Oxo-rhenium-catalyzed deoxydehydration of polyols with hydroaromatic reductants

Article abstract of DOI:10.1021/acs.organomet.5b00226

Several dihydroaromatic compounds are shown to be effective reducing agents in the oxo-metal-catalyzed deoxydehydration of diols and polyols to produce olefins and the corresponding arenes. NH₄ReO₄ and MeReO₃ are active catalysts for the reactions. The most effective of the hydroaromatic reductants is indoline, which is oxidized to indole. Yields for a variety of diols and polyols range from 35% to 99%. Two hydrogen donors, 1,3-cyclohexadiene and dihydroanthracene, engage in tandem DODH/cycloaddition reactions. Competition experiments show that indoline is more reactive than representative alcohols in H-transfer. Indoline is shown to reduce MeReO₃ to MeReO₂ via an isolable adduct, MeReO₃(indoline) (4), which has been structurally characterized and is suggested to be an intermediate in the catalytic DODH process.

Full text of DOI:10.1021/acs.organomet.5b00226

Article
pubs.acs.org/Organometallics
Oxo-Rhenium-Catalyzed Deoxydehydration of Polyols with
Hydroaromatic Reductants
Camille Boucher-Jacobs and Kenneth M. Nicholas*
Department of Chemistry and Biochemistry, Stephenson Life Sciences Research Center, University of Oklahoma, 101 Stephenson
Parkway, Norman, Oklahoma 73019, United States
S
* Supporting Information
ABSTRACT: Several dihydroaromatic compounds are shown
to be effective reducing agents in the oxo-metal-catalyzed
deoxydehydration of diols and polyols to produce olefins and
the corresponding arenes. NH4ReO4 and MeReO3 are active
catalysts for the reactions. The most effective of the
hydroaromatic reductants is indoline, which is oxidized to indole. Yields for a variety of diols and polyols range from 35% to
99%. Two hydrogen donors, 1,3-cyclohexadiene and dihydroanthracene, engage in tandem DODH/cycloaddition reactions.
Competition experiments show that indoline is more reactive than representative alcohols in H-transfer. Indoline is shown to
reduce MeReO3 to MeReO2 via an isolable adduct, MeReO3(indoline) (4), which has been structurally characterized and is
suggested to be an intermediate in the catalytic DODH process.
has been employed stoichiometrically together with MeReO3 or
NH4ReO4 for the DODH of organic-soluble glycols.13
Shiramizu and Toste found 1-butanol to be effective as a
reducing agent and solvent in combination with perrhenic acid
as catalyst.14 The mechanistic details of the alcohol-driven
DODH have been the subject of some debate regarding which
Re-intermediates react with the reductant and how the
hydrogens are transferred.15
Hydroaromatics, several of which are abundant in fossil
resources, have been employed as liquid organic hydrogen
carriers for hydrogenation reactions and for hydrogen storage.16
The catalytic activity of redox-active solid metal oxides, e.g.,
V2O5 and FexOy, for the dehydrogenation of alkyl aromatics,
used for the production of styrene from ethylbenzene,17
suggested to us the potential use of these reductants in
homogeneous oxo-metal-catalyzed reactions. However, rather
little is known about their reactivity with soluble oxo-metal
species.18 In the first report of ostensibly heterogeneous
catalytic DODH reactions using perrhenate supported on
carbon,19 the primary reductant employed was hydrogen, but a
few examples of H-transfer reductants, e.g., benzyl alcohol and
tetrahydronaphthalene, were also noted. Here we report the
development of efficient homogeneous catalytic DODH
reactions with hydroaromatic hydrogen donors and provide
insights into the nature of the reactive intermediates involved.
INTRODUCTION
■
The finite supply, fluctuating prices, and uneven worldwide
distribution of fossil-derived resources have stimulated interest
in new processes for the chemical conversion of renewable
feedstocks. Renewable plant-derived cellulose and triglycerides
can be transformed into carbohydrates and polyols by
hydrolysis and hydrogenation processes.1 Three oxygen-altering
processes have been considered to increase the value and
diversity of these biorefinery inputs: dehydration,2 hydro-
deoxygenation,3 and, most recently, deoxydehydration
(DODH). In the DODH reaction vicinal diols (glycols) are
converted into alkenes by the action of a reducing agent and an
oxo-metal catalyst (Scheme 1).4
Scheme 1. DODH Catalyst/Reductant Systems
Several studies have focused on the development of catalysts
and reductants for the DODH reaction. Oxo-rhenium
compounds have largely been employed as catalysts, e.g.,
Cp*ReO3, TPB*ReO3, MeReO3, and Z+ReO4 , in combination
−
with triphenylphosphine,5 hydrogen,6 sulfite,7 and elements,
e.g., zinc, iron, manganese, and carbon,8 as reductants. Recently,
oxo-molybdenum9 and oxo-vanadium10 complexes have been
reported as inexpensive DODH catalysts with phosphine or
sulfite reductants.
RESULTS AND DISCUSSION
■
A set of seven potential hydrogen donors were tested for
efficacy under typical DODH conditions with diethyl tartrate
(DET) and 1,2-octanediol as representative glycols. In these
reactions (Table 1) we obtained moderate to excellent yields of
The use of alcohols as H-transfer reductants in DODH has
also received attention.11−14 Typically, secondary alcohol-
reducing agents have been used, often as the reaction
solvent.11,12 The activated primary alcohol, benzyl alcohol,
Received: March 18, 2015
© XXXX American Chemical Society
A
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78−90% yield depending on the solvent used. In the second
case (entry 6a), 9,10-dihydroanthracene was employed as the
reductant for DET, expecting to generate anthracene and
fumarate, which would engage in cycloaddition. Indeed, the
trans-adduct 221 was formed, albeit in a modest 40% yield, with
about 25% of fumarate remaining.
Table 1. Screening Hydroaromatic Reductants for DODH
Tetrahydronaphthalene (THN) from coal has been studied
as a hydrogen donor.22 An attractive feature of this abundant
hydroaromatic is that it could provide two equivalents of
hydrogen. We further investigated its efficacy using a half or
one equivalent on diethyl tartrate and octanediol (Table 1,
entries 2a−d). After only 17 h the yields were moderate using
half an equivalent of THN, improving somewhat after 48 h; use
of one equivalent of THN increased the yield further.
The results in Table 1 show that indoline is the most
effective hydroaromatic hydrogen donor for DODH of the
model substrates. Not only did it provide the highest yields of
olefin, but its coproduct, indole, is easily detected and formed
with high efficiency; typically the ratio of alkene to indole is
close to 1:1.
Accordingly, indoline was utilized in DODH reactions with a
broader range of polyols (Table 2). While the DODH reactions
of diols are efficient employing NH4ReO4 (APR) or MeReO3
(MTO)/indoline in aromatic solvents (Table 1, entries 3a,b),
to effectively convert poorly soluble higher polyols, we turned
to 1-butanol as a solvent.14 The reaction utilizing indoline in
combination with APR or MTO as catalysts in 1-butanol
solvent proved to be effective for the DODH of several polar
diols and polyols (Table 2, entries 2−10). We were pleased to
obtain a quantitative yield (99%; 90% isolated) of N-allyl purine
323 from diprophylline (Table 2, entry 2). By comparison, a
70% yield of 3 was obtained from this substrate using the
benzyl alcohol/NH4ReO4 system.13 N-Allyl heterocycles such
as 3 have diverse biological activity, including as modulators of
drug-induced sleep and spontaneous activity,24 as selective
binders to the A2 adenosine receptor,25 and as anticancer and
antiviral agents.26 Glycerol and its ester and ether derivatives
(Table 2, entries 3−5) gave similarly high yields. Erythritol and
xylitol (Table 2, entries 6 and 7) afforded moderate yields of
the corresponding diene products. The three polyhydroxy
carboxylic acids glyceric acid, tartaric acid, and mucic acid
(Table 2, entries 8, 9, and 10) each provided an esterified
alkene product; tartaric acid yielded 78% dibutyl fumarate (60%
isolated), while mucic acid gave 57% of the corresponding
trans,trans-diene ester. Glyceric acid gave a fairly low yield of
butyl acrylate, which may be the result of competing reactions.
The same reaction as Table 2, entry 8, using APR as catalyst
showed no acrylate product, and GC-MS analysis revealed
indoline- and indole/fumarate-coupled products (Scheme 4).27
Several observations indicate that indoline is a superior H-
transfer agent relative to primary and secondary alcohols. In the
indoline-driven reactions with 1-butanol as solvent, neither
butanal nor the butanal/1-butanol acetal were detected in
appreciable amounts. Two competition experiments were
conducted to determine the relative H-transfer reactivity of
indoline vs sec-alcohol reductants (Scheme 5). Conducting the
same reaction as entry 3 (Table 2) with 2-butanol as solvent
provided a lower yield of allyl alcohol, 46%, but a 1:1 ratio of
allyl alcohol to indole and no detectable amount of butanone.
In a fairer competition using conditions of entry 3a Table 1 the
reaction of DET and 1.1 equivalents each of indoline and of 3-
pentanol provided an 82% yield of diethyl fumarate with an
appreciable amount of 3-pentanol remaining.
a
b
Yield determined by 1H NMR with internal standard. 17 h reaction
c
time with 0.55 equiv of reducing agent. MeReO3 used as catalyst and
the reductant as solvent. The oxidized product of fluorene not
characterized.
d
the alkene, from 70% to 99% from DET and 40% to 70% from
octane diol. In the cases of entries 5a and 6a (Table 1), we
could couple the DODH product, diethyl fumarate (DEF), in a
tandem Diels−Alder (DA) reaction14 (Schemes 2 and 3). In
Scheme 2. Tandem DODH/DA with DET and 1,3-
Cyclohexadiene
Scheme 3. Tandem DODH/DA with DET and 9,10-
Dihydroanthracene
the first case (entry 5a) 1,3-cyclohexadiene serves both as the
reducing agent for the DODH reaction and as the diene for the
Diels−Alder reaction, providing the trans-cycloadduct 120 in
B
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e
Table 2. DODH with Indoline as Reductant
a
b
c
d
e
Yield determined by 1H NMR. 100% conversion. Isolated yield. 65%. Conditions: (A) NH4ReO4, benzene (0.2 M), 48 h, 150 °C; (B) MeReO3
(MTO), 1-butanol (0.3 M), 24 h, 150 °C; (C) MTO, 1-butanol (0.3 M), 4 h, 170 °C (entry 7, 4.5 h); (D) NH4ReO4, 1-butanol (0.3 M), 24 h, 150
°C; (E) MTO 2.5 mol %, 1-butanol (0.3 M), 24 h, 150 °C.
Scheme 4. DODH of Glyceric Acid Using Indoline as
Reducing Agent
Scheme 5. Reducing Agent Competition: (A) Indoline vs 2-
Butanol; (B) Indoline vs 3-Pentanol
Several of the hydroaromatics in Table 1 have been used in
H-transfer reactions, including heterogeneous hydrogena-
tion,28,29 hydrogenolysis of aralkyl− and aryl−oxygen and
−nitrogen bonds,30 and light-driven alkane dehydrogenation.31
In most of these studies indoline also showed high activity as a
hydrogen donor. Computational and thermodynamic studies
indicate an energetic driving force for the indoline/indole
conversion ascribed to the gain of heteroaromatic stabilization,
but little is known about the mechanism of H-transfer.16,30
The high efficiency of indoline-driven DODH reaction and
the established affinity of amines for MTO32 prompted us to
C
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investigate the interaction of MTO with indoline. The addition
of a stoichiometric amount of indoline to a MTO solution at
room temperature results in a rapid color change from colorless
to yellow or green (λ 656 nm) depending on the concentration.
The 1H NMR spectrum of the mixture shows significant
shifting of the indoline and Me-Re resonances, and the ReO
IR absorption is also shifted lower by 33 cm−1. The MTO-
indoline adduct 4 was isolated and structurally characterized by
X-ray diffraction (Figure 1). The structure of 4 features a
Scheme 7. Possible Mechanism of MTO-Indoline H-
Transfer Redox Reaction
Scheme 8. Potential MTO Catalytic Cycle with Indoline as
Reductant
Figure 1. X-ray ORTEP diagram of MeReO3(indoline) (4). Selected
bond lengths (Å) and angles (deg): Re−O2 1.719(2), Re−O1
1.719(2), Re−O3 1.724(2), Re−C1 2.102(3), Re−N1 2.395(2), N−H
0.85(4); O2−Re−O1 105.01(10), O2−Re−O3 117.32(10), O1−Re−
O3 106.26(10), O2−Re−C1 116.28(10), O1−Re−C1 90.03(11),
O3−Re−C1 116.53(11), O2−Re−N1 78.62(9), O1−Re−N1
168.12(9).
distorted trigonal bipyramidal Re center with the N-
coordinated indoline unit occupying an apical position and
the Me-Re in an equatorial position.
1
After heating a solution of 4 for an hour at 150 °C, the H
NMR spectrum showed the formation of indole (ratio
indoline:indole, 1.0:0.4); the indoline was entirely consumed
after 4.5 h. From these observations it is clear that indoline
coordinates to MTO at room temperature and, upon heating, is
oxidized to indole. The less basic indole does not appreciably
associate with MTO, given that after 20 h at room temperature
there was no change in the 1H NMR spectrum of an equimolar
mixture of the two.
and fragmentation, while in path B the rhenium(VII) species
first forms the glycolate, which is then reduced and fragments
to the alkene. The affinity of MTO for indoline and the
subsequent H-transfer redox reaction that proceeds at moderate
temperatures indicate that path A (Scheme 8), involving initial
reduction of MTO by indoline, is viable. To assess the relative
affinity of indoline vs the diol substrate for MTO, we prepared
an equimolar mixture of these reactants to which was added an
equivalent of MTO at room temperature. The resulting NMR
spectrum showed a 1.0:0.1 ratio of Me-ReO3-(indoline) (4) to
Re-glycolate (6)7 (Scheme 8, R = H) and remained unchanged
after 20 h, showing that indoline has a greater binding affinity
than the glycol for MTO. This result also suggests a preference
for pathway A, but does not exclude the operation of path B.
In conclusion we have demonstrated the ability of hydro-
aromatic compounds to serve as reductants in oxo-metal-
catalyzed deoxydehydration of polyols. From a practical
perspective the value of this transformation will depend on
the cost/availability of the particular polyol and hydroarene co-
reactants and the respective olefinic and aromatic co-products.
Indoline is particularly efficient for these reactions, being more
reactive than representative primary and secondary alcohols. Its
effectiveness may be ascribed to its ability to coordinate to
electrophilic oxo-metal species, to then effect entropically
favored intramolecular H-transfer, and to form the stable,
weakly coordinating aromatic, indole. Atom-economical
tandem DODH/Diels−Alder reactions are also illustrated.
The use of 1-butanol as solvent allows expansion of the range of
Because the corresponding reduced Re-species, MeReO2
(MDO), will be highly reactive under the reaction conditions,33
to test for its intermediacy, we conducted a trapping reaction
with 3-hexyne.12 Heating a mixture of indoline, 3-hexyne, and
MTO (5:2:1) at 100 °C for 19 h fully consumed the MTO;34
1
the H and 13C NMR spectra of the resulting mixture showed
the formation of MeReO2(3-hexyne) (5) (Scheme 6).
Scheme 6. MDO-Trapping Experiment
In Scheme 7 we suggest a possible mechanism for the
formation of MDO and indole involving complexation, proton
transfer of the acidified N−H to a ReO unit, α-C-H transfer
with H2O loss, and isoindoline tautomerization.35
In the DODH system two pathways (A, B, Scheme 8) are
viable, differing in the sequence of the reduction/condensation
steps. In sequence A the rhenium(VII) species is first reduced,
followed by condensation with glycol to form the Rev-glycolate
D
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Hz), 7.02 (t, 1 H, J = 7.8 Hz). 13C NMR (C6D6, 100 MHz): 150.7,
130.8, 127.8, 125.2, 120.9, 111.9, 47.7, 30.1, 19.3.
polyols that are effectively converted to unsaturated products.
The efficacy and availability of the hydroaromatic reductants
could lead to their use in other oxo-metal-catalyzed reductions.
X-ray Crystal Analysis of 4. A yellow, plate-shaped crystal of
dimensions 0.460 × 0.240 × 0.050 mm was selected for structural
analysis. Intensity data for this compound were collected using a
diffractometer with a Bruker APEX ccd area detector38 and graphite-
monochromated Mo Kα radiation (λ = 0.710 73 Å). The sample was
cooled to 100 K. Cell parameters were determined from a nonlinear
least-squares fit of 6810 peaks in the range 2.32° < θ < 28.32°. A total
of 21 419 data were measured in the range 2.318° < θ < 28.333° using
ϕ and ω oscillation frames. The data were corrected for absorption by
the empirical method,39 giving minimum and maximum transmission
factors of 0.069 and 0.575. The data were merged to form a set of 2432
independent data with R(int) = 0.0395 and a coverage of 100.0%.
The monoclinic space group P21/c was determined by systematic
absences and statistical tests and verified by subsequent refinement.
The structure was solved by direct methods and refined by full-matrix
least-squares methods on F2.40 The positions of hydrogens bonded to
carbons were initially determined by geometry and were refined using
a riding model. The hydrogen bonded to the nitrogen was located on a
difference map, and its position was refined independently. Non-
hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atom displacement parameters were set to 1.2
(1.5 for methyl) times the isotropic equivalent displacement
parameters of the bonded atoms. A total of 131 parameters were
EXPERIMENTAL SECTION
■
General Information. All reactants and catalysts were obtained
commercially and used without further purification. All solvents were
ACS grade and were used directly (unless otherwise described in the
1
2
procedures). H, H, and 13C NMR spectra were collected on Varian
VX300 MHz or VNMRS 400 MHz instruments. The NMR data were
processed using SpinWorks36 and ACD37 software. GC-MS-EI
analyses were performed on a Thermo-Finnigan instrument using a
Stabilwax capillary column.
Representative Procedure for DODH Reactions. Diethyl
tartrate (1.00 mmol, 0.17 mL), NH4ReO4 (0.10 mmol, 26.8 mg),
indoline (1.10 mmol, 0.123 mL), and benzene (5 mL) were added to a
thick-walled Ace glass reactor tube. A N2 flow was bubbled into the
mixture for at least 60 s before the Teflon seal was closed. The purge
was made to avoid any oxidation of the hydrogen donors by O2. The
reactor was placed in an oil bath at 150 °C for 48 h while stirring
magnetically. After cooling to rt, a 100 μL aliquot of the reaction
mixture was removed and added to CDCl3 and 2.0 μL of DMSO as
internal standard for NMR analysis. This product and the others listed
were characterized and quantified using 1H NMR spectroscopy. Some
of them were isolated and analyzed by NMR. The isolation of selected
products was accomplished by column or preparative TL chromatog-
raphy. The eluents varied according to the different polarity of the
alkene product; hexane/ethyl acetate or chloroform/ethyl acetate
mixtures were used. All olefinic products from the DODH reactions
have been previously reported and were identified by comparison of
their NMR spectra with authentic samples or published data (see the
Supporting Information).
refined against 2432 data to give wR(F2) = 0.0476 and S = 1.012 for
2
weights of w = 1/[σ2(F2) + (0.0280P)2 + 1.4600P], where P = [Fo
+
2Fc2]/3. The final R(F) was 0.0162 for the 2340 observed, [F >
4σ(F)], data. The largest shift/su was 0.003 in the final refinement
cycle. The final difference map had maxima and minima of 0.840 and
−0.873 e/Å3, respectively. Further details on crystal data, data
collection, and refinements are summarized in Table S1 (Supporting
Information).
DODH/Diels−Alder Tandem Reaction with Diethyl Tartrate
and 1,3-Cyclohexadiene. Diethyl tartrate (0.33 mmol, 0.057 mL),
NH4ReO4 (0.033 mmol, 9.0 mg), and 1,3-cyclohexadiene (1.1 mL)
were added to a thick-walled Ace glass reactor tube. The Teflon seal
was closed, and the reactor was placed in an oil bath at 150 °C for 24 h
while stirring magnetically. After cooling to rt, a 100 μL aliquot of the
reaction mixture was removed and added to CDCl3 and 2.0 μL DMSO
as internal standard for NMR analysis. The product was identified and
quantified by 1H NMR spectroscopy and by comparison with an
authentic sample of the separately prepared D−A adduct from the
corresponding diene and diethyl tartrate (see the Supporting
Information).
DODH Reaction of Glyceric Acid with Indoline. This reaction
was conducted in the same way as the other DODH reactions. The
NMR and GC-MS spectra of the crude product mixture after solvent
evaporation indicated the presence of indole- and indoline-acrylate
adducts (see the Supporting Information).
MTO/Indoline Reactivity Study. MTO (0.04 mmol, 10 mg) was
dissolved in 800 μL of d6-benzene in a high-pressure NMR tube.
Indoline (0.04 mmol, 4.5 μL) was added, and the tube was flushed
1
with N2. H NMR spectra were collected before and after placing the
tube in an oil bath at 150 °C for 1 h. The reduction of MTO by
indoline was set up similarly at a 0.20 mmol scale with protio-benzene
in a thick-walled Ace glass reactor tube. A N2 flow was bubbled in the
mixture for 60 s, and after sealing the reactor the tube was placed in an
oil bath at 150 °C for 4.5 h while stirring. After cooling at room
temperature, the mixture was transferred to an NMR tube for analysis
by no-D solvent NMR (see the Supporting Information). The two
CH2 group absorptions from indoline (3.1 and 3.6 ppm) were absent
after the reaction, being replaced by the peaks for indole (with no
overlap; 6.5 and 7.8 ppm). The mixture changed from colorless to
yellow upon the addition of indoline at room temperature, then turned
green. At 150 °C the solution turned blue then brown-black.
MDO Trapping. In a high-pressure NMR tube, 63 mg of MS 5 Å
powder and 20 mg of MTO (0.08 mmol) were added to an 800 μL
solution of 3-hexyne (2 equiv, 0.16 mmol, 18 μL) in d-chloroform
Reducing Agent Competition between 3-Pentanol vs
Indoline. Diethyl tartrate (1.0 mmol, 0.17 mL), NH4ReO4 (0.10
mmol, 27 mg), 3-pentanol (1.1 mmol, 0.12 mL), indoline (1.1 mmol,
0.12 mL), and benzene (5 mL) were added to a thick-walled Ace glass
reactor tube. A nitrogen flow was bubbled into the mixture for at least
60 s before the Teflon seal was closed, and the reactor was placed in an
oil bath at 150 °C for 24 h while stirring. After cooling to rt, a 100 μL
aliquot of the reaction mixture was removed and added to CDCl3 and
2.0 μL of DMSO as internal standard for NMR analysis. The product
1
(dried over 4 Å molecular sieves). The tube was flushed with N2. H
and 13C NMR spectra were collected at rt and after placing the tube in
an oil bath at 100 °C. The reaction was completed after a total of 19 h.
Competition between Indoline and Ethylene Glycol for
MTO. MTO (0.02 mmol, 5 mg) was added to 800 μL of a 0.03 M
1
equimolar solution of indoline and ethylene glycol in d6-benzene. H
1
composition was determined and quantified using H and 13C NMR
NMR spectra were collected directly and confirmed after standing
overnight at room temperature.
spectroscopy.
Preparation of the MeReO3(η1-indoline) (4). MTO (0.040
mmol, 10 mg) was added to 3 mL of a 0.05 M solution of indoline in
hexanes; the color change to yellow-green occurred instantaneously.
The mixture was warmed to 40 °C in order to dissolve all the material.
At 3 °C, a yellow precipitate formed in 10 min. Crystals for X-ray
analysis were obtained by slow evaporation of the solvent at room
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra of the reaction mixtures and isolated products. X-
ray crystallographic files (CIF) for complex 4. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.organomet.5b00226.
1
temperature. IR: ν(ReO) 933 cm−1. H NMR (C6D6, 400 MHz):
1.28 (s, 3 H), 2.60 (t, 2 H, J = 8.2 Hz), 2.93 (t, 2 H, J = 8.2 Hz), 6.59
(d, 1 H, J = 7.8 Hz), 6.77 (t, 1 H, J = 7.4 Hz), 6.96 (d, 1 H, J = 7.2
E
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: knicholas@ou.edu.
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Res. 2011, 44, 36−46. Karki, M.; Araujo, H.; Magolan, J. Synlett 2013,
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for the support provided by the U.S.
Department of Energy (DE-11ER16276). We thank Dr.
Douglas Powell for determination of the X-ray structure of 4.
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DOI: 10.1021/acs.organomet.5b00226
Organometallics XXXX, XXX, XXX−XXX