Hydrogen Activation and Hydrogenolysis Facilitated by Late-Transition-Metal-Aluminum Heterobimetallic Complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b01359

Previously reported heterobimetallic rhodium-aluminum and iridium-aluminum alkyl complexes are shown to activate hydrogen, generating the corresponding alkane. Kinetic data indicate a mechanistic difference between the iridium- A nd rhodium-based systems. In both cases the transition metal is an active participant in the release of alkane from the aluminum center. For iridium-aluminum species, experimental mechanistic data suggest that multiple pathways occur concomitantly with each other: One being the oxidative addition of hydrogen followed by proton transfer resulting in alkane generation. Computational data indicate a reasonable barrier to formation of an iridium dihydride intermediate observed experimentally. In the case of the rhodium-aluminum species, hydrides are not observed spectroscopically, though a reasonable barrier to formation of this thermodynamically unstable species has been calculated. Alternative mechanistic possibilities are discussed and explored computationally. Cooperative hydrogenolysis mechanisms are computed to be energetically unfeasible for both metal centers.

Full text of DOI:10.1021/acs.inorgchem.9b01359

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hydrogen Activation and Hydrogenolysis Facilitated By Late-
Transition-Metal−Aluminum Heterobimetallic Complexes
R. Malcolm Charles III,† Timothy W. Yokley,† Nathan D. Schley,‡ Nathan J. DeYonker,†
and Timothy P. Brewster*,†
†Department of Chemistry, The University of Memphis, 3744 Walker Avenue, Smith Chemistry Building, Memphis, Tennessee
38152, United States
‡Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
S
* Supporting Information
ABSTRACT: Previously reported heterobimetallic rhodium−alumi-
num and iridium−aluminum alkyl complexes are shown to activate
hydrogen, generating the corresponding alkane. Kinetic data indicate a
mechanistic difference between the iridium- and rhodium-based
systems. In both cases the transition metal is an active participant in
the release of alkane from the aluminum center. For iridium−
aluminum species, experimental mechanistic data suggest that multiple
pathways occur concomitantly with each other: one being the
oxidative addition of hydrogen followed by proton transfer resulting
in alkane generation. Computational data indicate a reasonable barrier
to formation of an iridium dihydride intermediate observed
experimentally. In the case of the rhodium−aluminum species,
hydrides are not observed spectroscopically, though a reasonable barrier to formation of this thermodynamically unstable
species has been calculated. Alternative mechanistic possibilities are discussed and explored computationally. Cooperative
hydrogenolysis mechanisms are computed to be energetically unfeasible for both metal centers.
We are particularly interested in exploring the chemistry of
late-metal−aluminum systems, as the enhanced Lewis acidity
of aluminum, relative to boron, would be beneficial in
heterolytic bond activation. Fewer examples of heterobimetal-
lic aluminum complexes have been reported, likely due to their
high reactivity and relative synthetic difficulty. Early examples
of such complexes were reported by Fryzuk,32 Lu,12,18,33−35
Emslie,29 Cramer,36 and Bourissou.25,37−39 Of the late-
transition-metal−aluminum complexes that are known, few
are reported to cooperatively activate chemical bonds.
Bourissou reported the ability of a Pt−Al Z-type complex to
activate CO2, CS2, H2, and benzamide.25 However, this system
was not found to be catalytically useful. The first catalytic late-
metal−aluminum systems were reported by Cramer,36
Iwasawa17 and Nakao.16 The observed cooperative reactivity
is reminiscent of that reported for aluminum−phosphorus
frustrated Lewis pairs (FLPs).40−43
Recently, our laboratory reported a series of iridium and
rhodium complexes featuring a pendant organoaluminum
moiety (1a,b and 2a,b, Figure 1; note that throughout this
paper odd-numbered complexes contain Rh, even-numbered
complexes contain Ir, complexes denoted “a” contain ethyl
substituents, and complexes denoted “b” contain isobutyl
substituents).27 In this new report, we detail our continued
INTRODUCTION
■
There has been a recent explosion of interest in bimetallic and
multimetallic systems as researchers seek to harness the
properties of multiple metal sites for applications ranging from
molecular magnets and molecular electronics to small molecule
activation and biomimetic catalysis.1−15 In these applications,
the metal sites are intended to act concertedly rather than
independently to achieve a desired function that would be
otherwise unachievable. Of particular interest to our laboratory
are systems which incorporate both a transition metal and a p-
block metal.11,16−23 The Lewis acidity of a group 13 metal is
expected to be beneficial in a variety of catalytic systems.17,22,23
For example, heterobimetallic Zr−Al systems were developed
by Roesky24 for ethylene polymerization with the expectation
that the aluminum functionality would serve as an internal
surrogate for the common activator methylalumoxane (MAO).
Though a few examples have been published recently,
relatively unexplored are bimetallic complexes featuring an
electron-rich late transition metal and a p-block ele-
ment.11,12,18,25−31 The vast electronic difference between the
two metal sites opens up the possibility of cooperative
activation of polar, or polarizable, small molecules.11,25 Of
these types of bimetallic systems, late-transition-metal−boron
systems are the best studied, with applications in catalytic
nitrogen fixation22 and olefin hydrogenation.20 Alkynes,
amines, alcohols, and ketones have also been found to react
stoichiometrically with late-metal−boron complexes.11
Received: May 9, 2019
© XXXX American Chemical Society
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their iridium analogues [Ir(PPh3)2(CO)(2-pyridone-AlEt2)]-
[NO3] (2a) and [Ir(PPh3)2(CO)(2-pyridone-Al(iBu)2)]-
[NO3] (2b, Figure 1) were pressurized with hydrogen in a J.
1
Young NMR tube. Post-reaction H NMR spectra of each
complex displayed a new signal in the alkane region. Reactions
of 1a and 2a led to the appearance of a singlet at δ 0.80 ppm,
corresponding to ethane gas.44 Product spectra of the reactions
of 1b and 2b with hydrogen contained a new doublet at δ 0.86
ppm corresponding to the methyl protons of isobutane (Figure
2; for the full spectra of all complexes, see Figures S6−S9 in
the Supporting Information). In all cases, the corresponding
Figure 1. Rhodium and iridium complexes containing alkylaluminum
moieties.
exploration of the interesting chemistry afforded by this unique
heterobimetallic system. Reaction of 1a,b and 2a,b with H2
resulted in hydrogenolysis of the aluminum alkyl, yielding the
corresponding hydrocarbon gas. Mechanistic studies, sup-
ported by a corresponding computational investigation, suggest
a role for both the transition metal and aluminum center
during the reaction for both the iridium and rhodium
complexes. The observed reactivity stands in sharp contrast
to the report from Bourissou, where the product of bimetallic
H2 activation was stable and unreactive toward the alkyl groups
present at aluminum.25 To our knowledge, hydrogenolysis of
an aluminum alkyl facilitated by a late-transition-metal−
aluminum system has not been previously reported.
1
loss of diagnostic H NMR signals arising from the diagnostic
diastereotopic aluminum alkyl protons in the starting material
suggests hydrogenolysis of the alkyl has occurred. The reaction
proceeds slowly, reaching completion, as indicated by the
complete loss of aluminum alkyl resonances over the course of
several hours at ambient temperature. On the basis of alkane
yield, both alkyl groups at each aluminum center are found to
undergo hydrogenolysis.
To validate that the alkane produced originates from the
reaction of the aluminum alkyl with hydrogen gas and not an
alternative proton source, an analogous hydrogenolysis
reaction was attempted using D2. Protio-toluene solutions of
complexes 1a and 2a were pressurized with D2 and allowed to
react at ambient temperature for 24 h. The resulting mixtures
were then analyzed by 2H NMR spectroscopy. A peak at δ 0.8
ppm indicated the formation of C2H5D, the expected
isotopomer of deuterium hydrogenolysis (Figure 3). This
result confirms that an alkane is formed as a result of the
reaction of our starting bimetallic complexes with hydrogen.
RESULTS AND DISCUSSION
■
To investigate the reactivity of our previously reported
heterobimetallic complexes, we first exposed them to hydrogen
gas. Benzene solutions of rhodium−aluminum heterobimetallic
complexes [Rh(PPh3)2(CO)(2-pyridone-AlEt2)][NO3] (1a)
and [Rh(PPh3)2(CO)(2-pyridone-Al(iBu)2)][NO3] (1b) and
Figure 2. Truncated post-reaction 1H NMR spectra of reactions of 1a (red), 1b (dark green), 2a (navy), and 2b (purple) with H2. Signals indicate
the formation of alkane gas. The triplet in 2a and shoulder in 2b result from a pentane solvent impurity.44
B
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2
Figure 3. H NMR spectra of 1a (red, bottom) and 2a (navy, top) following a 24 h reaction with D2. Chemical shifts are referenced to residual
solvent signals (labeled as “deuterated toluene”).
Mechanistic Studies. With this exciting reactivity in hand,
we then began to interrogate potential hydrogenolysis
mechanisms. Because the alkane originates from the aluminum
alkyl, mechanisms based solely around reactivity at the late
transition metal should be readily excluded. To verify this, we
reacted hydrogen with [Ir(PPh3)2(CO)(2-pyridone)][NO3]
(3) and [Rh(PPh3)2(CO)(2-pyridone)][NO3] (4) (Figure 4).
As expected,45 no alkane was formed. Thus, all mechanisms
considered below include a contribution from the alkylalumi-
num moiety.
of facilitating hydrogenolysis via a σ-bond metathesis-like
pathway or an aluminum−pyridone cooperative pathway that
does not involve the late transition metal. In short, the
activation of H2 could occur at the transition metal
(mechanism 1), across the two metal sites (mechanisms 2
and 3), or at aluminum (mechanism 4). Protonolysis is
expected to be quite rapid due to the known acidity of Ir and
Rh hydrides derived from Vaska’s complex.46 As such, our
mechanistic investigation has focused on the key hydrogen
activation step.
We are able to eliminate mechanism 4, wherein the
transition metal does not participate in the reaction. This
type of reactivity has been reported for pyridonate boranes;
thus, we initially thought it may be possible for our system.47
To investigate the possibility of FLP-like behavior involving
the pyridone ligand and the aluminum center, we reacted the
ligated aluminum species AlEt2(2-pyridone) with H2. No
alkane formation was observed. Investigation of the reactivity
of the anionic species [AlEt2(2-pyridone)(NO3)]− was then
attempted. Unfortunately, synthesis and isolation of a benzene-
soluble derivative of this salt proved elusive. However, the
combination of the control data acquired and the fact that we
observe vastly different kinetic profiles for rhodium complex 1b
and iridium complex 2b (vide infra) makes us quite confident
in our assertion that the late transition metal is intimately
involved in the reaction.
Figure 4. Pyridone complexes of iridium and rhodium.27
Four general mechanistic processes have been considered
(Scheme 1). The first begins with a transition-metal-based
oxidative addition of H2. In a series of subsequent steps, the
aluminum alkyl would fully deprotonate the late-transition-
metal dihydride complex to yield the alkane gas. The second
potential mechanism involves a concerted, cooperative
substrate activation to eliminate the first equivalent of alkane,
followed by protonolysis of the second aluminum alkyl. The
third involves an initial dissociation of pyridone from the
transition metal,27,45 followed by H2 activation.41 Subsequent
protonolysis would then release the alkane product. Finally, we
considered the possibility that the aluminum center is capable
To distinguish among potential mechanisms 1, 2, and 3, a
joint experimental/theoretical investigation was carried out.
Experimentally, the hydrogenolysis reaction kinetics for
representative complexes 1b and 2b were monitored using
1H NMR and 31P{1H} NMR spectroscopy. For each complex,
the 1H data and 31P{1H} data were found to be self-consistent.
The 1H NMR data were found to be the most informative with
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Scheme 1. Considered Mechanisms for Reaction of IridiumAluminum and RhodiumAluminum Bimetallics with H2
1
regard to speciation and will be discussed in detail below.
31P{1H} NMR data can be found in the Supporting
Information (Figures S26 and S27). Samples of 1b and 2b
were first dissolved in benzene-d6 in a J. Young NMR tube.
Hydrogen gas was added via a Schlenk manifold, and spectra
were then recorded at regular intervals. Peak integrations were
reported relative to a 1,3,5-trimethoxybenzene internal stand-
ard (for full procedure, see Experimental Details). We were
surprised to observe different kinetic data for 1b and 2b,
suggesting that different mechanisms may be involved
depending on the nature of the transition-metal center. Each
complex will be discussed individually below.
equivalent triphenylphosphine resonances are present and. H
NMR signals corresponding to the pyridone ligand are not
observed, suggesting that 2-pyridone has precipitated along
with the aluminum moiety and is not a ligand to Rh. To
maintain charge balance, it is likely that nitrate is present. An
X-ray structure of a single crystal of this complex is presented
in the Supporting Information (see Figure S32). Our fully
refined coordinates have been added to the Cambridge
Crystallographic Data Centre to augment the unit cell
parameters reported previously.48,49
To account for mass balance, we ascribe the fluffy white
precipitate to be an aluminum−pyridone-based species. In
1
conjunction with the lack of pyridone resonances in the H
Hydrogenolysis of 1b. Under our reaction conditions,
rhodium complex 1b was found to react slowly with hydrogen
over the course of several days. At the conclusion of the
reaction, a pale yellow solution and a fluffy white precipitate
were present, in addition to the isobutane product. On the
NMR spectrum of the product solution, elemental analysis of
the precipitate supports this assertion. The presence of
nitrogen in the precipitate strongly suggests the presence of
pyridone. The relatively low percentage of carbon in the
analyzed precipitate indicates that aluminum is present.
Unfortunately, simple combinations of aluminum and 2-
pyridone did not sufficiently match with elemental analysis
data. As a result, we cannot be more specific about the exact
formulation of the precipitate at the present time.
1
basis of the observed color of the solution and H NMR data
of the product mixture, we assert that the solution contains a
rhodium(I) bis-phosphine complex. While rigorous character-
ization of the solution product has remained elusive, we believe
the product is most likely to be known complex Rh-
(PPh3)2CO(NO3).48 1H and 31P{1H} NMR spectra of the
product mixture support this assertion. Signals attributable to
To learn more about this interesting reaction, we monitored
the reaction progress over time. A plot of concentration of 1b
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and the isobutane product over time can be found in Figure
S22 (Supporting Information). The concentration of the
starting material decreases exponentially as the product gas is
produced. The lower-bound yield for isobutane production, on
the basis of NMR integration, is approximately 60% (assuming
a 2:1 ratio of alkane to heterobimetallic starting material). This
yield only accounts for isobutane present in solution; it is likely
that the actual yield is much higher when gaseous product
present in the headspace of the NMR tube is accounted for.
Attempts to accurately quantify isobutane present in the
headspace using GC/MS were not fruitful. However, the
alkane yield exceeds 50%, which is the maximum allowable
yield should only one alkyl group per aluminum center
undergo hydrogenolysis. This confirms our expected reaction
stoichiometry.
undertaken to determine the rates under different pressures
of hydrogen gas.53 NMR tubes were pressurized with hydrogen
to 1, 2, and 4 atm using a custom-built NMR tube
pressurization apparatus (see Figure S1 in the Supporting
Information). The reaction was found to be first order in H2
(see Figure S25 in the Supporting Information), suggesting an
o v e r a l l r a t e l a w f o r h y d r o g e n o l y s i s o f
rate = kreaction[Rh−Al][H2]. From the slopes of the kinetic
plots, first-order rate constants for consumption of the starting
heterobimetallic complex are calculated to be kobs = 4.38 ×
10−5 s−1 at 1 atm of H2, 7.67 × 10−5 s−1 at 2 atm of H2, and
1.76 × 10−4 s−1 at 4 atm of H2, where kobs= kreaction[H2]. Using
the known solubility of H2 gas in benzene at 298 K,54 the
average kreaction value is calculated to be 0.0144 M−1 s−1
(standard deviation 0.0012). Applying this value to obtain
Interestingly, no readily observable intermediates formed
over the course of this reaction by either 1H or 31P{1H} NMR
spectroscopy. This stands in contrast to the behavior of
complex 2b (vide infra). Additionally, hydrides are not
observed as products in this reaction. This suggests that a
rhodium-based dihydride (complex 5, mechanism 1) cannot be
a stable intermediate. To further support this claim, we
revisited the reaction of complex 4 with hydrogen. As
kobs for the reaction of our bimetallic species with 1 atm of H2
(2.98 × 10−5 s−1) allows for estimation of the standard free
energy of activation: ΔG°⧧ = 22.9 kcal/mol at 298 K. First-
order dependence on both Rh and H2 is consistent with rate-
limiting activation of H2 (mechanism 1, 2, or 3) followed by
rapid protonolysis.
To distinguish among mechanisms 1, 2 and 3, a computa-
tional study was undertaken to investigate the initial steps in
each potential reaction pathway. Considering similarities in the
NMR product spectra (Figure 2), it was expected that
complexes 1a and 1b should have similar thermodynamic
and kinetic behavior. Therefore, proposed mechanisms were
only computed for 1a, drastically reducing the conformational
space and required time for the computations. Figure 6
displays the free energy diagram for the possible first
elementary steps along the hydrogenation pathways for
mechanisms 1 and 3 (Scheme 1). The reference free energy
value (center of the plot) in the free energy diagram is the sum
of 1a and H2 at infinite separation. Proceeding to the left along
pathway 1, the supermolecular complex of 1a with the added
H2 substrate in the second coordination sphere is 6.8 kcal
mol−1 higher in energy due to unfavorable entropic effects as
well as the enthalpic penalty of posing the H2 in a presumably
local minimum. The effective free energy of activation for the
formation of dihydride 5a (mechanism 1) is 24.6 kcal mol−1.
This activation barrier is consistent with our experimental data.
Additionally, complex 5a is calculated to be thermodynamically
unstable, with a free energy 13.7 kcal mol−1 higher than the 1a
+ H2 supermolecule, explaining our inability to observe this
species experimentally. We were unfortunately unable to locate
a simple low-energy transition state for intramolecular proton
transfer from 5a to release alkane. However, unmodeled
multimolecular or dissociative pathways may be accessible.
Thus, mechanism 1 is consistent with the computational data
and is a plausible mechanism.
1
confirmed via H NMR spectroscopy, 4 is unreactive toward
H2; hydride formation is not observed at 1 atm pressure. This
result is consistent with literature surrounding rhodium
analogues of Vaska’s complex, which are not observed to
form monomeric dihydrides under 1 atm of H2 due to the
thermodynamically disfavored nature of H2 oxidative addi-
tion.50−52 However, though we have no direct evidence of
hydride formation, we cannot exclude the possibility of a
transient monometallic hydride species forming in concen-
trations below the NMR detection limit and rapidly being
consumed via protonolysis as a potential mechanism
(mechanism 1).
A detailed analysis of the kinetics plot tracing the
disappearance of 1b was then undertaken. The reaction was
found to be first order with respect to the bimetallic starting
material (Figure 5). This excludes rate-limiting bimolecular
hydrogen activation as a potential mechanism. To obtain
thermodynamic parameters and to ascertain the reaction order
with respect to hydrogen, a series of experiments were
After multiple attempts, no cooperative hydrogen activation
transition state (TS) for mechanism 2 could be identified, even
when substituting Rh for Ir in the optimized TS converting 2a
+ H2 to 8a + intermediates (vide infra). This may be, in part,
due to steric repulsion between the alkyl groups on aluminum
and the triphenylphosphine ligands. In the solid-state structure
of 2a, these functionalities are positioned to minimize this
interaction.27 We then turned our attention to potential
mechanism 3. Our computations suggest that the activation of
H2 occurs either after, or in concert with, dissociation of the
Al(pyridone)Et2(NO3) ligand from Rh. However, no direct
transition state from the 1a + H2 complex could be located that
leads to hydrogen activation and/or dissociation of Al-
Figure 5. First-order linearization of kinetic data for rhodium
consumption obtained from room-temperature hydrogenolysis of
complex 1b.53
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Figure 6. Free energy diagram of proposed mechanisms 1 (left) and 3 (right) for complex 1a. The baseline free energy (in kcal/mol) is complex 1a
+ the free energy of H2 at infinite separation.
(pyridone)Et2(NO3). Only the monohydride complex 7a +
Al(pyridone-H)Et(NO3) has been theoretically characterized,
and the 7a + Al(pyridone-H)Et2(NO3) supermolecule is 15.0
kcal mol−1 higher in free energy than the initial 1a + H2
complex, a lower energy than was calculated for 5a. However,
the effective activation energy for subsequent proton transfer is
36.9 kcal mol−1, which does not agree with the experimental
observation of half-life on the time scale of hours. Given this
information, we believe that mechanism 1 is most likely of the
considered mechanisms to be in operation.
result from further thermal decomposition of Ir(PPh3)(CO)-
(NO3)(H)2, as observed by Eisenberg for Vaska’s complex.50
Similar to the case of rhodium complex 1b, the precipitate was
found to contain nitrogen, suggesting that it is an aluminum−
pyridone byproduct.
The observation of iridium dihydride in the product spectra
suggests that an additional mechanism, proton transfer from a
product dihydride, may occur in the case of Ir. To test the
validity of this hypothesis, we attempted a stepwise generation
of alkane via an iridium dihydride. Complex 3 (Figure 4) was
first exposed to 1 atm of hydrogen in the presence of an
internal integration standard. As noted above, an iridium
dihydride forms readily under these conditions.45 The
hydrogen pressure was then released inside a glovebox and
excess triethylaluminum was added. This resulted in formation
of 3 equiv of ethane (relative to Ir) and disappearance of the
hydride signals. The 3 equiv of ethane can be accounted for as
originating from the two iridium hydrides and the acidic
pyridone O−H proton. This strongly suggests that the
generated product dihydride can serve as an intermediate
along one pathway toward alkane production from 2b. This
potential is unsurprising, given the potential acidity of the
dihydride.46
Further information regarding the reaction mechanism was
obtained by monitoring the reaction progress over time by 1H
NMR spectroscopy. Isobutane was found to form immediately
at ambient temperature concomitant with the loss of signals
derived from parent complex 2b. The major dihydride product
also begins to form at early stages of the reaction (see Figures
S19 and S21 in the Supporting Information). After
approximately 4 h (ambient temperature), a new set of
diastereotopic proton signals can be observed downfield (δ
0.40−0.57) from the diastereotopic proton resonances of 2b (δ
0.15−0.33). On the basis of the coupling pattern and chemical
shift, these can only be attributed to a new tetrahedral
aluminum diisobutyl species. Growing in simultaneously are a
new pair of iridium hydride resonances (δ −8.49 and −18.50).
The hydride peaks are doublets of triplets with small two-bond
coupling constants, indicative of a cis-dihydride iridium bis-
Hydrogenolysis of 2b. As in the case of 1b, the total yield of
isobutane in solution was found to be approximately 60%,
implying that both alkyl groups react productively to form
alkane. Interestingly, despite the similar overall yield, the
reaction profile for complex 2b was found to deviate from that
of 1b. Though both reactions resulted in a yellow solution, a
white precipitate, and the release of isobutane, the nature of
1
the transition-metal product in solution is distinct. The H
NMR spectrum of the product mixture indicates the formation
of several iridium hydride species. The major product in
solution is an iridium bis-phosphine cis-dihydride, as indicated
by a pair of doublet of triplets signals observable at δ −6.53
and −22.23 (see Figure S9 in the Supporting Information).
The observation of hydride products is not surprising, as Ir(I)
complexes in the family of Vaska’s complex are well-known to
oxidatively add hydrogen.52 While the exact formulation of this
product is unknown, we posit that Ir(PPh3)(CO)(NO3)(H)2
is likely. Coordinated pyridone is not observed in the 1H NMR
product spectrum of the reaction of 2b and H2, eliminating
Ir(PPh3)(CO)(2-pyridone)(H)2 as a potential product.
Importantly, dihydrides with identical chemical shifts were
observed after the reaction of 3 with H2, suggesting that
pyridone and nitrate may exchange under these conditions.
This is not surprising, due to the known weakness of the bond
between iridium and the nitrogen of the coordinated pyridone,
and is consistent with observations by Johnson for the related
complex [Ir(PPh3)(CO)(2-pyridone)][OTf−], which crystal-
lized as a dihydrido solvent complex (acetonitrile) after
exposure to H2.27,45 The additional hydride products may
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phosphine in which the phosphines are mutually trans.
Notably, these new hydride resonances integrate with a ratio
of 1:4 with respect to the new set of diastereotopic methylene
protons. This relative ratio remains consistent as the peaks
grow over time. On the basis of these data, we can
unambiguously identify this species as [Ir(PPh3)2(CO)(2-
pyridone-Al(iBu)2)(H)2][NO3] (6b, Figure 7), the product of
hydrogen oxidative addition to 2b (mechanism 1).
Figure 7. Compound 6b, [Ir(PPh3)2(CO)(2-pyridone-Al(iBu)2)-
(H)2][NO3].
As the concentration of 2b decreases, the concentration of
isobutane produced, increases. Isobutane continued to be
generated after all of 2b was consumed. Excitingly, this
continued formation of isobutane corresponded to a decrease
in the intensity of NMR resonances attributable to 6b. The
consumption of 6b was found to be extremely slow at ambient
temperature. Hydrogenolysis of 2b was repeated at 40 °C to
increase the overall rate of the reaction. Under these
conditions, a similar overall kinetic profile was obtained. 6b
was fully consumed after 13 h, and no additional alkane was
produced. This confirmed that 6b serves as an intermediate
along one reaction pathway toward alkane production. A plot
of the concentrations of 2b, isobutane, and 6b over time at
ambient temperature and 40 °C are presented in Figure 8.
The complicated nature of the reaction profile for 2b
prevents derivation of a quantitative rate law. However,
important conclusions can be reached. First, multiple reaction
pathways are likely available that can account for the observed
hydrogenolysis, and they occur in parallel. The slow rate of
disappearance of 6b after 2b is consumed, relative to the initial
rate of alkane production, supports this hypothesis. The
product dihydride may also participate in proton transfer
reactions, as it is formed rapidly over the course of the
reaction. Second, dihydride intermediate 6b is involved in at
least one of the parallel reaction paths. This observable
difference between Ir and Rh is driven by the relative
thermodynamics for dihydride formation in the presence of
H2. Thus, we conclude that, at minimum, mechanism 1
(Scheme 1) is reasonable for this system and that it is likely
occurring in conjunction with at least one other mechanistic
pathway.
Figure 8. Kinetics of reaction of 2b with hydrogen, focusing on
formation of isobutane and intermediate 6b at (A) ambient
temperature and (B) 40 °C.
unsuccessful. Interestingly, a direct transition state leading
from the 2a + H2 supermolecule to 8a + products was located
and computed, but the extraordinarily high ΔG⧧ value of 55.7
kcal mol−1 kinetically rules out this pathway. Given no obvious
computational route to products from the dihydride
intermediate 6a, we must again propose that protonolysis
could proceed following ligand dissociation, with solvent
assistance, via substantial conformational reorganization of
intermediates and fragments, or in a bimolecular fashion.
Because proton transfer occurs after the rate-limiting addition
of hydrogen to the transition metal, we are unable to
experimentally distinguish between these possibilities.
CONCLUSION
■
Figure 9 depicts the computed free energy diagram for
potential initial mechanistic steps along reaction pathways 1
and 3 for the Ir−Al bimetallic system (2a). In this case, the
thermodynamics of dihydride formation (mechanism 1) agree
quite well with experiment, showing an exergonic reaction free
energy (−2.1 kcal mol−1) and a reasonable effective activation
free energy of 25.1 kcal mol−1. Much like the Rh-centered case,
intramolecular transition states leading from the dihydride
species 6a to hydrogenolysis products could not be computa-
tionally located. In contrast to the results for hydrogenation of
1a/1b, efforts to find transition states leading to the
dissociative hydrogen activation of 2a (mechanism 3) were
In conclusion, a unique aluminum−alkyl bond hydrogenolysis
facilitated by late-transition-metal−aluminum heterobimetallic
complexes has been demonstrated. A detailed investigation of
the reaction has demonstrated that the course of the reaction is
dependent on the transition metal employed in the
heterobimetallic structure. While they are exciting as a
standalone demonstration of new reactivity of late-metal−
aluminum bimetallic complexes, the results reported herein
also serve as an important warning for those interested in
catalyst design. Aluminum-containing heterobimetallic com-
plexes are promising systems for the reduction of CO2;17
however, great care must be taken in designing the ligand set at
G
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Inorganic Chemistry
Article
Figure 9. Free energy diagram of proposed mechanisms 1 (left) and 3 (right) for complex 2a. The baseline free energy (in kcal/mol) is complex 2a
+ the free energy of H2 at infinite separation.
ppm (ethane) or a doublet at 0.86 ppm (isobutane) indicated the
presence of alkane.
aluminum, for inclusion of alkyl ligands may lead to unwanted
side reactions and catalyst decomposition.
Hydrogenolysis Control Experiments.
1. In the glovebox, ∼5 mg (0.006 mmol) of [M(PPh3)2(CO)(2-
pyridone)][NO3] was suspended in benzene-d6 in a J. Young
NMR tube. The tube was removed from the glovebox and
evacuated via three freeze−pump−thaw (FPT) cycles using a
homemade J. Young NMR tube pressurizer (see picture on
page S5 in the Supporting Information). The sample was then
pressurized with 1 atm of H2 and an initial spectrum recorded.
The NMR tube was then returned to the glovebox and opened,
releasing the hydrogen pressure. Trialkylaluminum was then
added to the tube, and the sample was evaluated for alkane
production.
EXPERIMENTAL DETAILS
■
Syntheses and manipulations were performed in a nitrogen-filled Inert
Technologies glovebox or using standard Schlenk techniques unless
otherwise specified. Deuterated solvents were purchased from
Cambridge Isotope Laboratories, dried over molecular sieves or
calcium hydride, and stored in the glovebox over molecular sieves
prior to use. Ir(PPh3)2Cl(CO) and Rh(PPh3)2Cl(CO) were
synthesized according to methods established in the literature.55
[Ir(PPh3)2(CO)(2-pyridone)][NO3], [Rh(PPh3)2(CO)(2-
pyridone)][NO3], [Ir(PPh3)2(CO)(2-pyridone-AlEt2)][NO3], [Rh-
(PPh3)2(CO)(2-pyridone-AlEt2)][NO3], [Ir(PPh3)2(CO)(2-pyri-
done-Al(iBu)2)][NO3], and [Rh(PPh3)2(CO)(2-pyridone-Al-
(iBu)2)][NO3] (complexes 1−4) were synthesized by the method
of Brewster et al.27 All other reagents and solvents used were
commercially available and used without further purification unless
specified. Aluminum reagents and 2-pyridone were stored in a
2. In a manner directly analogous to the hydrogenolysis
procedure above, the ability of Al(Et)2(2-pyridone) to facilitate
hydrogen activation was ascertained. In the glovebox, ∼5
mmol of Al(Et)2(2-pyridone) was dissolved in benzene-d6 in a
J. Young NMR tube. The tube was removed from the glovebox
and evacuated and back-filled with hydrogen via three freeze−
pump−thaw cycles using a homemade J. Young NMR tube
pressurizer. The sample was then pressurized with 1 atm of H2
1
nitrogen glovebox prior to use. H NMR spectra were recorded on a
400 MHz JEOL or 500 MHz Varian spectrometer and referenced to
the residual solvent peak.44
1
and evaluated using H NMR spectroscopy.
Synthesis of Al(Et)2(2-pyridone). In the glovebox, 951 mg (10
mmol) of 2-pyridone was suspended in 15 mL of THF. Separately,
0.686 mL of Al(Et)3 was dissolved in 5 mL of THF. The pyridone
suspension was added dropwise to the Al(Et)3 and stirred at room
temperature in the glovebox. Rapid bubbling yielded a pale yellow
solution after 10 min. The solvent was removed in vacuo to give a
white sticky solid. The solid was dried, washed with pentane, and then
dried overnight. The product was recrystallized from benzene layered
with excess pentane in the glovebox freezer (−35 °C). Yield: 0.656 g,
69%. 1H NMR (400 MHz, benzene-d6): δ 7.58 (dd, J = 7.7 Hz, 1H),
6.87−6.81 (m, 1H), 6.51 (d, J = 8.5 Hz, 1H), 6.03−5.98 (m, 1H),
0.40 (q, J = 8.1 Hz, 4H). 13C{1H} NMR (101 MHz, Benzene-d6): δ
166.79, 143.30, 142.95, 118.02, 114.66, 9.61, −0.37 (br).
General Hydrogenolysis Procedure. In the glovebox, ∼5 mg
(0.005 mmol) of [M(PPh3)2(CO)(2-pyridone-AlR2)][NO3] (M = Ir,
Rh; R = ethyl, isobutyl) was suspended in benzene-d6 in a J. Young
NMR tube. The tube was removed from the glovebox and evacuated
and back-filled with hydrogen via three freeze−pump−thaw cycles
using a homemade J. Young NMR tube pressurizer or Schlenk
manifold. The sample was then pressurized with H2 and evaluated
using 1H NMR spectroscopy. Formation of a new singlet peak at 0.80
3. For deuterium labeling, in the glovebox, ∼5 mg (0.005 mmol)
of [M(PPh3)2(CO)(2-pyridone-AlR2)][NO3] (M = Ir, Rh; R
= ethyl) was suspended in freshly dried toluene (dried over
CaH2) in a J. Young NMR tube. The tube was removed from
the glovebox and evacuated and back-filled with D2 via three
freeze−pump−thaw cycles using a homemade J. Young NMR
tube pressurizer. The sample was then pressurized with 1 atm
2
of D2 and evaluated using H NMR spectroscopy. Formation
of a new singlet peak at 0.80 ppm (ethane) indicated the
presence of deuterated alkane.
Reaction Kinetics. In the glovebox, ∼5 mg (0.005 mmol) of 1b
and 2b were suspended in benzene-d6 in separate J. Young NMR
tubes. A 20 μL aliquot of a solution of trimethoxybenzene (∼5 mg) in
benzene-d6 was added as an internal integration standard. The tubes
were removed from the glovebox and evacuated via three freeze−
pump−thaw (FPT) cycles using a homemade J. Young NMR tube
pressurizer. Following the third FPT cycle, the samples were
pressurized with 1 atm of hydrogen gas and kept frozen to prevent
the reaction from beginning. The samples were then placed in an
NMR spectrometer at ambient temperature and monitored by either
H
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Inorganic Chemistry
Article
1H or 31P{1H} NMR spectroscopy. Scans were recorded at 5 min
intervals overnight (15 h) at ambient temperature; subsequent spectra
were recorded at 20, 22, 64, and 136 h post-pressurization.
X-ray Crystallography. A suitable crystal was selected for analysis
and mounted in a polyimide loop. All measurements were made on a
Rigaku Oxford Diffraction Supernova Eos CCD with filtered Mo-Kα
radiation at a temperature of 100 K. Using Olex2,56 the structure was
solved with the ShelXT structure solution program using direct
methods and refined with the ShelXL refinement package57 using
least-squares minimization.
work. The authors thank Prof. Davita Watkins and Mr. Frank
Wiggers of the University of Mississippi for assistance with
acquisition of kinetic data and Mr. Drake Williams of the
University of Memphis for assistance with acquisition of mass
spectrometry data. The High Performance Computing Center
and the Computational Research on Materials Institute at The
University of Memphis (CROMIUM) also provided generous
resources for this research.
Computational Details. All computations were performed using
the Gaussian16 software package.58 Gas-phase energies, optimized
geometries, and unscaled harmonic vibrational frequencies were
obtained using density functional theory (DFT).59 The hybrid B3LYP
functional was used with default grid parameters.60 The basis set for
rhodium and iridium was the Hay and Wadt basis set and effective
core potential (ECP) combination (LANL2DZ)61,62 as modified by
Couty and Hall,63 where the two outermost p functions of Rh and Ir
have been replaced by a split of the optimized 5p/6p function,
respectively. Computations with phosphorus and aluminum used the
standard LANL2DZ basis set/ECP combination. All carbon, nitrogen,
oxygen, and hydrogen atoms utilized the 6-31G(d′) basis set.64 The
Hessian of the energy was computed at all stationary points to
designate them as either minima or transition states (first-order saddle
points). Zero-point energies (ZPE) and thermal enthalpy/free energy
corrections were computed at 1 atm and 298.15 K. Aqueous solvation
energies were computed using the CPCM model with standard cavity
parameters for benzene.65,66
REFERENCES
■
(1) Lin, P.; Burchell, T. J.; Ungur, L.; Chibotaru, L. F.; Wernsdorfer,
W.; Murugesu, M. A Polynuclear Lanthanide Single-Molecule Magnet
with a Record Anisotropic Barrier. Angew. Chem., Int. Ed. 2009, 48,
9489−9492.
(2) Murugesu, M.; Habrych, M.; Wernsdorfer, W.; Abboud, K.;
Christou, G. Single-Molecule Magnets: A Mn-25 Complex With a
Record S = 51/2 Spin for a Molecular Species. J. Am. Chem. Soc. 2004,
126, 4766−4767.
(3) Hernandez Sanchez, R.; Bartholomew, A. K.; Powers, T. M.;
Menard, G.; Betley, T. A. Maximizing Electron Exchange in a [Fe-3]
Cluster. J. Am. Chem. Soc. 2016, 138, 2235−2243.
(4) Cao, Z.; Xi, B.; Jodoin, D. S.; Zhang, L.; Cummings, S. P.; Gao,
Y.; Tyler, S. F.; Fanwick, P. E.; Crutchley, R. J.; Ren, T. Diruthenium-
Polyyn-diyl-Diruthenium Wires: Electronic Coupling in the Long
Distance Regime. J. Am. Chem. Soc. 2014, 136, 12174−12183.
(5) Ying, J.; Liu, I. P.; Xi, B.; Song, Y.; Campana, C.; Zuo, J.; Ren, T.
Linear Trimer of Diruthenium Linked by Butadiyn-Diyl Units: A
Unique Electronic Wire. Angew. Chem., Int. Ed. 2010, 49, 954−957.
(6) Paul, F.; Lapinte, C. Organometallic Molecular Wires and Other
Nanoscale-Sized Devices. An Approach Using The Organoiron
(dppe)Cp*Fe Building Block. Coord. Chem. Rev. 1998, 178, 431−509.
(7) Miller, T.; Ahmed, K.; Wrighton, M. Complexes of Rhenium
Carbonyl Containing Ferrocenyl-Derived Ligands - Tunable Electron-
Density at Rhenium by Control of the Redox State of the Ferrocenyl
Ligand. Inorg. Chem. 1989, 28, 2347−2355.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01359.
■
S
NMR spectra of product mixtures and kinetics experi-
ments, mechanistic data, and Cartesian coordinates of all
computed structures (PDF)
(8) Karunananda, M. K.; Vazquez, F. X.; Alp, E. E.; Bi, W.;
Chattopadhyay, S.; Shibata, T.; Mankad, N. P. Experimental
Determination of Redox Cooperativity and Electronic Structures in
Catalytically Active Cu-Fe and Zn-Fe Heterobimetallic Complexes.
Dalton Trans. 2014, 43, 13661−13671.
Accession Codes
CCDC 1936522 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(9) Mankad, N. P. Non-Precious Metal Catalysts for C-H Borylation
Enabled by Metal-Metal Cooperativity. Synlett 2014, 25, 1197−1201.
(10) Anderson, J. S.; Moret, M.; Peters, J. C. Conversion of Fe-NH2
to Fe-N2 with Release of NH3. J. Am. Chem. Soc. 2013, 135, 534−537.
(11) Barnett, B. R.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S.
Cooperative Transition Metal/Lewis Acid Bond-Activation Reactions
by a Bidentate (Boryl)iminomethane Complex: A Significant Metal-
Borane Interaction Promoted by a Small Bite-Angle LZ Chelate. J.
Am. Chem. Soc. 2014, 136, 10262−10265.
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.P.B.: tbrwster@memphis.edu.
■
ORCID
(12) Rudd, P. A.; Planas, N.; Bill, E.; Gagliardi, L.; Lu, C. C.
Dinitrogen Activation at Iron and Cobalt Metallalumatranes. Eur. J.
Inorg. Chem. 2013, 2013, 3898−3906.
R. Malcolm Charles III: 0000-0002-1919-7507
Nathan D. Schley: 0000-0002-1539-6031
Nathan J. DeYonker: 0000-0003-0435-2006
Timothy P. Brewster: 0000-0002-7654-4106
(13) Tsui, E. Y.; Kanady, J. S.; Agapie, T. Synthetic Cluster Models
of Biological and Heterogeneous Manganese Catalysts for O2
Evolution. Inorg. Chem. 2013, 52, 13833−13848.
Notes
(14) Gloaguen, F.; Lawrence, J.; Rauchfuss, T. Biomimetic
Hydrogen Evolution Catalyzed By an Iron Carbonyl Thiolate. J.
Am. Chem. Soc. 2001, 123, 9476−9477.
The authors declare no competing financial interest.
(15) Ghosh, P.; Ding, S.; Chupik, R. B.; Quiroz, M.; Hsieh, C.;
Bhuvanesh, N.; Hall, M. B.; Darensbourg, M. Y. A Matrix of
Heterobimetallic Complexes for Interrogation of Hydrogen Evolution
Reaction Electrocatalysts. Chem. Sci. 2017, 8, 8291−8300.
(16) Hara, N.; Saito, T.; Semba, K.; Kuriakose, N.; Zheng, H.;
Sakaki, S.; Nakao, Y. Rhodium Complexes Bearing PAIP Pincer
Ligands. J. Am. Chem. Soc. 2018, 140, 7070−7073.
ACKNOWLEDGMENTS
■
This work was supported in full or in part by the National
Science Foundation under Grant No. CHE-1531466 and a
grant from The University of Memphis College of Arts and
Sciences Research Grant Fund. This support does not
necessarily imply endorsement by the University of research
conclusions. T.P.B. and N.J.D. also thank the University of
Memphis for start-up funding which partially supported this
(17) Takaya, J.; Iwasawa, N. Synthesis, Structure, and Catalysis of
Palladium Complexes Bearing a Group 13 Metalloligand: Remarkable
I
DOI: 10.1021/acs.inorgchem.9b01359
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Effect of an Aluminum-Metalloligand in Hydrosilylation of CO2. J.
Am. Chem. Soc. 2017, 139, 6074−6077.
(37) Sircoglou, M.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Bourissou,
D. A Zwitterionic Gold(I) Complex From an Ambiphilic
Diphosphino-Alane Ligand. Organometallics 2008, 27, 1675−1678.
(38) Devillard, M.; Nicolas, E.; Appelt, C.; Backs, J.; Mallet-Ladeira,
S.; Bouhadir, G.; Slootweg, J. C.; Uhl, W.; Bourissou, D. Novel
Zwitterionic Complexes Arising from the Coordination of an
Ambiphilic Phosphorus-Aluminum Ligand to Gold. Chem. Commun.
2014, 50, 14805−14808.
(39) Layh, M.; Uhl, W.; Bouhadir, G.; Bourissou, D. Organo-
aluminum Compounds and Lewis Pairs. In The Chemistry of
Organoaluminum Compounds; Micouin, L., Marek, I., Rappoport, Z.,
Eds.; Wiley: Hoboken, NJ, 2017; pp 379−424.
(40) Menard, G.; Stephan, D. W. C-H Activation of Isobutylene
Using Frustrated Lewis Pairs: Aluminum and Boron σ-Allyl
Complexes. Angew. Chem., Int. Ed. 2012, 51, 4409−4412.
(41) Stephan, D. W.; Erker, G. Frustrated Lewis Pair Chemistry:
Development and Perspectives. Angew. Chem., Int. Ed. 2015, 54,
6400−6441.
(18) Rudd, P. A.; Liu, S.; Gagliardi, L.; Young, V. G., Jr.; Lu, C. C.
Metal-Alane Adducts with Zero-Valent Nickel, Cobalt, and Iron. J.
Am. Chem. Soc. 2011, 133, 20724−20727.
(19) Burlitch, J. M.; Leonowicz, M. E.; Petersen, R. B.; Hughes, R. E.
Coordination of Metal Carbonyl Anions to Triphenylaluminum,
-gallium, and -indium and the Crystal Structure of Tetraethylammo-
nium Triphenyl((η5-cyclopentadienyl)dicarbonyliron)aluminate (Fe-
Al). Inorg. Chem. 1979, 18, 1097−1105.
(20) Fong, H.; Moret, M.; Lee, Y.; Peters, J. C. Heterolytic H2
Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane.
Organometallics 2013, 32, 3053−3062.
(21) Suess, D. L. M.; Peters, J. C. H-H and Si-H Bond Addition to
Fe NNR2 Intermediates Derived from N2. J. Am. Chem. Soc. 2013,
135, 4938−4941.
(22) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of
Nitrogen to Ammonia by an Iron Model Complex. Nature 2013, 501,
84−87.
(42) Stephan, D. W. Frustrated Lewis Pairs: From Concept to
Catalysis. Acc. Chem. Res. 2015, 48, 306−316.
(43) Menard, G.; Stephan, D. W. H2 Activation and Hydride
Transfer to Olefins by Al(C6F5)3-Based Frustrated Lewis Pairs. Angew.
Chem., Int. Ed. 2012, 51, 8272−8275.
(23) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. Homogeneous CO
Hydrogenation: Dihydrogen Activation Involves a Frustrated Lewis
Pair Instead of a Platinum Complex. J. Am. Chem. Soc. 2010, 132,
3301−3303.
(24) Bai, G.; Singh, S.; Roesky, H.; Noltemeyer, M.; Schmidt, H.
Mononuclear Aluminum Hydroxide for the Design of Well-Defined
Homogeneous Catalysts. J. Am. Chem. Soc. 2005, 127, 3449−3455.
(25) Devillard, M.; Declercq, R.; Nicolas, E.; Ehlers, A. W.; Backs, J.;
Saffon-Merceron, N.; Bouhadir, G.; Slootweg, J. C.; Uhl, W.;
Bourissou, D. A Significant but Constrained Geometry Pt -> Al
Interaction: Fixation of CO2 and CS2, Activation of H2 and
PhCONH2. J. Am. Chem. Soc. 2016, 138, 4917−4926.
(26) Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.;
Howard, J. A. K.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Ambiphilic
Diphosphine-Borane Ligands: Metal -> Borane Interactions Within
Isoelectronic Complexes of Rhodium, Platinum and Palladium. Chem.
- Eur. J. 2008, 14, 731−740.
(27) Brewster, T. P.; Nguyen, T. H.; Li, Z.; Eckenhoff, W. T.; Schley,
N. D.; DeYonker, N. J. Heterobimetallic Aluminum Complexes of
Iridium and Rhodium: Synthesis, Characterization, and Reactivity.
Inorg. Chem. 2018, 57, 1148−1157.
(28) Cowie, B. E.; Emslie, D. J. H. Nickel and Palladium Complexes
of Ferrocene-Backbone Bisphosphine-Borane and Trisphosphine
Ligands. Organometallics 2015, 34, 4093−4101.
(29) Cowie, B. E.; Tsao, F. A.; Emslie, D. J. H. Synthesis and
Platinum Complexes of an Alane-Appended 1, 1 ‘-Bis(phosphino)-
ferrocene Ligand. Angew. Chem., Int. Ed. 2015, 54, 2165−2169.
(30) Fischer, R. A.; Weiss, J. Coordination Chemistry of Aluminum,
Gallium, and Indium at Transition Metals. Angew. Chem., Int. Ed.
1999, 38, 2830−2850.
(31) Bauer, G.; Nieger, M.; Gudat, D. Heterobimetallic Catechol-
Phosphine Complexes with Palladium and a Group-13 Element:
Structural Flexibility and Dynamics. Dalton Trans. 2014, 43, 8911−
8920.
(32) Fryzuk, M.; Gao, X.; Joshi, K.; MacNeil, P.; Massey, R.
Synthesis and Reactivity of the Coordinatively Unsaturated
Methylene Complex Ir = CH2[N(SiMe2Ch2PPh2)2]. J. Am. Chem.
Soc. 1993, 115, 10581−10590.
(33) Cammarota, R. C.; Clouston, L. J.; Lu, C. C. Leveraging
Molecular Metal-Support Interactions for H2 and N2 Activation.
Coord. Chem. Rev. 2017, 334, 100−111.
(44) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR
Chemical Shifts of Trace Impurities: Common Laboratory Solvents,
Organics, and Gases in Deuterated Solvents Relevant to the
Organometallic Chemist. Organometallics 2010, 29, 2176−2179.
(45) Forrest, S. J. K.; Manojveer, S.; Johnson, M. T. Cooperative or
Oxidative Hydrogen Addition to 2-Hydroxypyridonate Iridium
Complexes: Dependence on Oxidation State. Eur. J. Inorg. Chem.
2017, 2017, 3239−3243.
(46) Morris, R. H. Bronsted-Lowry Acid Strength of Metal Hydride
and Dihydrogen Complexes. Chem. Rev. 2016, 116, 8588−8654.
(47) Gellrich, U. Reversible Hydrogen Activation by a Pyridonate
Borane Complex: Combining Frustrated Lewis Pair Reactivity with
Boron-Ligand Cooperation. Angew. Chem., Int. Ed. 2018, 57, 4779−
4782.
(48) Heaton, B. T.; Iggo, J. A.; Jacob, C.; Blanchard, H.; Hursthouse,
M. B.; Ghatak, I.; Harman, M. E.; Somerville, R. G.; Heggie, W.; Page,
P. R.; Villax, I. Structural and Spectroscopic Studies of Rhodium-(I)
and -(III) Nitrato Complexes. J. Chem. Soc., Dalton Trans. 1992,
2533−2537.
(49) Cambridge Crystallographic Data Centre Deposition Number
1289270, 1993.
(50) Duckett, S. B.; Eisenberg, R. Observation of H2 Oxidative
Addition to Chlorocarbonylbis(triphenylphosphine)rhodium(i) using
Parahydrogen-Induced Polarization. J. Am. Chem. Soc. 1993, 115,
5292−5293.
(51) de Charentenay, F.; Osborn, J. A.; Wilkinson, G. Interaction of
Silanes with Tris(triphenylphosphine)chlororhodium(1) and Other
Rhodium Complexes - Hydrosilation of Hex-1-Ene by use of
Trichlorosilane. J. Chem. Soc. A 1968, 787−790.
(52) Vaska, L. Reversible Activation of Covalent Molecules by
Transition Metal Complexes. The Role of the Covalent Molecule. Acc.
Chem. Res. 1968, 1, 335−344.
(53) In the experiment described above, the hydrogen pressure was
not directly measured. Reliable rate information with respect to the
hydrogen concentration could not be obtained.
(54) Hydrogen and Deuterium. International Union of Pure and
Applied Chemistry Solubility Data Series; Young, C. L., Ed.; Pergamon
Press: Elmsford, NY, 1981; Vol. 5/6, p 646.
(55) Collman, J.; Sears, C.; Kubota, M. Trans-Carbonylchlorobis-
(triphenylphosphine)iridium. Inorg. Synth. 2007, 28, 92−94.
(56) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.
K.; Puschmann, H. OLEX2: A Complete Structure Solution,
Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42,
339−341.
(34) Moore, J. T.; Smith, N. E.; Lu, C. C. Structure and Dynamic
NMR Behavior of Rhodium Complexes Supported by Lewis Acidic
Group 13 Metallatranes. Dalton Trans. 2017, 46, 5689−5701.
(35) Cammarota, R. C.; Lu, C. C. Tuning Nickel with Lewis Acidic
Group 13 Metalloligands for Catalytic Olefin Hydrogenation. J. Am.
Chem. Soc. 2015, 137, 12486−12489.
(36) Donets, P. A.; Cramer, N. Diaminophosphine Oxide Ligand
Enabled Asymmetric Nickel- Catalyzed Hydrocarbamoylations of
Alkenes. J. Am. Chem. Soc. 2013, 135, 11772−11775.
J
DOI: 10.1021/acs.inorgchem.9b01359
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(57) Sheldrick, G. A Short History of SHELX. Acta Crystallogr., Sect.
A: Found. Crystallogr. 2008, 64, 112−122.
(58) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson,
G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.;
Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J.
V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;
Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson,
T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.;
Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Throssell, K.; Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark,
M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.;
Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell,
A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.;
Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. GAUSSIAN 16,
Revision B.01; Gaussian Inc., Wallingford, CT. 2016.
(59) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(60) Becke, A. Density-Functional Thermochemistry. 3. the Role of
Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(61) Hay, P.; Wadt, W. Abinitio Effective Core Potentials for
Molecular Calculations - Potentials for K to Au Including the
Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299−310.
(62) Hay, P.; Wadt, W. Abinitio Effective Core Potentials for
Molecular Calculations - Potentials for the Transition-Metal Atoms Sc
to Hg. J. Chem. Phys. 1985, 82, 270−283.
(63) Couty, M.; Hall, M. Basis sets for transition metals: Optimized
outer p functions. J. Comput. Chem. 1996, 17, 1359−1370.
(64) Hariharan, P. C.; Pople, J. A. Influence of Polarization
Functions on Molecular-Orbital Hydrogenation Energies. Theor.
Chim. Acta 1973, 28, 213−222.
(65) Barone, V.; Cossi, M. Quantum Calculation of Molecular
Energies and Energy Gradients in Solution by a Conductor Solvent
Model. J. Phys. Chem. A 1998, 102, 1995−2001.
(66) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies,
Structures, and Electronic Properties of Molecules in Solution with
the C-PCM Solvation Model. J. Comput. Chem. 2003, 24, 669−681.
K
DOI: 10.1021/acs.inorgchem.9b01359
Inorg. Chem. XXXX, XXX, XXX−XXX