Molecular Coordination-Switch in a New Role: Controlling Highly Selective Catalytic Hydrogenation with Switchability Function

Article abstract of DOI:10.1021/acscatal.6b00150

A molecular coordination-switch controlled by acid-base input has been developed and utilized in switchable catalysis. The molecular switch consists of a hybrid pyridylidene-benzimidazole ligand bound to an Ir^IIICp? moiety wherein the benzimidazole functionality has been utilized for acid/base controlled reversible coordination, switching between an Ir^III-benzimidazole species (form I; neutral imino-type N-coordination) and an Ir^III-benzimidazolate species (form II; anionic amido-type N-Ir bonding). Owing to the distinctly different nature of the metal-ligand bonding, it has been demonstrated that while the form I is almost inactive (TOF 1 h^-1) in catalytic hydrogenation of imine under ambient pressure and temperature, the form II is greater than an order of magnitude more efficient (TOF 15.8 h^-1) in the same catalysis. Moreover, the catalysis could be switched OFF and ON efficiently for several cycles with the addition of acid and base, respectively. Spectroscopic studies and kinetics have been performed to understand the switching activity.

Full text of DOI:10.1021/acscatal.6b00150

Letter
pubs.acs.org/acscatalysis
Molecular Coordination-Switch in a New Role: Controlling Highly
Selective Catalytic Hydrogenation with Switchability Function
Shrivats Semwal and Joyanta Choudhury*
Organometallics & Smart Materials Laboratory, Department of Chemistry, Indian Institute of Science Education and Research
Bhopal, Bhopal 462 066, India
S
* Supporting Information
ABSTRACT: A molecular coordination-switch controlled by
acid−base input has been developed and utilized in switchable
catalysis. The molecular switch consists of a hybrid
pyridylidene−benzimidazole ligand bound to an IrIIICp*
moiety wherein the benzimidazole functionality has been
utilized for acid/base controlled reversible coordination,
switching between an IrIII-benzimidazole species (form I;
neutral imino-type N-coordination) and an IrIII-benzimidazo-
late species (form II; anionic amido-type N−Ir bonding).
Owing to the distinctly different nature of the metal−ligand
bonding, it has been demonstrated that while the form I is almost inactive (TOF 1 h−1) in catalytic hydrogenation of imine under
ambient pressure and temperature, the form II is greater than an order of magnitude more efficient (TOF 15.8 h−1) in the same
catalysis. Moreover, the catalysis could be switched OFF and ON efficiently for several cycles with the addition of acid and base,
respectively. Spectroscopic studies and kinetics have been performed to understand the switching activity.
KEYWORDS: molecular switch, switchable catalysis, acid−base, iridium(III), hydrogenation
might be partly due to less efficient switching and/or
olecular property and function depend on the molecular
Mdesign, which, in turn involves a certain level of insufficient stability required to run catalytic turnover in
solution. Significantly, the concept of protonation−deprotona-
tion modulated catalysis by utilizing the tunable electronic
influence of pH-responsive ligands has been explored
previously by the groups of Fujita,4a Himeda,4b Periana,4c
Szymczak,4d,e and Papish,4f,g although a “true” molecular
coordination-switch was not involved in these systems. We
anticipated that the two interconvertible coordination environ-
ments around a metal center, generated by simple acid/base
input, might be a prospective platform to design and operate a
catalysis-switch. Of course, a delicate balance of electronic
characteristics around the metal−ligand core should be able to
influence the substrate activation step(s), the most relevant
bond-breaking/bond-making processes, and subsequently the
reaction kinetics markedly to achieve entirely variable final
outcome as required for a prospective switch. Herein we
present a molecular design that not only switches its metal-
coordination modes reversibly with acid/base input displaying
two distinct properties but also operates an efficient switchable-
catalysis leading to a great control of ON/OFF output in terms
of catalytic efficiency. The molecular design was based on a
hybrid pyridylidene−benzimidazole ligand bound to an IrIIICp*
moiety wherein the benzimidazole functionality was utilized for
acid/base controlled coordination-switch (forms I and II) while
sophistication, smartly incorporated into the structure of the
molecule concerned. Nature fascinates us with demonstration
of several signal-controlled desirable enzymatic activities, from
energy harvesting to communication, growth, and reproduc-
tion, through a high level of sophisticated trigger-induced
features.1 Inspired by the control mechanisms of the natural
systems, it is understood that switching the property, function,
and activity of a molecule with simple chemico-physical stimuli
(acid/base, metal ion, light, sound, electric potential,
mechanical force, etc.) is achievable only through judicious
incorporation of a reversibly responsive regulatory feature.
Eventually, designing artificial switchable catalysts has become
an emerging research area to confer advanced functions with
controllable outcome in terms of rate of the reaction and/or
chemo-, regio-, stereoselectivity of the product(s), for instance.2
An acid/base-controlled molecular coordination-switch in-
volving two interconvertible metal-coordination modes has
been known to chemists for a long time (Figure 1).3 For
example, Constable, Wolf, and others demonstrated the
reversible switching between the (Y,S) and (Y,C) coordination
modes of thiophene-derived ligands (Y = a pendant pyridine or
phosphine ligand or a metal center) with Ru, Os, or Rh metal
centers by reaction with acid or base. However, the
phenomenon was used merely to achieve a variable
spectroscopic, electrochemical, and/or fluorescence excited
state property. No real application, such as switching a chemical
catalysis, has been demonstrated so far with this concept. This
Received: January 15, 2016
Revised: March 3, 2016
© XXXX American Chemical Society
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Scheme 1. Synthesis of the Two Forms (1 and 2) of the
Molecular-Switch and the Molecular Structures of 1 and 2
Figure 1. (Top) Example of acid−base controlled molecular
coordination-switch and (bottom) a catalysis-switch developed on a
molecular coordination-switch platform.
the pyridylidene-based remote NHC ligand was utilized for
providing a strong and robust metal−ligand platform to carry
out the catalytic reaction (Figure 1). Mayer et al. previously
showed that a RuII-coordinated pendant imidazole ligand could
be deprotonated to imidazolate for binding via an anionic
fashion to the metal center and with the treatment of an acid
the binding mode could be restored as well.5 We hypothesized
that the mode of H2 activation/cleavage with the IrIII-
benzimidazolate species (form II; anionic amido-type N−Ir
bonding) and the IrIII-benzimidazole species (form I; neutral
imino-type N-coordination) of the designed bifunctional
system would be mechanistically distinct due to distinctly
different nature of the metal−ligand bonding which in turn
could affect the efficiency of subsequent catalytic hydrogenation
of unsaturated organic molecules.6 For example, the form II of
the complex may cleave H2 in a cooperative manner with the
help of relatively basic amido-type ligand and an electrophilic
IrIII center to generate an Ir−H species quite efficiently and
thereby may facilitate catalytic hydrogenation. The proof of
concept was realized by performing catalytic hydrogenation of
imines7 at ambient pressure and temperature wherein it was
found that the form II (TOF of 15.8 h−1) was greater than an
order of magnitude more efficient than the form I (TOF of 1
h−1) under identical reaction conditions, and the catalysis was
switched OFF and ON efficiently with the addition of acid and
base, respectively.
ment in the two complexes were characterized by 1H NMR and
13C{1H} NMR spectroscopy and confirmed unambiguously by
single-crystal X-ray diffraction studies (Scheme 1 and
Supporting Information). Solution-phase interconversion be-
1
tween 1 and 2 with acid−base inputs was studied by H NMR
and UV−vis spectroscopic techniques and was established to be
highly efficient (Figure 2; see Supporting Information for
details). The stability and robustness of the above coordination-
switch was also tested in solution with consecutive acid−base
switching starting from 2. The value of the extinction
To demonstrate the newly developed proof-of-concept
switchable catalyst system, the hybrid pyridylidene−benzimi-
dazole ligand, L was designed, synthesized, and metalated with
[Cp*IrIII] motif, as shown in Scheme 1, to provide the imino-
coordinated Ir(III) complex 1 and the amido-bound Ir(III)
complex 2. The reaction conditions for synthesizing 1 and 2
were essentially identical, but the use of Cs2CO3 was necessary
for 2. Successful synthesis of 1 from 2 and 2 from 1 in good
yields was also achieved with the use of CF3CO2H and Cs2CO3,
respectively (Scheme 1). The variable coordination environ-
Figure 2. (A) Interconversion of 1 and 2 with base and acid. (B),(C)
Consecutive switching between 1 and 2 in solution.
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Letter
coefficient of the band at 380 nm was found to be almost
constant after several regenerative cycles (Figure 2B,C),
suggesting the high stability of the switch in solution under
operating conditions.
The two complexes 1 and 2 of the switchable catalyst system
were utilized for catalytic hydrogenation of imines with
molecular hydrogen under ambient conditions (H2 balloon,
35 °C) in 2,2,2-trifluoroethanol (TFE) (Table 1A). It was
and water oxidation catalysis.4c,d,g The scope of the present
catalysis was further extended successfully via performing the
hydrogenation reaction by using an equimolar mixture of
various aldehydes and amines as substrates (Table 1B). The
trend of the activity of 1 and 2 was again proved to be the same
as earlier. Moreover, selectivity toward hydrogenation of imine
over other unsaturated organic substrates such as alkene,
alkyne, ketone, and enone, were tested via competitive
reactions, and it was found to be highly selective with almost
no conversion of the competing substrates (Table 1C).
Table 1. Examples of 2 and 1-Catalyzed Hydrogenation of
(A) Imines, (B) Equimolar Mixture of Aldehydes and
Amines, and (C) Imines in the Presence of Competing
After demonstrating that the imino-bound complex 1 is
catalytically inactive (OFF; red curve, Figure 3) and the amido-
a
Substrates
Figure 3. Plot of yield (%) versus time for the hydrogenation of N-
benzylideneaniline catalyzed by 1 (red curve) and 2 (green curve);
blue curve represents the ON/OFF switching of the catalytic activity
when started with catalyst 1 followed by consecutive addition of base
(NEt3) and acid (CF3CO2H); maroon curve represents the ON/OFF
switching of the catalytic activity when started with catalyst 2 followed
by consecutive addition of acid (CF3CO2H) and base (NEt3).
bound complex 2 is active (ON; green curve, Figure 3), the
potential of switching the catalysis between ON and OFF
during the actual progress of the reaction was explored. Thus,
when the hydrogenation catalysis was started with the OFF
catalyst 1, there was virtually no yield of the product after 2 h.
At this point of time, addition of 1 equiv (with respect to the
catalyst) of NEt3 switched the catalysis ON, and after 1 more
hour, the yield was found to be 10%. Addition of CF3CO2H
terminated the ongoing catalysis. However, the ON state could
be regenerated again by the addition of NEt3. Seven subsequent
cycles were performed without any significant loss of catalytic
activity, showing ∼11.4% yield in each ON step and ∼1.1%
yield in each OFF state over the entire course of the reaction
(Figure 3, blue curve). Similarly, the reversible switching of
ON/OFF catalysis was also proved efficient when the catalysis
was started with the ON catalyst 2 and subjected to the
addition of acid and base in consecutive steps during the course
of the reaction (Figure 3, maroon curve).
The dramatic difference in catalytic activity of 1 and 2 might
be ascribed to the nature of the ligand sphere and its effect on
the key step involving the activation of H2. Although the imino
ligand in 1 is inert in playing any additional role in H2
activation step, it is reasonable to predict that the basic
amido ligand in 2 can facilitate to activate H2 in a bifunctional
manner to generate Ir−H species quite easily, promoting the
hydride insertion and subsequent hydrogenation efficiently.6
The above suggestion for the plausible differential reactivity of
a
Conditions: (A) 0.2 mmol of imine, H2 balloon, 1 mol % catalyst, 2
mL of TFE, 35 °C, 6 h. (B) 0.2 mmol of aldehyde, 0.2 mmol of amine,
H2 balloon, 1 mol % catalyst, 2 mL of TFE, 35 °C, 6 h. (C) 0.2 mmol
of N-benzylideneaniline, 0.2 mmol of unsaturated substrate, H2
balloon, 1 mol % catalyst, 2 mL of TFE, 35 °C, 6 h. Yields reported
in parentheses represent isolated yields with catalyst 2 and 1,
respectively.
observed that for all the substrates, catalyst 2 was active (yield
83−90%), while catalyst 1 was almost inactive (yield 8−11%),
indicating the effect of variable coordination platform in 1 and
2. Notably, in the previously reported pH-responsive catalytic
systems, ligand deprotonation by base or at higher pH also led
to enhanced activity in CH activation, nitrile hydroboration,
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H2 with 1 and 2 leading to the formation of σ-Ir(H2) species
AUTHOR INFORMATION
Corresponding Author
*E-mail: joyanta@iiserb.ac.in.
Notes
■
1
from 1 but Ir−H species from 2 was verified by VT H NMR
(500 MHz) spin−lattice relaxation time (T1, ms) measure-
ments (Figure 4).8 A short T1 (min) of 49 ms at 233 K in case
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.C. thanks DST and IISER Bhopal for generous financial
support. S.S. is a recipient of UGC doctoral fellowship. The
authors thank Mr. Rajbeer Singh for help in T1 measurements.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00150.
■
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Experimental details; spectra; additional text and figures
(PDF)
CIF file (CIF)
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