Mechanism of hydrogenolysis of an iridium-methyl bond: Evidence for a methane complex intermediate

Article abstract of DOI:10.1021/ja310982v

Evidence for key σ-complex intermediates in the hydrogenolysis of the iridium-methyl bond of (PONOP)Ir(H)(Me)⁺ (1) [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] has been obtained. The initially formed η²-H₂ complex, 2, was directly observed upon treatment of 1 with H₂, and evidence for reversible formation of a σ-methane complex, 5, was obtained through deuterium scrambling from η²-D₂ in 2-d₂ into the methyl group of 2 prior to methane loss. This sequence of reactions was modeled by density functional theory calculations. The transition state for formation of 5 from 2 showed significant shortening of the Ir-H bond for the hydrogen being transferred; no true Ir(V) trihydride intermediate could be located. Barriers to methane loss from 2 were compared to those of 1 and the six-coordinate species (PONOP)Ir(H)(Me)(CO)⁺ and (PONOP)Ir(H)(Me)(Cl).

Full text of DOI:10.1021/ja310982v

Communication
pubs.acs.org/JACS
Mechanism of Hydrogenolysis of an Iridium−Methyl Bond: Evidence
for a Methane Complex Intermediate
Jesus Campos,† Sabuj Kundu,‡ Dale R. Pahls,§ Maurice Brookhart,*,‡ Ernesto Carmona,†
́
and Thomas R. Cundari§
†Departamento de Química Inorgan
Investigaciones Científicas, Avda. Amer
́
ica, Instituto de Investigaciones Químicas (IIQ), Universidad de Sevilla, Consejo Superior de
́
ico Vespucio 49, 41092 Sevilla, Spain
‡Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
§Center for Advanced Scientific Computing and Modeling (CASCaM), Department of Chemistry, University of North Texas, Box
305070, Denton, Texas 76203-5070, United States
S
* Supporting Information
Scheme 1. Two Possible Pathways for Hydrogenolysis of a
ABSTRACT: Evidence for key σ-complex intermediates
M−CH3 Bond
in the hydrogenolysis of the iridium−methyl bond of
(PONOP)Ir(H)(Me)+ (1) [PONOP = 2,6-bis(di-tert-
butylphosphinito)pyridine] has been obtained. The
initially formed η2-H2 complex, 2, was directly observed
upon treatment of 1 with H2, and evidence for reversible
formation of a σ-methane complex, 5, was obtained
through deuterium scrambling from η2-D2 in 2-d2 into the
methyl group of 2 prior to methane loss. This sequence of
reactions was modeled by density functional theory
calculations. The transition state for formation of 5 from
2 showed significant shortening of the Ir−H bond for the
hydrogen being transferred; no true Ir(V) trihydride
intermediate could be located. Barriers to methane loss
from 2 were compared to those of 1 and the six-coordinate
species (PONOP)Ir(H)(Me)(CO)+ and (PONOP)Ir(H)-
(Me)(Cl).
the basic hydrogenolysis reaction, cleavage of a M−CH3 bond
by H2. Pathway (a) involves the formation of oxidative addition
intermediate A followed by reductive elimination of CH4.
Pathway (b) involves the formation of a σ-H2 complex,
conversion to a σ-methane (σ-CH4) complex, and release of
methane. While these are two “simplified” schemes, exper-
imentally relevant pathways are certainly more complex. σ-
Complex intermediates likely intervene in the formation and
decay of A in pathway (a). In pathway (b), a higher-energy
oxidative addition intermediate such as A may connect the σ-H2
and σ-CH4 intermediates or the interconversion of these
species may be a concerted process. The latter case, in which
there is no higher-oxidation-state intermediate along the
pathway, has been termed σ-complex-assisted metathesis (σ-
CAM).6 The extent of the M−H interaction in this transition
state may also vary substantially.
Here we report the hydrogenolysis of the Ir−CH3 bond of a
cationic iridium complex wherein the σ-H2 intermediate can be
observed spectroscopically. Deuterium labeling experiments
provided evidence for interconversion of the σ-H2 complex
with the σ-CH4 complex prior to release of methane. DFT
calculations revealed the energy difference between the σ-H2
and σ-CH4 complexes and the nature and structure of the
transition state connecting these species.
σ-Bond metathesis is a fundamental transformation exhibited
by complexes of metals across the periodic table and plays an
important role in many catalytic and stoichiometric processes.1
Hydrogenolysis of metal−alkyl bonds is a particularly
significant reaction for both early and late transition metal
alkyls.2 For example, hydrogenolysis of d0 metal−alkyl bonds
provides an important method for controlling polymer
molecular weights in olefin polymerization reactions.3 In both
early- and late-metal systems, hydrogenolysis of metal−alkyl
bonds releases the alkane in hydrogenation reactions in which
the catalytic cycle involves olefin insertion into a metal
monohydride.
In the case of d0 metal−alkyl complexes, the σ-bond
metathesis reaction cannot occur via an oxidative addition/
reductive elimination process.4 Density functional theory
(DFT) calculations suggest that σ-complex intermediates
intervene along the pathway, but such species are sufficiently
high in energy that, to date, they have escaped observation. In
the case of late-transition-metal systems, there are multiple
possibilities for the exact pathways followed during σ-bond
metathesis, as revealed by both DFT studies and experimental
observations.5 A simplified scheme is shown in Scheme 1 for
As previously reported, exposure of the cationic, coordinately
unsaturated Ir(III) methyl hydride complex (PONOP)Ir(H)-
(Me)+ (1) [PONOP = 2,6-bis(di-tert-butylphosphinito)-
Received: November 7, 2012
Published: January 11, 2013
© 2013 American Chemical Society
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Communication
pyridine] to H2 at −100 °C results in the formation of the η2-
7
1
H2 complex 2 in equilibrium with 1 (Scheme 2). The key H
Scheme 2. Hydrogenolysis of Complex 1 at Low
Temperature
positions results in equal rates of deuterium scrambling into the
two sites.
Scheme 3. Hydrogenolysis of Complex 1 and Formation of
σ-CH4 Complex 5
NMR data for 2 show a 2H signal at −1.98 ppm for the σ2-H2
ligand (JHD = 34 Hz for the HD complex) and a terminal
hydride at −13.4 ppm. Warming to −50 °C results in loss of
methane and formation of dihydride complex 3, which under
H2 forms tetrahydride complex 4.8 The equilibrium ratio of 1
and 2 at known H2 concentrations was examined over the
temperature range from −110 to −60 °C. A van’t Hoff plot
revealed ΔH° = −6.0 kcal/mol and ΔS° = −18 eu.9 With this
correction for the equilibrium between 1 and 2, we were able to
determine the first-order rate constant for loss of methane from
2 as 3.7 × 10−5 s−1 at −50 °C, corresponding to ΔG⧧ ≈ 17.9
kcal/mol.
Is a σ-CH4 complex involved as an intermediate in the loss of
methane from 2? To probe this question, we prepared complex
2-d2 possessing an η2-D2 ligand by exposure of 1 to a limited
amount of D2 at −120 °C in CDCl2F. Monitoring of this
solution at −90 °C showed deuterium scrambling into both the
methyl group and the terminal hydride at equal rates. The
results of this scrambling reaction are shown in Figure 1. The
half-life of the scrambling reaction is ca. 1 h, corresponding to
ΔG⧧ ≈ 13.6 kcal/mol.
To gain further insights into this σ-bond metathesis reaction,
this transformation was investigated using DFT. We modeled
our system using the PBE0 functional12 with the Stuttgart basis
set including an additional f polarization function with an
exponent of 0.685 for Ir and the 6-311G** basis set13 for all
other elements. Recent publications on similar systems7,14 have
found success with this functional using basis sets of double-ζ
quality. Calculations using the M06 functional were also
examined [see the Supporting Information (SI)], but the PBE0
calculations were found to be in better agreement with
experiment. All of the structures were optimized in the gas
phase with a single-point solvent correction using the SMD
solvation model15 (solvent = CH2Cl2) as implemented in
Gaussian 09.16
The DFT-computed structures of 2 and 5 are shown in
Figure 2. The H−H distance of 0.84 Å in the σ2-H2 ligand is
consistent with the observed JHD value of 34 Hz,17 and the Ir−
H distances of 1.57 and 1.80 Å are in the expected ranges for a
terminal Ir−H and an η2-H2, respectively. In the σ-CH4
complex 5, CH4 is bound primarily through one C−H bond
(M−H distance of 1.78 Å, in contrast to next-nearest geminal
These results point to a mechanism involving reversible
formation of the σ-CH4 complex 5 prior to methane loss, as
shown in Scheme 3.10,11 Rapid tumbling in the σ-CH4 complex
coupled with the equivalence of the two terminal hydride
Figure 2. DFT-calculated structures of (a) (PONOP)Ir(H)(Me)-
+
(H2)+ (2) and (b) (PONOP)Ir(CH4)(H)2 (5). The tert-butyl
substitutents have been omitted for clarity. Ir, purple; C, black; H,
Figure 1. Deuterium scrambling in the σ-D2 complex 2-d2 at −90 °C.
gray; O, red; P, orange; N, blue.
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C−H···Ir distance of 2.58 Å). The σ2-H2 complex 2 was
calculated to be 8.6 kcal/mol more stable than the σ-CH4
isomer 5 in a continuum CH2Cl2 solution. This value provides
an estimate of 17.9 − 8.6 = 9.3 kcal/mol for ΔG⧧ for
dissociation of methane from 5.
We considered that σ-H2 complex 2 could be converted into
σ-CH4 complex 5 by either a two-step or a one-step mechanism
(Scheme 4). In the two-step mechanism, the H2 ligand would
Scheme 4. Two Potential Mechanisms for Interconversion of
2 and 5
Figure 3. (a) Transition state for hydrogen transfer with an Ir−H
bond length of 1.6 Å. (b) Transition state for hydrogen transfer with a
restricted Ir−H distance of 2.0 Å. The tert-butyl substitutents have
been omitted for clarity.
obtained. Instead, the optimized structure had an Ir−H
distance of 1.6 Å, similar to the structure discussed previously.
This preference for the shorter Ir−H distance suggests that the
Ir−H interaction is important in stabilizing the transition state
and is thus consistent with a σ-CAM mechanism. The
reactivities of other isomers of 2 were considered, but the
reaction pathways required to access those intermediates were
deemed too high in energy to be chemically significant (see the
SI).
Trapping of 1 by CO yields the six-coordinate Ir(III) cation
(PONOP)Ir(CO)(CH3)(H)+ (9), while treatment of
(PONOP)IrCH3 with HCl yields the neutral, six-coordinate
Ir(III) complex (PONOP)Ir(H)(CH3)Cl (8).11b When these
species are warmed, they cleanly eliminate CH4. The measured
first-order rate constants for elimination at 80 °C are 1.6 × 10−4
s−1 (ΔG⧧ = 26.9 kcal/mol) for 8 and 7.1 × 10−4 s−1 (ΔG⧧ =
27.5 kcal/mol) for 9. The elimination of CH4 from 9 is not
inhibited by CO, indicating that reductive elimination occurs
from the six-coordinate complex. In the case of 8, examination
of deuterium-labeled (PONOP)Ir(H)(CD3)Cl showed scram-
bling into the iridium hydride site, indicating reversible
formation of a CH4 complex (t1/2 ≈ 90 min at 55 °C) prior
to loss of CH4. These data allow an interesting comparison of
the barriers to methane loss for the series of complexes 1,11b 2,
8, and 9, as summarized in Figure 4.
undergo oxidative addition to give a seven-coordinate
trihydride complex, 6, which in turn would undergo reductive
elimination to form the CH4 adduct. The one-step concerted
mechanism would involve a direct hydrogen transfer from the
H2 ligand to the CH3 ligand to form the CH4 adduct. The
single hydrogen-transfer step could proceed via transition state
TS2−5 with either substantial or little involvement of the metal.
We were not able to locate a minimum on the potential
energy surface corresponding to 6. All attempts to locate this
trihydride complex optimized to the σ-H2 complex 2. The lack
of a stable iridium trihydride complex is strong evidence against
an oxidative addition/reductive elimination pathway for the
interconversion of the σ-H2 and σ-CH4 complexes.
The free energy of the transition state for the one-step
conversion of H2 to CH4 was calculated to be ΔG⧧ = 13.3 kcal/
mol in CH2Cl2, which is in close agreement with the
experimental free energy barrier of 13.6 kcal/mol derived
from the H/D scrambling experiments. In the transition state,
the Ir−H distance for the hydrogen being transferred is 1.60 Å
(Figure 3). This short metal−hydride distance (which can be
compared to the distances of 1.80 Å for the η2-H2 ligand in 2)
indicates a very significant interaction of the metal with the
hydrogen being transferred in the transition state, which
therefore seems closer to a trihydride structure. To determine
whether this interaction with the metal was necessary for
reactivity, we searched for a transition state with a reduced Ir−
H interaction. This was accomplished by freezing the distance
between the Ir center and the H being transferred and
optimizing the rest of structure. With this procedure, a second
transition state with a frozen Ir−H distance of 2.00 Å was
located (Figure 3b). This structure does not represent a true
saddle point on the potential energy surface because one
coordinate was not allowed to optimize, and therefore, the
energy of this structure is not physically meaningful. When the
constraint was removed and the full system was allowed to
optimize, a structure with an Ir−H distance of ∼2.0 Å was not
Consistent with numerous previous studies, reductive
elimination from the five-coordinate complex has a significantly
lower barrier (by ca. 5 kcal/mol) relative to reductive
Figure 4. Experimentally determined ΔG⧧ values for CH4 elimination
from Ir complexes 1, 2, 8, and 9.
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elimination from the six-coordinate complexes 8 and 9.18 The
barrier for reductive elimination from cationic 9 possessing a π-
acidic carbonyl ligand differs little from that for neutral 8 in
which the CO ligand has been replaced by a chloride ligand.
The barrier for release of CH4 from six-coordinate 2 is ∼9 kcal/
mol lower than those from the six-coordinate complexes 8 and
9. This is clearly the result of the operation of a σ-bond
metathesis mechanism in the case of 2 wherein the oxidation
state of Ir is unchanged.
(9) See the Supporting Information for details.
(10) An alternate explanation for scrambling would involve
deprotonation/reprotonation of 2 coupled with reversible loss of H2
from 2. Under the conditions for generation of 2-d2, a small excess of 1
remained. No deuterium scrambling into 1 was observed during the
scrambling described in Scheme 3, ruling out the involvement of 1.
(11) A Rh−CH4 complex, (PONOP)Rh(CH4)+, has been directly
observed by low-temperature NMR spectroscopy, and the iridium
analogue (PONOP)Ir(CH4)+ was proposed as the intermediate
responsible for 1H site exchange between the iridium hydride and
the methyl hydrogens in (PONOP)Ir(H)(CH4)+. These methane
complexes are square-planar Rh(I) and Ir(I) species, in contrast to 5,
which is a six-coordinate Ir(III)−CH4 complex. See: (a) Bernskoetter,
W. H.; Schauer, C. K.; Goldberg, K. I.; Brookhart, M. Science 2009,
326, 553. (b) Bernskoetter, W. H.; Hanson, S. K.; Buzak, S. K.; Davis,
Z.; White, P. S.; Swartz, R.; Goldberg, K. I.; Brookhart, M. J. Am.
Chem. Soc. 2009, 131, 8603. (c) Walter, M. D.; White, P. S.; Schauer,
C. K.; Brookhart, M. New J. Chem. 2011, 35, 2884.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, kinetic experiments, computations,
complete ref 16 (as ref S14), and the crystal structure of 9
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(12) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,
865.
AUTHOR INFORMATION
■
(13) (a) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114,
3408. (b) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72,
Corresponding Author
mbrookhart@unc.edu
5639. (c) Andrae, D.; Haußermann, U.; Dolg, M.; Stoll, H.; Preuß, H.
̈
Notes
Theor. Chim. Acta 1990, 77, 123.
The authors declare no competing financial interest.
(14) Ghosh, R.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J.
Am. Chem. Soc. 2008, 130, 11317.
ACKNOWLEDGMENTS
(15) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B
2009, 113, 6378.
(16) Frisch, M. J.; et al. Gaussian 09, revision A.1; Gaussian, Inc.:
■
The authors acknowledge support of this work by the National
Science Foundation through Grant CHE-1205189, Center for
Enabling New Technologies through Catalysis. Financial
support (FEDER support) from the Spanish Ministerio de
Wallingford, CT: 2009.
(17) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R.
H.; Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc.
1996, 118, 5396.
(18) (a) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. J. Am. Chem.
Soc. 2000, 122, 1456. (b) Grice, K.; Scheuermann, M.; Goldberg, K.
Top. Organomet. Chem. 2011, 503, 1. (c) Puddephatt, R. J. Coord.
Chem. Rev. 1993, 123, 247. (d) Stahl, S. S.; Labinger, J. A.; Bercaw, J.
E. J. Am. Chem. Soc. 1995, 117, 9371.
Ciencia e Innovacion (Project CTQ2010-17476) and Con-
́
solider-Ingenio 2010 (CSD2007-0000) and from the Junta de
Andalucia (Projects FQM-119 and P09-FQM-4832) is grate-
́
fully acknowledged. J.C. thanks the Ministerio de Educacion for
a research grant (ref AP20080256).
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