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Journal ArticleDOI

Synthesis, stability, and (de)hydrogenation catalysis by normal and abnormal alkene- and picolyl-tethered NHC ruthenium complexes

21 Jun 2019-Organometallics (American Chemical Society)-Vol. 38, Iss: 13, pp 2624-2635

AbstractA series of p-cymene and cyclopentadienyl Ru(II)-aNHC complexes were synthesized from 2-methylimidazolium salts with either an N-bound alkenyl (1, 3) or picolyl tether (6, 7). The C(5)-Me substitut...

Topics: Cyclopentadienyl complex (60%), Ruthenium (55%), Alkene (54%)

Summary (1 min read)

Introduction

  • N-heterocyclic carbenes (NHCs) have the ability to exhibit both innocent and non-innocent behavior in metal-mediated transformation reactions.
  • 4,5(a) However, the formation of C(4)-bound aNHCs via transmetallation of the corresponding Ag-aNHC intermediate is generally limited because of redox reactions of the imidazolium salt with the strong oxidant Ag2O.3.
  • The need for more facile routes to access these desirable aNHCs remains relevant, as the rational preparation of aNHC metal complexes continues to be a synthetic challenge.
  • Abnormal coordination selectivity has also been related to steric control imparted by the tether length and the bite angle, as well as to the nature of the anion of the aNHC precursor.
  • Here the authors report the synthesis of eight new abnormally bound NHC half-sandwich Ru(II) complexes and demonstrate the strong binding of these aNHC ligands through acid stability studies.

Results and Discussion

  • Formation of the C(2)-isopropyl functionalized Ru(II)-aNHC complexes 9 and 10, also known as 10   Scheme 4.
  • Nonetheless, complexes 1 and 3 performed considerably better than the precursor salts (entries 9,10), revealing a direct impact of the tethered aNHC ligand on the catalytic activity.
  • Conversions were lower with substrates containing electron-withdrawing groups such as 4'-chloroand 4'-nitro-acetophenone (entries 2, 3).

Conclusions

  • Variation of the arene ligand (p-cymene vs. cyclopentadienyl) and of the chelating tether of aNHC ligands (alkenyl vs. picolyl) provided access to six unique half-sandwich aNHC Ru(II) complexes.
  • In addition, Ag-mediated C(2)-demethylation resulted in the identification of two normally-bound NHC Ru(II) side-products.
  • Symmetrization of the N-alkene substituents of the aNHC ligand, as well as employing an iPr-group on the C(2)-position of the imidazolium precursor, prevented C(2)dealkylation and allowed for the selective C(4)-ruthenation for both the p-cymene and cyclopentadienyl Ru(II) precursors.
  • Preliminary catalytic studies involving transfer hydrogenation suggest a greater impact of vacant coordination sites available via halide substitution (p-cymene Ru(II) complexes) than via reversible alkene and/or phosphine dissociation (cyclopentadienyl Ru(II) complexes).
  • The transfer hydrogenation results indicate that chelating aNHC ligand systems provide a dynamic platform for the development of active, selective, and long-lived catalysts.

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source: https://doi.org/10.7892/boris.131935 | downloaded: 10.8.2022
1
Synthesis, stability, and (de)hydrogenation catalysis by normal and abnormal alkene- and
picolyl-tethered NHC ruthenium complexes
Frederick P. Malan,
Eric Singleton,
Petrus H. van Rooyen,
Martin Albrecht
* and Marilé
Landman
*
Department of Chemistry, University of Pretoria, 02 Lynnwood Road, Hatfield, Pretoria, 0002, South
Africa.
Department of Chemistry and Biochemistry, Universität Bern, Freiestrasse 3, 3012 Bern, Switzerland.
Email: marile.landman@up.ac.za; martin.albrecht@dcb.unibe.ch
Abstract
A series of p-cymene and cyclopentadienyl Ru(II)-aNHC complexes have been synthesized from 2-
methylimidazolium salts with either an N-bound alkenyl (1, 3) or picolyl tether (6, 7). The C(5)-Me
substituted alkenyl-tethered analogues (2, 4) have also been synthesized. Ag-mediated C(2)-
dealkylation was a prominent side reaction that led to the formation of normally bound NHC Ru(II)
complexes, which in selected cases have been isolated (5, 8). A C(4)- over C(2)-selectivity for
ruthenium binding has been established by protecting the C(2)-position with an iPr group on the
imidazolium precursor, for which unique p-cymene (9) and cyclopentadienyl (10) Ru(II)-aNHC
derivatives have been synthesized. All complexes were applied in the transfer hydrogenation of
ketones and in secondary alcohol oxidation, with higher catalytic activity for the p-cymene over the
cyclopentadienyl systems, as well as the alkenyl- over the picolyl-containing aNHC complexes.

2
Introduction
N-heterocyclic carbenes (NHCs) have the ability to exhibit both innocent and non-innocent behavior
in metal-mediated transformation reactions.
1–3
Therefore it is not surprising that these stabilizing
ligands have been coordinated to most of the transition metals in a range of different oxidation states.
NHCs are known to form some of the strongest M–C bonds,
4,5
and hence attempts have been directed
towards the synthesis of highly electron-donating NHCs in order to better stabilize sensitive transition
metal species, as well as to access high oxidation state metal species.
2,3
It has also been shown that
C(4)/C(5)-bound imidazolylidenes (a subclass of so-called abnormal NHCs, aNHCs) are significantly
stronger donors compared to their classical C(2)-bound imidazolylidene counterparts (Figure 1).
3
The
enhanced donor properties imparted by aNHCs has been shown to further improve the catalytic
efficiency of these metal complexes in a variety of reactions.
2,4
A range of different backbone- and
N-functionalized aNHCs have been reported with tunable donor properties that are significantly
influenced by both the number and positions of heteroatoms within the heterocycle (Figure 1), with
subsequent catalytic implications.
5
For example, the pronounced mesoionic character of the aNHC
ligand allows for the usual carbene-type stabilization when coordinated to low-valent electron-rich
metals, whereas a carbanionic character may be dominant with electron-poor, high-oxidation state
metal centers.
2,6
Figure 1: Order of donor strength of imidazolylidene and pyrazolylidene-based NHCs.
Despite these attractive advantages of aNHCs, the organometallic chemistry of NHCs remains
dominated by normally bound NHC complexes.
4
A major limitation of aNHC complexes is the
synthetic accessibility because of the low acidity of the imidazolium C(4)/C(5) proton,
7
which
requires specific protocols to direct metalation at this position.
7,8

3
Protection of the imidazolium C(2) position with alkyl or aryl substituents is probably the most
rational and useful route that has been developed for selective metal coordination to the
imidazolylidene C(4) atom.
4,5(a)
However, the formation of C(4)-bound aNHCs via transmetallation
of the corresponding Ag-aNHC intermediate is generally limited because of redox reactions of the
imidazolium salt with the strong oxidant Ag
2
O.
3
The need for more facile routes to access these
desirable aNHCs remains relevant, as the rational preparation of aNHC metal complexes continues
to be a synthetic challenge.
4,9,10
One attractive strategy, particularly for aNHCs, is chelate-assisted C–
H bond activation.
11
In this approach a variety of donor groups such as alkenyl, picolyl, pyridyl,
thioether, amine, oxo, or phosphine substituents are grafted onto the C(2)-protected NHC scaffold,
usually through N-alkylation, essentially forming a bidentate aNHC ligand precursor. Abnormal
coordination selectivity has also been related to steric control imparted by the tether length and the
bite angle, as well as to the nature of the anion of the aNHC precursor.
12
In addition, such N-
substituted chelators may exhibit hemilability and non-innocent behavior in solution, which enhances
the set of metal-specific steps within a potential catalytic cycle.
13
In light of the latter, the lack of
exploitation of potentially hemilabile bidentate aNHC ligand systems prompted us to evaluate the
catalytic potential of chelating aNHC ligands when bound to a ruthenium metal center. Even though
only a small number of well-defined aNHC Ru complexes have been reported to date, most of these
complexes exhibit superior catalytic activities compared to their normally bound analogues.
3(a),14
Here
we report the synthesis of eight new abnormally bound NHC half-sandwich Ru(II) complexes and
demonstrate the strong binding of these aNHC ligands through acid stability studies. In addition, the
novel complexes have been evaluated for their application in transfer (de)hydrogenation catalysis.
Each of these Ru(II) complexes is comprised of either an alkene- or picolyl-substituent as one of two
possible N-functionalized chelating moieties. In addition, selectivity and yield issues typically
encountered with Ag
2
O-assisted carbene transfer reactions are addressed.
Results and Discussion
Synthesis of the NHC ligands. The four aNHC ligand precursors [HL1]Cl–[HL4]Cl were
synthesized from the respective C(2)-substituted imidazoles (Scheme 1). For example, 1,2-
dimethylimidazole reacts with either 3-chloro-2-methyl-propene or 2-(chloromethyl)pyridine in a
quaternization reaction to produce the C(2)-protected imidazolium chloride salts in excellent yields
(93%, [HL1]Cl; 88%, [HL2]Cl). The tetra-substituted imidazolium salt [HL3]Cl was accessed by
reacting 2,4-dimethylimidazole first with acetic anhydride, followed by methyl iodide to selectively
yield the 1,2,5-trimethyl imidazole. It was found previously that direct deprotonation and alkylation

4
of 2,4-dimethylimidazole gave mixtures of the 1,2,4-, and 1,2,5-trisubstituted imidazoles.
15
The 1,2,5-
trimethylimidazole precursor was further reacted with 3-chloro-2-methyl-propene to obtain [HL3]Cl
in moderate yield (53%). In an attempt to address selectivity issues in the metalation (see below), the
symmetrical aNHC precursor [HL4]Cl was synthesized in high yield by sequential treatment of 2-
isopropylimidazole with KOH and 3-chloro-2-methylpropene, followed by another equivalent of 3-
chloro-2-methyl-propene (90%).
Scheme 1: Synthesis
of the aNHC ligand precursors.
The
1
H NMR spectra of [HL1]Cl, [HL3]Cl, and [HL4]Cl all showed characteristic singlets for the 2-
methylpropenyl moiety at δ
H
1.53–1.78 (CH
3
), 4.60–4.82 (NCH
2
), and 4.51–5.06 (two singlets,
=CH
2
). Depending on the symmetrical nature of the ligand, the imidazolium backbone protons either
appeared as a singlet at δ
H
7.47 ([HL4]Cl) or 7.14([HL3]Cl), or as two singlets (δ
H
7.51 and 7.78 for
[HL1]Cl, 7.46 and 7.64 for [HL2]Cl).
16
Although only
13
C,
15
N,
31
P, and
77
Se NMR techniques are
classically used to compare σ-donor and π-accepting properties of various NHC ligands,
17
it is
interesting to compare the values of the aromatic imidazolium
1
H NMR signals of L1L4 to gauge
σ-donor strength: L1 < L2 < L4 < L3. According to this series, ligand L3 bearing a C(5)-Me group
is more donating than its C(5)-H counterpart, L1, as might be expected from the inductive effect of a
NN
CH
3
CN, reflux
NN
NN
Cl
Cl
NHN
NN
Cl
NN
(ii) MeI, reflux
(i) Ac
2
O, benzene
reflux
CH
3
CN, reflux
NN
Cl
CH
3
CN, refluxCH
2
Cl
2
,KOH
reflux
NHN
[H
L1
]Cl
Cl
N
CH
3
CN, reflux
NN
Cl
N
Cl
ClCl
1
2
3
45
1
2
3
4
5
1
2
3
45
[HL2]Cl
[HL3]Cl
[HL4]Cl

5
methyl substituent. This approximate donor strength series is supported when comparing the
increasing resonance frequency of the pre-carbenic C(4)-signal in the
13
C NMR spectra in the series
122.4 ppm ([HL2]Cl) 122.9 ppm ([HL1]Cl), 123.7 ppm ([HL4]Cl), 129.7 ppm ([HL3]Cl).
Synthesis of the normal and abnormal NHC Ru(II) complexes. Each of the ligands L1-L4
contains a C(2)-alkyl substituent to inhibit metal coordination at the normal position of the NHC.
Metallation was carried out using standard literature procedures
3,18
which involves reacting the
imidazolium salt with Ag
2
O under exclusion of light to form the carbene silver complex in situ, and
subsequently treating the reaction mixture with the appropriate ruthenium(II) precursor (Scheme 2).
This procedure afforded the aNHC ruthenium(II) complexes 1 and 2 from the reaction of [HL1]Cl
and [HL3]Cl, respectively, with [(p-cymene)RuCl
2
]
2
in moderate yield (49% and 38%,
respectively).
19
Transmetallation with [CpRuCl(PPh
3
)
2
] gave the aNHC complexes 3 and 4,
respectively. While complex 3 was the only detectable product when starting from the trisubstituted
imidazolium salt [HL1]Cl, the tetrasubstituted imidazolium salt [HL3]Cl afforded, in addition to the
aNHC complex 4, also the normal NHC complex 5. Similar C(2)-C
alkyl
bond cleavage was observed
previously,
3(b)
in particular with methyl, ethyl, and benzyl substituents at C(2).
4(b)
Conversely, no
dealkylation occurs when the C(2) substituent is a bulky, secondary alkyl or an aryl group such as
iso-propyl or phenyl.
1(d),3(b)
The yields of complexes 15 were only moderate (19-49%), and
complexes 1 and 2 formed slightly better (>38% yields) owing to the higher reactivity of [(p-
cymene)RuCl
2
]
2
vs. [CpRuCl(PPh
3
)
2
].
Scheme 2: Synthesis of aNHC half-sandwich Ru(II) complexes with an alkenyl chelating group.

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Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Synthesis, stability, and (de)hydrogenation catalysis by normal and abnormal alkene- and picolyl-tethered nhc ruthenium complexes" ?

In this paper, a series of p-cymene and cyclopentadienyl Ru ( II ) -aNHC derivatives have been synthesized from 2methylimidazolium salts with either an N-bound alkenyl ( 1, 3 ) or picolyl tether ( 6, 7 ).