Effect of Substituents on the Bond Strength of Air-Stable Dicyanomethyl Radical Thermochromes

Article abstract of DOI:10.1021/acs.joc.7b01188

A series of substituted aryl dicyanomethyl radicals were synthesized, and the bonding thermodynamic parameters for self-dimerization were determined from van't Hoff plots obtained from variable-temperature electron paramagnetic resonance and ultraviolet-v

Full text of DOI:10.1021/acs.joc.7b01188

Note
Effect of Substituents on the Bond Strength of Air
Stable Dicyanomethyl Radical Thermochromes
Joshua P. Peterson, Margarita R. Geraskina, Rui Zhang, and Arthur H. Winter
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 30 May 2017
Downloaded from http://pubs.acs.org on June 1, 2017
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Effect of Substituents on the Bond Strength of Air Stable Dicy-
anomethyl Radical Thermochromes
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Joshua P. Peterson, Margarita R. Geraskina, Rui Zhang, Arthur H. Winter*
Department of Chemistry, Iowa State University, 2101d Hach Hall, Ames, Iowa, 50011, United States
Supporting Information Placeholder
ABSTRACT: A series of substituted aryl dicyanomethyl radicals were synthesized and the bonding thermodynamic parameters for
selfꢀdimerization were determined from Van ’t Hoff plots obtained from variable temperature EPR and UVꢀVis spectroscopy. At
low temperatures, the radicals dimerize, but the colored, airꢀstable free radicals are returned upon heating. Heating and cooling
cycles (5ꢀ95 oC) can be repeated without radical degradation and with striking thermochromic behavior. We find a linear free enerꢀ
gy relationship between the Hammett para substituent parameter and the dimerization equilibrium constant, with para electron
donating substituents leading to a weaker bond and withdrawing substituents leading to stronger bonds, following a captodative
effect. DFT investigations (B98D/6ꢀ31+G(d,p)) reveal that the dimers prefer a slipꢀstacked geometry and feature elongated bonds.
Dynamic materials that adapt their properties to environmental
cues or external stimuli are increasingly being pursued. We
have been interested in achieving such stimuliꢀresponsive beꢀ
havior by exploiting changes in spin state upon a stimulus due
to the unique properties of radicals.1ꢀ3 However, this approach
is made challenging because many radicals lack air and therꢀ
mal stability.4ꢀ9 We were intrigued by reports of stable dicyꢀ
anomethyl radicals and related radicals in the literature,4,6,7 as
well as a recent report by Kobashi and coworkers describing
stable thermochromic triarylaminoꢀdicyanomethyl radicals.9
We thus set out to perform a systematic investigation of the
effect of the structure of the dicyanomethyl radical on the
strength of the radical bonding interaction, with an eye toꢀ
wards identifying potential new building blocks for stimuliꢀ
responsive materials.
radical grows in the EPR and UVꢀVis spectrum. The radicals
have absorption bands in the visible region of the optical specꢀ
trum. Example variable temperature EPR and UVꢀVis data are
shown in Figure 1 for radicals 1, 3, 4, and 11 (all data, includꢀ
ing a video demonstrating the thermochromic behavior, can be
found in the Supporting Information). Each radical in Figure 1
has a different color, leading to striking thermochromic behavꢀ
ior that is highly dependent on the radical structure. In order
to test the durability of the radical species, temperature cycling
studies were performed where a sample was monitored for 30
minutes at 5 °C followed by 30 minutes at 95 °C. After 6 cyꢀ
cles, there was no decrease in absorbance at 95 °C confirming
that the radical was thermally stable (see UVꢀVis insets in
Figure 1).
A set of 11 aryl dicyanomethyl radicals were synthesized
(Table 1) and the thermodynamic parameters were evaluated
using Van’ T Hoff plots generated from variable temperature
EPR and UVꢀVis spectroscopy experiments. The radicals were
synthesized via palladium cross coupling reactions of varying
aryl bromides with malononitrile, followed by oxidation to the
radical using potassium ferricyanate. Remarkably, these radiꢀ
cals/radical dimers can be purified by a lowꢀtemperature biꢀ
phasic separation between acetonitrile and hexane under air.
All of the radicals exhibit qualitatively the same behavior.
At low temperatures, the radical dimerizes, as evidenced by a
lack of an EPR signal, yielding a yellow or colorless solution.
As the temperature is raised, a signal corresponding to the
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Figure 1. (Top row) Structure of example dimer species. (Second row) Variable temperature EPR study. (Third row) Variable temperature
UVꢀVis study. (Inset)– Temperature cycling at 5 °C and 95 °C for 30 minutes each. (Bottom row) Pictures demonstrating thermochromꢀ
ism open to air (Pictures are shown to display full thermochromic behavior and may not represent all temperatures studied via EPR).
The thermodynamic bonding parameters for all the radicals
were obtained from Van ‘T Hoff plots following the growth of
the EPR signal as a function of temperature and are shown in
Table 1. All thermodynamic data are the average of three
separate runs and fits. Inspection of the binding constants
shows that better electronic donors, such as compounds 1-4,
have weaker binding constants (Ka = 105 – 106 Mꢀ1) than radiꢀ
cals bearing either weaker donating substituents or withdrawꢀ
ing substituents, such as compounds 5-10 (Ka >107 Mꢀ1).
Table 1. (Top) Thermodynamic binding parameters.
(Bottom) Plot of the log of the binding constant versus
Hammett sigma parameter.
To test if substituent effects were additive, we synthesized
11, a trimethoxy derivative. Indeed, this radical showed the
weakest dimer bond of all molecules studied (Ka = 1.9 x 105
Mꢀ1), suggesting that substituent effects are additive. The
compounds with less electron donation exhibit stronger bindꢀ
ing. Compounds 5-8, are interesting because the halogenated
derivatives 7 and 8 bind less tightly than the alkyl substituted
derivatives 5 and 6. The ability for halogens to be both pi elecꢀ
tron donors and sigma withdrawing groups may play a role in
weaker dimerization. Derivatives containing electron withꢀ
drawing groups (e.g. CN, CO2Me) were found to have the
highest binding constants (~108 Mꢀ1), and even have resolved
NMR spectra at room temperature. The other dimers only
exhibit NMR signals at low temperature due to fast exchange.
A plot of the Hammett sigma para parameter10 for each
monomer unit against the log of the binding constant showed a
positive, relatively linear trend (Table 1). The existence of the
captodative effect has been debated11,12 but this model predicts
that radicals containing both donating and withdrawing groups
would be stabilized. Given that donating substituents lead to
the weakest binding (and presumably, most stabilized radiꢀ
cals), our data are consistent with the captodative model.
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structure was obtained on a BRUKER APEX II Difractometer
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equipped with an APEX II CCD Detector. All reactions were
performed under an argon atmosphere in oven dried glassꢀ
ware. Xylenes were dried under activated molecular sieves
(0.4 nm). Flash column chromatography was performed with
silica gel 40ꢀ63 micrometer (230ꢀ400 mesh) from
SILICYCLE. All other chemical reagents were purchased
from commercial sources and used without purification.
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General procedure for malononitrile coupling to aryl bro-
mide. To a 50 mL three neck round bottom flask with a magꢀ
netic stir bar, NaOtBu (13 mmol) was added under ambient
atmosphere. The flask was sealed and placed under an argon
atmosphere. To the flask was then added 20 mL of xylenes
(dried by 0.4 nm activated molecular sieves), and the flask
was stirred for 5 min to reach a uniform solution. To the flask
was next added CH2(CN)2 (6.6 mmol) via syringe and the
resulting mixture was stirred for 60 min. In a separate 100 mL
conical bottom flask was added the aryl bromide of the reacꢀ
tion (4.4 mmol) under ambient atmosphere. The flask was
pumped into a glove box to add 0.2 g [Pd(PPh3)2]Cl2 catalyst
(6.5 mol%). To the conical bottomed flask was then added 10ꢀ
15 mL of dried xylenes and stirred for 5ꢀ10 min. The conical
flask was cannulated into the round bottom flask. The reaction
was stirred at 130 °C and was monitored by TLC (CHCl3 as
eluent) until completion. The resulting reaction mixture was
then quenched with 10% HCl aq. solution. The mixture was
then filtered through celite and extracted with ethyl acetate (3
x 50 mL). The organic layers were combined, dried with
Na2SO4, and the solvent was removed by rotary evaporation.
The crude product was then purified by flash chromatography
(CHCl3 as eluent) to afford the product.
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Figure 2. Computed structure (B98D/6ꢀ31+G(d,p)) of dimer speꢀ
cies 1 (A – side view, B – top view), Visualized by IboView.14,15
Computed Mulliken atomic spin densities for compounds 1 (C), 9
(D), 8 (E), 6 (F) and visualized total spin density (below).
Computational studies were performed on select dimer speꢀ
cies to better understand the orientation of the two units with
respect to the sigma bond formed between two radicals (Figꢀ
ure 2).13 Surprisingly, the dihedral angle between the two molꢀ
ecules is only ~60 degrees. Only minor variance was observed
between differentlyꢀsubstituted compounds and the calculated
dihedral angle. Figure 2 shows the computed structures, at two
different viewpoints, for dimer 1, as well as the Mulliken spin
densities for compounds 1, 9, 8, and 6. Each dimer features an
exceptionally long computed sigma bond of ~1.63 .
Procedure for oxidation of 1-11. To a 50 mL schlenk flask
was added the aryl malonitrile (0.3 mmol) dissolved in 15 mL
CH2Cl2 along with a magnetic stir bar. The flask was flushed
with argon and kept under inert atmosphere. Separately in a 50
mL round bottom flask was placed 15 mL of 0.3 M KOH soluꢀ
tion that was purged with argon. To the round bottom was
added K3[Fe(CN)6] (1.5 mmol) as the oxidant. The solution
was allowed to mix for 5 min before cannulating the solution
into the schlenk flask containing the aryl malonitrile. The reꢀ
action was mixed under vigorous stirring for 10 min. The mixꢀ
ture was then exposed to air and the organic layer was separatꢀ
ed, dried with Na2SO4, and the solvent was removed by rotary
evaporation. The crude product was then purified by cold exꢀ
traction (ꢀ78° C) between hexanes and acetonitrile. The dimer
product was kept in the solid acetonitrile while impurities reꢀ
mained in the hexanes.
In conclusion, we have synthesized a set of stable aryl dicyꢀ
anomethyl radicals that exhibit thermochromic properties and
have a bond strength that is highly dependent on the substituꢀ
ents. The bond strengths can be tuned from 105 Mꢀ1 to 108 Mꢀ1
based on the nature of the para substituent. It is possible that
these radicals could be incorporated into polymeric materials
leading to bulk materials with temperatureꢀresponsive behavꢀ
ior, or find use in dynamic covalent chemistries.
EPR studies. All EPR studies were performed on samples
purged with argon to view the hyperfine coupling of the radiꢀ
cal species. Samples were allowed to equilibrate at each temꢀ
perature for 5 minutes before acquiring data. The following
EPR parameters were utilized for all samples unless otherwise
noted; Modulation Frequency: 100 kHz, Receiver Gain: 50
dB, Modulation Amplitude: 0.5, Time Constant: 0.01 seconds,
Center Field: 3300 G, Sweep Width: 200 G, Microwave Atꢀ
tenuation: 20 dB, Microwave Power: 2 mW, Number of Data
Points: 2048, Average Number of Scans: 8. The sweep time
and conversion time varied from sample to sample and is menꢀ
tioned below for each compound.
Experimental
General Methods:
1H Nuclear Magnetic Resonance (1H
NMR) and 13C Nuclear Magnetic Resonance (13C NMR) specꢀ
tra were recorded on a Bruker DRXꢀ500. NMR spectra were
obtained in CDCl3 at room temperature except where noted.
The NMR data were presented accordingly: chemical shift in
ppm with deuterated chloroform (CDCl3, δ = 7.26 ppm) for
1HꢀNMR and the chloroform residue (δ = 77.0 ppm) for 13Cꢀ
NMR as internal standards. Multiplicity labels (s = singlet; d =
doublet; t = triplet; m = multiplet), J splitting (Hz), integration.
EPR spectra were collected on a Bruker FTꢀEPR. UVꢀVis
spectra were collected on an Agilent 8453 spectrometer. Mass
spectra were recorded on an Agilent QTOF 6540. Melting
points were determined on a DigiMelt SRS apparatus. Crystal
UV-Vis Studies. To a 3 mL quartz cuvette was added a
known concentration of dimer species dissolved in toluene for
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study. Identical results were obtained whether samples were 129.1, 128.7, 128.3, 127.7, 127.3, 125.0, 111.9, 27.9. HRMS
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purged with argon or performed under air. All samples were
studied in the temperature range of 5 °C ꢀ 95 °C. A temperaꢀ
ture controlled water circulating bath was used to monitor the
temperature and at each data point a time of 5 minutes was
given to equilibrate the sample to the current temperature.
(ESIꢀTOF) m/z: calcd. for C15H9N2, 217.0771; Found:
217.0776 [MꢀH]; 6D: Following the general oxidation proceꢀ
dure a clear organic solution was isolated as a white powder.
For EPR spectroscopy a modulation amplitude of 2.0, a conꢀ
version time of 2.56 milliseconds, and a sweep time of 5.24
seconds was used.
Compound characterizations (H represents 1-11 and D repreꢀ
sents oxidized radical dimer of that compound):
7H: General procedure was followed to yield a pale orange
solid (0.536 g, 69 %). m.p. 72ꢀ75 °C; H NMR (500 MHz,
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1H: General procedure was followed to yield a green/yellow
solid (0.383 g, 47 % yield). m.p. 168ꢀ170 °C; H NMR (500
CDCl3) δ 7.50 (d, J = 8.7 Hz, 2H), 7.46 (d, J = 8.7 Hz, 2H),
5.05 (s, 1H). 13C NMR (500 MHz, CDCl3) δ 137.2, 130.7,
129.0, 125.0, 111.7, 28.0; 7D: Following the general oxidation
procedure a yellow organic solution was isolated as a pale
yellow powder.
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MHz, CDCl3) δ 7.98 (d, J = 7.6 Hz, 2H), 6.70 (d, J = 7.6 Hz,
2H), 3.16 (s, 1H). 13C NMR (500 MHz, CDCl3) δ 151.4,
128.2, 112.8, 112.6, 112.4, 40.3, 27.6; 1D: Following the genꢀ
eral oxidation procedure a brilliant blue organic solution was
isolated as a dark red solid. For EPR spectroscopy a converꢀ
sion time of 2.56 milliseconds and sweep time of 5.24 seconds
was used.
8H: General procedure was followed to yield a white crystalꢀ
line solid (0.218 g, 31 %). 1H NMR (500 MHz, CDCl3) δ 7.51
(dd, J = 7.0 Hz, 2H), 7.21 (t, J = 7.0 Hz, 2H), 5.06 (s, 1H). 13C
NMR (500 MHz, CDCl3) δ 163.8, 129.4, 122.2, 117.4, 111.7,
27.6. HRMS (ESIꢀTOF) m/z: calcd. for C9H4FN2, 159.0359;
Found: 159.0366 [MꢀH]; 8D: Following the general oxidation
procedure a yellow organic solution was isolated as a pale
yellow powder. For EPR spectroscopy a modulation amplitude
of 2.0 was used.
2H: General procedure was followed to yield a pale yellow
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powder (0.151 g, 20 % yield). H NMR (500 MHz, CDCl3) δ
7.37 (d, J = 8.35 Hz, 2H), 7.30 (d, J = 8.5 Hz, 2H), 5.04 (s,
1H), 2.50 (s, 3H). 13C NMR (500 MHz, CDCl3) δ 142.4,
127.6, 127.0, 122.2, 111.9, 27.7, 15.2. HRMS (ESIꢀTOF) m/z:
Calcd. For C10H7N2S, 187.0335; Found: 187.0339 [MꢀH]; 2D:
Following the general oxidation procedure a green organic
solution was isolated as a yellow powder. For EPR spectrosꢀ
copy a conversion time of 2.56 milliseconds and sweep time
of 5.24 seconds was used.
9H: General procedure was followed to yield a slightly pink
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crystalline solid (0.331 g, 45 %). m.p. 98ꢀ100 °C; H NMR
(500 MHz, CDCl3) δ 7.83 (d, J =10 Hz, 2H), 7.68 (d, J = 10
Hz, 2H), 5.21 (s, 1H). 13C NMR (500 MHz, CDCl3) δ 133.8,
131.1, 128.3, 117.4, 114.9, 110.9, 28.2. HRMS (ESIꢀTOF)
m/z calcd. for: C10H4N3, 166.0411; Found: 166.0411 [MꢀH];
9D: Following the general oxidation procedure a clear organic
solution was isolated as a white powder. For EPR spectroscoꢀ
py a modulation amplitude of 2.0 was used.
3H: General procedure was followed to yield a pink solid
(0.124 g, 15 % yield). 1H NMR (500 MHz, CDCl3) δ 7.38 (d,
J = 8.3 Hz, 2H), 6.97 (d, 7.6 Hz, 2H), 5.03 (s, 1H), 3.82 (s,
3H). 13C NMR (500 MHz, CDCl3) δ 160.9, 128.6, 117.9,
115.3, 112.3, 55.5, 27.4; 3D: Following the general oxidation
procedure a pink organic solution was isolated as a pale yelꢀ
low solid.
10H: General procedure was followed to yield a white crystalꢀ
line solid (0.088 g, 10 %). 1H NMR (500 MHz, CDCl3) δ 8.14
(d, J = 7.5 Hz, 2H), 7.59 (d, J = 7.5 Hz, 2H), 5.22 (s, 1H), 3.94
(s, 3H); 10D: Following the general oxidation procedure a
clear organic solution was isolated as a white powder. For
EPR spectroscopy a modulation amplitude of 2.0 was used.
13C NMR (500 MHz, CDCl3) δ 165.2, 134.4, 130.9, 128.7,
128.6, 109.9, 53.0, 52.6 HRMS (ESIꢀTOF) m/z: calcd. for
C11H7N2O2, 199.0508; Found: 199.0511 [M/2].
4H: General procedure was followed to yield a beige powder
(0.120 g, 12 %). m.p. 153ꢀ155 °C; 1H NMR (500 MHz,
CDCl3) δ 7.34 (d, J = 7.3 Hz, 2H), 6.94 (2H, J = 7.3 Hz, d),
5.00 (s, 1H), 3.84 (t, J = 4.0 Hz, 4H), 3.20 (t, J = 4.0 Hz, 4H).
13C NMR (500 MHz, CDCl3) δ 152.5, 128.3, 116.1, 116.0,
112.3, 66.7, 48.4, 27.5. HRMS (ESIꢀTOF) m/z: calcd. for
C13H14N3O, 228.1131; Found: 228.1136 [M+H]; 4D: Followꢀ
ing the general oxidation procedure a yellow/brown organic
solution was isolated as a brown solid. For EPR spectroscopy
a conversion time of 2.56 milliseconds and sweep time of 5.24
seconds was used. This sample used a receiver gain of 40 dB
and a modulation amplitude of 1.0
11H: General procedure was followed to yield white crystalꢀ
1
line solid (0.092 g, 9 %). m.p. 106ꢀ108 °C; H NMR (500
MHz, CDCl3) δ 6.15 (s, 2H), 5.52 (s, 1H), 3.90 (s, 6H), 3.82
(s, 3H). 13C NMR (500 MHz, CDCl3) δ 163.4, 158.3, 112.5,
95.7, 91.0, 56.2, 55.6, 16.6. HRMS (ESIꢀTOF) m/z: calc. for
C12H13N2O3, 233.0921; Found: 233.0919 [M+H]; 11D: Followꢀ
ing the general oxidation procedure a dark red organic solution
was isolated as an orange powder.
5H: General procedure was followed to yield a pale yellow
powder (0.570 g, 83 % yield). m.p. 56ꢀ58 °C; H NMR (500
1
MHz, CDCl3) δ 7.38 (d, J = 8.0 Hz, 2H), 7.30 (d, J = 8.0 Hz,
2H), 5.02 (s, 1H), 2.40 (s, 3H). 13C NMR (500 MHz, CDCl3) δ
140.7, 130.7, 127.1, 123.2, 111.9, 27.8, 21.2; 5D: Following
the general oxidation procedure a yellow organic solution was
isolated as a pale yellow powder.
ASSOCIATED CONTENT
Supporting Information
All Van ‘T Hoff plots, variable temperature EPR and UVꢀVis
spectra, and computational absolute energies and Cartesian coorꢀ
dinates. The Supporting Information is available free of charge on
the ACS Publications website.
6H: General procedure was followed to yield a white crystalꢀ
1
line solid (0.288 g, 30 %). m.p. 105ꢀ108 °C; H NMR (500
MHz, CDCl3) δ 7.71 (d, J = 8.3 Hz, 2H) 7.58 (m, J = 8.3, 7 8
Hz, 4H), 7.49 (t, J = 7.8, 7.2 Hz, 2H), 7.43 (t, J = 7.2 Hz, 1H),
5.12 (s, 1H). 13C NMR (500 MHz, CDCl3) δ 143.5, 139.4,
AUTHOR INFORMATION
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Corresponding Author
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*Email: winter@iastate.edu
Notes
The authors declare no competing financial interest.
Acknowledgements
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Financial support from NSF (CHEꢀ 1464956) and ACS PRF
(PRF55820ꢀND4) are gratefully acknowledged. We thank Sarah
Cady and the Iowa State Chemical Instrumentation Facility for
assistance with the variable temperature EPR measurements.
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