Crystal Structure, Supramolecular Organization, Hirshfeld Analysis, Interaction Energy, and Spectroscopy of Two Tris(4-aminophenyl)amine-Based Derivatives

Abstract

The use of tris(4-aminophenyl)amine (TAPA) as central to the synthesis of both polyimines and polyimides and covalent organic frameworks and inorganic cages, among others, has grown in the last few years. The resulting materials exhibit high performance in their area of application. In this contribution, the crystal structures of two TAPA derivatives, triethyl (nitrilotris(benzene-4,1-diyl))tricarbamate (1) and triethyl 2,2′,2″-((nitrilotris(benzene-4,1-diyl))tris(azanediyl))tris(2-oxoacetate) (2), are described. The molecular and supramolecular structures of both compounds were compared between them and with analogous compounds. The analyses of their vibrational and ¹³C-CPMAS NMR spectroscopies, as well as their thermal stability, were included and corelated with the crystal structure. Hirshfeld surface analysis on the crystal structures of both TAPA derivatives revealed the stabilization of the crystal network via the amide N—H∙∙∙O interactions of dispersive nature in the carbamate, whereas dispersive carbonyl–carbonyl interactions also played a competitive role in the supramolecular arrangement of the oxamate. Interaction energy DFT calculations performed at the B3LYP/6-31G(d,p) level allowed us to estimate the energy contributions and nature of several interactions in terms of the stability of both crystal lattices.

1. Introduction

Tris(4-aminophenyl)amine (TAPA) is a tritopic compound widely used in material science for the synthesis of covalent organic frameworks (COFs), polymers, and inorganic cages, among others. Its use has been exponentially growing since its first report appeared in 1967 [1]; in sum, 873 publications to date, with patents amounting to half of them, and 81% have been published in the last ten years [2]. TAPA and some of its derivatives have been reported as versatile building blocks for organic electronic applications [3]. In this context, covalent organic frameworks (COFs) based on TAPA are promising crystalline materials [4,5] that have been applied in the preparation of permeable membranes with high separation performance [6,7,8,9], semiconductor thin films [10], energy storage [11,12], and photophysic applications [13,14,15]. Moreover, TAPA-based porous polymers have been synthesized as both polyimines and polyimides, the latter being much more stable. Therefore, TAPA-based polymers have been used as a base for the selective capture of toxic heavy metal ions from water [16], for photocatalyst applications [17], and for gas separation [18,19,20,21,22,23,24,25], among other applications. The main function of TAPA, as a core starting structure in COFs, porous polymers, and inorganic cages, is to provide helicity and planarity, as well as to obtain hyperbranched structures.

TAPA is triphenylamine (TPA) substituted with an amino group in the para-position of each of the three phenyl groups (tris(4-aminophenyl)amine). The central nitrogen atom is a tertiary amine with an almost-planar geometry because of the resonance delocalization of the N-electron lone pair on the whole structure. The crystal structure of TPA, although not published, has been deposited in the CCDC with the snem01 identification code [26], with four independent molecules in the asymmetric unit. Computational calculations are in accordance with the C–N bond length of 1.42 Å, the C–N–C bond angle of 120°, and the torsion CCNC angle of 42° for tilting the phenyl rings [27]. The complex bis-TAPA-(acetonitrile)chlorocopper(I) is the first metal complex containing TAPA as a ligand whose structure has been determined [28]. In this fashion, the crystal structure of N,N′,N″-[nitrilotris(4,1-phenylene)]tributanamide 1,4-dioxane solvate has also been reported [29].

On the other hand, our research group has devoted strong attention to the study of molecular and supramolecular structures of phenylene oxalic acid derivatives, such as oxamates, oxalamides, and thiooxalamides, whose structures include functional groups, such as N-H, C=X (X=O, S), and aromatics, capable of forming hydrogen bonding interactions. These studies cover the nature and strength of the three-centered hydrogen bond (TCHB), solid-state polymorphism, the effect of steric restraints, host–guest complexes, and supramolecular self-assembly [30,31,32,33,34,35]. Moreover, a considerable amount of work has been published on the metal coordination chemistry of oxalic acid derivatives as ligands [36,37,38,39], metal–organic frameworks [40,41,42], and metallosupramolecular chemistry [43,44].

Oxamates are bifunctional compounds whose supramolecular architectures are driven by N—H∙∙∙O=C and C=O∙∙∙H—O supramolecular synthons. The formation of intramolecular hydrogen bonds between the amide NH and the CO₂R carboxy–oxygen atom makes the oxamic fragment flat, whose relative disposition in relation to the phenyl ring rises to antiperiplanar (ap) ±180–±150°, anticlinal (ac) ±150–±90°, synclinal (sc) ±90–±30°, and synperiplanar (sp) ±30–±0° conformations [32].

In this contribution, the molecular and supramolecular structures, IR spectroscopy, ¹³C-CPMAS NMR, and thermal analysis of two derivatives of TAPA (namely triethyl (nitrilotris(benzene-4,1-diyl))tricarbamate (1) and triethyl 2,2′,2”-((nitrilotris(benzene-4,1-diyl))tris(azanediyl))tris(2-oxoacetate) (2)) are described, notably in Figure 1. The nature and energetics of the non-covalent interactions involved in the stability of the crystal lattice were explored through computational methods, including the evaluation of Hirshfeld surfaces (HSs), energy framework diagrams, and the quantitative analysis of intermolecular solid-state interactions. This contribution aims to bring to light the role of carbamate and oxamate functional groups in the conformational stability of the crystal networks of TAPA.

2. Materials and Methods

2.1. Crystallography

The single crystal data of compounds 1 and 2 were recorded on a Bruker D8 VENTURE diffractometer using a graphite monochromator (Mo Kα, λ = 0.71073) at 150 K (1) and 100 K (2). Cell refinement and data reduction were carried out with the APEX2 v2012 and SAINT V8.27B [45] software, respectively. The structures were solved by direct methods using the SHELXS-97 program [46] of the WINGX package [47]. The final refinement was performed by full-matrix least-squares methods using the SHELX97 ver. 2018/3 program [46]. The H atoms attached to the carbon atoms were included in geometrically calculated positions based on their hybridization and isotropically refined, with C—H distances of 0.95 (aromatic), 0.98 (methyl), and 0.99 Å (methylene). The positions of the H atoms bonded to N were found on a Fourier difference map and freely refined. Platon [48] and Mercury [49], from the WINGX 2021.3 platform, were used to prepare the material for publication. For compound 1, two disorder positions [0.479(6) and 0.521(6)] were refined for molecules N3 (O29A,B, C31A,B) and N4 (C41A,B). For compound 2, two disorder positions [0.264(1) and 0.736(1)] were refined for molecule N1 (C28A,B, O28A,B); two others [(0.45(3) and 0.55(3)] were refined for molecule N2 (C49A,B, O58A,B); and two more [0.312(17) and 0.688(17)] were refined for molecule N3 (O88A,B, C89A,B, C91A,B, C92A,B). Positional disorder was treated with commands DFIX, PART 1 and PART 2, and additionally SIMU, SAME, SADI, and ISOR of the SHELXL-97 Program, for 1 and 2, respectively. General crystallographic data for compounds 1 and 2 were deposited in the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication numbers 2260080 and 2260079, respectively.

2.2. Hirshfeld Surfaces, Interaction Energies, and Energy Framework Diagrams

The Hirshfeld surfaces (HSs), shape index (SI), curvedness (CS) [50], intermolecular interactions, and their reciprocals’ fingerprints [51,52] were determined by previous procedures [35] in the CrystalExplorer 21.5 software [53]. The energy components E_elect (electrostatic potential), E_disp (dispersion), E_pol (polarization), E_rep (repulsive), and E_total (total energy) were calculated linking the software with the Gaussian 09 software [54] using the B3LYP/6-31G (d,p) theory. Scale factors were applied [55], and the % contribution of individual components to the stabilization energy were also calculated. The energy distribution in the crystal package was represented as frameworks [55,56] by independent energy components E_elect, E_disp, and E_tot in a 2³ unit cells, and a 5.0 Å radius, with a tube size of 150 kJ mol⁻¹ and a cut-off value of 10 kJ mol⁻¹, was used as the limit.

2.3. Instrumental

FT-IR spectra were recorded neat at 25 °C using a Perkin Elmer FT-IR Spectrum Two, using the Miracle ATR Single Reflection Diamond (PIKE Technologies) device and are reported in terms of wavenumber of absorption (cm⁻¹). To indicate the intensity of the signals, the following abbreviations are used: w for weak, m for medium, and s for strong. Melting points were determined with an Electrothermal Mel-Temp melting point apparatus (Cole Parmer) and are uncorrected. ¹H and ¹³C chemical shifts were acquired on Varian NMR spectrometer Mercury-300 using DMSO-d6 as the solvent. All chemical shift values (δ) are reported in parts per million (ppm), using as reference the residual solvent peak (DMSO-d6: ¹H, δ 2.50; ¹³C, δ 39.52), and the coupling constants (ⁿJ) are in Hz. The following abbreviations are used to indicate the multiplicity of the signals: s for a singlet, d for a doublet, t for a triplet, and q for a quadruplet. ¹³C CPMAS spectra were recorded on a Bruker Avance DPX-400 (101 MHz). The following conditions were applied: spectral width, 30.242 kHz; acquisition time, 33.8 ms; contact time, 2000 ms; rotation rate, 8 kHz; and relaxation delay, 5 s. Up to 256 scans for each spectrum were collected. TG measurements were performed in a Thermobalance Q5000 IR of TA instruments using approximately 3.0–5.0 mg of sample and a gradient of 5.00 °C/min from R.T. to 350 °C under air flux of 50 mL/min in open panels.

2.4. Synthetic Methodology

TAPA was synthesized as reported [57,58], through reduction of tris(4-nitrophenyl)amine with hydrazine and Pd/C. All reagents were analytical grade, purchased from Sigma-Aldrich, and used as received.

2.4.1. Triethyl (Nitrilotris(benzene-4,1-diyl))tricarbamate (1)

In a round-bottom flask, 50 mL of dry THF, 1.50 g (5.17 mmol) of tris(4-aminophenyl)amine, and 5 mL (35.9 mmol) of triethylamine were added; to this mixture, 2.21 mL (23.3 mmol) of ethyl chlorocarbonate were added under vigorous stirring and a cold bath. The reaction mixture was left at room temperature for 2 h under stirring. At the end, the solvent was evaporated under vacuum; then, 50 mL of distilled water was added, the resulting suspension was stirred during 30 min, and the filtered solid was washed with water, left to dry at r.t., and recrystallized from ethyl alcohol to obtain 2.1 g (80% yield) of beige solid that melts at 193–195 °C. ¹H NMR (DMSO-d6) δ: 9.48 (s, 1H), 7.30 (d, 2H, ³J = 8.8, H-3), 6.83 (d, 2H, ³J = 8.8, H-2), 4.07 (q, 2H, ³J = 7, CH₂), 1.20 (t, 3H, ³J = 7, CH₃). ¹³C NMR (DMSO-d6) δ: 158.8 (NCO), 147.50 (C1), 139.1 (C4), 128.9 (C2), 124.8 (C3), 65.2 (CH₂), 19.7 (Me). ${\nu }^{¯}$ (cm⁻¹) = 3305 (br, NH), 1694 (m, C=O), 1593 (w, Ar), 1528 (m), 1502 (m), 1224 (s), 1058 (s). Single crystals suitable for X-ray were obtained as follows: 60 mg of 1 were dissolved in 50 mL of hot ethyl alcohol; after cooling at RT, the resulting solution was filtered and left to stand for slow evaporation until crystals appeared.

2.4.2. Triethyl 2,2′,2″-((Nitrilotris(benzene-4,1-diyl))tris(azanediyl))tris(2-oxoacetate) (2)

This was prepared as described for 1 but using 2.60 mL (23.3 mmol) of ethyloxalylchloride instead. The reaction mixture was additionally refluxed for 1 h. After solvent evaporation, the solid was washed with distilled water and left to dry at r.t. to quantitatively obtain 3.05 g (2.17 mmol) of a yellow-greenish solid that melts at 196–198 °C. ¹H NMR (DMSO-d6) δ: 10.75 (s, 1H, NH), 7.63 (d, 2H, ³J = 8.8, H-3), 6.94 (d, 2H, ³J = 8.4, H-2), 4.27 (q, 2H, ³J = 7.1, CH₂), 1.28 (t, 3H, ³J = 7.1, Me). ¹³C NMR (DMSO-d6) δ: 161.2 (C=O), 155.6 (NC=O), 144.1 (C1), 133.0 (C4), 124.1 (C2), 122.3 (C3), 62.8 (CH₂), 14.3 (Me). ${\nu }^{¯}$ (cm⁻¹) = 3308 (br, NH), 1762 (vw, C=O_asym), 1731 (w, C=O_asym), 1692 (s, C=O_sym), 1683 (s, C=O_sym), 1593 (Ar), 1529 (m), 1504 (s), 1289 (s), 1268 (s), 1176 (s), 1155 (s). Single crystals suitable for X-ray were obtained as already described for 1, starting from 10 mg of 2 and 10 mL of methyl alcohol.

3. Results and Discussion

3.1. Molecular and Supramolecular Architecture of Compound 1

Compound 1 crystallized as a trigonal crystal system in the space group P-3. Crystal data, data collection, and refinement details of the structure are summarized in

Table 1. Crystallographic data of compounds 1 and 2.

	1	2
Crystal Data
Chemical formula	C27H30N4O6	C30H30N4O9
Molecular weight [g/mol]	506.55	590.59
Crystal system	Trigonal	triclinic
Space group	P-3 (No.147)	P-1 (No. 2)
a, b, c [Å]	17.536 (2), 17.536 (2), 19.103 (2)	12.8325 (2), 13.6938 (3), 26.7504 (5)
α, β, γ [deg]	90, 90, 120	91.535 (2), 90.038 (2), 113.120 (2)
V [Å3]	5087.333	4321.320
Z	8	6
D (calc) [g/cm3]	1.323	1.362
µ (MoKα) [/mm ]	0.095	0.102
F (000)	2144	1860
Crystal size [mm], color	0.42 × 0.25 × 0.15, purple	0.33 × 0.28 × 0.25, orange
Data Collection
Temperature (K)	150	100
Radiation [Å]	Mo Kα 0.71073	Mo Kα 0.71073
Theta Min-Max [Deg]	2.3, 25.0	2.3, 27.9
Dataset	20:−20; 20:−20; 22:−22	16:−16; 17:−17; 34:−34
Tot., Uniq. Data, R(int)	147,529; 5790; 0.127	124,561; 20,056; 0.084
Observed Data [I > 0.0 sigma (I)]	4049	11,684
Refinement
Nref, Npar	5790; 490	20,056; 1301
R, wR2, S	0.063, 0.147, 1.042	0.060, 0.142, 1.031
Max. and av. shift/error	0.00, 0.00	0.001, 0.000
Min. and max. resd. Dens. [e/Å3]	−0.645, 0.676	−0.064, 0.55

. The asymmetric unit contains one third of the fragment of four independent molecules labeled as 1N1, 1N2, 1N3 and 1N4. The other two parts of each molecule were generated by C₃ rotations and a crystallographic inversion center; then, a total of eight molecules in the unit cell (Z = 8) were found. In Figure 2, only four independent molecules are shown for clarity.

A first analysis of the four complete molecules of 1 evidences the agreement of aromatic C—C, C—N, C=O, and O—Et bond lengths with reported values. The complete parameters inherent to the geometry of compound 1 are listed in Table S1. The amide conformation is typical for a secondary amide, with the NH almost anti positioned to the amide carbonyl with HNCO torsion angle values H(n7)—N(n7)—C(n7) —O(n7) (n = 0, 1, 2, 3) of −178(3)°, −172(2)°, 142(4)°, and −176(4)°, with the molecule 1N3 (with n = 2) being the most distorted. The amide nitrogen is tilted away from the plane of the aromatic ring with Car—Car—Nn7—Cn8 (n = 0–3, ar = aromatic) angles of 48.6(5)°, 13.5(4)°, −29.9(6)°, and 28.6(7)°. Nevertheless, the lone pair of the amide nitrogen is more delocalized to the amide carbonyl (mean bond length Nn7—Cn8 = 1.344(10) Å) than to the aryl ring (mean bond length Nn7—Car = 1.414(6) Å).

In spite of the central nitrogen (Nc) being close to planarity, with a mean Car—Nc—Car angle of 119.8(3)°, the mean Nc—Car distance value of 1.419(7) Å is in close correspondence with single Car—Nsp³ bonds of 1.426(11) Å [59]. The three ethyl benzenecarbamate rings adopt a three-bladed propeller structure around the central nitrogen with values of the angles between the NPhNHCO₂Et planes of 86.8(4)°, 63.2(4)°, 77.7(4)°, and 81.2(4)° for 1N1, 1N2, 1N3, and 1N4, respectively—wider than the average measured in TPA (74(5)°) (CCDC: snem01) [26]. This result indicates that the carbamoyl group increases the steric repulsion of the neighboring phenyl rings of TPA, which is minimized by tilting the phenyl rings away from the plane.

In TAPA [60], the Nc is planar, and this can be judged by the sum of angles around the Nc (ΣNc), which is 360(1)°, with a mean Car-Nc-Car angle value of 120(1)°. Compared to reported analogous compounds A–E [29,61,62,63], this geometric parameter remains almost unchanged with amidation, while the mean angle between the three NcPh planes, as well as the mean angle between the Ph and the CNCOR planes, become wider as the steric demand of the R substituent increases. The angles between the NcPhs planes and between the Ph and the CNCOR planes are listed in

Table 2. Mean angles between selected planes of compounds 1 and A–E.

Comp.	Angle/° between NcPh Planes (σ)	Angle/° between Ph and CNCOR Planes (σ)
TAPA [60]	67(8)
A [61]	60.8(3.1)	8(3)
B [62]	63.0(4.5)	15.2, 17.2, 65.2
1	67.5(4.1)	39(7)
C [29]	76.06(0.03)	28.9(1)
D [63]	83.3	53.5
E [63]	82.6	60.5

for 1 and A–E, sorted from the lowest to highest steric effect. Compound A with an isonicotinic acid residue shows the smallest steric effect, probably due to the planarity of the ring and it best fitting into the crystal network. Nevertheless, in the Cu and Cd complexes of A [61], the angles between the NcPhs planes and between the Ph and CNCOR planes become wider compared to A; they take values in the 60–78° and 2.5–41° ranges, respectively, by a complexation effect.

The primary structure that shapes the supramolecular architecture is an infinite chain formed by the sequence of [1N4∙∙∙1N2∙∙∙1N3∙∙∙1N1]_n molecules, bonded together through N—H∙∙∙O=C hydrogen bonding (HB) interactions, between the NH and the carbonyl of the carbamate group. These HB interactions, running in chains along the direction of the c axis, are favored by anti disposition between the NH and CO groups. The union between the ending molecules 1N1 and 1N4 is given by three N—H∙∙∙O=C hydrogen bonds, forming a dimer bonded together through the participation of the three arms of each molecule, as shown in Figure 3. The dimer 1N1∙∙∙1N4 acts as a node to develop the second and third dimensions also through N—H∙∙∙O=C hydrogen bonds and C—H∙∙∙π interactions that link three primary chains [1N4∙∙∙1N2∙∙∙1N3∙∙∙1N1]_n. Layers of molecules of 1N3 and 1N2 alternate in the ab plane. A schematic representation is shown in Figure 4. A summary of the geometric parameters of intermolecular interactions is listed in

Table 3. Geometric parameters associated with intermolecular interactions of compound 1.

Interaction	Symmetry Code	D—H/Å	H∙∙∙A/Å	D∙∙∙A/Å	D—H∙∙∙A/°
N7—H7∙∙∙O28	[−x + y, −x, z]	0.90(3)	2.10(3)	2.969(3)	162(3)
N17—H17∙∙∙O38	[x, y, z]	0.86(5)	2.20(4)	3.010(3)	159(4)
N27—H57∙∙∙O18	[x − y, x, z]	0.88(4)	2.08(4)	2.934(4)	165(4)
N37—H37∙∙∙O8	[x − y, x, 1−z]	0.83(4)	2.15(94)	2.956(3)	164(4)
C31BB—H31FB∙∙∙Cg(3)	[−y, x − y, z]		2.53	3.392(6)	147
C31BB—H31DB∙∙∙Cg(4)	[−x + y, 1 − x, z]		2.50	3.453(6)	165
C31AA—H31AA∙∙∙Cg(2)	[−x + y, 1 − x, z]		2.74	3.507(104)	135

Cg(2) = C131–C136, Cg(3) = C101–C106, Cg(4) = C111–C116.

.

3.2. Molecular and Supramolecular Architecture of Compound 2

Compound 2 crystallizes as a triclinic system in the space group P-1 with three independent molecules in the asymmetric unit and, therefore, six molecules in the unit cell, labeled as 2N1, 2N2, and 2N3. Positional disorder remains in the OEt fragments. A summary of crystallographic data is listed in Table 1, and the ORTEP and numbering scheme are shown in Figure 5.

The central nitrogen atom Nc is almost planar, with a mean Car—Nc—Car (n = 1, 2, 3) angle value of 120(3)°; however, the mean Nc-Car distance value of 1.430(16) Å is in close correspondence with the Car—N(sp³) bond of 1.426(11) Å [59]. The selected bond lengths and angles are listed in Table S2. The three ethyl phenyloxamate rings adopt a three-bladed propeller structure around the Nc, with two rings lying almost perpendicular to each other, whereas the third ring is 45.3° (2N1), 49.8 (2N2), and 64.4° (2N3) from the other two. A complete list of planes and angles are listed in Table S3. This arrangement is the consequence of the steric restraints imposed by the oxamate group (vide infra).

The oxalamide conformation is typical for a secondary amide with the NH almost anti positioned to the amide carbonyl, with mean values of the torsion angles H(n7)—N(n7)—C(n7)—O(n7) (n = 0–8) of 178.3(9)° (six fragments) and 165(7)° (three fragments). The oxamate nitrogen is almost in the plane of the aromatic ring, with absolute mean values of the torsion Car—Car—Nn7—Cn8 (n = 0–8) angle of 15(13)° and 5(3)° in molecules 2N1 and 2N2, respectively, but twisted away from the plane of the aromatic ring in molecule 2N3, with values of the torsion angles of 18.4(2)°, 34.1(3)°, and 42.4(3)°. As shown for compound 1, the lone pair of the amide nitrogen is more delocalized to the amide carbonyl (mean bond length Nn7—Cn8 = 1.350(11) Å, n = 0–8) than to the aryl ring (mean bond length Nn7—Car = 1.424(10) Å). The coplanarity of the oxamate groups with the phenyl rings increases the steric demand to release the repulsion between them. Thus, two of the blades are perpendicularly disposed between each other to leave space for the third blade at the expense of tilting it away from the phenyl plane.

Oxalyl carbonyls are in anti-disposition, with a mean OCCO torsion angle of 173(10)°, and one arm of molecule 2N3 was not considered because of disorder. The C=O and OC-CO bond lengths are in the characteristic range for this functional group [64]. According to the coplanarity between the oxamic arms and the phenyl ring, a full or partial intramolecular hydrogen bonding (HB) scheme is observed [30]. The full characteristic intramolecular HB of oxamates, given through Car—H∙∙∙O8, N7—H∙∙∙O9 and Csp³H∙∙∙O9 contacts, is formed by two of the three arms of 2N2, rising three adjacent set of rings S(6)S(5)S(6) [65]. The other arm of 2N2 and two arms of 2N1 form the S(6)S(5) assembly, whereas for the molecule 2N3 only the N7H∙∙∙O9 (S(5)) HB interaction is observed. The geometric parameters of intramolecular HB are listed in Table S4, and the intramolecular HB patterns are depicted in Figure 6.

In contrast to compounds 1 and A–E, where the Car—NCO bond length remains unchanged upon complexation with metals, in compound 2 the complexation has the effect of lengthening this bond from 1.424(8) Å in 2 to 1.46(3) Å (mean length) in the complexes [41]. Furthermore, the pyramidalization of the Nc occurs with the ΣNc angles in compound 2 of 359.4(4)° (mean Car-Nc-Car angle values of 120(3)°), whereas the ΣNc angles is 346.2(3)° (mean Car-Nc-Car angle values of 115.4(9)°) in the complexes. These changes are in agreement with the electronic delocalization to the oxamate ending by complexation.

On the other hand, the supramolecular architecture is ruled by NH∙∙∙O HB and C=O∙∙∙C dipolar interactions, assisted by C=O∙∙∙π and CH∙∙∙π dispersive interactions. Intermolecular D—H∙∙∙A and CO∙∙∙Cg interactions in compound 2 are listed in

Table 4. Intermolecular D—H∙∙∙A and CO∙∙∙Cg interactions in compound 2.

Interaction	Symmetry Code	D—H/Å	H∙∙∙A/Å	D∙∙∙A/Å	D—H∙∙∙A/°
N7—H7∙∙∙O59	[x, 1 + y, z]	0.88(3)	2.35(3)	3.181(3)	158(2)
N17—H17∙∙∙O9	[x−1, y−1, z]	0.88(3)	2.45(3)	3.212(3)	145(3)
N27—H27∙∙∙O8	[1 + x, y, z]	0.79(3)	2.32(3)	3.105(3)	172(3)
N37—H37∙∙∙O29	[x, y−1, z]	0.85(2)	2.44(2)	3.252(3)	160(2)
N47—H47∙∙∙O39	[1 + x, 1 + y, z]	0.83(3)	2.55(3)	3.254(3)	143(2)
N57—H57∙∙∙O38	[1 + x, y, z]	0.85(3)	2.19(3)	3.019(3)	164(2)
N57—H57∙∙∙O40	[1 + x, y, z]	0.85(3)	2.58(2)	3.134(2)	123(2)
N67—H67∙∙∙O38	[1 + x, y, z]	0.90(3)	2.48(3)	3.299(3)	151(2)
N87—H87∙∙∙O78	[1 + x, y, z]	0.88(4)	2.09(4)	2.945(4)	161(3)
C71—H71B∙∙∙O28BA	[1−x, 1 − y, 1 − z]	0.99	2.33	2.908(14)	116
C105—H105∙∙∙O59	[x, 1 + y, z]	0.95	2.56	3.350(3)	140
C123—H123∙∙∙O8	[1 + x, y, z]	0.95	2.56	3.374(3)	144
C205—H205∙∙∙O29	[x, y − 1, z]	0.95	2.56	3.405(3)	148
C213—H213∙∙∙O38	[1 + x, y, z]	0.95	2.51	3.316(3)	142
C316—H316∙∙∙O88	[x, y, z]	0.95	2.45	3.406(3)	174
C322—H322∙∙∙O89AA	[2−x, −y, −z]	0.95	2.56	3.438(7)	154
C323—H323∙∙∙O88AB	[2−x, −y, −z]	0.95	2.52	3.372(11)	149
C61—H61B∙∙∙Cg(3)	[x, y, z]		2.96	3.833(4)	148
C81—H81A∙∙∙Cg(4)	[1−x, 1 − y, −z]		2.75	3.651(3)	152
C126—H126∙∙∙Cg(9)	[x, y, z]		2.93	3.821(3)	156
C216—H216∙∙∙Cg(5)	[x, y, z]		2.97	3.568(2)	157

Cg(3) = C321–C326, Cg(4) = C101–C106, Cg(5) = C111–C116, Cg(9) = C221–C226.

. Then, infinite tapes (R²₁(6)R¹₂(6); symmetry code: 1 + x, y, z) and chains (C(15); symmetry code: 1 + x, 1 + y, z) of 2N2 molecules develop the fundamental [1 −1 34] family of planes. Three carbonyl–carbonyl interactions of a sheared parallel type [66] reinforce the following observed motifs: C59O59∙∙∙C38, C59O59∙∙∙C39, and C49AO49∙∙∙C39, Figure 7a. Similarly, tapes (R²₁(6); symmetry code: −1 + x, y, z) and chains (C(15); symmetry code: −1 + x, −1 + y, z) of 2N1 molecules, assisted by C19O19∙∙∙C9 interactions, are developed in alternated parallel planes to 2N2, as shown in Figure 7b. In both cases, R⁴₄(46) rings are observed. Lastly, infinite chains of 2N3 molecules running in the direction of the a axis, are developed through N87—H87∙∙∙O78 HB interactions, as shown in Figure 8a. Two of these chains are held together through the participation of two C79O79∙∙∙C78 sheared-type carbonyl interactions and C323—H323∙∙∙O88 and C322—H322∙∙∙O89AA interactions forming a ribbon, as shown in Figure 8b. Two NH∙∙O HB interactions (N7H7∙∙∙O59 and N37H37∙∙∙O29) hold 2N1 and 2N2 molecules together, as shown in Figure 9a. N67H67∙∙∙O38 links 2N2 to 2N3, whereas the following four carbonyl interactions hold 2N1 and 2N3 together: C18O18∙∙∙C89A, C18O18∙∙∙C88, C68O68∙∙∙C28B, and C88O88B∙∙∙C18. This supramolecular arrangement in 3D is shown in Figure 9b. The geometric parameters associated with CO∙∙∙CO interactions are listed in

Table 5. Intermolecular CO∙∙∙A (A = Cg, CO) interactions in compound 2.

Interaction	Symmetry Code	C∙∙∙A	O∙∙∙A	C—O∙∙∙A
C58—O58∙∙∙Cg(1)	x, y, z	3.676(2)	3.428(3)	68.78(14)
C18—O18∙∙∙C89A	−1 + x, −1 + y, z	3.578(8)	3.094(8)	103.2(2)
C18—O18∙∙∙C88	−1 + x, −1 + y, z	3.226(5)	3.089(8)	85.2(3)
C68—O68∙∙∙C28B	1 + x, 1 + y, z	4.023(19)	2.971(17)	93.5(16)
C88—O88B∙∙∙C18	1 + x, 1 + y, z	3.226(5)	3.113(2)	84.5(18)
C49—O49∙∙∙C39	1 + x, 1 + y, z	3.982(4)/3.723(4)	3.185(3)	122.85(6)
C59—O59∙∙∙C38	1 + x, y, z	4.018(16)	3.013(3)	139.03(16)
C59—O59∙∙∙C39	1 + x, y, z	4.221(4)	3.213(3)	139.77(17)
C19—O19∙∙∙C9	−1 + x, −1 + y, z	3.857(4)	3.036(3)	124.9(2)
C78—O78∙∙∙C79	3 − x, 1 − y, −z	3.415(4)	2.937(4)	102.07(17)
C78—O78∙∙∙C78	3 − x, 1 − y, −z	3.240(4)	3.096(4)	85.45(15)
C29—O29∙∙∙C9	−1 + x, y, z	4.160(4)	3.195(3)	136.17(17)
C29—O29∙∙∙C8	−1 + x, y, z	4.046(4)	3.050(3)	138.70(15)

Cg(1) = C301–C306.

.

Herein, it is worth noting that both compound form helices, but oxamate 2 shows conformational diversity, rendering it more suitable for the design of materials where this property is required [39].

3.3. Quantitative Interaction Energy Analysis

Interaction energy DFT calculations were performed at the B3LYP/6-31G(d,p) level of theory, using the CrystalExplorer 21.5 software [53]. Selected results are listed in

Table 6. Calculated interaction energies ^a (kJ mol⁻¹); %E_comp contributions ^b to stabilization energy for selected HBs and close contacts in compounds 1 and 2.

Interaction	Molecules	−Eelec	−Epol	−Edisp	Erep	−Etot	%Eelec	%Edisp	R c/Å
Comp. 1
N7—H7∙∙∙O28	N1∙∙∙N3	24.7	3.6	16.0	15.6	28.8	50.1	39.4	11.45
N17—H17∙∙∙O38	N2∙∙∙N4	22.7	4.0	20.6	17.2	30.2	42.5	46.8	11.08
N27—H27∙∙∙O18	N3∙∙∙N2	27.1	4.8	20.0	19.5	32.4	46.5	41.7	11.12
N37—H37∙∙∙O8	N4∙∙∙N1	101.8	16.7	81.7	85.4	114.8	45.3	44.1	4.84
Comp. 2
N7—H7∙∙∙O59 C105—H105∙∙∙O59 N37—H37∙∙∙O29 C205—H205∙∙∙O29	N1∙∙∙N2 N2∙∙∙N1	33.2	5.5	38.1	30.0	46.8	38.1	53.0	11.34
N87—H87∙∙∙O78	N3∙∙∙N3	31.2	6.6	31.1	0.0	68.9	39.8	48.2	12.83
N27—H27∙∙∙O8 C29—O29∙∙∙C9 C29—O9∙∙∙C8 C123—H123∙∙∙O8	N1∙∙∙N1	35.2	6.2	29.3	0.0	70.7	44.2	44.6	12.83
N57—H57∙∙∙O38 C59—O59∙∙∙C38 C59—O59∙∙∙C39 C213—H213∙∙∙O38	N2∙∙∙N2	35.2	6.2	29.3	0.0	70.7	44.2	44.6	12.83
N67—H67∙∙∙O38	N3∙∙∙N2	7.7	2.7	16.6	6.6	20.5	24.3	63.5	9.30
N17—H17∙∙∙O9 C19—O19∙∙∙C9	N1∙∙∙N1	14.4	2.4	28.5	0.0	45.2	27.5	66.1	14.64
N47—H47∙∙∙O39 C49—O49∙∙∙C39	N2∙∙∙N2	14.4	2.4	28.5	0.0	45.2	27.5	66.1	14.64
C126—H126∙∙∙Cg(9) C216—H216∙∙∙Cg(5)	N1∙∙∙N2 N2∙∙∙N1	6.7	1.5	57.8	18.4	47.6	8.4	88.9	3.94
C78—O78∙∙∙C79 C78—O78∙∙∙C78	N3∙∙∙N3	11.2	1.6	26.8	0.0	39.6	24.4	70.9	14.25
C18—O18∙∙∙C89A C18—O18∙∙∙C88 C68—O68∙∙∙C28B C88—O88B∙∙∙C18	N1∙∙∙N3 N1∙∙∙N3 N3∙∙∙N1 N3∙∙∙N1	38.5	11.1	88.8	41.3	97.1	23.7	66.5	7.49
C58—O58∙∙∙Cg(1) C316—H316∙∙∙O58	N2∙∙∙N3 N3∙∙∙N2	29.3	6.1	75.7	34.5	76.6	22.5	70.7	8.87

^a Scaling factors (k_comp) were applied (E_comp = k_comp × E’_comp) according to reference [55]. ^b The % contribution of E_comp (%E_comp) is calculated through (E_comp/E_stab) × 100 where E_stab = E_ele + E_pol + E_disp. ^c R = distance between centroids in Å. Cg(n) is the centroid of the Cm-Cm+5 ring: Cg(1) = C301–C306, Cg(5) = C111–C116, and Cg(9) = C221–C226.

, and complete lists and pictures are shown in Figures S1 and S2. In compound 1, the amide N37—H37∙∙∙O8 interaction is the main contributor to stabilization energy (E_tot = −114.8 kJ mol⁻¹), similar to the reported value for a ureic compound (−104.2 kJ mol⁻¹) [67]. This amide’s HB links molecules 1N4 and 1N1, forming the R²₂(16) motif depicted in Figure 3b. The other three N—H∙∙∙O interactions are approximately 3.8 times weaker in energy than N37—H37∙∙∙O8; however, all of them are almost equally weighted in the %E_disp/%E_elect nature. The calculated energies for the abovementioned N—H∙∙∙O interactions are comparable to other systems (E_tot = −41.4 kJ mol⁻¹) [68].

In compound 2, single N—H∙∙∙O motifs are observed, as well as those composed of N—H∙∙∙O, CO∙∙∙CO and C—H∙∙∙O interactions, which differ in both strength and nature. The single amide–carbonyl interaction N87—H87∙∙∙O78 (2N3∙∙∙2N3) exhibits a total energy (E_Tot) value of −68.9 kJ mol⁻¹ at R = 12.83 Å, corresponding to the shortest N—H∙∙∙O interaction, whereas N67—H67∙∙∙O38 (2N3∙∙∙2N2) is the weakest (E_tot = −20.5 kJ mol⁻¹, R = 9.30 Å), corresponding to the longest geometric parameters associated with the HB; see Table 4. They also differ in nature, with the %E_disp/%E_elect values being 1.2 and 2.6, respectively. As far as the composed N—H∙∙∙O HB interactions are concerned, the calculated energy (E_tot = −70.7 kJ mol⁻¹, R = 12.83 Å) is the same as for 2N1∙∙∙2N1 and 2N2∙∙∙2N2, which are equally apart in comparison to 2N3∙∙∙2N3, at R = 12.83 Å, but they are assembled by a set of N—H∙∙∙O and two CO∙∙∙CO and C—H∙∙∙O interactions, which are equally weighted in %E_disp/%E_elect components. The E_tot becomes weaker and more dispersive when none (E_tot = −46.8 kJ mol⁻¹, R = 11.34 Å, %E_disp/%E_elect = 1.4) or only one (E_tot = −45.2 kJ mol⁻¹, R = 14.64 Å, %E_disp/%E_elect = 2.4) CO∙∙∙CO interaction takes part in the assembly, such as in 2N1∙∙∙2N2 or 2Nn∙∙∙2Nn (n = 1, 2), respectively. These energies are comparable to the reported value for a hydrazide compound (E_tot = −41.4 kJ mol⁻¹) [68].

Furthermore, CO∙∙∙CO and C—H∙∙∙A (A = O, π) interactions largely participate in stabilization energy, with the assembly of 2N1∙∙∙2N3 occurring through four CO∙∙∙CO interactions, the strongest set of interactions with E_tot = −97.1 kJ mol^{−1, at} R = 7.49 Å and %E_disp/%E_elect = 2.8; followed by the 2N2∙∙∙2N3 assembly through C58—O58∙∙∙Cg(1) and C316—H316∙∙∙O58 interactions with E_tot = −76.6 kJ mol⁻¹, at R = 8.87 Å and %E_disp/%E_elect = 3.1; then the 2N1∙∙∙2N2 assembly through C—H∙∙∙Cg(n) (n = 5, 9) interactions with E_tot = −47.6 kJ mol⁻¹, at R = 3.94 Å and %E_disp/%E_elect = 10.6; and, lastly, the 2N3∙∙∙2N3 assembly through two CO∙∙∙CO interactions with E_tot = −39.6 kJ mol⁻¹ at R = 14.25 Å and %E_disp/%E_elect = 2.9. The calculated energy values for CO∙∙∙CO interactions (n→π*) are more stabilizing than those reported values of −22.3 kJ mol⁻¹ for antiparallel CO⋯CO [66], probably due to synergic effects.

Thereby, the crystal network of compound 1 is stabilized by N—H∙∙∙O interactions of an equal dispersive/electrostatic nature, showing a synergic effect when forming intermolecular rings as in compound 1. In the case of compound 2, the E_tot values of single N—H∙∙∙O interactions seem to depend on the geometric parameters as associated with the HB, even though the intermolecular amide HB in compound 2 is clearly more energetically favored and dispersive than in compound 1. This assessment results after comparing N7—H7∙∙∙O28 and N87—H87∙∙∙O78 HBs of compounds 1 and 2, respectively, since both interactions are singles and show similar geometric parameters. The energy values and nature of the composed N—H∙∙∙O HB in 2 are dependent upon the other interactions participating in the assembly. In general, the participation of CO∙∙∙CO interactions largely contribute to stabilization. Nevertheless, the dispersive contribution to the total energy is increased as long as the CO∙∙∙CO and C—H∙∙∙O dispersive interactions participate, changing from being almost equal to being 2.4 times more important than the electrostatic component. At this point, it is worth noting that in compound 2, the amide proton (Nn7Hn7) is also engaged in the intramolecular HB with the carbonyl oxygen atom of the ester group (On9). Since this interaction is absent in compound 1, both the calculated energy differences and the large dispersive nature of the intermolecular assemblies in compound 2 could then be attributed to intramolecular hydrogen bonding.

Finally, the assemblies of CO∙∙∙CO and C—H∙∙∙A (A = O, Cg)’s dispersive interactions largely participate in the stabilization energy with even better energy values than the assemblies where N—H∙∙∙O HBs participate.

3.4. Hirshfeld Surface (HS) Analysis and Energy Framework Diagraxms

The contributions of several interactions to the HSs are differentiated between the independent molecules of the asymmetric unit in both compounds. Therefore, the values per molecule and the average values of the percent contribution of the most significant interactions and their reciprocals are shown in Figure 10 (1) and Figure 11 (2), respectively. The whole picture of non-covalent interactions is appreciated through the HSs of compounds 1 (molecule 1N4) and 2 (molecule 2N1), which are depicted in Figure 12 (HSs, mapped in the color range of −0.48 to 0.48 Å). The HS of only one molecule is shown, and the other molecules of compounds 1 and 2 are depicted in Figures S3 and S4, respectively. They show the corresponding interactions mapped in different colors according to the distances between atoms inside (d_i) or outside (d_e) the surface. The bright red areas in the HSs correspond to hydrogen bonding (O∙∙∙H/H∙∙∙O) interactions (average values (a.v.): 20 ± 1%, 1; 28 ± 3%, 2), whereas H∙∙∙C/C∙∙∙H contacts (a.v.: 19 ± 5, 1; 16 ± 3%, 2) are visible as small and light red spots, and the H∙∙∙H contacts (a.v.: 56 ± 5%, 1; 44 ± 2, 2) represent the largest contribution interaction in each surface.

Other colored HSs are also displayed, as well as the shape index (SI, mapped in the color range of −1.0 to 1.0 Å) and curvedness (CS, in the range of −4.0 to 0.4 Å). Blue and red areas in the SIs indicate the presence of complementary interactions in crystal packing. Blue colored areas are the convex region in the surface that are in contact with the concave red colored areas of another surface. This bump-hollow representation of 1N4 shows π···alkyl interactions with 1N3 outside the surface; meanwhile, the SI of 2N1 exhibits T-shaped interactions with 2N2 outside the surface, as shown in Figure 12. SIs are in agreement with the absence of π-stacking interactions, characterized by the presence of adjacent red- and blue-colored triangles. The CSs shows small green-colored flat areas spaced by blue edges, also indicating the absence of π-stacking interactions. Nevertheless, the large green areas in the CS of molecules 1N1 and 1N2 of compound 1, around the central nitrogen, are in agreement with the flattest region of the molecule (CNC angle of 119.5(3)°), as shown in Figure S3.

The energy frameworks of compounds 1–2 were obtained by DFT calculations at the B3LYP/6-31G (d, p) level of theory. The magnitude of the energy is proportional to the tube size (150 kJ mol⁻¹). The distribution of each energy component is more esthetic in 1 compared with 2, due to the large molecular symmetry in the first, showing a honeycomb shape, whereas the energies in compound 2 showed zigzagging distribution. The energy propagation in each crystal is more evident in the dispersion component, as depicted in Figure 13.

3.5. PXRD, Thermal, and SEM Analyses

Compounds 1 and 2 herein reported are single phases confirmed by PXRD. The pale gray color of compound 1 is the powder isolated from the reaction mixture, contrasted with the dark blue of its corresponding crystal. Nevertheless, the powder and crystals of 1 correspond to the same crystalline solid phase, as shown in Figures S5 and S6, whereas compound 2 results in a yellow powder after isolation from the reaction, which crystallizes from EtOH as bright yellow crystals in the triclinic system and the space group P-1 as the powder. SEM images of 1 and 2, in both powder and crystal forms, are shown in Figure 14, where the great similitude between the powder and smashed crystals are appreciated.

Both compounds are thermally stabile in their solid forms; compound 1 melts in the 193–195 °C range, and compound 2 melts in the 196–198 °C range. Nevertheless, both decompose after melting. TGA recordings are shown in Figure S7, and they show the loss of 43.3% weight in 1 and 45.2% weight in 2, corresponding to the CO₂Et and CO₃Et fragments, respectively. The higher thermal stability of 2 compared to compound 1 is in agreement with its larger density measured from solid and molecular weights.

3.6. Vibrational Analysis

The infrared spectra (FT-IR) of both compounds showed the characteristic amide N―H and C=O stretching bands, and the IR spectra of compounds 1 and 2 are depicted in Figures S8 and S9. The N―H stretching band of oxalamide 2 is broader and shifted to smaller wavenumbers (3400–3220 cm⁻¹) than amide 1 (3305 cm⁻¹). Deconvolution of the first band, as seen in Figure 15, allowed for a view of the following contributing bands at 3386 (N17—H17∙∙∙O9), 3370 (N37—H37∙∙∙O29), 3349 (N47—H47∙∙∙O39, N57—H57∙∙∙O40), 3328 (N7—H7∙∙∙O59, N67—H67∙∙∙O38), and 3289 (N27—H27∙∙∙O8, N77—H77···O79 intramolecular, N87—H87∙∙∙O78) cm⁻¹, in agreement with the involvement of oxamate N―H in several hydrogen bonding arrangements, given by the conformational diversity of the —NHCOCO₂Et moiety in the crystal. The match was performed according to the empirical equation previously reported by our group, where the flattest CarCarNn7Cn8 torsion angle (φ) is associated with the largest stretching N―H wavenumber (ν). The correlation (R = 0.975) between the predicted and experimental φ values are depicted in Figure S10. The dependence of the νNH to the φ values are explained because of the oxalyl torsion, and the N lone pair of the amide is compromised with the aromatic ring resonance, developing a partial positive charge on the N atom; therefore, the νNH band is shifted to large wavenumbers [35]. This phenomenon has been used in the determination of conformational changes in peptides [69,70,71] and in the identification of crystal polymorphs [72,73,74].

Anti-symmetric (ν_asym) and symmetric (ν_sym) C=O stretching vibrations are usually observed in oxamic derivatives. In the case of compound 2, two pairs of bands are observed in the IR spectrum at 1762 (ν_asym) and 1692 (ν_sym), and 1731 (ν_asym) and 1683 (ν_sym) cm⁻¹, presumably corresponding to the conformational diversity of the -NHCOCO₂Et fragment, but the symmetric stretching is shifted to shorter wavenumbers, 1692 (ν_sym), than in compound 1. This result is in agreement with the involvement of oxamate carbonyls in non-covalent interactions.

3.7. NMR in Solution and in the Solid State

In the DMSO-d6 solution, the stronger acid character of the oxamate NH in comparison to the carbamate NH is evident; it appears to be shifted to higher frequencies by 1.27 ppm (δ_NH = 9.48, 1; δ_NH = 10.75, 2). The chemical shifts of the NH signals in compounds 1 and 2 showed a small variation with the temperature in the DMSO-d6 solution, which is in agreement with similar mobilities (Δδ_NH/ΔT/(ppb K⁻¹): 4.9 (±0.1), comp 1; 4.8(±0.1), comp 2) [75]. This result suggests that the formation of a regular hydrogen bond of similar energy in both compounds occurs [30]. A complete list of δ_NH values at several temperatures are listed in Table S5.

By contrast, the H3 proton in 1, as well as the ¹³C chemical shifts of the aromatic ring, appears at higher frequencies in 1 than in 2, in agreement with the stronger electro-withdrawing (EW) character of the carbamate group than the oxamate. Furthermore, the carbamate carbonyl appears at δ = 158.8, in contrast to oxamate carbonyl, which appears at δ = 155.6, according to the electronegativity of the -OEt vs. -CO₂Et groups.

The ¹³C-CPMAS spectra of compound 1 is in agreement with one third of the fragment of four independent molecules found in the asymmetric unit (vide supra). The chemical shifts differences between C3 and C5, because of the deshielding effect of the amide carbonyl positioned in the same side of this last carbon atom, are also revealed. C3 and C5 are shifted by 2 ppm to lower and higher frequencies, respectively, compared to the solution. In addition, C4 is shifted to lower frequencies, as expected for more delocalization of the lone pair of amide nitrogen into the aromatic ring in the solid. ¹³C-CPMAS data of compounds 1 and 2 are listed in

Table 7. ¹³C-CPMAS and ¹³C in DMSO-d6 solution; chemical shifts of compounds 1 and 2.

Comp.	CO	NCO	C1	C2/6	C3	C4	C5	CH2	Me
1 (DMSO-d6)		158.8	147.5	128.9	124.8	139.1	124.8	65.2	19.7
1		158(4)	147(1) 146(3)	129(1) 127(7)	124(2) 123(1) 121(1)	136(1) 135(3)	126(4)	63(1) 62(3)	18(2) 17(1) 15(1)
2 (DMSO-d6)	161.2	155.6	144.1	124.1	122.3	133.0	122.3	62.8	14.3
2	165(3) 164(6)	157(5) 156(1) 154(3)	146(5) 148(1) 144(3)	128(4) 125(14)	121(7) 118(2)	138(1) 135(2) 131(6)	123(9)	66(6) 64(3)	18(2) 17(1) 16(3) 15(2) 12(1)

, and the ¹³C-CPMAS spectra are shown in Figures S11 and S12, respectively.

In the case of compound 2, several closely related signals are present in the ¹³C-CPMAS spectrum, in agreement with three independent molecules in the asymmetric unit. The chemical shifts of C1 and C4 are spread in the 144–148 ppm and 138–131 ppm ranges, respectively. The more shielded values correspond to the more coplanar structures between the phenyl ring and the central nitrogen atom or the oxalamide fragment, respectively. δC3 and δC5 are differentiated, in agreement with the spatial closeness to the amide carbonyl; then, the chemical shift value of 118 ppm is associated with C3. Finally, the chemical shift of the amide carbonyl in 2 is in the 157–154 ppm range, in agreement with its participation in N–H⋯OC HB interactions of several strengths, including the low frequency value (154 ppm) assigned to N77–H77 not involved in HB interactions [35].

4. Conclusions

In this work, structures in the solid state of ethyl carbamate and ethyl oxamate derivatives of tris(4-aminophenyl)amine were compared. Several independent molecules were found in the asymmetric unit, but more symmetry was attained in the carbamate than in the oxamate derivative. Both compounds adopted a three-bladed propeller structure around a planar central nitrogen atom, where the blades were twisted according to the steric demand of the pendant ethyl carbamate or ethyl oxamate groups but modulated by the conformational flexibility of the ethyl oxamate groups.

The crystal network of compound 1 was stabilized by N—H∙∙∙O interactions of an equal dispersive/electrostatic nature, showing synergic effects when forming intermolecular rings. The presence of a second carbonyl in compound 2 strongly modified the supramolecular architecture. It was ruled by NH∙∙∙O and C=O∙∙∙C dipolar interactions and assisted by C=O∙∙∙π and CH∙∙∙π interactions whose dispersive nature was the predominant contributor to the stabilization energy of the lattice. In both structures, the O∙∙∙H/H∙∙∙O interactions were the second largest contributors in the HS, followed by H∙∙∙C/C∙∙∙H contacts, but O∙∙∙C/C∙∙∙O contacts, which represented a small part of the HS, strongly participated in the stabilization energy of compound 2. The distribution of each energy component was more esthetic in 1 compared with 2 due to the large molecular symmetry in the first, showing a honeycomb shape, whereas the energies in compound 2 showed zigzagging distribution. We demonstrate that the conformational versatility of the oxamate pendant group, accurately given by the X-ray analysis, finely tuned the IR-vibrational and ¹³C-CPMAS spectra, allowing us to predict the conformation in the solid state. Carbamate and oxamate formed a single homogeneous phase in both powder and crystal solid forms; they are thermally stabile solids that melt in the 193–195 °C and 196–198 °C ranges, respectively, by losing the pendant group. Overall, the results presented show the potential that carbamate and oxamate TPA derivatives, herein synthesized and structurally analyzed, have for the design of functional materials, where the desired property depends upon their conformation or helicity.