Ruthenium complexes with PYA pincer ligands for catalytic transfer hydrogenation of challenging substrates

Article abstract of DOI:10.2533/chimia.2019.299

Here we highlight the potential of a series of ruthenium complexes with tridentate N,N,N pincer-type ligands featuring two pyridylidene amide (PYA) moieties in the ligand skeleton. They were successfully applied in transfer hydrogenation of ketones and C=C double bonds. Rational ligand design was key for increasing the catalytic performance in the reduction of challenging substrates such as potentially chelating acetylpyridines. The specific reaction profiles indicate catalyst poisoning via imine coordination as well as N,O-bidentate coordination of the substrate or the product. Approaches to mitigate this inhibition are presented. Furthermore, these PYA pincer ruthenium complexes accomplish the selective reduction of the C=C over C=O bond of α,β-unsaturated ketones such as benzylideneacetone, while other α,β-unsaturated ketones such as trans-chalcone predominantly underwent oxidative C=C bond cleavage.

Full text of DOI:10.2533/chimia.2019.299

Laureates: Junior Prizes of the sCs faLL Meeting 2018
CHIMIA 2019, 73, No. 4 299
doi:10.2533/chimia.2019.299
Chimia 73 (2019) 299–303 © Swiss Chemical Society
Ruthenium Complexes with PYA
Pincer Ligands for Catalytic Transfer
Hydrogenation of Challenging Substrates
Philipp Melle§ and Martin Albrecht*
§SCS-DSM Award for best poster presentation in Inorganic & Coordination Chemistry
Abstract: Here we highlight the potential of a series of ruthenium complexes with tridentate N,N,N pincer-type
ligands featuring two pyridylidene amide (PYA) moieties in the ligand skeleton. They were successfully applied
in transfer hydrogenation of ketones and C=C double bonds. Rational ligand design was key for increasing the
catalytic performance in the reduction of challenging substrates such as potentially chelating acetylpyridines.
The specific reaction profiles indicate catalyst poisoning via imine coordination as well as N,O-bidentate co-
ordination of the substrate or the product. Approaches to mitigate this inhibition are presented. Furthermore,
these PYA pincer ruthenium complexes accomplish the selective reduction of the C=C over C=O bond of α,β-
unsaturated ketones such as benzylideneacetone, while other α,β-unsaturated ketones such as trans-chalcone
predominantly underwent oxidative C=C bond cleavage.
Keywords: Homogenous catalysis · Ligand design · Pyridylidene amides · Transfer hydrogenation
Philipp Melle studied chemistry at the curs via the amide nitrogen, which has to be deprotonated with
University Hamburg. His bachelor thesis a base either in situ or prior to metalation. With varying groups
on the development of UV-cross-linkable on the amide either monodentate,[5] bidentate (N,N and C,N)[6] or
adhesives was written in cooperation with tridentate[7] N,N,N pincer-type bis-PYA ligands have been suc-
tesa SE. He went for an Erasmus stay at the cessfully synthesized.
University College Dublin to the group of
One of the main features of PYAs is their donor flexibility,
Prof.Albrecht, studyingtransferhydrogena- derived from limiting resonance structures due to the highly con-
tion with abnormal dicarbene rhodium(iii) jugated pyridinium-amide system. The para PYA ligand has been
complexes. Afterwards he joined the group shown to adapt to different solvent polarities, toggling between the
of Prof. Fröba for his master project about formally neutral diene-imine resonance structure (L-type coordi-
the synthesis and characterization of hierarchically structured nation mode, weak donor) and the zwitterionic pyridinium-amide
aerogels consisting of periodic mesoporous organosilica nanopar- resonance structure (X-type coordination mode, strong donor).[8]
ticles. In 2016 he moved to Bern for a PhD in the group of Prof. Also the comparison of ortho, meta and para PYAs showed a strong
Albrecht, focusing on the synthesis, characterization and catalytic effect on the electronic properties, with the mesoionic[9] meta PYA
applications of transition metal complexes bearing N-mesoionic being the strongest donor.[7,10] Their structural variety and donor
ligands.
flexibility make PYAs a promising class of ligands that may adapt
their electronic configuration depending on the requirements of the
1. Introduction
Pyridylidine amides (PYAs) are a relatively new class of li-
gands[1] that consist of an alkylated pyridine ring, which is con-
nected to an amide functionality.A wide structural variety of PYAs
is accessible taking into account the facile synthetic procedure
that normally includes the coupling of aminopyridines with acyl
chloride derivates. Depending on the nature of the acyl chloride
reagent, different substituents can be introduced to the ligand sys-
tem, resulting in e.g. alkyl-,[2] pyridyl-[3] or phenyl-substituted[4]
M
M
N
N
ortho PYA :
meta PYA :
N
R
N
R
O
O
M
M
N
N
N
N
O
O
PYAs. The choice of aminopyridines on the other hand leads to
R
R
the formation of different pyridine substitution patterns, includ-
ing ortho (2-aminopyridine), meta (3-aminopyridine) and para
(4-aminopyridine) PYAs (Fig. 1). Pyridine alkylation is usually
straightforward, i.e. using standard methylation reagents such as
methyl iodide or methyl trifluorosulfonate, resulting in the cor-
responding pyridinium salts. Coordination to the metal center oc-
M
M
para PYA :
N
N
N
N
R
O
O
R
X-type, anionic
L-type, neutral
*Correspondence: Prof. M. Albrecht
Fig. 1. Limiting resonance structures of ortho/meta/para-PYA ligand sys-
tems, featuring the zwitterionic X-type anionic coordination mode on the
left and the L-type neutral coordination mode on the right.
E-mail: martin.albrecht@dcb.unibe.ch
Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3,
CH-3012 Bern

300 CHIMIA 2019, 73, No. 4
Laureates: Junior Prizes of the sCs faLL Meeting 2018
metal center, and therefore these ligands have the potential to sta- plex 2b were readily replaced with a variety of ancillary ligands,
bilize several transition states on a putative catalytic cycle. These including PPh in complex 3 and 1,2-bis(diphenylphosphino)ethane
properties have led to a number of potent homogeneous catalysts (dppe) in comp3 lex 4 (Scheme 1).
containing rationally designed PYA ligands.[11]
We have shown that the catalytic activity in the hydrogenation
The pincer platform[12]
provides robustness of metal bonding of ketones is strongly dependent on a variety of parameters includ-
ing the PYA moiety, the ancillary ligands, and the reaction condi-
(catalyst longevity) and great potential for variability (Fig. 2). They
are widely used ligand motifs for numerous potent homogeneous tions (Fig. 3).[7] Here we will highlight their activity towards chal-
catalysts, with the commercially available Ru-MACHOor Milstein’s lenging substrates including heteroaromatic and α,β-unsaturated
catalyst as prominent examples.[13] We have therefore incorporated ketones containing two potential hydrogenation sites.
PYA donor sites into a pincer framework based on the well-known
pyridine-2,6-dicarboxamide scaffold.[14] Accordingly, the dicationic 2. Results and Discussion
ligand precursors 1a,b containing two PYA units were prepared and
metalated with [RuCl (cym)]2 under mildly basic conditions, af- 2.1 Transfer Hydrogenation of Heteroaromatic
fording the correspond2ing N,N,N pincer-type bis-PYA ruthenium(ii) Substrates
complexes 2a and 2b (Scheme 1). The acetonitrile ligands in com-
While precatalyst 3 was successfully applied in transfer hy-
drogenation of a variety of substrates, including diaryl, aryl-alkyl
and dialkyl ketones, the conversion of heteroaromatic substrates
bearing nitrogen functionalities remained difficult. In particular
pyridyl-containing ketones were only slowly and never fully con-
verted (Fig. 4, Table 1). For example, di(2-pyridyl)ketone was
initially a suitable substrate with high catalytic turnovers (entry
1, 40% conversion after 15 min). At later stages, however, the
activity drops dramatically and the conversion plateaus at around
60% after 3 h reaction time. This profile is indicative for product
inhibition. The substrates 2-, 3- and 4-acetylpyridine (entries 3,
5 & 7) are converted only slowly already at early reaction times,
Fig. 2. Complex design: possible modifications of the catalyst for in-
creased activity in transfer hydrogenation.
indicative for a substrate-mediated inhibition (Fig. 4).
-
-
PF6
PF6
NCCH3
O
(PF6)2
N
N
N
N
N
N
Ru
NH HN
N
(a)
O
NCCH3
O
O
N
NCCH3
(PF6)2
PPh3
O
1a
2a
N
N
N
N
Ru
(b)
O
NCCH3
-
-
PF6
PF6
N
NCCH3
3
NCCH3
O
(PF6)2
N
N
N
N
N
N
Ru
NH HN
N
(a)
O
NCCH3
O
O
N
NCCH3
2b
(PF6)2
NCCH3
O
1b
N
N
N
N
Ru
O
PPh2
(c)
N
Ph2P
4
Scheme 1. Synthesis of ruthenium(ii) N,N,N pincer-type bis-PYA catalysts. Reaction conditions: (a) 0.5 equiv. of [RuCl2(cym)]2; 3 equiv. of Na2CO3 in
MeCN, reflux, 16 h. (b) 1 equiv. of PPh3 in EtOH, reflux, 16 h. (c) 1 equiv. of dppe in EtOH, reflux, 16 h.
Fig. 3. Evolution of catalytic performance in transfer hydrogenation of benzophenone as the model substrate.

Laureates: Junior Prizes of the sCs faLL Meeting 2018
CHIMIA 2019, 73, No. 4 301
Table 1. Catalytic activity of complexes 3 and 4 in transfer hydrogenation
Three potential catalyst inhibition pathways are most conceiv-
able with complex 3 (Scheme 2):
of heteroaromatic substrates.a
1) Imine N-coordination (which should be stronger in the product
entry substrate catalyst
yield [%]b
5 min 1 h
TOFini [h–1]
imine than the starting material due to the electron-withdraw-
ing nature of the carbonyl group in the substrate)
2) N,O-bidentate chelation of the substrate via the pyridine nitro-
6 h
63
1
2
3
4
5
6
7
8
3
4
3
4
3
4
3
4
25
26
< 2
10
9
52
300
gen and the carbonyl oxygen
3) N,O-bidentate chelation of the deprotonated product alcohol,
either after migratory insertion of the ketone into the Ru–H
bond or as alkoxide under basic conditions
> 99 > 99 310
10 < 10
> 99 > 99 120
4
PPh3
22
38
17
23
37
110
N
N
N
4
> 99 50
Ru
N
O
R
N
9
28
69
110
50
O
4
O
R
N
R
A
1.
aGeneral reaction conditions: substrate (1 mmol), KOH (0.1 mmol, 10 mol%),
[Ru] (0.01 mmol, 1 mol%), iPrOH (5 mL), reflux temperature. bdetermined by
1H NMR spectroscopy using hexamethylbenzene as internal standard and aver-
aged over at least two runs, conversions correspond to yields.
PPh3
PPh3
O
N
N
N
N
N
Ru
R
N
N
N
2.
Ru
O
NCCH3
N
R
NCCH3
OH
3.
3
B
R
PPh3
N
N
N
Ru
O
N
R
C
Scheme 2. Possible inhibition pathways of catalyst 3 with heteroaro-
matic substrates, including imine N-coordination (1.), N,O-bidentate
chelation of substrate (2.) or N,O-bidentate chelation of the product
alcohol (3.).
These pathways take into account that both MeCN ligands
are very labile and dissociate under catalytic conditions. N,O-
chelationismuchmorefavoredfordi(pyridyl)ketoneand2-acetyl-
pyridine substrates, since the spatial arrangement of the pyridine
nitrogen and the oxygen atom is preset for bidentate coordination
and formation of a stable 5-membered metallacycle. Moreover,
di(pyridyl)ketone is known to act also as a N,N-bidentate coordi-
nating ligand.[15] With 3- and 4-acetylpyridine as substrate, on the
other hand chelation is not possible and inhibition pathways 2 and
3 are of minor relevance.
The initial rates of di(pyridyl)ketone and 2-acetylpyridine, the
two potential chelators, are substantially different, which may be
due to their specific electronic properties. 2-acetylpyridine shows
a strong affinity for N,O-bidentate bonding via pathways 2 and
3, i.e. binding as N,O-chelate with the ketone oxygen directly or
as alkoxide after migratory insertion. Taking into account the +I
effect of the methyl group, which increases the electron density
on the carbonyl group, the oxygen lone pair is much more avail-
able for metal bonding. The di(pyridyl)ketone on the contrary
features two electron-withdrawing pyridyl substituents on the
carbonyl group which leads to substantially lower electron den-
sity on the oxygen atom and prevents direct chelation of the sub-
strate via ketone bonding. The base-mediated alkoxide formation
of product inhibition pathway 3 however is expected to be more
pronounced for alcohols with electron-deficient substituents. The
higher acidity of the OH-group in di(pyridyl)methanol facilitates
Fig. 4. Time conversion profiles for the transfer hydrogenation of
2-acetylpyridine (a) and 3-acetylpyridine (b) with catalyst 3 (green lines)
and catalyst 4 (blue lines) respectively. Reaction conditions: substrate
(1 mmol), KOH (0.1 mmol, 10 mol%), [Ru] (0.01 mmol, 1 mol%),
iPrOH (5 mL), reflux temperature.
N,O-chelation after formation of a substantial amount of the hy-

302 CHIMIA 2019, 73, No. 4
Laureates: Junior Prizes of the sCs faLL Meeting 2018
O
O
O
drogenated product as seen in the conversion plateau after ca. 30
minutes reaction time. Hence, the reaction with di(pyridyl)ketone
is product-inhibited, while the conversion of 2-acetylpyridine is
substrate-inhibited.
Inhibition pathway 1, viz. the formation of a Ru–N bond to the
pyridine nitrogen, is much more favored for sterically unshielded
pyridine substrates. In this model the substrate mediated catalyst
cat. 3 (1 mol%)
a)
b)
Ph
Ph
Ph
Ph
KOH, i-PrOH, reflux
> 99%
OH
cat. 3 (1 mol%)
KOH, i-PrOH, reflux
> 99%
inhibition will increase in the following order: di(pyridyl)ketone Scheme 3. Selective transfer hydrogenation of benzylideneacetone
catalyzed with 3 gives exclusively benzylacetone, even though benzyl-
acetone is converted to the fully saturated product in independent runs.
≈ 2-acetylpyridine < 3-acetylpyridine < 4-acetylpyridine, which
explains the higher conversion of 3-acetylpyridine (37% after 6 h)
compared to 4-acetylpyridine (28% after 6 h).
widely used in the fragrance industry as a perfume ingredient.[19]
Catalyst inhibition via N,O-bidentate chelation of the sub-
strate or the product to the ruthenium center may be prevented
when substituting the monophosphine ligand in complex 3 with
a diphosphine ligand, which reduces the number of substitution-
The main industrial production processes rely on heterogeneously
catalyzed hydrogenation reactions.[20]
This selectivity pattern is particularly remarkable when con-
sidering that the benzylacetone is readily transfer hydrogenated
labile MeCN ligands from two to only one. Thus, reaction of com-
when used as a substrate (Scheme 3b). In a separate catalytic run
under the exact same conditions benzylacetone is quantitatively
hydrogenated within one hour, giving 4-phenyl-2-butanol as the
only product. Based on these results we assume deactivation of
plex 2b with dppe affords complex 4 featuring only one potential
coordination site for substrate coordination (cf Scheme 1).
Complex 4 indeed transfer hydrogenates 2-acetylpyridine
and di(pyridyl)ketone quantitatively and with appreciable reac-
the catalytically active species after C=C bond hydrogenation,
tion rates (Fig. 4, Table 1), demonstrating the potential of rational
which prevents further reduction. Catalyst deactivation is sup-
ported by the results obtained from reduction of benzylideneac-
etone, followed by addition of acetophenone after 30 min to the
reaction mixture. Negligible hydrogenation of acetophenone was
noted after 6 h, although this substrate was fully converted after
30 min when applied on its own.
catalyst design and also supporting chelation as the major cata-
lyst inhibition pathway with these two substrates. A closer look at
the time-conversion profiles shows that the potentially chelating
substrates are converted >20 times faster (2-acetylpyridine, en-
tries 3, 4) and to completion without any detectable product in-
hibition (di(pyridyl)ketone, entries 1, 2). However, the turnover
frequency increased less dramatically for the pyridyl derivatives
A different reactivity pattern was observed when using trans-
chalcone as substrate, which afforded a mixture of products con-
with the acyl group in more remote position (entries 5–8). The less
sisting of the saturated ketone, acetophenone, benzaldehyde, and
their hydrogenated products (Scheme 4). The selective formation
of the 1,3-diphenyl-1-propanone without further reduction of the
substantial enhancement of catalytic activity is in agreement with
monodentate imine coordination to the metal center, which is not
prevented by the bidentate phosphine. Imine coordination is much
less favored for 2-acetylpyridine due to the steric shielding of the
carbonyl group parallels the selectivity pattern observed for ben-
zylideneacetone (see above). The formation of the other products
acyl group in ortho positon. The increased activity of complex 4
is more intriguing and suggests an oxidative cleavage of the C=C
bond to yield benzaldehyde and acetophenone, which themselves
are substrates for transfer hydrogenation and afford benzyl alco-
towards the transfer hydrogenation of these substrates as compared
to complex 3 is rationalized by the fact that imine coordination is
less competitive with complex 4 because of the high trans effect
of the phosphine, whereas in complex 3, imine coordination may
occur trans to the central pyridyl site of the PYA pincer ligand and
hence be kinetically much less labile and more inhibiting. Indeed,
1H NMR spectroscopic investigations on the lability of the two
MeCN ligands in complex 3 revealed that the MeCN ligand trans
to the pyridyl unit is exchanged more than 2 orders of magnitude
slower (t = 8 h) than the ligand trans to the phosphine (t1/2 < 2
min). He1n/2ce, imine coordination trans to pyridyl in complex 3 is
much more effective than trans to the phosphine in complex 4 and
thus decelerates the catalytic activity of complex 3 towards the
conversion of heteroaromatic substrates that are otherwise little
shielded.
hol and 1-phenylethanol. Monitoring the reaction progress using
1H NMR spectroscopy allows for time-dependent quantification
of the various products (Fig. 5). Accordingly, benzaldehyde and
acetophenone are formed simultaneously at the early stages and at
much faster rates (TOFini = 130 h–1) than transfer hydrogenation of
the substrate takes place (TOFini
= 50 h–1). These initial observa-
tions imply that ruthenium-catalyzed oxidative C=C bond cleav-
age is substantially faster than the hydrogenation of this bond.
Moreover, time-dependent monitoring clearly indicates that alde-
hydes are converted much faster than ketones (cf the rapid con-
sumption of benzaldehyde compared to the slow hydrogenation
of acetophenone). Notably, the very slow formation of 1-phenyl-
ethanol and benzylalcohol may probably be related to the same
catalyst deactivation as observed in the selective hydrogenation
of benzylideneacetone.
The same reactivity and selectivity patterns were observed
when 1-phenyl-2-buten-1-one was used as α,β-unsaturated ke-
tone substrate. However, the presence of a methyl rather than a
phenyl group at the ene terminus increased the propensity for
2.2 Selectivity in Transfer Hydrogenation of
α,β-Unsaturated Ketones
Michael systems, i.e. α,β-unsaturated ketones, constitute an-
other class of challenging substrates for (transfer) hydrogenation
as both the C=C and C=O double bonds may undergo reduction.
Depending on the substrate, complex 3 induces different reactiv-
Michael aldol condensation reactions and produced a mixture of
ity patterns. Transfer hydrogenation of benzylideneacetone un-
products from C–C bond formation in addition to selective C=C
hydrogenation and oxidative C=C bond cleavage products.
der aerobic conditions produced selectively the saturated ketone
as the exclusive product (Scheme 3a). Complex 3 achieves full
conversion in just about 30 min (1 mol% catalyst loading) and
even upon prolonged reaction times, no hydrogenation of the C=O
bond was observed, thus featuring one of the rare homogeneous
catalytic systems that selectively reduces the C=C bond in α,β-
unsaturated ketones. Other catalysts typically show selectivity for
hydrogenation of the carbonyl bond[16] or produce the fully hydro-
genated product.[17] Of note, benzylacetone is a natural product
found in wild tobacco or cocoa beans to attract melon flies[18] and
3. Conclusions
Here we introduced two electronically flexible PYA moieties
into the well-known pincer coordination motif and demonstrated
that the corresponding Ru(ii) complexes are excellent catalyst
precursors for transfer hydrogenation. Catalyst tailoring by intro-
ducing ancillary phosphine ligands induces a substantial enhance-
ment of the catalytic performance and allows for the efficient

Laureates: Junior Prizes of the sCs faLL Meeting 2018
CHIMIA 2019, 73, No. 4 303
Scheme 4. Product distribution
after 6 h reaction time for the sub-
strate trans-chalcone (TH = trans-
O
TH
Ph
Ph
+
33%
O
fer hydrogenation).
O
cat. 3 (1 mol%)
KOH, i-PrOH, reflux
Ph
Ph
O
OH
OH
TH
+
oxidative
cleavage
< 2%
12%
21%
12%
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