Oxidative addition of boron trifluoride to a transition metal

Article abstract of DOI:10.1002/anie.201103226

A difficult break-up: Boron trifluoride was activated by oxidative addition to the transition-metal complex [(Cy₃P)₂Pt]. The product of this addition, trans-[(Cy₃P)₂Pt(BF₂) (FBF₃)], was characterized by NMR spectroscopy and crystal structure determination. Furthermore, the degradation product trans-[(Cy ₃P)₂Pt(H)(FBF₃)] and the stable derivative trans-[(Cy₃P)₂Pt(BF₂)(Cl)] were fully characterized. Copyright

Full text of DOI:10.1002/anie.201103226

DOI: 10.1002/anie.201103226
ꢀ
B F Activation
Oxidative Addition of Boron Trifluoride to a Transition Metal**
Jꢀrgen Bauer, Holger Braunschweig,* Katharina Kraft, and Krzysztof Radacki
Dedicated to Professor Gerhard Bringmann on the occasion of his 60th birthday
Organic synthesis, at its most fundamental level, is the study
of carbon–carbon and carbon–element bond formation. As
these bonds are rather stable and many organic compounds
lack accessible coordination sites, the first step of a reaction
has to be the cleavage of a bond. In catalysis, this is often
carried out by a transition metal by means of an oxidative
addition.
Consequently, boronfluorides have hitherto eluded oxidative
addition to a metal center, thus precluding direct metal
mediated fluoroboration of unsaturated organic molecules.
Such a process would be a potentially convenient way to
install two functional groups of opposite polarity at adjacent
carbon atoms, thereby creating two controllable access points
for sequential functionalization. Also of direct relevance to
ꢀ
The synthesis of pharmaceutical, agricultural, and other
fine chemicals is the most crucial and demanding arena for
any new homogeneous catalytic process. In this setting,
selective and protecting-group-free organic synthesis under
mild conditions, and with a minimum of waste, has always
been an ideal.[1,2] With convenience and atom-efficiency in
mind, mild functionalization of the (particularly unreactive)
B F bond activation is the growing interest in organofluor-
oborates as substrates for the widely-used Suzuki–Miyaura
coupling protocol and their attendant possibilities for appli-
cation in organic and pharmaceutical chemistry.[10]
In the course of our studies on the zerovalent platinum
species [(Cy3P)2Pt] (1) and related electron-rich late-transi-
tion-metal complexes, we have explored the limits of the
ꢀ
C H bond has become a prominent endeavor, albeit one that
Lewis basicity of 1 and its pronounced propensity to
[11,12]
ꢀ
ꢀ
rests heavily on the non-trivial ability of the transition metal
to cleave this bond.[3] Similarly, transition-metal-catalyzed
processes involving initial element–element bond oxidative
addition steps are also established, including hydrogenation,[4]
hydrosilylation,[5] and hydroboration,[6] leading to transfer of
(H)(H), (R3Si)(H), or (R2B)(H) groups to an unsaturated
organic substrate.
oxidatively add B Cl and B Br bonds.
naturally, we became interested in its reactivity towards
fluoroboranes. We hereby report the hitherto unknown
By extension,
ꢀ
oxidative addition of a B F bond, namely that of borontri-
fluoride (BF3), to a metal center.
Reaction of [(Cy3P)2Pt] (1) with equimolar amounts of
gaseous BF3 instantaneously affords a mixture of remaining
starting material and two new products, as determined by
31P{1H} NMR spectroscopy. However, use of two equivalents
of BF3 leads to complete conversion of the starting material
into the two new products. In the 19F{1H} NMR spectra, a
platinum-bound difluoroboryl ligand could be identified by its
broad resonance at ꢀ33.3 ppm, flanked by characteristic 195Pt
satellites (2JF–Pt = 1230 Hz). These findings are in good agree-
ment with cis-[(Ph3P)2Pt(BF2)2] (2; d(19F{1H}) = ꢀ17.4 ppm;
2JF–Pt = 1040 Hz).[13]
The search for strongly reactive and Lewis basic transi-
tion-metal complexes by inorganic and organometallic chem-
ists has been in part motivated by the desire to activate ever
more recalcitrant element–element bonds. Tools might
thereby be developed to create bonds using more abundant
(for example alkanes instead of alkenes) or less reactive
reagents (such as aryl chlorides instead of aryl iodides), or to
functionalize unreactive sites of organic molecules that would
ꢀ
be otherwise inaccessible. Much progress in the area of C C
bond activation (bond dissociation energy (BDE):
Another resonance, assigned to a tetrafluoroborate
346 kJmolꢀ1) has been made by Milsteinꢀs group,[7] and even
moiety, was detected at ꢀ168 ppm. In the 11B{1H} NMR
spectra, only the resonance for the latter could be detected at
0.3 ppm, and thus in the typical region for four-coordinate
boron. The absence of a detectable resonance for the BF2
ligand has much precedence in platinum boryl chemistry and
is due to unresolved coupling to the platinum and phospho-
rous nuclei, and in this case additional unresolved coupling to
the fluorine.[14] In the 31P{1H} NMR spectra, the two new
ꢀ1
ꢀ
strong C F bonds (BDE: 485 kJmol ) have been activated,
both by oxidative addition to transition metals[8] and tran-
sition-metal-mediated processes,[9] leading to important ap-
plications in the area of pharmaceutical chemistry.
ꢀ
The significantly stronger and highly polar B F bond
(BDE: 651 kJmolꢀ1) ranks among the most stable s bonds.
1
products give rise to resonances at 48.1 (3, JP–Pt = 2828 Hz)
1
[*] J. Bauer, Prof. Dr. H. Braunschweig, Dr. K. Kraft, Dr. K. Radacki
Institut fꢀr Anorganische Chemie
and 44.0 ppm (4, JP–Pt = 2595 Hz), respectively. These data
confirm the presence of two square-planar platinum(II)
complexes in solution.[15] Moreover, and along with the
typical resonances for the phosphorous-bound cyclohexyl
groups, a significantly upfield-shifted triplet (2JH–P = 24 Hz) at
ꢀ31.64 ppm flanked by 195Pt satellites (1JH–Pt = 1806 Hz) is
detected in the 1H NMR spectra, indicating the presence of a
platinum-bound hydrogen.
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[**] We are grateful to the German Science Foundation (DFG) for
financial support. J.B. is grateful to the Fonds der Chemischen
Industrie for a Ph.D. scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103226.
Angew. Chem. Int. Ed. 2011, 50, 10457 –10460
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10457

Communications
On account of these data, we proposed the formation of
solution, we were able to grow single crystals suitable for X-
ray analysis, which finally confirmed the oxidative addition of
BF3 to the zerovalent platinum center with formation of trans-
[(Cy3P)2Pt(BF2)(FBF3)] (4). It should be mentioned that the
constitution of the degradation product trans-[(Cy3P)2Pt(H)-
(FBF3)] (3) was further confirmed by its alternative synthesis
by reaction of [(Cy3P)2Pt] (1) with HBF4/Et2O and compar-
ison of the spectroscopic data of an authentic sample with
those observed for 3 in the reaction mixture.
two products, trans-[(Cy3P)2Pt(H)(FBF3)] (3) and trans-
[(Cy3P)2Pt(BF2)(FBF3)] (4), according to Scheme 1. Monitor-
ing the reaction mixture in benzene by multinuclear NMR
spectroscopy disclosed an initial ratio between 3 and 4 of
approximately 1:2. The formation of 3 in significant amounts
is apparently a nonstoichiometric reaction (with regard to
Scheme 1) but could be explained by formation of HF owing
to hydrolysis and the unavoidable presence of trace amounts
of water (for example on glass surfaces).[16] However, it should
be noted that BF3 is the most stable haloborane with respect
to hydrolysis,[17] and under the same reaction conditions (very
low temperatures, short reaction time, and rigorous inert
conditions; see the Experimental Section) the related com-
plex trans-[(Cy3P)2Pt(BBr2)(Br)] (5) was easily synthesized in
our laboratories from 1 and the far more sensitive borane
BBr3.[12c] Nevertheless, to provide further information regard-
ing a possible hydrolysis of BF3 to HF and concomitant
formation of HBF4 we repeated the reaction in wet (that is,
not dried but degassed) solvents, which led again to the
formation of 3 and 4 in approximately the same initial ratio of
1:2 (see the Supporting Information). Therefore, hydrolysis
processes seem not to be responsible for the presence of 3.
The fast degradation of trans-[(Cy3P)2Pt(BF2)(FBF3)] (4)
is somewhat surprising. Although difluoroboryl complexes
are scarce, the few species such as cis-[(Ph3P)2Pt(BF2)2] (2) or
fac-[(Ph3P)2Ir(CO)(BF2)3] (8), which were fully characterized
in solution and in the solid state,[13] and [(h5-C5Me5)-
(Me3P)Ir(H)(BF2)] (9),[18] which was ascertained by multi-
nuclear NMR spectroscopy, display no increased tendency to
degrade under ambient conditions. Likewise, fluoroboryl
complexes of Fe and Ru, which were characterized in
solution, did not reveal an enhanced instability.[19] Thus, we
sought to transfer trans-[(Cy3P)2Pt(BF2)(FBF3)] (4) into a
more stable derivative by replacing the presumably loosely
coordinated BF4ꢀ ligand in trans position to the difluoroboryl
group. To this end, the synthesis of 4 was carried out in the
presence of NBu4Cl, which indeed furnished the correspond-
ing chloro complex trans-[(Cy3P)2Pt(BF2)(Cl)] (10) in good
yields of 85% as a colorless solid. Compound 10 reveals a
markedly increased stability in comparison to 4, and showed
no signs of degradation in benzene, toluene, or dichloro-
methane solutions over extended periods of time at ambient
temperature and could be purified by filtration over a short
plug loaded with alumina. The complex trans-[(Cy3P)2Pt-
(BF2)(Cl)] (10) was fully characterized in solution by multi-
nuclear NMR spectroscopy, and data thus obtained match
those of complexes of the type trans-[(Cy3P)2Pt(BX2)(Br)]
(X = Br, NMe2).[12c] However, it should be noted that 10, in
contrast to 4, gave rise to a detectable, broad 11B NMR
resonance at 30 ppm, indicative to the BF2 ligand (Table 1).
Single crystals of 4 and 10 suitable for X-ray diffraction
analyses were obtained as described in the Experimental
Section. Both complexes have a square-planar coordination
with the two PCy3 ligands in trans position (Figure 1). While
the data confirm the constitution of 10, a significant disorder
of the two anionic ligands precludes a detailed discussion of
Scheme 1. Reaction of 1 with gaseous BF3 leading to the complexes 3
and 4.
While mechanistic details for the formation of the latter
species are yet elusive, it should be mentioned that the related
haloboryl complexes trans-[(Cy3P)2Pt(X)(BX2)] convert
slowly into the corresponding hydrido species trans-
[(Cy3P)2Pt(X)(H)] (6, X = Cl; 7, X = Br) after elongated
storage in solution and under strictly inert conditions. Like-
wise, in case of the fluoro species 4 and 3, the initial ratio of
2:1 was found to be reversed after about 1 h at room
temperature in solution, and continuously shifted towards
full conversion of 4 into 3. This indicates that the latter
hydride complex represents the degradation product of the
former boryl species. However, we could not verify the source
of the hydrogen in Scheme 1. Of note, the 1H NMR resonance
of the hydric hydrogen in 4 is, compared to 6 or 7,[15]
significantly shifted towards higher field.
Further proof for the proposed constitution of 3 and 4 was
provided when [(Cy3P)2Pt] (1) was treated with two equiv-
alents gaseous BF3 in hexane. Here, immediate precipitation
of a colorless solid occurred. Analysis by solid-state MAS-
31P{1H} NMR spectroscopy disclosed two resonances, again
in a 1:2 ratio at 47.7 (3, 1JP–Pt = 2741 Hz) and 42.0 ppm (4, 1JP–
Pt = 2550 Hz), which match those derived from NMR spec-
troscopy in solution. Likewise, elemental analysis supported
the formulation of the solid as a mixture of 3 and 4 in a 1:2
ratio. Despite the instability of the difluoroboryl complex 4 in
ꢀ
the bond lengths of the fluoroboryl moiety. However, the Pt
B separation of 1.965(3) ꢁ in 4 is comparable to that
(1.963(6) ꢁ) found in trans-[(Cy3P)2Pt(BBr2)(Br)] (5),[11]
ꢀ
while the Pt B distance in cis-[(Ph3P)2Pt(BF2)2] (2)
(2.052(6) ꢁ) is about 0.09 ꢁ greater, as expected for a
mutual trans position of the boryl and the phosphine
ligand.[20] All other structural parameters of the fluoroboryl
ligand in 4 are in very good agreement with previous findings.
Table 1: NMR parameters of compounds 3, 4 and 10.[a]
1
d(31P) [ppm] JP–Pt [Hz] d(19F) [ppm] 2JF–Pt [Hz] d(11B) [ppm]
4
3
44.0
48.1
2595
2828
2604
ꢀ33.3, ꢀ167 1230
–, 0.4
0.2
30
ꢀ169
–
10 30.7
ꢀ24.8
958
[a] Values in italics are assigned to the BF4 ligand.
10458 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 10457 –10460

BCat ligand (Cat = catecholato). These results are in good
agreement with previous findings, which indicated a decrease
of the trans influence with increase of the electronegativity of
the boron-bound substituents.[22]
In summary, we have presented the first oxidative
addition of one boron–fluorine bond of BF3 to a transition
metal. As the product of the oxidative addition (4) is not
stable in solution, the fluoroboryl ligand could be preserved in
a subsequent reaction rendering the complex to the more
stable trans-chloro derivative 10; single-crystal X-ray diffrac-
tion was applied to both complexes. Furthermore, the
degradation product 3 could be obtained by an alternate
reaction pathway. All of the complexes were examined by
multinuclear NMR spectroscopy, both in the solid state and in
solution, and by IR spectroscopy, elemental analysis, and
DFT calculations.
Figure 1. Molecular structures of 4 and 10. Relevant bond lengths [ꢂ]
and angles [8]: 4: Pt–P1 2.325(1), Pt–P2 2.327(1), Pt–B1 1.965(3), Pt–
F3 2.272(2), B1–F1 1.327(4), B1–F2 1.336(3), B2–F3 1.441(3), B2–F4
1.379(4), B2–F5 1.369(4), B2–F6 1.375(4); P1-Pt-P2 170.1(1), P1-Pt-B1
90.6(1), P1-Pt-F3 90.0(1), F1-B1-F2 112.0(2), F3-B2-F4 107.0(2), F3-B2-
F5 107.0(4), F3-B2-F6 107.0(2), F4-B2-F5 111.8(2), F4-B2-F6 111.4(2),
F5-B2-F6 111.8(3). 10: Pt–P2.3187(6)); symmetry-related positions
(ꢀx, ꢀy, ꢀz) are labeled with ’. Ellipsoids set at 50% probability;
ellipsoids of the ligands, solvent molecules, and hydrogen atoms
omitted for clarity.
Experimental Section
General considerations regarding the experimental procedures, X-ray
diffraction, and computational studies are provided in the Supporting
Information.
4: In a Schlenk flask equipped with a Teflon valve, 1 (100 mg,
0.13 mmol) was dissolved in hexane (5 mL), cooled to ꢀ1968C, and
the flask was evacuated. At the same time, a gas trap with two Teflon
valves was filled with gaseous BF3 (18 mg, 0.26 mmol). After melting
of the reaction mixture, the BF3 was immediately added by
connection of the gas trap to the Schlenk flask (by opening the first
Teflon valve) and finally the vacuum was equalized with argon (by
opening the second valve of the gas trap). Under warming to room
temperature, the mixture was stirred for 15 min; meanwhile a
colorless precipitate was formed. After decanting off the solvent the
precipitate was washed two times with hexane, all volatiles were
removed in vacuo, yielding 102 mg of a colorless powder. The
constitution of the product was determined by solid-state NMR
spectroscopy as a 2:1 mixture of the product 4 and the degradation
product 3. Despite the instability in solution, a small amount of
crystals suitable for X-ray diffraction could be obtained instanta-
neously after addition of BF3 to a benzene solution of 1 in a Young
NMR tube. The crystals were found after 1 h at room temperature.
1H NMR (400.1 MHz, C6D6): d = 2.53ꢀ2.41 (m, 6H, Cy),
ꢀ
For example, the B1 F separations (1.327(4)/1.336(3) ꢁ) and
F1-B1-F2 angle (112.0(2)8) correspond with those (1.327(6)/
1.33(7) ꢁ; 110.8(5)8) reported for 2. The tetrafluoroborate
ligand reveals distortion from ideal tetrahedral geometry
ꢀ
owing to coordination to the Pt center. Thus, the B2 F3
separation of the bridging fluorine is increased (1.441(3) ꢁ)
compared to the mean separation of the terminal fluorine
substituents (1.374 ꢁ). Likewise, the F3-B2-F angles amount
to about 1078, whereas the other F-B2-F angles are about
1128. Overall, the BF4 ligand displays the typical structure for
h1-coordinated fluoroborates, as for example reported for
[(ItBu)(h3-C3H3)Pd(BF4)] (11).[21]
ꢀ
Recent work has shown that the length of the Pt Cl or the
Pt Br bond can be correlated with the degree of trans
ꢀ
influence exerted by boryl ligands in square-planar platinu-
m(II) complexes, but owing to disorder in the structure of 10,
this approach cannot be applied here.[12c,22] Therefore, density
functional calculations were carried out to compare the title
compounds with related complexes bearing different boryl
ligands. The optimized structures of the complexes
[(Cy3P)2Pt(X)(Br)] (X = BtBuBr (12), BCl2 (13), BBr2 (5),
BF2 (14), and Bcat (15)) were used to assess the relative trans
influence of the different boryl ligands (Table 2). According
to the calculated bond lengths, the fluoroboryl ligand reveals
a very weak trans influence, which is even smaller than that of
the bromoboryl ligand and only slightly larger than that of the
2.14ꢀ1.08 (m, 60H, Cy); 11B{1H} NMR (128.4 MHz, C6D6): d =
1
0.3 ppm; 13C{1H} NMR (100.6 MHz, C6D6): d = 35.4 (vt, N =j JP–C
+
2
3JP–C j= 27 Hz, C1 Cy), 30.6 (s, C3,5 Cy), 27.5 (vt, N =j JP–C + 4JP–C j=
11 Hz, C2,6 Cy), 26.8 ppm (s, C4 Cy); 19F{1H} NMR (376.5 MHz, C6D6):
d = ꢀ33.3 (vbr s, 2JPt–F = 1230 Hz), ꢀ167.0 ppm (vbr s); 31P{1H} NMR
(162.0 MHz, C6D6): d = 44.0 ppm (s, 1JPt–P = 2595 Hz); 31P HPDec/
MAS NMR (162.0 MHz): d = 42.0 ppm (s, 1JPt–P = 2550 Hz). IR: 1147,
1215 cmꢀ1 (BF2), 1115, 1174 cmꢀ1 (BF4). C,H analysis calcd. [%] for a
2:1 mixture of C36H66B2F6P2Pt and C36H67BF4P2Pt: C 49.38, H 7.64;
found: C 49.71, H 7.77.
3: In a Schlenk flask, 1 (100 mg, 0.13 mmol) was dissolved in 5 mL
(Et2O), and an excess of HBF4 (1 mL, 50% in Et2O) was added. After
stirring for 30 min, all volatiles were removed in vacuo, the residue
was extracted with toluene and again all volatiles were removed
in vacuo yielding 3 (93.4 mg, 0.11 mmol, 85%) as a colorless powder.
Table 2: Selected bond lengths [ꢂ] of DFT-optimized complexes.[a]
1H NMR (400.1 MHz, C6D6): d = 2.29ꢀ2.20 (m, 6H, Cy),
1
12
13
5
14
15
2.14ꢀ1.08 (m, 60H, Cy), ꢀ31.64 ppm (t, 2JP–H = 24 Hz, JPt–H
=
1806 Hz);
11B{1H} NMR
(128.4 MHz,
C6D6):
d = 0.2 ppm;
ꢀ
Pt Br
ꢀ
Pt B
2.739
2.013
2.698
1.987
2.695
1.980
2.692
2.002
2.689
2.013
1
13C{1H} NMR (100.6 MHz, C6D6): d = 34.8 (vt, N =j JP–C + 3JP–C j=
27 Hz, C1 Cy), 30.9 (s, C3,5 Cy), 27.5 (vt, N =j JP–C + 4JP–C j= 11 Hz,
2
[a] DFT methods using the 6-31G(d,p) basis set for H, B, C, Cl, F, O, P,
6-311G(d,p) for Br, and “Stuttgart Relativistic Small Core” ECP Basis Set
for Pt.
C2,6 Cy), 26.8 ppm (s, C4 Cy); 19F{1H} NMR (376.5 MHz, C6D6): d =
ꢀ169.0 ppm (vbr s); 31P{1H} NMR (162.0 MHz, C6D6): d = 48.1 ppm
(s, 1JPt–P = 2828 Hz); 31P HPDec/MAS NMR (162.0 MHz): d =
Angew. Chem. Int. Ed. 2011, 50, 10457 –10460
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Communications
47.7 ppm (s, 1JPt–P = 2741 Hz). IR: 1113, 1169 cmꢀ1 (BF4). C,H analysis
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calcd. [%] for C36H67BF4P2Pt: C 51.25, H 8.00; found: C 50.90, H 8.04.
10: Using the same apparatus as in synthesis of 4, complex 1
(100 mg, 0.13 mmol) and NBu4Cl (36.7 mg, 0.13 mmol) were reacted
with BF3 (18 mg, 0.26 mmol) in toluene (5 mL). The mixture was
stirred with warming to room temperature for 60 min. The reaction
mixture was then filtered over a short plug with alumina (activity
grade 1) and all volatiles were removed in vacuo. The colorless
precipitate was washed twice with hexane, and again all volatiles were
removed in vacuo, yielding 10 (88.4 mg, 0.11 mmol, 85%) as a
colorless powder. Crystals suitable for X-ray diffraction were grown
from a benzene solution at room temperature.
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1H NMR (400.1 MHz, C6D6): d = 2.71ꢀ2.60 (m, 6H, Cy),
2.23ꢀ2.15 (m, 12H, Cy), 1.81ꢀ1.20 (m, 48H, Cy); 11B{1H} NMR
(128.4 MHz, C6D6): d = 30 ppm; 13C{1H} NMR (100.6 MHz, C6D6):
1
d = 35.8 (vt, N =j JP–C + 3JP–C j= 28 Hz, C1 Cy), 30.5 (s, C3,5 Cy), 28.0
2
(vt, N =j JP–C + 4JP–C j= 11 Hz, C2,6 Cy), 27.0 ppm (s, C4Cy);
2
19F{1H} NMR (376.5 MHz, C6D6): d = ꢀ24.8 ppm (vbr s, JPt–F
=
=
1
958 Hz); 31P{1H} NMR (162.0 MHz, C6D6): d = 30.7 ppm (s, JPt–P
2604 Hz). IR: 1136, 1171 cmꢀ1 (BF2). C,H analysis calcd. [%] for
C36H66BClF2P2Pt: C 51.46, H 7.92; found: C 52.12, H 8.11.
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Revised: July 1, 2011
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ꢀ
Keywords: B F activation · boron trifluoride ·
fluoroboryl ligands · oxidative addition · transition metals
.
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