Highly regioselective hydride transfer,
oxidative dehydrogenation, and hydrogen-atom
abstraction in the thermal gas-phase chemistry
of [Zn(OH)]+
/C3H8†‡
Xiao-Nan Wu, Hai-Tao Zhao, Jilai Li, Maria Schlangen* and Helmut Schwarz*
The thermal reactions of [Zn(OH)]+ with C3H8 have been studied by means of gas-phase experiments
and computational investigation. Two types of C–H bond activation are observed in the experiment, and
pertinent mechanistic features include inter alia: (i) the metal center of [Zn(OH)]+ serves as active site in
the hydride transfer to generate [i-C3H7]
+ as major product, (ii) generally, a high regioselectivity is
accompanied by remarkable chemoselectivity: for example, the activation of a methyl C–H bond results
mainly in the formation of water and [Zn(C3,H7)]+. According to computational work, this ionic product
corresponds to [HZn(CH3CHQCH2)]+
. Attack of the zinc center at a secondary C–H bond leads preferentially
to hydride transfer, thus giving rise to the generation of [i-C3H7]
; (iii) upon oxidative dehydrogenation (ODH),
liberation of CH3CH2QCH2 occurs to produce [HZn(H2O)]+
. Both, ODH as well as H2O loss proceed through
the same intermediate which is characterized by the fact that a methylene hydrogen atom from the substrate
is transferred to the zinc and one hydrogen atom from the methyl group to the OH group of [Zn(OH)]+
. The
combined experimental/computational gas-phase study of C–H bond activation by zinc hydroxide provides
mechanistic insight into related zinc-catalyzed large-scale processes and identifies the crucial role that the
Lewis-acid character of zinc plays.
1. Introduction
Enhancing the efficiency for the selective activation of carbon–
hydrogen bonds is linked to the success in generating new or
improving existing catalysts.1–3 To this end, great efforts have been
undertaken to reveal the mechanisms of bond activation processes at
a molecular level.3–8 Among the various catalysts so far applied in
industry, quite a few employ transition metals. Zinc-based catalysts
are also in use, for example in the oxidative conversion of CH4, C2H6
and C3H8.
9 Further, zinc-doped zeolites are known to be effective
catalysts for promoting dehydrogenation and aromatization of light
alkanes, and Zn species including [Zn(OH)]+ are believed to play a key
role in these mechanistically rather complex transformations.10–12
Also, pure or Li doped zinc oxides act as catalyst for the C–H bond
activation of light alkanes, e.g. in the oxidative coupling of methane
or the oxidative dehydrogenation of ethane and propane.13 While in
all these reactions, Zn species are considered as the active
ingredients, the reaction mechanisms as well as the precise
structure and exact composition of the active sites of the catalysts
are still under debate.12,14,15 In this respect, gas-phase experiments
have proven useful because they provide in an unperturbed way
rather detailed insight into the elementary steps of numerous
transformations mediated by zinc-containing catalysts.16–18
There have been several experimental and theoretical stu￾dies on the gas-phase reactions with various zinc species.19–23
As shown by Georgiadis and Armentrout, C–C bond cleavage of
alkanes can be achieved by atomic [Zn]+
24 Further, the interaction
of neutral [ZnO] with CH4 has been investigated by theoretical
methods, and possible pathways yielding syngas, CH2O, and
CH3OH, respectively, have been identified.25 Kretschmer et al.
reported N–H bond activation of NH3 by [Zn(OH)]+
17 and CO2
activation has been brought about in the reaction of [LnZn(OH)]+
(L = imidazole and pyridine; n = 1, 2) in analogy with the Lipscomb
mechanism for carbonic anhydrase.18 However, the activation of
C–H bonds of light alkanes with zinc hydroxide is much less
investigated. This is rather surprising given the fact that well￾designed gas-phase processes of transition-metal fragments using
advanced mass-spectrometric techniques in conjunction with
theoretical studies have greatly helped in uncovering mechanistic
aspects underlying C–H bond activation.3,5,7,8,26–33
Herein we present a combined experimental/theoretical
investigation of the gas-phase reactions of cationic zinc hydroxide
Institut fu¨r Chemie, Technische Universita¨t Berlin, Straße des 17. Juni 135,
10623, Berlin. E-mail: [email protected],
[email protected]
† Dedicated to Professor A. W. Castleman, Jr., in recognition of his inspiring work
on gas-phase catalysis.
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp02139h
Received 16th May 2014,
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with alkanes. While [Zn(OH)]+ does not activate methane and
ethane, mechanistically rather remarkable processes with propane
are observed. As will be shown, studying the mechanistic aspects of
C–H bond activation by zinc species proves helpful to understand
the role of zinc in catalysis in a broader context.
2. Methods
Experiments were performed with a VG BIO-Q mass spectrometer of
QHQ configuration (Q: quadrupole, H: hexapole) equipped with an
ESI source, as described previously in detail.34 To this end, Q(1) is
used for mass-selection of the ion of interest and then, in the rf-only
hexapole, the ion/molecule reactions are conducted. Ionic products
are analyzed by scanning Q(2). Further, for ESI, millimolar solutions
of Zn(NO3) in pure methanol were introduced through a fused-silica
capillary to the ESI source by a syringe pump (ca. 4 mL min1
) to
produce the [Zn(OH)]+ cations. Nitrogen was used as a nebulizing
and drying gas at a source temperature of 80 1C. Maximal yields of
the desired complexes were achieved by adjusting the cone voltage
(Uc) to 80 V; Uc determines the degree of collisional activation of the
incident ions in the transfer from the ESI source to the mass
spectrometer. The identity of the ions was confirmed by comparison
with the calculated isotope patterns, which also assisted in the
choice of the adequate precursor ion to avoid coincidental mass
overlaps of isobaric species in the mass-selected ion beam.35 Here,
we selected [64Zn(OH)]+ as the reactant ions by means of Q(1). In the
hexapole, the ion/molecule reactions with CH4, C2H6, C2D6, C3H8,
C3D8, CD3CH2CD3, and CH3CD2CH3 were probed at a collision
energy (Elab) set to nominally 0 eV; this, in conjunction with
the kinetic energy width of about 0.4 eV of the parent ion at peak
half-height, allowed the investigation of quasi-thermal reactions,
as demonstrated previously.18
Since absolute rate constants cannot readily be determined
by using the experimental setup of the VG BIO-Q mass spectro￾meter, the rate constant and the branching ratios of the reaction of
[Zn(OH)]+ with C3H8 have been determined by using a Spectrospin
CMS 47X Fourier Transform Ion Cyclotron Resonance (FT-ICR)
mass spectrometer; details of the instrument have been described
previously.36,37 Atomic [Zn]+ ions were generated by laser ablation
of pure Zn metal disks using a Nd:YAG laser operating at 1064 nm
in the presence of helium carrier gas. The [64Zn]+ isotope was
isolated and allowed to react with a mixture of N2O and H2O
(ca. 5 : 1) to give [64Zn(OH)]+
. The so-formed product ions are
subsequently quenched by collisional thermalization with the
buffer gas (argon, ca. 2 108 mbar). After collisional thermaliza￾tion, the [64Zn(OH)]+ species were mass-selected and exposed to react
with C3H8 by introducing the substrate through a leak-valve. For the
thermalized ions a temperature of 298 K was assumed.36,37 The
branching ratios have been determined by extrapolating the ratios at
different reaction times to t = 0 s. Note, that somehow different
branching ratios are obtained by using the two types of mass
spectrometers applied in this study, cf. Fig. 1a (branching ratios of
reactions (a), (b) and (c) are 78%, 12% and 10%) and eqn (a)–(c);
these differences may reflect the lack of proper collisional
thermalization in the experiments using the VG BIO-Q mass
spectrometer.
Calculations were carried out by using the Gaussian 09
program suite.38 Potential energy surfaces (PESs) are calculated
by using the Møller–Plesset second-order perturbation MP2
method39,40 employing a triple-zeta level basis set with diffuse
and polarization functions 6-311++G(2d,2p) for all atoms.41 To
obtain even more accurate energies of the relevant structures,
the coupled-cluster CCSD(T) method42,43 with single, double,
and perturbative treatment of triple excitations in conjunction
with the correlation-consistent polarized valence triple-zeta basis
sets cc-pVTZ was used.44,45 The MP2/6-311++G(2d,2p) optimized
geometries were employed for the single-point coupled cluster
calculations without reoptimization at the CCSD(T)/cc-pVTZ
levels, and the results are in line with the MP2 calculations
(Table 1). All geometries were fully optimized without symmetry
constraints. Vibrational frequency calculations were performed
to identify the nature of reaction intermediates, transition states
(TSs) and products. To corroborate which minima are linked by the
considered transition states, normal coordinate analyses were
performed on these TS structures by intrinsic reaction coordinate
(IRC) routes in both reactant and product directions.46–48
Additional geometry optimizations starting from the last IRC
structures were carried out when the IRC calculations did not
converge. Unscaled vibrational frequencies were used to calculate
zero-point energy (ZPE) corrections. To demonstrate the applicability
of the MP2 method selected for this study, test calculations were
performed at the MP2/6-311++G(2d,2p) level of theory (Table S1,
ESI‡); the results are in agreement, within 0.36 eV, of experimental
values.21,22,49
The relative energies of P1 and TS3/4 in Fig. 2 have also
been calculated by DFT using different functionals50–59 and the
6-311++G(2d,2p) basis set, as well as by single-point energy
calculations using the CCSD(T) method; the results are listed in
Table S2 (ESI‡). MP2 calculated relative energies of high-spin
and low-spin products of the reaction of [Zn(OH)]+ with C3H8, C2H6,
Fig. 1 Mass spectra showing the ion/molecule reactions of mass-selected
64Zn(OH)]+ with C3H8 (a), C3D8 (b), CD3CH2CD3 (c), and CH3CD2CH3 (d) at
a pressure of 1.0 103 mbar in the VG BIO-Q mass spectrometer.
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and CH4, respectively, are shown in Tables S3–S5 (ESI‡) with
respect to the particular ground state separated reactant pair.
3. Results and discussion
In the thermal reaction of [Zn(OH)]+ with propane (Fig. 1a), the
generation of [C3H7]
+ by hydride transfer60–64 corresponds to the
main process (reaction (a)); in competition, one observes oxidative
dehydrogenation, (reaction (b)) as well as the generation of
[Zn(C3,H7)]+ accompanied by the elimination of water (reaction
(c)). The reaction pathways are confirmed in labeling experiments
in which C3D8, CD3CH2CD3, and CH3CD2CH3 have been employed
as substrates (Fig. 1b–d). The structural assignments of the neutral
and/or cationic products described in eqn (a) to (c) are based on
theoretical results (see below). The labeling experiments are quite
instructive regarding the reaction mechanisms: (i) in the hydride
transfer reaction, a secondary C–H bond is preferentially activated. A
best fit with the data obtained in Fig. 1 is obtained by assuming an
average kinetic isotope effect (KIE) of 1.1 and a specificity of 1: 45 in
favor of the activation of a secondary C–H bond of propane.64 In
contrast, it is the primary C–H bond of C3H8 which exclusively ends
up as water in reaction (c). At the detection limit, only [Zn(C3D5H2)]+
and [Zn(C3H5D2)]+ are formed in the ion/molecule reactions with
CD3CH2CD3 and CH3CD2CH3, respectively. Thus, both hydrogen￾transfer processes, i.e. reactions (a) and (c), do not share a common
intermediate like [Zn(H2O)(C3H7)]+ as might be anticipated. Also,
based on the labeling experiments, ODH proceeds via a specific
transfer of HD in the reactions of [Zn(OH)]+ with CD3CH2CD3 and
CH3CD2CH3, respectively. Finally, the branching ratios given in
eqn (a)–(c) as well as the rate constant k([Zn(OH)]+
/C3H8) of 3.2
108 cm3 s
1 molecule1 have been measured by using the FT-ICR
mass spectrometer; the rate constant corresponds to an efficiency of
30%, relative to the collision rate.65,66
To obtain additional insight in the mechanisms of the
reactions of [Zn(OH)]+ with C3H8, MP2/6-311++G(2d,2p) calcu￾lations have been performed, and the corresponding PESs are
shown in Fig. 2 and Fig. S1 (ESI‡). Overall, these reactions are
controlled by the Lewis-acid character of the metal center
interacting with the electron donating C–H bonds of propane.
Some pertinent details of the most favorable PESs are shown in
Fig. 2a and b; possible pathways which involve the OH moiety
interacting with C–H bonds of propane have also been tested
(Fig. 2c) but turned out to be higher in energy. Four different
encounter complexes have been located on the PES for the
initial interaction of the metal center with C–H bonds of C3H8.
In the iso-energetic I1 and I3, the metal interacts with two
hydrogen atoms of the secondary position, and H–Zn bond
lengths amount to 187/185 pm for I1 and 193 pm for I2,
respectively. In I6 two hydrogens from one methyl group
participate while in I7, one C–H bond of each of the two methyl
groups is involved. All four intermediates form remarkably
stable ion/molecule complexes [Zn(OH)(C3H8)]+
, and these
complexes profit from the electron donation from the C–H
bonds into the empty 4s–4p hybrid orbital of zinc (see also
below). These interactions are indicated by the fact that the
coordinating C–H bonds are slightly elongated (from 109 pm in
free propane to 112 pm in I1 and I3, and to 110–111 pm in
I6 and in I7, respectively). Regarding the hydride transfer from
a methyl group in intermediate I6 to the Zn atom, no barrier
has been located in this step I6 – P4 ([n-C3H7]
/[HZn(OH)]); in
contrast, the hydride transfer from I1 to P1 ([i-C3H7]
[HZn(OH)]) proceeds via transition structure TS1/2 and inter￾mediate I2. However, TS1/2 and I2 are almost iso-energetic, i.e.
this pathway proceeds in a quasi barrier-free process. Notably,
while the formation of product P1 is exothermic (0.20 eV) and
thus accessible under thermal conditions, product P4 is much
higher in energy (0.06 eV); this is in line with the labeling
experiments clearly favoring the former reaction (see above). As
shown in Fig. S1a (ESI‡), formation of [i-C3H7]
+ is also accessible via
the more complex pathway R – I10 – TS10
/20 – I20 – P1. In the
formation of [i-C3H7]
+ (Fig. 2a and Fig. S1a, ESI‡), neutral HZn(OH)
is co-generated; the alternative to produce a neutral water complex
Zn(H2O) is less favorable both kinetically and thermodynamically
(product P6 of path 3, Fig. 2c). Likewise, the combined formation of
Zn(H2O) and [n-C3H7]
+ is more endothermic than generating
HZn(OH) and [n-C3H7]
; the former product pair is 0.30 eV above
the entrance channel (P8, Fig. S1c, ESI‡). Thus, the interaction
of the OH group of [Zn(OH)]+ to C–H bonds of C3H8 (Fig. 2c and
Fig. S1c, ESI,‡ respectively) to form neutral Zn(H2O) cannot
compete with the initial coordination of the metal center to
secondary or primary C–H bonds of propane, respectively (Fig. 2a
and b). These findings are also confirmed in single point energy
calculations using the CCSD(T) method.
For the regiospecific ODH process, the sequence I3 – TS3/4 -
I4 – P2 (Fig. 2a) constitutes the energetically most favorable
pathway. Formation of propene takes place via TS3/4, in which
neutral HZn(OH) interacts with [i-C3H7]
+ resulting in intermediate
I4; the NBO atomic charge of the HZn(OH) moiety in TS3/4
corresponds to 0.06 |e|, and the associated IRC paths are shown
Table 1 The relative energies (in eV) of intermediates, transition states and
products obtained by MP2/6-311++G(2d,2p) and CCSD(T)/cc-pVTZ (relative
to separated C3H8 and [Zn(OH)]+)
Path 1 I1 TS1/2 I2 P1 I3 TS3/4 I4 P2
MP2 1.70 0.38 0.39 0.20 1.70 0.40 1.78 1.12
CCSD(T) 1.65 0.31 0.32 0.13 1.65 0.38 1.62 1.19
TS4/5 I5 P3 Path 2 I6 P4 I7 TS7/8 I8
1.70 2.22 1.23 MP2 1.64 0.06 1.82 0.12 2.61
1.75 2.23 1.12 CCSD(T) 1.58 0.19 1.80 0.17 2.51
P5 Path 3 I9 TS9/10 I10 P6 TS1 TS2
1.07 MP2 0.17 0.16 0.16 0.06 0.16 0.30
0.89 CCSD(T) 0.17 0.06 0.07 0.08 0.18 0.20
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in Fig. S2 (ESI‡). The weakly bound propene ligand in I4 can easily
be liberated to yield the experimentally observed ODH product
[HZn(H2O)]+ (P2). In competition, I5 is generated in which the Zn
atom is coordinated to the CQC double bond of C3H6. The weakly￾bound H2O group can be eliminated yielding the cationic product
[HZn(CH3CHQCH2)]+ (P3), which is experimentally observed in
reaction (c). A kinetically less favorable pathway for the elimination
of water and formation of [Zn(n-C3H7)]+ of reaction (c) is shown in
Fig. 2b (see below). The energetic requirement to produce P2 and
P3 are comparable (1.12 versus 1.23 eV); this is consistent with
similar branching ratio of reactions (b) and (c) as observed
experimentally. As to the energetics of the competitive productions
of P1 versus P2 and P3, the relative energy of P1 (0.20 eV) is higher
than that of TS3/4 (0.40 eV). The same order of relative energies of
P1 and TS3/4 are also obtained by DFT calculations using different
functionals as well as single point energy calculations using the
CCSD(T) method (see Table S2, ESI‡). Taking into account
the errors as well as previous work,67–69 our calculations are
in agreement with the branching ratio of the production of
[i-C3H7]
/[HZn(OH)] (reaction (a)) versus reactions (b) and (c)
(ca. 54% : 46% by using the FT-ICR mass spectrometer). In
addition, the direct dissociation I1 – P1 is kinetically favored
over a more complex rearrangement/dissociation path proceeding
via the tight transition state TS3/4.
For the alternative water formation pathway of reaction (c),
as shown in Fig. 2b, the generation of [Zn(n-C3H7)]+ occurs
along the route I7 – TS7/8 – I8 – P5 with the intermediate
formation of a propyl–water complex I8, [Zn(C3H7)(H2O)]+
Fig. 2 MP2-calculated potential-energy profiles for the reactions of [Zn(OH)]+ with C3H8. (a) Path 1 and (b) path 2 show the channels starting with the
initial coordination of a secondary and primary C–H bond of C3H8, to the Zn site; (c) path 3 depicts the channel starting with C–H bond activation by the
OH ligand. Color code: blue Zn, red O, gray C, and white H. Selected bond lengths are given in pm; relative DH0K energies (in eV) are given with reference
to the separated reactant pair.
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mechanistically, this is a s bond metathesis reaction. The
relative energy of TS7/8 amounts to 0.12 eV, which is higher
in energy as compared to TS3/4 (0.40 eV), and thus unlikely to
compete efficiently under thermal conditions. A brief comparison
of structural features of the transition states for the s-metathesis
reaction, i.e. TS7/8 versus TS1 (Fig. S1b, ESI‡) is indicated. In TS7/8,
the metal center is not only a constituent of the four-membered
ring which is essential for a s-bond metathesis but is also
coordinated by a C–H bond of the distal methyl group of propane;
this results in a slight but clearly discernible elongation of the C–H
bond from 109 pm to 113 pm. Quite likely, this agostic interaction
is beneficial for the stabilization of TS7/8 versus TS1; the latter one
lacks this stabilization.
Finally, with regard to the C–H bond activation in [Zn(OH)]+
the 4s orbital hybridizes with a 4p orbital of Zn leading to the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO), respectively (Fig. 3a and
b). While the former is used to bind to the OH ligand, the latter
can accept two electrons from a C–H bond of propane resulting
in hydride transfer and carbocation formation; this view is
supported by the similarity of the HOMO of neutral [HZn(OH)]
with the LUMO of [Zn(OH)]+ (Fig. 3b and c).
On the triplet PES, all reaction channels described above for
the [Zn(OH)]+
–C3H8 system have been calculated to be
endothermic (Table S3, ESI‡); therefore, this spin state has
not been considered in further calculations.70,71
As mentioned above, the reactions of [Zn(OH)]+ with ethane
and methane have also been studied in the experiments; here,
only adduct formations are observed for [Zn(OH)]+
–C2H6 (Fig. S3,
ESI‡). In agreement with these findings, all reactions involving
C–H bond activation for the [Zn(OH)]+
–C2H6 and [Zn(OH)]+
–CH4
systems are endothermic according to MP2 calculations (see
Tables S4 and S5, ESI‡).
With regard to catalysis, [Zn(OH)]+ species have been con￾jectured to play a role in Zn/Na-ZSM5 catalysts for the conver￾sion of propane to propene and aromatic compounds.14,72–75 As
shown in this study, the metal Zn and the resulting strong
Lewis-acid character in [Zn(OH)]+ are of crucial importance for
the hydride transfer from propane to generate both [i-C3H7]
and propene.72,74 Thus, the identification of the active sites of
ZnOH species helps to unravel part of the enigma associated
with the conversion of alkane by zinc catalysts.72–75
4. Conclusion
Here, we report and analyze the thermal gas-phase reactions of
[Zn(OH)]+ with C3H8 by using experimental and theoretical meth￾ods. The reactivity of cationic zinc hydroxide [Zn(OH)]+ toward C3H8
is characterized by C–H bond activation; the main reaction channel
corresponds to a hydride transfer from the hydrocarbon to the
Lewis acid metal center resulting in the generation of [HZn(OH)]/
[i-C3H7]
+
. Homolytic C–H bond activation give rise to an ODH
channel (generation of propene) as well as the competitive for￾mation of [HZn(CH3CHQCH2)]+
/H2O. Our study may prove helpful
to further understand the industrially relevant, catalytic conversion
of small alkanes by Zn species.
Acknowledgements
This work is supported by the Fonds der Chemischen Industrie,
the Deutsche Forschungsgemeinschaft (DFG), and the Cluster
of Excellence ‘‘Unifying Concepts in Catalysis’’ (coordinated by
the Technische Universita¨t Berlin and funded by the DFG). For
computational resources, the Institut fu¨r Mathematik at the
Technische Universita¨t Berlin is acknowledged. Dr Xiaonan Wu
is grateful to the Alexander von Humboldt-Stiftung for a post￾doctoral fellowship. We thank Dr Robert Kretschmer, Dr Patricio A.
Gonza´lez-Navarrete, Dr Shiya Tang, Dr Shaodong Zhou, and
Dr Nicole Rijs for helpful suggestions and discussions. Andrea
Beck is to be thanked for technical assistance, and the Reviewer
for thoughtful comments.
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