Pyridine-Based PCP-Ruthenium Complexes: Unusual Structures and Metal-Ligand Cooperation

Article abstract of DOI:10.1021/jacs.9b02669

Metal-ligand cooperation (MLC) by dearomatization/aromatization provides a unique way for bond activation, which has led to the discovery of various acceptorless dehydrogenative coupling reactions. However, most of the studies are based on pincer complexes with a central nitrogen donor. Aiming at exploration of the possibility of MLC by PCP-type pincer complexes, we report herein the synthesis, characterization, structure, and reactivity of pyridine-based PCP-Ru complexes. X-ray structures and DFT calculations indicate a carbenoid character of quaternized pyridine-based PCP-Ru complexes. These complexes undergo dearomatization by direct deprotonation, and the dearomatized complex can react with hydrogen, alcohols, or nitriles to regain aromatization via MLC.

Full text of DOI:10.1021/jacs.9b02669

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Article
Pyridine-based PCP-Ruthenium Complexes:
Unusual Structures and Metal-Ligand Cooperation
Shan Tang, Niklas von Wolff, Yael Diskin-Posner, Gregory Leitus, Yehoshoa Ben-David, and David Milstein
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02669 • Publication Date (Web): 19 Apr 2019
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Pyridine-based PCP-Ruthenium Complexes: Unusual Structures and
Metal-Ligand Cooperation
Shan Tang,†§ Niklas von Wolff,†§ Yael Diskin-Posner,‡ Gregory Leitus,‡ Yehoshoa Ben-David,† David
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Milstein†*
†Department of Organic Chemistry and ‡Chemical Research Support, the Weizmann Institute of Science, Rehovot 76100,
Israel
ABSTRACT: Metal-ligand cooperation (MLC) by dearomatization/aromatization provides a unique way for bond activation, which
has led to the discovery of various acceptorless dehydrogenative coupling reactions. However, most of the studies are based on pincer
complexes with a central nitrogen donor. Aiming at exploration of the possibility of MLC by PCP-type pincer complexes, we report
herein the synthesis, characterization, structure and reactivity of pyridine-based PCP-Ru complexes. X-ray structures and DFT cal-
culations indicate a carbenoid character of quaternized pyridine-based PCP-Ru complexes. These complexes undergo dearomatiza-
tion by direct deprotonation, and the dearomatized complex can react with hydrogen, alcohols or nitriles to regain aromatization via
MLC.
complexes, and what effects it would have on the structure and
reactivity of such complexes.
INTRODUCTION
Scheme 1. Metal-Ligand Cooperation (MLC) of Pyridine-
Developing new approaches for bond activation plays a key role
based Pincer Complexes via Dearomatization / Aromatiza-
in the discovery of novel catalytic reactions. Over the past dec-
tion.
ades, metal complexes with non-innocent ligands, which can
cooperate with the metal center during substrate activation,
have demonstrated unique catalytic reactivity in the activation
of inert chemical bonds.1-3 Pincer complexes are a class of metal
complexes containing tridentate ligands with meridional geom-
etry.4-5 Besides the importance in organometallic chemistry, the
possibilities of structure and electronic modifications of pincer
ligands have provided opportunities for developing highly effi-
cient and selective catalysts in organic synthesis.6-9 Arene-based
PCP pincer complexes and pyridine-based PNP pincer com-
plexes are among the most common pincer complexes in organ-
ometallic chemistry.
Since 2005, our group has studied the metal-ligand coopera-
tion (MLC) of pyridine-based PNP/PNN pincer complexes via
Besides the dearomatization of pyridine-based PNP/PNN
pincer complexes, our group also reported on the dearomatiza-
tion of hydroquinone PCP-type pincer complexes by deproto-
nation or oxidation.22-23 More recently, Ozerov and co-workers
reported on the dearomatization of a high-valent benzene-based
PCP-Re(V) complex by deprotonation.24 We are unaware of
other reports on dearomatization of PCP pincer complexes. Alt-
hough the methylene group at the meta position of pyridine has
a somewhat higher pKa value compared to the methylene group
at the ortho position of pyridine,25 we envisioned that 3,5-
lutidine-based pincer complexes might also undergo similar
dearomatization/aromatization as 2,6-lutidine-based pincer
complexes (Scheme 1b). Due to the increased trans effect of the
carbon metal moiety (as compared to nitrogen donors), new re-
dearomatization/aromatization of the pincer ligand (Scheme
1a).10 Pyridine-based pincer complexes can be deprotonated by
base at the benzylic carbon, resulting in dearomatization of the
pyridine ring.11-13 Cooperation between the ligand and the metal
center of the dearomatized complex can result in the activation
of chemical bonds (H-H, O-H, N-H, C-H etc.) with consequent
aromatization. The process can be reversible, resulting in prod-
uct formation. This bond activation approach has led to the dis-
covery of a series of unprecedented, highly efficient and envi-
ronmentally friendly dehydrogenative coupling reactions of al-
cohols with hydrogen evolution.10,14-15 It is noteworthy that the
formal oxidation state of the central metal remains unchanged
during the whole reaction process.16-21 We set out to explore
whether this MLC approach could be expanded to PCP-type
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activity and catalysis are expected. In particular, the dearomati-
demonstrating the significant electronic effect of the nitrogen
atom in the lutidine backbone. The P-Ru-P bond angles of 2a
and 2b are both smaller than 180˚ ((157.623(14)˚) and
158.15(2)˚, respectively).
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zation of the pyridine-based PCP ligand might involve the for-
mation of metal carbene species with the potential of unique re-
activity in catalytic acceptorless dehydrogenation reactions.
Herein, we report the synthesis, unique structures, and reactiv-
ity of pyridine- and pyridinium - based PCP-Ru complexes, in-
cluding metal-ligand coorperation based on dearomatiza-
tion/aromatization.
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RESULTS AND DISCUSSION
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In 1996, we reported the synthesis and characterization of pyri-
dine-based PCP-Pd/Rh complexes.26 Nevertheless, deprotona-
tion and possible dearomatization of these complexes were not
explored at that time. In this work, two 3,5-lutidine-based
bisphosphine ligands (1a and 1b) were synthesized from 3,5-
pyridinedicarboxylic acid (See Supporting Information for de-
tailed procedure). Reaction of 1a or 1b with [RuCl2(COD)]n in
ethanol at 120 oC for 48 h resulted in the formation of a yellow
precipitate. After cooling to room temperature, the precipitate
was collected and dissolved in pyridine upon heating at 120 oC
for 30 min. The pyridine solution was then reacted with CO (1
atm) at room temperature for 24 h. The reaction with 1a af-
forded 2a as a pale yellow solid in 76% yield while the reaction
with 1b furnished 2b as a yellow solid in 70% yield (Scheme
2). The 31P{1H} NMR spectra of 2a and 2b showed character-
istic singlets at 74.53 and 91.12 ppm, respectively. The IR spec-
tra of 2a and 2b exhibited in each case two strong absorptions
bands of the CO ligands with relative absorbance of 1:1, indic-
ative of a 90° angle between the two CO ligands in the gener-
ated pyridine-based PCP-Ru complexes. In the 13C{1H} NMR
spectra, the ipso-carbons of 2a and 2b gave rise to singlets at
181.11 and 182.49 ppm, respectively.
2b
2a
Figure 1. X-ray crystal structures of 2a and 2b (thermal ellipsoids set
at 50% probability level, i-propyl and t-butyl groups are presented as
wireframe, and hydrogen atoms are omitted for clarity). Selected bond
distances (ꢀ) and angles (˚) for 2a: Ru1-C1 2.106(2), Ru1-C24
1.949(2), Ru1-C25 1.850(2), Ru1-Cl1 2.4589(6), Ru1-P1 2.3787(6),
Ru1-P2 2.3840(7), C3-C2 1.510(3), C6-C7 1.510(3), C1-C3 1.403(3),
C1-C6 1.400(3), C3-C4 1.391(3), C5-C6 1.392(3), P1-Ru1-P2
157.62(2), C1-Ru1-Cl1 87.33(6), C1-Ru1-C24 172.58(10), C24-Ru1-
C25 93.9(1); 2b: Ru1-C1 2.082(2), Ru1-C24 1.960(3), Ru1-C25
1.983(4), Ru1-Cl1 2.446(1), Ru1-P1 2.4408(6), Ru1-P2 2.4332(6), C2-
C3 1.505(4), C6-C7 1.508(4), C1-C3 1.400(3), C1-C6 1.409(3), C3-C4
1.387(4), C5-C6 1.387(4), P1-Ru1-P2 158.15(2), C1-Ru1-Cl1 88.00(7),
C1-Ru1-C24 177.1(1), C24-Ru1-C25 94.3(1).
Scheme 2. Synthesis of Pyridine-based PCP-Ru Complexes
Figure 2. X-ray crystal structure of 3a (thermal ellipsoids set at 50%
probability level, isopropyl groups are presented as wireframe, and hy-
drogen atoms are omitted for clarity). Selected bond distances (ꢀ) and
angles (˚) for 3a: Ru1-C1 2.095(1), Ru1-C21 1.953(2), Ru1-C22
1.851(3), Ru1-Cl1 2.4530(6), Ru1-P1 2.3832(4), Ru1-P2 2.3791(4),
C2-C3 1.505(2), C6-C7 1.508(2), C1-C3 1.410(2), C1-C6 1.409(2),
C5-C6 1.376(2), C3-C4 1.387(2), N1-C8 1.479(2), P1-Ru1-P2
159.80(1), C1-Ru1-Cl1 89.27(4), C1-Ru1-C21 175.80(6), C21-Ru1-
C22 93.15(7).
As was observed in our previous report,26 the free pyridine
nitrogen atoms in pyridine-based pincer complexes might fur-
ther bind to other metal centers to form coordination polymers.
In order to avoid this possibility, and also to enhance the acidity
of the benzylic position, we sought to quaternize 2a and 2b.
Mixing a stoichiometric amount of methyl triflate with these
complexes in dichloromethane at room termperature,28 the
quaternized pyridine-based PCP-Ru complexes were obtained
in quantitative yields (Scheme 2, 3a and 3b). Only slight chem-
ical shifts of the peaks in the 31P{1H} NMR spectra of these
Single crystals of 2a and 2b were obtained by solvent evap-
oration of their pyridine solutions under vacuum, exhibiting oc-
tahedral configurations with mutually cis CO ligands (Figure 1).
The Ru-Cl bond distances of 2a and 2b are quite similar, while
the ipso Ru-C bond distances in 2a and 2b (2.1059(15) ꢀ and
2.082(2) ꢀ, respectively) are slightly different. They are both
significantly longer than the Ru-C bond in a similar benzene-
based PCP-Ru complex (1.896(4) ꢀ) reported by our group,27
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complexes as compared with 2a and 2b were observed. How-
C5-C6 bond lengths with longer C1-C3 and C1-C6 distances
and contraction of the N1-C1 bond length from 2.830 ꢀ in 2a
to 2.778 ꢀ in 3a.
ever, the 13C{1H} NMR spectrum of 3a and 3b showed signifi-
cant changes in the chemical shifts of the ipso-carbons, from
181.11 to 206.41 ppm (3a) and from 182.49 to 207.96 ppm (3b).
These results might be rationalized by the strong electron-with-
drawing effect of the quaternized pyridine nitrogen, resulting in
electronic deshielding of the ipso carbons. X-ray quality crys-
tals of 3a were obtained by slow evaporation of its benzene so-
lution (Figure 2). Noteworthily, the bond distance of ipso C-Ru
in 3a (2.0948(13) ꢀ), is shorter than that of 2a (2.1059(15) ꢀ).
Moreover, in 3a the C3-C4 and C5-C6 bonds (about 1.38 ꢀ)
differ significantly from the C1-C3 and C1-C6 bonds (about
1.41 ꢀ), as opposed to that in 2a (1.39-1.40 ꢀ). An outer-sphere
triflate counter anion was also determined in the crystal struc-
ture.
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Following characterization of the pyridine-based PCP-Ru
complexes, we tried to deprotonate them with an equimolar
amount of base such as KOtBu or KHMDS. Mixing 3a with
KOtBu in dioxane resulted in a dark purple solution. After
workup, dearomatized complex 4a was isolated in 70% yield
(Scheme 3). A similar result was observed when KHMDS was
used instead of KOtBu. The 31P{1H} NMR spectrum showed a
clear AB system at 72.03 ppm and 63.64 ppm with a coupling
constant of 2JPP = 239.0 Hz, indicating non-equivalent phospho-
rus nuclei coordinated to the metal center. The non-equivalent
two protons of the pyridine ring gave rise to peaks at 5.60 ppm
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and 5.32 ppm in the H NMR spectrum. The vinylic proton at
the deprotonated benzylic position exhibited a low-field shift to
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3.60 ppm, showing a dd resonance (2JHP = 5.5 Hz, JHP = 3.8
Hz). In the 13C{1H} NMR spectrum, the deprotonated benzylic
carbon was observed at 65.78 ppm with strong C-P coupling
(1JCP = 67.2 Hz). The ipso-carbon of the dearomatized pyridine-
based PCP-Ru complex exhibited an up-field chemical shift to
180.59 ppm. The IR absorption of two CO ligands exhibit bands
at 2012 cm-1 and 1943 cm-1 in a ratio of 1:1, indicating that the
two CO ligands still maintain an angle of 90o in the dearoma-
tized complex.
Scheme 3. Dearomatization of 3a by Deprotonation
Figure 3. Calculated bonding picture of 3a.
In order to understand the bonding pattern in 3a more pre-
cisely, we performed DFT calculations (see SI for details) and
NBO analysis. Calculated bond lengths fit well with the exper-
imental values. Interestingly, the observed bond lengths within
the aromatic core upon quaternization of 2a show dissymmetry
and are not well explained by the canonical Lewis structure de-
picted in Scheme 2. Also, the shortening of the Ru-C bond from
2.106(2) ꢀ in 2a to 2.095(1) ꢀ in 3a is difficult to rationalize
using the classical bonding picture in Scheme 2. Instead, NBO
analysis suggests a partial carbenoid character of the ipso-car-
bon in 3a (Figure 3). The carbenoid lone pair (Figure 3a, 1st
NBO) is involved in 3c4e bonding with the adjacent Ru-CO
bond. Nevertheless, the carbenoid character is strongly attenu-
ated by donation of electron density into the vacant orbital on
carbon from the adjacent double bonds (Figure 3a, 2nd and 3rd
NBO) with only little back donation from ruthenium-centered
orbitals (Figure 3a, 4th NBO) explaining the relatively long Ru-
C bond distance. Another way of thinking about bonding in 3a
would be a formal “Dewar-pyridinium”-Lewis structure (Figure
3b, right), which allows us to rationalize the short C3-C4 and
Figure 4. X-ray crystal structure of 4a (thermal ellipsoids set at 50%
probability level, isopropyl groups are presented as wireframe, and hy-
drogen atoms are white spheres except for the side arm are omitted for
clarity). Selected bond distances (ꢀ) and angles (˚) for 4a: Ru1-C1
2.106(2), Ru1-C21 1.949(2), Ru1-C22 1.859(2), Ru1-Cl1 2.4751(6),
Ru1-P1 2.3871(6), Ru1-P2 2.3769(5), C2-C3 1.399(3), C6-C7 1.504(3),
C1-C3 1.440(3), C1-C6 1.339(3), C3-C4 1.421(3), C5-C6 1.406(3),
N1-C8 1.478(3), P1-Ru1-P2 161.23(3), C1-Ru1-Cl1 85.64(6), C1-
Ru1-C21 175.08(8), C21-Ru1-C22 89.9(1).
X-ray quality crystals of 4a were obtained by layering its di-
oxane/benzene solution with pentane (Figure 4), exhibiting an
octahedral complex bearing a bound chloride and two mutually
cis CO ligands. The ipso Ru-C bond distance in 4a of 2.106(2)
ꢀ is similar to that of 2a. As expected, the C-C bond of the
deprotonated side arm (1.399(3) ꢀ) is significantly shorter than
that of the non-deprotonated side arm (1.504(3) ꢀ), indicating
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partial double bond character, in conjugation with the pyridine
To gain a preliminary insight into the reactivity of the
dearomatized complex, we explored the reaction of 4a with di-
hydrogen (Scheme 4a). Exposure of a benzene solution of 4a to
1 atm H2 led to an instantaneous color change from dark blue to
yellow. Full conversion to the ruthenium hydride complex 5a
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ring. Importantly, a carbenoid resonance structure (which
would imply a formal 20 electron species) can be excluded due
to the 6-coordinate nature of 4a. NBO calculations on 4a indeed
confirmed the absence of carbenoid character at the ipso-posi-
tion (Figure 5a). Instead, 4a might be best described in terms of
Lewis structure by the resonance form on the right-hand side of
Figure 5b. The partially occupied lone pair in the meta-position
is highly delocalized into the adjacent double bond antibonds
(Figure 5b, left). Again, the ipso-C-Ru bond can be described
as a 3c4e interaction between a sp2-type lone pair on carbon do-
nating into the adjacent Ru-CO antibond (Figure 5b, right).
Given the absence of carbenoid character, low lying empty or-
bitals on the ipso-carbon are lacking, limiting the possibility of
backbonding from ruthenium and explaining the elongated Ru-
C bond of 2.106(2) ꢀ in 4a compared to 3a (2.082(2) ꢀ).
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was achieved by heating the solution at 80 C for 3 h. The
31P{1H} NMR spectrum of 5a showed a characteristic singlet at
87.96 ppm, indicating the formation of two equivalent phos-
phine groups. The ruthenium hydride exhibited a clear triplet in
the 1H NMR spectrum at -7.55 ppm while the pyridine ring pro-
tons were found at 8.12 ppm. In the 13C{1H} NMR spectrum,
the ipso-carbon shifted downfield from 180.59 ppm to 208.53
ppm. The mechanism of dihydrogen addition to the dearoma-
tized complex 4a was examined by DFT calculations (Scheme
4b, see SI for details). The aromatized species 4a is fully satu-
rated at ruthenium and a first step might thus be the generation
of a pentacoordinate species. We hence investigated the ener-
getics of four different mechanistic proposals:(1) loss of CO, (2)
a concerted SN2@Ru mechanism, where H2 replaces the chlo-
ride ligand, (3) Loss of the chloride ligand, (4) opening of either
of the phosphine side-arms. Both pathways (3) and (4) can be
excluded due to higher reaction barriers (> 36.3 kcal.mol-1, see
SI for details), as well as an activation across the Ru-C bond
(>34.1 kcal.mol-1, see SI for details). On the other hand, loss of
a CO-ligand from 4a might be feasible. Indeed, although the
generation of the pentacoordinate species I is 13.3 kcal.mol-1
higher than the starting material, it allows for the addition and
splitting of H2 (TS1, 32.0 kcal.mol-1) to generate hydride 5a.
Pathway (2) is only slightly higher in energy (TS2, 33.0
kcal.mol-1) and can thus not be ruled out. Here, H2 undergoes a
nucleophilic substitution at ruthenium in a highly concerted
manner, being directly deprotonated by the leaving chloride lig-
and. Reprotonation by HCl then leads to the formation of 5a.
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Figure 5. Calculated bonding picture of 4a.
Scheme 4. Aromatization of 4a upon H2 Addition
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Addition of 10 equiv of benzyl alcohol to a benzene solution of
The dearomatization of 2a, 2b and 3b by deprotonation was
also explored. Some color change was observed upon addition
of KOtBu or KHMDS into their solutions in THF or dioxane.
However, the dearomatized complexes could not be observed
by NMR analysis. We thought that the problem was not depro-
tonation of the benzylic position of these complexes, but rather
the stability of the deprotonated complexes. One way to prove
this assumption was to trap the deprotonated complexes in situ.
Our group has reported that dearomatized PNP pincer com-
plexes can undergo cooperative activation of the C≡N triple
bonds of nitriles via [1,3]-addition to form ketimido or enamido
complexes.29-30 Thus, we tried to deprotonate 2a in the presence
of excess amount of benzonitrile. Similar to the reactions of
dearomatized PNP-Re/Mn complexes, a ketimido complex 6a
was formed upon addition of KOtBu to a THF solution of 2a
and benzonitrile at room temperature and stirring for 24 h
(Scheme 5). A dearomatized anion intermediate 4a′ is proposed
to be formed in this process. The 31P{1H} NMR spectrum of 6a
exhibited a clear AB system at the characteristic chemical shifts
of 124.27 ppm and 83.62 ppm with 2JPP = 257.2 Hz. In the 1H
NMR spectrum, the deprotonated benzylic CH appears as a
doublet shifted to 5.61 ppm (2JHP = 8.7 Hz). Moreover, the
deprotonated benzylic carbon was downfield shifted to 63.20
ppm, exhibiting strong C-P coupling (1JCP = 32.1 Hz) in the
13C{1H} NMR spectrum.
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the dearomatized 4a led to an instantaneous color change. After
heating at 80 C for 3 h, 4a was quantitatively converted into
the ruthenium hydride complex 5a (Scheme 6), and only an
equivalent amount of benzaldehyde was observed by 1H NMR
analysis. Similar reactivity was also observed with ethanol.
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Scheme 6. Aromatization of 4a by Benzyl Alcohol
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The dearomatized complex 4a (1 mol%) was an efficient cat-
alyst for the acceptorless dehydrogenative coupling of benzyl
alcohol and benzylamine to form the corresponding imine in 92%
NMR yield after 48 h in refluxing toluene (eq. 2). Hydrogen gas
was detected by GC analysis.
Since the dearomatized complexes expected to be formed
upon deprotonation of complexes 2a, 2b and 3b could not be
isolated, we tried the reaction of the aromatized complexes with
base and alcohol. Addition of a stoichiometric amount of
KOtBu to a THF solution of 2a and excess amount of methanol
(50 equiv), the ruthenium alkoxide complex 7a was obtained in
quantitative yields after 24 h at room temperature (Scheme 7).
A clear shift of the 31P resonance was observed by 31P{1H}
NMR (singlet at 78.51 ppm). As expected, heating the THF so-
lution of 7a led to full conversion to the ruthenium hydride com-
plex 8a. The 31P{1H} NMR spectrum of 8a showed a singlet at
87.18 ppm while the ruthenium hydride gave rise to a triplet at
Scheme 5. Reaction of 2a with Benzonitrile by Deprotona-
tion
1
-7.29 ppm in the H NMR spectrum. Moreover, complex 8a
could also be obtained in 63% NMR yield by the reaction be-
tween ketimido complex 6a and benzyl alcohol (50 equiv). Ben-
zonitrile was observed by GC-MS, which indicated the reversi-
bility of MLC of 4a′ with nitrile.
Interestingly, complex 6a catalyzed the Michael addtion of
benzyl nitrile to ethyl acrylate under mild conditions, in the ab-
sence of added base. Similar to the PNP pincer complexes, a
nucleophilic enamido complex was proposed to be formed in
this transformation.29-30 A mixture of single and double addition
products was obtained when benzyl nitrile and ethyl acrylate
was used in 1:1 ratio. Importantly, the double addition product
could be isolated in quantitative yield when benzyl nitrile and
ethyl acrylate were used in 1:2 ratio (eq. 1).
Scheme 7. Reaction of 2a with Methanol
Over more than a decade, we have studied the MLC of pyri-
dine-based PNP-Ru and PNN-Ru complexes via dearomatiza-
tion/aromatization. One of the applications of this bond activa-
tion approach was the acceptorless dehydrogenative coupling
reactions of alcohols to form esters. We hence explored the re-
action of the pyridine-based PCP-Ru complexes with alcohols.
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Different from the reaction of 2a with methanol, the ruthe-
ꢀ, which is longer than that of 2b. The quaternized ruthenium
hydride complex 5b (2.107 ꢀ) exhibited a slightly shorter ipso
C-Ru bond than 8b. However, the Ru-H bond of 5b (1.74(3) ꢀ)
is much longer than that of 8b (1.5578 ꢀ).
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nium hydride complex 8b was directly obtained after reaction
at room temperature for 24 h in the case of 2b (Scheme 8a).
Similarly, the reaction with 3b afforded the ruthenium hydride
complex 5b at room temperature (Scheme 8b). The 31P{1H}
NMR spectra of 8b and 5b showed characteristic singlets at
104.07 ppm and 104.81 ppm, respectively. The hydride ligands
1
appear as triplets in their H NMR spectra at -7.27 ppm and -
7.45 ppm, respectively. In the 13C{1H} NMR spectra, the ipso-
carbons of 8b appeared at 183.41 ppm, while for 5b it appeared
at 214.30 ppm, indicating a strong electron-withdrawing effect
of the quaternized pyridine nitrogen.
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Scheme 8. Reactions of 2b and 3b with Methanol
8b
5b
Figure 6. X-ray crystal structures of 8b and 5b (thermal ellipsoids set
at 50% probability level, isopropyl and t-butyl groups are presented as
wireframe, counter anion and hydrogen atoms except for ruthenium hy-
dride are omitted for clarity). Selected bond distances (ꢀ) and angles
(˚) for 8b: Ru1-C1 2.116(1), Ru1-C24 1.917(2), Ru1-C25 1.944(2),
Ru1-H 1.5578, Ru1-P1 2.3556(4), Ru1-P2 2.3517(4), C6-C7 1.510(2),
C2-C3 1.506(2), C1-C6 1.401(2), C1-C3 1.405(2), C3-C4 1.397(2),
C5-C6 1.391(2), P1-Ru1-P2 154.59(1), C1-Ru1-H 88.9, C1-Ru1-C24
173.60(6), C24-Ru1-C25 101.95(6); 5b: Ru1-C1 2.107(2), Ru1-C25
1.923(2), Ru1-C26 1.940(3), Ru1-H 1.74(3), Ru1-P1 2.3797(5), Ru1-
P2 2.3635(5), C2-C3 1.503(2), C6-C7 1.506(2), C1-C3 1.410(2), C1-
C6 1.415(2), C3-C4 1.379(2), C5-C6 1.383(2), N1-C8 1.477(2), P1-
Ru1-P2 156.76(2), C1-Ru1-H 81(1), C1-Ru1-C25 164.72(9), C25-
Ru1-C26 98.5(1).
X-ray quality crystals of 8b were obtained by slowly evapo-
rating its toluene/pentane solution, and X-ray quality crystals of
5b were also obtained by slowly evaporating its benzene/pen-
tane solution (Figure 6). The ipso C-Ru bond in 8b is 2.116(1)
Scheme 9. Free energy pathways for alcohol dehydrogenation
(ethanol) might proceed after CO-loss and alcohol addition via
the deprotonation step TS4 (18.0 kcal.mol-1, Scheme 9a) to gen-
erate alkoxide species IV (4.7 kcalmol-1). Once the alkoxide is
generate, chloride can dissociate to allow for beta-hydride elim-
ination (TS5, 30.3 kcal.mol-1). Replacement of the aldehyde by
CO then regenerates the hydride 5a. Slightly higher in energy
The mechanism of alcohol dehydrogenation was also inves-
tigated by DFT calculations (Scheme 9). As for the activation
of alcohol, the first step from 4a must be the generation of a free
coordination site. Again, chloride dissociation and side-arm
opening will not be treated in detail due to high activation bar-
riers (> 37.7 kcal.mol-1, see SI). Deprotonation of an alcohol
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(8) Kumar, A.; Bhatti, T. M.; Goldman, A. S. Dehydrogenation of
is the concerted deprotonation/hydride abstraction pathway
(TS6, 32.6 kcal.mol-1, Scheme 9b), where again CO dissocia-
tion creates the open coordination site at ruthenium.
Alkanes and Aliphatic Groups by Pincer-Ligated Metal
Complexes, Chem. Rev. 2017, 117, 12357-12384.
(9) Benito-Garagorri, D.; Kirchner, K. Modularly Designed Transition
Metal PNP and PCP Pincer Complexes based on
Aminophosphines: Synthesis and Catalytic Applications, Acc.
Chem. Res. 2008, 41, 201-213.
(10) Gunanathan, C.; Milstein, D. Metal-Ligand Cooperation by
Aromatization-Dearomatization: A New Paradigm in Bond
Activation and "Green" Catalysis, Acc. Chem. Res. 2011, 44, 588-
602.
(11) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Facile
Conversion of Alcohols into Esters and Dihydrogen Catalyzed by
New Ruthenium Complexes, J. Am. Chem. Soc. 2005, 127, 10840-
10841.
(12) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. Metal-
Ligand Cooperation in C-H and H2 Activation by an Electron-
Rich PNP Ir(I) System: Facile Ligand Dearomatization-
Aromatization as Key Steps, J. Am. Chem. Soc. 2006, 128, 15390-
15391.
(13) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Efficient
homogeneous catalytic hydrogenation of esters to alcohols,
Angew. Chem., Int. Ed. 2006, 45, 1113-1115.
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In summary, we have disclosed a new coordination mode for
the MLC of pincer complexes through dearomatization/aroma-
tization. A series of pyridine-based PCP-Ru complexes were
synthesized and characterized. Interestingly, DFT calculations
of quaternized pyridine-based PCP-ruthenium complexes indi-
cate the carbenoid character of the ipso-carbon. We were able
to observe the dearomatization of a quaternized pyridine-based
PCP-Ru complex by direct deprotonation. NBO calculations on
the dearomatized complex show the absence of carbenoid char-
acter at the ipso-position. Importantly, the dearomatized PCP-
Ru complex reacts with hydrogen or alcohols to generate the
aromatized ruthenium hydride complex via MLC. This concep-
tually new MLC mode provides an alternative choice for the
development of acceptorless dehydrogenation reactions.
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ASSOCIATED CONTENT
(14) Gunanathan, C.; Milstein, D. Applications of Acceptorless
Dehydrogenation and Related Transformations in Chemical
Synthesis, Science 2013, 341, 249.
(15) Gunanathan, C.; Milstein, D. Bond Activation and Catalysis by
Ruthenium Pincer Complexes, Chem. Rev. 2014, 114, 12024-
12087.
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
Experimental details of synthetic procedures, NMR spectra, X-ray
data, and computational details (PDF)
Crystallographic data for 2a, 2b, 3a, 4a, 5b, 8b (CIF)
(16) Prokopchuk, D. E.; Tsui, B. T. H.; Lough, A. J.; Morris, R. H.
Intramolecular C-H/O-H Bond Cleavage with Water and Alcohol
AUTHOR INFORMATION
Using
a Phosphine-Free Ruthenium Carbene NCN Pincer
Complex, Chem. Eur. J. 2014, 20, 16960-16968.
Corresponding Author
(17) Sun, Y.; Koehler, C.; Tan, R.; Annibale, V. T.; Song, D. Ester
hydrogenation catalyzed by Ru-CNN pincer complexes, Chem.
Commun. 2011, 47, 8349-8351.
*david.milstein@weizmann.ac.il
Author contributions
§Shan Tang and Niklas von Wolff contributed equally.
(18) Fogler, E.; Garg, J. A.; Hu, P.; Leitus, G.; Shimon, L. J. W.;
Milstein, D. System with Potential Dual Modes of Metal–Ligand
Cooperation: Highly Catalytically Active Pyridine-Based PNNH–
Ru Pincer Complexes, Chem. Eur. J. 2014, 20, 15727-15731.
(19) Filonenko, G. A.; Cosimi, E.; Lefort, L.; Conley, M. P.; Copéret,
C.; Lutz, M.; Hensen, E. J. M.; Pidko, E. A. Lutidine-Derived Ru-
CNC Hydrogenation Pincer Catalysts with Versatile Coordination
Properties, ACS Catal. 2014, 4, 2667-2671.
(20) Balaraman, E.; Gnanaprakasam, B.; Shimon, L. J. W.; Milstein, D.
Direct Hydrogenation of Amides to Alcohols and Amines under
Mild Conditions, J. Am. Chem. Soc. 2010, 132, 16756-16758.
(21) Li, H.; Zheng, B.; Huang, K.-W. A new class of PN3-pincer
ligands for metal–ligand cooperative catalysis, Coordin. Chem.
Rev. 2015, 293-294, 116-138.
(22) Ashkenazi, N.; Vigalok, A.; Parthiban, S.; Ben-David, Y.; Shimon,
L. J. W.; Martin, J. M. L.; Milstein, D. Discovery of the First
Metallaquinone, J. Am. Chem. Soc. 2000, 122, 8797-8798.
(23) Dauth, A.; Gellrich, U.; Diskin-Posner, Y.; Ben-David, Y.;
Milstein, D. The Ferraquinone–Ferrahydroquinone Couple:
Combining Quinonic and Metal-Based Reactivity, J. Am. Chem.
Soc. 2017, 139, 2799-2807.
(24) Kosanovich, A. J.; Komatsu, C. H.; Bhuvanesh, N.; Pérez, L. M.;
Ozerov, O. V. Dearomatization of the PCP Pincer Ligand in a ReV
Oxo Complex, Chem. Eur. J. 2018, 24, 13754-13757.
(25) Bordwell, F. G. Equilibrium acidities in dimethyl sulfoxide
solution, Acc. Chem. Res. 1988, 21, 456-463.
(26) Weisman, A.; Gozin, M.; Kraatz, H.-B.; Milstein, D. Rhodium and
Palladium Complexes of a 3,5-Lutidine-Based Phosphine Ligand,
Inorg. Chem. 1996, 35, 1792-1797.
(27) van der Boom, M. E.; Iron, M. A.; Atasoylu, O.; Shimon, L. J. W.;
Rozenberg, H.; Ben-David, Y.; Konstantinovski, L.; Martin, J. M.
L.; Milstein, D. sp3 C–H and sp2 C–H agostic ruthenium
complexes: a combined experimental and theoretical study, Inorg.
Chim. Acta 2004, 357, 1854-1864.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This research was supported by the European Research Council
(ERC AdG 692775). D. M. holds the Israel Matz Professorial Chair
of Organic Chemistry. S.T. is thankful to the Israel Planning and
Budgeting Committee (PBC) for a postdoctoral fellowship. N.v.W.
is supported by the Foreign Postdoctoral Fellowship Program of the
Israel Academy of Sciences and Humanities.
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