2H and 6H). 13C{1H} NMR (126 MHz, C6D6) δ 236.03 (t, 2JPC
=
4.3 Hz, 1C), 106.35 (vt, |2JPC + 3JPC| = 13.8 Hz, 2C and 6C), 41.32 (vt,
|1JPC + JPC| = 23.2 Hz, C(CH3)3), 39.98 (vt, |1JPC + JPC| = 25.0 Hz,
C(CH3)3), 28.97 (vt, |2JPC + 4JPC| = 7.0 Hz, C(CH3)3), 28.74 (vt, |2JPC
+ 4JPC| = 6.8 Hz, C(CH3)3), 20.68 (s, 4C). 31P{1H} NMR (202 MHz,
C6D6) δ 179.48 (s). Anal. Calcd for C22H44ClIrO2P2: C, 41.93; H,
7.04. Found C, 42.14, H, 6.86.
3
3
Formation of Compound 15. The experiment was conducted in
a way similar to synthesis of complex 13 described above, starting from
POCyOP ligand (0.521 g, 1.288 mmol) and [Ir(COD)Cl]2 (0.433 g,
0.644 mmol). After crystallization, in addition to dark red crystals of
13 (0.191 g, 23%), large yellow crystals of 15 (0.255 g, 41% based on
1
Ir) were obtained, which were separated manually. H NMR (500
3
3
MHz, C6D6) δ 4.17 (td, 1H, JHH = 7.3 Hz, JHH = 3.1 Hz, −CH
CH− in COD), 4.13−4.10 (m, 1H, −CHCH− in COD), 4.07−
4.00 (overlapping, 2H, −CHCH− in COD + 2H), 3.95 (td, 1H,
3JHH = 7.3 Hz, JHH = 3.3 Hz, −CHCH− in COD), 3.30−3.24 (m,
3
1H, 6H), 2.59 (dd, 1H, 3JHH = 11.6 Hz, 3JHH = 8.1 Hz, 1H), 2.17−2.00
(m, 4H, −CH2− in COD), 1.92−1.84 (m, 1H, 5HA), 1.73 (d,
overlapping, 9H, 3JPH = 12.6 Hz, C(CH3)3), 1.72 (d, overlapping, 9H,
3JPH = 12.6 Hz, C(CH3)3), 1.58−1.53 (m, 1H, 3HA), 1.43 (d, 9H, 3JPH
= 12.7 Hz, C(CH3)3), 1.41−1.34 (overlapping, 1H, 3HB), 1.37−1.31
(overlapping, 1H, 4HA), 1.36 (d, 9H, 3JPH = 12.7 Hz, C(CH3)3), 1.24−
1.07 (overlapping, 6H, −CH2− in COD + 5HB + 4HB), −27.11 (dd,
2JPH = 16.9 Hz, 2JPH = 12.1 Hz, 1H, Ir−H). 13C{1H} NMR (126 MHz,
C6D6) δ 88.46 (apparent t, 2JP1C = 2JP2C = 2.7 Hz, 6C), 83.99 (dd, 2JPC
= 8.7 Hz, 3JPC = 2.7 Hz, 2C), 61.88 (s, CC in COD), 61.85 (s, C
C in COD), 61.74 (s, CC in COD), 61.27 (s, CC in COD),
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00846.
Further experimental details, NMR spectra, and
computational results (PDF)
Crystallographic information files for compounds 6, 8,
13, and 15 (CIF)
Computed molecule Cartesian coordinates (XYZ)
1
3
1
44.27 (dd, JPC = 9.6 Hz, JPC = 8.3 Hz, C(CH3)3), 41.52 (dd, JPC
=
13.8 Hz, 3JPC = 8.7 Hz, C(CH3)3), 40.43 (dd, 1JPC = 20.9 Hz, 3JPC = 5.0
1
3
Hz, C(CH3)3), 39.88 (dd, JPC = 29.3 Hz, JPC = 4.3 Hz, C(CH3)3),
32.78 (br s, 1C), 32.13 (s, −CH2− in COD), 32.04 (s, −CH2− in
COD), 31.78 (s, −CH2− in COD), 31.75 (s, −CH2− in COD), 30.21
AUTHOR INFORMATION
Corresponding Author
*E-mail: ola.wendt@chem.lu.se.
■
2
2
4
(d, JPC = 4.5 Hz, C(CH3)3), 29.83 (dd, JPC = 5.2 Hz, JPC = 1.1 Hz,
C(CH3)3), 29.55 (d, 2JPC = 4.0 Hz, C(CH3)3), 29.39 (d, 2JPC = 4.6 Hz,
ORCID
3
3
C(CH3)3), 27.63 (d, JPC = 10.8 Hz, 5C), 26.83 (d, JPC = 10.6 Hz,
3C), 18.47 (s, 4C). 31P{1H} NMR (202 MHz, C6D6) δ 155.36 (dd,
Ola F. Wendt: 0000-0003-2267-5781
Notes
The authors declare no competing financial interest.
2JPP = 366.1 Hz, JPH = 16.9 Hz), 150.18 (dd, JPP = 366.1 Hz, JPH
=
2
2
2
12.1 Hz). Anal. Calcd for C30H58Cl2Ir2O2P2: C, 37.22; H, 6.04. Found
C, 37.46, H, 5.92.
Crystallography. Intensity data were collected with an Oxford
Diffraction Xcalibur 3 system using ω-scans and Mo Kα (λ = 0.71073
Å). The CCD data were extracted and integrated using Crysalis
RED.39 The structures were solved using direct methods and refined
by full-matrix least-squares calculations on F2 using SHELXL40 and
ACKNOWLEDGMENTS
■
Financial support from the Swedish Research Council, the Knut
and Alice Wallenberg Foundation, and the Crafoord
Foundation is gratefully acknowledged.
I
DOI: 10.1021/acs.organomet.6b00846
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(18) Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem.
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K
DOI: 10.1021/acs.organomet.6b00846
Organometallics XXXX, XXX, XXX−XXX