C-H activation of alkanes and arenes catalyzed by an O-donor bis(tropolonato)iridium(III) complex

Article abstract of DOI:10.1002/anie.200462065

Stable and swift: The title complex is stable to air and water under thermal conditions. It efficiently activates the C-H bonds of alkanes and arenes at rates 50 times higher than does the only previously reported O-donor Ir ^III complex (see pi

Full text of DOI:10.1002/anie.200462065

Communications
À
C H Activation Catalysis
À
C H Activation of Alkanes and Arenes Catalyzed
by an O-Donor Bis(tropolonato)iridium(iii)
Complex**
Gaurav Bhalla and Roy A. Periana*
À
Catalysts based on the C H activation reaction show
potential for the development of selective hydrocarbon-
oxidation reactions.[1] One of the key challenges in this field is
À
developing catalysts based on C H activation that yield
functionalized products such as alcohols.[1g] We have been
interested in O-ligated late-transition-metal complexes as a
starting point for the development of new hydrocarbon-
oxidation catalysts. Although O-donor ligands have been
investigated with early and late transition metals,[2] to our
knowledge, the only well-defined O-ligated late-transition-
À
metal complexes that activate alkane and arene C H bonds
have been reported recently by our group.[3–5] Compared to
the N-, C-, or P-donor ligands generally utilized for homoge-
neous catalysts,[6] O-donor ligated complexes may have the
potential for higher stability under thermal, protic, and
oxidation conditions given the lower basicity and higher
electronegativity of O atoms. Another important reason for
the study of this class of complex is that the unique
combination of the higher electronegativity with the p-
donor[7] and “hard” properties of O-donor ligands could
allow access to higher oxidation states during catalysis that
À
may be required for the C H activation as well as for the
generation of functionalized products.[1g]
Recently, we demonstrated that the bis-bidentate O-
donor complex [Ir(CH3)(py)(acac-O,O)2] (acac-O,O = k2-
À
O,O-acetylacetonate, py = pyridine) catalyzes the C H acti-
vation of alkanes[3] and functionalization of arenes by the
intermolecular anti-Markovnikov hydroarylation of olefins to
selectively generate n-alkyl benzene derivatives.[4] Experi-
ments and theoretical[5] calculations reveal that this O-donor
octahedral d6 late-transition-metal complex is stable to air
and protic media under thermal conditions and activates the
À
C H bonds of alkanes and arenes by a transition state that
has oxidative-addition or insertion character. This transition
state is intriguing as the use of ligands with electronegative O-
À
donor atoms to facilitate this mode of C H activation may
[*] G. Bhalla, Prof. Dr. R. A. Periana
University of Southern California
Department of Chemistry
Loker Hydrocarbon Research Institute
Los Angeles, CA 90089 (USA)
Fax: (+1)213-821-2656
E-mail: rperiana@usc.edu
[**] This work was supported by NSF Grant CHE-0328121 and by the
ChevronTexaco Energy Technology Co. We thank Dr. William
Schinski of ChevronTexaco for helpful discussions. We also
acknowledge Dr. Jonas Oxgaard and Prof. W. A. Goddard III for their
useful insight.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200462065
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Angewandte
Chemie
seem contrary to the common guiding principle that such
reactions require electron-rich metal centers.[8] However, one
possibility is that the p-donor properties of O-donor ligands
ever, in addition to this material a red–black solid (1) was also
isolated, which was insoluble in dichloromethane. Attempts
to purify 1 or obtain reproducible analytical data for this
material were unsuccessful and we presume that the material
is polymeric.[15] The solid 1 is soluble in coordinating solvents
such as THF, CH3CN, DMSO, and pyridine, but NMR
spectroscopic studies suggest that multiple species are pres-
ent. Attempted separation of these species by chromatogra-
phy was also unsuccessful.[16] However, the addition of
[Zn(CH3)2] to a solution of 1 in THF followed by the addition
of pyridine, resulted in a black organometallic complex
[Ir(CH3)(py)(trop-O,O)2] (2-Me), which could be obtained
in 10% overall yield after column chromatography
(Scheme 1). This material has been fully characterized by
1H and 13C NMR spectroscopy, elemental analysis, and single-
crystal X-ray diffraction.[17] An ORTEP drawing of 2-Me is
shown in Figure 1.
À
may facilitate this mode of C H activation, whereas the s-
acceptor properties could help to stabilize the complex. This
À
combination of stability and C H bond reactivity is very
attractive for the development of oxidation catalysts based on
À
C H activation and we are currentlx exploring the extension
of this chemistry to other readily available O-donor ligands
such as aryloxides,[9] tropolones,[10] catechols,[11] and hydroxy-
acetophenones,[11c]
.
The O-donor ligand, trop-O,O (trop-O,O = k2-O,O-tropo-
lonato) has often been speculated to be an analogue of the
acac-O,O ligand as they share many common features. Both
ligands are bidentate, mono-anionic, and both bond through
delocalized chelate rings that are formed through two oxygen
atoms. Importantly, however, significant differences in reac-
tivity could be anticipated from the smaller bite angle[12] as
well as increased delocalization over the larger tropolonato
aromatic system. Thus, for example, trop-O,O complexes may
more readily accommodate an increase in the coordination
number at the metal center[10] or potentially—because of
changes in the extent of p donation that results from the
differences in bite angle—change the reactivity of the metal
center. Rate changes related to differences in ligand bite
angle have been reported.[12a,13] Herein, we report the syn-
thesis and chemistry of the bis-bidentate, O-donor tropolon-
ato iridium complex 2-Me [Ir(CH3)(py)(trop-O,O)2]. Impor-
tantly, we find that this O-donor complex is more active for
Figure 1. ORTEP drawing of 2-Me. Thermal ellipsoids are at the 50%
probability level. Hydrogen atoms omitted for clarity. Selected bond
lengths [AL] and angles [8]: Ir1-C20 2.046(10), Ir1-N1 2.180(8); O1-Ir1-
O2 78.4(3).
We find from 1H NMR spectroscopic studies that the
methyl group bound to the iridium center in 2-Me (2.08 ppm,
À
À
the C H activation of alkanes and arenes than is the
analogous bis-acac-O,O iridium complex.
Ir CH3) is located downfield compared to that in the acac-
À
O,O analogue [Ir(CH3)(py)(acac-O,O)2] (1.65 ppm, Ir
To obtain the bis trop-O,O complex of iridium, the
original synthesis of the tris complex [Ir(trop-O,O)3] was
reinvestigated in anticipation that the bis trop-O,O complex
could be isolated as an intermediate in this synthesis.[14] As
reported by Griffith et al.,[14] heating IrCl3 with an excess of
tropolone and sodium acetate in water resulted in the
formation of [Ir(trop-O,O)3] in 40% yield (Scheme 1). How-
CH3),[18] which suggests possible significant electronic differ-
ences at the metal centers. A similar trend is also observed in
the 13C NMR spectrum, with the chemical shift of the Ir CH3
À
group at À23.3 ppm for 2-Me compared to À27.1 ppm for the
acac-O,O analogue. However, the most significant difference
between the two complexes can be seen in the crystal
structures of these complexes. As anticipated, the bite angle
O1-Ir-O2 in 2-Me (78.4(3)8) is much smaller than that in the
À
acac-O,O analogue (95.17(16)8). The Ir N bond length
(2.180(8) AL) is comparable to that in the analogous acac
complex (2.181(4) AL).
À
To investigate the stoichiometric C H activation chemis-
try of this new O-donor complex we examined the reaction of
the complex in various hydrocarbon solvents. Thus, heating 2-
Me in neat mesitylene at 1308C for 1 h cleanly yielded the
corresponding mesityl complex 2-Mes and methane as shown
in Scheme 2. 1H NMR spectroscopic analysis of the crude
reaction mixture, after solvent removal and dissolution in
CDCl3, showed that the reaction was essentially quantitative,
as in the case of the acac-O,O analogue,[3] and only the
Scheme 1. Synthesis of 2-Me.
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Communications
À
C H activation chemistry as well as the thermal
stability to air and protic media of O-donor late-
transition-metal complexes are not unique to
the acac-O,O complex and 2) the chemistry of
O-donor late-transition-metal complexes can be
significantly changed by ligand modification.
This observation of ligand-dependent reactivity
is an important requirement for this class of O-
donor ligand–metal complex to be useful. These
results raise the expectation that further inves-
tigation of O-donor complexes of late-transi-
tion-metal complexes could lead to a broad class
of complex with desirable stability, reactivity, and ligand
control properties.
À
Scheme 2. Stoichiometric C H activation reactions of 2-Me with RH to generate 2-R.
À
benzylic C H bond of mesitylene was activated. Other
hydrocarbon substrates that react by C H activation with 2-
À
Me are shown in Scheme 2. Thus, heating a solution of 2-Me
in benzene or acetone at 1208C for 1 h results in the formation
of the corresponding hydrocarbyliridium derivatives, 2-Ph
and 2-Ace, respectively, in almost quantitative yield. Sim-
ilarly, heating 2-Me in cyclohexane resulted in the corre-
sponding cyclohexyliridium complex 2-Cy, which was purified
by flash chromatography and isolated in 35% yield. All these
hydrocarbyliridium derivatives, 2-R (R = Cy, Mes, Ph, and
In conclusion, we have synthesized a stable O-donor late-
À
transition-metal complex that activates the C H bonds of
alkanes and arenes more rapidly than does the only pre-
viously reported O-donor metal complex. Theoretical calcu-
lations and further experimental study of these complexes are
underway to understand the basis for the increased reactivity
and to determine its scope.
Ace) were fully characterized by H and 13C NMR spectros-
copy as well as by elemental analysis. Importantly, as was the
case for the acac-O,O analogues, these hydrocarbyl O-donor
iridium derivatives are all air, water, and thermally stable.
1
Received: September 21, 2004
Published online: January 26, 2005
À
Keywords: C H activation · homogeneous catalysis · iridium ·
O ligands
.
À
Significantly, the stoichiometric C H activation reactions of
2-Me are faster than those of the acac-O,O analogue. Thus,
the half-life (t1/2) for the reaction of 2-Me with benzene at
1208C is less than 5 minutes versus about 50 minutes for the
acac-O,O analogue.
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Having established that 2-Me can stoichiometrically
À
activate the C H bonds of alkanes and arenes, we examined
À
the catalytic C H activation of this complex as a first step
towards attempting to develop stable hydrocarbon-oxidation
catalysts. The relative rates of the H/D exchange reaction with
a C6H6/[D8]toluene mixture (1:1 v/v),[19] catalyzed at 1208C by
2-Me (0.1 mol%) and the acac-O,O analogue were used to
compare these complexes. As can be seen in Figure 2, the
trop-O,O complex, 2-Me, is at least 50 times faster than the
acac-O,O analogue. Critically, both complexes are stable over
the time period studied (ca. 5 h; turnover number (TON)
ca. 140 and turnover frequency (TOF) = 80 ꢀ 10À4 sÀ1 for 2-
Me, and TON ca. 2 and TOF = 1 ꢀ 10À4 sÀ1 for the acac-O,O
analogue). This is an important result as it shows that: 1) the
À
Figure 2. Comparison of catalytic C H activation of trop-O,O and
acac-O,O complexes.
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Chemie
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the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
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group P2(1), a = 8.463(2), b = 11.060(3), c = 9.924(3) A, aL = 90,
b = 94.721(4), g = 908, V= 925.6(4) A3, FL (000) = 508, 1calcd (Z =
2) = 1.896 mgmÀ3, m = 0.7236 mmÀ1, approximate crystal dimen-
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