Extended Spoke Adducts

The donor behaviour of the terminal halide atom of numerous spoke -like XY-adducts of chalcogen-donor molecules towards additional XY molecules (XY is mainly I2) has been demonstrated by carrying out the reactions with an excess of XY with respect to the 1 1 molar ratio between the rea-gents.i 20,26.30.37,46.48,49 gtructural features of adducts having a 1 2 

However, the interaction of the terminal iodine atom of the Se-l2 system with the second I2 molecule (d l--I2) = 3.29(7) (mean value) and 3.175(1) A for btseMe 2I2 and (Me2N)3PSe 2I2, respectively) is not strong enough to form a triiodide ion. For these systems a bent-shaped n -l2 sequence, 

According to its structural features, the compound (R2 N)3PSe 2.512 (R2 N = A-morpholyl), having higher di-iodine content, can be described, as in the case of (Me2N)3PSe 2I2, as an ionic compound of the type LE-I I2, 

The local environment of 1 in this system is structurally reminiscent of some lj ions that feature a central 1 interacting with three di-iodine molecules with an approximate local C y symmetry. 

Structurally characterised XY-adducts with a di-halogen content higher than 1 1 are still quite rare and so we cannot generalise about their structural archetypes. Apart from the cases already discussed, the adduct of trip-henylphosphine sulphide (TPPS) with I2 reported by E.A. Mayers and coworkers and having a 2 3 donor/acceptor stoichiometry, is characterised by 

All these extended spoke adducts structurally resemble Z -shaped X82-polyhalides (X = I, Br) in which two asymmetric X3 trihalides are bridged by an X2 molecule.15 This further supports the above-mentioned similarity in the chemical nature of asymmetric X3 trihalides and linear E-X-Y systems in C.-T. adducts. 

More complicated structural situations are observed in di-halogen-rich adducts of chalcogen-donor molecules when further soft-soft halogen halogen interactions intervene, thus generating multidimensional networks (see Chapter 13.1). A clear example is represented by the structure of the compound (dithizone)2 7I2 46 Based on structural data the authors have described the repeating unit in this polymeric compound as a 

A polymeric structure is also observed in the adduct LE 2I2 (LE = 4,5-6z s(bromomethyl)l,3-dithiole-2-thione). LE-I2- I2 bent-shaped units, which are typical in discrete extended spoke adducts with 1 2 (LE/I2) stoichiometry, are joined head-to-tail by soft-soft I- -I interactions involving the terminal iodide atom of the LE-I2 moiety and the terminal iodide atom of the second coordinated di-iodine molecule.37 

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For some strong electron donor molecules the polarization of the X2 molecule may be sufficient that the X atom not complexed to B serves as an electron donor to a second X2 molecule, i.e., the dihalogen is amphoteric , acting as a Lewis acid to Lewis base B, and as a Lewis base to the second X2 molecule, acting as a Lewis acid. For a 1 1 B X2 X2 ratio, an extended adduct (Fig. 1, mode AA) is formed, as illustrated in Fig. 2c for 4,5-bis(bromomethyl)-l,3-dithiole-2-thione-diiodine diiodine (HAMCAA) [58]. This is often referred to as an extended spoke structure. If the second X2 acts as Lewis acid acceptor at either end of the molecule, then a bridged amphoteric adduct (Fig. 1, mode BA) is formed, as illustrated for (acridine I2)2 I2 (QARGIZ) [31] in Fig. 2d. 

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