The hydride-ion affinity of borenium cations and their propensity to activate H2 in frustrated Lewis pairs

Article abstract of DOI:10.1002/chem.201203318

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu ₃)]⁺ has a lower hydride-ion affinity (HIA) than B(C ₆F₅)₃. This resulted in H₂ activation being energetically unfavourable in a FLP with the strong base PtBu₃. However, ligand disproportionation of CatBH(PtBu₃) at 100 °C enabled trapping of H₂ activation products. DFT calculations at the M06-2X/6-311G(d,p)/PCM (CH₂Cl₂) level revealed that replacing catechol with chlorides significantly increases the chloride-ion affinity (CIA) and HIA. Dichloro-borenium cations, [Cl ₂B(amine)]⁺, were calculated to have considerably greater HIA than B(C₆F₅)₃. Control reactions confirmed that the HIA calculations can be used to successfully predict hydride-transfer reactivity between borenium cations and neutral boranes. The borenium cations [Y(Cl)B(2,6-lutidine)]⁺ (Y=Cl or Ph) form FLPs with P(mesityl) ₃ that undergo slow deprotonation of an ortho-methyl of lutidine at 20 °C to form the four-membered boracycles [(CH₂{NC ₅H₃Me})B(Cl)Y] and [HPMes₃]⁺. When equimolar [Y(Cl)B(2,6-lutidine)]⁺/P(mesityl)₃ was heated under H₂ (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation. DFT and experimental studies indicated that [Cl ₂B(2,6-lutidine)]⁺ has a greater hydride-ion affinity (HIA) than B(C₆F₅)₃. Heating the frustrated Lewis pair formed from [Cl₂B(2,6-lutidine)]⁺ and P(mesityl)₃ under H₂ resulted in activation of H ₂ and deprotonation of lutidine to form the boracycle [(CH ₂{NC₅H₃Me})BCl₂] (see scheme). Copyright

Full text of DOI:10.1002/chem.201203318

DOI: 10.1002/chem.201203318
The Hydride-Ion Affinity of Borenium Cations and Their Propensity to
Activate H2 in Frustrated Lewis Pairs
Ewan R. Clark, Alessandro Del Grosso, and Michael J. Ingleson*[a]
Abstract: A range of frustrated Lewis
pairs (FLPs) containing borenium cat-
with chlorides significantly increases
the chloride-ion affinity (CIA) and
and neutral boranes. The borenium cat-
ions [Y(Cl)B(2,6-lutidine)]+ (Y=Cl or
A
HIA.
[Cl2B
Dichloro–borenium
cations,
Ph) form FLPs with PACTHUGNTERNNU(G mesityl)3 that
G
undergo slow deprotonation of an
ortho-methyl of lutidine at 208C to
form the four-membered boracycles
G
have considerably greater HIA than
BACHTUGNTREN(NNGU C6F5)3. Control reactions confirmed
U
resulted in H2 activation being energet-
ically unfavourable in a FLP with the
strong base PtBu3. However, ligand dis-
that the HIA calculations can be used
to successfully predict hydride-transfer
reactivity between borenium cations
[(CH
[HPMes3]+.
[Y(Cl)B(2,6-lutidine)]+/P
ACTHNUGTRENUN(G mesityl)3 was
G
and
equimolar
proportionation of CatBH
1008C enabled trapping of H2 activa-
tion products. DFT calculations at the
G
heated under H2 (4 atm), heterolytic
cleavage of dihydrogen was competi-
tive with boracycle formation.
Keywords: borenium cations · bo-
AHCTUNGTRENNUNG
M06-2X/6-311G
N
level revealed that replacing catechol
Introduction
librium [Eq. (1)] drastically towards the neutral starting ma-
terials, even when using a strong LB.[23–25]
Sterically frustrated Lewis pairs (FLPs) utilise unquenched
Lewis acidity and basicity for the concerted activation of
small molecules.[1,2] This field has developed rapidly since
Stephanꢀs breakthrough publication in 2006,[3] with a range
of impressive transformations reported including the revers-
Borenium cations are three coordinate borocations[26] that
can possess considerable Lewis acidity engendered by the
positive charge.[27] Recently, this high electrophilicity has
been utilised for applications in organic transformations.[28,29]
We have previously synthesized borocation-based FLPs
based on catechol and thiocatechol (o-C6H4S2) ligated bo-
ACHTUNGTRENNUNGible activation of H2 and the catalytic reduction of unsatu-
rated and aromatic moieties.[4–16] A plethora of Lewis bases,
ranging from moderately basic (2,4,6-trimethylphenyl)3P
(PMes3) to strongly basic N-heterocyclic carbenes (NHCs),
have been successfully employed in FLP H2 activation. In
contrast, excluding a limited number of notable excep-
tions,[17–20] the Lewis acidic component has largely been lim-
AHCTUNGTRENNUNG
ited to highly fluorinated aryl boranes, particularly B
G
N
ACHTUNGTRENNUNG
This is due to a minimum cumulative Lewis acid (LA)/base
(LB) strength being a prerequisite for H2 activation by a
Lewis acidity towards hydride ions should enable the exten-
sion of FLP activation of H2 towards weaker LBs. Recogniz-
ing the potential of highly electrophilic cationic LAs in
FLPs, Mꢁller et al. have recently reported that a silicenium
cation [(Me5C6)3Si]+ formed an FLP with PMes3 that rapidly
FLP.[9,21,22] Studies using FLPs containing alternative bo
ACHTUNGTRENNUNG
ACHTUNGTRENUNG
cleaved H2. However, this FLP did not activate H2 with
[17]
weakly basic PACTHNUGTRNE(UNG C6F5)3. Borenium cations are valence iso-
AHCTUNGTREGeNNUN lectronic to silicenium cations, and a first indication that
[a] Dr. E. R. Clark, A. Del Grosso, Dr. M. J. Ingleson
Department of Chemistry, University of Manchester
Manchester, M13 9PL (U.K.)
they could activate H2 came from the work of Vedejs et al.,
who observed deuterium incorporation on heating 1 under a
D2 atmosphere (Figure 1, top).[31] Although the heteroge-
E-mail: Michael.ingleson@manchester.ac.uk
Supporting information for this article contains experimental details,
full crystal structure determination data, additional NMR spectro-
scopic discussion and explicit spectra, as well as details of the quan-
tum chemical calculations including total energies and xyz coordi-
nates. It is available on the WWW under http://dx.doi.org/10.1002/
chem.201203318.
AHCTUNGTREGnNNNU eous nature of this system precluded mechanistic elucida-
tion, H2 activation by a FLP was one possibility. Insight into
the hydride-ion affinity (HIA) of borenium cations, particu-
larly relative to BACTHNUTRGNE(UNG C6F5)3, is limited. One exception comes
from the work of Stephan et al., which synthesized [CatB-
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FULL PAPER
affinity (FIA), which is greater than that of B
of SbF5 =489 compared to a FIA of 444 kJmolÀ1 for
(C6F5)3).[36] Clearly, there is a distinction between the
Lewis acidity of [CatB
(amine)]+ towards highly electroneg-
ACHTNUGRTNE(NGNU C6F5)3 (FIA
BACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ative atoms and towards less electronegative moieties (e.g.,
hydrides). Presumably this is due to the large electrostatic
component to bonding between boron and FÀ or Et3POÀ,
consistent with Pearsonꢀs classic hard-soft acid-base
ACTHNUTRGNEUNG
theory.[37] Hence, borocations, such as [CatB(NEt3)]+, which
have a high calculated natural bond order (NBO) positive
charge at boron (+1.365e at the MPW1K/6-311+ +GACHTUNGTRENNUNG(d,p)
level)[30] are highly Lewis acidic towards hard bases, but less
so towards hydrides. This is related to Britovsekꢀs observa-
tion on the reversal of the relative Lewis acidity of the
Figure 1. Recent examples of H2 activation by borocations and the appli-
cation of 2 as a catalyst for the hydrogenation of enamines.
BACHTNUTGRENNG(U C6F5)3ÀxACHTUGNTREN(NUGN OC6F5)x (x=0–3) series with respect to Et3PO
and Et3PS LBs.[38]
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ported the first definitive FLP-based activation of H2 by
using a borenium cation 2 (Figure 1, bottom). Compound 2
also has a lower HIA than BACHTUNRGTNE(UNG C6F5)3 based on hydride-trans-
fer reactivity.[33] Borenium cations are particularly attractive
for catalysis, because they are inexpensive and simple to
produce whilst being easily modified to vary electrophilicity
and steric factors. Their catalytic utility has been recently
demonstrated by Stephan et al. with 2 proving a highly ef-
Although catechol–borenium cations are less Lewis acidic
towards hydride than BACHTUNRGTNE(UGN C6F5)3, it remained feasible that
they would activate H2 in a FLP with a strong LB analogous
to the borenium cation 2.[33] Because stable FLPs obtained
by using [CatB
ture of PtBu3/AHCTUNTGREG[NNNU CatBACHTUNRTEGGN(NNU PtBu3)]ACTHNUGTREN[UNNG AlCl4] (3) was investigated. It
(NEt3)]+ were not accessible,[39] the 1:1 mix-
ACHTUNGTRENNUNG
fective hydro
with improved selectivity and functional-group tolerance
compared to B
(C6F5)3 and its derivatives.[33]
A
displayed broad 31P{1H} and 11B NMR resonances (d=34.9
À
N
and 21.5 ppm, respectively) both devoid of B P coupling in-
dicative of fluxionality involving B P bond cleavage.[39] Mul-
À
Herein, we report our recent studies into the incorpora-
tion of borenium cations into FLPs for H2 activation. This
includes combined experimental and computational studies
demonstrating that borocations possess an extremely wide
range (>30 kcalmolÀ1) of HIAs. Significantly, dichloro-ligat-
ed borenium cations were found to have considerably great-
tinuclear NMR spectroscopy at À408C revealed the pres-
ence of two major species, the borenium cation 3 and free
[39]
PtBu3.
Thus, compound 3 and PtBu3 exist as a FLP in
equilibrium with its Lewis adduct [CatBACTHUNGTRENNUNG
(PtBu3)2]+. The ad-
dition of H2 (ca. 4 atm) to equimolar 3/PtBu3 in C6H4Cl2
(+10% C6D6) led to no reaction at 258C (18 h). However,
er HIA than that of B
ACHTUNGTRENNUNG
ed from phosphine- and amine-ligated bo
N
heating this soACTHNGUTRENNUlG uCAHTUNGTRNEtNUNG ion to 1008C for 24 h resulted in the forma-
accessible including examples that activate H2.
tion of products consistent with H2 activation (Figure 2).
The expected initial products from heterolytic H2 cleavage
by 3/PtBu3 are [HPtBu3]+ and CatBH
ACTHNUTRGNEU(NG PtBu3), but the latter
Results and Discussion
is in equilibrium with CatBH and PtBu3. This system is fur-
ther complicated by CatBH/PtBu3 disproportionating rapidly
FLPs containing catecholato-ligated borenium cations:
[CatBACHTUNGTRENNUNG
(amine)]+ cations have HIA lower than that of
B
B
ACHTUNGTRENNUNG
A
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG(C6F5)3] (amine=Et3N and 4,N,N-trimethylaniline). This is
in contrast to the relative Lewis acidities determined by the
Gutmann–Beckett method.[34,35] The greater downfield
chemical shift of the phosphorus resonance of Et3PO upon
coordination to [CatB
cation is more Lewis acidic towards Et3PO than B
[Eq. (2)]. Furthermore, we have previously reported that
[CatB
(NEt3)]+ abstracts fluoride from [SbF6]À,[30] indicating
that this borenium cation has an extremely high fluoride-ion
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
Figure 2. Schematic representation of the equilibrium with 3/PtBu3 and
activation of H2 with subsequent ligand disproportionation.
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2463

M. J. Ingleson et al.
at 1008C[40] to form products including B2cat3 and
(tBu3P)BH3 (determined by 11B NMR spectroscopy).[41,42]
Unambiguous confirmation of dihydrogen activation was
provided by the exposure of equimolar 3/PtBu3 to D2. This
led to the formation of [DPtBu3]+ (a 1:1:1 triplet in the
the respective NBO charges at boron of 1.34 and 0.40 e;[30]
2) the enhanced steric demand of chloride versus hydride.
This is particularly noticeable with [Cl2B(2,6-lutidine)]+,
([4]+), in which the CIA is significantly lower than expected
based on the HIA value. The discrepancy arises from unfa-
ACHTUNGTRENvNGNU ourable steric interactions between the ortho methyl
groups of lutidine, which are directed towards the boron
centre, and one of the three chlorides in (lutidine)BCl3.
31
1
À
P{ H} NMR spectrum with P D coupling of 66 Hz) and
11
À
broad B NMR resonances devoid of B H coupling.
The requirement for high temperatures before H2 activa-
tion observed with 3/PtBu3 is attributed to the activation of
H2 being energetically unfavourable due to the lower HIA
À
Steric crowding in four coordinate (at boron) (iPr
NHC)BH
A
G
of [CatB
(PtBu3)]+ (relative to B
N
of cation [2]+, which is not stabilised by any p-donor ligands
concentration of H2 activation products is presumably pres-
ent and the rapid ligand disproportionation of CatBH/PtBu3
that takes place at 1008C and drives H2 activation to com-
pletion. This hypothesis is supported by the reverse reaction,
(Table 1).
Table 1. Relative hydride- and chloride-ion affinities.
HIA [kcalmolÀ1
M06-2X/6-311G(d,p)
/PCMACHTUNGTREG(NNNU CH Cl ) DH
]
CIA [kcalmolÀ1
M06-2X/6-311G
/PCMACHTUNGTREG(NNNU CH Cl ) DH
]
that is, the addition of [HPtBu3][B
(PtBu3), evolving H2 at
([B
(C6H3Cl2)4]À is termed [BArCl]À from here on).
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG(d,p)
ACHTUNGTRENNUNG
2
2
2
2
ACHTUNGTRENNUNG
neutral boranes
BEt3
0.0
À41.0
–
–
B
A
+
+
À
[CatB PtBu3] , [3]
À33.0
À37.9
À41.6
À43.6
À45.8
À47.8
À61.0
À61.7
À65.8
À67.8
–
27.1
30.2
–
21.8
27.7
26.8
17.3
20.7
8.3
+
À
[CatB NEtiPr2]
[2]+
+
À
[CatB NEt3]
+
À
AHCTUNGTRENNUNG
Assessment of relative HIAs and chloride-ion affinities
(CIAs): Seeking a borenium cation that possesses a greater
A
ACHTUNGTRENNUNG
+
À
[Cl2B NEtiPr2]
HIA than B
cations was calculated. On the basis of a benchmarking
study, M06-2X/6-311G(d,p) was chosen as an optimal com-
ACHTUNGTRENNUNG(C6F5)3, the relative HIA of a range of borenium
A
ACHTUNGTRENNUNG
+
À
[Cl2B NEt3]
+
À
U
6.3
0.0
AHCTUNGTRENNUNG
promise between accuracy and computational time.[39,43] Rel-
ative HIAs were calculated from the isodesmic reactions be-
tween [HBEt3]À, and the boron LA [Eq. (4) and (5)] analo-
gous to DuBoisꢀs approach.[44] The addition of a polarisable
continuum model (PCM) to approximate solvation was es-
sential to prevent over estimation of borenium-cation HIA
due to charge neutralization. An analogous affect has been
reported when calculating the fluoride-ion affinities (FIA)
of phosphenium cations, which led to >100 kcalmolÀ1 over-
estimation of FIA.[45] CIAs relative to AlCl3 were also esti-
mated by using an approach based on the transfer of chlo-
Anion-transfer reactions: To determine if the calculated DH
values for CIA and HIA allow prediction of solution-phase
reactivity, a number of control reactions were investigated.
First, all CIAs were positive relative to AlCl3 consistent
with the chloride abstraction experimentally observed on
combination of X2BCl(L) and AlCl3, forming [X2BL]-
AHCTUNGTRENNUNG
A
N
A
ACHTUNGTRENNUNG
A
BACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
BACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Although there is a general correlation between CIA and
N
N
ACHTUNGTRENNUNG
HIA, deviations were observed in a number of cases, and
these are attributable to two main phenomena: 1) enhance-
ment of relative CIA (compared to relative HIA) due to the
increased magnitude of positive charge localized at boron.
For example, [CatBNEt3]+ has an enhanced CIA versus the
thiocatechol analogue [(o-C6H4S2)BNEt3]+ consistent with
BACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Chem. Eur. J. 2013, 19, 2462 – 2466

Hydride-Ion Affinity of Borenium Cations
FULL PAPER
this level of theory, the HIA of B
G
as the major product. However, resonances consistent with
identical to [CatB
N
H2 activation (specifically BHCl2ACHTUNGETRNNU(G lutidine)) as a minor prod-
tively). Relative HIA calculations are therefore only useful
to predict reactivity when there was a considerable differ-
ence in DH values (dDH) between the two LAs, as was
uct were also observed by multinuclear NMR spectroscopy.
Confirmation of dihydrogen activation was forthcoming
from the reaction of D2 with [4]+/PMes3, which produced
[Mes3PD]+ (a 1:1:1 triplet in the 31P{1H} NMR spectrum).
The 11B NMR spectrum now showed the resonance for 5
and a broad signal centred at d=1 ppm attributable to (luti-
dine)BDCl2. Importantly, 5 did not react with H2 at 1008C
(4 atm, 18 h),[39] confirming that H2 activation is by the FLP
formed from the borenium cation [4]+ and PMes3.
found for [4]+ and B
ACTHNUTRGEN(UNG C6F5)3.
FLPs containing phenyl-ligated borenium cations: Mecha-
nistically, extensive computational work indicated that the
FLPs containing di-chloride-ligated borenium cations:
Cation [4]+ was selected to incorporate into FLPs for H2 ac-
tivation because it possesses a higher HIA than BACHTNUTRGNEUNG(C6F5)3
activation of H2 by BACTHUNGTREN(UNG C6F5)3/LBs proceeds through a con-
certed process involving “encounter complexes”.[50] Howev-
er, an alternative mechanism involving intramolecular heter-
À
but has a diminished Lewis acidity towards LBs more steri-
cally demanding than HÀ (e.g., ClÀ). The relatively low CIA
allows facile access to [4]+ by abstraction of halide from (lu-
olytic cleavage of H2 across a B C bond followed by subse-
quent intermolecular deprotonation cannot currently be pre-
cluded.[51] Furthermore, Piers et al. have recently demon-
strated that the highly electrophilic perfluoropenta
phenylborole activates H2 without an additional equivalent
tidine)BCl3 by AlCl3 or Na
sible on addition of PtBu3, PCy3 or PPh3 to [4]+. Instead,
[Cl2B(2,6-lutidine)
(PR3)]+ cations, which did not activate H2
[BArCl].[47] FLPs were not acces-
ACHTUNGTRENNUNG
À
G
of LB, with H2 heterolytically cleaved across the B C
even on heating to 1008C under H2 at 4 atm, were formed.
A FLP is formed on addition of extremely bulky PMes3 to
[4]+, but this FLP did not activate H2 (4 atm in
o-C6H4Cl2) at 208C. At this temperature, equimolar mix-
tures of [4]+/PMes3 displayed a slow decrease in the
11B NMR resonance associated with [4]+ and a concomitant
increase in a sharp 11B NMR resonance at d=6.4 ppm con-
sistent with a four-coordinate boron environment. Full con-
version to this new boron species can be achieved by heat-
ing equimolar [4]+/PMes3 to 608C for 1 h [Eq. (8)]. The
phosphonium cation [Mes3PH]+ was the only new phospho-
rus-containing species observed in the 31P{1H} NMR spec-
trum, indicating deprotonation of lutidine. The carbanion
produced by deprotonation is stabilized by coordination to
boron forming the four-membered boracycle [(CH2-
bond.[52] To probe if H2 cleavage would occur with a boreni-
À
um cation containing a B Caryl moiety, PhBCl
N
synthesized, and halide abstraction effected with AlCl3 or
NaACHTNUTRGENNUG HCATUNGTRENNGUN
[BArCl]. Whilst pure [PhBCl(lutidine)]+ ([6]+) was not
isolable despite numerous efforts, [6]+ can be produced in
situ as the major species (>90% by multinuclear NMR
spectroscopy). In the absence of a LB, [6]+ displayed no re-
action towards H2 at 208C or 1008C (4 atm of H2, o-
C6H4Cl2). The addition of one equivalent of PMes3 to [6]+
produced a FLP that also undergoes slow deprotonation of
a methyl group of lutidine leading to boracycle 7 [Eq. (9)].
At high temperatures (1008C), D2 activation was observed
with [6]+/PMes3 to form (luti
ACTHNUTRGNEdNUG ine)BD(Ph)Cl and
[DPMes3]+, but again the major product was the boracycle
7, formed by deprotonation of lutidine. With no observed
difference in the propensity of [Cl2B
A
(lutidine)]+ and
ACHTUNGTRENNUNG
highly Lewis acidic {BCl2}+ fragment significantly reduced
the pKa of the ortho-methyl protons enabling deprotonation
by the bulky, but only moderately basic phosphine. Com-
pound 5 can be independently synthesised, but requires the
strong base nBuLi to deprotonate 2,6-lutidine; subsequent
transmetalation with BCl3 provides 5, analogous to the for-
[PhBCl
ACHTUNGTRENNUNG
FLPs activate H2 through a similar mechanism, presumably
involving an encounter complex.
mation of [(CH2ACHTUNGTRENNUNG
{NC5H4})BCy2].[49]
Conclusion
Borenium cations that possess a wide range of HIAs are
easily accessible, which makes them readily tuneable cata-
lysts for hydrogenations. The calculated relative HIA is a
useful tool for assessing hydride-transfer ability, a key step
in ionic hydrogenation.[53] The HIA of borenium cations is
Because the formation of 5 is slow at 208C, the ability of
[4]+/PMes3 to activate H2 at elevated temperatures was in-
vestigated. Heating [4]+/PMes3 to 1008C under 4 atm of H2
(in o-C6H4Cl2) resulted in the formation of the boracycle 5
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M. J. Ingleson et al.
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the four-coordinate neutral borane (HBX2L) and the elec-
trophilicity of the borenium cation. Borenium cations with
high (relative to BACHTUNGTRENNUNG(C6F5)3) HIAs can be readily accessed by
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high-HIA borenium cations that are stable to deactivation
reactions, such as ligand deprotonation, are currently under
investigation.
Acknowledgements
We gratefully acknowledge the Royal Society (M.J.I. for the award of a
University Research Fellowship), the Leverhulme Trust (E.R.C.) and the
University of Manchester for funding (A.D.G.). The authors would also
like to acknowledge the use of the EPSRC U.K. National Service for
Computational Chemistry Software (NSCCS) at Imperial College
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A
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Received: September 17, 2012
Published online: January 7, 2013
2466
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