Mechanistic insights into catalytic carboxylic ester hydrogenation with cooperative Ru(II)-bis{1,2,3-triazolylidene}pyridine pincer complexes

Article abstract of DOI:10.1016/j.jorganchem.2017.01.003

Transmetallation of newly designed lutidine-based CNC or CNN ligands L, featuring flanking 1,2,3-triazolylidene (tzNHCs) moieties, from Ag(I) to Ru(II) provided access to well-defined cationic [Ru^II(CO)(H)(L)(PPh₃)]⁺ complexes 2 and 5. Spectroscopic investigations confirm that, in both complexes, the tridentate ligand binds in a rare facial mode to the metal center. The complexes, that exhibit ligand-based reversible deprotonation/dearomatization reactivity, are active in catalytic ester hydrogenation in the presence of KOtBu (≥20 mol%) as an exogenous base. The beneficial effect of the base on catalytic activity relates to transesterification of substrates to the corresponding tert-butyl ester derivatives, which are hydrogenated considerably faster than methyl esters. The mechanistic findings in this work confirm that this transformation is very complex, with this transesterification, metal-ligand cooperative reactivity, base strength and possibly product inhibition all playing a role. Furthermore, relevant Ru(CNC)(hydride) species have been observed by NMR spectroscopy under near-catalytic conditions.

Full text of DOI:10.1016/j.jorganchem.2017.01.003

Journal of Organometallic Chemistry xxx (2017) 1e8
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Mechanistic insights into catalytic carboxylic ester hydrogenation
with cooperative Ru(II)-bis{1,2,3-triazolylidene}pyridine pincer
complexes
*
**
Soraya N. Sluijter, Ties J. Korstanje, Jarl Ivar van der Vlugt , Cornelis J. Elsevier
van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 November 2016
Received in revised form
Transmetallation of newly designed lutidine-based CNC or CNN ligands L, featuring flanking 1,2,3-
triazolylidene (tzNHCs) moieties, from Ag(I) to Ru(II) provided access to well-defined cationic
II
þ
[
Ru (CO)(H)(L)(PPh )] complexes 2 and 5. Spectroscopic investigations confirm that, in both complexes,
3
5
January 2017
the tridentate ligand binds in a rare facial mode to the metal center. The complexes, that exhibit ligand-
based reversible deprotonation/dearomatization reactivity, are active in catalytic ester hydrogenation in
the presence of KOtBu (ꢀ20 mol%) as an exogenous base. The beneficial effect of the base on catalytic
activity relates to transesterification of substrates to the corresponding tert-butyl ester derivatives, which
are hydrogenated considerably faster than methyl esters. The mechanistic findings in this work confirm
that this transformation is very complex, with this transesterification, metal-ligand cooperative reac-
tivity, base strength and possibly product inhibition all playing a role. Furthermore, relevant Ru(CN-
C)(hydride) species have been observed by NMR spectroscopy under near-catalytic conditions.
© 2017 Published by Elsevier B.V.
Accepted 6 January 2017
Available online xxx
Keywords:
Pincer ligand
Ruthenium
Carboxylic ester
Hydrogenation
Catalysis
1. Introduction
pyrazolate [5] switching. Also reversible cyclometalation has been
reported as concept for cooperative bond activation [6]. Reversible
The design of rigid tridentate ligands with a preference for a
meridional coordination mode, commonly referred to as ‘pincer’
ligands, is still drawing much attention, nearly four decades after
the initial reports on their coordination chemistry [1]. The original
dearomatization of heteroatom donor sidearm functionalized
picoline or lutidine designs has emerged as ‘privileged’ concept for
the (intramolecular) dehydrogenative coupling as well as hydro-
genation of a wide variety of substrates [7]. Typically, these scaf-
folds are decorated with phosphines or amines, whereas N-
heterocyclic carbene side-groups have been relatively underex-
plored to date.
Carboxylic ester hydrogenation (Scheme 1) is an industrially
important transformation to produce alcohols [8]. Biomass feed-
stocks, such as vegetable fats and oils, contain many ester func-
tionalities. Considering the depletion of fossil fuels, ester
hydrogenation may therefore play a pivotal role in the transition to
a biobased chemical industry. Currently, non-selective heteroge-
neous catalysts (typically based on copper chromite) that require
high pressures and temperatures are used to perform this reaction
[9]. Homogeneous catalysts may allow both better chemoselectivity
and milder reaction conditions. Early reports on ruthenium-based
catalysts for the hydrogenation of esters required additives, harsh
conditions and/or were only suitable for activated esters [10].
The hydrogenation of carboxylic esters to alcohols, formally
called hydrogenolysis, is far more difficult compared to ketone
‘design’ of a monoanionic alkyl- or arene-based carbon donor with
two flanking heteroatom donors (‘ECE’) has been significantly
expanded, not only with respect to the ‘core’ donor, but also the
overall charge of the ligand in coordination complexes and the ri-
gidity of the overall framework. Amidst the plethora of pincer
ligand designs that have been developed, platforms that allow for
active ligand participation in bond activation and functionalization
processes have emerged in the last decade. These ‘reactive’ ligand
classes often operate via some type of internal basic moiety that can
undergo reversible proton-transfer. This may involve reversible
alcohol-alcoholate
[2],
secondary
amine-amide
[3],
sulfonaminephosphine-sulfonamidophosphine [4] or pyrazole-
*
Corresponding author.
** Corresponding author.
E-mail addresses: j.i.vandervlugt@uva.nl (J.I. van der Vlugt), c.j.elsevier@uva.nl
(
C.J. Elsevier).
http://dx.doi.org/10.1016/j.jorganchem.2017.01.003
022-328X/© 2017 Published by Elsevier B.V.
0
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S.N. Sluijter et al. / Journal of Organometallic Chemistry xxx (2017) 1e8
Scheme 1. Generic reaction for ester hydrogenation (top), a selection of Ru pre-catalysts for this reaction and the generic structure of the target complexes disclosed herein
bottom).
(
hydrogenation because the ester C]O double bond is a weak
electrophile that is stabilized by resonance. Our group was among
the first to convert esters to alcohols at reasonable temperature and
H/OR metathesis [16] and c) ligand assisted hydride transfer [17]. In
order to facilitate the hydrogen transfer, electron-rich ligands can
be employed to enhance the nucleophilicity of the hydride towards
the substrate. This is nicely illustrated by the rate enhancement
upon replacing a phosphine donor for a more electron-donating
NHC moiety [18]. The ligand can also play a role in hemiacetal
decomposition (Scheme 2, III), by promoting C-O bond cleavage of
the intermediate, which might lead to the alternatively proposed
Ru(alkoxide) species [19]. The cooperative reactivity of deproto-
nated N-heterocyclic carbene-based CNC ligands is more pro-
nounced compared to phosphine-containing PNP systems, which
has been attributed to a combination of electronic properties and
extended flexibility of the larger CNC chelate [20]. Alternatively, an
outer-sphere bifunctional mechanism has been suggested to be
operational for these systems [21].
ꢁ
pressure (100 C and 70 bar) using a homogeneous ruthenium(-
triphos) catalyst (triphos ¼ 1,1,1-tris(diphenylphosphinomethyl)
ethane; Scheme 1) [11]. In the last decade, significant progress in
ester hydrogenolysis catalyzed by well-defined transition metal
complexes based on lutidine-derived pincer ligands has been
achieved (Scheme 1) [12]. In several cases, the reactivity can be
attributed to the existence of accessible metal-ligand cooperative
(
MLC) pathways [13].
Ester hydrogenation mechanisms are still under debate and
have thus far not been proven unequivocally [14]. A plausible
proposed catalytic cycle for ester hydrogenation catalyzed by a
cooperative Ru(hydride) complex is depicted in Scheme 2. The rate
limiting step of this reaction has generally been considered to be
hydrogen transfer to the stabilized ester carbonyl moiety to
generate a hemiacetal (Scheme 2, I). On the basis of DFTcalculations
three mechanisms have been proposed for this step: a) carbonyl
insertion (via a 4-membered ring Ru-O-C-H transition state) [15], b)
In this contribution, the synthesis and catalytic application of
novel CNC and CNN pincer ligands bearing flanking 1,2,3-
triazolylidene (tzNHC) [22] moieties and a central pyridine donor
II
are described. The corresponding Ru complexes have been applied
in catalytic ester hydrogenolysis. Considering the correlation
Scheme 2. Proposed catalytic cycle for the Ru-catalyzed hydrogenation of carboxylic esters to alcohols using metal-ligand cooperative systems. Box: proposed mechanisms for
hydrogen transfer (I): a) carbonyl insertion, b) H/OR metathesis and c) ligand assisted hydride transfer.
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S.N. Sluijter et al. / Journal of Organometallic Chemistry xxx (2017) 1e8
3
between electron density and catalytic ester hydrogenation activ-
ity, it is hypothesized that the strong tzNHC donors in these ligands
may enhance the hydricity of a Ru(hydride) fragment, leading to
higher catalyst activity. Furthermore, we have investigated the
mechanistic aspects of the ester hydrogenation such as the role of
the base, additives, MLC reactivity and the nature of potentially
relevant species in the catalytic reaction.
coordination mode of ligand L1 to the ruthenium center was
1
31
1
13
1
deduced from the combined H, P{ H} and C{ H} NMR spec-
troscopic data [28]. As a consequence of the inherent nonsym-
metrical character of the ligand binding, separate signals for each
1
13
1
hydrogen and carbon atom are observed by H and C{ H} NMR
spectroscopy. Most indicative are the distinctive signals for both
2
triazolylidene carbons at
1
d
172.2 ( JCP ¼ 7.2 Hz, cis to PPh
3
) and
) in the C{ H} NMR
ligands were observed as a doublet
2
13
1
64.9 ppm ( JCP ¼ 75.7 Hz, trans to PPh
3
spectrum. The hydrido and PPh
3
2
. Results and discussion
2
1
at
6.7 ppm in the P{ H} NMR spectrum, respectively. These data are
consistent with the only previous report of a fac-Ru(CNC) complex
21]. Meridional coordination of ligand L1 was observed in Pd(II)
complex 3 (see SI), which is also accessible via transmetalation of
d
ꢂ7.04 ppm ( JPH ¼ 28.9 Hz) in the H NMR and a singlet at
31 1
4
2.1. Synthesis of the CNC and CNN ligands
[
The desired CNC ligand L1 was efficiently synthesized in two
steps from commercially available starting materials (Scheme 3)
23]. To the best of our knowledge, only one tridentate (pincer)
I
2 2
Ag complex 1 with [Pd(PhCN) Cl ] [29].
[
For the corresponding Ag-complex 4 of CNN-ligand L2 (Scheme
ligand bearing two triazolylidene moieties is known [24] and L1 is
the first to be tested in catalysis. We were also interested to obtain
the CNN analogue L2, because the potential hemilability of the
triazole moiety might have a positive effect on the catalytic ester
hydrogenolysis. When only one equivalent of methylating agent
was reacted with intermediate A, a mixture of starting material,
mono- and bis(methylated) product was obtained. The three
different species were easily separated by column chromatography,
providing access to the CNN pincer ligand L2 (Scheme 3).
5
), cold-spray ionization (CSI-)HR-MS indicated the presence of the
þ
[Ag(CNN)
2
]
ion, suggesting that two ligands are coordinated to
one silver center. Coordination is presumably only occurring via the
triazolylidene donors, with the triazole groups remaining uncoor-
dinated. Transmetalation to ruthenium using [RuCl(CO)(H)(PPh
generated fac-[Ru(CO)(H)(L2)(PPh )]BF 5, with the facial coordi-
nation supported by NMR spectral features. Compared to complex
, the hydrido and PPh ligand appeared as lower frequency signals
3 3
) ]
3
4
2
3
2
1
at
40.9 ppm in the P{ H} NMR spectrum, respectively. In the
NMR spectrum, the large coupling of the tzNHC carbon (166.0 ppm;
d
ꢂ13.0 ppm ( JPH ¼ 28 Hz, cis to PPh
3
) in the H NMR and
3
1
1
13
d
C
2.2. Synthesis of Ru(II) complexes
2
J
CP ¼ 76.9 Hz) with the phosphorus atom indicates that the
Silver(I) complex 1 was obtained by stirring the CNC ligand L1 in
the presence of Ag O in MeOH for two days (Scheme 4) [25]. The
2
strongly donating NHC is located trans to the PPh ligand. This
3
leaves the position of the weak-field triazole-nitrogen coordinating
trans to the hydride.
formation of the desired complex was confirmed by the disap-
1
pearance of the triazolium hydrogen signal in the H NMR spec-
Both Ru complexes are susceptible to ligand-centered dear-
omatization upon reaction with one equivalent of KOtBu, marked
by a characteristic color change from brown-yellow to dark red. In
the corresponding NMR spectrum of 2′ an upfield shift of the pyr-
idine hydrogens and the appearance of the vinylic proton
trum. The high resolution mass spectrum (HR-MS) corresponds to a
1
:1 ratio of the silver and ligand, which suggests the presence of
þ
mononuclear structure [AgL1] with the two tzNHC groups coor-
dinating to the silver center in a linear fashion [26]. Unlike what
was reported for Ru-complexes with bis(NHC)pyridine systems
(5.52 ppm) is observed compared to 2. The dearomatization is fully
[27], direct deprotonation of the triazolium fragments of L1 using
reversible, as addition of hydrochloric acid (1 M in dioxane) led to
rearomatization of the pyridine ring. This chemoresponsive
behaviour of the bis-triazolylidene and triazolylidene-triazole
systems may be relevant for metal-ligand bifunctional ester
hydrogenolysis reactivity. Furthermore, the remarkable facial
various bases in the presence of various ruthenium precursors was
unsuccessful. However, silver complex 1 proved to be a useful re-
agent for transmetalation to generate fac-[Ru(CO)(H)(L1)(PPh )]BF
3 4
(
complex 2; fac ¼ facial) by reaction of 1 and [RuCl(CO)(H)(PPh
3 3
) ]
ꢁ
in THF at 55 C for 2 days (Scheme 4). The surprising fac-
$
4
Scheme 3. Synthesis of CNC and CNN ligands L1 and L2. i) NaN
3
, Na
2
3
CO , CuSO
5H
2
O, sodium ascorbate, DMF/H
2
O (4:1), ii) two equiv. Me
3
O$BF
4
, iii) one equiv. Me
3
O$BF
4
, followed
by separation using column chromatography.
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S.N. Sluijter et al. / Journal of Organometallic Chemistry xxx (2017) 1e8
ꢁ
Scheme 4. Synthesis of Ag(I) complex 1 and Ru(II) complex 2. i) Ag
2
O, MeOH, 48 h; ii) [Ru(CO)HCl(PPh
3
)
3
], THF, 55 C, 48 h.
ꢁ
Scheme 5. Synthesis of Ru(II)(CNN) complex 5 via Ag(I) complex 4. i) Ag
2
O, MeOH, 48 h; ii) [Ru(CO)HCl(PPh
3
)
3
], THF, 55 C, 48 h.
coordination mode of these novel tridentate ligands is reminiscent
of the coordination mode of the bis(thioether)amine SNS ligand in a
Ru catalyst for ester hydrogenation [30].
explained by the acidity of the formed phenol, neutralizing the
required base [33]. Gratifyingly, the sterically hindered tert-butyl
benzoate was hydrogenated to benzyl alcohol within 2 h (entry 10).
In fact, the activity of 2 for this substrate is higher than for methyl
benzoate (vide infra, Table 2).
2.3. Ruthenium catalyzed hydrogenolysis of esters
For complex 5, dissociation of the speculated hemilabile triazole
donor in the CNN motif could result in a vacant site on the Ru
center. Table 2 contains the results of methyl and tert-butyl ben-
zoate hydrogenation using complex 5 vs. complex 2. As no signifi-
cantly improved activity of pre-catalyst 5 compared to 2 was found,
we conclude that either the triazole donor is more strongly bound
than anticipated (hence not displaying hemilability) or this feature
does not play a critical role in the rate determining step of this
catalytic reaction [34].
The application of 2 in the catalytic hydrogenolysis of esters was
initially studied using methyl benzoate as the substrate, following
the protocol described by Beller and co-workers [31]. When
applying 20 mol% of KOtBu, full conversion was observed within 2 h
ꢁ
at 100 C under 50 bar of H
2
pressure in 1,4-dioxane, using a
catalyst loading of 0.75 mol% (Table 1, entry 1) [32]. At lower
pressure (5 bar) the substrate was still converted, albeit at a
significantly slower rate (77% conversion in 18 h). Other esters
could also be hydrogenated using this system (Table 1). The
aliphatic n-butyl benzoate was smoothly hydrogenated by complex
2.4. The role of KOtBu in the hydrogenolysis of esters
2
under the same reaction conditions (entry 2). Longer alkyl chains
did not pose a problem either, as methyl stearate was nearly
completely reduced to octadecanol within 2 h (entry 3). Methyl
oleate, which contains both a carbon-carbon double bond and ester
functionality, was considered an ideal substrate to test the che-
moselectivity of the system (entry 4). Almost full conversion of the
unsaturated fatty carboxylic ester was observed with formation of
both the saturated and unsaturated product in a 2.6: 1 ratio.
Following the reaction over time clearly indicated that ester
hydrogenolysis is kinetically favored over C]C bond hydrogena-
tion. The triglyceride triolein (not shown) was also converted, but
several (partly unidentified) products, including octadecanol (~5%)
and oleyl alcohol (~4%), were detected by GC analysis.
Catalyst 2 shows good activity in the hydrogenolysis of aromatic
esters, but only when a minimum amount of 20 mol% of KOtBu is
used (Table 3, entry 1 vs. 2). This base dependence is also apparent
for aliphatic esters (93% conversion of methyl butyrate to n-butanol
using 20 mol% KOtBu vs 4% conversion using 10 mol%). The
remarkable activity of complex 2 for tert-butyl benzoate led us to
investigate whether transesterification of the ester substrates,
induced by KOtBu, to generate tert-butyl benzoate could play a role
in the mechanism. Stirring methyl benzoate with 20 mol% of KOtBu
ꢁ
at 100 C (without catalyst) led to formation of 8% tert-butyl ben-
zoate, whereas with 10 mol% of base only traces (~1%) were
observed. Thus, transesterification followed by hydrogenation of
tert-butyl benzoate could be a plausible explanation for the need
for 20 mol% of KOtBu. From Table 3 it becomes apparent that the
hydrogenation of tert-butyl benzoate reaches significantly higher
conversion in the same reaction time than the corresponding
methyl ester (entry 1 vs 3). Furthermore, methyl benzoate was not
converted when KHMDS was applied as base, whereas the tert-
butyl analogue was fully consumed (entry 5 vs. 6) [32]. This might
be explained by tert-butoxide being a better leaving group than
methoxide or by a previously observed inhibiting effect of
Hydrogenation of g-valerolactone led to a moderate yield of 1,4-
pentanediol (entry 5). Not surprisingly, given the basic reaction
conditions, carboxylic acids were incompatible with the system
(
entries 6 and 7). Even the activated trifluoroacetic acid (TFA) was
not converted at all. As expected the methyl trifluoroacetate was
completely converted to trifluoroethanol (entry 8). When phenyl
benzoate (entry 9) was used as substrate, only some trans-
esterification products (tert-butyl benzoate and benzyl benzoate)
but no benzyl alcohol were detected by GC analysis. This might be
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S.N. Sluijter et al. / Journal of Organometallic Chemistry xxx (2017) 1e8
5
Table 1
Substrate scope for hydrogenation catalyzed by complex 2.
PPh3
N
BF
pTol
4
N
N
H ,
Cat. 2,
KOtBu
2
O
R'
N
Ru CO
R
O
R
OH + R'OH
H
N
1
,4-dioxane
pTol
1
00 °C
2
N N
Entry
1
Substrate
Conv. (%)a
53
Product
Yield (%)a
53
2
3
93
93
93
93
4
98
71
2
7
5
6
54
0
54
e
7
8
9
0
e
100
17
100
0
10
100
100
ꢁ
2
Conditions: 0.5 mmol ester substrate, 0.75 mol% of 2, 20 mol% of KOtBu, 50 bar of H in 1,4-dioxane, 100 C, 2 h.
a
19
Yields were determined by GC analysis with p-xylene as internal standard (entries 1e6, 9 and 10) or by F NMR spectroscopy using 1,3-bis(trifluoromethane)benzene as
internal standard (entries 7 and 8).
methanol [19]. Unfortunately, decreasing the amount of base for
the substrate tert-butyl benzoate also resulted in a large drop in
conversion (entry 4). Thus, although transesterification seems to
play a role in the hydrogenation of esters with 2, it does not fully
explain the need for the critical amount of base.
visible at 59.6, 22.9, 14.9 and ꢂ5.5 ppm, the latter being charac-
1
3
teristic for free PPh . The hydride region of the H NMR spectrum is
depicted in Fig. 1 and suggests the presence of three hydride-
containing Ru complexes (2'a, 2b and 2'c in a ~1:1:1 ratio).
31
Proton-coupled P NMR spectroscopy revealed that the doublet
1
at ꢂ6.90 ppm in the H NMR spectrum, with a large JPH coupling
constant of 122 Hz, correlates with the resonance at 23 ppm in the
2.5. NMR investigations of complex 2 under near-catalytic
31
1
P{ H} NMR spectrum. This J value indicates a mutual trans
arrangement of the hydride and PPh , similar to the values reported
for a related mer-[Ru(CNN)(CO)(H)(PPh )] complex [19]. Hence, the
conditions
3
1
3
To gain more insight in the catalytically active species, H NMR
investigations were performed. A solution of complex 2 in THF-d
was studied under near-catalytic conditions (20 equiv. of KOtBu,
bar of H ) in a J. Young pressure tube. At room temperature, the
CNC ligand likely coordinates in a meridional fashion, which leads
to the proposed structure of 2'a in Fig. 1. Most likely, the ligand
backbone is deprotonated in this complex, considering the large
amount of base present in the mixture. Apparently, fac-mer isom-
erization of the flexible CNC chelate is possible under these con-
ditions, for instance via a five-coordinated Ru species. Attempts to
induce this isomerization thermally in absence of base were
8
5
2
1
dearomatized complex 2′ was present in solution, as deduced by H
NMR spectroscopy. After the reaction mixture was heated at 100 C
ꢁ
for 2 h, several products were detected by NMR spectroscopy at
31
1
room temperature. In the P{ H} NMR spectrum four signals were
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S.N. Sluijter et al. / Journal of Organometallic Chemistry xxx (2017) 1e8
Table 2
Catalytic hydrogenation of methyl vs. tert-butyl benzoate with complexes 2 and 5.
BF
pTol
PPh3
BF4
pTol
PPh3
N
4
N
N
H ,
Cat.,
KOtBu
2
N
N
O
N
R
N
Ru CO
N
Ru CO
O
OH + ROH
H
N
H
N
1
,4-dioxane
pTol
pTol
1
00 °C
2
N N
5
N N
Entry
R
Cat
Yield (%)
1
2
3
4
Me
Me
tBu
tBu
2
5
2
5
53
50
99
100
ꢁ
Conditions: 0.5 mmol ester, 0.75 mol% of catalyst, 20 mol% of KOtBu, 50 bar H
2
in 1,4-dioxane at 100 C for 2 h. The yield was determined by GC analysis with p-xylene as
internal standard.
unsuccessful.
ratio of approximately 2:1, which suggests that either 2b or 2'c
contain a mer-coordinated CNC ligand, assuming that no fac-mer
isomerization occurs at room temperature. However, the exact
stereochemistry around the metal centers of 2b and 2'c has not
1
H- H COSY combined with 1H{31P} NMR spectroscopy indi-
1
cated that the hydride signals at ꢂ11.77 and ꢂ15.09 ppm (d,
2
J
HH ¼ 8.4 Hz) belong to a cis Ru(dihydride) bearing no phosphine
ligand (2b in Fig. 1). Considering the diamagnetic nature of this
complex and the prevalence of (active) Ru(II) species, we assume
that 2b contains the protonated ligand. The remaining two signals
2
been verified. When applying H pressure and 10 equivalents of
KOtBu to 2 the same species 2'a-c were observed in solution, albeit
in a slightly different ratio (2'a: 2b: 2'c ¼ 1.5: 2: 1). These NMR
experiments provide initial insight in the structure of the catalyt-
ically relevant species during the hydrogenation of esters. These
experiments also suggest that installment of a hemilabile donor on
the ligand is not required, as the phosphine (or carbonyl group) can
dissociate under (near-)catalytic conditions. Additionally, the mer-
isomer 2′ appears to be thermally accessible.
in the hydride region at ꢂ7.53 (integrating to
1 H) and
between ꢂ8.67 and ꢂ9.11 ppm (integrating to 2 H's) relate to a
31
1
signal at 59.6 ppm in the P{ H} NMR spectrum. Based on inte-
gration and 1H- H COSY, H({ P}) and P({ H}) spectra, T
1
1
31
31
1
mea-
1
surements (T [ 300 ms) and the observed patterns, which are
1
indicative of an ABMX spin system, this species presumably con-
cerns the trihydride species 2'c that contains a deprotonated CNC-
ligand but no CO ligand (Fig. 1). However, based on these data we
cannot deduce which ligand arm is deprotonated.
2.6. Mechanistic considerations
Once the H
2
pressure was released, the mixture of species
The mechanistic implications of the experiments described
above, including aspects related to i) metal-ligand cooperativity, ii)
converted to the mer- and fac-ruthenium complexes 2'a and 2′ in a
Table 3
Catalytic hydrogenation of methyl vs. tert-butyl benzoate with complex 2.
PPh3
N
BF
pTol
4
N
N
H ,
Cat. 2,
KOtBu
2
O
R
N
Ru CO
O
OH + ROH
H
N
1
,4-dioxane
pTol
1
00 °C
2
N N
Entry
R
Base
Yield (%)
53
1
2
3
4
5
6
7
8
Me
Me
tBu
tBu
Me
tBu
Me
Me
KOtBu
KOtBu (10 mol%)
KOtBu
KOtBu (10 mol%)
KHMDS
KHMDS
a
2
99
13
0
100
28
0
KOH
K CO
2 3
ꢁ
2
Conditions: 0.5 mmol ester, 0.75 mol% of 2, 20 mol% of KOtBu (except for entry 6), 50 bar H in 1,4-dioxane at 100 C for 2 h. The yield was determined by GC analysis with p-
xylene as internal standard.
a
Yield after 5 h.
Please cite this article in press as: S.N. Sluijter, et al., Journal of Organometallic Chemistry (2017), http://dx.doi.org/10.1016/
j.jorganchem.2017.01.003

S.N. Sluijter et al. / Journal of Organometallic Chemistry xxx (2017) 1e8
7
Fig. 1. Postulated structures (top e 2'a and 2'c contain a deprotonated monoanionic version of the CNC ligand L1; 2b contains the neutral ligand L1) and hydride region of 1H NMR
ꢁ
spectrum (bottom) of complexes formed from 2 and KOtBu under 5 bar H
2
after 2 h at 100 C.
role of the base and iii) substrate transesterification, are discussed
here. Like many other ester hydrogenation catalysts, complexes 2
and 5 have a cooperative functionality incorporated in their ligand
design that shows the expected reversible deprotonation/dear-
omatization reactivity upon addition of one equivalent of KOtBu.
The proposed active species 2'a and 2'c contain a deprotonated
ligand, whereas 2b does not, but this species is possibly in equi-
librium with the other two species. It is therefore likely that the
triazole-based ligand participates in the catalytic cycle, e.g. in the
hydrogen transfer and/or hemiacetal decomposition (Scheme 2).
This has also suggested been suggested for a similar fac-SNS system
ester hydrogenation mechanism is complex. It participates in
deprotonation of the ligand, inducing metal-ligand cooperativity, as
well as in transesterification of the substrate, and possibly also in
enhancing product expulsion from the catalyst.
3. Conclusions
Novel lutidine-derived CNC and CNN pincer ligands, L1 and L2,
have been developed. Coordination of the ligands via trans-
II
metalation of the corresponding Ag(I) complex led to Ru com-
plexes 2 and 5, with rare facial coordination of the tridentate
ligands. The complexes showed (reversible) deprotonation/dear-
omatization reactivity upon addition of one equivalent of KOtBu.
[15].
It is remarkable that a minimal amount of KOtBu (ꢀ20 mol%) is
II
necessary to achieve high conversions in the ester hydrogenation
with our Ru-systems. Although the requirement for a base or the
accelerating effect of a base was previously noted [14,21,32], the
exact role of the base in ester hydrogenation has hardly been
investigated. For the Co-catalyzed ester hydrogenation of alkyl
alkanoate substrates, the basic conditions generate enolate de-
rivatives that are susceptible to hydrogenolysis [35]. However, non-
enolizable esters (e.g. methyl benzoate and methyl trifluoracetate)
were not converted by this system, which is in contrast with our
observations. Thus this mechanism cannot be operative for our
The Ru species are active pre-catalysts for the hydrogenolysis of a
ꢁ
range of aromatic and aliphatic esters at 100 C in the presence of
20 mol% KOtBu. Potentially active Ru(CNC)(hydride) species have
been detected by NMR spectroscopy under near-catalytic condi-
tions. The mechanistic findings in this work confirm that the
catalysis is very complex, with a combination of metal-ligand
cooperativity, transesterification, effects of base strength and
product inhibition at work.
Acknowledgements
system.
Alkoxide
intermediates
of
the
trans-[Ru((R)-
BINAP)(H)
2
(R,R)-dpen)] lactone/ester hydrogenation catalyst were
We thank the Dutch National Research School Combination
Catalysis Controlled by Chemical Design (NRSC-Catalysis) for
financial support within project CJE_2009-13 and Ed Zuidinga and
Jan-Meine Ernsting for valuable technical assistance.
spectroscopically observed by Bergens et al. They argued that the
base facilitates elimination of the alkoxide for H on the metal
through deprotonation of the bifunctional group in their proposed
2
mechanism.3
1,49
Such a mechanism might be plausible for our
system, considering the similar facial coordination of the ligand
around the metal center and the results discussed below.
Appendix A. Supplementary data
Our results suggest that the beneficial effect of KOtBu on the
ester hydrogenation activity may relate to transesterification of
substrates to the corresponding tert-butyl ester derivatives [26]. We
found that the bulky substrates are hydrogenated considerably
faster than methyl esters. This might be explained by tert-butoxide
being a better leaving group than methoxide. Another explanation
may be that methanol has an inhibiting effect by reacting with the
deprotonated complex to form a Ru-alkoxide [32]. tert-Butanol is
likely too bulky to show this undesirable reactivity and additional
base has proven to push the equilibrium towards to the formation
of the alcohol(s) [27,32]. Lastly, the tert-butyl ester might inhibit the
inner-sphere mechanism (too bulky) and be transformed via the
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2017.01.003.
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