Facile dehydrogenation of ethers and alkanes with a β-diiminate Pt fragment

Article abstract of DOI:10.1021/ja074488g

Reaction between a neutral β-enamineimine ligand (nacnacH) and Me₄Pt₂(μ-SMe₂)₂ in diethyl ether results in dehydrogenation of the solvent to form the Pt(II) olefin hydride complex (nacnac)Pt(ethyl vinyl ether)(H). Activation of ethers is kinetically favored over activation of alkanes. The KIE for ether activation (2.2) is significantly lower than that for pentane activation (3.8), indicating that initial coordination through oxygen is the likely pathway for ether activation. Marginal catalytic transfer dehydrogenation of diethyl ether and THF was achieved by heating (nacnac)Pt(neohexene)(H) at 50 °C for several days with excess neohexene acting asa hydrogen acceptor. Copyright

Full text of DOI:10.1021/ja074488g

Published on Web 09/21/2007
Facile Dehydrogenation of Ethers and Alkanes with a â-Diiminate Pt Fragment
Nathan M. West, Peter S. White, and Joseph L. Templeton*
Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
Received June 19, 2007; E-mail: joetemp@unc.edu
Many transition metal complexes activate C-H bonds of alkanes,
but few lead to functionalized products.1 Recent successes use d8
transition metals for catalytic dehydrogenation of alkanes,2 including
their use in alkane metathesis.2f Progress in dehydrogenation of
functionalized hydrocarbons has lagged behind alkane dehydroge-
nation,3 and reports of C-H activation of ethers by d8 metals are
rare in comparison to alkane activation reactions.4
We report herein Pt complexes that dehydrogenate diethyl ether,
THF, and alkanes, with dehydrogenation of ethers kinetically
favored relative to dehydrogenation of alkanes.
Recently, â-diiminate (nacnac) ligands with bulky ortho-sub-
stituents on the N-phenyl groups have become popular due to both
their ease of synthesis and their ability to shield the metal center.
Goldberg has used this type of ligand to stabilize five-coordinate
Figure 1. X-ray structure of (nacnac)Pt(Me)(ethylene) 1a. Thermal
ellipsoids at 30%.
Scheme 1. Dehydrogenation of Et2O
Pt(IV) trimethyl species.5a Thermolysis of these complexes results
in reductive elimination of ethane and dehydrogenation of either
the ortho-substituent or the alkane solvent.5b,c,6
Hypothesizing that a (nacnac)Pt(olefin)(H) species with simple
N-phenyl rings would facilitate associative olefin displacement, we
attempted to synthesize a monomethyl Pt(II) species capable of
C-H activation that would not require the use of bulky substituents
on the ligand for stability. Note that thermolysis at 85 °C of p-Cl
dehydrogenation of diethyl ether with platinum, [(TMEDA)Pt(ethyl
vinyl ether)(H)]+, isomerizes over time to the Fischer-type carbene.4e,f
Conducting the reaction in THF led to the formation of the 2,3-
or p-OMe (nacnac)Pt(Me)(SMe2) in benzene has been shown to
form (nacnac)Pt(Ph)(SMe2).7a
dihydrofuran adduct with the double bond adjacent to the oxygen
in (nacnac)Pt(H)(2,3-dihydrofuran) (2b). Metal hydride complexes
of 2,3-dihydrofuran have been elusive, but here dehydrogenation
of THF leads to an olefin adduct rather than to the isomeric
heteroatom carbene.
Reaction of nacnacH and Me4Pt2(µ-SMe2)2 in n-pentane solvent
yielded (nacnac)Pt(H)(1-pentene), 2c. Only the 1-pentene isomer
adduct was observed.11 Acyclic alkanes yield only terminal olefins,
indicating a preference for terminal C-H bonds in alkanes. This
is in contrast to ethers which favor activation of the C-H bond
adjacent to oxygen. The differing pathways are likely due to
coordination through oxygen preceding C-H activation.
Competition experiments in which mixtures of two solvents were
used displayed a kinetic preference for activation of ethers over
alkanes. Diethyl ether is favored 3.9:1 over pentane, Et2O is favored
1.2:1 over THF, and n-pentane is favored 3.2:1 over cyclopentane.
The kinetic preference for ethers is compatible with a mechanism
involving associative displacement of methane from (nacnac)Pt-
(CH3)(CH4). Presumably ether binds more quickly than pentane to
form the unobserved intermediate (nacnac)Pt(Me)(OEt2) which then
undergoes intramolecular C-H activation at the R position. A
similar intramolecular C-H activation of ether was observed by
Bercaw in (TMEDA)Pt(Me)(OEt2).4f Our results also show that
straight chain alkanes are favored over cyclic alkanes. However,
no similar effect favoring primary C-H activation is seen in ethers,
no doubt reflecting their ability to initially bind through oxygen.
Kinetic isotope effects were determined using 1:1 ratios of protio
and deutero substrates. Diethyl ether and THF both yielded kH/kD
values of 2.2(1), while for pentane, the kH/kD ratio was 3.8(4). The
The targeted monomethyl Pt(II) species was formed by stirring
Li(nacnac) with (Me)(Cl)Pt(SMe2)2 followed by addition of L (CO
or ethylene), yielding (nacnac)Pt(Me)(L) 1 (L ) C2H4 1a, CO 1b).7
Both 1a and 1b were crystallographically characterized (1a is shown
in Figure 1).8 Although the bound ethylene in 1a is static at room
temperature, olefin rotation can be observed at higher temperature
1
by H NMR and reflects a barrier of 16.7 kcal/mol.
The crystal structures indicate that the metal is indeed more
exposed than that in related structures with ortho-substituents on
the nacnac phenyl rings. Although we had the (nacnac)PtII(Me)
framework in place, our efforts to isolate sp3 C-H activation
products by displacing L either photolytically or thermally failed.
In spite of the high CO stretching frequency of 1b (2065 cm-1
,
CH2Cl2), complexes 1a,b were inert under the conditions we
explored.
We turned to a one-pot method for generating a (nacnac)PtII
species capable of C-H activation. The neutral â-enamineimine
ligand (nacnacH) has an acidic proton available to react with Me4-
Pt2(µ-SMe2)2 to form CH4 and generate a reactive Pt(II) methyl
fragment. Stirring these reagents in diethyl ether did not yield
(nacnac)Pt(L)(Me) (L ) SMe2, OEt2),9,10 but rather the downstream
dehydrogenation product (nacnac)Pt(H)(ethyl vinyl ether), 2a, was
isolated (Scheme 1). Reaction with CH3CD2OCH2CH3 yields a
mixture of CH4 and CH3D as well as both (nacnac)Pt(H)(C4H6D2O)
(2a) and (nacnac)Pt(H)(C4H7DO) (2a-d1), indicating that the R-C
is the site of initial C-H activation. Accordingly, no Pt-D product
was observed in this experiment. The other reported product of
9
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J. AM. CHEM. SOC. 2007, 129, 12372-12373
10.1021/ja074488g CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Proposed Mechanism for Et2O Dehydrogenation
In conclusion, simple ethers and alkanes undergo facile activation
by reaction with nacnacH and Me4Pt2(µ-SMe2)2. Activation of ethers
is kinetically favored due to the associative displacement of
methane, but activation of the R C-H bond is rate-limiting as
demonstrated by the kinetic isotope effect. Selectivity for ether
activation facilitates its dehydrogenation using an alkyl hydrogen
acceptor. The lack of bulky ortho-substituents on the nacnac
nitrogens exposes the metal center to associative olefin exchange
and allows transfer dehydrogenation to occur. Resistance to hydride
migration and thermal instability limits the utility of this catalyst
for transfer dehydrogenation.16
Acknowledgment. This research was supported by funding from
the National Science Foundation (CHE-0717086).
Supporting Information Available: Experimental details for the
synthesis of complexes 1a,b and 2a-d, solvent competition reactions,
kinetic isotope effect studies, transfer dehydrogenation reactions, and
crystallographic data for 1a,b are available. This material is available
free of charge via the Internet at http://pubs.acs.org.
significantly lower value for the ethers is consistent with initial
coordination through oxygen followed by rate-limiting C-H
activation at the R position (Scheme 2). The use of CH3CD2OCH2-
CH3 also yielded an intramolecular KIE of 2.2(1). Rate-limiting
C-H activation may explain the preference for terminal activation
of alkanes. Several d8 systems have been found to favor coordina-
tion of secondary C-H bonds,12 though actiVation of primary C-H
bonds is favored.13
Transfer dehydrogenations of Et2O, THF, and n-pentane using
t-butyl ethylene (TBE) as a hydrogen acceptor were attempted using
a catalytic amount of (nacnac)Pt(H)(TBE) (2d).14 Formation of free
olefin product (ethyl vinyl ether, 2,3-dihydrofuran, and pentenes,
respectively) and t-butyl ethane (TBA) was observed with (nacnac)-
Pt(H)(TBE) as the only Pt species visible by NMR.15 A 7:1 ratio
of TBE to 2d in diethyl ether yielded 1.3 turnovers (19% yield)
after 120 h at 50 °C. Similar conditions in THF yielded 1.1
turnovers. The low yields are due to catalyst decomposition over
the course of the reaction. This is a rare example of catalytic
dehydrogenation with Pt and the first reported transfer dehydro-
genation of Et2O.
References
(1) (a) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437. (b) Dick,
A. R.; Sanford, M. S. Tetrahedron 2006, 62, 2439.
(2) (a) Jensen, C. M. Chem. Commun. 1999, 2443. (b) Zhu, K.; Achord, P.
D.; Zhang, X.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc.
2004, 126, 13044. (c) Gottker-Schnetmann, I.; Brookhart, M. J. Am. Chem.
Soc. 2004, 126, 9330. (d) Kuklin, S. A.; Sheloumov, A. M.; Dolgushin,
F. M.; Ezernitskaya, M. G.; Peregudov, A. S.; Petrovskii, P. V.; Koridze,
A. A. Organometallics 2006, 25, 5466. (e) Yamakawa, T.; Fujita, T.;
Shinoda, S. Chem. Lett. 1992, 905. (f) Goldman, A. S.; Roy, A. H.; Huang,
Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science 2006, 312, 257.
(3) Dehydrogenation of THF: (a) Gupta, M.; Kaska, W. C.; Jensen, C. M.
Chem. Commun. 1997, 461. Dehydrogenation of tertiary amines: (b)
Zhang, X.; Fried, A.; Knapp, S.; Goldman, A. S. Chem. Commun. 2003,
2060. (c) Lu, C. C.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 15818.
(4) (a) Luecke, H. F.; Arndtsen, B. A.; Burger, P.; Bergman, R. G. J. Am.
Chem. Soc. 1996, 118, 2517. (b) Carmona, E.; Paneque, M.; Poveda, M.
L. Dalton Trans. 2003, 4022. (c) Owen, J. S.; Labinger, J. A.; Bercaw, J.
E. J. Am. Chem. Soc. 2006, 128, 2005. (d) Alvarez, E.; Paneque, M.;
Petronilho, A. G.; Poveda, M. L.; Santos, L. L.; Carmona, E.; Mereiter,
K. Organometallics 2007, 26, 1231. (e) Holtcamp, M. W.; Labinger, J.
A.; Bercaw, J. E. J. Am. Chem. Soc. 1997, 119, 848. (f) Holtcamp, M.
W.; Henling, L. M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg.
Chim. Acta 1998, 270, 467.
(5) (a) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001,
123, 6423. (b) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124,
6804. (c) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc.
2003, 125, 15286.
(6) Similar results observed with (nacnac)Ir: Bernskoetter, W. H.; Lobkovsky,
E.; Chirik, P. J. Organometallics 2005, 24, 6250.
(7) An analogous procedure was used to synthesize (nacnac)Pt(Me)(SMe2)
where the nacnac ligands contain p-Cl or OMe substituents: (a) Iverson,
C. N.; Carter, C. A. G.; Scollard, J. D.; Pribisko, M. A.; John, K. D.;
Scott, B. L.; Baker, R. T.; Bercaw, J. E.; Labinger, J. A. ACS Symp. Ser.
2004, 885, 319.
(8) The structure of 1b is given in the Supporting Information.
(9) Products of this nature are observed from the reaction of 2-(N-arylimino)-
pyrrole and Me4Pt2(µ-SMe2)2 in: Iverson, C. N.; Carter, C. A. G.; Baker,
R. T.; Scollard, J. D.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2003, 125, 12674.
Hydride migration appears to be the rate-limiting step since olefin
exchange is facile. Equilibrium binding ratios were measured for
the olefins from worst to first: ethyl vinyl ether, neohexene,
cyclopentene, 2,3-dihydrofuran, n-pentene, ethylene: 1:1.8:2.1:4.35:
7.0:58. Addition of olefin to the platinum hydride species results
in a rapid shift to the equilibrium concentration of bound olefin.
The ethylene complex (nacnac)Pt(H)(C2H4) (2e) displays rapid
olefin rotation at room temperature, with a barrier to rotation of
(10) A little (nacnac)Pt(Me)(SMe2) is formed as a byproduct.
(11) Pentene isomers were observed after 48 h at 50 °C.
1
only 9.2 kcal/mol determined by low-temperature H NMR. This
is 7.5 kcal/mol lower than the corresponding ethylene rotation
barrier in the methyl complex and indicates a much less sterically
hindered metal center which helps to explain why ligand exchange
is rapid. The facile olefin exchange is in stark contrast to the ortho-
substituted nacnac complexes reported by Goldberg; they underwent
rapid hydride migration but failed to undergo efficient olefin
exchange.5c
(12) (a) Chen, G. S.; Labinger, J. A.; Bercaw, J. E. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6915. (b) Periana, R. A.; Bergman, R. G. J. Am. Chem.
Soc. 1986, 108, 7332.
(13) Vetter, A. J.; Flaschenriem, C.; Jones, W. D. J. Am. Chem. Soc. 2005,
127, 12315.
(14) Compound 2d was synthesized by adding excess TBE to 2c.
(15) Olefins and TBA observed by 1H NMR and GC.
(16) Decomposition occurs rapidly above 60 °C.
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