The Acetate Proton Shuttle between Mutually Trans Ligands

Article abstract of DOI:10.1021/acs.organomet.8b00417

This work addresses a counterintuitive observation in the reactivity of the well-known ruthenium complexes [Ru(X)H(CO)(PiPr₃)₂], according to which the 5-coordinate chloro complex (X = Cl, 1) is less reactive toward phenylacetylene than its 6-coordinate acetate analogue (X = O₂-OC(O)Me, 3), since 3 undergoes a hydride-to-alkenyl-to-alkynyl transformation, whereas the reaction of 1 stops at the alkenyl derivative. The experimental kinetics of the key alkenyl-to-alkynyl step in the acetate complex are compared to the results of DFT calculations, which disclose the ability of the acetate not only to assist the alkyne C-H activation step via a CMD mechanism but also to subsequently deliver the proton to the alkenyl ligand. Possible consequences of this mechanistic resource connecting mutually trans ligands are briefly discussed on the basis of reported chemoselectivity changes induced by carboxylate ligands in 1-alkyne hydrosilylations catalyzed by this type of ruthenium complexes.

Full text of DOI:10.1021/acs.organomet.8b00417

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
The Acetate Proton Shuttle between Mutually Trans Ligands
†,‡
Adiran de Aguirre,† Silvia Díez-Gonzalez,
Feliu Maseras,*,† Marta Martín,§
́
and Eduardo Sola*,§
†Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Catalans 16,
43007 Tarragona, Spain
‡Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London SW7 2Az, U.K.
§
́
́
Instituto de Síntesis Química y Catalisis Homogenea, CSIC − Universidad de Zaragoza, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: This work addresses a counterintuitive obser-
vation in the reactivity of the well-known ruthenium
complexes [Ru(X)H(CO)(PiPr3)2], according to which the
5-coordinate chloro complex (X = Cl, 1) is less reactive
toward phenylacetylene than its 6-coordinate acetate analogue
(X = κO2-OC(O)Me, 3), since 3 undergoes a hydride-to-
alkenyl-to-alkynyl transformation, whereas the reaction of 1
stops at the alkenyl derivative. The experimental kinetics of
the key alkenyl-to-alkynyl step in the acetate complex are compared to the results of DFT calculations, which disclose the ability
of the acetate not only to assist the alkyne C−H activation step via a CMD mechanism but also to subsequently deliver the
proton to the alkenyl ligand. Possible consequences of this mechanistic resource connecting mutually trans ligands are briefly
discussed on the basis of reported chemoselectivity changes induced by carboxylate ligands in 1-alkyne hydrosilylations
catalyzed by this type of ruthenium complexes.
shuttle),14 has been recognized key in regioselective catalytic
INTRODUCTION
■
hydrations of terminal alkynes via ligand-facilitated vinylidene
tautomers.15 Besides these possibilities, other destinations for
the proton remain still conceivable. In particular, its transfer to
a neighboring ligand seems a likely process that might unlock
new reaction sequences and trigger more elaborate catalytic
transformations.
This work shows that acetate can indeed play such a
sophisticated role as functional ligand by offering a lone pair to
break C−H bonds and subsequently delivering the proton to a
neighboring ligand. These observations have been extracted
from the behavior of the known ruthenium(II) hydride
complex [Ru{κO2-OC(O)Me}H(CO)(PiPr3)2]16 and its
alkenyl17 and alkynyl derivatives,18 whose acetates drive the
exchange of protons between ligands coordinating mutually
trans. Such a process overcomes a typical reactivity limitation
in octahedral coordination environments, which results from
the preferred location of vacant coordination sites trans to
strongly σ-donor ligands such as hydrides or alkenyls.19
Accordingly, carboxylate ligands may not only enhance
reactivity and change regioselectivity10,12,14 but also modify
chemoselectivity.20
The lone electron pairs in ligands in the first or second
coordination sphere often play a role in the activation of σ
bonds by unsaturated transition metal complexes.1 In fact, with
the exception of some electron-rich derivatives able to offer at
the metal both an empty orbital for reagent coordination and
suitable electron density to break bonds via oxidative addition,
most complexes must bring into play electrons outside the
metal to complete bond cleavages.2 Depending on where in the
complex these electrons are found, the bond cleavage may
receive different mechanistic descriptors. In the particular field
of C−H bond activations, for example,3 1,2-additions, σ-bond
metathesis, and σ-CAM (σ-coordination-assisted metathesis)4
mechanisms utilize electrons formerly in metal−ligand bonds,
while the so-called electrophilic substitutions5 and the CMD
(concerted metalation deprotonation)6 or AMLA (ambiphilic
metal ligand activation)7 alternatives turn on lone pairs in
ligands or external bases. All of these processes are very similar
and resemble bond cleavages accomplished by cooperating
Lewis pairs,8 the metal being the Lewis acid.9
Carboxylates are frequently used as assistant ligands for
bond activations,10 particularly in C−C forming reactions
catalyzed by palladium complexes via direct arylation.11 While
the role of the carboxylate during the C−H activation step has
been analyzed in depth,12 it is commonly assumed that the
resulting carboxylic acid merely transfers the proton to a
sacrificial base.13 Yet, this may not occur if, for instance, the
proton returns to the original ligand to cause its tautomeriza-
tion. Such a process, termed LAPS (ligand-assisted proton
RESULTS AND DISCUSSION
■
The five-coordinate chloro complex [Ru(Cl)H(CO)(PiPr3)2]
(1) was reported to form the alkenyl derivative [Ru(Cl)(E-
Received: June 16, 2018
© XXXX American Chemical Society
A
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CHCHPh)(CO)(PiPr3)2] (2) after insertion of just 1 equiv
The subsequent transformation of 5 into the alkenyl ester
complex 6 was found to be slow, taking several days at room
temperature. All complexes in Scheme 1 had been previously
reported,16−18 although 4−6 were synthesized by procedures
other than the present reaction sequence. This hydride →
alkenyl → alkynyl sequence is quite common in the chemistry
of ruthenium and frequently ends in butenynyl complexes
formed after the coupling of two alkyne moieties.22,23 Actually,
this is the case for the analogue of 2 bearing PPh3 instead of
PiPr3 ligands, which was found to form a five-coordinate
butenynyl derivative under forcing reaction conditions.24
Complex 2, however, did not undergo reaction even in the
presence of excess alkyne at high temperatures.
of phenylacetylene (Scheme 1).21 In contrast, the six-
Scheme 1
To rationalize the counterintuitive reactivity difference
between 2 and 4, their reactions with phenylacetylene were
studied by DFT methods. Initially, we computed a series of
free energy profiles on model complexes with PMe3 as
phosphine (calculations labeled with the sufix m) to explore
different mechanistic scenarios. After, we calculated the most
likely option with the experimental PiPr3 system, for a
straightforward comparison with the obtained kinetic data.
Some of the computed profiles are presented here, while others
are included as Supporting Information.
The insertion of phenylacetylene into the Ru−H bond of
this type of complexes has been previously calculated and
discussed by Eisenstein, Caulton, and co-workers.25 As detailed
for 1m in Figure 2, the reaction is feasible after distortion of
the square-pyramidal ground-state geometry of the complex to
allow the incoming alkyne coordinate cis to the hydride. The
most favorable option is opening the vacancy trans to CO to
attain intermediate I1m. Then, the alkyne would insert
smoothly into the Ru−H bond, resulting in alkenyl complex
2m, 24.5 kcal mol−1 below the reactants. The calculated profile
in Figure 2 continues with the nonobserved alkenyl-to-alkynyl
metathesis step in 2m, which would be exergonic by 11.7 kcal
mol−1, but would require a hardly attainable activation barrier
of at least 27.3 kcal mol−1, probably higher for the more
sterically encumbered real complex 2.
The profile for the model acetate complex 3m in this
mechanistic scenario based on distortions of the complexes
ground-state geometries is qualitatively similar to that of the
chloro analogue, although activation barriers increase by about
6−8 kcal mol−1 because of the extra oxygen atom coordination
to the sixth position of the stable complexes. The initial
phenylacetylene insertion has an overall activation barrier of
16.2 kcal mol−1, the highest point corresponding to the first
transition state TS3m‑I3m. This value still corresponds to a
reaction possible at room temperature. On the contrary, the
calculated barrier for the (experimentally observed) alkenyl-to-
alkynyl step is as high as 34.0 kcal mol−1 from intermediate 4m
coordinate acetate analogue [Ru{κO2-OC(O)Me}H(CO)-
(PiPr3)2] (3) was observed to further react with a second,
and even a third equivalent of this reagent. Monitoring of the
latter by 31P NMR in toluene-d8 solution indicated that
formation of alkenyl complex [Ru{κO2-OC(O)Me}(E-CH
CHPh)(CO)(PiPr3)2] (4) and its consumption to form
alkynyl derivative [Ru{κO2-OC(O)Me}(CCPh)(CO)-
(PiPr3)2] (5) occur at comparable rates. As detailed in Figure
1, the kinetic features of the transformation of 4 into 5 are
consistent with the expected second-order reaction between
the alkenyl precursor and the alkyne.
to transition state TSI4′m‑5m
.
An alternative evolution of 4 based on the possible
assistance of the acetate in the activation of the second
molecule of phenylacetylene was hence explored. As a result,
we found not only the expected facile alkyne C−H activation
via concerted metalation deprotonation (CMD)12 but also a
feasible pathway for the subsequent transfer of the proton to
the alkenyl ligand, forming styrene and the alkynyl complex 5.
This transfer is formally similar to previously reported
examples of ligand-assisted proton shuttle (LAPS),14,15
although, in this case, the shuttle connects different ligands
instead of different carbons of the same ligand. The structures
and free energies calculated for this new reaction sequence in
Figure 1. (above) Dependence upon phenylacetylene concentration
of the experimental pseudo-first-order rate constants (kobs) for the
transformation of 4 into 5. (below) Eyring analysis of the second-
order rate constants.
B
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Figure 2. Free energy profiles (in kcal mol−1) for the hydride-to-alkenyl-to-alkynyl sequence in PMe3 models of 5-coordinate chloro complex 1m
(green) and 6-coordinated acetate complex 3m (black), through a direct mechanism that generates vacant sites cis to hydride and alkenyl ligands.
Figure 3. Free energy profile (in kcal mol−1) for the alkenyl-to-alkynyl transformation in 4, through a mechanism featuring C−H activation (CMD)
and proton shuttle (LAPS) steps assisted by the acetate ligand. Energy relative to the precursor hydride complex 3.
the real PiPr3 system are summarized in Figure 3. A further key
difference between this mechanism and that in Figure 2 is the
initial alkyne coordination, which can now take place at the
most readily accessible position of 4, trans to the alkene ligand.
The energy profile of Figure 3 features two high lying
transition states, TSI6‑I7 and TSI7‑5, with similar barriers (from
4) of 25.5 and 26.4 kcal mol−1, respectively. Both barriers are
only slightly above the experimental estimation: 23.7 ( 2.1)
kcal mol−1, thus being good candidates for the rate-limiting
step. Decomposition of free energies in enthalpic and entropic
terms suggests that both TS are equally compatible with
experiment. Experimental activation parameters are ΔH⧧
=
16.2 ( 1.3) kcal mol−1 and TΔS⧧ = 7.1 ( 0.8) kcal mol−1,
while the respective calculated values are 12.0 and 13.6 kcal
mol−1 for TSI6‑I7, and 12.8 and 13.6 kcal mol−1 for TSI7‑5
.
Clearly, entropic effects are overestimated in the calculation,
but the signs of the contributions are correct. A further piece of
evidence coming from experiment is the kinetic isotopic effect
observed using PhCCD as reagent: kobsH/kobsD = 1.6. The
value seems too close to 1 for a rate-limiting C−H or O−H
C
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bond breaking step,12,26 thus discarding TSI4‑I5 and TSI7‑5
especially after confirming that their geometries are not
particularly early.27 Accordingly, the KIE points to TSI6‑I7 as
the highest energy point of the mechanism.
,
expand the reactivity of complexes and modify their catalytic
performance.
EXPERIMENTAL SECTION
■
This TSI6‑I7, associated with the return of the acetic acid to
the ruthenium coordination sphere, constitutes the only
qualitative discrepancy between the reaction profile obtained
from the real system and that derived from the PMe3 model. In
the latter (Figure S1), the acetic acid fragment of intermediate
I5m can rotate around the Ru−O bond to bring the proton
close to the alkenyl group, whereas this is not feasible in the
real intermediate I5 because of steric reasons. Alternative
mechanisms where the acetate “slips” in the metal coordination
sphere without rotation were explored without success.
Instead, it is better for the acetic acid to separate from
ruthenium and return with the proper orientation in I7.
Calculations in the PMe3 model produced an overall activation
barrier much lower than the experimental (19.9 kcal mol−1),
while calculated structures close to the TS for acetic acid
rotation seemed sterically crowded and likely sensitive to the
phosphine size. The steric properties of the real PiPr3 system
were also found to increase the calculated free energy barrier
for the first alkyne insertion in 3 to form 4, up to 23.2 kcal
mol−1 (Figure S2), although the shape of the energy profile
does not change with respect to that in Figure 2. We also
explored the possible occurrence of the CMD/LAPS sequence
in complex 2 (Figure S3). The barrier is prohibitively high
(34.8 kcal mol−1) because the chloride ligand is much less
basic than acetate, and thus less efficient as proton acceptor.
Viewed as a mere hydrogen transfer, the overall process of
Figure 3 has several well-characterized precedents in solvent-,
counteranion-, and ligand-assisted tautomerizations and related
reactions.14,15,28 However, when considering that the origin
and destination of the hydrogen atom are two trans ligands, the
case emerges as a simple paradigm to circumvent the
ubiquitous energy-consuming distortions required to accom-
modate reactive ligands in relative cis positions of catalytically
active complexes. Consequences of this extra mechanistic
resource of the acetate can be inferred from the reported
catalytic behavior of this type of complexes in 1-alkyne
hydrosilylations.29,30 The replacement of chloride by acetate in
[Ru(X)H(CO)(PPh3)n] catalyst precursors was observed to
change the chemoselectivity in the addition of triethylsilane to
phenylacetylene, favoring products of dehydrogenative silyla-
tion (PhCCSiEt3 and styrene) over those of hydrosilylation
(Z- and E-alkenylsilanes).20 Such a selectivity change is an
expected result of ease alkenyl-to-alkynyl metathesis, since the
reaction produces styrene together with alkynyl complexes:
proposed key intermediates in dehydrogenative silylations.31 In
agreement with this, we have confirmed (by 1H and 31P NMR)
that both alkenyl 4 and alkynyl 5 react with excess
triethylsilane to regenerate hydride 3, thus closing competing
catalytic cycles for phenylacetylene hydrosilylation and
dehydrogenative silylation, respectively.
All manipulations were carried out with exclusion of air by using
standard Schlenk techniques. Solvents were obtained from a solvent
purification system (MBraun). Deuterated solvents were dried with
appropriate drying agents and degassed with argon prior to use. NMR
spectra were recorded on a Bruker Avance 300 MHz spectrometer.
GC−MS analysis was carried out in an Agilent 6890 Series GC
System, equipped with an Agilent 5973 Network Mass Selective
Detector, using a cross-linked methyl silicone gum capillary column.
Complex 4 was prepared from 2 and sodium acetate, as detailed in
ref 17. Complex 5 was prepared by treatment of 4 (191 mg, 0.31
mmol) with phenylacetylene (73 μL, 0.66 mmol) in 5 mL of toluene.
After 4 h of reaction at 353 K, the solution was taken to dryness and
the residue was treated with cool hexane, decanted, and dried in vacuo
(yield 152 mg, 80%). The 1H, 31P{1H}, and 13C{1H} NMR spectra of
the resulting yellow solid matched those previously reported for 5 in
ref 18. The reaction between 4 and triethylsilane excess in toluene-d8
at room temperature was observed by H and 31P NMR to cleanly
1
produce the E-alkenylsilane and complex 3, though this outcome
slowly evolved into other unidentified products. Similarly, complex 5
and triethylsilane excess reacted to initially form 3 (NMR) and
PhCCSiEt3 (GC−MS), though these products readily disappeared
to form an intricate mixture of several other products.
Kinetics. The reactions of 4 with phenylacetylene excess were
followed by monitoring the decrease in the intensity of the 31P{1H}
NMR signal of 4 in toluene-d8 solution. Samples were prepared from
aliquots of a 4.14 × 10−2 M common solution of 4 in this solvent.
After addition of the corresponding phenylacetylene excess, sample
volumes were adjusted to 0.5 mL. kobs values (Table 1) were obtained
Table 1. Experimental Kinetic Data for the Reactions of
Complex 4 with Phenylacetylene
T (K) [PhCCH] (M)
kobs (s−1
)
k
obs/[PhCCH] (M−1 s−1
)
313
323
323
331
331
331
343
1.48
1.64 × 10−4
3.71 × 10−4
2.36 × 10−4
3.56 × 10−4
7.43 × 10−4
1.09 × 10−3
1.72 × 10−3
1.11 × 10−4
2.51 × 10−4
1.48
1.48 (PhCCD)
0.73
1.48
2.19
1.48
5.02 × 10−4
1.16 × 10−3
from exponential fittings of the intensity vs time data. Errors in the
magnitudes obtained from the Eyring regressions were estimated
through conventional error propagation formulas,32 assuming 1 K
error in the temperature and a 10% error in the rate constant.
Computational Details. All calculations were carried out using
the Gaussian09 (Rev. D01) package.33 The ωB97x-D level of theory
was used for all calculations.34 All the structures were optimized in
solution (toluene: ε = 2.3741), using the SMD implicit solvation
model.35 Vibrational frequency calculations for all stationary points
were carried out in order to assign their nature as minima (zero
imaginary frequencies) or as transition state (one imaginary
frequency). The 6-31+G(d) basis set36 for H, C, P, O, and Cl
atoms and SDD for Ru atom and its corresponding pseudopotential37
were used for optimizations and frequency calculations. For the real
system (PiPr3 ligand), potential energies were further refined through
single point calculations in solution using the 6-311++G(d,p) basis
set38 for H, C, P, O, and Cl atoms. All the reported energies are free
energies in solution at 298 K and 1 atm. The accurate calculation of
free energies of reaction in solution is still a topic of discussion,39,40
and a number of refinements have been proposed, some including
scaling factors for specific functionals.41 The issue is not critical for
the interpretation of the results in the current paper. A data set
collection of all computational data is available in the ioChem-BD
CONCLUSION
■
Our results show that acetate ligands can not only assist C−H
bond activations at adjacent metal coordination positions but
also subsequently deliver the H atom to other neighboring
ligands. In particular, this can enable reactions between ligands
mutually trans to each other, not feasible through conventional
concerted elementary steps. This additional resource can
D
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Article
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A set of additional free energy profiles and the
corresponding discussion (PDF)
All computed Cartesian coordinates (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: fmaseras@iciq.es (F.M.).
*E-mail: sola@unizar.es (E.S.).
■
ORCID
Adiran de Aguirre: 0000-0001-7991-6406
́
Silvia Díez-Gonzalez: 0000-0003-3950-5156
Feliu Maseras: 0000-0001-8806-2019
Marta Martín: 0000-0001-7819-670X
Eduardo Sola: 0000-0001-5462-6189
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors would like to dedicate this work to Professor
Ernesto Carmona on the occasion of his 70th birthday. The
authors are thankful for financial support from Ministerio de
́
Economıa, Industria y Competitividad (MINECO)/Fondo
Europeo de Desarrollo Regional (FEDER) (Grants CTQ2015-
64486-R and CTQ2017-87792-R), MINECO-Severo Ochoa
Excellence Accreditation 2014-2018 (Grant SEV-2013-0319),
́
and Gobierno de Aragon/FEDER (E08_17R). A. de A. thanks
MINECO for a FPI fellowship (BES-2015-073012).
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