840
J. Am. Chem. Soc. 1997, 119, 840-841
Catalytic Dehydrogenation of Cycloalkanes to
Arenes by a Dihydrido Iridium P-C-P Pincer
Complex
Mukta Gupta,† Chrystel Hagen,† William C. Kaska,*,‡
Roger E. Cramer,† and Craig M. Jensen*,†
Department of Chemistry, UniVersity of Hawaii
Honolulu, Hawaii 96822
Department of Chemistry, UniVersity of California
Santa Barbara, California 93106
ReceiVed July 25, 1996
Figure 1. Projection of IrH2{C6H3-2,6-(CH2PBut2)2} (1) with the
thermal ellipsoids at 50% probability. Selected bond distances (Å) and
angles (deg): Ir-C(1), 2.12(1); Ir-P, 2.308(2); P-Ir-C(1), 82.41(6);
P-Ir-P(a), 164.8(1). The hydrogen atoms are omitted for clarity.
The dehydrogenation of alkanes through catalytic reforming
is the leading commercial method of producing arenes.1
Platinum (often with a rhenium promoter) on alumina catalysts
employed in this reaction require temperatures of 450-550 °C.1
During the past decade, there has been steady progress in the
development of soluble transition metal complexes as catalysts
for the dehydrogenation of alkanes to alkenes at moderate
conditions.2-4 However, there have been few extensions of
these systems to the production of arenes. Pioneering studies
by Shilov showed that K2PtCl4 catalyzes the reaction of H2-
PtCl6 with cycloalkanes to produce a mixture of arenes, alcohols,
and chlorinated species at 100-120 °C in aqueous trifluroacetic
acid.2 Pd(O2CCF3)2 has been reported to catalyze the dehy-
drogenation of cyclohexene to benzene6 but reacts with cyclo-
hexane only stoichiometrically in the presence of CF3COOH
to produce benzene.3 Ru(styrene)2(PPh3)2, [Ir(COD)(PPh3)2]+,
and [Ir(COD)(P(p-FC6H4)3)2]+ catalyze the dehydrogenation of
cyclohexene to benzene but are unreactive with cyclohexane,8,9
while PdSO4 in the presence of H2SO4 effects only the
stoichiometric convertion of cyclohexane to benzene.10 Crabtree
has found an oxidative addition type system in which IrH2-
{O2CCF3}(PPri3)2 effects the dehydrogenation of alkanes to
arenes at 150 °C in the presence of the hydrogen acceptor, tert-
butylethylene (tbe).5 This system fails to turnover catalytically
because hydrogenolysis of the phosphine P-C bonds occurs at
Scheme 1
† University of Hawaii.
‡ University of California at Santa Barbara.
the temperatures of 135 °C or above required for the release of
the arenes from the intermediate complexes. We recently found
that the iridium P-C-P pincer complex IrH2{C6H3-2,6-(CH2-
PBut2)2} (1) is a highly active, homogeneous catalyst for the
transfer dehydrogenation of cyclooctane with unusual long-term
stability at temperatures as high as 200 °C.4 This reactivity
has now been extended to the catalytic transfer dehydrogenation
of cycloalkanes to arenes. We report this novel catalytic activity
as well as the results of an X-ray structure determination of 1.
As seen in Scheme 1, treatment of a pentane solution of the
hydrochloride complex IrHCl{C6H3-2,6-(CH2PBut2)2} (2) under
1 atm of hydrogen with LiBEt3H at 25 °C produces the white
tetrahydrido complex IrH4{C6H3-2,6-(CH2PBut2)2} (3) in 85%
yield.11 The brown dihydrido complex 112 is obtained quanti-
tatively upon heating finely powdered 3 to 130 °C en Vacuo.
(1) Wiseman P. Petrochemicals; Ellis Horwood: Chichester, England,
1986; pp 90-91.
(2) (a) Gol’dshleger, N. F.; Es’kova, V. V.; Shilov, A. E.; Shteinman,
A. A. Russ. J. Phys. Chem. 1972, 46, 785. (b) Shilov, A. E. ActiVation of
Saturated Hydrocarbons by Transition Metal Complexes; Reidel: Dordrecht,
The Netherlands, 1984; pp 164-165.
(3) (a) Baudry, D.; Ephritikhine, M.; Felkin, H. J. Chem. Soc., Chem.
Commun. 1983, 788. (b) Felkin, H.; Fillebeen-Khan, T.; Holmes-Smith,
R.; Zakrzewski, J. Tetrahedron Lett. 1984, 25, 1279. (c) Felkin, H.;
Fillebeen-Khan, T.; Holmes-Smith, R.; Lin, Y. Tetrahedron Lett. 1985, 26,
1999. (d) Burk, M. W.; Crabtree, R. H.; McGrath, D. V. J. Chem. Soc.,
Chem. Commun. 1985, 1829. (e) Burk, M. W.; Crabtree, R. H. J. Am. Chem.
Soc. 1987, 109, 8025. (f) Nomura, K.; Saito, Y. J. Chem. Soc., Chem.
Commun. 1988, 161. (g) Nomura, K; Saito, Y. J. Mol. Catal. 1989, 54, 57.
(h) Sakakura, T.; Sodeyama, T.; Tanaka, M. New J. Chem. 1989, 13, 737.
(i) Maguire, J. A.; Boese, W. T.; Goldman, A. S. J. Am. Chem. Soc. 1989,
111, 7088. (j) Fujii, T.; Satio, Y. J. Chem. Soc., Chem. Commun. 1990,
757. (k) Sakakura, T.; Sodeyama, T.; Abe, F.; Tanaka, M. Chem. Lett. 1991,
297. (l) Maguire, J. A.; Petrillo, A.; Goldman, A. S. J. Am. Chem. Soc.
1992, 114, 9492. (m) Maguire, J. A.; Goldman, A. S. J. Am. Chem. Soc.
1991, 113, 6706. (n) Fujii, T.; Higashino, Y.; Satio, Y. J. Chem. Soc., Dalton
Trans. 1993, 517. (o) Aoki, T.; Crabtree, R. H. Organometallics 1993, 12,
294. (p) Miller, J. A.; Knox, L. K. J. Chem. Soc., Chem. Commun. 1994,
1449. (q) Belli, J.; Jensen, C. M. Organometallics 1996, 15, 1532.
(4) Gupta, M.; Hagen, C.; Kaska, W. C.; Flesher, R.; Jensen, C. M. J.
Chem. Soc., Chem. Commun. 1996, 2083.
(11) The spectroscopic and analytical data previously reported4 for 1 are
for the tetrahydride complex 3.
(12) For 1: 1H NMR (400 MHz, cyclohexane-d12) δ 7.11 (d, JHH ) 7.7
Hz, 2H, m-H), 6.89 (t, JHH ) 7.7 Hz, 1H, p-H), 3.56 (vt, JPH ) 3.5 Hz, 4H,
CH2), 1.28 (vt, JPH ) 6.2 Hz, 18H, CH3), -18.89 (t, JPH ) 8.1 Hz, 2H,
IrH); 31P{1H} NMR (161.9 MHz, cyclohexane-d12) δ 86.1 (s).
(13) Dark orange crystals of 1 were obtained from slow evaporation of
a pentane solution: tetragonal P42, Z ) 2, a ) b ) 11.710(4) Å, c )
9.701(5) Å, V ) 1330.2(9) Å3; 137 parameters were refined on 1744
reflections having I > 2σ(I); R (Rw) ) 3.05 (6.21)%, GOF ) 0.920. The
refined Flack parameter suggested twinning, and twin component was
included with the twinning law of 1,0,0,0,-1,0,0,0,-1. The twin component
refined to a ratio 733:267(4), and its inclusion lowered R and decreased
the dispersion in chemically equivalent carbon-carbon distances. Chirality
is established by the final Flack parameter of 0.02(3). The hydride could
not be reliably located.
(5) Crabtree, R. H.; Parnell, C. P.; Uriarte, R. J. Organometallics 1987,
6, 696.
(6) Trost, B. M.; Metzner, P. J. J. Am. Chem. Soc. 1980, 102, 3572.
(7) Goldshleger, N. F.; Khidekel, M. L.; Shilov, A. E.; Shteinman, A.
A. Kinet. Katal. 1974, 15, 261.
(8) Chaudret, B. N.; Cole-Hamilton, D. J.; Wilkinson, G. Acta Chem.
Scand., Ser. A. 1978, 32, 763.
(9) Rudakov, E. S.; Zamashchikov, V. V.; Belyaeva, N. P.; Rudakova,
R. I. Zh. Fiz. Khim. 1973, 47, 2732.
(10) Crabtree, R. H.; Parnell, C. P. Organometallics 1985, 4, 519.
S0002-7863(96)02560-7 CCC: $14.00 © 1997 American Chemical Society