A remote coordination booster enhances the catalytic efficiency by accelerating the generation of an active catalyst

Article abstract of DOI:10.1039/c6cc00157b

A remote Ru^II(terpy)₂ unit incorporated in conjugation with the [NHC-Ru^II(para-cymene)] catalytic site, acts as a "coordination booster" for enhancing the catalytic efficiency to achieve excellent performance in selective oxidative scission of various carbon-carbon multiple bonds to the corresponding aldehydes, ketones and diketones. Generation of an active catalyst via oxidative loss of para-cymene from the precatalyst was found to be accelerated by the "coordination booster" through the electronic effect.

Full text of DOI:10.1039/c6cc00157b

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A remote coordination booster enhances the
catalytic efficiency by accelerating the generation
Cite this:DOI: 10.1039/c6cc00157b
of an active catalyst†
Received 7th January 2016,
Accepted 19th January 2016
Suraj K. Gupta and Joyanta Choudhury*
DOI: 10.1039/c6cc00157b
www.rsc.org/chemcomm
II
II
+
A remote Ru (terpy)
2
unit incorporated in conjugation with the of para-cymene from [(NHC)Ru (para-cymene)Cl] complexes
II
[
NHC-Ru (para-cymene)] catalytic site, acts as a ‘‘coordination (NHC = N-heterocyclic carbene) to generate active catalysts is
booster’’ for enhancing the catalytic efficiency to achieve excellent the key step in the catalytic selective oxidation of olefins and
8
II
2
performance in selective oxidative scission of various carbon– alkynes to carbonyl compounds and a [Ru (terpy) ] unit (terpy =
0
0
00
carbon multiple bonds to the corresponding aldehydes, ketones 2,2 : 6 ,2 -terpyridine) placed remotely from the catalytically
and diketones. Generation of an active catalyst via oxidative loss of active (NHC)Ru center acts as a ‘‘coordination booster’’ which
para-cymene from the precatalyst was found to be accelerated by triggers and accelerates the loss of para-cymene efficiently to
II
the ‘‘coordination booster’’ through the electronic effect.
enhance the catalytic activity. Although a remote [Ru (terpy)
2
]
unit within a catalyst backbone was previously utilized as a
9
Enzymes modulate their catalytic activity through the allosteric photo-trigger, its use as an electronic-trigger is not known. Few
effect wherein a remote activator triggers necessary changes examples are known involving the use of B(C F ) or Al(C F5) as
6
5 3
6
3
1
enabling the active site with modified reactivity. Learning this an electronic trigger through Lewis-acid binding to a metal-
concept from Nature, chemists have shown tremendous efforts coordinated ligand, for accelerating polymerization and reductive
10,11
for remote modification of the activity of man-made catalytic elimination reactions.
systems via a stimuli-triggered strategy. One important prerequisite
2
II
2
The assembled pre-catalyst, [Ru(terpy) –(NHC)Ru (para-
for the exercise of modifying the activity of a synthetic catalyst cymene)Cl] , 2 was designed by attaching the remote activator
3
+
is the knowledge of the mode of activation of the pre-catalyst Ru(terpy) through the NHC-backbone, and synthesized from
2
and subsequently the key steps of the catalytic reaction. For example, its precursor complex, 1 via a simple procedure (for details, see
6
3
III
4
the activation mode of many [M(Z -arene)] and [Ir Cp*]-based
ESI†) for demonstrating the proposed boosting effect. A model
II
pre-catalysts involved the initial labilization/dissociation of the complex, free of any remote modification, [(NHC)Ru (para-
arene and oxidative loss of the Cp* ligand, respectively, to generate cymene)Cl] , 3 was utilized to compare with the activity of 2
+
12
active catalysts; and therefore an oxidative stress was utilized to (Fig. 1).
1
enhance the catalytic activity. A very similar class of complexes,
The H NMR spectrum of 2 showed the absence of an NCHN
II
[
Ru (para-cymene)]-based one, was well studied as a catalyst proton in high chemical shift values (9.5–11.5 ppm) when
5
precursor in many reactions including C–H functionalisation,
compared with precursor 1. The presence of four doublet peaks
6
7
C-heteroatom coupling and transfer hydrogenation; however, with the integration of one proton each in the range of 4.5–6.0 ppm,
it is believed that the para-cymene ligand remained coordinated one septet peak at 2.21 ppm with the integration of one proton,
during the catalytic cycle. To the best of our knowledge, there is and two doublet peaks with the integration of three protons
no report on the generation of active catalysts under oxidizing each in the range of 0.84–0.98 ppm suggested the ruthenium-
conditions via loss of para-cymene from [Ru (para-cymene)]- coordinated para-cymene backbone in 2. In addition, the H NMR
based catalyst precursors. Herein, we show that dissociative loss spectrum of 2 also featured the NHC backbone protons with
expected number and expected chemical shift values. Complex 2
II
1
Organometallics and Smart Materials Laboratory, Indian Institute of Science
Education and Research Bhopal, Bhopal 462 066, India.
E-mail: joyanta@iiserb.ac.in
showed an intense absorption band centered at 480 nm (Fig. S24,
II
ESI†) which is the characteristic of the presence of a Ru (terpy)
2
13
unit (due to MLCT), as confirmed by the presence of a similar
†
Electronic supplementary information (ESI) available: Synthetic procedures,
absorption maximum at 480 nm in the case of precursor complex 1.
characterization data and spectra, additional schemes, tables, figures, and
crystallographic data. CCDC 1445612 and 1445613. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c6cc00157b
II
Similarly, the presence of the Ru (terpy)
2
unit in 2 was realized
via electrochemical analysis. The reversible peak at 1.24 V vs. SCE
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Fig. 1 Ruthenium complexes used in this study.
II
III
was confirmed to be due to the Ru /Ru couple of the Ru(terpy)2
1
Fig. 3 H NMR studies with complex 2 (top) and complex 3 (bottom) in
the presence of 10 eq. of NaIO (acetone-d : D O; 3 : 2). A = only
1
3
unit which was present in both 2 and 1 (Fig. S25, ESI†). Most
4
6
2
importantly, the proposed electronic effect of this remote complex, B = 5 min (after addition of NaIO ), C = 10 min, D = 15 min,
4
E = 20 min, F = 30 min. * = resonance peaks related to ruthenium coordinated
2
Ru(terpy) booster on the redox potential of the catalytic Ru
center, [that is Ru (para-cymene) center in 2], was evaluated by
II
para-cymene. # = resonance peaks related to free para-cymene.
differential pulse voltammetry (Fig. 2). Interestingly, the redox
II
III
II
potential of the Ru /Ru couple in the Ru (para-cymene) site in
was found to be shifted to more positive potential by 100 mV
the complete loss of para-cymene under similar oxidizing
conditions even after 30 min (Fig. 3).
2
II
III
as compared to that in 3 [E1/2(Ru /Ru ) in 3 = 1.336 V vs. SCE
and in 2 = 1.436 V vs. SCE]. This shift suggested a more electron-
deficient Ru (para-cymene) center in the modified complex 2
than in the model unmodified complex 3. Such electron-deficiency
at the ruthenium center is supposed to promote the dissociation
of para-cymene under oxidizing conditions due to a poor Ru-
para-cymene back-bonding effect.
The preliminary evidence that the bimetallic complex loses
para-cymene extremely faster prompted us to utilize complex 2
for anticipated enhanced catalytic efficiency in the selective
oxidation of alkenes and alkynes to the corresponding oxygen
inserted products. The results of catalysis are summarized
in Table 1, which indeed showed an augmented activity of 2
II
(
Fig. 4). A similar activity of complex 2 observed under both
The consequence of the above electronic effect on the
II
normal and dark conditions suggested that the Ru (terpy) unit
2
II
oxidative activation of the Ru (para-cymene) pre-catalyst center
did not act as a photo-trigger in our assembly (entry 2, Table 1).
Complex 1 did not show any catalytic activity under similar
conditions, as expected, since it did not possess any catalytic
site. The unmodified complex 3 was found to be comparatively
less active as compared to complex 2 when monitored in the
model reactions with styrene and 4-methylstyrene as substrates
under similar oxidative reaction conditions (entries 1 and 2,
Table 1 and Fig. 4). The correlation between the loss of para-
1
was examined by H NMR spectroscopy. Addition of 10 eq. of
NaIO4 to the solution containing complex 2 resulted in the
complete decoordination of para-cymene in just 5 min (Fig. 3).
Resonance peaks related to free para-cymene appeared at
expected chemical shift values with an appropriate integration
ratio (confirmed by comparing with the authentic sample). At
the same time, the model unmodified complex 3 did not show
II
cymene and the observed catalytic activity at the Ru (para-
cymene) center of 2 and 3, as reflected from the above results,
suggested the significance of the generation of an active catalyst
to provide vacant sites for the substrate to be transformed
during catalysis (vide Scheme 1). Sub-stoichiometric reactions
of complex 2 with 4-methylstyrene in the presence of NaIO
2 : 4-methylstyrene : NaIO ; 1 : 10 : 25) were carried out to get
4
(
4
some information about the species present during the catalysis.
GC analysis of the reaction mixture confirmed the formation of
the product, 4-methylbenzaldehyde. At the same time, mass
spectral analysis of the same reaction mixture showed a number
of clusters in the mass spectrum (Fig. S38, ESI†). An intense
peak at m/z = 601.5567 was assigned to the parent complex.
Fig. 2 Differential pulse voltammograms of complexes 1, 2, and 3 (for
details see ESI†).
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a
Table 1 Catalytic oxidation of carbon–carbon multiple bonds to carbonyl compounds
ꢀ1
Yield (%)/TOF (min
)
#
Substrate
Product
2
3
4
b
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Styrene
Benzaldehyde
80/2.67, 58/1.93
30/1.00
25/0.83
18/0.60
14/0.70
42/0.35
35/0.29
20/0.66
61/2.03
65/3.25
18/0.30
68/9.06
42/1.40
40/2.00
35/1.75
68/2.27
51/2.04
75/2.50
70/2.80
46/0.76
60/3.00
82/1.82
82/10.93
84/11.20
70/1.17
85/11.33
78/3.90
68/3.40
65/3.25
b
c
4-Methylstyrene
4-Chlorostyrene
4-Fluorostyrene
a-Methyl styrene
4-Vinylanisole
4-Bromostyrene
trans-Stilbene
cis-Stilbene
Allylbenzene
trans-a-Methyl stilbene
Diphenylacetylene
1-Phenyl-1-butyne
1-Phenyl-1-propyne
4-Methylbenzaldehyde
4-Chlorobenzaldehyde
4-Fluorobenzaldehyde
Acetophenone
4-Methoxybenzaldehyde
4-Bromobenzaldehyde
Benzaldehyde
78/3.90, 55/1.83, 73/3.65
81/2.70
78/3.90
88/0.73
60/0.50
79/2.63
85/2.83
92/4.60
60/1.0
91/12.13
74/2.47
Benzaldehyde
0
1
2
3
4
2-Phenylacetaldehyde
Acetophenone, benzaldehyde
Benzil
1-Phenylbutane-1,2-dione
1-Phenylpropane-1,2-dione
78/3.90
80/4.00
a
ꢀ
c
ꢀ
Reaction conditions: Halide (2Cl and Br ) salt of 2 (0.5 mol%), starting material (0.4 mmol), acetone + water (1 : 1, 6 mL), NaIO
4
(1.0 mmol),
salt of 2. Under dark conditions. The yields of the products were calculated by GC analyses. TOF = mmol (product)/
mmol (cat) ꢁ time (min)}.
b
room temperature. PF
{
6
Fig. 4 Comparison of the catalytic activity of 2 and 3.
Along with this peak, the mass spectrum also showed peaks
II
II
3+
31 9
at m/z = 296.0180 for [C44H N Ru Ru –H] , m/z = 306.6811
II
VI
3+
Scheme 1 Proposed ‘‘boosting’’ effect of a remote [Ru(terpy) ] unit on
the catalytic activity of the Ru(NHC) center.
2
for [C44
H
31
N
9
Ru Ru –H + (O)
2
]
and m/z = 339.3787 for
II
II
3+
44 31 9 3 3 2 2
[C H N Ru Ru (CH COCH )(H O) Cl] fragments. The presence
of all these fragments showed that the removal of para-cymene
and the formation of cis-dioxo ruthenium(VI) species are the
necessary steps for this kind of transformation as depicted in
Scheme 1. To check whether para-cymene dissociation is a key
step, oxidation of 4-methylstyrene was carried out in the presence
of added para-cymene. A decrease in the yield in the oxidation of
4-methylstyrene to the corresponding aldehyde was observed
(Fig. 5). A decrease in the yield was also observed with the
addition of other electron rich arenes (Fig. 5), due to the expected
strong coordination of these molecules to the high-valent metal
center. Electron-deficient arenes, being poor donors and efficient
acceptors, were ineffective to decrease the yield; instead a slight
Fig. 5 Effect of added para-cymene (left) and other arenes (right).
enhancement was observed as expected. We also confirmed that the pre-catalyst and thereafter for the catalytic oxidation of
para-cymene did not undergo any transformation under similar carbon–carbon multiple bonds with complex 2 (or 3).
catalytic conditions (Scheme S4 ESI†). These studies suggested that
the removal of para-cymene is a necessary step for the activation of derivative of complex 2, [Ru (terpy) –(NHC)Ru (MeCN) ] , 4,
2 4
These controlled studies encouraged us to synthesize a
II
II
4+
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silver triflate (1.2 eq.) in acetonitrile under reflux conditions for
6
0 h (Scheme 2). Complex 4 was fully characterized by NMR
1
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7
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9
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3
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1
J. C. sincerely thanks DST and IISER Bhopal for generous
financial support. S. K. G. thanks CSIR for doctoral fellowship.
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