Rotating Catalysts Are Superior: Suppressing Product Inhibition by Anchimeric Assistance in Four-Component Catalytic Machinery

Article abstract of DOI:10.1021/jacs.8b04437

Three distinct four-component supramolecular nanorotors, prepared by varying the rotator's structure and keeping all other components constant, exhibit rotational frequencies that differ by almost 2 orders of magnitude. When the rotors were used as catalyst for two click reactions, the product yield correlated with the speed of the machine, e.g., 20% at 0.50 kHz, 44% at 20 kHz and 62% at 42 kHz. The kinetic effect on the product yield is attributed to the ability of the rotating catalysts to displace the product more efficiently from the active site at higher speed (anchimeric assistance). This mechanistic hypothesis was convincingly corroborated by a linear correlation between product yield and product liberation.

Full text of DOI:10.1021/jacs.8b04437

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Communication
Rotating Catalysts Are Superior – Suppressing Product Inhibition
by Anchimeric Assistance in Four-Component Catalytic Machinery
Pronay Kumar Biswas, Suchismita Saha, Thomas Paululat, and Michael Schmittel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04437 • Publication Date (Web): 22 Jun 2018
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Rotating Catalysts Are Superior – Suppressing Product Inhibition by
Anchimeric Assistance in Four-Component Catalytic Machinery
Pronay Kumar Biswas,a Suchismita Saha,a Thomas Paululat,b Michael Schmittela,*
a Center of Micro- and Nanochemistry and Engineering, Universität Siegen, Organische Chemie I, Adolf-Reichwein-Str. 2,
D-57068 Siegen, Germany. b Universität Siegen, Organische Chemie II, Adolf-Reichwein-Str. 2, D-57068 Siegen, Germany.
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ABSTRACT: Three distinct four-component supramolecular nanorotors, prepared by varying the rotator’s structure and keeping all other compo-
nents constant, exhibit rotational frequencies that differ by almost two orders of magnitude. When the rotors were used as catalyst for two click reac-
tions, the product yield correlated with the speed of the machine, e.g. 20% at 0.50 kHz, 44% at 20 kHz and 62% at 42 kHz. The kinetic effect on the
product yield is attributed to the ability of the rotating catalysts to displace the product more efficiently from the active site at higher speed (anchi-
meric assistance). This mechanistic hypothesis was convincingly corroborated by a linear correlation between product yield and product liberation.
Inspired by biological motor proteins,1,2 researchers have probed sever-
respond to the complexed phenanthroline station, i.e. Npy
[Cu(phenAr2)]+, while the upfield signals originate from the unloaded
phenAr2. After addition of one equiv of [Cu(CH3CN)4]PF6 and 5 min
of sonication, the pre-rotor complex ROT-1 afforded nanorotor ROT-
1' = [Cu2(1)(4)(DABCO)]2+ in quantitative yield. The rotor was un-
al strategies to mimic Nature’s sophisticated multi-component ma-
chinery. While significant progress has been achieved in artificial (i)
multi-component assembly,3 (ii) light-,4-7 electricity-,8,9 proton10 and
chemical-fuel11,12 driven covalent motors,13 (iii) multi-component
rotors,14,15 and (iv) networking machinery,16,17 finally insights from all
these areas need to be amalgamated to build artificial multi-component
machinery competitive to that in nature, e.g. ATP synthase.18
1
1
ambiguously characterized by H-NMR, H-1H-COSY NMR, ESI-MS
and 1H-DOSY NMR.
Herein we demonstrate that simple mixing of four distinct components
leads to stochastically rotating machinery that is both catalytically ac-
tive and equipped with advanced features for suppressing product
inhibition. The linear correlation between the machine’s rotational rate
and efficiency to catalyze a reaction is reminiscent of the situation in
ATP synthase.19
The present four-component nanorotors15 are made from rotator 1-3,
stator 4, DABCO as hinge and copper(I) ion(s) (Figure 1a), suggest-
ing to alter the rotor’s speed by varying the component(s). For the
rotor axis, the stator’s and rotator’s porphyrin moieties are hetero-
sandwiched together by DABCO. The heteroassembly is favored be-
cause at the periphery the stator’s phenanthroline station and the rota-
tor’s nitrogen-terminated arm are connected via a copper(I) ion in a
so-called HETPYP (HETeroleptic PYridine and Phenanthroline)20
complex (Figure 1b). Rotation requires Npy[Cu(phenAr2)]+ bond
cleavage, i.e. detachment of the rotator’s pyridine from the copper(I)
phenanthroline of the stator, in the rate determining step. Since rotators
1-3 differ by the flexibility of the rotator arm and the type of terminal
binding unit, distinct rotational rates are expected for rotors ROT-1'–
ROT-3'. Synthetic details with regard to rotators are discussed in the
supporting information (Scheme S1), whereas stator 4 is known from
previous work.21
The pre-rotors were prepared as described earlier15 by reacting one
equiv of both stator 4 and [Cu(CH3CN)4]PF6 with one equiv of both
rotator 1 and DABCO in CD2Cl2. After sonication for 5 min, the pre-
rotor ROT-1 = [Cu(1)(4)(DABCO)]+ (Figure 1b) formed quantitati-
vely. The complex was fully characterized by 1H-NMR, 1H-1H-COSY
NMR, ESI-MS and elemental analysis. In the 1H-NMR spectrum, char-
acteristic multiplets21 for DABCO-H at –[4.50-4.46] and –[4.44-4.40]
ppm clearly indicate the formation of the hetero-bisporphyrin sand-
wich complex (SI, Figure S9). The upfield-shifted doublet at 6.63 ppm
Figure 1. (a) Structural formulas and cartoon representations of the
rotor components. (b) Cartoon representation of ROT-1 and ROT-1';
(c) Partial 1H NMR (400 MHz, CD2Cl2, 25 °C) of ROT-1 and ROT-1'
where C, u and Cu refer to the corresponding HETPYP complexed,
uncomplexed and Cu+-loaded 15-H and 16-H signals; (d) Partial VT
1H-NMR (600 MHz, CD2Cl2) of ROT-1', showing splitting of 16-H
into two sets (1:1 ratio).
for pyridine proton d-H strongly validates formation of a Npy

[Cu(phenAr2)]+ (HETPYP)complexation between stator and rotator
(SI, Figure S9).
The phenanthroline protons (e.g. 15-H, 16-H) appear as two sets in
1:1 ratio (Figure 1c and S31). The more downfield shifted signals cor-
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In ROT-1' the phenAr2 stations furnish two separate sets of signals;
(Table 1). The same trend was seen in the click reaction with 5b and
6b furnishing 7b (Table 1). Notably, for both reactions the yield linear-
ly correlates with the rotational frequency of the nanorotors (Figure
3); the faster the rotational exchange at the two copper(I)-loaded phe-
nanthroline stations the higher the catalytic activity.
evidently rotation is slow on NMR time scale (vide infra). Both, the
complexed Npy [Cu(phenAr2)]+ and the Cu+-loaded phenanthroline
protons appear as coalescence-broadened sets of signals (Figure 1d). In
1
the variable temperature (VT) H-NMR (+30 to –10 °C) the two sets
of signals started to separate further upon lowering the temperature
(Figure 1d). At –10 °C they are fully split into two singlets (1:1). We
have chosen the signal corresponding to proton 16-H for calculating
the exchange speed because it is well resolved throughout the tempera-
ture range (SI, Figure S31). The exchange frequency was determined
using the WinD-NMR software22 to k25 = 0.50 kHz at 25 °C.
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Preparation of the pre-rotors ROT-2 = [Cu(2)(4)(DABCO)]+ and
ROT-3 = [Cu(3)(4)(DABCO)]+ was accomplished in a similar man-
ner as that of ROT-1. Addition of one equiv of [Cu(CH3CN)4]PF6 to
ROT-2 or ROT-3 afforded rotors ROT-2' = [Cu2(2)(4)(DABCO)]2+
and ROT-3' = [Cu2(3)(4)(DABCO)]2+, respectively. Different from
ROT-1', all HETPYP-complexed and Cu+-loaded phenanthroline
protons merge into one set of 1H-NMR signals, indicating that rotation
in ROT-2' and ROT-3' is faster than NMR time scale (SI, Figures S15
Figure 2. Reactions catalyzed by nanorotors.
1
& S19). Analysis of the VT H-NMR (SI, Figures S32 & S34) reveals
an exchange frequency k25 = 20.0 kHz for ROT-2' and k25 = 42.0 kHz
for ROT-3'. With the activation data of nanorotors at hand (Table 1),
one can rule out (see SI, page S45) that the exchange process proceeds
via dissociation and reassociation of components favoring intramolecu-
lar rotation.
Table 1. Activation data of nanorotors and catalytic reaction yields.
Figure 3. Yield of 7a (black) and 7b (red) vs rotational frequency.
k25 / s−1
Yield of Yield of
ΔG⧧
/
Nano-
rotor
25
7aa
7ba
kJmol−1
57.7±0.7
48.6±0.2
46.6±0.3
For comparison, the catalytic performance of simple copper(I) phe-
nanthroline complexes was studied (Figure 4a). We determined –
using 20 mol% of copper(I) – the yield of product 7a in the click reac-
tion (4 h, 55 °C) catalyzed by (a) C1=[Cu(8)]+ (19±1%; SI, Figure
S47), (b) C2=[Cu(8)(9)]+ (2%; SI, Figure S48) and (c) the dynamic
mixture (1:1) of C1=[Cu(8)]+ and C2=[Cu(8)(9)]+ (13±1%; SI,
Figure S45). The mixture C1+C2 experiences fast intermolecular ex-
change of 9 (vide infra) and serves as a mimic of the dynamics in the
nanorotors where the pyridine terminal swaps between two identical
copper(I) phenanthrolines. The mixture C1+C2 = 1:1, obtained
from[Cu(CH3CN)4]PF6 and ligands 8, 9 (2:2:1) in CD2Cl2, shows
0.50103
20.0103
42.0103
ROT-1'
ROT-2'
ROT-3'
20±1%
44±2%
62±2%
27±2%
42±2%
56±3%
a Yields determined from 3 independent runs. For conditions, s. text.
Apparently, the exchange frequency depends not only on the rotator’s
head group but also to some extent on strain release in the transition
state, as revealed by a comparison of ROT-1' and ROT-2'. Although
the rotator terminal in both rotors is pyridine the exchange frequency
in ROT-1' with its rather flexible rotator arm is significantly lower than
that of ROT-2' (Table 1), most likely due to more strain release in the
transition state of rotor ROT-2'. On shifting from ROT-2' to ROT-3'
the exchange frequency increases (Table 1). Here, the change is readily
1
only one set of H NMR signals for protons 3-H to 6-H indicating fast
1
exchange. The VT H-NMR (25 to –75 °C) reveals rapid exchange at
k25 = 125 MHz (SI, Figure S36). As a comparison for ROT-3', the 2:2:1
mixture of [Cu(CH3CN)4]PF6, 2,9-dimesitylphenanthroline (8) and
4-methylpyrimidine (10) furnishing C1+C3 = 1:1 was investigated.
justified from the differential binding strength of pyridine (log Kpy
=
3.20) and pyrimidine (log Kpym = 2.78) to [Cu(phenAr2)]+, the Cu+-
loaded phenanthroline station.23 The corresponding free energy differ-
ence (ΔΔG25) of 2.4 kJmol−1 is almost fully preserved in the activation
energy difference ΔΔG⧧ = 2.0 kJmol−1 of ROT-2' and ROT-3'
25
(Table 1). Thus, the thermodynamic difference finds almost full reflec-
tion in the kinetic change suggesting that both ROT-2' and ROT-3' are
comparable with regard to strain.
Since copper(I) phenanthroline complexes24 are catalysts for click
transformations, two obvious questions do arise: (a) will the four-
component nanorotors be active as catalysts although rotation may
interfere with the free “copper(I) site”? (b) Will there be an influence
of rotational speed on the catalytic activity?
Figure 4. (a) Model complexes C1 and C2 (1:1). (b) Yields of 5a+6a
(each 25.6 mM) 7a obtained with various copper(I) catalytic sys-
When the rotors ROT-1'–ROT-3' were used as catalyst (10 mol%) for
the reaction between 9-(azidomethyl)anthracene (5a) and (prop-2-yn-
1-yloxy)benzene (6a) (CD2Cl2:CDCl3 = 15:1, 55 °C, 4 h), it was ob-
served that nanorotor ROT-3' furnished the highest yield of click
product 7a (62%) followed by rotor ROT-2' (44%) and ROT-1' (20%)
tems (same amount of copper(I) ion, 55 °C, 4 h, CD2Cl2:CDCl3
=
15:1).
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While this catalytic system produced 7a in 16±1% yield (SI, Figure
S46), nanorotor ROT-3' furnished almost four-fold the yield. Complex
C3 = [Cu(8)(10)]+ as catalyst afforded 4% (SI, Figure S49) of 7a.
Thus, among the reference systems the two exchanging catalytic sys-
tems (C1/C2 and C1/C3) as well as C1 perform far less than the relat-
ed nanorotors (Figure 4b).
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Due to its chelate character, 7a binds rather strongly to both copper(I)
centers in [Cu2(4)]2+ (log  = 8.2±0.2) (SI, Figure S59). Thus a rea-
sonable hypothesis for the increased catalytic efficiency of the nano-
rotors claims that product 7a, when bound to the catalytically active
copper(I) station(s), is exchanged in an intramolecular SN2-type reacti-
on by the rotator head with the displacement being more efficient at
faster rotating speed.
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Figure 5. a) Yield of 7a and initial ratev0(5a+6a) correlate linearly with
the corrected product release of 7a in nanorotors (s. Table 2). b) Ki-
netic traces used for the determination of the initial ratev0(5a+6a).
Convincing proof for reduced product inhibition with increasing speed
was obtained by fully loading all rotors with 7a and measuring the
amount of 7a liberated into solution. As a reference, two equiv of 7a
were uploaded onto the copper(I)-filled stator [Cu2(4)]2+ furnishing
[Cu2(4)(7a)2]2+ (SI, Figure S26). Incidentally, proton h-H of free 7a in
solution is downfield shifted by 0.84 ppm in comparison to that in
[Cu2(4)(7a)2]2+ (Table 2). In the loaded rotors [(ROT-1'–ROT-
3')(7a)2] the single signal for proton h-H of 7a indicates rapid ex-
change of free and bound 7a. We thus expect an increasing downfield
shift of this dynamically averaged 1H NMR signal with higher rotation-
al speed, because the loaded rotors should increasingly liberate 7a into
solution. Indeed, in the series ROT-1'ROT-3' the resonance of pro-
ton h-H shifts from 4.40 to 4.67 ppm (Table 2), thus approaching 4.97
ppm (the shift of free 7a) at higher speed.
In conclusion, simple mixing of four distinct components leads to cata-
lytic machinery equipped with advanced features for suppressing pro-
duct inhibition. Significantly, the nanorotors’ catalytic activity, ex-
pressed by the yield in two click reactions, is linearly correlated with
their rotational frequency and their ability to suppress product inhibi-
tion. Apparently, by nanomechanical motion of the rotator’s arm, the
product formed at the catalytically active metal ion is displaced clearing
the site for the next turn-over.
Analogous to multicomponent enzymes, such as ATP synthase,18 the
catalytic activity depends on the liberation of the product, most likely
by an SN2-type nanomechanical displacement. Among non-biological
catalytic machinery, which rarely show a correlation of machine speed
and catalytic activity,26 such mechanism is without precedent.
Table 2. NMR shift and amount of 7a liberated from rotors.
■ ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at.
Compounds
δ / ppma
4.13
7ab / % 7ac / %
9
[Cu2(4)]2+ + 7a (2 equiv)
ROT-1' + 7a (2 equiv)
ROT-2' + 7a (2 equiv)
ROT-3' + 7a (2 equiv)
7a
4.40
38
57
29
48
58
Synthetic procedures and characterization data, NMR and ESI-MS
spectra, binding constants and catalytic studies (PDF).
4.57
4.67
67
■ AUTHOR INFORMATION
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4.97
(100)
Corresponding Author
a NMR shift of proton h-H of 7a. b Amount of 7a liberated from rotor
(at 2.56 mM). c Corrected product release = release (%) of 7a – 9%.
* schmittel@chemie.uni-siegen.de
■ ACKNOWLEDGMENTS
From the stability constant of [Cu2(4)(7a)]2+ (log  = 8.2) and spe-
ciation analysis (SI, Figure S60) it was calculated that only 9% of
[Cu2(4)(7a)]2+ is dissociated (at 2.56 mM). Using the NMR shift of
free 7a and that of 7a in[Cu2(4)(7a)]2+, the liberated amount of 7a was
determined for each rotor from the NMR shift (Table 2, SI: page S36).
As 9% of 7a is liberated without rotation, the product release was cor-
rected accordingly. The plot of the yield of 7a vs corrected product
release of the nanorotors reveals a linear correlation (red line in Figure
5a). Moreover, we have determined the initial rate v0(5a+6a) (SI, page
S55) in presence of all three nanorotors from the kinetic traces (Figure
5b). The initial rate equally correlates in a linear fashion with the cor-
rected product release (blue line in Figure 5a). Thus experimental
evidence convincingly suggests that the improved catalytic activity is
due to reduced product inhibition.
We are grateful to the DFG (Schm 647/19-2 and 20-1) and Universität
Siegen for financial support.
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