Nickel and palladium silyl pincer complexes: Unusual structural rearrangements that involve reversible Si-C(sp3) and Si-C(sp 2) bond activation

Article abstract of DOI:10.1002/anie.200904570

Feeling the pinch: Remarkably facile SiC(sp³) and Si-C(sp ²) bond cleavage processes occur in [(Cy-PSiP)M(alkyl)] species (M = Ni, Pd; see picture); in the case of Ni, these Si-C bond activation processes are reversible on the NMR timescale in solution. Such examples of metalmediated cleavage of an unstrained SiC(sp³) bond are extremely rare and are unprecedented for Ni.

Full text of DOI:10.1002/anie.200904570

Communications
DOI: 10.1002/anie.200904570
Bond Cleavage
Nickel and Palladium Silyl Pincer Complexes: Unusual Structural
3
2
À
À
Rearrangements that Involve Reversible Si C(sp ) and Si C(sp ) Bond
Activation**
Samuel J. Mitton, Robert McDonald, and Laura Turculet*
Transition-metal pincer complexes have been the subject of
numerous reactivity studies that range from explorations of
catalytic activity to investigations of challenging bond-acti-
vation reactions.[1] With the goal of extending the versatility of
metal pincer chemistry, significant effort has been devoted to
the development of new pincer-like tridentate ligands by
variation of the steric and electronic properties of the donor
fragments and/or the ligand backbone.[1,2] In this context,
pincer ligands that feature a bis(o-phenylene) backbone have
emerged as particularly attractive candidates for the synthesis
of highly reactive early- and late-transition-metal com-
plexes.[3–6] Such o-phenylene linkers provide a relatively
rigid ligand architecture, and the lack of b-hydrogen atoms
in the resulting complexes circumvents potential ligand
degradation by b-hydride elimination. A variety of such
phenylene-bridged pincer ligands have been reported, includ-
ing examples where the central donor is an amido,[1a,3]
phosphido,[4] carbene,[5] or silyl group.[6,7] In this regard, we
have recently introduced silyl pincers of the type [k3-(2-
bond-activation processes are reversible on the timescale of
2
À
NMR spectroscopy in solution. Although Si C(sp ) bond
activation is well-documented,[8] examples of unstrained Si
À
C(sp3) bond cleavage within the coordination sphere of a
mononuclear metal complex are extremely rare,[9,10] and are
unprecedented for Ni.[11] By comparison, this ligand rear-
À
rangement was not observed in our previously reported [(R’
PSiP)PtII] chemistry.[6c]
À
In the pursuit of new Ni and Pd alkyl complexes, the [(Cy
PSiP)MCl] species (M = Ni, 1; M = Pd, 2) were treated with
alkyl lithium and Grignard reagents. In the case of 2,
treatment with 1 equivalent of MeLi led to the formation of
À
[(Cy PSiP)PdMe] (3), which was isolated in 78% yield
(Scheme 1).[12] The NMR spectra of isolated
3 (in
À
[6a–c]
À
R’2PC6H4)2SiMe] ((R’ PSiP), R’ = Ph, cyclohexyl (Cy));
we considered that the strongly electron-donating and trans-
labilizing central Si donor may promote the formation of
electron-rich and coordinatively unsaturated complexes that
exhibit aggressive reactivity with s bonds. Indeed, we have
À
demonstrated that [(Cy PSiP)Ir] species undergo facile arene
[6b]
À
C H bond-activation chemistry.
We have recently begun to investigate Group 10 metal
À
(R’-PSiP) complexes and have reported the synthesis of [(R’
PSiP)PtII] species, including examples of square-planar com-
plexes that feature alkyl, aryl, or silyl substitution trans to the
pincer Si donor.[6c] In expanding this chemistry to Ni and Pd,
À
we uncovered an unusual ligand rearrangement for [(Cy
À
Scheme 1. Synthesis and reactivity of [(Cy PSiP)M] complexes.
PSiPM)(alkyl)] species (M = Ni, Pd) that involves remarkably
3
2
À
À
facile Si C(sp ) and Si C(sp ) bond-cleavage processes; these
À
results are reported herein. Notably, for M = Ni, these Si C
[D6]benzene) are consistent with a CS-symmetric complex,
as indicated by the presence of a single resonance in the
1
[*] S. J. Mitton, Prof. Dr. L. Turculet
Department of Chemistry, Dalhousie University
Halifax, Nova Scotia B3H 4J3 (Canada)
Fax: (+1)902-494-1310
31P NMR spectrum at d = 60.5 ppm. The H NMR spectrum
of 3 features a singlet resonance at d = 0.72 ppm that
corresponds to the SiMe protons of the pincer ligand, as
well as a triplet resonance at d = 0.56 ppm (3JHP = 4 Hz) that
corresponds to the PdMe group. Interestingly, upon standing
at room temperature, 31P NMR spectroscopic analysis of a
benzene solution of 3 revealed the appearance of a new C1-
symmetric species 4, as indicated by two new resonances at
d = 68.3 (d, JP–P = 19 Hz) and À39.2 ppm (d, JP–P = 19 Hz).
Quantitative conversion to 4 was attained after heating (658C,
7 h), and complex 4 was isolated in 90% yield. The 1H NMR
spectrum of isolated 4 features a doublet resonance at d =
E-mail: laura.turculet@dal.ca
Dr. R. McDonald
X-Ray Crystallography Laboratory, Department of Chemistry
University of Alberta, Edmonton, Alberta T6G 2G2 (Canada)
[**] We are grateful to NSERC of Canada and Dalhousie University for
their support of this work. We also thank Dr. M. Lumsden (NMR3,
Dalhousie) for assistance in the acquisition of NMR data.
2
2
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904570.
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1.01 ppm (6H, 4JH–P = 3 Hz) that correlates to a resonance in
the 29Si NMR spectrum at d = 34.4 ppm (d, 2JSi–P = 157 Hz) in
ment of 1 with 1 equivalent of MeMgBr resulted in the
quantitative consumption of 1 and the clean formation of two
new products, 6 and 7 (1:2 ratio, characterized by 31P NMR
spectroscopy; Scheme 1). Complex 6 gives rise to a single
resonance in the 31P NMR spectrum at d = 60.0 ppm, while
complex 7 appears to have C1 symmetry in solution and as a
consequence exhibits two resonances in the 31P NMR spec-
trum at d = 68.4 (d, 1P, JP–P = 9 Hz) and À32.8 ppm (d, 1P,
1
a H–29Si HMBC experiment. On the basis of these data, we
proposed that 4 is a C1-symmetric complex that features cis-
À
phosphane ligands, as well as a Pd SiMe2R substituent in
which the silyl group is positioned trans to a phosphane unit.
We envisioned that such a species could arise from a
rearrangement of 3 that involves net transfer of the PdMe
2
À
group to Si and cleavage of a Si C(sp ) bond in the pincer
JP–P = 9 Hz). Repeated attempts to alkylate 1 with MeMgBr
ligand backbone to yield a four-membered Pd-C-C-P metal-
under modified conditions led to reaction mixtures with an
identical ratio of 6:7, and heating of these reaction mixtures
(up to 1008C) did not result in increased conversion to either
product. Attempts to separate 6 and 7 by precipitation or
crystallization were not successful. Analysis of this product
lacycle (Scheme 1). The solid-state structure of 4 (Figure 1)
1
mixture by H NMR spectroscopy indicates two SiMe reso-
nances at d = 1.02 (d, JHP = 2 Hz) and 0.69 ppm that correlate
to resonances in the 29Si NMR spectrum at d = 35.0 (d, 2JSiP
=
132 Hz) and 65.2 ppm, respectively, in a 1H–29Si HMBC
3
experiment, as well as a resonance at d = 0.43 ppm (t, JHP
=
6 Hz) that corresponds to a terminal NiMe group. By analogy
with the features of the NMR spectra recorded for 3 and 4,
À
compound 6 is assigned as the CS-symmetric complex [(Cy
PSiP)NiMe], while 7 is formulated as the Ni analogue of 4
(Scheme 1). Unlike the clean formation of 3 en route to 4, it
appears that either 6 and 7 are formed independently in the
case of Ni, or that by analogy with the Pd system, 6 is the
product that is formed first, and in turn establishes an
equilibrium with 7.
Figure 1. The crystallographically determined structures of 4, 8, and
9·OEt2. Thermal ellipsoids are set at 50% probability; selected H
atoms and the Et2O solvate have been omitted for clarity. Interatomic
distances [ꢀ] for 4: Pd–Si 2.3037(4), Pd–C21 2.087(1); 8, Pd–Si1
2.3211(4), Pd–Si2 2.4721(4). 9·OEt2: Ni–Si 2.2344(4), Ni–C2 2.123(1),
Ni–C3 2.015(1), Ni–C4 2.102(1).
In an effort to probe the interconversion of 6 and 7,
variable-temperature 1H and 31P NMR spectroscopy stud-
ies of the 6/7 mixture were performed ([D8]toluene); no
appreciable changes in the ratio of 6 to 7 were observed in the
range from À80 to 908C. However, 31P–31P EXSY NMR
spectra of the 6/7 mixture (708C; mixing times = 0.75 and
1.5 s) revealed chemical exchange between the magnetically
was confirmed by single-crystal X-ray diffraction analysis, and
is consistent with the metallacycle formulation.[12b] While
alternative pathways can be envisioned, a possible mechanism
for the formation of 4 could involve the intermediacy of a Pd0
2
À
species (5-Pd), which undergoes Si C(sp ) oxidative addition
(Scheme 2). Given that direct reductive elimination from 3 to
non-equivalent phosphorus environments in 7 (consistent
2
À
with reversible Si C(sp ) bond cleavage), as well as off-
diagonal cross-peaks that are indicative of exchange that
involves 6 and 7 (in keeping with reversible Si C(sp ) bond
3
À
cleavage). Under similar conditions, no chemical exchange
between the magnetically non-equivalent phosphorus envi-
ronments in 4 was observed. The interconversion of 6 and 7
(possibly via the intermediate 5-Ni or a related s complex,[13]
Scheme 2) was further confirmed by 1H–1H EXSY NMR
spectroscopy experiments (708C; mixing times = 0.75 and
1.5 s), which revealed chemical exchange between the SiMe
À
Scheme 2. Rearrangement of [(Cy PSiP)MMe] species.
and NiMe environments in 6 and 7.[14] Given the rarity of well-
3
À
documented Si C(sp ) bond activation processes that involve
first-row transition metals,[10b,15] the facile and reversible
nickel-mediated Si C(sp ) bond cleavage reaction required
for the interconversion of 6 and 7 is remarkable, especially in
3
À
afford 5-Pd is unlikely because of the trans-disposed Pd–Si
and Pd–Me groups, it is plausible that Si C(sp ) bond
3
À
3
À
formation is preceded by Pd–P dissociation or a tetrahedral
light of the robust nature of the Si C(sp ) linkage (bond
distortion in 3. Support for the viability of a Pd0 species such
dissociation energy of ca. 90 kcalmolÀ1). The observed trans-
2
À
as 5-Pd, which undergoes Si C(sp ) oxidative addition to
formation of 3 into 4, and the direct formation of a 6/7 mixture
À
form 4, was obtained by the quantitative generation of 4 from
the reaction of (2-Cy2PC6H4)2SiMe2 with 0.5 equivalents of
[Pd2(dba)3] (dba = dibenzylideneacetone; Scheme 1).
are in stark contrast to the reactivity of [(Cy PSiP)PtMe],
which does not undergo a similar rearrangement.[6c] In light of
the heightened propensity of Pt for s-bond activation
(relative to Pd and Ni), it is possible that the divergent
reactivity observed for these species may correlate with the
Intrigued by the unusual rearrangement of 3 to 4, we
sought to prepare a NiMe derivative analogous to 3. Treat-
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barrier to accessing 5-M (M = Ni, Pd, or Pt) from related
[D6]benzene).[17] Upon heating at 708C for 40 minutes, 66%
1
[(Cy PSiP)MMe] precursors.
conversion to 11 was observed (characterized by H, 13C, 31P,
À
The reactivity of 3, 4, and 6/7 was probed in order to gain
further understanding of the possible interconversion of the
terminal methyl complexes 3 and 6 with the rearrangement
products 4 and 7 (Scheme 1). Treatment of isolated 3 with
and 29Si NMR spectroscopy), where 11 is proposed as an
analogue of 4 in which a SiMe fragment is replaced by an allyl
group. Decomposition of 11 was observed with subsequent
heating at this temperature. The relatively facile rearrange-
ment observed for 10 relative to the h3-allyl Ni complex 9
suggests that allyl h3-coordination confers stability to the
latter complex.
À
1 equivalent of Ph2SiH2 led to the formation of [(Cy
PSiP)PdSiHPh2] (8). The crystal structure of 8 (Figure 1) is
consistent with distorted square-planar coordination geome-
try at Pd in which the SiHPh2 ligand is coordinated trans to the
pincer Si donor. No reaction was observed upon heating of
isolated 8 (1008C, 24 h; reaction monitored by 31P NMR
spectroscopy). Compound 3 also reacted with 1 equivalent of
In summary, we have uncovered an unusual ligand
À
rearrangement for [(Cy PSiP)M(alkyl)] species (M = Ni,
3
À
Pd). The rearrangement involves remarkably facile Si C(sp )
2
À
and Si C(sp ) bond-cleavage processes. In the case of Ni,
these Si C bond-activation processes are reversible on the
timescale of NMR spectroscopy in solution. Such examples of
metal-mediated cleavage of an unstrained Si C(sp ) bond are
À
À
Ph2SiHCl to form [(Cy PSiP)PdCl] (2) and Ph2SiHMe
1
(characterized by H and 31P NMR spectroscopy). Surpris-
3
À
ingly, treatment of 4 with 1 equivalent of Ph2SiH2 resulted in
50% conversion to 8 after heating in benzene solution (96 h,
1008C; characterized by 1H, 31P and 29Si NMR spectroscopy).
Similarly, exposure of 4 to 1 equivalent of Ph2SiHCl led to
quantitative formation of 2 and Ph2SiHMe after heating in
benzene (96 h, 1008C; characterized by 1H and 31P NMR
spectroscopy). Throughout the course of these reactions, no
evidence of 3 was observed (by 31P NMR spectroscopy).
While the conversion of 3 to 2 and 8 can be viewed as
extremely rare, and are unprecedented for Ni. Preliminary
investigations of the scope of this reaction have indicated that
1
À
the rearrangement is most facile for [(Cy PSiP)M(h -alkyl)]
(M = Ni, Pd) species, with no reactivity of this type observed
3
À
À
for [(Cy PSiP)Ni(h -C3H5)] (9) or [(Cy PSiP)PdSiHPh2] (8).
While the rearrangements documented herein are certainly
relevant to silyl pincer systems,[6,7] ancillary ligand reactivity
of this type may also play a role in the chemistry of alternative
classes of pincer-like tridentate ligands that feature o-
phenylene backbone fragments.
À
proceeding with retention of connectivity within the (Cy
PSiP) ligand, the conversion of 4 to 2 and 8 requires cleavage
3
À
of a Si C(sp ) linkage within the rearranged species 4 in order
to reform the (Cy PSiP) framework. From a mechanistic
À
Received: August 16, 2009
Published online: September 30, 2009
perspective, although we cannot rule out the possibility that 3
and 4 may traverse independent reaction pathways in the
synthesis of 2 and 8, it is possible that 4 transiently regenerates
the terminal methyl complex 3 under the reaction conditions
and/or that 3 and 4 access a common reactive intermediate
such as 5-Pd in these transformations. A similar rationale may
apply in the Ni system, where the 6/7 product mixture is
Keywords: nickel · palladium · phosphanes · pincer complexes ·
.
À
Si C activation
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À
cleanly transformed into [(Cy PSiP)NiCl] (1) with loss of
Ph2SiHMe upon exposure to 1 equivalent of Ph2SiHCl (258C,
12 h); the possible intermediacy of 5-Ni is supported by 31P–
31P EXSY NMR spectroscopy data (see above). Monitoring
of this transformation did not provide evidence for inter-
mediates, and the ratio of 6:7 did not vary during the reaction
(monitored by 31P NMR spectroscopy).
À
In order to survey the scope of these unusual Si C bond
activation processes, the synthesis of Ni and Pd allyl
complexes was attempted. Treatment of 1 with 1 equivalent
3
À
of (C3H5)MgBr led to the formation of [(Cy PSiP)Ni(h -
C3H5)] (9, 94%).[16] Complex 9 has C1 symmetry in solution
(278C, [D6]benzene), as evidenced by an AB pattern centered
at 49.2 ppm in the 31P{1H} NMR spectrum; the X-ray structure
of 9 is consistent with the room-temperature NMR spectros-
copy data for this complex (Figure 1). No reaction was
observed upon heating of isolated 9 (1008C, 96 h; charac-
terized by 31P NMR spectroscopy). By comparison, treatment
of 2 with 1 equivalent of (C3H5)MgBr led to the formation of
1
À
the CS-symmetric complex [(Cy PSiP)Pd(h -C3H5)] (10,
88%), which features a resonance in the 31P NMR spectrum
at 59.1 ppm. The h1-coordination of the allyl ligand is invoked
on the basis of the allyl resonances in the 13C NMR spectrum
observed at d = 135.2, 87.3, and 25.3 ppm (278C,
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À
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[11] To the best of our knowledge, reports of well-documented Si C
À
bond cleavage by Ni are limited to an example involving Si
C(sp) bond activation: B. L. Edelbach, R. J. Lachicotte, W. D.
Jones, Organometallics 1999, 18, 4660.
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[12] a) Experimental procedures and characterization data for all
new compounds can be found in the Supporting Information;
b) CCDC 741388 (1), 741390 (4), 741387 (8), and 741389
(9·OEt2) 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.
2
À
[8] For selected examples of Si C(sp ) bond cleavage by mono-
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3
3
À
À
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À
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´
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3
À
[10] For selected examples of Si C(sp ) bond cleavage in strained
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