Lutidine-derived Ru-CNC hydrogenation pincer catalysts with versatile coordination properties

Article abstract of DOI:10.1021/cs500720y

Lutidine-derived bis-N-heterocyclic carbene (NHC) ruthenium CNC-pincer complexes (Ru-CNC's) were prepared. Depending on the synthetic procedure, normal (1, 2) or mixed normal/abnormal NHC-complexes (3) are formed. In the presence of phosphazene base, Ru-CNC complexes activate nitriles to give ketimino compounds 4-6. Nitrile adduct 4 shows reactivity toward strong bases to yield dearomatized complex 7, which heterolytically activates H₂ to form the bis-hydrido complex 8. Finally, these Ru-CNC's are active in catalytic hydrogenation of CO₂ to formate salts, and unlike the phosphine-containing Ru-PNP counterpart, they also catalyze the selective hydrogenation of esters to alcohols.

Full text of DOI:10.1021/cs500720y

Letter
pubs.acs.org/acscatalysis
Lutidine-Derived Ru-CNC Hydrogenation Pincer Catalysts with
Versatile Coordination Properties
Georgy A. Filonenko, Elena Cosimi, Laurent Lefort, Matthew P. Conley, Christophe Coper
́
et,⊥
†
,‡
§
§
⊥
∥
†,‡
,†,‡
Martin Lutz, Emiel J. M. Hensen, and Evgeny A. Pidko*
†
‡
Inorganic Materials Chemistry Group, Schuit Institute of Catalysis, Institute for Complex Molecular Systems, Eindhoven University
of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
§
DSM Innovative Synthesis BV, P.O. Box 18, 6160 MD Geleen, The Netherlands
⊥
̈ ̈
Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir Prelog Weg. 2 CH-8093 Zurich, Switzerland
∥
Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The
Netherlands
*
S Supporting Information
ABSTRACT: Lutidine-derived bis-N-heterocyclic carbene
NHC) ruthenium CNC−pincer complexes (Ru-CNC’s)
were prepared. Depending on the synthetic procedure, normal
1, 2) or mixed normal/abnormal NHC-complexes (3) are
(
(
formed. In the presence of phosphazene base, Ru-CNC
complexes activate nitriles to give ketimino compounds 4−6.
Nitrile adduct 4 shows reactivity toward strong bases to yield
dearomatized complex 7, which heterolytically activates H to
2
form the bis-hydrido complex 8. Finally, these Ru-CNC’s are
active in catalytic hydrogenation of CO to formate salts, and
2
unlike the phosphine-containing Ru-PNP counterpart, they also catalyze the selective hydrogenation of esters to alcohols.
KEYWORDS: ruthenium, hydrogenation, metal−ligand cooperation, N-heterocyclic carbene, nitrile activation
uthenium lutidine-derived pincer complexes containing
Scheme 1. Selected Lutidine-Derived Ru Pincer Complexes
R
PNX-type ligands are efficient catalysts for a wide range of
important chemical transformations. They show catalytic
1
activity in acceptorless dehydrogenation, hydrogenation of
2,3
4
carboxylic acid derivatives, synthesis of amides from esters,
5
,6
and hydrogenation of CO2. Reversible ligand dearomatiza-
tion is often invoked to explain the catalytic properties of Ru-
PNX catalysts. Reacting Ru-PNX complexes with a strong base
triggers ligand-assisted cooperative activation of H , carbonyl
2
7
−12
compounds, and CO2.
N-Heterocyclic carbene (NHC)
complexes have attracted considerable attention in catalysis
13−18
because they offer several advantages over phosphines.
In
the context of pincer complexes, replacement of phosphines in
Ru-PNN with NHC ligands yields Ru-CNN catalysts that
19−22
match or outperform their phosphine-based counterparts.
In addition, Ru-CNN pincers reported by Song and co-workers
show similar chemical reactivity toward ligand dearomatization
and heterolytic H activation. Although many examples of
addition of LiBr during complexation. The reactivity toward
2
strong bases and H draws a parallel between Ru-CNC’s and
pyridine-based ruthenium bis-NHC pincers have been
2
2
3−26
the thoroughly studied Ru-PNX pincers developed by the
Milstein group (Scheme 1). Deprotonation of Ru-CNC’s yields
dearomatized species that promote heterolytic cleavage of H2.
reported,
ruthenium complexes based on potentially
2
2,27,28
cooperative CNC ligands are rare.
Consequently, no
comparison to existing PNP analogues has been offered.
Here, we report the synthesis of Ru-CNC complexes and
explore their reactivity and catalytic properties. We demon-
strate that the coordination mode of the NHC fragments
Received: May 27, 2014
Revised: July 7, 2014
Published: July 8, 2014
(Scheme 1) can be controlled by the sterics of the ligand or the
©
2014 American Chemical Society
2667
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ACS Catalysis
Letter
Ru-CNC catalysts are active in CO hydrogenation to formates,
The reaction of L2 containing 2,6-diisopropylphenyl (dipp)
2
and unlike structurally analogous pyridine-based Ru-PNP
pincers, they also catalyze ester hydrogenation, also referred
to as ester hydrogenolysis.
The Ru-CNC pincer complex 1 was synthesized by reacting
bis-imidazolium ligand L1 containing mesityl (Mes) substitu-
ents, the base 2-tert-butylimino-2-diethylamino-1,3-dimethyl-
perhydro-1,3,2-diazaphosphorine (BEMP), and RuHCl(CO)-
substituents with RuHCl(CO)(PPh ) in the presence of LiBr
3
3
resulted in complex 2, containing normally bound NHC groups
(Scheme 2). Exchange of the bromide for a triflate by treatment
of 2 with silver triflate yielded the more soluble 2*OTf,
allowing the detection of the Ru−C resonances at 191.3 ppm in
the 13C NMR spectrum. In contrast, complexation of L2 with
RuHCl(CO)(PPh ) in the absence of LiBr led to 3 (Scheme
3
3
(
PPh ) in THF (Scheme 2). Alternative approaches involving
2), in which one of the NHC arms was coordinated abnormally
3
3
30−32
to Ru through the C4 carbon.
The hydride ligand in 3 is
Scheme 2. Complexation of Ru with bis-NHC Ligands in
THF
trans to the PPh as evident from the large J (100 Hz). The
3
PH
signature C2 imidazolium proton of the abnormally bound
NHC moiety appears at 9.66 ppm, while the remaining C5
imidazolium proton is significantly shifted upfield to 4.46 ppm.
The solid-state structure of 3 is shown in Figure 2. Complex 3
Figure 2. X-ray crystal structure of 3 (50% probability ellipsoids,
solvent molecules, hydrogens, and the halide anion are omitted for
clarity). Selected bond lengths (Å): Ru1−P1, 2.4353(5); Ru1−C9,
2.0672(16); Ru1−C23, 2.1124(17); Ru1−C56, 1.8283(18); Ru1−N1,
2.2034(14).
transmetalation or using strong anionic bases were unsuccess-
ful. Initially, 1 was obtained as a bromide/chloride mixture. The
molecular structure in the crystal is shown in Figure 1. The
is a rare example of bis-NHC pincers with mixed normal/
3
3,34
abnormal composition.
Our findings are consistent with
previous reports on Os and Ir NHC’s, for which counterion and
steric effects were shown to control the NHC coordination
3
5−38
mode.
In particular, it has been shown that in the presence
of Br− anions, the C−H heterolysis at C2 position is
accelerated, resulting in the preferential normal NHC
coordination. The possibility of selecting the NHC binding
mode of the CNC pincer ligand reported here provides yet
another useful tool for tuning properties of Ru-CNC’s.
38
When CH CN was used as a solvent, the complexation of the
3
Ru precursor with L1 and L2 led to the formation of CH CN
3
1
adducts 4 and 5, respectively (Scheme 3). The H NMR of 4
Figure 1. X-ray crystal structures of 1 and 4 (50% probability
ellipsoids, solvent molecules, hydrogen atoms on the ligand, and the
bromide anion in 4 are omitted for clarity). Selected bond lengths (Å):
Scheme 3. Complexation of Ru with bis-NHC Ligands in the
Presence of Nitriles
(
1
1) Ru1−C9, 2.0886(12); Ru1−C22, 2.0751(12); Ru1−C32,
.8080(13). (4) Ru1−N6, 2.1614(16); Ru1−C9, 2.0470(18); Ru1−
C22, 2.0737(18); Ru1−C34, 1.833(2); Ru1−N1, 2.1667(15).
pure bromide 1 was isolated when the reaction was conducted
19
in the presence of LiBr. The product is stable to ambient
atmosphere in the solid state. In CD Cl , the NHC backbone
2
2
3
protons appear as two doublets at 7.17 and 6.66 ppm ( J = 2
HH
Hz), and the pyridine protons appear as a triplet and doublet
3
(
7.82 and 7.46 ppm, J = 8 Hz). Upon ligand coordination,
HH
the ortho-CH groups and aromatic protons of the mesityl
3
substituents are no longer equivalent and appear as separate
singlets. The Ru−H signal is shifted significantly upfield to
1
9
−
15.6 ppm, similar to that observed in related Ru pincers.
Methylene protons in 1 appear as a broad peak at 5.1 ppm.
29
2
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ACS Catalysis
Letter
contains a high frequency imino proton signal at 9.89 ppm.
Three methylene bridge protons appear as two sharp doublets
at 5.40 and 5.30 ppm ( JHH = 14 and 8 Hz) and a sharp singlet
the pyridine ring is rearomatized, and all four methylene
protons are observed as doublets at 5.59 and 5.33 ppm.
Similar to the structurally analogous Ru-PNP catalysts, Ru-
2
at 8.53 ppm, indicating nitrile insertion into the C−H bond of
CNC’s were active in CO hydrogenation to formate salts (see
Table S1 in the Supporting Information). The conversion of
2
6
the methylene group. The Ru-NHC resonances of 4 appear at
1
3
1
91.9 and 191.2 ppm in the C NMR spectrum. Complex 5
an equimolar H /CO mixture (40 bar) at 70 °C in the
2
2
with dipp groups on the NHC’s has an NMR spectrum similar
to that of 4. The crystal structure analysis of 4 and 5 (Figure 1
and Supporting Information) confirms the nitrile addition
across the metal center and the methylene bridge of the ligand.
The coordination of the CNC ligand in 4 and 5 is similar to
that in 1.
presence of DBU resulted in near full base consumption within
1 h, giving a TON up to 2600. Nevertheless, under these
conditions, Ru-PNP complex 9 (RuHCl(CO)( BuPNP))
t
outperforms bis-NHC pincers (see Table S1, Supporting
Information). This may be due to the stronger donor properties
of the PNP ligand, resulting in a more electron-rich Ru center
in 9 compared with that in Ru-CNC’s 1−6, evident from the
slight difference in the position of the CO stretching band in
Organonitrile activation resulting in the formation of a new
34
35
C−C bond was reported for Rh and Ir complexes. The
nitrile binding mode in 4 and 5 resembles that in Re-PNP
ketimido and enamido adducts, with the difference that the
addition to Re-PNP occurs only after the deprotonation of the
−1
these complexes (ν(CO) = 1906 cm for 9 vs 1916−1927
−1
cm for 1−6).
Hydrogenation of esters to alcohols, which can also be
referred to as ester hydrogenolysis, is usually more challenging
39
ligand with a strong base. Similar nitrile addition with
subsequent coordination of an imine group to the metal center
was previously described for macrocyclic complexes of Fe, Co,
3,45−48
compared with hydrogenation of CO2
The Ru-PNP
complex 9 is inactive in ester hydrogenation. In sharp contrast,
Ru-CNC complexes 1−6 hydrogenate a wide range of esters to
the corresponding alcohols (Table 1). Ru-CNC catalysts
effectively hydrogenate aromatic esters, including chloro- and
methoxy-functionalized derivatives (Table 1, entries 23−26),
aliphatic esters, and lactones. The hydrogenation of methyl 10-
undecenoate (Table 1, entries 21, 22) resulted in the
predominant formation of the fully saturated undecanol
product. Good to quantitative yields were obtained in 4−16
4
0−42
W, and Mo.
In addition, iron complexes with tetradentate
nitrogen ligands were shown to attack nitriles and form similar
43
adducts in the presence of base (NEt3). Interestingly, 1 can
also undergo a direct transformation to nitrile adducts in the
presence of BEMP. In CH CN, ∼47% of 1 was transformed to
3
4
overnight. In the presence of benzonitrile, 1 is converted to 6
in 91% yield (see the Supporting Information). Note that the
related phosphine-based pincer (Ru-PNP, 9, Scheme 1) did not
react with nitriles under these conditions.
h at 70−100 °C and 50 bar H in the presence of KOMe or
2
t
Catalysis with lutidine-derived pincer complexes is often
triggered by activation with a base. The strong base
deprotonates the ligand side arm and generates five-coordinate
active species. The reactivity of Ru-CNC’s with strong bases
was probed with NMR spectroscopy. The reaction of 1 with
KO Bu base promoters. This activity was achieved at a relatively
low catalyst loading (0.1−0.5 mol % Ru, Table 1 and
49−52
Supporting Information). Mercury poisoning
(315 equiv
per Ru, 850 rpm stirring) does not affect the hydrogenation of
methyl benzoate catalyzed by 1 or 3 (entries 2 and 5, Table 1),
thus showing that the active catalyst is molecular and not
associated with heterogeneous Ru species. Postcatalytic ESI-MS
measurements (Figure S40, Supporting Information) indicate
the preservation of the Ru-CNC moiety in complex 3 under the
ester hydrogenation conditions. Molecular ions corresponding
t
KHMDS or KO Bu at room temperature led to incomplete
conversion, providing impure mixtures containing dearomat-
ized complex 7 within hours.
Alternatively, 7 can be prepared quantitatively from 4 by a
t
reaction with KO Bu (Scheme 4). Relatively unstable 7 was
+
+
to mononuclear [RuCl(CNC)] , [Ru(BnO)(CNC)] , [RuH-
+
+
Scheme 4. Generation and Reactivity of Complex 7
(CO)(PPh )(CNC)] , and [RuCl(PPh )(CNC)] species
3
3
were observed in the mass spectrum of the reaction mixture
corresponding to entry 5, Table 1.
At elevated substrate/catalyst ratios (S/C = 1000, 0.1% Ru),
1
converts methyl benzoate quantitatively to methanol and
benzyl alcohol at 70 °C and 50 bar H within 16 h (see section
2
S4 of Supporting Information). The kinetics of hydrogenation
was unaffected by the base concentration. For base loadings of
2
−10% , the reactions showed very similar time−conversion
mol
profiles, characterized by initial turnover frequencies (TOF) of
150−160 h (see Figure S41 in the Supporting Information).
Decreasing the reaction temperature to 40 °C or H pressure to
−1
characterized in situ by NMR spectroscopy. Dearomatization of
the pyridine ring is evidenced by a significant upfield shift of the
corresponding H NMR signals to ∼6.20 and 4.81 ppm. The
methylene group resonances appear at 4.90 and 4.74 ppm as
doublets, and the signal from the deprotonated bridge appears
at 5.83 ppm. These results point to a more facile transformation
of the nitrile adducts 4−6 to catalytically active dearomatized
Ru-pincer complexes in the presence of a strong base compared
2
1
5 bar strongly reduced the catalytic performance of 1
(Supporting Information Figure S41). The obtained rates of
ester hydrogenation are comparable to those reported for other
NHC-based catalysts and superior to the rates attainable with
2
,3,19,53
the lutidine-derived Ru-PNN and Ru-CNN systems.
The
catalytic reactions can be carried out under conditions
with the parent Ru-CNC’s 1 and 2. Similar to Ru-PNP
significantly milder than in the case of the TriPhos and
7
54−56
analogues, 7 reacts with H to form the dihydrido complex 8
TriSulph Ru catalysts.
However, Ru-CNC catalysts display
2
1
(
Figure S36 in the Supporting Information). The H resonance
lower activity than the state-of-the-art aliphatic Ru pincer
46,57 58,59
of the Ru−H in 8 is shifted downfield to −5.94 ppm, the typical
position for Ru-dihydrido complexes. Upon addition of H ,
complexes;
the Noyori-type catalysts;
and, in particular,
44
the most active catalytic system based on aliphatic Ru-SNS
2
2
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Letter
Table 1. Catalytic Ester Hydrogenation to Alcohols with
RuCNC’s.
ASSOCIATED CONTENT
Supporting Information
Experimental details, hydrogenation procedures, X-ray data for
■
a
*
S
1
, 3−6. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: e.a.pidko@tue.nl.
■
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Prof. Hans de Vries (Leibniz Institute for
Catalysis, Rostock, Germany) for useful discussions on the
potential reactivity of Ru-CNC in ester hydrogenation. G.A.
̈
thanks Henk Janssen and Serge Sontjens (TU/e) for useful
discussions. E.A.P. thanks the Technology Foundation STW
and The Netherlands Organization for Scientific Research
(
NWO) for his personal VENI grant. The X-ray diffractometer
has been financed by NWO.
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