Acceleration of nucleophilic CH activation by strongly basic solvents

Article abstract of DOI:10.1021/ja102518m

(IPI)Ru(II)(OH)_n(H₂O)_m, 2, where IPI is the NNN-pincer ligand, 2,6-diimidizoylpyridine, is shown to catalyze H/D exchange between hydrocarbons and strongly basic solvents at higher rates than in the case of the solvent alo

Full text of DOI:10.1021/ja102518m

Published on Web 08/24/2010
Acceleration of Nucleophilic CH Activation by Strongly Basic Solvents
Brian G. Hashiguchi,‡ Kenneth J. H. Young,† Muhammed Yousufuddin,§ William A. Goddard III,| and
Roy A. Periana*,‡
The Scripps Energy Laboratories, The Scripps Research Institute, Scripps Florida, Jupiter, Florida 33458,
Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089, Center for
Nanostructured Materials, The UniVersity of Texas at Arlington, Arlington, Texas 76019, and Materials and Process
Simulation Center, California Institute of Technology, Pasadena, California 91125
Received March 26, 2010; E-mail: rperiana@scripps.edu
Abstract: (IPI)Ru(II)(OH)n(H2O)m, 2, where IPI is the NNN-pincer
ligand, 2,6-diimidizoylpyridine, is shown to catalyze H/D exchange
between hydrocarbons and strongly basic solvents at higher rates
than in the case of the solvent alone. Significantly, catalysis by 2
is accelerated rather than inhibited by increasing solvent basicity.
The evidence is consistent with the reaction proceeding by base
modulated nucleophilic CH activation.
There is ongoing interest in metal mediated CH activation1 reactions
as a platform for designing hydrocarbon functionalization catalysts.
We recently suggested that the various named CH activation reactions
can be broadly classified on a continuum of electrophilic, ambiphilic
Figure 1. Conceptual diagram showing how ligand protonation by acids
or nucleophilic character depending on the net direction of charge
transfer between the CH bond and the metal fragment in the transition
state for CH cleavage.2 The most effective catalysts reported for the
selective, facile conversion of alkanes to oxygenates require strongly
acidic solvents and operate by electrophilic CH activation followed
by functionalization of the M-R intermediates.1d An important basis
for the efficiency of these systems is that both CH activation and
functionalization are compatible with and accelerated by strong acid
solvents. The key issue with these electrophilic systems, as with most
known CH activation systems, is inhibition by coordinating species
(such as H2O and alcohols) through formation of stabilized ground
states that inhibit hydrocarbon coordination.3
(A) and deprotonation by bases (B) can increase reactivity for electrophilic
and nucleophilic CH cleavage reactions, respectively.
high electron count nonbonding orbitals and (2) the ligands are strong
electron donors, e.g., OH-. Significantly, these systems show much
less sensitivity to inhibition by coordinating species such as water and
could provide a basis for the design of CH conversion catalysts that
can directly and efficiently generate coordinating products such as
alcohols, amines, carboxylates, etc.
We have shown that an essential role of the strong acid solvent
is ligand protonation that facilitates both CH actiVation and
functionalization.4 In the case of M-R functionalization, this
increases the electrophilicity of the R group and facilitates reductive
functionalization. In the case of CH activation, ligand protonation
facilitates both coordination of the alkane by generating, new, less
stable catalyst ground states (e.g., by protonation of coordinated
water) and CH cleavage by increasing catalyst electrophilicity.4 The
basis for this increase in electrophilicity is shown conceptually in
Figure 1A. Ligand protonation removes electron density at the metal
center, leading to a lower energy metal based LUMO that facilitates
interaction with the low lying CH HOMO.
Our theoretical calculations show that these nucleophilic systems
can cleave CH bonds by 4-centered transition states involving the
basic hydroxide group (eq 1).7 Recently, we also showed that M-R
complexes from this nucleophilic class cannot be functionalized
by pathways that operate for strongly electrophilic, oxidizing
metals.8 Instead, new functionalization pathways were identified
with these (nucleophilic) M-R complexes that, significantly,
require basic aqueous OH- solVents for high efficiency.9 In these
pathways, coordination of OH- to M-R increases the nucleophi-
licity of R, facilitating the reaction with weakly electrophilic O-atom
donors to generate alcohols (eq 2). Consequently, we expect that
designing new catalysts by coupling nucleophilic CH activation
reactions with base-assisted functionalization of the resulting
nucleophilic M-R intermediates will require that the CH activation
reaction be compatible and ideally accelerated by basic solvents.
Recently, we and others reported the first examples of efficient
ambiphilic or nucleophilic CH cleavage2 by d6 IrIII and RuII ligated
metal cations with alkoxo5 and hydroxo6 ligands. The two key
distinguishing features of nucleophilic systems are that (1) the metal
centers utilized are both weakly electrophilic and oxidizing and
characterized by good π-donor properties that result from high lying,
‡ Scripps Florida.
† University of Southern California.
§ The University of Texas at Arlington.
| California Institute of Technology.
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C O M M U N I C A T I O N S
Low activation barriers for CH activation to generate M-R
intermediates require both low energy transition states for cleavage of
the coordinated CH bond and high energy, reactive ground states that
are sufficiently labile to facilitate CH coordination. Analogous to how
ligand protonation by acid solvents accelerates electrophilic CH
cleavage (Figure 1A), we considered that reVersible ligand deproto-
nation of protic ligands by strongly basic solVents could also accelerate
CH cleavage by systems that operate via nucleophilic CH activation
(Figure 1B).10 As shown, increasing electron donation to the metal
center by ligand deprotonation with a basic solvent could enhance the
π-nucleophilicity of metal complexes by raising the energy of metal-
based, filled, nonbonding, π-donor orbitals (the HOMO) and facilitate
interaction with the high lying, antibonding orbitals of the CH bond
(the LUMO). Significantly, this effect should be exhibited by depro-
tonation of any protic ligand and show a dependence on the basicity
of the solvent when the deprotonation is reversible.11 Aqueous KOH
could be an ideal basic solvent and could facilitate in situ generation
of the active M-OH catalyst.
Key to utilizing this concept is the requirement that KOH should
not inhibit substrate coordination by generating highly stable M-OH
ground states. Since H2O is known to inhibit most CH activation
systems by generation of stable ground states3 and OH- is generally
considered to be much more coordinating than H2O, this may seem to
be implausible. However, we reasoned that this type of ground state
inhibition could be prevented or minimized with nucleophilic metals
since the M-OH complexes of these metals are known to be basic.12
Thus, a strongly basic M-OH could be less stable than the corre-
sponding M-H2O complex and use of KOH solvent should not lead
to inhibition. Indeed, in such systems, along with increasing the
π-nucleophilicity (Vide supra), deprotonation of an aquo complex (e.g.,
M(OH2)2) in strongly basic solvents could generate more labile and
reactive metal hydroxo (e.g., M(OH)(H2O)) ground states. This
expected effect is related to the long known observations by Basolo
and others that bases can catalyze ligand substitution by decreasing
the electrophilicity of the metal center through reversible deprotonation
of protic spectator ligands.13 These considerations led us to investigate
whether (HL)M(OH)n(H2O)m type complexes, where M is a strong
π-nucleophile and HL is a protic, polydentate ligand, could be
compatible with and ideally accelerated by aqueous KOH for nucleo-
philic CH activation.
stability. Complex 1 was synthesized as shown in eq 3 and was
fully characterized by elemental analysis, X-ray crystallography
(Figure 2), IR, and HR-MS. 1 is not soluble in neutral water but is
readily soluble in >1 M KOH at room temperature. The chlorides
of 1 are quite labile, and recrystallization from wet DMSO can
lead to O-bridged species.15 Treatment of brown-red KOH solutions
of 1 at room temperature with Zn dust led to a rapid color change
to a deep purple solution. Other reductants such as NaBH4, H2PO3,
H2, Al, Mg, etc. showed varying levels of reduction but were not
1
as clean as Zn. As expected H and 13C analysis of the reaction
mixture showed that several diamagnetic, IPI coordinated, putative
(IPI)RuII(OH)n(H2O)m species were formed, designated as 2,
presumably due to various levels of ligand deprotonation (Vide
supra) by solvent.
Consistent with 2 as a mixture of labile, equilibrating, (IPI)RuII
species, addition of several equivalents of KCN at room temperature
led to an immediate color change and formation of a single diamag-
netic, IPI coordinated species. Quantitative 13C NMR analysis of the
crude reaction mixture prepared with K13CN shows a single species
with a fully coordinated, symmetrical IPI ligand and two distinct
cyanides in a 2 to 1 ratio. The material was crystallized from aqueous
methanol and identified as K[(IPI)RuII(CN)3] on the basis of 1H and
13C NMR, IR, X-ray crystallography, elemental analysis, and mass
spectrometry.
Figure 2. ORTEP representation of 1 (50% ellipsoids; the hydrogens and a
DMSO molecule are removed for clarity); see Supporting Information (SI).
To examine whether 2 would catalyze CH activation in basic
solvents, the rates of H/D exchange were examined using a 4 mL total
internal volume PTFE-lined reactor equipped with a magnetic stir bar.
Varying concentrations of KOD/D2O with 3.0 mM of 2 prepared by
in situ reduction of 1 with zinc dust were used at temperatures up to
160 °C. As expected, benzene is immiscible with aqueous KOH but
H/D exchange of this two-phase system could be observed if rapidly
stirred. To avoid mass transfer issues and simplify kinetic studies, we
focused our studies on water-soluble hydrocarbons substrates. Use of
phase transfer agents were not examined as these would likely
complicate kinetic interpretation. Importantly, any H/D exchange due
to reversible deprotonation by the basic solvent (hydrocarbons are very
weak carbon acids) in the absence of catalyst (background reaction)
was subtracted from the reported catalytic rates.
We report herein the first observation that aqueous KOH can
accelerate, rather than inhibit, nucleophilic CH activation by a d6
(HL)RuII(OH)n(H2O)m complex. A key challenge in designing such
base modulated catalysts was to identify nucleophlic M-OH com-
plexes that would be soluble and thermally stable in strongly basic
aqueous solvents and with protic ligands that could be reversibly
deprotonated by strong bases. Among several systems investigated,
(IPI)RuIIICl3, 1, where IPI is the tridentate, NNN-pincer ligand, 2,6-
diimidizoylpyridine (IPI),14 was selected as a good candidate to observe
base modulated catalysis because (1) in situ 1e- reduction should
generate a desired high electron count d6, RuII center; (2) reversible
reaction with aqueous KOH should generate a range of highly labile,
strong π-donor water-soluble complexes (L)RuII(OH)n(H2O)m by
chloride displacement and deprotonation of the IPI, aquo, and/or
hydroxo ligands; and (3) the tridentate IPI could impart catalyst
1 mol % 2
R-CH + D2O
8 R-CD + DHO
(4)
3.7 M KOD/D2O,
90 or 160 °C, 1 h
As shown in Chart 1, 2 catalyzes the relatively fast H/D exchange
between 3.7 M KOD in D2O and the water-soluble substrates (eq
4).16 Consistent with the reduced species, 2, as the active catalyst
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control experiments showed that 1 was inactive without reduction.
Additionally, neither RuCl3 (without IPI) nor free IPI ligand in the
presence or absence of Zn catalyzed the H/D exchange. The IPI
ligand is critical to the stability of 2 since addition of RuCl3 to
KOD/D2O in the presence or absence of Zn immediately led to
formation of insoluble, inactive, black precipitates (presumably Ru
oxides) not observed upon reduction of 1.
Chart 1. Selected Aromatic and Aliphatic Substrates Showing
Relative Percent H/D Exchange with 3.7 M KOD/D2O at 90 and
160 °C in the Presence and Absence (in brackets) of 1
Figure 3. (a) Rate of H/D exchange as a function of [1] with [IA] ) 0.3 M
at 80 °C for 1 h in 3.7 M KOD/D2O (where rate ) ([IA]0 - [IA]Final)time-1
)
and (b) Rate of H/D exchange as a function of [IA] with [2] ) 3.0 mM at 80
°C for 1 h in 3.7 M KOD/D2O.
To test for the possibility of base modulated catalysis, the rate
of H/D exchange was examined as a function of [KOD]. Isophthalic
acid (IA) was utilized for this study as it was soluble from 1.5 M
to a maximum of 8 M [KOH] whereas the other substrates were
insoluble above ∼4 M [KOH]. Significantly, as shown in Figure
4, the rate of H/D exchange catalyzed by 2 is accelerated rather
than inhibited by increasing [KOD] over the entire 1.5 to 8 M range.
We consider that this data is consistent with H/D exchange
proceeding by reversible CH activation involving coordination of the
CH bond and concerted, rate-determining nucleophilic CH cleavage
to generate Ru-R intermediates. Consistent with a mechanism
involving substrate coordination, addition of 5 equiv of water-souble
sodium nicotinate or potassium cyanide (both poor σ-donors and
relatively weak π-acids) relative to 2 (∼6 mM) inhibits all catalytic
H/D exchange at 90 °C even though the solution remains homoge-
neous. Significantly, as shown in Chart 1, the less acidic, stronger
aromatic CH bonds (benzene: pKa ) 43, BDE ) 110.9 kcal/mol)19b
are more reactive than the more acidic, weaker benzylic CH bonds
(toluene: pKa ) 41, BDE ) 88.0 kcal/mol).19b If the reaction proceeded
by simple reversible C-H deprotonation by the RuII-OH species, as
observed with analogous amido complexes by Bergman and others,12
then the more acidic CH bonds should show higher H/D exhange rates
(as observed for the background acid-base reactions, Vide supra).
Higher rates of reaction at the stronger aromatic CH bonds would also
rule against free radical, radical cationic, and anionic pathways. The
well behaved kinetics, ligand inhibition, good reproducibility, and
reaction with water-soluble alkyl substates are also consistent with a
CH activation mechanism.
The increase in H/D exchange rate with increasing [KOH] would
require that [KOH] is involved in the rate-determining step(s) for CH
activation. This would be the case if reversible deprotonation of any
ligand20 by OH- facilitated both substrate coordination (see SI for
proposed rate law) and/or CH cleavage by increasing the nucleophi-
licity of the metal center (Vide supra). Another possibility is that the
outer sphere, solvent OH-, could facilitate rate-determining cleavage
of CH bonds coordinated to the Ru(II) center. A proposed mechanism
based on reversible ligand deprotonation is shown in Figure 5.
As discussed above, we could not examine [KOD] above 8 M due
to substrate insolubility. However, we anticipate that since reactivity
likely correlates with the activity of OH- (see derived rate law in SI)
a TOF ) [producttotal - background][1]-1 time-1. See SI for more data and
other substrates.
The kinetics of the H/D exchange reactions are well behaved,
showing a high degree of reproducibility and clean first order
dependence on both 2 and the added substrate (IA) (Figure 3). To
minimize the possible influence of steric interactions and chelation
control on the CH activation reactions, only the CH bonds meta to
the carboxylate groups of isophthalic acid (IA, Chart 1) were used in
these kinetics studies. In general, aromatics (Chart 1) are substantially
more reactive and were examined at 90 °C, where 2 is stable for at
least 4 h. As can be seen, the water-soluble alkyl substrates were less
reactive and examined at 160 °C. At this temperature the catalyst is
stable for ∼1 h. At low substrate conversion only monodeuteration of
the methyl groups was observed. As discussed above, H/D exchange
is observed in a two-phase system with benzene at 90 °C, but no
detectable exchange was observed with methane or soluble long chain
alkyl carboxylates at temperatures up to 160 °C for 1 h. We believe
this is due to the expected lower reactivity of the alkanes. As noted
above, higher temperatures could not be examined due to the thermal
instability of 2 above 160 °C.
The emphasis in this communication is neither the absolute rates
of H/D exchange nor a comparison to reported systems. The key focus
was to determine whether CH activation catalysts could be designed
that were not inhibited in basic solvents and whether strongly basic
solvents would lead to base modulated catalysis and acceleration.
However, some comparison is useful to begin to access the potential
of this approach. To our knowledge there are no reports of CH
activation systems that benefit from the use of basic solvents and the
closest comparisons are with H/D exchange studies in D2O as the
solvent.17 Of these, the most analogous system is the use of water-
soluble Cp*Ir(PMe3)Cl2 reported by Bergman17a that catalyzes H/D
exchange between water-soluble aryl and alkyl carboxylates and D2O
with TOFs of 1.6 × 10-4-8.8 × 10-4 s-1 obtained at 135 °C over
40 h.18 Figure 3 shows that the rates of H/D exchange for arenes
catalyzed by 2 compare favorably to this system and are faster in some
cases.
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C O M M U N I C A T I O N S
Energy Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences under
Award Number DE-SC0001298. The authors acknowledge helpful
discussions with Dr. Daniel H. Ess, Dr. Robert J. Nielsen, Dr. Kapil
S. Lokare, and Steven M. Bischof. We thank Chevron Corporation
for partial financial support for this research.
Supporting Information Available: Synthetic procedures, spec-
troscopic details, crystallographic data, and catalysis procedures for 2.
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
Figure 4. Plot of TOF vs [KOD] for the H/D exchange reaction between
the meta-position of isophthalic acid and KOD/D2O at 50 °C for 1 h.
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(15) Based upon X-ray analysis, see SI for details.
(16) In a typical experiment a 0.3 M solution of substrate in 3.7 M KOD was heated
with 1 mol % of 2 in the presence of excess zinc at 90 °C for 1 h with sp2 substrates
and 160 °C for 1 h with sp3 substrates. After a cooling step, addition of an internal
standard (0.2 mL of 1.05 M CH3CO2K/KOD solution) and 1H NMR analysis were
used to monitor the incorporation of deuterium into the substrates. 2H NMR
spectroscopy showed deuterium incorporation into IA.
Figure 5. Proposed reaction mechanism for H/D exchange involving
reversible ligand deprotonation followed by water loss, substrate coordina-
tion, reversible CH activation, and loss of product.
the reactivity could continue to accelerate since the activity of OH-
of concentrated aqueous KOH is known to increase exponentially at
higher concentrations.21 Other possibilities for continued acceleration
could be that, as with electrophilic catalysts in neat strong acidic
solvents,4 very concentrated basic solvents might generate, new, less
stable, more reactive ground states (e.g., by complete deprotonation
of a protic ligand) or open new, lower energy CH cleavage pathways,
involving, e.g., Ru-O- species. These possibilities are being examined
theoretically and experimentally.
In summary, (IPI)RuII(OH)n(H2O)m, 2, catalyzes facile H/D ex-
change reactions between water-soluble hydrocarbons and strongly
basic KOD/D2O solvent at rates faster than in the case of the solvent
alone. We propose that catalysis proceeds via reversible nucleophilic
CH activation. Significantly, the reaction is accelerated by increasing
the solvent basicity which we propose to result from reversible ligand
deprotonation and the resulting increase in ligand lability and π-nu-
cleophilicity of the RuII catalyst. We are continuing to study this and
related systems with the ultimate goal of understanding how to couple
base accelerated CH activation with base accelerated functionalization
of the M-R intermediates in an effort to design new hydrocarbon
functionalization systems that are not inhibited by coordinating
substrates. We believe that this concept of base modulated catalysis
through ligand deprotonation in strongly basic solvents could be
extended to activate other weak π-acceptors (e.g., N2, CO32-, etc.),
and these studies are underway.
(17) (a) Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R. G. J. Am. Chem.
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Theyseen, N.; Loschen, R.; Milstein, D.; Leitner, W. Angew. Chem., Int.
Ed. 2007, 119, 2319. and citations therein. (c) Meier, S. K.; Young, K. J. H.;
Ess, D. H.; Tenn, W. J., III; Oxgaard, J.; Goddard, W. A., III; Periana,
R. A. Organometallics 2009, 28, 5293. and citations therein. (d) Feng, Y.;
Jiang, B.; Boyle, P. A.; Ison, E. A. Organometallics 2010, 29, 2857.
(18) TOF ) [product][catalyst]-1 time-1 is approximated by TONtotal/time, where
TONtotal ) [product][catalyst]-1
.
(19) (a) Fulton, J. R.; Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem. Soc.
2000, 122, 8799–8800. (b) Fulton, J. R.; Sklenak, S.; Bouwkamp, M. W.;
Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 4722.
(20) The pKa of free IPI is ∼14, but that of the coordinated IPI will depend on
the other species coordination to Ru.
(21) Rochester, C. H. Q. ReV. Chem. Soc. 1966, 20, 511.
Acknowledgment. The mechanistic studies were supported as
part of the Center for Catalytic Hydrocarbon Functionalization, an
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