[(IMes)2Pt(H)(ClBC5H4SiMe3)]: A borabenzene-platinum adduct with an unusual Pt-Cl-B interaction

Full text of DOI:10.1002/anie.200902870

Angewandte
Chemie
DOI: 10.1002/anie.200902870
Lewis Acid Adducts
[(IMes)2Pt(H)(ClBC5H4SiMe3)]: a Borabenzene–Platinum Adduct
with an Unusual Pt-Cl-B Interaction**
Andrꢀ Languꢀrand, Stephanie S. Barnes, Guillaume Bꢀlanger-Chabot, Laurent Maron,
Philippe Berrouard, Pierre Audet, and Frꢀdꢀric-Georges Fontaine*
Since the first report of a metallaboratrane by Hill et al. in
1999,[1] there has been a quest for transition metal complexes
containing a dative interaction with Group 13 Lewis acids. In
a recent report, Braunschweig and co-workers demonstrated
that [(PCy3)2Pt0] (Cy = cyclohexyl) interacts with alanes to
form Lewis adducts [(PCy3)2Pt(AlX3)],[2] which are rare
examples of well-characterized alane (M!AlR3) com-
plexes.[3,4] However, the analogous reaction with haloboranes
does not yield Lewis adducts, but instead forms platinum
boryl complexes,[5] a class of anionic boron-containing species
of which numerous examples have been reported to date.[6]
The only compounds having an M!B dative interaction with
boron are supported by ambiphilic ligands.[7–9] The boron
atom of bis- or tris(methimazolyl)boranes coordinates selec-
tively to the transition metal in metallaboratranes.[10] In
contrast, in addition to interacting with a transition metal, the
Lewis acid moiety of phosphinoboranes prepared by Bour-
issou[11] can interact with an anionic ligand within the
coordination sphere, depending on the nature of the tran-
sition metal and the ambiphilic ligand. Such an interaction is
seldom observed with Lewis acids, and is supported by
ambiphilic ligands,[12] with the exception of BF4 adducts and
halocarboranes, in which the Lewis acidity of the boron atom
can no longer be considered.[13]
the coordination modes of these heterocyclic molecules with
transition metals are quite limited. A common feature of
borabenzenes[15] and boratabenzenes,[16] the analogue with
anionic nucleophiles, is their h6 coordination by the aromatic
ring in an analogous fashion to arene and cyclopentadienyl
ligands.[17] The exception is a phosphidoboratabenzene, which
binds transition metals by the phosphine moiety.[18] Using the
efficient and general strategy of forming borabenzene organic
adducts from boracyclohexadiene by the elimination of
Me3SiCl, which was developed by Fu et al. (Scheme 1),[19]
Scheme 1. Formation of borabenzene adducts from 1-chloro-2-(trime-
thylsilyl)boracyclohexadiene. L=Lewis base.
we examined the possibility of using a nucleophilic transition
metal complex to stabilize the Lewis acidic boron orbital on
borabenzene. By applying this strategy, the isolation of an h1-
borabenzene transition metal adduct, and consequently of an
unchelated borane having a dative interaction with a tran-
sition metal, could be possible. During these investigations,
Whereas borabenzene adducts are the subject of continu-
ing interest, and in particular for their electronic properties,[14]
ꢀ
we observed that the B Cl oxidative addition of the
[*] A. Languꢀrand, S. S. Barnes, G. Bꢀlanger-Chabot, P. Berrouard,
P. Audet, Prof. F.-G. Fontaine
boracyclohexadiene with platinum(0) species does not
occur, in contrast to previously observaions with haloboranes.
Instead, formation of a m-h1-Cl-borabenzene adduct, which is
an unusual bonding mode for an unchelated Lewis acid, was
observed.
Dꢀpartement de Chimie, Universitꢀ Laval
1045 Avenue de la Mꢀdecine
Citꢀ Universitaire, Quꢀbec, Qc, G1V 0A6 (Canada)
Fax: (+1)418-656-7916
E-mail: frederic.fontaine@chm.ulaval.ca
As seen in Braunschweig adducts (see above),
[(PCy3)2Pt0] appeared to be a promising and potent nucleo-
phile for borabenzene formation; however, initial experi-
ments with 1-chloro-2-trimethylsilyl-4-isopropylborabenzene
(1a) were problematic owing to phosphine dissociation.
Indeed, free phosphine was shown to interact with boracy-
clohexadiene precursors to form PCy3–borabenzene
adducts.[20] This result prompted us to use N-heterocyclic
carbene ligands instead of phosphines because they are
known to bind tightly to transition metals whilst offering good
steric protection.[21]
The addition of one equiv of boracyclohexadiene 1a to
one equiv of a yellow solution of [(IMes)2Pt0] (Scheme 2;
IMes = 1,3-di(2,4,6-trimethylphenyl)imidazolin-2-ylidene) in
[D6]benzene gave a colorless reaction mixture. After 5 min, it
was possible to observe one major compound, 2a, in solution
L. Maron
Universitꢀ de Toulouse, INSA, UPS, LPCNO
135 avenue de Rangueil, 31077 Toulouse (France)
and
CNRS, LPCNO, UMR 5215 CNRS-UPS-INSA
31077 Toulouse (France)
[**] We are grateful to NSERC (Canada), CFI (Canada), FQRNT
(Quꢀbec), and Universitꢀ Laval for financial support. L.M. is a
member of the Institut Universitaire de France and would like to
acknowledge CalMip and CINES for a generous grant of computing
time. We are grateful to Johnson Matthey for the loan of PtCl2, and to
Prof. S. A. Westcott, D. Bourissou, and R. W. Schurko for helpful
discussions. We also acknowledge one of the referees for proposing
the experiment with [Bu4N]Cl. IMes=1,3-di(2,4,6-trimethylpheny-
l)imidazolin-2-ylidene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902870.
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6695

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borane complex interaction reported by Bourrisou.[11c] The
final hypothesis involves an interaction between the chloride
and the borabenzene, which is quite unusual for a boron-
containing species other than BF4 and halocarboranes. To
gain more structural information, the synthesis of two other
analogues, using precursors 1b and 1c, was undertaken.
The addition of boracyclohexadienes 1b and 1c
(Scheme 2) to [(IMes)2Pt0] in [D6]benzene gave similar
reactivity to 1a, affording 2b and 2c. The feasibility of the
reaction with 2c confirms that the elimination of TMSCl is
not a requirement for the borabenzene adduct formation. It
can also be observed by 1H NMR that the borabenzene in 2c
is symmetric on an NMR timescale, as H2/H6 and H3/H5 are
equivalent. However, 1H NMR signals arising from the
hydride and the IMes-bound are, within the margin of error,
equivalent for all three complexes. Also essentially equivalent
are the 195Pt resonances for all borabenzene complexes (2a =
ꢀ4364 ppm, 2b = 2c = ꢀ4367 ppm), which are upfield from 3
(d = ꢀ4285 ppm). Although the acquisition of an 11B NMR
spectrum proved difficult to obtain, especially for 2c, which
decomposed during the acquisition, it was possible to observe
that both 2a and 2b have similar chemical shifts (d = 36.2 and
38.8 ppm, respectively), which are in the expected range for
borabenzene adducts.[19]
Scheme 2. Formation of borabenzene adducts 2a–c (1a: R1 =TMS,
R2 =iPr; 1b: R1 =TMS, R2 =H; 1c: R1 =H, R2 =H), and 3 upon formal
elimination of the borabenzene moiety.
by 1H NMR spectroscopy. Along with resonances correspond-
ing to the IMes carbene in the NMR spectrum, the presence
of a hydride signal at d = ꢀ22.41 ppm was observed with an
1
unprecedented JPt-H coupling constant of 1910 Hz.[22] Signals
were observed that are characteristic for a 2-trimethylsilyl-4-
isopropylborabenzene adduct (see the Experimental Sec-
tion), and the integration of the resonances corresponds to
two IMes ligands for one hydride and one borabenzene
moiety. A 2D ROESY NMR experiment was also carried out
on 2a. ROE correlations were observed between the bora-
benzene group and the {(IMes)2Pt(H)(Cl)} fragment. The
methyl groups in the ortho position of the mesityl group have
ROE correlations with both the resonances of TMS group
and the hydrogen at d = 5.75 ppm. Also, the methyl groups in
the isopropyl moiety correlate with the aromatic proton in the
meta position of the mesityl ring. Such correlations imply that
the substituents in ortho and meta positions are in close
proximity of the metal center.
Compound 2a is unstable in solution and decomposes
readily. Higher concentration, slight excess of 1a, or removal
of the solvent under reduced pressure increases the decom-
position rate dramatically. Under dilute conditions (ca.
10ꢀ6 m), compound 2a was the major component in solution
for up to three days. However, a single inorganic degradation
product, 3, was always present, and its concentration was
shown to be inversely proportional to 2a in solution. All
attempts to crystallize compound 2a failed, and only 3 was
isolated. Compound 3 has very similar 1H NMR spectroscopic
features to 2a, with the IMes resonances and the hydride (d =
ꢀ17.96 ppm, 1JPt-H = 1550 Hz) shifted slightly downfield,
although it contains no borabenzene moiety. Suitable crystals
for X-ray crystallography were obtained from a saturated
solution of 3 in benzene, which allowed its identification as
[(IMes)2Pt(H)(Cl)].[23]
The similarity of the spectroscopic features of the
{(IMes)2Pt(H)(Cl)} core in 2a–c tends to favor the hypothesis
ꢀ
of a bridging chloride adduct. Indeed, if a Pt B interaction
were present, a more significant shift in the 195Pt and 1H
resonance should arise depending on the nature of the
borabenzene. DFT calculations were carried out to credit or
ꢀ
ꢀ ꢀ
discredit the presence of a Pt B or a Pt Cl B interaction.
Modeling of 2b show no minimum energy state that would
ꢀ
ꢀ ꢀ
account for any Pt B interaction; however the Pt Cl B
model did show an energy minimum (Figure 1).
In the complex 2b, a marked interaction between the
ꢀ
chlorine and boron atoms is obtained (B Cl distance of
1.93 ꢀ). An NBO analysis indicates that a covalent bond is
formed between these two atoms, and thus that a chlorobor-
ꢀ
atabenzene ligand is obtained. At the same time, the Pt Cl
bond is elongated (2.47 ꢀ in [(IMes)2Pt(H)(Cl)] and 2.56 ꢀ in
The similarity of the spectroscopic features of 2a and 3
suggest that the former complex should also be a hydrido-
chloroplatinum(II) species, but with a borabenzene bound to
it. It is also evident that the aromatization of the borabenzene
does not involve the elimination of TMSCl, but instead occurs
0
ꢀ
via C H bond activation by the Pt precursor, forming the
stable hydride complex. To account for the observed data,
three different structures were proposed for the adduct; none
of these structures has ever been reported with borabenzene.
The first proposed structure involves an interaction between
the hydride and the borabenzene, but can be ruled out, as no
broadening caused by the quadrupolar boron atom is
observed for the hydride. Our second hypothetic structure
has a Pt!B interaction, which is similar to the phosphino-
Figure 1. Optimized structure of the chloroboratabenzene complex 2b.
Hydrogen atoms and methyl groups of the 2,4,6-trimethylphenyl
groups are omitted for clarity.
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Chemie
ꢀ
2b), and the Pt H bond is slightly shortened (1.53 ꢀ in 2b
present in solution, 3 and [NBu4]Cl, are mixed together,
suggests formation of a tetrabutylammonium salt of 1-
chloro(2-trimethylsilyl)boratabenzene (5).
Compounds 2a–c are, to our knowledge, the first exam-
ples of unsupported M-X-BR3 complexes in which the Lewis
acidity of a borane is still available. The stability of the
and 1.56 ꢀ in [(IMes)2Pt(H)(Cl)]), which could account for
the large coupling constant between these two elements
observed in the borabenzene complexes. At the NBO level, in
2b, a remaining interaction between chlorine and platinum is
found at the second-order donor–acceptor level (circa
20 kcalmolꢀ1). For comparison,
a
NBO analysis of
platinum–NHC bond relative to usual Pt L dative bonds
ꢀ
[(IMes)2Pt(H)(Cl)] already reveals a mainly ionic bond
between platinum and chlorine (second-order interaction of
more than 90 kcalmolꢀ1). NMR calculations of the 11B
nuclear shift were carried out for the complex 2b and also
for the “free” chloroboratabenzene. In complex 2b, a 11B
nuclear shift of 35 ppm is obtained, whereas a value of 51 ppm
is found for the “free” chloroboratabenzene. These values are
in excellent agreement with the experimental data, and
clearly indicate that the chloroboratabenzene is coordinated
to the platinum center.
seems to be
a
requirement to prevent the weak
{(IMes)2Pt(H)(Cl)}–borabenzene adducts from dissociating
to form Lewis base/borabenzene adducts. Although it might
be economically more viable to use the Fu strategy to
generate borabenzene adducts, the synthetic pathway we
present will favor the presence of additional functional groups
on the borabenzene by not requiring a TMS group, and makes
hypothetically possible the interaction with less donating
donor ligands to form new and unusual borabenzene adducts,
which we are currently examining.
It was not possible to determine the nature of the organic
fragment resulting from the dissociation of the borabenzene
fragment from complex 2.[24] However, the addition of 2 equiv
or 10 equiv of pyridine to 2b leads to the formation of the
borabenzene adduct 4 in which the trimethylsilyl group is still
present (Scheme 3). Formation of the same compound was
Experimental Section
[(IMes)2Pt0],[25] 1a,[14a] 1b,[19] and 1c[26] were synthesized according to
literature procedures. 2a–c, 3: In a typical experiment, a dilute
solution of 1 (0.004 mmol) in C6D6 is added dropwise to a yellow
solution of [(IMes)2Pt0] (0.0035 g, 0.00425 mmol) in C6D6. NMR
spectra indicate the formation of species 2, which decomposes to give
3 over time. This decomposition is greatly accelerated if 1 is present
even in slight excess.
2a: 1H NMR (400 MHz, [D6]benzene): d = 7.78 (d, 4JH-H = 2.1 Hz,
1H, H3), 7.25 (dd, 3JH-H = 10.3 Hz, 4JH-H = 2.1 Hz, 1H, H5), 6.76 (s, 8H,
HmetaMes), 5.94 (s, 4H, Himid), 5.75 (d, JH-H = 10.3 Hz, 1H, H6), 3.25
3
3
(sept, JH-H = 6.9 Hz, 1H, CH(CH3)2), 2.32 (s, 12H, MeparaMes), 1.85
(s, 24H, MeorthoMes), 1.64 (d, 3JH-H = 6.9 Hz, 6H, CH(CH3)2), 0.59 (s,
9H, TMS), ꢀ22.42 ppm (s, 1JPt-H = 1910 Hz, 1H, Pt-H). 13C{1H} NMR
(75.4 MHz, [D6]benzene): d = 137.8 (CparaMes), 137.3 (C3), 136.5
(CipsoMes), 135.7 (CorthoMes), 133.6 (C5), 129.3 (CmetaMes), 123.7 (C6),
121.7 (Cimid), 26.6 (CH(CH3)2), 21.3 (MeparaMes), 18.6 (MeorthoMes),
2.1 ppm (TMS). C2, C4, and the carbenic carbon were not observed.
1
195Pt NMR (85.99 MHz, [D6]benzene): d = ꢀ4364 ppm (d, JPt-H
=
1918 Hz). 11B NMR (128.34 MHz, [D6]benzene): d = 36.2 ppm.
2b: 1H NMR (400 MHz, [D6]benzene): d = 8.00 (dd, J = 7.2,
1.3 Hz, 1H, H3), 7.41 (ddd, J = 10.3, 6.7, 1.4 Hz, 1H, H5), 6.98 (td, J =
6.8, 0.7 Hz, 1H, H4), 6.75 (s, 8H, HmetaMes), 5.95 (s, 4H, Himid), 5.80
(dt, J = 10.1, 0.7 Hz, 1H, H6), 2.30 (s, 12H, MeparaMes), 1.86 (s, 24H,
MeorthoMes), 0.58 (s, 9H, TMS), ꢀ22.42 ppm (s, 1JPtꢀH = 1915 Hz, 1H,
Pt-H). 13C{1H} NMR (75.4 MHz, [D6]benzene): d = 139.3 (C3), 137.8
(CparaMes) 136.4 (CipsoMes), 135.6 (CorthoMes), 135.3 (C5), 129.3
(CmetaMes), 123.4 (C6), 121.6 (Cimid), 112.5 (C4), 21.3 (MeparaMes),
18.5 (MeorthoMes), 2.0 ppm (TMS). C2 and the carbenic carbon were
not observed. 195Pt NMR (85.99 MHz, [D6]benzene): d ꢀ4367 (d,
1JPtꢀH = 1960 Hz). 11B NMR (128.34 MHz, [D6]benzene): d =
38.8 ppm.
Scheme 3. Nucleophilic displacement of 2-(trimethylsilyl)borabenzene
from 2b.
observed in the addition of pyridine to 1-chloro-2,6-bis(tri-
methylsilyl)boracyclohexa-2,4-diene by the elimination of
TMSCl. The connectivity of 4 was confirmed using X-ray
crystallography. It can be speculated that the boron is the
subject of a nucleophilic attack, leading to the displacement of
3 by pyridine to form 4. If such a mechanism occurs, it should
be possible to use a chloride anion as a nucleophile to
synthesize a chloroboratabenzene salt. Indeed, five minutes
after the addition of [NBu4]Cl to 2b in [D6]benzene, it was
possible to observe both the clean formation of 3 as the major
inorganic compound (ca. 90%) and a new borabenzene
2c: 1H NMR (400 MHz, [D6]benzene): d = 7.76 (dd, J = 10.5,
7.1 Hz, 2H, H3/H5), 7.00 (tt, J = 7.0, 1.1 Hz, 1H, H4), 6.77 (s, 8H,
HmetaMes), 6.31 (dd, J = 10.5, 1.1 Hz, 2H, H2/H6), 5.93 (s, 4H, Himid),
2.30 (s, 12H, MeparaMes), 1.88 (s, 24H, MeorthoMes), ꢀ22.44 ppm (s,
1JPt-H = 1905 Hz, 1H, Pt-H). 13C{1H} (HSQC) NMR (75.4 MHz,
[D6]benzene): d = 133.6 (C3 and C5), 128.8 (CmetaMes), 121.5 (Cimid),
120.4 (C2 and C6), 111.9 (C4), 21.1 (MeparaMes), 18.2 ppm
(MeorthoMes). CipsoMes, CorthoMes, CparaMes, and the carbenic carbon
were not observed. 195Pt NMR (85.99 MHz, [D6]benzene): d =
ꢀ4367 ppm (d, 1JPt-H = 1910 Hz).
1
fragment (5) in the H NMR spectrum. Although it was not
possible to isolate the new borabenzene product, a significant
upfield shift of the a and b methylene signals of the butyl
chains of the tetrabutylammonium cation, which integrates
for a 1:1 ratio with the borabenzene fragment, was observed.
The absence of such a shift when the only two other species
3: 1H NMR (400 MHz, [D6]benzene): d = 6.80 (s, 8H, HmetaMes),
6.08 (s, 4H, imidazol), 2.33 (s, 12H, MeparaMes), 2.07 (s, 24H,
MeorthoMes), ꢀ17.97 ppm (s, 1JPt-H = 1542 Hz, Pt-H).13C{1H} NMR
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Communications
(75.4 MHz, [D6]benzene): d = 180.1 (carbene), 137.3 (CipsoMes), 136.9
Howard, P. W. Dyer, K. Miqueu, D. Bourissou, Chem. Eur. J.
2008, 14, 731 – 740; d) M. Sircoglou, S. Bontemps, G. Boudhadir,
N. Soffon, K. Miqueu, W. Gu, M, Mercy, C.-H. Chen, B. M.
Foxman, L. Maron, O. V. Ozerov, D. Bourissou, J. Am. Chem.
Soc. 2008, 130, 16729 – 16738; e) S. Bontemps, G. Bouhadir, D. C.
Apperley, P. W. Dyer, K. Miqueu, D. Bourissou, Chem. Asian J.
2009, 4, 428 – 435.
(CparaMes), 136.1 (CorthoMes), 129.0 (CmetaMes), 120.8 (Cimid), 21.4
(MeparaMes), 18.9 ppm (MeorthoMes). 195Pt NMR (85.99 MHz,
1
[D6]benzene): d = ꢀ4285 ppm (d, JPt-H = 1523 Hz).
Complete characterization and spectroscopic data of all new
compounds can be found in the Supporting Information.
[12] To our knowledge, all compounds having an M-X-B interaction
are supported by ambiphilic ligands. See Ref. [7a] and references
within.
Received: May 29, 2009
Revised: July 2, 2009
Published online: August 5, 2009
[13] According to a SciFinder search, more than 840 BF4 and 64
halocarborane adducts have been reported.
Keywords: borabenzene · boron complexes · Lewis acids ·
N-heterocyclic carbenes · platinum
.
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[20] The addition of 1a to [(PCy3)2Pt0] in [D6]benzene leads to a
ꢀ
complex mixture in which only 4-iPrC5H4B PCy3 and
[(PCy3)2Pt(H)(Cl)] were isolated (see the Supporting Informa-
tion).
[9] Other reports of complexes having a M!B interaction have
been discredited: a) H. Braunschweig, T. Wagner, Chem. Ber.
1994, 127, 1613 – 1614; b) H. Braunschweig, T. Wagner, Z.
Naturforsch. B 1996, 51, 1618 – 1620; c) H. Braunschweig, C.
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[22] Elsevier claimed to have the largest reported JPtꢀH coupling of
1
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[23] Crystallographic data are given in the Supporting Information.
CCDC 724579 (C), CCDC-724499 (3), and CCDC-737237 (4)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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