Structural identification of a novel axially chiral binaphthyl fluorene based salen ligand in solution using electronic circular dichroism: A theoretical-experimental analysis

Article abstract of DOI:10.1021/jp2112507

A novel axially chiral binaphthyl fluorene based salen ligand, AFX-155 [2,2′-(1E,1′E)-(R)-1,1′-binaphthyl-2,2′-diylbis(azan-1- yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-((7-(diphenylamino)-9, 9-dihexyl-9H-fluoren-2-l)ethynyl)phenol)], with potential ap

Full text of DOI:10.1021/jp2112507

Article
pubs.acs.org/JPCA
Structural Identification of a Novel Axially Chiral Binaphthyl Fluorene
Based Salen Ligand in Solution Using Electronic Circular Dichroism: A
Theoretical−Experimental Analysis
Carlos Diaz,† Andrew Frazer,† Alma Morales,† Kevin D. Belfield,†,‡ Supriyo Ray,§
and Florencio E. Hernandez*,†,‡
́
†Department of Chemistry, University of Central Florida, P.O. Box 162366, Orlando, Florida 32816-2366, United States
‡The College of Optics and Photonics, CREOL, University of Central Florida, P.O. Box 162366, Orlando, Florida 32816-2366,
United States
§Department of Physics, University of Central Florida, P.O. Box 162385, Orlando, Florida 32816-2385, United States
S
* Supporting Information
ABSTRACT: A novel axially chiral binaphthyl fluorene based
salen ligand, AFX-155 [2,2′-(1E,1′E)-(R)-1,1′-binaphthyl-2,2′-
diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-
((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-l)ethynyl)-
phenol)], with potential applications in homogeneous
catalysis, biophotonics, and sensing was synthesized. A full
comparative theoretical−experimental analysis of the UV−vis
and electronic circular dichroism (ECD) spectra of the 10
primary isomers, comprising stereoisomers and optical
isomers, revealed the presence of the unique structure in
tetrahydrofuran (THF) solution, the trans-R-intra//trans-R-extra. A proposed route of attack of the (R)-(+)-2,2′-diamino-1,1′-
binapthalene onto a salicaldehyde 5-(2-(2-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)ethynyl)-2-hydroxybenzaldehyde
followed by a consecutive attack of the resulting species onto another salicaldehyde, both via Burgi:Dunitz trajectory, validates
the unambiguous formation of the established isomer. Steric hindrances seem to be the determinant factor that defines the 3D
structural conformation of this particular stereoisomer of AFX-155 with triple axial chirality.
The determination of every optimal structure and the dominant conformers of AFX-155 were calculated evaluating, in
CONFLEX, their steric energies using force fields at MMFF94S (2006-11-24HGTEMP) level in gas phase. The geometry of the
conformers was optimized in THF (using PCM) using Gaussian 09 at the DFT/B3LYP level of theory and 6-31G* basis set. The
first 100 electronic excited states were calculated using the same level of theory and basis set.
Although a large number of ligand structures have been
studied during recent decades,8−13 there is still a relative
modest understanding about salen-type ligands with multiple
sources of chirality into the ligand backbone to enhance the
reaction possibility and selectivity of these catalysts.14,15
Furthermore, the challenges involved in the synthesis of
helical-type salen complexes have not permitted the study of
twisted structures. In this respect, Wiznycia et al. have recently
reported on the synthesis and structural characterization of the
first monohelical salen complexes with predetermined handed-
ness.16−18 These authors were able to produce, for the very first
time, a multidentate ligand by the condensation of (R)-(+)-1,1-
binaphthyl-2,2-diamine with 1,1-binaphthyl-2,2-diamine in
ethanol. Using X-ray crystallography, Wiznycia et al. accurately
determined the helicity as well as the handedness of a crystal of
the monohelical complex of FeII and ZnII and demonstrated
INTRODUCTION
■
The discovery of privileged chiral ligands such as (R)-BINAP,
(R)-BINOL, and (R,R)-salen, which exhibit high selectivity
over a broad range of reactions,1−4 has enabled the develop-
ment of practical applications for catalytic asymmetric processes
in fundamental and applied research as well as in industry.5,6
The main advantage of using the first two ligands resides on
their extraordinary selectivity imposed by the atropisomerism
on the C2 symmetry framework with axial chirality, which limits
the racemization. The major benefit of using the latter in
asymmetric reactions is the typical high yield synthetic
accessibility of enantiopure derivatives by the condensation of
salicylic aldehydes with diamines of definite chirality. The
precise catalyst steric and electronic property of specifically
designed chelating ligands that bridges metal ions with two
nitrogen and two oxygen atoms (salen ligands) for a broad
range of catalytic asymmetric reactions and with important
applications in the pharmaceutical and chemical industries7
strongly depends on their structure.
Received: November 22, 2011
Revised: February 7, 2012
Published: February 13, 2012
© 2012 American Chemical Society
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Scheme 1. Steps for the Synthesis of AFX-147, AXF-155, and AXF-169
that the 1,1-binaphthyl backbone is an effective helix-forming
unit. This unique work shows that the synthesis and structural
study of new salen ligands with multiple chirality and helicity is
in great need to truly understand their full ability in catalytic
asymmetric processes important in industry. Exploring further
the potential of using 1,1-binaphthyl backbone in the synthesis
of these specific ligands as well as determining their 3D
structural conformation in solution is in great demand.
SYNTHESIS OF AFX-155
■
We synthesized a new chiral chromophore (see Scheme 1) of
the D−π−A−π−D type, where A represents an electron-
withdrawing moiety while −π− represents a conjugated π−
electron system. D symbolizes a well-known donor group, a
fluorene moiety attached to a diphenylamino group. On the
opposite arm of the fluorene, in the 2 position, there is an
alkyne bond connected to a phenyl ring. The fluorene hexyl
groups in position 9 augment the solubility in hydrophobic
environments.
A fluorene derivative was specifically chosen for this synthesis
because of its extended π−electron delocalization that confers
(a) high nonlinear absorption cross section,20−25 (b) rigid
fluorene core that grants exceptional thermal- and photo-
stability,26−28 (c) high fluorescence quantum yield (>0.5)
required for bioimaging,23 (d) very low cytotoxicity necessary
for practical biomedical applications,28,29 and (e) a typical linear
absorption maxima between 350 and 450 nm exceptional for
multiphoton excitation.30−32
The key step for the synthesis of this fluorenyl probe, AFX-
155, was a Pd catalyzed Sonogashira coupling reaction (Scheme
1), which comprises the coupling of the previously described 5-
ethynyl-2-hydroxybenzaldehyde (3) with a 7-bromo-9,9-
dihexyl-N,N-diphenyl-9H-fluoren-2-amine (4) to yield the
salicaldehyde 5-(2-(2-(diphenylamino)-9,9-dihexyl-9H-fluoren-
7-yl)ethynyl)-2-hydroxybenzaldehyde (AXF-147). The con-
densation of the novel fluorene based salicaldehyde with the
R enantiomer of the binapthyl diamine (R)-(+)-2,2′-diamino-
1,1′-binapthalene resulted in the formation of this new
binapthyl based salen compound in good overall yield (63%).
Herein, we report on the structural characterization of a
recently synthesized axially chiral binaphthyl fluorene based
salen ligand, AFX-155 [2,2′-(1E,1′E)-(R)-1,1′-binaphthyl-2,2′-
diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-
((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-l)ethynyl)-
phenol)], with potential applications in homogeneous catalysis,
biophotonics, and sensing. The full comparative theoretical−
experimental analysis of the UV−vis and electronic circular
dichroism (ECD) spectra of the primary isomers, comprising
stereoisomers and optical isomers, revealed the presence of the
unique trans_R-intra//trans_R-extra. The analysis of the
theoretical−experimental results has permitted us to determine
the 3D structural conformation of AXF-155 in tetrahydrofuran
(THF) solution and to propose a preferential route of attack of
the (R)-(+)-2,2′-diamino-1,1′-binapthalene onto a salicaldehyde
5-(2-(2-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)ethynyl)-
2-hydroxybenzaldehyde followed by a consecutive attack of the
resulting species onto another salicaldehyde both via
Burgi:Dunitz trajectory.19 The substantiation of the important
steric hindrances, throughout the condensation phase, validates
the unambiguous formation of the particular stereoisomer of
AFX-155 with triple axial chirality and distorted helicity.
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All compounds were fully characterized by 1H NMR, 13C
NMR, and elemental analysis or HR-MS.
g, 63%), mp 112−113 °C. 1HNMR CDCl3 (500 MHz):
δ(ppm) = 12.37(s, 2H, O−H), 8.72(s, 2H, HCN), 8.16(d,
2H), 8.01(d, 2H), 7.70(d, 2H), 7.55−7.37(m, 20H), 7.26−
7.22(m, 6H), 7.12−7.20(m, 10H), 7.03−6.98(m, 6H), 6.72(d,
2H), 2.05(m, 8H), 1.26(m, 16H), 0.79(t, 8H). 13CNMR
CDCl3 (500 MHz): δ(ppm) = 158.21, 158.16, 149.75,
147.95,145.21, 144.81, 140.57, 140.39, 138.37, 133.22, 132.82,
132.61, 130.52, 129.99, 127.82, 127.52, 127.24, 126.49, 125.68,
124.50, 123.76, 123.51, 122.90, 121.21, 120.74, 117.96, 117.92,
116.56, 116.38, 116.30, 114.90, 113.84, 111.20, (86.56, 85.77)
C≡C, 52.38, 37.53, 28.84, 26.93, 21.04, 19.88, 11.35. MS (HR-
ESI) calculated for C112H106N4O2 (M + H+) 1538.83; found
(M + Na+) 1560.79.
Materials and Synthetic Procedures. The syntheses of 5-
ethynyl-2-hydroxybenzaldehyde (3) and 7-bromo-9,9-dihexyl-
N,N-diphenyl-9H-fluoren-2-amine (4) have been described
previously.33−35 Pd catalyzed reactions were performed in
high-pressure Schlenk tubes under N2 atmosphere. All reagents
and solvents were used as received from commercial suppliers
unless otherwise noted. 1H and 13C NMR spectroscopic
measurements were performed using a Varian 500 NMR
spectrometer at 500 MHz with tetramethysilane (TMS) as
internal reference: 1H (referenced to TMS at δ = 0.0 ppm) and
13C (referenced to CDCl3 at δ = 77.0 ppm). High-resolution
mass spectrometry (HR-MS) analysis was performed in the
Department of Chemistry, University of Florida, Gainesville,
FL.
Synthesis of (E)-4-((7-(Diphenylamino)-9,9-dihexyl-
9H-fluoren-2-yl)ethynyl)-2-((naphthalen-2-ylimino)-
methyl)phenol AXF-169. To a slurry of 5-((7-(diphenyla-
mino)-9,9-dihexyl-9H-fluoren-2-yl)ethynyl)-2-hydroxybenzal-
dehyde (AXF-147) (100 mg, 0.16 mmol) in ethanol (4.0 mL),
we slowly added at room temperature 3.0 mL of a solution of
naphthalen-2-amine (22 mgs, 0.16 mmol) in ethanol. After-
ward, the temperature of the reaction mixture was raised to 70
°C and was held constant for 18 h under Dean−Stark
conditions. After the solid was dissolved, a yellow solution
was formed. Then, the mixture was halted and was brought to
ambient temperature. A dark-orange powder was recovered and
was washed with hexane. The final product was dried in
vacuum and was characterized with no further purification. An
orange solid was finally isolated (yield 87 mgs, 73%). mp 71−
Synthesis of Salicaldehyde 5-(2-(2-(diphenylamino)-
9,9-dihexyl-9H-fluoren-7-yl)ethynyl)-2-hydroxybenzal-
dehyde AXF-147. To 30 mL of a solution of the 7-bromo-9,9-
dihexyl-N,N-diphenyl-9H-fluoren-2-amine (D) (1.24 g, 2.14
mmol) in THF:triethylamine (50:50), we added 15.0 mg of
CuI (0.08 mmol) and Pd(PPh3)2Cl2 (0.07 mmol). Then, 20
mL of a solution of 5-ethynyl-2-hydroxybenzaldehyde (0.48 g,
3.29 mmol) in THF was added to the mixture. The resultant
slurry was then heated at 80 °C for 8 h. Afterward, the reaction
mixture was allowed to cool to ambient temperature before
filtering it through Celite. The Celite was then washed twice
with 30 mL of THF. The combined filtrates had all volatile
residues removed under reduced vacuum. The recovered final
product was a red oil. The purification of this product was then
performed using a silica gel column as the stationary phase and
eluting with hexane and ethylacetate 1:9 (mobile phase). The
isolated pure orange oil was subject to constant vacuum over a
period of 24 h to yield an orange solid (yield 0.80 g, 58%), mp
1
73 °C. HNMR CDCl3 (500 MHz): δ(ppm) = 13.64(s, 1H,
O−H), 8.77(s, 1H, HCN), 7.93−7.86(m, 3H), 7.73−7.69
(m, 2H), 7.61−7.45 (m, 9H), 7.23−7.04(m, 12H), 1.88(m,
4H), 1.08(m, 12H), 0.80(t, 6H), 0.66(m, 4H). 13CNMR
CDCl3 (500 MHz): δ(ppm) = 158.21(NC), 149.75(C−
OH), 147.95, 145.21, 144.81, 140.57, 140.39, 138.37, 133.22,
132.82, 132.61, 130.52, 129.99, 127.82, 127.52, 127.24, 126.49,
125.68, 124.50, 123.76, 123.51, 122.90, 121.21, 120.74, 117.96,
117.92, 116.56, 116.38, 116.30, 113.84, 111.20, (86.56, 85.77)
C≡C, 52.38 (C9), 37.53, 28.84, 26.93, 21.04, 19.88, 11.35. MS
(HR-ESI) calculated for C56H54N2O (M + H+) 771.42; found
(M + H+) 771.42.
1
59−61 °C. HNMR CDCl3 (500 MHz): δ(ppm) = 11.14(s,
1H, O−H), 9.92(d, 1H, HCO), 7.81(d, 1H), 7.72(dd, 1H),
7.61−7.56(m, 2H), 7.50−7.45, 2H), 7.29−7.26(m, 6H), 7.15−
7.12(m, 5H), 7.05−7.01(m, 4H), 1.91−1.85(m, 4H), 1.17−
1.07(m, 12H), 0.82(t, 6H), 0.67(m, 4H). 13CNMR(500 MHz):
δ(ppm) = 196.35 (HCO), 161.52 (C−OH), 152.70, 150.98,
148.11, 147.91, 141.71, 140.00, 136.99, 135.56, 130.84, 129.44,
125.92, 124.20, 123.62, 122.92, 120.97, 120.79, 120.33, 119.29,
119.22, 118.38, 115.77, 90.44, 87.59, 55.34, 40.47, 31.77, 29.86,
23.98, 22.80, 14.27; MS (HR-ESI) calculated for C46H47NO2
(M + H+) 646.36; found (M + H+).
EXPERIMENTAL METHODS
■
All measurements were performed in THF solution. The linear
absorption spectra were taken using a single-beam spectropho-
tometer (Agilent 8453 Diode Array UV−vis) from 190 to 600
nm in a 0.1 cm quartz cell and at a concentration of 1.5 × 10−4
M. ECD spectra were recorded on a J-810 CD spectropo-
larimeter (Jasco Corp., Tokyo, Japan) under the following
conditions: concentration 1 × 10−3 M; temperature 25 °C;
quartz cuvette length 4 mm; wavelength range 190−600 nm; 1
nm step and scan speed 50 nm/min. Contributions from the
solvent and the quartz cells were subtracted from all the
spectra.
Synthesis of Chiral Fluorene Compound 2,2′-(1E,1′E)-
(R)-1,1′-Binaphthyl-2,2′-diylbis(azan-1-yl-1-ylidene)bis-
(methan-1-yl-1-ylidene)bis(4-((7-(diphenylamino)-9,9-
dihexyl-9H-fluoren-2-yl)ethynyl)phenol) AXF-155. To a
slurry of 5-((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)-
ethynyl)-2-hydroxybenzaldehyde (AXF-147) (200 mg, 0.31
mmol) in ethanol (7.0 mL), we added slowly and at room
temperature 5.0 mL of a solution of (R)-(+)-2,2′-diamino-1,1′-
binapthalene (44 mgs, 0.15 mmol) in ethanol. The reaction
mixture temperature was then increased to 70 °C and was kept
constant for 18 h using Dean−Stark conditions. A yellow
solution was recuperated after the solid was dissolved.
Subsequently, the reaction mixture was halted and was taken
back to ambient temperature. After filtering the precipitate, we
recovered a dark-orange powder. The purification of this final
product was performed using a silica gel column as the
stationary phase and was eluted with hexane: ethylacetate 2:1
(mobile phase). A bright orange solid was isolated (yield 0.112
THEORETICAL METHODS
■
Structures of all isomers were optimized at the Becke’s three
parameter exchange Lee, Yang, and Parr correlation
(B3LYP)36−38 level of theory using the 6-31G*39 basis set
under a C1 geometry. Excited-state energies, oscillator
strengths, and velocity rotatory strengths for AXF-155 were
calculated using time-dependent density functional theory
(TD-DFT),40,41 specifically DFT/B3LYP and 6-31G* basis
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Figure 1. (no hydrogen bonding (NHB)) trans_S-extra conformer of AXF-169′ (center). (a) (NHB) cis_M-extra conformer of AXF-169′, (b)
(NHB) trans_R-intra conformers of AXF-169′, (c) (hydroden bonding (HB)) trans_S-extra conformer of AXF-169′, and (d) (HB) trans_R-extra
conformers of AXF-169′.
set, over the first 100 electronic excited states. Calculation on
AXF-169′ was performed over the first 60 electronic excited
states. The polarizable continuum model (PCM),42,43 using
universal force field (UFF) radii with a multiplicative factor of
1.1, was employed to include solvent effects for both the
geometry optimizations and the TD-DFT calculations. We used
the nonequilibrium regime for THF with a static dielectric
constant of 7.4257 and an optical dielectric constant of 1.9740.
All the calculated electronic transitions were broadened using
Gaussian shaped functions with 3500 cm−1 (HW(1/e)M) for
the absorption spectra and 0.35 eV (HW(1/e)M) for the ECD
spectra. The values of εi(v) were calculated using the oscillator
strength according to41
Here, the i subscript refers to the particular excited state; vi is
the transition frequency (cm−1) of the excited state of interest,
v is the incident radiation frequency (in cm−1), σ is the standard
deviation of the simulated band in wavenumber (HW1/eM)
(cm−1), fi is the oscillator strength (dimensionless), and Ri is
the velocity rotatory strength (erg·esu·cm/Gauss) for the
corresponding transition. The values of εi(v) and Δεi(v) have
units of l·mol−1·cm−1.45,46
All the theoretical calculations were performed with Gaussian
09 computational chemistry software.47
RESULTS AND DISCUSSION
■
To accurately determine the structure of our novel axially chiral
binaphthyl fluorene based salen ligand in THF solution, we
performed a systematic comparison of the theoretical ECD
spectra of all the potential conformers with the experimental
spectrum. For that purpose, we optimized first the molecular
structures of half-AXF-155 (AXF-169′) in THF according to
the procedure explained previously in the theoretical methods.
In AXF-169′, we replaced one-half of the molecule by a methyl
group with the purpose of recognizing the relative orientation
of the remaining fluorene moiety with respect to the detached
half (see Figure 1).
n
ε(v) = ∑ ε (v)
i
i=1
2
)
⎛
⎞
⎟
n
v−v
i
f
−
i
= ∑ ⎜⎜
e
(
σ
−9
⎟
⎠
7.653619415 × 10
σ
⎝
i=1
(1)
The Δεi(v)(ECD) was obtained from the velocity rotatory
strength applying the Harada-Nakanishi equations:44
n
From these preliminary calculations, we identified the
following four significant features on AFX-169′: (a) The
existence of geometric isomers around the CN bond at the
center of the molecule (Figure 1a), (b) the relative orientation
of the salicaldehyde moiety with respect to the detached half
(pseudo-endo and pseudo-exo isomerism, hereafter, intra and
extra, respectively) (Figure 1b), (c) the possibility of formation
Δε(v) = ∑ Δε (v)
i
i=1
2
)
⎛
⎞
⎟
n
v−v
i
R
v
−
i
σ
i
= ∑ ⎜⎜
e
(
σ
−39
⎟
⎠
2.296 × 10
π
⎝
i=1
(2)
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of intramolecular hydrogen bonding (HB) (Figure 1c), and (d)
the presence of an additional axial chirality around the CN
bond at the center of the molecule (Figure 1d). Despite the
existence of the cis isomers, we only show the trans in Figure 1
to illustrate clearly these four features. A similar type of variant
was found in the cis isomers.
reveals the impact of solvent effects in solution and serves to
reduce the number of possible conformers of AXF-169 from 16
to only 8. Nevertheless, the theoretical analysis was still
performed on all 16 conformers.
Since the absorption spectra of two optical isomers are
identical,49 in Figure 2a, we show the theoretical absorption
spectra of only one enantiomer (R and M for the trans and cis
isomers, respectively) of AXF-169, that is, (i) trans_R-extra, (ii)
trans_R-intra, (iii) cis_M-extra, and (iv) cis_M-intra, with and
without HB. The first observation is that the theoretical spectra
for all the conformers match well with the experimental within
the uncertainty. The observed similarity indicates that the
electronic transitions are primarily determined by the
conjugated fluorene-alkyne_bond-phenyl_ring branch. The
second observation is that the theoretical molar extinction
coefficient is slightly larger than the experimental. This small
difference is justified considering the size of the molecule under
study.
In Figure 2b, we present the theoretical ECD spectra of the
same conformers. Because the ECD spectra of optical isomers
are mirror images, only the spectrum of one enantiomer is
shown. The spectra of equal geometric isomers look alike
independently of their relative orientation intra or extra. This
indicates that the substitution of half of AXF-155 in AXF-169′,
by a small methyl group, does not distort substantially the 3D
configuration of the latter by changing its relative orientation.
Another aspect to be noticed in Figure 2b is the remarkable
differences between the ECD spectra of similar structures with
and without HB. The clear difference between both reveals the
potential of using ECD for the accurate determination of the
existence (partial or total) or not of HB in the final 3D
molecular structure of AXF-169′ and, therefore, of AXF-155.
With the intention of corroborating these theoretical results, we
attempted to make a comparison between them and the
experimental. However, the experimental ECD spectrum was
not measurable because of the formation of a racemic mixture
of AXF-169 through its synthesis, which was impossible to
separate. Nevertheless, the optimization of all conformers
revealed strong steric hindrances in the cis isomers. Therefore,
one should expect the trans isomer to define the final 3D
structural configuration of AXF-155 in solution.
While geometric isomerism was entirely anticipated, the
existence of an intra isomer was a little surprising keeping in
mind that the potential steric effects between the two halves in
the whole AXF-155 would keep the structure open in an extra
arrangement. However, upon replacing one of the halves by a
methyl group in AXF-169′, these effects can be considered
substantially reduced thus opening possibilities for the
formation of the intra conformers. The formation of intra-
molecular HB was also expected because of the close distance
between the nitrogen attached to the naphthalene and the
hydroxyl group. However, the presence of the additional axial
chirality was totally unexpected considering the typical rigidity
that the strong π-conjugation conferred to the whole molecular
system. This effect, in principle, should maintain the structure
planar. Nevertheless, a dihedral angle of ≈32° for the trans_R-
intra and ≈53° for the trans_R-extra isomer, for example,
showed the contrary. Similar results were also obtained for the
cis isomer but with M and P chirality. As a result, there were 16
different distinguishable conformers of AXF-169′ to study
(Scheme 2). To simplify the classification of such a big number
Scheme 2. Descriptive diagram for the determination of the
16 conformers of AXF-169′. The diagram includes two
geometric isomers (trans and cis), four axial chiralities [two
for trans (R and S) and two for cis (M and P)], two relative
orientations (intra and extra isomerism), and the formation
or not of intramolecular hydrogen bonding, HB and NHB,
respectively
With this information in hand, we proceeded to construct all
the possible conformers of AXF-155 combining the different
configurations obtained for AXF-169′. Then, we completed the
structural optimization of all its conformers and calculated their
excited states. Not surprisingly, we found in AXF-155 the same
different features already discussed for AXF-169′ but in either a
symmetric or a dissymmetric fashion with respect to the (R)-
(+)-1,1′-binapthalene moiety at the center of the molecule
(Figure 3). Following this approach, we were able to recognize
40 different possible conformers of AXF-155 (see Table 1), 20
with HB and 20 NHB. According to the spectroscopic
of molecules, we utilized the following systematic rule: first, we
identify the geometric isomer; second, we classify the
conformers by optical isomers; and third, we differentiate the
relative orientation of the fluorene moiety. For instance, the
molecule at the center of Figure 1 would be thus called trans_S-
extra. The classification with HB or without hydrogen bonding
(NHB) is done separately from this methodical assignation of
names to the conformers.
1
characterization of AXF-155 using HNMR, the presence of
intramolecular HB at the center of the molecule is now an open
possibility since a signal with a chemical shift of 12.37 ppm,48
for the hydroxyl hydrogen, is too low to completely discard this
possibility.
To corroborate our theoretical predictions, we synthesized
AXF-169 (see Materials and Synthetic Procedures). According
to the HNMR characterization of this molecule, the single
signal with a chemical shift of 13.64 ppm for the hydroxyl
hydrogen is definitely an indication of the absence of
intramolecular HB in AXF-169.48 This piece of information
To have a better initial idea of the most probable 3D
conformation of AXF-155, we first ran in collaboration with
professor Prasad Polavarapu (Professor of Chemistry, Depart-
ment of Chemistry, Vanderbilt University) molecular dynamic
calculations using CONFLEX in gas phase.49−51 This first
attempt revealed that out of the 90 conformers generated by
1
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Figure 2. (a) Theoretical (broken lines) and experimental (solid line) UV−vis spectra of AXF-169′ conformers. (i) trans_R-extra, (ii) trans_R-intra,
(iii) cis_M-extra, and (iv) cis_M-intra with (HB) and without hydrogen bonding (NHB). Solid vertical bars represent the different electronic
transitions with their corresponding oscillator strengths (scale on the right). (b) Theoretical ECD spectra of AXF-169′ conformers. (i) trans_R-extra,
(ii) trans_R-intra, (iii) cis_M-extra, and (iv) cis_M-intra with (HB) and without hydrogen bonding (NHB). Solid vertical bars represent the different
transitions with their corresponding rotatory strengths (scale on the right).
this program for AXF-155, with fixed (R)-axial chirality at the
center, the trans_R-extra//trans_R-extra (90.2749%) should be
the predominant conformer of AXF-155 followed by the
trans_R-intra//trans_R-extra (3.9273%) and the trans_R-
intra//trans_R-intra (0.0566%) in that order (Figure 4a) (the
double bar, //, at the center of the name represents the (R)-
(+)-1,1′-binapthalene moiety). However, our spectroscopic
theoretical and experimental results, shown later in this article,
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Figure 3. Molecular diagram used for the determination of the 40 conformers of AXF-155, including geometric isomers (trans and cis), relative
orientation (intra and extra isomerism), axial chirality, and the formation or not of intramolecular hydrogen bonding (HB and NHB).
Table 1. Symmetric Trans and Cis Conformers of AXF-155
electrostatic interactions).52 Therefore, the geometry optimiza-
tion essentially depends on the input structure (in this
particular case AXF-155) and not on how other effects such
as intermolecular steric hindrances between the interacting
parts (AFX-147 and (R)-(+)-2,2′-diamino-1,1′-binapthalene)
can favor the formation of a particular conformer throughout
the chemical reaction. Hence, the synthesis of a conformer such
as the trans_R-extra//trans_R-extra with a relative lower energy
in the potential energy surface (PES) is impeded by the
mentioned effect thus favoring the formation of another
conformer with a relative higher energy in PES, that is, the
trans_R-intra//trans_R-extra. This is consistent with our
empirical examination of the different possibilities for the
nucleophilic addition of (R)-(+)-2,2′-diamino-1,1′-binapthalene
to the carbonyl carbon of salicaldehyde AXF-147 shown at the
end of this article.
trans−trans conformers
cis−cis conformers
trans_R-intra//trans_R-extra
trans_S-intra//trans_S-extra
trans_R-intra//trans_S-extra
trans_S-intra//trans_R-extra
trans_R-intra//trans_R-intra
trans_S-intra//trans_S-intra
trans_S-intra//trans_R-intra
trans_R-extra//trans_R-extra
trans_S-extra//trans_S-extra
trans_S-extra//trans_R-extra
cis_M-intra//cis_M-extra
cis_P-intra//cis_P-extra
cis_M-intra//cis_P-extra
cis_P-intra//cis_M-extra
cis_M-intra//cis_M-intra
cis_P-intra//cis_P-intra
cis_P-intra//cis_M-intra
cis_M-extra//cis_M-extra
cis_P-extra//cis_P-extra
cis_P-extra//cis_M-extra
clearly demonstrate that the only species present in the THF
solution is the trans_R-intra//trans_R-extra.
To explain this discrepancy is good to remember that
calculations of steric energies using molecular dynamics only
consider intramolecular interactions (bond stretching, bond
bending and bond torsional forces, van der Waals and
An additional interesting outcome from the molecular
dynamic calculations is the presence of intramolecular HB
between the hydroxyl group and the adjacent nitrogen of the
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Figure 4. continued
extra//trans_R-extra with HB in only one side of the molecule
and in trans-R-intra//trans_R-extra with HB in only one side of
the molecule.
diphenylamino on each half of AXF-155 in the gas phase. This
result was not unexpected since the proximity between both
groups allows for such an interaction. However, in solution, this
type of interaction can become weaker by solvent effects, for
example, solvation of AXF-155. The discussion about the
characterization of AXF-169 supports this statement. None-
theless, we still considered the three conformers with HB at
both sides (Figure 4a), the three with NHB (Figure 4b), and
the four asymmetric HB species, that is, with HB in only one
side of AXF-155 (Figure 4c). An additional structure not
included in the discussion but with 5.6733% probability,
according to CONFLEX calculations, is the trans_S-intra//
trans_S-extra. This specific and unexpected conformer was
totally discarded because its theoretical ECD spectrum was
totally inverted with respect to the experimental (see
Supporting Information).
In Figure 5a, we show the experimental absorption spectrum
of AXF-155 and the theoretical absorption spectra of the
trans_R-intra//trans_R-intra and the trans_R-extra//trans_R-
extra 3D structures in THF solution. It can be noticed that the
spectral shapes and the molar extinction coefficients of all of
them are very similar for the same reasons explained before in
the discussion of Figure 2a. Therefore, one cannot rely on this
data to identify the actual configuration of AXF-155. Never-
theless, the shape and the differences in amplitude observed
between the experimental absorption spectra of AXF-169 and
that of AXF-155 (approximately 2 folds greater for AXF-155)
lead us to think that because the π-electron conjugation is
broken at the center of AXF-155, the electronic transitions of
this molecule are mostly determined by the electronic
transitions of AXF-169. In an effort to establish the 3D
structural configuration of AXF-155 in solution, we collected
more theoretical−experimental spectroscopic data. In Figure
5b, we present the experimental ECD spectrum of AXF-155 in
THF solution and the theoretical spectra of the same six
conformers.
Because of the less steric hindrances found in AXF-169, the
open configuration with NHB in that molecule was favored.
However, in AXF-155, the constrains imposed by the presence
of two bulky AXF-169, attached together by the naphthalene
moieties to form AXF-155, impose more restrictions to the
molecule’s degree of freedom thus favoring the formation of
intramolecular HB in the gas phase. However, in THF, this
interaction seems to become weaker thus permitting the
formation of species with NHB.
Using ECD, a technique that has been proven to be very
sensitive to 3D molecular structural conformations,44 we were
able to discard the trans_R-extra//trans_R-extra and the
trans_R-intra//trans_R-intra. This was done through the
cautious comparison of the theoretical and experimental ECD
spectra of the AXF-155 shown in Figure 5b. As it can be seen,
while in all the trans_R-extra//trans_R-extra the principal
theoretical peak−valley feature between 350 and 450 nm is
inverted, in the trans_R-intra//trans_R-intra there is (a) no
valley between 400 and 450 nm for the conformer with HB at
both sides of the molecule (Figure 5b, i); (b) a strong spectral
Figure 4. (a) Optimized 3D molecular structure of AXF-155 with HB
at both sides, trans-R-intra//trans_R-intra, trans-R-extra//trans_R-
extra, and trans-R-intra//trans_R-extra. (b) Optimized 3D molecular
structure of AXF-155 in trans-R-intra//trans_R-intra, trans-R-extra//
trans_R-extra, and trans-R-intra//trans_R-extra conformation with
NHB at both sides of the molecule. (c) Optimized 3D molecular
structure of AXF-155 in trans-R-intra//trans_R-intra and trans-R-
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Figure 5. (a) Theoretical (broken lines) and experimental (solid line) UV−vis spectra of the two symmetric AXF-155 conformers, trans-R-intra//
trans_R-intra and trans-R-extra//trans_R-extra, with (i) HB at both sides, (ii) NHB, and (iii) HB in only one side of the molecule. Solid vertical bars
represent the different transitions with their corresponding oscillator strength (scale on the right). (b) Theoretical (broken lines) and experimental
(solid line) ECD spectra of the two symmetric AXF-155 conformers, trans-R-intra//trans_R-intra and the trans_R-extra//trans_R-extra, with (i) HB
at both sides, (ii) NHB, and (iii) HB in only one side of the molecule. Solid vertical bars represent the different transitions with their corresponding
rotatory strength (scale on the right).
blue-shift of the longer wavelength positive band accompanied
by the appearance of a negative band at ca. 290 nm for
conformer with NHB (Figure 5b, ii); and (c) no strong features
between 250 and 450 nm and a significant peak/valley
amplitude disproportion in the red region of the spectral
range for the dissymmetric conformer with HB in only one-half
of the molecule (Figure 5b, iii). (In the Supporting
Information, we show the oscillator and rotatory strength for
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Figure 6. Theoretical (broken lines) and experimental (solid line) UV−vis and ECD spectra of the asymmetric AXF-155 conformer, trans-R-intra//
trans_R-extra, with (i) HB at both sides, (ii) NHB, (iii) HB only in the trans_R-extra side, and (iv) HB only in trans-R-intra side. The oscillator
strength and the rotatory strength scales are on the right of the graphs.
the first 20 transitions of these six conformations.) These clear
theoretical−experimental differences are evidence to discard
the 6 symmetric conformers from the list of 10 possible 3D
structural conformations of AXF-155 in THF solution. It also
suggests that the ultimate structure should be a combination of
trans_R-intra and trans_R-extra. In fact, there is only one
remaining candidate, the trans_R-intra//trans_R-extra con-
former.
Table 2. Calculated Energies and Entropies and Boltzmann
Weighted Average of the Four Asymmetric Conformers of
AXF-155 (trans-R-intra//trans_R-extra) with HB at Both
Sides, NHB, HB Only in the trans_R-extra Side, and HB
Only in trans-R-intra Side
a
entropy
Boltzmann
a
conformer
energy (Ha) (J/mol.K) weighted average
i
(HB) trans_R-intra//
trans_R-extra (HB)
−3915.004
−3,914.977
−3914.985
−3914.993
1900.477
1910.117
1899.331
1791.731
0.999991
In Figure 6, we show the experimental and theoretical
absorption and ECD spectra of the four asymmetric trans_R-
intra//trans_R-extra configurations in THF solution with and
without HB. First, one can notice that the theoretical
absorption spectra of all four arrangements are very similar
among them and are similar to the experimental as discussed
above. However, discerning between the four different
configurations of this conformer, with and without HB, using
ECD is a difficult task because the theoretical and experimental
spectra match satisfactorily for all of them. Noticeably, the best
theoretical−experimental pairing is observed for the config-
uration with NHB (Figure 6ii) and that with HB on trans_R-
intra (Figure 6iv). The strong spectral resemblance leads us to
think that these should be the preferred 3D structural
conformation of AXF-155 in THF solution. However, the
other two configurations of the same trans_R-intra//trans_R-
extra conformer might be present in this solution. Con-
sequently, one cannot reject the presence of any of the other
two in THF and at room temperature. In an effort to discern
between all four possibilities, we calculated the energies and
entropies of these four structures (see Table 2). As one can see
in Table 2, the energies are virtually identical for all four
possibilities and are only slightly lower for that with HB at both
sides (Figure 6i).
ii
trans_R-intra//trans-
R-extra
3.74908 × 10−13
1.80187 × 10−9
8.66010 × 10−6
iii trans_R-intra//
trans_R-extra (HB)
iv (HB) trans_R-intra//
trans_R-extra
a
ni/N = e−ε /kT/∑ie−ε /kT
.
i
i
Boltzmann probability of occurrence suggests that the con-
former with HB at both sides (Figure 6i) should have an
overwhelming contribution of 99.9991%. However, this result is
not conclusive since the ECD spectra of the other three
conformers resemble much better the experimental ECD
spectrum of AXF-155. In addition, other effects such as the
important steric hindrances between the interacting parts and
solvation effects could favor the formation of configurations ii
and iv of the trans_R-intra//trans_R-extra conformer through
the condensation phase of the reaction.
Having determined the structural configuration of AXF-155,
we inspected the different possibilities for the nucleophilic
addition of (R)-(+)-2,2′-diamino-1,1′-binapthalene to the
carbonyl carbon of salicaldehyde AXF-147 following the well-
known Burgi:Dunitz trajectory.19 This empirical examination,
supported by our theoretical−experimental results, confirms
the formation of the sole trans_R-intra//trans_R-extra con-
former.
To obtain a better grasp of the actual fractions of all probable
species in THF solution, we calculated the Boltzmann weighted
average for all of them using the energies in Table 2. The
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In a first stage, the steric impediment coming from one of the
naphthalene amino moieties in the nucleophile forces the attack
to take place at approximately 105° 5° with respect to the
CO through the electron pair of the most accessible
nitrogen, that is, from below (Figure 7a, left side). Although
this attack can take place through either side of the molecule, it
always ends forming the trans_R-intra conformer. The
alternative possibilities of attack from above were found to be
not viable because of the strong steric hindrances imposed by
the nucleophile or AFX-147, which restricts the interaction
between the two molecules (Figure 7b, right side).
In a second stage, the consecutive attack of the generated
species in stage 1 to another AXF-147 materializes following
the same type of trajectory.19 Since the steric effects are now
more obstructive than in the first stage, the attack is more likely
to happen from below through the left side of the molecule and
from the above through the right (Figure 7b). This molecular
positioning only generates the trans_R-extra on that side
(Figure 7b). As a result, the unequivocal generated conformer
is indeed the trans_R-intra//trans_R-extra. Remarkably, these
two steps result in the same conformer independently of the
chances of starting with an AXF-147 with or without HB.
Therefore, there is an apparent possibility of finding all four
different configurations (with and without HB) of this
conformer in solution.
In an attempt to explain the discrepancy between the
preferential configuration of the trans_R-intra//trans_R-extra
conformer, yielded by the Boltzmann weighted average (Figure
6i) and that obtained by comparing the overlapping between
the theoretical and experimental ECD spectra (Figure 6iv), we
examined the influence of solvent effects throughout the
reaction. It is known that intramolecular hydrogen bonding on
the carbonyl oxygen can increase the electrophilicity of the
carbonyl group thus making it more reactive.53 On the basis of
this statement, the presence of this type of interaction in AXF-
147 should favor the formation of the trans_R-intra//trans_R-
extra conformer with HB at both sides. However, in protic
solvents, this interaction can be counteracted by the solvation
of the hydroxyl proton on the electrophile.54 Consequently,
having used ethanol as a solvent in the last step of the synthesis
of AXF-155 and considering the solvation of the hydroxyl
proton on AXF-147, one should attain a preferential formation
of the trans_R-intra//trans_R-extra conformer of AXF-155 with
NHB. Therefore, the conformer with NHB at both sides should
be the dominant configuration in solution. However, the
spectral resemblance between the theoretical and the
experimental ECD spectra of all four configurations indicates
the presence of a mixture of all of them in solution.
Figure 7. (a) Representation of the naphthalene amino moiety
(nucleophile) attack to AXF-147 from different flanks of the
salicaldehyde derivative following a Burgi:Dunitz trajectory. Top-left:
attack through the left side of AXF-147 with the nitrogen that is
attached to the naphthalene placed underneath; bottom-left: attack
through the right side of AXF-147 with the nitrogen that is attached to
the naphthalene placed underneath; top-right: attack through the left
side of AXF-147 with the nitrogen that is attached to the naphthalene
placed above; bottom-right: attack through the right side of AXF-147
with the nitrogen that is attached to the naphthalene placed above.
Thin arrows show the attack site. Thick semitransparent double arrows
indicate steric hindrances. (b) 3D description of the attack of the
naphthalene amino-AXF-147 fragment to another AXF-147 from
different flanks and following a Burgi:Dunitz trajectory. Top-left:
attack through the left side of AXF-147 with the available nitrogen
attached to the naphthalene placed underneath; bottom-left: attack
through the left side of AXF-147 with the available nitrogen that is
attached to the naphthalene placed above; top-right: attack through
the right side of AXF-147 with the available nitrogen that is attached to
the naphthalene placed underneath; bottom-right: attack through the
right side of AXF-147 with the available nitrogen that is attached to the
naphthalene placed above. Thin arrows show the attack site. Thick
semitransparent double arrows indicate steric hindrances.
Although discerning between the four different configu-
rations of this conformer in solution is very challenging, we can
be certain of two things: (a) the only conformer of AXF-155,
present in THF solution, is the trans_R-intra//trans_R-extra
and (b) the most favored configuration in THF solution should
be the trans_R-intra//trans_R-extra conformer with NHB
(Figure 6ii).
CONCLUSIONS
■
Throughout the theoretical−experimental analysis of the ECD
spectrum of this novel axially chiral binaphthyl fluorene based
salen ligand and by inspecting the different possibilities for the
nucleophilic addition of (R)-(+)-2,2′-diamino-1,1′-binapthalene
to the carbonyl carbon of salicaldehyde AXF-147 following the
well-known Burgi:Dunitz trajectory, we were able to determine
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(18) MacMillan, S. N.; Jung, C. F.; Shalumova, T.; Tanski, J. M.
the 3D structural conformation of AXF-155 in a THF solution.
ECD has been proven to be a powerful tool to provide accurate
information not only about the structure of organic molecules
with extended π−electron conjugation but also of molecules
with multiple axial chiralities in solution. AXF-155 is expected
to find potential applications in homogeneous catalysis,
biophotonics, and sensing. Work in the sensing applications
is in progress.
Inorg. Chim. Acta 2009, 362, 3134−3146.
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ASSOCIATED CONTENT
(23) Belfield, K. D.; Morales, A. R.; Kang, B. S.; Hales, J. M.; Hagan,
D. J.; Van Stryland, E. W.; Chapela, V. M.; Percino, J. Chem. Mater.
2004, 16, 4634−4641.
■
S
* Supporting Information
3D molecular structure, UV−vis, and ECD spectra of the trans-
S-intra//trans_S-extra conformer. Tables with the oscillator and
rotatory strengths of the first 20 excited states of the 10 main
conformers of AXF-155, i.e., trans-R-extra//trans_R-extra,
trans-R-intra//trans_R-intra, and trans-R-intra//trans_R-extra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(24) Belfield, K. D.; Schafer, K. J.; Mourad, W.; Reinhardt, B. A. J.
Org. Chem. 2000, 65, 4475−4481.
(25) Hales, J. M.; Hagan, D. J.; Van Stryland, E. W.; Schafer, K. J.;
Morales, A. R.; Belfield, K. D.; Pacher, P.; Kwon, O.; Zojer, E.; Bredas,
J.-L. J. Chem. Phys. 2004, 21, 3152−3160.
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(27) Belfield, K. D.; Bondar, M. V.; Przhonska, O. V.; Schafer, K. J.
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AUTHOR INFORMATION
■
(28) Belfield, K. D.; Bondar, M. V.; Przhonska, O. V.; Schafer, K. J.
Photochem. Photobiol. Sci. 2004, 3, 138−141.
Notes
The authors declare no competing financial interest.
(29) Schafer, K. J.; Belfield, K. D.; Yao., S.; Frederiksen, P. K.; Hales.,
J. M.; Kolattukudy, P. E. J. Biomed. Opt. 2005, 10, 051402-1−051402-
8.
ACKNOWLEDGMENTS
■
(30) Cohanoschi, I.; Toro, C.; Hernandez, F. E. J. Chem. Phys. 2006,
This work was partially supported by the National Science
Foundation through grant no. CHE-0832622. The authors
would like to acknowledge Dr. Suren Tatulian, Assistant
Professor, Physics Department at UCF, for kindly allowing
them to measure the experimental ECD spectrum of AXF-155
in his J-810 CD spectropolarimeter. F.E. Hernandez would like
to thank Professor Prasad Polavarapu, Professor of Chemistry,
Department of Chemistry, Vanderbilt University, for his
valuable support performing the molecular dynamic calcu-
lations of AXF-155 in gas phase using CONFLEX.
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