Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270100004194/jz1394sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270100004194/jz1394Isup2.hkl |
CCDC reference: 147643
Compound (I) was synthesized according to the literature method of Hino et al. (1969), recrystallized from benzene and vacuum dried to give colourless prisms [thin layer chromatography with benzene/n-hexane (2:1): Rf = 0.15; m.p. 455 K].
The quadrants ±h+k+l and ±h-k-l were measured. H atoms were located by a difference Fourier synthesis and refined with isotropic displacement parameters, with the exception of the methyl-H atom positions, which were idealized by geometrical considerations and allowed to rotate but not tip.
Data collection: DIF4 (Stoe & Cie 1991); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie 1991); program(s) used to solve structure: SHELXS86 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP/PC (Siemens 1990); software used to prepare material for publication: SHELXL97.
Fig. 1. The molecular structure and atomic numbering of (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as spheres of arbitrary radii. |
C18H16N2S2 | F(000) = 1360 |
Mr = 324.45 | Dx = 1.307 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
a = 17.635 (5) Å | Cell parameters from 80 reflections |
b = 8.418 (2) Å | θ = 7.6–13.9° |
c = 22.591 (5) Å | µ = 0.32 mm−1 |
β = 100.50 (3)° | T = 293 K |
V = 3297.5 (14) Å3 | Prism, yellow |
Z = 8 | 0.49 × 0.21 × 0.15 mm |
Stoe STADI4 four-circle diffractometer | Rint = 0.036 |
Radiation source: fine-focus sealed tube | θmax = 25.0°, θmin = 1.8° |
Planar graphite monochromator | h = −20→20 |
scan width (ω) = 1.05–1.20, scan ratio 2θ:ω = 1.00 | k = −10→10 |
5790 measured reflections | l = −26→26 |
2895 independent reflections | 3 standard reflections every 60 min |
1664 reflections with I > 2σ(I) | intensity decay: 3.4% |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.042 | Hydrogen site location: difmap except H9A to H9C and H18A to H18C geom |
wR(F2) = 0.106 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.99 | Calculated w = 1/[σ2(Fo2) + (0.057P)2] where P = (Fo2 + 2Fc2)/3 |
2895 reflections | (Δ/σ)max = 0.023 |
241 parameters | Δρmax = 0.19 e Å−3 |
0 restraints | Δρmin = −0.27 e Å−3 |
C18H16N2S2 | V = 3297.5 (14) Å3 |
Mr = 324.45 | Z = 8 |
Monoclinic, C2/c | Mo Kα radiation |
a = 17.635 (5) Å | µ = 0.32 mm−1 |
b = 8.418 (2) Å | T = 293 K |
c = 22.591 (5) Å | 0.49 × 0.21 × 0.15 mm |
β = 100.50 (3)° |
Stoe STADI4 four-circle diffractometer | Rint = 0.036 |
5790 measured reflections | 3 standard reflections every 60 min |
2895 independent reflections | intensity decay: 3.4% |
1664 reflections with I > 2σ(I) |
R[F2 > 2σ(F2)] = 0.042 | 0 restraints |
wR(F2) = 0.106 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.99 | Δρmax = 0.19 e Å−3 |
2895 reflections | Δρmin = −0.27 e Å−3 |
241 parameters |
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. Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane) 8.0116 (0.0103) x − 7.3936 (0.0047) y + 1.4381 (0.0217) z = 2.7772 (0.0124) * 0.0249 (0.0020) C1 * −0.0211 (0.0021) C2 * −0.0039 (0.0023) C3 * −0.0097 (0.0025) C4 * −0.0100 (0.0026) C5 * 0.0024 (0.0029) C6 * 0.0117 (0.0026) C7 * 0.0042 (0.0024) C8 * 0.0015 (0.0020) N1 0.0011 (0.0044) C9 0.1001 (0.0026) S1 Rms deviation of fitted atoms = 0.0126 14.9230 (0.0102) x + 0.7081 (0.0057) y + 8.2036 (0.0164) z = 11.7156 (0.0106) Angle to previous plane (with approximate e.s.d.) = 67.11 (0.06) * 0.0383 (0.0019) C10 * −0.0187 (0.0019) C11 * −0.0218 (0.0020) C12 * −0.0209 (0.0023) C13 * −0.0218 (0.0024) C14 * 0.0141 (0.0025) C15 * 0.0258 (0.0025) C16 * −0.0017 (0.0021) C17 * 0.0067 (0.0018) N2 − 0.0229 (0.0040) C18 0.0994 (0.0024) S2 Rms deviation of fitted atoms = 0.0214 |
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. |
x | y | z | Uiso*/Ueq | ||
S1 | 0.47085 (4) | 0.22738 (10) | 0.54668 (3) | 0.0618 (3) | |
S2 | 0.36455 (5) | 0.21374 (12) | 0.75862 (3) | 0.0778 (3) | |
N1 | 0.33494 (11) | 0.0865 (3) | 0.51114 (9) | 0.0465 (5) | |
N2 | 0.38333 (12) | −0.0958 (3) | 0.73988 (9) | 0.0537 (6) | |
C1 | 0.38766 (13) | 0.1490 (3) | 0.55488 (11) | 0.0426 (6) | |
C2 | 0.35741 (13) | 0.1339 (3) | 0.61356 (11) | 0.0408 (6) | |
C3 | 0.28249 (13) | 0.0468 (3) | 0.59512 (10) | 0.0416 (6) | |
C4 | 0.27082 (13) | 0.0229 (3) | 0.53347 (11) | 0.0447 (6) | |
C5 | 0.20720 (16) | −0.0520 (4) | 0.50240 (14) | 0.0632 (9) | |
C6 | 0.15350 (17) | −0.1055 (5) | 0.53550 (15) | 0.0734 (10) | |
C7 | 0.16355 (16) | −0.0840 (4) | 0.59643 (14) | 0.0663 (9) | |
C8 | 0.22813 (14) | −0.0070 (4) | 0.62719 (12) | 0.0534 (7) | |
C9 | 0.34157 (16) | 0.0814 (4) | 0.44748 (11) | 0.0649 (9) | |
H9A | 0.3550 | 0.1849 | 0.4349 | 0.097* | |
H9B | 0.2932 | 0.0492 | 0.4238 | 0.097* | |
H9C | 0.3809 | 0.0067 | 0.4421 | 0.097* | |
C10 | 0.38775 (14) | 0.0556 (4) | 0.72262 (11) | 0.0500 (7) | |
C11 | 0.41652 (14) | 0.0553 (3) | 0.66338 (11) | 0.0418 (6) | |
C12 | 0.43032 (13) | −0.1171 (3) | 0.65277 (11) | 0.0424 (6) | |
C13 | 0.40882 (14) | −0.2024 (4) | 0.69935 (11) | 0.0503 (7) | |
C14 | 0.41488 (19) | −0.3667 (4) | 0.70240 (17) | 0.0699 (9) | |
C15 | 0.4454 (2) | −0.4420 (5) | 0.65781 (17) | 0.0757 (10) | |
C16 | 0.46747 (18) | −0.3597 (4) | 0.61193 (16) | 0.0657 (9) | |
C17 | 0.45990 (16) | −0.1957 (4) | 0.60819 (13) | 0.0527 (7) | |
C18 | 0.35434 (19) | −0.1465 (5) | 0.79337 (13) | 0.0831 (11) | |
H18A | 0.3575 | −0.0596 | 0.8212 | 0.125* | |
H18B | 0.3849 | −0.2335 | 0.8120 | 0.125* | |
H18C | 0.3016 | −0.1796 | 0.7821 | 0.125* | |
H2 | 0.3504 (12) | 0.244 (3) | 0.6289 (10) | 0.042 (7)* | |
H5 | 0.2030 (15) | −0.075 (3) | 0.4617 (13) | 0.070 (9)* | |
H6 | 0.1078 (16) | −0.151 (4) | 0.5161 (12) | 0.074 (9)* | |
H7 | 0.1247 (15) | −0.118 (3) | 0.6195 (12) | 0.071 (8)* | |
H8 | 0.2383 (14) | 0.016 (3) | 0.6715 (12) | 0.061 (8)* | |
H11 | 0.4623 (13) | 0.116 (3) | 0.6684 (10) | 0.042 (7)* | |
H14 | 0.4002 (15) | −0.410 (3) | 0.7368 (13) | 0.066 (9)* | |
H15 | 0.4516 (19) | −0.565 (5) | 0.6604 (15) | 0.109 (12)* | |
H16 | 0.4909 (16) | −0.409 (4) | 0.5790 (14) | 0.082 (10)* | |
H17 | 0.4765 (16) | −0.131 (4) | 0.5803 (13) | 0.070 (10)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
S1 | 0.0543 (4) | 0.0662 (6) | 0.0715 (5) | −0.0201 (4) | 0.0286 (4) | −0.0055 (4) |
S2 | 0.0879 (6) | 0.0898 (7) | 0.0590 (5) | 0.0057 (5) | 0.0217 (4) | −0.0261 (5) |
N1 | 0.0435 (12) | 0.0581 (15) | 0.0393 (12) | −0.0040 (11) | 0.0115 (9) | 0.0028 (11) |
N2 | 0.0493 (13) | 0.0694 (18) | 0.0417 (12) | −0.0046 (12) | 0.0060 (10) | 0.0072 (12) |
C1 | 0.0438 (15) | 0.0372 (15) | 0.0495 (15) | −0.0004 (11) | 0.0157 (12) | 0.0024 (12) |
C2 | 0.0401 (14) | 0.0405 (17) | 0.0437 (14) | −0.0017 (12) | 0.0123 (11) | −0.0037 (12) |
C3 | 0.0340 (13) | 0.0485 (16) | 0.0428 (14) | 0.0024 (11) | 0.0084 (10) | 0.0056 (12) |
C4 | 0.0357 (13) | 0.0561 (18) | 0.0428 (15) | −0.0036 (12) | 0.0083 (11) | 0.0071 (13) |
C5 | 0.0493 (17) | 0.093 (3) | 0.0449 (17) | −0.0145 (16) | 0.0020 (14) | −0.0024 (17) |
C6 | 0.0418 (16) | 0.110 (3) | 0.063 (2) | −0.0250 (19) | −0.0031 (14) | 0.0098 (19) |
C7 | 0.0389 (16) | 0.097 (3) | 0.064 (2) | −0.0143 (16) | 0.0094 (14) | 0.0180 (19) |
C8 | 0.0394 (15) | 0.076 (2) | 0.0458 (17) | 0.0009 (14) | 0.0110 (12) | 0.0095 (15) |
C9 | 0.0658 (19) | 0.090 (2) | 0.0427 (16) | −0.0089 (17) | 0.0192 (14) | 0.0044 (16) |
C10 | 0.0424 (14) | 0.067 (2) | 0.0394 (14) | −0.0039 (14) | 0.0037 (11) | −0.0057 (15) |
C11 | 0.0372 (14) | 0.0464 (17) | 0.0414 (14) | −0.0062 (13) | 0.0060 (11) | −0.0063 (12) |
C12 | 0.0351 (13) | 0.0427 (17) | 0.0459 (15) | −0.0032 (12) | −0.0017 (11) | −0.0016 (13) |
C13 | 0.0432 (15) | 0.055 (2) | 0.0480 (16) | −0.0042 (14) | −0.0027 (12) | 0.0027 (15) |
C14 | 0.073 (2) | 0.060 (3) | 0.070 (2) | −0.0113 (18) | −0.0059 (18) | 0.019 (2) |
C15 | 0.081 (2) | 0.049 (2) | 0.086 (3) | 0.0014 (18) | −0.0129 (19) | −0.010 (2) |
C16 | 0.064 (2) | 0.057 (2) | 0.071 (2) | 0.0088 (17) | −0.0029 (16) | −0.0142 (19) |
C17 | 0.0477 (16) | 0.055 (2) | 0.0524 (18) | 0.0017 (14) | 0.0005 (13) | −0.0113 (16) |
C18 | 0.083 (2) | 0.118 (3) | 0.0520 (18) | −0.008 (2) | 0.0221 (16) | 0.020 (2) |
S1—C1 | 1.649 (2) | C4—C5 | 1.365 (4) |
S2—C10 | 1.650 (3) | C5—C6 | 1.384 (4) |
N1—C1 | 1.335 (3) | C6—C7 | 1.367 (4) |
N1—C4 | 1.424 (3) | C7—C8 | 1.382 (4) |
N1—C9 | 1.465 (3) | C10—C11 | 1.515 (3) |
N2—C10 | 1.339 (4) | C11—C12 | 1.498 (4) |
N2—C13 | 1.413 (3) | C12—C13 | 1.383 (3) |
N2—C18 | 1.459 (3) | C12—C17 | 1.384 (4) |
C1—C2 | 1.522 (3) | C13—C14 | 1.388 (4) |
C2—C3 | 1.502 (3) | C14—C15 | 1.379 (5) |
C2—C11 | 1.537 (3) | C15—C16 | 1.361 (5) |
C3—C8 | 1.379 (3) | C16—C17 | 1.388 (4) |
C3—C4 | 1.385 (3) | ||
C1—N1—C4 | 112.14 (19) | C7—C6—C5 | 121.6 (3) |
C1—N1—C9 | 124.7 (2) | C6—C7—C8 | 120.8 (3) |
C4—N1—C9 | 123.2 (2) | C3—C8—C7 | 118.5 (3) |
C10—N2—C13 | 111.9 (2) | N2—C10—C11 | 107.6 (2) |
C10—N2—C18 | 124.5 (3) | N2—C10—S2 | 126.3 (2) |
C13—N2—C18 | 123.5 (3) | C11—C10—S2 | 126.2 (2) |
N1—C1—C2 | 107.7 (2) | C12—C11—C10 | 103.5 (2) |
N1—C1—S1 | 126.02 (18) | C12—C11—C2 | 114.1 (2) |
C2—C1—S1 | 126.27 (19) | C10—C11—C2 | 110.7 (2) |
C3—C2—C1 | 103.3 (2) | C13—C12—C17 | 119.8 (3) |
C3—C2—C11 | 116.0 (2) | C13—C12—C11 | 107.8 (2) |
C1—C2—C11 | 112.03 (19) | C17—C12—C11 | 132.3 (3) |
C8—C3—C4 | 119.4 (2) | C12—C13—C14 | 121.7 (3) |
C8—C3—C2 | 132.5 (2) | C12—C13—N2 | 109.1 (3) |
C4—C3—C2 | 108.2 (2) | C14—C13—N2 | 129.2 (3) |
C5—C4—C3 | 122.8 (2) | C15—C14—C13 | 117.3 (3) |
C5—C4—N1 | 128.6 (2) | C16—C15—C14 | 121.8 (4) |
C3—C4—N1 | 108.6 (2) | C15—C16—C17 | 121.0 (3) |
C4—C5—C6 | 116.9 (3) | C12—C17—C16 | 118.4 (3) |
C4—N1—C1—C2 | −2.1 (3) | C13—N2—C10—S2 | −179.02 (18) |
C9—N1—C1—C2 | 178.5 (2) | C18—N2—C10—S2 | 2.1 (4) |
C4—N1—C1—S1 | 177.6 (2) | N2—C10—C11—C12 | −2.5 (3) |
C9—N1—C1—S1 | −1.8 (4) | S2—C10—C11—C12 | 178.55 (18) |
N1—C1—C2—C3 | 2.7 (3) | N2—C10—C11—C2 | 120.2 (2) |
S1—C1—C2—C3 | −176.9 (2) | S2—C10—C11—C2 | −58.8 (3) |
N1—C1—C2—C11 | 128.2 (2) | C3—C2—C11—C12 | 49.3 (3) |
S1—C1—C2—C11 | −51.4 (3) | C1—C2—C11—C12 | −68.9 (3) |
C1—C2—C3—C8 | 178.2 (3) | C3—C2—C11—C10 | −67.0 (3) |
C11—C2—C3—C8 | 55.3 (4) | C1—C2—C11—C10 | 174.8 (2) |
C1—C2—C3—C4 | −2.4 (3) | C10—C11—C12—C13 | 2.1 (3) |
C11—C2—C3—C4 | −125.3 (2) | C2—C11—C12—C13 | −118.3 (2) |
C8—C3—C4—C5 | 0.1 (4) | C10—C11—C12—C17 | −176.6 (3) |
C2—C3—C4—C5 | −179.4 (3) | C2—C11—C12—C17 | 63.0 (3) |
C8—C3—C4—N1 | −179.2 (2) | C17—C12—C13—C14 | −1.0 (4) |
C2—C3—C4—N1 | 1.3 (3) | C11—C12—C13—C14 | −179.9 (2) |
C1—N1—C4—C5 | −178.8 (3) | C17—C12—C13—N2 | 177.8 (2) |
C9—N1—C4—C5 | 0.6 (5) | C11—C12—C13—N2 | −1.0 (3) |
C1—N1—C4—C3 | 0.5 (3) | C10—N2—C13—C12 | −0.7 (3) |
C9—N1—C4—C3 | 179.9 (2) | C18—N2—C13—C12 | 178.2 (2) |
C3—C4—C5—C6 | −0.3 (5) | C10—N2—C13—C14 | 178.1 (3) |
N1—C4—C5—C6 | 178.9 (3) | C18—N2—C13—C14 | −3.0 (4) |
C4—C5—C6—C7 | 0.1 (5) | C12—C13—C14—C15 | 1.8 (4) |
C5—C6—C7—C8 | 0.2 (6) | N2—C13—C14—C15 | −176.9 (3) |
C4—C3—C8—C7 | 0.3 (4) | C13—C14—C15—C16 | −1.2 (5) |
C2—C3—C8—C7 | 179.6 (3) | C14—C15—C16—C17 | −0.2 (5) |
C6—C7—C8—C3 | −0.4 (5) | C13—C12—C17—C16 | −0.3 (4) |
C13—N2—C10—C11 | 2.0 (3) | C11—C12—C17—C16 | 178.2 (3) |
C18—N2—C10—C11 | −176.8 (2) | C15—C16—C17—C12 | 0.9 (5) |
Experimental details
Crystal data | |
Chemical formula | C18H16N2S2 |
Mr | 324.45 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 293 |
a, b, c (Å) | 17.635 (5), 8.418 (2), 22.591 (5) |
β (°) | 100.50 (3) |
V (Å3) | 3297.5 (14) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 0.32 |
Crystal size (mm) | 0.49 × 0.21 × 0.15 |
Data collection | |
Diffractometer | Stoe STADI4 four-circle diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 5790, 2895, 1664 |
Rint | 0.036 |
(sin θ/λ)max (Å−1) | 0.594 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.042, 0.106, 0.99 |
No. of reflections | 2895 |
No. of parameters | 241 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.19, −0.27 |
Computer programs: DIF4 (Stoe & Cie 1991), DIF4, REDU4 (Stoe & Cie 1991), SHELXS86 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), XP/PC (Siemens 1990), SHELXL97.
S1—C1 | 1.649 (2) | N2—C13 | 1.413 (3) |
S2—C10 | 1.650 (3) | C1—C2 | 1.522 (3) |
N1—C1 | 1.335 (3) | C2—C11 | 1.537 (3) |
N1—C4 | 1.424 (3) | C10—C11 | 1.515 (3) |
N2—C10 | 1.339 (4) | ||
N1—C1—C2 | 107.7 (2) | N2—C10—C11 | 107.6 (2) |
N1—C1—S1 | 126.02 (18) | N2—C10—S2 | 126.3 (2) |
C1—C2—C11 | 112.03 (19) | C10—C11—C2 | 110.7 (2) |
C3—C2—C11—C12 | 49.3 (3) | C3—C2—C11—C10 | −67.0 (3) |
C1—C2—C11—C12 | −68.9 (3) | C1—C2—C11—C10 | 174.8 (2) |
1,1'-Dimethyl-3,3'-biindoline-2,2'-dithione, (I), a central intermediate in our synthetic work (Schroth et al., 2000), was obtained by oxidative dimerization of indolinethione, (II), using iodine in methanolic solution (Hino et al., 1969). The product is, in principle, able to exist in two diastereomeric forms differing in the chirality at the 3- and 3'-positions. Apart from the (3R*,3'R*) configuration, as formulated for (I), the (3R*,3'S*) counterpart must also be considered {the prefix (3R*,3'R*) corresponds to dl, and (3R*,3'S*) [(3R,3'S) = (3S,3'R)] to meso in the older IUPAC recommendations}. According to NMR spectroscopic characterization and thin layer chromatography findings, the product is homogeneous. Furthermore, its regeneration from the dithiolate, (III), by acidification provides evidence that the (3R*,3'R*) configuration is thermodynamically favoured with respect to the steric alternative (3R*,3'S*) (Schroth et al., 2000). An exact assignment, however, was lacking until now. \sch
A comparison with 1,1'-dimethyl-3,3'-bipyrrolidine-2,2'-dithione, (IV), is useful. This compound arose from the corresponding thiopyrrolidone, (V), via deprotonation and treatment with iodine (Tamaru et al., 1978). Here, the assignment of the (3R*,3'R*) configuration was based on chemical transformations and the assumption that a Cope-type rearrangement of the disulfide formed as the major product, (VI), should proceed through a chair-like arrangement.
In order to classify unequivocally the configuration of compound (I), an X-ray analysis has been performed. Its result confirms the presence of the (racemic) (3R*,3'R*) diastereomer.
The molecular structure of (I) and the atomic numbering used are shown in Fig. 1. The molecule has a strongly folded shape and exhibits C2 symmetry to a good approximation; the (non-crystallographic) twofold axis is a perpendicular bisector of the C2—C11 bond. With respect to the bond lengths, the only significant deviation from the C2 symmetry is shown by C4—C5 and C13—C14, which differ by 0.023 (4) Å. The two indoline ring systems are planar to a good approximation, the r.m.s. deviations of the fitted endocyclic atoms being 0.013 (N1,C1—C8) and 0.021 Å (N2,C10—C17), respectively. The maximum deviations of the S and methyl-C atoms from the corresponding least squares planes are not greater than 0.1 Å. The dihedral angle subtended by the two planes is 67.11 (6)° and the antiperiplanar conformation is illustrated by the torsion angle C1—C2—C11—C10 = 174.8 (2)°.
Most of the bond lengths and angles in (I) agree well with expectation but some details are worthy of discussion. The N atoms in (I) are sp2-hybridized (the bond angles sum to 360.0° at N1 and 359.9° at N2). Due to their different environment, the non-equivalent endocyclic N—C bonds have significantly unequal lengths. The standard values for an Nsp2—Csp2 single or double bond are given as 1.40 and 1.29 Å, respectively, in the literature (Rademacher, 1987). Compared with these values, N1—C4 [1.424 (3) Å] and N2—C13 [1.413 (3) Å] are slightly longer than a normal single bond and N1—C1 [1.335 (3) Å] and N2—C10 [1.339 (4) Å] are near the average between a single and a double bond. On checking the 1998 version of the Cambridge Structural Database (Allen & Kennard, 1993), it appeared that no crystal structure containing a bis(thioxindole) moiety is known. Of the corresponding bis(oxindoles), the racemic (Suyama et al., 1994) and meso (Kato et al., 1985) forms of 1,1',3,3'-tetramethylleucoisoindigo are the most suitable for structural comparison. In their crystal structures the above discussed N—C bond lengths also differ, but less remarkably, by about 0.04 Å. The C2—C11 bond linking the molecular halves has a normal length of 1.537 (2) Å in (I), whereas in the bis(oxindoles) it is markedly lengthened (1.581 and 1.575 Å, respectively).