Room temperature dehydrogenation of ethane to ethylene

Article abstract of DOI:10.1021/ja202316m

The transient titanium alkylidyne, (PNP)Ti≡C^tBu (PNP = N[2-PⁱPr₂-4-methylphenyl]₂^-), activates a C-H bond of ethane at room temperature, and a β-hydrogen of the resulting ethyl ligand is subsequently transferred to the adjacent alkylidene ligand to form an ethylene adduct of titanium. Treatment of the ethylene complex with two-electron oxidants such as organic azides results in extrusion of ethene concomitant with formation of a mononuclear titanium imido complex.

Full text of DOI:10.1021/ja202316m

COMMUNICATION
pubs.acs.org/JACS
Room Temperature Dehydrogenation of Ethane to Ethylene
Vincent N. Cavaliere, Marco G. Crestani, Balazs Pinter, Maren Pink, Chun-Hsing Chen,
Mu-Hyun Baik,* and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405, United States
S Supporting Information
b
dehydrogenation of ethane to ethene can be achieved by A at
room temperature with good yield.
ABSTRACT: The transient titanium alkylidyne, (PNP)Tit
t
i
À
2
C Bu (PNP = N[2-PPr -4-methylphenyl] ), activates a CÀ
A cyclohexane solution of the alkylidyne precursor complex,
2
t
t
H bond of ethane at room temperature, and a β-hydrogen of
the resulting ethyl ligand is subsequently transferred to the
adjacent alkylidene ligand to form an ethylene adduct of
titanium. Treatment of the ethylene complex with two-
electron oxidants such as organic azides results in extrusion
of ethene concomitant with formation of a mononuclear
titanium imido complex.
(PNP)TidCH Bu(CH Bu), undergoes a clean conversion in
2
the presence of ethane at 21 °C over 24 h to give a single product.
Moderately elevated ethane pressure (>400 psi) is required to
improve yield and avoid the side-reactions of A with the
14
solvent. At these higher pressures, the transformation is
31
quantitative within the detection limit of P NMR spectroscopy
(45% isolated yield). The isolated brown solid gives, by the
31
1
solution P{ H} NMR spectrum, two doublets at 27.8 and
2
2
5.9 ppm and having a J
= 22.2 Hz. A spectroscopically
PÀP
hereas methane received much attention as a potential
identical compound was synthesized independently by treating a
diethyl ether solution of (PNP)TidCH Bu(OTf) with 1 equiv of
t
W
carbon feedstock, the chemistry of ethane remains rela-
tively unexplored. This hydrocarbon, which makes up the second
largest component of natural gas, after methane, is particularly
ClMgC H (or 0.5 equiv of Mg(C H ) ) at À35 °C.
2
5
2 5 2
Unfortunately, single crystals suitable for X-ray diffraction
studies could not be obtained. In lieu of this, the structure of
complex 1 was unambiguously assigned as the ethylene adduct
interesting as a potential source of more reactive C products,
2
such as ethene. Although practical, the most common industrial
method for the conversion of ethane to ethene, referred to as
steam cracking, is very energy-intensive (Temp. >800 °C) and
t
2
(PNP)Ti(CH Bu)(η -CH CH ) (1) and not the ethyl inter-
2
2
2
t
1
mediate, (PNP)TidCH Bu(CH CH ) (B) on the basis of H,
C, HSQC, and COSY NMR spectra. A salient feature is the
presence of a titanium-neopentyl group, evidenced by correlation
of a C resonance at 110.0 ppm with two diasterotopic signals in
the H NMR spectrum at 1.26 and 0.02 ppm arising from the
methylene protons. The latter H NMR signal appears as a
doublet of triplets with J
the former signal is overlaying with other resonances from the
PNP backbone. In addition to this neopentyl group, the foremost
feature is an ethylene moiety, which was identified by C NMR
spectroscopy as two moderately broad singlets at 72.8 (Δν1/2
2
3
13
rather inefficient given the detrimental buildup of CO (1.5À
2
1
3
tons of CO per ton of ethene produced). The use of ethane in
2
13
organometallic processes under mild conditions is fundamentally
difficult because of its inferior binding affinity to typical transi-
tion metal catalysts, the inherent strength of the CÀH bond of
1
1
2
3
2
3
∼
101 kcal/mol and extraordinarily high pK in the range of 50.
= 11.9 Hz and J
= 2.9 Hz, but
a
HÀH
HÀP
Another factor often inhibiting the dehydrogenation of ethane to
4
ethene is the accumulation of hydrogen, which can serve as a
1
3
ligand or hydride source, thus, impeding catalytic turnover.
Homogeneous dehydrogenation catalysts were developed in
=
5,6
4,7,8
9
the past using iridium, rhodium,
and rhenium, but these
16.1 Hz) and 66.8 (Δν1/2 = 15.8 Hz) ppm. These resonances are
significantly upfield shifted when compared to free ethylene
require high temperatures, limiting their utility to less volatile
1
5
2
paraffins (C and heavier and usually in the presence of a H2
(122.9 ppm) and to the analogous complex Cp* Ti(η -C H )
2 2 4
16
6
acceptor). Solid-state ethane dehydrogenation catalysts are also
reported by Bercaw, but are roughly in line with the titaniumÀ
1
0
2
2 2 4
17
known and degenerate metathesis and cross-metathesis of
ethylene complex, (ArO) Ti(η -C H ) published by Rothwell.
ethane have been reported in surface-based organometallic
In addition, an HSQC NMR experiment reveals correlation of
1
1
13
catalysts, but the conversion of ethane to value-added chemi-
cals like ethene has not yet been demonstrated under mild
each ethylene 1 C NMR signal with a set of two multiplet reso-
nances in the H NMR spectrum: the more downfield-shifted
12
1
conditions—either catalytically or stoichiometrically —or in a
manner allowing for a clear understanding of the underlying
mechanism.
carbon (72.8 ppm) with signals at 2.30 (1H, J = 151 Hz) and
CH
1
13
0.94 ppm (1H, J = 148 Hz), and the more upfield C signal
(66.8 ppm) with a broad resonance at 1.96 ppm (2H, J
CH
1
=
CH
Previously, we established that the transient titanium alkyli-
151 Hz). The COSY NMR spectrum shows clearly that the latter
resonance arises from two overlaying multiplets, and reveals
coupling between all four proton resonances, allowing for the
t
i
dyne complex (PNP)TitC Bu (A) (PNP = N[2-P Pr -4-
2
À
18
methylphenyl]2 ) can activate CÀH bonds of arenes and other
hydrocarbons (RÀH) to give alkylideneÀalkyl complexes of the
conclusive assignment of the titanium-ethylene moiety.
t
type (PNP)TidCH Bu(R), in which the alkylidene hydrogen
13
derives from a 1,2ÀCH bond activation of the hydrocarbon.
More recently we demonstrated that methane is also a suitable CH-
Received: March 22, 2011
Published: June 16, 2011
1
4
activation substrate. It is herein reported that the stoichiometric
r 2011 American Chemical Society
10700
dx.doi.org/10.1021/ja202316m
|J. Am. Chem. Soc. 2011, 133, 10700–10703

Journal of the American Chemical Society
COMMUNICATION
Scheme 1. Synthesis of the Ethane Complex 1 via Dehydro-
genation of CH CH and Independent Synthesis of This
3
3
Complex via Salt Metathesis
Figure 1. Computed reaction profile for CÀH activation of ethane and
conversion to the titanacyclopropane 1.
lifetime. We have made significant efforts to do so, but were
1
8,21
unsuccessful.
Complex 1 is calculated to adopt a trigonal-bipyramidal
geometry in which the C H ligand is oriented parallel to the
2
4
plane defined by the PNP ligand (Figure 2). Although it is
reasonable to interpret the short TiÀC1 and TiÀC2 distances of
2
1
.13 and 2.14 Å, respectively, and the long CÀC distance of
2
2
.44 Å as a strong indicator of a Ti(IV)-cyclopropane moiety,
which we denote as 1a, the alternative assignment of a Ti(II)À
ethene π-complex 1b is equally plausible, as highlighted in
Scheme 1. Distinguishing these two resonance structures is
potentially helpful, as fundamentally different reactivities may
be expected: The titanacyclopropane 1a may undergo migratory
insertion reactions, while in the π-complex 1b, the olefin ligand
may be preactivated toward nucleophilic attack in addition to the
Figure 2. Optimized structures of the transition state B-TS and
titanacyclopropane 1.
To gain insight about the mechanism of forming 1, we
computed several possible reaction trajectories, and the most
likely pathway is illustrated in Figure 1. As anticipated, the tita-
nium alkylidyne intermediate A and ethane first form the adduct
22
metal being more vulnerable toward oxidation. One plausible
way of estimating the weight of these resonance extremes is to
quantify the metal character in the molecular orbital correspond-
ing to the metal to etheneÀπ* interaction. Our calculations
indicate that the HOMO represents a metalÀπ* interaction
À1
A-CH CH , a σ-complex calculated to be 12.1 kcal mol
3
3
3
higher in energy than A and free ethane. A 1,2-addition of the
CÀH bond across the highly reactive TitC linkage yields the
ethyl-neopentylidene intermediate B with a relative solution
1
8
consisting of a 56% contribution by the metal-d orbitals, thus
positioning the electronic structure around the median of 1aÀ1b
range. Consequently, we expect 1 to display chemical reactivities
reminiscent of both canonical forms.
À1
phase free energy of À13.5 kcal mol . Finally, a concerted
3
β-hydrogen migration from the ethyl moiety to the neopentyli-
dene ligand gives the final product complex 1, traversing the
transition state B-TS, which is best envisioned as containing a
Complex 1 is thermally stable up to 60 °C, but transforms
slowly over several days to intractable products. Unfortunately,
ethylene was not found to be one of the products. Two-electron
oxidation of 1 by N R (R = SiMe or 1-adamantyl) at room
[
4,3]-metallabicyclic framework. This last step is associated with
À1 19
a barrier of 25.9 kcal mol . The computed structure of B-TS,
3
depicted in Figure 2, adopts a trigonal-bipyramidal geometry in
which the migrating hydrogen H1 is located midway between the
donor carbon C1 and the acceptor carbon C3 with CÀH
distances of 1.59 and 1.53 Å, respectively. The relatively short
3
3
temperature, however, rapidly and quantitatively afforded the
t
titaniumÀneopentyl imido complexes (PNP)TidNR(CH Bu)
2
shown in Scheme 2 (2: R = SiMe ; 3: R = 1-adamantyl). Ethylene
3
Ti H1 distance of 1.72 Å, a Wiberg bond order of 0.23, and
Natural Population Analysis electron density of 0.74 at H1 in this
extrusion via oxidation of a titaniumÀethylene complex Cp* Ti-
3
3 3
2
2
(η -C H ) with N Ph was previously observed by Bergman and
2
4
3
23
transition state suggest that this step can be described as a metal
Andersen. When these reactions are performed in a sealed,
20
mediated hydrogen atom transfer. The ethylene product 1 lies
J-Young NMR tube in THF-d , free ethylene (∼0.5 equiv
8
À1
4
.9 kcal mol lower in energy than intermediate B, rendering
measured in solution using an internal standard) is formed
3
1
the last step irreversible within plausible reaction conditions
and in good agreement with our experimental observations.
The computed reaction profile suggests that intermediate B
should in principle be detectable, as it should display a finite
displaying a characteristic H NMR signal at 5.36 ppm. The
presence of free ethylene was further confirmed by bubbling it
into a THF-d solution having compounds 2 or 3. Imido
8
complexes 2 and 3 are both stable at room temperature which
1
0701
dx.doi.org/10.1021/ja202316m |J. Am. Chem. Soc. 2011, 133, 10700–10703

Journal of the American Chemical Society
COMMUNICATION
Scheme 2. Oxidative Elimination of Ethylene with Organic
Azides To Give Titanium Imido Alkyls 2 and 3
ethane and then abstracting an adjacent CÀH bond to give a
titaniumÀethylene adduct, under ambient temperatures. Two-
electron oxidation by N R or N O quantitatively releases ethy-
3
2
lene. Although the term “oxidative” dehydrogenation of ethane is
often used in the literature to illustrate the activation of the
catalyst by an oxidant, the oxidant used in the present study is not
involved in either activation of the titanium species or involved in
the activation of two CÀH bonds. Instead, the oxidant is solely
used for product liberation. Therefore, this dehydrogenation
process can be best described as a R,β-double CÀH activation
involving a combination of 1,2-CH bond addition and β-hydro-
gen migration. Formation of free ethylene most likely occurs by
an oxidatively induced reductive elimination, that is, oxidation of
26
the metal to promote product release. The latter process may
also involve insertion and retrocycloaddition steps rather than
direct oxidation of the metal. To the best of our knowledge, this is
the only example of a well-defined homogeneous system capable
of performing this type of transformation at room temperature
12,27
with a volatile paraffin such as ethane.
’
ASSOCIATED CONTENT
Supporting Information.
S
Experimental procedures,
b
Figure 3. Single crystal X-ray diffraction structure of 3. Hydrogen
atoms, tolyl methyls, and isopropyl methyls are omitted for clarity.
X-ray crystallographic information, computational information
and spectral data are provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
is unsurprising given that a closely related imido-alkyl com-
i
t
’ AUTHOR INFORMATION
pound, (PNP)TidN[2,6- Pr C H ](CH Bu) was found to be
2
6
3
2
24,25
remarkably stable in solution and solid state.
Likewise,
Corresponding Author
mindiola@indiana.edu
exposure of 1 to an atmosphere of N O also promotes ethylene
2
elimination cleanly, but the metal-based product, presumably a
metastable titanium oxo complex of the formula (PNP)TidO-
t
2
18
(
CH Bu), could not be isolated.
’
ACKNOWLEDGMENT
More precise information about the structure of 3 was
obtained via X-ray diffraction studies of single crystals of complex
Financial support of this research was provided by the
National Science Foundation (CHE-0848248, CHE-0645381).
D.J.M. acknowledges support from the Alexander von Humboldt
Stiftung for a Friedrich Bessel Research Award. M.G.C. acknowl-
edges CONACYT for a postdoctoral fellowship. The authors
acknowledge Dr. Jonathan A. Karty for assistance obtaining mass
spectra, and Dr. Xinfeng Gao for assistance obtaining NMR
spectra.
3
grown from Et O at À35 °C (Figure 3). Overall, the structural
2
parameters for the first coordination sphere of the metal in 3
resembles the previously characterized complex (PNP)TidN-
i
t
24
[2,6-Pr C H ](CH Bu). Formation of a terminal titanium-imido
2 6 3 2
ligand is further confirmed by the short TidN(2) bond distance
of 1.714(3) Å and nearly linear TidN(2)ÀC angle of 174.7(3)°.
The titanium-neopentyl has a TiÀC bond distance of 2.125(4) Å
3
and a TiÀC1ÀC angle of 131.5(3)°, indicative of an sp hybridized
alkyl carbon singly bonded to titanium (Figure 3). The connectivity
’
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31
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1
0702
dx.doi.org/10.1021/ja202316m |J. Am. Chem. Soc. 2011, 133, 10700–10703

Journal of the American Chemical Society
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1
(
(18) See Supporting Information.
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(
21) The reaction profile was re-examined at À35 °C using Et
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2
O as
2
barrier shown in Figure 1. As a result, DFT might be overestimating the
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3
1
1
complex 1 in solution (by P NMR or H NMR) at low temperature via
t
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2 4
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1
0703
dx.doi.org/10.1021/ja202316m |J. Am. Chem. Soc. 2011, 133, 10700–10703