Through-space 19F-19F spin-spin coupling in ortho-fluoro Z -azobenzene

Article abstract of DOI:10.1002/mrc.4327

We report through-space (TS) ¹⁹F-¹⁹F coupling for ortho-fluoro-substituted Z-azobenzenes. The magnitude of the TS-coupling constant (^TSJ_FF) ranged from 2.2-5.9 Hz. Using empirical formulas reported in the literature, these coupling constants correspond to non-bonded F-F distances (d_FF) of 3.0-3.5 ?. These non-bonded distances are significantly smaller than those determined by X-ray crystallography or density functional theory, which argues that simple models of ¹⁹F-¹⁹F TS spin-spin coupling solely based d_FF are not applicable. ¹H, ¹³C and ¹⁹F data are reported for both the E and Z isomers of ten fluorinated azobenzenes. Density functional theory [B3YLP/6-311++G(d,p)] was used to calculate ¹⁹F chemical shifts, and the calculated values deviated 0.3-10.0 ppm compared with experimental values.

Full text of DOI:10.1002/mrc.4327

Research article
Received: 4 May 2015
Revised: 22 July 2015
Accepted: 30 July 2015
Published online in Wiley Online Library: 1 September 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4327
19 19
Through-space F– F spin–spin coupling in
ortho-fluoro Z-azobenzene
Shiva K. Rastogi,a Robert A. Rogers,a Justin Shi,a Christopher T. Brown,a
Cindy Salinas,a Katherine M. Martin,a Jacob Armitage,a Christopher Dorsey,a
Gao Chun,b Peter Rinaldib and William J. Brittaina*
We report through-space (TS) 19F–19F coupling for ortho-fluoro-substituted Z-azobenzenes. The magnitude of the TS-coupling
constant (TSJFF) ranged from 2.2–5.9 Hz. Using empirical formulas reported in the literature, these coupling constants correspond
to non-bonded F–F distances (dFF) of 3.0–3.5 Å. These non-bonded distances are significantly smaller than those determined
by X-ray crystallography or density functional theory, which argues that simple models of 19F–19F TS spin–spin coupling solely
1
based dFF are not applicable. H, 13C and 19F data are reported for both the E and Z isomers of ten fluorinated azobenzenes.
Density functional theory [B3YLP/6-311++G(d,p)] was used to calculate 19F chemical shifts, and the calculated values deviated
0.3–10.0ppm compared with experimental values. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: NMR; 19F; 1H; 13C; azobenzene; through-space coupling; fluorinated azobenzene; cis-azoenzene
photoisomerization of p,p′-disubstituted tetrafluoroazobenzene; we
Introduction
adopted their synthesis for compound 10. Compounds 9 and 10
have been previously reported.[11] As prepared, the configuration of
Azobenzene (AB) is arguably the most studied organic chromo-
phore in chemistry.[1] AB exists as two geometric isomers
these compounds is principally the more stable E isomer, which
(Scheme 1) with the E conformation 50 kJ/mol more stable than
the Z isomer. Excitation wavelengths for AB photoisomerization
are dependent on the substituents, but generally, irradiation with
320–380 nm promotes E to Z isomerization, and the process is re-
versed with λ ~400–450-nm exposure or thermally by placing the
sample in the dark.[2] Z-AB adopts a bent conformation with the
phenyl groups twisted 53° out of the plane of the azo group (this
corresponds to the C2C1NN′ dihedral angle in Table 1).[3] There are
two pathways for isomerization: (i) in-plane inversion centered at
one azo nitrogen and (ii) out-of-plane rotation about the N=N
bond.[4] Depending on substituents and experimental conditions,
either pathway can be operative. The isomerization occurs with lit-
tle photodegradation; the increased dipole moment and smaller
molecular cross section of Z-AB have been exploited by using AB
compounds as local environmental probes and stimuli in hybrid
materials.[5] The focus of this report is the structure of the Z isomer
and not the photoisomerization process. We have observed that
through-space (TS) 19F–19F coupling in ortho-fluoro Z-ABs, which
we argue, provides an additional probe of conformation of Z-AB
in hybrid systems through the use of 19F NMR.[6]
can be converted to a mixture of Z and E within minutes using UV
irradiation. The thermal Z to E isomerization is sufficiently slow so that
isomer separation is possible by flash chromatography (Rf-Z> Rf-E
for hydrocarbon mobile phase and silica absorbant).
NMR studies
The geometric isomers of 1–11 exhibit distinct 1H, 13C and 19F NMR
spectra.[12] Table 2 lists the 19F chemical shifts for 1–11, and the
data fall within a relatively narrow range chemical shift range. The
fluorines on the methoxy-substituted ring are designated FA. The
19F chemical shifts are slightly solvent dependent with ~2-ppm
shifts to higher frequency in toluene-d8 compared with CDCl3. Nu-
clei in the Z isomer are more shielded in the 1H and 13C NMR except
for the 13C shifts of the ipso ring. Steric deshielding has been previ-
ously reported for ABs[13] and related systems,[14] where there is
close proximity of two carbon atoms. Based on the X-ray analysis
of our systems, the ipso carbons are the only atoms having non-
bonded distances less than the sum of the van der Waals radii.
The X-ray distance between ipso carbons ranges from 2.71–3.12Å
Results and discussion
*
Correspondence to: William Brittain, Department of Chemistry and Biochemistry,
Texas State University, 601 University Drive, San Marcos, TX 78666, United States.
E-mail: wb20@txstate.edu
Synthesis
Azobenzene compounds 1–8 (Scheme 1) were prepared via the dia-
zonium salt intermediate[7] followed by iodomethane reaction with
phenol. Compound 1 has been previously reported.[8] Compounds
9 and 11 were prepared from 2-nitrosobenzene derivatives and the
corresponding aniline.[9] Belger et al.[10] reported the synthesis and
a Department of Chemistry and Biochemistry, Texas State University, 601 University
Drive, San Marcos, TX, 78666, United States
b Department of Chemistry, The University of Akron, Akron, OH, 44325, United
States
Magn. Reson. Chem. 2016, 54, 126–131
Copyright © 2015 John Wiley & Sons, Ltd.

Through-space 19F–19F spin–spin coupling in ortho-fluoro Z-azobenzene
provided in Table S1. The difference between calculated and ob-
served 19F chemical shifts ranged from 0.3 to 10.0 ppm. There was
no correlation between calculated and experimental 19F chemical
shifts, and the difference between theory and experiment exceeds
the observed chemical shift differences between Z and E isomers
or the differences due to fluorine substitution patterns. DFT has
shown more predictive value for 19F chemical shifts in rigid
systems.[17,19] Contreras[24] and Ono[25] reported similar multi-ppm
differences between calculated and experimental 19F chemical shifts
in aliphatic fluorocarbons. Further refinement of DFT calculations is
outside the scope of this paper.
Scheme 1. Cis–trans isomerization of ortho-substituted azobenzene.
Table 1. Calculated and X-ray structural parameters for Z8
Through-space coupling
For all E isomers, peaks appeared as singlets, and there was no ev-
idence of TS coupling (TSJFF). For compounds with F on both rings
(5–8, 11), TS coupling is observed in the Z isomer. The 19F{1H}
spectra of 1-(2,6-difluoro-4-methoxyphenyl)-2-(2,6-difluorophenyl)
diazene (8) and 1-(2,6-difluorophenyl)-2-(2-fluorophenyl)diazene
(11) are shown in Fig. 1. Z8 exhibits two triplets with TSJFF = 4.6Hz,
and Z11 exhibits a doublet and triplet with TSJFF = 5.9 Hz. Splitting
in Z8 and Z11 was not affected by solvent (CDCl3, toluene-d8 and
CD3CN). Normally, coupling constants are not strongly solvent de-
pendent, although Mele et al.[26] reported a JFF solvent dependence
for fluoronucleosides, which was related to hydrogen-bonding abil-
ity of solvent. Intrinsic coupling constants are insensitive to solvent,
and changes in observed values can be largely attributed to
changes in conformational equilibria.[27] The 19F homonuclear
J-resolved NMR spectrum and 19F–19F COSY spectrum of Z8 are
shown in Fig. 2a and b and are consistent with FA–FB coupling.
X-ray
DFTa
%Deviation
R(N,N′)
1.257
1.435
1.418
58.55
58.45
10.25
3.70
1.243
1.421
1.426
51.80
56.00
10.7
1.1
1.0
R(N,C1)
R(N′C1′)
C2C1NN′
C2′C1′N′N
C1NN′C1′
0.6
11.5
4.2
4.4
b
dFF
3.70
0.1
4.72
4.93
7.9
R, bond length; Å, dihedral angles in degrees; DFT, density functional
theory.
DFT method – B3YLP/6-311++G(d,p), gas phase.
aValues are for optimized ground-state geometry.
bMolecule exhibits two unique distances for intramolecular F–F.
19
Table 2. F chemical shifts, ppma
Compound
FA
FB
Compound
FA
FB
E2
ꢀ121.68
ꢀ116.86
—
—
E3
—
—
ꢀ125.48
ꢀ122.72
Z2
E4
—
Z3
ꢀ122.68
ꢀ120.41
E5
ꢀ121.31 ꢀ125.30
ꢀ118.10 ꢀ122.26
ꢀ121.27 ꢀ122.80
ꢀ119.62 ꢀ120.82
Z4
E6
—
Z5
ꢀ117.59 ꢀ124.79
ꢀ117.77 ꢀ122.69
ꢀ117.63 ꢀ122.53
ꢀ117.77 ꢀ119.95
E7
Z6
E8
Z7
E9
ꢀ121.70
ꢀ118.16
—
—
Z8
E10
Z10
Z9
ꢀ124.57
ꢀ122.03
—
—
E11b
Z11
ꢀ121.73 ꢀ124.10
ꢀ119.75 ꢀ122.89
aExperimental δ relative to CFCl3 internal standard (δ = 0.00).
bFor compound 11, FA corresponds to ring with two Fs.
compared with a 3.40-Å sum of van der Waals radii. The 19F chem-
ical shifts for the Z isomer appear upfield relative to E isomer except
for the FA fluorines in Z8.
We evaluated density functional theory (DFT) as a tool to interpret
chemical shift differences between the E and Z isomers. We used the
6-311++G(d,p)[15–18] and cc-pVTZ[19] basis sets and continuous set of
gauge independent atomic orbitals (GIAO) for NMR calculations.[20,21]
We note that application of DFT to conjugated, electron-rich systems
is not without controversy. Klug and Burcl[22] published DFT results
for AB photoisomerization, which were subsequently challenged by
Meier on the basis that electron correlation was not fully included.[23]
A comparison of calculated with experimental 19F chemical shifts is
Figure 1. 19F{1H} NMR spectrum in CDCl3 at 28 °C for E8 and Z8 (a) and E11
and Z11 (b).
Magn. Reson. Chem. 2016, 54, 126–131
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

S. K. Rastogi et al.
coupling observed in the Z isomer is not a through-bond coupling;
this statement is qualified by recognition that π-conjugation can
produce larger contributions from the spin dipole term in
Ramsey’s[28] isotropic contributions to the spin–spin coupling
constant (SSCC).[29] Based on the dihedral angle between the azo
linkage and the phenyl groups, we estimate that only 38% effective
p conjugation exists in Z isomers. This difference in conjugation
may make comparison of E with Z TS coupling less valid.
For a non-bonded F–F distance (dFF) ≤ 3.20 Å and for fluorines
separated by four or more bonds, coupling is mediated primarily
by ‘TS’ coupling.[15,30–32] For compounds 5,8–11, the fluorines
are separated by seven bonds, and most F–F distances exceed
3.20 Å. It is generally understood that ‘TS’ coupling occurs through
a Fermi contact mechanism, but the key orbitals involved in TS
coupling are still not known with confidence. In a study of
thirteen 1,8-difluoronaphthalene compounds, Mallory et al.[15]
reported an empirical equation for dFF (pm) versus the SSCC:
À
Á
1:703 ꢁ 107 eꢀð4:96d . Similarly, Ernst and Ibrom[33]
FFÞ
JFF
¼
studied TS coupling in rigid cyclophanes and reported a similar
TS
¼ ð275000Þeꢀð0:0321d Þ . Mallory
FF
exponential relationship –
J
FF
based his analysis on overlapping in-plane lp(π) orbitals on fluorine
to explain the distance dependence of SSCC. Mallory’s and Ernst’s
empirical formulas have two deficiencies: (i) The empirical correla-
tion was based on organofluorine compounds with either rigid or
structurally constrained systems, and 2) the relative orientation of
C–F bonds is not included. The limitations of these empirical formu-
las are recognized.[34] Based on computations, Tuttle et al.[29] con-
cluded that lp(σ) and lp(σ)–lp(σ) contributions are larger than lp(π)
contributions and that TS coupling is maximized for a head-to-head
orientation of F atoms. Contreras et al.[35] reported that the Fermi
contact term is the largest contributor to JFF SSCCs; the second larg-
est is the paramagnetic spin–orbit term. The paramagnetic spin–
orbit term shows an unsymmetric second rank tensor characterized
by the ‘geometric effect’. The literature does not have sufficient
guidance on proper inclusion of the ‘geometric effect’ in our data
analysis. Because we have observed a range of C-F–F-C dihedral
angles, we expect the geometry effect between fluorines to be
important, and we regard our conclusions as qualitative.
Figure 2. 2D NMR spectroscopy of E8 and Z8: 19F–19F COSY (a) and 2D J-
resolved (b); asterisk (*) denotes unidentified impurities.
Table 3 contains the experimentally observed TS coupling values,
and they lie within a narrow range except for Z5 whose lower value
is a consequence of conformations having substantially different
dFF. Multiplicity followed the (n +1) rule for TS 19F–19F coupling.
No splitting was observed for E isomers, which argues that the
Structural analysis
Table 3. Comparison of through-space (TSJFF) versus non-bonded dis-
tance for Z–AB compoundsa
Assessment of the SSCC–dFF empirical relationships requires
independent determination of dFF. We have measured this intramo-
lecular distance by both X-ray crystallography and DFT. In solution-
state studies of non-bonded interactions like TS coupling, DFT data
are preferred because X-ray interatomic distances can be distorted
by crystal packing forces.[6,33] Our X-ray data are limited to three Z
structures (Z7–9) because Z isomers typically form oils and suitable
X-ray crystals were difficult to obtain. Figure 3 depicts the crystal
structures of Z7 and Z8. The parent Z-AB (Z-1,2-phenyldiazene) is
dFF, Å
Compound
7JFFd (Hz)
Calculated from
TS
Exp.
Malloryb
J
FF
X-ray
DFT
Ernstc
Z5
2.2 (d, d)
4.4 (d, t)
4.1 (t, d)
4.6 (t, t)
5.9 (d, t)
nd
nd
4.30
4.82
4.37
4.31
4.10
3.20
3.06
3.07
3.03
2.99
3.65
3.44
3.46
3.40
3.35
Z6
Z7
3.47
4.22
nd
Z8
Z11
DFT, density functional theory; nd, not done.
aValues are the average F–F distance.
bMallory et al.[15]
cErnst and Ibrom[33]
dMultiplicity given in parentheses where the first letter corresponds to
peak at higher frequency.
Figure 3. X-ray structures of Z8 (a) and Z7 (b).
wileyonlinelibrary.com/journal/mrc
Copyright © 2015 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2016, 54, 126–131

Through-space 19F–19F spin–spin coupling in ortho-fluoro Z-azobenzene
distorted from planarity with a C-N=N-C dihedral angle of 8° and
phenyl ring torsion of 53.3.[3] The crystal structures of Z8 and Z7
show similar distortions. The data in Tables 1, S1 and S2 show that
compared with X-ray, DFT underestimates the torsion angle of the
aromatic ring to azo group (C2-C1-N=N) and dFF. The unsubstituted
phenyl ring exhibits a higher torsion angle, which is due to
para-methoxy rather than ortho-fluorine substitution. The DFT-
minimized structure of Z-1-(4-methoxyphenyl)-2-phenyldiazene
exhibits a 43° torsion for the substituted aromatic ring. Introduction
of fluorine into the ortho positions does not significantly alter twist
angles, which is consistent with the work by Mazzanti et al.[36] on
biphenyl rotational barriers that indicated that the steric size of
fluorine is similar to hydrogen.
containing materials near the coil in order to avoid the interference
from the background fluorine signals that usually exist in the stan-
dard probes. The high-frequency channel on this probe is doubly
tuned to 1H and 19F to produce short 90° pulse widths needed to
excite large fluorine spectral windows. A duplexer with low inser-
tion loss provides the capability of combining the signals from
1
the H and 19F rf channels and directing them to the dual-tuned
1H/19F high-frequency channel of the probe. The returning 1H and
19F signals from the probe can be separated by this duplexer again
and the desired signal (1H or 19F) directed to the receiver. The sam-
ple was prepared by dissolving approximately 10 mg of material in
a solvent mixture of CDCl3/CH2Cl2/CCl4 (27/60/13) (v/v/v) with a
trace of CFCl3 added as a chemical shift reference. The 19F 1D
NMR spectra were collected with a 5.2-kHz spectra window, 1.0-s
relaxation delays, 2.0-s acquisition time and 3.5-μs (30°) pulse width;
64 transients were averaged with continuous 1H decoupling using
WALTZ-16 modulation (γHBH/2π = 2.7kHz). The data were zero filled
to 256k and exponentially weighted with a line broadening of
0.5 Hz before FT.
Conclusions
We have observed TS 19F–19F coupling in Z-AB and supported our
TS assignment using 19F–19F COSY and 2D J-resolved NMR spec-
troscopy. The empirical equations of Mallory and Ernst predict
much shorter F–F distances when applied to our experimental
values of TS coupling. DFT and X-ray structures indicate F–F dis-
tances (dFF) greater than 3.47 Å, which would not be expected to
show TS coupling. We did not include the geometrical relationship
between interacting C–F groups and are currently developing more
inclusive formulas for dFF versus SSCC. Cormanich et al.[37] found
that the calculated C-F–F-C distances are very sensitive to the
choice of DFT basis set and this is also another current area of inves-
tigation. Attempts to use DFT for NMR calculations were unsuccess-
ful with the deviation between theory and experiment in excess of
changes due to geometrical isomerism or fluorine substitution
pattern.
Computational details
Density functional theory[39] calculations were performed with the
6-311++G(d,p) basis set and the restricted B3YLP functional and
cc-pVTZ basis set implemented in Gaussian 09.[40] Geometry
optimizations, frequency calculations and NMR calculations were car-
ried without solvent. Continuous set of gauge transformations[20,21]
method was used for NMR calculations and isotropic values con-
verted to ppm using the equation: 19F δ: δcalc =(ꢀ0.914* δisotropic
)
+ 142.63. T. Lectka, private communication.
General synthesis
Synthesis of 1–8 followed the procedure of Siewierski et al.[6] where
the diazonium salt was formed under acidic conditions and reacted
with a basic aqueous solution of the phenol. The crude product was
then treated with 2-equivalent iodomethane in acetonitrile with 2-
equivalent K2CO3 to afford crude product. Compounds 9 and 11
synthesis followed the procedure of Dong et al.,[9] and synthesis
of 10 followed the report of Belger et al.[10] All compounds were
purified by flash chromatography using mixtures of toluene and
hexane. Z and E isomers were assigned by comparison of spectra
for E versus E/Z mixture. For the description of general synthesis
methods, see the study of Merino.[41] Spectra are provided in
Supplementary Information.
Experimental
General information
Unless otherwise noted, all reagents were obtained from commer-
cial sources and used without purification. Flash chromatography
used Agela 60-Å gel, and TLC was performed using EMD Millipore
(Billerica, MA, USA) silica plates with fluorescent indicator. Absorp-
tion measurements were performed using either a Cary 100 Bio
UV–vis spectrometer or an Ocean Optics (Dunedin, FL, USA) com-
ponent system with a HR2000+ detector, qpodTM sample holder,
DH 2000 source and Spectra Suite software for data collection.
Solutions were degassed via three freeze–pump–thaw cycles.
Photochemical kinetic experiments used Oriel (Newport Corpora-
tion, Irvine, CA, USA) 66901 50–200-W Research Arc Lamp; the output
was cooled with a water filter and passed through a bandpass filter
(350-nm center, 70-nm FWHM). X-ray crystallography was performed
on a Rigaku (Rigaku Americas, The Woodlands, TX, USA) SCX-Mini
Single Crystal X-ray Diffractometer.[38]
1-(2-Fluoro-4-methoxyphenyl)-2-phenyldiazene E2. 1H NMR (400.13 MHz,
CDCl3, δ) 3.87 (s, 3H, OCH3), 6.77 (m, 1H), 6.79 (m, 1H), 7.5 (m, 3H),
7.83 (m, 1H), 7.95 (m, 2H). 13C NMR (100.61 MHz, CDCl3, δ) 56.2
(Me3O), 102.5 (C3), 111.0 (C5), 1181.8 (C6), 123.1 (C8, 12), 129.4
(C9, C11), 131.1 (C10), 135.3 (C1), 161.9 (C2), 153.4 (C7), 163.7 (C4).
19F NMR (376.50 MHz, CDCl3, δ) ꢀ121.68. UV (hexane) λmax, nm
1
(ε): 347 (19 000). Z2: H NMR (400.13 MHz, CDCl3, δ) 3.75 (s, 3H,
Routine NMR spectra were obtained using a Bruker (Billerica, MA,
USA) Advance III 400 spectrometer. 1H and 13C NMR shifts are
reported relative to TMS, and 19F shifts are relative to CFCl3
(0.00ppm). All 19F spectra were collected with broadband proton
decoupling.
OCH3), 6.49 (m, 1H), 6.52 (m, 1H), 6.77 (m, 1H), 6.90 (m, 2H), 7.18
(m, 1H), 7.27 (m, 2H). 13C NMR (100.61 MHz, CDCl3, δ) 55.8 (Me3O),
102.3 (C3), 110.1 (C5), 122.5 (C6), 123.1 (C8, 12), 128.1 (C9, C11),
129.1 (C10), 135.5 (C1), 152.4 (C2), 154.5 (C7), 161.1 (C4). 19F NMR
(376.50MHz, CDCl3, δ) ꢀ116.86.
19F homonuclear 2D J NMR spectra were collected from a Varian
(Palo Alto, CA, USA) Direct Drive 500-Mhz spectrometer equipped
with five broad band rf channels and a 5-mm 1H/19F/13C triple
resonance pulse field gradient probe. This probe was made special
for detecting fluoro-materials, because there are no fluorine-
1-(2-Fluorophenyl)-2-(4-methoxyphenyl)diazene E3. 1H NMR (400.13 MHz,
CDCl3, δ) 3.89 (s, 3H, OCH3), 7.02 (m, 2H), 7.22 (m, 2H), 7.40 (m,
1H), 7.76 (m, 1H), 7.97 (m, 2H). 13C NMR (100.61 MHz, CDCl3, δ)
55.9 (Me3O), 114.6 (C2, C6), 117.2 (C9), 118.1 (C12), 124.5 (C11),
Magn. Reson. Chem. 2016, 54, 126–131
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

S. K. Rastogi et al.
1
125.5 (C2, C6), 132.0 (C10), 141.1 (C7), 147.6 (C1), 160.1 (C8), 162.7
ꢀ117.63, ꢀ122.53. UV (hexane) λmax, nm (ε): 331 (14000). Z8: H
(C4). 19F NMR (376.50MHz, CDCl3, δ) ꢀ125.48. UV (hexane)
NMR (400.13 MHz, CDCl3, δ) 3.75 (s, 3H, OCH3), 6.48 (d, 2H), 6.96
(t, 2H), 7.29 (m, 1H). 13C NMR (100.61 MHz, CDCl3, δ) 56.3 (Me3O),
98.7 (C3, C5), 112.4 (C9, C11), 126.3 (C1), 129.8 (C10), 132.6 (C7),
152.2 (C8, C12), 153.3 (C2, C6), 161.4 (C4). 19F NMR (376.50 MHz,
CDCl3, δ) ꢀ117.77, ꢀ119.95.
λ
max, nm (ε): 336 (16000). Z2: 1H NMR (400.13 MHz, CDCl3, δ) 3.77
(s, 3H, OCH3), 6.77 (m, 2H), 6.96 (m, 2H), 6.98 (m, 2H), 7.09 (m, 1H).
13C NMR (100.61 MHz, CDCl3, δ) 55.7 (Me3O), 114.0 (C3, C5), 116.8
(C9), 122.2 (C12), 123.5 (C2, C6), 124.9 (C11), 128.7 (C10), 142.4
(C7), 147.4 (C1), 150.1 (C8), 160.0 (C4). 19F NMR (376.50 MHz, CDCl3,
δ) ꢀ122.72.
1-(2-Fluorophenyl)-2-phenyldiazene E9. 1H NMR (400.13MHz, CDCl3, δ)
7.23 (m, H5), 7.29 (m, H3), 7.45 (m, H4), 7.53 (m, H9, H11), 7.79 (m,
H6), 7.98 (m, H8, H12). 13C NMR (100.61 MHz, CDCl3, δ) 117.3 (C3),
118.0 (C6), 123.4 (C8, C12), 124.5 (C5), 129.3 (C9, C11), 131.7 (C10),
132.7 (C4), 140.9 (C1), 153.0 (C7), 161.5 (C2). 19F NMR (376.50 MHz,
CDCl3, δ) ꢀ121.70. Z9: 1H NMR (400.13 MHz, CDCl3, δ) 6.88 (m, H5),
6.90 (m, H3), 6.95 (m, H9, H11), 7.04 (m, H6), 7.11 (m, H4), 7.24 (m,
H10), 7.27 (m, H8, H12). 13C NMR (100.61 MHz, CDCl3, δ) 116.5 (C3),
120.1 (C9, C11), 122.0 (C5), 124.5 (C6), 128.3 (C10), 128.9 (C8, C12),
129.0 (C4), 142.0 (C1), 149.1 (C7), 152.9 (C2). 19F (376.50 MHz, CDCl3,
δ) ꢀ118.16.
1-(2,6-Difluorophenyl)-2-(4-methoxyphenyl)diazene E4. 1H NMR (400.13 MHz,
CDCl3, δ) 3. 90 (s, 3H, OCH3), 7.01 (d, 4H), 7.27 (m, 1H), 7.94 (d, 2H).
13C NMR (100.61 MHz, CDCl3, δ) 56.0 (Me3O), 112.7 (C9, C11), 114.6
(C3, C5), 125.4 (C2, C6), 129.7 (C10), 131.9 (C7), 148.1 (C1), 156.0
(C8, C12), 163.1 (C4). 19F NMR (376.50 MHz, CDCl3, δ) ꢀ122.68.
1
UV (hexane) λmax, nm (ε): 311 (22 000). Z4: H NMR (400.13 MHz,
CDCl3, δ) 3.77 (s, 3H, OCH3), 6.80 (d, 4H), 6.90 (d, 2H), 7.13 (m,
1H). 13C NMR (100.61 MHz, CDCl3, δ) 55.7 (Me3O), 112.3 (C9, C11),
114.1 (C3, C5), 122.5 (C2, C6), 128.4 (C10), 131.8 (C7), 148.3 (C1),
151.7 (C8, C12), 160.5 (C4). 19F NMR (376.50 MHz, CDCl3, δ)
ꢀ120.41.
1,2-bis(2-Fluorophenyl)diazene E10. 1H NMR (400.13 MHz, CDCl3, δ)
7.05 (t, H3, H5), 7.31 (m, H10), 7.54 (m, H4, H9, H11), 7.95 (m, H8,
H12). 13C NMR (100.61 MHz, CDCl3, δ) 112.8 (C3, C5), 123.3 (C8,
C12), 129.4 (C9, C11), 130.1 (C10), 131.7 (C1), 132.2 (C4), 153.5 (C7),
156.0 (C2, C6). 19F NMR (376.50 MHz, CDCl3, ꢀ124.57. Z10: 1H
NMR (400.13 MHz, CDCl3, δ) 6.78 (m, H3, H5), 6.92 (d, H8, H12),
7.08 (m, H4), 7.20 (m, H10), 7.25 (m, H9, H11). 13C NMR
(100.61MHz, CDCl3, δ) 112.0 (C3, C5), 119.0 (C8, C12), 128.8 (C4),
128.9 (C10), 131.6 (C1), 151.1 (C2, C6), 155.2 (C7). 19F NMR
(376.50MHz, CDCl3, δ) ꢀ122.03.
1-(2-Fluoro-4-methoxyphenyl)-2-(2-fluorophenyl)diazene E5. 1H NMR
(400.13 MHz, CDCl3, δ) 3.88 (s, 3H, OCH3), 6.76 (m, 2H), 7.22 (m,
2H), 7.42 (m, 1H), 7.77 (t, 1H), 7.84 (t, 1H). 13C NMR (100.61 MHz,
CDCl3, δ) 56.0 (Me3O), 102.3 (C3), 111.3 (C4), 117.3 (C9, C11),
118.6 (C6), 119.2 (C12), 132.5 (C10), 132.5 (C10), 135.7 (C7), 141.3
(C1), 160.4 (C2), 162.2 (C8), 164.2 (C4). 19F NMR (376.50 MHz, CDCl3,
δ) ꢀ121.31, ꢀ125.30. UV (hexane) λmax, nm (ε): 335 (14 000). Z5: 1H
NMR (400.13 MHz, CDCl3, δ) 3.77 (s, 3H, OCH3), 6.53 (2H), 6.80 (1H),
6.98 (2H), 7.08 (1H), 7.15 (1H). 13C NMR (100.61 MHz, CDCl3, δ) 56.0
(Me3O), 102.4 (C3), 110.5 (C5), 116.7 (C9), 121.8 (C11), 121.9 (C12),
122.7 (C6), 129.4 (C10), 135.8 (C7), 142.6 (C1), 150.5 (C8), 152.8 (C2),
161.1 (C4). 19F NMR (376.50 MHz, CDCl3, δ) ꢀ118.10, ꢀ122.26.
1-(2,6-Difluorophenyl)-2-(2-fluorophenyl)diazene E11. 1H NMR (400.13 MHz,
CDCl3, δ) 7.19 (t, 2H), 7.40 (m, 2H), 7.51 (m, 1H), 7.64 (m, 1H), 7.71 (m,
1H). 13C NMR (100.61 MHz, CDCl3, δ) 113.2 (C3, C5), 113.9 (C9), 121.6
(C11, C12), 125.8 (C1), 132.9 (C4), 142.0 (C7), 156.6 (C2, C6), 161.0 (C8).
19F NMR (376.50 MHz, CDCl3, δ) ꢀ121.73, ꢀ124.10. Z11: H NMR
1
1-(2,6-Difluoro-4-methoxyphenyl)-2-(2-fluorophenyl)diazene E6. 1H NMR
(400.13 MHz, CDCl3, δ) 3.89 (s, 3H, OCH3), 6.62 (d, 2H), 7.27 (m,
1H), 7.47 (m, 2H), 7.72 (t, 1H). 13C NMR (100.61 MHz, CDCl3, δ)
56.4 (Me3O), 99.2 (C3, C5), 117.3 (C9), 117.9 (C11), 124.6 (C12),
126.0 (C1), 134.0 (C10), 141.9 (C7), 158.0 (C2, C6), 160.2 (C8),
162.4 (C4). 19F NMR (376.50 MHz, CDCl3, δ) ꢀ117.59, ꢀ124.79.
(400.13 MHz, CDCl3, δ) 6.97 (t, 2H), 7.15 (m, 2H), 7.31 (m, 3H). 13C
(100.61 MHz, CDCl3, δ) 113.1 (C3, C5), 113.7 (C9), 121.6 (C11, C12),
125.6 (C1), 131.7 (C4), 132.9 (C10), 143.4 (C7), 152.0 (C2, C6), 152.2
(C8). 19F (376.50 MHz, CDCl3, δ) ꢀ119.75, ꢀ122.89.
1
Acknowledgements
UV (hexane) λmax, nm (ε): 335 (14 000). Z6: H NMR (400.13 MHz,
CDCl3, δ) 3.77 (s, 3H, OCH3), 6.36 (d, 2H), 7.01 (m, 2H), 7.10 (t, 1H),
7.20 (m, 1H). 13C NMR (100.61 MHz, CDCl3, δ) 54.2 (Me3O), 98.5
(C3, C5), 116.8 (C9), 117.6 (C11), 124.5 (C12), 125.6 (C1), 130.0
(C10), 143.1 (C7), 152.6 (C2, C6), 151.0 (C8), 160.9 (C4). 19F NMR
(376.50 MHz, CDCl3, δ) ꢀ117.77, ꢀ122.69.
W. J. B. thanks T. Lectka for the helpful advice and the National
Science Foundation (PREM Center for Interfaces, DMR-1205670)
and the ACS Petroleum Research Fund (51997UR7) for the financial
support of this research. The authors also acknowledge the support
of the National Science Foundation for X-ray and NMR instrumenta-
tion (CR1IF:MU-0946998 and MRI-0821254) and the Welch Founda-
tion (AI-0045).
1-(2,6-Difluorophenyl)-2-(2-fluoro-4-methoxyphenyl)diazene E7. 1H NMR
(400.13 MHz, CDCl3, δ) 3.89 (s, 3H, OCH3), 6.75 (m, 1H), 6.77 (m,
1H), 7.02 (m, 2H), 7.29 (t, 1H), 7.78 (t, 1H). 13C NMR (100.61 MHz,
CDCl3, δ) 56.3 (Me3O), 102.2 (C3), 111.4 (C5), 112.7 (C9, C11),
118.7 (C6), 131.9 (C20), 132.2 (C7), 136.1 (C1), 156.2 (C8, C12),
162.3 (C2), 164.7 (C4). 19F NMR (376.50 MHz, CDCl3, δ) ꢀ121.27,
ꢀ122.80. UV (hexane) λmax, nm (ε): 335 (14 000). Z7: 1H NMR
(400.13 MHz, CDCl3, δ) 3.77 (s, 3H, OCH3), 6.56 (m, 2H), 6.85 (m,
2H), 6.90 (m, 1H), 7.20 (m, 1H). 13C NMR (100.61 MHz, CDCl3, δ)
56.1 (Me3O), 102.5 (C3), 110.2 (C5), 112.3 (C9, C11), 122.6 (C6),
1232.1 (C10), 132.1 (C7), 136.45 (C1), 151.7 (C8, C12), 153.2 (C2),
161.4 (C4). 19F NMR (376.50 MHz, CDCl3, δ) ꢀ119.62, ꢀ120.82.
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