Room temperature dehydrogenation of ethane, propane, linear alkanes C4-C8, and some cyclic alkanes by titanium-carbon multiple bonds

Article abstract of DOI:10.1021/ja4060178

The transient titanium neopentylidyne, [(PNP)Ti≈C^tBu] (A; PNP^-≈N[2-PⁱPr₂-4-methylphenyl] ₂^-), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η²-H₂C-CH₂)(CH ₂^tBu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C ₄-C₆ to form isolable α-olefin complexes of the type, [(PNP)Ti(η²-H₂C-CHR)(CH₂ ^tBu)] (R = CH₃ (2), CH₂CH₃ (3), ⁿPr (4), and ⁿBu (5)). Complexes 1-5 can be independently prepared from [(PNP)Ti-CH^tBu(OTf)] and the corresponding alkylating reagents, LiCH₂CHR (R = H, CH₃(unstable), CH ₂CH₃, ⁿPr, and ⁿBu). Olefin complexes 1 and 3-5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including ¹H- ³¹P HOESY, and in the case of the α-olefin adducts 2-5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C-H coupled and decoupled ¹H-¹³C multiplicity edited gHSQC, ¹H- ³¹P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C ₇ and C₈) are also dehydrogenated by A to form [(PNP)Ti(η²-H₂C-CHⁿPentyl)(CH ₂^tBu)] (6) and [(PNP)Ti(η²-H ₂C-CHⁿHexyl)(CH₂^tBu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with D ₂C-CD₂ with concomitant release of H₂C-CH ₂. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH₂^tBu)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d₁₂ can be dehydrogenated to 4 and 4-d₁₂ with comparable rates (KIE = 1.1(0) at ～29 C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N₂O, organic azides (N₃R; R = 1-adamantyl or SiMe₃), ketones (O-CPh₂; 2 equiv) and the diazoalkane, N₂CHtolyl₂. For complexes 3-7, oxidation with N₂O also liberates the α-olefin.

Full text of DOI:10.1021/ja4060178

Article
pubs.acs.org/JACS
Room Temperature Dehydrogenation of Ethane, Propane, Linear
Alkanes C4−C8, and Some Cyclic Alkanes by Titanium−Carbon
Multiple Bonds
Marco G. Crestani, Anne K. Hickey, Xinfeng Gao, Balazs Pinter, Vincent N. Cavaliere, Jun-Ichi Ito,
Chun-Hsing Chen, and Daniel J. Mindiola*,†
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States
S
* Supporting Information
ABSTRACT: The transient titanium neopentylidyne, [(PNP)Ti
−
CtBu] (A; PNP−N[2-PiPr2-4-methylphenyl]2 ), dehydrogenates
ethane to ethylene at room temperature over 24 h, by sequential 1,2-
CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η2-
t
H2CCH2)(CH2 Bu)] (1). Intermediate A can also dehydrogenate
propane to propene, albeit not cleanly, as well as linear and volatile
alkanes C4−C6 to form isolable α-olefin complexes of the type,
[(PNP)Ti(η2-H2CCHR)(CH2 Bu)] (R = CH3 (2), CH2CH3 (3), Pr
t
n
n
(4), and Bu (5)). Complexes 1−5 can be independently prepared from
[(PNP)TiCHtBu(OTf)] and the corresponding alkylating reagents,
n
n
LiCH2CHR (R = H, CH3(unstable), CH2CH3, Pr, and Bu). Olefin complexes 1 and 3−5 have all been characterized by a
diverse array of multinuclear NMR spectroscopic experiments including 1H−31P HOESY, and in the case of the α-olefin adducts
2−5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally
established. The latter has been spectroscopically elucidated by NMR via C−H coupled and decoupled H−13C multiplicity
1
edited gHSQC, 1H−31P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C7 and C8) are also dehydrogenated by A to
t
t
form [(PNP)Ti(η2-H2CCHnPentyl)(CH2 Bu)] (6) and [(PNP)Ti(η2-H2CCHnHexyl)(CH2 Bu)] (7), respectively, but
these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with
D2CCD2 with concomitant release of H2CCH2. In addition, deuterium incorporation is observed in the neopentyl ligand as
a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of
t
cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH2 Bu)] fragment to form 1. Dehydrogenation of the alkane is not rate-
determining since pentane and pentane-d12 can be dehydrogenated to 4 and 4-d12 with comparable rates (KIE = 1.1(0) at ∼29
°C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric
repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers.
The olefin in 1 can be liberated by use of N2O, organic azides (N3R; R = 1-adamantyl or SiMe3), ketones (OCPh2; 2 equiv)
and the diazoalkane, N2CHtolyl2. For complexes 3−7, oxidation with N2O also liberates the α-olefin.
In this context, large-scale industrial processes, such as steam
reforming,11 steam cracking,12 and Fischer−Tropsch,3d,13 are
vastly important given the ever growing world energy demand
in order to convert volatile alkanes to more synthetically useful
products such as liquid fuels or unsaturated compounds,14 but
in all cases these transformations are thoroughly energy and
capital intensive.
Steam cracking is especially useful worldwide for these
purposes, being a chief-industrial transformation that converts
vast resources such as ethane to ethylene and other more
reactive hydrocarbons.15,16 In this process, a stream of light
alkanes (ethane, propane, butane) are heated using high
velocities and diluted with steam to elicit C−C bond homolysis.
The resulting alkanes undergo a series of radical reactions that
INTRODUCTION
■
In the face of a world energy crisis,1 spurred by rising global
temperatures and dwindling accessible petroleum reserves,2 the
quest for new methods to better convert vast resources such as
natural gas into more useful commodity reagents is an attractive
research endeavor.2−4The chemistry of alkanes has been one of
the most intriguing challenges to chemists in the 20th
century,4,5 but despite research efforts few examples for
selective conversions of the most volatile alkanes6 (and hence
the most abundant) are known.7,8 Consequently, one of the
most important challenges facing chemists, both fundamentally
and practically speaking, is the efficient and mild activation and
functionalization of the major components of natural gas:
methane,8 9 ethane, and other alkanes; many of which
t,
constitute some of the most kinetically inert components, not
Received: June 15, 2013
ideal for use as liquid fuels.10
© XXXX American Chemical Society
A
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Scheme 1. Divergent Reactivity of n-Hexane with Pincer-Iridium Complexes: Dehydroaromatization (above) and Alkane
Metathesis via Tandem Dehydrogenation/Olefin Metathesis (below)
ultimately lead to the corresponding olefins (ethylene, propene,
butenes, etc.) and a fraction of dienes (e.g., 1,3-butadiene).
However, the process, which accounts for over 65% of ethylene
production in the U.S. and over 90% of ethylene production in
North America, requires temperatures in excess of 800 °C and
current technology, on average, produces 1−3 tons of CO2 per
ton of ethylene formed.15
As a result, finding a method that would selectively convert
alkanes to alkenes under mild conditions10b,17 would be of
significant practical utility, since an olefin such as ethylene is the
second-most produced chemical in the world after sulfuric acid.
Notably, ethylene is also the starting material for other
industrially important raw materials such as 1,2-dichloroethane,
ethylene oxide, and styrene among many other products.
Terminal olefins such as propene, 1-butene, and 1-hexene are
equally important, but their low-yield production in the steam
cracking process, coupled with the energy needed to produce
these monomers, has triggered the pursuit of alternative
pathways as a result of the rising costs of crude oil.
One attractive transformation for mildly converting linear
alkanes to olefins is the process denoted transfer dehydrogen-
ation.9 Crabtree originally showed that low-valent and electron-
poor diphosphino-iridium complexes could dehydrogenate
cyclooctane, proceeding via a series of oxidative addition and
β-hydride elimination steps.8a Due to the inherent endother-
micity of this reaction, high temperatures and a sacrificial olefin
must be used to abstract the dihydrogen equivalents.18 Indeed,
tandem methods based on olefin metathesis and concurrent
hydrogenation that eliminate the requirement of a sacrificial
olefin have been developed recently.19 Organometallic catalysts
incorporating iridium,9,20 rhenium,20 rhodium,21 and ruthe-
nium22 have been developed for such methods, being highly
active toward alkane dehydrogenation, in some cases with
excellent selectivity.
proven to be versatile and efficient, reactions that could operate
under even milder conditions (especially using cheaper metals)
are desirable, since the least reactive alkanes could be converted
to their respective terminal olefin. Likewise, converting alkanes
to terminal olefins without the need of a β-hydride elimination
step would be attractive since the microscopic reverse step,
migratory insertion of an olefin, would be avoided. Under
conditions where allylic C−H bond activation is unfavorable,26
this feature would block detrimental isomerization pathways of
the terminal olefin to the more thermodynamically favored (but
less synthetically useful) internal olefin.
In recent years, our group reported that the complex
[(PNP)TiCHtBu(CH2 Bu)] decays in solution with t1/2
=
t
3.1 h at room temperature (kavg = 5 × 10−5 s−1) to generate an
unprecedented transient and terminal titanium alkylidyne
[(PNP)TiCtBu] (A).27 This complex then activates the
C−H bond of benzene by 1,2-addition across the alkylidyne
moiety to generate [(PNP)TiCHtBu(Ph)].27 Later it was
found that this complex could activate multiple sp3 C−H bonds
in SiMe4, Me3SiCCSiMe3, Me3CCCCMe3, 1,3,5-Me3C6H3
and MeC6F5 by virtue of (1) 1,2-CH bond addition, (2) α-
hydrogen abstraction, and (3) tautomerization steps to yield
[(PNP)TiCHR(CH2R)] (R = SiMe3, SiMe2CCSiMe3,
CMe2CCCMe3, C6H3Me2, C6F5), in an overall dehydrogen-
ation process also referred to as an alkylidene-alkyl metathesis
reaction (Scheme 2).27b Species A also activates a C−H bond
of methane to generate [(PNP)TiCHtBu(CH3)],10
a
relatively stable methyl complex that can undergo slow
exchange with the neopentylidene ligand via a possible titanium
methylidene species (Scheme 2). More recently, we extended
Scheme 2. Dehydrogenation of SiMe4, Me3SiCCSiMe3,
Me3CCCCMe3, 1,3,5-Me3C6H3, CH3C6F5, CH4, and
CH3CH3 by Transient A
To date, the most efficient and versatile catalyst is the pincer-
iridium complex developed by Kaska,23 Jensen,24 and Gold-
man.19 This catalyst system, [(PCP)IrH2] (PCP− = 2,6-
i
(R2PCH2)2C6H3, R = Pr or tBu) has been shown to
dehydrogenate cyclooctane with unprecedented turnover
frequency (TOF) and turnover numbers (TON).9 A variation
of this complex also selectively converts linear alkanes such as
n-octane to 1-octene, but prolonged exposure to the catalysis
conditions converts the terminal olefins to the thermodynami-
cally favored internal olefins.9a Another variation of ancillary
ligand gave rise to dehydroaromatization of n-hexane, n-
heptane, and n-octane to give benzene, toluene, and a mixture
of o-xylene and ethylbenzene, respectively (Scheme 1).25 One
notable accomplishment of this catalyst architecture has been
tandem catalysis with a Schrock-type olefin metathesis catalyst,
which accomplishes an overall alkane metathesis cycle,
converting n-hexane to an array of alkanes, C2−C20 (Scheme
1).19 While the current series of dehydrogenation catalysts has
B
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the chemistry of A to the C−H activation of ethane which
results in formation of an η2-ethylene complex of the type,
ligand (the aryl moieties are not coplanar). Unfortunately
complex 2 is unstable and undergoes decomposition over
minutes, thus preventing us from fully characterizing this
species. Despite this limitation, this complex can be prepared
[(PNP)Ti(η2−H2CCH2)(CH2 Bu)] (1), by a stepwise
t
double α,β C−H bond activation process.28 Linear ethers can
be also dehydrogenated by A, albeit in competition with a
dehydroalkoxylation pathway.29 In the current work we report
the first complete and systematic study of the room
temperature dehydrogenation of ethane, propane, and linear
alkanes ranging from C4−C8 by A, to form terminal olefins
exclusively. We combine a battery of NMR spectroscopic
experiments and DFT to understand the structure of the olefin
compounds formed in solution and showcase a new mechanism
of alkane dehydrogenation that includes those of cyclohexane
and methylcyclohexane; the first of which decays in solution via
formation of an unstable titanium(II) species with concomitant
release of cyclohexene. Reactivity studies involving the titanium
olefin complexes are also presented.
1
independently and characterized spectroscopically by H and
31P{1H} NMR spectroscopy (vide infra).31 Treating 1 with n-
butane (neat or in cyclohexane solution), n-pentane, and n-
hexane cleanly gives rise to the α-olefin complexes, [(PNP)-
Ti(η2-H2CCHCH2CH3)(CH2 Bu)] (3, 1:3 mixture of
t
t
diastereomers), [(PNP)Ti(η2-H2CCHnPr)(CH2 Bu)] (4,
1:4 mixture of diastereomers), [(PNP)Ti(η2-H2CCHnBu)-
t
(CH2 Bu)] (5, 1:4 mixture of diastereomers), respectively
(Scheme 3). As opposed to complex 2, which is unstable at
room temperature, complexes 3−5 can be isolated as solids that
can be stored at −35 °C. These species, however, are not as
long-standing as compound 1, which decomposes only on
heating above 65 °C. Lastly, treatment of [(PNP)Ti
t
CHtBu(CH2 Bu)] with heavier linear alkanes such as n-heptane
and n-octane afforded the longer olefin adducts [(PNP)Ti(η2-
RESULTS AND DISCUSSION
■
t
H2CCHnPentyl)(CH2 Bu)] (6, 1:3 mixture of diaster-
Synthesis of the Titanium-Olefin Complexes by
Alkane Dehydrogenation. As shown in an earlier
communication,28 a cyclohexane (C6H12) solution of complex
t
eomers) and [(PNP)Ti(η2-H2CCHnHexyl)(CH2 Bu)] (7,
1:4 mixture of diastereomers),31 respectively (Scheme 2).
Unfortunately, like 2, these compounds are unstable and
decompose within hours (compound 6) or even minutes
(compound 7) to a myriad of titanium products, along with the
corresponding free α-olefin: 1-heptene or 1-octene, respec-
tively.
[(PNP)TiCHtBu(CH2 Bu)] under ethane pressure (400−
t
700 psi), in a sealed, stainless steel reactor at room temperature
for 24 h, gives rise to an η2-ethylene complex, [(PNP)Ti(η2-
t
H2CCH2)(CH2 Bu)] (1) (Scheme 2), on the basis of
multinuclear and multidimensional NMR spectroscopy (vide
infra). Complex 1 is formed in quantitative yield via a stepwise
double α β C−H bond activation process through a titanium-
neopentylidene-ethyl intermediate, [(PNP)TiCHtBu-
(C2H5)] (B) (Scheme 2). The structure of 1 can be described
as a metallacyclopropane where significant backbonding with
the olefin occurs (resonance 1a), or by a canonical form having
an ethylene-bound ligand that is purely a σ-donor (resonance
1b) in accordance with the Dewar−Chatt−Duncanson model
in metal-olefin bonding.30 Complex 1 gradually decomposes
above 65 °C releasing ethylene and forming a myriad of PNP-
based products.
Complexes 1−5 can be independently prepared via salt
metathesis reaction of [(PNP)TiCHtBu(OTf)] in Et2O with
the corresponding alkylating reagents LiCH2CH2R (R = H,
29,32
n
CH3, CH2CH3, Pr,
nBu) as shown in Scheme 3, therefore
suggesting that formation of these species most likely traverses
through the alkylidene-alkyl intermediate, [(PNP)TiCHtBu-
(CH2CH2R)]. Noteworthy, compound 2 is unstable even by
this synthetic method, for which reason complete NMR
spectroscopic assignment was not possible (vide infra).
Complex 2 is too unstable to explore its reactivity, but in the
case of 6 and 7, we resorted to exposing these systems to
ethylene to promote olefin exchange and also to form the more
stable ethylene species, 1. In fact, attempts to prepare 2 from
[(PNP)TiCHtBu(OTf)] and LiCH2CH2CH3 (generated in
situ from ICH2CH2CH3 and LitBu),31,32 at −78 °C, resulted in
some formation of the propene complex, although workup
could not avoid rapid decomposition. Gratifyingly, treating 6
and 7 with 1 atm of ethylene afforded 1, along with the
corresponding olefins 1-heptene and 1-octene, respectively. We
are presently unsure of why complex 2 is unstable (or whether
an impurity is promoting its decomposition), but in the case of
6 and 7, the bulkier (or more flexible) group on the olefin may
be kinetically destabilizing such species. However, we have
found theoretically that formation of the allyl containing
complex [(PNP)Ti(η3-CH2CHCH2)] is significantly exother-
mic (−17.1 kcal/mol) with respect to the propylene adduct 2.
Therefore, one possible mode for decomposition of the latter
complex might be the abstraction of the propylene methyl in 2
to form the allyl ligand and free neopentane. Theoretical
studies of [(PNP)Ti(η3-CH2CHCH2)] reveal this species to
adopt a square-planar geometry with the triplet ground state
lying ∼7 kcal/mol below the most stable singlet state, and thus,
making its detection difficult by standard NMR spectroscopic
techniques.31
t
When [(PNP)TiCHtBu(CH2 Bu)] is treated with propane
(120 psi) in cyclohexane, the propene complex [(PNP)Ti(η2-
t
H2CCHCH3)(CH2 Bu)] (2) is formed but not cleanly given
the observation of other titanium products including free
(PNP)H, when the mixture is gauged by 31P{1H} NMR
spectroscopy (Scheme 3). Unlike 1, complex 2 exists as a
mixture of diastereomers in a 1:3 ratio due the asymmetry of
the α-olefin, as well as the C2 symmetric nature of the PNP
Scheme 3. Dehydrogenation of Propane and Linear C4−C8
Alkanes by Transient A as well as Independent Preparation
of Complexes 2−7 and Olefin Exchange with Complexes 3−
7
Previously we reported that complex [(PNP)TiCHtBu-
t
(CH2 Bu)] decomposes to a myriad of products in C6H12 and
C
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that the rate of decay (pseudo-first-order on titanium) was
suggestive of A being formed in the course of the reaction (kavg
= 4.97(2) × 10−5 s−1, at 29.6 °C).27,29 However, when
atm of ethylene forms 1 and the free terminal olefin (Scheme
5).
t
compound [(PNP)TiCHtBu(CH2 Bu)] is allowed to decay
Scheme 5. Selective Dehydrogenation of Methylcyclohexane
and Trapping of the Ti2+ Proposed intermediate E with
Ethylene To Form 1
in C6H12 over 12 h and the volatiles are examined by GC-MS,31
we observe formation of cyclohexene exclusively, thus
suggesting that A must be dehydrogenating the solvent
(Scheme 4). We therefore propose that A is activating C6H12
Scheme 4. Proposed Pathway Involving the
Dehydrogenation of C6H12 by Transient A and Formation of
1 via the Reaction of Ethylene with Intermediates D or E
Computational Studies of the Titanium Olefin
Complexes. The formation of the titanium olefin complexes
2−7 formally follows a mechanism analogous to that of
dehydrogenation of ethane to ethylene.28 As Figure 1 shows for
butane, intermediate A and free alkane first form a σ-complex
(A-Buα) followed by the 1,2-addition of the terminal C−H
bond across the reactive TiC linkage (A-Bu-TSα) to give rise
to the alkyl-neopentylidene intermediate, B-Buα, which finally
allows the formation of the dehydrogenated olefin adduct 3
through a concerted metal-mediated β-hydrogen migration step
(B-TSα). In the cases of long-chain alkanes however, internal
C−H bonds are also potentially prone of activation, e.g.,
leading to β-olefin complexes. Figure 1 clearly reveals why
terminal olefin adducts are exclusively observed experimentally:
the initial activation of a β-C−H bond of butane is ∼6 kcal
mol−1 higher in energy than the activation of a terminal C−H
bond. Additionally, the olefin adduct 3β is less stable than 3 by
4.5 kcal mol−1. Moreover, according to our results, B-Buβ forms
through initial β-C−H activation and inevitably undergoes a
subsequent α-C−H cleavage resulting in compound 3, rather
than the γ-bound olefin adduct 3β, a result of the greater
stability exhibited by B-Bu-TSx over B-Bu-TSβ. In this light, the
study shows that formation of terminal alkenes is both
kinetically and thermodynamically more favorable than
formation of internal ones in the reaction of A with linear
alkanes.
We have calculated the structures of compounds 1−5 and
shown in Figure 2 is that for the parent olefin 1. Complex 1
adopts a quasi-trigonal bipyramidal geometry with the olefin
oriented perpendicular to the equatorial plane (dihedral angles
= 7.6°).31 The simplified Newman projection of this compound
(Figure 2) describes the orientation of the olefin as being
approximately parallel to the P−Ti−P vector. The latter also
indicates clearly how dissymmetry around the titanium center is
created as a result of the puckering of the PNP backbone. The
C−C distance composing the metallacyclopropane motif is 1.44
Å in 1, which is significantly elongated to that of free ethylene
(1.337(2) Å);33 comparable to early transition-metal ethylene
complexes including Bercaw’s (η5-Cp*)2Ti(η2-H2CCH2)
(1.438(5) Å),34 a series of related metallocene derivatives,35
and Rothwell’s (2,6-diphenylphenoxide)2Ti(η2-H2CCH2)-
(PMe3) (1.425(3) Å)36,37 ethylene complex (Table 1).
Likewise, the computed Ti−Cethylene distances of 2.13 and
2.14 Å are not isometric but are quite similar to the distances
observed in structurally characterized titanium ethylene
complexes.
to form a putative cyclohexyl intermediate, [(PNP)Ti
CHtBu(c-C6H11)] (C), which then undergoes β-hydrogen
abstraction to form the unstable cyclohexene adduct, [(PNP)-
t
Ti(CH2 Bu)(η2-c-C6H10)] (D) (Scheme 4). In fact, treating
[(PNP)TiCHtBu(OTf)] with Li(c-C6H11) also leads to
formation of cyclohexene and innumerable titanium-based
products within several hours, as gauged by 31P{1H} NMR
spectroscopy (Scheme 4). Given that the olefin is sterically
crowed (and most likely adopts a chair conformation),
intermediate D presumably undergoes elimination of cyclo-
hexene to form an unstable titanium(II) species, [(PNP)Ti-
t
(CH2 Bu)] (E), which then inexorably decomposes. To test
whether a species such as D or E was likely to form in the
aforementioned transformation, we examined the reaction of 1
in C6H12 in the presence of ethylene, since the latter substrate
reacts sluggishly with A but could rapidly trap the electron-rich
putative complex E, or exchange with D. Indeed, dissolving
t
[(PNP)TiCHtBu(CH2 Bu)] in C6H12 under a headspace of
ethylene (1 atm) produced complex 1 along with cyclohexene
over 12 h (observed by GC-MS and 1H NMR spectroscopy).31
When 1 atm of ethylene is used, some minor decomposition
products were observed by 31P{1H} NMR spectroscopy,
suggesting that ethylene does not entirely trap the titanium(II)
species E or cleanly exchanges with D (Scheme 4). Exposing
complex [(PNP)TiCHtBu(CH2 Bu)] to high-pressure ethyl-
t
ene (500 psi) in C6H12 using a reactor vessel31 still results in
some formation of 1 but accompanied by another titanium(IV)
complex that has eluded characterization.
Treatment of [(PNP)TiCHtBu(CH2 Bu)] with methyl-
t
cyclohexane yields methylenecyclohexane as the major product,
confirmed by GC-MS analysis of the crude reaction mixture
after 12 h. Also, akin to cyclohexane, exposing a methyl-
cyclohexane solution of [(PNP)TiCHtBu(CH2 Bu)] to 1
t
D
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Figure 1. Computed reaction profiles for the dehydrogenation of butane with intermediate A leading to terminal and internal olefin complexes.
Transition states are shown above the reaction coordinate in brackets.
interactions assuming C2v symmetry where the d-orbitals
2
2
transform to b1 (dxz), a2 (dyz), a1 (dx −y ), b2 (dxy) and a1
38
2
(dz ), as discussed by Burdett and co-workers. Since the
electron-donating π-orbital of the ethylene fragment has no
orientation selectivity when interacting with the metal d-
orbitals, it is only the π*-orbital that determines the orientation
of the olefin with respect to the metal containing fragment.
This π*-orbital can either interact with the low-lying b1 to make
the ethylene fragment align along the axial phosphine ligands
(eq∥, along z, left in Figure 3) or with the b2 orbital to confine
the ethylene perpendicular to the z-axis (eq⊥, along y, right in
Figure 3). The latter interaction is significantly more advanta-
geous than the former, due to the better energy match of π*
orbital of ethylene with the hybridized dxy + py titanium
orbitals.38 Note that in the case of a d2 metal ion like Ti2+, only
b1 is filled and accordingly, the stabilization of this orbital in the
eq∥ arrangement (ΔE∥) is directly manifested in the
stabilization of the complex, whereas the eq⊥ arrangement is
only stabilized by as much as the energy of b2 dropping below
b1 (ΔE⊥) as a result of the interaction with the π* of the
ethylene. Because of this, the eq⊥ type of orientation is more
Figure 2. (Left) Optimized geometry of complex 1. Isopropyl groups
and PNP aryl peripherals have been omitted for clarity. (Right)
Newman projection of 1 (looking down the Ti−N bond, also with the
aryl framework of the PNP backbone omitted for clarity), depicting
the orientation of the olefin and pendant neopentyl group in relation
to the PNP pincer ligand.
The analysis of the electronic structure of 1 clearly reveals
why the investigated titanium olefin product adopts a trigonal
bipyramidal geometry with the olefin being parallel to the P−
Ti−P vector (Figure 2) and not a quasi-octahedral geometry
with C1−C2 perpendicular to the P−Ti−P axis. Figure 3 shows
these two plausible arrangements and the relevant metal-olefin
a
1
Table 1. H and 13C{1H} NMR Spectroscopic Comparisons of Titanium(η2-ethylene) Compounds
1
compound
1H NMR δ (m)
13C{1H} NMR δ (m, JCH
)
CC (Å)
ref
free ethylene
Cp*2Ti(ethylene)
5.25
2.02 (s)
122.96 (s)
1
1.337(2)
1.438(5)
1.425(3)
1.442(9)
1.427(5)
1.454(9)
1.446(4)
33
34
105.1 (t, JCH = 143.6 Hz)
78.0 (1JCH = 147.6 Hz), 67.0 (1JCH = 149.6 Hz)
(ArO)2Ti(PMe3)(ethylene)
(C5Me4SiMe3)2Ti(ethylene)
Cp′2Ti(ethylene)
1.72 (td), 0.51 (t)
2.34 (s)
36, 54
35a
104.3 (s)
97.8 (s)
2.94 (s)
35b
35c
t
(C5Me4 Bu)2Ti(ethylene)
2.08 (s)
101.8 (s)
102.7 (s)
(C5Me4H)2Ti(ethylene)
2.03 (s)
35c
b
1
2.3 (br m), 1.98 (br m), 1.91 (br m), 73.2 (1JCHa = 153 Hz, 1JCHb = 147 Hz), 67.2 (1JCHa = 150 Hz,
1.44
28 and this
work
1JCHb = 150 Hz)
0.93(br m)
a
b
Cp* = C5Me5; ArO = 2,6-diphenylphenoxide; Cp′ = [(1,3-(tBu)2C5H3)], NMR spectroscopic data reported in benzene-d6. Distance based on
DFT calculations.
E
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Scheme 6. Axial Chirality Gives Rise to Four Chemically
Inequivalent Positions of the Alkyl Chain (right) for
Complexes 2−7, Whereby C2 is a Stereogenic Center
more orientations of the neopentyl group, a total of eight
different diastereomers is possible, each with its corresponding
enantiomer (16 combinations in total). Scheme 7 depicts the
Newman projection for these eight possible diastereomers
based on the orientation of the aryl groups (with the aryl
backbone removed for clarity), the neopentyl group and the
location of R group on the olefin. As a point of reference, the
first syn or anti abbreviation refers to the orientation of the
neopentyl group with respect to the olefin, R or S represents
the configuration generated by the twisted aryl moieties, while
the second syn or anti abbreviations represent the orientation of
the R group of the olefin with respect to the neopentyl ligand.
To scrutinize the effect of R and how this affects the energy
of the conformers we calculated the solvent phase free energies
of the possible isomers with various substituents (R = H, CH3,
31
n
n
n
CH2CH3, Pr, Bu, Pentyl). The relative free energies of the
various conformers as a function of R, respective to A and free
alkane, are given in Figure 4. Note that although the energy
difference between structures is only a few tenths of kcal mol−1
in some cases (which is beyond the error of our computational
protocol), a few chemically meaningful trends can be inferred
from Figure 4. For example, it is easy to observe that shifting
from ethylene to longer alkenes, the olefin complex becomes
less stable by about 5 kcal mol−1. Also, in the case of 1 (R = H),
the energy difference between the syn and anti isomers,
denoting the orientation of the neopentyl group with respect to
the olefin (syn-1 versus anti-1) is about 2 kcal mol−1, preferring
the syn arrangement. This difference diminishes if the R group
of the olefin also points into the syn position. Both of these
trends can be explained by the steric repulsion of the bulky
alkyl groups on the PNP and the bound olefins, the effects of
which are greater with increasing length of the R group on the
latter, i.e., going from ethylene to longer alkenes, the R group
Figure 3. Simplified MO diagram for a π-acceptor ethylene ligand
coordinated to the titanium(II) framework (PNP)Ti(CH2 Bu) in
either an eq∥ or eq⊥ fashion.
t
commonly observed for dx configurations where x ≥ 6, because
one would then stabilize the populated b2 orbital as in the case
of classical molecule Fe(CO)4(η2-H2CCH2).39 Nevertheless,
our data clearly converge to the eq∥ state being 23.7 kcal mol−1
more stable than the eq⊥ state, as indicated in Figure 3.40,41
Moreover, the eq⊥ structure was found to be a transition state
corresponding to the rotation of the ethylene fragment,
allowing its slow rotation in 1, at room temperature by
traversing a barrier of ∼24 kcal mol−1.31
In line with computations, in the case of complex 1, there is
no spectroscopic evidence for the formation of isomers even at
lower temperatures. However, when propane and linear alkanes
C4−C8 are dehydrogenated by A to form 2−7, two titanium-
containing complexes are clearly observed spectroscopically by
31P{1H} and 13C{1H} NMR (vide infra). For each alkane, we
propose these two resulting species to be a mixture of
diastereomers which arise from different orientations of the
neopentyl ligand and the R group of the olefin relative to the
PNP backbone. Since the PNP pincer ligand is akin to C2-
symmetric, chiral phosphine ligands such as DuPhos42 or
BINAP,43 which are commonly used in asymmetric catalysis, it
is not surprising that α-olefin adducts would give rise to
stereoisomers. These isomers, having axial chirality,44 can adopt
either Ra or Sa configurations defined by the handedness of the
axis that relates them. Scheme 6 shows these orientations for
PNP in a five-coordinate complex where the other two
equatorial ligands, R1 and R2, are inequivalent. The two
orientations Ra and Sa shown in Scheme 6 are enantiomers and
thus are not discernible by standard NMR spectroscopic
techniques. Since there are two orientations of the skewed aryl
groups of PNP, along with two possible orientations of the
bound olefin (both oriented along the P−Ti−P axis) and two
i
collides with the Pr groups of the PNP backbone resulting in
an overall destabilization of the complexes. Moreover,
whenever the neopentyl substituent and the R group of the
olefin occupy the syn positions (syn-R-syn or syn-S-syn), they
clash into each other, further abating their stability.
n
Only three stable isomers were calculated for R = Bu and
nPentyl, due to the high computational demand of these
structures. However, the trend observed for these species fits
nicely with that of shorter alkenes. Thus, based on the relative
energies of the conformers, we propose that the experimentally
detected two-product species most likely correspond to the syn-
R-anti and anti-S-syn arrangements. Then again, it is easy to
judge why the R group of the olefin, the bulky iPr groups of the
PNP backbone, and the neopentyl substituent elicit the most
advantageous arrangements in these two isomers, minimizing
the overall steric repulsion and resulting in the more stable
structures. It is also important to note that the relationship of
the conformers is probably even more complex than illustrated
here, since interconversion might take place via the rotation of
F
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n
n
n
Scheme 7. Computed Diastereomers for the α-Olefin Complexes of Titanium Where R = CH3, CH2CH3, Pr, Bu, Pentyl
1 exhibits two doublets at 28.06 and 26.05 ppm with 2JPP = 22.2
1
Hz (Figure S5), while the H NMR spectrum, along with the
1
selective- and fully decoupled 1H{31P} NMR, and H−31P
HMBC spectra (Figures S6−S12) clearly assigns the two
diastereotopic methylene protons for the neopentyl fragment at
1.28 (d, 2JHH = 11.5 Hz) and 0.021 ppm (dt, 2JHH = 12.0, 3JHP
=
2.6 Hz). Of these, the more shielded resonance exhibits
coupling to both the geminal proton and the two phosphorus
atoms in the PNP backbone giving rise to a doublet of triplets,
whereas the deshielded resonance at 1.28 ppm exhibits solely
geminal proton coupling and resolves only into a doublet, albeit
overlapping with the isopropyl-methyl resonances of the PNP.
The fact that complex 1 could not be obtained as single crystals
led us to fully investigate its structural features in solution with
the aid of numerous NMR spectroscopic experiments. In
addition to 1D (1H, H{31P}, 13C{1H}) NMR spectra, the
1
Figure 4. Relative stabilities (ΔGsol) of the possible conformers of the
complete connectivity of 1 was elucidated by a battery of state-
of-the-art 2D NMR techniques including double quantum
filtered (dqf) COSY, DEPT-135, and C−H coupled/C−H
t
olefin complexes [(PNP)Ti(η2-H2CCHR)(CH2 Bu)], in kcal mol−1,
n
n
n
as a function of the R group (H, CH3, CH2CH3, Pr, Bu, Pentyl).
Stabilities are given respective to A and free alkane.
1
decoupled gradient H−13C HSQC experiments. Each hydro-
gen in the ethylene unit of complex 1 is inequivalent (1H NMR:
2.3, 1.98, 1.91, 0.93 ppm) and should thus be diastereotopic.
Unfortunately these resonances are broad and featureless in the
1H NMR spectrum and are often difficult to assign due to
overlap with isopropyl resonances from the PNP. Gratifyingly,
the dqfCOSY spectrum (Figure 5, left) clearly discerns the
the olefin fragment. Such rotation has been calculated to have
an activation barrier of about 20−23 kcal mol−1 with R = CH3
and CH2CH3, depending on the position of the R group.
NMR Spectroscopic Characterization of the Titanium
Olefin Complexes. The 31P{1H} NMR spectrum of complex
Figure 5. Expanded absolute-phase dqfCOSY (left) and C−H coupled 1H−13C gHSQC (right) NMR spectra of complex 1. Numeric assignments of
t
the olefin hydrogen or carbons are arbitrary. Spectra have been cropped (the large Bu resonance in the 1H NMR has been cropped). *Represents
1
residual solvent in the spectrum. Numbers in parentheses for the right spectrum represent chemical shifts for the H and 13C spectra.
G
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Table 2. Summarized Key NMR Spectroscopic Data for Olefin Compounds 1−7
a
b
c
1
2
3
1
Compound decomposes very rapidly. No 13C{1H} NMR could be recorded. JHH and JHP measured from the H NMR spectra. JCH values
d
measured from C−H coupled H−13C gHSQC experiments. Only chemical shifts of the major diastereomer are listed.
1
cross-correlation resonances that arise from the three spin
systems: the correlation between the diasterotopic protons in
the Ti-CH2 Bu moiety and the cross-peaks due to the
parameters for the few known titanium-ethylene complexes are
listed in Table 1.
t
Lastly, we wanted to gather information regarding the spatial
orientation of the neopentyl and ethylene fragments with
respect to the phosphines of the PNP ligand. Accordingly, we
resonances of the four protons in the ethylene ligand of 1. As
t
with the diastereotopic protons on Ti-CH2 Bu (red trace), the
1
collected a H−31P HOESY spectrum of 1 (Figure 6), which
protons on the bound ethylene fragment are nonfluxional and
also diastereotopic (blue trace), with each of these protons
giving individual resonances which is consistent with the
olefinic carbons being locked in a distorted geometry around
titanium (Figure 5, left). The C−H decoupled 1H−13C gHSQC
spectrum of 1 further corroborates the above conclusion with
the individual resonances for the diastereotopic protons on the
methylene carbons appearing clearly resolved (Figure S15).
However, some of the most informative NMR spectroscopic
1
data were extracted from a C−H coupled H−13C gHSQC
experiment (Figure 5, right). In addition to being able to clearly
assign the CH2 groups in the 13C-DEPT trace (shown as
negative resonances), this experiment allowed us to precisely
1
extract the magnitudes of the JCH coupling constants for the
t
Ti-CH2 Bu (∼99 Hz) and the H2CCH2 (∼150 Hz) moieties
on complex 1 (Tables 1 and 2). Noteworthy, the magnitudes of
the coupling constants of the diastereotopic methylene protons
t
1
on Ti-CH2 Bu are unusually small respective to typical JCH
values of unpolarized alkane bonds, H-CHRR′ (R = H, Me; R′
1
= Me), which range from ∼119 to 125 Hz, whereas the JCH
Figure 6. Expansion of the 1H−31P HOESY (400 MHz, 25 °C)
spectrum of complex 1, highlighting in a red circle the spatial
correlation between the more shielded resonance of the Ti-CH2 Bu
moiety (0.02 ppm) and the PNP-phosphorus resonance at 26.05 ppm.
A dotted line in the scheme indicates our rationale for this spatial
correlation.
values of the bound H2CCH2 are only slightly smaller from
the ones reported for free ethylene (156.2 Hz).33a The low 1JCH
value for the neopentyl-methylene protons is a strong evidence
of a large electropositive character inflicted by the titanium
center, with the magnitude of these constants being comparable
to that observed in methyllithium (1JCH = 98 Hz).45 The fact
that the values of the 1JCH constants of the ethylene moiety in 1
are affected to a much lesser extent by the titanium center
tantalizingly implies little distortion of the s-character on the
sp2 hybridized carbons of the bound olefin, which could in turn
suggest weak back-donation to form a metallacyclopropane
t
revealed that the more shielded diastereotopic proton on the
t
Ti−CH2 Bu ligand (0.02 ppm) clearly correlates through space
to only one phosphorus resonance at 26.05 ppm. This result
suggests that the corresponding proton is somewhat aligned
with one phosphorus and/or points in that direction.
Conversely, the fact that the other methylene proton in Ti-
1
resonance, 1a (Scheme 2, vide supra). Comparison of the H
and 13C{1H} NMR spectroscopic data and crystallographic
CH2 Bu gives no tangible correlation to either phosphorus
t
H
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1
Figure 7. Expanded C−H coupled H−13C gHSQC NMR spectrum of complex 3 (left). Right is the C−H decoupled gHSQC NMR spectrum of
complex 5. Spectra have been cropped (the large tBu resonance in the 1H NMR spectrum has been cropped). *Represents residual solvent. Numbers
1
in parentheses represent chemical shifts for the H and 13C spectra.
resonance suggests that it is locked in a geometrical
conformation pointing away from both phosphorus atoms. As
a result, it is quite possible that the neopentyl group in 1 is not
oriented strictly in the forward position or that the P−Ti−P
angle is highly distorted due to the Ti−P distances not being
diastereomers are not interconverting with each other (at least
not in the temperature range tested). Akin to 1, the H NMR
1
spectrum of 3 reveals a shielded doublet of triplets at 0.59 ppm
3
(2JHH = 11.6, JHP = 2.8 Hz), consistent with a diastereotopic
methylenic proton that is simultaneously coupled to both
phosphorus atoms in the PNP backbone and the other geminal
1
isometric and far from a transoid orientation. From the H,
t
13C{1H}, and 13P{1H} NMR spectroscopic data it can be
generalized that all hydrogens on the ethylene ligand are
proton in the Ti-CH2 Bu moiety. Unlike 1 however, the
chemical shift of the latter resonance is considerably obscured
by peak overlap with the PNP ligand and the aliphatic moiety
of the bound olefin.31
1
shielded when compared to free ethylene. In addition, JCH
values fall in the 143−153 Hz range. Overall, NMR
spectroscopic data of Rothwell’s complex (ArO)2Ti(η2-H2C
CH2)(PMe3) (ArO− = 2,6-diphenylphenoxide)36,37 are similar
to that observed for 1, presumably due to these systems being
We relied on C−H decoupled and coupled (Figure 7)
1H−13C gHSQC spectra to fully assign the connectivity around
1
the titanium centers as well as accurately measure the JCH
t
coupling constants derived from the Ti-CH2 Bu and Ti(η2-
1
more electronically unsaturated. Salient H and 13C{1H} NMR
H2CCHCH2CH3) core fragments and the pendant aliphatic
spectroscopic data for complex 1 are included in Table 2.
Since complex 2 is unstable in solution over several hours, we
1
chain, Ti(η2-H2CCHCH2CH3). These JC−H coupling
1
provide only 1D H and 31P{1H} NMR spectra (Table 2). For
constants are listed in Table 2. In Figure 7 (left) it can be
observed how the protons on all CH2 moieties of 3 are
diastereotopic. The contours for the methylene protons on Ti-
this reason, we focus our attention on complexes 3−5 since
these systems are much more stable and should not
significantly differ from 2. The NMR spectroscopic character-
ization of the α-olefin complexes 3−5 follows a similar trend to
that for 1, with the exception that the two isomers can be
observed by both 31P{1H} and 13C{1H} NMR spectroscopy.
t
CH2 Bu appear as distinct blue, negative contours, at virtually
the same chemical shifts as the ones observed in compound 1.
The methylene carbon exhibits a resonance at ∼110 ppm (C−
1
H decoupled H−13C gHSQC; Figure 7, left). In addition, the
1
The H NMR spectra do not reveal the presence of isomers,
negative contours derived from the protons on the Ti(η2-
H2CCHCH2CH3) moiety are aligned with the carbon
resonance of this fragment at 78.9 ppm, similar to the
respective one associated to Ti(η2-H2CCH2) in 1, at 73
ppm. Unlike 1, the contour for the vinylic C−H proton in
Ti(η2-H2CCHCH2CH3) is considerably deshielded with its
carbon resonance located as a positive peak at 86.7 ppm on the
DEPT-135 trace shown in red (labeled 5) in Figure 7, left. Such
unique pattern is also observed for the longer α-olefin adducts,
4 and 5 (Figures S34 and S46). In terms of the magnitude of
suggesting these to have coincidental chemical shifts (as in the
case of 2). Compound 2 can only be observed spectroscopically
as a mixture of two isomers by 31P{1H} NMR due to rapid
decomposition (Table 2). However, complexes 3−5 are
relatively stable and have been unambiguously established by
solution NMR spectroscopy. Table 2 reports the most
important spectroscopic features for all these species, including
the longer and more unstable olefins 6 and 7. For comparative
purposes we will focus our attention on complex 3.
The 31P{1H} NMR spectrum of complex 3 clearly reveals a
ratio of two diastereomers of ∼5:1 with the major diastereomer
the 1JCH coupling constant on the Ti-CH2 Bu and Ti(η2-H2C
t
CHCH2CH3) core fragments in 3, these values are in exact line
with the ones determined for such fragment in compound 1
2
exhibiting resonances at 27.1 (d, JPP = 21 Hz) and 21.1 ppm
1
(d, 2JPP = 21 Hz). The minor diastereomer is observed at 26.64
(Table 2). Notably, the JCH constant for the allylic CH group
2
2
(d, JPP = 22.2 Hz) and 24.82 ppm (d, JPP = 22.2 Hz). The
relative ratio of these two species does not change upon heating
(>50 °C) of the solution, and this is also the case with all the
higher olefin adducts (4 and 5). This indicates that the
has a slightly smaller magnitude of 134 Hz, likely indicative of a
more polarized fragment. The magnitudes for the diastereo-
topic methylene protons in the −CH2CH3 substituent of the
bound butene in 3 are all within normal values for sp3
I
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Table 3. Summary of Alkane Dehydrogenation Kinetics
a
b
entry
substrate
T (°C)
conv. (%)
k × 10−5 (s−1
)
σ × 10−5
t1/2 (h)
KIE
1
2
pentane
pentane-d12
hexane
29.7
29.8
29.7
29.6
29.65
29.7
29.6
28.9
29.7
29.9
90
90
83
79
82
74
89
88
84
88
6.0
5.6
0.5
0.5
3.2
3.5
3.5
3.8
3.9
4.9
3.9
4.6
3.9
3.4
1.1
3
5.48
5.115
5.0
0.01
0.005
0.6
4
pentane:hexane (1:1)
heptane
5
6
octane
4.0
0.7
7
cyclohexane
cyclohexane-d12
methylcyclohexane
1-hexene
4.97
4.2
0.02
0.1
1.2
8
9
5.0
0.2
10
5.7
0.4
a
b
All reactions were performed in duplicates. KIE = kH/kD
1
Figure 8. Overlaid expansions of the H NMR spectra (400 MHz, 25 °C) of the partially deuterated (bottom) and fully protiated (top) pentene
adducts 4-d12 and 4, respectively.
hybridized carbons and in the range of 118 to 122 Hz. In this
groups in addition to the neopentyl ligand. In Figure 7 (right),
the C−H decoupled 1H−13C gHSQC NMR spectrum of
complex 5 exposes the analogous pattern to complex 3 for
hydrogens numbered 1−5, the olefinic carbons b and c, and the
allylic carbon d. Likewise, the minor diastereomer is also
observed in the vertical DEPT-135 NMR trace.
context, the fact that the bound olefin exhibits both stereogenic
1
carbon centers and distinct values for the JCH coupling
constants for the allylic groups in H2CCHCH2CH3 is
indicative of a highly rigid metallacyclopropane moiety as
proposed for resonance 1a with considerably greater degree of
covalency between the titanium center and the olefin. As the
alkyl chain becomes longer, the chemical shifts of the more
distal methylene groups become essentially equivalent in the 1H
NMR spectrum, hence no longer being diastereotopic. In fact,
the diastereotopic character of the methylenic protons on the
bound α-olefin decreases sharply after the δ-carbon, with the
protons of this and the ε-carbon of the hexene adduct 5 already
displaying nearly single-type contours indicative of their
equivalence (Figure 7, right). The C−H decoupled and
Mechanistic Studies Involving the Dehydrogenation
of Alkanes by Transient A. Complex [(PNP)TiCHtBu-
t
(CH2 Bu)] decays cleanly in neat solutions of n-pentane, n-
hexane, n-heptane, and n-octane with nearly identical rates
(Table 3). These pseudo-first-order decay rates are very similar
to those measured for reactions with benzene (6.5(4) × 10−5
s−1 at 27 °C),27 methane (7.9 × 10−5 s−1 at 31 °C, 1150 psi),10c
and cyclohexane (5.86 × 10−5 s−1 at 31 °C).10c Therefore,
formation of intermediate A is most likely the slowest step in
the conversion of alkanes C2−C8 to the corresponding titanium
olefin adducts. In fact, the rates for decay of [(PNP)Ti
1
coupled H−13C gHSQC spectra for 4 and 5 clearly portray
these subtle differences as a function of alkyl chain growth
t
(Figures S33, S34, S45, and S46). The 13C{1H} NMR spectra
CHtBu(CH2 Bu)] are similar in cyclohexane versus methyl-
1
of 3−5 in the H−13C gHSQC experiment clearly show the
cyclohexane, and mixtures of alkanes (hexane/pentane) did not
result in unequal formation of the olefin complexes. To test if
other steps such as 1,2-CH bond addition or β-hydrogen
abstraction were competitive with α-hydrogen abstraction in
other diastereomer present in solution (what appear as small
impurities next to each 13C resonance), and the DEPT trace
reveals this species to contain the same set of vinylic and allylic
J
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[(PNP)TiCHtBu(CH2 Bu)] to A, we examined the rate of
ethylene to form B (Figure 8). Rothwell reported reversible
homo coupling of ethylene with (ArO)2Ti(η2-H2CCH2)-
(PMe3) to form the metallacyclopentane species (ArO)2Ti-
(H2CCH2CH2CH2).36,37 For our case, we propose olefin
exchange in 1 to occur via a dissociative mechanism and
theoretical studies suggest the S = 1 intermediate E to be only
4.6 kcal/mol higher in energy than the ethylene adduct. The
results of this study have been published elsewhere.29
t
decay of this precursor in pentane-d12 and found that the KIE
was close to unity (1.1 at ∼29 °C, Table 3).31 In addition, H
1
2
and H NMR spectra confirm formation of the isotopologue
[(PNP)Ti(η2-D2CCDCD2CD2CD3)(CD2 Bu)] (4-d12),
t
where complete deuteration of the alkylidyne carbon in
transient A has occurred. Figure 8 depicts the 1H NMR
spectrum of 4 overlaid with 4-d12 thus showing full deuteration
of the neopentyl (labels 1 and 2 in blue) and olefin residues
(labels for the methylenes 3, 4, and 6−9 (all in blue) as well as
the vinylic 5 (red) and terminal CH3, 10, in red). This result
indicates that transfer of the hydrogens must be occurring at
the alkylidyne moiety of A. In a sense, the conversion of an
alkane to the olefin in our system involves an intramolecular
transfer dehydrogenation pathway, whereby the alkylidyne
ligand is the hydrogen acceptor.
Two-Electron Oxidation Reactions of the Titanium
Olefin Complexes. Although 1 and 3−7 can sluggishly
extrude the olefin above 50 °C, no titanium product(s) could
be isolated. For this reason, we explored two-electron oxidants
that could promote not only elimination of the olefin but also
form a stable titanium byproduct. Based on studies by Bergman
and Andersen involving the reactivity of (η5-Cp*)2Ti(η2-C2H4)
with various oxidants to form TiNR,48 TiO,49 TiCR2,50
and TiS51 functionalities and free ethylene, we decided to
explore similar reactivity with our titanium olefin complexes.
These reactions would also provide an indirect method to
characterize the olefin moiety as well as the site of
dehydrogenation in the case of compounds 3−7. As noted
previously,28 exposing 1 to a bed of N2O in C6H6 or C6D6
immediately lead to a color change from brown to wine-red,
Exposure of 4-d12 to 1 atm of ethylene (slight excess) slowly
formed [(PNP)Ti(η2-H2CCH2)(CD2 Bu)] (1-d2) and the
t
terminal olefin, 1-pentene-d10.46 Likewise, addition of ethylene
to a solution of [(PNP)TiCHtBu(CH2 Bu)] in C6D12 also
t
formed 1-d2 and C6D10. Comparison of the rates of decay
(pseudo-first order) of 1 in C6H12 versus C6D12 yielded a KIE
of 1.2 at 29 °C (Table 3). Therefore, all our data suggest that
t
[(PNP)TiCHtBu(CH2 Bu)] forms A slowly, while 1,2-CH
t
concurrent with formation of [(PNP)TiO(CH2 Bu)] (8)
and free ethylene (Scheme 9). As shown in Scheme 9,
bond addition of the alkane to form the alkylidene-alkyl,
[(PNP)TiCHtBu(CH2CHR)], as well as β-hydrogen ab-
straction to form the olefin-bound diastereomers, [(PNP)Ti-
Scheme 9. Oxidation of 1 with N2O, N3R (R = 1-adamantyl
a
t
(η2-H2CCHR)(CH2 Bu)], are all post-rate determining
or SiMe3), 2 OCPh2, and N2Ctolyl2
steps.
To establish whether complex 1 liberates the olefin, we
treated this compound with a slight excess of D2CCD2 (1
atm) in C6H6 or C6D6. Over 72 h at 25 °C, complex 1 does
exchange with D2CCD2 to form [(PNP)Ti(η2-D2C
t
CD2)(CH2 Bu)] (1-d4) and free H2CCH2 but does so not
cleanly giving the formation of another titanium product which
we have been unable to characterize. In addition, we also see
slow incorporation of deuterium in the neopentyl ligand, thus
implying that β-hydrogen abstraction might be a reversible
process and that 1 and intermediate B could equilibrate slowly
(Scheme 8). Because the reaction between 1 and D2CCD2 is
not clean, we cannot exclude other processes by which the
deuterium becomes incorporated in the neopentyl ligand of 1,
such as α-elimination47 in E to form an alkylidene-hydride
(PNP)TiCHtBu(H) followed by migratory insertion of
a
Oxidation of 3−7 with N2O to release the terminal olefin are also
illustrated.
compound 8 has been previously reported and is unstable over
several hours in solution, at room temperature. However,
complex 8 can be trapped with B(C6F5)3 to form the stable
Scheme 8. Exchange of Ethylene-d4 with 1 via Dissociation
of Ethylene and Tautomerization or by α-Elimination and
Insertion of the Olefin
t
zwitterion [(PNP)Ti{OB(C6F5)3}(CH2 Bu)] (9) (Scheme 9),
which has been characterized by 1H and 31P{1H} NMR
spectroscopy, in addition to a solid-state crystal structure.31,40,52
Treating 1 with N3R′ also leads to ethylene formation along
t
with the stable titanium imidos [(PNP)TiNR′(CH2 Bu)] (R′
= 1-adamantyl, 10; SiMe3, 11). Compounds 8, 10, and 11 have
been structurally characterized and were discussed in an earlier
communication.28 Likewise, addition of N2Ctolyl2 at low
temperatures (−110 °C) leads to clean formation of the
t
diazoalkane [(PNP)TiNNCtolyl2(CH2 Bu)] (12) in 74%
yield, along with free ethylene (Scheme 9). The reaction must
be performed at low temperature, otherwise N2Ctolyl2 further
reacts with 12 to give other titanium species which will be
omitted from this discussion. Unlike metallocene-diazoalkane
complexes,51,53 complex 12 does not extrude N2 to form
alkylidene intermediates, nor do we have any spectroscopic
K
dx.doi.org/10.1021/ja4060178 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
evidence for the N2Ctolyl2 ligand engaging in a side-on fashion
with the titanium center. For example, the chemical shift of one
diastereotopic TiCH2 Bu proton, δ = 1.36 ppm, does not shift
downfield as it does with another titanium species having side-
on ligands.31 To more accurately compare the reactivity of 1
with other titanium ethylene complexes, we turned our
attention to benzophenone since this substrate is known to
produce 3-, 5-, and even 7-metallacycles.37 Accordingly,
are exposed to oxidants such N2O, N3R, and N2Ctolyl2.
Complex [(PNP)Ti(η2-H2CCH2)(CH2 Bu)] also reacts with
t
t
two equiv of benzophenone to afford the pinacol coupled
product, concomitant with release of ethylene. Cyclohexane
and methylcyclohexane can be also dehydrogenated by A, at
room temperature, to cyclohexene and methylenecyclohexane
respectively, and the putative Ti(II) species formed from
decoordination of the olefin can be trapped with ethylene.
Kinetic and isotopic labeling studies suggest the dehydrogen-
ation steps not to be the rate-determining while reactions using
D2CCD2 reveal exchange of the olefin in [(PNP)Ti(η2-
exposure of 1 with 1 equiv of benzophenone results in 50%
formation of complex (PNP)Ti(CH2 Bu)(TPP) (13) (TPP2−
=
t
tetraphenylpinacolato) and ethylene along with titanium
starting material, as a consequence of coupling of two
benzophenone molecules. In addition to NMR spectroscopic
data, the connectivity of 13 was elucidated by X-ray diffraction
(using a poorly diffracting single crystal), confirming the
formation of the pinacolato framework.31,40,54 Addition of 2
equiv of the ketone forms 13 in better yield (∼80% isolated
yield, Scheme 9). The reactivity of 1 toward benzophenone
differs completely from Rothwell’s (ArO)2Ti(η2-H2CCH2)-
(PMe3) complex with two equiv of OCPh2 to release PMe3 and
t
H2CCH2)(CH2 Bu)], in addition to a scrambling phenom-
enon involving the olefin and the α-hydrogens of the neopentyl
group. The fact that the alkylidyne precursor [(PNP)Ti
CHtBu(CH2 Bu)] reacts sluggishly with ethylene suggests that
t
dehydrogenation of the alkane to olefin is preferred over
dehydrogenation of an alkene to an alkyne or activation of the
more thermodynamically vulnerable allylic C−H bonds.
Preliminary studies have revealed that when [(PNP)Ti
CHtBu(CH2 Bu)] is treated with 1-hexene for 12 h, followed by
t
f o r m
a
s e v e n - m e m b e r e d r i n g i n ( A r O ) 2 T i -
quenching of the reaction with N2O, some formation of 1,5-
hexadiene is observed in addition to some other hydrocarbon
products.31 This result, although premature, suggests that A is
somewhat selective toward the dehydrogenation of alkanes and
that a linear alkane can be converted, stepwise and stoichio-
metrically, to the corresponding terminal diene. This trans-
formation holds great promise since one could envision the
direct conversion of a hydrocarbon such as butane to butadiene
or transforming a branched alkane such as 2-methylbutane to
isoprene. Both of these dienes are heavily used in the
production of synthetic or natural rubber, respectively.
(OCPh2CH2CH2CPh2O).37 Unlike Cp*2Ti(η2-H2CCH2)
or (ArO)2Ti(η2-H2CCH2)(PMe3), which do not release
free ethylene in solution, the mechanism leading to formation
of compounds 8−13 might not involve an insertion pathway.
This is especially true since compound 1 slowly releases
ethylene and is more coordinatively saturated. However, the
possibility of insertion chemistry involving the ethylene ligand
in 1 cannot be ruled out since the labile phosphine arms in
PNP could be dissociating in these reactions.
We also explored the reactivity of the α-olefin complexes 3−
7. In all cases, addition of a 1 atm of N2O to these compounds
elicited rapid formation of complex 8 and corresponding free
olefin (Scheme 9). Only formation of the linear, terminal olefin
C4−C8 was observed based on 1H NMR spectroscopy and GC-
MS. Reactions with N2O are quantitative and cleaner than
olefin exchange reactions with ethylene (vide supra), Scheme 3.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic information (CIF), isotopic and kinetic
studies, kinetic analysis, spectral data, reactions, and computa-
tional studies. This material is available free of charge via the
Internet at http://pubs.acs.org.
CONCLUSIONS
■
AUTHOR INFORMATION
Corresponding Author
mindiola@indiana.edu and mindiola@sas.upenn.edu
In this work we have studied the reactivity of transient A with
the volatile alkanes C2−C8. In all cases dehydrogenation takes
place to afford species of the type [(PNP)Ti(η2-H2C
■
t
n
n
n
CHR)(CH2 Bu)] (R = H, CH3, CH2CH3, Pr, Bu, Pentyl,
Present Address
n
and Hexyl). When C4−C8 alkanes are dehydrogenated, only
†Department of Chemistry, University of Pennsylvania, 231
South 34th Street, Philadelphia, Pennsylvania 19104-6323.
the α-olefins are observed. The ethylene complex is the most
stable form, and in the case of R = Me, nPentyl, and nHexyl, the
olefin adducts are too unstable to fully characterize. The
dehydrogenation of alkanes by A takes place through a two-step
mechanism starting with a 1,2-addition of the terminal C−H
bond to the Ti≡C functionality followed by a metal-mediated
β-hydrogen migration. DFT studies have revealed that the
exclusive formation of α-olefins has kinetic and thermodynamic
origins. A combination of theory and multidimensional
spectroscopic data suggests the olefin in [(PNP)Ti(η2-H2C
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support of this research was provided by the National
Science Foundation (CHE-0848248 and CHE-1152123).
M.G.C. acknowledges CONACYT for a postdoctoral fellow-
ship. The authors thank Prof. Allen Siedle and Prof. Mu-Hyun
Baik for insightful discussions. J.-I.I. acknowledges financial
support from the JSPS (Japan Society for the Promotion of
Science).
t
CHR)(CH2 Bu)] to be oriented along the P−Ti−P vector,
where the neopentyl group points opposite the aryl groups of
the PNP ligand. This characteristic feature was rationalized
within the MO framework with the interaction of the π* of the
olefin with the corresponding metal d-orbitals. A product
distribution was proposed based on the relative stabilities of the
possible conformers. Compound [(PNP)Ti(η2-H2CCHR)-
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L
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(52) Unit cell parameters of 9: triclinic, P-1, a = 14.1509 Å, b =
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(32) n-Propyllithium solid was synthesized via addition of 2 equiv of
t
1.7 M BuLi (7 mL, 0.0118 mol) to 0.9995 g (0.0059 mol) of
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then taken to dryness in vacuo. Pentane was then added to the solid.
The solution was allowed to stir, followed by decanting of the pentane
to remove impurities. After drying in vacuo, 0.9012 g of white powdery
solid was recovered. Coordination of two ether molecules is evident in
1
the H NMR spectrum.
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