*3.2.* β*-(BEST)3(HOC2H4SO3)2*·*H2O (2)*

Compound **2** has similar cell parameters to **1**, as shown in Table 1, but has a different space group (*P*21) and is not isomorphous with **1**. There are three BEST molecules, two isethionate anions and one water molecule, in the asymmetric unit. The crystal structure is shown in Figure 2a. The unit cell consists of alternating donor/anion and water layers along the *c* axis. The donor layer (Figure 2b) has a β-type packing motif with both intra-and interstack short S···S contacts, suggesting that the salt is a 2D conductor. The ratio of donor to anion is 3:2, indicating that the BEST molecules form into trimers with two holes. In turn, the holes are arranged into spin dimers, which suggests that the salt is a band insulator. The resistivity measurements (Figure S2) indicate that the salt is, in fact, a semiconductor with higher conductivity than **1**. Salt **2** has a room temperature resistivity that is approximately two orders of magnitude smaller, ρRT of 4.47 Ω·cm, and approximately a half of the activation energy, *E*<sup>a</sup> of 108 meV, when compared to **1**.

**Figure 2.** Crystal structure (**a**); arrangement of the donor layer (**b**), where dashed lines indicate short S···S, Se···S and Se···Se contacts (<3.70, 3.75 and 3.80 Å, respectively); and structure of the anionic layer (**c**) of **2**.

Figure 2c shows the structure of the anionic layer. The present R value of 11.6% is relatively high, so we cannot discuss some details of the structure. Several covalent bonds are distorted; therefore, bond length and angle restraints were applied. The space group, *P*21, is polar, so there are no inversion centres in the anionic layer. However, the anions are arranged to cancel almost all the dipole moments. The only small dipole moment seems to exist along the *b* axis (see Figure 2c). In addition, the X-ray diffraction measurement at 150 K gave a much more distorted structure, so we have not shown these data.
