Tandem transfer hydrogenation-epoxidation of ketone substrates catalysed by alkene-tethered Ru(ii)-NHC complexes

Article abstract of DOI:10.1039/c9nj01220f

A series of nine cyclopentadienyl Ru(ii)-NHC complexes (1-9) have been synthesised by systematically varying the ligand and/or ligand substituents: η⁵-C₅H₄R′ (R′ = H, Me), EPh₃ (E = P, As), NHC (Im, BIm), where NHC = Im(R)(R′) (R, R′ = Me, Bn, 4-NO₂Bn, C₂H₄Ph, C₄H₇). Each of the Ru(ii)-NHC complexes features an N-alkenyl tether to attain bidentate NHC ligands. All complexes found application as catalysts in the tandem transfer hydrogenation and epoxidation reactions of carbonyl substrates. The catalytic activity of the complexes was shown to be similar, with efficiencies of up to 69% conversion after 18 hours and varying alcohol:epoxide selectivity for a variety of electronically diverse carbonyl substrates. Complex 3, with a nitro-containing substituent on the NHC ligand, was the only complex that showed preference for the alcohol product over the epoxide after 18 hours of reaction time.

Full text of DOI:10.1039/c9nj01220f

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Tandem transfer hydrogenation-epoxidation of ketone substrates catalysed by
alkene-tethered Ru(II)-NHC complexes
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Frederick P. Malan, Eric Singleton, Petrus H. van Rooyen, and Marilé Landman*
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Abstract: A series of nine cyclopentadienyl Ru(II)-NHC complexes (1-9) have
been synthesised by systematically varying the ligand and/or ligand
substituents: ƞ5-C5H4R' (R' = H, Me), EPh3 (E = P, As), NHC (Im, BIm), where
NHC = Im(R)(R') (R, R' = Me, Bn, 4-NO2Bn, C2H4Ph, C4H7). Each of the Ru(II)-
NHC complexes features an N-alkenyl tether to attain bidentate NHC ligands.
All complexes found application as catalysts in the tandem transfer
hydrogenation and epoxidation reactions of carbonyl substrates. The
catalytic activity of the complexes was shown to be similar, with efficiencies
of up to 69% conversion after 18 hours and varying alcohol:epoxide
selectivity for a variety of electronically diverse carbonyl substrates.
Complex 3, with a nitro-containing substituent on the NHC ligand, was the
only complex that showed preference for the alcohol product over the
epoxide after 18 hours of reaction time.
multi-functional properties are exploited in addition to the tailored
electronic and steric environments they provide.4 Bidentate NHCs
introducing potentially hemilabile donor groups, enabling
reversible dissociation from the metal centre, include
phosphine,3(a), 3(f) pyrimidine,7 ether,8 thioether,6(c) carboxylate,6(c)
indenyl,9 oxazoline,10 amine,11 and pyridine12 moieties. However,
the alkene functional group as secondary donor group received
much less attention, despite it featuring in one of the first classical
donor NHC complexes synthesised.3(d),13 To date, only Lavigne,3(c)
Bera2(c) and Albrecht6(c) have successfully employed alkene-
tethered NHCs in the synthesis of a total of four ruthenium-NHC
complexes (Figure 1) for application in catalysis studies.
Introduction
Alkene epoxidation is recognised as an important reaction in
organic synthesis, able to provide enantiopure epoxides, which are
important precursors in amongst other, drug synthesis.1 However,
non-atom-economical oxidants such as alkyl hydroperoxides and
hypochlorites are often employed in catalytic epoxidation
reactions.1(b) Much less attention has been devoted to ‘greener’
oxygen-containing precursors, such as halohydrins. This is mainly
because of the undesirable equilibrium that exists between the
vicinal halohydrin and corresponding epoxide since the expelled
mineral acid provides a nucleophilic halide to ring-open the formed
epoxide. This problem could, however, be circumvented using acid
scavengers or anion exchangers.1 By making use of tandem
hydrogenation and epoxidation reactions of the more readily
available acyl halide precursors, a more efficient catalytic reaction
is being devised. This would necessitate the use of highly versatile,
transfer-hydrogenation active catalysts that withstands
deactivation under oxidative, acidic conditions.2
Figure 1. Previously reported alkene-functionalised Ru-NHC complexes, along with the
new alkene-tethered Ru-NHC complexes of this study.
The lack of research on potentially hemilabile bidentate NHC
ligands as part of half-sandwich ruthenium complexes, prompted
us to investigate possible stability and catalytic enhancement
effects that these chelating NHC ligands may provide. Half-
sandwich ruthenium complexes containing NHC ligands are of
special interest, given the rich synthetic and catalytic applications
that they provide, especially with regard to transfer hydrogenation
and epoxidation catalysis.2(c),3(e),14 Here we report the synthesis of
nine new alkene-tethered half-sandwich Ru(II)-NHC complexes,
evaluate their application in tandem transfer hydrogenation-
epoxidation catalysis, and relate the respective catalytic activities
in a DFT study.
During the last two decades an increasing amount of impetus has
been placed on harnessing the synthetic and catalytic advantages
that two independent research areas combine: Highly stabilising N-
heterocyclic carbenes (NHCs), and the hemilability of multi-dentate
hybrid ligand systems when coordinated to transition metals.3
NHCs constitute an ever-expanding ligand class capable of
stabilising most transition metals in a range of different oxidation
states.4-6 In addition, NHCs have been shown to act as ‘smart’
ligands, where their non-innocent, cooperative, switchable, and/or
Results and Discussion
Synthesis of the NHC ligands and metal precursors
The NHC ligand precursors of this study were synthesised from N-
substituted 3-chloro-2-methyl-propene imidazole (Scheme 1).
Quaternisation reactions of the functionalised imidazole with the
respective alkyl halides produce the corresponding alkene-
tethered imidazolium chloride salts in excellent yields (94%,
[HL1]Cl; 91%, [HL2]Cl; 67%, [HL3]Cl; 86%, [HL4]Cl; 88%, [HL5]Cl;
92%, [HL6]Cl). The purification of ligand precursors L1-L6 entailed
concentration of the crude reaction mixtures and washing with
ethyl acetate to remove unreacted starting material. Removal of
residual solvent in vacuo to resulted in the pure imidazolium salts
Department of Chemistry, University of Pretoria, 2 Lynnwood road,
Hatfield, Pretoria, 0002, South Africa.
E-mail: marile.landman@up.ac.za
† Supporting information for this article is given via a link at the end of the
document.
1

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as white to light yellow crystalline solids or oils. The 1H-NMR
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spectra of [H(L1-L6)]Cl all exhibited characteristic signals for the 2-
methylpropenyl moiety: δH 1.39-1.73 ppm (singlet, methyl protons),
4.69-5.22 ppm (singlet, methylene protons), and 4.65-5.07 ppm
(two singlets, alkene protons). The asymmetrical nature of the
imidazolium salts resulted in two singlets for the imidazolium
backbone protons at δH 6.66-7.81 ppm ([H(L1-L4)]Cl, [HL6]Cl). The
only symmetrical ligand precursor, [HL5]Cl (Figure 2), exhibited one
singlet at δH 7.38 ppm for these two protons. The acidic C(2)-H
proton signals for the salts [H(L1-L6)]Cl varied between δH 9.40-
10.82 ppm. The pseudo-carbenic C(2)-signals in the 13C-NMR
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Scheme 2. Synthesis of the ruthenium(II) precursors P1-P3.
Figure 3. Perspective views of ruthenium precursors P2 and P3. Thermal ellipsoids are
drawn at 50% level. The phenyl moieties of the pnictogen moieties have been shown
as wireframe presentations, as well as the omission of one molecule of CH2Cl2 within
each structure have been done for clarity purposes.
Synthesis of the Ru(II)-NHC complexes
Each of the ligand precursors [H(L1-L6)]Cl contains an N-alkenyl
substituent. After ruthenation of the ligand (L1-L6) by one of the
precursors P1-P3, a normal (C2-substituted) NHC chelate complex
(1-8) is formed in each case. Ruthenation was carried out using
standard literature procedures2(c),6(c) which involves reaction of the
imidazolium salt with Ag2O, with the exclusion of light. A biscarbene
silver complex was formed, to which the appropriate half-sandwich
ruthenium(II) precursor (P1-P3) was added in situ. Subsequent
anion exchange with NH4PF6 in acetone yielded complexes 1-8
(Scheme 3). Complexes 1-6 resulted from the reaction of ligand
precursors [H(L1-L6)]Cl with [CpRuCl(PPh3)2] (P1), respectively, via
in situ silver transmetallation. After purification, complexes 1-6
were obtained as air and light stable yellow to dark yellow solids.
The yields of complexes 1-6 (after purification) were moderate (46-
66%), with 3 being the lowest. The decreased reactivity of [HL3]Cl
is ascribed to the presence of an electron-withdrawing 4-NO2Bn
substituent on one of the N-atoms of the NHC ligand, which renders
the ligand less donating than the others. Reactions of
[HIm(Bn)(C4H7)]Cl (L2 precursor) with [(ƞ5-C5H4R')RuCl(EPh3)2] (R' =
Me, E = P, P2; R' = H, E = As, P3) in a similar reaction procedure,
yielded 7 (43 %, dark yellow solid) and 8 (62 %, brown solid),
respectively. Spectroscopic evidence of NHC-coordination for 1-8
was obtained from their NMR spectra: Disappearance of the 1H-
NMR signal of [HL1-L6]Cl corresponding to the C(2)-H proton
between δH 9.40-10.82 ppm, along with the concomitant
appearance of the 13C-NMR carbene signal between δC 174.8-192.7
ppm were observed. 1H-NMR signals for the cyclopentadienyl (1-6,
8) and methyl-cyclopentadienyl (7) moieties generally appeared in
an equimolar ratio to the respective ligand L1-L6. Interestingly,
other general NMR trends on comparing the imidazolium salts and
the coordinated imidazolylidene ligands included upfield shifts of
the N-alkyl signals, where for example shifts from δH 3.89 ppm
(NMe, L1) to 2.94 ppm (1); 4.83 ppm (NCH2, L2) to 4.71 ppm (2),
and 4.91 ppm (NCH2, L5) to 3.59 ppm (5), have been observed.
spectra ranges from 136.9 ([H(L2)]Cl) to 144.3 ppm ([H(L6)]Cl).
Scheme 1. Synthesis of ligand precursors [H(L1-L6)]Cl from the corresponding N-
functionalised imidazoles.
Figure 2. Perspective view of [HL5]Cl. Thermal ellipsoids are drawn at 50% level. The
omission of one molecule of H2O has been done for clarity purposes.
The half-sandwich precursors P1-P3 were synthesised according to
a modified version of the literature procedure (Scheme 2). In this
modified procedure, an ethanolic solution of RuCl3.xH2O is reacted
with 2.5 equivalents of the respective pnictogen EPh3 (E = P, As),
and excess (> 2 equivalents) of the respective cyclopentadiene
C5H5R' (R' = H, Me). Reaction mixtures were allowed to heat under
reflux overnight, which upon cooling allowed for the orange [(ƞ5-
C5H4R')RuCl(EPh3)2] (R' = H, Me; E = P, As; P1-P3) complexes to
precipitate. The solid-state molecular structures of P215(a) and P3
(Figure 3) are similar in that both assume the classical piano-stool
structure with Ru1-Cg = 1.844(3) (P2), 1.813(4) (P3) (Cg = centroid
of Cp ring); as well as Ru1-E1 = 2.3222(7) (P2, E = P), 2.4229(2) (P3,
E = As). All of the Ru-Cg and Ru-E (E = P, As) bond lengths
correspond well with related CpRuCl(PPh3)2 complexes.15
2

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spectra were obtained from a (CD3)2CO solution containing 4 and
recorded between -50°C and +50°C (Supplementary Information).
In the higher temperature spectra (> 20°C), decoalescence is
observed only for the multiplet at 3.02 ppm, assigned to the
ethylene linker protons of the phenethyl substituent, in the -50°C
spectrum into two multiplets at 2.93 and 3.09 ppm. No coalescence
is seen for the signals related to the alkenyl tether. The signals
related to the alkenyl tether only underwent an upfield shift of ca.
0.48 ppm (d, 2JHH = 12 Hz, NCH2 protons) and 0.19 ppm (d, 2JHH = 15
Hz, =CH2 protons) at low temperatures. In an attempt to further
probe the hemilability and subsequent reactivity of the N-alkenyl
moiety, C=C bond cleavage via halogenation was attempted.
Reaction of a DCM solution of 1 with a slight excess of I2 resulted in
-
an anion exchange with PF6 to form the complex
[CpRu(PPh3){Im(Me)(C4H7)}]I3 (9, Scheme 4). The presence of
minute amounts of moisture is thought to have mediated a
reversible disproportionation reaction between I2 and H2O.16(a),16(b)
Furthermore, no reaction was observed after heating the sample
for several hours. This indicates a level of robustness in the Ru-
alkene bond, and hence a lower degree of hemilability that exists
in solution than originally anticipated.
Scheme 3. Syntheses of complexes 1-8 from the respective Ru(II) precursors P1-P3,
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and ligand precursors [H(L1-L6)]Cl.
In addition, upfield shifts of the C(4)-H and C(5)-H protons in L1-L5
after complexation have been observed, with the shift of L1 being
the most notable: an upfield shift occurs from δH 7.74 and 7.81 ppm
(L1) to 6.92 and 7.03 ppm (1) respectively. This is indicative of a
significant degree of electron density perturbation on the C(4) and
C(5) carbon atoms after complexation due to higher electron
density on the C(2) position.2 The signal relating to the PPh3 ligand
in the 31P-NMR spectra of complexes 1-7 was observed in a narrow
Scheme 4. Reaction of 1 with I2 to form 9. Inset: Perspective view of 9. Thermal
ellipsoids are drawn at 50% level. The phenyl moieties of PPh3 have been shown as
wireframe presentations for clarity purposes.
The proposed molecular structures of the novel Ru-NHC complexes
were supported by single crystal X-ray diffraction (SCXRD) studies.
C(2)-carbene coordination for complexes 1, 2, 4, 5, 8 and 9 were
confirmed (Scheme 4, Figure 4). All of the half-sandwich Ru(II)
complexes assumed pseudo three-legged piano stool structures,
where the η2-alkene moiety is considered as one leg.2(c),6(c) Selected
bond lengths and angles are summarised in Table S4 in the
Supplementary Information. If chelation of the alkene-tether in
complexes 1-4 is considered to form pseudo-five membered
ruthenacyles, the resulting acute bite angles are almost identical
for all complexes, and varies between 89.38(1)° (8) and 89.71(1)°
(4). While the tether and imidazolium planes are essentially co-
planar in related five-membered metallacycles,6(c) the alkene-
containing complexes exhibit anticlinal torsion angles (between the
alkene and imidazolylidene mean plane) of either between 24.7(3)°
and 28.3(3)° (1, 2, 4, 5), or -22.6(7) (8) and -26.8(3) (9).
1
region between δP 56.5-58.1 ppm. H-NMR signals for the alkenyl
moiety were in general very similar for complexes 1-8: the methyl
protons resonated between δH 1.74-2.18 (s) ppm, the
3
unsymmetrical alkene protons between δH 1.89-2.51 (d, JHP = 15
Hz) and 3.73-4.28 (s) ppm, and the methylene protons between δH
2
2
1.13-2.08 (d, JHH = 12 Hz) and 3.51-4.00 (d, JHH = 12 Hz) ppm. In
complex 5, containing both free and bound N-alkenyl moieties,
signals pertaining to the bound tether were observed more upfield
(2.07 and 3.85 for =CH2 protons) as compared to the free tether
(4.50 and 4.97 for =CH2 protons), which has little to no interaction
with the metal centre.
A slight degree of fluxionality of the alkenyl tether of complexes 1-
8 was inferred from the variable temperature 1H-NMR spectra of 4
(bearing both a coordinated alkenyl tether, as well as a free
phenethyl N-substituent). The variable temperature 1H-NMR
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Figure 4. Perspective views of Ru(II)-NHC complexes 1, 2, 4, 5, and 8. Thermal ellipsoids are drawn at 50% level. For clarity purposes, the phenyl moieties of the pnictogen moieties
have been shown as wireframe presentations, as well as the omission of one molecule of PF6 within each structure, including the omission of one molecule of CH2Cl2 (2 and 8).
mixtures in the presence of anisole (internal standard) taken over a
It is also interesting to note that the Ru-C(2) bond distances of the
complexes have a narrow distribution which fall between 2.033(2)
Å (1) and 2.042(2) Å (2). This is indicative of the negligible electronic
effect the different NHC ligands have on the Ru-NHC bond.
However, the Ru-C bond distances are in good agreement with
similar Ru-C bond distances of related normally bound Ru(II)-NHC
complexes.6(c),14(a),15
period of 18 hours, suggested a tandem reaction, whereby a
mixture of secondary alcohol (halohydrin) and the ring-closed
epoxide products were observed. No induction time was observed
for all reactions under optimised conditions.
All complexes except 3 showed similar activity (Table 1), with initial
TOFs of 8-12 h-1 (total conversion) were observed (determined
after the first 15 minutes reaction time), under typical conditions.
The latter TOFs compare poorly with some of the best-performing
half-sandwich ruthenium-based catalysts,17(a),18 where TOFs of up
to 3000 h-1 have been reported for transfer hydrogenation catalysis.
However, the TOFs of 1-8 fall within the lower range of related half-
Catalysis Studies
The ability of catalysts to mediate tandem reactions are highly
attractive since multistep transformations allow for the
minimisation of follow-up batch reactions, employment of less
catalyst(s), and a rapid increase in molecular complexity from
readily available starting materials.16(c) Transfer hydrogenation in
particular has been widely studied by numerous groups over the
years, although the added benefits of employing metal-NHC
complexes as catalysts were only realised in the last two
decades.2(a) Very few studies have been concerned with the one-
pot functionalisation of post-hydrogenation products.
sandwich bidentate NHC-Ru(II) analogues.12,17
A change of
pnictogen ligand (As vs. P) has very little effect on the catalytic
activity (compare entry 2 with entry 8 in Table 1). Chelation
generally stabilises catalysts to render them more robust and
therefore less prone to catalyst deactivation by means of catalyst
poisons. However, chelation may also inhibit fast substrate
conversion through the occupation of required vacant sites, and
hence could reduce the overall yield.19 This effect might be
prevalent in the catalytic activities of 1-8, where a slow transfer
hydrogenation reaction of BPAB occurs during the initial few hours
of reaction, followed by the concomitant epoxidation reaction of
the alcohol intermediate product.
We were therefore interested in the base-mediated epoxidation of
the formed halohydrins as a secondary reaction. Similar to
numerous other reported half-sandwich Ru(II) complexes,12,17 1-8
were all found to be transfer hydrogenation active. Standard
conditions were employed, i.e. excess isopropanol as a hydrogen
donor, base (KOH or KOtBu) as activator, and 4'-bromophenacyl
bromide (BPAB) as substrate. 1H-NMR analyses on reaction
After reaction optimisation (Table S5 in the Supplementary
Information), complexes 1-8 were screened for their catalytic
activity. After two hours’ reaction time, complexes 1, 2, 4, 5, and 7
all exhibited comparable activity, having converted between 27%
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Table 1. Tandem transfer hydrogenation-epoxidation of BPAB using 1-8 as catalyst.
(5) and 34% (1) of the substrate. Complex 3, bearing the N-4-NO2-
benzyl NHC ligand, fared the worst, with a conversion of 23% (TOF15
= 4 h-1). The efficiency of the transfer hydrogenation-epoxidation
reaction was found to be temperature and base-dependent. At
room temperature, the reaction proceeds sluggishly (9%
conversion after 18 hours). Without addition of a base, low (< 31 %)
conversions were obtained. Furthermore, the low conversions
obtained when using KOH (31% after 6h) as base necessitated the
use of the stronger KOtBu base, for which better conversions were
obtained (56% after 6 hours).
Conversiona (%)
Selectivityb
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Entry
Complex
(%, alc:epox)
2h
34
32
23
33
27
26
29
37
6h
52
48
36
49
43
46
48
56
18h
60
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39
55
45
51
52
62
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
35:65
43:57
62:38
40:60
38:62
29:71
38:62
27:73
In general, it is seen that similar N-substituents (methyl, benzyl,
phenethyl) on the NHC ligand have little effect on the catalytic
activity (entries 1, 2 and 4). The lower conversion of complex 5 is
attributed to the second N-alkenyl group on the NHC ligand: the
presence of a potential chelating group competes with substrate
coordination once an available site on the ruthenium centre is
generated. Although comparable catalytic activities (within a 3%
error range) have been observed for complexes 1-8, these result
appear to indicate that steric effects may play a role in reducing the
reaction rate, especially if an inner sphere mechanism is considered.
Both the N-substituent and the backbone moiety of the NHC ligand
may contribute to the overall steric bulk of the resulting Ru-NHC
complex, for which complex 1 (N-Me) is sterically less demanding
than 4 (N-(CH2)2Ph) or 6 (benzimidazole vs. imidazole). The
resulting effect is seen in the final substrate conversions observed
(60% (1) vs. 55% (4) and 51% (6)). Electronic properties of the
substrate marginally affected the conversions (Table 2), i.e. more
electron-donating substituents (4'-Me vs. 4'-Br; entry 2) resulted in
slightly higher conversions (69% (Me, entry 2, Table 2) vs. 62% (Br,
entry 8, Table 1)). The substrates with 4'-OH and 4'-NH2 groups
showed lower conversion, probably due to coordination
interferences. The presence of the electron-withdrawing moieties
(Br, NO2) appears to deactivate the carbonyl moiety due to
resonance effects.
General conditions: 4'-bromophenacyl bromide (BPAB, 0.6 mmol),
iPrOH (4 mL), base (1.2 eq.), anisole as internal standard (65 μL,
0.6 mmol), [Ru] (2 mol%), 110 °C. a Determined by 1H-NMR, based
b
on the average of at least two runs. Selectivity (%): alc:epox =
alcohol:epoxide.
Table 2. Substrate screening in the tandem transfer hydrogenation-epoxidation.
Conversiona (%)
Selectivityb
Entry
R1
R2
(%,
alc:epox)
18
h
2h
6h
1
2
3
4
5
H
H
H
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32
24
19
20
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38
31
28
56
69
48
41
41:59
36:64
39:61
34:66
22:78
A lack of selectivity is observed in the catalytic conversions using
complexes 1-8. The selectivity between the halohydrin and
corresponding epoxide is shifted towards the halohydrin during the
initial two hours’ reaction time (69-84%(alcohol):17-31%(epoxide)).
After six hours’ reaction time, near equimolar amounts of the
halohydrin and corresponding epoxide are observed (48-
Me
H
OH
H
NH2
NO2
H
36
61%(alcohol):39-52%(epoxide)).
A shift towards the formed
General conditions: substrate (0.6 mmol), PrOH (4 mL), KOtBu
i
epoxide is observed after 18 hours’ reaction time (27-
43%(alcohol):57-73%(epoxide)). This shift is more pronounced for
substrates with electron-withdrawing substituents (NO2,
22%(alcohol):78%(epoxide)) where the corresponding epoxide is
more readily formed after a slow(er) transfer hydrogenation step.
(1.2 eq.), anisole (65 μL, 0.6 mmol), 8 (2 mol%), 110 °C.
Determined by 1H-NMR, based on the average of at least two
a
runs. b Selectivity (%): alc:epox = alcohol:epoxide.
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A mechanism for the tandem transformation is proposed in Figure
5, using complex 8. Analogous to similar CpRu derivatives, it is
reasonable to assume that catalytic transfer hydrogenation occurs
via a classic monohydride mechanism.17(b) Dissociation of an AsPh3
ligand (a) allows for isopropoxide coordination (b), which
subsequently dissociates as acetone after β-hydride elimination.
The latter process leads to the formation of the active hydride
species (c) needed for hydrogen transfer. By reversible alkene
dissociation, an additional vacant site is created by which an
incoming carbonyl substrate (d) is reduced to the alkoxide-moiety
(e) by means of hydrogen transfer. The functionalised alkoxide
intermediate is then substituted for isopropoxide (f). Finally, a
tandem intramolecular nucleophilic attack of the oxo-group of the
deprotonated halohydrin occurs to eliminate a bromido anion,
along with the desired epoxide (g).
such that the LUMO is distributed over the ruthenium and NHC
ligand. This might be indicative of the Ru-NHC bond being prone to
reduction, especially in the presence of harsh reducing conditions.
In contrast, the HOMO of all the complexes except complex 3
exhibit ruthenium and pnictogen character. The HOMO of complex
3 is exclusively positioned over the 4-NO2-benzyl moiety of the NHC
ligand. This may suggest that a further oxidation process could
involve this moiety.
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Upon reaction of P1, P2 or P3 with the respective NHC ligands, the
corresponding HOMO and LUMO energies of 1-8 are lowered. This
is indicative of the stabilisation effect the chelating NHC exerts on
the resulting complex. Furthermore, the energy gaps of all
complexes except 3 are larger by around 0.34-0.57 eV. The smallest
energy gap of 3.25 eV (3) corresponds to the least stable (and more
reactive) complex of complexes 1-8. This observation corresponds
to the experimental catalytic activities of complexes 1-8.20 Complex
3 being the least active catalyst, exhibits the smallest energy gap, is
the least stable, and is therefore the most susceptible to fast
catalyst deactivation. The energy gaps of the remainder of the
complexes (4.31-4.55 eV) are comparable, and correspond to
similar catalytic activities observed for these complexes.
Relating to the Principle of Maximum Hardness (PMH) with respect
to molecular orbital (MO) theory, the hardness of a molecule (η) is
defined as half the energy gap between the HOMO and the LUMO
of that molecule.20(b) A chemical system therefore is inclined to
maximise the hardness of its respective molecules since a hard
molecule tends to have a large(r) energy gap.20(b) Comparing the
hardness of complexes 1-8, all complexes (2.16 ≤ η ≤ 2.27) except
complex 3 (η = 1.63) exhibit relatively hard character, and are
therefore more robust against catalyst deactivation than complex
3.
Figure 5. Proposed mechanism for the tandem transfer hydrogenation and
epoxidation transformation reaction.
Conclusions
Variation of the N-alkyl group of the NHC, the ancillary pnictogen
ligand, and (substituted) cyclopentadienyl ligand, provided access
to eight unique half-sandwich Ru(II)-NHC complexes. The NHC
ligands of these complexes were tailored to feature an alkenyl-
group to act as a hemi-labile group for catalysis purposes. Catalytic
studies involving the tandem transfer hydrogenation and
epoxidation of a range of electronically diverse phenacyl bromides
suggest a low degree of hemilability provided by the alkenyl-tether.
The catalytic activities of complexes 1-8 were found to be similar
while selectivities favoured the epoxide product. The only
exception was complex 3, which favoured halohydrin formation
after 18 hours’ reaction time (62:38 alc:epox). Furthermore, a slight
deactivating effect has been observed for the phenacyl bromide
substrates bearing electron-withdrawing moieties.
DFT studies
In order to probe the stability, reactivity and subsequent catalytic
applicability of complexes 1-8, a DFT study of the frontier orbitals is
presented (Figure 6). In Figure
6 the HOMO and LUMO
representations, the energies of the HOMO and LUMO, as well as
the corresponding energy gap (energy difference between the
HOMO and LUMO) of all complexes are presented. In terms of the
molecular orbitals, similarities among complexes 1-8 could be
drawn: Precursor complexes P1-P3 all exhibit a LUMO distributed
mainly over the ruthenium centre with a minor contribution from
the cyclopentadienyl ligand. The HOMO of P1-P3 is uniquely
positioned mainly on the pnictogen moieties. After NHC-
functionalisation, electron density within complexes 1-8 is shifted
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Figure 6. Frontier orbitals of the precursor complexes P1-P3, and CpRu-NHC complexes 1-8. Representations are ordered according to descending LUMO energy (1-8) from the
respective precursor (P1-P3)
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However, enhanced selectivities towards the epoxide product were
observed for the latter substrates.
chromatography, using gradient elution with Et2O/CH2Cl2. Yellow to
dark-yellow solids were obtained.
1
[CpRu(PPh3)(L1)]PF6 (1): Yield: 66%. H NMR (CDCl3): δH = 1.14 (d,
2JHH = 12 Hz, 1H, NCH2), 1.75 (s, 3H, CCH3), 1.89 (d, 3JHP = 15 Hz, 1H,
=CCH2), 2.94 (s, 3H, NCH3), 3.52 (d, 2JHH = 12 Hz, 1H, NCH2), 3.76 (s,
1H, =CCH2), 5.03 (s, 5H, C5H5), 6.92 (s, 1H, CimiH), 7.03 (s, 1H, CimiH),
7.21-7.38 (m, 15H, HPh). 13C{1H} NMR (CD3CN): δC = 32.7 (s, CCH3),
37.8 (s, NCH3), 48.2 (s, =CCH2), 57.5 (s, NCH2), 81.1 (s, =CCH2), 88.7
(s, C5H5), 118.3 (s, CimiH), 121.4 (s, CimiH), 126.0 (s, CPh), 129.5 (s, CPh),
131.4 (s, CPh), 134.4 (s, ipso CPh), 175.8 (s, C-Ru). 31P{1H} NMR
(CDCl3): δP = -144.1 (septet, 1JPF = 704 Hz, PF6), 58.1 (s, PPh3). CHN
found (calcd) for [C31H32F6N2P2Ru]×0.5C3H6O: C, 53.17 (52.85), H,
4.71 (4.78), N, 3.93 (3.79)%. HR-MS (ESI): m/z 565.1344 (1+) calcd
for C31H32N2PRu 565.1347.
Experimental Section
General Procedures
All experiments were carried out under an argon atmosphere using
standard Schlenk techniques. Solvents were dried and distilled
from appropriate drying agents prior to use. The new imidazolium
chloride ligands [H(L1-L6)]Cl, as well as the metal precursors
[CpRuCl(PPh3)2] (P1), [MeCpRuCl(PPh3)2] (P2), and [CpRuCl(AsPh3)2]
(P3) were synthesised and purified according to standard literature
procedures. All other chemicals were purchased from commercial
1
suppliers and used without further purification. H (300 MHz) and
1
[CpRu(PPh3)(L2)]PF6 (2): Yield: 58%. H NMR (CDCl3): δH = 1.54 (d,
13C{H} (76 MHz) NMR spectra were recorded on a Bruker Avance-
400 spectrometer using CDCl3. All measurements were performed
at ambient temperature (298 K), unless otherwise noted. Chemical
shifts were referenced to the internal residual solvent resonances.
Microanalytical analyses (%CHNS) were obtained using a Thermo
Scientific Flash 2000 elemental analyser fitted with a TCD detector.
2JHH = 12 Hz, 1H, NCH2), 1.91 (s, 3H, CCH3), 2.08 (d, 3JHP = 15 Hz, 1H,
=CCH2), 3.71 (d, 2JHH = 12 Hz, 1H, NCH2), 3.88 (s, 1H, =CCH2), 4.71 (s,
2H, NCH2), 4.87 (s, 5H, C5H5), 6.94 (s, 1H, CimiH), 6.96 (s, 1H, CimiH),
6.96-7.07 (m, 5H, HPh), 7.19-7.44 (m, 15H, HPh). 13C{1H} NMR (CDCl3):
δC = 32.8 (s, CCH3), 48.4 (s, =CCH2), 57.0 (s, NCH2), 65.8 (s, NCH2),
82.4 (s, =CCH2), 87.5 (s, C5H5), 126.1 (s, CimiH), 127.9 (s, CimiH), 128.2
(s, CPh), 128.8 (s, CPh), 128.9 (s, CPh), 129.1 (s, CPh), 129.9 (s, CPh),
133.3 (s, CPh), 133.5 (s, ipso CPh), 133.7 (s, ipso CPh), 176.0 (s, C-Ru).
31P{1H} NMR (CDCl3): δP = -144.2 (septet, 1JPF = 713 Hz, PF6), 57.5 (s,
PPh3). CHN found (calcd) for [C37H36F6N2P2Ru]×0.5CH2Cl2: C, 54.54
(54.39), H, 4.42 (4.50), N, 3.17 (3.38)%. HR-MS (ESI): m/z 641.1656
(2+) calcd for C37H36N2PRu 641.1660.
Electrospray mass spectra (ESI-MS) were recorded on
MicromassQuatro LC instrument.
a
General synthesis of half-sandwich Ru(II)-NHC complexes (1-8):
A suspension of the appropriate imidazolium chloride salt (4 mmol)
in CH2Cl2 (30 mL) containing Ag2O (0.93 g, 4 mmol) was stirred at
30°C for 12 hours in the absence of light. The mixture was filtered
to which the appropriate half-sandwich Ru(II) precursor
([CpRuCl(PPh3)2] P1, [MeCpRuCl(PPh3)2] P2, or [CpRuCl(AsPh3)2] P3,
4 mmol) was added and heated under reflux for 36 hours. The crude
reaction mixture was filtered, concentrated in vacuo (5 mL), to
which acetone (15 mL), and NH4PF6 (0.65 g, 4 mmol) was added.
The reaction mixture was stirred for a further hour after which the
mixture was evaporated in vacuo, and purified by silica gel column
1
[CpRu(PPh3)(L3)]PF6 (3): Yield: 46%. H NMR (CDCl3): δH = 1.82 (d,
2JHH = 12 Hz, 1H, NCH2), 2.18 (s, 3H, CCH3), 2.36 (d, 3JHP = 15 Hz, 1H,
=CCH2), 3.98 (d, 2JHH = 12 Hz, 1H, NCH2), 4.28 (s, 1H, =CCH2), 5.25 (s,
5H, C5H5), 5.61 (s, 2H, NCH2), 6.88 (s, 1H, CimiH), 6.90 (s, 1H, CimiH),
6.93-7.16 (m, 4H, HPh), 7.26-7.32 (m, 9H, HPh), 7.69 (s, 2H, HPh), 8.12
(m, 4H, HPh). 13C{1H} NMR (CDCl3): δC = 34.3 (s, CCH3), 49.9 (s, =CCH2),
57.6 (s, NCH2), 59.6 (s, NCH2), 81.6 (s, =CCH2), 87.5 (s, C5H5), 119.7
(s, CimiH), 123.9 (s, CimiH), 128.2 (s, CPh), 128.6 (s, CPh), 129.0 (s, CPh),
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130.0 (s, CPh), 130.5 (s, CPh), 133.8 (s, CPh), 137.3 (s, ipso CPh), 143.3
2.51 (d, JHP = 15 Hz, 1H, =CCH2), 3.72 (d, JDHHO=I: 1102.1H03z9,/1CH9,NNJ0C1H222)0,F
(s, ipso CPh), 178.2 (s, C-Ru). 31P{1H} NMR (CDCl3): δP = -144.2 (septet, 4.18 (s, 1H, =CCH2), 4.36 (d, JHH = 8 Hz, 2H, NCH2), 4.82 (s, 2H,
2
1JPF = 716 Hz, PF6), 57.2 (s, PPh3). CHN found (calcd) for
[C37H35F6N3O2P2Ru]×0.5H2O: C, 52.95 (52.92), H, 4.55 (4.32), N, 5.03
(5.00)%. HR-MS (ESI): m/z 686.1208 (3+) calcd for C37H35N3O2PRu
686.1510.
C5H4CH3), 5.00 (s, 2H, C5H4CH3), 6.81 (s, 1H, CimiH), 6.82 (s, 1H, CimiH),
6.82-7.00 (m, 5H, HPh), 7.14-7.45 (m, 15H, HPh). 13C{1H} NMR (CDCl3):
δC = 12.2 (s, C5H4CH3), 33.3 (s, CCH3), 53.3 (s, =CCH2), 56.9 (s, NCH2),
57.7 (s, NCH2), 78.3 (s, C5H4CH3), 80.3 (s, =CCH2), 87.9 (s, C5H4CH3),
101.5 (s, C5H4CH3), 121.2 (s, CimiH), 124.7 (s, CimiH), 128.1 (s, CPh),
128.8 (s, CPh), 129.0 (s, CPh), 129.2 (s, CPh), 130.8 (s, CPh), 133.5 (s,
CPh), 133.7 (s, ipso CPh), 135.8 (s, ipso CPh), 176.1 (s, C-Ru). 31P{1H}
NMR (CDCl3): δP = -144.2 (septet, 1JPF = 713 Hz, PF6), 58.1 (s, PPh3).
CHN found (calcd) for [C38H38F6N2P2Ru]×0.7CH2Cl2: C, 54.06 (54.23),
H, 4.51 (4.63), N, 3.49 (3.27)%. HR-MS (ESI): m/z 655.1816 (7+) calcd
for C38H38N2PRu 655.1816.
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1
[CpRu(PPh3)(L4)]PF6 (4): Yield: 55%. H NMR ((CD3)2CO): δH = 1.60
(d, 2JHH = 12 Hz, 1H, NCH2), 1.95 (s, 3H, CCH3), 2.15 (d, 3JHP = 15 Hz,
3
1H, CH2), 3.07 (dddd, JHH = 6, 10, 14, 36 Hz, 2H, NCH2CH2), 3.67
3
3
(dddd, JHH = 6, 10, 14, 77 Hz, 2H, NCH2CH2), 3.85 (d, JHH = 12 Hz,
1H, NCH2), 4.03 (s, 1H, =CH2), 5.27 (s, 5H, C5H5), 7.06 (s, 1H, CimiH),
7.08 (s, 1H, CimiH), 7.10-7.24 (m, 5H, HPh), 7.25-7.57 (m, 15H, HPh).
13C{1H} NMR (CDCl3): δC = 32.8 (s, CCH3), 48.3 (s, =CCH2), 50.9 (s,
NCH2CH2), 56.8 (s, NCH2), 65.8 (s, NCH2CH2), 82.4 (s, =CCH2), 87.5 (s,
C5H5), 121.3 (s, CimiH), 122.9 (s, CimiH), 128.4 (s, CPh), 128.8 (s, CPh),
129.0 (s, CPh), 130.5 (s, CPh), 131.4 (s, CPh), 133.4 (s, CPh), 133.9 (s,
ipso CPh), 137.0 (s, ipso CPh), 174.7 (s, C-Ru). 31P{1H} NMR (CDCl3): δP
= -144.2 (septet, 1JPF = 713 Hz, PF6), 57.4 (s, PPh3). CHN found (calcd)
for [C38H38F6N2P2Ru]×0.5CH2Cl2: C, 54.59 (54.91), H, 4.40 (4.67), N,
3.15 (3.33)%. HR-MS (ESI): m/z 655.1877 (4+) calcd for C38H38N2PRu
655.1816.
1
[CpRu(AsPh3)(L2)]PF6 (8): Yield: 62%. H NMR (CDCl3): δH = 1.86 (s,
3H, CCH3), 2.08 (d, 2JHH = 12 Hz, 1H, NCH2), 2.15 (s, 1H, =CCH2), 3.73
(s, 1H, =CCH2), 3.80 (d, 2JHH = 12 Hz, 1H, NCH2), 4.43 (dd, 2JHH = 16,
48 Hz, 2H, NCH2), 4.81 (s, 5H, C5H5), 6.68 (s, 1H, CimiH), 6.70 (s, 1H,
CimiH), 6.85-7.05 (m, 5H, HPh), 7.18-7.41 (m, 15H, HPh). 13C{1H} NMR
(CDCl3): δC = 30.9 (s, CCH3), 46.0 (s, =CCH2), 57.3 (s, NCH2), 65.8 (s,
NCH2), 79.2.4 (s, =CCH2), 85.0 (s, C5H5), 121.0 (s, CimiH), 122.2 (s,
CimiH), 128.2 (s, CPh), 128.6 (s, CPh), 129.2 (s, CPh), 130.7 (s, CPh), 132.5
(s, CPh), 134.8 (s, CPh), 135.4 (s, ipso CPh), 139.4 (s, ipso CPh), 176.1 (s,
C-Ru). 31P{1H} NMR (CDCl3): δP = -144.2 (septet, 1JPF = 713 Hz, PF6).
CHN found (calcd) for [C37H36F6N2AsPRu]×0.5CH3CN: C, 53.35
(53.68), H, 4.58 (4.45), N, 4.24 (4.12)%. HR-MS (ESI): m/z 685.1140
(8+) calcd for C37H36N2AsRu 685.1140.
1
[CpRu(PPh3)(L5)]PF6 (5): Yield: 61%. H NMR (CDCl3): δH = 1.53 (d,
2JHH = 12 Hz, 1H, NCH2), 1.54 (s, 3H, CCH3), 1.91 (s, 3H, CCH3), 2.07
(d, 3JHP = 15 Hz, 1H, =CCH2), 3.59 (m, 2H, NCH2), 3.69 (d, 2JHH = 12 Hz,
1H, NCH2), 3.85 (s, 1H, =CCH2), 4.50 (s, 1H, =CCH2), 4.98 (s, 1H,
=CCH2), 5.00 (s, 5H, C5H5), 7.08 (s, 1H, CimiH), 7.10 (s, 1H, CimiH),
7.37-7.67 (m, 15H, HPh). 13C{1H} NMR (CDCl3): δC = 30.9 (s, CCH3),
32.9 (s, CCH3), 48.9 (s, =CCH2), 55.0 (s, =CCH2), 57.0 (s, NCH2), 65.8
(s, NCH2), 79.9 (s, =CCH2), 82.5 (s, =CCH2), 87.6 (s, C5H5), 121.0 (s,
CimiH), 124.6 (s, CimiH), 128.4 (s, CPh), 130.8 (s, CPh), 131.9 (s, CPh),
140.8 (s, ipso CPh), 175.6 (s, C-Ru). 31P{1H} NMR (CDCl3): δP = -144.3
Synthesis of [CpRu(PPh3)(L1)]I3 (9): To a solution of complex 1 (0.36
g, 0.5 mmol) in CH2Cl2 (30 mL) was added I2 crystals (0.26 g, 1 mmol).
The resulting mixture was stirred upon which an immediate
darkening of the light yellow solution was observed. After 2 hours
reaction time, the mixture was concentrated in vacuo, extracted
with hexane (3  20 mL). The residue was taken up in CH2Cl2, passed
through a plug of silica, and cooled. Dark yellow crystals formed
overnight. Yield: 77%. 1H NMR ((CD3)2CO): δH = 1.13 (d, 2JHH = 12 Hz,
1H, NCH2), 1.74 (s, 3H, CCH3), 1.89 (d, 3JHP = 15 Hz, 1H, =CCH2), 2.93
(s, 3H, NCH3), 3.51 (d, 2JHH = 12 Hz, 1H, NCH2), 3.75 (s, 1H, =CCH2),
5.03 (s, 5H, C5H5), 6.91 (s, 1H, CimiH), 7.01 (s, 1H, CimiH), 7.22-7.47
(m, 15H, HPh). 13C{1H} NMR (CD3CN): δC = 32.7 (s, CCH3), 37.8 (s,
NCH3), 48.1 (s, =CCH2), 57.5 (s, NCH2), 81.1 (s, =CCH2), 88.8 (s, C5H5),
118.3 (s, CimiH), 121.4 (s, CimiH), 126.0 (s, CPh), 129.5 (s, CPh), 131.4
(s, CPh), 134.3 (s, ipso CPh), 175.8 (s, C-Ru). 31P{1H} NMR ((CD3)2CO):
δP = 58.1 (s, PPh3). HR-MS (ESI): m/z 565.1344 (1+) calcd for
C31H32N2PRu 565.1347.
1
(septet, JPF = 713 Hz, PF6), 57.5 (s, PPh3). CHN found (calcd) for
[C34H36F6N2P2Ru]×0.25CH2Cl2: C, 53.70 (53.36), H, 4.59 (4.77), N,
3.63 (3.66)%. HR-MS (ESI): m/z 605.1715 (5+) calcd for C34H36N2PRu
605.1660.
1
[CpRu(PPh3)(L6)]PF6 (6): Yield: 51%. H NMR (CDCl3): δH = 1.67 (d,
2JHH = 12 Hz, 1H, NCH2), 2.01 (s, 3H, CCH3), 2.23 (d, 3JHP = 15 Hz, 1H,
=CCH2), 4.00 (d, 2JHH = 12 Hz, 1H, NCH2), 4.07 (s, 1H, =CCH2), 4.67 (s,
2H, NCH2), 4.93 (s, 5H, C5H5), 7.21-7.24 (m, 4H, HPh), 7.34-7.37 (m,
4H, HPh), 7.41-7.46 (m, 12H, HPh), 7.52 (m, 2H, HPh), 7.61-7.66 (m,
3H, HPh). 13C{1H} NMR (CDCl3): δC = 30.9 (s, CCH3), 49.0 (s, =CCH2),
50.7 (s, NCH2), 54.4 (s, NCH2), 80.6 (s, =CCH2), 88.2 (s, C5H5), 125.0
(s, CimiH), 126.9 (s, CimiH), 128.0 (s, CimiH), 128.4 (s, CPh), 128.5 (s,
CimiH), 128.9 (s, CPh), 129.4 (s, CPh), 130.5 (s, CPh), 131.9 (s, CPh), 132.1
(s, CPh), 133.6 (s, ipso CPh), 133.7 (s, ipso CPh), 192.7 (s, C-Ru). 31P{1H}
NMR (CDCl3): δP = -144.2 (septet, 1JPF = 713 Hz, PF6), 56.5 (s, PPh3).
CHN found (calcd) for [C41H38F6N2P2Ru]×0.9CH2Cl2: C, 55.19 (55.26),
H, 4.25 (4.40), N, 2.95 (3.08)%. HR-MS (ESI): m/z 691.1312 (6+) calcd
for C41H38N2PRu 691.1816.
Transfer hydrogenation catalysis
To a round-bottom flask containing the carbonyl substrate (0.6
mmol), base (1.2 equivalents), anisole (65 μL, 0.6 mmol), and
ruthenium complex (2 mol%, 12 μmol) was added 2-propanol (4
mL) and the subsequent reaction mixture was heated under reflux
at 110°C for the time indicated. Aliquots (0.05 mL) diluted with
CDCl3 was analysed with 1H-NMR through which conversions were
1
[MeCpRu(PPh3)(L2)]PF6 (7): Yield: 43%. H NMR (CDCl3): δH = 1.50
(d, 2JHH = 12 Hz, 1H, NCH2), 1.90 (s, 3H, CCH3), 1.94 (s, 3H, C5H4CH3),
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determined relative to anisole as the internal standard. All yields
are based on the average of at least two runs.
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X-ray crystallography of [HL5]Cl, P2, P3, 1, 2, 4, 5, 8, and 9
Single crystal diffraction studies of the compounds were done using
Quazar multi-layer optics monochromated Mo Kα radiation (k =
0.71073 Å) on a Bruker D8 Venture kappa geometry diffractometer
with duo Is sources, a Photon 100 CMOS detector and APEX II
control software.22 X-ray diffraction measurements were
performed at 150(2) K. Data reduction was performed using
SAINT+22 and the intensities were corrected for absorption using
SADABS.22 The structures were solved by direct methods using
SHELXT,23 using the SHELXL-2014/724 program. The non-hydrogen
atoms were refined anisotropically. All H atoms were placed in
geometrically idealised positions and constrained to ride on their
parent atoms. For a table containing the data collection and
refinement parameters, see Tables S1-S4 in the Supplementary
Information. Crystallographic data for all structures have been
deposited with the Cambridge Crystallographic Data Centre (CCDC)
as supplementary publication numbers) 1901387 ([HL5]Cl),
1901389 (P3), 1901386 (1), 1901388 (2), 1901385 (4), 1901391 (5),
1901390 (8), and 1901384 (9).
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Conflict of interest
There are no conflicts to declare.
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Acknowledgements
This work has received support from the South African National
Research Foundation (ML Grant nr. 93638 and 115694, FPM Grant
nr. 88594) and the Central Research Fund of the University of
Pretoria (ML). Selected single crystal X-ray diffraction collections
and further assistance by Mr. D. Liles are gratefully acknowledged.
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Keywords: ruthenium • N-heterocyclic carbene • transfer
hydrogenation • epoxidation • catalysis
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Eight novel alkene-tethered Ru(II)-NHC complexes were employed as catalysts in tandem
transfer hydrogenation-epoxidation reactions using phenacyl bromide derivatives as substrates.