Electrophilic Organoiridium(III) Pincer Complexes on Sulfated Zirconia for Hydrocarbon Activation and Functionalization

Article abstract of DOI:10.1021/jacs.9b00896

Single-site supported organometallic catalysts bring together the favorable aspects of homogeneous and heterogeneous catalysis while offering opportunities to investigate the impact of metal-support interactions on reactivity. We report a (^dmPhebox)Ir(III) (^dmPhebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complex chemisorbed on sulfated zirconia, the molecular precursor for which was previously applied to hydrocarbon functionalization. Spectroscopic methods such as diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS), dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and X-ray absorption spectroscopy (XAS) were used to characterize the supported species. Tetrabutylammonium acetate was found to remove the organometallic species from the surface, enabling solution-phase analytical techniques in conjunction with traditional surface methods. Cationic character was imparted to the iridium center by its grafting onto sulfated zirconia, imbuing high levels of activity in electrophilic C-H bond functionalization reactions such as the stoichiometric dehydrogenation of alkanes, with density functional theory (DFT) calculations showing a lower barrier for β-H elimination. Catalytic hydrogenation of olefins was also facilitated by the sulfated zirconia-supported (^dmPhebox)Ir(III) complex, while the homologous complex on silica was inactive under comparable conditions.

Full text of DOI:10.1021/jacs.9b00896

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Article
Electrophilic Organoiridium(III) Pincer Complexes on Sulfated
Zirconia for Hydrocarbon Activation and Functionalization
Zoha Syed, David Kaphan, Frédéric A. Perras, Marek Pruski, Magali S. Ferrandon, Evan C. Wegener,
Gokhan Celik, Jianguo Wen, Cong Liu, Fulya Dogan, Karen I. Goldberg, and Massimiliano Delferro
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00896 • Publication Date (Web): 22 Mar 2019
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Electrophilic Organoiridium(III) Pincer Complexes on Sulfated Zirconia
for Hydrocarbon Activation and Functionalization
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Zoha H. Syed,a,⊥ David M. Kaphan,a,* Frédéric A. Perras,b Marek Pruski,b,c Magali S. Ferrandon,a Evan
C. Wegener,a Gokhan Celik,a Jianguo Wen,d Cong Liu,a Fulya Dogan,a Karen I. Goldberg,e Massimili-
ano Delferroa,*
a Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, United States
b U.S. DOE Ames Laboratory, Ames, IA 50011, United States
c Department of Chemistry, Iowa State University, Ames, IA 50011, United States
d Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL 60439, United States
e Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, United States
KEYWORDS Surface Organometallic Chemistry, Hydrocarbon Functionalization, Sulfated Oxides, Electrophilic Activation, Pin-
cer
ABSTRACT: Single-site supported organometallic catalysts bring together the favorable aspects of homogeneous and heterogeneous
catalysis while offering opportunities to investigate the impact of metal-support interactions on reactivity. We report a
(dmPhebox)Ir(III) (dmPhebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complex chemisorbed on sulfated zirconia, the
molecular precursor for which was previously applied to hydrocarbon functionalization. Spectroscopic methods such as diffuse re-
flectance infrared Fourier transformation spectroscopy (DRIFTS), dynamic nuclear polarization (DNP)-enhanced solid-state nuclear
magnetic resonance (SSNMR) spectroscopy, and X-ray absorption spectroscopy (XAS) were used to characterize the supported spe-
cies. Tetrabutylammonium acetate was found to remove the organometallic species from the surface, enabling solution-phase analyt-
ical techniques in conjunction with traditional surface methods. Cationic character was imparted to the iridium center by its grafting
onto sulfated zirconia, imbuing high levels of activity in electrophilic C–H bond functionalization reactions such as the stoichiometric
dehydrogenation of alkanes, with density functional theory (DFT) calculations showing a lower barrier for β–H elimination. Catalytic
hydrogenation of olefins was also facilitated by the sulfated zirconia-supported (dmPhebox)Ir(III) complex, while the homologous
complex on silica was inactive under comparable conditions.
group catalysts, analogous to noncoordinating counterions in ho-
mogeneous catalysis, such as triflate or perfluorotetraphenyl-
borate.31 Sulfated zirconia has been proposed to contain Brønsted
acid sites stronger than 100% H2SO4,32 although the nature and
strength of the acid sites remain debated in the literature.32-35 This
solid acid has also previously been shown to exhibit interesting re-
dox activity that facilitates the isomerization of alkanes.36-40 The
application of this material as a support for organotransition metals
(such as Zr and Hf) was pioneered by Marks and coworkers, result-
ing in the generation of single-site supported catalysts active for
selective arene hydrogenation and olefin polymerization (Figure
1A).41-47 Conley and coworkers have also reported olefin polymer-
ization catalysts using late transition metal Ni(II) and Pd(II) α-
Introduction
The ability to stereoelectronically modulate the local environment
of a catalytic metal center is paramount to enhancing chemical re-
activity and selectivity for desired mechanistic pathways. Hetero-
geneous catalysts are employed in an estimated 90% of industrial
chemical processes,1 due to their ease of separation, recyclability,
and robustness. However, these catalysts generally lack the activity
and selectivity associated with homogeneous catalysts. Recently,
the use of single-site supported organometallics has emerged as a
promising strategy for combining the positive attributes of hetero-
geneous and homogeneous catalysts.1-5 Studies have focused on
transition metal complexes grafted on unmodified metal oxides,
such as silica and alumina, for hydrocarbon functionalizations in-
cluding alkane dehydrogenation,6-9 olefin hydrogenation,10-14 ole-
fin polymerization,15-24 and olefin metathesis.25-30 The relatively
high Lewis basicity of these surfaces after chemisorption of an or-
ganometallic complex generally leads to strong catalyst-surface in-
teractions, decreasing the propensity for electrophilic bond activa-
tion mechanisms. More recently, modified acidic metal oxide sup-
ports, in particular sulfated zirconia (SO4/ZrO2), have been used to
imbue electrophilic character to both transition metal and main
+
diimine species48,49 (Figure 1B) as well as SiR3 supported on
SO4/ZrO2 for C–F bond activation.50
Recently, our group reported the chemisorption of
Cp*(PMe3)IrMe2 on SO4/ZrO2 and SO4/Al2O3 to create a highly
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corresponding olefin.59 It was later shown that Lewis acid additives
A
Marks and coworkers
such as NaBArF4 could be used to enhance the rate of β–H elimina-
tion, aiding in the opening of a coordination site on the metal cen-
ter.62 The electrophilicity of the metal center could also be in-
creased through the addition of a Lewis acid. Computational inves-
tigation demonstrated that more electrophilic (dmPhebox)Ir metal
centers, generated via modulation of the pincer ligand backbone
could decrease the barrier for C–H activation by several kcal/mol,
affording more active alkane activation systems, though variation
of the X-type carboxylates did not result in this effect.63 When mod-
ulation of the carboxylates ligated to the metal center was studied,
the results were in agreement with previous experimental and com-
putational studies, in that variation of the carboxylates affected the
barrier for β–H elimination, similar to Lewis acids, whereas that of
C–H activation was unaffected.64
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Conley and coworkers
While Goldman, Celik, and coworkers have investigated the chem-
isorption of phosphine-ligated iridium systems onto silica and alu-
mina via a linkage on the pincer-ligand backbone and their subse-
quent proclivity for C–H activation via Ir(I),65,66 the catalytic activ-
ity observed was similar to that obtained with the molecular pre-
cursor. Mezzetti and coworkers were able to graft similar com-
plexes via the metal center onto SBA-15, resulting in supported cat-
alysts more active for olefin hydrogenation than the analogous ho-
mogeneous reaction.11-13 We envisioned that grafting 1 onto
SO4/ZrO2 by acetate protonolysis would offer an opportunity not
only to heterogenize this system but also to study the effect on ele-
mentary steps in alkane dehydrogenation, in particular C–H activa-
tion and β–H elimination. We hypothesized that an increase in rate
may occur through the increase in electrophilicity at the metal cen-
ter imparted by the weakly coordinating surface. The propensity for
stoichiometric alkane dehydrogenation with the supported
(dmPhebox)Ir complexes was investigated in addition to the cata-
lytic activity for olefin hydrogenation, the microscopic reverse of
alkane dehydrogenation.
Figure 1. (A) Examples of Zr and Hf complexes supported on sul-
fated zirconia studied by Marks and coworkers.41-47 (B) Ni(II) and
Pd(II) α-diimine complexes supported on sulfated zirconia studied
by Conley and coworkers.48,49
electrophilic Ir(III) metal center for catalytic H/D exchange of me-
thane under mild conditions.51 We envisioned expanding this meth-
odology to pincer-ligated iridium complexes, thereby generating
thermally-robust organometallic species that could facilitate hydro-
carbon functionalization via enhanced electrophilicity at the metal
center.52,53 Previously, organometallic iridium complexes such as
the (tBu4PCP)Ir(I) system (tBu4PCP = κ3-C6H3-2,6-(CH2PtBu2)2)52,53
have been studied, which can facilitate highly endothermic trans-
formations such as non-oxidative alkane dehydrogenation52-56 un-
der mild conditions with high levels of selectivity.52,53 For this set
of catalysts, C–H activation of the alkane occurs via oxidative ad-
dition to a reactive Ir(I) fragment, which is inhibited by N2, H2O,
and the alkene product.52,53 Consequently, significant effort has
also been devoted to the development of catalysts proceeding
through alternative alkane activation pathways such as the con-
certed-metalation-deprotonation (CMD) mechanism.57 In this sce-
nario, C–H activation at the Ir(III) center58 is facilitated by an in-
ternal base at the metal center, such as a carboxylate, through a six-
membered transition state, as shown in Scheme 1.57,59 CMD-type
Results and Discussion
Synthesis and Characterization of the Supported Ir(III)-Pincer
Complex on Sulfated Zirconia. The chemisorption of 1 onto sul-
fated zirconia (sieved to 125 µm, 120 mesh, with a surface area of
140 m2/g and sulfur loading of 1.58% w/w51) was initially investi-
gated in order to establish the viability of this material as a weakly-
coordinating support for the Ir center through protonolysis of an X-
type67 acetate ligand, forming (dmPhebox)Ir(OAc) on SO4/ZrO2 (3)
(Scheme 2). Following addition of sulfated zirconia to a solution of
1 in benzene-d6, resonances corresponding to 1 began to decrease
in intensity as referenced to an internal standard by 1H nuclear mag-
netic resonance (NMR) spectroscopy (Figure S1). 93% of the final
saturation loading of 1 was chemisorbed at approximately 1 h, with
the remaining 7% deposited over the next 15 h, after which no fur-
ther changes were observed and 3 could be isolated by filtration.
Evolution of acetic acid, typically associated with protonolysis of
an X-type acetate ligand by a Brønsted-acidic surface site was not
Scheme 1. CMD-type Activation of Alkanes with 1
1
detected over time by H NMR spectroscopy, likely due to phy-
sisorption of acetic acid on the surface. The iridium fragment could,
in principle, bind the surface through either an L-type bonding in-
teraction67 by displacement of water at the labile sixth coordination
site,63 or by an X-type interaction via protonolysis of an acetate
group (Scheme 2). To investigate the possibility of L-type binding,
3 was washed with tetrahydrofuran (THF), a donating solvent, to
displace any datively-bound complexes from the surface (Scheme
2). Inductively coupled plasma-atomic emission spectroscopy
(ICP-AES) of 3 prior to THF washing showed 2.26% Ir deposited
on the surface by weight. Analysis of the THF-washed
bond activation has been investigated extensively with
(dmPhebox)Ir systems (dmPhebox = 2,6-bis(4,4-dimethyloxazoli-
nyl)-3,5-dimethylphenyl) (Scheme 1).59-61 Previously, it was re-
ported that the activation of n-octane occurred in the presence of a
base at 160 °C using (dmPhebox)Ir(OAc)2(OH2) (1), generating
(dmPhebox)Ir(OAc)(n-octyl) in 78% yield after 70 h.59,60 Goldberg
and coworkers showed that β–H elimination occurs in the absence
of a base at 200 °C, generating (dmPhebox)Ir(OAc)(H) (2) and the
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Scheme 2. Grafting experiment of 1 onto SO4/ZrO2 resulting in a mixture of X-type and L-type bound species, the latter of which
can be washed away with THF to give 4
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(dmPhebox)Ir(OAc) on SO4/ZrO2 (4) by ICP-AES revealed a de-
crease in deposited surface Ir from 2.26% to 1.32%.68 Further
washing with THF did not remove any additional iridium, suggest-
ing that the material immediately after chemisorption and benzene
washes constituted a mixture of L- and X-type binding modes,
while that after washing with THF was constituted by only the X-
type species. Suspension of 3 in THF-d8 also led to desorption of
complex (dmPhebox)Ir(OAc)2(THF) (~79% of expected iridium
from ICP analysis), corroborating the X-type binding mode of the
species left on the surface post-washing (Figure S2, see the Sup-
porting Information for further discussion).
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) was employed to provide structural insight into the na-
ture of the surface-supported iridium fragment. C–H stretching vi-
brations around 3000 cm-1 were observed in 4, which were qualita-
tively similar to those found in the pincer ligand manifold for aro-
matic and aliphatic C–H stretches. Other C–H stretching vibrations
corresponding to the pincer ligand were also observed at lower fre-
quencies between 1000 cm-1 and 1700 cm-1, though these over-
lapped with stretching vibrations associated with sulfated zirconia
(see Figure S3 for full DRIFTS spectrum of 4).
Dynamic nuclear polarization (DNP)-enhanced cross-polarization
magic-angle spinning (CPMAS) NMR spectroscopy was next used
to obtain detailed structures of the supported iridium species.69-73
The DNP-enhanced 13C CPMAS NMR spectrum of 3 yielded res-
onances with chemical shifts in agreement with those of the molec-
ular precursor 1, suggesting that the iridium center had maintained
its pincer ligand during the chemisorption process (Figure 2A,B).
To further confirm this structure and gain additional details con-
cerning the spatial arrangement of the complex's ligands we addi-
tionally performed 2D 13C{1H} heteronuclear correlation
(HETCOR) spectroscopy. A spectrum acquired using a CP contact
time of 100 µs shows predominantly one-bond C-H correlations, as
expected, as well as two weaker two-bond correlations to quater-
nary carbon 'H' (Figure 2A). With a contact time lasting 1000 µs, a
number of new through-space correlations appeared (Figure 2B).
Notably, the sole correlation to the rather isolated carbon 'B' is to
the methyl protons of the acetate ligand ('J'). This confirms that
both ligands are simultaneously coordinated to Ir and further pro-
Figure 2. DNP-enhanced 13C CPMAS (top traces) and HETCOR
spectra of 3 acquired with CP contact times of 100 µs (A) and 1000
µs (B), and of 4 acquired with a contact time of 1000 µs (C). On
the right-hand side of (A) and (B), the observed through-space cor-
relations for 3 are indicated with dashed red lines. The peak labels
are assigned to the structure shown in (C). An asterisk denotes the
solvent resonance from TCE-d2. In (C) 'x' symbols are placed in the
spectrum to indicate new correlations to THF not seen in (B), while
some new intensity, which may originate from THF, is also circled.
vides conformational information about the complex, namely that
the acetate ligand is bent over the pincer ligand, possibly due to
steric interactions with the support surface. No correlation between
the acetate carboxyl moiety (‘A’) and an acidic hydrogen could be
detected, strongly suggesting that the acetate is non-protonated and
thus remains an X-type ligand post-grafting. Once the complex was
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This can be more clearly seen in Figure S8 in the Supporting Infor-
mation, which shows the enlarged views of (B) and (C) for 0 ≤
δ(13C) ≤ 100 ppm.
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washed with THF to form species 4 we observed a new 13C reso-
nance (‘L’) as well as an increase in intensity which we attributed
to carbon 'M' of THF. Again, 2D 13C{1H} HETCOR NMR spec-
troscopy was performed to decipher the location of the THF mole-
cule. Cross-peaks between the 'L' protons of the THF and the 'K'
and 'C' carbon sites of the pincer ligand were observed (Figure 2C)
suggesting that, like the acetate, the THF molecule is adjacent to
the pincer ligand in the iridium coordination sphere. The 2D NMR
spectroscopy thus provided compelling evidence that the pincer lig-
and adopts a coordination that is trans to that of the support. Alt-
hough this differs from what was observed in the homogeneous
complex, it is unsurprising from both a steric and electronic point
of view, given the repulsion between the bulky pincer ligand and
the surface, as well as the favorability of positioning the weakly
donating surface coordination trans to the strongly donating pincer-
carbon. The steric demand of the surface similarly plays a role in
enforcing the close proximity of the pincer ligand and the THF and
acetate ligands on either side, as evidenced by the observed
through-space cross-peaks. This proximity between the acetate and
the pincer ligands was not observed in the molecular complex
where such sterics are absent.61 We note that such precise confor-
mational details on catalytic surfaces are difficult to obtain by other
surface characterization techniques. Similar studies utilizing DNP
have also been able to characterize the three dimensional spatial
orientation of ligands around a metal.72-73
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X-ray absorption spectroscopy (XAS) was also used to further in-
terrogate the structural integrity of the iridium-pincer after chemi-
sorption. Complete fitting of the X-ray absorption spectra was not
possible due to the difficulty in deconvoluting the scattering pat-
terns obtained for the supported Ir samples. Nonetheless, similar
qualitative features were observed in the extended X-ray absorption
fine structure (EXAFS) for 1 and 4, supporting the presence of the
intact iridium-pincer on the surface after chemisorption. Addition-
ally, X-ray absorption near-edge structure (XANES) analysis sug-
gested that the Ir(III) was the predominant oxidation state (Figure
S9). Moreover, no evidence of iridium nanoparticle formation was
observed. To further characterize the bonding between the iridium
center and the sulfated zirconia surface as a polarized X-type ligand
interaction, 4 was suspended in a solution of tetrabutylammonium
acetate (NBu4OAc) in methylene chloride-d2 (see the Supporting
Information for stripping procedure) in an attempt to displace the
iridium complex from the surface (Figure 3). Monitoring the reac-
tion by 1H NMR spectroscopy revealed that after 10 min, 1 is ob-
served in solution. Minor asymmetric species were also detected by
1H NMR as doublet of doublets around δ 4.65 ppm. One of these
may be consistent with the κ1/κ2 asymmetric analog of 1 being pre-
sent in solution, in which one acetate can bind in κ2 fashion, dis-
placing the L-type ligand (Figure 3B). When the stripping experi-
ment was performed in the presence of residual THF, only the sym-
metric κ1/κ1 analog was observed. The other minor species was
assigned as (dmPhebox)Ir(Ph) on SO4/ZrO2 (5), generated after sur-
face activation of benzene over the course of the grafting reaction,
vide infra. The appearance of 1 following treatment with NBu4OAc
was significant as it demonstrated the possibility of removing the
organometallic species from the surface for characterization via so-
lution-phase techniques such as 1H NMR, the employment of
which is complementary to surface analytical techniques.74,75 The
resultant solid material was analyzed by DRIFTS following isola-
tion by filtration (Figure 3A). C–H stretching vibrations around
3000 cm-1 as well as the C=O and C=N
Figure 3. Tetrabutylammonium acetate can be used to strip 1 from
the surface, leaving behind tetrabutylammonium bound to the sur-
face acidic sites. (A) This was confirmed by DRIFTS. Bottom spec-
trum shows 4 as prepared with associated IR stretches of 4 indi-
cated (*). Top spectrum shows the material post-stripping with as-
1
sociated IR stretches of tetrabutylammonium indicated (ǂ). (B) H
NMR spectrum of stripping experiment with 4 over time in meth-
ylene chloride-d2 with characteristic peaks highlighted. A mixture
of asymmetric and symmetric 1 and (dmPhebox)Ir(OAc)(Ph) (6)
come off the surface, the latter from room temperature activation
of benzene solvent during loading. “L” may represent a labile site
with THF or aquo bound to the metal center.
stretches from 1200 to 1700 cm-1 associated with the pincer ligand
t
backbone became attenuated, replaced with NBu4+ C–H stretches
(Figure 3A). This was further confirmed by comparison to a
DRIFTS spectra of 1 diluted with KBr, NBu4OAc diluted with
KBr, and NBu4OAc physisorbed onto SO4/ZrO2 (Figure S20). ICP-
AES analysis of the support material after stripping with NBu4OAc
also provided evidence of removal of the Ir from the surface as only
0.156% iridium by weight was remained after acetate stripping,
representing a minority of strongly-bound surface sites where ace-
tate could not facilitate removal.
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Figure 4. (A) Reaction of 4 with neat benzene at 80 °C for 24 h
generates Ir-Ph on the surface. (B) Reaction of 4 with neat nonane
at 120 °C for 24 h generates Ir-H on the surface.
Stoichiometric Reactivity of Supported Ir(III)-Pincer Complex
on Sulfated Zirconia Towards Aryl and Alkyl. By recapitulating
the stoichiometric reactivity first described by Nishiyama and
coworkers61 and Goldberg and coworkers,59 further insight could
be gained regarding the impact of the support on reactivity of the
metal center. First, 4 was suspended in benzene at 80 °C for 24 h
in order to assess whether the supported material could effect CMD
C–H activation of the solvent to form 5 by analogy to the homoge-
neous precedent (dmPhebox)Ir(OAc)(Ph) (6); see Figure 4A.59,61
DRIFTS of the isolated material post-benzene-treatment revealed
new aromatic C–H stretching vibrations between 3000 cm-1 and
3100 cm-1 consistent with new Csp –H bonds expected upon gener-
2
ation of 5 (Figure 5A). Furthermore, stripping of the organometal-
lic species from the surface with NBu4OAc in methylene chloride-
d2 resulted in the appearance of resonances associated with 6 by 1H
NMR spectroscopy,59,61 along with 1 (Figure 5B). Analysis of 5 by
DNP-enhanced 13C CPMAS NMR spectroscopy76 revealed the ap-
pearance of two new aromatic resonances from Carom-H (“P”) and
Carom-Ir (“N”) sites from a phenyl ligand. This assignment was then
confirmed by the appearance of a new correlation between carbon
Figure 5. (A) Infrared absorbance spectra of materials following
reaction of 4 (blue, as prepared), with benzene for 24 h (black, IR
stretches associated with aromatic C–H stretches indicated with *)
and with nonane for 72 h (red, IR stretch associated with Ir–H in-
dicated with ǂ) (B) 1H NMR spectra enlarged to show the aryl and
hydride regions of post C–H activation materials treated with tet-
rabutylammonium acetate after 20-24 h in CD2Cl2. The aromatic
proton resonances for Ir-Ph are indicated with * and the resonance
associated with Ir–H is indicated with ǂ.
1
“B” and the H aromatic region in the corresponding HETCOR
spectrum (Figure 6). An attenuated acetate signal in the solid state
13C NMR spectrum further substantiated the loss of acetate associ-
ated with CMD-type C–H activation.
When 4 was suspended in neat n-nonane for 72 h at 120 °C (Figure
4B), analysis of the recovered supported material by DRIFTS re-
vealed a new IR stretching vibration at approximately 2220 cm-1
(Figure 5A), similar in wavenumber to the stretching frequency ob-
served for authentically prepared hydride 2 (2194 cm-1), suggesting
formation of iridium-hydride in the supported complex. In contrast,
the formation of 2 in 96% yield was observed upon heating 1 in a
neat solution of n-octane at 200 °C for 72 h,59 but when the temper-
ature was lowered to 180 °C, only a small amount of iridium-hy-
dride was observed after five days and at 150 °C no reaction was
observed even after heating for six days.59 To confirm the charac-
terization of the IR stretch at 2220 cm-1 as an iridium-hydride, an
authentic sample of (dmPhebox)Ir(H) on SO4/ZrO2 (7) was prepared
by grafting 2 onto sulfated zirconia (see the Supporting Infor-
mation). A similar IR signal at 2220 cm-1 was observed upon direct
chemisorption of 2, supporting the formation of an iridium-hydride
after treatment with n-nonane under relatively mild conditions.
Solid-state 1H NMR spectroscopy also revealed a resonance around
-35 ppm consistent with an iridium hydride signal,59 with a
similar chemical shift to that observed in the 1H NMR spectrum of
the iridium-hydride complex 2 (Figure S24, S26). The DNP-
enhanced 13C CPMAS NMR spectrum also featured a weakened
iridium acetate signal consistent with the dissociation of acetic acid
following C–H activation. To further determine organometallic
speciation and quantify the amount of iridium-hydride formed, the
iridium species present after treatment with n-nonane were stripped
from the surface with NBu4OAc in methylene chloride-d2 (Figure
5B). After 10 min, resonances associated with 2 were observed by
1H NMR spectroscopy, which continued to increase in intensity un-
til approximately 40 min.77 Of the product mixture subsequently
obtained, 70% corresponded to 2 by 1H NMR spectroscopy based
on the CAr–H signal integration.78 Formation of the iridium-hydride
by the putative C–H activation, β–H elimination mechanism was
also observed at shorter reaction times, representing 58% of the
product mixture after 24 h at 120 °C,79 and 34% of the product mix-
ture after2 h at 120 °C.80 The diacetate starting material (1) was
also present in the product mixtures.77 The formation of iridium-
hydride in appreciable yield under these conditions is notable in
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ethane dehydrogenation were calculated for three systems:
(dmPhebox)Ir(OAc)2 1, the supported complex 4 on a cluster model
of SO4/ZrO2, and (dmPhebox)Ir(OAc)(OTf), which was used as a
homogeneous analog for the weakly-donating nature of the sulfated
zirconia X-type coordination mode (Figure 7). A SZr cluster model
was used for the SZr supported Ir complex. The SZr cluster (See
Figure S27 in SI), was cleaved from a periodic model that was de-
veloped in a previous study. The cluster consists of 3 Zr and 10 O
atoms for the ZrO2 support and a tripodal SO42-. The O atoms in the
ZrO2 cluster were terminated by protons and one extra proton was
added to charge balance the negative charge of SO42-(the other pos-
itive charge comes from the Ir center). During geometry optimiza-
tion, the three Zr atoms were frozen while the other atoms in the
cluster were allowed to relax.46 The calculated reaction pathway of
ethane dehydrogenation undergoes a C–H activation, followed by
a β-H elimination. β-H elimination was found to be the rate-limit-
ing step of the reaction for all three iridium catalysts. Both the
cluster model of 4 and the triflate complex had a lower apparent
barrier than the bis-acetate molecular analog 1 (∆∆G‡ = 5.1 and 3.7
kcal/mol respectively, Figure 7). The relative acceleration of the
isolated β-H elimination step was even more extreme, with 4 and
the triflate complex undergoing β-H elimination with an intrinsic
barrier of 18.9 and 16.9 kcal/mol as compared to 31.0 kcal/mol for
1, however this effect is partially obscured by the increased relative
energy of the iridium-alkyl intermediate for 4 and the triflate com-
plex.
1
2
3
4
5
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7
8
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55
56
57
58
59
60
Figure 6. DNP-enhanced 13C CPMAS (top trace) and HETCOR
NMR spectrum of 5. An asterisk denotes the solvent resonance
from TCE-d2.
comparison to the activity observed with molecular precursor 1 for
stoichiometric n-octane dehydrogenation. For 1, C–H activation
proceeded at 160 °C and higher temperatures (200 °C) were re-
quired in order to observe appreciable amounts of β–H elimination
to form the iridium-hydride.59 The effect of the surface on the metal
center, in particular its tuning of electrophilicity, is similar in nature
to what may occur when a Lewis acid interacts with a chelated ac-
etate or if the carboxylate contains electronegative moieties as in
the case of the homogeneous reactions,62,64 which may offer sup-
port for the observed rate enhancement in β–H elimination.
The calculated barriers for the C−H activation step were also found
to be decreased in the presence of the more weakly-donating spec-
tator ligands, albeit to a lesser extent than the β-H elimination, how-
ever this energetic barrier is kinetically irrelevant to the overall sys-
tem due the endergonic and reversible nature of this step prior to
the rate controlling transition state of β-H elimination.
In order to gain further insight into the experimentally observed
rate enhancement in the activation and functionalization of nonane
with 4, a computational investigation was undertaken (see the Sup-
porting Information regarding methods). The catalytic pathways of
L
O
N
L
O
Ir
H
N
O
L
N
O
O
Ir
Ir
N
N
O
O
O
H
N
O
HO
35.5
34.6
27.4
52.2
48.5
47.1
31.7
28.2
21.3
40.3
38.9
37.8
C2H4
L
L
39.7
32.3
26.9
O
Ir
O
N
N
L
AcOH
O
Ir
N
O
N
N
O
O
O
Ir
N
H
C2H6
O
0
L = AcO, TfO and SZr
Figure 7. Calculated reaction pathways of C-H activation and β-H elimination using an ethylene model. The values in the figure are relative
free energies in kcal/mol at 120 °C using the PBE0 functional with CEP-121G basis set. Computations where carried out using the Gauss-
ian09 software.
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plug flow reactor with silica as a diluent (propylene/H2 = 1/4.5, see
Catalytic Hydrogenation of Propylene and 1,3-Butadiene with
Supported Ir(III)-Pincer Complexes on Sulfated Zirconia. Hav-
ing investigated the stoichiometric reactivity of 4, studies were un-
dertaken to determine its performance under catalytic conditions.
At temperatures required to thermodynamically-enable observable
dehydrogenation (>300 °C), extrusion of SO2 and SO3 from sul-
fated zirconia can occur,81 thus low temperature acceptorless dehy-
drogenation of nonane under vigorous reflux (T = 151 °C) and gas-
phase transfer dehydrogenation of propane with ethylene as a sac-
rificial olefin (T = 50 °C to 300 °C) were undertaken. In the case of
acceptorless dehydrogenation, no olefins were observed after heat-
ing nonane with 4, possibly due to deleterious reactions with resid-
ual Brønsted acid sites on sulfated zirconia.75 Similarly, upon at-
tempting gas-phase transfer dehydrogenation of propane in a plug
flow reactor, turnover was too slow to achieve appreciable conver-
sion under conditions compatible with the catalyst. Thus, in lieu of
dehydrogenation, hydrogenation of propylene was used as the
model transformation, representing the microscopic reverse of the
dehydrogenation of propane. In addition to 4, 1 was also grafted
onto silica dehydroxylated at 700 °C (SiO2-700) and then washed
with THF (see the Supporting Information for details) to form
(dmPhebox)Ir(OAc) on SiO2-700 (8; %Ir w/w by ICP-AES = 0.48%).
In contrast to 4, this silica-supported species is stereoelectronically
more similar to 1 due to the lack of significantly enhanced electro-
philicity created at the metal center by sulfated zirconia. Spectro-
scopic characterization of 8 (see Supporting Information) was also
consistent with deposition of the intact iridium-pincer fragment alt-
hough with decreased iridium surface density after THF washing,
presumably due to a minority of the silanol sites having sufficient
reactivity to displace an acetate ligand (see the Supporting Infor-
mation for details).
the Supporting Information for further details). Sulfated zirconia
supported catalyst 4 was found to display significantly higher hy-
drogenation activity than the silica-supported analog (8). At 100
°C and 120 °C, an induction period could be observed for 4 in gen-
eration of the active catalyst species, with complete conversion ob-
served for higher catalyst loadings (Figure 8). No deactivation of 4
was observed from 100 °C to 120 °C after approximately 52 hours
on stream, while slight deactivation was observed at 150 °C (Figure
8). With 2.5 mg (0.00017 mmol) of 4, over 20,000 turnovers were
observed after 52 h at 120 °C. Negligible C-C bond cleavage and/or
oligomerization were observed during catalysis. In addition, under
these conditions, the sulfated zirconia support is inactive towards
the hydrogenation of olefin.
1
2
3
4
5
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8
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Conversely, for 8, little to no activity was observed at 100 °C and
only marginal activation at 120 °C (Figure 8). At 150 °C, the cata-
lyst began to show more significant activity, and propylene conver-
sion was observed (Figure 8). As nanoparticle formation from or-
ganometallic iridium precursors is well-explored in the literature,82-
84 the possibility of in-situ nanoparticle formation was then evalu-
ated by analysis of the post-catalysis materials. 4 and 8 were sub-
jected to catalytic conditions and then isolated for characterization
by high-angle annular dark-field (HAADF) scanning transmission
electron microscopy (STEM) to determine whether iridium nano-
particle formation had occurred. For catalyst 4, no iridium nano-
particles were observed by STEM in post-catalysis samples at 120
°C (Figure 9A) and single ions could be observed (Figure 9A, cir-
cled in red), offering further support for the presence of single-site
organoiridium species. In contrast, for catalyst 8 exposed to reac-
tion conditions at 120 °C, at which it is minimally active, no iridium
nanoparticles were observed by STEM (Figure S44). However, for
the higher temperature post-catalysis sample of 8, for which the
temperature was raised to 200 °C, STEM revealed formation of
iridium nanoparticles on the surface (Figure 9B).
In an initial series of experiments, 4 (2.5 mg, 0.00017 mmol Ir, 5
mg, 0.00034 mmol Ir, and 10 mg, 0.00069 mmol Ir) and 8 (30 mg,
0.00075 mmol Ir) were evaluated for propylene hydrogenation in a
150 °C
100 °C
120 °C
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
100
90
80
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10
0
0
20
40
60
0
20
40
60
0
20
40
60
Time on stream (h)
Time on stream (h)
Time on stream (h)
Figure 8. Hydrogenation of propylene with 4 (2.5 mg = green, 5 mg = blue, 10 mg = red) and 8 (30 mg = black). Iridium catalyst 4 is active
from 100 °C to 150 °C whereas iridium catalyst 7 is not active until at least 150 °C. Conditions: 100, 120, and 150 °C, propylene/H2 ratio =
1/4.5, 5% propylene/Ar = 1 mL/min, 5.188% H2/Ar = 4.34 mL/min per flow reactor (see the Supporting Information for more details on
conditions and experimental setup).
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Table 1. Hydrogenation of 1,3-Butadiene at 80 °C
Page 8 of 14
A
1
2
3
4
Sel. for
Butenes
(%)
Ir loading
(%w/w)a
Initial conv.
(%)
Ir Cat.
5
6
7
8
1.32
55
94
29
4
8
0.481
3.8
9
Conditions: 5 mg of each catalyst, 1.01% 1,3-butadiene/Ar = 5.5
mL/min, 4.008% H2/Ar = 6.3 mL/min, activation of 4 at 120 °C for
6 h and activation of 9 at 200 °C for 6 h then temperature is dropped
to 80 °C. Deactivation of the catalyst is observed over time thus
initial conversions are presented. aBy ICP-AES. Selectivity = (con-
centration of butenes / sum of product concentrations) * 100.
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B
Mechanistic Studies into Propylene Hydrogenation Using Sup-
ported Ir(III)-Pincer Complexes. Having established the bench-
mark for hydrogenation reactivity of 4, our subsequent efforts fo-
cused on gaining mechanistic insight into this catalytic process.
First, the role of the ancillary ligands on the Ir center in 4 was as-
sessed. To determine the necessity of the acetate moiety on the
metal center, (dmPhebox)Ir(OAc)(Cl) was grafted onto SO4/ZrO2
and washed with THF (9), leading to an iridium-chloride species
on the sulfated surface, which would be incapable of CMD-type
activation. Notably, the activity of 9 (TOF = 62 2 h-1) for propyl-
ene hydrogenation was similar to that of 4 (TOF = 75 2 h-1) (Table
2), indicating that the acetate is not necessary for hydrogenation
and that both may be converging to the same active intermediate
over the course of the reaction.85
Figure 9. STEM images of (A) post-catalysis sample of 4 at 120
°C and (B) post-catalysis sample of 8 with temperature ramped
from 150 °C (held for 7 h) to 200 °C (held for 7 h) then back down
to 100 °C (held for 10 h).
Table 2. Effect of Ancillary Ligand Modification on Turno-
ver Frequency (TOF) for Hydrogenation of Propylene at 50
°C.
The presence of nanoparticles may explain the onset of activity of
the silica-supported samples at higher temperatures, as iridium na-
noparticles are known to be very efficient hydrogenation cata-
lysts.82-84 This provides some evidence that a supported organome-
tallic species was catalytically active at those temperatures on sul-
fated zirconia, whereas the onset of catalytic activity of the silica
supported material correlated with the appearance of nanoparticles,
suggesting that the molecular species on the electronically-neutral
support is relatively inactive. As achieving product selectivity in
complex or multi-step processes is a benefit of supported organo-
metallic species relative to traditional heterogeneous catalysts, the
hydrogenation of 1,3-butadiene was then investigated (Table 1).
When 4 was treated with 1,3-butadiene and H2 in flow (see the Sup-
porting Information for discussion of experimental setup and cata-
lytic data), conversion of 1,3-butadiene to butenes and n-butane oc-
curred following activation at 120 °C. Approximately 94% selec-
tivity for butenes was observed at 80 °C (initial conversion = 55%,
Table 1) with catalyst 4. In contrast, with catalyst 8, 29% selectivity
for butenes was observed at 80 °C, following activation at 200 °C
(initial conversion = 3.8%, Table 1). In both cases, catalyst deacti-
vation was observed over time possibly due to coke and/or polymer
formation. The higher selectivity with 4, were indicative of a sup-
ported organometallic complex as the active species, as lower prod-
uct selectivity values are expected with Ir nanoparticles generated
with 8 under reducing conditions.
Ir loading
(%w/w)
Ir/ZrS Catalyst
TOF (hr-1)
1.32
75
62
2
2
4
9
1.82
Conditions: 5 mg of each catalyst, 5% propylene/Ar = 2 mL/min,
3.573% H2/Ar = 12.6 mL/min, activation at 120 °C for 6 h then
temperature is dropped to 50 °C. TOFs are an average of three in-
dependent runs with standard deviations listed.
In order to obtain further information regarding the active iridium
species, more detailed kinetic analyses were performed. Following
activation of 4 at 120 °C for 24 hours, catalytic hydrogenation was
studied over a range of temperatures (50 °C, 60 °C, 70 °C, and 80
°C) to gain insight into the activation energy (see the Supporting
Information for catalytic data). From the resulting Arrhenius plot,
an activation energy of 29.8 kcal/mol was found for the reaction
(Figure 10). When varying the H2 concentration at 50 °C following
activation at 120 °C, a first-order rate dependence on H2 was ob-
served (Figure 11), confirmed by a double log plot with a slope of
1.02 (Figure S31, S32).86 Conversely, when varying the propylene
concentration at 50 °C, a fractional kinetic dependence between
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H
2 Dependence
Propylene Dependence
zero and first order was observed (Figure 11, S34, S35). This ki-
netic dependence was consistent with a catalytic resting state where
the iridium sites are partially saturated with olefin as an iridium-
1
2
3
4
5
6
7
8
100
80
60
40
20
0
100
80
60
40
20
0
alkyl or an iridium olefin π-complex. Alternatively, the olefin could
be incorporated after the rate-determining step of the reaction.
Arrhenius Plot
-2
-3
y = 60.17x + 30.42
R² = 0.97
y = 34.11x - 6.87
R² = 0.99
9
y = -15x+ 38.84
R² = 0.98
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-4
0
1
2
3
4
0.0 0.1 0.2 0.3 0.4 0.5
[propylene] (% volume)
-5
-6
-7
-8
-9
[H2] (% volume)
Figure 11. Dependence of H2 and olefin in the hydrogenation of
propylene with 4 (5 mg). Activation conditions: 120 °C for 6 hours,
propylene/H2 ratio = 1/4.5, 5% propylene/Ar = 1 mL/min, 5.188%
H2/Ar = 4.34 mL/min per flow reactor. H2 dependence ratios
screened propylene/H2 = 1/6 (2/11.58 mL/min), 1/5 (2/9.64
mL/min), 1/4 (2/7.72 mL/min), 1/2 (2/3.68 mL/min). Propylene de-
pendence ratios screened propylene/H2 = 0.5/4 (1/7.72 mL/min),
1/4 (2/7.72 mL/min), 1.5/4 (3/7.72 mL/min), 2/4 (4/7.72 mL/min).
See the Supporting Information for individual data points and
standard deviations and discussion of error.
2.8
2.85
2.9
2.95
3
3.05
3.1
3.15
1/T * 10-3 (K-1)
Figure 10. Elucidation of activation energy in propylene hydro-
genation with 4 (1.5 mg). Activation conditions: 120 °C for 24
hours, propylene/H2 ratio = 1/4.5, 5.001% propylene/Ar = 1
mL/min, 5.188% H2/Ar = 4.34 mL/min per flow reactor. Tempera-
tures screened were 50 °C, 60 °C, 70 °C, and 80 °C at 2 mL/min
5.001% propylene/Ar and 8.7 mL/min 5.188% H2/Ar. See the Sup-
porting Information for individual data points and standard devia-
tions and discussion of error.
To interrogate these possibilities, the reaction of supported iridium-
hydride 7 with flowing propylene was monitored by DRIFTS (Fig-
ure 12, see the Supporting Information for experimental setup).
Upon exposure of 7 to propylene at room temperature, a weakening
of the characteristic iridium-hydride IR stretch for 7 was observed
concomitant with the appearance of a new signal at 2120 cm-1 (Fig-
ure 12A). This new species persists at approximately the same ratio
under flowing propylene. The in situ DRIFTS cell was then purged
with N2, which resulted in the gradual disappearance of the new
peak at 2120 cm-1 and the restoration of the original absorption in-
tensity of the hydride stretch associated with 7 (Figure 12B). When
the gas flow was cycled back to propylene, the iridium-olefin spe-
cies once again increased in intensity while that of iridium-hydride
A
Propylene 1st cycle
B
Nitrogen Flush
C
Propylene 2nd cycle
Figure 12. Infrared absorbance spectra of iridium-hydride 7 reversibly reacting with propylene to form an iridium-olefin species. (A) pro-
pylene flow cycle 1, 3% propylene/Ar = 51.8 mL/min, black = initial no flow, red = 2 min under propylene flow, blue = 5 min under
propylene flow, green = 10 min under propylene flow. (B) N2 flush, N2 = 51.5 mL/min, black = initial 10 min propylene flow, red = 2 min
N2 flush, blue = 5 min N2 flush, green = 10 min N2 flush, orange = 83 min N2 flush. (C) propylene flow cycle 2, 3% propylene/Ar = 51.8
mL/min, black = initial 83 min N2 flush, red = 2 min under propylene flow, blue = 5 min under propylene flow, green = 10 min under
propylene flow, orange = 30 min under propylene flow.
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Page 10 of 14
7 decreased (Figure 12C). The species formed in the presence of
calculations also confirming the lower barrier. Organometallic spe-
gaseous propylene may correspond to the olefin binding at the la-
bile sixth coordination site on the metal center, forming an Ir-olefin
π complex on the surface, potentially explaining the partial kinetic
order in propylene. The binding of the olefin is in dynamic equilib-
rium under these conditions and can reversibly occur as evidenced
by peak intensity changes when alternating between flowing pro-
pylene and N2 atmosphere.
ciation can also be assessed and quantified using solution-phase
NMR techniques by removal of generated species from the surface,
providing a powerful tool for the characterization of surface organ-
ometallic complexes in this system. Moreover, these materials were
shown to effect olefin hydrogenation, the microscopic reverse of
alkane dehydrogenation, with an organometallic species as the ac-
tive catalyst on sulfated zirconia whereas iridium nanoparticles
were likely the active species in silica-supported samples. Tuning
the electrophilicity of metal centers to bolster the activation of al-
kanes is a concept widely applied in homogeneous catalysis. The
application of this concept by grafting onto a highly acidic oxide
surface offers advantages through the coalescence of properties in-
herent to homogeneous and heterogeneous catalysis.
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ASSOCIATED CONTENT
Supporting Information. Supplemental data includes general dis-
cussion of synthetic and experimental procedures, instrumentation,
and characterization of materials. The supporting information is
available free of charge at the ACS publications website at DOI:
AUTHOR INFORMATION
Corresponding Author
*delferro@anl.gov
*kaphand@anl.gov
Present Addresses
Figure 13. DNP-enhanced 13C CPMAS NMR spectrum of post-
hydrogenation Ir on sulfated zirconia at 200 °C and 120 °C. An
asterisk denotes the solvent resonance from TCE-d2. § indicates lo-
cation of the attenuated signal for an acetate resonance.
⊥ Department of Chemistry, Northwestern University, 2145 Sher-
idan Road, Evanston, IL 60208.
Author Contributions
Further spectroscopic characterization of the post-catalysis samples
of 4 was undertaken using DNP-enhanced 13C CPMAS NMR to
gain structural insight. The resonance corresponding to the acetate
carboxyl is absent in the CPMAS spectrum (Figure 13) thus indi-
cating its removal during the catalysis. No resolvable alkyl reso-
nances were observed by 13C solid-state NMR spectroscopy. The
organometallic species were then removed from the surface of 4
post-catalysis by stripping with NBu4OAc (vide supra). Treatment
of the post-catalysis material with acetate resulted in the desorption
of iridium hydride 2, indicating that it is likely present in the resting
state of the catalyst (Figure S46)87 The post-catalysis samples were
also examined by XAS, revealing a qualitatively similar EXAFS
region and XANES when compared to catalyst 4 as prepared (Fig-
ure S47).
The manuscript was written through contributions of all authors. /
All authors have given approval to the final version of the manu-
script.
Funding Sources
This work was supported by the U.S. Department of Energy (DOE),
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences, under Contract DE-AC-02-
06CH11357 (Argonne National Laboratory) and DE-AC-02-
07CH11358 (Ames Laboratory). Use of the Advanced Photon
Source is supported by the U.S. Department of Energy, Office of
Science, and Office of the Basic Energy Sciences, under Contract
DE-AC-02-06CH11357. MRCAT operations are supported by the
Department of Energy and the MRCAT member institutions. Use
of the STEM at the Center for Nanoscale Materials at Argonne Na-
tional Laboratory is supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Contract
No. DE-AC02-06CH11357. The calculations were performed us-
ing the computational resources provided by the Laboratory Com-
puting Resource Center (LCRC) at Argonne and National Energy
Research Scientific Computing Center (NERSC). Z.H.S. thanks the
U.S. Department of Energy (DOE), Office of Science and Office of
Workforce Development for Teachers and Scientists (WDTS) un-
der the Science Undergraduate Laboratory Internships (SULI) Pro-
gram for financial support.
Conclusions
The ability to modulate the reactivity of a metal center through tai-
lored metal-support interactions aids in the design of supported or-
ganometallic complexes with catalytic activity for industrially-rel-
evant processes. Dehydrogenation of alkanes is one such challeng-
ing transformation, hence the development of strategies for alkane
conversion to alternative chemical building blocks such as olefins
remains ongoing. In this study, (dmPhebox)Ir organometallic com-
plexes grafted onto sulfated zirconia proved competent for hydro-
carbon functionalization in stoichiometric and catalytic processes,
showing a heightened propensity toward bond activation and elim-
ination reactions in comparison to their molecular precursors. Sul-
fated zirconia-supported samples showed increased reactivity in the
stoichiometric dehydrogenation of n-nonane, demonstrating the
rate-enhancing impact of the surface for β-H elimination, with DFT
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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Delferro, M. Nuclearity effects in supported, single-site Fe(II) hydro-
The authors thank Dr. Ce Yang for collecting TEM images of sup-
genation pre-catalysts. Dalton Trans. 2018, 47, 10842-10846.
(15) Chen, E.Y.; Marks, T.J. Cocatalysts for Metal-Catalyzed Olefin
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