Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes

Article abstract of DOI:10.1021/jacs.9b05577

The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy₂PCH₂CH₂PCy₂)(η: η-(H₃C)CH(CH₃)₂][BAr^F₄] and [Rh(Cy₂PCH₂CH₂PCy₂)(η: η-C₆H₁₂)][BAr^F₄] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D₂ occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H₂. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [k_H/k_D = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large k_H/k_D for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step.

Full text of DOI:10.1021/jacs.9b05577

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Article
Room Temperature Acceptorless Alkane
Dehydrogenation from Molecular #–Alkane Complexes
Alasdair I. McKay, Alexander J. Bukvic, Bengt E. Tegner, Arron L. Burnage, Antonio
J. Martinez-Martinez, Nicholas H. Rees, Stuart A. Macgregor, and Andrew S. Weller
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05577 • Publication Date (Web): 27 Jun 2019
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Journal of the American Chemical Society
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Room Temperature Acceptorless Alkane Dehydrogenation from Molec-
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Alasdair I. McKay,1‡ Alexander J. Bukvic,1‡ Bengt E. Tegner,2‡ Arron L. Burnage,2 Antonio J. Mar-
tinez–Martinez,1 Nicholas H. Rees,1 Stuart A. Macgregor,2* Andrew S. Weller1*
1 Chemistry Research Laboratories, University of Oxford, Oxford OX1 3TA, United Kingdom. 2 Institute of Chemical Sciences,
Heriot Watt University, Edinburgh EH14 4AS, United Kingdom.
ABSTRACT: The industrially important non–oxidative catalytic dehydrogenation of light alkanes via C–H activation is a highly
endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable ther-
modynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C–H activation
is disfavored. We demonstrate that by biasing the pre–equilibrium of alkane binding, by using solid–state molecular organometallic
chemistry (SMOM–chem), well–defined isobutane and cyclohexane s–complexes, [Rh(Cy2PCH2CH2PCy2)(h:h–
(H3C)CH(CH3)2][BArF ] and [Rh(Cy2PCH2CH2PCy2)(h:h–C6H12)][BArF ] can be prepared by simple hydrogenation in a solid/gas
4
4
single–crystal to single–crystal transformation of precursor alkene complexes. Solid–gas H/D exchange with D2 occurs at all C–H
bonds in both complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid–
state. These are probed by variable temperature SSNMR experiments and periodic DFT calculations. These alkane s–complexes
undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes,
by simple application of vacuum or Ar–flow to remove H2. These processes can be followed temporally, and modelled using classical
chemical, or Johnson–Mehl–Avrami–Kologoromov (JMAK), kinetics. When per-deuteration is coupled with dehydrogenation of
cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the
rate determining steps involve C–H activation. Periodic DFT calculations predict overall barriers of 20.6 kcal/mol and 24.4 kcal/mol
for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify
significant C–H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation
results from a pre-equilibrium involving C–H oxidative cleavage and a subsequent rate-limiting b-H transfer step.
been observed using low temperature in situ (–80 ºC or lower)
INTRODUCTION
The “on-purpose” non–oxidative catalytic dehydrogenation
of abundant, unreactive and low value light alkanes to produce
NMR19-22 or in situ diffraction techniques,23 or on very short
timescales (µs to s) using Time Resolved Infrared experiments
(TRIR).24-26 An additional challenge for catalytic alkane dehy-
alkenes, which are key chemical intermediates, is of significant
drogenation is thus one of pre–equilibrium prior to C–H
industrial importance;1,2 that is amplified by the recent move-
ment in feedstocks from naphtha to shale gas. Dehydrogenation
Scheme 1. (A) Non–Oxidative Dehydrogenation of cyclohex-
s
ane and isobutane. (B) –alkane complexes: pre–equilib-
rium, C–H oxidative cleavage and dehydrogenation; L = lig-
and or solvent.
is an energy intensive process, due to the high positive enthalpy
of reaction (e.g. isobutane, cyclohexane: ΔHrº ~ 118 kJmol–1,
Scheme 1A),3 and high temperatures are thus required to drive
the reaction (commonly 550–750 ºC using a heterogeneous cat-
alyst) which present challenges for catalyst decomposition, cok-
ing and process selectivity.4 In molecular homogenous dehy-
drogenation systems a sacrificial alkene H2–acceptor is com-
monly used at operating temperatures of 120–200 ºC,5-7 or
lower with more exotic acceptors.8 In the absence of an acceptor
dehydrogenation can be driven photolytically,9-11 or by contin-
uous removal of H2 at elevated temperatures (~150 ºC) to bias
the thermodynamics.12-14
Non–Oxidative Dehydrogenation
+ H2
A
+ H2
ΔHr = ~118 kJ mol–1
σ–alkane complexes and C–H activation
R = CH2R’
H
B
H
H
– L
+ L
M
M
M
L
H
C
C
H
CH2R
R
Key, but undetected, first–formed intermediates in both ho-
mogenously and heterogeneously catalyzed alkane dehydro-
genation are s–alkane complexes, in which the C–H bond of an
alkane interacts with the metal center, in a 3–center 2–electron
s–interaction, prior to C–H oxidative bond cleavage and b–hy-
drogen elimination (Scheme 1B).15-18 As C–H bonds in alkanes
are strong, non–polar and relatively sterically crowded, alkanes
are very poor ligands (M···H–C bond enthalpies less than 60
kJmol–1), meaning that such complexes have generally only
R
H
H
– H2
CH2
σ–alkane complex
In situ generation,
short–lived in solution at
low temperatures (< –80ºC)
M
CHR’
activation, as solvent or other ligands will generally outcompete
any weak s–interaction from the alkane under normal condi-
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Page 2 of 14
tions.21,27-31 However, C–H activation can be a rather facile pro-
cess once a s–complex is formed.7,26,32-34 Combined, all these
factors make observing intermolecular dehydrogenation pro-
cesses directly from s–alkane complexes very challenging, and
many experimental contributions have thus focused on the over-
all thermodynamics and catalytic efficiencies of such processes,
as well as kinetic studies of catalytic systems, including iso-
topic–substitution.5,35 Such work has also been supported by nu-
merous computational studies.25,36,37
process: heterogeneous Pt/Sn catalyst at 525–700 ºC). Our re-
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sults provide definitive structural and reactivity data for the key
intermediates in both heterogenous and homogeneous catalytic
dehydrogenation processes. They also demonstrate the potential
for SMOM systems to mediate low temperature dehydrogena-
tion by biasing both the pre–equilibrium towards s–complexes,
and the overall dehydrogenation by straightforward removal of
H2.
RESULTS AND DISCUSSION
Synthesis and Characterization of Isobutane and Cyclo-
We have recently reported that s–alkane complexes can be
prepared using so–called solid–state molecular organometallic
(SMOM) chemistry techniques. By operating under single–
crystal to single–crystal conditions (SC–SC),38 addition of H2 to
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s
hexane –Alkane complexes. Alkene precursors to the s–al-
kane complexes, namely isobutene
[Rh(Cy2PCH2CH2PCy2)(C4H8)][BArF ] [1–C4H8][BArF ] and
precursor
norbornadiene
complexes,
e.g.
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cyclohexadiene [Rh(Cy2PCH2CH2PCy2)(h4–C6H8)][BArF ] [1–
i
[Rh(R2PCH2CH2PR2)(h2h2–C7H8)][BArF ] (R = Pr, Cy, Cyp;
4
4
C6H8][BArF ] were prepared in good yield as crystalline mate-
ArF = 3,5–(CF3)2C6H3) generates the corresponding s–alkane
(i.e. norbornane) complexes directly in the solid–state. Some of
these show remarkable stability at room temperature, which we
postulate is due to the, albeit non–porous, octahedral nanoreac-
4
rials (Figure 1A).46 While [1–C6H8][BArF ] is prepared using
4
traditional solution routes, [1–C4H8][BArF ] is best accessed
4
via SC–SC solid/gas reactivity by addition of gaseous isobutene
to [1–NBA][BArF ] and displacement of NBA,47 followed by
tor39 environment provided by the [BArF ]– anions (Scheme 2,
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4
R = Cy, [1–NBA][BArF ]).40-43
recrystallization from a solution saturated with isobutene. Sin-
gle crystal X-ray diffraction, low temperature solution and
SSNMR spectroscopy confirm the formulations as alkene com-
plexes.46 The solid–state structure of the isobutene complex [1–
4
s
Scheme 2 SMOM approach to the synthesis of stable –al-
kane complexes in the solid–state.
C4H8][BArF ] has a bound alkene fragment that also has an ad-
4
Solid–state Molecular OrganoMetallic Chemistry: SMOM
ditional supporting agostic Rh···H3C interaction, and so fea-
tures an h2p:h2C-H-binding mode, similar to the recently reported
propene analogue.47 The isobutene is disordered over two su-
perimposed positions that are related by a non–crystallographic
apparent C2 axis, which means that discussion of the detailed
bond metrics is not appropriate. The cyclohexadiene complex,
norbornane
–
[BArF
]
4
Cy2
+ H2
P
C2
Rh
Rh
P
Cy2
SC–SC
298 K
C1
diene precursor
ArF = (CF3)2C6H3
A
H
Cy2
Cy2
[1–NBA][BArF
]
H
H
P
4
CH2
P
Rh
P
Rh
P
Stable σ–alkane complex
Cy2
SC–SC
– NBA
H
Cy2
CH3
H
–
[BArF
]
4
[1–C4H8][BArF
]
This Work: Acceptorless Dehydrogenation
4
F
CH2
–H2
H
F
C6H8
M
M
Cy2
Cy2
P
C
P
CHR’
298 K
R
Rh
Rh
P
H
P
CH2Cl2
H
Cy2
Cy2
isobutane, cyclohexane
[1–C6H8][BArF
]
4
· Crystalline solid–state organometallics
· 298 K, acceptorless dehydrogenation
· H/D exchange at the bound alkane
· Structures, mechanism, kinetics
B
C
Anion Oh–microenvironment
We now report that by using this methodology the synthesis
of s–complexes of the light alkanes cyclohexane and isobutane
can be achieved at Rh(I) centers, that allow for their detailed
characterization by single crystal X-ray diffraction, solid–state
NMR (SSNMR) spectroscopy and periodic DFT calculations.
These complexes are shown to undergo rapid H/D exchange at
all the C–H bonds of the bound alkane on addition of D2, and a
remarkable acceptorless dehydrogenation at 25 ºC by simple re-
moval of H2 under flow or vacuum, for which significant kinetic
isotope effects can be directly measured for the dehydrogena-
tion of cyclohexane. The products of dehydrogenation, cyclo-
hexene and isobutene, are key intermediates in the chemical
manufacturing chain (nylon production and gasoline addi-
tives/butyl rubber respectively).44,45 In particular isobutene is
currently produced commercially using a high temperature
non–oxidative dehydrogenation of isobutane (e.g. the Oleflex
P2
C3
Rh1
C2
P1
C1
C4
Figure 1. (A) Synthesis of [1–C4H8][BArF ] and [1–
4
C6H8][BArF ]. (B) Solid–state structure of [1–C4H8][BArF ]. Dis-
4
4
placement ellipsoids shown at the 30% probability level. [BArF ]–
4
anions and most hydrogen atoms omitted for clarity. One disor-
dered component shown. Rh1–P1, 2.2238(9); Rh1–P2, 2.2400(9);
Rh1–C1, 2.262(6); Rh1–C2, 2.136(8); Rh1–C3, 2.368(9); C1–C2,
1.320(12); C2–C3, 1.474(13), 1.513(13). (C) Packing diagram of
[1–C4H8][BArF ] (van der Waal radii) showing the Oh arrangement
4
of [BArF ]− anions.
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[1–C6H8][BArF ] adopts the expected h4 diene binding mode
genation of the alkene to form the corresponding s–alkane com-
plexes, [1–C4H10][BArF ] and [1–C6H12][BArF ], via SC–SC
4
(Fig. S95). Both [1–C4H8][BArF ] and [1–C6H8][BArF ] have
1
2
3
4
5
6
7
8
4
4
4
4
extended solid–state structures in which the organometallic cat-
transformations, Scheme 3A, in which the Oh arrangement of
ion is surrounded in a pseudo–Oh cavity defined by the [BArF ]–
[BArF ]– anions is retained (Fig. S94,96). Analysis of the isobu-
4
4
anions (Fig. S93,95),46 and Figure 1C shows that for [1–
tane s–complex [1–C4H10][BArF ] by single crystal X–ray dif-
4
C4H8][BArF ]. [1–C4H8][BArF ] is a rare example of a crystal-
fraction (R = 9.5%, two independent molecules in the unit cell)
shows the Rh(I)–center has two h2–Rh···H–C interactions42
from adjacent methyl (C1) and methine (C2) groups in the al-
kane [e.g. Rh···C1, 2.362(14); Rh···C2, 2.442(7) Å for one of
the independent molecules in the unit cell], Figure 2. These dis-
4
4
lographically characterized isobutene complex.48
Like its propene analogue,47 the isobutene complex [1–
C4H8][BArF ] exhibits fluxional processes at 298 K in both so-
4
lution and the solid–state that exchange the methyl and meth-
ylene hydrogens. This symmetry in the cation is demonstrated
by a single environment being observed in the 298 K 31P{1H}
NMR solution spectrum [δ 95.3, d, J(RhP) = 179 Hz] while no
distinct alkene resonances are observed in the 13C{1H} NMR
(solid–state or solution) or 1H NMR spectra (solution). We pro-
pose a 1,3–shift via an methallyl hydride intermediate,49 cou-
pled with a further exchange of two methyl groups by libration.
These can be slowed at low temperature, i.e. 183 K solution,
158 K solid–state. Thus, in solution two mutually coupled en-
vironments are now observed in the low temperature 31P{1H}
NMR spectrum at δ 97.6 [dd, J(RhP) = 201, J(PP) = 26 Hz],
93.6 [dd, J(RhP) 158, J(PP) = 26 Hz]. The 31P{1H} SSNMR
spectrum shows two overlapping environments centered at δ
94.8. The solution 13C{1H} NMR spectrum shows two signals
9
tances are similar to those in [1–NBA][BArF ] that also shows
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a 1,2–h2:h2–coordination motif,50 albeit through two methylene
groups [2.389(3), 2.400(3) Å].41 This description is also fully
supported by electronic structure analyses (see Supporting Ma-
terials). The C–C distances in the alkane show single bonds
[1.516(13)–1.551(13) Å]. The Rh–P distances in [1–
C4H10][BArF ] are shorter by ~0.04
Å than in [1–
4
C4H8][BArF ], reflecting the weaker trans influence of the al-
4
kane ligands. A chemically identical disordered component is
related by a small rotation of the alkane (25º) around C2 (Fig.
S94). Hydrogen atoms were placed in calculated positions in
the final refinement. Addition of H2 is also signaled by a change
in geometry around the tertiary C–atom (C2) from sp2 in [1–
C4H8][BArF ] to sp3 in [1–C4H10][BArF ]: angles around C2 =
4
4
360.0º and 335.1º respectively. The 13C{1H} SSNMR spectrum
shows a featureless alkene region (d 110–50), while in the
31P{1H} SSNMR spectrum a major new broad signal is shifted
12 ppm to lower field compared to the starting alkene complex
(d 106.8). Notably, under these conditions a small amount of
starting material and alkane–loss decomposition product in
1
due to coordinated alkene [d 111.5, 72.6], and the H NMR
spectrum shows a signal that can be assigned to the alkene
groups and an agostic Rh···H3C interaction [d –0.15], although
the low temperature limit was not reached (Figs. S1 to 7). The
13C{1H} SSNMR spectrum shows alkene signals at δ 108.6 and
70.6. The agostic Rh···H3C signal could not be unambiguously
identified, but a resonance at d 15.7 that is absent in the 298 K
spectrum is consistent with such an interaction.47 In contrast [1–
which the [BArF ]– anion is coordinated with the metal center,
4
[1–BArF ],41 are also observed (~ 10% total). Longer times for
4
C6H8][BArF ] does not show fluxional behavior, and its NMR
H2 addition (90 mins, 298 K) resulted in complete loss of crys-
4
tallinity to give [1–BArF ],51 Scheme 3B, as measured by
4
spectra are unremarkable.
31P{1H} SSNMR spectroscopy.
Scheme 3. Synthesis and stability of [1–C4H10][BArF ] and
4
[1–C6H12][BArF ]; Time for ~10% decomposition in the
a
4
A
B
solid–state under 1 atm H2 (by 31P{1H} SSNMR spectros-
copy) = 15 minutes and 90 minutes respectively.
C5
C4
A
C4
Synthesis of ο–alkane complexes
C1
P1
C6
Rh
P1’
Rh
H
H
Cy2
Cy2
C3
CH2
CH3
P
H2 (1 bar)
P
P1
CH2
CH3
P2
C2
C1
Rh
Rh
P
P
Cy2
Cy2
H
C2
C3
CH3
Solid–State
< 5 mins
298 K
[1–C4H8][BArF
]
4
[1–C4H10][BArF
]
4
[1–C4H10][BArF
]
[1–C6H12][BArF
]
4
4
Cy2
Cy2
P
H2 (1 bar)
P
H
Rh
Rh
P
P
H
H
Cy2
Figure 2. Solid–state structures of [1–C4H10][BArF ] and [1–
H
Cy2
4
C6H12][BArF ]. Displacement ellipsoids shown at the 30% proba-
4
[1–C6H12][BArF
]
[1–C6H8][BArF
]
bility level. [BArF ]– anions and most hydrogen atoms omitted for
4
4
4
clarity. Only one disordered component shown. (A) [1–
C4H10][BArF ] (one of the independent cations): Rh1–P1,
4
Stabilty under H2 (298K)
B
2.1830(14); Rh1–P2, 2.1914(14); Rh1–C1, 2.362(14); Rh1–C2,
2.442(7); C1–C2, 1.551(13); C2–C3, 1.528(13); C2–C4, 1.516(13).
90 minsa
BArF
[1–C4H10][BArF
]
3
4
(B) [1–C6H12][BArF ]: Rh1–P1, 2.191(2); Rh1–C1, 2.62(2); Rh1–
Cy2
4
P
Rh
F3C
[1–BArF
P
C3, 2.53(2); C1–C2, 1.529(15); C2–C3, 1.531(15).
Cy2
CF3
For [1–C6H12][BArF ] the cyclohexane ligand is disordered
[1–C6H12][BArF
]
4
4
24 hrsa
]
4
over two positions (Figure 2 and Scheme 4A), related by a crys-
tallographically–imposed C2 axis which, when coupled with the
reduction in data quality inherent in SC–SC transformations (R
Addition of H2 (298 K, 1 bar, 15 mins) to single crystalline sam-
ples of each of the alkene complexes resulted in rapid hydro-
3
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= 10.3%), meant that the C–C distances in the alkane were nec- at 158 K, retains the fidelity of the diastereotopic methylene
1
2
3
4
5
6
7
8
essarily restrained. Nevertheless, the coordination geometry is
fully consistent with a s–alkane ligand interacting via two C–
H···Rh interactions in a 1,3-motif.52 The Rh···C distances
groups and does not exchange 1,3,5 and 2,4,6 positions; and a
higher energy chair–chair ring flip that makes all the carbon po-
sitions equivalent (Scheme 4B). This latter fluxional process
mirrors the observed disorder in the solid–state structure. Low
energy fluxional processes in the solid–state have been reported
for other s–alkane, or related, complexes.42,53,56 While these two
processes make all the carbon environments equivalent on the
NMR timescale, they do not exchange all the axial and equato-
rial C–H groups in the ring, and this model for the fluxional
process leads to six C–H bonds that contact the metal center
(highlighted in red, Scheme 5) and another set of six C–H bonds
of the cyclohexane ligand that are always remote from the metal
(blue). SSNMR calculations (periodic-DFT, GIPAW method)
on the nearest-neighbor ion-pair derived from the optimized
[2.62(2), 2.53(2) Å] are longer than in [1–C4H10][BArF ], but
4
similar to those in [1–pentane][BArF ] [2.514(4), 2.522(5) Å]
4
that also shows a 1,3–coordination mode for the alkane.53 The
31P{1H} and 13C{1H} NMR spectra are consistent with this for-
mulation, and are similar to [1–C4H10][BArF ]. Notably C=C
4
environments that are observed in the 13C{1H} SSNMR spec-
9
trum of [1–C6H8][BArF ] (96–81 ppm) disappear on addition of
4
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26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H2 (Fig. S22, S25). Despite the longer Rh···C distances, [1–
C6H12][BArF ] is significantly less sensitive to displacement by
4
H2 than [1–C4H10][BArF ] and after 90 minutes under H2 only
4
10% decomposition is observed by 31P{1H} SSNMR (Scheme
3B and Fig. S34).51 This may reflect the weak, multiple, stabi-
lizing dispersive interactions between the surface of cyclohex-
structure of [1–C6H12][BArF ] reveal significant high field
4
shifts for the C4–Hax and C6–Hax hydrogens that interact di-
rectly with the metal center,20,21,41,56 with smaller high field
shifts computed for the remote C1–Hax and C3–Hax positions.
The latter are likely due to ring current effects from the nearby58
anion aryl groups (Fig. S33).41 The computed average chemical
shift for the Hax and Heq hydrogen at the C2, C4 and C6 positions
is –4.3 ppm and 0.8 ppm, respectively, in reasonable agreement
with the values observed at 158 K.
ane and the proximal [BArF ]– in the anion–microenvironment
4
as we have previously commented on for other alkane–com-
plexes.42
For both [1–C4H10][BArF ] and [1–C6H12][BArF ] addition
4
4
of MeCN to the crystalline solids results in liberation of the free
alkane as determined by 1H NMR spectroscopy of the vacuum
transferred volatiles. Neither [1–C4H10][BArF ] or [1–
4
C6H12][BArF ] are stable in solution, and zwitterionic [1–
Scheme 5. Computed chemical shifts for [1–C6H12][BArF ]
4
4
BArF ], [1–(CH2Cl2)n][BArF ]54 and free alkane are observed
and [1–C4H10][BArF ].
4
4
4
by NMR spectroscopy on dissolving in cold (183 K) CD2Cl2.
On warming these solutions decompose to give a mixture of
products, as identified by ESI–MS, some of which come from
C–Cl activation of the solvent.55
Scheme 4. (A) Crystallographically–imposed cyclohexane
disorder in [1–C6H12][BArF ]. (B) Proposed fluxional pro-
4
cess in the solid–state.
C5
A
C1’
P1’
[1–C6H12][BArF4]: crystallographically
Rh
imposed C2 symmetry
For the isobutane ligand in [1–C4H10][BArF ] two environ-
P1
4
C1
C5’
ments are observed in the 203 K NQS spectrum in the aliphatic
region, at d ~21 and ~15. At 158 K these signals disappear, sug-
gesting an arrested low energy motion for the isobutane ligand
B
5
3
5
6
chair–chair
flip
4
1,3,5–
rotation
6
4
P
2
4
1
in the solid–state. A H/13C HETCOR FSLG SSNMR experi-
Rh
P
P
P
Rh
Rh
P
P
3
3
5
1
ment at 158 K shows a correlation between the signal at d ~21
2
(13C) and d –3.4 (1H projection), similar to [1–C6H12][BArF ],
2
6
4
Proposed cyclohexane mobility by SSNMR (numbered C1-C6)
signaling a Rh···H–C interaction (Figs. S11–13). However,
these experimental data do not map directly onto computed
The alkane ligands in both the s–complexes undergo motion
chemical shift averages for [1–C4H10][BArF ] (Scheme 5).
4
in the solid–state, as we have noted previously for the nor-
bornane ligand in [1–NBA][BArF ] and related systems.42,53,56
Given our recent success in calibrating computational and ex-
4
In the H/13C HETCOR FSLG SSNMR59 spectrum of [1–
1
perimentally determined chemical shifts in s–alkane complexes
53,56
C6H12][BArF ] at 158 K a distinct correlation is observed be-
in the solid–state
this discrepancy may point to a flux-
4
tween d(13C) 19.7 and two signals in the H projection at d –
1
ional/equilibrium process that is occurring at low temperature
that remains to be determined.
1.6/1.2, consistent with diastereotopic methylene groups in cy-
clohexane (i.e. axial and equatorial, Figs. S29–S32). At 198 K
these signals disappear, suggesting the onset of a fluxional pro-
cess. A 158 K 13C–NQS experiment, that probes motion of
(CHn) groups in a frequency range similar to or greater than the
1H−13C dipolar coupling,57 shows two signals at d 21.4 and 19.7
that are assigned to the cyclohexane ligand. At 198 K only one
signal is observed at d 21.4 (Figs. S27,28). These observations,
combined with the disorder in the single–crystal X-ray struc-
ture, lead us to propose a combination of two low energy flux-
ional processes are occurring: a 1,3,5–ring walk, that operates
Short–lived, cyclic and branched s–alkane complexes have
been characterized in solution at low temperature (173 K or
lower) by in situ NMR spectroscopy, e.g. (h5–
C5H5)Re(CO)2(cyclohexane)60
and
(h5–
C5H5)Mn(CO)2(isopentane),61 or TRIR experiments, (h5–
C5H5)Rh(CO)(cycloalkane).62 In such species the alkanes bind
with the metal centers through M···H–C interactions in an en-
semble of interconverting isomers, and similar to that postu-
lated to occur for cyclohexane here, these interconvert by chain
or ring walking, or axial/equatorial isomerization.
4
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Journal of the American Chemical Society
s
H/D exchange in –alkane complexes. The isolation of [1–
For [1–C4H10][BArF ] the limited stability of the isobutane s–
4
C4H10][BArF ] and [1–C6H12][BArF ] in the solid–state in syn-
alkane complex under H2 (D2) meant that H/D exchange exper-
iments started from the isobutene complex for experimental ex-
pediency. This formed a mixture of isobutane isotopologues,
C4HxD(10–x) (x = 0 – 4, Fig. S41), after 90 minutes, as measured
by GC-MS of the volatiles after vacuum transfer into CD2Cl2.
This distribution of isotopologues also increases monotonically.
Beyond this time complete decomposition by loss of alkane oc-
1
2
3
4
5
6
7
8
4
4
thetically meaningful amounts (up to 0.15 g) offers an oppor-
tunity to study C–H activation processes in s–alkane complexes
in the absence of competing pre–equilibria. Catalytic H/D ex-
change in alkanes using D2 probes such processes by reversibly
intercepting the corresponding metal–alkyl hydride intermedi-
ate that arises from C–H bond cleavage (Scheme 1B).30 We
curs to form [1–BArF ]. The 13C{1H} NMR spectrum of vola-
have recently shown that [1–NBA][BArF ] undergoes a remark-
ably selective exo-H/D exchange at the bound alkane on addi-
tion of D2 in a solid/gas SC–SC reaction.56 Addition of D2 (298
K) to either [1–C4H10][BArF ] or [1–C6H12][BArF ] results in
4
4
tiles liberated on addition of MeCN shows that H/D incorpora-
tion at both methine (C–H) and methyl (CH3) groups under the
timescale of the experiment, the former signaled by the obser-
vation of an apparent 1:1:1 triplet [d, 22.5; J(CD) = 20 Hz, Fig.
3]. Initial deuteration of isobutene places D in this position.63
The methyl groups present a more complicated set of overlap-
ping resonances that have been simulated with
CD3/CD2H/CDH2 in a 62:30:8 ratio (Scheme 6). Two environ-
ments in a relative 1:9 ratio [d 1.86, 0.85] are observed in the
2H NMR spectrum, and are assigned to the d–methine and d–
methyl respectively (Fig. S39). Again, there must be a fluxional
process in the solid–state that allows for all the C–H bonds of
the methyl groups to undergo H/D exchange, however decom-
position of the alkane complex under D2 (H2) atmosphere makes
studying this less straightforward than for its cyclohexane ana-
log. Nevertheless the rotational disorder observed in the solid–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
4
relatively rapid H/D exchange at all the C–H bonds of the bound
alkane. This is best shown for crystalline [1–C6H12][BArF ],
4
where 3 successive additions of D2 results in perdeuteration of
the cyclohexane (optimized, 90 minutes total, 10% decomposi-
tion). This is conveniently measured by liberating the alkane on
addition of MeCN to the crystalline solid (Scheme 6). GC–MS
analysis shows the formation of only one isotopologue, C6D12
(m/z = 96.17), confirmed by the 13C{1H} NMR spectrum, which
shows a quintet [d 25.3; J(CD) = 19 Hz], and the 2H NMR spec-
trum that shows a single environment for cyclohexane at d 1.37.
Shorter exposure times (3 × 5 minutes) resulted in a mixture of
isotopologues for which D–incorporation increases monoton-
ically, as measured by GC-MS (Fig. S48). Interestingly, despite
the shorter reaction times, the isotopologue distribution is dom-
inated by 6-fold H/D exchange (i.e. C6H6D6) and above. As the
perdeuteration observed indicates all 12 C–H bonds are in-
volved in H/D exchange an additional fluxional process that ex-
changes the faces of the cyclohexane under conditions of exog-
enous D2 is necessary that, in combination with the 1,3,5–ring
walk/chair–chair flip already described (Scheme 4), allows the
metal center to access to all the methylene C–H positions. The
distribution of isotopologues at short exposure times suggests
this face exchange process is likely higher in energy than the
other two process (1,3,5–rotation and chair–chair flip). These
processes have been defined computationally, see later.
state structure of [1–C4H10][BArF ], coupled with the mobility
4
suggested by NQS experiments and deuteration levels ap-
proaching C4D10, indicates that all methyl groups can contact
the Rh–center.
For both cyclohexane and isobutane s–alkane complexes
stepwise H/D exchange with D2 could occur either by oxidative
addition of D2 followed by s–CAM64 with a Rh···H–C bond, or
via oxidative cleavage of an alkane C–H bond to form Rh–H
species that are intercepted by D2. The alternative pairwise ex-
change would involve dehydrogenation to form an alkene
which is then deuterated, as we and others have commented
upon previously.21,56 While the monotonic increase in partially
deuterated isotopologues for both alkanes suggests stepwise ex-
change, as we show next alkane dehydrogenation is a remarka-
bly facile process, and thus we cannot rule out either mechanism
– or if both operate contemporaneously. Computational studies
are underway to probe the precise mechanism of H/D exchange
and will be reported in a future contribution.
s
Scheme 6. H/D exchange in the –alkane complexes, and as-
sociated 13C{1H} NMR (and simulated) spectra of the liber-
ated alkane. * = pentane impurity.
H/D exchange
D
D
D
D
D
D
1. D2 1 bar
3 × 30 mins
D
D
D
D
Cy2
P
s
Acceptorless dehydrogenation of –alkane complexes. In
[Rh(L)(NCMe)2]+
H
+
Rh
P
H
H
298 K
solid–state
the absence of H2 or D2, acceptorless dehydrogenation of the
bound s–alkane ligand occurs in the solid–state to reform the
corresponding alkene complex. Although for both free isobu-
tane and cyclohexane this is an endothermic process – and this
remains the case when these are bound to a metal center (see
computational section) – removal of the generated H2 results in
a remarkably fast (minutes to hours) dehydrogenation in the
solid–state to reform the alkene complexes (Figure 3). This pro-
H
Cy2
D
D
[1–C6H12][BArF
]
d12–cyclohexane
BArF
2. MeCN
4
3
H
D2 1 bar
90 mins
Cy2
Cy2
P
P
CH2
Rh
+
P
Rh
P
Cy2
298 K
solid–state
d10 – d6
Cy2
F3C
CF3
CH3
[1–C4H8][BArF
]
4
[1–BArF
]
4
cess is so facile that isolated [1–C4H10][BArF ] and [1–
4
C6H12][BArF ], and their deuterated analogues, show measura-
C6D12
13C{1H} NMR
C4HxD(10-x)
4
ble dehydrogenation under an Ar atmosphere after only 5
sim.
minutes at 298 K. Isolation of pure [1–C4H10][BArF ], espe-
4
cially, is finely balanced: under an H2 (or D2) atmosphere com-
plete alkane loss occurs over 90 minutes to form [1–BArF ]
4
while under Ar, or mild vacuum (10–2 mbar), dehydrogenation
occurs on a comparable timescale. [1–C6H12][BArF ] is more
4
26
25
robust to alkane loss, meaning the dehydrogenation process is
more reliably followed.
25
24
23
22
J(CD) = 19 Hz
5
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Journal of the American Chemical Society
Page 6 of 14
H
H
b
H
Cy2
A
– H2
15 mins
solid–state
[1–C6H8][BArF
]
4
[1–C6H12][BArF
]
B
Cy2
1
2
3
4
4
Time
P
P
H
H
Rh
Rh
P
Cy2
H
P
Cy2
a
H
298 K
H
[1–C6H8]+
+
[1–C6H10
]
k1obs
[1–C6H12][BArF
]
[1–C6H10][BArF
]
4
4
5
6
7
[1–C6D8]+
– H2
D2 1 bar
k2obs
18 hrs
298 K
+
[1–C6D10
]
3 × 30 mins
8
9
D12
– 2 × D2
Cy2
P
P
Cy2
Cy2
P
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7 days
Rh
Rh
P
298 K
Cy2
H8/D8
31P{1H} (CD2Cl2, 183 K)
solid–state
[1–C6H8][BArF
]
]
4
4
[1–C6D12][BArF
]
Each time point an individal experiment
4
or
[1–C6D8][BArF
kH/kD k1obs = 3.6 ± 0.5; k2obs = 10.8 ± 0.6
not isolated
Figure 3. (A) Dehydrogenation of crystalline [1–C6H12][BArF ] or in situ formed [1–C6D12][BArF ] under Ar flow or vacuum (10–2 mbar).
4
4
(B) Temporal plot of the solid–state dehydrogenation under vacuum, as measured by quantitative 31P{1H} NMR spectroscopy of dissolved
sample (CD2Cl2, 183 K). Signals due to [1–(CH2Cl2)n][BArF ] are taken as a proxy for [1–C6H12][BArF ] (not shown). Each time point is
4
4
an individual experiment, calibrated to an internal standard of PPh3 of known concentration in a flame-sealed capillary (d6–acetone). Solid–
lines are simulated plots (COPASI65) for two consecutive first order processes. Inset shows dehydrogenation of [1–C6D12][BArF ].
4
The dehydrogenation of [1–C6H12][BArF ] can be monitored by
HSQC) and a resonance in the high field region of the 1H NMR
spectrum characteristic of a Rh···H–C agostic interaction (2H,
d –1.01). We propose a low energy libration of the alkene to
account for this Cs symmetry observed in solution that ex-
changes Ca and Cb (Figure 3 and S69), as has been proposed for
4
solid–state and solution NMR spectroscopies by running multi-
ple solid–state experiments in which the time of reaction is var-
ied before dissolving in CD2Cl2 at 183 K by vacuum transfer of
solvent onto the sample. For consistency, finely–ground micro-
crystalline powder was used (10 mg), a dynamic vacuum was
applied (10–2 mbar) to remove H2 and low temperature (183 K,
CD2Cl2, internal standard) quantitative 31P{1H} NMR spectros-
copy of the dissolved samples was deployed to track progress.
Under these low temperature measurement conditions the al-
kane complexes form the solvent adducts, [1–
(CH2Cl2)n][BArF ], alongside [1–BArF ], both of which act as
the closely associated [1–(cis–2–butene)][BArF ] analogue
4
where the calculated barrier to libration is 3 kcalmol–1.47 Warm-
ing solutions resulted in decomposition to [1–C6H8][BArF ],
4
the benzene complex [1–C6H6][BArF ] (independently synthe-
4
sized) and [1–BArF ].
4
The
corresponding
perdeuterated
analogue,
[1–
C6D12][BArF ], also undergoes dedeuteration in the solid–state,
4
4
4
a proxy for the s–alkane complexes.47 For [1–C6H12][BArF ]
4
but much more slowly, taking 7 days to afford [1–
C6D8][BArF ], as shown by ESI–MS, 1H, 2H and 31P{1H} solu-
these experiments show complete dehydrogenation to the diene
4
[1–C6H8][BArF ] in 16 hrs, which was fully characterized by
tion NMR spectra (Figure 3). In the 13C{1H} NMR spectrum
three environments are observed at d 94.7 [1:1:1 triplet, J(CD)
= 26 Hz], d 82.3 [1:1:1 triplet, J(CD) = 23 Hz], d 21.3 [1:2:3:2:1
quintet, J(CD) ~ 20 Hz] assigned to the two pairs of =CD and
CD2 groups respectively (Fig. S87). That the bound, deuterated,
4
solution NMR spectroscopy.46 The dehydrogenation can also be
tracked using in situ 31P{1H} and 13C{1H} SSNMR spectros-
copy (Fig. S67-S68), but as long–range order is lost in the pro-
cess, likely due to crystal cracking,66 attempts to follow this by
SC–SC X-ray diffraction experiments were not successful. The
material does retain microcrystallinity, however,67,68 as demon-
strated by a powder X-ray diffraction experiment on the dehy-
drogenated sample.
diene is also seen after addition of D2 to [1–C6H12][BArF ],
4
shows that the s–alkane interactions must persist on H/D ex-
change prior to undergoing dehydrogenation.
By using a solution–based kinetics model the temporal evo-
This temporal profile shows that after 15 minutes the princi-
pal component (~95%) is a new complex that can be fully char-
acterized using low temperature solution NMR spectroscopy
(183 K) to be the result of a single dehydrogenation, i.e. the cy-
lution of the dehydrogenation of [1–C6H12][BArF ] to give first
4
[1–C6H10][BArF ] and then [1–C6H8][BArF ] in the solid–state
4
4
as measured by the individual trapping experiments can be sim-
ulated, using COPASI,65 by two consecutive first–order pro-
cesses with k1obs = 3.1(2)×10–3 s–1 and k2obs = 4.2(2)×10–5 s–1.69
These correspond to ΔG‡ (298 K) of 21 and 24 kcalmol–1 re-
spectively for these two overall C–H activation processes. De-
deuteration from the – not isolated – perdeuterated s–alkane
clohexene complex [1–C6H10][BArF ]. Addition of CO(g) in a
4
solid/gas reaction after 15 minutes displaces the cyclohexene
allowing for full characterization by NMR spectroscopy and
GC/MS (Fig. S79-S82). By analogy with other mono–alkene
complexes we propose the cyclohexene in [1–C6H10][BArF ]
4
complex [1–C6D12][BArF ] can also be modelled by two
4
adopts an h2 :h2C-H binding mode in which the p-interaction is
p
(slower) first order processes, and this allows for a significant
kinetic isotope effect (KIE) to be determined for these two over-
all dehydrogenation processes: KIE (k1obs) = 3.6(5) and KIE
(k2obs) = 10.8(6). The first dehydrogenation (k1obs) has a KIE
similar to other cyclohexane mono–dehydrogenations, e.g.
photo-dehydrogenation using trans–Rh(PMe3)2(CO)Cl (kH/kD =
5.3)9 and photo– or transfer–dehydrogenation using
supported by a agostic interaction from an adjacent methylene
group. This structure is also located computationally (see be-
low). Notable data include two mutually coupled environments
in the 31P{1H} spectrum [d 98.0, J(RhP) = 207 Hz; d 91.5,
J(RhP) = 159 Hz], while in the 1H NMR spectrum a single al-
kene environment is observed (2H, d 5.23, confirmed by
6
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Journal of the American Chemical Society
solid–state
[1–C4H8][BArF
]
[1–C4H10][BArF
]
4
4
– H2
H
1
2
3
4
H
Cy2
Cy2
Time
CH2
CH3
P
P
Cy2
P
P
Cy2
4 hrs
CH2
Rh
Rh
H
298 K
CH3
P
Rh
CH3
[1–C4H10][BArF
[1–C4H8][BArF
]
P
4
]
4
5
6
7
8
9
10
11
(~90%)
C1
C3
C2
C4
D2 1 bar
30 mins
– D2
22 hrs
R = 10.8%
Two SC–SC transformations
from [1–C4H8][BArF
[1–C4DxH(10-X)][BArF
]
[1–C4DxH(8–X)][BArF
]
4
4
298 K
x = 0 – 4
x = 0 – 2
]
4
Figure 4. Dehydrogenation of crystalline [1–C4H10][BArF ] under Ar flow or vacuum (10–2 mbar). Solid–state structure of [1–C4H8][BArF ]
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
4
formed in a SC–SC transformation (ball and stick). Temporal plot of the solid–state dehydrogenation under vacuum. Details as in Figure 4.
Lines are simulated plots (COPASI65) for first order process (dashed), second order process (solid).
Ir(PR3)2(H)2(O2CF3) (kH/kD = 4.4–7.7).6 The second dehydro-
genation of the cyclohexene shows a larger kinetic isotope ef-
fect. While this may indicate a small tunnelling contribu-
tion,similarly large KIEs have been reported for photochemi-
cally promoted C–H activations at Cp*Rh(CO)2,26,33 or C–H ac-
tivation of methane in Cp*2ScCH2CMe3.70 The details of these
dehydrogenation mechanisms are discussed in the computa-
tional study.
30 mins addition of D2 to [1–C4H10][BArF ], to give [1–
4
C4DxH(8–x)][BArF ] (x = 0 – 4). This partial deuteration meant
4
that experiments to determine a KIE were not attempted.
The dehydrogenation of these s–alkane complexes in the
solid–state has also been modelled using modified Johnson–
Mehl–Avrami–Kologoromov (JMAK) kinetics;71-73 that ex-
press the progress (i.e. conversion) of solid–state reactions in
terms of a nucleation and growth model (Equation 1, Figure 5):
where k is the growth rate constant and n is the Avrami expo-
nent. Exponents close to n = 4, 3 and 2 are suggestive of 3–D,
2–D and 1–D growth, respectively, while n = 1 is indicative of
a non–cooperative transformation that occurs throughout the
crystal, and can be related to a classical first order process in
homogenous systems.74 Pertinently, JMAK analysis has been
used to describe SC–SC photoreactions in the solid–state;75-77
while Finke has discussed the relationship between solid–phase
reaction progress and classical chemical kinetics, especially the
connections between k/n and rate constants/order in reaction.73
Given the small number of data points for the first rapid dehy-
A remarkably straightforward dehydrogenation process also
occurs from the isobutane complex [1–C4H10][BArF ], so that
4
after 4 hours complete H2 loss has occurred in the solid–state to
give [1–C4H8][BArF ] (Figure 4). This occurs under a mild dy-
4
namic vacuum (10–2 mbar), as for the cyclohexane analogue,
and can be followed using solution trapping or solid–state NMR
spectroscopy. This provides data suitable for a quantitative
analysis, by measuring the concentrations of [1–C4H8][BArF ]
4
for different samples where the time of dehydrogenation is var-
ied. This also occurs in an Ar–flow, resulting in a similar tem-
poral profile. Unlike for [1–C6H12][BArF ] this SC–SC process
4
drogenation of [1–C6H12][BArF ], only the second dehydro-
retains enough long–range order to confirm the structure of the
isobutene complex by single crystal X-ray diffraction, and this
is essentially identical to that prepared independently (see ear-
lier), albeit with a poorer structural solution (R = 10.8%,
twinned crystals), Figure 4. Surprisingly to us, this dehydro-
genation process is best modelled as following overall second
4
genation was modelled using JMAK reaction kinetics, and this
yielded n = 1.02(3) with an associated growth rate constant, k,
of 4.2(2) × 10–5 s–1 which is also an excellent fit with that deter-
mined using classical chemical kinetics (Figure 5, k2obs), i.e. first
order. We interpret this as each lattice point in the crystalline
material acting independently for this second dehydrogenation
order kinetics (i.e. second order in [1–C4H10][BArF ]), k(obs)
=
4
1.6(2) × 10–4 M–1s–1, and Figure 4 shows a COPASI modelled
step. For isobutane dehydrogenation in [1–C4H10][BArF ], dif-
4
fit to both 1st and 2nd order processes. The same process occurs
ferent solid–state kinetics are determined: n = 0.55(3) with an
associated growth rate constant, k, of 1.6(6) × 10–3 s–1 – which
is not directly relatable to a classical rate constant given that n
≠ 1.74 It has been suggested that such non–integer Avrami con-
stants point to the kinetics being diffusion controlled.72 It is in-
teresting to note that this process can also be modelled using
2nd–order classical kinetics (vide supra), which may point to a
cooperative process for H2 loss in the single–crystal. While we
currently are reluctant to overinterpret these observations, they
could be related to a reaction front (i.e. H2 loss) that moves
through the crystal from outside to in, as we have previously
demonstrated empirically by CO addition to an analogue of [1–
from partially deuterated [1–C4DxH(10–x)][BArF ], formed from
4
n
conversion = 1–e–(kt)
(Equ. 1)
JMAK Plots
[1–C6H12][BArF
]
[1–C4H10][BArF
]
4
4
C6H8][BArF ].78 Differences in the second69 dehydrogenation
4
process between [1–C6H12][BArF ] (n
= 1) and [1–
4
C4H10][BArF ] (n ~ 0.5) may be related to the loss of long–
4
Figure 5. Modified JMAK plot71 of conversion versus time for the
range order in the former on dehydrogenation, likely via crystal
degradation that exposes new crystal surfaces,66 that may result
in H2 loss processes being less important to reaction progress.
second dehydrogenation of [1–C6H12][BArF ] and dehydrogena-
4
tion of [1–C4H10][BArF ]. k = growth rate constant, n = Avrami
4
exponent. Details as in Figure 3.
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Computational Studies. Thermodynamics and Mechanism Thus both dehydrogenation processes are endergonic, but still
1
2
of Dehydrogenation. The thermodynamics of H2 loss from [1–
C6H12][BArF ], [1–C6H10][BArF ] and [1–C4H10][BArF ] were
accessible thermodynamically upon removing H2 from the sys-
tem.
4
4
4
computed with periodic DFT calculations with the PBE-D3
functional. Extended solid-state structures were fully optimized
in all cases based on experimental crystallographic data, with
3
4
5
6
7
8
9
For the mechanisms of the sequential dehydrogenations of
[1–C6H12][BArF ] the experimental KIE data clearly signal sig-
4
nificant C-H bond extension in the rate-determining steps for
H2 loss, however, they do not allow us to discriminate between
C–H oxidative cleavage or b–H transfer as being rate limiting.
Periodic DFT calculations were therefore employed to con-
struct free energy profiles for these processes. These calcula-
tions used our previously published protocol,56 i.e. for [1–
the exception of [1–C6H10][BArF ] where an initial geometry
4
was constructed from [1–C6H12][BArF ] via removal of H2 from
4
each cyclohexane ligand whilst maintaining the space group
symmetry.79 Optimized geometries for [1–C6H12][BArF ] and
4
[1–C4H10][BArF ] provided good agreement with the experi-
4
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mental structures and, moreover, showed lengthening of the C–
H bonds in contact with Rh (to 1.14 – 1.16 Å) that is consistent
with s-complex formation. This was also confirmed by elec-
tronic structure analyses (see SupportingMaterials).50 Including
the solid-state environment in these calculations is essential.
For example, optimizations on the isolated [1-C6H12]+ cation
show cyclohexane to prefer a 1,2-binding mode in which it lies
parallel to the Rh coordination plane, while in the solid-state
this structure is strongly disfavored (see Fig. 9 and the discus-
C6H12][BArF ] dehydrogenation at one of the Rh cations within
4
the unit cell is considered while the remaining cell contents
were free to relax within a unit cell that was constrained at its
experimental dimensions.
sion below). In [1–C6H10][BArF ] the cyclohexene ligand binds
4
to the Rh center in an h2 :h2C–H mode consistent with the NMR
p
data measured for this species.
Figure 6 shows the computed free energies for dehydrogena-
tion expressed both in terms of DG, the free energy change for
dehydrogenation of a complete unit cell, and DGRh, the average
free energy loss per Rh center (i.e DG/Z). DGRh = +6.3 kcal/mol
for [1–C6H12][BArF ] and +6.7 kcal/mol for [1–C6H10][BArF ].
Figure 6. Computed thermodynamics of H2 loss (kcal/mol) from
(A) [1–C6H12][BArF ] and (B) [1–C4H10][BArF ] expressed as
4
4
DG, the overall free energy change per unit cell, and DGRh, the free
energy change per Rh center. See Supporting Materials for a com-
parison of computed and observed metrics.
4
4
Figure 7. Free energy profiles (kcal/mol) for the dehydrogenation of (A) cyclohexane at one Rh center within the [1–C6H12][BArF ] unit
4
cell and (B) cyclohexene at one Rh center within the [1–C6H10][BArF ] unit cell. Selected distances (Å) within the reacting Rh cations are
4
shown, where [Rh]+ = [(Cy2P(CH2)2PCy2)Rh]+ and the remaining cell contents are omitted for clarity. Distances to delocalized p-ligands are
to the centroid of the carbons involved. Also shown are the computed structures of the rate-limiting transition states for each profile, with
the reacting Rh cation (ball and stick mode) set against the nearby unit cell contents (space-filling mode): Rh (teal); P (orange); C (charcoal);
H (silver) and F (green).
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Figure 7A shows the computed free energy profile for dehydro-
coordination such that the intermediate 3,5-chair structure (co-
genation in [1–C6H12][BArF ], denoted I in the computational
incidentally at 0.0 kcal/mol) has the cyclohexane moiety ori-
ented as seen in the second disordered component defined crys-
tallographically (although note that in this calculation only one
of the four Rh centers is accessing this geometry). To higher
energy is a ‘ring flip’ process by which the axial and equatorial
hydrogens on one face of the cyclohexane are exchanged. This
proceeds through a twist-boat bis-s-complex at +5.7 kcal/mol
that is bound through the C–H3 and C–H6 bonds; this reflects a
coupling of the half-chair transition state with a counter-clock-
wise rotation of the cyclohexane moiety. From this intermediate
a further half-chair transition state can be located that retrieves
a chair conformation and establishes a Rh…H4–C s-interaction.
This entails a clockwise rotation and again moves the cyclohex-
ane above the Rh coordination plane84 to the ‘disordered’ struc-
ture.
1
2
3
4
5
6
7
8
4
study. This commences with oxidative cleavage of the C–H1
bond via TS(I-II) at +20.6 kcal/mol to give the hydrido alkyl
intermediate II at +15.0 kcal/mol.80 A facile rearrangement then
brings the C–H2 bond into contact with the Rh center (III, +11.0
kcal/mol) and permits b-H transfer via TS(III-IV) at +13.6
kcal/mol. This forms the Rh(III) dihydride intermediate IV at
+8.3 kcal/mol in which the cyclohexene engages in an addi-
tional agostic interaction via the C–H5 bond. H–H reductive
coupling then provides h2-H2 complex V from which H2 disso-
ciates via TS(V-VIa) at +16.7 kcal/mol to give the cyclohexene
adduct VIa which, once H2 molecule is removed from the lat-
tice,81 has a free energy of +2.8 kcal/mol.82 The overall dehy-
drogenation barrier of 20.6 kcal/mol agrees well with the value
derived from experiment (21 kcal/mol) and the rate limiting
transition state features a C…H1 distance of 1.70 Å that is con-
sistent with a significant kH/kD KIE.80 The computed structure
of TS(I-II) (Figure 7A, right) also highlights the proximity of
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the [BArF ] anion within the solid-state environment and indeed
4
this and other stationary points along the profile all exhibit a
number of H…F contacts below the sum of the van der Waals
radii (2.7 Å). A comparison of the solid-state profile in Figure
7A with that computed with the isolated cation (see Figure S97–
S104) reveals several important differences. In the latter, facile
rearrangement to more stable alternative 1,2-bis s-cyclohexane
complexes is computed from which C–H oxidative cleavage
can proceed through a transition state at +9.7 kcal/mol. With
this model the final H2 loss becomes rate-limiting with DG‡
span
= +19.6 kcal/mol. Thus, although the overall barrier is reasona-
ble, a simple molecular model fails to account for the observed
KIE and even predicts the wrong geometry for the alkane com-
plex (see also the discussion of cyclohexane rearrangements in
Figure 9).
Figure 7B shows the equivalent free energy profile for the
Figure 8. Computed pathways for cyclohexane rearrangements in
dehydrogenation of cyclohexene in the full [1-C6H10][BArF ]
[1-C6H12][BArF ] via (A) 1,3,5-Ring Walk and (B) Ring Flip
4
4
unit cell. Starting from this species (denoted VIb), initial C–H3
bond activation forms allyl hydride VII at +6.7 kcal/mol which
features an exo-orientation of the allyl ligand (i.e, with the cen-
tral C–H oriented away from the Rh–H bond).83 This allows the
C–H6 bond to engage in an agostic interaction cis the Rh–hy-
dride and so permits H-transfer via a s-CAM mechanism to
form h2-H2 cyclohexadiene species VIII at +12.4 kcal/mol. H2
dissociation and expulsion from the lattice forms IX at +6.7
kcal/mol. The overall barrier to dehydrogenation is 24.4
kcal/mol via TS(VIII-IX) and so provides excellent agreement
with the activation barrier derived from experiment (24
kcal/mol). TS(VIII-IX) again exhibits significant C–H bond
elongation (C…H6 = 1.83 Å), but in this case this rate determin-
ing transition state is preceded by a pre-equilibrium involving
reversible C–H oxidative cleavage. We therefore suggest that
the observed large isotope effect of 10.8 ± 0.6 arises from a
combination of an equilibrium isotope effect and a KIE. A sim-
ilar scenario has been offered for the isotope effect measured in
photochemically–driven cyclohexane dehydrogenation using
trans–Rh(PMe3)2(CO)Cl.9
a
mechanisms, with free energies indicated in kcal/mol. The SCF
electronic energy of this 4,6-chair structure places it 0.7 kcal/mol
above the 1,3-chair, however a large entropic stabilization gives
this anomalously low free energy.80
Finally a mechanism for exchanging all 12 C–H positions
was investigated, as required by the observation of per-deuter-
ated C6D12 experimentally. This requires a ‘face-flip’ process
whereby the set of six C–H bonds accessible via the 1,3,5-ring
walk and ring flip processes are exchanged with the six C–H
bonds that are initially remote from the metal center. In princi-
ple this could proceed via initial formation of a 1,2-bis s-com-
plex featuring Rh…Heq–C2 Rh…Hax–C1 interactions followed by
rotation around the C1–C2 vector (see upper pathway, Figure 9).
Such a process is readily accessible when computed in the iso-
lated cation, however in the solid state none of these structures
is a minimum and attempts to compute the central bis equatorial
s-complex gave energies at least 30 kcal/mol above the 1,3-re-
actant. This reflects the proximity of the [BArF ]– anion in the
4
solid state that does not permit the perpendicular orientation of
the cyclohexane demanded by this pathway and again empha-
sizes the importance of taking the full solid-state environment
into account when modelling these SMOM systems.
Calculations also probed the fluxional processes involving
the cyclohexane ligand in [1-C6H12][BArF ]. The most accessi-
4
ble of these involves exchange of the three axial sites interacting
with Rh via a 1,3,5-‘ring walk’ process and occurs with a very
low barrier of 3.7 kcal/mol (see Figure 8A). This rotation also
involves movement of the cyclohexane ring relative to the Rh
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H/D exchange using D2, indicating that C–H oxidative cleavage
1
of the bound alkane must also be a relatively low energy pro-
cess. When followed by b-H-elimination alkane dehydrogena-
tion occurs – an overall endothermic process that normally re-
quires very high temperatures, or (at lower temperatures) a sac-
rificial acceptor. The SMOM approach thus promotes both (i)
alkane complex formation and (ii) the easy removal of liberated
H2 by simple application of vacuum or Ar–flow: two consecu-
tive processes that are necessary for the observed reactivity.
With the cyclohexane s–complex dehydrogenation occurs via
a cyclohexene intermediate to give the corresponding cyclohex-
adiene product. Coupling these dehydrogenations with prior
per-deuteration allows for kH/kD KIEs of 3.6(5) and 10.8(6), re-
spectively, to be determined. Periodic DFT calculations identify
rate-limiting C–H oxidative cleavage (for cyclohexane dehy-
drogenation) and b-H transfer (for cyclohexene dehydrogena-
tion). The large KIE of the latter arises from the combination of
significant C–H bond elongation in rate-limiting transition state
with a pre-equilibrium that also involves C–H oxidative cleav-
age. The importance of solid–state computational studies, that
capture the holistic microenvironment, compared with those on
an isolated cation (i.e. so–called “gas phase”) is reflected by the
excellent agreement between computation and experiment in
probing the rate–limiting step – which are not captured in the
absence of the solid–state environment.
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Figure 9. Pathways for the cyclohexane face-flip in [1-
C6H12][BArF ]. The upper pathway shows potential 1,2-bis s-in-
4
termediates as Newman projections looking down the C2–C1 bond
but which proved inaccessible in the solid-state. The lower pathway
shows the proposed H2-facilitated pathway with free energies in
kcal/mol. See text for details.
Instead we found that a face-flip process could be accessed
upon addition and oxidative cleavage of H2. The resultant
Rh(III) dihydride intermediates allows more flexibility for cy-
clohexane movement including access to additional s-interac-
tions in the axial sites (see lower pathway, Figure 9). The face-
flip transition state, TSFF, again involves rotation about the C1–
C2 vector, but now that the ligand can access space above the
Rh coordination plane this proves to be accessible within the
solid-state pocket and proceeds with an overall computed bar-
rier of 24.6 kcal/mol. Thus access to the remote (‘blue’) face of
the cyclohexane ligand has a considerably higher barrier than
rearrangements between the closer (‘red’) C–H bonds and this
is consistent (assuming facile H/D exchange mechanisms) with
the very rapid formation of C6H6D6 upon exposure of [1-
While driving catalytic (acceptorless) dehydrogenation by re-
moval of H2,13 working in the solid–phase,85 or under continu-
ous-flow gas phase conditions at high temperatures,86 are not
new concepts, that stoichiometric dehydrogenation occurs at
such well–defined s–alkane complexes in the solid–state at 25
ºC suggests opportunities to develop this process catalytically
at lower temperatures. Fine–tuning of metal ligand coordination
environment in the single–crystalline phase,42 coupled with the
possibilities offered by expediently removing H2, offer potential
solutions to move from stoichiometric to catalytic regimes in
the single–crystalline state. Encouraging this approach we have
recently shown that SMOM–systems are highly effective
solid/gas alkene–isomerization catalysts.47 Overcoming the
acknowledged problems of product (alkene) inhibition,5 and un-
derstanding how gaseous reagents/products move in and out of
the non–porous crystalline lattice are future challenges that we
are currently focused on resolving.
C6H12][BArF ] to D2, but the somewhat slower rate of formation
4
of the higher C6HxD(12-x) isotopologues (x = 0-5).
CONCLUSIONS
We report here the industrially relevant, low temperature, ac-
ceptorless, dehydrogenation of the light alkanes isobutane and
cyclohexane when bound as s–complexes to a Rh(I) center.
This demonstrates the advantages of solid–state organometallic
chemistry (SMOM–chem) for the synthesis, characterization
and subsequent reactivity of well–defined s–complexes. Such
species are traditionally short–lived when synthesized using in
situ solution techniques at very low temperature, due to facile
displacement of the weakly bound alkane by solvent or other
exogenous ligand,60,61 making onward exploration of structure
and reactivity very challenging. It is, without doubt, the micro-
ASSOCIATED CONTENT
Supporting Information
Full details of experimental methods, characterization data, details
of computational methods and experiments and single crystal X-
ray diffraction collection and refinement data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
andrew.weller@chem.ox.ac.uk
S.A.Macgregor@hw.ac.uk
environment provided by the [BArF ]– anions in the solid–state
4
that allows for this chemistry of M···H–C alkane interactions
described here to be developed.
Author Contributions
‡These authors contributed equally.
By biasing the pre–equilibrium completely to the side of al-
kane binding in the solid–state, a number of important observa-
tions can be made. Experiment and computation show that both
alkane ligands can access low energy fluxional processes in the
solid–state that allow all the C–H bonds to come into contact
with the metal center. This, in turn, permits per–deuteration by
ACKNOWLEDGMENT
We thank the EPSRC (EP/M024210, EP/K035908, EP/ K035681),
the Leverhulme Trust (RPG-2015-447) and SCG Chemicals Co.,
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Journal of the American Chemical Society
(18) Zhu, Q.; Wang, G.; Liu, J.; Su, L.; Li, C. Effect of Sn on
Ltd, Thailand for funding. This work used the ARCHER UK Na-
Isobutane Dehydrogenation Performance of Ni/SiO2 Catalyst:
Adsorption Modes and Adsorption Energies of Isobutane and
Isobutene. ACS App. Mat. Interface. 2017, 9, 30711-30721.
(19) Calladine, J. A.; Duckett, S. B.; George, M. W.; Matthews,
S. L.; Perutz, R. N.; Torres, O.; Vuong, K. Q. Manganese Alkane
Complexes: An IR and NMR Spectroscopic Investigation. J. Am.
Chem. Soc 2011, 133, 2303-2310.
(20) Bernskoetter, W. H.; Schauer, C. K.; Goldberg, K. I.;
Brookhart, M. Characterization of a Rhodium(I) -Methane Complex in
Solution. Science 2009, 326, 553-556.
(21) Walter, M. D.; White, P. S.; Schauer, C. K.; Brookhart, M.
Stability and Dynamic Processes in 16ve Iridium(III) Ethyl Hydride
and Rhodium(I) s-Ethane Complexes: Experimental and
Computational Studies. J. Am. Chem. Soc. 2013, 135, 15933-15947.
(22) Yau, H. M.; McKay, A. I.; Hesse, H.; Xu, R.; He, M.; Holt,
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1
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4
5
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tional Supercomputing Service (http://www.archer.ac.uk) and the
Cirrus UK National Tier-2 HPC Service at EPCC (http://www.cir-
rus.ac.uk) funded by the University of Edinburgh and EPSRC
(EP/P020267/1). We thank Dr Graham Tizzard (UK National Crys-
tallographic Service) for data collection on [1–C6H8][BArF ], and
4
Dr Hamish Yeung for valuable discussions in solid–state kinetics.
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(69) A consecutive 2nd/1st order process also fits the data, but less
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Weller, A. S.; Krämer, T.; Macgregor, S. A. Solid-State Synthesis and
Characterization
of
s-Alkane
Complexes,
[Rh(L2)(h2,h2-
C7H12)][BArF ] (L2 = Bidentate Chelating Phosphine). J. Am. Chem.
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Macgregor, S. A.; Weller, A. S. Modulation of s-Alkane Interactions
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(46) See Supporting Materials
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Rees, N. H.; Krämer, T.; Macgregor, S. A.; Weller, A. S. Solid-State
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Tridentate Diamido Ligands [(I-Prn-O-C6H4)2]2- ([I-Prnon]2-) and
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(50) See Supporting Materials for QTAIM, NBO and NCI plot
data. A full account of the bonding in these and related alkane
complexes will be the subject of a separate publication
(51) Under these conditions an intermediate is observed on the
way to [1–BArF4], δ ~100, that we have not been able to idenfity but
propose is is the initial product of alkane loss (see Fig. S14).
(52) QTAIM, NBO and NCI analyses suggest an interaction
intermediate between the extremes of η2:η2 and η1:η1 (see Figures
S118-120 for details)
(53) Chadwick, F. M.; Rees, N. H.; Weller, A. S.; Krämer, T.;
Iannuzzi, M.; Macgregor, S. A. A Rhodium–Pentane Sigma-Alkane
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(73) Finney, E. E.; Finke, R. G. Is There a Minimal Chemical
(81) A geometry for VI in which H2 was retained in the lattice
was also derived from the characterisation of TS(V-VI). This had a free
energy of +9.7 kcal/mol and had essentially the same coordination
geometry around Rh. The drop in free energy to +2.8 kcal/mol reflects
the entropy gain upon releasing free gaseous H2. See Supporting
Materials for details, Figure S101.
Mechanism Underlying Classical Avrami-Erofe’ev Treatments of
Phase-Transformation Kinetic Data? Chem. Mat. 2009, 21, 4692-4705.
(74) Khawam, A.; Flanagan, D. R. Solid-State Kinetic Models:ꢀ
Basics and Mathematical Fundamentals. J. Phys. Chem. B 2006, 110,
17315-17328.
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9
(82) This value differs from DGRh (+6.3 kcal/mol) as the two
models employed are not the same: here one Rh-cyclohexene moiety is
computed in the presence of three Rh-cyclohexane cations (and four
(75) Benedict, J. B.; Coppens, P. Kinetics of the Single-Crystal
to Single-Crystal Two-Photon Photodimerization of Α-Trans-
Cinnamic Acid to Α-Truxillic Acid. J. Phys. Chem. A 2009, 113, 3116-
3120.
[BArF ]- anions), whereas DGRh is based on dehydrogenation of all four
4
(76)
Jarvis, A. G.; Sparkes, H. A.; Tallentire, S. E.; Hatcher, L.
Rh centers
E.; Warren, M. R.; Raithby, P. R.; Allan, D. R.; Whitwood, A. C.;
Cockett, M. C. R.; Duckett, S. B., Clark, J. L.; Fairlamb, I. J. S.
Photochemical-Mediated Solid-State [2+2]-Cycloaddition Reactions
of an Unsymmetrical Dibenzylidene Acetone (Monothiophos-Dba).
CrystEngComm 2012, 14, 5564-5571.
(77) Hatcher, L. E.; Skelton, J. M.; Warren, M. R.; Stubbs, C.; da
Silva, E. L.; Raithby, P. R. Monitoring Photo-Induced Population
Dynamics in Metastable Linkage Isomer Crystals: A Crystallographic
Kinetic Study of [Pd(Bu4dien)NO2]BPh4. Phys. Chem. Chem. Phys.
2018, 20, 5874-5886.
(78) Pike, S. D.; Krämer, T.; Rees, N. H.; Macgregor, S. A.;
Weller, A. S. Stoichiometric and Catalytic Solid–Gas Reactivity of
Rhodium Bis-Phosphine Complexes. Organometallics 2015, 34, 1487-
1497.
(79) Under this constraint there are two structures that can be
generated depending on which C–C is dehydrogenated. The more
stable option is used here and details of the alternative structure are
provided in the Supporting Materials.
(83) An alternative pathway based on the initial formation of an
endo-Rh–H/allyl intermediate, a trans-cis isomerisation prior to H2 loss
was ruled out on the basis of calculations on the isolated cation that
indicated a high energy isomerisation process. See Supporting
Materials for details, Figure S100.
(84) A related transition state at +16.5 kcal/mol involves counter-
clockwise rotation of the cyclohexane and generates a 2,6-structure that
is geometrically equivalent to the initial 1,3-chair. (Supporting
Materials, Figure S107).
(85) Kumar, A.; Zhou, T.; Emge, T. J.; Mironov, O.; Saxton, R.
J.; Krogh-Jespersen, K.; Goldman, A. S. Dehydrogenation of N-
Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High
Yields of Α-Olefin Product. J. Am. Chem. Soc. 2015, 137, 9894-9911.
(86) Boris Sheludko, B.; Cunningham, M.T.; Goldman, A. S.; Celik,
F. E. Continuous-Flow Alkane Dehydrogenation by Supported Pincer-
Ligated Iridium Catalysts at Elevated Temperatures ACS Catal. 2018,
8, 7828−7841
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(80) The alternative pathway based on initial C–H3 bond
activation has a slightly larger barrier of 23.9 kcal/mol and is generally
somewhat less accessible than the pathway in Figure 7. This reflects
the asymmetry of the cyclohexane binding pocket which renders the
C–H1 and C–H3 bonds inequivalent. Full details of this alternative
pathway are in the Supporting Materials
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Acceptorless Dehydrogenation at Isolated σ–alkane Complexes
· Solid–state Molecular OrganoMetallics (SMOM)
· Isobutane & cyclohexane σ–complexes: X-ray diffraction & SSNMR
· Complete H/D exchange at the bound alkane with D2
· 298 K, acceptorless, dehydrogenation
CH2
–H2
H
M
M
C
CHR’
298 K
R
H
H
· Kinetics and isotope effects in the solid–state
· Periodic DFT defines fluxional processes and captures KIE.
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