Frustrated Lewis pairs beyond the main group: Cationic zirconocene- phosphinoaryloxide complexes and their application in catalytic dehydrogenation of amine boranes

Article abstract of DOI:10.1021/ja201989c

The cationic zirconocene-phosphinoaryloxide complexes [Cp ₂ZrOC₆H₄P(t-Bu)₂][B(C ₆F₅)₄] (3) and [Cp*₂ZrOC ₆H₄P(t-Bu)₂][B(C₆F₅) ₄] (4) were synthesized by the reaction of Cp₂ZrMe ₂ or Cp*₂ZrMe₂ with 2-(diphenylphosphino) phenol followed by protonation with [2,6-di-tert-butylpyridinium][B(C ₆F₅)₄]. Compound 3 exhibits a Zr-P bond, whereas the bulkier Cp* derivative 4 was isolated as a chlorobenzene adduct without this Zr-P interaction. These compounds can be described as transition-metal-containing versions of linked frustrated Lewis pairs (FLPs), and treatment of 4 with H₂ under mild conditions cleaved H ₂ in a fashion analogous to that for main-group FLPs. Their reactivity in amine borane dehydrogenation also mimics that of main-group FLPs, and they dehydrogenate a range of amine borane adducts. However, in contrast to main-group FLPs, 3 and 4 achieve this transformation in a catalytic rather than stoichiometric sense, with rates superior to those for previous high-valent catalysts.

Full text of DOI:10.1021/ja201989c

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Frustrated Lewis Pairs beyond the Main Group: Cationic
ZirconoceneꢀPhosphinoaryloxide Complexes and Their
Application in Catalytic Dehydrogenation of Amine Boranes
Andy M. Chapman, Mairi F. Haddow, and Duncan F. Wass*
School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
S Supporting Information
b
here the synthesis and structural characterization of cationic
ABSTRACT: The cationic zirconoceneꢀphosphinoarylox-
ide complexes [Cp2ZrOC6H4P(t-Bu)2][B(C6F5)4] (3) and
[Cp*2ZrOC6H4P(t-Bu)2][B(C6F5)4] (4) were synthesized
by the reaction of Cp2ZrMe2 or Cp*2ZrMe2 with 2-(diphenyl-
phosphino)phenol followed by protonation with [2,6-di-
tert-butylpyridinium][B(C6F5)4]. Compound 3 exhibits a
ZrꢀP bond, whereas the bulkier Cp* derivative 4 was
isolated as a chlorobenzene adduct without this ZrꢀP
interaction. These compounds can be described as transi-
tion-metal-containing versions of linked frustrated Lewis
pairs (FLPs), and treatment of 4 with H2 under mild
conditions cleaved H2 in a fashion analogous to that for
main-group FLPs. Their reactivity in amine borane dehy-
drogenation also mimics that of main-group FLPs, and they
dehydrogenate a range of amine borane adducts. However,
in contrast to main-group FLPs, 3 and 4 achieve this
transformation in a catalytic rather than stoichiometric sense,
with rates superior to those for previous high-valent catalysts.
zirconoceneꢀphosphinoaryloxide complexes that activate H2.
As a model catalytic reaction, we show that these complexes
mediate dehydrogenation (dehydrocoupling) of amine borane
adducts in a manner analogous to that for metal-free FLPs11 and
that in contrast to purely main-group FLPs, they achieve this
transformation in a catalytic rather than stoichiometric sense.
We initially targeted a zirconocene species14 with a 2-(di-tert-
butylphosphino)phenoxide ligand, reasoning that this would
incorporate favorable design features such as a strong ZrꢀO
bond and an intramolecular phosphine donor that could serve as
a potent but sterically encumbered Lewis base. We recently
described related phosphinoꢀborinate ester Lewis pairs.15 The
neutral complex [Cp2Zr(Me)OC6H4P(t-Bu)2] was prepared by
reaction of the previously reported16 HOC6H2P(t-Bu)2 with
Cp2ZrMe2 (Scheme 1) in hexanes under ambient conditions.
The novel compound 1 was isolated as an analytically pure, air-
and moisture-sensitive crystalline solid in high yield (89%). The
phosphine showed no sign of coordination to the zirconium
center, as evidenced by NMR spectroscopy [see the Supporting
Information (SI)].17,18 In an entirely analogous fashion, the
bulkier pentamethylcyclopentadienyl (Cp*) derivative 2 was
obtained in 90% yield.
We next sought to access the base-free cationic complexes 3
and 4 by protonolysis of the remaining methyl ligand. Reaction of
equimolar quantities of 1 and [DTBP][B(C6F5)4] (DTBP = 2,6-
di-tert-butylpyridinium) in fluorobenzene (Scheme 1) instanta-
neously resulted in a bright-yellow solution. 31P NMR spectroscopy
revealed that protonation of the pendant phosphine preceded
protonation of the methyl ligand, as evidenced by a characteristic
doublet at 23.8 ppm with J = 478 Hz. Large orange crystals of 3
grown from a layered fluorobenzene/hexane solution at room
temperature were used for structure determination (Figure 1).
Complex 3 displays the expected pseudotetrahedral geome-
try at Zr and a distorted five-membered ring. Noteworthy is
the presence of a ZrꢀP bond, although at 2.8826(5) Å it is long
for a Zr(IV)ꢀalkylphosphine bond {e.g., 2.693(4) Å for
[Cp2Zr(η2-py)PMe3][BPh4]}19 From the large downfield shift
in the 31P NMR spectrum relative to 1 and the lack of
observable evidence for coordinated fluorobenzene in the 19F
NMR spectrum, it appears that the ZrꢀP interaction persists in
solution, even at temperatures up to 80 °C. We note that there
are other examples of main-group FLPs that exhibit the char-
acteristic reactivity but for which structural characterization
olution-phase combinations of sterically hindered (“frus-
S
trated”) Lewis acidꢀLewis base pairs have been the subject of
recent interest, not least because of the high latent reactivity of such
species in the activation of small molecules, the reversible heterolytic
cleavage of H2 being the archetypal example.1ꢀ3 The promise of
such species lies in the possibility of metal-free catalysis, and
catalytic hydrogenations of bulky imines, enamines,4 and other
specific substrates5 have been reported. Numerous other inter-
esting reactivity patterns6 have been observed, including hetero-
lytic cleavage of disulfides, reversible CO2 binding7 and reduction to
methanol,8,9 ring opening of cyclic ethers,10 and amine borane
dehydrogenation.11 However, none of these transformations has
been achieved in a catalytic sense, usually because the resulting
products are too stable. We reasoned that replacing the Lewis
acid component of such frustrated Lewis pairs (FLPs), usually a
polyfluorinated aryl borane, with an electrophilic transition-
metal center such as Zr(IV) might offer a solution to this
drawback. Clearly, the implicit notional novelty of metal-free
catalysis could not be claimed for such a zirconiumꢀphosphine
pair; however, combining the ability of transition-metal com-
plexes in catalysis with the capability of FLPs to activate substrate
molecules via ditopic activation offers exciting possibilities for
exploiting new activation pathways and reactivity patterns in
catalytic processes. Our approach12 is mirrored in the activation
of small molecules using a combination of a low-valent transition-
metal center with a pendant Lewis acid function.13 We report
Received: March 4, 2011
Published: May 06, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Compounds 3 and 4
Scheme 2. Dihydrogen Activation by 4
Figure 2. ORTEP representation of the molecular structure of 5. The
borate anion and H atoms except for ZrꢀH and PꢀH have been omitted
for clarity; thermal ellipsoids are drawn at the 50% probability level.
Selected bond lengths (Å) and angles (deg): Zr1ꢀH1, 2.23(1);
Zr1ꢀO1, 2.038(2); C21ꢀO1, 1.324(4); H1ꢀZr1ꢀO1, 87.8(4);
Zr1ꢀO1ꢀC21, 158.8(2).
Compound 5 was also obtained when the neutral complex
[Cp*2Zr(H)OC6H4P(t-Bu)2] (Scheme 2; also see the SI) was
treated with 1 equiv of [DTBP][B(C6F5)4]. Crystals of 5 suitable
for X-ray diffraction were grown from a chlorobenzene solution
layered with hexane, and the obtained structure is shown in
Figure 2. Although the hydride was not unambiguously located
by X-ray crystallography, its presence was supported by NMR
analysis,23 the results of the independent synthesis (Scheme 2),
and deuterium-labeling experiments (see the SI for detailed dis-
cussion). Despite the apparent chemical similarity between 5 and
the intermediate [Cp*2Zr(Me)OC6H4PH(t-Bu)2] (Scheme 1),
5 was resistant to the reverse reaction (i.e., intramolecular
protonolysis) even at elevated temperatures (1 week, 120 °C)
but immediately liberated H2 upon addition of THF or CH2Cl2.
With these promising initial results, we investigated utilizing 3
and 4 for the dehydrogenation (dehydrocoupling) of a series of
amine borane adducts AꢀC (Figure 3). This reaction has
attracted significant attention because of the potential for use
of amine borane as a hydrogen storage material and as a controlled
route to new inorganic polymers.24 It is also an ideal test reaction
in that it is known only in a stoichiometric sense for main-group
FLPs11 and, despite numerous other transition-metal-based
catalysts,25ꢀ33 is at best extremely sluggish (reaction over the
course of weeks) for high-valent Zr(IV) systems.34 When a
fluorobenzene solution of complex 3 was treated with 10 equiv
of dimethylamine borane (A), the evolution of H2 was observed,
and the expected dehydrocoupling reaction proceeded smoothly
Figure 1. ORTEP representation of the molecular structure of 3. All H
atoms and the borate anion have been omitted for clarity; thermal
ellipsoids are drawn at the 50% probability level. Selected bond lengths
(Å) and angles (deg): Zr1ꢀO1, 1.9972(13); Zr1ꢀP1, 2.8826(5);
C16ꢀP1, 1.816(2); O1ꢀC11, 1.360(3); O1ꢀZr1ꢀC11, 124.51(11);
O1ꢀZr1ꢀP1, 70.18(4).
revealed a persistent Lewis acidꢀbase bond.20,21 With a similar
protocol, protonolysis of 2 with [DTBP][B(C6F5)4] in chloro-
benzene afforded an orange solution. In this case, a 31P NMR
resonance at 7.0 ppm suggested the absence of a ZrꢀP
interaction, and other spectroscopic and elemental analysis
data indicated a chlorobenzene adduct. This is a result of the
substantially bulkier Cp* ligands, which we presume make the
related ZrꢀP species too sterically congested.
In many ways, the “standard” reaction of main-group FLPs is
the heterolytic cleavage of H2, and we were keen to evaluate the
behavior of 3 and 4 in this regard. While compound 3 did not react
with 1 bar H2 at 25 °C, 4 rapidly cleaved H2 in a heterolytic
fashion1 across the Zr and P centers to give ZrꢀH phosphonium
complex 5 in high (93% isolated) yield (Scheme 2). This reactivity
is related to that of certain hydrogenation catalysts,22 where in
particular H2 addition across RuꢀN bonds (i.e., with N acting as
an internal base) is well-established.
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Figure 3. Amine borane dehydrogenation.
in the presence of 1 mol % 3 to give 100% conversion of A after
only 10 min at room temperature. 11B NMR analysis of the
reaction mixture revealed nearly quantitative conversion to the
cyclic dimer A1 along with small quantities (8ꢀ10% yield)35
of A2, which slowly converted into A1. In contrast to the
Cp2TiCl2/n-BuLi system reported by Manners and co-
workers,36 none of the linear dimer was observed at any point
during the reaction. The activity of our catalyst [in this case, a
turnover frequency (TOF) of 600 hꢀ1] was exceptional in
comparison with all known group 4 systems.37 Compound 4
facilitated the same catalytic reaction, although the reaction was
more sluggish in this case, affording 99% yield after 8.5 h with a
catalyst loading of 5 mol %. The isostructural Hf analogue of 3
(for full details, see the SI) was also active (99% yield after 1 h at
1% catalyst loading). Encouraged by these results, we repeated
Figure 4. ORTEP representation of the molecular structure of 6. The
borate anion and H atoms except BꢀH have been omitted for clarity;
thermal ellipsoids are drawn at the 50% probability level. Selected bond
lengths (Å) and angles (deg): Zr1ꢀH38A, 2.23(2); Zr1ꢀH38B,
2.31(2); Zr1ꢀH38C, 3.32(3); Zr1ꢀB2, 2.728(3); Zr1ꢀO1, 1.983(2);
B2ꢀN1, 1.588(3). C21ꢀO1ꢀZr1, 160.1(1); O1ꢀZr1ꢀH38A,
116.0(6); O1ꢀZr1ꢀH38B, 78.1(6); O1ꢀZr1ꢀH38C, 110.0(5).
Scheme 3. Proposed Catalytic Cycle
the experiment with diisopropylamine borane and 5 mol % 3. 11
B
NMR analysis of the reaction mixture after 3 h showed 100%
conversion to the previously reported dehydrocoupled amino-
borane B1.37 Repeating the reaction with 1 mol % 3 gave B1 in
100% yield after 19 h. To complete the series, we investigated the
utility of 3 and 4 in the catalytic dehydrocoupling of ammonia
borane (C). Treatment of C with 5 mol % 3 or 4 gave traces
(∼5%) of borazene (C1) and products consistent with polymers
C2 of the type (BH2NH2)n.25 When the same reaction was
carried out in the presence of a 10-fold excess of cyclohexene
relative to C, 50% conversion to the trapped hydroboration
product Cy2BNH2 was observed, suggesting the intermediacy of
monomeric H2BNH2.38 It is noteworthy that the existing group 4
catalyst Cp2TiCl2/n-BuLi is completely inactive toward C.
In an effort to probe the catalytic cycle, we carried out the
stoichiometric reaction of 3 with the fully deuterated isotopomer
dehydrogenation, would allow isolation of compounds structu-
rally similar to the proposed intermediates, as demonstrated by
Weller and co-workers.32 Although 3 showed no reaction with
Me3NBH3 at room temperature,42 4 reacted immediately, and
the desired complex [Cp*2Zr(H3BNMe3)OC6H4P(t-Bu)2][B-
(C6F5)4] (6) was isolated in nearly quantitative yield. The
structure of 6 is shown in Figure 4. The intermediacy of both 5
and 6 in the catalytic cycle is supported by the fact that each acts
as a catalyst for the transformation of A to A1.
On the basis of these experiments, we propose that the catalysis
proceeds as depicted in Scheme 3. Despite the superficial similarity
of previous group 4 metallocene catalysts for amine borane
dehydrogenation, our catalysts are fundamentally different. Cata-
lysts based on titanocene fragments have been shown in detailed
mechanistic studies36 to be based on Ti(II) species formed either
in situ from Cp2TiCl2/n-BuLi or as well-defined Cp2Ti(PMe3)2
complexes. Catalysis then proceeds via a metal-centered oxidative
addition (to yield Cp2TiH2)/reductive elimination manifold. In
contrast to these highly active titanium systems, the analogous
zirconium species exhibited at best only very low activity, requiring
high catalyst loadings and extended reaction times to achieve even
modest conversion (e.g., 65% conversion of A after 20 h with
2
of A (AD).39 11B{1H}, 31P{1H}, and H NMR analysis of the
reaction mixture immediately after mixing (∼5 min) revealed
only 3, A1D, A2D, and D2. Adding a further 10 equiv gave after 10
min >99% conversion to A1D and A2D, 3, and traces of a species
with a singlet resonance at 8.9 ppm in the 31P{1H} NMR
spectrum, which we attributed to the σ-complex intermediate
[Cp2Zr(D3BNDMe2)OC6H4P(t-Bu)2]þ. Carrying out an ana-
logous set of experiments with 4 and AD gave only 5D,40 A1D, and
A2D. Approximately 1 h after treatment with a further 10 equiv of
AD, the 31P NMR spectrum showed only a singlet resonance at
8.0 ppm, while the 11B NMR spectrum showed ∼75% conver-
sion to A1D and a broad resonance at ꢀ14.0 ppm,41 which we
again attributed to the intermediate [Cp*2Zr(D3BNDMe2)OC6-
H4P(t-Bu)2]þ. After 16 h, 5D and A1D were the only species
observable by NMR analysis. Because of the inherent reactivity
of complexes 3 and 4 with A, only tentative spectroscopic
evidence for the σ-complex adducts could be obtained. We
reasoned that the use of Me3NBH3, which cannot undergo
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10 mol % catalyst), while the Hf system was found to be totally
inactive.36 In our systems, only high-valent zirconium is present,
and our observations point to a mechanism similar to that of the
stoichiometric dehydrogenation with the metal-free Lewis pair
P(t-Bu3)/B(C6F5)3,11,43 with the crucial difference that H2 elim-
ination is facile under these conditions, regenerating the active
catalyst and leading to catalytic turnover (Scheme 3). All of the
illustrated intermediates were observed and structurally character-
ized. Critically, the essential nature of the phosphine fragment was
highlighted when we attempted the dehydrogenation of A using
the known complex [Cp2ZrOR][B(C6F5)4] (R = t-Bu).44 Even
with loadings of this species as high as 50 mol %, no detectable
quantities of A1 or A2 were observed by 11B NMR analysis.
In conclusion, we have synthesized cationic zirconoceneꢀ
phosphinoaryloxide complexes that under various conditions
activate H2 and dehydrogenate amine boranes. We suggest that
drawing an analogy between our systems and frustrated Lewis
pairs is useful. Although much of the previous literature high-
lights the metal-free nature of FLPs, we see value in extending
this concept to frustrated combinations of electrophilic transis-
tion-metal species and Lewis bases. Certainly, all of the reactivity
we have observed is well-described as FLP chemistry, with the
crucial addition that our systems achieve amine borane dehy-
drogenation in a catalytic sense.
(12) During the preparation of this manuscript, N2O activation by a
nonlinked zirconocene phosphine Lewis pair was reported. See: Neu,
R. C.; Otten, E.; Lough, A.; Stephan, D. W. Chem. Sci. 2011, 2, 170.
(13) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, selected
b
NMR spectra, and crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
Duncan.Wass@bris.ac.uk
’ ACKNOWLEDGMENT
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(11B{1H} NMR: 27.8 ppm) and [H2B(μ-H)(μ-NMe2)BH2] (11B{1H}
NMR: ꢀ19.0ppm).See:Whittell, G. R.;Balmond, E. I.;Robertson, A. P. M.;
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(39) The deuterated analogue of A1 (A1D) reduced the reaction rate
and facilitated the unambiguous observation of [PꢀD]þ intermediates.
(40) Unlabeled 5 was also present as a result of incomplete
deuteration of AD.
We thank the University of Bristol for funding and Prof.
Ian Manners, Dr. George Whittell, Mr. James Vance, and Mr.
Alasdair Robertson for helpful discussions and the provision of
some of the amine borane adducts.
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