1-(Diamino­methyl­ene)thio­urea: a tautomer of 2-imino-4-thio­biuret

Crystals of the title compound, C₂H₆N₄S, are built up from nonplanar 1-(diamino­methyl­ene)thio­urea mol­ecules. Pairs of mol­ecules related by inversion are linked by N—H...N hydrogen bonds to form dimeric units, and weak N—H...S inter­actions link these dimeric units into a three-dimensional framework.

S1. Comment

Continuing our investigations into the characterization of compounds that form multiple and different hydrogen-bonding systems in solids (Janczak & Perpétuo, 2008; Perpétuo & Janczak, 2007, 2008; Janczak & Kubiak, 2005a,b), we report here the crystal structure of the title compound, (I). The commercially available crystalline product described as 2-imino-4-thiobiuret (amidinothiourea) by Aldrich (CAS No. 2114–02–05) is in fact the tautomeric form 1-(diaminomethylene)thiourea, as shown by the present X-ray structure determination. Recrystallization from water of the product purchased from Aldrich yielded crystals of 1-(diaminomethylene)thiourea, not the imino tautomer.

Both tautomers of (I), i.e. 1-(diaminomethylene)thiourea and 2-imino-4-thiobiuret, are potentially interesting componds that can be used in crystal engineering to built up extended frameworks, since they contain complementary arrays of hydrogen-bonding sites (MacDonald & Whitesides, 1994; Steiner, 2002; Desiraju, 1990, 2002). In addition, both tautomers have several potential coordination modes, since they can act as N,N- or N,S-ligands and can form several different types of complex with metal ions. The coordination of metals by these tautomers is feasible with both neutral and deprotonated (anionic) forms. A search of the Cambridge Structural Database (CSD, Version 5.29, November 2007; Allen, 2002) for structures containing 1-(diaminomethylene)thiourea or its tautomer, 2-imino-4-thiobiuret, yielded only three structures, all of them platinum complexes (Doxiadi et al. 2003). In these complexes, the Pt is coordinated by 2-imino-4-thiobiuret or its deprotonated form. Since the crystal structures of (I) and its tautomer are not known, we therefore decided to investigate the crystal structure of 1-(diaminomethylene)thiourea, (I). In addition its X-ray geometry is compared with that in the gas phase, as predicted for isolated molecules of (I) and its tautomeric form 2-imino-4-thiobiuret using density functional theory (DFT). Ab initio molecular orbital calculations were performed at the B3LYP/6–31+G* level (Frisch et al. 1998) and the results are illustrated in Fig. 1.

The asymmetric unit of (I) with the atom-labelling scheme is shown in Fig. 2. A s revealed by X-ray structure analysis, the molecule of (I) is not planar. However, the two arms of (I) containing the N1/C2/N3/N4 and N1/C1/N2/S1 fragments are planar. The deviation of the non-H atoms from the weighted mean planes defined by atoms N1/C2/N3/N4 and N1/C1/N2/S1 is less than 0.017 (2) and 0.024 (2) Å, respectively. Thus, the conformation of the molecule is twisted, with the two planar fragments rotated around the C1—N1 bond.

The C1—S1 bond is longer by ~0.08 Å than typical C═S double bonds in thiourea derivatives (mean value 1.663 Å; Allen et al., 1997). In the gas phase, thioformaldehyde, CH₂═S, has a C═S bond distance of 1.6109 (8) Å (Johnson et al., 1971) and this represents 100% double-bond character, while C—S bond lengths of ca 1.74 Å, as observed in dithiolate anions (Johnson et al., 1971; Fausto et al., 1989), represent 50% double-bond character.

The three C—NH₂ bond distances of (I) are in the range 1.326 (2)–1.334 (2) Å, slightly shorter than typical Csp²—NH₂ bond lengths of 1.341–1.363 Å (Allen et al., 1987). The central atom N1 links both arms of the molecule of (I) by a shorter bond to atom C2 and a longer bond to atom C1 (i.e. N1—C2 < N1—C1). These C—N bond lengths involving the central atom N1, the C═S bond which is slightly longer than a typical double bond, and the almost planar NH₂ groups, point to partial delocalization of the π electrons of the C2═N1 and C1═S1 double bonds over almost the whole molecule. The planarity of the NH₂ groups in (I) indicates that the lone pairs of electrons on the N atoms occupy the p orbitals, which are perpendicular to the planes of the amine groups. Due to symmetry of the p orbital at the N atoms and at the C and S atoms, partial delocalization of the electrons of the π bonds (C2═N1 and C1═ S1) is possible, giving in effect an elongation of the double bonds C1═S and C2═N1 and a shortening of the other single C—N bonds. Thus, the bond order of C1≐S1 is less than 2, and the bond orders of all the C—N bonds have almost similar values, being greater than 1. The interaction of the amine group at N4 with atom S1 is stronger than the steric effect of a lone-pair of electrons at the central atom N1, which is localized on the hybridized orbital sp² (Gillespie, 1963, 1992) and which makes the N4—C2—N1, C2—N1—C1 and N1—C1—S1 angles greater than 120°, as expected for sp² hybridization. Additionally, steric interaction between atoms N4 and S1 is responsible for the opposite rotation of both rigid arms of the molecule around the C—N bonds involving atom N1 (C2—N1 and C1—N1).

Comparing the X-ray diffraction results with the molecular orbital (MO) calculations for the gas phase of both tautomers of (I), it should be stated that the correlation between the calculated bond lengths and angles for 1-(diaminomethylene)thiourea give a similar pattern to that obtained by X-ray analysis. However, the small differences between the geometric parameters from the X-ray analysis and those obtained by MO calculations arise from the intermolecular interactions present in the crystal structure. The geometric parameters of the second tautomer of (I), i.e. 2-imino-4-thiobiuret, obtained by MO calculations and X-ray analysis show significantly greater differences, especially between the MO calculated and experimental X-ray values of the C2—N1, C2—N4, C2—N3 and C1—S1 bonds and the C1—N1—C2, N1—C1—S1 and N1—C2—N4 angles. Moreover, the amine group containing atom N3 of 2-imino-4-thiobiuret has almost pyramidal geometry, in contrast with 1-(diaminomethylene)thiourea, in which the MO optimized geometry of all NH₂ groups is almost planar, similar to what is observed in the crystal structure. Additionally, the calculated total energy of the 1-(diaminomethylene)thiourea tautomer has a lower value than 2-imino-4-thiobiuret by 58.77 kJ mol^-1 (11.99 kcal mol^-1). Thus, the crystalline product of commercially available 2-imino-4-thiobiuret, as well as the crystals obtained by its recrystallization from water, are present in the other tautomeric form, i.e. 1-(diaminomethylene)thiourea.

In the crystal structure of (I), pairs of molecules related by an inversion interact via N—H···N hydrogen bonds to form a dimeric unit (Fig. 3). The dimers are arranged along the b axis forming a stacking structure. Between the dimers, weaker N—H···S interactions are observed. The S atom contains two lone pairs of electrons that can be involved as acceptors in N—H···S hydrogen bonds. A non-bonded S···H contact requires that the distance between the S and H atoms (d_SH) should be less than the sum of the van der Waals radii [r_S = 1.80 Å (Bondi, 1964) and r_H = 1.10 Å (Rowland & Taylor, 1996)]. Thus, Table 2 contains the S···H contacts shorter than 2.9 Å. Allen et al. (1997) classified the intermolecular hydrogen-bond S···H distances to a C═S acceptor on the basis of over 600 structures in the CSD and found that the majority of these structures (over 85%) contain S···H distances within the range 2.28–2.72 Å, and only about 2% have S···H distances of ~2.90 Å. In these structures, the overall mean S···H—N(or O) angle of 157 (1)° shows a strong tendency to linearity in C═S···H—N(O) systems. Thus, in the present structure, the intermolecular S···H interactions, with distances in the range 2.677 (2)–2.885 (2) Å and with an average C═S···H(N) angle of 154°, are relatively weak, and they are weaker than analogous C═O···H(N) interactions, since the electronegativity of S [2.44 (Alled & Rochow, 1958); 2.58 (Pauling, 1960)] is much lower than that of O (3.50 and 3.44, respectively) while being very similar to that of C (2.50 and 2.55, respectively). For this reason, hydrogen bonds with C═S acceptors exhibit an H···A range which is much wider than that for the C═O analogues (Allen et al., 1997).

Related literature

For related literature, see: Alled & Rochow (1958); Allen (2002); Allen et al. (1987, 1997); Bondi (1964); Desiraju (1990, 2002); Doxiadi et al. (2003); Fausto et al. (1989); Frisch (1998); Gillespie (1963, 1992); Janczak & Kubiak (2005a, 2005b); Janczak & Perpétuo (2008); Johnson et al. (1971); MacDonald & Whitesides (1994); Pauling (1960); Perpétuo & Janczak (2007, 2008); Rowland & Taylor (1996); Steiner (2002).

S2. Experimental

Crystals of (I) were obtained by recrystallization of 2-imino-4-thiobiuret (Aldrich) from aqueous solution.

S3. Refinement

H atoms were located in difference Fourier maps and were then refined, giving a range of N—H distances of 0.824 (14)–0.860 (13) Å.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2005); cell refinement: CrysAlis CCD (Oxford Diffraction, 2005); data reduction: CrysAlis CCD (Oxford Diffraction, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg & Putz, 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. Results of the optimized molecular orbital calculations (B3LYP/6–31+G*) for (a) the 1-(diaminomethylene)thiurea and (b) the 2-imino-4-thiobiuret tautomers of (I) (Å, °).
	Fig. 2. A view of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 3. A view of the crystal packing in (I), showing the stacking structure. The N—H···N hydrogen bonds forming the dimers are represented by light dashed lines and the N—H···S interactions linking the dimers are represented by dark dashed lines.

1-(Diaminomethylene)thiourea

Crystal data

C2H6N4S	F(000) = 496
Mr = 118.17	Dx = 1.537 Mg m^−3 Dm = 1.53 Mg m^−3 Dm measured by flotation
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1086 reflections
a = 13.505 (2) Å	θ = 2.9–28.3°
b = 5.380 (1) Å	µ = 0.50 mm^−1
c = 14.111 (3) Å	T = 295 K
β = 95.05 (2)°	Rectangular, colourless
V = 1021.3 (3) Å^3	0.54 × 0.25 × 0.23 mm
Z = 8

Data collection

Kuma KM-4 with CCD area-detector diffractometer	1261 independent reflections
Radiation source: fine-focus sealed tube	1086 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.014
Detector resolution: 1024x1024 with blocks 2x2 pixels mm^-1	θmax = 28.3°, θmin = 2.9°
ω scans	h = −16→18
Absorption correction: analytical face-indexed (SHELXTL; Sheldrick, 2008)	k = −7→7
Tmin = 0.769, Tmax = 0.879	l = −18→18
5622 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.022	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.051	All H-atom parameters refined
S = 1.00	w = 1/[σ^2(Fo^2) + (0.0267P)^2 + 0.2215P] where P = (Fo^2 + 2Fc^2)/3
1261 reflections	(Δ/σ)max = 0.001
88 parameters	Δρmax = 0.14 e Å^−3
0 restraints	Δρmin = −0.15 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
S1	0.575368 (17)	0.16219 (5)	0.651139 (19)	0.03541 (10)
C1	0.65895 (6)	−0.00282 (16)	0.58865 (6)	0.02487 (19)
N1	0.75846 (5)	0.01172 (14)	0.59411 (5)	0.02664 (18)
H1	0.6583 (9)	−0.272 (2)	0.4972 (8)	0.037 (3)*
H2	0.5599 (10)	−0.190 (2)	0.5186 (8)	0.042 (3)*
C2	0.81074 (7)	0.19836 (17)	0.63575 (6)	0.02574 (19)
N2	0.62239 (6)	−0.18171 (17)	0.53092 (7)	0.0364 (2)
N3	0.90899 (7)	0.1660 (2)	0.64744 (8)	0.0420 (2)
H3	0.9312 (9)	0.028 (2)	0.6353 (9)	0.039 (3)*
H4	0.9476 (10)	0.286 (3)	0.6649 (9)	0.050 (4)*
N4	0.77528 (8)	0.41517 (17)	0.66206 (7)	0.0374 (2)
H5	0.8121 (10)	0.523 (3)	0.6910 (9)	0.047 (4)*
H6	0.7147 (10)	0.436 (3)	0.6605 (9)	0.048 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S1	0.02903 (12)	0.03694 (15)	0.04028 (17)	0.00479 (10)	0.00305 (10)	−0.00922 (11)
C1	0.0225 (4)	0.0248 (4)	0.0271 (4)	0.0003 (3)	0.0006 (3)	0.0025 (4)
N1	0.0213 (4)	0.0268 (4)	0.0316 (4)	−0.0010 (3)	0.0009 (3)	−0.0045 (3)
C2	0.0257 (4)	0.0275 (5)	0.0238 (4)	−0.0023 (4)	0.0008 (3)	0.0012 (4)
N2	0.0220 (4)	0.0418 (5)	0.0450 (5)	−0.0036 (4)	0.0008 (4)	−0.0165 (4)
N3	0.0238 (4)	0.0359 (5)	0.0651 (7)	−0.0039 (4)	−0.0028 (4)	−0.0096 (5)
N4	0.0334 (5)	0.0291 (4)	0.0489 (6)	−0.0032 (4)	−0.0011 (4)	−0.0122 (4)

Geometric parameters (Å, º)

S1—C1	1.7364 (9)	N2—H1	0.860 (13)
C1—N2	1.3277 (12)	N2—H2	0.848 (13)
C1—N1	1.3416 (11)	N3—H3	0.826 (13)
N1—C2	1.3338 (11)	N3—H4	0.853 (15)
C2—N4	1.3263 (13)	N4—H5	0.843 (14)
C2—N3	1.3341 (13)	N4—H6	0.824 (14)
N2—C1—N1	113.20 (8)	C1—N2—H2	118.3 (8)
N2—C1—S1	117.32 (7)	H1—N2—H2	117.1 (11)
N1—C1—S1	129.33 (7)	C2—N3—H3	117.8 (8)
C2—N1—C1	123.91 (8)	C2—N3—H4	120.9 (9)
N4—C2—N1	126.53 (9)	H3—N3—H4	121.3 (13)
N4—C2—N3	117.38 (9)	C2—N4—H5	121.7 (9)
N1—C2—N3	115.99 (9)	C2—N4—H6	119.7 (10)
C1—N2—H1	123.6 (8)	H5—N4—H6	117.4 (13)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H1···N1^i	0.860 (13)	2.200 (14)	3.0584 (13)	176.5 (11)
N2—H2···S1^ii	0.848 (13)	2.885 (12)	3.5406 (13)	135.6 (10)
N3—H3···S1^iii	0.826 (13)	2.762 (13)	3.5180 (12)	153.0 (11)
N3—H4···S1^iv	0.853 (15)	2.677 (15)	3.4868 (12)	159.0 (12)
N4—H5···S1^v	0.843 (14)	2.691 (13)	3.4419 (12)	149.0 (11)
N4—H6···S1	0.824 (14)	2.385 (14)	3.0153 (11)	133.9 (12)

Symmetry codes: (i) −x+3/2, −y−1/2, −z+1; (ii) −x+1, −y, −z+1; (iii) x+1/2, y−1/2, z; (iv) x+1/2, y+1/2, z; (v) −x+3/2, y+1/2, −z+3/2.