Self-assembly in organoarsenic compounds

The As S secondary bonds become shorter with halogen removal, i.e. the interactions between the cations are stronger. The As- -S distance is reduced to 

523 A in the monochloro dimer and to 2.442 A in the unsubstituted dithiaarsoli-dinium dimer. Secondary bonds to chlorine atoms of the anions (As- -Cl 3.180-3.772 A and between sulfur and chlorine, S- -Cl 3.437-3.637 A) are also present in the crystal of dithiaarsolidinium tetrachlorogallate, 147. Similar self-assembly occurs in the nitrogen analog, 148, with As- - -N 2.103 A [376]. 

An unexpected self-assembly, 149, with dimer formation, was observed in phen-oxarsine dithiophosphinates, 0(C6H4)2AsS2PR2- With R = Ph [377] and R = Et [378] dimers, 149, are formed by secondary As- - -S bonding (primary As-S 2.315 A, intermolecular secondary As---S 3.402 A, intramolecular secondary As- - -S 3.402 A when R = Ph primary As-S 2.318 A, intermolecular secondary As- - -S 3.440 A and intramolecular secondary As- - -S 3.505 A when R — Et). The methyl derivative (R = Me) is, however, an rmassociated monomer. The secondary As- - - S bonds observed in these compounds are longer (weaker) than the intramolecular secondary bonds measured in a phenylarsenic dithiophosphate, PhAs[S2P(OPr )2]2, investigated earlier (primary As-S 2.310 and 2.317 A intramolecular secondary As---S 3.135 and 3.187 A) [379]. 

In the solid state, the dinuclear anions [Me2As2Cl5] are self-assembled dimeric supermolecules, 150, with As- -Cl secondary bonds (3.126 and 3.365 A) [380]. 

A supramolecular self-assembly, 151, has been reported for dimethylarsenic cyanide, Me2AsCN, which forms chains via As- - -N secondary bonds (As- - -N 3.18 A) [381]. Methylarsenic dicyanide, MeAs(CN)2, is also associated into supramolecular arrays [382]. 

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