S—H...S hydrogen-bond chain in thio­salicylic acid

In crystalline thio­salicylic acid (2-mercapto­benzoic acid), C₇H₆O₂S, the carboxyl­ic acid groups form hydrogen-bonded dimers, whereas the S—H groups form an infinite S—H...S—H...S—H hydrogen-bond chain, with an S...S distance of 3.986 (3) Å.

S1. Comment

Although S—H is a classical hydrogen-bonding functional group, there is surprisingly little structural information on S—H···X hydrogen bonds. Even for the most common variant, S—H···O, the published material is scarce (Allen et al., 1997). For all the other cases like S—H···S, S—H···N, S—H···Cl⁻, S—H···Ph, etc., there are only a handful of relevant crystal structures (surveyed by Desiraju & Steiner, 1999; also see Steiner, 1998; Rozenberg et al., 1999). In this context, the crystal structure of thiosalicylic acid, (I), was determined. Initially, it was unclear which kind of hydrogen bond the S—H group would form (inter- or intramolecular S—H···O, S—H···S). \sch

The molecular structure of (I) in the crystal is shown in Figure 1. The mercapto and carboxyl groups have normal geometries [C2—S1 = 1.766 (3), S1—H1 = 1.30 (5) Å, C2—S1—H1 = 95 (2)°; O1═C7 = 1.219 (4), O2—C7 = 1.306 (4) Å]. No intramolecular S—H···O hydrogen bond is formed, but the S—H group is oriented away from the carboxylic acid moiety [torsion angle C1—C2—S1—H1 = 178 (2)°]. This contrasts the oxygen analogue salicylic acid, where an intramolecular O—H···O═C hydrogen bond is formed with O···O = 2.62 Å (Sundaralingam & Jensen, 1965). The molecule of (I) is significantly non-planar with the carboxylic acid group rotated 10.0 (2)° out of the aromatic plane. This way, O1 is displaced by 0.131 (5) Å from the ring plane. S1 is displaced from that plane in opposite direction by −0.016 (4) Å. The angle C1—C2—S1 is widened to 122.1 (2)°. These distortions indicate significal steric repulsion between O1 and S1 [distance 2.752 (3) Å]. When thinking about why no intramolecular hydrogen bond is formed, one must consider that S···O distances in S—H···O hydrogen bonds have been found only longer than 3.2 Å (Desiraju & Steiner, 1999). A hypothetical S—H···O interaction with an S···O separation around 2.75 Å would (if relatively linear) have an H···O distance so short that it is deep in the repulsive regime of the hydrogen-bond potential.

The hydrogen-bond scheme that is actually formed in (I) is shown in Figure 2, geometrical parameters are listed in Table 1. The carboxylic acid groups form dimers with standard geometry [O···O = 2.657 (4) Å]. The S—H groups form an infinite hydrogen-bond chain S—H···S—H···S—H in direction of the y axis with distances S···S = 3.986 (3) and H···S = 2.72 (5) Å. Since the C—S—H group has refined almost to typical neutron-diffraction geometry [e.g. S—H = 1.338 (2) Å and C—S—H = 96.9 (2)° in the neutron diffraction study of N-acetyl-l-cysteine, Takusagawa et al., 1981], the geometry in Table 1 is already realistic and normalization is unnecessary. The distances are similar as in other S—H···S hydrogen bonds between poorly activated S—H groups like in thiols and thiophenols (they can be considerably shorter between more polar S—H groups). The acceptor directionality is described by the angle H···S1—C2 = 76.1 (9)° and the torsion angle H···S1—C2—C1 = −81 (1)°, showing that the hydrogen bonds are directed at the `sides' of the C—S—H acceptors. Two weak C—H···O hydrogen bonds are also formed (Table 1).

S2. Experimental

Thiosalicylic acid, (I), was obtained from Aldrich, and was recrystallized from MeOH by slow evaporation of the solvent.

S3. Refinement

H atoms bonded to C were treated in the default riding model, the H atoms bonded to O2 and S1 were located in difference Fourier calculations and refined isotropically. All H atom displacement parameters refined to realistic values [H atoms bonded to C: 0.056–0.072 Ų; H atoms at O and S: 0.11 (1) and 0.09 (1) Ų, respectively].

Computing details

Program(s) used to solve structure: SHELXS86 (Sheldrick, 1986); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Molecular structure of (I). Displacement ellipsoids are drawn at the 30% probability level.
	Fig. 2. Section of the crystal packing illustrating the system of O—H···O and S—H···S hydrogen bonds. The S—H···S chain runs in direction of the y axis.

2-mercaptobenzoic acid

Crystal data

C7H6O2S	F(000) = 320
Mr = 154.18	Dx = 1.489 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 7.856 (7) Å	Cell parameters from 34 reflections
b = 5.963 (5) Å	θ = 7.2–16.2°
c = 14.940 (11) Å	µ = 0.40 mm^−1
β = 100.69 (7)°	T = 293 K
V = 687.7 (10) Å^3	Plate, yellow
Z = 4	0.25 × 0.20 × 0.03 mm

Data collection

Stoe four circle diffractometer	Rint = 0.043
Radiation source: fine-focus sealed tube	θmax = 27.5°, θmin = 2.6°
Graphite monochromator	h = −10→7
ω–scans	k = −7→7
1692 measured reflections	l = −19→14
1580 independent reflections	3 standard reflections every 90 min
1188 reflections with I > 2σ(I)	intensity decay: 5%

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)] = 0.057	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.171	H atoms treated by a mixture of independent and constrained refinement
S = 1.03	w = 1/[σ^2(Fo^2) + (0.1042P)^2 + 0.1754P] where P = (Fo^2 + 2Fc^2)/3
1580 reflections	(Δ/σ)max < 0.001
103 parameters	Δρmax = 0.53 e Å^−3
0 restraints	Δρmin = −0.30 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	0.51689 (9)	0.11696 (13)	0.16353 (5)	0.0462 (3)
H1	0.495 (6)	0.301 (8)	0.206 (3)	0.089 (14)*
O1	0.5120 (3)	−0.2870 (4)	0.07396 (16)	0.0513 (6)
O2	0.2759 (3)	−0.4588 (4)	0.00178 (18)	0.0567 (7)
H2	0.355 (7)	−0.542 (11)	−0.021 (3)	0.108 (18)*
C1	0.2377 (3)	−0.1588 (5)	0.09739 (18)	0.0364 (6)
C2	0.2970 (3)	0.0356 (5)	0.14519 (18)	0.0368 (6)
C3	0.1783 (4)	0.1685 (6)	0.1799 (2)	0.0495 (8)
H3	0.2164	0.2970	0.2129	0.072 (12)*
C4	0.0063 (4)	0.1121 (6)	0.1661 (2)	0.0516 (8)
H4	−0.0712	0.2041	0.1889	0.056 (10)*
C5	−0.0528 (4)	−0.0779 (6)	0.1191 (2)	0.0501 (8)
H5	−0.1694	−0.1161	0.1106	0.065 (11)*
C6	0.0623 (4)	−0.2121 (6)	0.0845 (2)	0.0437 (7)
H6	0.0221	−0.3405	0.0520	0.057 (10)*
C7	0.3548 (4)	−0.3061 (5)	0.05711 (19)	0.0400 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S1	0.0355 (4)	0.0408 (5)	0.0627 (5)	−0.0039 (3)	0.0104 (3)	−0.0065 (3)
O1	0.0319 (10)	0.0506 (13)	0.0726 (14)	−0.0017 (9)	0.0126 (9)	−0.0245 (11)
O2	0.0367 (11)	0.0542 (14)	0.0810 (17)	−0.0036 (10)	0.0160 (11)	−0.0309 (12)
C1	0.0335 (13)	0.0366 (14)	0.0406 (13)	0.0022 (11)	0.0104 (10)	−0.0005 (11)
C2	0.0323 (13)	0.0387 (14)	0.0401 (13)	0.0018 (11)	0.0088 (10)	0.0001 (11)
C3	0.0440 (16)	0.0448 (17)	0.0615 (19)	0.0036 (13)	0.0147 (14)	−0.0144 (14)
C4	0.0418 (16)	0.056 (2)	0.0611 (19)	0.0086 (14)	0.0199 (14)	−0.0111 (15)
C5	0.0329 (14)	0.063 (2)	0.0569 (18)	−0.0003 (13)	0.0139 (12)	−0.0068 (15)
C6	0.0347 (14)	0.0460 (16)	0.0515 (16)	−0.0029 (12)	0.0114 (11)	−0.0088 (13)
C7	0.0369 (14)	0.0369 (14)	0.0478 (15)	−0.0007 (12)	0.0122 (11)	−0.0051 (12)

Geometric parameters (Å, º)

S1—C2	1.766 (3)	C2—C3	1.396 (4)
S1—H1	1.30 (5)	C3—C4	1.370 (4)
O1—C7	1.219 (4)	C3—H3	0.9300
O2—C7	1.306 (4)	C4—C5	1.368 (5)
O2—H2	0.91 (6)	C4—H4	0.9300
C1—C6	1.392 (4)	C5—C6	1.377 (4)
C1—C2	1.395 (4)	C5—H5	0.9300
C1—C7	1.478 (4)	C6—H6	0.9300
C2—S1—H1	95 (2)	C5—C4—H4	119.6
C7—O2—H2	110 (3)	C3—C4—H4	119.6
C6—C1—C2	119.2 (3)	C4—C5—C6	119.3 (3)
C6—C1—C7	118.9 (3)	C4—C5—H5	120.3
C2—C1—C7	121.9 (2)	C6—C5—H5	120.3
C1—C2—C3	118.6 (3)	C5—C6—C1	121.2 (3)
C1—C2—S1	122.1 (2)	C5—C6—H6	119.4
C3—C2—S1	119.2 (2)	C1—C6—H6	119.4
C4—C3—C2	120.9 (3)	O1—C7—O2	122.7 (3)
C4—C3—H3	119.6	O1—C7—C1	122.9 (3)
C2—C3—H3	119.6	O2—C7—C1	114.4 (3)
C5—C4—C3	120.8 (3)
C6—C1—C2—C3	1.1 (4)	C4—C5—C6—C1	0.7 (5)
C7—C1—C2—C3	178.9 (3)	C2—C1—C6—C5	−0.8 (5)
C6—C1—C2—S1	−179.3 (2)	C7—C1—C6—C5	−178.7 (3)
C7—C1—C2—S1	−1.5 (4)	C6—C1—C7—O1	−171.1 (3)
C1—C2—C3—C4	−1.2 (5)	C2—C1—C7—O1	11.1 (5)
S1—C2—C3—C4	179.2 (3)	C6—C1—C7—O2	9.0 (4)
C2—C3—C4—C5	1.1 (5)	C2—C1—C7—O2	−168.8 (3)
C3—C4—C5—C6	−0.8 (5)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O1^i	0.91 (6)	1.75 (6)	2.657 (4)	174 (5)
S1—H1···S1^ii	1.30 (5)	2.72 (5)	3.986 (3)	164 (3)
C5—H5···O1^iii	0.93	2.66	3.584 (5)	171
C6—H6···O2^iv	0.93	2.62	3.362 (5)	137

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