Ruthenium(II) complexes of hemilabile pincer ligands: Synthesis and catalysing the transfer hydrogenation of ketones

Article abstract of DOI:10.1039/c6dt02459a

A series of Ru(ii) complexes were synthesised based on a hemilabile pyrazole-N-heterocyclic carbene (NHC)-pyrazole (C₃N₂H₃)CH₂(C₃N₂H₂)CH₂(C₃N₂H₃) NCN pincer ligand 1. All complexes were fully characterised using single crystal X-ray crystallography and multinuclear NMR spectroscopy. Hemilabile ligands provide flexible coordination modes for the coordinating metal ion which can play a significant effect on the efficiency and mechanism of catalysis by the resulting complex. Here we observed and isolated mono-, bi- and tri-dentate complexes of both Ag(i) and Ru(ii) with 1 in which the resultant coordination mode was controlled by careful reagent selection. The catalytic activity of the Ru(ii) complexes for the transfer hydrogenation reaction of acetophenone with isopropanol was investigated. The unexpected formation of the pentaborate anion, [B₅O₆(OH)₄]^-, during the synthesis of complex 6a was found to have an unexpected positive effect by enhancing the catalysis rate. This work provides insights into the roles that different coordination modes, counterions and ligand hemilability play on the catalytic activity in transfer hydrogenations.

Full text of DOI:10.1039/c6dt02459a

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10.1039/C6DT02459A.
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DOI: 10.1039/C6DT02459A
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ARTICLE
Ruthenium(II) Complexes of Hemilabile Pincer Ligands: Synthesis
and Catalysing the Transfer Hydrogenation of Ketones
Received 00th January 20xx,
Accepted 00th January 20xx
Ashwin G. Nair,a,b Roy T. McBurney,b D. Barney Walker,a,b Michael J. Page,a Mark R. D. Gatus,a,b
Mohan Bhadbhade,c and Barbara A. Messerlea,b,†
DOI: 10.1039/x0xx00000x
A series of Ru(II) complexes were synthesised based on a hemilabile pyrazole-N-heterocyclic carbene (NHC)-pyrazole (C-
3N2H5)CH2(C3N2H2)CH2(C3N2H5) NCN pincer ligand 1. All complexes were fully characterised using single crystal X-ray
crystallography and multinuclear NMR spectroscopy. Hemilabile ligands provide flexible coordination modes for the
coordinating metal ion which can play a significant effect on the efficiency and mechanism of catalysis by the resulting
complex. Here we observed and isolated mono-, bi- and tri-dentate complexes of both Ag(I) and Ru(II) with 1 in which the
resultant coordination mode was controlled by careful reagent selection. The catalytic activity of the Ru(II) complexes for
the transfer hydrogenation reaction of acetophenone with isopropanol was investigated. The unexpected formation of the
pentaborate anion, [B5O6(OH)4]-, during the synthesis of complex 6a was found to have an unexpected positive effect by
enhancing the catalysis rate. This work provides insights into the roles different co-ordination modes, counterions and
ligand hemilability play on the catalytic activity in transfer hydrogenations.
www.rsc.org/
or k3 complex, demonstrated enhanced catalytic turnovers for
transfer hydrogenation (Chart 1a) in comparison to other Ir(I)
NHC complexes.2a Similar to the ligand system under
investigation here, Shreeve and co-workers reported Pd(II)
complexes with bidentate ligands containing the strongly
donating carbene donor as well as the weakly coordinating
pyrazole donors as catalysts for the Heck and Suzuki reactions
in ionic liquids, (Chart 1b).3 The hemilability of this ligand
facilitates the weakly coordinating pyrazole to dissociate from
the metal centre, allowing the catalyst to pass through the key
Pd(0) intermediate for enhancing catalysis.
Introduction
In an effort to create organometallic complexes with diverse
coordination modes pincer ligands which incorporate
hemilability with a mix of strongly donating donors and weakly
donating pendant arms that may or may not coordinate to a
metal ion have received significant attention. Complexes with
pincer ligands have been used successfully as catalysts for
promoting a wide range of transformations, and selected
coordination motifs may significantly improve catalysis rates
and/or selectivities.1 Our research focuses on pincer ligands
containing a central N-heterocyclic carbene (NHC) donor2
together with labile pendant pyrazole donors. NHCs are an
increasingly common motif, competing with the central aryl
unit prominent in the work of van Koten1b,c and others. The
carbene moiety should provide a strong
s
-donor,1a therefore
stabilising metal complexes of these ligands giving access to
different coordination modes. Complexes of such hemilabile
pincer ligands are known to display enhanced catalytic activity
over analagous complexes where the pendant arms are
strongly binding. For instance, an Ir(I) carbene-based complex
with two pendant pyridyl donor groups, hence potentially a k2
Chart 1 Comparison of other carbene-based ligands with pendant N-heterocyclic donor
groups. This work is (c) where n = 1.
We have recently reported the coordination chemistry and
catalytic activity of Rh(I) and Ir(I) complexes containing a
pincer ligand with a central NHC and a pair of weakly
coordinating pyrazole pendant donors (Chart 1c).4 As with
many other hemilabile carbene containing ligands, the
complexes exhibited varied and diverse coordination
chemistry, where even the less flexible ligands (n = 1)
underwent conformational exchange. The Ir(I) and Rh(I)
complexes with the ligand featuring a methylene bridge (n = 1)
were shown to be good catalysts for both hydroamination and
hydroacyloxylation but not hydrosilylation reactions. It was
found that an increase in hemilability of the pendant donors,
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achieved using a longer linker between the pendant donor and that would be able to select for different coordination motifs
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DOI: 10.1039/C6DT02459A
the central NHC donor (ethylene vs methylene bridge), with the ligand. We attempted numerous approaches to
resulted
hydrocarboxylation reactions, though a similar reactivity was transmetallation via Ag(I), as detailed in Scheme 2.
observed in hydroamination reactions. Deprotonation of the imidazolium salt with NEt3, followed by
Herein, we report the co-ordination chemistry of Ru(II) complexation with RuCl3.xH2O in refluxing ethanol led to a bis-
with the hemilabile pincer ligand , an NHC-based bis-pyrazole 3-NCN complex in a good 71% yield. Treatment of with
NCN pincer, (Chart 1c, n = 1).4 The most efficient hemilable Ru(H)Cl(CO)(PPh3)3 in refluxing methanol resulted in the
Ru(II) pincer catalyst reported to date for the transfer production of complex in a moderate yield of 52%.
hydrogenation of acetophenone is a Ru(II) based complex with In an effort to achieve better complexation yields we
tridentate pyrazole-pyridyl-benzimidazole ligand which attempted a slightly different approach to the final Ru(II)
achieves substrate conversions of 95% in 1 minute under very complexes where an intermediate bis- 1-NCN Ag(I) complex,
mild conditions (room temperature, in air, catalyst loading of was synthesised by reaction of with Ag2O in CH2Cl2 with the
only 0.2 mol%).5 We were interested in how Ru(II) catalysts intention of using complex as a carbene transfer reagent.6
with a central strong -donor carbene pincer ligand would Treatment of with {(C6H6)}RuCl2}3 in CH2Cl2 at room
impact on the efficiency of catalysed transfer hydrogenation temperature led to the Ru(II) 2-NCN η-6-benzene chloro
in 48% yield. It was expected that treatment of
in
much
poorer
catalytic
activity
for achieving complexation to Ru(II), including carbene
1
1
k
2
1
3
a
k
4,
1
4
s
4
k
catalysis of ketones. During the synthesis of the complexes we complex
5
4
encountered formation of a rare tetrahydroxypentaborate with [Ru(η6-p-cym)Cl2]2 (where p-cym = p-cymene) would led
[B5O6(OH)4]- anion. The pentaborate anion had an unexpected to a pure BPh4 complex. Instead, upon elucidation of the solid
effect during the transfer hydrogenation where its presence state structure of 6a, it was found that rather than the
actually increased the catalysis rate compared to just the expected
BPh4
anion
there
was
a
rare
tetraphenylborate [BPh4]- anion.
tetrahydroxypentaborate [B5O6(OH)4]- anion present. This was
then revealed to be a mixed counterion system in the bulk
sample, with both BPh4 and a rare tetrahydroxypentaborate
[B5O6(OH)4]- anion present in an 86:14 ratio as determined by
1H NMR analysis.7 11B NMR proved unable to verify this ratio
due to extremely broad resonances, see ESI. The ratio of
boron-based anions varied in each synthesis batch and it is
unclear as to how exactly the pentaborate anion formed as our
conditions vary from those reports in which the synthesis of
this molecule is reported.8 In an effort to obtain the Ru(II)
complex containing solely the [BPh4]- counterion, an in situ
Results and Discussion
Synthesis and Characterisation
The imidazolium pro-ligand
1 was synthesised according to our
previous report from chloromethylpyrazole and half an
equivalent of trimethylsilyl imidazole in refluxing toluene, see
Scheme 1.4 After 16 h a viscous brown oil had formed; upon
anion exchange with NaBPh4, NCNH.BPh4,
1, was isolated as
colourless crystals in 42% yield. Crystals suitable for
crystallographic analysis were grown by slow evaporation of an
silver transmetallation method was utilised where
1 was
reacted with [Ru(η6-p-cym)Cl2]2 in the presence of Ag2O.
Complex 6b was isolated in 51% yield and the purity of the
anion was confirmed by NMR and elemental analysis, with no
pentaborate anion being detected.
acetone solution of 1. As expected the solid state structures
confirmed the chemical structure with the pro-ligand
essentially in a straight conformation, see Figure 1.
The 1H NMR spectrum of a bis-
k3-NCN Ru(II) complex 2
contained two sets of ligand proton resonances, which were
attributed to the formation of two isomers. Variable NMR
temperature experiments revealed that upon heating the
complex to 40 °C the two sets of NMR resonances approached
pairwise coalescence, indicating that a single species was
present (see ESI). The ratio of ligand to BPh4 proton resonances
Scheme 1
Synthesis of imidazolium bis-pyrazole pro-ligand
1
from
chloromethylpyrazole and trimethylsilyl imidazole.4
was 1:1 indicating that
2 was likely to be a Ru(II) NCN
homoleptic complex, this was confirmed in the solid state by
crystallographic analysis (Figure 3a). The solid state structure
showed the expected octahedral coordination geometry of
Ru(II) with the NCN ligands occupying tridentate fac
orientations, with the carbene groups located trans to each
other. It is likely that the pincer ligand can coordinate to the
Ru centre in either fac or mer coordination modes, although
only the fac structure was observed in the solid state using X-
ray crystallography. Mass spectrometry showed the presence
of a single molecular ion and elemental analysis was in
Figure 1 X-ray crystal structure of the imidazolium bis-pyrazole pro-ligand 1. Ellipsoids
shown with 50% probability, hydrogen atoms and BPh4 anions have been omitted for
clarity.
Our experience with this imidazolium pro-ligand (1) has
proved it to be capricious during attempts to introduce a metal
agreement with the calculated value of complex 2.
ion.4 We wanted to create a general and robust methodology
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Scheme 2
Complexation routes from pro-ligand 1 to generate complexes 2 to 6.
Figure 2 X-ray crystal structures of complexes (a) 2, (b) 3, (c) 4, (d) 5, and (e) 6a. Complex 6a crystallised with the [B5O6(OH)4]- anion, the BPh4 anion was not observed in the
solid state. Ellipsoids shown with 50% probability, hydrogen atoms and BPh4 anions have been omitted for clarity, as have the Ph3 groups present in complex 3.
1
The H NMR spectrum of complex 39 contained a triplet
2
Table 1 Selected bond lengths(Å) and angles(°) for 2, 3, 5, and 6a.
resonance at -6 ppm due to a Ru(II) bound hydride with JH-P
Atoms
Ru(1)-C(1)
Ru(1)-C(2)
Ru(1)-N(2)
Ru(1)-N(3)
Ru(1)-Cl
Ru(1)-P(1)
Ru(1)-P(2)
C(1)-Ru(1)-N(3)
C(1)-Ru(1)-N(2)
2
3
5
6a
2.046(2)
----
coupling of 21.9 Hz to two chemically equivalent phosphine
atoms (31P NMR,
d
= -48 ppm). The Ru-H triplet and a single 31
P
2.049(5)
----
2.076(4)
2.088(4)
-----
-----
-----
84.2(2)
82.1(2)
2.117(6)
1.810(5)
-----
2.122(5)
-----
2.368(1)
2.369(1)
86.6(2)
-----
2.04(1)
----
-----
2.096(7)
2.399(2)
-----
-----
83.3(3)
-----
NMR resonance indicated the presence of two phosphine
ligands trans to one another.10 The IR spectrum contained two
distinct strong signals for the Ru-H (1606.2 cm-1) and Ru-CO
-----
2.085(2)
2.408(1)
-----
-----
83.58(9)
-----
(1938.3 cm-1) stretches. The solid state structure of
3
confirmed the trans geometry of the phosphine groups and
revealed that the CO unit lies trans to N3 contained in the NCN
ligand which was bound to the Ru centre in a bidentate fashion
though N3 and C1 atoms.
being that the NCN ligand is bound to Ru(II) in a bidentate co-
ordination mode in complex while in complex the ligand
The solid state structures of Ru(II) complexes
exhibit an octahedral geometry, with the main difference
2 and 3 each
3
2
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adopts a tridentate co-ordination mode to the metal centre coordination geometry proved to be the most acVtiievweAratinclde Oanllsinoe
DOI: 10.1039/C6DT02459A
(Figure 3). The Ru-C(1) and Ru-N(3) bond lengths are slightly gave the highest yield of hydrogenated products, see Figure 3.
longer in complex
lengths in , indicating stronger bonding interaction between counterion achieves over 90% conversion of acetophenone to
Ru and pyrazole in as strong π back-donation from Ru to CO 1-phenylethan-1-ol in less than 30 minutes at 83 C, and
in weakens the Ru-pyrazole bond located in a trans position, complete conversion on heating for four hours. The efficacy of
see Table 1. The carbene carbons and pyrazole nitrogens (N2, 6a as a catalyst for transfer hydrogenation is comparable to
3 in comparison to the analogous bond The mixed anion catalyst, 6a, containing the unusual boron
2
2
°
3
N3) in complex
atom lies on an inversion centre (C-Ru-C/N-Ru-N: 180.00(2)
However, similar alignments in complex (X-Ru-Y, where X=N,
Y=C, or X=Y=P)) exhibit heavy distortion from linearity (P-Ru-P:
168.77(2) ) likely due to the steric bulk of the two PPh3 groups.
The proton NMR spectra of complexes 6a and 6b all
3 show no distortion from linearity as the Ru some of the best Ru(II) catalysts reported for this reaction,
°
). such as [Ru(OTf)(PCP)PPh3].13
3
°
5
,
exhibited a large number of proton resonances, indicative of
complexes with a single, asymmetrically bound ligand. The
solid state structures for complexes
5 and 6a confirmed the
initial assignment, revealing that both complexes featured
bidentate coordination of the NCN ligand to the Ru centre,
with a bound chloride and a η-6-aromatic ligand completing
the first coordination sphere. As in complex
coordinates to the metal centre through though its N3 and C1
atoms in both complexes and 6a. Complexes and 6a each
3 the NCN ligand
5
5
adopt a psuedo-octahedral geometry and are similar in
structure, for example, the Ru-C(1) bond lengths are the same
within 1 esd. Structural similarity of complexes
5 and 6a is
further confirmed by comparison of the Ru-N(3) or Ru-Cl bond
lengths, which fall within 0.01 Å of each other, as well as the
Figure 3 Catalytic transfer hydrogenation of ketones. % Conversion refers to
C(1)-Ru-N(3) angle which differs by only 0.2
° between
conversion of acetophenone to 1-phenylethan-1-ol.
complexes and 6a. The Ru-C1 bond lengths of complexes 3a
5
-
6a are comparable to those of similar Ru complexes reported
in the literature.11 The boron counterion [B5O10H4]- in 6a was
identified using X-ray crystallography. The counterion is
composed of a central tetrahedral boron as the apex of two 6-
membered rings composed of alternating boron and oxygen
atoms, the counterion is terminated by four hydroxyl groups.
Comparing the efficacy of complex 6a containing a mixed
anion with that of complex 6b containing pure BPh4 anion, we
found 6a was more active with conversion of substrate at 0.5
hours 91% vs. 39% for 6b, (Figure 3 and Table 2). This indicates
that the presence of [B5O6(OH)4]- enhances the overall
catalytic activity for the transfer hydrogenation reaction and is
therefore non-innocent, acting as a catalyst in its own right.
Polyborate anions have been reported in the literature for
promoting transfer hydrogenation reactions.14 Thus it seems
very likely that the increase in catalytic activity for 6a is as a
result of the pentaborate anion.
As expected the crystal structure of Ag(I) complex
4
showed two NCN ligands coordinated the Ag(I) ion through the
carbene unit. In the solid state the pendant pyrazole arms are
directed toward the same face such that the complex has
centre of inversion through the silver centre. The Ag-C1 and
Ag-C2 each have a bond length of 2.08 Å typical of other
Catalysts
the similarity in structure of the two complexes, with the only
difference being a change of the bound co-ligand (benzene,
vs. p-cym, 6b). Whilst complex 6b achieves complete (>99%)
substrate conversion within four hours at 83 C, complex
5 and 6b display different catalytic rates despite
similar silver bis
C2 is 4o off linearity (180
of the NHC ligands out of the plane relative to the other giving
-
NHC complexes and the angle between C1-Ag-
°
).12 There is also slight rotation of one
5
,
1
a torsion angle of 17°. The H NMR spectra was simpler, with
°
5
only one species present and a 1:1 ratio of ligand to BPh4
counterion.
only reaches a maximum of 80% conversion. The stronger
electron donating properties of the p-cym co-ligand may
stabilise 6b more effectively than the benzene co-ligand of
Transfer Hydrogenation Catalysis
complex
higher conversion of the substrate.
Catalyst catalysed the formation of the 1-phenylethan-1-
5, resulting in a longer catalyst lifetime and hence
The Ru(II) complexes 2, 3, 5, 6b, and the complex containing
the mixture of BPh4 and [B5O6(OH)4] counterions, 6a, were
tested as catalysts for the transfer hydrogenation of
acetophenone using a catalyst loading of 1.5 mol% in propan-
2-ol and KOH. The catalysis reactions were followed both at
room temperature and under reflux conditions using GC-MS
and 1H NMR spectroscopy to monitor reaction progress.
Interestingly the complexes with the pseudo-octahedral
3
ol product with only 24% conversion after four hours at reflux,
increasing to >99% conversion after 24 hours at reflux. This
was unexpected as the complex contains a pre-existing
hydrido group which should enable more efficient transfer
hydrogenation reaction, as one of the key steps in the catalytic
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cycle is the transfer of a hydride from the catalyst to the reduced due poor solubility. Addition of THF as aVcieow-sAortilcvleeOnntlinine
substrate.
The bis-NHC dimer complex
a propan-2-ol:THF ratio of 9:1 aided solDuObiIl:i1t0y.1o0f39t/hCe6DcTa0t2a4ly5s9tA.
was expected to possess a The catalytic activity of complex 6b in either neat propan-2-ol
2
low catalytic activity due to the metal centre being enclosed by or in the propanol-2-ol/THF mix for the transfer hydrogenation
two pincer ligands which occupy all six available co-ordination of acetophenone proved to be nearly identical (45% at 0.5 h,
sites. Indeed, complex
hydrogenation reaction to a conversion of 14% after 4 hours at affecting the reaction, complex
83 C, and after 24 hours to approximately 30% (see Figure 3). hydrogenation of acetophenone using the 9:1 propan-2-ol:THF
As complex is coordinatively saturated, any conversion of the solvent ratio. The addition of THF to the reaction mixture
substrate must result from the hemilability of the pyrazole side resulted in complete conversion of the substrate (87% after
arms. 0.5 h, >99% at 4 hours at reflux), the catalytic rate remains
2
only promoted the transfer >99% at 4 h). For exact comparison and to ensure THF was not
5
was tested for the transfer
°
2
When the high activity of catalyst 6a, which we attribute in relatively constant in comparison to using neat propan-2-ol as
part to the pentaborate anion, is set aside, it is clear that the solvent. It is likely that the complete conversion achieved by
“pure” organometallic catalysts
5
and 6b outperform catalyst
3
catalyst 5 was a result of co-ordination of THF to the Ru metal
for the transfer hydrogenation of acteophenone. Thus the centre, stabilising the complex and thus prolonging the
reason for the difference in catalytic efficiency between lifetime of the catalyst.
catalysts
between
3
3
, and
5
and 6b must lie in the structural features
The most active catalysts, 6a and 6b, were investigated for
, that contains well defined carbonyl and hydride the transfer hydrogenation reaction of a number of different
ligands, and the pseudo-octahedral, chloride bearing catalysts ketone substrates (both aromatic and aliphatic, see Table 2).
and 6b. The mechanisms in operation must therefore Both catalysts successfully catalysed the reactions at reflux and
proceed via slightly different initiation stages. Complexes were able to achieve complete conversion of substrate in each
5
5
and 6b could be operating via an inner-sphere mechanism case with the exception of the p-nitroacetophenone (Table 2,
similar to that postulated by Bhattacharjee, where an initial entry 3). Again, catalyst 6a was more efficient for the
chloride exchange with alkoxide is followed by
b-hydride conversion of all substrates in comparison to 6b. Catalyst 6a
elimination to give a ruthenium hydride species and acetone.15 was tested for the transfer hydrogenation reaction of
Catalyst
follow
3
a
, despite possessing a pre-existing hydride, must acetophenone, benzophenone and cyclohexanone at room
similar inner-sphere mechanism, however, the temperature and reasonable yields of 51%, 41% and 59% were
reaction proceeds at a slower rate as there is no chloride achieved respectively after 24 hours. (Table 2, entries 1, 2 and
which can be lost to allow binding of the alkoxide.16 4).
Additionally, a recent study has shown that half-sandwich
Table 2. Catalytic transfer hydrogenation of ketones using complexes 6a and 6b.
Ru(II) NHC complexes containing a hemilabile picolyl sidearm
are able to catalyse the transfer hydrogenation reaction
efficiently.17 This is achieved through a stepwise process in
which the hemilabile arm is lost generating a 16 e- Ru centre,
before coordination of the alkoxide and delivery of the hydride
to the metal centre.17
Of all the catalysts tested here the mixed anion complex 6a
was the only active catalyst at room temperature (25 °C),
resulting in a 51% conversion within 24 hours (Figure 3).
entry
product
catalyst
% conv. at time (h)
0.5b
91
2b
4b
24c
51
6a
6b
6a
6b
6a
96
>99
1
39
91
69
0
82
91
97
0
>99
>99
>99
0
-
41
-
2
3
Complexes 2, 3, and 5 and 6b did not catalyse the reaction at
room temperature even after 24 hours. This would suggest
that the pentaborate anion does indeed play a non-innocent
role in the transfer hydrogenation and likely has an entirely
separate mechanism from the carbene pincer based catalyst.
All efforts to isolate the pentaborate anion (via salt exchange
and chromatography) failed, otherwise a potassium or sodium
pentaborate salt would have provided an interesting set of
control experiments. While most Ru(II) complexes have been
reported to catalyse the transfer hydrogenation reaction of
-
6b
0
0
0
-
6a
6b
92
72
>99
>99
>99
>99
59
-
4
5
6a
6b
51
44
94
78
>99
>99
-
-
o
ketones to alcohols at elevated temperatures (> 50 C), there
a Reaction conditions: 1.5 mol% catalyst, isopropanol, KOH; 40 °C; 25 °C. %
b
c
are relatively few reports of Ru(II) complexes capable of
catalysing the reaction at room temperature. The catalyst
containing the pentaborate anion (6a) is indeed a unique
system which requires further mechanistic investigation.
Whilst using complex 6b for the transfer hydrogenations it
transpired that this complex was not completely soluble in
propan-2-ol, indicating that the catalytic activity may be
conversion refers to conversion of aldehyde substrate to product alcohol shown.
Conclusions
The coordination chemistry of NCN pincer ligand precursor
1
with Ru(II) and Ag(I) salts was investigated. The Ru(II)
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Synthesis of [Ru(NCN)2](BPh4)2 (2)
NCNH.BPh4 (
0.092 mmol) and NEt3 (1 mL) were suspended in 15 mL of dry
ethanol. The mixture was refluxed for 16 hours. The resulting
dark green solid, [Ru(NCN)2](BPh4)2 was separated from a pale
yellow solution using glass fibre (GF/C) filter paper. Yield: 65%.
Single crystals were grown by slow evaporation of a saturated
complexes were tested as catalysts for transfer hydrogenation
reactions. A series of coordination motifs were observed in the
solid state, ranging from a fully saturated tridentate Ru(II)
dimer to a mono-dentate Ag(I) dimer, all of which could be
preferentially selected for by careful reagent choice. It was
through crystallographic analysis that the rare pentaborate
anion was observed in complex 6a. Indeed, this mixed anion
complex was found be the most active catalyst for the transfer
hydrogenation reaction, promoting the conversion of a range
of aromatic and aliphatic ketone substrates to alcohols with
high yields in reasonable times at elevated temperature. The
catalysed transfer hydrogenation reaction could also be
conducted at room temperature using catalyst 6a. Complexes
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DOI: 10.1039/C6DT02459A
) (0.05 g, 0.092 mmol) and RuCl3.xH2O (0.024 g,
1
acetone solution of
2
. 1H NMR (400 MHz, (CD3)2CO): δ 8.14 (d,
1), 7.85 (s, 4H, 5), 7.28 (m, 16H, o-BPh4),
3J = 2.6 Hz, 4H,
H
H
3
3
6.88 (t, J = 7.3 Hz, 16H, m-BPh4), 6.85 (d, 4H,
H
H
4), 6.73 (t, J =
3), 6.17 (br dd,
4) ppm. 13C{1H} NMR (100
6), 164.3-162.3 (q, 1J = 49.8 Hz, ipso-
1), 136.1 ( 3), 135.5 (o-BPh4), 125.2 (m-
BPh4), 121.4 (p-BPh4), 120.8 ( 5), 107.2 ( 2), 62.4 ( 4) ppm.
3
7.2 Hz, 8H, p-BPh4), 6.52 (d, J = 1.6 Hz, 4H,
3
4H,
MHz, (CD3)2SO): δ 195.4 (
of BPh4), 147.0 (
H2), 5.97 (d, J = 13.7Hz, 4H, H
C
2, 3a, 3b, 5, and 6b were catalytically inactive at room
C
C
C
C
temperature. Comparison of the catalytic activities of the
mixture 6a and complex 6b revealed that the presence of the
[B5O6(OH)4]- counterion enhanced the rate of reaction, most
probably by catalysing transfer hydrogenations via a separate
mechanism from the organometallic catalyst. Despite not
being amongst the most active hemilabile Ru(II) in the
literature, the discovery and catalytic activity of the
pentaborate anion warrants further investigation.
C
C
Elemental analysis found: C, 70.17; H, 5.50; N, 14.05. Calc for
Ru1C70H64N12B2: C, 70.18; H, 5.55; N, 14.03. ESI MS: (m/z
279.08) [M-2BPh4]2+ amu.
Synthesis of [Ru(H)CO(NCN)(PPh3)2]BPh4 (3a)
NCNH.BPh4 (
1) (0.040 g, 0.073 mmol) and [Ru(H)COCl(PPh3)3]
(0.070 g, 0.073 mmol) were suspended in 20 mL of dry MeOH
and NEt3 (1 mL). The mixture was refluxed for 2 hours. The
resulting pale yellow solution was filtered using glass fibre
(GF/B) filter paper. The filtered solution was reduced to half its
initial volume under reduced pressure resulting in the
precipitation of [Ru(H)CO(NCN)(PPh3)2]BPh4 as a white solid.
The solvent was removed and the product was washed with
diethylether (2 x 20 mL) and dried under vacuum. Yield: 40%.
Single crystals were grown by vapour diffusion of diethyl ether
Experimental Section
Materials and Methods
All manipulations were carried out under
a nitrogen
atmosphere using standard schlenk techniques or in a nitrogen
or argon filled glovebox unless otherwise stated. Imidazolium
pro-ligand
1
was prepared according to literature method.4
1
into saturated dichloromethane solution of 3a. H NMR (300
Commercially available reagents were purchased from Sigma-
Aldrich or Alfa Aesar Inc. and used as received. Solvents were
acquired using a solvent purification system. Propan-2-ol,
methanol and THF were freshly distilled from standard drying
reagents. 1H, 13C and 31P NMR spectra were recorded on
Bruker DPX300, DMX500 and DMX400 spectrometers.
MHz, (CD3)2CO): δ 7.60 (d, 3J = 1.8 Hz, 1H, 1), 7.57 (d, 3J = 2.1
H
Hz, 1H,
BPh4, PPh3,
(br tt, 3J = 7.2 Hz, 4H, p-BPh4), 6.26 (dd, 1H,
8), 5.53 (dd, 1H, 4), -6.03 (t, 3J = 22.0 Hz,
10) 5.30 (br s, 2H,
1H, Ru-
) ppm. 13C{1H} NMR (75 MHz, CD2Cl2): δ 206.7 (Ru-
O), 189.3 ( 7), 149.3 ( 9), 141.6 ( 3), 136.3 ( 1), 134.7-133.7
) 130.7 (3 x ), 130.2 ( 11), 129.3-129.0 (3 x ) 126.2 (m-
5), 122.4 (o- of BPh4), 122.0 ( 6), 107.5
10), 62.1 ( 8), 61.8 (
4) ppm.31P NMR (121 MHz,
H
5), 7.49 (d, 3J = 2.1 Hz, 1H,
3,
9), 6.92 (t, 3J = 7.2 Hz, 9H, m-BPh4,
2), 5.67 (br s, 2H,
H
6), 7.48-7.30 (m, 43H, o-
H
H
H
11), 6.77
H
H
H
H
H
1
Chemical shifts (δ) are quoted in ppm. H, 13C and 31P NMR
C
C
C
C
C
chemical shifts were referenced internally to residual solvent
resonances. IR spectra were recorded using an Avatar 370 FTIR
spectrometer. Elemental analyses were carried out at the
Campbell Microanalytical Labaoratory, University of Otago,
New Zealand and the Elemental Analysis Unit, The Research
School of Chemistry, Australian National University. Single
crystal X-ray analyses were carried out at the Mark Wainwright
Analytical Centre, University of New South Wales, Sydney. X-
ray diffraction measurements were carried out on a Bruker
Kappa APEXII CCD diffractometer using graphite-
monochromated Mo-Kα radiation (λ = 0.710723 Å). All
structures were solved by direct methods and the full-matrix
least-square refinements were carried out using SHELXL.
Absorption correction was performed using Multi-scan
SADABS and H-atom parameters were treated as constrained.
CCDC 1480997 – 1481002 contains supplementary X-ray
(4 x
C
C
C
C
C
of BPh4) 124.7 (
2) 107.2 (
C
C
C
C
(
C
C
C
(CD3)2CO): δ 47.7 (P
-Ru) ppm. ESI-MS: (m/z 883.26) [M-BPh4]+
amu. IR (Solution): Ru-H: 1606.2 cm-1, Ru-CO: 1938.3 cm-1.
Synthesis of [Ag(NCN)2]BPh4 (4)
The imidazolium salt NCNH.BPh4 (
1) (0.155 g, 0.287 mmol) and
Ag2O (0.090 g, 0.388 mmol) were suspended in 20 mL of dry
CH2Cl2. The suspension was stirred for 16 hours at room
temperature. The colourless solution was filtered using glass
fibre (GF/C) filter paper and the solvent removed under
vacuum yielding a white precipitate. Yield: 81%. Single crystals
were grown by vapour diffusion of pentane into saturated
1
dichloromethane solution of complex
4
. H NMR (300 MHz,
3
3
CD2Cl2): δ 7.73 (d, J = 2.4 Hz, 4H,
3), 7.38 (m, 8H, o-BPh4), 7.20 (s, 4H,
8H, m-BPh4), 6.89 (t, 3J = 7.2 Hz, 4H, p-BPh4), 6.38 (dd, 4H,
6.36 (s, 8H,
4) ppm. 13C NMR (75 MHz, CD2Cl2): δ 183.9 (
H1), 7.58 (d, J = 1.8 Hz, 4H,
crystallographic data for complexes 1, 2, 3, 4, 5 and 6a. The
yields of hydrogenation products were determined using
GCMS - QP2010 ULTRA.
3
H
H
5), 6.98 (t, J = 7.0 Hz,
2),
6,
H
H
C
6 | J. Name., 2012, 00, 1-3
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Page 7 of 9
PleaDsealdtoonnTortaandsjuascttimonasrgins
Journal Name
ARTICLE
2
identified through HMBC), 141.9 (
C
1), 136.3 (o-
of BPh4), 122.3 (p- of BPh4), 122.2 (
4) ppm.
C
of BPh4), 3J = 2.0 Hz, 1H,
5), 1H,
5.90 (d, 3J = 6.2 Hz, 1H,
(d, 3J = 6.2 Hz, 1H, 21), 5.24 (d, 3J = 6.1 Hz, 1H,
= 14.1 Hz, 1H,
8), 4.64 (d, 2J = 14.1 Hz, 1H, 8), 2.64 (sept, 3J =
7.0 Hz, 1H, 17), 2.07 (s, 3H, 12), 1.22-1.21 (2 x d, 2H, 18 &
19) ppm. 13C {1H} NMR (100 MHz, CD2Cl2): δ 177.7 (
7),
1), 142.2
9), 126.3 (m- of
H
6), 6.53 (d, J = 13.8 Hz, 1H,
H
4)V,ie6w.4A2rtic(lbe rOndlidne,
2
130.5 (
108.1 (
C
C
3), 126.1 (m-
2), 65.3 (
C
C
C
H
2), 6.38 (br dd, 1H,
H
10), 6.09 (d,DOJ I=: 1103.1.0839H/Cz6, D1TH0,2459A
H
4),
14), 5.47
H
15), 4.75 (d, 2J
C
H H
20), 5.78 (d, 3J = 5.8 Hz, 1H,
H
Synthesis of [Ru(η6-C6H6)(NCN)Cl]BPh4 (5)
[Ag(NCN)2]BPh4 (
) (0.060 g, 0.068 mmol) and [Ru(η6-C6H6)Cl2]2
H
H
1
H
H
H
(0.034 g, 0.068 mmol) were dissolved in 20 mL of CH2Cl2. The
mixture was stirred overnight at room temperature under an
atmosphere of Argon. The resulting yellow solution was
filtered and pentane (40 mL) was slowly added to the filtrate
resulting in the precipitation of the complex [Ru(η6-
H
C
1
164.8-163.4 (q, J = 49.3 Hz ipso-
11), 136.4 (o- of BPh4), 134.8 (
BPh4), 123.4 ( 5), 122.7 (p- of BPh4), 121.2 (
108.6 ( 2), 107.8 ( 10), 102.6 (
86.6 ( 15), 64.5 ( 8), 61.8 (
19), 21.4 (
C
of BPh4), 148.4 (
C
(
C
C
C
3), 131.2 (
C
C
C
C
C
6), 112.5 (
C
C
16),
21),
C
C
C
13), 89.1 (
C
20), 86.9 (
C6H6)(NCN)Cl]BPh4 (
5) as a yellow powder. Yield: 48%. Single
C
14), 84.3 (
C
C
C
4), 31.9 (C17), 23.7
crystals were grown by slow evaporation of a saturated
(
C
C
18), 18.9 (
C
12) ppm. Elemental Analysis found: C,
1
methanol solution of 5. H NMR (300 MHz, (CD3)2CO): δ 8.19
65.86; H, 5.77; N, 9.83 %. calculated for Ru1C45H46N6B1:
C,65.97; H, 5.78; N, 10.26. ESI MS: (m/z 499.19) [M-
BPh4/B5O6(OH)4]+ amu.
(d, 3J = 2.2 Hz, 1H,
H
1), 8.16 (d, 3J = 2.4 Hz, 1H,
H
11), 8.14 (d, 3J
= 2.8 Hz, 1H,
Hz, 1H,
H
3), 7.68 (d, 3J = 1.8 Hz, 1H,
H
9), 7.63 (d, 3J = 2.2
3
H
5), 7.40 (d, J = 2.2 Hz, 1H,
H
6), 7.37-7.30 (m, 8H, o-
BPh4), 6.92 (t, 3J= 7.3 Hz, 8H, m-BPh4), 6.86 (d, 2J = 13.9 Hz, 1H,
General Procedure for Transfer Hydrogenation Reactions
3
2
H
H
4), 6.77 (t, J= 7.3 Hz, 4H, p-BPh4), 6.75 (d, J = 13.9 Hz, 1H, The transfer hydrogenation experiments were carried out
8), 6.55 (t, 3J = 2.4 Hz, 1H, 2), 6.49 (d, 2J = 13.9 Hz, 1H,
8), under standard schlenk conditions. The substrates (0.25
10), 6.21 (s, 6H, Ru-C6H6), 6.08 (d, 2J = mmol), catalyst (1.5 mol%) and base (KOH, 0.045 mmol) were
4) ppm. 13C{1H} NMR (75 MHz, (CD3)2CO): δ mixed in 10 mL of 2-propanol. The mixture was heated to
H
H
6.40 (d, 3J = 2.1 Hz, 1H,
H
13.9 Hz, 1H,
177.8 ( 7), 165.9-164.0 (q, JC-B = 48.9 Hz, ipso-
149.4, ( 1), 142.1 ( 9) 137.0 (o- of BPh4), 135.4 (
11), 126.1 (m-BPh4), 124.1 ( 5), 122.3 (p-BPh4 &
2), δ 107.7 ( 10), 89.7 (Ru-C6H6), 64.9 ( 8), 63.3 (C4) ppm. drops) were diluted in dichloromethane and collected for GC-
H
1
°
C
C of BPh4), reflux (82 C) for 24 hours. Aliquots were taken at regular
C
C
C
C
3), 132.2 intervals, which were quenched with cold isopropanol (1 mL)
(
C
C
C6), 108.8 and filtered through a plug of silica. The crude products (2-3
C
C
C
Elemental Analysis found: C, 64.69; H, 5.42; N, 10.85. Calc for MS analysis. Selected aliquots were also analysed using 1H
Ru1C41H38N6B1: C, 64.83; H, 5.44; N, 10.80. ESI MS: (m/z NMR spectra. Integration of selected resonance signals were
443.03) [M-Cl]+ amu.
compared in quantitative ratio between substrates and
respective products. The GC-MS product yield values were
consistently within 2% of the 1H NMR analysed yield values.
Synthesis of [Ru(η6-C10H14)(NCN)Cl]BPh4.[B5O6(OH)4] (6a)
[Ag(NCN)2]BPh4
C10H14)Cl2]2 (0.035 g, 0.056 mmol) were dissolved in 20 mL of
(4)
(0.050 g, 0.056 mmol) and [Ru(η6-
dry CH2Cl2. The mixture was left stirring overnight at room Acknowledgements
temperature under an N2 atmosphere. The resulting yellow
This work was supported under Australian Research Council’s
solution was filtered using glass fibre (GF/C) filter paper and
reduced to 10 mL. 40 mL of diethylether was slowly added to
the solution resulting in the precipitation of [Ru(η6-
C10H14)(NCN)Cl]BPh4.[B5O6(OH)4] as a yellow powder. Yield:
51%. Elemental Analysis found: C, 40.05; H, 4.08; N, 11.64.
Calc. for [Ru(η6-C10H14)(NCN)Cl]14%BPh4.86%[B5O6(OH)4]: C,
40.01; H, 4.44; N, 11.49 ESI MS: (m/z 499.19) [M-
BPh4/B5O6(OH)4]+ amu.
Discovery Projects funding scheme (project number
DP110101611). AGN is grateful for an Australian Postgraduate
Award (APA) awarded by the Australian Government,
administered by the University of New South Wales and
Macquarie University. We thank the University of New South
Wales and the Australian Government for financial support.
We acknowledge Dr Samantha Binding for useful discussions
on the preparation of this manuscript. We acknowledge the
Campbell Microanalytical Laboratory, University of Otago, New
Zealand, and the Research School of Chemistry, Australian
National University, for elemental analyses.
Synthesis of [Ru(η6-C10H14)(NCN)Cl]BPh4 (6b)
Ag2O (0.1g, 0.432 mmol), NCN.BPh4 (1) (0.061g, 0.12mmol)
and [Ru(η6- C10H14)Cl2]2 (0.035g, 0.056mmol) were dissolved in
20 mL of dry CH2Cl2. The mixture was left stirring overnight at
room temperature under an N2 atmosphere. The resulting dark
suspension was filtered using glass fibre (GF/C) filter paper
producing a yellow solution which was reduced to 10 mL. 40
mL of diethyl ether was slowly added to the solution resulting
in the production of [Ru(η6-C10H14)(NCN)Cl]BPh4 as a yellow
Notes and references
1
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(b) M. H. P. Rietveld, D. M. Grove and G. van Koten, New J.
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Poverenov, M. Gandelman, L. J. W. Shimon, H. Rozenberg, Y.
Ben-David and D. Milstein, Organometallics, 2005, 24, 1082; (g)
1
powder. Yield: 51%. H NMR (400 MHz, CD2Cl2): δ 7.87 (br d,
3
1H,
H
1), 7.77 (d, J = 2.4 Hz, 1H,
H
9), 7.71 (br d, 1H,
H
3), 7.40
(br t, 8H, o-BPh4), 7.09 (d, 3J = 2.6 Hz, 1H, H11), 7.06 (t, 3J = 7.5
Hz, 8H, m-BPh4), 6.95 (t, 3J = 7.5 Hz, 5H, p-BPh4 &
H5), 6.64 (d,
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8
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The ratio of BPh4- to [B5O6(OH)4]- was determined using 1H NMR
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upfield from -6 ppm to -13 ppm (2JH-P = 21 Hz). Whilst the
hydride chemical shift changes, the coupling constants and the
chemical shifts of the resonances in the 31P NMR spectrum
remain the same, which suggests that the two phosphine
ligands remain trans to one another in the new isomer. See ESI
for VT NMR spectra and catalysis results of this isomer.
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8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6DT02459A
Journal Name
ARTICLE
TOC Entry
A series of hemilabile NHC Ru(II) complexes were synthesised and assessed for their catalytic activity for transfer
hydrogenation reactions.
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