1-(2-Hydroxyethyl)imidazolidine-2-thione

Abstract

1-(2-Hydroxyethyl)imidazolidine-2-thione (1) was obtained as a product from an in situ reaction between N-(2-hydroxyethyl)ethylenediamine, carbon disulfide, potassium hydroxide, and di(4-fluorobenzyl)tin dichloride. Compound 1 was characterized by IR, UV, ¹H, ¹³C{¹H}, and 2D (COSY, NOESY, HSQC, and HMBC) NMR spectroscopies. The cyclic molecular structure was confirmed by single crystal X-ray crystallography which showed the five-membered ring to be non-planar and the π-electron density to be localized over the CN₂S chromophore. In the crystal, thioamide–N–H^…O(hydroxy) and hydroxy–O–H^…S(thione) hydrogen bonds lead to supramolecular layers in the bc-plane.

1. Introduction

The title compound, 1-(2-hydroxyethyl)imidazolidine-2-thione (1), as shown in Scheme 1, is in fact a known compound and was probably first described/authenticated in the mid-1950s [1]. Despite there being considerable and long-term chemotherapeutic interest [2,3] in this compound, in more recent times interest has related to its utility in the electroplating of metals [4,5] and as a corrosion inhibitor [6]. Yet, the crystal structure determination has not been described and a detailed spectroscopic analysis is lacking [7]. In the present study, 1 was isolated as a side product from the in situ cyclization reaction of N-(2-hydroxyethyl)ethylenediamine, carbon disulfide, and potassium hydroxide in the presence of di(4-fluorobenzyl)tin dichloride. Such cyclization reactions have been documented in dithiocarbamate chemistry as being a convenient method to form 1-alkyl-2-imidazolidinethiones [8,9], suggesting the organotin reagent is not required for the synthesis. The motivation for the original reaction was to generate bioactive organotin dithiocarbamate compounds inspired by their biological activity, primarily anti-cancer and anti-microbial [10], and as a result of the recent reports of the potential anti-cancer (gold [11], bismuth [12], and zinc [13]) and anti-microbial (copper, silver [14] and gold [15,16]) activities of metal hydroxyethyl-substituted dithiocarbamate compounds, the biological activity of metal dithiocarbamates has been reviewed [17]. Herein, the spectroscopic characterization and X-ray crystal structure determination of 1 are reported.

2. Results and Discussion

Compound 1 was obtained from an in situ reaction of N-(2-hydroxyethyl)ethylenediamine, carbon disulfide, and potassium hydroxide (normally required to prepare dithiocarbamate anions) with substituted dibenzyltin (IV) chlorides (see Section 3.2); no other compounds were isolated from these reactions. Several closely related cyclic compounds have been observed in the literature as a result of the decomposition reaction between sodium N-isopropyl-N-hydroxyethyldithiocarbamate with a cadmium salt [18], benzimidazole based thiones [19,20], and as a side product from the cyclization of (2-t-butyl)ethanol dithiocarbamate [21]. It is likely the presence of a strong alkaline medium that promotes the formation of these cyclic compounds [22].

IR, UV, ¹H, and ¹³C NMR spectroscopies were employed to characterize 1; see the Supplementary Materials for original spectra. The IR spectrum showed a prominent and broad absorption band due to OH and NH stretching at 3241 cm⁻¹. Other prominent absorption bands were also observed: 1508(s) ν(–NC=S), 1312(m) and 1257(m) ν(C–N), and 1061(s) ν(C–O). In the UV spectrum recorded in methanol solution, a band at 284 nm is assigned to a thione π→π* transition; a similar result was noted in an acetonitrile solution. The assignment of the resonances in the ¹H and ¹³C{¹H}-NMR spectra was not straightforward owing to the similarity in the magnetic environments for the four methylene groups. Therefore, 2D NMR (COSY, NOESY, HSQC, and HMBC) experiments were conducted. From the COSY spectrum, a correlation between the OH (δ 2.38) and H5 (δ ~3.87) protons was established, and possibly also between H2 (δ ~3.64) and H3 (δ 3.85). In order to confirm the correlation between the H2 and H3 protons, a NOESY experiment was conducted which indeed confirmed this correlation. Confirmation of the assignments was achieved through HSQC and HMBC experiments, and final assignments are given in Section 3.2.

On the basis of the literature, it is noted that the chemical shifts of O- and N-bound protons can occur virtually anywhere in ¹H-NMR spectra. This is because the value does not depend on the exact molecular structure of a compound but is also dependent on the solvent used and the possibility of hydrogen bonding [23,24]. In the ¹H spectrum of 1, the chemical shifts for the O- and N-bound protons were shielded and appeared as singlets δ 2.38 and 5.88 ppm, respectively. The relatively high-field shifts for these protons suggest there is only weak or most likely no intramolecular hydrogen bonding in the molecule.

The molecular structure of 1, as determined by single-crystal X-ray crystallography, is shown in Figure 1, and selected geometric parameters are included in the caption. The C1=S1 bond length of 1.6996(13) Å is indicative of a thione bond. The C1–N1 and C1–N2 bond lengths are systematically shorter than the other C–N bonds consistent with some delocalization of π-electron density over the CN₂S atoms. The internal C1–N–C angles are both similar and smaller than the C–N2–C4 angles. The same trend is observed for the angles subtended at the C1 atom with the external S1–C1–N angles being approximately 15° wider than the internal N1–C1–N2 angle. The r.m.s. deviation of the five-members comprising the ring is 0.0337 Å with the maximum deviations above and below the least-squares plane being 0.0451(8) and 0.0416(8) Å for the C2 and C3 atoms, respectively, consistent with the ring being twisted around the C2–C3 bond. The S1 (0.0122(19) Å) and C4 (0.025(2) Å) atoms lie to either side of the plane, that is, to the same side as the C2 and C3 atoms, respectively.

A view of the unit cell contents for 1 is shown in Figure 2a. The most prominent features of molecular packing in the crystal are the formation of thioamide–N–H^…O(hydroxy) and hydroxy–O–H^…S(thione) hydrogen bonds which combine to form supramolecular layers in the bc-plane (Figure 2b) via the formation of 12-membered {^…HO^…HNCS}₂ synthons which have the form of a chair (Figure 2c); in Graph Set terminology [25], the rings are labeled R⁴₄(12). The layers stack along the a-axis without directional interactions between them.

The most closely related structure to 1 in the literature is one with a benzene ring fused across the C2–C3 bond which, with a bond length of 1.387(3) Å, now has a significant double bond character [18]. The C1–S1, C1–N1, and C1–N2 bond lengths are 1.6801(18), 1.356(3), and 1.362(2) Å, respectively, with the crucial C1–N bond lengths being marginally longer than the equivalent bonds in 1, a result consistent with the significant shortening of the C2–N1 and C3–N2 bond lengths, that is, to 1.379(2) and 1.398(2) Å, compared with 1, and substantial delocalization of π-electron density over the entire ring which is strictly planar [r.m.s. deviation = 0.009 Å].

As intimated above, imidazolidine-2-thiones form an important class of compound and with derivatization possible at both nitrogen atoms and in the ethylene link, it is therefore not surprising there is considerable X-ray crystallographic data available for these derivatives. Possibly, the simplest di-nitrogen substituted compound with an ethylene link is the N,N′-dimethyl-2-imidazolidinethione compound (2) [26]. The substituents at nitrogen can be large and chiral as in the recently report of the (R)-5,6,7,8-tetrahydro-[1,2′-binaphthalen]-8-yl) di-nitrogen substituted derivative (3) [27], again with an ethylene bridge. Substitution can also occur in the ethylene bridge, and related to 1, in the hydroxyethyl chain, with phenyl groups, as in the case of 1-(2-hydroxy-1-phenylethyl)-3-methyl-4-phenylimidazolidine-2-thione (4), implying the formation of two chiral centers [28]. Examples of substantial substitution at all positions around the ring are also exemplified by (E)-3-n-butyl-4-butylidene-1′-methyl-1-phenyl-2-thioxo-1′H-spiro[imidazolidine-5,3′-indol]-2′-one (5) [29] in which one ethylene-carbon is di-substituted, being incorporated in a spiro link, and the other participates in a double bond to an exocyclic carbon atom. The C=S bond lengths in 2, 3, and 5 span the range 1.659(3) Å (5) to 1.684(3) Å (3) with the longer bonds in 1 and 4 (1.697(3) Å) being correlated with their thione sulfur atoms participating in significant hydrogen bonds in their respective crystals. As for 1, the C1–N1, N2 bond lengths are experimentally equivalent in each of 2 (1.342(5) and 1.334(5) Å), 3 (1.361(4) and 1.338(4) Å), 4 (1.345(4) and 1.348(4) Å), and 5 (1.354(4) and 1.358(3) Å).

In conclusion, this study enabled the assignment of the ¹H and ¹³C-NMR resonances for 1 through an analysis of 2D NMR spectra. X-ray crystallography established the non-planar nature of the five-membered ring and π-electron density localized over the CN₂S chromophore. In the crystal, supramolecular layers are formed, being sustained by thioamide–N–H^…O(hydroxy) and hydroxy–O–H^…S(thione) hydrogen bonds.

3. Materials and Methods

3.1. General Information

All chemicals were purchased from commercial sources: Potassium hydroxide, tin powder (Merck, Darmstadt, Germany), 4-fluorobenzyl chloride (Sigma-Aldrich, St. Louis, MI, USA), N-(2-Hydroxyethyl)ethylenediamine (Alfa Aesar, Haverhill, MA, USA), and carbon disulfide (Pancreac, Barcelona, Spain) and used as received with further purification. The solvents used were of EMSURE grade (Merck, Darmstadt, Germany) and used without further purification. The melting point was determined using a Biobase automatic melting point apparatus (Biobase Group, Jinan, Shandong, China) and was uncorrected. The IR spectrum was measured on a Bruker Vertex 70v FTIR (Billerica, MA, USA) spectrophotometer from 4000 to 400 cm⁻¹. Elemental analyses were performed on a Leco TruSpec Micro CHN Elemental Analyzer (Leco, Saint Joseph, MI, USA). The optical absorption spectra were obtained from methanol and acetonitrile solutions of 1 (2 × 10⁻⁴ M) in the range 200–800 nm on a Shimadzu UV-3600 plus UV/VIS/NIR spectrophotometer (Shimadzu Corporation, Kyoto Prefecture, Japan). ¹H, ¹³C{¹H}, and 2D-NMR experiments (COSY, NOESY, HSQC, and HMBC) were recorded on a Bruker Ascend 400 MHz NMR (Billerica, MA, USA) spectrometer. Compound 1 was dissolved in deuterated chloroform and the chemical shifts were recorded in ppm relative to tetramethylsilane.

3.2. Synthesis and Characterization

Carbon disulfide (0.12 mL, 2 mmol) was slowly added to a stirred solution of N-(2-hydroxyethyl)ethylenediamine (0.20 mL, 2 mmol) in acetone at 0 °C. The solution was stirred for 30 min. Next, potassium hydroxide (50% w/v, 0.23 mL) was added dropwise into the solution which was stirred for another 30 min. Next, di(4-fluorobenzyl)tin dichloride (0.41 g, 1 mmol) in acetone (10 mL) was added into the mixture and stirring was continued for 3 h. The solvent was gradually removed by evaporation until a light brown solid was obtained. The precipitate was recrystallized from acetone to yield pale orange crystals. Similar crystals were isolated when di(4-fluorobenzyl)tin dichloride was substituted with di(4-methylbenzyl)tin dichloride and di(4-chlorobenzyl)tin dichloride. M.p.: 122–124 °C cf. 136.5–137.5 °C [1]. IR (ATR, cm⁻¹) 3241(b) ν(OH, NH), 1508(s) ν(–NC=S), 1480(m) ν(C–C), 1312(m) ν(C–N), 1257(m) ν(C–N), 1061(s) ν(C–O), 932(m) ν(C–C). UV: (methanol): λ_abs = 284 nm, ε = 5585 L·cm⁻¹·mol⁻¹ and (acetonitrile): λ_abs = 283 nm, ε = 5850 L·cm⁻¹·mol⁻¹. ¹H-NMR (refer to Figure 1 for atom labels): δ 3.88 (H5, ³J_HH = 5.08 Hz, ³J_H-OH = 4.60 Hz), 3.85 (H3, ³J_HH = 9.64 Hz, ⁴J_H-NH = 6.76 Hz), 3.81 (H4, ³J_HH = 5.48 Hz, ⁴J_H-OH = 5.20 Hz), 3.63 (H2, ³J_HH = 9.52 Hz, ³J_H-NH = 8.52 Hz). ¹³C{¹H}-NMR: δ 184.34 (C1), 61.23 (C5), 49.97 (C3), 49.21 (C4), 41.68 (C2).

3.3. X-ray Crystallography

Intensity data for 1 were measured at T = 100(2) K on an Agilent Technologies SuperNova Dual AtlasS2 diffractometer (Rigaku, The Woodlands, TX, USA) fitted with Cu Kα radiation (λ = 1.54184 Å) so that θ_max was 67.1°. Data reduction, including absorption correction, was performed with CrysAlis Pro [30]. Of the 8573 reflections measured, 1236 were unique (R_int = 0.025), and of these, 1181 data satisfied the I ≥ 2σ(I) criterion of observability. The structure was solved by direct methods [31] and refined (anisotropic displacement parameters and C-bound H atoms in the riding model approximation) on F² [32]. The O- and N-bound H atoms were refined without restraint. A weighting scheme of the form w = 1/[σ²(F_o²) + (0.039P)² + 0.272P] was introduced, where P = (F_o² + 2F_c²)/3. On the basis of the refinement of 90 parameters, the final values of R and wR (all data) were 0.024 and 0.066, respectively. The molecular structure diagram was generated with ORTEP for Windows [33] and the packing diagrams using DIAMOND [34].

Crystal data for C₅H₁₀N₂OS (1): M = 146.21, monoclinic, P2₁/c, a = 6.99980(10), b = 10.3937(2), c = 9.5692(2) Å, β = 95.764(2)°, V = 692.68(2) ų, Z = 4, D_x = 1.402 g cm⁻³ and μ = 3.509 mm⁻¹. CCDC deposition number: 1876988.