Reactivity of a Cl-boratabenzene Pt(ii) complex with Lewis bases: Generation of the kinetically favoured Cl-boratabenzene anion

Article abstract of DOI:10.1039/c1dt11756d

Complex [(IMes)₂Pt(H)(ClBC₅H₄SiMe ₃)] (IMes = 1,3-di(2,4,6-trimethylphenyl)imidazolin-2-ylidene) reacts with Lewis bases (L = pyridine, trimethylphosphine, acetonitrile, tert-butylisocyanide) to generate the kinetically favoured ion pairs [(IMes)₂Pt(H)(L)][ClBC₅H₄SiMe₃]. Over time, the formation of the thermodynamically favoured borabenzene-L adducts is observed with L = pyridine and trimethylphosphine. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11756d

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Reactivity of a Cl-boratabenzene Pt(II) complex with Lewis bases: generation
of the kinetically favoured Cl-boratabenzene anion†
Stephanie S. Barnes,a Marc-Andre´ Le´gare´,a Laurent Maronb and Fre´de´ric-Georges Fontaine*a
Received 1st September 2011, Accepted 15th September 2011
DOI: 10.1039/c1dt11756d
Complex [(IMes)2Pt(H)(ClBC5H4SiMe3)] (IMes
=
1,3-
(MeCN), and tert-butylisocyanide (2-BuNC)) to generate the ionic
pair [(IMes)2Pt(H)(L)]+[ClBC5H4SiMe3]-. Coordination isomers
[(IMes)2Pt(H)(Cl)] and 1-L-2-SiMe3-borabenzene were calculated
to be thermodynamically favoured compared to the ion pair,
and were observed in the presence of pyridine and PMe3. It is
the first report of the synthesis of borabenzene adducts from a
boratabenzene precursor, and gives valuable insight on the stability
of the chloroboratabenzene anion.
di(2,4,6-trimethylphenyl)imidazolin-2-ylidene) reacts with
Lewis bases (L = pyridine, trimethylphosphine, acetonitrile,
tert-butylisocyanide) to generate the kinetically favoured
ion pairs [(IMes)2Pt(H)(L)][ClBC5H4SiMe3]. Over time, the
formation of the thermodynamically favoured borabenzene-
L adducts is observed with L = pyridine and trimethylphos-
phine.
In our investigation on the Lewis acidity of the boron centre of
1, we observed that the borabenzene fragment could be transferred
to pyridine to generate a pyridine-borabenzene adduct (4-Py)
24 h after the addition of 2 equivalents of pyridine in benzene-
d6.13 However, close monitoring of the reaction mixture allowed
the observation of a transient species, 2-Py (Scheme 1). Using
1H NMR spectroscopy, we observed that the hydride of 2-Py
Derivatives and metal complexes of borabenzene, and its anionic
analogue boratabenzene, have a large range of applications,1
notably as sources of chirality,2 as catalysts for cyclotrimerization,3
alkene polymerization4 and C–H bond activation,5 and for
their optoelectronic properties.6 A large variety of derivatives of
borabenzene and boratabenzene have been synthesized,7 but in
order to form a thermally stable adduct,8,9 a strong nucleophile
needs to be present on boron, thus limiting the range of possible
substituents. A class of boratabenzene species that would be of
particular interest for further functionalization are haloborataben-
zenes (X = Cl-, Br-, or I-). Indeed, unlike most boratabenzene
species reported with alkyl, amido, or phosphido substituents, the
B–X bonds can undergo a large range of reactivities including
transmetallation, oxydative cleavage, or reduction, which could
lead to species unobtainable by other synthetic pathways. Al-
though the most common precursors for borabenzene synthesis
are chloroboracyclohexadienes, only one serendipitous report of
a fluoroboratabenzene species exists,10 resulting from an unusual
rearrangement of an iridium metallabenzene.11
1
was upfield (d -20.10) and that the JPt–H was significantly lower
(1348 Hz) than for 1 (d -22.42, 1JPt–H = 1915 Hz). In addition to
the signals for the N-heterocyclic ligands, a new set of resonances
was observed at d = 8.05, 7.81, 7.03, and 6.85 with the H–H
coupling constants expected for borabenzene species. Monitoring
this reaction mixture over a 24 h period did show that the
resonances associated with 2-Py disappeared at a similar rate to
the formation of [(IMes)2Pt(H)(Cl)] (3).
As part of our program aimed at the exploration of novel bond-
ing modes for the borabenzene moiety – notably to generate novel
Z-type interactions with transition metals12 – we recently reported
the synthesis of species [(IMes)2Pt(H)(ClBC5H4SiMe3)] (1),13 one
of the rare borabenzene metal complexes where the p system of
the heterocycle is not involved in binding the transition metal.14
We wish to report that species 1 reacts unexpectedly with Lewis
bases (L = pyridine (Py), trimethylphosphine (PMe3), acetonitrile
Scheme 1 Generation of 2-L and 4-L from 1.
aDe´partement de chimie & Centre de recherche sur les interfaces et la catalyse,
Universite´ Laval, 1045 avenue de la Me´decine, Que´bec, QC, G1V 0A6,
Canada. E-mail: frederic.fontaine@chm.ulaval.ca
Analogues of 2-Py were observed in the presence of ace-
tonitrile (2-MeCN), trimethylphosphine (2-PMe3), and tert-
butylisocyanide (2-BuNC). Although the resonances for the Pt–
H shifted significantly for the three other complexes (d -19.13,
1JPt–H = 1551 Hz (2- MeCN); d -5.65, 1JPt–H = 1060 Hz (2-PMe3); d
bUniversite´ de Toulouse, INSA, UPS, LPCNO, CNRS, LPCNO, UMR
5215 CNRS-UPS-INSA, 135 avenue de Rangueil, 31077, Toulouse, France
† Electronic supplementary information (ESI) available: Synthetic pro-
cedures, NMR, MS, and DFT data for all products. See DOI:
10.1039/c1dt11756d
1
1
-7.79, JPt–H = 1101 Hz (2-BuNC)), the H and 13C resonances
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Table 1 Reaction coordinates minimized using DFT
C6D6
DE1→2L (kcal mol-1)
DE2L+3→4L (kcal mol-1)
DE2L+3→4L
(kcal mol-1)
distPt–H (A)
˚
L
CNtBu
PMe3
Py
MeCN
PCy3
-22.7
-5.6
-50.5
-12.2
26.0
-6.6
-8.4
-9.6
-1.1
-32.7
-0.3
-3.2
-4.8
+3.7
-21.8
1.60
1.59
1.56
1.56
N.A.
associated to the borabenzene moiety were very similar from
one compound to the next, although some small variations were
observed.15 The coordination of the phosphine in 2-PMe3 was
With time, species 2-L generally degrades to generate 3.
However, in the case of the pyridine analogue, the generation
of the adduct 4-Py was observed.13 Species 4-PMe3 was also
observed from 2-PMe3, based on a comparison with independently
synthesized 1-PMe3-2-SiMe3-borabenzene, albeit in very low yield
because of several side reactions. The transformation of 2-L to
3 and 4-L was also investigated using DFT (Table 1). In the gas
phase, the neutral adducts were favoured thermodynamically over
the ion pair, whereas the formation of 4-MeCN was calculated
to be endothermic in benzene. As can be expected from the
experimental results, 4-Py gives the most stable neutral system
with reaction coordinates at -4.8 kcal mol-1 in benzene followed
by 4-PMe3 (-3.2 kcal mol-1). Although the formation of 1-PCy3-
2-SiMe3-borabenzene is favoured, its absence in solution seems
to indicate that the formation of 2-L is a prerequisite for its
generation.
1
1
confirmed using 31P{ H} and H NMR spectroscopy, where the
1JPt–P and 2JH–P of 1060 and 187 Hz, respectively, are characteristic
1
of phosphine coordination trans to a hydride. The 11B{ H} NMR
chemical shifts for all 2-L were, within experimental error, at 38
ppm. It should be noted that species 2-PCy3 was not observed
when tricyclohexylphosphine was added to 1.
We propose that 2-L consists of the ion pair
[(IMes)2Pt(H)(L)][ClBC5H4SiMe3]. To support this hypothesis,
the generation of [(IMes)2Pt(H)(L)]BF4 was carried out by
the addition of AgBF4 to a solution of [(IMes)2Pt(H)(Cl)] in
benzene-d6 in the presence of excess L (see ESI†). With all L, the
products displayed the same resonances associated to the cationic
centre of species 2-L, although solubility issues did not allow for
clean isolation of the ionic species. ESI-MS or HRMS studies in
benzene also gave probing evidences of the existence of 2-L (see
ESI†). The ionic species [(IMes)2Pt(H)(L)]+ for 2-PMe3 (m/z =
881.2), 2-Py (m/z = 884.1), and 2-BuNC (m/z = 887.1) were
observed, although in all samples the principal ion corresponded
to [(IMes)2Pt(H)]+ (m/z = 804.6). The chloroboratabenzene anion
was also observed in the HRMS anionic mode at m/z = 183.0694.
It was possible to model these complexes using DFT (see ESI for
computational details†). The structure of 2-Py is presented in Fig.
1 and those of the other complexes are available in the ESI.† The
formation of the ion pair 2-L from 1 was shown to be exothermic
for all species, with the exception of 2-PCy3 where the reaction
is disfavoured by 26.0 kcal mol-1 (Table 1). All species have the
platinum atom in a square planar environment, with the L ligand
trans to the hydride. The biggest distortion from ideal geometry,
in all cases, is being caused by steric repulsion between L and the
IMes ligands. The 1-Cl-2-SiMe3-boratabenzene anion is identical
in all 2-L complexes and does not have any significant interaction
with the cation.
To obtain more information on the mechanism of the trans-
formation of 1 to 4-Py, solutions with various equivalents of
pyridine were monitored using 1H NMR spectroscopy. When one
equivalent of pyridine was added to the solution, the complete
generation of 2-Py was observed within 30 min. Monitoring
of the solution over a period of one week did not reveal the
presence of a significant concentration of 4-Py. However, when 5
equivalents of pyridine were added to 1, it was possible to observe
the gradual conversion of 2-Py to 4-Py over a period of a few
hours. These results suggest that an excess of pyridine is required
to generate 1-pyridine-2-SiMe3-borabenzene. More interestingly,
thirty minutes after the addition of half an equivalent of pyridine,
all of it was consumed and both 2-Py and 1 were observed
in solution. Whereas the resonances for the [(IMes)2Pt(H)(py)]+
were sharp, the [ClBC5H4SiMe3]- resonances for 1 and 2-Py
were significantly broadened (see Fig. S16†). Variable temperature
NMR spectroscopy did show that the peaks associated to both
chloroboratabenzene species become sharper on cooling and
coalesce into a single set of resonances upon warming. Such
a phenomenon points towards a fluxional process where the
chloroboratabenzene anion of 2-L can undergo ligand exchange
with the chloroboratabenzene of 1, as seen in Fig. 2. Modelling
of the bandwidth of the TMS resonances with WINDNMR-Pro16
allowed the determination of the exchange rates and activation
parameters (DH‡ = 12.8
0.3 kcal mol-1 and DS‡ = -8
1 cal mol-1 K-1).
The first step of this transformation, the formation of 2-L, is
expected to occur by a mechanism similar to the [ClBC5H4SiMe3]-
exchange between 1 and 2-L, where in this situation the chlorob-
oratabenzene anion acts as the Lewis base. The negative value
for the entropy of the transition state of -8 1 cal mol-1 K-1
suggests an associative pathway, but the relatively small values
cannot discredit a dissociative pathway. Despite our efforts, no
Fig. 1 B3PW91/SDDALL(Pt,Cl,Si)/6-31g(d,p (N,C,H,B)) optimized
structure 2-Py. Hydrogen atoms are omitted for clarity.
12440 | Dalton Trans., 2011, 40, 12439–12442
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traditional nucleophilic substitution reactions on neutral boraben-
zenes. Indeed, the evidence shows that it is a thermodynamically
unstable species compared to the most common adducts. The
rich and unusual chemistry of 1 with other substrates, and the
coordination chemistry of the chloroboratabenzene anion are
currently underway.
Acknowledgements
We are grateful to NSERC (Canada), CFI (Canada), FQRNT
(Que´bec) and CCVC (Que´bec) for financial support. M.-A. L.
and S. S. B. would like to acknowledge CRSNG and FQRNT for
scholarships. L.M. is a member of the Institut Universitaire de
France and would like to acknowledge CalMIp and CINES for
computing time. We acknowledge R. T. Baker and C. Diaz-Urrutia
with their help with MS experiments. We acknowledge Prof. H.
J. J. Reich for helpful comments. F.-G. F. would like to thank the
CCRI for hosting him while writing this manuscript.
Fig. 2 Experimental (red) and simulated (black) 1H NMR spectra
of the -SiMe3 region of 1 and 2-Py at various temperatures with
the associated rate constant. Eyring plot for the exchange of the
1-Cl-2-TMS-boratabenzene moieties in 1 and 2-Py.
Notes and references
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Scheme 2 Associative mechanism for the dissociation of Cl- from 2-L to
generate 4-L.
Conclusions
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PMe3 and pyridine, an associative substitution at boron can
occur since these Lewis bases are known to strongly stabilize
borabenzene adducts. It exposes the difficulties associated with
the generation of this interesting boratabenzene moiety using
8 A N2 adduct has been observed using IR at 10 K in a nitrogen matrix: G.
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12442 | Dalton Trans., 2011, 40, 12439–12442
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©