Reductive elimination from cyclometalated platinum(IV) complexes to form Csp2-Csp3 bonds and subsequent competition between Csp2-H and C sp3-H bond activation

Article abstract of DOI:10.1021/om300303a

Reductive elimination reactions of the cyclometalated platinum(IV) compounds [PtMe₂Br{C₆H₄CH=NCH ₂(4-ClC₆H₄)}L] (L = SMe₂, PPh ₃) to form C_sp³_-Csp² bonds, followed by either exclusive C_sp²-H bond activation (L = SMe₂) or competition between C_sp² _-H and C_sp³-H bond activation (L = PPh ₃) are reported. Reductive elimination to form a C-Br bond is also reported.

Full text of DOI:10.1021/om300303a

Communication
pubs.acs.org/Organometallics
Reductive Elimination from Cyclometalated Platinum(IV) Complexes
2
3
To Form Csp −Csp Bonds and Subsequent Competition between
2
3
Csp −H and Csp −H Bond Activation
Margarita Crespo,*,† Craig M. Anderson,*,‡ Nicole Kfoury,‡ Merce Font-Bardia,§,∥ and Teresa Calvet§
̀
†
́
́
Departament de Quımica Inorganica, Facultat de Quımica, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain
̀
‡Department of Chemistry, Bard College, 30 Campus Road, Annandale-on-Hudson, New York 12504, United States
§Departament de Crystal·lografia, Mineralogia i Dipos
Spain
̀
its Minerals, Universitat de Barcelona, Martí i Franques s/n, 08028-Barcelona,
̀
∥Unitat de Difraccio
́ ̀
de Raigs X, Centre Científic i Tecnologic de la Universitat de Barcelona (CCiTUB), Universitat de Barcelona,
́
Sole i Sabarís 1-3, 08028-Barcelona, Spain
S
* Supporting Information
ABSTRACT: Reductive elimination reactions of the cyclo-
metalated platinum(IV) compounds [PtMe2Br{C6H4CH
3
2
NCH2(4-ClC6H4)}L] (L = SMe2, PPh3) to form Csp −Csp
2
bonds, followed by either exclusive Csp −H bond activation (L
2
3
= SMe2) or competition between Csp −H and Csp −H bond
activation (L = PPh3) are reported. Reductive elimination to
form a C−Br bond is also reported.
he reductive elimination reaction is often the product-
[PtMe2X(C6H4CHNCH2R)SMe2] (II),13 respectively
T
forming step in both catalytic cycles and stoichiometric
transformations leading to formation of C−C, C−H, or C−X
bonds. Much attention has been paid recently to reductive
elimination from d6 octahedral complexes, in particular those
containing Pt(IV). These compounds readily undergo reductive
elimination, often with initial loss of a ligand to generate a five-
coordinate intermediate. Most reported studies are concerned
(such as those shown in Chart 1).
Chart 1
3
3
with reductive Csp −Csp elimination leading to ethane either
from trimethylplatinum(IV)1 or tetramethylplatinum(IV)2
compounds. Competition between C−C bond formation and
C−O,3 C−N,4 and C−I5 bond formation has also been studied
for trimethylplatinum(IV) compounds containing O-donor, N-
donor, or halo ligands. Reductive elimination of methane from
platinum(IV) compounds containing both methyl and hydrido
ligands6 has also been addressed. Analogous reductive
elimination studies have been carried out for arylplatinum(IV)
These compounds display a fac-PtC3 arrangement and are
stable at room temperature both in solution and in the solid
state. Although these compounds provide an opportunity to
analyze the competition between different reductive elimination
processes, such reactions have not yet been studied. In this
communication we present preliminary results for the
thermolysis reactions of platinum(IV) compounds containing
the cyclometalated ligand 2-BrC6H4CHNCH2(4-C6H4Cl).
7
2
compounds, including Csp −H bond formation and competi-
8
2
2
2
tion between Csp −Csp and Csp −halide bond formation.
2
2
Moreover, Csp −Csp coupling from cyclometalated platinum-
(IV) compounds leading to five-, six-, or seven-membered
platinacycles containing a biaryl linkage has also been reported.9
3
As shown in Scheme 1, these compounds could lead to Csp −
3
3
3
2
2
Examples of Csp −Csp reductive elimination are scarce.10
Csp (path a), Csp −Br (path b), Csp −Csp (path c), or Csp -Br
(path d) reductive elimination processes. In the two cases
involving cleavage of the metallacycle (paths c and d),
subsequent cyclometalation at the available ortho position of
the aryl ring (with reductive elimination of methane) can be
anticipated.12,13
2
3
2
However, a catalytic process for conversion of a Csp −F bond
2
3
into a Csp −Csp bond involving reductive elimination from
platinum(IV) compounds has been reported.11
Intramolecular oxidative addition of C−X bonds (X = Br, Cl,
F) of appropriately designed nitrogen ligands to [Pt2Me4(μ-
SMe2)2] gives terdentate [C,N,N′] or bidentate [C,N]
cyclometalated platinum(IV) compounds with the general
formulas [PtMe2X(C6H4CHNCH2CH2NMe2)] (I)12 and
Received: April 16, 2012
Published: June 14, 2012
© 2012 American Chemical Society
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dx.doi.org/10.1021/om300303a | Organometallics 2012, 31, 4401−4404

Organometallics
Communication
a
Scheme 1. Possible Reaction Paths from Cyclometalated Platinum(IV) Compound 2
a
3
Isolated compounds are shown within a frame. Legend: (a−d) reductive elimination reactions from cyclometalated platinum(IV) complexes, Csp −
3
3
3
2
2
Csp (a), Csp −Br (b), Csp −Csp (c), Csp −Br (d); (e) cyclometalation step with methane elimination; (f) substitution reaction of SMe2 for PPh3.
The reactivity of the [C,N,N′] platinum(IV) compound
[PtMe2Br(C6H4CHNCH2CH2NMe2)] (1)12a was initially
tested. The compound was recovered unaltered after refluxing
in a toluene solution for 8 h. The lack of reactivity, under these
conditions, was taken as an indication that ligand dissociation
to yield a five-coordinate species, prior to the reductive
elimination process, is required. Therefore, our next target was
the compound [PtMe2Br{C6H4CHNCH2(4-ClC6H4)}-
SMe2] (2) containing the labile SMe2 ligand, which was
prepared as a single isomer with a fac-PtC3 arrangement, as
previously reported for analogous compounds.13
compounds.12,13 This reaction sequence is analogous to that
reported for biaryl formation (sp2−sp2 coupling) from
cyclometalated platinum(IV) compounds in which a final
cyclometalation step occurs with release of an arene molecule.9
In order to explore this reaction type as a function of the
stereoelectronic features of the ancillary ligand, the thermal
behavior of [PtMe2Br{C6H4CHNCH2(4-ClC6H4)}PPh3]
(5) in refluxing toluene solution was also studied. As shown
in Scheme 2, compound 5 was obtained from compound 2 in a
substitution reaction of SMe2 for PPh3. Recrystallization of the
resulting crude product gave, in addition to compound 5,
orange crystals of compound 6, in a molar ratio of 3:1 for 5 and
6. The NMR spectra of compound 5 indicate a fac-PtC3
arrangement with the PPh3 trans to a methyl ligand, as
reported for analogous compounds.12,13
When a toluene solution of compound 2 was refluxed for 4 h,
the cyclometalated platinum(II) compound [PtBr{2-
MeC6H3CHNCH2(4-ClC6H4)}SMe2] (3) shown in Scheme
1 was obtained as a single product. A one-pot procedure from a
mixture of [Pt2Me4(μ-SMe2)2] and the ligand 2-BrC6H4CH
NCH2(4-C6H4Cl) gave the same result. In the 1H NMR
spectrum, a single methyl resonance was observed at 2.42 ppm
and showed NOESY interactions with both the imine and
aromatic protons. A displacement reaction of dimethyl sulfide
for triphenylphosphine produced the compound [PtBr{2-
MeC6H3CHNCH2(4-ClC6H4)}PPh3] (4) (see Scheme 1),
which was also characterized using one- and two-dimensional
NMR spectroscopic techniques.
[PtMe{2-BrC6H3CHNCH2(4-ClC6H4)}PPh3] (6),
formed as a byproduct in the synthesis of compound 5, was
fully characterized, including single-crystal X-ray diffraction
analysis (Figure 1). The formation of this compound suggests
2
that Csp −Br reductive elimination and subsequent cyclo-
metalation take place along with the substitution process.14
When a toluene solution of pure compound 5 was refluxed
for 4 h, a mixture of compounds 4 and 7 in the ratio 1:2.5 was
3
2
obtained. Compounds 4 and 7 both arise from Csp −Csp
3
These results indicate that, under the conditions tried, Csp −
reductive elimination from the platinum(IV) compound 5,
2
Csp reductive elimination is the most favored of the four
which is followed by subsequent cyclometalation and loss of
2
possible processes (paths a−d indicated in Scheme 1). We
speculate that the methyl reductively eliminates with the aryl
ring but the ligand is still tethered to the metal by the nitrogen
atom. Therefore, the aryl ring would be in close proximity to
the metal and allow for a fast cyclometalation to occur, with
subsequent loss of methane, as described for analogous
methane. Competition between activation of either a Csp −H
bond, leading to a five-membered platinacycle (compound 4),
3
or activation of a Csp −H bond in the methyl group previously
reductively eliminated, leading to a novel six-membered
platinacycle (compound 7), takes place in the cyclometalation
process. White crystals of compound 7 were obtained from the
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Organometallics
Communication
Scheme 2. Synthesis (Step a) and Thermolysis (Step b) of
a
the Cyclometalated Platinum(IV) Compound 5
a
Yields of compounds 5 and 6 (see text) are given in parentheses.
Figure 2. Molecular structure of compound 7. Selected bond lengths
(Å) and angles (deg) with estimated standard deviations: Pt(1)−C(1),
2.059(3); Pt(1)−N(1), 2.114(3); Pt(1)−P(1), 2.2191(11); Pt(1)−
Br(1), 2.5328(7); N(1)−C(8), 1.278(4); N(1)−C(9), 1.477(4);
C(1)−C(2), 1.509(4); C(2)−C(7), 1.411(4); C(7)−C(8), 1.459(4);
C(1)−Pt(1)−N(1), 84.38(12); C(1)−Pt(1)−P(1), 88.65(9); N(1)−
Pt(1)−P(1), 172.95(8); C(1)−Pt(1)−Br(1), 174.24(9); N(1)−
Pt(1)−Br(1), 90.41(8); P(1)−Pt(1)−Br(1), 96.60(3).
bond activation have been reported.16 The results presented in
this communication have some noteworthy features. First, the
activated methyl was the methyl group initially bound to
platinum that was transferred to the imine ligand through a
3
reductive elimination process. Second, Csp −H bond activation
was only observed when PPh3 was coordinated as the ancillary
ligand to the platinum(IV) center, thus indicating that
stereoelectronic factors are decisive in the competition between
aromatic and aliphatic C−H bond activation. A recent example
of a related process with formation of a six-membered
platinacycle also involves a platinum(IV) precursor containing
a PPh3 ligand.17 Moreover, it has been reported that the
preference for aromatic versus aliphatic C−H bond activation
in the reactions of methylbenzenes with platinum(II)
complexes can be overridden under different conditions.18
In summary, our results show that reductive elimination from
cyclometalated platinum(IV) compounds 2 and 5 takes place
Figure 1. Molecular structure of compound 6. Selected bond lengths
(Å) and angles (deg) with estimated standard deviations: Pt−C(1),
2.040(8); Pt−C(15), 2.074(8); Pt−N(1), 2.141(7); Pt−P(1),
2.313(2); N(1)−C(7), 1.286(11); N(1)−C(8), 1.486(11); C(1)−
C(6), 1.378(12); C(6)−C(7), 1.427(12); C(1)−Pt−C(15), 92.5(3);
C(1)−Pt−N(1), 78.4(3); C(15)−Pt−N(1), 169.8(3); C(1)−Pt−
P(1), 173.3(2); C(15)−Pt−P(1), 91.5(2); N(1)−Pt−P(1), 98.02(19).
3
2
selectively to produce exclusively Csp −Csp coupling, which is
followed by cyclometalation and subsequent loss of methane.
2
The latter metalation process produced Csp −H bond activation
initial crude product. The compound was fully characterized,
including NMR spectroscopy and an X-ray diffraction study
(Figure 2).
for the compound containing a smaller SMe2 ligand (2) and
3
2
competition between Csp −H and Csp −H bond activation for
the bulkier triphenylphosphine analogue (5). In addition, a C−
Br reductive elimination process followed by cyclometalation
has also been observed. Further work is in progress with the
aim of exploring the scope and the mechanism of both the
Formation of a six-membered platinacycle as a result of the
3
activation of a Csp −H bond is remarkable, since (i) a strong
tendency to form five-membered rings over six-membered rings
and (ii) a preference for the activation of sp2 over sp3 C−H
bonds are generally observed in cyclometalation reactions.15
Few examples of platinacycles formed through aliphatic C−H
3
2
2
Csp −Csp and Csp −Br reductive elimination processes as well
as the factors governing the competition between aliphatic and
aromatic C−H bond activation.
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Organometallics
Communication
́ ́
T.; Crespo, M.; Font-Bardia, M.; Gomez, K.; Gonzalez, G.; Martinez,
ASSOCIATED CONTENT
* Supporting Information
Full experimental details and characterization data for
compounds 2−7, crystal data for compounds 6 and 7, and
CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
M. Organometallics 2009, 28, 5096. (e) Crespo, M.; Calvet, T.; Font-
Bardia, M. Dalton Trans. 2010, 39, 6936. (f) Crespo, M.; Font-Bardia,
M.; Calvet, T. Dalton Trans. 2011, 40, 9431.
S
(10) Madison, B. L.; Thyme, S. B.; Keene, S.; Williams, B. S. J. Am.
Chem. Soc. 2007, 129, 9538.
(11) (a) Wang, T.; Alfonso, B. J.; Love, J. A. Org. Lett. 2007, 9, 5629.
(b) Wang, T.; Love, J. Organometallics 2008, 27, 3290. (c) Buckley, H.
L.; Sun, A. D.; Love, J. A. Organometallics 2009, 28, 6622. (d) Wang,
T.; Keyes, L.; Patrick, B. O.; Love, J. Organometallics 2012, 31, 1397.
(12) (a) Anderson, C. M.; Crespo, M.; Jennings, M. C.; Lough, A. J.;
Ferguson, G.; Puddephatt, R. J. Organometallics 1991, 10, 2672.
(b) Anderson, C. M.; Crespo, M.; Ferguson, G.; Lough, A. J.;
Puddephatt, R. J. Organometallics 1992, 11, 1177.
AUTHOR INFORMATION
Corresponding Author
*M.C.: e-mail, margarita.crespo@qi.ub.es; tel, 34934039132;
fax, 34934907725. C.M.A.: e-mail, canderso@bard.edu; tel,
845-752-2356; fax, 845-752-2339.
■
(13) (a) Crespo, M.; Martinez, M.; Sales, J.; Solans, X.; Font-Bardia,
M. Organometallics 1992, 11, 1288. (b) Crespo, M.; Martinez, M.;
Sales, J. Organometallics 1993, 12, 4297.
Notes
The authors declare no competing financial interest.
(14) Activation of a C−H bond upon reaction of [Pt2Me4(μ-SMe2)2]
with the imine 2-BrC6H4CHNCH2(4-C6H4Cl), followed by
reaction with PPh3, could also lead to formation of compound 6.
However, activation of a C−H bond was not observed in the synthesis
ACKNOWLEDGMENTS
■
This material is based upon work supported by the Ministerio
́
de Ciencia y Tecnologia under project CTQ2009-11501 and by
1
of compound 2, as confirmed by H NMR spectra. Moreover, such a
the National Science Foundation under CHE-1058936
(C.M.A., PI). We also thank Bard College (BSRI) for support.
process has not been observed previously for related ligands
containing an o-bromophenyl group.12,13
(15) Albrecht, M. Chem. Rev. 2010, 110, 576.
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