Molecular complexes of diethyl N, N ′-1,3-phenyldioxalamate and resorcinols: Conformational switching through intramolecular three-centered hydrogen-bonding

Article abstract of DOI:10.1021/cg401500k

The mechanochemically induced complexation between diethyl N,N′-1,3-phenyldioxalamate tweezers and resorcinol, orcinol, 4,6-di-tert-butyl-1,3-benzenediol, and 4-hexyl-1,3-benzenediol is described. IR-spectroscopy, X-ray powder diffraction, ¹³C CPMAS, and single X-ray diffraction allowed establishing the structures of the complexes as hydrogen-bonded heterodimers and heterotetramers. Complexation occurs through O-H···O=C hydrogen-bonding interactions with the participation of phenolic OH and amide carbonyl groups. The initial conformation and steric factors coming from the 1,3-benzenediols exert a strong influence on the final structure of the complex formed. Complexation twists both oxalyl arms by 180°, strengthens the intramolecular (amide) CO···H(Csp²)···OC(amide) three-centered hydrogen bond, and moves apart the oxalyl arms to allow the accommodation of the 1,3-benzenediol inside the cavity. The supramolecular architectures of the complexes in 1-D are directed by R¹ ₂(6), R²₂(10), and R¹₂(6) adjacent hydrogen-bonding ring motifs; meanwhile, the 2-D and 3-D arrays are driven by multipolar interactions. Theoretical DFT calculations at the B3LYP/6-31G(d,p) level of theory were performed to support the experimental findings. The complexes herein reported constitute the first examples of molecular complexes with phenyldioxalamate.

Full text of DOI:10.1021/cg401500k

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Article
Molecular complexes of diethyl N,N´-1,3-phenyldioxalamate
and resorcinols: conformational switching through
intramolecular three-centered hydrogen-bonding
Juan Saulo González-González, Francisco J. Martínez-Martínez, Efrén V. García-Baez,
Alejandro Cruz, Luis Morín-Sánchez, Susana Rojas-Lima, and Itzia I. Padilla-Martinez
Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 06 Jan 2014
Downloaded from http://pubs.acs.org on January 11, 2014
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Crystal Growth & Design
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Molecular complexes of diethyl N,N´ꢀ1,3ꢀ
phenyldioxalamate and resorcinols: conformational
switching through intramolecular threeꢀcentered
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hydrogenꢀbonding
Juan Saulo GonzálezꢀGonzález,§ Francisco J. MartínezꢀMartínez,† Efrén V. GarcíaꢀBáez,‡
Alejandro Cruz, ‡ Luis M. MorínꢀSánchez, ‡ Susana RojasꢀLima,⊥ Itzia I. PadillaꢀMartínez.‡*
§ Universidad de la Cañada, Instituto de Farmacobiología, Carretera TeotitlánꢀSan Antonio
Nanahuatipán Km 1.7 s/n, C.P. 68540, Teotitlán de Flores Magón, Oaxaca, México.
† Facultad de Ciencias Químicas, Universidad de Colima, Km. 9 Carretera ColimaꢀCoquimatlán,
C.P. 28400, Coquimatlán, Colima, Mexico.
‡ Departamento de Ciencias Básicas, Unidad Profesional Interdisciplinaria de Biotecnología del
Instituto Politécnico Nacional, Av. Acueducto s/n Barrio la Laguna Ticomán, México D. F., C.P.
07340, Mexico.
⊥ Área Académica de Química, Universidad Autónoma del Estado de Hidalgo, Carretera
PachucaꢀTulancingo Km. 4.5, Ciudad Universitaria, C.P. 42184, Mineral de la Reforma Hidalgo,
Mexico.
ABSTRACT. The mechanochemically induced complexation between diethyl N,N´ꢀ1,3ꢀ
phenyldioxalamate tweezers and resorcinol, orcinol, 4,6ꢀdiꢀtertbutylꢀ1,3ꢀbenzenediol, and 4ꢀ
hexylꢀ1,3ꢀbenzenediol is described. IRꢀspectroscopy, Xꢀray powder diffraction, 13C CPMAS and
single Xꢀray diffraction allowed establishing the structures of the complexes as hydrogenꢀbonded
heterodimers and heterotetramers. Complexation occurs through O—HꢁꢁꢁO=C hydrogenꢀbonding
interactions with the participation of phenolic OH and amide carbonyl groups. The initial
conformation and steric factors coming from the 1,3ꢀbenzenediols exert a strong influence on the
final structure of the complex formed. Complexation twists both oxalyl arms by 180º,
strengthens intramolecular (amide)COꢁꢁꢁH(Csp2)ꢁꢁꢁOC(amide) threeꢀcentered hydrogen bond and
moves apart the oxalyl arms to allow the accommodation of the 1,3ꢀbenzenediol inside the
cavity. The supramolecular architectures of the complexes in 1ꢀD are directed by R1 (6), R2 (10)
2
2
and R1 (6) adjacent hydrogenꢀbonding ring motifs; meanwhile, the 2ꢀD and 3ꢀD arrays are driven
2
by multipolar interactions. Theoretical DFT calculations at the B3LYP/6ꢀ31 G (d, p) level of
theory were performed to support the experimental findings. The complexes herein reported
constitute the first examples of molecular complexes with phenyldioxalamate.
* I. I. P.ꢀM., Departamento de Ciencias Básicas, Unidad
Profesional Interdisciplinaria de Biotecnología del
Instituto Politécnico Nacional, Av. Acueducto s/n Barrio
la Laguna Ticomán, México D. F., C.P. 07340, Mexico,
eꢀmail: ipadillamar@ipn.mx.
Phone: (55)ꢀ57296000 ext. 56324
Fax: (55)ꢀ57296000 ext. 56305
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Molecular complexes of diethyl N,N´ꢀ1,3ꢀ
phenyldioxalamate and resorcinols: conformational
switching through intramolecular threeꢀcentered
hydrogenꢀbonding
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Juan Saulo GonzálezꢀGonzález,§ Francisco J. MartínezꢀMartínez,† Efrén V. GarcíaꢀBáez,‡
Alejandro Cruz, ‡ Luis M. MorínꢀSánchez, ‡ Susana RojasꢀLima,⊥ Itzia I. PadillaꢀMartínez.‡*
§ Universidad de la Cañada, Instituto de Farmacobiología, Carretera TeotitlánꢀSan Antonio
Nanahuatipán Km 1.7 s/n, C.P. 68540, Teotitlán de Flores Magón, Oaxaca, México.
† Facultad de Ciencias Químicas, Universidad de Colima, Km. 9 Carretera ColimaꢀCoquimatlán,
C.P. 28400, Coquimatlán, Colima, Mexico.
‡ Departamento de Ciencias Básicas, Unidad Profesional Interdisciplinaria de Biotecnología del
Instituto Politécnico Nacional, Av. Acueducto s/n Barrio la Laguna Ticomán, México D. F., C.P.
07340, Mexico.
⊥ Área Académica de Química, Universidad Autónoma del Estado de Hidalgo, Carretera
PachucaꢀTulancingo Km. 4.5, Ciudad Universitaria, C.P. 42184, Mineral de la Reforma Hidalgo,
Mexico
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KEYWORDS. Threeꢀcentered hydrogen bond, resorcinol complexation, mechanochemistry,
conformational switch, 13C CPMAS, molecular tweezers.
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ABSTRACT. The mechanochemically induced complexation between diethyl N,N´ꢀ1,3ꢀ
phenyldioxalamate tweezers and resorcinol, orcinol, 4,6ꢀdiꢀtertbutylꢀ1,3ꢀbenzenediol, and 4ꢀ
hexylꢀ1,3ꢀbenzenediol is described. IRꢀspectroscopy, Xꢀray powder diffraction, 13C CPMAS and
single Xꢀray diffraction allowed establishing the structures of the complexes as hydrogenꢀbonded
heterodimers and heterotetramers. Complexation occurs through O—HꢁꢁꢁO=C hydrogenꢀbonding
interactions with the participation of phenolic OH and amide carbonyl groups. The initial
conformation and steric factors coming from the 1,3ꢀbenzenediols exert a strong influence on the
final structure of the complex formed. Complexation twists both oxalyl arms by 180º,
strengthens intramolecular (amide)COꢁꢁꢁH(Csp2)ꢁꢁꢁOC(amide) threeꢀcentered hydrogen bond and
moves apart the oxalyl arms to allow the accommodation of the 1,3ꢀbenzenediol inside the
cavity. The supramolecular architectures of the complexes in 1ꢀD are directed by R1 (6), R2 (10)
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2
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and R1 (6) adjacent hydrogenꢀbonding ring motifs; meanwhile, the 2ꢀD and 3ꢀD arrays are driven
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by multipolar interactions. Theoretical DFT calculations at the B3LYP/6ꢀ31 G (d, p) level of
theory were performed to support the experimental findings. The complexes herein reported
constitute the first examples of molecular complexes with phenyldioxalamate.
1. INTRODUCTION
Among nonꢀcovalent interactions, hydrogen bonding has been established as effective tool for
constructing sophisticated assemblies because of the strength and directionality.1 In
supramolecular chemistry,2 the design and synthesis of model molecules, for donorꢀacceptor
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interaction studies, continues to be an area of interest3 in which, additionally, the reversibility,
specificity and cooperativity of hydrogen bonding are particularly useful.
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Mechanochemical methods have been successfully applied to form supramolecular systems,4
among which 1,3ꢀbenzenediols are highlighted in the formation of hydrogenꢀbonding
interactions.5 Mechanochemical synthesis consists of the application of mechanical energy to
induce a chemical reaction, and its use and applications have been recently reviewed.6
Resorcinol has proved to be a very good hydrogenꢀbonding cocrystallizing agent, mostly through
O―HꢁꢁꢁA (A = N, O) interactions. From the 76 hits for neutral molecular complexes of several
resorcinols found in the CSD,7 83% correspond to amine or pyridine acceptors and 17% to
carbonyl group acceptors. Hydrogen bonding of resorcinols with pyridines,5a and more recently
with isoquinolinꢀ3(2H)ꢀone,8 has been used to suitably align olefins for photodimerization in the
solid state. In general, benzenediols have lately been used as cocrystal formers for active
pharmaceutical ingredients in order to improve their physicochemical properties.9 In this context,
rigid resorcinolꢀbased macrocycles have been used as hosts10 for a variety of polyols,10a
sugars10b,c and carboxylic acids.10d
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In previous works we have reported on the design and synthesis of oxalamide derivatives
whose molecular and supramolecular structures are stabilized by hydrogen bonding.11
Oxalamides have recently been used as model compounds to design supramolecular βꢀsheets for
a better understanding of supramolecular interactions in protein quaternary structures and in
amyloids,12 as well as in the design of compounds with antiꢀHIV activity.13 Oxalamides
cocrystrallize with a wide variety of molecules such as alkynylꢀ and alkenylꢀpyridines,
carboxylic acids, amines, halobenzenes, phenols, alcohols and other oxalamides (CSD).7 In
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contrast, no reported cocrystal was found in the CSD for the oxalamate moiety, which is the
combined amide and ester derivative of oxalic acid.
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The phenyldioxalamate moiety has become particularly important while versatile
supramolecular building block and has been exploited for the formation of supramolecular
architectures such as columns, sheets, tapes, layers and helices.14 A previous report on several
diethyl N,N´ꢀ1,3ꢀphenyldioxalamates with different conformations has shown that they form
molecular clefts that can be controlled by steric hindrance and hydrogenꢀbonding effects.15
Therefore, 1,3ꢀphenyldioxalamates are good candidates to be used in molecular recognition
processes since they bear NH and C=O groups able to form hydrogenꢀbonding interactions with
other molecules, and a phenyl ring spacer with the possibility of πꢀstacking. Herein, we explored
the molecular complexation between diethyl N,N´ꢀ1,3ꢀphenyldioxalamate (1) and several 1,3ꢀ
benzenediols (2a-d) using mechanochemistry (Scheme 1). There are two known polymorphs of
1: in the first the oxalyl arms exhibit an exo conformation (1-exo),14c meanwhile, in the second,
an endo conformation (1-endo),14b is adopted. In addition, the OH groups of 1,3ꢀbenzenediols
exhibit rotation flexibility which leads to conformational isomerism. In order to support the
experimental findings DFT calculations were performed.
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Scheme 1. Chemical structures of diethyl N,N´ꢀ1,3ꢀphenyldioxalamate (1) and 1,3ꢀbenzenediols
(2a-d).
HO
OH
O
O
OEt
H
H
EtO
N
N
N
N
R1
R3
O
O
O
O
H
H
R2
O
O
OEt
OEt
1-exo
1-endo
2
a R1 = R2 = R3 = H
b R1 = R3 = H, R2 = CH3
c R1 = R3 = C(CH3)3, R2 = H
d R1= (CH2)5CH3, R2 = R3 = H
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2. EXPERIMENTAL SECTION
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Compounds. 1,3ꢀBenzenediol (resorcinol 2a), 5ꢀmethylꢀ1,3ꢀbenzenediol (orcinol 2b), 4,6ꢀdiꢀ
tertbutylꢀ1,3ꢀbenzenediol (2c) and 4ꢀhexylꢀ1,3ꢀbenzenediol (2d), were obtained from
commercial sources and used as received. Diethyl N,N′ꢀ(1,3ꢀphenylene)dioxalamate (1) was
prepared as previously reported.14b
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Crystalline powders of 1:1 mixtures of 1 with resorcinol (1·2apowd), orcinol (1·2bpowd), 4,6ꢀdiꢀ
tertbutylꢀ1,3ꢀbenzenediol (1·2cpowd), 4ꢀhexylꢀ1,3ꢀbenzenediol (1·2dpowd) and 4:3 mixture of 1
with resorcinol (14·2a3powd), were obtained after grinding in a glass mortar. At the beginning and
after every five minutes of grinding, 0.1 mL of chloroform was added to approximately 200 mg
of mixture, to complete twenty minutes of grinding.
Single crystals suitable for Xꢀray analysis were obtained from an ethyl acetate solution of
1·2apowd mixture and from chloroform solutions of the complexes 1·2bpowd, and 1·2cpowd, to obtain
the compounds 14·2a3, 1·2b and 1·2c, respectively. Efforts to crystallize 1·2dpowd were
unsuccessful.
Instrumental. IR spectra were recorded neat at 25 ºC using a Varian 3100 FTꢀIR with ATR
system Excalibur Series spectrophotometer. Room temperature Xꢀray powder diffraction data
were collected on a PAN Analytical X´Pert PRO diffractometer with Cu Kα1 radiation (λ =
1.5405 Å, 45 kV, 40 mA) or on a D8 Focus Bruker AXS instrument using Cu Kα1 radiation (λ =
13
1.542 Å, 35 kV, 25 mA). C CPMAS spectra were recorded on a Bruker Avance II 400. The
following conditions were applied: spectral width 30.242 kHz, acquisition time 33.8 ms, contact
time 2000 ms, rotation rate 8 kHz, relaxation delay 5 s and up to 256 scans for each spectrum
were collected. Single crystal Xꢀray diffractions data were collected on a CrysAlis CCD area
detector diffractometer at 293K with Cu Kα radiation λ = 1.54184 Å. Crystal data, collection and
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refinement parameters are listed in Table 1. The cell refinement and data reduction were carried
out with the CrysAlis RED software.16 The structures were solved by direct methods using the
SHELXS9717 program of the WINGX package.18 The final refinement was performed by fullꢀ
matrix least square methods using the SHELX9717 program. H atoms on C, N and O were
positioned geometrically and treated as riding atoms with C—H 0.93ꢀ0.98 Å, Uiso(H) = 1.2
eq(C) or 1.5 eq(C); OꢀH = 0.82 Å, Uiso(H) = 1.5 eq(O); NꢀH = 0.86 Å, Uiso(H) = 1.2 eq(N).
Platon19 and Mercury20 were used to prepare the material for publication. The hydrogen atoms of
the methyl group of orcinol in cocrystal 1·2b have two occupancies; each disordered hydrogen
atom was constrained to 50%, according to the imposed crystallographic symmetry. The chlorine
atoms of the cocrystal chloroform solvate 1·2c are disordered over two positions, which were
modeled using the PART instruction. The occupancies of the two sites converged at 0.446(4) and
0.554(4).
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Table 1. Crystal data, collection and refinement parameters.
Cocrystal
14·2a3
1·2b
1·2c
CCDC no.
Formula
953677
953679
953678
4(C14H16N2O6),
3(C6H6O2)
C14H16N2O6,
C7H8O2
C14H16N2O6, C14H22O2,
CHCl3
Formula Weight
Crystal System
Space group
a (Å)
1563.48
432.42
649.97
Triclinic
Monoclinic
P21/m (No. 11)
4.9410(2)
Monoclinic
P21/c (No. 14)
15.8182(7)
18.9181(6)
Pꢀ1 (No. 2)
15.1465(4)
17.4664(6)
b (Å)
18.7803(5)
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c (Å)
17.4943(5)
11.2062(4)
90
11.3683(4)
90
113.765(3)
108.654(2)
98.054(2)
α (°)
97.223(3)
90
92.369(4)
90
β (°)
γ (°)
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V (Å3)
3815.3(2)
2
1031.61(6)
2
3399.1(2)
4
Z
D(calc) (g/cm3)
ꢀ(CuKα) (mmꢀ19
F(000)
1.361
1.392
1.270
0.902
0.908
2.840
1644
456
1368
Crystal Size (mm)
0.33x0.30x0.28
2.9, 73.7
0.32x0.28x0.24
4.0, 73.5
0.34x0.32x0.30
3.6, 73.8
Θ
MinꢀMax (°)
Dataset
ꢀ18:17; ꢀ21:21; ꢀ21:21 ꢀ5:5; ꢀ23:23; ꢀ ꢀ19:19; ꢀ22:22;
13:13
ꢀ14:14
Tot., Uniq. Data, R(int) 58427, 14934,
0.026
10094, 2092,
0.025
32434, 6757,
0.029
11304
1668
4542
Observed data (I > 2.0 σ
(I))
Nref, Npar
R, wR2, S
14934,1009
0.089, 0.277,
1.013
2092, 150
0.042, 0.108,
1.039
6757, 408
0.09, 0.279,
1.041
Max.
Shift/Error
and
Av. 0.022, 0.02
0.00, 0.00
0.076, 0.005
Min. and Max. Resd. 1.76, ꢀ0.67
Dens (e/Å3)
0.194, ꢀ0.262
0.812,ꢀ0.633
Theoretical calculations. Geometry optimizations at the B3LYP/6ꢀ31 G (d, p) level of theory
were performed on 1,3ꢀbenzendiols 2a-d without any symmetry restraints using the Gaussian 09
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package.21 Relaxed linear potential energy surface scans for the OH group rotation were
performed using direct inversion of iterative subspace (GDIIS).22
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3. RESULTS AND DISCUSSION
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Solid cocrystalline phases of 1 and 1,3ꢀbenzenediols 2a-d in 1:1 ratios were prepared using the
solvent drop grinding method, a variant of the mechanochemical method,23 to form the
complexes 1·2a-dpowd. The resulting powders were analyzed by IR spectroscopy and Xꢀray
powder diffraction. Crystals suitable for singleꢀcrystal Xꢀray diffraction analysis of the
composition 14·2a3 were grown from crystalline powders of 1·2apowd mixture in ethyl acetate and
single crystals of the compositions 1·2b and 1·2c were grown from chloroform solutions at room
temperature, using the crystalline powders 1·2bpowd and 1·2cpowd. Efforts to crystallize the
crystalline powder 1·2dpowd were unsuccessful. Finally, crystalline powders 14·2a3powd and 1·2b-
dpowd were analyzed by 13CꢀCPMAS NMR (Cross Polarization Magic Angle Spinning).
3.1. IR spectroscopy. The IR spectra of 1, 1,3ꢀbenzenediols 2a-d, crystalline powders of
1·2apowd mixture and 1·2b-dpowd, and single crystals of 14·2a3 and 1·2b-c were recorded neat and
their relevant frequencies are listed in Table 2. The IR data confirm the formation of the
complexes since the IR spectra were different from the simple superposition of the individual
components, as is shown in Figure 1 for the complex between 1 and 2a. The IR spectrum of the
solid phases 1·2apowd, Figure 1(c), and 14·2a3powd (not shown) are exactly the same but they look
different from the crystal of 14·2a3, Figure 1(d), further XRPD experiments confirmed the same
solid phase in 1·2apowd, 14·2a3powd and 14·2a3 (vide infra).
The IR data confirm the formation of the complexes since the IR spectra were different from
the simple superposition of the individual components, as is shown in Figure 1 for 1·2apowd
mixture and complex 14·2a3. The O―H stretching bands of resorcinol 2a and orcinol 2b are in
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the expected range of hydrogen bonding resulting from self association,24 whereas 4,6ꢀdiꢀ
tertbutylꢀ1,3ꢀbenzenediol and 4ꢀhexylꢀ1,3ꢀbenzenediol 2c and 2d, respectively, show bands for
free and hydrogenꢀbonded O―H stretching frequencies (Figures S1ꢀS3). In the IR spectra of the
crystalline powders and single crystals, the stretching bands for both free and hydrogenꢀbonded
O―H are shifted to the 3416ꢀ3329 cmꢀ1 range, indicating that the protons remain localized at the
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phenolic oxygen atoms.25 The frequency shifts (±ꢂ
ν (OH)) are in agreement with a
rearrangement of the intermolecular hydrogenꢀbonding scheme involving the phenolic protons.
(OH) is similar to the value measured in the resorcinolꢀurea complex.26
The magnitude of ꢂ
ν
The IR spectrum of 1 shows a strong C=O symmetric stretching band at 1693 cmꢀ1 for both the
amide and ester carbonyls. In addition, a very weak vibration at 1723 cmꢀ1, corresponding to the
asymmetric stretching C=O vibrations, is hardly noticed. The association of 1 and 2a-d through
the amide C=O acceptor group and hydroxylic proton donor, has the effect of intensifying this
last frequency from very weak (vw) to a medium vibration band (m).27a The shifts of both C=O
bands are very small; most of them are in the range of the experimental error of ꢁν = ±2 cmꢀ1.
Usually, low frequency shifts of the C=O symmetric stretching band are associated to the
C=OꢁꢁꢁHO complex formation and highꢀfrequency shifts are associated to the free C=O bond.27
Thus, in this case the small shifts can be associated with the preservation of the carbonyl
hydrogenꢀbonding interactions in 1 upon association.
The N―H stretching band is slightly shifted to lower frequencies in the IR spectra of all
complexes, when compared to 1, indicating that the N―H group is not involved in the complex
formation.
Table 2. O―H, N―H and C=O stretching frequencies (cmꢀ1) of 1, 2a-d and their complexes in
crystalline powders, 1·2a-dpowd, and single crystals, 14·2a3 and 1·2b-d, solid phases.
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ν
(OH)
ꢂν
(OH)
ν
(NH)
ꢂν
(NH) ν (C=O)
1723 (vw)
ꢂν (C=O)
3344
1
7
8
1693 (s)
9
3177
2a
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1·2apowd 3421
244
3326
3326
ꢀ18
ꢀ18
1726 (sh)
1701 (s)
1746 (m)
1695 (s)
ꢀ3
8
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165
23
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1·2bpowd 3374
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3327
3330
ꢀ17
ꢀ14
1729 (m)
1686 (s)
1728 (m)
1688 (s)
6
ꢀ7
5
3375
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1·2b
3347
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3520, 3434, 3317
2c
1·2cpowd 3421
ꢀ99, ꢀ13, 104
3325
3327
ꢀ19
ꢀ17
1722 (sh)
1705 (s)
1730 (w)
1701 (s)
ꢀ1
12
7
3386
ꢀ134, ꢀ48, 69
ꢀ170, 84, ꢀ33
1·2c
3350
8
3417, 3336
2d
1·2dpowd 3421
4, 85
3331
ꢀ13
1724 (m)
1697 (s)
1
4
(s) strong, (m) medium, (vw) very weak, (sh) shoulder
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Figure 1. IR spectra of (a) 1, (b) 2a, (c) 1·2apowd and (d) 14·2a3 crystals.
3.2. Powder X-ray diffraction. There are two known polymorphs of diethyl N,N´ꢀ1,3ꢀ
phenyldioxalamate 1ꢀexo and 1ꢀendo. Herein, the first was used as the starting material, which
was evidenced by comparison of the XRPD pattern of the starting crystalline powder of
compound 1 with the simulated pattern from the monocrystal data of each polymorph, Figure 2.
The conformer 1ꢀexo is not transformed into 1ꢀendo under the solvent drop grinding conditions
herein used.
Comparative depictions of the XRPDꢀpatterns are shown in Figures 3ꢀ6. In general 1,3ꢀ
benzendiols can exist in three possible conformations, taking into account the spatial disposition
of both OH groups relative to H2 (Scheme 2). The XRPD pattern of the starting resorcinol
(Figure 3(b)) allowed indentifying it with the RESORA03 form, which corresponds to the antiꢀ
anti rotamer C. On the other hand, the XRPD pattern of the starting orcinol 2b (Figure 4(b))
exhibited preferential orientation effects, which alter the intensity sequence, but after grinding,
the pattern of the EWAMAR polymorph (CSD)7 was displayed (Figure 4(e)). Thus, the starting
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orcinol has the synꢀanti conformation B. To the best of our knowledge, there are no reports of
XRPD patterns nor single crystal Xꢀray structures of 1,3ꢀbenzendiols 2c (Figure 5(b)) or 2d
(Figure 6(b)); thus, the starting conformation of these compounds is unknown.
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Figure 2. Simulated XRPD patterns of (a) 1ꢀexo, (b) 1ꢀendo and (c) experimental powder of 1.
Scheme 2. Conformers of 1,3ꢀbenzenediols.
H
H
H
H
O
O
O
O
O
O
O
O
H
H
H
H
R1
R3
R1
R3
R1
R3
R1
R3
R2
R2
R2
R2
syn-syn
anti-syn
syn-anti
anti-anti
A
C
B'
B
The XRPD patterns of 1·2b-dpowd exhibit a new set of peaks in comparison to the starting
compounds 1 and the corresponding 1,3ꢀbenzenediol 2b-d, indicating the formation of a new
solid form (Figures 3(c)ꢀ6(c)). Simulations of the powder patterns of 14·2a3 (Figure 3(d)), 1·2b
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(Figure 4(d)) and 1·2c (Figure 5(d)) from the singleꢀcrystal data were performed and then
compared with the experimental XRPD diffractograms. The crystalline powder 1·2apowd and the
singleꢀcrystals 14·2a3 contain the same crystalline phase, in spite of the apparent differences
observed in the IR spectra. This result was confirmed from the XRPD pattern of a crystalline
powder mixture of composition 14·2a3powd. The same was concluded for 1·2cpowd and 1·2c. On the
other hand, the XRPD diffractograms of 1·2bpowd and 1·2b are different from each other albeit
their IR spectra were very similar. The polymorphism of 1·2bpowd and 1·2b could be explained by
the high symmetry exhibited by the crystal that could not be attained in the powder; however, the
presence of different rotamers in the ethylꢀside group can not be disregarded.
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Figure 3. XRPD patterns of (a) 1ꢀexo, (b) resorcinol 2a, (c) 14·2a3powd and (d) 14·2a3 crystals.
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Figure 4. XRPD patterns of (a) 1ꢀexo, (b) orcinol 2b, (c) 1·2bpowd, (d) 1·2b crystals and (e) 2b
ground crystals.
Figure 5. XRPD patterns of (a) 1ꢀexo, (b) 4,6ꢀdiꢀtertbutylꢀ1,3ꢀbenzenediol 2c, (c) 1·2cpowd and
(d) 1·2c crystals.
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Figure 6. XRPD patterns of (a) 1ꢀexo, (b) 4ꢀhexylꢀ1,3ꢀbenzenediol 2d and (c) 1·2dpowd
.
3.3. Single crystal X-ray diffraction analysis. The structures of the molecular complexes
were established by Xꢀray diffraction, which revealed the 4:3 stoichiometry of the cocrystal
14·2a3, and confirmed the 1:1 stoichiometry of 1·2b and 1·2c. Cocrystals 14·2a3 are triclinic,
space group Pꢀ1 with seven independent molecules in the asymmetric unit; whereas cocrystals
1·2b and 1·2c, the last as chloroform solvate, crystallized in the monoclinic space groups P21/m
and P21/c with two 1·2b and four 1·2c complexes in the asymmetric unit, respectively. Complex
1·2b has a crystallographic plane of symmetry and thus only one half of the molecule is
observed. The crystal structures of 1ꢀexo,14c 1ꢀendo,14b 2a (CCD refcode: RESORA05)7 and 2b8
are known. Bond distances and angles of 1ꢀendo, 2a and 2b remained almost unchanged upon
complexation (Tables S1ꢀS3). Differences between 1ꢀexo and 1ꢀendo have been discussed
elsewhere.14f
3.3.1. Structure of the molecular complexes 14·2a3, 1·2b and 1·2c. The structure of the
heptameric complex 14·2a3 is featured by two distinct supramolecular subunits, one being
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trimeric with 2:1 stoichiometry (12·2a), Figure 7(a), and the other tetrameric with 2:2
stoichiometry (12·2a2), Figure 8(a), respectively. In the subunit 12·2a, two molecules of 1 and one
molecule of 2a are almost coplanar (angles between the mean phenyl planes: C1ꢀC6 and C23ꢀ
C28 of 12.24(14)º, C1ꢀC6 and C91ꢀC96 of 12.61(12)º), whereas in the subunit 12·2a2, both
resorcinol molecules are tilted from the phenyl plane of 1, one of which upwards and the other
downwards regarding the plane (angles between the mean 1 and 2a phenyl planes: C31ꢀC36 and
C53ꢀC58 of 72.12(13)º, C31ꢀC36 and C83ꢀC88 of 76.77(13)º, C61ꢀC66 and C53ꢀC58 of
74.03(12)º, C61ꢀC66 and C83ꢀC88 of 78.80(13)º). Meanwhile, complexes 1·2b, Figure 9(a), and
1·2c, Figure 10(a), are heterodimeric with 1:1 stoichiometry of 1 and orcinol 2b, and 4,6ꢀdiꢀ
tertbutylꢀ1,3ꢀbenzenediol 2c, respectively. Both 1 and the corresponding 1,3ꢀbenzenediol are
almost planar. The torsion angles between the mean phenyl planes of 1·2b and 1·2c are of
5.16(6)º and 19.64(8)º, respectively. It is worth noting that, among the several possible
conformations of 2a-c, the conformation exhibited by the corresponding 1,3ꢀbenzenediol relative
to Hꢀ2, in the subunit 12·2a of complex 14·2a3 and in complexes 1·2b and 1·2c is synꢀsyn whereas
in the subunit 12·2a2 it is synꢀanti, conformers A and B, respectively (Scheme 2).
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1,3ꢀBenzenediols and 1, in the subunit 12·2a of complex 14·2a3 and in complexes 1·2b and 1·2c
are linked by two hydrogenꢀbonding interactions between the amide carbonyls and phenolic
hydrogen atoms: O23—H23ꢁꢁꢁO8 and O25—H25ꢁꢁꢁO18. The mean values of the HOꢁꢁꢁO
distances and O—HꢁꢁꢁO angles of 2.87(6) Å and 178(2)º are in the range of moderate hydrogenꢀ
bonding interactions29 and form R2 (10) motifs, according to the graph set notation.30 In the case
3
of the subunit 12·2a of complex 14·2a3, two sets of threeꢀcentered N17—H17ꢁꢁꢁO109, C4—
H4ꢁꢁꢁO109 and N107—H107ꢁꢁꢁO19, C94—H94ꢁꢁꢁO19 interactions, which involve the ester C=O
as acceptor, link both complexed and free molecules of 1 (mean values of the NꢁꢁꢁO distances and
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N—HꢁꢁꢁO angles of 3.166(14) Å and 160(2)º, respectively; mean CꢁꢁꢁO distances and C—HꢁꢁꢁO
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angles of 3.33(3) Å and 146.5(5)º, respectively). They form R1 (6), R2 (10) and R1 (6) adjacent
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ring motifs, which were also observed in the supramolecular structure of ethyl Nꢀ
phenyloxalamate,31 as well as in the supramolecular architecture of the other complexes herein
described, vide infra.
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Figure 7. (a) Molecular structure at 30% of probability of heterotrimeric subunit 12·2a of
complex 14·2a3. Selected torsion angles (º): C2C1N7C8 = 9.5(6), C2C3N17C18 = 2.5(5),
C92C91N97C98 = ꢀ6.3(6), C92C93N107C108 = ꢀ8.5(6). (b) 1ꢀD tapes along the [11 1 ꢀ1]
direction formed by N7—H7ꢁꢁꢁO99, C6—H6ꢁꢁꢁO99; and N97—H97ꢁꢁꢁO9, C96—H96ꢁꢁꢁO9
interactions (mean values of: NꢁꢁꢁO distances and N―HꢁꢁꢁO angles of 3.13(2) Å and 161.5(1)º,
respectively; CꢁꢁꢁO distances and C―HꢁꢁꢁO angles of 3.322(16) Å and 146.5(1)º, respectively).
(c) C=Oꢁꢁꢁπ, C—Hꢁꢁꢁπ interactions between stacked duplex of 12·2a complex.
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Figure 8. (a) Molecular structure at 30% of probability of heterotetrameric subunit 12·2a2 of
complex 14·2a3. Selected torsion angles (º): C32C31N37C38 = ꢀ3.3(6), C32C33N47C48 =
2.4(5), C62C61N67C68 = ꢀ12.5(6), C62C63N77C78 = ꢀ17.6(5). (b) Supramolecular architecture
in the bc plane built by N67—H67ꢁꢁꢁO49, C66—H66ꢁꢁꢁO49; N47—H47ꢁꢁꢁO69, C34—H34ꢁꢁꢁO69;
N37—H37ꢁꢁꢁO79, C36—H36ꢁꢁꢁO79 and N77—H77ꢁꢁꢁO39, C64—H64ꢁꢁꢁO39 interactions (mean
values of: NꢁꢁꢁO distances and N―HꢁꢁꢁO angles of 3.15(3) Å and 159(5)º, respectively; CꢁꢁꢁO
distances and C―HꢁꢁꢁO angles of 3.31(2) Å and 148º, respectively).
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Figure 9. (a) Molecular structure at 30% of probability of heterodimeric complex 1·2b.
Selected torsion angle (º): C2C1N7C8 = 13.5(3). (b) Weaving hydrogenꢀbonding tapes along the
direction of the b axis formed by N7—H7ꢁꢁꢁO9 and C4—H4ꢁꢁꢁO9 interactions (N7ꢁꢁꢁO9 distance
of 3.1969(19) Å and N7―H7ꢁꢁꢁO9 angle of 162º; C4ꢁꢁꢁO9 distance of 3.379(2) Å and
C4―H4ꢁꢁꢁO9 angle of 142º). (c) Stacked columns developed by dipolar interactions O=CꢁꢁꢁO
along the direction of the c axis.
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Figure 10. (a) Molecular structure at 30% of probability of heterodimeric complex 1·2c,
chloroform molecule was omitted for clarity. Selected torsion angles (º): C2C1N7C8 = 0.5(5),
C2C3N17C18 = ꢀ2.8(5). (b) 1ꢀD tapes along the [0 1 0] direction built by N7—H7ꢁꢁꢁ O19, C6—
H6ꢁꢁꢁO19 and N17—H17ꢁꢁꢁO9, C4—H4ꢁꢁꢁO9 threeꢀcentered hydrogenꢀbonding interactions
(mean values of: NꢁꢁꢁO distances of 3.22(4) Å and N―HꢁꢁꢁO angles of 158(3)º; CꢁꢁꢁO distances of
3.28(2) Å and C―HꢁꢁꢁO angles of 149.5(1)º). (c) Family of planes along the (0 7 0) direction
linking 1ꢀD tapes through C―Hꢁꢁꢁπ interactions.
In the tetrameric subunit 12·2a2 of complex 14·2a3, the mean of the four HOꢁꢁꢁO distances is
2.89(1) Å, a value similar to that found in the dimers, but the O—HꢁꢁꢁO angle depends on the
conformation of the OH group: it is almost linear, 172(1)º, when the OH is in anti conformation
and slightly bent, 161º, when syn. In this case a larger ring, described by the R4 (20) motif, is
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In order to properly focus the discussion, the hydrogenꢀbonding motifs relevant for the
organization of 1,3ꢀbenzenediols with carbonyl moieties were systematized. The analysis of the
related cocrystals deposited in the CSD exhibiting the OHꢁꢁꢁO=C motif was performed and the
resulting relevant motifs IꢀVII are depicted in Scheme 3. In regards to the 1,3ꢀbenzenediol, it
adopts the rotamer B, synꢀanti form in five of them, the antiꢀanti rotamer C is observed in four,
while the synꢀsyn rotamer A is only observed in two structures (Scheme 2). This behavior
contrasts with the OHꢁꢁꢁN motif in molecular crystals of 1,3ꢀbenzenediols with pyridines where
this last rotamer is the most frequently observed.28 Only cocrystals with 2a and 2b were
considered for comparison purposes because, to the best of our knowledge, cocrystals of 2c or 2d
with carbonyl moieties are not known.
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60
Scheme 3
O
(a)
Syn-anti
H
O
H
H
O
O
H
H
R = CH3, A = OH
R = H, A = CO
N
N
O
H
H
O
O
H
H
H
H
H
H
O
O
O
O
O
O
N
N
O
O
O
H
H
O
H
H
H
H
H
A
H
O
O
H
Motif I
Motif III
Motif II
(b)
Syn-syn
O
O
H
H
O
O
O
O
O
N
O
N
RO
H
N
H
H
O
H
O
N
OR
H
O
O
O
H
O
Motif IV
Motif V
(c)
Anti-anti
N
O
H
H
O
H
H
H
O
O
H
H
O
O
O
O
H
H
O
H
N
O
O
O
O
Motif VI
N
O
H
H
O
H
H
O
O
O
O
H
H
H
H
O
N
Motif VII
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Crystal Growth & Design
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9
In this context two new motifs, VIII and IX, are described. In the first motif, the 1,3ꢀ
benzendiol adopts the synꢀsyn conformation whereas in the last the synꢀanti conformation is
adopted. Motif VIII is observed in the subunit 12·2a of complex 14·2a3, and in complexes 1·2b
and 1·2c. Motif IX is observed in the subunit 12·2a2 of complex 14·2a3. In both cases the 1,3ꢀ
benzenediol acts only as OH donor (Scheme 4).
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14
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60
Scheme 4
syn-anti
syn-syn
O
OEt
O
H
EtO
H
H
N
O
O
H
H
O
N
N
N
H
O
O
O
H
H
O
O
H
O
N
EtO
OEt
H
O
H
H
H
O
H
N
O
O
O
OEt
H
O
EtO
O
Motif IX
Motif VIII
3.3.2 Intramolecular three-centered hydrogen bond. Complexation modifies the
conformation of both oxalyl arms, which are twisted by almost 180º, from the exoꢀanticlinal
conformation in the starting polymorph 1ꢀexo, to an endoꢀsynperiplanar conformation in the
three complexes herein reported. The torsion angles C2C1N7C8 (subunit 12·2a of complex
14·2a3, and complexes 1·2b and 1·2c), C2C3N17C18 (subunit 12·2a of complex 14·2a3, and
complex 1·2c), Cn2Cn1Nn7Cn8 and Cn2Cn3N(n+1)7C(n+1)8 (n = 3, 6 for subunit 12·2a2 of
complex 14·2a3 and n = 9 for subunit 12·2a of complex 14·2a3) are in the mean range of ±0.0(5)º
to ±17.6(5)º (See the captions of Figures 7ꢀ10). Thus, in the three complexes, both oxalyl arms
are almost coplanar with the phenyl ring, allowing the formation of a full intramolecular
hydrogenꢀbonding scheme through N—HꢁꢁꢁO (mean values of NꢁꢁꢁO distances of 2.705(16) Å
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1
2
and N―HꢁꢁꢁO angles of 109.6(5)º) and threeꢀcentered (amide)COꢁꢁꢁH(Csp2)ꢁꢁꢁOC(amide)
interactions, described as a whole by the graph set notation as S(5)S(6)S(6)S(5) motif (Figures
7(a)ꢀ10(a)). In Table S4, a summary of the geometry and graph set descriptors associated with
intramolecular and intracomplex hydrogenꢀbonding interactions is listed.
3
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7
8
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60
The intramolecular hydrogenꢀbonding scheme in 1 is strengthened as a consequence of
coplanarity. The mean value of the C2H2ꢁꢁꢁO8 distances is 2.28(2) Å (subunit 12·2a in complex
14·2a3, and complexes 1·2b-c), C2H2ꢁꢁꢁO18 (subunit 12·2a in complex 14·2a3, and complex 1·2c),
Cn2Hn2ꢁꢁꢁOn8 and Cn2Hn2ꢁꢁꢁO(n+1)8 (n = 3, 6 for subunit 12·2a2 in complex 14·2a3) and the
mean value of the corresponding C—HꢁꢁꢁO angles is 122(1)º, thus being shorter and wider than
the corresponding values in uncomplexed 1ꢀendo with values of 2.450(12) Å and 108.8(8)º,
respectively.
The mean OꢁꢁꢁO distance between amide carbonyls of complexed 1 in the subunit 12·2a of
complex 12·2a2, and in complexes 1·2b and 1·2c (motif VIII), is 3.88(4) Å, which is shorter than
the value exhibited in uncomplexed 1ꢀendo of 4.638(2) Å. This OꢁꢁꢁO distance is also shorter
than the values measured in other 1,3ꢀbenzendiolꢁcarbonyl motifs such as IV (open, 4.855 Å) or
V (closed, 5.031 Å) and even shorter than the NꢁꢁꢁN mean distance of 4.13(17) Å observed in 1:1
molecular crystals between 1,3ꢀbenzenediols and pyridines (CSD). The approaching between
carbonyl amides is in agreement with the strengthening of intramolecular
(amide)COꢁꢁꢁH(Csp2)ꢁꢁꢁOC(amide) threeꢀcentered hydrogenꢀbonding scheme in 1 due to
complexation, overcoming the amide oxygen–oxygen repulsive interactions. This result is
opposite to that caused by the steric effects of the substituents in the phenyl ring of 1ꢀendo,
which are responsible for twisting the oxalyl arms, moving away the amide carbonyls from each
other to give rise to the conformer 1ꢀexo.15
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9
Complexation of 1 with 1,3ꢀbenzenediols also exerts the effect of moving apart the oxalyl
arms. The distance O10ꢁꢁꢁO20, between the ester carbonyls is enlarged from 8.397(3) Å in the
free form of 1ꢀendo, to 8.887(4) Å (O10ꢁꢁꢁO20) in the subunit 12·2a of complex 14·2a3, 8.929(5)
Å (O40ꢁꢁꢁO50) and 8.864(4) Å (O70ꢁꢁꢁO80) in the subunit 12·2a2 of complex 14·2a3, 8.706(2) Å
(O10ꢁꢁꢁO’10) in 1·2b and 9.074(3) Å (O10ꢁꢁꢁO20) in 1·2c in order to fit the steric demand of the
1,3ꢀbenzenediol inside the cavity. The distance O100ꢁꢁꢁO110 in the subunit 12·2a of complex
14·2a3 (Figure 7(a)) is also enlarged (8.818(4) Å) in spite of the empty cavity because of
intercomplex interactions (vide infra). The involvement of amide carbonyls in intramolecular
threeꢀcentered hydrogen bonding32 would play a positive role in the association process, because
of the amide moieties NH and C=O, in 1ꢀendo are already polarized. This cooperative effect has
also been observed in other amideꢀbenzenediol complexes.33
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Following with the preꢀorganization proposal and cooperative polarization effects, the
hydrogenꢀbonding scheme in the subunit 12·2a of complex 14·2a3, and in complexes 1·2b and
1·2c can be described as ADA·DD system and that found in the subunit 12·2a2 of complex 14·2a3,
as (ADA·D2)2 system (Scheme 5), taking into account the donor (D) and acceptor (A) elements.
The donor in ADA (C(sp2)H) links both acceptors (amide C=O), not only facilitating their
disposition into the plane for the proper association with the DD or D2 (OH) complementary
partners, but also enhancing their subsequent association by polarization. It is worth highlighting
that an arrangement of hydrogenꢀbonding donors and acceptors, similar to that herein described,
has been used as a model to study the cooperativity associated with the formation of
intermolecular hydrogen bonding in solution.34 These findings contrast with those found in
trisamides of N,Nꢀbis(2ꢀaminoethyl)ethaneꢀ1,2ꢀamine where the formation of intramolecular
hydrogen bonds has been proposed to avoid their association with nꢀoctylꢀglucose.35
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1
2
3
4
Scheme 5
5
6
7
8
9
D
D
A
A
D
A
A
D
D
D
10
11
12
13
14
15
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17
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D
A
A
D
D
.
.
ADA
DD
(ADA D2)2
3.3.3 Supramolecular architecture of complexes 14·2a3, 1·2b and 1·2c. The values of the
geometric parameters associated with intercomplex hydrogen bonding are listed in Table S5,
mean values for each complex are in the captions of Figures 7(b)ꢀ10(b) and the geometric
parameters associated with πꢀinteractions are listed in Table S6.
A comparison among the selfꢀassembled lattice structures of the complexes herein reported,
have revealed remarkable similarities in the hierarchy of their structures. Motif X, is common to
the three complexes herein reported (Scheme 6). Motif X can be described as a zigꢀzagging
homopolymeric planar tape of R1 (6), R2 (10) and R1 (6) adjacent ring motifs, where the
2
2
2
moderate intermolecular N—H(amide)ꢁꢁꢁOC(ester)ꢁꢁꢁH―C(sp2) hydrogen bonds are involved as
supramolecular heterosynthons.
Scheme 6
EtO
OEt
EtO
OEt
O
O
O
O
H
H
N
H
H
O
O
O
O
1
(6)
R
N
H
N
H
N
H
N
1
2
H
H 2
HR2
(10) H
2
H
H
H
NS(6)
(6)
R
H S(6)
O
O
O
O
EtO
OEt
Motif X
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Crystal Growth & Design
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9
Duplex of motif X, held together through C=Oꢁꢁꢁπ and C—Hꢁꢁꢁπ interactions, develops the 1ꢀD
of the subunit 12·2a along the [11 1 ꢀ1] direction, Figures 7(b) and 7(c). Motif X interlinks the
subunits 12·2a2 in complex 14·2a3 to develop the second dimension as layers in the bc plane,
Figure 8(b). Finally, the subunits of complex 14·2a3 are held together through C—Hꢁꢁꢁπ
interactions to develop the third dimension along the a axis, Figure 11.
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60
Figure 11. 3ꢀD assembly of subunits 12·2a and 12·2a2 in complex 14·2a3.
Motif X, form 1ꢀD weaving tapes in complex 1·2b and planar tapes in 1·2c that propagate
along the b axis, Figures 9(b) and 10(b), respectively. The curvature effect in 1·2b is induced by
the symmetry of the complex and the out of the mean phenyl plane disposition of the oxalyl arm
(C2C1N7C8 = 13.5(3)º and C12C11O10C9 = 85.5(2)º). The second and third dimensions in 1·2b
and 1·2c are developed by C—Hꢁꢁꢁπ and dipolar interactions, Figures 9(c) and 10(c), among
which the dipolar threeꢀcentered36 HOꢁꢁꢁC=O interactions: O23ꢁꢁꢁC9=O9 3.076(3) Å, 99.54(15)º
and O23ꢁꢁꢁC8=O8 3.142(3) Å, 99.87(15)º, where the partial positive charge, residing on the C
atom of the intrinsically polar C=O unit, represents an electrostatic center of attraction for the
partially negatively charged O atom of the OH fragment of a neighboring orcinol, in complex
1·2b.
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2
It is worth noting that the robust heterosynthon N–H(amide)ꢁꢁꢁOC(ester) (R2 (10) ring motif),
3
4
2
5
6
7
8
responsible of the mesoꢀhelix supramolecular structure of uncomplexed 1ꢀexo, prevails upon
complexation with 1,3ꢀbenzenediols, in spite of the 180º twist in the conformation. A similar
finding is observed in the cocrystal of resorcinol with the secondary amide isoquinolinꢀ3(2H)ꢀ
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one,8 whose homosynthon N–H(amide)ꢁꢁꢁOC(amide) (R2 (8) ring motif), is preserved. This result
2
indicates that, at least with 1,3ꢀbenzenediols, oxalamateꢁꢁꢁoxalamate interactions are as strong as
amideꢁꢁꢁamide ones, in contrast to the predicted weakness of oxalamideꢁꢁꢁoxalamide
interactions.37
13
3.4. C CPMAS NMR of crystalline powders. In order to gain more information about the
structure of the crystalline powdered complexes, 13C CPMAS NMR spectra of 1ꢀexo, 2a-d,
14·2a3powd and 1·2b-dpowd were recorded. The signals of the 13C CPMAS spectra were assigned in
agreement with solution data for uncomplexed 1 and 2a-d. Complete isotropic chemical shifts
concerning 1 and 2a-d in both free and complexed forms are listed in Tables 3 and 4,
13
respectively. The C CPMAS spectrum of 1ꢀexo showed the same number of signals that it did
in solution because of the C2 molecular symmetry in the crystal, with the two fold axis running
through the Cꢀ2 and Cꢀ5 atoms of the benzene ring. Six and twelve signals appear in the 13C
CPMAS spectra of 2a and 2d, respectively, in agreement with the number of crystallographically
different carbon atoms in the molecule; nevertheless, most of the 13C NMR resonances of 2b are
split into doublets as expected for two inequivalent molecules in the asymmetric unit (CCD
refcode: EWAMAR)7. At last, four distinctly inequivalent signals for Cꢀ2 were observed in the
13C CPMAS spectrum of 2c, in agreement with four independent molecules in the solid powder.
The data for 1ꢀexo, 2a and 2b are in accordance with the reported Xꢀray structures (vide supra).
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Crystal Growth & Design
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Table 3. 13C CPMAS isotropic chemical shifts (δ) of diethyl N,N′ꢀ(1,3ꢀphenylene)dioxalamate in
5
free 1ꢀexo and complexes 14·2a3powd and 1·2b-dpowd
.
6
7
8
5
9
6
4
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12
13
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O
O
9
15
O
O
12
8
3
18
N
1
N
14
2
11
17
O
H
H
O
Compound Cꢀ1,3
1ꢀexo 140.3
Cꢀ2
Cꢀ4,6 Cꢀ5
Cꢀ8,14
Cꢀ9,15
Cꢀ11,17 Cꢀ12,18
117.0 121.0 131.3 156.8
114.8 116.1 127.6 158.1
164.6
159.9
67.5
66.1
64.8
64.7
62.7
17.2
14·2a3powd 137.7
12.9
(2C)
137.0
1·2bpowd
1·2cpowd
1·2dpowd
138.1
(2C)
111.2 117.5 126.1 158.7
158.7
(2C)
12.5
(2C)
(2C)
115.9
138.3
137.4
142.0
140.2
108.9 116.2 129.1 160.9
154.0
(2C)
64.7
(2C)
14.5
13.5
(2C)
114.9
112.5 120.1 133.9 159.1
163.1
162.1
68.4
67.4
17.3
(2C)
(2C)
118.4
The signals for the carbon atoms of 1 and the corresponding 1,3ꢀbenzenediol of 1·2b-dpowd, are
consistent with the proposal of a single complex in the crystalline powder. In the particular case
of 14·2a3powd, all signals are broader than those observed for the other complexes, as expected for
the seven independent molecules found in the unit cell of 14·2a3 cocrystal.
Table 4. 13C CPMAS isotropic chemical shifts (δ) of the 1,3ꢀbenzenediol in free 2a-d and
complexes 14·2a3powd and 1·2b-dpowd
.
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