J. Am. Chem. Soc. 2000, 122, 11017-11018
11017
Addition of C-H Bonds to the Catalytically Active
Complex (PCP)Ir (PCP ) η3-2,6-(tBu2PCH2)2C6H3)
Mira Kanzelberger, Bharat Singh, Margaret Czerw,
Karsten Krogh-Jespersen, and Alan S. Goldman*
Department of Chemistry, Rutgers UniVersity
Unlike the five-coordinate precursor, 2 does not undergo arene
exchange and is stereochemically rigid at ambient temperature.
Addition of CO to the mixture of three isomers/rotamers of 1-tolyl
in toluene gives the three corresponding CO adducts, while each
reaction with the m-xylene and 2-chloro-m-xylene adducts gives
a single adduct.
Piscataway, New Jersey 08854-8087
ReceiVed May 12, 2000
The development of catalysts for the selective functionalization
of unactivated C-H bonds is presently one of the major goals of
inorganic and organic chemistry. The oxidative addition of C-H
bonds to transition metal centers is clearly of great interest in
this context. Studies of systems that undergo observable C-H
additions1 have yielded deep insight into this reaction over the
past two decades; however, there has existed for the most part a
disparity between such systems and those that effect catalytic
conversions. In the past several years, complexes containing the
(PCP)Ir moiety (PCP ) η3-2,6-(R2PCH2)2C6H3); typically R )
tBu) have been reported to be efficient and, in some cases,
selective catalysts for thermochemical alkane dehydrogenation.2
Herein we report the observation of complexes resulting from
the addition of vinyl and aryl C-H bonds to the Ir(PCP) unit,
and characterization of their dynamic behavior. To our knowledge,
these are the first observable examples of thermal C-H addition
products in which the alkyl hydride analogues are presumed
intermediates in a catalytic alkane functionalization cycle.
When (PCP)IrH2 is reacted with either tert-butylethene (TBE)
or norbornene, either in benzene solvent or in mesitylene with
added benzene, disappearance of the dihydride is observed in the
31P NMR spectrum, accompanied by the appearance of a single
peak at δ 67.2 (δ 67.5 in mesitylene). The 1H NMR spectrum at
room temperature reveals signals characteristic of only a PCP
ligand in a fully symmetrical (C2V) environment. At lower
temperatures (< ca. -10 °C), however, a hydride resonance at δ
-45.56 (t, JP-H ) 13.4 Hz) is observed, strongly indicative of a
five-coordinate d6 species.3 In addition, resonances attributable
to an η1-phenyl ligand appear,4-6 the PCP t-butyl and benzylic
protons are each resolved as two inequivalent sets, and P-H
coupling appears in the selectively decoupled 31P NMR spectrum.
These data are all consistent with characterization of the product
as (PCP)Ir(Ph)H (1).
Most reported products of C-H addition to late-metal centers
lack a vacant coordination site necessary for â-H-elimination and
therefore cannot directly serve as potential intermediates for alkane
dehydrogenation, or as models thereof. An outstanding exception
to this, closely related to 1, was reported by Werner: Ir(PiPr3)2-
Cl(Ph)H (3),6 which was formed via benzene addition.8 Complex
3 has a 16-valence-electron count and, more specifically, it has
the five-coordinate, d6, M(III) configuration which has been shown
to be critical in olefin hydrogenation9 as well as alkane dehy-
drogenation (though not directly observed in either case). Ac-
cordingly, complex 3 attracted our interest several years ago, and
we have reported kinetic10 and calorimetric11 studies of it; both
approaches illustrate very high stability. The kinetic barrier to
elimination of benzene from 3 is found to be quite high (only
slow arene exchange with concomitant decomposition at 120 °C);
this is in agreement with the calorimetric studies which indicate
that Ph-H addition is ca. 32 kcal/mol exothermic.11 Complexes
1 and 3 are closely related, at least formally. We have previously
calculated, however, that C-H addition to Ir(PMe3)2X is ca. 30
kcal/mol more favorable for X ) Cl than for X ) Ph (Ph held
coplanar with P-Ir-P so as to model the PCP ligand).12 In
accordance with this prediction, we find that C-H elimination
from 1 is far more facile than elimination from 3.
Indeed, in contrast to the behavior of 3, a broad range of
observations demonstrate that intermolecular arene exchange with
1 occurs rapidly eVen on the NMR time scale. Addition of excess
C6D6 to mesitylene solutions of 1-(phenyl-h5) results in disap-
pearance of the phenyl and hydride resonances. As noted above,
warming above ca. -10 °C results in reversible loss of phenyl
(1) For an introduction to stoichiometric C-H activation by late-metal
complexes, see, for example: (a) Arndtsen, B. A.; Bergman, R. G.; Mobley,
T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154-162. (b) Jones, W. D.;
Feher, F. J. Acc. Chem. Res. 1989, 22, 91-100. (c) Harper, T. G. P.; Desrosiers,
P. J.; Flood, T. C. Organometallics 1990, 9, 2523-2528.
(2) (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M.
Chem. Commun. 1996, 2083-2084. (b) Xu, W.; Rosini, G. P.; Gupta, M.;
Jensen, C. M.; Kaska, W. C.; Krogh-Jespersen, K.; Goldman, A. S. Chem.
Commun. 1997, 2273-2274. (c) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C.
M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086-4087.
(3) For example, (PCP)IrHCl has a hydride NMR signal at δ -43.0 (JP-H
) 12.0 Hz): Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020-1024.
Analogous experiments with other arenes further support this
characterization. With toluene, low-temperature 1H NMR reveals
three addition products in a ratio of ca. 1:1:1.4 Since the addition
of arene C-H bonds ortho to a methyl group is generally much
less favorable than meta and para addition,5 we attribute the
signals to two m-tolyl rotamers and the p-tolyl isomer.6,7
Consistent with the lack of formation of an ortho-substituted tolyl
complex, m-xylene and 2-chloro-m-xylene each give only one
(4) See Supporting Information for details. All reactions were conducted
under an argon atmosphere.
(5) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106, 1650-1663.
(6) Werner, H.; Ho¨hn, A.; Dziallas, M. Angew. Chem., Int. Ed. Engl. 1986,
25, 1090-1092.
(7) Hindered rotation about M-phenyl bonds is well precedented; see, for
example, ref 6.
(8) For other closely related examples, see: (a) Desrosiers, P. J.; Shimomoto,
R. S.; Flood, T. C. J. Am. Chem. Soc. 1986, 108, 1346-1347. (b) Renkema,
K. B.; Bosque, R.; Streib, W. E.; Maseras, F.; Eisenstein, O.; Caulton, K. G.
J. Am. Chem. Soc. 1999, 121, 10895-10907 and references therein.
(9) Halpern, J. Inorg. Chim. Acta 1981, 50, 11-19 and references therein.
(10) Rosini, G. P.; Wang, K.; Patel, B.; Goldman, A. S. Inorg. Chim. Acta
1998, 270, 537-542.
(11) Rosini, G. P.; Liu, F.; Krogh-Jespersen, K.; Goldman, A. S.; Li, C.;
Nolan, S. P. J. Am. Chem. Soc. 1998, 120, 9256-9266.
(12) Krogh-Jespersen, K.; Goldman, A. S. Transition State Modeling for
Catalysis; ACS Symposium Series 721; American Chemical Society: Wash-
ington, DC, 1998; pp 151-162.
1
signal in the low-temperature 31P NMR and upfield H NMR
spectra, while p-xylene and mesitylene give no observable C-H
addition products.4
Addition of CO to 1-phenyl in benzene at 6 °C results in the
appearance of a single resonance in the 31P NMR spectrum at δ
52.3 (selectively decoupled: d, JP-H ) 17.3 Hz) and a hydride
1H NMR signal at δ -8.83 (t, JP-H ) 17.8 Hz). A full spectral
characterization4 indicates the formation of 2-phenyl (and its 13CO
isotopomer).
10.1021/ja001626s CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000