Activation of H2 by frustrated Lewis pairs derived from mono- and bis-phosphinoferrocenes and B(C6F5)3

Article abstract of DOI:10.1039/b819525k

Mono- and bis-phosphinoferrocenes are shown to react as frustrated Lewis pairs (FLPs) to effect nucleophilic aromatic substitution on B(C ₆F₅)₃ and/or activate H₂ to form ferrocenylphosphonium borates. The Royal

Full text of DOI:10.1039/b819525k

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Activation of H2 by frustrated Lewis pairs derived from mono- and
bis-phosphinoferrocenes and B(C6F5)3w
Alberto Ramos, Alan J. Lough and Douglas W. Stephan*
Received (in Cambridge, UK) 3rd November 2008, Accepted 10th December 2008
First published as an Advance Article on the web 12th January 2009
DOI: 10.1039/b819525k
Mono- and bis-phosphinoferrocenes are shown to react as fru-
strated Lewis pairs (FLPs) to effect nucleophilic aromatic
substitution on B(C6F5)3 and/or activate H2 to form ferrocenyl-
phosphonium borates.
the steric congestion required for FLP behavior and the ability
to heterolytically cleave H2.
The reaction of the phosphinoferrocene [(Z5-C5H5)Fe-
(Z5-C5H4PtBu2) with B(C6F5)3 proceeds in CH2Cl2 in a few
hours to yield an orange solid 1 in 92% isolated yield. The
31P NMR spectrum of 1 shows a triplet of triplet resonance
at 64.3 ppm, while the 11B{1H} NMR displays a doublet
at ꢀ0.5 ppm. These data support the formation of phosphonium
and borate centers. The 19F NMR spectrum shows signals
at ꢀ117.8 and ꢀ130.1 ppm attributable to a C6F4 fragment
in addition to resonances attributable to C6F5 and
BF units. These data support the formulation of 1 as
[(Z5-C5H5)Fe(Z5-C5H4PtBu2(C6F4)BF(C6F5)2)] (Scheme 1),
in which the phosphinoferrocene has effected attack at the
para-carbon of a C6F5 ring, prompting fluoride transfer to B.
This nucleophilic attack is presumably a result of steric bulk
that precludes classical dative bond formation between the
B and P centers.
In the last two years, we have unveiled the concept of
frustrated Lewis pairs (FLPs) which are combinations of
Lewis acids and bases where the Lewis acidity and
basicity remain unquenched by the intervention of
steric bulk in adduct formation.1 One such system has been
shown to be capable of the reversible activation of H2
by interconversion of (C6H2Me3)2PH(C6F4)BH(C6F5)2 and
(C6H2Me3)2P(C6F4)B(C6F5)2.2 This finding led to the devel-
opment of the first metal-free hydrogenation catalysts.3 Sub-
sequently, we showed that nitriles, aziridines and imines could
act as both the substrate and the base of an FLP thus leading
to hydrogenated products by the addition of catalytic amounts
of B(C6F5)3 under an H2 atmosphere.4 This was followed by
the development of related FLP catalysts using linked phos-
phonium borates by Erker and coworkers,5,6 while the Rieger
group has described related H2 activations using amines and
boranes.7,8 Using simple combinations of bulky trialkyl- or
triarylphosphines and boranes, we have demonstrated that
FLPs can effect the ring opening of THF,9,10 nucleophilic
aromatic substitution, P–B addition to olefins,11 the activation
of B–H bonds12 as well as heterolytic activation of H2.13
Simultaneously, we and the Tamm research group have also
extended FLPs to include combinations of boranes with NHC-
carbenes.14,15 Such systems activate H2 and NH bonds,
although the steric demands of the ligand substituents proved
critical for FLP reactivity. Most recently, we have shown that
monomeric phosphido-boranes R2PB(C6F5)2 also activate
H2 as a result of the poor p-overlap between the donor P
and the acceptor B.16
The related reactions of bis-phosphinoferrocene of
[(Z5-C5H4PR2)2Fe] (R = tBu, iPr) were also investigated.
The 1 : 1 stoichiometric reaction with B(C6F5)3 proceeds to
give pentane insoluble orange solids 2 and 3, in 96% and 91%
yields, respectively. In each case, the 31P{1H} NMR spectra
show two well separated resonances. The downfield signal
exhibits three-bond P–F coupling while the other signal is a
singlet. Similar to 1 the 19F NMR spectra showed signals
attributable to both C6F4 and C6F5 fragments. In addition,
11B{1H} doublet resonances at approximately ꢀ0.6 ppm were
consistent with the presence of one-bond BF coupling in both
The utility of the ferrocene backbone in ligand design has
prompted Erker and coworkers to probe ferrocenophane-based
FLPs.17 In these cases, reaction with H2 affords an unfunctio-
nalized ferrocenophane product and a phosphine–borane
adduct. Herein, we focused our attention on systems incor-
porating mono- and bis-ferrocenylphosphines. These systems
provide robust FLPs that illustrate both unique approaches to
Department of Chemistry, University of Toronto, 80 St George St,
Toronto, Ontario, Canada M5S 3H6.
E-mail: dstephan@chem.utoronto.ca
w Electronic supplementary information (ESI) available: Supple-
mentary data including experimental and spectroscopic details
have been deposited. CCDC number for 7 is 707707. For ESI
and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b819525k
Scheme 1 Reactions of bis-phosphinoferrocenes with B(C6F5)3.
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2 and 3. These data, together with the 1H NMR data,
were consistent with formulation of and as
2
3
[(Z5-C5H4PR2)Fe(Z5-C5H4PR2(C6F4)BF(C6F5)2)] (Scheme 1).
In the case of 2, addition of a second equivalent of B(C6F5)3
resulted in no further reaction at room temperature and led
to multiple decomposition products upon heating at 90 1C in
BrC6D5 for several days. In contrast, addition of a further
equivalent of B(C6F5)3 to 3 resulted in the formation of a new
species 4. This species was isolated in 96% yield. It exhibits
only a single 31P{1H} NMR triplet of triplets with P–F
couplings of 6 and 11 Hz. The form of the 19F and 11B{1H}
NMR spectra of 4 is similar to that seen for 1–3 with
resonances attributable to both C6F4 and C6F5 fragments
and BF units, respectively. These data as well as the
1H NMR data were consistent with formulation of 4 as
[(Z5-C5H4PiPr2(C6F4)BF(C6F5)2)]2Fe (Scheme 1). The con-
trasting reactivities of 2 and 3 with additional B(C6F5)3
suggest that in the case of 2 the increased steric congestion
precludes further reaction with B(C6F5)3.
Scheme 3 Activation of H2 to give 7.
reaction by NMR spectroscopy even on standing for 7 h at
room temperature. Thus, the steric demands resulting from the
C5Ph5 ring preclude Lewis acid–base adduct formation giving
rise to a FLP. However, exposure of this mixture to H2 results
in the immediate formation of a red solid 7 in 83% isolated
yield (Scheme 3). The 31P NMR spectrum of 7 comprises a
doublet at 49.0 ppm with a P–H coupling constant of 446 Hz,
while the 11B NMR signal is a doublet at ꢀ25.4 ppm with B–H
coupling of 90 Hz. These data, together with the H, 19F and
1
13C{1H} NMR data, are consistent with the formulation of 7
as [(Z5-C5Ph5)Fe(Z5-C5H4PHtBu2)][HB(C6F5)3]. This was
subsequently confirmed by X-ray crystallography (Fig. 1).z
The metric parameters are unexceptional, although it is note-
worthy that the closest approach of the BH to PH fragment in
the solid state is observed to be 2.927 A. Preliminary electro-
chemical studies of 7 showed reversible oxidation in CH2Cl2
using [Bu4N][PF6] as the electrolyte at a potential of 0.56 V
relative to the ferrocene/ferrocenium couple. In addition, an
irreversible wave at ꢀ2.23 V was attributed to reduction of the
proton of the phosphonium moiety to give H2. This view is
consistent with previously published electrochemical data for
other ferrocenyl phosphonium derivatives.18 The growth of a
reversible wave at 0.12 V was attributable to the presence of
parent phosphine, [(Z5-C5Ph5)Fe(Z5-C5H4PtBu2)].19
Subsequent reaction of 2 with Me2SiHCl afforded its stoi-
chiometric conversion to [(Z5-C5H4PtBu2)Fe(Z5-C5H4PtBu2-
(C6F4)BH(C6F5)2)] 5. The latter species was isolated in
75% yield and the formulation is evident from NMR
data, in particular the 11B NMR signal at ꢀ25.0 ppm
with B–H coupling of 87 Hz. Exposure of
5 and
B(C6F5)3 to H2 resulted in a rapid reaction to effect
the formation of the corresponding phosphonium borate,
[(Z5-C5H4PHtBu2)Fe(Z5-C5H4PtBu2(C6F4)BH(C6F5)2)][HB(C6F5)3]
(6). This species was isolated in 71% yield (Scheme 2). Thus,
the remaining P center of 5 is sterically encumbered enough to
preclude interaction with B(C6F5)3 and this unquenched Lewis
basicity and acidity prompts the activation of H2 affording 6.
An alternative strategy to similar sterically encumbered ferro-
cenylphosphines involves the use of [(Z5-C5Ph5)Fe(Z5-C5H4PtBu2)].
Reaction of this species with B(C6F5)3 showed no evidence of
Collectively, these data demonstrate the unique reactivity of
bulky phosphinoferrocenes with B(C6F5)3. Steric demands
about P preclude reaction with B(C6F5)3 to give simple Lewis
acid–base adducts. Instead, nucleophilic aromatic substitution
can occur with concurrent F transfer to B. Alternatively, in
cases where the steric congestion is more severe, no reaction is
seen with borane. However, such sterically frustrated Lewis
pairs react rapidly with H2 effecting the heterolytic cleavage,
yielding stable ferrocenylphosphonium borates. This reactivity
establishes the viability of the ferrocene-scaffold for the
Fig. 1 POV-ray drawing of one ion pair in the asymmetric unit of 7.
Scheme 2 Formation of 5–6.
One conformation of the disordered anion is shown.
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development of FLPs. Synthetic and reactivity studies of these
and related systems are underway.
7 V. Sumerin, F. Schulz, M. Atsumi, C. Wang, M. Nieger,
M. Leskela, T. Repo, P. Pyykko and B. Rieger, J. Am. Chem.
Soc., 2008, 130, 14117.
8 V. Sumerin, F. Schulz, M. Nieger, M. Leskela, T. Repo and
B. Rieger, Angew. Chem., Int. Ed., 2008, 47, 6001.
9 G. C. Welch, J. D. Masuda and D. W. Stephan, Inorg. Chem.,
2006, 45, 478–480.
Financial support from NSERC of Canada is gratefully
acknowledged.
Notes and references
10 G. C. Welch, R. Prieto, M. Dureen, A. J. Lough, S. Labeodan,
T. Holtrichter-Rossmann and D. W. Stephan, Dalton Trans., 2008,
¨
¨
accepted for publication.
z Crystallographic data for 7: C66H49BF15FeP, MW = 1224.68,
ꢀ
T = 150 K, space group triclinic, P1, a = 11.9830(2) A, b =
11 J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem.,
Int. Ed., 2007, 46, 4968–4971.
12 M. A. Dureen and D. W. Stephan, Chem. Commun., 2008, 4303.
13 G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129,
1880–1881.
21.9410(4) A, c = 22.8040(4) A, a = 71.3220(11)1, b = 84.4670(11)1,
g = 80.923(1)1, V = 5601.96(17) A3, Z = 4, m = 0.390 mmꢀ1
,
measured reflections = 25 496, independent reflections = 1663, para-
meters = 186, Rint = 0.0602, for observed data: R = 0.0635, wR =
0.1482, GOF = 1.004. Disorder of the borate anion was modeled, while
slight disorder of the tBu substituents in the cation was not.
14 P. A. Chase and D. W. Stephan, Angew. Chem., Int. Ed., 2008,
47, 7433.
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2 G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan,
Science, 2006, 314, 1124.
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¨
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16 S. Geier, T. M. Gilbert and D. W. Stephan, J. Am. Chem. Soc.,
2008, 130, 12638.
17 D. P. Huber, G. Kehr, K. Bergander, R. Frohlich, G. Erker,
¨
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