β-Diiminate Platinum Complexes for Alkane Dehydrogenation

Article abstract of DOI:10.1021/ja037781z

New five-coordinate Pt(IV) complexes [{(o-R₂-p-R′-C₆H₂)NC(R″)}₂CH]PtMe₃ (R, R′, R″ = alkyl or H) are reported. The complex with R = Me, R′ = ^tBu, R″ = Me generates unsaturated Pt(II) species capable of alkane C?H bond activation and stoichiometric dehydrogenation. Copyright

Full text of DOI:10.1021/ja037781z

Published on Web 11/21/2003
â-Diiminate Platinum Complexes for Alkane Dehydrogenation
Ulrich Fekl,† Werner Kaminsky, and Karen I. Goldberg*
Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700
Received August 6, 2003; E-mail: goldberg@chem.washington.edu
Scheme 1
Saturated hydrocarbons represent abundant and inexpensive
chemical feedstocks, and the selective synthesis of valuable
functionalized products directly from alkanes remains a prominent
research goal. Significant progress has been made in this area during
the last several years, particularly with late transition metal
homogeneous catalysts.1,2 Two very promising types of function-
alization reactions are functionalizations which introduce hetero-
atoms, for example, conversions of alkanes to alcohols, and
dehydrogenation reactions which produce olefins from alkanes.
Generally, dehydrogenation reactions have been carried out with
the metals rhodium3 and iridium,4 and heteroatom functionalizations
mostly with the metal platinum.2 This is perhaps somewhat
accidental, because the difference in reactivity could be due to the
ligands employed rather than to intrinsic properties of the metals.5
Stoichiometric6 and catalytic7 alkane dehydrogenation reactions
involving platinum in homogeneous solution are known but rare.
In this contribution, the first significant steps toward using platinum
complexes of â-diiminate ligands for alkane dehydrogenations are
presented.
Five-coordinate alkyl Pt(IV) species have been proposed as short-
lived intermediates in platinum-catalyzed alkane functionalization
cycles for many years. A â-diiminate ligand was recently used to
prepare the first example of a stable five-coordinate Pt(IV) alkyl
complex (Scheme 1, 1a).8,9 Upon thermolysis, 1a undergoes ethane
reductive elimination, followed by C-H activation of the ligand,
methane elimination, and â-hydrogen elimination (1 f 2 f 3 f
4 f 5) to form an olefin(hydrido) Pt(II) complex, where the olefin
is part of the chelating ligand (5a).10 Deuterium from deuterated
alkane solvent is incorporated into 5a selectively into the Pt-H,
isopropyl, and isopropenyl positions, at elevated temperatures. It
was proposed that this alkane activation occurs via the reversible
sequence 5 a 4 a 6 a 7 (Scheme 1).11 In this previous study
using 1a as a precursor, no alkane functionalization products, apart
from deuterium/hydrogen exchange into the alkane, were observed.
Although an olefin hydride 8a might be formed reversibly, such a
species was not detected. The intramolecular dehydrogenation
product 5 is apparently the thermodynamic sink when alkyls which
can undergo â-hydrogen elimination are present in ortho-positions
on the arenes. Herein, we report on the generality of our synthetic
approach to five-coordinate Pt(IV) alkyls of type 1 and on
investigations of the chemistry of a five-coordinate Pt(IV) complex
where the ligand cannot be dehydrogenated by â-hydrogen elimina-
tion.
2
exhibited a characteristic singlet (with JPtH ) 74 Hz) integrating
to three methyls, showing that all Pt-bonded methyls are equivalent
on the NMR time scale, as was previously observed for 1a.8 The
Pt(IV) complex 1b having a new chelate ligand (prepared by
standard methods)12 which bears methyl groups in the ortho-
positions of the arenes is of particular interest.13 If similar to the
thermolysis of 1a, a species 4 is generated upon thermolysis of
1b, dehydrogenation of the ligand will not be possible. In this case,
stoichiometric alkane activation and dehydrogenation (4 f 6 f
7 f 8) may be feasible.
The major product of thermolysis of 1b in the alkane solvent
neohexane is indeed the Pt(II) olefin hydride complex 8b (Scheme
1, R ) tBu). A hydride signal at δ -21.5 in the 1H NMR spectrum
shows platinum coupling of 1JPt-H ) 1185 Hz, suggestive of a Pt-
(II) hydride with a hard donor atom in the trans position.12,14 The
signals for the Pt-coordinated neohexene are also observed by NMR,
as well as all other expected signals.12 A single-crystal X-ray
structure determination15 of 8b confirmed the molecular structure,
shown in Figure 1 (selected parameters in the legend). In a similar
thermolysis reaction, the use of cyclohexane as the solvent leads
to the analogous cyclohexene hydride, unambiguously characterized
by NMR spectroscopy.12 These thermolysis reactions in alkane
solvent are accompanied by some decomposition, and the highest
yields of the olefin hydride products (40-50%) are obtained when
In a series of NMR experiments, the potassium salts of various
diiminate ligands were allowed to react with the Pt(IV) precursor
[Me3PtOTf]4 (OTf ) F3CSO3) in alkane solvent. In addition to 1a,
related five-coordinate complexes [{(o-R1 -p-R2C6H2)NC(R3)}2CH]-
2
t
PtMe3 (1) were prepared: 1b (R1 ) Me, R2 ) Bu, R3 ) Me), 1c
(R1 ) Me, R2 ) H, R3 ) Me), and 1d (R1 ) Et, R2 ) H, R3 )
tBu). The room temperature 1H NMR spectra for all compounds 1
† Current address: University of Toronto, UTM, Mississauga, ON, Canada L5L
1C6.
9
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10.1021/ja037781z CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
catalytic turnovers were observed with the 8b (or 8b′) system and
cyclohexane with neohexene as a potential hydrogen acceptor.
To investigate the reason for the lack of catalysis, the cyclohexene
hydride 8b′ was heated with an excess of neohexene in C6D6. No
reaction in the presence of 4 equiv of neohexene (0.05 M) was
observed over a period of 20 h at 50 °C. Under more forcing
conditions, 440 equiv of neohexene (5 M) at 90 °C for 1 h, starting
material was recovered along with unidentified decomposition
t
products, but no olefin substitution product 8b (R ) Bu) was
Figure 1. Thermal ellipsoid representation (30% probability, except
hydrogens) for 8b (R ) tBu). Selected distances and angles (Å, deg; centroid
X defined halfway between C1 and C2): Pt1-N1, 2.12(1); Pt1-N2, 1.99-
(1); Pt1-C1, 2.10(1); Pt1-C2, 2.17(2); Pt-X, 2.02(2); Pt1-H1, 1.6; C1-
C2, 1.40(2); N1-Pt1-N2, 90.3(4).
detected. The reluctance of 8b′ to undergo olefin substitution is
likely due to the sterically demanding groups on the olefin and on
the ligand. Olefin substitution at Pt(II) is normally associative in
nature and thus hindered by steric bulk.17
In conclusion, we have shown that novel five-coordinate plati-
num(IV) alkyl complexes can be made with a variety of â-diiminate
ligands and that they are useful precursors to unsaturated Pt(II)
species for alkane activation. Stoichiometric alkane dehydrogenation
was observed using either a five-coordinate Pt(IV) precursor or an
olefin hydride complex of Pt(II). Although alkane functionalization
with this system has not been made catalytic as yet, such catalysis
may be feasible if the olefin substitution rate at these Pt(II)
complexes can be substantially increased.
Scheme 2
Acknowledgment. We thank the National Science Foundation
for support of this work, Deutscher Akademischer Austauschdienst
for a fellowship award to U.F., and Prof. G. W. Coates and Scott
Allen (Cornell) for a sample of the ligand used to prepare 1d.
Supporting Information Available: Synthetic details for the
compounds discussed (PDF) and crystallographic data for 8b (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Recent general reviews on C-H bond activation and functionalization:
(a) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (b) Crabtree, R.
H. J. Chem. Soc., Dalton Trans. 2001, 2437.
(2) Recent reviews on C-H bond activation and functionalization emphasizing
platinum catalysts: (a) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003,
54, 259. (b) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic
Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes;
Kluwer: Boston, 2000. (c) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E.
Angew. Chem., Int. Ed. 1998, 37, 2180.
the reactions are performed at only slightly elevated temperature
(35 °C, 110-200 h).
(3) Wang, K.; Goldman, M. E.; Emge, T. J.; Goldman, A. S. J. Organomet.
Chem. 1996, 518, 55.
The above experiments show that intermolecular dehydrogenation
is possible using a five-coordinate Pt(IV) precursor complex with
a suitably designed â-diiminate ligand. In addition, if the Pt(II)
olefin hydride product 8b reacts in a fashion analogous to that of
the cyclometalated olefin hydride 5a, this complex should be able
to undergo reversible olefin insertion into the Pt-H bond, creating
an open site for further alkane activation. Indeed, heating the
neohexene hydride complex 8b (R ) tBu, Scheme 1) in cyclohexane
(78 °C, 1 h) leads to the corresponding cyclohexene hydride 8b′.
The most reasonable mechanism for this stoichiometric transfer
dehydrogenation reaction is depicted in Scheme 2. Following olefin
insertion to generate an open coordination site (8b f 7b),
cyclometalation (7b f 6b) and successive reductive elimination
and oxidative addition reactions lead to a three-coordinate Pt(II)
cyclohexyl complex (7b′) which undergoes â-hydrogen elimination
to form 8b′. The involvement of cyclometalation is supported by
deuterium labeling.16
(4) (a) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman, A. S. J. Am.
Chem. Soc. 1999, 121, 4086. (b) Burk, M. J.; Crabtree, R. H. J. Am. Chem.
Soc. 1987, 109, 8025.
(5) Eisenstein, O.; Crabtree, R. H. New J. Chem. 2001, 25, 665.
(6) (a) Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.;
Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467. (b) Vedernikov, A. N.;
Huffman, J. C.; Caulton, K. G. New J. Chem. 2003, 27, 665.
(7) Yamakawa, T.; Fujita, T.; Shinoda, S. Chem. Lett. 1992, 905.
(8) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123,
6423.
(9) Simultaneously, a five-coordinate dihydrido(silyl) Pt(IV) complex was
reported: Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J.
Am. Chem. Soc. 2001, 123, 6425.
(10) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804.
(11) “Three-coordinate” intermediates (2, 4, and 7) could be stabilized by
agostic C-H bond interactions with the open site. A nickel analogue of
7: Wiencko, H. L.; Kogut, E.; Warren, T. H. Inorg. Chim. Acta 2003,
345, 199.
(12) See Supporting Information.
(13) Complex 1b has an advantage over 1c in that the tert-butyl groups in the
para-positions of 1b impart an increased solubility in alkane solvents.
(14) Puddephatt, R. J. Coord. Chem. ReV. 2001, 219-221, 157.
(15) C35H54N2Pt, MW ) 697.9, clear prism, triclinic, space group ) P1h, T )
130(2) K, a ) 9.6220(8) Å, b ) 9.9680(11) Å, c ) 18.557(3) Å, R )
81.019(8)°, â ) 81.772(8)°, γ ) 69.642(6)°, Z ) 2, R1 (I > 2σ(I)) )
0.0597, wR2 (all data) ) 0.1415, GOF(F2) ) 1.01.
Also shown in Scheme 2 is that catalytic transfer dehydroge-
nation should be possible if the olefin substitution reaction to
regenerate the neohexene hydride (8b′ f 8b) was facile. This is
interesting because in this proposed cycle, unlike in virtually all
currently known transfer dehydrogenation systems, neither dihy-
drogen complexes nor dihydrides would be involved. However, no
(16) Reaction of 8b (R ) tBu) with C6D12 to form the cyclohexene-d10 deuteride
(52% yield by 1H NMR) liberates all-protio neohexane, supporting that
cyclometalation occurs as an intermediate step.
(17) Saito, K.; Kashiwabara, K. J. Organomet. Chem. 1987, 330, 291.
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