N,N′-Dibutyloxamide

Abstract

N,N′-dibutyloxamide (1) was prepared by reacting diethyloxalate with n-butylamine in ethyl alcohol and characterized by microanalytical techniques, FT-IR, and NMR spectroscopy. Crystals suitable for single crystal X-ray diffraction (SC-XRD) were obtained from an acetonitrile solution of 1, and the structural characterization showed the presence of intermolecular hydrogen bonding interactions.

1. Introduction

Oxamic acid derivatives are an important class of compounds with applications in fields as varied as medicine [1,2], organic synthesis [3,4], and cultural heritage [5,6,7,8]. In particular, the oxalamide fragment can be found in various compounds of synthetic relevance, including biologically active compounds [9,10], precursors of widely used agents such as ethylene glycol [11,12], and ligands [13,14].

In this context, N,N′-dibutyloxamide (1) was employed in recent years as a bridging ligand in polynuclear rhenium, ruthenium, and manganese complexes [15,16,17,18,19], several of which have been structurally characterized. Despite this, the crystal structure of N,N′-dibutyloxamide has not been reported to date.

Herein, we report on the spectroscopic and structural characterization of N,N′-dibutyloxamide (1), which was synthesized from diethyloxalate and n-butylamine.

2. Results and Discussion

N,N′-disubstituted oxalamides can be prepared through the reaction of oxalyl chloride with the desired amine [20,21], and in more recent years, N,N-dibutyloxamide (1) and other derivatives were also synthesized by other methods, such as the catalytic carbonylation of amines [22,23]. In the present work, however, compound 1 was prepared through a modification of the classical synthetic route developed by Rice and co-workers in the 1950s [24] by reacting diethyloxalate with n-butylamine in ethanol (Scheme 1).

Compound 1 was fully characterized by means of elemental analysis, melting point determination, FT-IR, ¹H-, and ¹³C-NMR spectroscopy.

The middle infrared spectrum shows the characteristic peaks for the N–H stretching mode at 3298 cm⁻¹, in agreement with previous reports [22,23,25], while a series of peaks between 2850 and 3000 cm⁻¹ can be assigned to the C–H stretching modes of the butyl chain, and those of the carbonyl groups fall at 1659 cm⁻¹ (Figure S1).

The ¹H-NMR spectrum in CDCl₃ (Figure S2) shows the NH signal as a broad singlet falling at 7.46 ppm and the quadruplet signal of the –CH₂– groups bound to the nitrogen atoms at 3.31 ppm, while those of the remaining protons of the n-butyl chain fall between 0.9 and 1.6 ppm. The ¹³C{¹H} spectrum in the same solvent features a signal at 160 ppm assigned to the carbonyl moieties, while the remining four signals resonating between 13 and 40 ppm can be attributed to the alkyl carbon atoms (Figure S3).

The recrystallization of compound 1 from acetonitrile yielded colorless block-shaped crystals that were structurally characterized by SC-XRD analysis.

Compound 1 crystallizes in the triclinic space group P-1 (Tables S1–S4), with half a molecule in the asymmetric unit (Figure 1). The two carbonyl groups are in the typical antiperiplanar conformation, with an O=C–C=O torsion angle of 180° (Table S4), the oxalamide core being perfectly planar. The C–C, C=O, and C–N bond lengths (1.542, 1.238, and 1.327 Å, respectively; Table S2) are very close to the corresponding average values of 1.533, 1.226, and 1.328 Å, respectively, found for the 258 differently substituted free oxalamides retrieved from the Cambridge Crystallographic Data Centre (CCDC) [26]. The two n-butyl groups lie on opposite sides of the oxalamide core and are oriented perpendicularly to it in an extended conformation (C1–C2–C3–C4 dihedral angle: 179°; Figure 1, Table S4). This is in analogy with the conformation adopted by compound 1 in metal complex [{(CO)₃Re(μ-η⁴-1)Re(CO)₃}₂(μ-pbin)₂] (pbin = phenyl-1,4-bisisonicotinate) [16]. In contrast, in [I(CO)₃Ru(μ-η⁴-1)Ru(CO)₃I], the n-butyl groups of 1 assume a folded conformation (dihedral angle within the alkyl chain: 67°) [18], and in the remaining complexes [(CO)₃M(μ-η⁴-1)(μ-bpp)M(CO)₃] [M = Mn, Ru; bpp = bis-(4-pyridyl)propane] [17], [L(CO)₃Mn(μ-η⁴-1)Mn(CO)₃L] (L = 4-picoline, 4-phenylpyridine), and [{(CO)₃Mn(μ-η⁴-1)Mn(CO)₃}₂(μ-pby)₂] (pby = 4,4′-bipyridine) [19], the n-butyl chains are on the same side of the oxalamide core. The conformational variability in the abovementioned complexes can be ascribed to templating effects exerted by the metal ion, steric hindrance induced by other ligands, and other packing effects.

In the crystal packing of compound 1, N–H···O hydrogen bonding interactions generating ${\mathrm{R}}_2^2\left(10\right)$ motifs [27,28] can be observed between adjacent molecules whose oxalamide cores are coplanar (Figure 2a and Figure S4,

Table 1. Intermolecular interactions of compound 1.

Interaction		dD–H (Å)	dH∙∙∙A (Å)	dD∙∙∙A (Å)	αD–H···A (°)
a	N1ii–H1ii∙∙∙O1	0.86(2)	2.12(2)	2.8727(9)	146.9(16)
b	C4iii–H4iii∙∙∙O1	0.99	2.61	3.499(1)	149.0

Symmetry operations: ⁱⁱ = x, –1 + y, z; ⁱⁱⁱ = 1 + x, –1 + y, z.

), forming ribbons running along the b-axis. These interactions are similar to those observed for other oxalamides featuring alkyl [25,29,30] or aryl [31,32,33] substituents. The ribbons formed through the N–H···O hydrogen bonds are held together by C–H···O interactions between the carbonyl oxygens and the hydrogen atoms of the –CH₂- groups bound to the nitrogen atoms (Figure 2b and Figure S4, Table 1); they are thus approximately stacked along the a-axis (Figure 2c).

3. Materials and Methods

3.1. General Methods

Solvents and reagents were purchased from TCI (Chennai, India), FluoroChem (Hadfield, UK), and Merck (Rahway, NJ, USA) and were used without further purification. Deuterated solvents were purchased from Eurisotop (Saint-Aubin-des-Bois, France) and stored over molecular sieves prior to use. FT-IR measurements were recorded at room temperature on a Thermo-Nicolet 5700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using KBr pellets with a KBr beam splitter and KBr windows (4000−400 cm⁻¹, resolution 4 cm⁻¹). ¹H and ¹³C NMR spectra were carried out in CDCl₃ at room temperature using a Bruker Avance III HD 600 spectrometer (Bruker, Billerica, MA, USA). Chemical shifts are reported in ppm (δ) and were calibrated to the solvent residue. Elemental analysis was performed with a CHNS/O PE 2400 series II elemental analyzer (T = 925 °C). Uncorrected melting points were determined in capillaries on an FALC mod. C melting point apparatus.

3.2. X-ray Diffraction Analysis

X-ray diffraction data for compound 1 were collected at 120(2) K on a Rigaku FRE+ diffractometer (Rigaku, Tokyo, Japan), equipped with VHF Varimax confocal mirrors, an AFC12 goniometer, and a Saturn 724+ detector. The structure was solved with the ShelXT [34] solution program using dual methods and the model was refined with ShelXL 2018/3 [35] using full-matrix least-squares minimization of F². Olex2 1.5 [36] was used as the graphical interface. All hydrogen atoms were included in the model at geometrically calculated positions and refined using a riding model, except for those of the N–H groups, which were refined freely.

3.3. Synthesis of N,N′-Dibutyloxamide (1)

Compound 1 was synthesized by a modification of a procedure described in the literature [24]: diethyloxalate (1.24 g; 8.50 mmol) in 5 mL of ethanol and n-butylamine (1.24 g; 17.0 mmol) in 5 mL of the same solvent were refluxed overnight. The mixture was cooled at room temperature and the white solid obtained was isolated by filtration on a Gooch funnel, washed with n-hexane, and air-dried. Colorless block-shaped crystals suitable for SC-XRD analysis were grown via the slow evaporation of an acetonitrile solution of the product (2.1 g; 10 mmol; yield = 63 %) M. p. = 152.8–153.5 °C (Lit.: 153–154 °C [24]). Elemental analysis calculated (%) for C₁₀H₂₀N₂O₂: C 59.97, H 10.07, N 13.99. Found: C 59.61, H 10.38, N 13.52. FT-IR (KBr, 4000–400 cm⁻¹): 3298 vs, 3062 w, 2955 s, 2930 m, 2873 m, 2862 m, 1650 vs, 1532 s, 1466 m, 1438 m, 1369 m, 1307 m, 1244 m, 1218 s, 1147 m, 1115 w, 977 w, 940 w, 759 vs, 557 s, 481 w, 465 w cm⁻¹. ¹H NMR (600 MHz, CDCl₃) δ: 7.46 (s, br, 2H, NH), 3.31 (q, 4H, CH₂CH₂CH₂CH₃), 1.54 (quin, 4H, CH₂CH₂CH₂CH₃), 1.37 (sex, 4H, CH₂CH₂CH₂CH₃), 0.93 (t, 6H, CH₂CH₂CH₂CH₃) ppm. ¹³C{¹H} NMR (151 MHz, CDCl₃) δ: 160.0, 39.5, 31.4, 20.1, 13.8 ppm.

4. Conclusions

The structural characterization of N,N′-dibutyloxamide (1), employed in recent years as a bridging ligand in the field of coordination chemistry, is reported in this paper. Compound 1 was synthesized by reacting diethyloxalate with n-butylamine and was fully characterized. Crystals suitable for SC-XRD were obtained by the slow evaporation of an acetonitrile solution of the compound, and the crystal structure shows two antiperiplanar amide groups and the n-butyl substituents protruding on opposite sides of the oxalamide plane. The crystal packing is governed by N–H···O hydrogen bonding interactions leading to the formation of ribbons, which are held together by C–H···O interactions.