Hydrogen activation by an aromatic triphosphabenzene

Article abstract of DOI:10.1021/ja5077525

Aromatic hydrogenation is a challenging transformation typically requiring alkali or transition metal reagents and/or harsh conditions to facilitate the process. In sharp contrast, the aromatic heterocycle 2,4,6-tri-tert -butyl-1,3,5-triphosphabenzene is shown to be reduced under 4 atm of H₂ to give [3.1.0]bicylo reduction products, with the structure of the major isomer being confirmed by X-ray crystallography. NMR studies show this reaction proceeds via a reversible 1,4-H₂ addition to generate an intermediate species, which undergoes an irreversible suprafacial hydride shift concurrent with P - P bond formation to give the isolated products. Further, para-hydrogen experiments confirmed the addition of H₂ to triphosphabenzene is a bimolecular process. Density functional theory (DFT) calculations show that facile distortion of the planar triphosphabenzene toward a boat-conformation provides a suprafacial combination of vacant acceptor and donor orbitals that permits this direct and uncatalyzed reduction of the aromatic molecule.

Full text of DOI:10.1021/ja5077525

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Article
Hydrogen Activation by an Aromatic Triphosphabenzene
Lauren E. Longobardi, Christopher A. Russell, Michael Green, Nell S. Townsend, Kun
Wang, Arthur J. Holmes, Simon B Duckett, John E. McGrady, and Douglas W. Stephan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5077525 • Publication Date (Web): 28 Aug 2014
Downloaded from http://pubs.acs.org on September 2, 2014
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Hydrogen Activation by an Aromatic Triphosphabenzene
Lauren E. Longobardi†, Christopher A. Russell‡, Michael Green‡, Nell S. Townsend‡, Kun Wang#$
Arthur J. Holmes&, Simon B. Duckett&, John E. McGrady# and Douglas W. Stephan†*
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†Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
‡School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS (UK)
# Department of Chemistry, South Parks Road, University of Oxford, OX1 3QR, Oxford (UK)
& Department of Chemistry, University of York, York, YO10 5DD (UK)
$
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081 (P. R.
China)
Supporting Information Placeholder
which undergoes an irreversible suprafacial hydride shift
concurrent with P-P bond formation to give the isolated
product. Further, para-hydrogen experiments confirmed the
addition of H2 to triphosphabenzene is a bimolecular process.
DFT calculations show that facile distortion of the planar
triphosphabenzene towards a boat-conformation provides
suprafacial combination of vacant acceptor and donor orbit-
als that permits this direct and uncatalyzed reduction of the
aromatic molecule.
ABSTRACT: Aromatic hydrogenation is a challenging trans-
formation typically requiring alkali or transition metal rea-
gents and/or harsh conditions to facilitate the process. In
sharp contrast, the aromatic heterocycle 2,4,6-tri-tert-butyl-
1,3,5-triphosphabenzene is shown to be reduced under 4 atm
of H2 to give [3.1.0]bicylo reduction products, with the struc-
ture of the major isomer being confirmed by X-ray crystallog-
raphy. NMR studies show this reaction proceeds via a re-
versible 1,4-H2 addition to generate an intermediate species,
systems33,37,39-41 shown to activate hydrogen present ground
state acceptor and donor orbitals that can interact with H2 to
effect the heterolytic cleavage of the H−H bond (Fig. 1(b)
1(c)). Based on this seemingly general criterion, aromatic
species would not be expected to activate H2. Indeed, these
notoriously stable molecules42 lack accessible donor-acceptor
orbitals needed to facilitate H2 cleavage, and thus aromatic
hydrogenations require a catalyst to pre-activate the H−H
bond.43-47
INTRODUCTION
The cleavage of the simplest molecule, dihydrogen (H2), is
the instrumental step to hydrogenation reactions used in a
myriad of industrial processes1-7 and is the key reaction in a
number of hydrogenase enzymes8-10 and related biological
systems.11 It was in the early 20th century that Sabatier12 un-
covered amorphous metals which catalyze hydrogenations,
while the emergence and evolution of organometallic chem-
istry provided initially homogeneous precious metal cata-
lysts13-22 and more recently earth-abundant metal systems.23-
25
Non-transition metal compounds including KOtBu26 and
calcium-hydride species27 have also been reported to catalyze
the hydrogenation of selected substrates. Non-metal, main
group elements were initially shown to react with H2 by low
temperature matrix28-30 and computational studies.31-32 Sub-
sequently, reaction with germynes33 were described but more
recently main group systems have been documented as effec-
tive metal-free hydrogenation catalysts34-36 with the advent of
frustrated Lewis pairs (FLPs)37.
Understanding these reactions with H2 in terms of the
orbitals involved, uncovers the underlying similarities of
transition metal and main group systems.38 For transition
metals, the combination of vacant d-orbitals accepting elec-
tron density from the H−H σ-bond and filled d-orbitals do-
nating electron density into the σ*-orbital of H2 enables H−H
bond cleavage (Figure 1(a)). In the same vein, main group
Figure 1 Activation of H2
Our recent finding that FLP hydrogenations using
B(C6F5)3 and sterically encumbered anilines48 resulted in the
corresponding saturated cyclohexylammonium salts prompt-
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ed us to explore the corresponding hydrogenation of 2,4,6-
The mechanism of this reduction was probed by
monitoring the reaction by 31P NMR spectroscopy at room
49
1
tri-tert-butyl-1,3,5-triphosphabenzene 1 (Scheme 1). Herein
we report that although the planar50 phosphorus-based het-
erocycle triphosphabenzene is aromatic51-53, it reacts with H2
in an uncatalyzed process. The nature of the products is es-
tablished and the unexpected mechanism of formation has
been established by para-hydrogen experiments and compu-
tational studies.
temperature. After 24 hours, about 14% of 1 is converted to a
species 3 (Scheme 1). Upon heating to 110 °C, both 1 and 3
were consumed, affording 2a/2b as the final products in 5
hours. The 31P NMR spectrum of this intermediate 3 shows
resonances at 240 ppm and -24.8 ppm (1JPH = 229 Hz) in a 2:1
ratio. This break in P atom symmetry taken with the large
one-bond P−H coupling constant is consistent with the for-
mulation of 3 as the product of 1,4-addition of H2 to 1. This
intermediate has been previously observed as one of the
products of hydrolysis of Hf complexes of P−C multiple
bonds.54 The subsequent migration of the PH hydride to the
adjacent carbon with concurrent P−P bond formation to give
2a/2b is thermally driven (Scheme 1). The existence of con-
formational isomers for 1,3,5-triphosphacyclohexa-1,4-dienes
3 provide an explanation for the observation of two isomers,
2a and 2b, as the final products.
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RESULTS AND DISCUSSION
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The aromatic molecule 151-53 was treated with 5 mol%
B(C6F5)3 under 4 atm of H2 at 110 °C for 5 hours in C6H5Br
and found to react quantitatively to give the isomeric bicyclic
compounds 2a and 2b.54-55 Interestingly, control experiments
revealed that reaction of 1 alone with 4 atm of H2 at 110 °C in
C6H5Br also led to the clean formation of 2a and 2b. Indeed,
monitoring this uncatalyzed hydrogenation over the course
of the reaction resulted in the consumption of 1 and the gen-
eration of 2a/2b at comparable rates in the presence or ab-
sence of B(C6F5)3. As compound 1 is prepared by the cyclot-
rimerization of tBuC≡P by Cl3V=NtBu,56-59 the hydrogenation
of 1 was explored in the presence of added Cl3V=NtBu. How-
ever, the rate of formation of 2a/2b was unaffected by this
additive. Collectively these data affirm the direct reaction of 1
with H2 affords the products 2a/2b. Previous literature has
reported the independent formation of 2a and 2b from reac-
tions of unsaturated P−C bonds at Hf and Ga centers.54-55
Scheme 1: Proposed mechanism for H2 activation
This reaction of 1 was also explored with parahydrogen
(p-H2)60, a technique which has been used very successfully
to study metal catalyzed reactions.61 When a sample of 1 was
warmed to 110 °C in C6D5Br in the presence of p-H2, two po-
larized proton signals appear due to compound 3 at 6.4 and
1.8 ppm (Figure 3). Importantly, the spin-encoding associated
with p-H2 is retained, which supports the suggested intramo-
lecular activation mechanism, and interestingly, a substantial
signal gain is evident for a period of many minutes, indica-
tive of the reversible formation of 3 from 1. The signal at 6.4
ppm exhibits a strong PH coupling of 227 Hz, and two small-
er P−P splittings of 6.5 Hz in addition to the anti-phase pro-
ton splitting of +1.5 Hz to the resonance at 1.95 ppm. A 2D
1H-31P HMQC located the 31P signals for this species at 237.2
and -30.6 with the selective decoupling experiments confirm-
ing that the PH signal arises from the high field resonance.
The observation of p-H2 addition to a non-metal system is
unusual, with the Koptyug’s observation of H2 addition to an
amino borane providing the only other metal-free example.62
When used in conjunction with the OPSY(only para-
hydrogen spectroscopy) protocol, a strong signal is still re-
tained thereby confirming the pairwise nature of this pro-
cess.63
Figure 2. POV-ray depiction of the molecular structure of 2a; color
scheme: P: orange, C: black, H: grey. Selected bond distances and
angles: five membered ring: C=P: 1.689(3) Å, C-P: 1.850(4) Å, 1.871(3)
Å, 1.832(3) Å, P-P: 2.196(1) Å; three membered ring: C-P: 1.830(4) Å,
1.865(4) Å. C-P-P: 54.3(1)°, 52.8(1)°, P-C-P: 72.9(1)°.
The isomeric products 2a and 2b were isolated by
slow concentration resulting in the formation of pale-yellow
crystals. 31P NMR data showed the intensity ratio of the reso-
nances attributed to 2a and 2b, respectively, was 4:1. In the
1
corresponding H NMR spectrum 2a exhibited resonances at
2.98, 1.49, 1.13, and 0.92 ppm while 2b gave peaks at 2.59, 1.46,
1.28, and 0.93 ppm. Assignment of these resonances and the
13C NMR data were confirmed with 31P-HMBC and HSQC
spectra and further supported by the corresponding charac-
terization of 2a-d2/2b-d2. The major isomer 2a was further
characterized by X-ray crystallography (Figure 2).
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At the transition state TS1’, the H−H bond is only mar-
1
ginally elongated from its equilibrium value (0.85 Å vs 0.75 Å
in H2) but nevertheless the P and C centers are distorted
significantly out of the plane such that the C3P3 unit adopts a
boat conformation.70 In the absence of H2, the boat distor-
tion of the triphosphabenzene ring corresponds to a very
low-frequency mode (92 cm-1), suggesting that the innate
flexibility of the ring is an important factor in facilitating the
activation process. The distortion of 1 to a boat conformation
has been previously observed experimentally in a cationic
Au(PR3) complex of 1.52 NICS-1 calculations revealed that a
high degree of aromaticity is maintained in this structural
arrangement, in contrast to corresponding carbocyclic ring
systems which become anti-aromatic with the same defor-
mation. A natural population analysis of 1’ reveals a substan-
tial polarization of the C-P bonds (q(C) = -0.78, q(P) = +0.74);
moreover, the negative charge on C1 increases at TS1’ (-0.85
vs -0.80 for the other two carbons in the ring), suggesting
that the deformation of 1’ to a boat conformation enhances
its zwitterionic character. An NBO analysis of the interac-
tions between the H2 and C3P3 units at the transition state
(TS1’) confirms that the dominant interactions are donation
of electron density (i) from a C−P π orbital localized on C to
H−H σ* and (ii) from H−H σ to a C−P π* orbital localized on
P (Figure 4 and SI, Figure S11). The overall picture of the acti-
vation process is therefore similar to that discussed in other
main-group systems, with acceptor/donor interactions be-
tween the σ and σ* orbitals of H2 and the polarized frontier
orbitals of the substrate.
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Transition state TS1’ connects the reactant to interme-
diate 3’a (Figure 2) which lies 7 kcal/mol above the reactants.
In 3’a the 1,3,5-triphosphacyclohexa-1,4-diene adopts the boat
conformation typical of the 1,4 cyclohexadienes, with the two
hydrogens that originate from H2 in apical positions. 3’a is
not, however, the most stable conformation of the system,
and in fact three further structures, 3’b-d, were located which
differ from 3’a in the configuration at phosphorus and/or at
the sp3 hybridized carbon. Of these, 3’b and 3’c, where the
hydrogen on phosphorus is equatorial, are the most stable,
lying 6-7 kcal/mol below 3’a and therefore isoenergetic with
the reactants. Interconversion of the various isomers of 3’,
either via inversion at the phosphorus center (TS3’ab, G‡ 15
kcal/mol) or via a planar transition state that connects two
boat structures (TS2’ac, G‡ 10 kcal/mol), is more facile than
the subsequent hydrogen migration step (vide infra). Inter-
mediate 3 should therefore reach an equilibrium distribution
of conformations dominated by 3b and 3c prior to rearrange-
ment to the product. To verify this structural assignment we
have computed 31P chemical shifts for the isomers at the
BP86-GD3BJ level (see supporting information, Figure S12
and Table S1). The computed values for 3’b and 3’c, are much
more consistent with the spectroscopic data detected for 3
than those for 3’a or 3’d.
Figure 3: p-H2 1D and 2D NMR data.
Further insights into this aromatic hydrogen activation
chemistry come from a survey of the computed potential
energy surface for the reaction of H2 with the simplified
model (PCMe)3 1’ (primes denotes the computational models
with Me substituents in place of tBu). A free energy surface
was computed using gradient-corrected density functional
theory64 with empirical dispersion corrections (BP86-
GD3BJ/6-311++G(3d,2p) (Figure 4).65-67 Parallel calculations
performed using Møller Plesset perturbation theory68
(MP2/cc-pVTZ) are summarized in the supporting infor-
mation (Figure S1): there are no significant differences be-
tween the two model chemistries. The optimized structure of
the model reactant 1’ is very similar to the previously report-
ed crystallographic data for 1, with three equivalent C=P
bond lengths of 1.75 Å (cf. 1.727 Å from X-ray50) and a planar
C3P3 core. The key hydrogen activation step occurs via a 1,4
addition wherein the H2 approaches from above the plane
with the H-H vector aligned parallel to the 1-4 axis of the
ring. The computed free energy barrier of 28 kcal/mol for
this step (at 298 K) is consistent with the elevated tempera-
tures and high pressures needed to accelerate the reaction: it
is somewhat larger than those reported by Frenking et al.69
for the reaction of digermynes with H2 which proceed at
room temperatures and ambient pressures.
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Figure 4 Reaction coordinate for the hydrogenation of 1’ (free energies at
298 K computed at the BP86ꢀGD3BJ level of theory). NLMOs of the
transition state TS1’ are boxed. (NB 3’d is not shown).
logue. This result demonstrates that access to a boat-shaped
conformation with an electrophilic phosphorus center
The most stable isomer of 3’ (3’b) is connected to the
minor product, 2’b, via TS4’b with a barrier of 28 kcal/mol.
The transition structure is typical of a suprafacial hydride
migration, with the migrating hydrogen located between the
C and P atoms in an almost perfectly planar C2P3 unit. The
incipient P−P bond is partially formed at the transition states
(P−P 2.90 vs 3.20 Å in 3’b and 2.25 Å in 2’b). The major prod-
uct, 2’a, can be reached only via the less stable 3’a and then
intermediate TS4’a, but the overall barrier (measured from
3’b) is the same in both cases. The data suggest, therefore,
that there is little difference, thermodynamically or kinetical-
ly, between the pathways that lead to the major and minor
isomers.
and a nucleophilic carbon center allows for the polar addi-
tion of H2. Subsequent thermally-driven hydrogen atom
transfer and concurrent P−P bond formation leads to the
observed products. This unexpected observation of dihydro-
gen activation by an aromatic heterocycle provides a new
strategy for the design of metal-free hydrogenation catalysts.
EXPERIMENTAL SECTION
General Considerations: All manipulations were per-
formed under an atmosphere of dry, oxygen-free N2 by
means of standard Schlenk or glovebox techniques (MBraun
glovebox equipped with a –35 ºC freezer). All glassware was
oven-dried and cooled under vacuum before use. Bromoben-
zene (D5) was purchased from Cambridge Isotopes Laborato-
ries, Inc., and was degassed and stored over 4 Å molecular
sieves prior to use. NMR spectra were obtained on a Bruker
Avance 400 MHz spectrometer or an Agilent DD2 600 MHz
spectrometer and spectra were referenced to residual solvent
In conclusion, the aromatic species 1 reacts with H2 in
the absence of a catalyst under relatively mild conditions,
cleanly forming the bicyclic species 2a/2b. The para-H2 ex-
periments and computational data are consistent with a di-
rect, reversible reaction with H2 across the 1,4 axis leading to
an intermediate 1,3,5-triphosphacyclohexa-1,4-diene ana-
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of C6D5Br (1H = 7.28 ppm for meta proton; 13C = 122.4 ppm for
ipso carbon). Chemical shifts (δ) listed are in ppm and abso-
lute values of the coupling constants are in Hz. Mass spec-
trometry was performed in house.
at 100 MHz) or a 500 MHz Avance HD system ((1H at 500.13
MHz, 13C at 125 MHz). NMR samples were made up in a
Young’s tap equipped 5 mm NMR tube and C6D5Br (sigma)
was used as the solvent. Parahydrogen was prepared by cool-
ing hydrogen gas over Fe2O3 at 30 K. The NMR sample was
warmed progressively from 298 K. Upon reaching 375 K,
weak PHIP was observed in the 1H NMR signals of 3 at ca 6.4
(JPH = 228 (d), 6.5 (tr) Hz and JHH = 2 Hz) and 1.8 (JPH = 9.5
(tr), 5.0 (d) Hz and JHH = 2 Hz) ppm. HMQC measurements
revealed coupling to two 31P signals at 273.2 and -30.6 ppm. A
common 31P splitting of 6.5 Hz is evident in the spectra.
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X-Ray Data Collection, Reduction, Solution and Re-
finement: A single crystal of 2 was coated in paratone-N oil
and mounted. The data were collected using the SMART
software package on a Siemens SMART System CCD diffrac-
tometer using a graphite monochromator with MoΚα radia-
tion (λ = 0.71073 Å). Data reduction was performed using the
SAINT software package and an absorption correction was
applied using SADABS. The structures were solved by direct
methods using XS and refined by full-matrix least-squares on
F2 using XL as implemented in the SHELXTL suite of pro-
grams. All non-hydrogen atoms were refined anisotropically.
Carbon-bound hydrogen atoms were placed in calculated
positions using an appropriate riding model and coupled
isotropic temperature factors.
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ASSOCIATED CONTENT
Supporting Information
Full metrical parameters for the solid-state structure of com-
pound 2a are available free of charge from the Cambridge
Crystallographic Data Centre under reference CCDC-1014778.
Experimental and crystallographic data are available free of
charge via the Internet at http://pubs.acs.org.
Synthesis of 1: 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene
was prepared by a variation on the literature method 49. To a
stirred solution of Cl3V=NtBu58-59 (0.10 g, 0.44 mmol) in tolu-
ene (2 mL) at -30 oC was added tBuCP (0.18 g, 1.76
mmol). The solvent was immediately evaporated under re-
duced pressure, the residue dissolved in n-hexane
and purified by flash chromatography on silica gel. Removal
of the solvent gave ca. 0.10 g (30%) of yellow crystalline 2,4,6-
tri-tert-butyl-1,3,5-triphosphabenzene.
AUTHOR INFORMATION
Corresponding Author
dstephan@chem.utoronto.ca
ACKNOWLEDGMENT
DWS gratefully acknowledges the financial support of
NSERC of Canada, the award of a Canada Research Chair.
LEL is grateful for the support of an NSERC-CGS-D Scholar-
ship. NST and CAR acknowledge the Bristol Chemical Syn-
thesis Doctoral Training Centre for funding. KW acknowl-
edges Prof. Jian-Guo Zhang and financial support from
SKLEST一ZDKT1203 of China.
Synthesis of 2: In a 1 dr vial 15.0 mg (0.05 mmol, 1 equiv) of
P3C3tBu3 was weighed and dissolved in C6D5Br (0.4 mL), re-
sulting in a bright yellow solution. The solution was trans-
ferred to a J-Young tube and sealed. The vessel was degassed
three times though a freeze-pump-thaw cycle and filled with
H2 (4 atm) at -196 °C. The J-Young was then heated to 110 °C
in an oil bath, and over five hours the reaction became a faint
yellow solution. The C6D5Br was removed slowly under re-
duced pressure yielding diffraction-quality crystals (13 mg).
NMR data shows isomers A and B in a 4:1 ratio. Isomers A3
and B4 have been previously reported in the literature. The
recorded 31P NMR data are in agreement with literature re-
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2
1H NMR (400 MHz, C6D5Br, 298K): δ 2.98 (dd, JP-H = 3.9, 1.7
2
Hz, 1H, A C2 H), 2.59 (t, JP-H = 8.2 Hz, 1H, B C2 H), 1.49 (d,
4
4JP-H = 1.6 Hz, 9H, A C1 tBu), 1.46 (d, JP-H = 2 Hz, 9H, B C1
tBu), 1.28 (s, 9H, B C2 tBu), 1.13 (s, 9H, A C2 tBu), 0.93 (s, 9H,
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2
2
298K): δ 310.2 (dd, JP-P = 36.4, 13.1 Hz, A P1), 287.5 (d, JP-P
=
1
2
30.0 Hz, B P1), -117.8 (dd, JP-P = 166.2 Hz, JP-P = 36.4 Hz, A
P2), -128.8 (ddd, 1JP-P = 166.2 Hz, 2JP-P = 13 Hz, 2JP-H = 3.2 Hz, A
P3), -135.4 (dd, 1JP-P = 167.0 Hz, 2JP-P = 29.4 Hz, B P2), -171.9 (d,
1JP-P = 167.1 Hz, B P3). HRMS (DART) Calcd. for [C15H30P3]+
([M+H]+) 303.1560, found 303.1562.
Para-Hydrogen derived measurements: Hyperpolarized
NMR measurements were made on either a Bruker Avance
III series 400 MHz NMR spectrometer (1H at 400.13 MHz, 13
C
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