A T-shaped platinum(II) boryl complex as the precursor to a platinum compound with a base-stabilized borylene ligand

Article abstract of DOI:10.1002/anie.200501588

(Chemical Equation Presented) T time: A cationic, three-coordinate T-shaped platinum boryl complex (see structure) serves as the precursor to a cationic platinum complex featuring a base-stabilized borylene ligand.

Full text of DOI:10.1002/anie.200501588

Angewandte
Chemie
(1) by reaction of [Pt(PCy3)2] with FcBBr2.[7b] A remarkable
À
structural feature of crystalline 1 is the rather long Pt Br
bond length (2.6183(8) Š), which provides an indication of
the high trans influence of the B(Fc)Br group. Indeed, on
account of its strong s-donor character, the boryl ligand exerts
a higher trans-labilizing influence[8] than that of other s
donors, such as hydrido[7a,9] and stannyl groups,[6a] or p
acceptors, such as CO.[10] Herein, we demonstrate how
strongly this property influences the reactivity of 1. Thus,
the reaction of 1 with Na[BArf4] (Arf = 3,5-C6H3(CF3)2) leads
to the T-shaped, three-coordinate platinum complex trans-
[(Cy3P)2Pt{B(Fc)Br}][BArf4] (2). Such a species constitutes
the precursor to the cationic, base-stabilized borylene com-
plex trans-[(Cy3P)2Pt(Br){B(Fc)(NC5H4-4-Me)}][BArf4] (3),
which is obtained upon addition of 4-methylpyridine, via an
overall 1,2-bromide migration from the boron center to the
platinum center.
Treatment of a solution of 1 in CD2Cl2 with Na[BArf4] led
to an immediate color change from bright orange to cherry
red, with concomitant precipitation of NaBr (Scheme 1, a).
Boron Ligands
DOI: 10.1002/anie.200501588
A T-Shaped Platinum(ii) Boryl Complex as the
Precursor to a Platinum Compound with a Base-
Stabilized Borylene Ligand**
Holger Braunschweig,* Krzysztof Radacki,
Daniela Rais, and David Scheschkewitz
Transition-metal boryl complexes constitute an abundant and
important class of boron-containing transition-metal com-
pounds,[1] in view of their intermediacy in the catalyzed hydro-
[2]
and diboration[3] of unsaturated organic substrates, as well
[4]
À
as the selective C H bond activation of alkanes and arenes.
A number of synthetic routes have been applied to the
generation of boryl complexes.[1] Among these, that involving
À
B X (X = Cl, Br) bond oxidative addition has seldom been
employed, and examples of rhodium and iridium,[5] palla-
dium,[6] and platinum[7] boryl complexes synthesized in this
fashion have only relatively recently appeared in the liter-
ature.
Scheme 1. a) Na[BArf4] in CD2Cl2; b) 4-methylpyridine in CD2Cl2.
The 31P{1H} NMR spectrum of the reaction mixture conclu-
sively revealed the quantitative formation of a new compound
within a few minutes, with the presence of a single signal at
d = 41.7 ppm (1J(P,Pt) = 2914 Hz), strongly downfield shifted
We previously reported the synthesis of trans-
[(Cy3P)2Pt(Br){B(Fc)Br}] (Fc = ferrocenyl, Cy = cyclohexyl)
1
with respect to that of 1 (d = 21.5 ppm, J(P,Pt) = 2892 Hz).
[*] Prof. Dr. H. Braunschweig, Dr. K. Radacki, Dr. D. Rais,
Dr. D. Scheschkewitz
In the 1H NMR spectrum, two broad signals for the
protons of the [BArf4]À ion appeared at d = 7.74 and 7.58 ppm,
alongside a new set of signals for the ferrocenylboryl ligand,
namely two multiplets at d = 4.71 ppm (2H) and d = 4.52 ppm
(2H), and a singlet at d = 4.26 ppm (5H). Surprisingly, no
resonance signal was observed in the 11B{1H} NMR spectrum,
probably due to broadening of the signal as a consequence of
unresolved coupling to the platinum and phosphorus
nuclei.[11]
Institut für Anorganische Chemie
Bayerische Julius-Maximilians-Universität Würzburg
Am Hubland 97074 Würzburg (Germany)
Fax: (+49)931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Engineering and Physical Sciences Research Council. D.R.
thanks the Alexander-von-Humboldt Foundation for a postdoctoral
fellowship. The authors thank Dr. G. R. Whittell, Bristol, for helpful
discussions.
Orange bricklike crystals of 2 were obtained by layering a
solution of the complex in CD2Cl2 with hexane and cooling
the solution to À368C. The X-ray diffraction study performed
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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on a suitable single crystal confirmed that complex 2 was a
three-coordinate platinum(ii) boryl compound that displayed
The absence of agostic interactions in 2 was supported by
density functional calculations[17] and variable-temperature
NMR studies. No appreciable change in the NMR spectra of 2
was observed upon cooling the sample to À608C. At lower
temperatures, the signal in 1H NMR spectrum at d = 4.52 ppm
for two of the ferrocenyl protons broadened. At À908C,
coalescence occurred, consistent with restricted rotation of
a
distorted T-shaped geometry (P1-Pt1-P2 162.96(3)8;
Figure 1).[12]
À
the ferrocenyl group around the B C(Fc) bond. Concomitant
broadening of the initial singlet at d = 41.7 ppm was observed
in the 31P{1H} NMR spectrum, and two separate broad signals
at d = 42.5 and 38.3 ppm could be detected at À908C.
However, none of these processes was accompanied by the
appearance of a high-field resonance signal that could be
assigned to an agostic hydrogen atom, suggesting the
decreased fluxionality of the molecule at low temperatures
as the likely origin of the spectral phenomena.
The bulky tricyclohexylphosphane ligands and the high
trans-labilizing boryl ligand cooperate to inhibit coordination
of more efficient nucleophiles, such as THF or CH3CN, at the
platinum center. Negligible decomposition of a solution of 2
in CD2Cl2 containing an excess (> 10 equiv) of THF or
CH3CN was observed by NMR spectroscopy over the course
of 24 h. Upon addition of the stronger Lewis base 4-
methylpyridine, however, gradual darkening of the solution
from cherry red to dark purple occurred within 2 h, and the
predominant formation of a new species, 3, which incorpo-
rated the added ligand was observed by NMR spectroscopy.
Although binding of the pyridine unit to the platinum center
might have been expected, on account of the electronic
unsaturation at the metal center and its affinity for nitrogen
donors,[13b–d] an X-ray diffraction study[12] performed on
purple crystals of 3 revealed a different outcome of the
reaction. As displayed in Figure 2, the constitution of complex
3 is that of a four-coordinate, cationic trans-bis(tricyclohex-
ylphosphino)platinum complex, with the two remaining sites
occupied by a bromide ligand and a 4-methylpyridine-
stabilized ferrocenylborylene moiety. As such, 3 represents
a unique example of a cationic[18] platinum(ii) borylene
complex and a rare instance of a base-stabilized borylene
Figure 1. Molecular structure of 2. Hydrogen atoms and the [BArf4]À
ion were omitted for clarity. Selected bond lengths [Š] and angle [8]:
À
À
À
À
Pt1 B1 1.966(4), Pt1 P2 2.3071(8), Pt1 P1 2.3323(8), B1 Br1
1.955(4); P1-Pt1-P2 162.96(3).
Despite the absence of a ligand trans to the boryl group,
À
the B1 Pt1 distance (1.966(4) Š) in 2 is, unexpectedly, only
slightly shorter than that in crystalline 1 (1.9963(34) Š).[7b]
This is presumably a consequence of the steric bulk of the
tricyclohexylphosphane ligands, which prevents a closer
approach of the boryl group to the metal center. The
threefold coordination at the platinum center in the cationic
portion of 2 is remarkable. It is well-established that the
highly unsaturated, Lewis acidic nature of the metal center in
14-electron d8 ML3 compounds greatly enhances the reactivity
of these species, leading to interactions even with weak
nucleophiles, such as solvent molecules, weakly coordinating
[14]
anions,[13] or agostic C H bonds. In 2, however, no such
À
interaction is observed. Particularly noteworthy is the
absence of any efficient agostic interaction between the
À
metal center and the C H bonds of the cyclohexyl groups.
À
À
The shortest Pt···H Cdistance is 2.542 Š (Pt···C H 3.117 Š),
which is considerably longer than corresponding distances in
À
the complexes [PtMe(PiPr3)2][1-H-closo-CB11Me11] (Pt H
[14g]
2.24, Pt C2.859 Š)
and [Pt{P(2,6-Me2C6H3)Cy2}{k2-P,C-
À
f
[14f]
À
P(2-Me-6-CH2-C6H3)Cy2}][BAr 4] (Pt C2.432 Š)
and in
the range of those observed in truly three-coordinate neutral
palladium complexes, in which the presence of agostic
interactions was excluded.[15] Indeed, theoretical calculations
on 14-electron d8 ML3 boryl complexes of the type [(R3P)2Rh-
À
(BX2)] support the reluctance of such compounds to bind C
H bonds in the site opposite to the boryl group, owing to the
extremely strong trans influence of the boryl ligand.[16] Thus,
compound 2 represents a structurally characterized example
of this class of complexes, and provides experimental
confirmation of what was predicted from theoretical studies.
Figure 2. Molecular structure of 3. The hydrogen atoms and the
[BArf4]À ion have been omitted for clarity. Selected bond lengths [Š]
À
À
À
and angles [8]: Pt1 B1 2.014(5), Pt1 P2 2.3452(11), Pt1 P1
À
À
2.3564(11), Pt1 Br1 2.6057(5), N1 B1 1.582(6); P2-Pt1-P1 171.83(4),
C10-B1-N1 114.4(4), C10-B1-Pt1 131.0(3), N1-B1-Pt1 113.6(3).
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Angewandte
Chemie
compound.[10b,19] Previously, Roper et al. described formation
of the only other reported organic base-stabilized borylene
compounds, upon reaction of the osmium dichloroboryl
precursor [Os(BCl2)Cl(CO)(PPh3)2] with 8-aminoquinoli-
ne[10b] or 2-aminopyridine.[19a] In analogy with the formation
of the osmium system, it can be inferred that formation of 3
occurs by a Lewis base induced, formal 1,2-halide shift from
complex 1 (2.6183(8) Š), in line with the high trans influence
of the boron-containing ligand.
The spectroscopic data of complex 3 were in agreement
with its solid-state structure. The 31P{1H} NMR spectrum
exhibited a singlet at d = 25.0 ppm (1J(P,Pt) = 2554 Hz),
strongly high-field-shifted with respect to the corresponding
signal of 2. In the room-temperature 1H NMR spectrum,
broad signals were observed at d = 7.90 and 7.82 ppm for two
of the 4-methylpyridine protons, as well as at d = 4.54 ppm for
two of the ferrocenyl protons and for some of the cyclohexyl
protons; this indicates slow hindered rotation of the methyl-
À
À
boron to platinum, by which a strong B N bond (B1 N1
1.582(6) Š) is formed within the borylene moiety and
concomitantly the coordinative unsaturation at the metal
center is relieved upon coordination of the bromide ligand.
The latter appears to be an important factor. For example,
intramolecular, particularly a-hydrogen, migrations within
organometallic compounds are thought to be kinetically
favorable, if through the shift a coordinatively unsaturated
species can be converted into coordinatively more strongly
saturated compound.[20] This principle, which plays a signifi-
cant role in organometallic chemistry,[21] provided a viable
route to silylene complexes of platinum.[22] The isolation of
complex 3 and of the osmium compounds demonstrates that
analogous 1,2-halide migrations might be similarly powerful
synthetic tools in borylene chemistry.[23]
À
pyridine ligands and of the ferrocenyl groups around the B N
À
and B Cbonds, respectively, with respect to the NMR time
scale. At À208C, the four pyridine protons gave rise to two
overlapping doublets at d = 10.45–10.42 ppm and two more
doublets at d = 7.92 and 7.79 ppm (3J(H,H) = 6 Hz). Only
after the sample had been cooled to À608Cwere four
separate signals for the four protons of the boron-bound
cyclopentadienyl ring observed at d = 4.92, 4.88, 4.73, and
4.04 ppm. Finally, as for complex 2, extreme broadening of the
resonance in the 11B{1H} NMR spectrum of 3 prevented
observation of a detectable signal.
À
In contrast to the very short Fe B distance (1.792(8) Š)
In conclusion, we have described the synthesis of the T-
shaped, three-coordinate boryl complex trans-[(Cy3P)2Pt-
{B(Fc)Br}][BArf4] (2) and its conversion into the cationic
base-stabilized borylene complex trans-[(Cy3P)2Pt(Br)-
{B(Fc)(NC5H4-4-Me)}][BArf4] (3) by an overall 1,2-migration
of bromide from the boron to the platinum center. Experi-
ments aimed at verifying the generality of this approach to the
synthesis of borylene complexes, as well as at exploiting the
high trans influence of the boryl ligand to access highly
unsaturated compounds are currently underway.
5
f
=
in
[(h -C5Me5)(OC)2Fe BMes][BAr 4]
(Mes = 2,4,6-
Me3C6H2),[18] the only other reported cationic terminal
borylene complex, the Pt1 B1 distance (2.014(5) Š) in 3 is
only slightly longer than that in the three-coordinate platinum
precursor 2 (1.966(4) Š) and comparable to that in the four-
coordinate boryl compound 1 (1.9963(34) Š).[7b] This is in line
with the increased coordination number at the metal center
and the enhanced steric congestion at the boron center.
À
À
Nonetheless, it must be stated that the comparable Pt B
bond lengths and angles at boron in the T-shaped complex 2
and the tetracoordinate complex 3 make the description of 2
as a boryl and of 3 as a borylene complex somewhat
ambiguous. A similar ambiguity was pointed out by Roper
et al. with regard to the “boryl” character of their base-
stabilized borylene compounds of osmium,[10b,19a] and found to
apply to base-stabilized silylene compounds, whose silylene
character was judged to be limited.[24a,c] Hence, while the
utilized description of complex 3 is based on the formalism
Received: May 10, 2005
Keywords: boron · boryl ligands · borylene ligands · platinum ·
.
structure elucidation
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À
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3
¯
orange brick, 0.093 ” 0.210 ” 0.250 mm , triclinic, space group P1,
a = 12.8443(9),
102.7860(10),
b = 17.7059(12),
b = 91.0560(10),
c = 19.5664(13) Š,
g = 91.9000(10)8,
a =
V=
4335.5(5) Š3, Z = 2, 1calcd = 1.566 gcmÀ3, T= 173(2) K; Bruker-
Apex platform with CCD detector, graphite-monochromated
MoKa radiation, 2qmax = 52.79; 39023 reflections, 17675 inde-
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molecules. All non-hydrogen atoms were refined anisotropically.
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(I > 2s), wR2 = 0.0885 (all data), GoF = 1.044, restrained GoF =
1.046, max/min residual electron density: 2.059/À0.485 ”
1030 emÀ3. Crystal data for 3: C84H94NB2BrF24FeP2Pt, red plate,
3
¯
0.06 ” 0.15 ” 0.30 mm , triclinic, space group P1, a = 14.2217(17),
b = 15.7319(19), c = 21.014(3) Š, a = 110.284(2), b = 103.702(2),
g = 93.827(2)8, V= 4226.9(9) Š3, Z = 2, 1calcd = 1.562 gcmÀ3, T=
173(2) K; Bruker-Apex platform with CCD detector, graphite-
monochromated MoKa radiation, 2qmax = 52.08; 35533 reflec-
tions, 16491 independent (Rint = 0.0287), absorption correction
SADABS (m = 2.432 cmÀ1). The structure was solved by direct
methods. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were refined on idealized positions. R1 = 0.0412
(I > 2s), wR2 = 0.0995 (all data), GoF = 1.068, max/min residual
electron density: 1.833/À1.362 ” 1030 emÀ3. CCDC-271343 (2)
and CCDC-271344 (3) contain the supplementary crystallo-
graphic 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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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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