Modulating photoswitch performance with halogen, coordinative and hydrogen bonding: A comparison of relative bond strengths

Article abstract of DOI:10.1039/d1cc01827b

The behavior of an enediyne photoswitch is modulated with halogen bonding, coordinative bonding and hydrogen bonding. Through NMR and computational studies we demonstrate that the relative strength of the secondary bonding directly influences the rate of photoisomerization and the photostationary state.

Full text of DOI:10.1039/d1cc01827b

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Modulating photoswitch performance with
halogen, coordinative and hydrogen bonding:
a comparison of relative bond strengths†
Cite this: Chem. Commun., 2021,
7, 6261
5
Received 6th April 2021,
Accepted 25th May 2021
a
a
b
a
Sofia Lindblad,
Daniel Sethio,
Orion B. Berryman* and Mate Erdelyi
´ ´
*
´
DOI: 10.1039/d1cc01827b
rsc.li/chemcomm
8
The behavior of an enediyne photoswitch is modulated with halo- proton via a three-center, four electron bond. From 1, the
gen bonding, coordinative bonding and hydrogen bonding. halogen bonded system 1-I, and the metal coordinated system
9
–11
Through NMR and computational studies we demonstrate that 1-Ag, were obtained following a literature protocol.
the relative strength of the secondary bonding directly influences
the rate of photoisomerization and the photostationary state.
Specifically, ligand 1 was mixed with AgBF
Sequential addition of I in dry conditions produced 1-I. To obtain
the hydrogen bonded system 1-H, 1-I was reacted with water, which
4
to obtain 1-Ag.
2
+
Controlling molecular conformation is critical to the function typically results in rapid decomposition of three-center [N–X–N]
of both synthetic and biological systems. For example, molecular halogen bonds
12,13
+
to the corresponding [N–H–N] complex. The
machines have received enormous recognition for their unique halogen bond of 1-I decomposed after 4 days, and thus, appears
ability to adopt multiple conformations that can access different unusually stable. Adding trifluoroacetic acid to 1-I immediately
1
,2
functions.
As such, using external stimuli to control the produced some 1-H, and after a day almost full conversion was
conformation of molecular machines and functional materials noted. The formation of 1-H was validated by LCMS analysis.
3
,4
has been critical to their development.
Using light as an Observation of the major peak corresponding to the mass of the
À1
+
external stimulus is especially desirable as it is a clean reagent ligand (M + 1, 231 g mol ) revealed that the I had not added
that does not leave residues in a reaction mixture, allowing for covalently to the enediyne moiety. The formation of 1-H was further
5
,6
simple work up and purification. Additionally, light can be validated by raising the pH with NaOH to produce 1. The control 2-I
1
0,14
used to control isomerization in both a temporal and spatial (Fig. 1) was synthesized as previously described.
1
5–18
19–22
manner. Thus, understanding new ways to control molecular
Hydrogen
and halogen bonding
as well as metal
have been used to control photoisomerization
23,24
switches will enable the further refinement of the function of coordination
molecular machines. Herein, we illustrate that halogen bonding, processes. However, they have not yet been evaluated for control-
coordinative bonding and hydrogen bonding can be used to ling enediyne photoswitches, and neither have been directly
modulate an enediyne photoswitch (Fig. 1). To study the photo- compared within a similar system. Additionally, incorporating
1
2,13,25
physical properties, the systems were non-selectively irradiated three-center bonds
into photoswitches has not been
with a Xe ARC light source, and the relative cis–trans isomeriza- reported, despite the strong nature of these halogen bonds that
tion rates and photostationary states were compared using NMR makes them useful to prepare spectacular supramolecular
2
6,27
spectroscopy.
complexes.
With the cis-compounds 1, 1-I, 1-Ag and 1-H in
+
For experimental evaluation of the ability of weak interac- hand, we were interested in assessing how the [N–X–N] halogen,
tions to influence photoisomerization, we synthesized ligand 1 [N–Ag–N] coordination and [N–H–N] hydrogen bonds, respec-
following a previously described route. This ligand contains an tively, influence the kinetics and thermodynamics of cis to trans
+
+
7
enediyne photoswitch with two appended pyridine rings, enediyne photoisomerization. For the photoirradiation experi-
appropriately spaced to interact with a halogen(I), silver(I) or a ments, B0.02 mmol compound was dissolved in 0.6 mL of
2 2
CD Cl in an NMR tube, and the solution was irradiated with
non-selective light from a 1000 W Xe ARC light source (Fig. S2,
ESI†).
Ligand 1 does not isomerize at room temperature, or upon
heating. However, photo-irradiating the noncoordinated ene-
diyne 1 caused the ligand to reach its photostationary state
(1 : 0.76, cis : trans) within r15 min (Fig. 2). In comparison, the
a
b
Department of Chemistry–BMC, Uppsala University, Uppsala SE-751 23, Sweden.
E-mail: Mate.Erdelyi@kemi.uu.se
Department of Chemistry and Biochemistry, 32 Campus Drive, University of
Montana, Missoula, Montana 59812, USA. E-mail: Orion.Berryman@mso.umt.edu
Electronic supplementary information (ESI) available: Experimental proce-
†
dures, NMR spectra, computational data. See DOI: 10.1039/d1cc01827b
This journal is © The Royal Society of Chemistry 2021
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1
Fig. 3 Stacked H NMR spectra (500 MHz, 25 1C, CD
after irradiation to reach its photostationary state, a mixture of its cis (1) and
trans (t-1) isomers (bottom), then (B) after addition of AgBF to that same
sample (second from bottom), and (C) after sequential irradiation for 10 min
second from top). Also shown is a reference spectrum of 1-Ag (D, top).
2 2
Cl ) of ligand 1 (A)
4
(
Complex t-1 is marked by brown lines, showing that the trans-isomer does
not interact with silver(I); the cis-isomer, however, coordinates silver(I)
forming 1-Ag (marked by grey lines). Upon irradiation, the signals of t-1
disappear, leaving behind only those of 1-Ag.
Fig. 1 The enediyne ligand 1 was used to compare how halogen bonding,
hydrogen bonding, and metal coordination modulate photoisomerization
(top). The halogen bonded 1,2-bis(pyridine-2-ylethynyl)benzene 2-I was
used as a control, to demonstrate that the halogen bond remains intact
upon photo irradiation (bottom). Here, X is iodine(I) for 1-I and t-1-I, silver(I)
for 1-Ag and t-1-Ag, proton (hydrogen(I)) for 1-H and t-1-H. The counterion
tetrafluoroborate, is identical for these complexes (apart from 1 that has no
counterion) and is omitted for clarity. The depiction of t-1-I and t-1-Ag is
schematic here, these complexes do not form (vide infra, Fig. S1, ESI†).
Protonation alters the photostationary state of 1 :t-1 (1 : 0.76) to 1-H : t-1-H
trans ratio of 1 :0.2 (Fig. S38 and S39, ESI†). Thus, the hydrogen
bond retards photoisomerization and biases 1 towards its cis
conformation at the photostationary state.
Remarkably, the silver(I) complex 1-Ag did not isomerize at
(1 :0.20), whereas silver coordination makes the isomerisation unidirectional
t-1-H - 1-H. The 1-I complex quickly decomposes upon isomerization to all upon irradiation (Fig. S35, ESI†). Some precipitate was
form t-1-H.
3
formed during the experiment, but re-dissolving it in CD CN
proved this to be the cis-isomer 1-Ag (Fig. S36, ESI†). Further
investigation showed that adding AgBF to a mixture of isomers
4
parent enediyne with phenyl rings instead of pyridines have
1
and t-1 produced 1-Ag and t-1, and irradiating this new
a nearly equal mixture of cis and trans isomers at the photo-
mixture converted t-1 into 1-Ag (Fig. 3). Thus, while hydrogen
bonding provides a means to modulate conformational equili-
bria and photoisomerization rate, silver(I) coordination affec-
tively turns off photoisomerization for the 1-Ag complex. This,
strategy could also be used as a method for isomeric resolution
in future applications.
2
8
stationary state, when irradiated with 366 nm light.
The hydrogen bond in 1-H caused the enediyne to photo-
isomerize at a significantly slower rate than 1, taking 54 minutes
to reach full conversion. Additionally, the photostationary state
deviated substantially from the parent ligand 1, reaching a cis :
Photoirradiation of the halogen bonded complex 1-I high-
lighted the sensitivity of this complex to conformation. When 1-I
was photoirradiated, it was converted to the hydrogen bonding 1-H
complex over the course of B31 minutes. This can be rationalized:
photoisomerization to the t-1-I complex would produce an
unstable dimer, owing to the instability of N-iodopyridinum
29,30
cation.
The fact that 1-I was converted to the 1-H complex,
however, indicates that isomerization was occurring, since control
experiments with the halogen bonded complex 2-I proved that
+
irradiation alone does not cause decomposition of an [N–X–N]
complex to any detectable extent (Fig. S34, ESI†).
To estimate the rotational barriers, and to understand how
the different bond strengths influence photoisomerization, we
performed calculations at the DLPNO-CCSD(T)/def2-TZVP//
o-B97X-D//aug-cc-pVDZ level of theory. The aug-cc-pVDZ basis
set was chosen as it is known to provide a good balance
1
Fig. 2 The stacked H NMR spectra (400 MHz, 25 1C, CD
2
Cl
2
) of 1 with
3
1–33
between accuracy and cost in predicting geometries.
We
indicated trans : cis ratios before (bottom), after 15 min (middle) and 25 min
top), using non-selective irradiation (Xe Arc 1000 W).
(
used the one nuclear coordinate with respect to torsional angle
6
262
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Conflicts of interest
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There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 2021
Chem. Commun., 2021, 57, 6261–6263 | 6263