Room temperature benzene C-H activation by a new [PSiP]Ir pincer complex

Article abstract of DOI:10.1039/b811811f

The synthesis and reactivity of coordinatively unsaturated Rh and Ir complexes supported by the new bis(phosphino)silyl pincer ligand [κ³-(2-Cy₂PC₆H₄) ₂SiMe]^- ([Cy-PSiP]^-) are re

Full text of DOI:10.1039/b811811f

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Room temperature benzene C–H activation by a new [PSiP]Ir pincer
complexw
Darren F. MacLean,a Robert McDonald,b Michael J. Ferguson,b
Andrew J. Caddella and Laura Turculet*a
Received (in Berkeley, CA, USA) 10th July 2008, Accepted 6th September 2008
First published as an Advance Article on the web 23rd September 2008
DOI: 10.1039/b811811f
The synthesis and reactivity of coordinatively unsaturated Rh and Ir
complexes supported by the new bis(phosphino)silyl pincer ligand
[j3-(2-Cy2PC6H4)2SiMe]ꢀ ([Cy-PSiP]ꢀ) are reported, including
the first example of facile, room temperature intermolecular arene
C–H bond activation mediated by a silyl pincer complex.
pincer complex. Although metal–silicon chemistry is well-
precedented,6 little attention has been given to the incorpora-
tion of silyl donor fragments into the framework of a
preformed tridentate ancillary ligand. A notable exception is
the work of Stobart and co-workers,7 who have reported late
transition metal complexes featuring multidentate phosphino-
silyl ligands. As well, Tilley and co-workers8 have recently
reported late metal complexes featuring a rigid, tridentate
NSiN ligand framework, including the high temperature
(120 1C) dehydrogenative silylation of arenes catalyzed by
[NSiN]Ir(III) species; notably, no isolable Ir products of C–H
bond activation were reported in this study. While it has been
proposed that the incorporation of strongly electron donating
and trans-labilizing silyl groups into pincer ligand architec-
tures may promote the formation of coordinatively unsatu-
rated complexes that can mediate aggressive bond activation
chemistry, beyond the aforementioned report by Tilley and
co-workers,8b the utility of such complexes in C–H bond
activation processes has not been demonstrated.
The metal-mediated activation and functionalization of
hydrocarbon C–H bonds represent a fundamental goal in
modern chemistry.1 Significant progress has been made in this
area during the past twenty years, particularly with respect to
the discovery of late transition metal complexes that can cleave
hydrocarbon C–H bonds under mild conditions. In this con-
text, much attention has been given to iridium complexes, due
in part to the pioneering work of Bergman1c,2 and Graham,3
who independently showed that photochemically generated
Cp*IrL (Cp* = Z5-C5Me5, L = R3P or CO) species can
oxidatively add alkane and arene C–H bonds. Subsequent
work also showed that Ir(III) species such as
Cp*(PMe3)IrMe+Xꢀ (X = OTf, B(C6F5)4) can undergo
thermal intermolecular C–H bond activation reactions under
mild conditions.4 Innovations in ancillary ligand design have
figured prominently in the continued advancement of iridium-
mediated C–H bond activation chemistry. Notably, the use of
phosphine-based ‘PCP’ pincer ligands has enabled the devel-
opment of cyclometalated [PCP]Ir ([PCP] = k3-2,6-(tBu2-
PCH2)2C6H3) complexes that are able to mediate the
catalytic dehydrogenation of alkanes.5 In this regard, the
identification of new ancillary ligation strategies for promoting
metal-mediated C–H bond activation represents an important
challenge, as such discoveries can serve as the starting point
for the rational development of efficient catalytic hydrocarbon
functionalization chemistry.
We have recently reported on the synthesis and
catalytic utility of cyclometalated [Ph-PSiP]ꢀ ([Ph-PSiP]ꢀ
=
[k3-(2-Ph2PC6H4)2SiMe]ꢀ) platinum group metal complexes.9
In an attempt to access more electron rich metal species, we
undertook the synthesis of the dicyclohexylphosphino deriva-
tive, [Cy-PSiP]ꢀ. The parent tertiary silane, [Cy-PSiP]H (1)
was obtained in 69% isolated yield by lithiation of
n
2-Cy2PC6H4Br with BuLi, followed by in situ treatment with
0.5 equiv. of MeSiHCl2. In contrast to the formation of
[Ph-PSiP]RhHCl(PPh3) upon treatment of [Ph-PSiP]H with
one equiv. of Rh(PPh3)3Cl,9 employing 1 under similar condi-
tions resulted in quantitative (by 31P NMR spectroscopy)
formation of the coordinatively unsaturated complex
[Cy-PSiP]RhHCl (2) with liberation of three equiv. of PPh3
(Fig. 1). Alternatively, 2 was also readily prepared (85%
isolated yield) by the reaction of 1 with half an equiv. of
[Rh(COE)2Cl]2 (COE = Z2-cyclooctene). The Ir analog
[Cy-PSiP]IrHCl (3) was prepared under similar reaction condi-
tions (75% isolated yield), employing half an equiv. of
[Ir(COE)2Cl]2. Both 2 and 3 exhibit Cs-symmetry in solution
(31P NMR spectroscopy) and the X-ray crystal structures of
In this contribution, we report new coordinatively
unsaturated group 9 pincer complexes supported by the
bis(phosphino)silyl ligand [k3-(2-Cy2PC6H4)2SiMe]ꢀ ([Cy-PSiP]ꢀ),
including the first example of facile, room temperature inter-
molecular C–H bond activation mediated by
a silyl
a Department of Chemistry, Dalhousie University, Halifax, Nova
Scotia, Canada B3H 4J3. E-mail: laura.turculet@dal.ca;
Fax: 1 902 494 1310; Tel: 1 902 494 6414
10
10
b X-Ray Crystallography Laboratory, Department of Chemistry,
University of Alberta, Edmonton, Alberta, Canada T6G 2G2
both 2ꢁ(OEt2)2 and 3ꢁ(OEt2)2 (Fig. 1)z confirm the forma-
tion of structurally analogous, Cs-symmetric five-coordinate
complexes in which the pincer phosphine donors are
trans-oriented. The geometry at the metal center in both com-
plexes can be described as distorted square-based pyramidal,
with Si occupying the apical coordination site.11 Notably, both
w Electronic supplementary information (ESI) available: Experimental
details and characterization data, including crystallographic data for
2ꢁ(OEt2)2 and 3ꢁ(OEt2)2. CCDC 693921–693922. For ESI and crystal-
lographic data in CIF or other electronic format, see DOI: 10.1039/
b811811f
ꢂc
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5146 | Chem. Commun., 2008, 5146–5148

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(ca. 3 : 2 ratio; d H = ꢀ11.70, ꢀ11.82; d 31P = 57.1, 57.0),
consistent with the formation of a mixture of para- and meta-
tolyl isomers (Scheme 1).14 2H NMR analysis of 6a,b-d8
(prepared via C–D activation of toluene-d8) revealed deuter-
ium incorporation at the Ir–H and Ir–aryl positions, as well as
two aryl–Me resonances (2.03 and 2.36 ppm, ca. 3 : 2 ratio).
No evidence for benzylic C–H bond activation was observed.
In an effort to understand the reactivity of 3 with MeLi in
the absence of arene solvents, the room temperature methyl-
ation of 3 was examined in cyclohexane-d12. 1H NMR analysis
of the reaction mixture upon addition of MeLi indicated the
quantitative consumption of 3 and the formation of methane,
accompanied by the appearance of broad aromatic and
[Cy-PSiP]–cyclohexyl resonances, as well as a broad Ir–H
resonance centered near ꢀ11.3 ppm. The 31P NMR spectrum
of this mixture exhibited a complex multiplet centered near
56.2 ppm, and both the 1H and 31P NMR spectral features did
not vary significantly between ꢀ90–90 1C (methylcyclo-
hexane-d14). While the observation of a new Ir–H resonance
may point to the formation of a cyclometalated variant of
[Cy-PSiP]Ir, we have thus far not been able to identify
unambiguously the product of this reaction. However, the
addition of an excess of benzene to this reaction mixture
resulted in the quantitative formation of 4.
1
Fig. 1 Synthesis of 2 and 3. The crystallographically determined
structure of 3ꢁ(OEt2)2, shown with 50% displacement ellipsoids;
selected hydrogen atoms and the diethyl ether solvates have been
omitted for clarity. Selected interatomic distances (A) and angles (1)
for 3ꢁ(OEt2)2 (and 2ꢁ(OEt2)2): M–Cl 2.414(1) (2.4109(6)); M–P
2.2980(9) (2.3017(5)); M–Si 2.274(1) (2.2616(7)); M–H1 1.55
(1.51(3)); P–M–P 166.07(4) (165.59(2)); Cl–M–H1 160.6(18)
(162.5(12)); Cl–M–Si 130.64(5) (131.68(2)); Si–M–H1 68.7(18)
(65.8(12)).
complexes feature relatively short Si–H1 distances (M = Rh,
2.14(3) A; M = Ir, 2.24(5) A) that fall within the sum of the van
der Waals radii (3.4 A). These distances are longer than those
typically observed for s-silane complexes, but do fall within the
range indicative of significant Si–H interaction.12 No evidence
of such an Si–H interaction is observed in solution, as indicated
2
by the small JSiH (o10 Hz) measured for 2 and 3.13
Facile intermolecular arene exchange was found to occur
with 4.15 Upon standing in benzene-d6 for 14 h at room
temperature or 1 h at 70 1C, 4 was cleanly converted to 4-d6
with liberation of benzene (1H NMR analysis); 2H NMR
analysis confirmed that deuterium incorporation occurred
exclusively at the Ir–H and Ir–Ph positions (Scheme 1).
Similarly, in toluene solution, 4 was transformed cleanly into
6a,b with liberation of benzene upon heating at 70 1C for 1 h.
By comparison with the Ir system, no intermolecular C–H
bond activation was observed for [Cy-PSiP]Rh species. Thus
far, we have observed that the reaction of 2 with MeLi in
either alkane or arene solvents leads to the quantitative
formation of the same as-yet-unidentified Rh-containing pro-
The addition of MeLi (1.6 M in Et2O) to a room tempera-
ture benzene solution of 3 resulted in quantitative conversion
to [Cy-PSiP]IrH(Ph) (4) with concomitant evolution of
methane (Scheme 1), as indicated by 31P and 1H NMR
analysis of the reaction mixture, which revealed a new Ir–H
2
resonance at ꢀ11.75 ppm (t, JPH = 17 Hz) as well as
resonances consistent with an Ir–Ph ligand. Spectroscopic
analysis of the reaction mixture immediately upon addition
of MeLi revealed the formation of 4, methane, as well as an
intermediate Ir–H species 5, which we tentatively assign as
[Cy-PSiP]IrH(Me). The formation of 4 is complete over the
course of an hour at room temperature. Compound 4 arises
from the Ir-mediated C–H bond activation of benzene. Con-
sistent with this scenario, when the addition of MeLi was
performed in benzene-d6 solution, deuterium incorporation
was observed at both the Ir–H and Ir–Ph positions to yield
4-d6. To further confirm this assignment, 4 was independently
generated by the reaction of 3 with PhLi.
1
duct 7 (d 31P = 62.9, JRhP = 162 Hz), with concomitant
1
elimination of methane. Compound 7 features broadened H
and 31P NMR resonances that are not resolved between
ꢀ90–100 1C (methylcyclohexane-d14), and no indication of a
Rh–H resonance was observed (1H NMR analysis) in this
temperature range.
Analogous to the reactivity observed with benzene, the
addition of MeLi to a room temperature toluene solution of
3 resulted in the formation of arene C–H activation products
(6a,b), as indicated by the formation of two new Ir–H species
In summary, we have prepared coordinatively unsaturated
Rh and Ir complexes supported by the new bis(phosphino)silyl
pincer ligand [Cy-PSiP]ꢀ, and have demonstrated for the first
time that Ir silyl pincer complexes can mediate intermolecular
aryl C–H bond activation processes, including arene exchange,
under mild conditions. Silyl pincer ligation represents a new
platform for hydrocarbon activation studies, as well as for
other aggressive bond activation chemistry that requires an
electron-rich metal center. Further studies aimed at elucidat-
ing the mechanism of C–H bond activation by [Cy-PSiP]Ir
species, as well as expanding the scope of E–H (E = main
group element) bond activation mediated by [PSiP]-ligated
complexes, are currently underway.
We are grateful to the Natural Sciences and Engineering
Research Council (NSERC) of Canada and Dalhousie
University for their support of this work. We also thank
Scheme 1 C–H Bond activation chemistry arising from 3.
ꢂc
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Chem. Commun., 2008, 5146–5148 | 5147

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Dr M. Lumsden (Atlantic Region Magnetic Resonance
Center, Dalhousie) for assistance in the acquisition of
NMR data.
5 (a) C. M. Jensen, Chem. Commun., 1999, 2443, and references
therein; (b) I. Gottker-Schnetmann, P. White and M. Brookhart,
¨
J. Am. Chem. Soc., 2004, 126, 1804.
6 J. Y. Corey and J. Braddock-Wilking, Chem. Rev., 1999, 99, 175.
7 For selected Ir examples, see: (a) R. A. Gossage, G. D. McLennan
and S. R. Stobart, Inorg. Chem., 1996, 35, 1729; (b) R. D. Brost, G.
C. Bruce, F. L. Joslin and S. R. Stobart, Organometallics, 1997, 16,
5669.
Notes and references
8 For Rh and Ir examples, see: (a) P. Sangtrirutnugul, M. Stradiotto and
T. D. Tilley, Organometallics, 2006, 25, 1607; (b) P. Sangtrirutnugul
and T. D. Tilley, Organometallics, 2007, 26, 5557.
9 M. C. MacInnis, D. F. MacLean, R. J. Lundgren, R. McDonald
and L. Turculet, Organometallics, 2007, 26, 6522.
10 The Rh–H in 2ꢁ(OEt2)2 and the Ir–H in 3ꢁ(OEt2)2 were each
located and refined; while no restraints were imposed on the
Rh–H, the Ir–H distance was fixed at 1.55 A.
11 Due to the acute Si–M–H1 angles, the geometry at the metal center
can also be viewed as Y-shaped; for a discussion of the electronic
influences on the geometry of five-coordinate d6 complexes, see:
(a) D. L. Thorn and R. Hoffmann, New J. Chem., 1979, 3, 39; (b)
J.-F. Riehl, Y. Jean, O. Eisenstein and M. Pelissier, Organometallics,
1992, 11, 729.
z Crystallographic data in CIF format have been deposited for
2ꢁ(OEt2)2 (CCDC 693922) and 3ꢁ(OEt2)2 (CCDC 693921). Selected
crystal data for 2ꢁ(OEt2)2 (C45H76ClO2P2RhSi, 877.45 g molꢀ1):
a = 17.141(2) A, b = 15.052(2) A, c = 18.351(2) A, V = 4734.7(8)
A3, space group = Pnma (orthorhombic), Z = 4, T = 193(ꢃ2) K,
independent reflections = 5619 (Rint = 0.0272), GOF = 1.093, R1 =
0.0279 (Fo2 4 2s(Fo2)), wR2 = 0.0766 (all data); selected crystal data
for 3ꢁ(OEt2)2 (C45H76ClIrO2P2Si, 966.74 g molꢀ1): a = 17.181(1) A,
b = 15.041(1) A, c = 18.333(1) A, V = 4737.6(6) A3, space group =
Pnma (orthorhombic),
Z
=
4,
T
=
reflections = 4684 (Rint = 0.0679), GOF = 1.049, R1 = 0.0278
193(ꢃ2) K, independent
2
(Fo 4 2s(Fo2)), wR2 = 0.0663 (all data).
1 (a) A. S. Goldman and K. I. Goldberg, in Activation and
Functionalization of C–H Bonds, ed. A. S. Goldman and K. I.
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E. Bercaw, Nature, 2002, 417, 507; (c) B. A. Arndtsen, R. G.
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1995, 28, 154.
12 S. Lachaize and S. Sabo-Etienne, Eur. J. Inorg. Chem., 2006, 2115.
13 It has recently been suggested that the observed JSiH is not a
definitive measure of the extent of Si–H interaction: S. K. Ignatov,
N. H. Rees, B. R. Tyrrell, S. R. Dubberley, A. G. Razuvaev, P.
Mountford and G. I. Nikonov, Chem.–Eur. J., 2004, 10, 4991.
14 This tentative assignment is based on the precedent that C–H
activation ortho to a methyl group is less favorable than meta- and
para-activation: W. D. Jones and F. J. Feher, J. Am. Chem. Soc.,
1984, 106, 1650.
2 A. H. Janowicz and R. G. Bergman, J. Am. Chem. Soc., 1982, 104,
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3 J. K. Hoyano and W. A. G. Graham, J. Am. Chem. Soc., 1982,
104, 3723.
4 (a) P. Burger and R. G. Bergman, J. Am. Chem. Soc., 1993, 115,
10463; (b) B. A. Arndtsen and R. G. Bergman, Science, 1995, 270,
1970.
15 For intermolecular arene exchange in related [PCP]Ir and [PNP]Ir
complexes see: (a) M. Kanzelberger, B. Singh, M. Czerw, K.
Krogh-Jespersen and A. S. Goldman, J. Am. Chem. Soc., 2000,
122, 11017; (b) Y. Zhu, C.-H. Chen, S. R. Finnell, B. M. Foxman
and O. V. Ozerov, Organometallics, 2007, 26, 6701.
ꢂc
This journal is The Royal Society of Chemistry 2008
5148 | Chem. Commun., 2008, 5146–5148