A pincer-type anionic platinum(0) complex

Article abstract of DOI:10.1002/anie.200705927

(Chemical Presented) Holding a Pt⁰ anion in a pincer: Reduction of the pincer-type Pt^II complex 1 results in the formation of the thermally stable anionic Pt⁰ complex 2. This complex adopts a T-shaped structure and exhibit

Full text of DOI:10.1002/anie.200705927

Angewandte
Chemie
DOI: 10.1002/anie.200705927
Coordination Chemistry
A Pincer-Type Anionic Platinum(0) Complex**
Leonid Schwartsburd, Revital Cohen, Leonid Konstantinovski, and David Milstein*
Pincer-type complexes constitute a large family of compounds
that have attracted much recent interest. Among these
compounds, aryl-anchored, d8 pincer complexes of the type
[MII(LCL’)] (M = Ni, Pd, Pt; L = neutral ligand such as
phosphine, amine, dialkyl sulfide) are a major group that
plays important roles in organometallic reactions and mech-
anisms, catalysis, and in the design of new materials.[1] In
contrast, and to our knowledge, no complexes of this type
with the metal in the zero oxidation state have been prepared.
Such d10 [M0(LCL’)] complexes with neutral L ligands and an
“anionic” aryl anchor would be anionic, and would be
expected to possess distinctly different properties to neutral
d10 (M0) complexes. We chose to utilize bulky bis-chelating
pincer-type ligands in this study as they have been shown to be
effective in stabilizing reactive species and have led to
unusual complexes.[1]
complex was not isolated but was trapped with a variety of
electrophiles to give a range of PtII complexes.[4,5]
Herein we report the preparation, characterization, and
computational study of the first thermally stable, monome-
tallic anionic Pt0 complex. The reactivity of this electron-rich,
16-electron PCP-type anionic Pt0 complex shows that it is a
Brønsted base and an effective electron-transfer reagent that
À
is capable of C F activation under exceedingly mild con-
ditions.
Reduction of the bulky PCP-PtII complex 1 (Scheme 1)[6]
with sodium in dry [D8]THF at room temperature overnight
led to a dramatic color change from colorless to dark red. A
We have recently shown that reduction of PCP-type PdII
complexes [Pd(X)C6H3(CH2PiPr2)2] (X = Cl, trifluoroace-
tate) with sodium metal results in collapse of the pincer
system, leading to formation of the diamagnetic binuclear
complex [Pd{C6H3(CH2PiPr2)2}2Pd], which contains a 14-
electron linear Pd0 moiety and a completely nonplanar
“butterfly”-type 16-electron PdII moiety.[2] Oxidation of the
binuclear complex, or its reaction with organic halides,
regenerates the original mononuclear framework.[2]
To avoid collapse of the pincer system upon reduction we
decided to use a PCP-PtII complex with the hope that the
more diffuse nature of the Pt orbitals (compared with Pd)
might stabilize the reduced metal center.[3] In addition,
increasing the steric bulk of the pincer phosphine ligand
might protect the reduced metal center against intermolec-
ular reactions and might lead to a rare monometallic Pt0
anionic complex. We are aware of only one monometallic
anionic Pt0 complex, namely [Pt(Me2NCS2)(PEt3)]À, which
was generated in situ at À788Cby proton abstraction from
the PtII hydride complex [PtH(Me2NCS2)(PEt3)].[4] This
Scheme 1. Reduction of complexes 1 and 3 to form the PCP-Pt0 anion
2 and its oxidation to regenerate the PtII complex. See text for details;
Cp=C5H5.
multinuclear NMR spectroscopy study of the reaction solu-
tion revealed that reduction of complex 1 had taken place to
give quantitative formation of the diamagnetic 16-electron
planar PCP-Pt0 anion 2 (Scheme 1). Thus, the resonance of 1
at d = 66.7 ppm (1JPt,P = 2893 Hz) in the 31P{1H} NMR spec-
trum is replaced by a signal for the new complex 2 at d =
120.5 ppm (1JPt,P = 3874 Hz). The 31P{1H} NMR spectrum of
the solution confirmed that this transformation is quantita-
tive. In addition, the 195Pt{1H} NMR spectrum revealed that
the diagnostic triplet of 1 at d = À4105 ppm (1JP, Pt = 2893 Hz)
[*] L. Schwartsburd, Prof. D. Milstein
Department of Organic Chemistry
Weizmann Institute of Science, 76100 Rehovot (Israel)
Fax: (+972)8-934-4142
E-mail: david.milstein@weizmann.ac.il
is replaced by a new triplet for 2 at d = À4034 ppm (1JP, Pt
=
Dr. R. Cohen, Dr. L. Konstantinovski
Unit of Chemical Research Support
Weizmann Institute of Science, 76100 Rehovot (Israel)
3874 Hz), thus showing that the two phosphorus donor atoms
stay bound to the metal center in 2.[7] The increase in the Pt P
À
[**] We thank Dr. Edward E. Korshin and Elena Poverenov for helpful
discussions. This work was supported by the Israel Science
Foundation, the Program for German–Israeli Cooperation (DIP),
and the Helen and Martin Kimmel Center for Molecular Design.
D.M. holds the Israel Matz Professorial Chair of Organic Chemistry.
À
coupling constant is indicative of a decrease of the Pt P bond
1
length upon going from 1 to 2. The H NMR spectrum of 2
exhibits a virtual triplet for the tert-butyl group at d =
1.36 ppm (3JP, H = 6 Hz) and a virtual triplet for the methylene
group at d = 3.56 ppm (2JP, H = 4 Hz); this pattern is character-
istic of strong phosphorus–phosphorus coupling between
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Selected bond lengths and angles for 1 and 2.
trans-bonded and magnetically equivalent phosphorus donor
atoms. In the 13C{1H} NMR spectrum, the ipso carbon gives
rise to a peak at d = 221.97 ppm with a one-bond coupling to
platinum (1JPt,C = 728 Hz). In addition, the 1H and
13C{1H} NMR spectra of 2 indicate conservation of the
aromaticity in the ligand backbone. We can therefore
conclude that the meridional PCP arrangement is maintained
in 2.[8] In accordance with the simple patterns in the 31P{1H}
and 195Pt{1H} NMR spectra, complex 2 appears to be a
monomer in THF solution rather than a dimer of the type
[PCP-Pt-Pt-PCP].[9] Coordination of dinitrogen to 2 can be
excluded since the same product was obtained under both
argon and nitrogen (as confirmed by 31P{1H} and
195Pt{1H} NMR spectroscopy).
X-ray
structure of
1[6b]
Computed
structure of
1
Computed
structure of
2
À
Pt P [Š]
2.291(2)
2.017(6)
167.35(5)
2.35
2.01
169.0
2.31
2.06
173.3
À
Cipso Pt [Š]
À À
P Pt P [8]
being delocalized over the metal center and the s-bound aryl
backbone (structure B).
Reduction of complex 1 with sodium metal in dry C6D6
(rather than THF) at room temperature overnight resulted in
formation of the anionic complex 2 as a dark-red precipitate
along with precipitation of sodium chloride (Scheme 1).
Similarly, reduction of the unsaturated cationic PtII complex
3[10] in dry C6D6 under the same conditions resulted in
precipitation of 2 and sodium tetrafluoroborate (Scheme 1).
The 31P{1H} NMR spectrum of the C6D6 reaction solution
revealed full consumption of complexes 1 or 3 in both cases,
while the 31P{1H} NMR spectrum of the dark-red precipitate
dissolved in dry THF confirmed quantitative formation of 2.
Compound 2 is stable at room temperature for a few days in
dry THF solution or in the solid state under an atmosphere of
dry argon (or nitrogen).
Clear evidence for the Brønsted basicity of the platinum
center in the anionic complex 2 was deduced from its reaction
with water. Thus, treatment of a THF solution of 2 with an
excess of H2O (D2O) resulted in quantitative formation of the
PtII hydride (deuteride) complex (4a and 4b, respectively;
Scheme 2) along with an immediate color change from dark
The planar T-shaped geometry proposed for 2 based on
the NMR spectroscopy study is supported by density func-
tional theory (DFT) calculations (Figure 1).[11] A comparison
of the computed structures of 1 and 2 (Table 1) reveals a
À
À À
shortening of the Pt P bonds and a flattening of the P Pt P
angle on going from 1 to 2.[12] Surprisingly, the Cipso Pt bond is
À
Scheme 2. Reaction of the anionic Pt0 complex 2 with water.
longer in the computed structure of 2 than in the computed
structure of 1 and the reported X-ray structure of 1[6b]
À
(Table 1). The aryl C Cbond lengths and angles in 2 are
quite similar to those of 1, hence the computational study
indicates that the negative charge is localized at the metal
center of the anionic complex 2 (structure A) rather than
red to colorless.[13] The resulting colorless THF solution was
found to be basic, which is compatible with formation of
sodium hydroxide.
Complex 4a, which has been reported previously,[13]
exhibits a doublet of triplets centered at d = À4776 ppm
due to one-bond coupling to the hydride ligand (1JH,Pt
=
746 Hz) and coupling to two phosphorus donor atoms
(1JP, Pt = 2918 Hz) in its 195Pt NMR spectrum (in THF). In
agreement with this, the 195Pt NMR spectrum of 4b
exhibits a triplet of triplets centered at the same chemical
shift arising from one-bond coupling to a deuteride ligand
(1JD,Pt = 114 Hz) and coupling to two phosphorus donor
1
atoms (1JP, Pt = 2918 Hz). H and 13CNMR spectroscopy
studies did not reveal any H/D exchange at the aryl
positions upon treatment of 2 with D2O, thus suggesting
that the negative charge in 2 is localized primarily at the
metal center rather than in the aromatic ring (struc-
ture A).
The PCP-Pt0 anion 2 can be re-oxidized to a PCP-PtII
complex. Thus, treatment of a THF solution of 2
(prepared from 3 in C6H6) with two equivalents of the
one-electron oxidant [FeCp2]BF4 is accompanied by an
Figure 1. Optimized structures of complex 1 (left) and the anion 2 (right).
Na+ counterion and hydrogen atoms have been omitted for clarity.
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immediate color change from dark red to yellow, thereby
indicating the formation of [FeCp2] (Scheme 1).[14] The
31P{1H} NMR spectrum of the reaction solution reveals
regeneration of the cationic PtII complex 3 (Scheme 1),[15]
which was isolated from the reaction mixture in 68% yield
after extraction of [FeCp2] with pentane. Oxidation of 2 to a
PtII complex is also observed upon reaction with a dihalogen.
Thus, treatment of a THF solution of 2 with one equivalent of
I2 leads to quantitative formation of the PtII iodo complex 5
(Scheme 3), which is accompanied by a color change from
give benzyl radicals, which can couple to yield dibenzyl.
Coordination of chloride to PtII would yield complex 1
(Scheme 5).[18]
Scheme 5. Proposed mechanism of electron transfer from complex 2
to benzyl chloride to form complex 1 and dibenzyl.
Scheme 3. Oxidation of complex 2 with iodine.
Much research effort has been devoted in recent years to
À
the activation of C F bonds by transition-metal complexes.
dark red to orange.[16a] A possible mechanism for this reaction
involves formation of an anionic intermediate containing an
end-on-bound iodine molecule, [PCP-Pt-I-I]À, followed by
two-electron oxidation of the metal and concomitant hetero-
Indeed, C F bond cleavage is now known in a number of
À
intermolecular systems involving transition-metal com-
plexes.[19–24] We found that the anionic, electron-rich Pt0
À
complex 2 is very active in C F activation. Thus, it reacts
À
lytic cleavage of the I I bond to quantitatively form complex
with one equivalent of C6F6 in THF at À358Cto form the Pt II
complex 6 and sodium fluoride within 2 h (Scheme 6). This is
5 and NaI. The h1-coordination of I2 to an NCN-PtII complex
has been reported previously.[16b]
The electron-rich Pt0 anion 2 is an effective electron-
transfer reagent, as demonstrated by the transformations
shown in Scheme 1. Significantly, the PtII complex 1 is
regenerated quantitatively in these reactions. In the first
reaction, reduction of p-toluenesulfonyl chloride with one
equivalent of 2 in THF results in the formation of a
corresponding amount of sodium p-toluenesulfinate and
complex 1 (Scheme 1; reaction (a). We assume that electron
transfer from 2 to the arenesulfonyl halides results in
formation of sulfonyl radicals (Scheme 4).[17] Since sulfonyl
À
Scheme 6. Facile C F activation of hexafluorobenzene by complex 2.
À
one of the fastest C F bond activation reactions reported to
date.[25] Note that C F activation by platinum complexes is
À
relatively rare.[21] Complex 6 was identified unambiguously by
multinuclear NMR spectroscopy. Thus, its 31P{1H} NMR
spectrum shows a singlet at d = 67.6 ppm flanked by platinum
satellites (1JPt,P = 2856 Hz) and its 195Pt{1H} NMR spectrum
exhibits a triplet of triplets centered at d = À4393 ppm which
arises from three-bond coupling to two fluorine atoms in the
ortho positions (3JF,Pt = 235 Hz) and coupling to two phos-
phorus donor atoms (1JP, Pt = 2856 Hz). The ipso carbon of the
Scheme 4. Proposed mechanism of electron transfer from complex 2
to arenesulfonyl halides to form complex 1 and sodium arylsulfinate.
C6F5 ligand gives rise to a triplet at d = 150.58 ppm (2JP, C
=
8 Hz) and the aryl backbone ipso carbon appears as a singlet
at d = 163.91 ppm with Pt satellites (1JPt,C = 710 Hz) in the
13C{1H} NMR spectrum.
radicals have a low tendency to dimerize (in comparison with
carbon-based radicals),[17] they can undergo additional reduc-
tion to yield sodium arylsulfinate (Scheme 4).
À
Two possible mechanisms for the observed C F activation
Treatment of half an equivalent of 2 in THF with benzyl
chloride results in a reductive coupling reaction that yields
half an equivalent of dibenzyl and complex 1 (Scheme 1;
reaction (b). This reaction can be explained by electron
transfer from 2 to benzyl chloride to form a benzyl chloride
radical anion and a PCP-PtI intermediate. The latter can also
transfer an electron to benzyl chloride to yield a cationic PtII
intermediate. Chloride loss from the radical anions would
by the electron rich anionic 2 are: 1) nucleophilic displace-
ment of fluoride ion from C6F6,[23] and 2) electron transfer to
[19a,24]
C6F6
to generate the radical anion [C6F6]CÀ and a PtI
intermediate, followed by fluoride loss and coupling of the
generated [C6F5]C radical with PtI. As 2 is a strong electron
donor and the metal center is sterically hindered by the bulky
tert-butyl phosphine groups, both of which retard nucleophilic
reactivity, we favor the electron-transfer mechanism.
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Communications
In summary, the first stable monometallic anionic Pt0
arising from three possible combinations of two platinum atoms
with different nuclear spins. T. Yoshida, T. Yamagata, T. H.
Tulip, J. A. Ibers, S. Otsuka, J. Am. Chem. Soc. 1978, 100, 2063.
[10] a) The unsaturated cationic PtII complex 3 was prepared from 1
by chloride ligand abstraction with AgBF4. For preparation and
characterization of 3 see reference [6b]; b) Reduction of 3 to 2
with sodium metal directly in THF is accompanied by formation
of unidentified by-products and thus is not applicable on a
preparative scale.
complex 2 has been prepared by reduction of a PCP-type
PtII complex. This PCP-Pt0 anion adopts a T-shaped structure
and exhibits diverse reactivity. Thus, it undergoes protonation
by water to give a PtII hydride complex and is an efficient
electron-transfer reagent, being re-oxidized quantitatively to
II
À
Pt . It is also capable of activating the strong C F bond of
hexafluorobenzene, even at À358C. Further studies on the
reactivity of this PCP-Pt0 anion and its catalytic potential are
underway.
[11] For full details of the optimization procedure and the xyz
coordinates of the optimized geometries of the complexes see
the Supporting Information.
[12] These findings are in agreement with the observed increase in
the Pt–P coupling constant (by almost 1000 Hz) in the 31P{1H}
and 195Pt{1H} NMR spectra upon transformation of 1 into 2.
[13] Complex 4a has been prepared previously from 1 by chloride
ligand nucleophilic substitution with hydride (see referen-
ce [6a]). For a modified procedure, see: B. F. M. Kimmich,
R. M. Bullock, Organometallics 2002, 21, 1504.
[14] For review of chemical redox agents for organometallic chemis-
try see: N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877.
[15] Similar oxidation of a THF solution of 2 prepared directly by
reduction of 1 in THF (in other words a THF solution containing
partially dissolved chloride anions) resulted in formation of a
mixture of several compounds, which included the cationic
complex 3 (30%) and the original complex 1 (40%) according to
NMR spectroscopy.
[16] a) Complex 5 has been prepared previously by a cyclometalation
reaction between [PtI2(cod)] (cod = cyclooctadiene) and the
ligand C6H4-2,6-(CH2PtBu2)2, see: M. A. Bennett, H. Jin, A. C.
Willis, J. Organomet. Chem. 1993, 451, 249; b) R. A. Gossage,
A. D. Ryabov, A. L. Spek, D. J. Stufkens, J. A. M. van Beek, R.
van Eldik, G. van Koten, J. Am. Chem. Soc. 1999, 121, 2488.
[17] V. Percec, B. Barboiu, H.-J. Kim, J. Am. Chem. Soc. 1998, 120,
305, and references cited therein.
[18] A mechanism involving oxidative addition of organic halide to
generate dibenzyl is also possible. See, for example: H. B.
Kraatz, M. E. van der Boom, Y. Ben-David, D. Milstein, Isr. J.
Chem. 2001, 41, 16.
Received: December 23, 2007
Published online: March 27, 2008
À
Keywords: C F activation · electron transfer · P ligands ·
platinum · reduction
.
[1] Reviews: a) M. Albrecht, G. van Koten, Angew. Chem. 2001,
113, 3866; Angew. Chem. Int. Ed. 2001, 40, 3750; b) M. E. van
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59, 1837; e) A. Vigalok, D. Milstein, Acc. Chem. Res. 2001, 34,
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[2] C. M. Frech, L. J. W. Shimon, D. Milstein, Angew. Chem. 2005,
117, 1737; Angew. Chem. Int. Ed. 2005, 44, 1709.
[3] In analogy to nonchelating systems, where reduction of [PtCl2L2]
(L = bulky tert-phosphine) with Na/Hg in THF affords PtL2
complexes while reduction of [PdCl2L2] (L = bulky tert-phos-
phine) with Na/Hg in THF results in the formation of metallic
palladium. The reduction was possible only when excess
phosphine was used. See: S. Otsuka, J. Organomet. Chem.
1980, 200, 191, and references therein.
[4] a) D. L. Reger, Y. Ding, Organometallics 1994, 13, 1047; b) L. M.
Rendina, T. W. Hambley in Comprehensive Coordination
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Meyer), Elsevier, Oxford, 2004, pp. 673 – 745.
[5] For non-isolated, anionic palladium(0) intermediates, and their
involvement in oxidative addition reactions, see: a) C. Amatore,
A. Jutand, Acc. Chem. Res. 2000, 33, 314; b) C. Amatore, A.
Jutand, F. Lemaitre, J. L. Ricard, S. Kozuch, S. Shaik, J.
Organomet. Chem. 2004, 689, 3728.
[6] a) For the first synthesis of 1, see: C. J. Moulton, B. L. Shaw, J.
Chem. Soc. Dalton Trans. 1976, 1020. b) A modified preparation
of 1 has also been reported: D. Vuzman, E. Poverenov, Y.
Diskin-Posner, G. Leitus, L. J. W. Shimon, D. Milstein, Dalton
Trans. 2007, 5692.
À
[19] For examples of catalytic C F activation see: a) M. Aizenberg,
D. Milstein, Science 1994, 265, 359; b) M. Aizenberg, D. Milstein,
J. Am. Chem. Soc. 1995, 117, 8674; d) T. Braun, R. N. Perutz,
M. I. Sladek, Chem. Commun. 2001, 2254; c) T. Schaub, M.
Backes, U. Radius, J. Am. Chem. Soc. 2006, 128, 15964.
[20] Reviews: a) J. L. Kiplinger, T. G. Richmond, C. E. Osterberg,
Chem. Rev. 1994, 94, 373; b) H. Torrens, Coord. Chem. Rev. 2005,
249, 1957.
[21] a) P. Hofmann, G. Unfried, Chem. Ber. 1992, 125, 659; b) S.
Hinterman, P. S. Pregosin, H. Ruger, H. C. Clark, J. Organomet.
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Soc. 2000, 122, 8685; d) N. A. Jasim, R. N. Perutz, A. C. Whit-
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[23] R. B. King, M. B. Bisnette, J. Organomet. Chem. 1964, 2, 38.
[24] O. Blum, F. Frolow, D. Milstein, J. Chem. Soc. Chem. Commun.
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[7] The 195Pt NMR spectroscopic data of two Pt0 phosphine
complexes (measured at 310 K) is presented here for compar-
ison: a) [Pt(PtBu3)2]: 195Pt{1H} NMR ([D8]THF): d = À6471 ppm
(t, JP, P t = 4420 Hz); b) Pt(PEt3)3: 195Pt{1H} NMR ([D8]toluene):
d = À4517 ppm (q, JP, Pt = 4216 Hz). Taken from: R. Benn, H. M.
Buch, R. D. Reinhardt, Magn. Reson. Chem. 1985, 23, 559.
[8] The meridional planar ML3 arrangement of anionic 2 can be
compared to the trigonal-planar ML3 arrangement of neutral 16-
electron Pt0 phosphine complexes. F. R. Hartley in Comprehen-
sive Organometallic Chemistry, Vol. 6 (Eds.: G. Wilkinson,
F. G. A. Stone, E. W. Abel), Pergamon, Oxford, 1982, pp. 471 –
762 and reference [4b].
À
[25] To compare with another anionic complex, C F activation of
hexafluorobenzene by Na[(C5H5)Fe(CO)2] was performed in
refluxing THF for 12 h.[23] C F activation of hexafluorobenzene
À
at room temperature by the highly reactive 14-electron fragment
[Pt0(dtbpm)]
(dtbpm = bis(di-tert-butylphosphanyl)methane)
has been reported.[21a]
[9] The 31P{1H} NMR spectrum of the Pt–Pt dimer [{Pt(tBu)2P-
(CH2)3P(tBu)2}2] in THF shows a complex resonance pattern
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